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.

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.

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.

1. 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.

2. 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.

3. 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%.

4. 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.

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.

According to the stoichiometric ratio of (Nd_0.015Re_0.02Sr_0.965)₅(PO₄)₃F_1.175, 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.

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 ⁴I_9/2 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.

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.

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.

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.

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 I_uc. 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) ⁴F_7/2 + ⁴I_11/2→⁴0_9/2 + ⁴F_9/2; 2) ⁴S_3/2 + ⁴I_15/2→⁴I_9/2 + ⁴I_13/2; and 3) ⁴I_13/2 + ⁴I_11/2→⁴I_15/2 + ⁴F_9/2.

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.

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.

A series of Ce,Ca:LuAG ceramics with the chemical composition of (Ce_xCa_{0.001 5}Lu_{0.998 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.

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.

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.

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.

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.

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.

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.

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.

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 Å). 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).

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 3T_2g, E_g and A_1g 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 . 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.

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.

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.