炭纤维表面处理对C/C复合材料显微结构的影响的电子显微学研究

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	炭纤维表面处理对C/C复合材料显微结构的影响的电子显微学研究
	刘叶群
学位类型	博士
导师	贺连龙
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料物理与化学
关键词	C/c复合材料炭纤维表面处理纳米炭纤维催化化学气相渗透 C/c Composites Carbon Fiber Surface Treatment Carbon Nanofiber Catalytic Chemical Vapor Infiltration
其他摘要	" 炭/炭（C/C）复合材料因其卓越的高温力学和物理性能，被广泛应用于航空航天等领域。然而，C/C复合材料有两个重大缺陷：其一，炭纤维（CF）和热解炭（PyC）基体的界面结合强度低；其二，极长的工艺时间导致C/C复合材料的生产成本非常高。通过在炭纤维表面原位生长纳米炭纤维（CNFs）可以改善C/C复合材料的界面结合性能，而通过在炭纤维表面化学镀Ni-P催化剂，利用催化剂对碳氢化合物的催化裂解作用则可以缩短其工艺时间。本论文利用透射电子显微术（TEM），通过与常规C/C复合材料的对比，研究了CNFs和Ni-P催化剂对C/C复合材料的显微结构和性能的影响，并分析了各种显微结构的形成机理。通过TEM观察和分析了CNFs和催化剂的形貌、结构以及二者之间的取向关系，并对CNFs的生长机理进行了探讨。结果表明原位生长的CNFs相互缠结，在炭纤维表面形成网状结构。主要存在直CNFs和竹节状CNFs，CNFs中存在残留棒状或锥状Ni催化剂。大部分CNFs的生长方向平行于[001]Ni晶带轴，Ni/CNFs侧壁界面为(220)Ni或(020)Ni晶面，Ni/CNFs端部界面为(002)Ni晶面。CNFs的生长过程伴随催化剂的拉伸和收缩，催化剂表面的纳米级台阶导致其断裂和分离。 T700炭纤维具有皮芯结构，芯部和皮层之间存在过渡层，其微晶尺寸大于芯部和皮层。微晶在炭纤维横截面内随机分布，但是在炭纤维轴向则高度择优取向，芯部、过渡层和皮层的定向角分别为54°、30°和26°。通过TEM对比研究了常规C/C复合材料和CNFs增强C/C复合材料的显微结构。结果表明常规C/C复合材料中，炭纤维被层状中织构（MT）热解炭同心包裹。对于CNFs增强C/C复合材料，其显微结构强烈依赖于CNFs的分布密度。在低CNFs分布密度区域，CNFs表面生成高织构（HT）热解炭，而炭纤维表面则生成MT热解炭；此外，在CF/MT界面上存在一层非常薄的HT热解炭（约20 nm）。热解炭基体总体上形成三层结构，最内层为HT热解炭薄层，中间层为MT热解炭、HT热解炭和CNFs组成的混合层，最外层为HT热解炭、CNFs和微米级孔隙组成的网状结构。然而，在高CNFs分布密度区域，炭纤维表面不存在MT热解炭，而且部分CNFs网状结构孔隙被各向同性（ISO）热解炭填充；另外，CNFs表面的HT热解炭层比低CNFs分布密度区域的更薄。热解炭基体总体上形成双层结构，内层为HT热解炭薄层，外层为HT热解炭、ISO热解炭、CNFs和纳米级孔隙组成的网状结构。三点弯曲实验表明CNFs增强C/C复合材料的弯曲强度和模量在垂直于炭纤维方向上分别提高30%和73%，在平行于炭纤维方向上分别提高60%和47%。这主要是因为原位生长CNFs可以提高CF/PyC界面结合强度和热解炭的层间结合强度，以及网状结构和复杂相界面带来的强度和韧性的提高。通过TEM详细研究了催化C/C复合材料中催化剂的分布、成分和结构。发现催化剂颗粒既分布于热解炭基体中，也迁移进入炭纤维表层。基体中的催化剂颗粒（100-800 nm）为Ni3P/Ni双相，Ni3P和Ni的取向关系为[111(-)]Ni//[11(-)0]Ni3P，[011]Ni//[110]Ni3P，[21(-)1]Ni//[001]Ni3P；而炭纤维表层的催化剂颗粒（<50 nm）为Ni3P单相。通过TEM研究了催化剂对C/C复合材料热解炭基体和炭纤维的显微结构的影响，并分析了各种显微结构的形成机理。结果表明基体中的催化剂颗粒被HT热解炭壳包覆，形成颗粒状热解炭，改变了常规C/C复合材料中热解炭的同心层状结构。热解炭壳中普遍存在缺口，当缺口位于炭纤维/催化剂界面时，液相Ni3P催化剂被优先挤出并向炭纤维表层迁移，炭纤维被催化石墨化，结构均匀性被破坏；催化剂颗粒的包覆以溶解/扩散/析出/包覆/挤出机理进行。当缺口位于气相/催化剂界面时，催化剂颗粒的包覆以溶解/扩散/析出/包覆/断裂/挤出机理进行。 "; " Carbon/carbon (C/C) composites are widely used in aeronautic and airspace fields due to their excellent high temperature mechanical and physical properties. However, there are two crucial shortcomings for C/C composites. Firstly, the interfacial bonding strength between carbon fiber (CF) and pyrocarbon (PyC) matrix is very low; secondly, production costs of C/C composites are very high because of their extremely long processing time. The interfacial adhesion property of C/C composites can be improved by in situ growing of CNFs on carbon fiber surface, and the processing time can be shortened by eletroless Ni-P catalysts on carbon fiber surface, taking advantage of the catalytic effect of the catalysts on hydrocarbon decomposition. In this paper, with comparison to conventional C/C composites, effects of CNFs and Ni-P catalysts on microstructures and properties of C/C composites were studied by transmission electron microscopy (TEM), and formation mechanisms of the microstructures were also analyzed. Morphologies and structures of the CNFs and the catalyst, as well as their orientation relationship, were investigated and analyzed by TEM, and growth mechanism of the CNFs was also discussed. The results indicate that the CNFs were entangled with each other, and they formed a network structure on the surface of carbon fibers. Mainly straight and bamboo-shaped CNFs were observed, with rod-like or conical catalysts remained in them. Most of the CNF growth directions are parallel to [001]Ni zone axis, with (220)Ni or (020)Ni as Ni/CNF sidewall interface and (002)Ni as Ni/CNF end interface. Catalyst prolongation and contraction occurred during the growth of CNFs, and the nanometer scale steps on the surface of the catalysts led to their break and splitting. T700 carbon fibers exhibit a core-skin structure, and a transition layer existed between the core and the skin, whose microcrystalline sizes are larger than the core and skin. Microcrystallines distributed randomly in the cross-section of carbon fibers, but they are highly oriented in the longitudinal direction of carbon fibers. Orientation angles (OAs) of the core, the transition layer and the skin are 54°, 30° and 26° respectively. Microstructures of the conventional C/C composites and the CNFs reinforced C/C composites were investigated comparatively by TEM. Results suggest that carbon fibers were concentrically surrounded by laminar medium-textured (MT) PyC. As to the CNFs reinforced C/C composites, their microstructures depend strongly on distribution density of the CNFs. In regions of low CNF distribution density, high-textured ( HT) PyC was formed on surfaces of the CNFs, and MT PyC was formed on carbon fiber surfaces. Furthermore, a thin layer of HT PyC (~20 nm) exists on the CF/MT interface. Generally, a tri-layer structure was formed in the PyC matrix, with the thin layer of HT PyC as inner layer, a mixed layer composed of MT, HT PyC and CNFs as middle layer and a network structure composed of HT PyC, CNFs and micropores as outmost layer. Nevertheless, in regions of high CNF distribution density, no MT PyC exists on carbon fiber surfaces, and some pores in the CNF networks were filled with isotropic (ISO) PyC; in addition, the HT PyC layer on the CNF surfaces are much thinner than in the low CNF distribution density regions. Generally, a bi-layer structure was formed in the PyC matrix, with the thin HT PyC layer as inner layer and a network structure composed of HT, ISO PyC, CNFs and nanopores as outer layer. Three-point bending test results indicate that the flexural strength and modulus of the CNFs reinforced C/C composites are promoted by 30% and 73% in the direction perpendicular to the carbon fibers respectively, and promoted by 60% and 47% in the direction parallel to the carbon fibers respectively. The promotions are attributed to increases of the CF/PyC interfacial bonding strength and the PyC interlaminar bonding strength due to the in situ grown CNFs, as well as increases of strength and toughness due to the network structure and complicated phase interfaces. Distribution, composition and structure of the catalysts in the catalyzed C/C composites were characterized detailedly by TEM. We found the catalyst particles were distributed not only in the PyC matrix, but also migrated into the carbon fiber surface layer. The catalyst particles (100-800 nm) in the matrix are composed of Ni3P/Ni bi-phase, and their orientation relationship is [111(-)]Ni//[11(-)0]Ni3P，[011]Ni//[110]Ni3P，[21(-)1]Ni//[001]Ni3P；However, the catalyst particles (<50 nm) in the carbon fiber surface layer are Ni3P single phase. Effects of the catalyst on microstructures of the PyC matrix and the carbon fiber of the catalyzed C/C composites were investigated by TEM, and formation mechanisms of the microstructures were also analyzed. Results indicate that the catalyst particles in the matrix were encapsulated by HT PyC shells, and particle-like PyC were formed, which changed the concentrical laminar structure of the PyC in the conventional C/C composites. Usually, there are openings in the PyC shells. When the openings exist at CF/catalyst interfaces, liquid Ni3P catalysts were preferentially extruded out of the shells and migrated into surface layer of the carbon fibers, and the carbon fibers were catalytically graphitized, so the structure homogeneity of the carbon fibers were seriously destroyed; encapsulation of the catalyst particles occurred in a dissolution/diffusion/precipitation/encapsulation/extrusion mechanism. When the openings exist at gas phase/catalyst interfaces, encapsulation of the catalyst particles occurred in a dissolution/diffusion/precipitation/encapsulation/extrusion mechanism. "
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64409
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	刘叶群. 炭纤维表面处理对C/C复合材料显微结构的影响的电子显微学研究[D]. 北京. 中国科学院金属研究所,2012.