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.

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 Φ50 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.

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.

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.

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.

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.

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.

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.

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 Gd_1.8Eu_0.2Zr₂O₇ transparent ceramics have an X-ray imaging resolution of up to 11 lp·mm^-1, having a potential for the imaging applications.

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. Gd_2-xEu_xZr₂O₇ could be obtained via sintering in a box furnace at 1 400 ℃ according to the reaction: Gd_2-XEu_XZr₂O₇

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.

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 Gd_1.8Eu_0.2Zr₂O₇ 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 O^2-—Eu³⁺. In addition, five excitation peaks at 362, 382, 393, 414 nm and 465 nm correspond to 4f-4f energy level transitions ⁷F₀→⁵D₄, ⁵G_J, 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 Gd_1.8Eu_0.2Zr₂O₇ ceramic samples of different materials show that the Gd_1.8Eu_0.2Zr₂O₇ 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.

A series of Eu³⁺ doped transparent ceramics were prepared by a vacuum sintering method. The crystal structure of Gd_2-xEu_xZr₂O₇ was gradually changed from defective fluorite to pyrochlore via adjusting the doping amount of Eu³⁺. At 630 nm, the Gd_2-xEu_xZr₂O₇ 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.

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. Eu_xLu_1.4—xGd_0.6O₃ 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 Eu_xLu_1.4—xGd_0.6O₃ 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 Eu_xLu_1.4-xGd_0.6O₃ ceramics is still unclear. In this paper, Eu_xLu_1.4-xGd_0.6O₃ (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.

In the synthesis of nano-powder, the obtained precursor was calcined in air at 1 100 ℃ for 4 h to obtain Eu_xLu_1.4-xGd_0.6O₃ (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 Eu_xLu_1.4-xGd_0.6O₃ 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).

The Eu_xLu_1.4-xGd_0.6O₃ (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 Å) or Gd³⁺(i.e., 0.938 Å) is larger than that of Lu³⁺(i.e., 0.848 Å). 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 Eu_xLu_1.4-xGd_0.6O₃ 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 Eu_xLu_1.4-xGd_0.6O₃ 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.

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 Eu_xLu_1.4-xGd_0.6O₃ 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.

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.

(Ca_0.98-xNa_xCe_0.02)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.

The CSS host has a cubic Garnet-type structure with the 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.

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.

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.

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.

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.

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.

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.

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 (R_a=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.

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.