碳/碳复合材料热解碳基体的织构界面形成机制Monte Carlo模拟

基于热解碳沉积的Particle-Filler（P-F）概念模型和Langmuir-Hinshelwood理论，提出了包含吸附/解吸附/脱氢的多步非均相反应动力学机制，实现了碳/碳复合材料制备中热解碳基体在碳纤维表面连续沉积及其织构形成过程的理论建模，并采用Gibbs系综Monte Carlo （MC）方法对化学气相渗透（CVI）工艺中热解碳基体的织构界面形成过程进行了数值模拟。研究表明：由于气相中小的芳香烃组分C₆的吸附比线性小分子烃组分C₂的吸附更容易受到抑制，因而限制了沉积表面的P-F双分子反应；随C₆与C₂浓度比值R的变化，热解碳的织构形成过程呈现双稳态分布，导致了2种不同的亚稳相碳即中织构（MT）和高织构（HT）热解碳的生成，并在碳/碳复合材料热解碳基体内部形成了鲜明的织构界面。进一步的计算表明：热解碳织构双稳态转变存在一个迟滞域，其大小受气相成分的组成参数R、线性小分子烃C₂的初始浓度及沉积温度T的影响；为了得到均一织构的热解碳，应当在迟滞域外的区域合理选取CVI的工艺参数。

Monte Carlo simulations of texture interface formation within pyrocarbon matrix of carbon/carbon composites

Based on the Particle-Filler model (P-F) and Langmuir-Hinshelwood mechanism of pyrocarbon deposition, a multistep heterogeneous reaction kinetic mechanism including adsorption/desorption/dehydrogenation reactions was proposed, by which continuous deposition of pyrocarbon on surface of the carbon fiber in the preparation of carbon/carbon composites and the formation process of texture interface were theoretically modeled. Then a Gibbs ensemble Monte Carlo (MC) method was used to simulate the pyrocarbon texture formation in chemical vapor infiltration (CVI) process. This study shows that P-F bimolecular reaction happening on the surface of deposition will be restricted due to the fact that the adsorption of the light aromatic component in gas phase represented by C₆ is more likely to be suppressed, compared to the adsorption of the light linear hydrocarbons represented by C₂. With the change of the concentration ratio R of C₆ to C₂ component, the texture formation process of pyrocarbon exhibits bi-stability distribution leading to the formation of two different metastable phases, i.e. medium-textured (MT) and high-textured (HT) pyrocarbons. And a sharp texture interface appears in the pyrocarbon of the carbon/carbon composites. The further simulation results indicate that the bistable transition of pyrocarbon textures is accompanied by a hysteresis cross domain, which is affected by the composition parameter R of gas phase, initial concentration of light linear hydrocarbons C₂ and deposition temperature T. CVI process parameters have to be selected outside the range of hysteresis cross domain to produce pyrocarbon with a pure composition.