Study on Vapor Phase Synthesis and Sintering of Ultrafine Silicon Nitride Composite Particles

1. (1)采用Si(CH_3)_4-NH_3体系（1200℃）气相反应和Si(CH_3)Cl_3-NH_3体系等离子体气相反应制备平均粒径为0.02 ~ 0.1 μ m的非晶态Si_3N_4-SiC复合粉体。复合颗粒的结构可分为SiC包覆Si_3N_4相、Si_3N_4包覆SiC相和Si_3N_4与SiC相混合三种类型。(2)对非晶复合粉末进行热处理，使α - si_3n_4和β - sic结晶。在1600℃的温度下，加热后的粉末由3mum以上的Si_3N_4粗颗粒和0.2mum左右的SiC细颗粒组成。对含有Y_2O_3-Al_2O_3助烧剂的复合粉体在1700℃的温度下热压得到了相对密度在90%以上的致密烧结体。烧结体的晶相为β - si_3n_4和β - sic。晶粒尺寸较小，约为0.2 μ m。SiC的加入抑制了Si_3N_4的晶粒长大。碳化硅的含量由于与氧化物的反应而降低。采用SiCl_4-BCl_3-NH_3体系（1400℃）气相反应制备了平均粒径为0.06 ~ 0.1mum的非晶态Si_3N_4-BN复合粉体。复合结构类型为Si_3N_4和BN相的混合物。在1550℃的温度下，热处理过程中未见结晶现象。当含助烧剂的复合粉末在1700℃的温度下热压时，α， β - si_3n_4结晶，而h-BN几乎没有结晶。烧结体的密度很低。建立了复合粉末的制备方法，并对其烧结性能进行了研究。其次要建立烧结技术，实现高强度、高韧性的复合材料。

1. Preparation and sintering of Si_3N_4-SiC composite powder (1) Amorphous Si_3N_4-SiC composite powder with an average particle size of 0.02-0.1mum was produced by the vapor phase reaction of Si(CH_3)_4-NH_3 system (1200ﾟC), or by the plasma gas phase reaction of Si(CH_3)Cl_3-NH_3 system. The structure of composite particles was classified into three types: SiC-coated Si_3N_4, Si_3N_4-coated SiC and mixture of Si_3N_4 and SiC phases. (2) alpha-Si_3N_4 and beta-SiC were crystallized by heat treatment of amorphous composite powder. At 1600ﾟC, the heated powder consisted of coarse Si_3N_4 particles with a size above 3mum and fine SiC particles with a size of about 0.2mum. Hot-pressing of composite powder containing Y_2O_3-Al_2O_3 sintering aid at 1700ﾟC gave a dense sintered body with a relative density above 90%. The crystal phases of sintered body were beta-Si_3N_4 and beta-SiC. The grain size was small as about 0.2mum. The inclusion of SiC suppressed the grain growth of Si_3N_4. SiC content was decreased owing to the reaction with oxides.2. Preparation and sintering of Si_3N_4-BN composite powder Amorphous Si_3N_4-BN composite powder with an average particle size of 0.06-0.1mum was produced by the vapor phase reaction of SiCl_4-BCl_3-NH_3 system (1400ﾟC). The type of composite structure seemed to be a mixture of Si_3N_4 and BN phases. No crystallization was observed in the heat treatment at 1550ﾟC. When composite powder containing sintering aid was hot-pressed at 1700ﾟC, alpha,beta-Si_3N_4 was crystallized but h-BN was little crystallized. The density of sintered body was very low. As described above, the preparation method of composite powder was established and the sintering behavior was elucidated. The sintering technique should be established next to realize high-strength and high-toughness composite materials.