Degradation of thermal transport properties in fine-grained isotropic graphite exposed to swift heavy ion beams

Isotropic polycrystalline graphite samples were irradiated with similar to 1 GeV Au-197 and U-238 ions of fluences up to 5 x 10(13) ions/cm(2). Beam-induced changes of thermophysical properties were characterized using frequency domain photothermal radiometry (PTR) and the underlying structural transformations were monitored by Raman spectroscopy. The ion range (similar to 60 mu m) was less than the sample thickness, therefore thermal diffusivity contributions of the irradiated as well as non-irradiated layer were considered when analyzing the PTR data. At the highest applied fluences, the thermal effusivity of the damaged layer degrades down to 20% of the pristine value and the corresponding calculated values of thermal conductivity decrease from 95 Wm(-1) K-1 for pristine material to 4 Wm(-1) K-1, a value characteristic for the glassy carbon allotrope. This technique provides quantitative data on thermal properties of ion-irradiated polycrystalline graphite and is very valuable for the prediction of lifetime expectancy in long-term applications in extreme radiation environments. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd.