Wearable instruments are functional devices that can be worn on human body, sensing, transmitting and processing body or environmental information in real time, and show broad application prospects in medical health, especially artificial intelligence, sports and entertainment. With the development of wearable instruments, various flexible sensors have emerged. Flexible mechanical sensors based on piezoelectric effect have attracted much attention because of their advantages of wide sensing frequency, fast response, good linearity, and self-power supply. However, traditional piezoelectric materials are mostly brittle ceramics and crystalline materials, which limit their application in flexible devices. With the deepening of research, more and more flexible piezoelectric materials and piezoelectric composites continue to emerge, injecting new development vitality into flexible wearable mechanical devices. This article mainly summarizes the cutting-edge progress of flexible wearable piezoelectric devices, including piezoelectric principle, preparation and performance improvement methods of flexible piezoelectric materials. In addition, the main application directions of flexible wearable piezoelectric devices, including medical health and human-computer interaction, as well as the challenges and opportunities encountered, are summarized.

With the progress of nuclear radiation technology in China, radiation detection has been developed rapidly in recent years for the wide usage in radiation safety monitoring, radioactive medicine diagnosis/treatment, X-ray security inspection, industrial non-destructive detection, microscopic particle track detection, and many other fields. Radio-photoluminescence (RPL), as a new radiation detection method, is a phenomenon in which a new luminescence center is generated inside a material under a ionizing radiation which can be excited by ultraviolet light to emit a special light. RPL materials usually have characteristics of storing radiation information, almost no attenuation of information, good linear dose response, high radiation sensitivity, low energy dependence, and repeatable reading, which can overcome the shortcomings in stability and reusability of optically stimulated luminescence (OSL) materials and thermally stimulated luminescence (TSL) materials. Since the RPL phenomenon was reported, RPL materials have emerged constantly, from the traditional materials as Ag-doped phosphate glass, Al2O3:C, Mg and LiF, to the novel materials such as Cu-doped RPL system, Sm-doped RPL system and undoped RPL system materials. Meanwhile, applications of RPL materials have also been explored, enabling them to become one of the indispensable materials in the field of radiation detection. Based on above aspects, this paper summarizes the latest development of RPL materials, focuses on the luminescence principle, performance characteristics and applications of traditional and novel RPL materials, and especially compares the performance of different RPL materials in radiation detection. Finally, advantages and disadvantages, accompanied by prospected development trend of RPL materials are summarized and analyzed

Natural bone has a unique micro-/nano-structure, which is composed of organic nanomaterials (collagen fibers) and inorganic nanomaterials (hydroxyapatite). Thus, compared with traditional synthetic materials, natural bone has incomparable advantages in biological, functional and mechanical properties. In the research of tissue engineering and regenerative medicine, biomaterial scaffold with micro-/nano-structures simulating the characteristics of natural bone tissue are one of the research focuses. In recent years, researchers have found that micro-/nano-structured biomaterials can effectively regulate cell proliferation, differentiation and migration, and have a strong ability to promote cell osteogenic differentiation, so as to promote bone tissue regeneration in vivo. In this article, we focus on reviewing recent research progress of biomaterial design on simulating the hierarchical characteristics of natural bone, analyzing the complicated interaction between micro-/nano-structured biomaterials and cells, and summarizing their applications in bone tissue engineering to provide new ideas for the design of biomaterials.

Bioceramic scaffolds with excellent osteogenesis ability and degradation rate exhibit great potential in bone tissue engineering. Akermanite (Ca2MgSi2O7) has attracted much attention due to its good mechanical property, biodegradability and enhanced bone repair ability. Here, akermanite (Ca2MgSi2O7) scaffolds were fabricated by an extrusion-type 3D printing at room temperature and sintering under an inert atmosphere using printing slurry composed of a silicon resin as polymer precursor, and CaCO3 and MgO as active fillers. Furthermore, the differences in structure, compressive strength, in vitro degradation, and biological properties among akermanite, larnite (Ca2SiO4) and forsterite (Mg2SiO4) scaffolds were investigated. The results showed that the akermanite scaffold is similar to those of larnite and forsterite in 3D porous structure, and its compressive strength and degradation rate were between those of the larnite and forsterite scaffolds, but it showed a greater ability to stimulate osteogenic gene expression of rabbit bone marrow mesenchymal stem cells (rBMSCs) than both larnite and forsterite scaffolds. Hence, such 3D printed akermanite scaffold possesses great potential for bone tissue engineering.

As an important raw material for synthesis of phosphor and laser gain medium, the sesquioxide of praseodymium (Pr2O3) receives few attentions due to its susceptibility to change of valence and hygroscopicity in air. Here, the reduction process from Pr6O11 to Pr2O3 and corresponding mechanism in air and Ar/H2 atmosphere as well as the crystal phases, morphologies, particle sizes and valence states of the two kinds of oxides were investigated. Furthermore, the photoluminescent properties of praseodymium oxide were analyzed in relation to the valence state of praseodymium. The results show that phase transition of Pr6O11 significantly varied in different atmospheres. Reduction of Pr6O11 is accelerated in Ar/H2 atmosphere and Pr2O3 can be obtained at 800 ℃. In addition to adsorption in ultraviolet region resulted from transition from valence to conductive band, the Pr2O3 containing Pr3+ also shows the absorption in visible light band caused by the f→f transition. Pr6O11 shows strong absorption to UV-Vis wavelengths over 320 nm, which is related to the charge transfer between Pr4+ and oxygen ion. Wide band in the fluorescence emission spectrum of Pr2O3 indicates that the lowest energy level of 4f5d orbital of Pr3+ is below 1S0. Fluorescence intensity of Pr6O11 containing Pr4+ decreases 63% compared to that of Pr2O3 at 404 nm, which can be attributed to the energy dissipation between Pr3+and Pr4+. This difference in fluorescence property can be used to analyze Pr valence in ceramics or crystals with high oxygen ion mobility. This research demonstrated that the transformation process and the related property from Pr6O11 to Pr2O3 in different atmosphere may promote the application of Pr2O3.

Ultraviolet (UV) nonlinear optical (NLO) crystals play an irreplaceable role as the key materials to realize the frequency conversion for all solid state lasers. To date, it is still difficult to design UV NLO crystals with large second harmonic generation (SHG) coefficients, moderate birefringences and wide band gaps. Benefiting from the large band gap, sulfate has become an important research direction of UV NLO crystals. However, since SO4 is isotropic tetrahedral building units with nearly nonpolar Td symmetry, it exhibits small microscopic second order polarizability and polarizability anisotropy, which tends to result in a weak SHG effect and small birefringence. In this work, we introduced Hg2+ ions that are easy to form distorted polyhedrons into the sulfates, resulting in a new NLO material, Rb3Hg2(SO4)3Cl. It crystallizes in a monoclinic space group (P21) with the lattice parameters a=0.78653(2) nm, b=0.97901(2) nm, c=1.00104(3) nm, and β=110.95(3) (Z=2). Structure of Rb3Hg2(SO4)3Cl consists of [SO4] tetrahedra, [HgO5] and [HgO4Cl] polyhedral, which connected by a common corner to form a spatial 3D network. All the Rb atoms reside in the cavity of 3D network. The powder SHG measurement proposed by Kurtz and Perry indicates that Rb3Hg2(SO4)3Cl is a phase-matchable material in the visible region and exhibits a moderate SHG response about 1.5 times that of KH2PO4 (KDP). In addition, the UV-Vis-NIR diffuse reflectance spectral measurement indicates that Rb3Hg2(SO4)3Cl has a short UV cut-off edge of 251 nm, corresponding to the band gap of 4.94 eV. Its polarizing microscope measurement reveals that Rb3Hg2(SO4)3Cl has a moderate birefringence (The birefringence of Rb3Hg2(SO4)3Cl crystal at 546.1 nm is 0.04). Moreover, first-principles calculations uncover that the distorted [HgO5], [HgO4Cl] and [SO4] polyhedral are responsible for its SHG effect. Our study shows that Rb3Hg2(SO4)3Cl may have potential applications as a UV NLO crystal.

Metallic Li is one of the ideal anodes for high energy density lithium-ion battery due to its high theoretical specific capacity, low reduction potential as well as abundant reserves. However, the application of Li anodes suffer from serious incompatibility with traditional organic liquid electrolyte. Herein, a gel complex electrolyte (GCE) with satisfactory compatibility with metallic Li anode was constructed via in situ polymerization. The double lithium salt system introduced into the electrolyte can cooperate with the polymer component, which broadens electrochemical window of the electrolyte to 5.26 V compared to 3.92 V of commercial electrolyte, and obtains a high ionic conductivity of 1×10-3 S·cm-1 at 30 ℃ as well. Results of morphology characterization and elemental analysis of Li anode surface show that GCE exhibits obvious protective effect on lithium metal under the condition of double lithium salt system, and volume effect and dendrite growth of Li anode are obviously inhibited. At the same time, the lithium metal full battery, assembled with commercial lithium iron phosphate (LiFePO4) cathode material, exhibits excellent cycling stability and rate performance. The capacity retention rate of the battery reaches 92.95 % after 200 cycles at a constant current of 0.2C (1C = 0.67 mA·cm-2) at 25 ℃. This study indicates that the GCE can effectively improve the safety, stability and comprehensive electrochemical performance of lithium-metal battery, which is expected to provide a strategy for universal quasi-solid electrolyte design.

Concentrated solar power plant needs to be equipped with large-scale high-temperature heat storage module. Metal oxides can store and release heat through reversible redox reaction. Manganese oxides is non-toxic and cheap, showing great potential for applying in solar power plant, but it displays poor reversibility. Here, a manganese-based oxides with high reversibility by deep eutectic solvent (DES) ionic thermal synthesis was proposed, and effects of synthesis parameters and iron doping on heat storage performance were studied. MnCO3, as the raw material, was used to produce manganese oxides by ionic thermal decomposition at high temperature and released CO2, which could also form abundant pore structure, providing great channels for oxygen transmission and diffusionfor redox. Although this Mn2O3 synthesis method showed better reactivity than commercial Mn2O3, its oxidation rate was low. It is worth noting that oxidation rate of manganese iron oxide doped with 20% Fe, synthesized at 150 ℃, was fast. Its heat storage density reached 300.66 J/g, and reversibility of the reaction was the best, which could realize long-term stable cycle. Our results demonstrated that ionic thermal synthesis can increase the lattice oxygen ratio in manganese oxides, promoting migration of oxygen vacancies, and further improving reversibility and cyclic stability.

Bi2Te3-based thermoelectric (TE) materials have already been commercialized, of which the hygrothermal stability has a direct impact on the service reliability of TE devices, but is still confronted many challenges. This work investigated the degradation behavior of commercial n-type Bi2Se0.21Te2.79 and p-type Bi0.4Sb1.6Te3 TE materials during storage in 85 ℃, 85% RH hygrothermal environment for 600 h. The surfaces of n-type Bi2Se0.21Te2.79 and p-type Bi0.4Sb1.6Te3 TE materials were oxidized with reaction process of Bi2Te3+O2→Bi2O3+TeO2 and Bi2Te3+Sb2Te3+O2→Bi2O3+Sb2O3+TeO2, respectively. The oxidation process creates nanoscale holes and even microcracks inside the material, which leads to an overall deterioration of the electrical and thermal properties. At room temperature, the electrical conductivity of the n-type Bi2Se0.21Te2.79 material drops from 9.45×104 S·m-1 to 7.79×104 S·m-1 after exposure, and ZT decreases from 0.97 to 0.79, while Seebeck coefficient of the p-type Bi0.4Sb1.6Te3 material declines from 243 μV·K-1 to 220 μV·K-1, correspondingly, ZT decreases from 1.24 to 0.97. In conclusion, Bi2Te3-based TE materials have extremely poor hygrothermal stability, and their corresponding micro-TE devices need to be strictly encapsulated in service to prevent complex redox reactions between the TE materials themselves and the environmental water vapor and air.

n-Type AgBiSe2-based compounds are considered as promising high-performance thermoelectric (TE) materials due to the low lattice thermal conductivity. However, their two phase transitions between 300 and 700 K limits their applications. Therefore, it is crucial to obtain AgBiSe2-based compounds with stable structures and optimized TE properties. In this work, the Pb-free group IV-VI compound SnTe is selected for alloying with AgBiSe2. Introduction of SnTe not only reduces the cubic phase transition temperature, but also effectively suppresses the reversible phase transition of AgBiSe2. At room temperature, reduction of the lattice thermal conductivity from 0.76 to 0.51 W·m-1·K-1 results from highly disordered distribution of atoms. Furthermore, Nb dopant to replace Ag, significantly improves carrier concentration of AgBiSe2-based compounds, which promotes the effective mass and increases the electrical conductivity from 77.7 S·cm-1 to 158.1 S·cm-1 at room temperature. Meanwhile, the defect scattering at high temperature is enhanced with the increase of impurity point defects, leading to the lattice thermal conductivity reduced. At 700 K, the lattice thermal conductivity is reduced from 0.56 to 0.43 W·m-1·K-1, obtaining stable cubic phase compound (Ag0.98Nb0.02BiSe2)0.75(SnTe)0.25 with a ZT of 0.32 at 650 K. These results indicate that the (AgBiSe2)0.75(SnTe)0.25 compound is a promising n-type TE compound with low lattice thermal conductivity and a stable cubic structure. Such efforts provide a scheme for the crystal structure regulation of high-performance TE materials with phase transition and promotion of its application.

High performance thermoelectric powders or bulks can be quickly obtained by self-propagating high- temperature synthesis (SHS) or its derivative methods. In the process of preparing skutterudite by SHS technology, unsteady SHS reaction is easy to occur, which leads to impurity phase in the powders compact after reaction. In this work, the SHS process of the skutterudite was investigated via regulating the power density of the laser (η) and preheating temperature (T0) by laser induction and preheating of powders compact, respectively. The change of combustion mode for the skutterudite of CoSb3 was observed and the process window for single phase was obtained. As a result, with fixed η and increased T0, the state of the SHS process changes correspondingly, i.e., reaction stop→unsteady spiral combustion→steady-state combustion→unsteady spiral combustion. With η=3.75 J·mm-2 and 250 ℃≤T0<370 ℃, the single-phase of CoSb3 can be obtained successfully.

Oxygen evolution reaction (OER) is the key reaction for water splitting, but its slow kinetics limitsits application. Therefore, rational design and construction of efficient OER catalysts are crucial for water splitting. Here, a Co2+ ion doped NiFe layered double hydroxides coupled Ti6C3.75 (NiFeCo-LDH-Ti6C3.75) catalyst was prepared by a simple one-step hydrothermal method using cobalt nitrate, nickel nitrate, iron nitrate, urea, and Ti6C3.75 as raw materials. NiFeCo-LDH-Ti6C3.75 catalyst showed a lamellar stacking structure, which is facilitating exposing more active sites. Introduction of Co2+ and Ti6C3.75 reduced the electronic density of Ni and Fe sites of NiFeCo-LDH-Ti6C3.75 catalyst. Benefiting from these features, the NiFeCo-LDH-Ti6C3.75 catalyst exhibits excellent OER activity with an overpotential of merely 290 mV at a current density of 20 mA·cm-2 and a Tafel slope of 87.84 mV·dec-1 with faster reaction kinetics. NiFeCo-LDH-Ti6C3.75 catalyst shows a relatively low charge transfer resistance, which means a fast charge transfer efficiency. Furthermore, after 6000 cycles of accelerated aging test at 20 mA·cm-2, the overpotential only increased ~7 mV, indicating excellent cycle stability of NiFeCo-LDH-Ti6C3.75.

Beta tricalcium phosphate (β-TCP) ceramic substituted materials have attracted a large amount of attention in the last decades because of their chemical similarity with bone inorganic components, good biocompatibility, and osteoconductivity. Such materials can be used for bone replacement and bone formation in various forms, such as nanoparticles, scaffolds and microspheres. In this study, five different microsphere materials of tricalcium phosphate/trimagnesium phosphate (TMP) (TCP, 25% TMP, 50% TMP, 75% TMP, and TMP) composites were prepared and characterized. With the increase of TMP content in the composite microspheres, the cumulative concentration of Mg2+ and Ca2+ released from the microspheres increased, indicating that TMP can regulate the degradation rate of the composite microspheres. The osteoblast precursor cell line (MC3T3-E1 cells) and human umbilical vein endothelial cells (HUVECs) were used as models to evaluate the biocompatibility, angiogenesis and osteogenesis of the composite microspheres. The results showed that compared with TCP, TMP and 75% TMP group, 25% TMP and 50% TMP composite microspheres had better cell compatibility and had a certain proliferative effect on HUVECs. Therefore, composite microspheres of 25% TMP and 50% TMP have more significant positive effects on angiogenesis and osteogenesis.

The fracture properties of ferroelectrics directly determine their processability and reliability of devices made of them. However, both experimentally and theoretically reported fracture toughness of piezoelectric ceramic materials remains nearly the same as that reported 30 years ago, limiting the application of piezoelectric devices in situation where high reliability is required. Here, we try to reveal the parameters that could be used to optimize the fracture performance of ferroelectrics. Specifically, stress-strain curves, intrinsic fracture toughness and long-crack fracture toughness of three typical PZT ceramics were measured by uniaxial compression method, crack-tip opening displacement (COD) technique and single-side V-notch beam (SEVNB) technique, respectively. It is shown that the intrinsic fracture toughness is positively correlated with the Young’s modulus of the material, which suggests that improving the Young’s modulus of ferroelectrics is an effective way to improve their intrinsic fracture toughness. The long-crack fracture toughness is related to the intrinsic toughness and extrinsic ferroelastic domain switching/phase transformation toughening, which also suggests that optimizing the ferroelastic switching behavior of piezoelectric ceramics can improve their extrinsic effect. Compared to the hard doped PZT, the soft doped PZT has low coercive stress, high remanent strain and high shielding toughness. The fracture patterns observed in different PZT materials are related to the different ferroelastic switching behavior of the materials. Soft PZT ceramics exhibit intergranular fracture, while hard PZT with weak ferroelastic switching behavior exhibits transgranular fracture. In conclusion, fracture toughness of ferroelectrics is enhanced by optimizing Young’s modulus and toughening of ferroelastic switching.