两种石墨类炭材料的显微结构研究

IMR OpenIR

	两种石墨类炭材料的显微结构研究
	林青云
学位类型	博士
导师	叶恒强 ; 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	掺杂石墨石墨化泡沫炭热解炭显微结构与缺陷透射电子显微学 Doped Graphite Graphitic Carbon Foam Pyrocarbon Microstructure And Defect Transmission Electron Microscope (Tem)
摘要	"石墨类炭材料具有低密度、高比强以及良好的传热、导电等优异性能，从而得到了深入研究和广泛应用。其中，良好的导热性能成为关注的重点之一，并由此开发了一系列新兴的高导热石墨类材料。众所周知，材料的微观结构决定其宏观性能，目前对于这类新型石墨材料的微观结构，特别是原子尺度的显微结构研究，还很不充分，有待进一步深入。本论文运用透射电镜相关技术(TEM)，细致研究了两种典型高导热石墨类炭材料的显微结构与缺陷，具体工作如下：一、掺杂石墨利用选区电子衍射(SAED)结合高分辨(HRTEM)技术首次发现了单独存在的菱方石墨相 (ABC...)，颠覆了传统意义上对菱方石墨存在形式的认识。同时，分析了菱方石墨的另外两种存在形式：与六方堆垛共存的ABC...+AB...结构、以及互为孪晶关系的两种菱方相与六方堆垛共存的ABC...+ACB...+AB...结构。此外，还首次发现了AA...周期堆垛的简单六方石墨结构，其局部存在于六方石墨结构中，这也从根本上打破了传统意义上对石墨基面堆垛形式的局限性认识。对三种石墨相分别进行了高分辨像模拟，证实所得高分辨像均对应其各自的真实结构，从而首次从实验上证实了单独菱方石墨晶粒及简单六方石墨的存在。利用SAED与HRTEM对掺杂组元Si、Ti的碳化物进行细致分析，从而探讨了其催化石墨化的机理。其中，SiC通过碳化物分解催化石墨化，催化完成后逸出基体，仅少量以片层形式夹杂在TiC晶粒之间或者镶嵌其中，两者之间较低能量的界面结合限制了SiC的分解逸出。与之前普遍认为的TiC仅液相转化催化石墨化的观点不同，除此之外，根据SAED与HRTEM分析，首次从实验角度提供了TiC分解催化机理的证据；并进一步归纳了残留TiC与石墨基体的多种共生关系，合理探讨了石墨析出时相对于TiC的取向关系对基面堆垛规律的影响。利用HRTEM分析了石墨晶体中由基面和非基面不全位错引起的基面堆垛层错。通过SAED和HRTEM研究了石墨晶体中的整体位移缺陷与旋转缺陷。此外，结合SAED细致分析了石墨中[1-100]、[11-20]型扭折的结构，建立了与实验结果相匹配的界面位错模型，分析了由扭折引起的六方石墨与菱方石墨相变、以及互为孪晶关系的菱方石墨间的相变，并建立了扭折形成过程中，位错萌生、运动及聚集形成扭折界面的结构模型。二、石墨化泡沫炭利用TEM细致分析了泡沫炭韧带与结点处的显微缺陷结构。其中韧带的石墨化较为完全，片层排列规则致密，但其内部存在层错、旋转缺陷、位移缺陷以及应力导致的局部区域晶格畸变等缺陷结构；同时韧带边缘形成竖直片层，从而否定了关于泡沫炭在双向气压下不可能形成竖直片层锚定状态的观点。而结点处的整体形貌为三角叉或多角叉形，平直延伸、石墨化程度较好的石墨片层仅存在于有限区域，同时其排列较为疏松且存在较多的分层、裂纹；炭层之间还存在扭折、弯曲、折叠等缺陷结构，并诱发基面的位移缺陷、裂纹以及孔洞的形成。对内壁沉积的热解炭结构及其与基体的界面结合进行了细致研究，并合理分析了热解炭织构与界面结构的区域性差异的具体原因。韧带上沉积的热解炭为双层平直炭层，其中内层的取向有序度更高；韧带边缘竖直片层之间的裂缝也被热解炭致密填充。结点本身结构的复杂性决定其上热解炭结构的多样性，大致存在三类：双层平直热解炭；存在过渡层的双重球状热解炭；没有过渡层的单一炭球，此种情况下，炭层收缩产生的应力导致裂缝的形成和应力石墨化效应，在内外层热解炭间形成约200nm厚的石墨片层结构。热解炭以泡沫炭基体表面为模板叠加沉积微晶面，此外，石墨晶粒的端面与侧面活性位浓度差导致端面上沉积有两层非晶炭结构；而侧面上直接沉积热解炭，且其微晶尺寸比端面的稍大。"
其他摘要	"Graphitic carbon materials have received thorough investigations and engaged extensive applications due to the advanced performance of low density, high specific strength and superior thermal conductivity. Over several decades, a series of new graphitic materials with high thermal conductivity have been developed, however the microstructures, especially at the atomic-scale, of these new graphitic materials are relatively less informed. It is well known that properties of materials depend on their microstructures. Thus, in this thesis, two kinds of graphitic materials with high thermal conductivity have been investigated in detail using transimission electron microscope (TEM) and other related techniques. The main results in the present investigation are as follows: 1. Doped graphite By recourse to slectcted area electron diffraction (SAED) and high-resolution TEM (HRTEM), pure rhombohedral graphite crystallites with homogeneous ABC… stacking sequence (3R) have been observed and verified evidently for the first time in graphite blocks. Meanwhile, 3R coexisted with AB...stacking sequence (2H) forming ABC...+AB... structures, and twins of 3R coexisted with 2H forming ABC...+ACB...+AB... structures have also been investigated in detailed. Moreover, the existence of simple hexagonal graphite with AA... stacked basal planes (1H) in 2H host has been confirmed for the first time by the abnormal SAED pattern and HRTEM image. HRTEM simulations about the three graphite structures have been carried out to validate the experimental HRTEM images of all stacking sequences. The mechanisms of the catalytic graphitization induced by the carbides of Ti and Si as doped catalysts have been investigated and analyzed using SAED and HRTEM. First, the formation and decomposition of SiC lead to the catalytic graphitization processes, and then it will evaporate from the graphite substrate. Due to the low interface energy between SiC and TiC, which inhibited the escape of the SiC, few SiC still coexisted with TiC crystallites. Second, different from the previous viewpoint that the solution of carbon into the catalyst particle followed by precipitation as a graphite material is the only catalytic mechanism of Ti, the evidences for the catalytic mechanism originating from the formation and decomposition of TiC have been also confirmed experimentally for the first time. Furthermore, three types of coexistence of TiC with graphite have been summarized, and based on the studies of the interface between TiC and graphite crystallites by SAED and HRTEM, it is proposed that the crystallographic orientations between precipitated graphite and catalysts TiC may influence the basal plane stacking sequence of graphite. Stacking faults of graphite basal planes induced by the basal and non-basal partial dislocations have been investigated by HRTEM. Meanwhile, the displacive defects and rotation defects have also been explored using SAED and HRTEM. Moreover, two kinds of kinks with [1-100] and [11-20] as axes and the dislocation models of their kink boundaries have been investigated in detail and developed reasonably. Furthermore, dislocation structure models are proposed to analyze the phase transformation from 2H to 3R, and 3R twins’ transformations induced by some kinks. Considering the complicated dislocation activities in the basal planes of graphite, the models about the formation of the kink structures have been advanced. 2. Graphitic carbon foam Depend on TEM, the microstructures and defects within the ligaments and nodes of graphitic foam have been investigated systematically. Firstly, ordered and dense graphitic layers with high graphitization degree are main microstructural features in the ligaments, while there are stacking faults, rotation defects, displacive defects and lattice distortions embedded in the microstructures; different from the previous conclusion that there are no edge-on graphitic layers in the foam structure, some edge-on orientations of the graphitic layers exist on the local surface of the ligaments. Secondly, in the typical triangular nodes, graphitic crystallites extend with limited parameters and there are many delaminations, nanocracks, kinks, as well as displacive defects among the crystallites. Not only the microstructures of deposited pyrocarbon (PC) and its interface with graphitic foam, but also the reasons for the difference of PC textures and interface structure in different areas have been analyzed in detail and discussed carefully. Two straight layers of PC deposited on the ligaments, while the inter carbon layers are more ordered arranged compared to the outer-layer; meanwhile the cracks between the edge-on graphitic layers on the ligaments are also densely filled with PC. Differently, according to the sophisticated morphologies, deposited PC can be divided into three categories in the nodes of the foam: 1) two straight layers, 2) dense carbon spheres with dual structures, 3) carbon spheres with cracks between the core and the PC. In the third case, the thermal stress after PC deposition leads to the stress graphitization and the formation of a 200nm-thicked graphitic layer between two layers of PC. The carbon layers of deposited PC are directly parallel to the outer boundary of the foam substrates. Due to the difference in the concentration of active sites between the edge side and the surface of graphite basal planes, there are two layers of amorphous carbon between the graphitic foam and PC on the edge side; while PC with larger carbon micro-crystallites is combined directly with the parallel surface of the foam."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64488
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	林青云. 两种石墨类炭材料的显微结构研究[D]. 北京. 中国科学院金属研究所,2012.