Abstract: To investigate the effects of the pore structures of carbon fiber preforms on the densification efficiency of carbon/carbon materials, three types of three-dimensional orthogonal preforms with different weaving parameters were investigated. The same chem-ical vapor infiltration (CVI) process was used to densify and study the diffusion behavior of gas in the pores within and between the fiber bundles, and the effects of different types of pores on the weight gain rate were explored. ImageJ software was used to calculate the evolution of the regularized model pores within the fiber bundles in the three types of preforms following CVI. In addition, the pore channel area and gas diffusion coefficient of the model were quantitatively calculated using a microscopic observation method. The results indicate that when gas diffuses into the pores of the fiber bundle, Fick diffusion occurs in the early stage and Knudsen diffusion dominates in the later stage. The number of pores in the bundle affects the weight gain rate in the early stage, which in turn leads to pore filling. The A3 preform with low fiber content has high inter-bundle porosity, a large-pore cross-sectional area, and a higher diffusion coefficient. The monofilaments inside the bundle are arranged in a concentric circle, which easily form closed pores during CVI. The gas mainly undergoes Fick diffusion in the pores between the fiber bundles, and the diffusion channel area and porosity affect the diffusion coefficient, which in turn affects the subsequent weight gain rate.

Key words: carbon ?ber preforms, pore structure, chemical vapor in?ltration (CVI) densi?cation, gas di?usion