Insight into the origin of oxidation behaviors of carbon fiber reinforced carbon aerogel composites with different porous skeletons

Carbon fiber reinforced carbon aerogel (C/CA) composites are promising lightweight insulators for the thermal protection of aerospace vehicles at ultra-high temperatures. However, it is seriously lack of understanding their oxidation behaviors despite its importance for the applications in oxygen-containing environments. Herein, we fabricate two kinds of C/CA using resorcinol-formaldehyde (RF) and phenolic resin (PR) as precursors and investigate the effects of original and heat-treated skeletons on their oxidation behaviors. The results show that the composites derived from RF have the lower mass losses at 600-800 degrees C after 35-min oxidation with an apparent activation energy of 50.80 kJ mol-1 than the counterparts derived from PR with the value of 40.14 kJ mol- 1. It can be explained by the existence of more impurities and macropores as well as the larger residual tensile stress in C/CA from PR. The temperature-dependent oxidation morphology evolution is similar to the time-dependent evolution, changing from the gradually enlarged pores, to the formed grooves and even islandlike structure. However, the composites from RF have reversely the higher mass losses when they were heat treated at 1600 and 2300 degrees C, which is attributed to its relatively lower crystallinity and larger mesopore surface area. The significant increase in crystallinity and decrease in impurities and specific surface area are the main explanations for the remarkably lower oxidation rates after heat treatment. The oxidation behaviors of C/CA are jointly governed by the carbon-oxygen reaction and diffusion within pores; and the residual stress exerts a role to different extent.