As an important optical material,transparent polycrystalline alumina (PCA) ceramic is widely used in aerospace,high-temperature windows,medical due to its high strength,corrosion resistance,high hardness,and excellent stability at high temperatures. Since Al2O3 belongs to the crystal structure of the trigonal crystal system (a=b≠c),light is prone to birefringence when passing through the interior of the ceramic,and its transmittance is difficult to reach that of cubic crystal materials such as Y2O3 even under almost fully dense conditions. Using a special sintering process to decrease the sintering temperature and improve the real in-line transmittance of PCA ceramics has always been a research hotspot in related fields. In this work,a comprehensive review is made from the point of view of the sintering process based on the research status of transparent PCA ceramics in decades.

Two-dimensionality of the properties of a volumetric object may be defined by its anisotropy. Perovskite-like compounds of the family of Aurivillius–Smolensky phases (ASPs) have exactly this property. By studying perovskite-like compounds of the family of Aurivillius–Smolensky phases and the changes in the volume of the crystal lattice,occurring in them,namely the increase or decrease in the phase transition temperature (Curie temperature),the preferred direction of grain growth with an increase in the sintering temperature and,as a consequence,the increase in the relative permittivity,the main factor in all cases is the change in the linear parameters a and b,lying in the plane perpendicular to the c-axis.

Decreasing the scale of vanadium dioxide (VO2) structures is one of the ways to enhance the switching speed of the material. We study the properties of VO2 films of altered thicknesses in the range of 20–170nm prepared on c-sapphire substrates with a TiO2 sublayer by pulsed laser deposition (PLD) method. The synthesis regime to design a TiO2 film was preliminarily optimized based on XRD data. XRD patterns reveal an epitaxial growth of the VO2 films with distortion of the monoclinic cell to hexagonal symmetry. The positions of the lattice vibration modes in Raman spectra are similar to those in bulk VO2 when the film thickness is greater than ∼30nm. For VO2 films thicker that ∼20nm,a lattice strain results in the modes’ positions and intensity change. However,the electrically triggered transition in a ∼50nm thick VO2 film reveals forward and reverse switching times as short as 20ns and 400ns,correspondingly.

In this work,solid-state reaction method and a two-step sintering procedure were successfully used to prepare neodymium-doped yttrium aluminum garnet (Nd:YAG) transparent dielectric ceramics. The effects of the microstructure and crystal structure of the ceramics on the microwave dielectric properties were investigated. Samples after vacuum sintered at 1600–1750∘C and further hot isostatic pressing (HIP) at 1700∘C had a pure YAG phase. The cell volume increased slightly after further HIP treatment compared to the vacuum-sintered ceramics. Combined with the disordered arrangement of the atoms in the HRTEM image,the lattice expansion could be explained by the entrance of larger ions into the YAG lattice. Moreover,the sintering drive provided by HIP could effectively eliminate the porosity in ceramics,increase the average grain size,and narrow the grain boundaries,which was conducive to the improvement of transmittance and quality factor (Qf) value. Finally,the sample that was vacuum sintered at 1700∘C and then exposed to HIP had outstanding microwave dielectric characteristics: εr=10.64,Qf=515,800GHz and τf=−21ppm/∘C.

New microwave dielectric ceramics of LiZnSbO4 were prepared by the reactive sintering method. Within the sintering temperature zone of 1350∘C–1385∘C,a single-phase LiZnSbO4 with an orthorhombic structure was identified by XRD refinement,which contains 10 molecular formula per unit cell (i.e.,Z=10). The relative density and average grain size had a main effect on εr and Q×f value of the LiZnSbO4 ceramics,whereas the phase assemblage was responsible for its τf. The 1375∘C-sintered LiZnSbO4 specimens owned comparable low εr∼6.5,but ultra-high Q×f∼74,500GHz (nearly three times) compared to LiZnPO4 and LiZnVO4 ultra-high Q×f∼74,500GHz (nearly three times) compared to LiZnPO4 and LiZnVO4 manufactured,which showed a return loss as low as 37.4dB,maximum gain of 5.7dB and a fractional bandwidth of 3.9% (11.95–12.43GHz).

The Mg2Al4Si5−xSnxO18 (0≤x≤0.20) ceramics were successfully synthesized via the solid-state reaction method. The XRD results show that the main phase of the ceramics is Mg2Al4Si5O18. When x=0,there is a second phase of Al2SiO5. With the increase of x,the content of Sn2+ gradually increased,the Al2SiO5 phase disappeared,and the SnO2 and MgAl2O4 phases appeared. In addition,the cell volume of the ceramic changes with the increase of x,which indicates that partial Sn2+ successfully enters the lattice with ion substitution and lattice distortion. The relative density and εr are highly correlated. The quality factor (Qf) is not only affected by the relative density but also by the symmetry of the [Si4Al2] ring. The bond strength as an evaluation of the stability of the crystal structure determines the magnitude of τf. Good microwave dielectric properties were achieved for samples at x=0.04 with sintering at 1400∘C: εr=4.86,Qf=17,000GHz and τf=−32ppm/∘C,demonstrating that Mg2Al4Si4.96Sn0.04O18 ceramics are an ideal candidate for microwave electronics.

A high performance of novel three-component composites with 2–1–2 connectivity is reported and discussed. Layers of the composites are parallel-connected,and each layer contains the ferroelectric (FE) component. The layer of the first type (LFT) represents domain-engineered single crystal poled along either [0 0 1] or [0 1 1]. The layer of the second type is described as a system of long FE ceramic rods that have the shape of an elliptic cylinder and are aligned in a polymer medium. Piezoelectric coefficients d3j∗ and g3j∗ and sets of figures of merit (FOM) (energy-harvesting d3j∗g3j∗,modified F3j∗σ for a stress-driven harvester and modified F3j∗ξ for a strain-driven harvester) are analyzed to show their large values and specifics of the anisotropy when varying volume fractions of components and a rotation angle of the ceramic rod bases. For the first time,the studied parameters are compared in two directions: (i) the composite based on [0 0 1]-poled single crystal versus the composite based on [0 1 1]-poled single crystal and (ii) the lead-free composite versus the lead-containing composite (both based on [0 0 1]-poled single crystals). The advantages of the high-performance lead-free composite are discussed. The 2–1–2 composites put forward in this paper are of interest as advanced materials suitable for piezoelectric sensors,actuators and energy-harvesting systems operating at constant stress or strain.

Ceramic samples of the (1−x)BaTiO3⋅xPbFe2/3W1/3O3,0≤x≤1 ((1−x)BT⋅xPFW) system were synthesized by solid-state reactions method. The samples were characterized by X-ray diffraction (XRD) and dielectric studies,as well as by the measurements of the thermally stimulated depolarization currents (TSDC). It was found that the predominant phase in the samples is presented by the (Ba1−xPbx)(Ti1−xFe2x/3Wx/3)O3 solid solutions with a perovskite structure,herewith the samples with 0≤x<0.25 are practically single-phase,and with 0.25≤x<1 contain the impurity phase BaWO4 (up to 15 mass.% at x=0.60–0.90). Information has been obtained about the changes in the structural and dielectric characteristics of the solid solutions with the change of their composition. It is established that the solid solutions crystal lattice symmetry at 296 K changes from tetragonal at x≤0.04 to cubic at x≥0.05. An increase in the PFW content in solid solutions causes a gradual change in their properties from ferroelectric at 0≤x<0.10 to relaxor ferroelectric at 0.10≤x≤0.25,and then to properties similar to those of the dipole glass with weak or zero correlation between dipoles at 0.25<x≲0.90. The addition of BT to PFW leads to rather quick degradation of the relaxor ferroelectric properties of PFW in the region x=0.9–1.0.

The paper presents the results of a study of the microwave absorption properties of ceramic materials based on bismuth ferrite containing rare earth elements,as well as systems of solid solutions (1−x)BiFeO3–xPbFe1/2Nb1/2O3 in a wide range of component concentrations. The methodology for measuring and calculating the parameters of samples of the materials under study is described. The influence of structural and microstructural factors on the average and maximum level of microwave absorption of the materials under study in a wide frequency range is analyzed. A comparison of the microwave absorbing properties of these materials with industrial absorbers has been carried out,and prospects for application in microwave technology have been shown.

Intrinsic SrTiO3 is a quantum paraelectric,but moderately Bi-doped SrTiO3 exhibits dielectric frequency dispersion similar to relaxor ferroelectrics. In this paper,detailed electron microscopic studies of the microstructures of Bi-doped SrTiO3 samples were presented. It was found that the Sr sites were replaced by off-central Bi,resulting in tensile strain in the strontium titanate (STO) lattice. In the Bi-doped SrTiO3 samples,the valence of titanium mainly showed the Ti4+ characteristic. According to the dielectric behavior and microstructure analysis,the polar nanoregions (PNRs) composed of strained SrTiO3 nanoclusters should be responsible for the ferroelectric relaxor behavior in samples with moderate Bi content.