The microstructural changes of carbon fiber pores in carbon–carbon composites during pyrolysis

Carbon/carbon (C/C) composites have been prepared from a phenolic resin and oxidized PAN felt with heat treatment to 1000, 1800, and 2400 °C. This paper explores the reasons for the formation of pores in the fibers and studies the microstructure of these pores. Most of the pores in the fiber with various sizes concentrate at a location 2 μm away from the fiber surface, and make a circle in the fiber. The structure of the carbon layer planes around the pores grows with increasing temperature, and has the feature of high orientation. The reasons for the formation of these pores might be: when the oxidized PAN fibers are heated, some low molecular weight species transforms into gases but do not leave the fibers due to the delicate fiber surface structure, so forms the small pores in the fibers. Through the temperature increase, the vibration of these low molecular weight molecules forms thermal stress, and thus results in the growth of the pores.