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Celsian was synthesized in situ and was used to sinter silicon carbide. The low temperature pressureless liquid sintering process for BAS/SiC complex phase ceramics was studied, and ceramic materials with high density were prepared. The effect of sintering temperature and content of BAS on density, structure and mechanical properties of the ceramics were investigated. The densification and microstructures of BAS/SiC ceramics were observed by scanning electron microscope. The crystalline phase of the composite ceramics was identified by X-ray powder diffraction. The results show that celsian is synthesized at the temperature of 1800 ℃, and BAS/SiC ceramics are prepared with density of 3.2 g/cm3. BAS is precipitated in hexagonal structure, and SiC particles are distributed uniformly in the composite ceramic. The flexural strength and fracture toughness of composite ceramics are decreased due to grains growth while the sintering temperature is above 1800 ℃. When the mass content of BAS reaches 30%, the typical flexural strength, elastic modulus and fracture toughness of the pressureless sintered BAS/SiC ceramics are 413 MPa, 210 GPa and 5.03 MPa?m1/2 at room temperature.

MgAl2O4 spinel powders were first applied for aluminum dihydrogen phosphate hydrosol curing reaction. The solidification of spinel powders to phosphate hydrosol was studied in middle and low temperature condition. Besides, composites were prepared with quartz textile as reinforcing part, aluminum hydrogen phosphate hydrosol as matrix and spinel MgAl2O4 as firming agent. Curing temperature of phosphate hydrosol and the additive content of spinel MgAl2O4 powders were measured by thermogravimetric analysis and differential scanning calorimeter. At the same time, the crystalline phase of curing products was identified by X-ray powders diffraction. Microstructures of the composites were observed by scanning electron microscope. Moreover, the mechanical and resistant moisture absorption properties of phosphate composites were investigated. The results showed that aluminum dihydrogen phosphate resin had occurred dehydration reaction and formed into cross-linkage net in the temperature near to 170 °C, while the resin was added spinel MgAl2O4 powders. And it was contributed to reduce original pores and flaws in the matrix with mild reaction process. Owing to the promoting effect of spinel, the mainly reaction products were aluminum phosphate with the style of quartz and magnesium phosphate with monoclinic structure. Meanwhile, the reaction that aluminum dihydrogen phosphate changed into aluminum polyphosphate was prevented, which was benefit to enhance the resistant moisture absorption property of the composite. When the content of spinel MgAl2O4 powders arrived at 18wt%, the composite obtained the highest properties, which the flexual strength at room temperature and at 800 °C were about 165 MPa and 75 MPa. And dielectric properties of the composite were remained stably during long time storage, which dielectric constant was about 3.5 to 3.6 and loss tangent was less than 0.01 at room temperature respectively. [ABSTRACT FROM AUTHOR]