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

采用气相反应法制备了SiCl_4-BCl_3-NH_3-H_2体系中的Si_3N_4-BN复合微粒和SiCl_4-TiCl_4-NH_3-H_2体系中的Si_3N_4-TiN复合微粒。粒径为0.05- 0.1 μ m。Si_3N_4-BN复合颗粒为非晶态。结果表明，Si_3N_4和BN在分子水平上是混合的。在Si_3N_4-TiN系统中，TiN晶粒尺寸小于10 nm，分散在非晶Si_3N_4颗粒中。（2）复合微粒的相变和晶化将复合微粒压坯在1200- 1600 ℃的N2气氛中进行热处理。烧结助剂为Y_2O_3（6wt%）-Al_2O_3（2wt%）体系。在Si_3N_4-BN体系中，虽然在16000 ℃下无烧结助剂时粉末仍为非晶态，但在16000 ℃下有烧结助剂时β- Si_3N_4晶化。红外光谱分析表明，随着烧结温度的升高，Si_3N_4和BN相分离，烧结助剂促进了晶化。XPS分析和TEM观察发现，在Si_3N_4-TiN体系中，TiN促进了β-Si_3N_4的晶化，而在Si_3N_4-TiN体系中，TiN促进了β- Si_3N_4的晶化。XPS分析表明，未添加助烧剂时，TiN颗粒偏聚在Si_3N_4颗粒表面，而添加助烧剂时，TiN颗粒被Si_3N_4颗粒包围。（3）纳米复合材料的力学性能采用热压法制备了Si_3N_4-BN和Si_3N_4-TiN纳米复合材料。BN在Si_3N_4-BN体系中的弥散分布提高了该体系的抗热震性。Si_3N_4-TiN系统的断裂韧性得到提高。结果表明，TiN的弥散分布促进了β Si_3N_4棒状晶粒的生长，从而提高了材料的断裂韧性。

(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.