Control of Nano-Phase Separated Structure in Fine Amorphou Composite Partucles

(1)复合微粒的结构用气相反应法制备了复合微粒：SiCl4-BCl3-NH3-H_2体系中的Si3N4-BN复合微粒和SiCl4-TiCl4-NH3-H2体系中的Si3N4-TiN复合微粒。粒径为0.05~0.1um。Si3N4-BN复合粒子为非晶态。推测Si3N4和BN在分子水平上是混合的。在Si_3N_4-TiN体系中，TiN微晶分散在非晶态的Si_3N_4颗粒中。(2)复合微粉的相变与晶化助烧剂为Y_2O_3(6wt%)-Al_2O_3(2wt%)体系。在Si_3N_4-BN体系中，虽然在未添加助烧剂的情况下，粉末在16000℃时仍为非晶态，但在添加助烧剂的情况下，β-Si3N_4在16000℃时已经晶化。红外光谱分析表明，随着温度的升高，Si3N4和BN相分离，助烧剂促进了晶化。X射线光电子能谱(XPS)分析和透射电子显微镜观察发现，在相分离过程中，BN在Si3N4颗粒表面发生了偏析，在Si3N4-TiN体系中，TiN促进了β-Si3N4的晶化。XPS分析表明，在没有助烧剂的情况下，TiN在Si_3N_4颗粒表面偏析，而在助烧剂作用下，TiN颗粒被Si3N_4颗粒包围。(3)纳米复合材料的力学性能。细小的BN分散提高了Si3N4-BN系统的抗热震性能。Si_3N_4-TiN体系的断裂韧性得到了提高。结果表明，TiN的细小分散促进了β-Si3N4棒状晶的生长，从而提高了断裂韧性。

(1)Structure of fine composite particlesFine composite particles were prepared by a vapor phase reaction method : Si_3N_4-BN composite particles in SiCI_4-BCl_3-NH_3-H_2 system and Si_3N_4-TiN composite particles in SiCI4-TiCl4-NH3-H2 system. The particle size was 0.05-0.1mum. The Si_3N_4-BN composite particles were amorphous. It was suggested that Si3N4 and BN were mixed on molecular level. In Si_3N_4-TiN system, TiN crystallites with a size below 10nm were dispersed in amorphous Si_3N_4 particles.(2)Phase change and crystallization of fine composite particlesThe powder compact of fine composite particles was heat-treated at 1200-16000C in N2. The sintering aid of Y_2O_3(6wt%)-Al_2O_3(2wt%) system was used. In Si_3N_4-BN system, although the powder was amorphous even at 1 6000C without sintering aid, beta- Si_3N_4 crystallized at 16000C with sintering aid. IR analysis indicated that Si_3N_4 and BN phases separated with a rise of temperature, and the crystallization was accelerated by sintering aid. It was found from XPS analysis and TEM observation that BN was segregated on the surface of Si_3N_4 particles during phase separation.In Si3N4-TiN system, TiN stimulated the crystallization of beta- Si_3N_4 with and without sintering aid. XPS analysis suggested that TiN was segregated on the surface of Si_3N_4 particles without sintering aid, whereas TIN particles were surrounded by Si_3N_4 particles with sintering aid.(3)Mechanical properties of nanocompositesThe nanocomposites of Si_3N_4-BN and Si3N4-TiN systems were fabricated by hot pressing of fine composite particles. Fine BN dispersion improved the thermal shock resistance in Si_3N_4-BN system. The fracture toughness was improved in Si_3N_4-TiN system. It was found that fine TiN dispersion accelerated the growth of beta Si_3N_4 rod-like grains, resulting in the improvement of fracture toughness.