丙烯在模型孔中CVI过程的热解炭沉积

Chemical vapor infiltration of pyrocarbon from propene into model capillaries

本文在温度为1 223 K,总压强40 kPa(N_2∶C_3H_6=9∶1)和停留时间0.56 s下,以丙烯为前驱体在内径为1.0 mm和长度31.0 mm的模型孔中裂解沉积生成热解炭。以15 h为沉积周期经75 h沉积实验得到最终试样,通过制样观察热解炭沿模型孔长度方向的沉积厚度,分析在基底水平和垂直方向上热解炭的沉积速率变化。结果表明,在该实验条件下热解炭沿模型孔长度方向由孔口到末端的沉积速率降低;而在垂直方向位置较低处的模型孔,热解炭沿长度方向的沉积速率梯度较小。通过耦合丙烯的均气相反应机理和总括反应机理,对该实验条件下丙烯在模型孔中的热解炭沉积过程进行模拟。在平推流反应器模型中,运用均气相反应机理对丙烯裂解的气相组分变化进行模拟,并将平推流反应器相应位置的气体组分浓度作为对应模型孔入口的初始浓度。运用总括反应机理的热解炭沉积模型,对丙烯在模型孔中的化学气相渗透过程进行模拟。对照热解炭在模型孔中沉积厚度的实验结果,证明该模型的合理性。模拟和实验结果表明通过减少气体停留时间可适当减缓热解炭表面封孔现象。

7dead end parallel capillaries1.0mm in diameter,31.0mm in length and10mm separation were aligned perpendicular to the axis of a gas conducting tube and infiltrated at1223K and40kPa(N2∶C3H6=9∶1)for75h with a gas residence time of0.56s within the tube.The thickness of the pyrocarbon layers produced was determined along each capillary at15h intervals.Results indicate that the pyrocarbon deposition rate decreases from the mouth to the end of the capillaries and the rate gradients are lower in the capillaries nearer to the gas inlet.Pyrocarbon deposition in the capillaries was simulated by a deposition model using the lumped reaction mechanism.The gas composition in the capillary mouths was obtained from computation by assuming that gas phase reactions are homogeneous and the tube is a plug flow reactor.The calculated deposition rate profiles agree well with the experimental ones.Both the experimental and theoretical results indicate that pore blockage is relieved by reducing the gas residence time within the conducting tube.