Experimental and Computational Thermal Conductivity Reduction in Single Crystal Thorium Dioxide From Lattice Defects

Actinide and lanthanide fluorite oxides form an important class of ceramic energy materials with applications ranging from nuclear fuels to solid oxide fuel cells. Potential application environments for ThO2 in particular include high radiation fields which directly generate lattice defects. Such defects drastically influence phonon thermal transport, a controlling safety and performance property. Here, we use a combination of ion beam irradiation and spatial domain thermoreflectance measurements to generate a defected region in single crystal ThO2 specimens in a variety of conditions and measure the resulting thermal conductivity from 77-300K. In parallel, defect evolution models are used with the linearized Boltzmann transport equation (BTE) and computed defect scattering cross sections to directly return lattice thermal conductivity over the same temperature range. The agreement shown between modeling and experiment is the first step towards a predictive thermal transport capability in fluorite oxides across a wide range of environmental conditions. The addition of light uranium doping in ThO2 is found to rapidly degrade thermal transport. This reduction is stronger than phonon scattering from mass and force constant differences alone would generate, clearly demonstrating the important role of 5f electrons on thermal performance in these systems.