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艰苦的应力、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一种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构成复合材料基体的均匀各向同性材料的增强。化学蒸汽渗透（CVI）是一种制备陶瓷基复合材料而不破坏用于增强的纤维预成型或晶须的方法。在此过程中，围绕纤维预制体的气态反应物或它们在气相中的分解产物通过孔隙运输，并通过化学气相沉积（CVD）在整个预制体中形成陶瓷基体。CVI导致产品具有非常好的机械性能，可以生产复杂形状和大尺寸的物体，并可用于许多陶瓷-陶瓷复合材料，包括氧化物和非氧化物。除了复合材料制造外，CVI还可用于部分致密化由陶瓷或其他材料制成的膜，并以这种方式生产用于特殊应用（如分离）的选择性透膜。计划进行理论和实验工作，以研究CVI致密化多孔预制体过程中传递过程（热量和质量）、反应和结构演变相互作用的基本原理。在这些研究中，甲基三氯硅烷沉积碳化硅将作为模型系统。实验工作将包括使用多孔和非多孔基底的沉积实验，薄膜和孔隙结构表征，以及质量传递系数测量。沉积实验将在热壁反应器中进行，以重量计的方式进行，而结构表征将使用显微镜，拉曼，XRD，电子探针，气体吸附和汞孔隙度测定法进行。理论工作将涉及模型的发展，总结在CVI期间发生的各种因素的相对重要性。除了有助于阐明CVI的基本原理外，该项目还将为任何涉及纤维结构介质运输的应用提供信息

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.***//