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Hexagonal BN particles (BNp) reinforced fused SiO2 matrix composite (BNp/SiO2) was regarded as a very important candidate material for potential applications on wave transparent thermal protection parts. In this study, pressurelessly sintered 15 vol% BNp/SiO2 composites were prepared by cold isostatic pressing (CIP) and gel-casting routes, respectively, using two kinds of fused silica powders with different particle sizes (5.82 and 3.24 μm in d50, respectively) as starting SiO2. It was demonstrated that the microstructure and mechanical properties of the composites were well dependent on the preforming techniques of green compacts. Gel-casting preforming route has an overwhelming superiority over CIP route on the thermal stability of fused SiO2 matrix against α-cristobalite crystallization, sinterability, and thus the mechanical properties. Using the finer fused silica (3.24 μm in d50) as the starting SiO2, the 15 vol% BNp/SiO2 composite sintered at 1375°C get a density of 2.05 g/cm3, the silica matrix primarily keep amorphous phase with uniformly dispersed nanosized α-cristobalite crystallites (<5 nm in diameter). The bending strength, fracture toughness and Young's modulus all reach the highest values, 101.5 ± 4.3 MPa, 1.57 ± 0.04 MPa·m1/2 and 61.3 ± 2.4 GPa, respectively. Microstructure characteristics, chemical bond information and mechanical properties of the as-prepared composites are correlated with their preforming routes, starting fused SiO2 particle size, sintering temperature, etc. based on X-ray diffractometry, scanning electron microscope, transmission electron microscope, high resolution transmission electron microscope, and FT-IR analysis. [ABSTRACT FROM AUTHOR]

6 H- SiC powder mixed with Si3N4, Al2O3, Y2O3, and AlN was sintered in a spark plasma sintering ( SPS) equipment. The 6 H→3 C SiC transformation was concerned. Research results indicated the polytype transformation completed in less than 30 s by imposing a 14 458 W (6.2 V, 2332 A) transitory DC current at 1700°C upon cooling down from 2000°C. The fast transformation kinetic and the significant 3 C- SiC grain growth accompanying the phase transformation were suspected to be correlated with the pulsed SPS direct current. [ABSTRACT FROM AUTHOR]

The ZrB2- BN composite without ZrC impurity was prepared successfully using reactive hot pressing via carbothermal reduction of ZrO2 with B4 C, C, and BN powders. The influences of additives and sintering temperature on the phase and microstructure of composites were investigated. The formation reaction of ZrC was inhibited by decreasing the evaporation of intermediate product B2 O3 due to forming a SiO2- B2 O3 eutectic system with the addition of SiO2 or SiO2 - CaF2. In comparison, SiO2- CaF2 exhibited better inhibition capacity of ZrC. ZrB2- BN composite without impure ZrC phase was achieved. In addition, the content of ZrO2 decreased with increasing sintering temperature, and finally disappeared when sintered at 1800°C. Moreover, the relative density of the ZrB2- BN composite reached 96.9%. [ABSTRACT FROM AUTHOR]