自蔓延法制备Si_3N_4粉时的氮气压力

Discussion of nitrogen pressure in the process of self-propagating high-temperature synthesis of Si_3N_4

探讨了硅粉在普通氮气和高纯氮气中的高温自蔓延合成反应过程,分析了稀释剂、氮气纯度与压力、成型坯体的气孔率等工艺参数对硅粉自蔓延过程的点火、最高燃烧温度及产物特征的影响.从热力学、动力学及Si3N4热分解过程几个方面分析了低氮气压力下燃烧合成Si3N4的可行性.研究结果表明:只要最高燃烧温度不高于相应氮气压力下Si3N4的热分解温度,就可以用SHS方法合成Si3N4;并在氮气压力为0.6～2.6 MPa时.以纯硅粉为起始原料燃烧合成出游离硅含量小于O.5％,β与α相混合,粒度为1～2 μm的Si3N4粉末;低氮气压力下硅粉的自蔓延合成反应,必须要引入Si3N4稀释荆,压坯气孔率控制在0～70％,否则反应不能进行;体系最高燃烧温度随着氮气压力和压坯气孔率的增加而升高;所需的最低氮气压力随硅粉粒度增大而提高;产物形态沿圆柱样径向有差异,由外到里β—Si3N4相明显增加.

Self-propagating high-temperature synthesis (SHS) process between silicon powder and nitrogen was investigated. Influences of parameters such as diluent content, nitrogen purity and pressure, porosity of green body on the ignition process, combustion temperature and the morphology of synthesized powder were analyzed. Feasibility for the combustion reaction to occur at low (0. 1 MPa) nitrogen pressure was analyzed regarding thermodynamics, kinetics and the thermal decomposition of silicon nitride. The analysis indicates that the SHS reaction between Si powder and nitrogen can proceed at low nitrogen pressure as long as combustion temperature no higher than the decomposition temperature of Si3N4 under response nitrogen pressure. Experimental results indicate that Si3N4 can be synthesized at 0. 6~2. 6 MPa nitrogen pressure. Mixture of α-and β- Si3N4 with grain size 1~2μm obtained from silicon powder as starting raw material and free silicon is less than 0. 5%. Combustion temperature increases with the increase of nitrogen pressure and porosity of green body. The minimum nitrogen pressure required increases with the Si particle size increases.