Thermal conductivity is a critical physical property of ceramic nuclear fuels such as uranium dioxide. While cerium dioxide is considered as a solid electrolyte in solid oxide fuel cells (SOFC), in this study it is used as surrogate material to study the properties of nuclear fuel. In nuclear fuel, these oxides are exposed to extreme environments such as high temperature and bombardment with heavy particles. The damage introduced by such conditions in the form of defects generated inside the material can be detrimental to the structural stability of the material and ability to transport heat efficiently. Also, thermal conductivity degradation impacts fuel performance negatively. As a result, thermal conductivity of ceria has been widely investigated as one of the critical properties. This study is aimed at understanding the interplay between these nanoscale defects and the thermal conductivity of ceria.
Polycrystalline ceria samples were irradiated at 600 deg C to the same dose but at different rates using protons accelerated to 2 MeV. These irradiation conditions were chosen to promote the generation of nanoscale defects and to investigate their impact on thermal conductivity of ceria. SRIM simulations were done to identify the peak and plateau damage region inside the sample. The calculations estimated the plateau damage to be ~0.14 dpa. The quantitative analysis of radiation induced dislocation loops including size and density was performed using transmission electron microscope (TEM) for which the samples were prepared using focused ion beam (FIB) system.
X-ray diffraction was used to confirm the stability of crystal structure and also revealed detectable lattice expansion caused by accumulation of nanoscale defects. In addition to this, thermal conductivity was measured using modulated thermoreflectance methods and showed a notable reduction in irradiated samples. In order to isolate the impact of different defects on thermal conductivity, measurements were done for ambient temperature range of 100 to 300 K. The changes in thermal conductivity were analyzed quantitatively using the classical thermal transport model based on Klemens-Callaway formalism that considers reduction of thermal conductivity by irradiation induced nanoscale defects.