多孔氮化硅/碳化硅复合材料制备的反应机理分析

为了探索碳热还原法制备多孔氮化硅/碳化硅(Si₃N₄/SiC)复合陶瓷材料在高温阶段的反应机理,采用固化的酚醛树脂为碳源,通过热解产生具有反应性的碳,使之在1300-1780℃等不同温度下与表面包裹的氮化硅粉反应,氩气为保护气氛.通过对试样的XRD、TEM分析和显微结构观察,结合反应的热力学和动力学结果计算推测,树脂裂解碳与Si₃N₄反应生成SiC的机理主要为Si₃N₄分解生成Si(l)与C进一步发生的液-固反应,和Si(l)与反应过程中的中间产物CO(g)之间发生的液-气反应.其他还包括C与Si₃N₄间直接进行的固-固反应;C与Si₃N₄表面的SiO₂间的气-固反应以及由SiO(g)、Si(g)参与的气-固反应.树脂裂解碳与Si₃N₄从1400℃左右开始发生反应形成SiC,温度升高对SiC层的生长有促进,保温时间的延长对SiC层的生长厚度影响较大.

Reaction Mechanism of Preparing Porous Silicon Nitride/Silicon Carbide Composite Ceramics

The reaction mechanism of preparing porous silicon nitride/silicon carbide (Si₃N₄/SiC) composite ceramics was investigated by using solidified phenolic resin as carbon source. Argon was used as protective atmosphere. The polymer can pyrolyze to provide free carbon, and farther react with Si₃N₄ powder coated at different temperatures ranged from 1300℃ to 1780℃. XRD, TEM, the microstructure of the samples, the thermodynamics and kinetics analysis show that the reaction mechanism mainly associates with liquid-solid reaction between C and Si(l) decomposed from Si₃N₄, and liquid-gas reaction between Si(l) and CO(g) which is the intermediate product during the reactions. Other reaction types involved are solid-solid reaction between C and Si₃N₄, gas-solid reaction between C and SiO₂ coated on the Si₃N₄ powder, and gas-solid reaction between C and SiO(g) or Si(g). The reactions between pyrolyzed carbon and Si₃N₄ begin at 1400℃, and form SiC layer. Keeping in higher temperatures can improve the formation of SiC layer and the prolongation of soaking time can increase the thickness of SiC layer significantly.