title: Ion-beam induced modifications of structural and thermophysical properties of graphite materials.
Creators: name: Prosvetov, Alexey
Publisher: name: UNSPECIFIED
Year: 2020

description: With further development of high-power accelerators and nuclear reactors, there is a strong demand for structural materials that can withstand extreme operational conditions, e.g., large heat loads, mechanical and thermal stresses, high intensity and long-term ion irradiation. Carbon-based materials show excellent thermal transport properties and thus efficient heat dissipation required for such applications. Under ion beam irradiation, graphite also has the advantage of lower stopping power and lower radiation induced activation compared to metals. For these reasons, polycrystalline graphite is commonly used in extreme radiation environments, e.g. beam dumps. In the past, neutron-irradiation effects in graphite have been studied in detail, whereas data on material behavior under exposure to high-energy ion beams is scarce. This thesis focuses on changes of structural and thermophysical properties induced by swift heavy ion irradiation in well-oriented flexible graphite (FG) and in fine-grained isotropic polycrystalline graphite (PG). To investigate radiation-induced degradation processes, graphite samples were irradiated with 4.8 and 5.9 MeV/u 12C, 48Ca, 129Xe, 197Au, and 238U ions with fluences up to ~2×10^14 ions/cm2 at the UNILAC accelerator of the GSI Helmholtz Centre for Heavy Ion Research, Darmstadt. Structural transformations along the ion range were monitored by Raman spectroscopy complemented by scanning electron microscopy. The corresponding modifications of thermophysical properties were characterized using laser flash analysis and frequency domain photothermal radiometry. The response of FG and PG to high-energy ion beams is shown to be quite different, which is ascribed to the different initial microstructure and microtexture of these graphite materials. Radiation damage in flexible graphite follows the trend of the nuclear energy loss and yields a weak or no correlation with the electronic energy loss up to 30 keV/nm. The density of point defects produced via elastic collisions monotonously increases along the ion range and causes a pronounced thermal diffusivity degradation from ~550 down to 50 mm2/s for the Au ion irradiation at a fluence of ~1×10^14 ions/cm2. In contrast, in polycrystalline graphite, both nuclear and electronic (above a certain threshold) energy loss contribute to material modifications. Raman spectroscopy reveals the same type of damage for low Z (Ca) and high Z (Au, U) ions at the end of the ion range (last ~15 µm), where the nuclear energy loss is maximal. Within the range section dominated by the electronic energy loss, the irradiation with Au and U ions (high energy loss of 20-25 keV/nm) causes significant disordering, crystallite refinement and misalignment as well as partial amorphization. At the highest applied fluence of 5×10^13 ions/cm2, this leads to a structure similar to glassy carbon. Exposure to Ca ions of the same fluence produces just a slight increase of the defect density. The corresponding drop of the thermal diffusivity is from 80 mm2/s (virgin) to ~5 mm2/s for Au ions and to ~70 mm2/s for Ca ions. Damage cross-sections in both graphite materials are calculated based on the evolution of the Raman parameters and thermophysi-cal properties as a function of fluence. Raman parameters assigned to the lattice disorder and thermal diffusivity values show a strong correlation, providing the possibility to estimate heat transfer properties of graphite materials by means of Raman spectroscopy.