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

doi:10.15541/jim20150528

Abstract

Abstract:

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^-1and 8.9 mm^-1near 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.

Key words: Monte Carlo simulation, carbon/carbon composites, pyrolytic carbon, honeycomb structure, surface area/volume, inlet gas pressure

CLC Number:

TQ174

XU Jian, TANG Zhe-Peng, PENG Yu-Qing, GU Chuan-Qing, KOYO Norinaga, LI Ai-Jun. Numerical Simulation for Influence of Surface Area/Volume Ratio and Inlet Gas Pressure on Pyrolytic Carbon Texture[J]. Journal of Inorganic Materials, 2016, 31(12): 1327-1334.

Figures/Tables 12

Fig. 1 Simplified reaction scheme for pyrocarbon deposition. GP: gaseous precursors; L: linear small molecule hydrocarbons; A: small aromatic hydrocarbons; PAHs: polycyclic aromatic hydrocarbons

Table1 Kinetic parameters of the heterogeneous surface reaction mechanism for carbon deposition and texture formation

k₁/(mol^-1·m³·s^-1)	k₂/s^-1	k₃/(mol^-6·m¹⁸·s^-6)	k₄/s^-1	k₅/s^-1	k₆/s^-1	k₇/(mol^-1·m³·s^-1)
1×10	5×10^-2	4.81×10⁴	2×10^-1	3.86×10^-2	6.57×10^-2	8.96×10

Fig. 2 Hexagon geometry model diagrammatic sketch for the microstru cture of pyrocarbon

Fig. 3 Rates of events happened per MCS on the surface of carbon matrix (times/MCS)

Table 2 Mole fraction profiles of C2 and C6 in the reactor near the intlet (No.2) with variation of AS/VR

A_S/V_R	0.31	0.79	1.6	2	3.2	4	5	6	7.4
C₂	0.094	0.065	0.033	0.034	0.031	0.025	0.022	0.015	0.015
C₆×100	0.28	0.32	0.41	0.45	0.51	0.50	0.43	0.37	0.31

Table 3 Mole fraction profiles of C2 and C6 in the reactor near the outlet (No.8) with variation of AS/VR

A_S/V_R	0.31	0.79	1.6	2	3.2	4	5	6	7.4
C₂	0.12	0.083	0.065	0.062	0.062	0.061	0.061	0.06	0.059
C₆×100	1.12	0.85	0.79	0.78	0.78	0.78	0.77	0.76	0.71

Fig. 4 C2 normalized concentration profiles (a) and R (b) as a function of [AS/VR] ratio at the No.2 sampling area (near the inlet) and the No.8 sampling area (near the outlet)

Table 4 Values of C2 normalized and the relative simulated catastrophe points at No.2 sampling area (near the inlet)

a	1	0.691	0.564	0.543	0.479	0.468	0.404	0.372	0.372
r₀	0.156	0.086	0.062	0.058	0.049	0.047	0.046	0.041	0.041

Table 5 Values of C2 normalized and the relative simulated catastrophe points at No.8 sampling area (near the outlet)

a	1	0.692	0.525	0.517	0.517	0.508	0.508	0.500	0.492
r₀	0.156	0.086	0.057	0.056	0.056	0.055	0.055	0.054	0.053

Fig. 5 R and r0 profiles as a function of [AS/VR] ratio^(a) No.2 sampling area (inlet); (b) No.8 sampling area (outlet)

Fig. 6 R and r0 profiles as a function of pressure^(a) No.2 sampling area (inlet); (b) No.8 sampling area (outlet); [AS/VR]=0.79 mm-1

Fig. 7 R and r0 profiles as a function of pressure^(a) No.2 sampling area (inlet); (b) No.8 sampling area (outlet). [AS/VR]=3.2 mm-1

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