Transparent spinel-type aluminum oxynitride (γ-AlON) ceramics have emerged as a highly promising material for military protection (e.g., infrared windows, armor materials) and civilian optics (e.g., lenses, semiconductor devices) due to their excellent optical properties (high transmittance, broad transmission band), outstanding chemical stability, and superior mechanical strength. Transparent ceramics require not only high optical transmittance but also high mechanical properties, which is the fundamental prerequisites for the practical application of AlON ceramics.The incorporation of sintering aids is a critical strategy in fabricating transparent AlON ceramics, as optimizing their type and content allows precise control over microstructural evolution during sintering, including grain nucleation and growth, phase distribution, and overall densification. Consequently, these microstructural modifications directly influence the ceramic’s properties, such as, optical transmittance, mechanical strength, and chemical stability, enabling their effective regulation and optimization for advanced applications. This paper comprehensively reviews the research progress on sintering aids for preparing high- quality AlON transparent ceramics. The sintering aids are categorized into rare earth oxides (Y₂O₃, La₂O₃, and Pr₂O₃), alkaline earth oxides (CaCO₃and MgO), and silicon - based compounds (SiO₂ and Si₃N₄). The review delves into the effects of these aids on the optical and mechanical properties of AlON transparent ceramics and details their mechanisms in promoting densification, optimizing grain size distribution, suppressing phase decomposition, and lowering sintering temperature. Optimal sintering aids facilitate pore elimination and suppress abnormal grain growth via liquid-phase formation or pinning effects, thereby enhancing optical transmittance while simultaneously improving mechanical properties (hardness, fracture toughness, and strength) through microstructural refinement. The mechanisms of sintering aids in AlON densification vary significantly depending on their chemical nature. Specifically, rare-earth additives predominantly facilitate liquid-phase sintering, alkali metals induce grain-boundary pinning effects, while silicon-based compounds primarily form solid solutions. This review also analyzes the existing problems and challenges in current research and looks forward to future research directions, aiming to provide theoretical guidance and technical reference for the preparation of high-performance AlON transparent ceramics.

The cold sintering process (CSP) is an advanced low-temperature sintering technology whose effectiveness is closely related to the selection of transient liquid phases (TLPs). While water serves as an ideal TLP for water-soluble ceramics, most water-insoluble materials necessitate acids, bases, or specialized solvents instead. This limitation has severely restricted the application of CSP, as many water-insoluble ceramics cannot be densified due to the lack of suitable TLPs. This study demonstrates a breakthrough approach that exploits nanoscale effects to enable water to act as an effective TLP for the densification of water-insoluble Li₂TiO₃ ceramics. A comparison of nano (19.71 nm) and microscale Li₂TiO₃ powders under identical sintering conditions revealed that despite the exceptionally low aqueous solubility of Li₂TiO₃, the nanopowders achieved 94.33% relative density at only 300 °C and 700 MPa, whereas the micropowders attained only 78% density. Further analysis revealed a distinctive densification mechanism that integrates dislocation-mediated plastic deformation with localized dissolution phenomena at nanoparticle interfaces. Compared with conventional sintering (1000 °C), the resulting nanoceramics exhibited superior Vickers hardness (905 HV) and enhanced electrical conductivity while maintaining a refined nanoscale grain structure (26.42 nm). This study established an effective strategy for the cold sintering of water-insoluble ceramics with layered structures using water as a TLP, significantly expanding the applicability of CSP technology and offering new pathways for the energy-efficient fabrication of advanced functional ceramics.

Investigating the dynamic mechanical behavior of single-crystal aluminum oxynitride (AlON) is fascinating and crucial for understanding material performance in relevant applications. Nevertheless, few studies have explored the dynamic mechanical properties of AlON single crystals. In this study, a series of nanoimpact experiments (representative strain rate ${\dot{\epsilon }}_{\text{r}}\text{≈}{\text{10}}^{\text{2}}{\text{s}}^{-\text{1}}$) were performed on three principal orientations ((010), (101), and (111)) of grains to extract the dynamic mechanical responses of AlON single crystals. Our results reveal that the dynamic plasticity of an AlON single crystal is governed by a combination of mechanisms, including dislocation motion and amorphization. Significantly, the localized amorphization induced by mechanical deformation has a softening effect (a lower dynamic hardness). The crystallographic orientation affects the dynamic hardness similarly to the static hardness. In particular, the (111) orientation results in the highest hardness, whereas the (010) orientation is the softest among the three principal orientations. This dependency aligns with the expectations derived from applying Schmid law. Furthermore, both the dynamic and static hardnesses exhibit typical indentation size effects (ISEs), which can be effectively described via the strain gradient theory associated with the geometrically necessary dislocations. In addition, the size and rate dependencies of the dynamic hardness can be decoupled into two independent terms.

Ultraviolet (UV) radiation poses risks to both human health and organics. In response to the urgent demand for UV-shielding across various applications, extensive endeavors have been dedicated to developing UV-shielding materials spanning from wide-bandgap semiconductors to organo-inorganic composite films. However, existing UV shielding materials, though suitable for daily use, cannot meet the demands of extreme conditions. In this work, we incorporated CeO₂ as a UV absorber into Y₂O₃ transparent ceramics for UV-shielding. The effect of CeO₂ concentration on the optical, mechanical, and thermal properties of Y₂O₃ ceramics was systematically investigated. These findings indicate that CeO₂ serves not only as a UV absorber but also as an effective sintering aid for Y₂O₃ transparent ceramics. The 5 at% Ce-doped Y₂O₃ transparent ceramics exhibit the optimal optical quality, with in-line transmittance of ~77% at 800 nm. The introduction of Ce shifted the UV cutoff edge of Y₂O₃ transparent ceramics from 250 to 375 nm, which was attributed to the visible band absorption of Ce⁴⁺. This shift grants UV shielding capabilities to Y₂O₃ transparent ceramics, resulting in 100% shielding for ultraviolet C (UVC, 100–280 nm) and ultraviolet B (UVB, 280–320 nm) and ~95% shielding for ultraviolet A (UVA, 320–400 nm). The service stability (optical properties) under various corrosive conditions (acid, alkali, UV irradiation, and high temperature) was investigated, confirming the excellent stability of this transparent ceramic UV-shielding material. A comparison of the performance parameters of transparent ceramics with those of traditional UV shielding materials such as glasses, films, and coatings was conducted. Our work provides innovative design concepts and an effective solution for UV-shielding materials for extreme conditions.

Pr-doped metal oxide polycrystalline transparent ceramics are highly desirable for photothermal window systems served in extreme environments; however, obtaining efficient photoluminescence (PL) together with high transparency in these ceramics is still posing serious challenges, which undoubtedly limits their applications. Here, Pr-doped Y₂Zr₂O₇ (YZO) transparent ceramics, as an illustrative example, are prepared by a solid-state reaction and vacuum sintering method. Owing to the elimination of defect clusters [ ${\text{Pr}}_{\text{Y}}^{4+}-{\text{O}}^{2-}-{\text{Pr}}_{\text{Y}}^{4+}$] and [ ${\text{Pr}}_{\text{Y}}^{4+}-{\text{e}}^{\prime }$] without the introduction of impurities and additional defects, the fabricated YZO:Pr ceramics exhibit high transparency (74%) and efficient PL (39-fold enhanced) after air annealing plus vacuum re-annealing treatment. Moreover, upon 295/450 nm excitation, the emission bands (blue, green, red, and dark red) from YZO:Pr ceramics present different temperature-dependent properties due to the thermal-quenching channel generated by the intervalence charge transfer (IVCT) state between Pr³⁺ and Zr⁴⁺ ions. Furthermore, a self-calibrated temperature feedback window with the same fluorescence intensity ratio (FIR) model (I₆₁₃/I₅₀₃, where I represents the intensity) under different excitation light sources (295 and 450 nm) is designed. The developed photothermal window operated in a wide temperature range (303–663 K) shows relatively high sensitivities (absolute sensitivity (S_a) and relative sensitivity (S_r) reach 0.008 K⁻¹ at 663 K and 0.47% K⁻¹ at 363 K, respectively), high repeatability (> 98%), and low temperature uncertainty (δT < 3.2 K). This work presents a paradigm for achieving enhanced PL along with elevated transparency of lanthanide (Ln)-doped ceramics through vacuum re-annealing treatment engineering and demonstrates their promising potential for photothermal window systems.