Multiple length-scale microstructural characterisation of four grades of fine-grained graphite

Multiple length-scale microstructural characterisation has been conducted on four different grades of unirradiated fine-grained graphite: CL 2020, IG 430, SGL R7650 and POCO ZXF-5Q, which are the potential candidates as high-power proton beam intercepting materials (secondary particle-production targets). Micrometre and sub-micrometre sized porosity structures have been characterised by X-ray micro-computed tomography (µXCT) and focused ion beam-scanning electron microscopy tomography (FIB-SEM tomography). Results show that these four grades of graphite have distinct porosity structures with a substantial amount of interconnected pores; POCO-ZXF-5Q has the most uniform porosity size and spatial distribution out of the four at these length scales examined. As a direct measurement of the total number and volume of nano-sized Mrozowski cracks is currently not readily feasible, the characteristic crystalline size L_a reflecting the coherent scattering length at nano-scale was evaluated using micro-Raman spectroscopy. It has been found that a significant reduction in average characteristic crystalline size had been induced by sample preparation. Further measurements on freshly fractured sample surfaces excluding these damages suggested that IG430 graphite has the largest average crystallite size (~290 nm) while POCO-ZXF-5Q has the smallest average crystallite size (~80 nm) with a narrow distribution. These results from un-irradiated graphite were used to further understand the proton irradiated behaviour of these four grades of graphite.