Introduction Deep sea equipment is an important guarantee for the exploration, development, and protection of deep-sea resources, including various types of deep-sea submersibles, landers, gliders, and their auxiliary equipment. In the deep-sea equipment above, the transparent protective cover for observation windows, cameras, and lighting must use high-strength transparent materials. The main transparent materials currently used are ultra-thick organic glass (observation window), inorganic glass, and sapphire (single crystal aluminum oxide). Their application in deep-sea equipment is possible due to the superior transparency of yttrium aluminum garnet (YAG) transparent ceramics like single crystals, as well as the characteristics of optical isotropy, high strength, high thermal conductivity, and stable physical and chemical properties. At present, there are many kinds of transparent ceramics, among which YAG transparent ceramics are the most representative. The existing researches on YAG transparent ceramics mainly focus on doping rare-earth elements to achieve optical functional applications, such as laser output, white LED luminescence, etc.. There are a few reports on the research of large-sized and complex shaped YAG transparent ceramics. In this paper, large-sized and complex shaped YAG transparent ceramics were prepared by a solid-state reaction sintering method, which could be applied to deep-sea equipment at 10 000 meter.Methods High-purity powders of α-Al2O3 (99.99% purity, Taimei Co., Japan), Y2O3 (self- synthesized by co-precipitation method) were used as starting materials. The starting powders were weighed according to a stoichiometric ratio of YAG, and then mixed with 0.5% tetraethyl orthosilicate (TEOS, 99.99%) and 0.05% MgO powder (analytical pure) as a sintering additive in ethanol by ball milling for 12 h. The mixtures were dried in an oven at 60 ℃ for 24 h and then sieved through 200-mesh screen. A portion of the dried powder was calcined at 900-1 500 ℃, and another part of the powder was pressed into circular discs with a diameter of 20 mm using a steel mold at 30 MPa, and then subjected to cold isostatic pressing at 200 MPa to obtain ceramic billets. The green body pellets were then sintered in a vacuum furnace with tungsten meshes as the heating elements under 10?3 Pa at 1?200-1 800 ℃ for 2-20 h. After sintering in vacuum, the pellets were further annealed in air at 1 400 ℃ for 10 h. Finally, the both surfaces of the as-prepared ceramic samples were mirror-polished with different grade of diamond slurries.The phase compositions of all the samples were qualitatively identified by a model D/Max-2550V X-ray diffractometer (Rigaku Co., Japan). The optical transmittances of all the samples of ceramics were measured by a model Carry 5000 UV-VIS-NIR spectrometer (Varian Co., USA). The surface microstructures of each group of samples after hot corrosion were characterized by a model JSM-6360 scanning electron microscope (JEOL Co., Japan).Results and discussion YAG transparent ceramics were fabricated by a solid-phase reaction sintering method. The phase evolution and microstructural changes during sintering were investigated. At high temperatures, Al2O3 and Y2O3 react to form intermediate phases YAM and YAP, eventually forming YAG. The effects of sintering temperature and holding time on the optical transmittance of YAG transparent ceramics were investigated. As the sintering temperature increases, the pores in YAG ceramics continue to discharge and the grains continue to grow. The optical transmittance firstly increases and then decreases, reaching its maximum value at 1 780 ℃. As the holding time prolongs, the transmittance of YAG ceramics gradually increases, but after more than 6 h, extending the holding time does not have a significant effect on improving the transmittance. The YAG ceramic dome is formed using a specially designed rubber mold and a stainless steel hemispherical mold by a cold isostatically press. The YAG ceramic dome is then vacuum sintered in the optimized process. The optical window under 127 MPa was analyzed by finite element analysis (i.e., a software named ANSYS). The analysis results show that the maximum compressive stress of the 120 mm diameter dome is 773.7 MPa and the maximum tensile stress is 21.9 MPa, both of which are less than the maximum allowable stress of YAG transparent ceramic material (i.e., approximately 1?000 MPa). The YAG transparent ceramic dome and titanium alloy cylinder are sealed and packaged. The transparent ceramic dome is undamaged without cracking or leakage at 127?MPa for 1.5 h. The high-power deep-sea LED lighting and deep-sea camera, using YAG transparent ceramics as protective covers, are installed on a deep-sea equipment “Canghai” lander. In November 2020, the lander sank to the depths of 10 000 meters in the Mariana Trench (the deepest part of the oceans) for several times, and a large amount of valuable video data were recorded. Conclusions Highly transparent YAG transparent ceramics were fabricated in vacuum via solid-state reaction sintering. Under high temperature conditions, Al2O3 and Y2O3 reacted to form intermediate phases YAM and YAP in sequence, and finally formed YAG at 1?500 ℃. As the sintering temperature increased, the pores in YAG ceramics continued to discharge and the grains continued to grow. The optical transmittance firstly increased and then decreased, reaching its maximum value at 1 780 ℃. As the holding time prolonged, the transmittance of YAG ceramics gradually increased. The transmittance of YAG ceramic samples sintered in vacuum at 1?780 ℃ for 20 h was 84.7% at 1 100 nm and 82.8% at 400 nm, respectively. The YAG transparent ceramic dome was prepared via optimization. It could be applied to high-power LED lighting and camera protection covers in the deep-sea equipment at 10?000?meters.

Introduction This paper prepared a series of Eu3+ doped Gd2Zr2O7 transparent ceramics by a vacuum sintering method, and charaterized the density and optical transmittance of cubic phase Gd2Zr2O7. The effects of doping ions and their concentration on the crystal structure, optical transmittance, fluorescence performance, and scintillation performance were investigated. The results show that the prepared transparent ceramic samples have a good optical transmittance. The spectral characterization reveals that 20% Eu3+ doped Gd2Zr2O7 has optimum fluorescence and irradiated luminescence intensity, and the most intense emission peak is located at 630 nm. The X-ray imaging shows that Gd1.8Eu0.2Zr2O7 transparent ceramics have an X-ray imaging resolution of up to 11 lp·mm-1, having a potential for the imaging applications.Methods High purity gadolinium oxide, europium oxide and zirconia (purity 4N) as raw materials were mixed according to a stoichiometric ratio (Gd3+,Eu3+):Zr4+ of 1:1. The mixed raw materials were ground in anhydrous ethanol and polyethylene glycol (PEG) in a planetary mill with zirconia grinding chamber and ballsat a ball:material mass ratio of 27:1 for 72 h. Afterwards, the ground mixture slurry was washed with anhydrous ethanol after drying. Gd2-xEuxZr2O7 could be obtained via sintering in a box furnace at 1 400 ℃ according to the reaction: Gd2-xEuxZr2O7 The ceramic blank was formed via dry pressing and cold isostatic pressing, sintered in a vacuum tungsten furnace at 1 800 ℃ for 10 h, and then annealed in a box furnace in air at 1 250 ℃for 6 h. Finally, a transparent ceramic with 1 mm thickness was obtained after double-sided polishing.Results and discussion A ceramic powder prepared by ball grinding was sintered in a vacuum tungsten wire furnace to prepare transparent ceramics with a high light transmission rate. The XRD results show that when 20% Eu3+ is added to Gd2Zr2O7, the crystal structure of Gd1.8Eu0.2Zr2O7 appears some superlattice diffraction peaks (331) of pyrochlore. The crystal structure changes from defective fluorite to pyrochlore. The optical transmittance spectra indicate that the optical transmittance decreases from 76% to 65% (@1 500 nm) when the doping content of Eu3+ increases from 0 to 28%. There exist 6 distinct excitation peaks between 230 nm and 500 nm. A wide excitation peak at 267 nm is attributed to the charge transfer band of O2-—Eu3+. In addition, five excitation peaks at 362, 382, 393, 414 nm and 465 nm correspond to 4f?4f energy level transitions 7F0→5D4, 5GJ, 5L6, 5D3 and 5D2 in Eu3+, respectively. Among six excitation peaks, a wide excitation peak at 267 nm has the most intense intensity. All the excitation peak intensities increase when Eu3+ concentration increases from 4% to 20%. When Eu3+ doping amount is 20%, the maximum excitation peak intensity appears. The further characterization of the scintillation properties of ceramic samples indicates that the X-ray irradiation intensity is a maximum value when the doping amount is 20%. At 630 nm, the radiation luminescence intensity value is 1.1×104, and the material has an X-ray absorption capacity equivalent to that of BGO crystals, and also has a good anti-radiation damage ability. Finally, X-ray imaging tests on Gd1.8Eu0.2Zr2O7 ceramic samples of different materials show that the Gd1.8Eu0.2Zr2O7 ceramic sample can clearly distinguish metal and plastic materials, and has a limit spatial resolution of 11 1p·mm-1, indicating that the modified material is a potential X-ray detection material.Conclusions A series of Eu3+ doped transparent ceramics were prepared by a vacuum sintering method. The crystal structure of Gd2-xEuxZr2O7 was gradually changed from defective fluorite to pyrochlore via adjusting the doping amount of Eu3+. At 630 nm, the Gd2-xEuxZr2O7 transparent ceramic sample showed the optimum fluorescence emission and X-ray irradiation intensity, and the material had a limit spatial resolution of 11 1p·mm-1, which could clearly distinguish metal and plastic materials, confirming an application potential of the A2B2O7 series of scintillating transparent ceramics X-ray imaging.

Introduction Lutetium aluminum garnet (Lu3Al5O12, LuAG) is investigated as a classical scintillation material. LuAG can be fabricated in the form of single crystals, optical ceramics, thin films and other morphologies to meet different scintillation application demands. Cerium-doped LuAG (Ce:LuAG) ceramics can be used as a promising candidate for detection material in shashlik calorimeter for high-energy physics (HEP) experiments due to their high density, fast decay, efficient scintillation and relatively low cost. For HEP experiments with a high event rate, however, a slow scintillation component of Ce:LuAG ceramics leads to a pileup effect that needs to be diminished as much as possible. Divalent, optically inactive, Ca2+ co-doping is an effective strategy for suppressing slow scintillation component according to the defect engineering theory. In addition, the coprecipitated nano-powder can be also used as a raw material of ceramics, and improving the consistency of structure, composition and luminescence within the ceramics is also an important strategy to optimize the performance of Ce:LuAG scintillation ceramics. In this paper, Ce,Ca:LuAG ceramics with different Ce concentrations were prepared with the coprecipitation nano-powders. The effect of activator ion concentration on the decay behavior, light yield (LY) at different shaping times and afterglow intensity of Ce,Ca:LuAG ceramics were investigated.Methods A series of Ce,Ca:LuAG ceramics with the chemical composition of (CexCa0.001 5Lu0.998 5-x%)3Al5O12; x=0.1, 0.2, 0.3, 0.4, 0.5 were prepared with coprecipitated nano-powders through vacuum pre-sintering and hot isostatic pressed (HIP) post-treatment. In the process of nano-powder preparation, high purity Lu2O3 (99.995%), CeO2 (99.999%), and CaCO3 powders were dissolved in nitric acid solution, and Al(NO3)3·9H2O (AR) powders were dissolved in deionized water to obtain various nitrate solutions. The metal nitrate solutions were mixed and further diluted to a specific concentration. Ammonium hydrogen carbonate (AHC, 99%) was dissolved in de-ionized water as a precipitant solution, and ammonium sulfate (99%) was added into the precipitant solution as a dispersant. The mixed metal nitrate solution was dripped into the AHC solution under mild agitation at room temperature (RT). The resulting suspension was aged for 1 h and then washed with deionized water and ethanol. The precursor was dried, sieved and calcined to obtain Ce,Ca:LuAG nano-powders with a pure garnet phase. The powders were uniaxially dry-pressed into pellets at 20 MPa and then cold-isostatically pressed at 250 MPa. The green bodies were vacuum pre-sintered at 1 825 ℃ for 10?h, and then HIP in argon atmosphere under 176 MPa at 1 600 ℃ for 3 h. The fabricated Ce,Ca:LuAG ceramics were annealed in air at 1 450 ℃ for 10 h to remove oxygen vacancies introduced via sintering in a reducing atmosphere. The final Ce,Ca:LuAG ceramics were polished on the both sides to the thickness of 1 mm for the coming characterization.Results and discussion The XRD patterns of all the ceramic samples are in reasonable agreement with the standard XRD patterns of LuAG (PDF 73-1368), indicating that the main phase of the prepared ceramics is a LuAG garnet phase. The lattice parameters of Ce,Ca:LuAG ceramics with different Ce concentrations increase gradually with increasing doping concentration. The presence of Ce3+ can act as a sintering aid to accelerate the migration of grain boundaries during sintering of garnet ceramics and increase the average grain size. The absorption spectra of Ce,Ca:LuAG ceramics indicate that the amplitudes of 4f→5d1 transition at 447 nm and the 4f→5d2 transition at 345 nm of Ce3+ increase with the starting content of Ce3+ that evidences its entering into the garnet lattice. An intense absorption below 350 nm is due to the charge transfer absorption band induced by Ce4+ in the garnet lattice. The photoluminescence decay curves of the 5d1→4f emission of Ce3+ (510 nm) under the 452 nm excitation show that the fast component intensity I1 of all Ce,Ca:LuAG ceramics is > 95%. The main decay time shows an acceleration at the Ce concentration of > 0.3% because the formation of closely spaced (Ce4+—Ca2+) pairs can increase with the increase of Ce concentration. The scintillation decay under γ-excitation slightly slows down with increasing Ce concentration, since the proportion of induced Ce4+ decreases with increasing total Ce concentration, and the competition in accelerating scintillation process becomes progressively weaker. The X-ray excited luminescence (XEL) intensity of the Ce,Ca:LuAG ceramics firstly increases and then decreases with the Ce concentration, reaching the maximum value as x=0.3. The integral scintillation efficiency of Ce,Ca:LuAG ceramics is 146%-190% of a BGO crystal. The variation of the LY and the integral scintillation efficiency obtained from XEL have a similar pattern, i.e., increasing and then decreasing with the increase of Ce concentration. The optimum energy resolution (ER) is obtained as x=0.3. The normalized afterglow intensity after continuous X-ray irradiation monotonically decreases with the Ce concentration due to the more intense competition ability of cerium centers for charge carrier capture.Conclusions Ce,Ca:LuAG scintillation ceramics with different Ce concentrations were prepared with coprecipitation nano-powders via vacuum pre-sintering and HIP post-treatment. A small amount of Al-rich secondary phase existed in the ceramics due to the loss of lutetium during coprecipitation and washing, which could be avoided by component design in the future ceramic preparation. At the Ce concentration of 0.3%, Ce,Ca:LuAG ceramic had the optimum XEL integral efficiency, LY and ER. This study demonstrated that Ce doping concentration had a certain effect on the PL decay, scintillation decay and afterglow of Ce,Ca:LuAG ceramics, providing a guidance for the subsequent selection of doping concentration in practical applications.

Introduction The lattice environment of low phonon energy of strontium fluoride can suppress the energy loss caused by non-radiation process of trivalent lanthanide ions, so it is an ideal matrix material for active-ions doping. The fabrication and spectroscopic properties of SrF2 transparent ceramics doped with trivalent lanthanide ions have attracted recent attention. Trivalent lanthanide ions tend to form clusters in SrF2 crystal lattice, and the distance between ions shortens, resulting in energy exchange process and concentration quenching within these clusters, especially for rare-earth ions with a rich energy level structure. The concentration quenching effect leads to a reduction in luminescence intensity, and their applications are limited to a certain extent. Co-doping buffer ions into Pr:SrF2 system is an effective solution to alleviate the concentration quenching of Pr3+ luminescence intensity. In this paper, Y3+ ions were selected as buffer ions. SrF2 raw powder was synthesized by a chemical precipitation method, and Pr3+ and Y3+ ions co-doped SrF2 transparent ceramic were fabricated in vacuum by a hot pressing (HP) sintering technology. The effect of Y3+ doping levels on the microstructure and luminescence spectroscopic characteristics of Pr, Y:SrF2 transparent ceramics was discussed to regulate the luminescence properties of Pr3+ in SrF2 transparent ceramics.Methods SrF2 particles were synthesized by a chemical precipitation method with commercial strontium nitrate and potassium fluoride reagents. Cationic (Sr2+) and anion (F-) solutions were prepared, and then separated via high-speed centrifugation after washing for three times. The as-synthesized SrF2 particles were mixed with commercial available PrF3 and YF3 particles, where the doping level of PrF3 was fixed at 3.0% and the level of YF3 was varied from 0 to 10.0%. Pr, Y:SrF2 transparent ceramics were fabricated in vacuum by a hot pressing sintering technology. The sintering temperature was 1 000 ℃, the pressure was 30 MPa and the holding time was 120 min. After the heat preservation, the ceramic samples were cooled to room temperature in the furnace, and the ceramics were polished on the both sides. The transmittance and absorption spectra of the ceramics were determined by a UV-vis-NIR spectrophotometer, and the phase composition of the transparent ceramics was characterized by a X-ray diffractometer. The polished transparent ceramics were immersed in 6 mol/L hydrochloric acid solution and corroded at room temperature for 10 min. The microstructure of transparent ceramics was measured by a phenom Prox model scanning electron microscope. The luminescence spectra and fluorescence lifetime of transparent ceramics were measured by a fluorescence spectrofluorometer. Results and discussion The diameter of Pr, Y:SrF2 transparent ceramics is16 mm and the thickness is 2 mm. The lattice parameters of ceramics are gradually decreased after Sr2+ are replaced by the smaller Y3+. Pr, Y:SrF2 ceramics co-doped with 0-5.0% YF3 have the similar transmittance, and the maximum transmittance of the ceramics is close to 90%. The ceramic co-doped with 5.0% YF3 has the maximum transmittance, and the transmittance at 400 nm wavelength is 90.1%, which is similar to the theoretical value. The transmittance of Pr, Y:SrF2 ceramics decreases significantly with the increase of YF3 co-doping level to 10.0%.After YF3 co-doping, the absorption capacity of the ceramic is enhanced, and the strongest absorption capacity is located at 443 nm. The ceramic has a dense microstructure without residual pores or impurity phase. The lattice distortion caused by the radius difference between Y3+ and Sr2+ promote the diffusion rate of ions at a high temperature and increase the average grain size of ceramics. 3P2 states of Pr3+ are directly populated from 3H4 ground states under light excitation at 443 nm, and then fall back to adjacent 3P1, 3P0 and 1D2 states through non-radiative relaxation, multiphonon relaxation and cross relaxation processes. These radiation transitions from generated high energy states to lower states forming different spectral bands. Y3+ break the cluster structure of Pr3+ , releasing more luminous centers, and reducing the energy transfer process within Pr3+, thus improving the luminous intensity. Y3+ co-doping causes the change of the local coordination environment around Pr3+, varying the transition probability between different energy states. The increase of the transition probability corresponds to the increase of the emission intensity. The proportion of emission band intensity to the total luminous intensity of Pr, Y:SrF2 ceramics varies with the co-doping levels of YF3, and the proportion of orange luminescence band intensity increases in the spectrum. The co-doping of Y3+ ions leads to serious distortion of SrF2 lattice, decreases the symmetry of matrix structure, and accelerates the energy transition rate. The fluorescence lifetime of Pr3+ energy transition decreases with the increase of Y3+ co-doping level.Conclusions SrF2 particles were synthesized by a chemical precipitation method, and Pr, Y:SrF2 transparent ceramics were fabricated by a hot pressing technique. The ceramic co-doped with 5.0% YF3 had the mximum transmittance, and the transmittance at 400 nm was 90.1%. As the co-doping level of YF3 increased to 10.0%, the transmittance of Pr, Y:SrF2 transparent ceramics decreased, and the average grain size increased to 200.1 μm. The co-doping of Y3+ ions into Pr, Y:SrF2 transparent ceramics improved the absorption capacity of the ceramics. The absorption cross-sections of Pr:SrF2 and Pr, Y:SrF2 transparent ceramics at 443 nm were 3.59×10-21 cm2 and 1.13×10-20 cm2, respectively. The luminescence spectral properties of Pr, Y:SrF2 transparent ceramics were regulated via changing the clusters and local coordination structures of Pr3+ in the matrix, and the luminescence of the ceramics gradually changed from reddish to orange-yellow. The lifetime of 3P0→3H4, 1D2→3H4 and 3P0→3F2 transition decreased from 114.2, 100.4 μs and 91.8 μs to 28.6, 24.4 μs and 30.3 μs, respectively.

Introduction The quality of the powder is crucial for the performance of transparent yttria ceramics. The powder characteristics, such as size, shape, distribution, and agglomeration, directly affect the densification and microstructure of the ceramics. Co-precipitation is a common method for preparing yttria powder, which has the advantages of simplicity, low cost, and scalability. However, it also has some drawbacks, such as uneven particle size distribution, severe agglomeration, and complicated post-treatment processes. The agglomeration of the powder not only reduces its activity, but also impairs its sinterability. Therefore, how to effectively disperse the powder is one of the key techniques for improving the quality of transparent yttria ceramics. Adding dispersant is a common method for dispersing the powder, among which PEG (polyethylene glycol) is a widely used dispersant, which can form strong hydrogen bonds with the surface of hydroxide colloids, and enhance the stability of the colloids by steric hindrance effect. Moreover, since PEG is an organic substance, it can be completely removed during the high-temperature calcination process, without causing any negative impact on the properties of the ceramics. In this study, we use PEG as the dispersant, and investigate its influence on the optical properties of yttria nanocrystalline powder and its sintered ceramics.Methods Zirconium oxynitrate and yttrium nitrate hexahydrate were dissolved in anhydrous ethanol to prepare dilute solutions as reaction materials. Two salt solutions were mixed according to a stoichiometric ratio of (Y0.97Zr0.03)2O3. Ammonium hydroxide was dissolved in 100 mL of anhydrous ethanol as a precipitant, and to investigate the effect of the addition amount of polyethylene glycol 4000 as a dispersant on the dispersion of samples, and PEG4000 with different molar amounts of 0, 0.1% (mole fraction), 0.3%, 0.6%, 0.9% of Y3+ content was added to the precipitant. The precipitant solution was gradually added to the reactants at 4 mL/min under vigorous stirring until the pH reached 9. The resulting precipitates were aged at room temperature for 4 h, then washed for 4 times with deionized water to remove impurity ions, and collected by centrifugation. The washed precipitates were freeze-dried for 10 h to obtain well-dispersed precursor powders. The precursor powders were calcined in a muffle furnace at 400, 600 ℃ and 850 ℃ for 4 h, respectively. The calcined yttria powders were wrapped with tantalum foil and then loaded into a customized graphite mold. The graphite mold was then placed in a vacuum hot-pressing furnace and sintered under vacuum. Another ceramic sample without tantalum foil shielding was also sintered using the same process for comparison. After the hot-pressing step, the Ta foil was removed, and the samples were annealed at 900 ℃ in air for 2 h and then ground and polished for the coming characterizations.Results and discussion The precursor was completely decomposed and crystallized into well-crystallized yttria powder with little organic and impurity residues after calcination at 850 ℃, as revealed by TG-DSC, FT-IR and XRD analyses. The average particle size of the yttria powder prepared with different dispersant concentrations was in the range of 20-30 nm, indicating that the addition of PEG had little effect on the grain growth. However, the addition of PEG changed the agglomeration state of the particles in the powder. At low PEG addition, the surface charge of the powder was unbalanced, and the particles tended to agglomerate due to van der Waals force or electrostatic force, because PEG could not form a uniform and thick adsorption layer on the particle surface. This agglomeration reduced the energy demand for high-temperature sintering of the nanopowder, but also resulted in the microstructural inhomogeneity of the green body. At high PEG addition, the particles were effectively isolated from the surface charge by the uniform and thick adsorption layer formed by PEG on the particle surface, which suppressed the agglomeration effect of the particles, and also provided steric hindrance and solvation effects, preventing the particles from contacting and bonding. Yttria transparent ceramics were obtained after sintering the yttria powder. The use of tantalum foil effectively shielded the carbon contamination. The sintering temperature of 1 500 ℃ was the optimal condition for the sample performance. The density and transmittance of the samples decreased when the PEG addition was low. The transmittance of the samples increased slightly when the PEG addition increased to 0.9% (the content of PEG in ethanol). This was because the addition of a small amount of PEG caused the agglomeration of yttria powder, which affected the forming, sintering and densification processes of the ceramics. With the increase of PEG content, the agglomeration degree of yttria powder decreased, and the ceramic grain size also gradually decreased. When the transmittance of the samples reached the maximum value, the ceramic grain size was 1-2 μm. However, the transmittance of all samples in the visible light range was still not high, which might be related to the oxygen vacancies and carbon impurities in the samples.Conclusions Yttria nanopowders were synthesized by a precipitation method in ethanol solvent with ammonia as a precipitant, PEG as a dispersant, and 3% ZrO2 as a sintering aid. The optimal calcination temperature of the precursor was determined to be 850 ℃ by thermal analysis, at which a high purity cubic yttria phase could be obtained. Y2O3 ceramics were prepared by a vacuum hot-pressing sintering technique at 1 450-1 600 ℃ under 30 MPa, and the samples were wrapped with tantalum foil to prevent carbon contamination. The results showed that 1 500 ℃ was the optimal sintering temperature, at which Y2O3 ceramic had the maximum optical transparency, with a linear transmittance of 48.4% at 1 100 nm wavelength for samples with the thickness of 2 mm. In addition, the PEG addition also affected the microstructure and optical properties of Y2O3 ceramic, and when the PEG content was 0.9%, Y2O3 ceramic had uniform and fine grains, which was beneficial to improving the transparency. This work could provide a reference for the effective preparation of a high-performance Y2O3 ceramic.

Introduction Tremendous efforts are devoted to improving the luminous performance of white light-emitting diodes (WLEDs) as so-called fourth generation lighting source. The most common approach for manufacturing WLEDs is to cover a blue LED chip with a yellow-emitting Y3Al5O12:Ce3+ (YAG:Ce3+) phosphor. This construction has an advantage of high lumen efficiency, but an insufficient red light component in the spectrum leads to a low color rendering index and a high color temperature. An effective solution is to enhance the red emission of YAG:Ce3+ via introducing red-emitting ions such as Pr3+, Eu3+, Sm3+, Mn2+, Mn4+ and Cr3+. In Y3Al5O12 and many other garnet-structured aluminates, Mn2+, Mn4+ and Cr3+ are suffered from serious luminescent thermal quenching, while Pr3+ and Sm3+ are severely limited by concentration quenching. In comparison, the luminescence of Eu3+ in YAG is highly anticipated because it has an ion radius similar to Y3+, which is beneficial to increasing doping concentration and reducing luminescence quenching. In this paper, a series of Ce3+-Eu3+ co-doped YAG transparent ceramics with different Eu concentrations were fabricated, and the effect of Eu3+-doping on the structure, optical transmittance and photoluminescence properties was also evaluated.Methods Ce0.01Y2.99-xEuxAl5O12 (0 < x < 0.75, Eu contents of 0-25%) fluorescent ceramics were synthesized in vacuum via high-temperature solid-state reaction. High purity Y2O3, α-Al2O3, Eu2O3, and CeO2 powders were used as starting materials. Oleic acid and tetraethoxysilane were used as a molding agent and a sintering aid, respectively. These materials were weighed in stoichiometric ratios, and ground in ethanol for 20 h. The obtained slurry was dried, ground and sieved through a 100?mesh sieve. The resulting powders were made into 20 mm discs under uniaxial pressure of 5-10 MPa, annealed in oxygen atmosphere at 800 ℃ for 10 h, and cold isostatically pressed under 200 MPa. Finally, the ceramics was mechanically thinned and polished to the thickness of 1mm for the subsequent structural and performance characterizations.Results and discussion For YAG:Ce3+/Eu3+ ceramics sintered at 1 600 ℃, a garnet phase appears when Eu content is in the range of 0-25%. However, for the ceramics sintered at 1700 ℃, the grains grow as Eu content (x) increases, and eventually become over-sintered and textured when x ≥ 15%. YAG:Ce3+/Eu3+ ceramics show some characteristic absorption/emission bands for both Ce3+ and Eu3+. For Ce3+, the PLE spectrum involves two broad bands at 320-370 nm and 400-520 nm, and the PL spectrum spans from 440 nm to 650 nm and is dominated by yellow-green emission. Unlike a broad band absorption/emission of Ce3+, the PLE/PL spectra of Eu3+ are composed of a series of lines originating form f-f transitions. There are some intense excitation lines around the maximum absorption locating at 395 nm, constructing a quasi-continuous broad band. It is therefore inferred that YAG:Ce3+/Eu3+ ceramics can be efficiently pumped by NUV LED chips. The PL spectrum of Eu3+ ions covers the spectral range from orange to deep red. The orange emission from 5D0→7F1 transition is more intense than the red emission from 5D0→7F2 transition because Eu3+ ions occupy the lattice sites with an inversion symmetry.The PL spectra of YAG:Ce3+/Eu3+ ceramics vary with excitation wavelength due to the difference in absorption spectra of Ce3+ and Eu3+. Therefore, the effect of excitation wavelength on the PL spectra of YAG:Ce3+/Eu3+ ceramics with various Eu3+ concentrations were investigated. Under the excitation at 363 nm and 466 nm, the emission bands/peaks of both Ce3+ and Eu3+ appear. The PL spectrum excited at 442 nm contains almost only a broadband emission of Ce3+ ions, as Eu3+ ions hardly absorb photons near this wavelength. In contrast, the PL spectrum under the excitation at 395 nm is dominated by Eu3+ emission due to a weak absorption of Ce3+. Clearly, the PL spectrum and color coordinates of YAG:Ce3+/Eu3+ exhibit an excitation wavelength dependence, which has a potential application. In addition, under excitation at 442 or 466 nm, the emission of Eu3+ is weak, while that of Ce3+ rapidly decays with increasing Eu content. As a result, the emission intensity of YAG: Ce3+/Eu3+ ceramics decreases rapidly with the increase of Eu content when excited by blue light. However, when excited by UV light (at 363 nm), the attenuation of Ce3+ emission is compensated by the enhancement of Eu3+ emission to some extent, and the overall photoluminescence intensity can be maintained at a high level. Especially, the samples with Eu contents of 1%-3% can emit an intense orange-yellow light, thus increasing the red component of YAG:Ce3+ effectively. This kind of phosphor has a certain practicability in WLEDs. Also, under the excitation at 395 nm, the PL spectrum originates mainly from the f-f transitions of Eu3+, and the quenching concentration of Eu3+ in YAG:Ce3+ ceramics is 9%, which is much higher than that of Pr3+ (i.e., 0.8%) and Sm3+ (i.e., 3%). For rare-earth ions with forbidden f-f transition, a high quenching concentration is crucial for achieving a high fluorescence emission intensity and meeting the inevitable requirements of LED applications.Thermal quenching is a key factor in evaluating luminescent materials. For the YAG:Ce3+/Eu3+ ceramics with Eu contents of 1% and 9%, the profile of PL spectrum does not change with temperature, leading to a good temperature stability of color coordinates. The relative PL intensity at 575 K remains 83% and 61% of that at room temperature (i.e., 300 K) for 1% and 9% Eu3+-doped samples, respectively, indicating that the photoluminescence thermal quenching in YAG:Ce3+/Eu3+ ceramics is rather weak. Furthermore, the relative PL intensity (IT/I0) and the ratio of emission arising from 5D0 to different 7FJ levels, including (5D0→7F1)/(5D0→7F2)、(5D0→7F4)/(5D0→7F2) and (5D0→7F1,2)/(5D0→7F4), vary linearly with temperature. This feature can be used in the field of fluorescent thermometers, which is one of the potential applications of YAG:Ce3+/Eu3+ transparent ceramics.Conclusions In YAG: Ce3+/Eu3+ transparent ceramics, Eu3+ could be excited by UV (at 363 nm), NUV (at 380-405 nm), and blue (at 466 nm) light, emitting a series of lines in the spectra range from 580 nm to 750 nm, i.e., from orange to deep red. These PL lines had an important application in lighting and display. Eu3+ showed the maximum emission when its concentration was between 5% and 9%. The high quenching concentration had a advantage in enhancing the red component in the emission spectrum of YAG:Ce3+. Furthermore, the red component in the spectrum could be regulated via adjusting the doping concentration of Eu3+. However, the increase in Eu content led to a decrease in Ce3+ emission intensity. The similar phenomena appeared in various dual/multi activators co-doped systems. The solution of this common problem was crucial for the application of this kind of luminescent materials in WLEDs. A highlight of YAG:Ce3+/Eu3+ transparent ceramics was that they could absorb NUV light from 380 nm to 405 nm and emit an orange-to-red light with a high quantum efficiency and a high thermal stability. Therefore, they could be used as red-emitting phosphors excited by NUV LED chips. Besides, the ratios of emission arising from 5D0 to different 7FJ levels varied linearly with temperature, having a promising potential application in the field of fluorescent thermometers.

Introduction Transparent aluminum oxynitride (AlON) ceramics have superior optical transparency properties, high strength and hardness, having great potential applications as window, dome, and other optical components requiring high strength combined with optical transparency. Recent efforts are made to fabricate transparent AlON ceramics, i.e., synthesis of high purity AlON powder, optimization of the particle size distribution of AlON powder, and doping with various additives in different amounts. In these works, sintering additives, such as Y2O3, Y2O3/La2O3, Y2O3/La2O3/MgO, etc., are commonly used to accelerate densification process of AlON. However, a long dwelling duration of ≥6 h is generally required to eliminate pores to obtain a high transmittance in pressureless sintering. Although the dwelling duration of pressureless sintering AlON ceramics can be shortened to 2.5 h, it is crucial to carefully match the doping amount of Y2O3 with the particle size of AlON powder. The excessive Y2O3 doping can result in a significant decomposition of AlON into α-Al2O3 and AlN, and the massive formation of α-Al2O3 can lead to particle aggregation/coarsening, and even separation between components during the early stage of sintering, which in turn retards the subsequent densification process. Studies on using CaCO3 as an additive to fabricate AlON ceramics indicate that it should be a promising way to fabricate highly transparent AlON ceramics via suppressing AlON decomposition during heating with sintering additive. In this paper, La2O3 was respectively doped to AlON powders with median particle size (D50) of 1.1 μm and 2.0 μm to prepared transparent AlON ceramics via pressureless sintering (PS) at 1 880 ℃ for 2.5 h. The effect of La2O3 doping amount on the transmittance of AlON ceramics was investigated, and the phase composition, microstructure and densification process of samples during heating were monitored to investigate the fast densification mechanism of AlON. Methods Pure AlON powder and La2O3 powder were used as starting materials. The AlON powders doped with 0-0.50% La2O3 were ground in absolute ethyl alcohol in a grinding mill with Si3N4 balls, and AlON powder with D50 of 1.1 μm and 2.0 μm were obtained, referred to as P1.1 and P2.0, respectively. The ground powders were loaded into a pellet of 13 mm in diameter at 50 MPa and then cold isostatically pressed at 120 MPa to prepare green bodies. The green bodies were pressureless sintered in N2 in a graphite furnace at a heating rate of 20 ℃/min. One group of samples were sintered at 1 880 ℃ for 2.5 h to prepare transparent AlON ceramics, and another group samples were respectively heated to 1 400-1 900 ℃ (no dwelling) to investigate the phase transformation, microstructure evolution and densification process during heating. The transparent AlON ceramics were ground and mirror polished on the both sides to a thickness of 2 mm for the optical transmittance measurement. The polished samples were hot etched at 1 640 ℃ for 40 min to determine their microstructure.Results and discussion After a 2.5 h dwell at 1 880 ℃, all the La2O3 doped AlON ceramics prepared (i.e., P1.1 and P2.0) show higher transmittance than their undoped counterparts, and the transmittances at 3 850 nm enhance from 17% and 0% to 80% and 84%, respectively. Moreover, the doping of La2O3 broadens the transmission wavelength range of the fabricated samples at 2 500-6 000 nm for P1.1 and the optical range at 220-6 000 nm for P2.0. Doping amount of La2O3 has a similar effect on the transmittance of AlON ceramics prepared by P1.1 and P2.0. When the doping amount of La2O3 ≤0.15% in P1.1 and ≤0.40% in P2.0, increasing the doping amount of La2O3 contributes to the enhanced transmittance of the AlON ceramics. Doping 0.15%-0.25% and 0.40% La2O3 into P1.1 and P2.0, respectively, results in the transmittances of AlON ceramics up to 80%-84% at 3 850 nm. The high transmittance of fabricated AlON ceramics is mainly due to thehigh relative density. For the two highly transparent AlON ceramics prepared by P1.1 doped with 0.20% La2O3 and P2.0 doped with 0.40% La2O3, their average grain sizes are 97.3 μm and 60.2 μm, and their Vickers hardness values are (16.00±0.31) GPa and (16.36±0.31) GPa.For the samples prepared by P1.1 doped with 0.20% La2O3 and by P2.0 doped with 0.40% La2O3, although AlON undergoes decomposition and reformation during heating. α-Al2O3 content in the samples does not exceed 25.1%. Moreover, a lower amount of α-Al2O3 appears in the sample P2.0, compared to the sample P1.1 at 1 400-1 700 ℃. Furthermore, compared to the samples without additive doping, the two samples above exhibit lower α-Al2O3 contents at ≤1 600 ℃. It is indicated that the decomposition of AlON during heating is slightly suppressed by doping La2O3 as an additive. No obvious particle aggregation/coarsening or no composition decomposition occur on the fracture surface of the samples due to the reduced α-Al2O3 content in the samples. It is beneficial for the subsequently densification process. Conclusions AlON ceramics with a high infrared transmittance of 80%-84% at 3 850 nm were fabricated via pressureless sintering at 1 880 ℃ for 2.5 h after doping 0.15%-0.25% and 0.40% La2O3 to AlON powders with D50 of 1.1 μm and 2.0 μm, respectively. During heating, La2O3 could suppress the decomposition of AlON and lead to a lower amount of α-Al2O3 being decomposed from AlON. This could reduce the particle aggregation/coarsening and even the separation of composition at the early stage of sintering. The obtained exceptional microstructure was beneficial for the subsequent fast densification. Moreover, La2O3 could provide sufficient sintering kinetics to effectively eliminate pores at the later stage of sintering, thus promoting the densification of AlON.

Introduction Transparent ceramics are polycrystalline inorganic nonmetallic materials that are transparent in the visible range and have some superior characteristics (i.e., high melting point, high strength, high insulation, corrosion resistance, high temperature resistance, and good light transmission). Diamond as a hardest substance in nature has an extremely high thermal conductivity, a wide spectral transmission range and a good chemical stability as an ideal transparent ceramic material in harsh and extreme environments. However, the single crystal of diamond is brittle and easily broken along the cleavage plane (111), greatly restricting the application of single crystal diamond. Nanocrystalline diamond (NPD) is a polymer of small diamond particles in nano-scale, and the grains are directly bonded by diamond to form a compact structure. The macroscopic properties of nanocrystalline diamond show an isotropy, and there is no directional and disfoliating plane due to the disordered accumulation between particles, having the better mechanical properties, compared to single crystal diamond. The nano-polycrystalline diamond can be prepared, but the preparation of transparent nano-polycrystalline diamond and its application in the field of light transmission are rarely reported. Little work on the transparent mechanism and defect formation mechanism of nano-polycrystalline diamond have been done yet. In this paper, a nanocrystalline diamond was prepared by a high-temperature and high-pressure method. In addition, the mechanical properties were also investigated.Methods A high purity graphite powder (99.999 9% in mass fraction) as a precursor was prepressed into a cylindrical shape ( = 2.5 mm, h = 2 mm), wrapped with aluminum oxide, and assembled. The pressure was uniformly increased to 15 GPa for 8?h. Afterwards, a rhenium (Re) heater was used for heating, and the temperature was uniformly increased to 1 800, 2 300 ℃, and 2?600 ℃ at 200 ℃/min. After holding for while, the temperature was slowly reduced to room temperature at 200 ℃/min, and the pressure was slowly relieved to normal pressure after 15 h. The polished sample was determined by an optical microscope. The phase composition of the sample was analyzed by a model Rigaku FR-X X-ray diffractometer (target Mo, wavelength λ = 0.709 3 ?). The Raman spectra were determined by a model Mono Vista CRS+ Raman spectrometer at an excitation wavelength of 532 nm. The surface morphology of the samples was analyzed by a model Hitachi SU-70 scanning electron microscope. The transmission samples were prepared by focusing ion beam (FIB), and the electron diffraction (SAED) and high-resolution transmission electron microscopic images of the samples were characterized by a model JEM-2200FS transmission electron microscope (HRTEM).Results and discussion A high-temperature and high-pressure method is an effective way to prepare high performance transparent ceramics. Graphite as a raw material can be transformed into a transparent nano-polycrystalline diamond at 15 GPa and 2 600 ℃ for 1 min. Also, the temperature gradient of the sample cavity leads to a non-uniform transparency of the sample, and the increase of temperature is conducive to the increase of the transparency of the nanocrystalline diamond. The results of morphology analysis show that the sample has a layer structure of martensitic phase transformation and a uniform particle structure of diffusion phase transformation. The layer thickness and grain size are approximately 100 nm. High pressure and stress cause a partial dislocation within the sample, improving the mechanical strength of the sample. Nano-polycrystalline diamond as the maximum hardness transparent ceramic is expected to be widely used in the field of special optical windows under extreme conditions.Conclusions Graphite powder was selected as a precursor material and pre-pressed into a 2.5 mm diameter and 2 mm high cylinder. The Kawai 6-8 press device (1 000 t) was used to synthesize a nano-polycrystalline diamond at 15 GPa and 1 800-2 600 ℃. The results showed that the partially transparent nano-polycrystalline diamond could be synthesized under the conversion boundary conditions of synthetic transparent diamond, and the complete transformation from graphite to diamond was completed at 15 GPa and 2 600 ℃, and the completely transparent nano-polycrystalline diamond was synthesized. The results by XRD and SEM analysis indicated that the prepared samples were a pure cubic phase, there was no preferred orientation, and the grain size was approximately 100 nm. Nano-polycrystalline diamond as the hardest transparent ceramic could be used to manufacture high-performance transparent ceramics, which have a promising application in military, industrial and other fields.

Introduction Gadolinium oxysulfide (Gd2O2S, GOS) scintillation ceramics as luminescent materials are widely used in nuclear medical imaging and security inspections due to their high light output, low afterglow, and strong X-ray stopping capability. However, the synthesis process of GOS powder is intricately complex, having some challenges for achieving accurate component ratios and controlling the second-phase impurities, thus leading to difficulties in producing high-quality GOS scintillation ceramics. In this paper, GOS precursor powders were firstly synthesized by a hot water bath reduction method with sub-micron and nano-scale gadolinium oxide (Gd2O3) as raw materials, and Gd2O2S:Pr scintillation ceramics were subsequently prepared by a two-step sintering method (i.e., atmospheric pressure pre-sintering and HIP post-treatment). In addition, the microstructure and optical properties of the powder and ceramics were also characterized.Methods Gd2O2S:Pr phosphors were synthesized by a hot water bath reduction method. Gd2O3 (99.999%, Ganzhou Berier New Materials Co., Ltd., China), Pr6O11 (99.995%, Ganzhou Berier New Materials Co., Ltd., China), and concentrated H2SO4 (AR, Shanghai Macklin Biochemical Co., Ltd., China) were used as high-quality raw materials. Gd2O3 and Pr6O11 were firstly weighed according to (Gd1-xPrx)2O2S (x=0.001-0.010), and then mixed with a diluted sulfuric acid solution at a molar ratio of Gd2O3:H2SO4 of 1:1. The suspension was stirred and uniformed by a magnetic stirrer. Also, the suspension was heated in a hot water bath at 90 ℃ for 150 min. After the reaction was completed, the suspension was cooled to room temperature, and then the suspension was filtered, washed/dried, ground and sieved. Finally, the ground powders were calcined in a tube furnace, The resulting mixture was subsequently calcined in a tube furnace under a 20% hydrogen-nitrogen mixture reduction atmosphere at 750 ℃for 6 h to yield the gadolinium oxide sulfide powders.The Gd2O2S:Pr powders obtained were formed in a 15 mm cylindrical mold under uniaxial dry pressing at 10 MPa and then treated by cold isostatic pressing (CIP) at 200 MPa. Subsequently, the green bodies were pre-sintered in a flowing hydrogen-nitrogen atmosphere at 1 350 ℃ for 2 h, and HIP post-treatment was performed in argon at 1 600 ℃ and 180 MPa for 2 h. The sintered ceramics were polished and annealed for coming characterizations.Results and discussion The microstructure of Gd2O2S:Pr powders prepared with submicron-sized particles of Gd2O3 and nano-sized particles of Gd2O3 was examined, respectively. The results reveal differences in particle size and morphology between the two powders. Gd2O2S:Pr powder derived from submicron-sized Gd2O3 powder has irregular columnar particles and sheet-like structures, whereas that prepared with nano-sized Gd2O3 powder is coralloid particles. The fluorescence emission spectrum of the ceramics is consistent with that of the powders although the excitation peak partially obscures by the absorption peak. High-temperature sintering gradually eliminates the differences in particle size and leads to the formation of dense sintered bodies in the ceramics. Consequently, the discrepancy in fluorescence intensity caused by differences in particle size is reduced. Specifically, ceramics prepared with Gd2O3 nano-powder display a low fluorescence intensity, while those produced with Gd2O3 submicron-sized powder exhibites a high fluorescence intensity. To further investigate the factors influencing the optical properties of the ceramics, the doping concentration of Pr3+ was explored. Fluorescence and XEL spectroscopy tests were conducted on Gd2O2S:xPr (x=0.1%, 0.2%, 0.4%, 0.6%, 0.8%, and 1.0%) ceramics prepared using Gd2O3 submicron-sized powder with different Pr3+ doping concentrations. The experimental results reveal that the fluorescence and XEL spectral intensities reach their maximum values when the Pr3+ doping concentration is 0.2%. The SEM images demonstrate that the ceramics prepared using submicron-sized Gd2O3 powder have a dense structure with well-defined grain boundaries and uniform grain sizes. The EDS element distribution results confirm the effective doping of Pr element into the host lattice. Gd2O2S:Pr ceramic prepared exhibits a high transmittance.Conclusions The utilization of Gd2O3 submicron-sized powder resulted in Gd2O2S:Pr powder and the scintillation ceramics displayed a higher fluorescence intensity, compared to that synthesized with Gd2O3 nano-sized powder. Furthermore, Gd2O2S scintillation ceramics pre-sintered in a weak reducing atmosphere and subsequently sintered in a hot isostatic pressing exhibited improved density and optical properties. The maximum fluorescence intensity of the scintillation ceramics was achieved at a Pr3+ doping concentration x of 0.2%. In addition, the scintillation ceramics achieved a transmittance of 31% at a wavelength of 513 nm. This study developed a simple and cost-effective ceramic preparation process without any byproduct pollution, thereby offering a promising potential for large-scale production of high-performance Gd2O2S scintillation ceramics.

Introduction Laser-driven white lighting has received much attention due to its high brightness, high efficiency, and compact size. Blue laser active inorganic color converter technology is a commonly used method to produce white laser lighting due to its cost-effectiveness and simple structure. However, the existing laser lighting endures a low color rendering index (CRI) because of insufficient cyan-green and red light components in the spectra of existing color converters. Therefore, there is a demand for the development of color converters with intense cyan-green and red emission to improve the optical performance of laser lighting.Methods (Ca0.98-xNaxCe0.02)3Sc2Si3O12 (x=0, 0.02, 0.04, 0.06, 0.08) (CSS) phosphors were synthesized by a solid-state reaction method. The prepared CSS phosphors were then blended with commercial CaAlSiN3: Eu2+ (CASN) phosphors at different weigh ratios (abbreviated as R/G), which were chosen to be 1/8, 1/12 and 1/20. An inorganic binder used was commercially available silica colloidal particles, while an organic binder was polyvinyl pyrrolidone (PVP). Firstly, phosphors, PVP and silica colloidal particles were prepared by thoroughly mixing at a mass ratio of 150:50:1. The resulting slurries were coated on the sapphire substrates via blade coating. Finally, the dried samples were sintered in a muffle furnace at 600 ℃ for 20 min to remove the organic residues.Results and discussion The CSS host has a cubic Garnet-type structure with the Iaˉ3d space group. Na+ ions are introduced into the CSS host to function as a charge compensator. This enables the elimination of the impurity phases of phosphors. Under the excitation of blue light at 450 nm, the CSS phosphors emit a broadband cyan-green light with a peak at 507 nm. As Na+ content increases, the emission intensity of CSS phosphors gradually increases due to the enhanced absorption efficiency (AE) and internal quantum efficiency (IQE). As x = 0.06, the emission intensity reaches its maximum, which is corresponding to internal/ external quantum efficiencies (IQE/EQE) of 86% and 55%, respectively.Dense CSS films were produced using colloidal silica. Clearly, the film is composed of tightly packed phosphors and the gaps between phosphor particles are also filled with SiO2. The film is bonded to the substrate tightly, which can be ascribed to the adhesion function of the colloidal SiO2. The surface of the film is sufficiently smooth without polishing, and its thickness is 135 μm. The SiO2 colloidal particles serve as an effective inorganic binder, a filler of gaps and a protective coating.The excitation and emission spectra, decay curves and quantum efficiency of the CSS film are similar to those of the related CSS phosphors. The unimpaired luminescence properties of the film indicate that sintering does not impair the intactness of CSS phosphors. The CSS film displays a thermal quenching at evaluated temperature, and the decrease of the integrated emission intensity of CSS film aligns with that of CSS phosphors, retaining 86% of its initial value at 200 ℃. When the incident laser power increases from 0.3 W to 5.30 W, the emission intensity of the film increases monotonously. However, when the laser power further increases to 6.20 W, the emission intensity of the film decreases sharply and a luminous saturation appears. The maximum luminous flux of 717?lm is achieved at an incident laser power of 5.30 W, and the corresponding luminous efficacy is 135 lm/W.Although the CSS film exhibits a high thermal stability and a high luminous efficacy, the CRI of this phosphor-converted white light is 57 due to the deficiency of red light components. To improve the CRI, a composite film comprising CASN and CSS phosphors was fabricated. When the mass ratio of CSS and CASN phosphors is 1/12, the spectral analysis of the composite film reveals a uniform distribution of cyan-green, yellow and red light components, with a half-height width of 176 nm. Also, when excited by 5.3 W blue laser, the composite film emits a white light with a high luminous flux of 647 lm, a luminous efficacy of 122?lm/W and a CRI of 88.Conclusions Highly efficient cyan-green CSS phosphors with an IQE/EQE of up to 86%/55% were prepared via introducing charge compensatory additives Na+. A composite phosphor film was developed on thermally conductive sapphire substrates using silica colloidal particles as an inorganic binder via combining the prepared cyan-green CSS and commercial red CaAlSiN3:Eu2+ phosphors. The phosphor film exhibited a superior thermal stability and it maintained 84% of its room-temperature luminescence intensity even at 200 ℃. The spectral analysis of the film showed an even distribution of cyan-green, yellow and red light components, with a half-height width of 176 nm. Under 5.3 W blue laser excitation, the composite film emitted a white light with a high luminous flux of 647 lm, a luminous efficacy of 122 lm/W and a CRI of 88. These results indicated that the composite phosphor film could have a promising potential for application in high-brightness laser lighting with superior color rendering.

Introduction In non-destructive testing of large devices, industrial computed tomography (CT) with high-energy radiation sources requires scintillators with a high performance of light yield, short decay time, high effective atomic number, and strong radiation resistance to achieve high-resolution imaging. Eu3+ doped Gd2O3-Lu2O3 solid solution ceramics have a high density and an effective atomic number, having high X-ray stopping power and radiation resistance. The ceramics have a high light yield that is beneficial to improving imaging quality. Moreover, Eu3+ emits a light in the red region, which matches well with the spectral sensitivity of silicon photodiodes. EuxLu1.4—xGd0.6O3 transparent ceramics are one of scintillators with great application prospects in the field of X-ray imaging. A recentl study managed to prepare commercial products of EuxLu1.4—xGd0.6O3 transparent ceramics with a high transparency and a high light output. However, such a research is still at the initial stage, and the optical quality of ceramics needs to be improved to meet practical application needs. There is also a lack of research on component design. Especially, the effect of Eu3+ concentration on the structure and spectral properties of EuxLu1.4-xGd0.6O3 ceramics is still unclear. In this paper, EuxLu1.4-xGd0.6O3 (x=0, 0.02, 0.06, 0.10, 0.14, 0.18) ceramics with different Eu3+concentrations were prepared via coprecipitation and subsequent vacuum sintering/hot isostatic pressing (HIP), and their microstructure and optical properties were analyzed.Methods In the synthesis of nano-powder, the obtained precursor was calcined in air at 1 100 ℃ for 4 h to obtain EuxLu1.4-xGd0.6O3 (x=0, 0.02, 0.06, 0.10, 0.14, 0.18) powders. The synthesized powders were molded in a 18 mm mold at 40 MPa, and then the ceramic green bodies were obtained by cold isostatic pressing at 250 MPa, pre-sintered in vacuum at 1 600 ℃ for 2 h and then sintered in argon atmosphere under 176 MPa at 1 750 ℃ for 2 h. The fabricated EuxLu1.4-xGd0.6O3 ceramics were annealed in air at 1 100 ℃ for 20 h to remove oxygen vacancies introduced by sintering in a reducing atmosphere.The microstructure of the sample was characterized by a model SU9000 field emission scanning electron microscope (Hitachi Co., Japan). The XRD pattern was detected by a model D8/DISCOVER DAVINCIX X-ray diffractometer (Brooke Co., Germany). The photoluminescence spectra (PL) and photoluminescence excited spectra (PLE) of ceramics were tested by a model FLS-980 fluorescence spectrometer (Edinburgh Instruments Ltd., UK). The performance of scintillating ceramics was characterized by an X-ray excitation emission spectrometer. The emission signals of the test samples were analyzed by a model QE65000 spectrometer (Ocean Optics Co., USA) excited by an X-ray tube operating at a voltage of 70 kV and a current of 1.5 mA. The transmittance curve was tested by a model Cary-5000 ultraviolet visible near-infrared spectrophotometer (Varian Co., USA).Results and discussion The EuxLu1.4-xGd0.6O3 (x=0, 0.02, 0.06, 0.10, 0.14, 0.18) powders are all cubic crystalline, and the diffraction peak position shifts towards a lower angle as Eu3+ doping concentration increases because the ion radius of Eu3+(i.e., 0.950 ?) or Gd3+(i.e., 0.938 ?) is larger than that of Lu3+(i.e., 0.848 ?). According to Scherrer’s formula, the average grain size of the powders gradually decreases from 63.7 nm to 55.0 nm as the concentration of Eu3+increases. The EuxLu1.4-xGd0.6O3 ceramics have uniform grain sizes, no obvious pores occur, and there is no second phase. The grain size measured by a linear intercept method decreases from 1.0 μm to 0.9 μm as the Eu3+doping concentration increases. This is because the lattice distortion caused by Eu3+doping suppresses the growth of ceramic grains in sintering. Compared to the prepared ceramics, the grain sizes of the ceramics after HIP increase. The average grain size of the ceramics after HIP decreases from 76.5 μm to 30.1 μm as the Eu3+doping concentration increases from 0 to 9%.The in-line transmittance of ceramics firstly increases and then decreases with the increase of Eu3+concentration. When x=0.1, the in-line transmittance of ceramics reaches a maximum of 71.4% at 611 nm. When the concentration of Eu3+is high, the lattice distortion inside the ceramic intensifies, leading to a decrease in the in-line transmittance. In the XEL patterns of EuxLu1.4-xGd0.6O3 ceramics, the emission spectrum is composed of Eu3+characteristic emission peaks. The main emission peak is located at 611 nm, which is an intense red light emitted from the 5D0→7F2 transition. When the Eu3+concentration is low, the emission intensity of the ceramic gradually increases with the increase of the number of emission centers. When the concentration of Eu3+exceeds 3%, the distance between the emission centers decreases. Also, the energy transfer between Eu3+ increases and the transition probability between Eu3+and defects increases due to the high energy of the excitation source, resulting in a decrease in scintillation luminescence intensity. The ceramic exhibits an intense red light emission matching well with silicon photodiodes.Conclusions The maximum in-line transmittance of the ceramic could be obtained at the Eu3+doping concentration of 5%, reaching 71.4% at 611 nm. The photoluminescence and photoluminescence excited spectra of EuxLu1.4-xGd0.6O3 ceramics demonstrated that increasing the concentration of Eu3+ could enhance the absorption and emission peak intensities of the ceramics. At the concentration of Eu3+ of 3%, the ceramic exhibited the most intense red light emission under X-ray excitation with the main emission peak located at 611 nm, corresponding to the 5D0→F2 transition of Eu3+. High transmittance and high red light emission intensity were of great significance for achieving a high-resolution X-ray imaging.

Introduction The defect pyrochlore ABB’O6 with a cubic crystal structure is widely used in photocatalysis, ion exchange, microwave dielectric ceramics and ion conductors. Recently, KNbTeO6 transparent ceramics with a high density and no secondary phase were prepared by a pseudo-hot isostatic pressing (PHIP) technology. The in-line transmittance of over 80% was obtained in this material, demonstrating potential applications in infrared imaging, windows, and optical lenses, etc.. The optical properties of transparent ceramics are closely related to their composition and structure. ANbTeO6 (A=K, Rb) compounds have a defect pyrochlore structure, and the alkali metal cations affect the crystal structure and optical bandgap of ANbTeO6. It is thus important to elucidate the impact of A-site ions on the structure and optical properties of the ANbTeO6 defect pyrochlore transparent ceramics. This paper was to investigate the composition, structure and optical properties of ANbTeO6 (A=K, Rb) transparent ceramics.Methods KNbTeO6 and RbNbTeO6 (abbreviated as KNT, RNT) powders were synthesized via solid-state reaction in air with K2CO3 (99.99%), Rb2CO3 (99.99%), Nb2O5 (99.99%), and TeO2 (99.99%) as raw materials in a ratio of 1:1:2. The synthesized and ball-milled powders were pressed into disks with 20 mm diameter and 3 mm thickness in a uniaxial pressure at 5 MPa, and then cold-isostatically pressed at 200 MPa for 5 min. The green bodies were sintered in air to obtain the pre-sintered bodies, and then the pre-sintered bodies were placed in a model HP D60/0 SPS apparatus (FCT Co., Germany) for PHIP processing. The obtained ceramic samples were ground and finally polished on the both sides.The phase composition of KNT and RNT powders and ceramics was examined by a model PANalytical X-ray diffractometer with Cu Kα radiation (XRD). The microstructure of powders and ceramics was determined by a model FEG250 field emission scanning electron microscope (FEI Quanta). The refractive indices of the ceramics in a wavelength range of 193?1 690 nm were measured by a model M-2 000 V elliptical polarization spectrometer. The in-line transmittance spectra were collected from the ceramics polished on the both sides with the thickness of 1 mm. The Raman vibration spectra of transparent ceramics were measured by a model LabRAM laser confocal microscopy Raman spectrometer. All the tests were conducted at room temperature.Results and discussion The XRD patterns of both powders exactly correspond to KNT and RNT with a cubic defect pyrochlore structure, respectively. The Rietveld method was used to refine the XRD patterns of the powders to obtain the information of crystal structure. Clearly, the lattice constant increases from 10.247 9 ? (KNT) to 10.258 0 ? (RNT) and the cell volume expands from 1 076.24 ?3 (KNT) to 1 079.41 ?3 (RNT) with little change in the length of (Nb/Te)-O bond. The composition and element distribution of the PHIP-ed ceramics with a few residual pores and an irregular grain boundary are not affected by sintering, according to the EDS spectra. The relative density of the both ceramics is 99.78% (KNT) and 99.55% (RNT), respectively. The grain size distribution is narrow, and the average grain size is 417 nm (KNT) and 378 nm (RNT), respectively. The optical transmission range of two transparent ceramics is 0.5-7.2 μm. The maximum in-line transmittances reach 80.25% (KNT, @ 2 677 nm) and 80.14% (RNT, @ 3 876 nm), which are similar to their theoretical values. The optical properties of KNT and RNT transparent ceramics were further discussed through valence electron band transitions, molecular electron polarizability, and lattice phonon vibrations. Compared with KNT, RNT transparent ceramic exhibits a slight red-shift in the absorption edge of visible light because of the reduction of the band gap from 2.23 eV (KNT) to 2.20 eV (RNT). The optical refractive index at 589.3 nm increases from 1.973 (KNT) to 2.013 (RNT) obtained from the fitted Sellmeier equation. The electron polarization of Rb+ is greater than that of K+, increasing the total molecular electron polarization, and K+ has a smaller radius than Rb+, expanding the lattice and increased the cell molar volume. The Abbé number of RNT (i.e., 18.86) is slightly lower than that of KNT (i.e., 19.45), indicating that the dispersion of RNT transparent ceramic is greater than that of KNT. Based on the Raman spectroscopy analysis, the high-frequency vibration in RNT shifts towards a low wavenumber, i.e., weakening in (Nb/Te)O6 octahedral vibration, which broadens the infrared cutoff wavelength of RNT by 0.3 μm.Conclusions Pure defect pyrochlores ANbTeO6 (A=K, Rb) ceramic powders were synthesized via solid-state reaction, and the transparent ceramics with the maximum in-line transmittance of 80% both were fabricated via pressure-less sintering and pseudo-hot isostatic pressing sintering. Based on the crystal structure analysis, the increase in the radius of A-site cations led to the cell expansion with a little change in the length of (Nb/Te)-O bond. Compared with KNT, RNT transparent ceramics had a red-shift in a visible light absorption edge, an increase in refractive index with a slightly higher dispersion, and a wider infrared cutoff wavelength. Defective pyrochlore ANbTeO6 (A=K, Rb) transparent ceramics had a potential application in miniaturization of mid-infrared lenses and devices with a good optical transparency and a high refractive index in the infrared area.

Introduction g-aluminum oxynitride (g-AlON) is widely used for optical lens, transparent arms, and infrared windows due to its high transparency from the ultraviolet to mid-infrared range. To obtain a high-performance AlON transparent ceramic, each stage of its manufacture process should be controlled with cares, i.e., powder synthesis, forming, sintering and annealing. Sintering aid (i.e., Y2O3, CaCO3, Y2O3-La2O3, etc.) is also used to improve the property of AlON transparent ceramic. However, the sintering temperature is still rather high, resulting in a high production cost. Recently, SiO2 is used in AlON system, leading to a decreased temperature of hot isostatic pressing (HIP) (i.e., 1 810 ℃). Unfortunately, however, the evaporation of SiO decomposed from SiO2 could increase the micro-pores in pre-sintered bodies, leading to the density variations and then optical inhomogeneity after HIP. It is thus necessary to explore new Si-contained additive that is stable at a high temperature for the preparation of high-performance AlON transparent ceramics. Mullite (3Al2O3·2SiO2) as a main phase of AlON has a superior high-temperature thermal stability, which is a promising application in AlON system. In this paper, AlON transparent ceramics were fabricated via pressureless sintering and hot isostatic pressing. In addition, the effect of mullite content on the microstructure of pre-sintered and hot isostatic pressed AlON ceramics and their corresponding optical property was also investigated. Materials and method An γ-AlON powder with 0.02%-0.20% mullite in alcohol was ground by a planetary mill with high-purity alumina balls. After drying, sieving and then calcining, the mixed powder was pressed into pellets and then cold isostatic pressed to obtain AlON green bodies. The green bodies were pressureless-sintered in a flowing N2 atmosphere at 1 880 ℃ for 4 h, and further hot isostatic pressed in a flowing Ar atmosphere at 1 800 ℃ and 200 MPa for 6 h to obtain AlON transparent ceramics.The phase analysis of specimens was identified by a model D8 Advance X-ray diffractometer (XRD, Bruker Co., Germany) using nickel-filtered Cu Kα radiation (λ =1.540 6 ?). The bulk density of AlON ceramics was determined by a water immersion method based on Archimedes’ principle. The chemically-etched and fracture surfaces of AlON ceramics were characterized by a model TM3000 scanning electron microscope (SEM, Hitachi Co., Japan). The average grain size was determined by a linear intercept line method with a correction factor of 1.56 via a software named Nano Measurer. The Raman spectra were recorded by a model XploRA One-532 Raman spectrometer (Horiba Co., Japan) using a 532 nm Ar+ laser. The optical in-line transmittance spectra of HIPed AlON ceramics were measured by a model V-770 UV-Vis-NIR spectrophotometer (JASCO Co., Japan) and a model FT/IR-4600 Fourier transform infrared spectrometer (FTIR, JASCO Co., Japan). Results and discussion The ground AlON powder mixed with mullite has a homogeneous distribution and the particle sizes are in the range of 0.2-1.6 μm with an average value of 0.7 μm. After pressureless sintering, all the XRD patterns of AlON ceramics with 0.02%-0.20% mullite match well with the standard cubic AlON phase (JCPDS 48-0686), and no extra peaks appear, indicating the dissolution of mullite in AlON lattice. The Raman spectraindicate the effect of the mullite content on the structure of the pre-sintered AlON ceramics. The phonon modes located at 316, 372, 628, 764 cm-1 and 920 cm-1 appear in all the samples, which are corresponded to the 3T2g, Eg and A1g modes of spinel. No shift of the Raman bands occurs and new bands do not appear, indicating the same structure and phases in AlON with different mullite contents.All the pre-sintered AlON ceramics have a milk-white color and are opaque. The microstructure of fracture surfaces indicates that pores easily appear in all the samples. Large pores appear and density decreases in AlON ceramics with 0.10%-0.20% mullite. The grain size increases steadily with increasing mullite content. For the low content and high-temperature stability of mullite, a reaction of solid-state sintering is proposed as 3Al2O3·2SiO2+8/3AlAl→2SiAl+VAl+13/3Al2O3 Furthermore, for an oxygen-rich AlON phase in the formation of AlON, the covalent bond strength of Al-N is higher than that of Al-O, and the diffusivity of oxygen is faster than that of nitrogen. AlON with a higher Al2O3 content is easy to sintering, leading to the grain growth.Al2O3 content increases and grains grow with increasing mullite content. The grains grow fast at a high mullite content, resulting in some trapped pores within the grains. All the hot isostatic pressed AlON ceramics have a density of > 99.2% of the theoretical value and a high transparency. The density, grain size and transmittance of hot isostatic pressed AlON ceramic firstly increase and then decrease with increasing mullite content. AlON transparent ceramic doped with 0.05% mullite exhibits the maximum transmittance of 81.9% (4 mm thickness) at 2 000 nm, with a density of 3.67 g/cm3 and an average grain size of 80.4 μm. Some pores in the grains appear in pre-sintered bodies at a higher mullite content, inhibiting the grain growth during the HIP process and desceasing the grain size. Also, some pores in the grains cannot eliminate and remain after HIP, degrading the transmittance. Conclusions AlON transparent ceramics were fabricated via pressureless sintering at 1 880 ℃ and hot isostatic pressing (HIP) at 1?800 ℃, respectively, with AlON powder and mullite., The same structure of AlON with different mullite contents of 0.02%-0.20% could be obtained. AlON transparent ceramic with 0.05% mullite exhibited the maximum transmittance of 81.9% (4 mm thickness) at 2 000 nm. At a mullite content of > 0.10%, some pores in grains appeared in pre-sintered bodies, inhibiting the grain growth in the HIP process.

Introduction Conventional transparent materials mainly include glass and polymers, and the materials both are widely used. However, they have shortcomings such as low mechanical strength, strong infrared spectral absorption, and unstable chemical properties, which restrict their applications. The degradation of the service environment for materials and the increasing strength requirements have generated an urgent demand for the development of transparent materials with the superior performance. The preparation of ceramic materials with superior optical properties becomes an important aspect in the development of inorganic materials, and the structure and function of the integration of transparent ceramics is a hot spot in materials research. The existing transparent ceramics with various functions can be prepared through the continuous optimization of the ceramic preparation process and the exploration of different ceramic systems. However, enhancing their optical properties is a challenge. A2B2O7 pyrochlore ceramics have excellent properties that can break through the limitations of conventional material design and broaden the application prospects. The preparation of non-stoichiometric ceramics has attracted recent attention, but research in this aspect is still in its nascent stage. In this paper, La1-xErZr2O7-3x/2 transparent ceramics were synthesized via combustion sintering with subsequent combustion. The impact of La content on the phase composition, morphology, and transmittance of ceramics was investigated, and the optical properties of the ceramics under non-stoichiometric conditions were analyzed.Materials and method La(NO3)3·6H2O, Er(NO3)3·5H2O, Zr(NO3)3·5H2O and glycine were used. The ceramic powder was prepared by a combustion method, and the powder was subsequently shaped into blocks through ball milling, freeze drying and cold isostatic pressing. After the samples were pre-sintered, they were sintered in vacuum (<10?3 Pa) at 1 825 ℃ for 6 h , and then subjected to subsequent treatment to obtain transparent ceramics.The phase compositions of powders and ceramics were analyzed by a model X′Pert Pro X-ray diffractometer (XRD) at Cu Kα rays λ of 0.154 426 nm with a scanning step size of 0.03°. The structure and chemical composition of the sample were determined by a model InVia Raman spectrometer. The hot corrosion surface and cross-sectional morphology of ceramics were examined by a model Ultra-55 scanning electron microscope (SEM). The 200 particle sizes of their thermally eroded surfaces were labeled and statistically analyzed by a software named Nano Measurer. The ceramic transmittance was tested in the wavelength range from 200 nm to 2 500 nm by a model Solidspec-3700 solid ultraviolet absorption spectrophotometer. The fluorescence spectra of La1-xErZr2O7-3x/2 transparent ceramics were characterized by a model F-4500 fluorescence spectrophotometer. The linear transmittance of La1-xErZr2O7-3x/2 transparent ceramics in the near-mid infrared band was measured by a model Spectrum Fourier transform infrared spectrometer (FT-IR). The up-conversion emission spectra of ceramics were determined by a model FLS980 steady-state transient fluorescence spectrometer.Results and discussion For the prepared non-stoichiometric La1-xErZr2O7-3x/2 transparent ceramics, changes in the crystal structure occur at different La contents in the A-site. For instance, the coexistence of the dual phase in the ceramic gradually transitions to single-phase when La content in the A-site decreases. The lattice distortions in the internal structure of ceramics appear as the nonstoichiometry in the A-site increases.La1-xErZr2O7-3x/2 transparent ceramics exhibit a good and stable transmittance in the infrared range of 2.5-7.0 μm. In the wavelength range of 200-1 400 nm, La0.8ErZr2O6.7 transparent ceramic has the maximum transparency with a transmittance of approximately 68.8% (1.0 mm in thickness). The main reason for the high transmittance of the ceramic is due to the presence of a few pores and impurities in the grain boundaries, resulting in a lower light scattering. The appropriate reduction of La content effectively eliminates the pores in the grain boundaries of the ceramic and alters the transmittance of the material. Under laser excitation at 980 nm, La0.8ErZr2O6.7 transparent ceramic exhibits three emission peaks, including two main emission peaks centered at 446 nm and 561 nm, which originate from the 2H11/2→4I15/2 and 4S3/2→4I15/2 transitions of Er3+, respectively. Besides, theintense red emission located at 684 nm originates from the 4F9/2→4I15/2 leap. The fluorescence emission intensity gradually weakens as La content decreases. The possible reason is that the decrease in La content leads to the lattice distortion and vacancies in the crystal structure, resulting in a decrease in the cell parameters and the distance between Er ions.Conclusions Non-stoichiometric La1-xErZr2O7-3x/2 transparent ceramics with the micron-sized particles were synthesized via combustion sintering and subsequent combustion. The results showed that the La content could have an impact on the ultimate phase composition, morphology and transmittance of the ceramics. The transmittance of ceramics in the visible light range firstly increased and then decreased as the La content decreased, and the moderate decrease of La content could favor the improvement of the optical quality. The ceramic with x of 0.2 had a superior transmittance in the visible to mid-infrared region. As the La content decreased, there existed a gradual transition from a coexistence of pyrochlore phase/defective fluorite phase to predominantly defective fluorite phase in the ceramic, and the average grain size was 96.6 μm at x of 0.5. The ceramic with the thickness of 1.0 mm and x of 0.1 in the infrared range of 2.5-7.0 μm exhibited a stable transmittance level (i.e., 74.8%).

Introduction The development of infrared imaging detection technology for new generation of ultra-high-speed vehicles has put forward harsh requirements for infrared windows. For ultra-low altitude (e.g., sea level) to perform ultra-high-speed flight mission, the surface of its infrared window is usually accompanied by ultra-high density heat flow. In the high-speed flight environment, the force and thermal environment introduced by the aerodynamic effect on the surface of infrared window materials is complicated, and the related engineering experiment testing costs are extremely high. Finite element simulation can predict the phenomena of window warming and thermal stress failure caused by aerodynamic effect to a certain extent, which can provide important information in engineering or applications. In this paper, two conventional commonly used infrared window materials (i.e., hexagonal single-crystal sapphire and cubic ZnS) with a low thermal conductivity were selected to compare with cubic diamond infrared window materials with a high thermal conductivity. The numerical simulations of the window temperatures, thermal stresses and their distributions were carried out under the action of ultra-high-density heat flow, and the results were compared with and without the cooling medium. This work could provide a theoretical reference for the design, selection and application of infrared windows of the new-generation ultra-high-speed vehicles.Materials and method In this work, numerical simulations (Finite Element Analysis, FEA) of stagnation temperatures and thermal stresses were carried out on some infrared materials (i.e., sapphire, ZnS and diamond) with a size of 50 mm×2 mm for window application when facing ultra-high heat flux (3-8 MW/m2) for 5 s at an angle range of 10°-90° with and without constant temperature circulating coolant. The service performance of the infrared materials as well as the property requirements of windows in environments with ultra-high heat flux, were analyzed. A contrast of the infrared materials between sapphire or ZnS and diamond was investigated.Results and discussion The results show that the conventional infrared materials cannot survive as a result of high temperature or thermal stress due to the poor thermal and mechanical properties. For sapphire window, the stagnation temperature is 378 ℃, a value that already exceeds its limiting operating temperature for infrared imaging (about 350 ℃) with a small angle (10°) under 3 MW/m2 for 5 s, possibly resulting in a failure to image properly. For ZnS window, the temperature reaches 600 ℃, a temperature at which the material oxidizes and fails to image applications. Their stagnation temperatures both continue to increase as the angle and heat flux increase. When the heat flux continues to increase (i.e., 5 MW/m2 for 5 s), the stagnation temperatures of sapphire and ZnS reach 500 ℃ and 1 000 ℃ or more, respectively, even with a small angle (10°). At the maximum heat flux (i.e., 8 WM/m2), the stagnation temperatures can even exceed the melting point of the material. Meanwhile, in terms of thermal stress, only sapphire can keep the structure from failing under 3 MW/m2 at 10°. With the addition of coolant, conventional infrared materials (i.e., sapphire and ZnS) may fare worse. The stagnation temperature almost does not change, and the thermal stresses rather increases. For diamond as the infrared window, a failure risk sharply decreases due to the ultrahigh thermal conductivity, low thermal stress and more even stress distribution. Although diamond is equally difficult to apply under large angle working conditions (i.e., 45°-90°) without coolant (stagnation temperature reaches 750 ℃,which is above the oxidation temperature of diamond.), the situation improves dramatically with the addition of coolant. Diamond for benefiting from the cooling system can even survive under a high thermal flux of 8 MW/m2 at ≤45°. This study can provide a reference for the design, material selection, and application of infrared windows for a new generation of hypersonic aircraft.Conclusions Conventional infrared materials such as sapphire and ZnS could hardly achieve the requirements of the working conditions operating at any angles (i.e., 10°-90°) under ultrahigh heat flux (i.e., 3-8 MW/m2) for 5 s due to the failure risk caused by excessively high temperatures or structural failure from high thermal stresses. However, diamond had a better performance. Diamond could only be used as an infrared optical window at a small angle (i.e., 10°) without coolant, but could be used at a heat flux of 3-5 MW/m2 and any angle, as well as at 8 MW/m2 and 45° and below with coolant. The surface temperature of diamond was higher than the oxidation temperature, and the high stresses with a possibility of destruction under 8 MW/m2 at 90° occurred. In addition, diamond showed a lower failure risk and some advantage of its outstanding properties rather than sapphire and ZnS under cooling conditions

Introduction Fluorescent materials have attracted much attention due to their application advantages in solar cells, biological diagnostics, infrared detection and solid-state laser. Up-conversion luminescence process refers to a special anti-stokes process that converts low-energy photons into high-energy photons. Rare-earth element doped fluoride nanoparticles are beneficial to obtaining a high up-conversion luminescence efficiency due to their low phonon energy. However, the lack of particle morphology and stability of the nanoparticles hinders their practical application. From the perspective of application analysis, rare-earth element doped fluoride crystal materials have a lower cost and a better stability, and are more suitable for complex occasions where bulk materials are needed. Er3+ ion is widely used in various up-conversion optical materials because it can emit green or red fluorescence under the excitation at 980 or 808 nm laser. From the perspective of luminescence, rare-earth element ion has a poor luminescence performance in the crystal environment with a high symmetry due to the prohibition of electric dipole transition. For the luminescence of doped ions, substances with a low symmetry system should have more potential advantages. Strontium fluorophosphates (Sr5(PO4)3F (referred to as S-FAP) crystal material is considered as a member of the hexagonal crystalline system fluorapatite family. The synthesis of high-quality single crystal has challenges. The single crystal growth process has frequent performance defects such as bubbles, clouds, cracks and impurity absorption. In this paper, a high quality strontium fluorophosphate transparent ceramic material was synthesized via conventional one-step hot pressing sintering as an economic way. In addition, the up-conversion luminescence properties of Er3+ in hexagonal strontium fluorophosphate asymmetric transparent polycrystalline ceramic material were also investigated.Materials and method The phase composition of 2% Er:S-FAP powder and ceramics was analyzed by a model D/Max-RB X-ray diffractometer (XRD) with Cu target at a tube voltage of 60 kV, tube current of 50 mA, scanning angle range of 20°-80°, and scanning step of 0.02°. The Rietveld refinement results of XRD were completed by a software named FullProf. The microstructure of powder and ceramic was determined by a model SU8010 field emission scanning electron microscope (SEM). For the SEM determination, the powder and ceramic samples were treated with gold spraying for 15 s and 20 s, respectively. The grain sizes of powder and ceramic samples were analyzed via softwares named nano Measure 1.2 and Image J. The optical linear transmittance and absorption spectra of 2% Er:S-FAP transparent ceramics were measured by a model UV-3600 UV-Visible-infrared spectrophotometer. The emission spectrum and fluorescence lifetime of ceramic samples at room temperature were measured by a model FLS1000 fluorescence spectrometer with excitation laser at 980 nm. All the tests were conducted at room temperature. The diameter and thickness of the ceramic samples were 16 mm and 2.2 mm, respectively.Results and discussion Based on the XRD patterns of 2% Er:S-FAP precursor powder and the XRD Rietveld refinement results of hot-pressing ceramics, the synthesized phase crystal structure is a hexagonal fluorapatite crystal structure. The SEM results of the powder show that the short rod-like fluorapatite nanoparticles with a high sintering activity and a well dispersion can be synthesized by a simple liquid-phase co-precipitation method. The average grain size is (19.10±1.6) nm, which is similar to the calculated value (i.e., 17.0 nm). The existence of compact and uniform surface and sectional structure is a basis of high optical quality 2% Er:S-FAP transparent ceramics, and the average grain size of the ceramics is (386.6±20.8) nm. The linear optical transmittance of the ceramic samples at 500 nm and 1 000 nm is 66.14% and 84.41%, respectively. The linear optical transmittance of 2% Er:S-FAP transparent ceramics is not close to the theoretical transmittance possibly due to some factors that reduce the transmittance (i.e., impurity scattering, porosity, and grain boundary birefringence). The actual average grain size of transparent ceramics is less than 1 μm. The scattering caused by coarse grain boundaries is small, and the pore and grain boundary birefringent scattering is a main reason of light scattering loss. The intensity of all emission peaks of 2% Er:S-FAP polycrystalline transparent ceramics increases with the increase of laser power intensity, and the intensity of emission peaks related to red emission at 661 nm is more obvious. The up-conversion luminescence process of Er3+ in S-FAP ceramic matrix, including red and green light emission, is dominated by a two-photon absorption process through fitting of excitation power P and up-conversion emission intensity Iuc. 2% of Er3+ emits an intense red light in S-FAP matrix when exciting Er3+ by a laser at 980 nm after Er3+ occupies Sr2+ site. The enhanced cross relaxation effect between Er3+-Er3+ leads to the relative enhancement of red emission relative to green emission. The specific cross relaxation process is as follows: i.e., 1) 4F7/2 + 4I11/2→409/2 + 4F9/2; 2) 4S3/2 + 4I15/2→4I9/2 + 4I13/2; and 3) 4I13/2 + 4I11/2→4I15/2 + 4F9/2.Conclusions The XRD and SEM results show that 2% Er:S-FAP nano-powder with a short rod-like morphology and an average grain size of (19.10±1.6) nm was synthesized. The superior linear optical transmittance of the ceramic was due to the high dense and uniform ceramic cross section and surface microstructure. The optical transmittance of the ceramic at 500-1 000 nm was 66.14 and 84.41%, respectively. The scattering factor of the ceramic was mainly grain boundary birefringent scattering. The absorption spectra at room temperature show that the absorption peak intensity of green light of Er3+ in S-FAP transparent ceramic was greater than that of red light. The intensity of the red emission peak of the ceramic was greater than that of the green light in the up-conversion emission spectrum as the laser pump power at 980 nm increases possibly due to the enhancement of cross relaxation phenomenon. Two-photon absorption dominates the up-conversion process of 2% Er3+ in S-FAP transparent ceramic matrix by means of the intensity of red-green upconversion light with the excitation power of the laser. It is indicated that Er3+:S-FAP transparent ceramic material is a kind of red up-conversion luminescent material with a promising application potential.

Introduction: Molding process is recognized as one of the most critical procedures in the preparation of Y3Al5O12 (YAG) transparent ceramics, affecting directly the densification of YAG transparent ceramics as well as their mechanical, thermal and optical properties. Various molding processes (i.e., cold isostatic pressing, slip casting, tap casting, and gel-casting) are applied to fabricate YAG transparent ceramics. However, the emergence of designing philosophy and applying technology promotes brand-new requirements on configurations or hybrid-functions of transparent ceramics. It is more difficult to realize the integrated molding of those transparent ceramics via traditional molding processes. Digital light processing (DLP) based 3D printing technology is a developed technology for ceramic forming. Some ceramic parts with complex shapes, high accuracy on sizes and high surface qualities can be obtained by this technology. The ceramic materials with a high performance are fabricated in recent years. Nevertheless, little work on YAG transparent ceramics have been reported yet. This paper dealt with DLP 3D printing of YAG transparent ceramics, and investigated the slurry preparation, molding, degreasing and sintering processes of the ceramics.Materials and method Commercial alumina (purity: 99.99%) and yttrium oxide (purity: 99.99%) powders with a small fraction of sintering additives (i.e., TEOS and MgO) were weighed stoichiometrically and mixed thoroughly in ethanol by ball-milling. The slurry was dried at 60 ℃ for 24 h, ground and sieved through a 200-mesh screen, subsequently calcined at 800 ℃ to obtain a homogenously distributed sub-micron-sized Al2O3-Y2O3 powder. The ceramic slurries with different solid loadings suitable for DLP 3D printing were prepared via mixing the mixed powder with photocurable resin, photoinitiator and defoamer. The green bodies with layers (layer thickness of 50 μm) were fabricated based on a computer-aided designed model in a model ADMAFLEX 130 instrument (ADMATEC Europe BV., the Netherlands). Afterwards, the green bodies were heat-treated in a muffle furnace at 600 ℃ for 5 h to eliminate organic components and further densified by sintering in vacuum at high temperatures. The specimens were double-surface polished to 1 mm for the coming characterizations.The rheological properties of the photocurable ceramic slurries were characterized by a model MCR302 stress-controlled rotational rheometer (Anton Par GmbH, Austria) with a cone (the diameter of 20 mm). The curing depth was determined via collecting the thickness of three independent printed samples. The thermal properties of the green body were determined in air by thermogravimetry (TG) and differential scanning calorimetry in a model STA449C thermal analyzer (NETZSCH Co., Germany) at a heating rate of 10 ℃/min. The microstructures of the powders, biscuits, and sintered YAG transparent ceramics were measured by a model SU8220 field emission-scanning electron microscope (FE-SEM) and a model TM3000 scanning electron microscope (SEM). The in-line transmittances of the ceramics were examined by a model V-770 UV-Vis-NIR spectrophotometer.Results and discussion The influence of solid loading on the rheological properties of the photocurable slurries was investigated by a stress-controlled rotational rheometer. The viscosity of the ceramic slurry increases from 7.9 to 11.7 Pa·s at 32 s-1 as the solid loading increases from 39.5% to 42.0%. The viscosities are suitable for the spreading of the slurries on the PET film. However, further increasing the solid loading probably leads to an increase of the viscosity and the slurry cannot spread out fluently. Thus, the ceramic slurries with solid loadings from 39.5% to 42.0% are prepared. The influence of photo-curing parameters on the curing depth, accuracy and strength of the single-layer ceramic was analyzed. The proper curing parameter is 50 mW/cm2-3 s. According to the TG-DTA analysis, the degreasing procedure is precisely controlled. Especially, a low temperature rising rate of 0.3 ℃/min is set at 300-520 ℃. The degreasing process is accomplished after holding at 600 ℃ for 5 h to remove all organic components. Little defects such as layer-layer separation and inner-layer cracks appear due to the microstructure. A vacuum sintering method is applied to further densify the ceramic. The YAG transparent ceramic with a high in-line transmittance of 82.9% is finally obtained by DLP 3D printing technology. This work can provide a foundation for the applications of YAG transparent ceramics.Conclusions Ceramic slurries with different solid loadings suitable for DLP 3D printing were prepared with a mixed Al2O3-Y2O3 powder. The influence of photo-curing parameters on the green bodies was discussed. The ceramic slices with superior comprehensive properties (i.e., curing depth, accuracy and strength) were obtained at the curing parameter of 50 mW/cm2-3 s. This processing parameter was used to print the green bodies of YAG transparent ceramics. According to the results by TG-DTA, degreasing processing was decided, and a low temperature rising rate of 0.3 ℃/min was set at 300-520 ℃, due to the integrity of microstructures of the ceramic biscuits. YAG transparent ceramics with a high optical quality were obtained via high temperature sintering in vacuum. Increasing the solid loading and the sintering temperature favored the mass transfer process. The YAG transparent ceramic with a high in-line transmittance of 82.9%@600 nm was finally obtained with the slurry with 42% solid loading at 1 780 ℃ for 3 h.

Introduction Transparent ceramics have been developed in the past decades due to their superior performance in optics, lenses, and armor. For their effective use, transparent ceramics exhibit a high optical transmittance from the ultraviolet-visible to the infrared range as well as superior mechanical and thermal properties. Metastable gamma-type aluminum oxide (γ-Al2O3) is one of the transition γ-Al2O3 crystals with a face-centered cubic crystal structure. γ-Al2O3 crystal can theoretically transmit s light over a substantial range of wavelengths (from deep ultraviolet to mid-infrared) and exhibit optical qualities comparable to those of single crystal sapphire. However, little work on this ceramic compound as a transparent bulk material has been conducted. Materials and method γ-Al2O3 powder was synthesized by a homogeneous-precipitation method. A solution containing Al3+ ions was prepared via dissolving aluminum nitrate nonahydrate (Al(NO3)3·9H2O, 99.99%) in distilled water. A precipitant solution was prepared with 1.5 mol/L ammonium hydroxide (NH4HCO3, analytical grade) and 0.5 mol/L ammonium hydroxide (NH4·H2O, analytical grade) in ethanol and distilled water (a molar ratio of 1:3). A nitrate salt solution was added in dropwise into the precipitant solution by a peristaltic pump at 2.5 mL/min under stirring at 18 ℃. After 24 h, the suspension at room temperature was filtered, and the resulting precipitate was washed with distilled water and ethanol. The precipitates were dried in a vacuum drier at 100 ℃ for 24 h, sieved through a 200-mesh screen, and then calcined in ambient air at 500-1 200 ℃ for 4 h. We carried out a series of high-pressure experiments at a temperature range from room temperature to 700 ℃ and at a pressure range from 3 GPa to 6 GPa. High-pressure sintering experiments were performed by a model mavo press LPR 1 000-400/50 Kawai-type apparatus with a Walker-module (Max Voggenreiter GmbH, Germany) and a model DS6&14 MN cubic press apparatus (China). The Kawai-type apparatus was used to explore the synthesis condition for the γ-Al2O3 transparent ceramics, and the cubic press apparatus was employed to acquire the large-size sample. The γ-Al2O3 powders prepared as described above were uniaxially dry-pressed at 20 MPa and finally the green bodies were cold-isostatically pressed at 200 MPa. The green bodies were then cold compressed at 3-6 GPa and subsequently heat-treated at 300-700 ℃ at a heating rate of 20 ℃/min and a cooling rate of 10 ℃/min. After maintaining this temperature for 20 min, the temperature decreased to room temperature at the same rate and then the pressure was released. The sample chamber temperature was measured directly by a thermocouple (PtRh 6%-PtRh 30%), and the pressure in the sample chamber was calibrated using the phase transitions of Bi, Tl, and Ba at a high pressure.Results and discussion Cubic gamma-alumina oxide transparent ceramic materials were prepared via high pressure sintering at a pressure of a certain GPa and a lower temperature. The result shows that the optimum optical properties of γ-Al2O3 transparent ceramic material with an average grain size of 20 nm sinterred at 5 GPa and 300 ℃ are achieved. The maximum transmittance of 86% can be obtained in the range of 0.6-1.2 μm, compared to that of single-crystal sapphire. The Vickers hardness of cubic γ-Al2O3 transparent ceramic is 17 GPa, which is similar to that of a conventional sapphire single crystal. In addition, the dielectric constant (i.e., 9.46) and dielectric loss (i.e., 0.00 11) are also comparable to those of sapphire in the c-axis direction. For γ-Al2O3 doped with Eu3+ ions, trivalent Eu3+ can be self-reduced to bivalent Eu2+. The most intense Eu2+ emission intensity is acquired in γ-Al2O3:Eu3+ materials heated at 300 ℃ and gradually decreases to a minimum value when further improving at 1 200 ℃, accompanied by the increase of trivalent Eu3+ ions emission intensity. The cubic alumina matrix has a structural advantage for self-reduction response, and hence is suitable for the incorporation of variable valence ions for acquirement of cubic aluminum oxide functional transparent ceramic materials.Conclusions The optimum optical properties of γ-Al2O3 transparent ceramic sinterred at 5 GPa and 300 ℃ were achieved. The optical transmittance of high quality γ-Al2O3 transparent ceramic reached 86% in visible and near-infrared wavelength region. The average grain size was 20 nm. The Vickers hardness measured at room temperature was 17 GPa. The dielectric constant and the dielectric loss were 9.46 and 0.001 1, respectively. When heated in air, γ-Al2O3 evolved into tetragonal δ-Al2O3 phase at 900 ℃, θ-Al2O3 phase at 1 000 ℃, and α-Al2O3 phase at 1 100 ℃, respectively. However, γ-Al2O3 phase started to transform into the mixed phase including metastable phase γ-AlOOH, Al2O3.H2O and stable α-Al2O3 phase after sintering at 5 GPa and 600 ℃. When the trivalent Eu3+ was added into γ-Al2O3 matrix, the trivalent Eu3+ could self-reduce to the bivalent Eu2+. Also, the most intense Eu2+ emission was obtained after heating at 300 ℃. However, the emission intensity of Eu2+ decreased with the increase of heating temperature to 500 ℃. After further heated at 1 200 ℃, the emission of Eu2+ in γ-Al2O3 matrix disappeared, accompanied by the increase in the emission intensity of trivalent Eu3+ ions. These results indicated that the cubic alumina matrix could be suitable for the incorporation of variable valence ions, which was conducive to the preparation of cubic aluminum oxide functional transparent ceramic materials.

Introduction The development of laser technology is related to laser materials. In particular, high-power solid-state lasers require the laser material to have the superior performance (i.e., lower pumping thresholds, large absorption and emission cross sections). Compared with glass and single crystal, transparent ceramics have many potential advantages that glass and single crystal are incomparable. In recent years, the breakthrough of ceramic preparation technology has gradually brought ceramics into the field of vision of the majority of scientific researchers. From the perspective of luminescence of doped rare-earth element ions, materials with a low symmetry system have more potential advantages. Strontium fluorophosphate has a hexagonal structure as a typical asymmetric system material. It can provide superior doping sites for rare-earth element ions, having broad potential application prospects in luminescence. Also, neodymium ion (Nd3+) is a commonly used active ion, its intense emission wavelength is mainly approximately 1 μm, which is widely used in atmospheric detection and infrared imaging and other fields. However, neodymium ion has a problem of concentration quenching, which will cause the reduction of luminous efficiency. To solve this problem, the commonly used solution is to co-dope inert regulatory ions to break the cluster phenomenon of neodymium ions in the matrix material. Therefore, Nd-doped strontium fluorophosphates transparent ceramics were prepared with S-FAP as a matrix material and Nd3+ as an activator ion. In addition, the effect of inert regulatory ions as a sensitizer on the microstructure and spectral properties of Nd:S-FAP was also investigated.Materials and method According to the stoichiometric ratio of (Nd0.015Re0.02Sr0.965)5(PO4)3F1.175, raw materials were weighed and cation was dropped into the anionic solution at a fixed titration rate, and then the suspension was separated in a centrifuge at 11 000 r/min for 4 times. Finally, the separated products were dried in an oven for 24 h. The dried powder was ground in a mortar. The dried powder of 3 g was put into a graphite mold, placed in a vacuum hot press furnace, and sintered at 900 ℃ for 2 h to obtain (Nd, Re):S-FAP transparent ceramics.The phase composition of the ceramic was analyzed by a model D8-Advance X-ray diffractometer. The Nd3+ and Re3+ concentrations of (Nd, Re): S-FAP were determined in argon at 0.3 MPa by a model Prodigy 7 inductively coupled plasma emission spectrometer. The transmittance of ceramics was measured by a model Lambda 750S spectrophotometer. The microstructure and pore distribution of ceramics were characterized by a model S4800 scanning electron microscope, and the emission spectrum and fluorescence lifetime of ceramic samples were measured by a model FLS920 spectrometer.Results and discussion After doping different kinds of inert regulated ions, each sample has no impurity and a high crystallinity. After high-temperature sintering, the ceramic still maintains a hexagonal structure with a pure phase S-FAP. Moreover, the position of the ceramic emission peak shifts to the right, indicating that the lattice constant gradually decreases The ceramic samples have a good optical transmittance, showing the absorption peaks of the 4I9/2 ground state level transition to the excited state level. For the samples doped with different kinds of inert regulated ions, there are differences in their microstructures. This is because the distortion caused by the doped S-FAP lattice is also different due to the different radii of inert ions. Y ion has the smallest radius and the greater lattice distortion degree. In sintering, the lattice distortion will provide part of the sintering energy, increase the diffusion rate of the grain boundary, eventually resulting in the larger grain size. The main reason affecting the overall transmittance of the sample is a difference in density, and the higher the density of the sample is, the higher the transmittance will be. The emission intensity of transparent ceramics is enhanced after co-doping inert regulated ions. The addition of inert regulated ions breaks the clusters of neodymium ions and increases the distance between different neodymium ions, thereby avoiding the phenomenon of cross relaxation between neodymium ions to a certain extent and thus improving the emission intensity. In addition, the emission peaks of the samples shift to the right to a certain extent after the addition of inert ions, indicating that the crystal field environment of neodymium ions can be changed by co-doped inert ions. The fluorescence lifetimes of ceramic samples doped with different kinds of inert rare-earth ions are 328.44, 331.82 and 355.68 μs, respectively. Compared with 1.5% Nd:S-FAP transparent ceramics, the fluorescence lifetime of ceramic samples doped with inert rare-earth ions increases. The neodymium ion clusters are broken, resulting in an increase in fluorescence intensity/lifetime. In addition, from doping of different kinds of inert rare-earth ions, the fluorescence life of Y ions is the longest after incorporation, reflecting that Y ions are more efficient in breaking clusters in S-FAP matrix materials, and are the most suitable inert regulatory ions for doping.Conclusions Inert rare-earth ions doped (Nd, Re):S-FAP transparent ceramics were prepared by a hot pressing sintering method. The lattice constant of rare-earth ion doping decreased, and the main factor affecting the change of lattice constant was rare-earth ion radius. The emission intensity and fluorescence lifetime of ceramic doped inert ions were increased, compared with that of ceramic undoped ions, and the regulation effect of Y ions was dominant. This indicated that the clusters between neodymium ions were broken and the luminous efficiency was improved.

Solid-state lighting is one of the most promising technologies in the 21st century due to its high luminous efficiency, faster responding, energy-saving, environmentally friendly, and longevity. It relies on solid-state electronic components to achieve the conversion of electrical energy to light energy. At present, solid-state lighting mainly includes light-emitting diodes (LEDs) and laser diodes (LDs). The luminous efficiency of existing LEDs decreases with the auger effect as the input power increases. They cannot achieve a higher light efficiency output. However, LDs have a higher efficiency, a stronger brightness and a longer lighting distance for the efficiency degradation of LEDs at high power densities. The existing color conversion materials that can be used for laser diodes are single crystals, phosphor ceramics, phosphor-in-glasses, phosphor films, etc.. Phosphor ceramics are the most promising color conversion materials for LDs with high thermal conductivity, good optical properties, and controllable microstructures.This review discussed the current research status of forming methods, sintering processes, component selection, and structural design of phosphor ceramics for laser diodes. This review summarized the design requirements of LDs for phosphor ceramics, such as thermal saturation characteristic, optical saturation characteristic, luminous efficiency, color rendering index, mechanical properties, etc.. This review also elaborated the performance requirements of phosphor ceramics for practical applications. The preparation technologies of phosphor ceramics were summarized. The preparation methods of the powders included chemical coprecipitation method, hydrothermal method, sol gel method, etc.. The forming methods of powder included dry pressing, cold isostatic pressing, molding by slip casting process, casting molding, gel casting, etc.. The sintering methods of ceramics include pressureless sintering, gas pressure sintering, vacuum sintering, spark plasma sintering, hot pressed sintering, hot isostatic pressing, etc.. For selecting and optimizing raw materials, sintering aids are one of the important factors affecting the sintering of phosphor ceramics. In the design of the composition of phosphor ceramics, the luminescence performance is adjusted via designing the types of matrix elements in oxide and nitrogen (oxide) phosphor ceramics. The luminescence behavior of phosphor ceramics is analyzed via introducing different luminescent centers into the same matrix. In addition, the introduction of a second phase is also chosen to improve both the blue light absorption rate and the heat dissipation performance of ceramics in order to further enhance the luminescent performance of phosphor ceramics. In the structural design of phosphor ceramics, some methods such as introducing pores, multi-layer structures, and surface modifications are used to further improve the luminescence efficiency and color rendering index of phosphor ceramics.Summary and prospects Lighting penetrates every aspect of life, i.e., daily lighting, projection displays, automotive lighting, and other industries. The requirements for lighting devices in various industries are increasing and gradually moving towards to high power and high brightness. This review represented recent research progress on phosphor ceramics in molding technology, sintering process, material design, and other aspects. At present, there are still shortcomings in the preparation of phosphor ceramics, such as single preparation method, limited variety, obvious thermal and optical saturation phenomena. Further improvements are needed in the following areas: 1) Preparation of highly active nano-powders and sintering additives are related to optical and mechanical properties of phosphor ceramics. Sintering aids promote liquid-phase mass transfer during ceramic sintering process and also impact the structure of the matrix lattice; 2) The forming method of ceramics directly determines the stacking situation of powder raw materials, which in turn affects the sintering density. The appropriate molding methods can obtain ceramic bodies with uniform composition, constant shape, and strength, making the particles tightly being connected and promoting facilitated transport during the sintering process; 3) The sintering of phosphor ceramics requires a high temperature and a certain sintering environment, such as reduction atmosphere, oxidation atmosphere, vacuum environment, etc.. The sintering conditions that thermal equipment can provide also affect the design and selection of sintering conditions for phosphor ceramics. The selection of thermal equipment has a significant impact on the sintering process of phosphor ceramics, and determines the range of sintering condition. Further promoting the technological progress of thermal equipment plays a decisive role in improving the performance of phosphor ceramics; 4) The emission spectrum of phosphor ceramics still lacks a red spectral component, so further expanding the types of red phosphor ceramics will inevitably effectively improve the color rendering index of phosphor ceramics; 5) The appropriate non- luminescent second phase preparation of multiphase phosphor ceramics is beneficial to improving the luminescence efficiency of phosphor ceramics; and 6) The intense heat dissipation and luminescence performance of luminescent materials are still a key to their widespread application.

Conventional materials generally have a relatively simple function, and it is difficult to meet the requirement of multi-functional materials in modern information society. It is necessary to develop multi-functional, high-sensitivity materials and devices. Transparent ferroelectric ceramic is an advanced multi-functional material, and it can couple optical functions with mechanical, electrical, and acoustical functions, becoming a research hotspot in materials engineering. Based on the performance characteristics and applications of transparent ferroelectric ceramics, they are divided into five categories, i.e., transparent piezoelectric ceramics, transparent electro-optic ceramics, transparent energy storage ceramics, transparent luminescent ceramics, and transparent photochromic ceramics. Recent efforts are made in the research of multifunctional transparent ferroelectric ceramics.High-performance electro-optical devices are developed based on the electro-optical effect of transparent ferroelectric ceramics, and applied to high-speed optical communication and high-power laser modulation. High-performance transparent transducers can be developed based on their piezoelectric effect and applied to medical photoacoustic imaging, transparent robots, and other fields. As a dielectric material, transparent ferroelectric ceramics can achieve a high dielectric energy storage performance, which has a great application potential in transparent supercapacitors, transparent photovoltaic windows, and transparent energy storage coatings. Transparent ferroelectric ceramics also exhibit good photoluminescence and photochromic properties and can be used in transparent information storage devices, 3D optical information storage, fluorescent labeling, multi-level encryption, transparent sensors and displays, smart windows, and other optoelectronic devices. Research work on multifunctional transparent ferroelectric ceramics give tremendous opportunities for innovation in information technology, medical and health care, new energy, smart technology, etc..Rare-earth elements (La, Sm) doped Pb(Mg1/3Nb2/3)O3-12PbTiO3 (PMN-PT) transparent ceramics have superior electro-optic outstanding electro-optic properties (i.e., high transmittance of 70%@NIR, large electro-optic coefficient of 66×10-16 m2/V2, low half wave voltage of 113 V and fast response speed of 10-100 ns). High-performance electro-optical devices, such as electro-optic switches, Q switches, variable optical attenuators, and tunable optical filters, are developed and commercialized. They can be used in free-space optical communication, dual-channel optical communication with continuous adjustment of laser intensity ratio, and continuous control of laser polarization state.Eu-doped PMN-PT transparent ferroelectric ceramics exhibit a high transmittance (i.e., T=68%) and a superior piezoelectricity (i.e., d33=1400 pC/N), which are greater than those of other transparent piezoelectric ceramics,. The high-performance transparent piezoelectric ceramics have promising application prospects in biomedical photoacoustic imaging, transparent robotics, and transparent loudspeakers. Er-doped and Pr-doped PMN-PT transparent ferroelectric ceramics are proven to have the superior properties in luminescence at visible and near-infrared wavelengths. Furthermore, the fluorescence intensity is linearally related to temperature, indicating a potential application of transparent luminescent ceramics in illumination and optical temperature sensors.(K0.5Na0.5)NbO3(KNN) based transparent ferroelectric ceramics exhibit a high transmittance (i.e., T of 69%) and a high energy storage density (i.e., 7.4 J/cm3), a large energy storage efficiency (i.e., 74%), and a great breakdown field strength (i.e., 750 kV/cm). This has a potential for application in high-voltage transparent pulse capacitors. Element-doped KNN transparent ceramics also have good photochromic properties. For instance, the maximum luminescence modulation ratio (?RL) reaches 92.6% in Tb-doped KNN transparent ceramics, and the maximum transmittance modulation ratio (?RT) is 70% in Er-doped KNLN transparent ceramics. At present, the storage life of optical information in KNN-based transparent ceramics prolongs to more than 7 d, indicating that it has a potential application in optical information storage.Summary and prospects Transparent piezoelectric ceramics can be used in biomedical photoacoustic imaging, transparent robotics, and transparent speakers. The existing lead-based transparent ceramics have superior transparency and piezoelectric performance. However, the microscopic mechanism and corresponding theoretical basis for the coexistence of these two properties are still unclear. A further research work is needed to provide a theoretical guidance for the development of lead-free high-performance transparent piezoelectric ceramics. The practical applications of transparent piezoelectric ceramics have not yet been reported, and The relevant research needs to promote its application in real life. At present, the piezoelectric properties of KNN-based lead-free transparent piezoelectric ceramics are still rather small and hard to meet the application requirements. Studies on component design, phase control, and grain manipulation need to develop lead-free transparent ceramics with a high transparency and a high piezoelectricity. High-performance electro-optical devices are developed based on transparent PMN-PT ceramics, and used in lasers, optical communications, and quantum optics. Doping rare-earth elements in the A-site is an effective method for regulating the electro-optical properties of transparent ferroelectric ceramics. However, only a few research work on the electro-optical properties of KNN and KTN-based lead-free transparent ceramics are reported, and their performance is far from the application requirements. It is indicated that improving the electro-optical properties of lead-free transparent ferroelectric ceramics is needed to meet the sustainability and environmental requirements. Transparent energy storage ceramics have some advantages in the miniaturization and integration of devices, potentially opening up opportunities in power electronic converters, new energy vehicles, and pulsed power systems. The energy storage density and efficiency of transparent ferroelectric ceramics are improved by component design and doping modification to reduce grain size, enhance breakdown field strength, and enhance relaxation characteristics. However, the subdivision of its specific application field is still unclear and needed to be explored. The extensive research of effectively combining light transmission properties and energy storage properties in practical applications should be carried out to develop new multifunctional devices and expand the application field of transparent energy storage ceramics.Transparent luminescent/photochromic ceramics have applications in fluorescent labeling and new optical information storage. Currently, researchers mainly focus on the enhancement of luminescence intensity, photochromic modulation ratio, and response speed by doping rare-earth luminescent ions or co-doping rare-earth elements with transition metal elements to increase the number of luminescent centers or vacancy defects. Modulating their luminescence/photochromic performance in electrical field has not done yet. This can further expand a frontier of transparent luminescent/photochromic ceramics.

High-entropy transparent ceramics (HETCs) are a type of ceramic material that is transparent and has high mechanical strength and thermal stability, which have attracted recent attention. The fabrication of HETCs involves the use of multiple elements in equal atomic proportions, leading to their unique properties. This review discussed the significance, progress, conclusion, and prospects of HETCs.HETCs have a potential to revolutionize several industries like aerospace, defense, and optoelectronics. HETCs can be used as a transparent armor in military vehicles, and transparent windows in aerospace vehicles, and as protective covers for electronic devices. The unique properties of HETCs (i.e., high mechanical strength, thermal stability, and transparency) make them ideal for these applications. Furthermore, HETCs can be used as a substrate for thin-film solar cells with the better light transmission, compared to glass substrates.The development of HETCs that are transparent in the visible and near-infrared regions of the electromagnetic spectrum is a significant achievement, as it allows for their use in optoelectronic applications. The high thermal stability of HETCs makes them suitable for high-temperature applications, which is essential in several industries.The high-entropy concept in these ceramics refers to the deliberate mixing of multiple elements in equal or nearly equal proportions. This random arrangement of elements results in a high degree of disorder, or entropy, within the crystal structure of the material. HETCs can achieve a unique combination of properties that surpass those of conventional ceramics via incorporating different elements.One of the key characteristics of HETCs is their ability to maintain transparency even under extreme conditions. Unlike conventional ceramics, which are typically opaque, HETCs exhibit a high degree of optical transparency, allowing a light to pass through them with minimal distortion. This property is particularly advantageous in applications on optical devices, windows, and lenses.Besides, one of the most significant advantages of HETCs is their exceptional mechanical strength. These ceramics exhibit great hardness, toughness, and resistance to fracture, making them suitable for high-stress environments. This property has some possibilities for their use in structural applications, such as protective coatings and armor.Another remarkable characteristic of HETCs is the superior thermal stability. These ceramics can withstand at high temperatures without undergoing significant structural or optical degradation. This thermal resilience makes them ideal for applications that require exposure to extreme heat, such as in aerospace and power generation systems.Furthermore, HETCs have a great potential in the field of optoelectronics. Their unique combination of transparency and electrical conductivity makes them suitable for various electronic and photonic devices. For instance, HETCs can be utilized in transparent electrodes, transparent conductive films, and even as substrates for flexible electronics.Despite their immense potential, the development of HETCs is still in its early stages, and there are numerous challenges that need to be addressed. The fabrication of these ceramics requires a precise control on the composition and processing conditions to achieve the desired properties. Also, the cost of production remains an obstacle that needs to be overcome for their widespread adoption.The fabrication of HETCs is a complex process that involves the use of high-temperature sintering techniques. HETCs are typically produced via a solid-state sintering process, in which the constituent powders are compacted and heated at high temperatures to form a dense, homogeneous material. In sintering, the constituent powders diffuse and react with one another, leading to the formation of a complex, multi-component material. Progress on the development of HETCs is made in recent years. One of the most significant achievements in this field is the development of HETCs that are transparent in the visible and near-infrared regions of the electromagnetic spectrum. HETCs with a high thermal stability is also developed for high-temperature applications.Summary and prospects High-entropy transparent ceramics represent a fascinating and promising material. Their unique combination of transparency, mechanical strength, thermal stability, and electrical conductivity makes them highly desirable for a wide range of applications. With their development, HETCs revolutionize various industries, having some possibilities in applications like optoelectronics, thermal management, and beyond.However, the development of HETCs is still in its early stages, and there are many challenges that need to be addressed before these materials can be widely adopted. One of the main challenges is an ability to produce HETCs in a reproducible and scalable manner. HETCs are typically produced via solid-state sintering process, in which is difficult to control and scale up to industrial levels.Another challenge is an ability to tailor the properties of HETCs for specific applications. The properties of these materials can be highly complex and difficult to predict decause HETCs are composed of multiple principal elements. Therefore, a fundamental understanding for the relationship among composition, processing and properties is critical for the development of HETCs.

In the response to laser induced damage, transparent ceramics are similar to other types of optical components (i.e., single crystals, fused quartz glass, etc.), such as surface and in vivo damage. However, they exhibit a particularity of damage due to its microstructure defects such as grain boundaries and micro pores. With the continuous development of high-power laser technology, transparent ceramics as laser gain media to resist laser damage are required. It is necessary to conduct in-depth research on the damage characteristics of transparent ceramics. In order to clarify the damage sources that induce damage to transparent ceramicsand the damage response caused by structural defects in transparent ceramics under intense laser irradiation as well as reveal the damage mechanism, the defect characterization and damage detection methods are also continuously developed and optimized. Among them, an online time-resolved imaging method represented via pump detection is developed. The evolution of stress waves and plasma, as well as their impact on damage to transparent ceramics are analyzed via time-resolved imaging. The damage characteristics and mechanism of transparent ceramics induced by strong laser are investigated by the online time-resolved imaging method. In addition, there are also many types of structural defects in transparent ceramics that are difficult to be accurately controlled. Therefore, it is possible to artificially regulate the source of damage, such as the size and density of micro pores. This can control uncontrollable micro-defects, greatly improving the efficiency of exploration. The damage research of transparent ceramics can be further innovated.This review represented the research methods of laser induced damage in transparent ceramics, the influencing factors of damage (i.e., surface structural defects, micropores, and ceramic grain boundaries), and some approaches to increase the damage threshold. Transparent ceramics are the gain medium in high power laser systems, and there are two main reasons for inducing laser damage. On the one hand, it is related to the input laser parameters, but more importantly, the main damage sources in transparent ceramics (i.e., surface roughness, ceramic grain boundaries, and micro-pores) lead to an irreversible damage under high power laser induction. This aspect becomes a recent research topic to explore the mechanisms and characteristics of various damage sources causing damage to transparent ceramics. To better explore the microscopic evolution process in the damage process, effectively analyze the damage mechanisms of transparent ceramics in macro-/meso- and micro-levels, and further explore some ways to improve the damage threshold of transparent ceramics, it is thus necessary to effectively combine line damage test, numerical simulation and theoretical calculation of multi-physical field coupling, and artificial defect control in multiple depths. A comprehensive analysis is carried out around the damage source in transparent ceramics to clarify the damage law, and gradually establish the correlation of “damage source-injury performance-injury mechanism”, and finally lay an important foundation for exploring a way to improve the damage threshold.A ultimate goal of research on laser damage to transparent ceramics is to explore effective ways to increase the damage threshold, in order to better ensure the application effect of transparent ceramics in practical engineering. This review thus discussed several possible implementation approaches for the damage threshold of transparent ceramics, i.e., laser pretreatment, annealing process, and surface micro/nano processing. The three approaches are effectively validated in improving the laser damage threshold of other types of optical components. However, based on the polycrystalline properties of transparent ceramics themselves, it is still necessary to conduct systematic experiments and explorations to determine whether they can have a positive effect in improving the laser damage threshold in transparent ceramics.Summary and prospects This review summarized three aspects i.e., the exploration methods of laser induced damage in transparent ceramics, the influencing factors of damage, and the ways to increase the damage threshold. The research progress on laser induced damage to transparent ceramics was elaborated, having an important reference significance in the related fields. To clarify the influence of structural defects such as micropores and grain boundary on the damage formation mechanism, a guidance for the control standards of structural defects in the preparation and processing of transparent ceramics was provided. This review also has an important reference value for the development of high damage threshold transparent ceramics and their reasonable application in high-power laser systems. The study of intense laser induced damage to transparent ceramics will continue to promote the expansion of related basic and applied science research, thereby providing a foundation/guidance for the design and manufacture of high-performance optical and optoelectronic devices

Infrared imaging and precision guidance systems are distinguished by their high imaging accuracy, robust concealment, and resilience to interference. These characteristics make them a pivotal area of contemporary and future warfare. The infrared window is an integral part of the infrared guidance system, being responsible for transmitting target signals, preserving aerodynamic configurations, and protecting internal precision optoelectronic components. Consequently, the window material must possess a high optical transmittance across the operational band and exhibit the superior strength and hardness. Hypersonic vehicles have challenges to infrared window materials, including extreme thermal shock, aerodynamic overload, degradation of optical and mechanical performance at high temperatures, and self-heat radiation that can interfere with target signals. At present, no material fully satisfies the demands of cutting-edge systems, making it imperative to develop a window material that offers a low radiation, a high strength, and a broadband transmittance at elevated temperatures.Commonly utilized infrared window materials encompass sapphire, spinel, AlON, magnesium fluoride, yttrium oxide, and Y2O3-MgO nanocomposite ceramic materials. Sapphire also as α-alumina monocrystal is characterized by its high strength and broad high transmittance properties. It boasts a well-established industrial chain and is extensively employed in various aircraft models. While polycrystalline alumina transparent ceramics offer lower production costs, compared to sapphire, and they have yet to reach the same level of application. Spinel and AlON both possess cubic structures and demonstrate great transmission rates from near-ultraviolet to mid-infrared spectrums. AlON exhibits superior mechanical properties akin to those of sapphire, making them popular choices for aircraft infrared windows. However, these three materials share a significant drawback, i.e., an excessively high heat emissivity at high temperatures. This can lead to thermal barrier complications due to spontaneous radiation interference with target signals under intense aerodynamic heating. In addition, their shorter infrared absorption cut-off edges also result in pronounced transmission rate reductions at 5 micrometers, following a high-temperature blue shift. Note that sapphire and spinel both present challenges related to their inadequate mechanical performance at high temperatures. Researches on zirconia ceramics as infrared windows are limited, but their high bending strength and decent transmission performance are noteworthy. Y2O3 and MgF2 with the cubic structure exhibit superior transmission in the 3-5 micrometer band due to their low phonon energy and minimal spontaneous radiation coefficient. Their further infrared absorption cutoff edge mitigates an impact of high temperature on the mid-infrared wave transmission. However, their mechanical properties do not withstand the intense heat shock encountered at a hypersonic speed. Y2O3-MgO nanocomposite ceramics, characterized by extremely small grain size from the pinning effect, combine a high transmission in mid-infrared, low radiation and high strength at high temperatures. This makes them an ideal material for infrared windows in hypersonic aircraft. Summary and prospects The development of future hypersonic vehicles necessitates the exploration of new infrared window materials. Among the materials under consideration, Y2O3-MgO nanocomposite ceramics have attracted recent attention due to its superior low radiation, thermal shock resistance, and transmission properties at a high temperature. The existing researches predominantly focus on powder preparation processes and sintering techniques, with relatively less emphasis on material doping modifications. An important research aspect is to further reduce the grain size, while maintaining a high density. Meanwhile, the incorporation of magnesium oxide into Y2O3-MgO nanocomposite ceramics introduces a degree of hygroscopicity. The enhancement of corrosion resistance in materials presents a challenge that requires resolution.Infrared and radar composite guidance technology, characterized by its robust anti-interference capabilities and superior guidance accuracy, represents a pivotal aspect for future aircraft development. This necessitates that window materials have a low dielectric constant in the radar band. While Y2O3-MgO nanocomposite ceramics exhibit a higher dielectric constant and significant radar attenuation, MgF2 has a commendable transmission performance within the radar band. However, its mechanical properties are suboptimal. Consequently, the pursuit of high-strength, low-dielectric constant window materials is of great importance.The over-the-horizon working distance in infrared optoelectronic systems, coupled with the need for a large angle reconnaissance range, necessitates infrared windows that have the superior optical uniformity. These windows should be also larger in dimensions and feature the specific window shape designs. Compared to crystals, ceramics are more readily available in large sizes and offer a more cost-effective approach for processing specific shapes. Hot isostatic pressing is an effective method for achieving these requirements, but the gas cost continues to be relatively expensive.In numerous studies, the primary focus of characterizing the mechanical properties of materials is on the strength and hardness. However, there is a need to integrate properties such as Poisson’s ratio, thermal conductivity, and thermal expansion coefficient more comprehensively to augment thermal shock resistance in materials. Furthermore, the existing researches of infrared materials predominantly center on the performance characterization in room-temperature environments. The optical and mechanical properties at high temperatures need to be further investigated.

Solid-state lasers have the advantages of realizing laser outputs with a high energy and a high peak power, having potential applications in civil and military fields (i.e., remote sensing, environmental monitoring, medical treatment and optoelectronic countermeasure). Rare-earth ions doped sesquioxide materials as one of the most promising gain media have attracted recent attention due to their low phonon energy, low thermal expansion coefficient and high thermal conductivity. However, large-size sesquioxide single crystals with a good optical quality are difficult to grow because of the high melting points (i.e., >2 400 ℃) and phase transition point at 2 280 ℃. Fortunately, the sintering of sesquioxide ceramics as an alternative way to prepare laser host materials. It is possible to obtain the materials with a large volume and a high doping concentration, showing a superiority in large-scale production, a feasibility of shape control and better mechanical properties.To achieve high-efficiency and high-power laser oscillation from sesquioxide ceramics, it is crucial to eliminate the main scattering centers inside microstructures, i.e., residual pores and secondary phases. Raw powders with less agglomeration and high sinterability, combined with appropriate molding methods are fundamental for producing compacts with small pore size and uniform microstructures. This is favorable to avoid differentiate densification, degrading the optical homogeneity and transparency. During the sintering process, it is essential for pores to remain at grain boundaries until the final full densification. This prevents the formation of intragranular pores that are hard to remove. The effective control of grain boundary migration can be achieved to prevent pore-boundary separation through the addition of suitable sintering additives, regulation of sintering atmospheres (i.e., vacuum, hydrogen and oxygen), and the use of advanced sintering techniques (i.e., microwave sintering, spark plasma sintering or two-step sintering, etc.). In addition, pressure assisted sintering, including hot pressing and hot isostatic pressing can also improve the densification rate and facilitate the effective removal of residual pores. This even enables the full densification of sesquioxide ceramics without using sintering additives. Zirconia is once widely used as a sintering aid for sesquioxide transparent ceramics. However, it can severely degrade the laser performance of sesquioxide ceramics due to the occurrence of photodarkening phenomenon under high-power pump beam excitation. This may be attributed to the charge imbalance between Zr4+ and host cation ions. Consequently, the introduced point defects act as acceptors for the electrons of excited laser ions, forming Zr3+ color centers that cause a broad absorption in a wavelength range of 400-700 nm. This seriously affects the laser oscillation efficiency. Therefore, in addition to achieving laser-grade quality compared to single crystals, minimizing the use of sintering additives or regulating the lattice defects are also considered in the fabrication process of sesquioxide ceramics.In general, chemical co-precipitation process for synthesizing well-dispersed powders, combined with vacuum sintering and hot isostatic pressing sintering, is considered as an effective way for fabricating high-quality sesquioxide transparent ceramics. This approach provides a great driving force for densification through well controlling grain boundary diffusion and migration. However, it is necessary to determine suitable sintering curves and microstructure morphology before hot isostatic pressing. Summary and prospects: At present, solid-state lasers based on sesquioxide ceramics primarily emphasize the near-infrared wavelength range. These lasers can be categorized into applications near 1 μm (doped with Yb3+ and Nd3+), 2 μm (doped with Tm3+ and Ho3+), as well as 1.6 μm and 3 μm (doped with Er3+), with significant achievements in both high power and ultra-short pulse laser outputs. It is noteworthy that Konoshima Chemical Co. and World-Lab Co. currently dominate the market with their high-quality sesquioxide laser ceramics. it is imperative to elucidate the effect of lattice defects on the laser performance in the future development of sesquioxide laser ceramics. Breakthroughs in fabrication technologies for large-size ceramics with a high optical homogeneity are crucial for laser engineering applications. Complex structure design is also essential to optimize thermal management in high-power laser systems. In addition, there should be also a focus on synthesizing single crystals through sintering methods, indicating a potential for obtaining new laser materials with heavily active ions doping and composite structures. A broad range of laser applications from sesquioxide transparent ceramics can be anticipated through the continuous improvement of powder synthesis, molding methods and sintering techniques.

With the development of economy and society, the fourth-generation light emitting diodes (LEDs) have gained a popularity for the large space and high illumination application in ocean, port and gymnasium due to their advantages (i.e., energy saving, eco-friendly, high luminous efficiency (LE) and long lifetime). The typical LEDs are assembled by blue LED chips and phosphor converters, such as yellow phosphor Y3Al5O12:Ce3+ (YAG:Ce) mixed with organic glue. For the popularization in large space lighting, high-power LEDs (hp-LEDs) become popular. Phosphors, phosphor in glasses (PiGs), phosphor crystals and phosphor ceramics are developed as color-converters. Among them, phosphor ceramics have attracted more attentions as an advanced color-converter for hp-LED and laser diode (LD) lighting due to their merits of high temperature resistance, high thermal conductivity, high LE and fine stability, which can solve some problems regarding the application bottleneck of heat dissipation difficulty and packaging failure as well as large light source volume. YAG:Ce ceramics and mass production of practical single-Kilowatt chip-on-board (COB) light source can be produced in a large scale. High quality hp-LED lighting with a high LE and a high color rendering index (CRI) is needed. In addition, RGB display modules for developed display technologies such as Mini-LED, and head up display (HUD) also require a higher LE and a more compact volume. Research on phosphor ceramics with a high luminescence performance is ascendant.Summary and prospects Ce3+-doped garnet (YAG:Ce-based) phosphor ceramic is an ideal convertor for hp-LED/LD lighting and display. To improve its luminescence performance, many efforts have been proposed. In general, there are five main strategies. 1) The doping of Lu3+, Ga3+, Sc3+, Gd3+ or Mg2+-Si4+ into YAG:Ce regulates the crystal field and changes the splitting of Ce3+ 5d energy levels, resulting in a shift of Ce3+ emission for compensating blue or red component. For instance, YLuAG:Ce3+ green-emitting ceramics show an obvious blue-shifting from 533 nm to 519 nm and a higher thermal stability as well as LE via the substitute of Y3+ with Lu3+. 2) Co-doping red-emitting ions, such as Mn2+, Pr3+ and Cr3+, is an effective method to broaden the spectrum and compensate more red emission to improve CRI. A significant enhancement of CRI for LuAG:Ce, Mn ceramics is achieved by regulating Mn2+ content (Ra=91.0, R9=37.9), which effectively adds the red emission peaked at 590 nm and 750 nm respectively. 3) Introducing secondary phase or pores into the matrix can prolong the photoluminescence pathway and induce an enhanced light-scattering effect due to the difference in refractive index. Al2O3, MgO, MgAl2O4, BaAl2O4, HA, CaF2 and AlN are usually chosen as secondary phases. Compared to Al2O3-free YAG:Ce ceramics, the LE of Al2O3-YAG:Ce composite is increased by 27.3%, and its thermal conductivity grows from 10.2 up to 32.5 W/(m·K). 4) Ce3+ is possibly oxidized to Ce4+, resulting in a decrease of emission intensity. Controlling sintered temperature and doping of Ba2+-Si4+ pair are beneficial to eliminating oxidation and maintaining Ce3+ value state. The replace of Lu3+-Al3+ by Ba2+-Si4+ pair is favorable for less Ce4+. Consequently, Lu3Al5O12:Ce3++Ba2+/Si4+ ceramic has a high LE of 216.9 lm/W. 5) Surface treatment by grinding and polishing can yield different roughness and increase a light extraction efficiency. Based on the strategies above, high performance YAG:Ce-based phosphor ceramics are developed. To address multifunctional and multiscenario applications of LEDs, other nitride, oxide or fluoride phosphor ceramics are being investigated, such as CaAlSiN3:Eu2+, (Ca, Sr, Ba)2Si5N8:Eu2+, Y2O3:Eu3+, Al2O3:Mn4+ and (K, Na, Cs)2(Si, Ti,)F6:Mn4+. These developed phosphor ceramics provide unique properties and good candidates for certain applications.In near future, phosphor ceramics, especially for YAG:Ce-based, will play an important role in LED/LD lighting and display. In response to the requirement of higher performance, involving power density, brightness and quality, it is also necessary to explore and optimize some phosphor ceramics. For phosphor ceramic material, several main methods are outlined to improve the luminescence performance, i.e., the introduction of secondary phases or pores; investigating and developing new phosphor ceramic system; utilizing theoretical calculation to accelerate research progress and optimize photoluminescence properties; extending the application to emerging lighting and display (i.e., laser projection and AR-HUD). In this review, recent research progress and application of phosphor ceramics for LED/LD lighting and display were summarized, and optimizing strategies and development trends of phosphor ceramics were also prospected.

As the “optical shield” for the future warfare, transparent armors exhibit high optical transparency, high hardness, high strength, and resistance to ballistic and explosive debris impacts. They are widely used on the equipment platforms such as armored vehicles, armed helicopters, and ship carriers. At present, the transparent armor with a superior elastic resistance is developed. However, the improvement of armor protection performance and lightweight armor design becomes a challenge, and it is manifested in the difficulty of balancing its protection performance and optical properties. Based on the research status and existing problems of transparent armor in the field of protective materials, this review represented the following aspects:1) The current development status of transparent protective materials are reviewed. It is important to introduce the physical and chemical properties, mechanism of action, and industrial status of hard protective materials and soft energy absorbing materials for in-service armor. The research status indicates that sapphire single crystal is the most suitable material for mass production of new armored projectile surface, and its mechanical properties and industrial advantages do not possess, compared to other hard and transparent protective materials such as float glass, AlON and MgAl2O4 ceramics. However, the cleavage fracture of sapphire single crystals restricts its application potential in transparent protection. Polyurethane adhesive can achieve the high-strength bonding of inorganic and organic materials in the armor fields, but its poor environmental weather resistance affects the service life of transparent armor. Polycarbonate board is an ideal soft energy absorbing material with superior optical properties, impact resistance, and creep resistance. However, it is prone to moisture absorption and fogging, affecting the observation field of transparent armor.2) The structural design strategy and the analysis of failure behavior are explored in transparent armor. A comparison is made between the design principles, performance differences, and failure mechanisms of conventional laminated glass structure (or similar-laminated glass structure) armors and the new structural armors with three functional layers. At present, the core of structure design is the synergistic matching of hard protective materials and soft energy-absorbing materials in optical properties and protective performance in the new structural transparent armor. The comparison results of different structural armors indicate that the weight of new structural armor is reduced by more than 30%, compared with that of conventional glass transparent armor under the same protection level. The conventional armor has a large damage surface with the first layer and the second layer being crushed and cracked, after being impacted by the projectile, resulting in the loss of its observation and combat capabilities. The damage surface of new structural armor shows granular crushing around the impact point, leading to the local area of the armor losing its observation and combat capabilities. The main causes of failure are the interface delamination cracking and projectile penetration damage in the optical application field of transparent armor.3) The evaluation methods of transparent armor are summarized. The comparison results of key evaluation items of transparent armor procurement indicate that European and American countries focus on the optical properties of armor, while China pays more attention to the protective performance of armor. The United States establishs a relatively complete transparent armor procurement specification, and incorporates the optical properties of armor such as transmittance, haze, and light distortion into the procurement standards. For special night vision application scenarios, the U.S. military focuses on the optical properties of transparent armor rather than protective performance. However, there is no scientific and effective normative document for armor evaluation methods in China, and their test and characterization are often based on the evaluation methods of non-transparent protective materials. In addition, the existing test standards for transparent bulletproof materials do not pay attention to the balanced evaluation between the optical properties and protective performance of the armor, as well as the adaptability of transparent armor to the environment.Summary and prospects A key to improve the protective performance and lightweight degree of transparent armor is to achieve the breakthroughs in the performance of transparent protective materials. It is important to establish and improve the scientific testing methods and assessment evaluation system of transparent armor for achieving practical application of transparent armor. This review provides a reference for the structural design and application evaluation methods of the new generation of transparent armor, and assists the high-quality development of armor industry.

Long afterglow luminescent materials have an intrinsic characteristic of emitting light continuously after being irradiated by excitation sources, which are widely used in the fields of emergency indicator, optical information storage, information encryption and bio-imaging, etc.. Although long afterglow phosphor powders are extensively investigated, their unstable physical and chemical properties and self-absorption induced low afterglow brightness strictly limit their long-term developments. These bottlenecks can be overcome via applying long afterglow glasses, but the limited ion doping concentration in glass matrix could not conducive to the improvement of their afterglow brightness and afterglow duration. Contrarily, long afterglow ceramics of dense structure integrate the merits of both phosphor powders and glasses, and develop rapidly in recent years. Among them, long afterglow transparent ceramics are easy to achieve a desired luminescence uniformity, a high luminescence brightness and a long afterglow duration since their afterglow brightness and afterglow duration can increase with increasing the ceramic thickness (i.e., volume effect), providing a considerable application prospect in the future. Therefore, it is of great significance to conduct systematic investigations towards the optimization of the optical and afterglow properties of long afterglow ceramics and long afterglow transparent ceramics as well as the development of their novel processing techniques.Long afterglow ceramics mainly include aluminate, garnet, sesquioxide and zinc gallate ceramics. Long afterglow aluminate ceramics are easy to achieve a long afterglow duration and a high afterglow brightness, and solid phase reaction method is a mainstream preparation technique of aluminate long afterglow ceramics. The impact of ion doping on the optical and afterglow properties is investigated. With the development of technology, new preparation techniques such as laser sintering and melt-quenching methods are also used to prepare aluminate long afterglow ceramics. Long afterglow garnet ceramics are widely concerned because of their advantages such as abundant doping ions and easy to achieve high concentration doping. Methods of vacuum sintering and ion sensitization are applied to regulate the trap concentration and trap depth of garnet ceramics, and pressure assisted sintering becomes a novel research focus to promote their densification behaviors. The cationic sites of long afterglow sesquioxide ceramics can accommodate a variety of red emitting rare-earth ions such as Eu3+ and Pr3+ ions, thus facilitating their red afterglow luminescence. Despite ion doping and charge compensation methods are utilized to enhance the red afterglow properties of sesquioxide ceramics, their afterglow luminescence performances are still needed to be improved. Long afterglow zinc gallate ceramics are characterized by flexible ion doping and easy formation of reverse defects, and the current research mainly focuses on the white afterglow performances after being irradiated by X-ray irradiations. Other long afterglow ceramics like stannate ceramics, germanate ceramics, phosphate ceramics and nitride ceramics are also reported recently.Long afterglow transparent ceramics are developed rapidly in recent years, and it is demonstrated that the afterglow property of transparent ceramics is superior to that the conventional long afterglow ceramics. The widely investigated long afterglow transparent ceramics include garnet ceramics, gallate strontium aluminate ceramics and zinc silicate ceramics. However, the preparation condition for long afterglow transparent ceramics is severe. Investigations toward long afterglow transparent ceramics are mainly the optimization of the optical qualities and defect states, and sintering techniques include vacuum sintering and hot isostatic pressing sintering are extensively applied to promote their sintering densification behaviors. Recently, novel preparation techniques for long afterglow transparent ceramics such as spark plasma sintering method and melt-quenching method are reported, and the obtained ceramics exhibit the superior optical and afterglow properties. Summary and prospects Although long afterglow ceramics and long afterglow transparent ceramics have achieved vigorous development, some problems (i.e., inadequate recognition of afterglow mechanism, unsatisfactory optical transmittance of long afterglow transparent ceramics, unsatisfactory afterglow performance of blue light excited long afterglow ceramics and lacking of investigations on red afterglow ceramics) are still needed to solve in order to accelerate their future developments. Therefore, developing advanced powder synthesis methods and novel ceramic fabrication techniques such as melt-queching method are crucial to improve the optical quality of long afterglow ceramics, and exploring large sized or complex shaped long afterglow ceramics is also necessary to promote their industrialization. In improving the afterglow performances of long afterglow ceramics, developing ceramic matrices with abundant lattice defects as well as the active or sensitize ions with long fluorescence lifetimes is significant in addition to further exploring the afterglow mechanisms for long afterglow ceramic systems. Also, utilizing the synergistic effect between ceramic matrix and doping ions, by means of regulating the energy difference between doping ions and conduction bands to manipulate trap depth of ceramics is another meaningful approach to optimize afterglow performances of ceramics. Finally, promoting the real applications of long afterglow ceramics is the ultimate destination of the research. The present mainstream application of long afterglow ceramics focuses on their visible light illuminations acting as indicators or decorations. In this regard, it is urgent to explore the near-infrared emitting long afterglow ceramics to accelerate their infrared imaging applications, which is conducive to the development of medical treatment and industrial production. Meanwhile, the development and application of mechano-luminescent long afterglow ceramics, deep trap long afterglow ceramics and photochromic long afterglow ceramics are important research aspects of long afterglow ceramic materials. In general, realizing the intrinsic optimizations of both optical qualities and afterglow performances of long afterglow ceramics is a key to accelerating their real applications, and the corresponding investigations are still needed to carry out in the future.

ZnS is a kind of material with superior optical, mechanical and thermal properties from visible light band to long-wave infrared band (i.e., 0.36-12 μm). It is widely used in infrared imaging, remote sensing, and guidance as well as other photoelectric systems. Also, ZnS is a semiconductor material with the widest band gap (3.6-3.9 eV) in the II-VI group compounds. ZnS materials can have superior luminescence properties by doping divalent transition metals (i.e., Co2+, Ni2+, Cr2+, and Fe2+). It is widely used in mid-infrared solid lasers, light-emitting diodes, radar screens and fluorescent lighting.Compared with ZnS single crystal, polycrystalline ZnS transparent ceramics can be used as main materials in commercial applications due to the advantages like short preparation cycle and easy preparation of coarse grain sizesas. The preparation methods of transparent ZnS ceramics mainly include hot pressing (HP), chemical vapor deposition (CVD), chemical vapor deposition combined with hot isostatic pressing (HIP), and spark plasma sintering (SPS). ZnS transparent ceramics prepared by different methods have certain optical and mechanical properties differences. The transmittance of ZnS ceramics prepared by HP in the visible light band is not as high as that of multi-spectral ZnS ceramics prepared by CVD and HIP post-treatment. However, the oriented grain size of multi-spectral ZnS ceramics is larger. The growth of grain size leads to a significant decrease in the mechanical properties, and the preparation cycle of CVD-prepared ceramics is also longer. Therefore, this review mainly represented development on the optical and mechanical properties of ZnS transparent ceramics prepared by different methods and introduced the application of ZnS transparent ceramics in infrared windows and mid-infrared lasers. In addition, the future development of ZnS transparent ceramics was also prospected.The physical and chemical properties of ZnS materials were described, including the phase structure, optical properties, and matrix characteristics of ZnS as a divalent transition metal (TM2+) doping. The main parameters of ZnS transparent ceramics prepared by different processes were summarized. The application scenarios of ZnS transparent ceramics in infrared window materials and mid-infrared lasers were described. The development of HP-ZnS transparent ceramics, CVD-ZnS transparent ceramics, and SPS-ZnS transparent ceramics applied in infrared windows were represented, and the preparation methods were briefly introduced. The preparation of ZnS transparent ceramics for mid-infrared lasers was introduced. The Cr2+ and Fe2+ doped ZnS transparent ceramics were discussed, and the spectral characteristics of two different ions in ZnS matrix were analyzed. The preparation of Cr2+/Fe2+:ZnS transparent ceramics were discussed. In addition, this review also prospected future development of ZnS transparent ceramics in different application fields.Summary and prospects:Although the long-wave infrared transmittance of HP-ZnS transparent ceramics is close to its theoretical value, the transmittance of its 1 064 nm laser band does not exceed 60%, which makes it difficult to realize the application in dual-mode guidance. Most of the HP-ZnS transparent ceramics still have a certain amount of hexagonal phase and impurities, leading to a decrease in the short-band transmittance of HP-ZnS transparent ceramics.Although the preparation process of CVD-ZnS transparent ceramics is relatively mature, which can be commercialized. The grain size of multi-spectral ZnS transparent ceramics increases to dozens of microns after HIP, resulting in low mechanical properties of the materials and poor corrosion resistance of multi-spectral ZnS.Most SPS-ZnS ceramics are still in the initial stage. Although SPS-ZnS ceramics have an advantage of maintaining small grain size, they have a low relative density and a poor long-wave transmittance of less than 70%.For ZnS transparent ceramics for mid-wave infrared laser, TM2+:II-VI can be mainly prepared by a thermal diffusion method. Cr:ZnS achieves a laser output, but Fe:ZnS does not achieve a laser output due to its low optical quality and lack of suitable pump source.Since the first semitransparent ZnS ceramic was prepared in the 1950s, ZnS transparent ceramics have been extensively studied in the past 70 years. With the increasing demand for infrared imagers and multi-spectral imagers in the fields of national defense, security, and civilian fields such as vehicle night vision systems, there is still a challenge for technological progress and development of ZnS transparent ceramics in the future. In terms of ZnS ceramics for infrared windows, it is necessary to continue to optimize the powder preparation process, improve the chemical purity and phase purity of the powders, and regulate the Zn/S value, combined with HP and HIP to prepare multi-band high-transparent ZnS transparent ceramics. In the future, the mechanical properties of multi-spectral ZnS transparent ceramics can be further improved by coating and other technologies, and its application range can be expanded. The further optimization of the preparation process of SPS-ZnS ceramics is needed. In terms of ZnS transparent ceramics for mid-infrared laser materials, the effective doping concentration and structural uniformity of active ions are improved without reducing the transmittance of the sample, and a further exploration is needed to prepare ZnS ceramics with a high power laser output.

Layered rare-earth hydroxides (LRHs) exhibit some characteristics of inorganic layered compounds (i.e., ion exchange, intercalation, and nanosheets exfoliation). LRHs possess the unique optical, electrical, magnetic, and catalytic properties of rare-earth ions. Therefore, LRHs as inorganic layered compounds have a wide range of potential applications and have attracted recent attention. The intercalated anions of LRHs readily exchange with various anions, and subsequently, nanosheets are exfoliated from the host layers. Two-dimensional nanosheets can maximize the physicochemical properties of the host layers under specific conditions. Under specific conditions, two-dimensional nanosheets can demonstrate the physicochemical properties of the host layers. LRHs nanosheets serve as effective building blocks for constructing novel functional materials.This review represented recent research progress on the LRHs, mainly on the structure characteristics, controllable synthesis methods, and nanosheet exfoliation. This review also highlighted the existing applications of LRHs in transparent ceramics and thin film fabrication. The structural and physicochemical properties of functional ceramics prepared using LRHs were summarized, and some insights into future research directions for efficient LRH synthesis and structural design were given, providing a reference for the potential applications of LRHs in various fields.LRHs are a novel class of anionic layered compounds, which can be synthesiszed via homogeneous precipitation and hydrothermal reaction as the most commonly used and effective synthesis techniques. A variety of LRHs pure phases are synthesized via precipitation. The hydrothermal method offers some advantages like high product purity, crystallinity, and minimal contamination. LRHs with different phases and morphologies can be prepared via controlling the hydrothermal synthesis conditions. A one-step synthesis technique for ultrathin LRHs nanosheets is developed by controlling the synthesis temperature.LRHs combine the characteristics of inorganic layered materials and rare-earth ions. The properties of ion exchange and nanosheet exfoliation in LRHs are consistently under scrutiny. NO3- ions in nitrate-type LRHs can exchange with F- and SO42- ions at room temperature. LRHs with an increased interlayer spacing due to ion exchange can be delaminated into two-dimensional nanosheets in aqueous or organic solutions. The use of NH4NO3 as a mineralizer expands the range of conditions for synthesizing LRHs, and also generates the larger-sized grains.The powder preparation process in ceramic manufacturing has a significant impact on the optical properties of transparent ceramics. LRHs prepared by wet-chemical methods can serve as precursors for oxide ceramic powders used in sintering and transparent ceramic production. LRHs are prepared by a one-step method at low temperatures and subsequently by ion exchange and calcination to produce oxide powders, which are more favorable for transparent ceramic production. The resulting transparent ceramics exhibit a fewer defects and a higher transparency. Compared to conventional manufacturing processes, the method of producing transparent ceramics through LRHs allows the acquisition of more uniform and stable ceramic particles, while effectively reducing impurity contamination. Moreover, it enhances the diversity of transparent ceramics functionality via the effective doping of rare-earth elements, thus simplifying the preparation process.A technology for preparing transparent ceramic fluorescent films is developed based on LRHs via utilizing their chemical properties and special interfacial reactions. LRHs undergo ion exchange and exfoliation treatment to obtain the dispersed nanosheets. The exfoliated nanosheets are firstly coated onto a hydrophilically treated substrate and then sintered to prepare a transparent ceramic film. This process can produce a variety of rare-earth-doped Y3Al5O12 and GdAlO3 films, exhibiting superior fluorescence properties and thermal stability of luminescence.Summary and prospects LRHs can be synthesized via precipitation and hydrothermal methods. However, the synthesis of doped LRHs for a few rare-earth elements remains elusive due to significant variations in the physicochemical properties of different rare-earth elements (i.e., primarily stemming from their distinct ionic radii). It is possible to effectively control the phase and morphology of LRHs via adjusting the synthesis parameters such as temperature and pH value, thereby expanding their application range. LRHs can increase interlayer spacing through anion exchange, allowing for the exfoliation of nanosheets. It is possible to effectively exfoliate large-sized nanosheets that retain the characteristics of host layer. However, the synthesis and exfoliation under hydrothermal conditions may not meet the production requirements for large-scale applications. Therefore, low-temperature, one-step synthesis of LRHs nanosheets could be a future research aspect.LRHs can be used as precursors to prepare oxide ceramic powders, meeting the strict requirements of transparent ceramics for sintering powders. The unique layered structure and physicochemical properties of rare-earth ions make LRHs highly promising for transparent ceramic production. Meanwhile, the nanosheets exfoliated from LRHs as ideal building units for transparent fluorescent films used in optoelectronic devices have attracted much attention in transparent ceramic film preparation. This can have a theoretical foundation for the future application of LRHs in the field of high-performance LEDs and laser lighting. The existing research primarily focuses on the application of LRHs in photonic materials. A further research on efficient synthesis and structural design can have a promising potential to enable the widespread application of LRHs in various fields.

The 3-5 μm mid-infrared laser can be used in infrared countermeasures, biomedicine, atmospheric remote sensing and environmental monitoring. All solid-state lasers using LD directly pumped laser materials become a main output method for mid infrared lasers due to their advantages like simple system, high conversion efficiency, and good beam quality. Polycrystalline transparent ceramics are commonly used as laser materials due to their preparation easiness, superior thermal and mechanical properties as well as the ability to achieve high doping concentrations. However, the development of high-power lasers requires overcoming the inevitable thermal effects during laser operation, i.e., thermal birefringence, thermal lensing, and depolarization, which arise from the temperature gradient inside the laser material. It is crucial to use laser materials with a high thermal conductivity to alleviate thermal effects and ensure the quality and stability of laser output. Moreover, laser host materials must also have a lower phonon energy to suppress non-radiative transition processes that compete with the luminescence processes. It is thus difficult for materials to simultaneously possess all of these properties. In recent years, a large number of reports have claimed that Y2O3-MgO nanocomposite ceramics have ideal properties required for laser host materials, including high infrared transmittance, low phonon energy, high thermal conductivity, and good mechanical properties. The microstructure and properties of Y2O3-MgO nanocomposite ceramics doped with rare-earth activation ions are investigated, indicating that nanocomposite ceramics can be used as potential host materials for mid infrared lasers.This review summarized the existing research results of Y2O3-MgO nanocomposite ceramics as a laser host material and also represented recent technologies and methods for preparing Y2O3-MgO nanocomposite ceramics.The existing studies by theoretical models and experiments indicate that Y2O3-MgO nanocomposite ceramics have a high thermal conductivity, compared with Y2O3 ceramics, and a low maximum phonon energy (591cm-1). This is due to a synergistic effect of composite properties, combined with a high thermal conductivity of the second phase MgO and a low phonon energy of Y2O3. However, the optical transmittance of Y2O3-MgO nanocomposite ceramics is mainly affected by grain size, porosity, and phase domain distribution, which makes it imperative to improve their microstructure. Therefore, some technologies and methods for preparing ceramics are emerged, i.e., colloidal forming, two-step sintering, high-pressure sintering, and reducing a difference in refractive index between the two phases. Moreover, the research results of rare-earth ion doped nanocomposite ceramics, such as Er：Y2O3-MgO and Ho：Y2O3-MgO, also confirm that the introduction of high thermal conductivity MgO can effectively improve the thermal conductivity of ceramics without affecting the photoluminescence properties of the luminescent components, indicating that Y2O3-MgO nanocomposite ceramics can be used as promising laser host materials.Summary and prospects Y2O3-MgO nanocomposite ceramics are promising laser host materials. However, there is still a gap between the optical quality requirements of Y2O3-MgO nanocomposite ceramics and infrared laser materials, even though researchers have made significant efforts to improve the microstructure of Y2O3-MgO nanocomposite ceramics. This leads to the preliminary characterization and analysis of the photoluminescence characteristics of rare-earth ion doped Y2O3-MgO nanocomposite ceramics, and has not yet achieved infrared laser output. There are still some aspects that need further research and exploration before its practical application: 1) Developing preparation methods and optimizing processing parameters are the main direction of future research to obtain the dense samples, while effectively suppressing their grain growth. If the grain size is small enough, it is expected to expand the transmittance range of Y2O3-MgO nanocomposite ceramics to the visible light band and apply it to the field of mid infrared lasers, 2) Different activation ions and pump light sources are needed to be selected for laser output at different wavelengths, and even sensitizers need to be added to match the existing pump light. As a result, it is necessary to investigate the influence of rare-earth dopants on the microstructure, thermal properties, optical properties, and laser output performance of nanocomposite ceramic systems, and 3) In order to expand the selection range of laser host materials, a further diversified research on nanocomposite ceramics needs to be carried out.

For the favorable physicochemical properties, high fluorescence conversion efficiencies, a relatively short fluorescence lifetime, and an efficient spectral congruence between excitation and ultraviolet/blue LED emission spectra, Ce3+-doped garnet-based fluorescent materials can be used in encompassing lighting, displays, medical imaging, etc.. For leveraging the diverse cation sites within a garnet structure, the emission wavelength of Ce3+ can be continuously adjusted from 460 nm to 610 nm via the incorporation of varied matrix chemical constituents. This versatile tunability broadens the horizons of their potential applications and augments the repertoire of materials available for advancing Ce3+ luminescence theory. However, recent work on Ce3+-doped garnet-based fluorescent materials has unveiled certain anomalies and even contradictions within the characteristics and the emission wavelength modulation theory, becoming one of the prominent bottlenecks hindering the development of this material. To address this issue, this review represented the relative theories of substrate chemical component substitution and the modulation of Ce3+ emission wavelengths. The review also summarized recent advancements in Ce3+-doped garnet-based fluorescent materials, and the influence of ion substitution on Ce3+ luminescence performance with the distortion factor of [CeO8]. This review discussed the Ce3+-doped yttrium aluminum garnet structure as a prototype and comprehensively analyzed the impacts of commonly used doping ions that occupy the [AO8], [BO6], and [CO4] sites on the emission wavelength, thermal quenching resistance, and the [CeO8] distortion factor of Ce3+.The ion substitution of a single lattice generally necessitates that the substituting ion and the substituted ion have the same valence state and exhibit a minimal disparity in atomic radius. In the case of [AO8] lattice substitution, Y3+ can be replaced with rare-earth ions, i.e., Lu3+, Tb3+, Gd3+, and La3+. When the radius of the doped rare-earth ions is greater that that of Y3+, the emission wavelength of Ce3+ shifts towards the red end of the spectrum. Conversely, when the radius is smaller, the shift occurs in the opposite direction. The ion substitution of octahedral lattice sites mainly involves the replacement of Al3+ by Ga3+, Sc3+, and In3+. With an escalation in the concentration of doped ions like Ga3+, Sc3+, and In3+, the peak wavelength of the Ce3+ emission spectrum has a blue shift. In addition, the incorporation of Sc3+ effectively enhances the thermal quenching resistance of Ce3+, while the inclusion of Ga3+ diminishes the thermal quenching resistance of Ce3+. The main substitutions of dodecahedral-octahedral lattice ion pairs include Ca2+-Hf4+ and Ca2+-Zr4+ replacing Y3+-Al3+. The emission and excitation peak wavelengths of Ce3+ gradually blueshifts as Ca2+-Hf4+ and Ca2+-Zr4+ doping contents increase. The substitution of dodecahedral-tetrahedral lattice ions is mainly achieved by M2+-Si4+ (M=Mg, Ca, Sr, Ba) replacing Y3+-Al3+, with alkaline earth metals occupying the dodecahedral lattice and Si4+ occupying the tetrahedral lattice. In the garnet system, when M2+-Si4+replaces Y3+-Al3+, the emission wavelength of Ce3+ undergoes a blue shift with the increase of M2+ ion radius, resulting in improving the thermal stability. The substitution of octahedral-tetrahedral lattice ions mainly involves Mg2+-Si4+/Ge4+ replacing Al3+-Al3+, where Mg2+ occupies the octahedral lattice and Si4+/Ge4+ occupies the tetrahedral lattice. Among them, Mg2+-Si4+ replacing Al3+-Al3+ is an effective method to achieve a large redshift of Ce3+ emission wavelength. The introduction of Mg2+-Ge4+ also leads to the redshift of Ce3+ emission wavelength, but the extent of this redshift is considerably less than that achieved by Mg2+-Si4+. The chemical components in dodecahedral-octahedral-tetrahedral lattice co-substitutions are more intricate, with Ca2+ and Mg2+ occupying the dodecahedral lattice, Mg2+, Sc3+, and Hf4+ occupying the octahedral lattice, and Si4+ and Ge4+ in the tetrahedral lattice. Different lattice ions on the co-substitution affect the luminescence performance of Ce3+.Summary and prospects The effect of ion substitution on the luminescence performance of Ce3+ is analyzed via utilizing Ce3+-doped yttrium aluminum garnet fluorescent material as a prototype. The comparative study reveals that the most effective redshift of Ce3+ emission wavelength appears when Mg2+-Si4+ occupies the octahedral-tetrahedral lattice configuration. This results in a possibility of red shifting the emission wavelength of Ce3+ at 610 nm, thereby significantly enhancing the color rendering capabilities of white LED/LD lighting systems. Conversely, an effective blue shift in Ce3+ emission wavelength appears in Ca2+-Zr4+ located in the dodecahedral-octahedral lattice, with a potential shifting at 460 nm. Furthermore, the inclusion of Sc3+ in the octahedral lattice and Ba2+-Si4+ in the dodecahedral-tetrahedral lattice markedly improves the thermal quenching resistance of Ce3+. Hence, this combination exhibits substantial advantages for high-power LED and LD lighting applications. In contrast, the introduction of Ca2+-Mg2+-Si4+/Ge4+ into the dodecahedral-octahedral-tetrahedral lattice configuration leads to a poor thermal quenching resistance for Ce3+, indicating a unsuitability for white light LED/LD illumination.Furthermore, the d88/d81 ratio is related to the Ce3+ emission wavelength and thermal stability, based on the degree of [CeO8] distortion. It is indicated that at an equivalent Ce3+ doping concentration, an increase in the d88/d81 ratio corresponds to a redshift in emission wavelength and a decrease in thermal stability. Among the research results available, little work on the impact of d88/d81 value on the luminescence performance has been done yet. A future research on the in-depth physical models and subsequent experimental validation for Ce3+-doped garnet-based fluorescent materials is needed.

With recent development of solid-state lasers, especially high-power fiber lasers in intelligent manufacturing, scientific research and the other fields are increasingly widespread. Eliminating the reflected light to protect the optical path system and stabilize the laser output becomes a critical issue. Magneto-optical materials with a high Verdet constant and a large size are an essential optical element for developing high-performance optical isolators, which can significantly shorten the length of magneto-optical medium, thus enabling the development of integrated and miniaturized optical isolators. Terbium (III) oxide has no absorption peak in the range 500?1 500 nm and its Verdet constant is 3.5 times greater than that of commercial terbium gallium garnet single crystals. Magneto-optical transparent ceramics are a new inorganic transparent optical functional material, and its optical transparency comparable to that of single crystals and glasses, and it has the advantages of adjustable composition and structure of polycrystalline ceramics.Tb2O3 is considered as a promising candidate medium for magneto-optical isolators due to its superior magneto-optical properties. However, challenges in the preparation are the oxidation of trivalent terbium oxide powder in air, and the reversible phase transition at 1 600 ℃, 2 167 ℃ and 2 370 ℃. To address the problems above, Tb2O3 crystals were grown via low-temperature flux growth, the optical floating zone method and the supercritical solvothermal method. However, Tb3+ ions are easily oxidized, undergoing multistage phase transitions at 1 550?2400 ℃ (i.e., the melting point, which makes it difficult to grow Tb2O3 single crystal from the melt. As a representative of ceramic process routes, rare-earth ions stabilized Tb2O3, TAG and TGG transparent ceramics are prepared via vacuum sintering, hot pressed sintering or NC-PLSH technology. This review represented the development, technologic process and key difficulties of Tb2O3 crystals and ceramics. Tb2O3 single crystals can be grown by flux and floating zone methods, but it is difficult to improve the crystal size, optical quality, and production efficiency to the application level. Tb2O3 crystal was prepared by a flux method, but its crystal size was only 5 mm×5 mm×2 mm. Compared with the crystal growth, a transparent ceramic technology has advantages in preparing large size and optical grade bulk materials. In order to stabilize the phase structure of terbium oxide, yttria-stabilized Tb2O3 ceramics was prepared by a self-propagating high-temperature method and vacuum sintering. Zirconia is used as a sintering aid to reduce the sintering temperature and accelerate the mass transport process in the ceramic sintering. Preparation schemes of transparent ceramics bring a prospect for Tb2O3 bulk materials.Summary and prospects Tb2O3 is a magneto-optical material with the maximum Verdet constant in visible bands. Its Verdet constant at 633 nm is 476 rad·T?1m?1, which is 3.5 times greater than that of commercial TGG single crystal. However, the ultra-high melting point and multistage reversible phase transition of Tb2O3 make it difficult to prepare Tb2O3 crystals and ceramics by a conventional method. This review compared solid-solution method combined with vacuum sintering and nano-crystalline pressure-less sintering in H2 atmosphere for preparing Tb2O3 ceramics involving yttrium-stabilized terbium oxide, lutetium-stabilized terbium oxide and zirconium oxide and lanthanum-doped terbium oxide ceramics, etc.. The existing progress in magneto-optical Tb2O3 transparent ceramics is described. The in-line transmittance of Tb2O3 ceramics with the diameter of 10 mm is close to 80%. Tb2O3 ceramics with the large diameter of >30 mm can be fabricated. The future work is to improve the optical quality of Tb2O3 ceramics, further develop the large-diameter ceramics, and evaluate the thermal effects of Tb2O3 ceramics in high power lasers.1) The sinterability (i.e., purity, particle size and homogeneity) of as-synthesized powder is a key factor in the preparation of highly transparent ceramics. This is a research aspect to further explore the process of wet chemical co-precipitation and accurately control the morphology and uniformity of powder.2) From the perspective of ceramic forming, wet molding is easy to obtain uniform green compact with connected pores, which is beneficial to producing large-size and high-quality transparent ceramics.3) Hot isostatic pressing sintering is an effective way to eliminate micro-pores. Terbium ion can be effectively maintained in +3 valence charge via sintering in H2 atmosphere. Optical grade Tb2O3 ceramics are expected to obtain by NC-PLSH technology and HIP post treatment.4) In order to meet the demands of high-power laser system, it is crucial to evaluate the thermal depolarization and thermal lensing effects of Tb2O3 ceramics used as an optical isolator medium in high energy lasers as well as the thermal compensation techniques adopted.

Conventional transparent materials can be primarily divided into four categories, i.e., glasses, polymers, single crystals, and transparent ceramics. Growing large single crystals with high melting points is difficult, and glass and polymers have poor mechanical and chemical stability. These shortcomings limit their applications. Transparent ceramics are favored because of their high refractive index, high hardness, high strength, high thermal conductivity, and high chemical stability. In 2005, La2Hf2O7 was fabricated into transparent ceramics with a high density and a high atomic number, having a great potential in the detection field of scintillators. Since then, A2B2O7 transparent ceramics has attracted much attention. The structure of A2B2O7 ceramics is mainly determined by the radius ratio of A-site and B-site ions (rA/rB), and includes three structures (i.e., fluorite structure (rA/rB1.78)). While, only the fluorite- and pyrochlore-structure A2B2O7 ceramics could be transparent monolith when sintered at proper temperature and pressure.A2B2O7 ceramics with fluorite and pyrochlore two-phase coexisting structures have the advantage of smaller grains, compared with conventional A2B2O7 transparent ceramics with single-phase structures. The mechanical properties of two-phase coexisting structures ceramics can be improved due to the smaller grains. LaLuZr2O7 transparent ceramics with two phases of fluorite and pyrochlore were fabricated in 2014. Two phases are uniformly distributed in the ceramic and the average grain size of the ceramic is only 2?3μm. The in-line transmittance of LaLuZr2O7 transparent ceramic is 73.4% at 1 100 nm due to the close refractive index between the two phases of fluorite and pyrochlore, and the scattering of light across the interface between the two phases is negligible. In 2019, (La0.2Nd0.2Sm0.2Gd0.2Yb0.2)2Zr2O7 was fabricated into transparent ceramics with a fluorite and pyrochlore two-phase coexisting structure. Compared with conventional transparent ceramics, high-entropy transparent ceramics may have better mechanical properties, corrosion resistance, and high-temperature resistance. The enhanced atomic disorder due to multi-element doping in high-entropy ceramics makes it a promising material for high-temperature windows with a lower thermal conductivity. However, there are some challenges in the preparation of two-phase coexistence transparent A2B2O7 ceramics. In some previous reports on A2B2O7 transparent ceramics with two-phase coexistence, the transmittance of the ceramics was severely reduced when there was a large difference in the grain sizes of the fluorite and pyrochlore phases.It is necessary to ensure that the grain growth rates of the pyrochlore and fluorite phases are similar during sintering. Transparent ceramic scintillators of La2Hf2O7:Ti4+ were fabricated in 2005. The transmittance of La2Hf2O7:Ti4+ transparent ceramics at 600-800 nm is close to 60%. La2Hf2O7:Ti4+ transparent ceramics gain a high X-ray stopping power due to its high density and high atomic number. The integrated emission intensity of La2Hf2O7:Ti4+ transparent ceramics is 1.5 times greater than that of Bi4Ge3O12 single crystals for the same size and the same excitation conditions. In 2019, Tb2Hf2O7 was fabricated into transparent ceramics. Tb2Hf2O7 transparent ceramic ceramics perform well in the Faraday magneto-optical effect tests, having higher measured Verdet constants than commercial Tb3Ga5O12 single crystals and a potential application as an excellent magneto-optical material. It is indicated that the magneto-optical properties of magneto-optical materials are related to the concentration of Tb3+ in the materials, and the higher the Tb3+ concentration is, the better the magneto-optical properties will be. Tb2Hf2O7 transparent ceramics can be doped with higher concentrations of Tb3+ due to the structural advantages of A2B2O7, thus having better magneto-optical properties rather than garnet-structured Tb3Ga5O12 single crystal.Summary and prospects This review represented the preparation methods, performances, and potential applications of A2B2O7 transparent ceramic. A2B2O7 transparent ceramics have the characteristics of a high melting point, a high density, and a high refractive index, which are potentially valuable in the fields of optical material. However, the optical loss and optical homogeneity of A2B2O7 transparent ceramics are difficult to guarantee, a further study of optical quality improvement for A2B2O7 transparent ceramics is still needed. At present, the preparation process of A2B2O7 transparent ceramics is still immature, there are many residual pores in the sintered A2B2O7 transparent ceramics, affecting the optical quality of the A2B2O7 transparent ceramics. It is necessary to optimize the raw powder preparation process and add sintering additives to reduce the pores in the sintered A2B2O7 transparent ceramics. Although A2B2O7 transparent ceramics have great application prospects in solid-state lasers, the low thermal conductivity of A2B2O7 transparent ceramics makes heat dissipation a challenge. If A2B2O7 transparent ceramics can be used in solid-state lasers, a superior heat dissipation system needs to be designed. In summary, A2B2O7 transparent ceramics is a promising transparent material, but there are still many problems to be solved before the application.