Response of ¹¹B enriched ZrB₂ ultra-high temperature ceramic to neutron irradiation at elevated temperatures

ZrB₂, an ultra-high temperature ceramic (UHTC) is being considered for use in fusion reactor first-wall structures, yet its response to irradiation remains poorly understood. This study employed scanning/transmission electron microscopy (S/TEM), synchrotron X-ray diffraction (XRD), finite element calculations, and thermal property measurements to thoroughly investigate the neutron-irradiation effects on ¹¹B-enriched ZrB₂. Neutron irradiations were conducted at 220 °C and 620 °C, with a neutron fluence of 2.2 × 10²⁵ neutron/m² (energy > 0.1 MeV), resulting in 3.9 dpa and 4200 appm He. The study revealed the unusual prevalence of prism loops and a > c anisotropic lattice swelling, likely linked to the low c/a ratio of ZrB₂, leading to grain boundary microcracking. Reducing the grain sizes was effective in reducing intergranular cracking and macroscopic swelling. The observation of cavities in ZrB₂ irradiated at 620 °C, as opposed to 220 °C, prompts questions about the temperature at which vacancies in ZrB₂ become mobile, and the role of neutron absorption by ¹⁰B in elevating irradiation temperatures. Isotopic enrichment in ¹¹B proves to be a viable strategy for mitigating helium production in transition-metal diborides, which is a critical consideration for nuclear applications. Irradiation-induced defects reduce the thermal diffusivity and conductivity of ZrB₂ by a factor of 4–9, which has important implications for its role as a plasma-facing material in fusion reactors that drive high heat fluxes through first-wall materials. This comprehensive study lays the foundation for understanding ZrB₂ behavior under neutron irradiation and highlights important phenomena to consider for various material applications.