比表面积与入口气体分压对热解炭微观结构影响的数值模拟研究

本研究在炭/炭复合材料热解炭基体织构形成与转化的模型基础上, 基于石墨微晶片层的表面结构特点, 建立了蜂窝结构的热解炭沉积表面几何模型, 并运用Monte Carlo方法模拟了在等温等压化学气相渗透(CVI)过程中热解炭基体沉积的动力学过程, 研究了预制体比表面积(A_S/V_R)和入口气体分压对热解炭微观结构的影响。通过数值模拟并结合已公开发表的实验结果发现, 在CVI工艺过程中一定的压力条件下, 通过控制A_S/V_R可以获得不同织构的热解炭, 预制体的A_S/V_R存在两个临界值, 靠近反应器入口处的临界值为1.45 m^-1和8.9 mm^-1, 靠近反应器出口处的临界值为0.3 mm^-1, 当A_S/V_R处于这两个临界值之间时, 系统主要沉积高织构热解炭; 在同一A_S/V_R且压强小于30 kPa的条件下, 通过控制反应气体压强的值也可以得到不同织构的热解炭, 并且压强也存在一个临界值, 当压强大于这个临界值时, 系统主要沉积高织构热解炭。

Numerical Simulation for Influence of Surface Area/Volume Ratio and Inlet Gas Pressure on Pyrolytic Carbon Texture

Based on the model of pyrolytic carbon matrix texture formation and transformation in CVI for densification of carbon/carbon composites, a new model was constructed to describe the surface of pyrolytic carbon deposited in a honeycomb structure pores. This new model is related with structure characteristics of graphite microchip. The Monte Carlo method is applied to simulate the pyrocarbon deposition kinetics and to analyze the influence of surface area/volume (A_S/V_R) and gas pressure on the texture of pyrocarbon in the process of isothermal-isobaric chemical vapor infiltration (ICVI). According to the numerical simulation results, under certain conditions of pressure, various textured carbon were obtained by controlling A_S/V_R in CVI process. Between two critical values (1.45 mm^-1 and 8.9 mm^-1 near the inlet, 0.3 mm^-1near the outlet) of A_S/V_R, the high-textured carbon can be obtained. Moreover, if A_S/V_R is certain value and pressure is less than 30 kPa, pyrolytic medium and high textured carbon texture can be obtained as a result of different pressure in CVI process. There is also a critical pressure value and the high-textured carbon can be obtained if the pressure is greater than the critical value.