为研究热解炭织态结构的生成规律,采用不同压强的甲烷为碳源,在1 100℃条件下进行了化学气相沉积和化学气相渗透实验.化学气相沉积以具有不同表面积/自由体积比([A/V]值)的直通方形多孔陶瓷为基体;化学气相渗透实验在直径为1mm细直孔内表面沉积和对炭纤维体积分数为7%的炭毡进行致密化.借助正交偏光显微镜(消光角)和透射电子显微镜(定向角)对在不同实验条件下制备的热解炭进行分析和定量表征.研究发现:热解炭的织态结构可以在两种不同的沉积条件下形成.当甲烷压强较低时为化学生长阶段;当甲烷压强较高时为物理形核阶段.在化学生长控制阶段,热解炭的织态结构可以利用之前提出的"颗粒填充模型(P-F模型)"加以解释.该模型假设高织构热解炭的沉积一定对应于气相中存在具有合适比例的芳香化合物(例如苯)和线性小分子(主要是C2H2),当二者的浓度比偏离该最优比(或者偏大,或者偏小),均将导致中织构甚至低织构热解炭的生成.在化学生成控制阶段,化学气相沉积和化学气相渗透对热解炭织态结构影响的差别,除了[A/V]值而外,还有氢气的作用.在化学气相渗透过程中,基体内部生成的氢气快速扩散至基体表面,使内外沉积速率和织态结构均发生较大变化.
Carbon textures, as a function of methane pressure, obtained at 1 100 ℃ by chemical vapor infiltration of a carbon fiber felt with a fiber volume fraction of 7 % and a capillary diameter of 1.1 mm, at different surface area/volume ratios of the substrate were characterized using the optical extinction and orientation angle. Significant texture changes with increasing methane pressure were attributed to the nucleation-growth mechanism of carbon formation with growth dominating at lower pressures, and the increasing influence of the nucleation mechanism of carbon formation at higher pressures. Carbon textures resulting from the growth mechanism perfectly agree with the particle filler model. It is postulated that high textured carbon is formed from a gas phase with an optimum ratio of aromatic hydrocarbons ( molecular particles) to small linear hydrocarbons (molecular filler), whereas medium and low textured carbon are deposited with an excess of either aromatic or small linear hydrocarbons. Most significant differences in texture formation between chemical vapor infiltration and deposition result from the much lower hydrogen partial pressures in infiltration because hydrogen diffuses to the surface of the porous substrate.
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