Aluminum nitride (AlN) is considered one of the most desirable materials for integrated circuits and electronic packaging substrates. However, raw AlN powder reacts easily with water, forming Al(OH)₃ or AlOOH on the surface and hindering the development of an aqueous tape-casting process for preparing AlN ceramic substrates. In this study, hydrolyzed polymaleic anhydride (HPMA) was used to modify AlN powder, which has good water solubility and dispersibility. The AlN powder was modified with 5 wt% HPMA remained stable in water for at least 90 h under magnetic stirring condition and 24 h under ball milling condition, indicating that HPMA-modified AlN powder has good resistance to hydrolysis. The action mechanism of HPMA is revealed. Firstly, –COOH of the HPMA polymer and the oxide layer on the surface of the AlN powder underwent a dehydration condensation reaction to form a compound. Secondly, long chains of the polymer further coated the surface of the AlN powder, forming an anti-hydration layer with a thickness of about 7.0 nm on the surface of the AlN particles. In addition, AlN green sheets were successfully prepared by aqueous tape casting using the HPMA-modified AlN powder without additional dispersants. Subsequently, AlN ceramic substrates were obtained by sintering at 1750 ℃ for 4 h under an N₂ atmosphere with a pressure of 0.2 MPa. The relative density and thermal conductivity were tested to be 97.3% and 122 W/(m·K), respectively.

The A₂B₂O₇ series of ternary oxides are derivatives of fluorite structure over a wide range of r_A/r_B. Competing by two rare-earths the A-site, La_2-xLu_xZr₂O₇ ceramics were found transparent only in pore-free microstructures with similar grain sizes of pyrochlore (PY) and defective fluorite (DF) phases. Mutual solubilities of Lu and La in both phases were found by imaging and energy-dispersive spectroscopy analysis in scanning electron microscope. The dual-phase microstructures were developed with liquid-phase resulted from the exothermal reactions, creating a miscibility gap between two structures to moderate their competing grain growth. Change in grain growth behaviors in liquid-phase is described by a nucleation line in La₂Zr₂O₇‒Lu₂Zr₂O₇ phase diagram. Variations of solution levels in DF grains and co-existing of dual-phase grain clusters in common orientation were revealed in transparent ceramics by electron backscattered diffraction, resulted by epitaxial relation of two phases promoted by the liquid-phase. Oxygen vacancies and various hole states common in both phases were revealed by characteristic cathodoluminescence peaks. The collective effects of pores, phase and grain boundaries, oxygen vacancies on scattering or absorption of visible light enables to establish a hierarchical microstructure‒transparency relationship in such complex oxide ceramics, which could be tailored or further optimized by controllable sintering.

A₂B₂O₇ system compounds, which usually present three phase structures mainly based on the ionic radius ratios of r_A and r_B (r_A/r_B), have been studied for potential applications in many fields, such as thermal barrier coatings, luminescence powders, fast-ion conductors, photocatalysts, and matrices for immobilization of highly active radionuclides. Since 2005, La₂Hf₂O₇ was fabricated into transparent ceramics and much more attentions were paid on A₂B₂O₇ transparent ceramics for new applications. In this review, the development of A₂B₂O₇ system transparent ceramics was described. The structure characteristics, powder synthesis method, and sintering techniques of the final A₂B₂O₇ transparent ceramics were summarized. After that, the mostly reported A₂Hf₂O₇, A₂Zr₂O₇, and A₂Ti₂O₇ system transparent ceramics were systematically introduced. The potential application fields and future development trends were also discussed, focusing on scintillators, optical elements, and other luminescent materials.