Theoretical Study on the Mechanism of the Initial Ammonolysis Reactions of the Main Raw Materials for the Formation of SiBCN Ceramics

摘要

A detailed mechanism investigation of the formation of B-N, Si-N and C-N bonds is conducive to the design and synthesis of siliconboroncarbonitride (SiBCN) ceramics to meet application requirements under extreme conditions. Boron trichloride (BCl3), methyldichlorosilane (MDCS) and methylvinyldichlorosilane (MVCS) are the most commonly used raw materials, and they can react with amino compounds, such as ammonia (NH3), methyl ammonium (CH3NH2), and hexamethyldisilazane (HMDZ, (CH3)(6)Si2NH) in the synthesis of SiBCN precursors. In this paper, quantum chemical calculations were used to study the ammonolysis mechanism of three raw materials with three amino compounds at the M06-2X/6-311G(d,p) level of theory. The structural properties, reaction pathways, energy barriers, reaction rates, and other aspects were discussed in detail. The results show that compounds containing B-N, Si-N and C-N bonds can be formed through a synergistic reaction mechanism with the release of ammonium chloride or trimethylchlorosilane (TMCS). The ammonolysis processes of BCl3 have absolute advantages in both kinetics and thermodynamics. The reaction of MDCS has the lowest energy barrier. All ammonolysis reactions are thermodynamically exothermic processes. The calculated results agree well with the experimental observations and provide an explanation for the initiation process of precursor synthesis.