原位气相分析研究长程CVI过程热解炭微结构控制与沉积机理

The majority of existing C/C composites CVI densification models were built based on deposition system with small size and uniform environment, which is quite different from the real deposition conditions of large-sized preforms. Thus, there have limitations of using these models to guide the large-sized C/C CVI processes in practice. This project will study the real-time changes in deposition conditions during large-sized preforms densification processes by using a self-designed system which can collect gaseous components from in situ deposition zone at different position. Based on these analyses, the mechanisms of gas long-distance transfer-infiltration process for pyrolytic carbon deposition will be studied. The evolution of pyrolysis products in free flow zone will be investigated, aiming to clarify the pyrolysis mechanism of carbon source in the long-distance transfer processes. The corresponding general pyrolysis kinetic model of carbon source will be established. Moreover, the surface deposition reactions of various pyrolysis intermediate compounds will be researched. Different surface reaction mechanisms with detailed characteristics existed in long-distance transfer process will be clarified. The relationship among microstructure of pyrolytic carbon, surface reaction mechanism, deposition species and deposition rates will also be revealed. Furthermore, the evolution of deposition rates and microstructure of pyrolytic carbon in long-distance infiltration process will be studied. The kinetic model of pyrolytic carbon deposition in radial infiltration processes will be built. The ratio design principle of gaseous compounds with optimum penetrating and deposition performance will be proposed. The findings of the project will provide new theoretical basis to the preparation of large-sized C/C composites with uniform and controllable microstructure and the development of large-scale CVI equipment.

现有C/C复合材料CVI致密化模型大多基于均匀沉积环境设定下的小区域沉积系统建立，与大尺寸预制体的实际沉积情况存在较大差异，导致其指导性存在局限。本项目利用自行设计的原位气相采集装置，对大尺寸预制体增密过程中微区沉积环境的变化进行实时分析，在此基础上解析长程CVI过程中气体的传输扩散及热解炭沉积机理。考察自由流动区碳源热解产物的变化规律，阐明碳源在长程传输过程中的气相热解机理，建立其热解集总动力学模型；研究各类型热解中间产物的表面沉炭反应，探明长程传输过程中表面沉炭机制种类，并详细描述其各自特征，构建热解炭微观结构与沉炭机制、沉炭基元种类及沉炭反应速率之间的联系；探索预制体内长程渗透过程中热解炭沉积速率和微观结构演变规律，建立热解炭径向渗透沉积动力学模型，提出具有最优渗透沉积能力的气相组分设计原则。本研究将为大尺寸C/C复合材料均匀化可控CVI制备及重大CVI沉积装备研制提供新的理论依据。

采用CVI技术制备的以热解炭为基体的C/C复合材料具有密度低、低缺陷、低烧蚀率等优点，已在航空航天、核能和生物等高技术领域得到了广泛应用。然而预制体在CVI致密化过程的“位置效应”和“结壳现象“一直是制约C/C复合材料性能提高和成本降低的瓶颈，这类问题在制备如航天应用的扩散段、帽头端等大尺寸复杂形状构件时显得尤为突出。本项目采用实验与软件模拟相结合的方法，对大尺寸沉积区域内甲烷的热解炭沉积过程进行了研究。通过原位气相分析实验，探讨了甲烷在长程传输过程中的热解历程，确立了甲烷在长程传输过程中的主要热解产物为乙烯、乙炔、乙烷、氢气以及多种类的芳香烃。随着甲烷在高温沉积区反应时间的延长，气相中间产物的含碳量逐渐增加，碳氢比也逐渐增加，碳原子更倾向于转化为大分子芳香烃化合物。通过FactSage软件分析了各中间产物的沉炭总括反应式，表明热解炭沉积过程受动力学因素控制，甲烷直接参与沉炭的速率不高，远低于烯烃和炔烃类物质，且该差距在低温下更为显著。在较低温度下，苯的沉炭能力最强。通过Comsol软件分析了垂直气流方向上预制体内孔隙的气体输送过程，结果表明沉积活性物质进入孔隙的驱动力为浓度差，沉积反应产物排出孔隙的驱动力为压力差。通过系列PECVD实验，探究了CCVD工艺条件下纳米炭材料的沉积过程，结果表明在催化剂作用下，气相中不同的中间产物将生成不同形貌的纳米炭沉积物，C2类中间产物有助于形成碳纳米管，C6或C6+类中间产物有助于形成纳米碳颗粒。

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