Abstract: Porous silicon nitride ceramics with a biomorphic microstructure were manufactured by
carbothermal reduction and nitridation of Y2O3/SiO2/charcoal composites in high concentration nitrogen atmospheres(0.6MPa) at 1600℃. These
composites were prepared from pine-derived charcoal impregnated with yttrium-incorporated silica mixed sol by vacuum/pressure infiltration
process. The influences of sintering temperature, reaction time, additive and sintering atmosphere on microstructure and phase composition were studied by XRD, SEM. The formation mechanism of Si3N4 was analyzed. XRD analysis shows that mainly \β-Si3N4 and minor glass component of
Y8Si4N4O{14 are detected. SEM observation reveals that the resulting porous Si3N4 retains the microcellular morphology of original pinewood tissue, and the remarkable rod-like \β-Si3N4 grains grow from the internal walls of macroscopic pores. The sintering process of porous \β-Si3N4 consists of three stages, namely, formation of α-Si3N4, transformation of α-\β and grain growth, respectively. Nitridation is based on gas-solid and gas-gas reaction among SiO(g), N2(g), C(s) or CO(g) for the samples treated at 1450℃. Porous ceramics with elongated fibrous \β-Si3N4 grain is developed by solution-reprecipitation mechanism at 1600℃.

Key words: biomimetic synthesis, porous Si3N4 ceramics, fibrous morphology grains, carbothermalreduction nitridation (CTN)