Superior molten FLiBe salt barrier properties of a mesophase-pitch-based carbon/carbon composite prepared via hot isostatic pressing

Carbon/carbon (C/C) composites are promising structural materials for molten salt reactors (MSRs) because of their exceptional high-temperature performance, resistance to fluoride salt corrosion, and low-neutron-absorption cross-section. However, fluoride salts can infiltrate the pores of C/C composites, potentially causing local hotspots and accelerating material degradation. In this study, a novel C/C composite was fabricated from mesophase-pitch-based carbon fibers and a pitch-based carbon matrix via hot isostatic pressing (HIP). To evaluate its performance under MSR conditions, the composite was exposed to molten FLiBe salt at 700 °C and 2–9 atm for a duration of 20 h. Weight gain, morphological characteristics, and crystallinity were characterized after the salt infiltration test. Compared with C/C composites fabricated via conventional chemical vapor infiltration (CVI), the HIP-C/C composite exhibited much better molten salt barrier properties owing to its compact structure and small pore diameter . The weight gain of the HIP-C/C composite was only 0.17 wt.% under 5 atm, significantly lower than the critical index proposed for carbon materials used in MSR. Morphological characterization revealed that FLiBe salt particles were rarely trapped in the small pores of the HIP-C/C composite. While the crystallinity of the salt-impregnated CVI-C/C composite increased, the HIP-C/C composite exhibited a slightly decreased degree of graphitization after salt infiltration. These findings provide insights into the design of high-performance C/C composites for applications in MSR systems.