Dissertation > Excellent graduate degree dissertation topics show

C / C composites impregnated with a matrix precursor of coal tar pitch

Author: ShenYiShun
Tutor: ZhangHongBo
School: Central South University
Course: Materials Science
Keywords: C/C composites coal tar pitch graphitization degree oxidation characteristic interface
CLC: TB332
Type: Master's thesis
Year: 2008
Downloads: 241
Quote: 1
Read: Download Dissertation

Abstract

Carbon-carbon composites(C/C) are advanced composites with excellent physical, chemical and mechanical behavior. However, the applicaion and development of C/C composites have been greatly obstructed due to its high price, long period and complicated process. Accordingly, studying on C/C composites with low pirce and satisfactory properties have been drawn attention to many countries. It is crucial to develop a matrix precursor that can markedly reduce the production cost and improve the properties of the composites.This article took coal tar pitches whose basic performance was analyzed as the research object. Coal tar pitch added with 3wt% boric acid, coal tar pitch added with 3wt% boron carbide and pure coal tar pitches were made of pitch dervied carbons. C/C composites were prepared with coal tar pitch as matrix precursor. The morphologies of coal tar pitch derived carbons were observed by optical microscope and Scanning Electron microscope (SEM). Oxidation resistance and graphitization degree of those carbons were analysed by using X-ray diffraction method (XRD), X-ray photoelectron spectroscopy (XPS) and simultaneous thermal analyzer etc. Meanwhile, the interfaces of C/C composites were analyzed.The main results are as follows:(1)Among the three kinds of pitches, high temperature pitch has the highest carbon yield, and impregnating pitch has the best rheological behavior. The thermogravimetric curve of the pitches may divid into three parts and weight loss is very rapid in the second part (from 300℃to 550℃). High temperature pitch has better thermal properties for the highest temperature of initial weightlessness and maximum weight loss rate, and the lowest weightlessness. Activation energy of coal tar pitches are calcuated by the thermal analysis data, activation energy of high temperature pitch is 142.28kJ/mol, that of medium temperature pitch is 178.78kJ/mol, that of impregnating pitch is 150.90kJ/mol.(2) The components of coal tar pitch influence the microstructure of carbons, lower quinoline insoluble (QI) content and higher toluene soluble are favor of flow domain type, in contrary, pitch will form grained mosaics type. The microstructure of pitch carbon is mainly flow domain type, while that of high temperature pitch carbon is less of grained mosaics type and medium temperature pitch carbon is less of domain type and grained mosaics type, impregnating pitch carbon mainly show flow domain type. Pitch carbon of flow domain type is more graphitiable, and grained mosaics type is hard to graphitization. Accordingly, impregnating pitch carbon has the highest graphitization degree at the same conditions.(3)The graphitization degree of pitch carbon increases as the temperature increases, its characteristics is two ends increasing slightly and the mean patch increasing fast. Boric acid and boron carbide plays a certain function for increasing graphitization. The promotion of boric acid is much better than that of boron carbide. Meanwhile, addition has an effect on the morphology of pitch carbon, and has an effect on the graphitization degree.(4) Oxidation of coal tar picth is controled by chemical reaction mechanism under lower temperature, and controled by gas diffusion mechanism under higher temperature. Oxidation resistance of the impregnating pitch carbon is better than that of the high temperature and medium temperature pitch carbon.The addition improves the oxidation resistance of the coal tar pitch carbon at different degree.The improvement degree is related to species of addition and heat treatment temperature. Its mechanism is, firstly, the addition promote graphitizing that made the defects of crystals fewer, secondly, boron oxide borned in boride hinder the attacking to active site of pith carbon.(5)When the pitch derived carbon grows around the carbon fiber layer by layer, it presents synusia and band structure and forming parallelly oriented graphite(POG) structure with carbon fiber, the interface between them is discontinuous. While pitch derived carbon grows around CVD carbon perpendicularly,it is transversely oriented graphite (TOG)structure, and exhibits strip structure,there are many defects wintin the interface. The CVD carbon grows around the carbon fiber, it presents synusia and band structure, it is parallelly oriented graphite(POG) structure and weak-bonding interface.Defects within the sample which were prepared by impregnating pitch is less than those of the sample made by high temperature coal pitch.

Related Dissertations

CLC: > Industrial Technology > General industrial technology > Materials science and engineering > Composite materials > Non-metallic composite materials
© 2012 www.DissertationTopic.Net Mobile