Near net size preparation of porous mullite ceramics with controllable shrinkage was achieved via gel casting and pore-forming agent process With Al₂O₃ and kyanite as raw materials, PMMA microspheres as a pore-forming agent, and isobutylene/maleic anhydride copolymer (Isobam104) as a gelling/dispersing agent. The effect of sintering temperature on the phase composition and the effect of solid loading on the microstructure, phase composition, shrinkage, porosity, and compressive strength of samples were investigated. The results show that the shrinkage of samples sintered at 1500 ℃ firstly decreases and then increases as the solid loading increases. At the solid loading of 30%(in volume fraction) and the content of pore forming agent of 30%(in mass fraction), the total shrinkage of the samples is close to zero, realizing the near net size preparation of porous mullite ceramics. The prepared porous mullite ceramics exhibite a higher porosity (i.e., 60.4%), a smaller average pore size (i.e., 3.75 μm) and a higher compressive strength (i.e., 8.3 MPa). The shrinkage of porous ceramics can be effectively controlled by the volume expansion effect in the preparation process, and the near net size preparation of porous mullite ceramics is of great significance for the preparation of large-size and complex porous ceramic parts and the reduction of processing cost.

Porous silicon nitride ceramics have attracted a considerable attention due to their excellent overall performance, but poor porosity homogeneity and structural shrinkage induced by prolonged high temperature sintering limit its further application. Herein, as a three-in-one solution for the above issues, for the first time we develop a novel approach that integrates the merits of gelcasting-SHS (self-propagating high-temperature synthesis) to prepare porous Si₃N₄ ceramics to simultaneously achieve high porosity, high strength, high toughness, and low thermal conductivity across a wide temperature range. By regulating the solid content, porous Si₃N₄ ceramics with homogeneous pore structure are obtained, where the pore size falls inbetween 1.61 and 4.41 μm, and the elongated grains are interlaced and interlocked to form micron-sized coherent interconnected pores. At the same time, porous Si₃N₄ ceramics with porosity of 67.83% to 78.03% are obtained, where the compressive strength reaches 11.79 to 47.75 MPa and fracture toughness reaches 1.20 to 6.71 MPa·m^1/2.

The α-Fe₂O₃@SiO₂ reddish pigments with core-shell structure were successfully prepared by hydrothermal and Stöber methods. The structure, morphology, and chromaticity of the synthesized pigments were characterized by XRD, SEM, TEM, FTIR, XPS, and colorimetry. The results indicated that the as-prepared pigments have the characteristics of narrow particle size distribution, high dispersion, and good sphericity. The α-Fe₂O₃@SiO₂ reddish pigments were uniform and well dispersed in solution. In addition, the pigments with different shell thickness were also prepared, and the effect of shell thickness on the color performance of the pigments was discussed.