Reaction, Transport, and Structure Evolution Phenomena in the Densification of Fibrous Substrates

在许多高温航空航天和能源应用中，需要具有高强度和在高度紧张的应力、温度、压力等条件下保持能力的低密度材料。纤维增强陶瓷基复合材料是满足低密度和可接受韧性要求的材料，因为纤维增强通常会导致形成复合材料基体的均匀各向同性材料的强化。化学气相渗透(CVI)是一种在不破坏用于增强的纤维或晶须预制体的情况下制备陶瓷基复合材料的方法。在此过程中，气态反应物或其在纤维预制件周围的气相中分解的产物通过气孔传输，并通过化学气相沉积(CVD)在整个预制件中形成陶瓷基质。CVI可以生产出力学性能非常好的产品，可以制造形状复杂、尺寸较大的物体，可以用于包括氧化物和非氧化物在内的多种陶瓷-陶瓷复合材料。除了复合材料的制造，CVI还可以用来部分致密陶瓷或其他材料制成的膜，并以这种方式为特殊应用(如分离)生产选择性渗透膜。理论和实验工作计划研究CVI致密化多孔预制体过程中传输过程(热和质量)、反应和结构演变的基本原理。这些研究将以甲基三氯硅烷沉积碳化硅作为模型体系。实验工作将包括使用多孔和非多孔衬底的沉积实验，薄膜和孔结构的表征，以及质量传输系数的测量。沉积实验将在热壁反应堆中进行，采用重量法操作，同时将使用显微镜、拉曼、X射线衍射仪、电子探针、气体吸附和压汞法进行结构表征。理论工作将涉及开发总结CVI期间发生的各种因素的相对重要性的模型。除了有助于阐明CVI的基本原理外，该项目还将为涉及纤维结构组成的介质中的传输的任何应用生成信息。*/

Low-density materials exhibiting high strength and an ability to hold up under highly strenuous conditions of stress, temperature, pressure, etc. are needed in many high- temperature aerospace and energy applications. Fiber- reinforced ceramic matrix composites are materials that satisfy the requirements of low density and acceptable toughness because fiber reinforcement usually leads to strengthening of the homogeneous isotropic material that forms the matrix of the composite. Chemical vapor infiltration (CVI) is a method that can be used to fabricate ceramic matrix composites without damaging the preform of fibers or whiskers used for reinforcement. During this process the gaseous reactants, or the products of their decomposition in the gas phase that surrounds the fibrous preform, are transported through the pores and form a ceramic matrix throughout the preform by chemical vapor deposition (CVD). CVI leads to products of very good mechanical properties, can produce objects of complex shape and large size, and can be used for many ceramic-ceramic composites including oxides and nonoxides. In addition to composite fabrications, CVI can be used to partially densify membranes made from ceramic or other materials and in this way produce selectively permeable membranes for special applications (such as separations). Theoretical and experimental work is planned to study the fundamentals of the interaction of transport processes (heat and mass), reaction, and structure evolution during densification of porous preforms by CVI. The deposition of silicon carbide from methyltrichlorosilane will be used as the model system in these studies. The experimental work will include deposition experiments using porous and nonporous substrates, film and pore structure characterization, and mass transport coefficient measurements. The deposition experiments will be carried out in a hot-wall reactor, operating in a gravimetric arrangement, while structure characterization will be carried out using microscopy, Raman, XRD, electron microprobe, gas adsorption and mercury porosimetry. Theoretical work will involve the development of models that summarize the relative importance of the various factors that occur during CVI. In addition to contributing to the elucidation of the fundamentals of CVI, this project will also generate information for any application involving transport in media consisting of fibrous structures.***//