First principles investigation of thermal transport of uranium mononitride

Abstract

We investigated the heat capacity and thermal conductivity of UN using first principles methods. The generalized gradient approximation (GGA) of the Perdew, Burke, and Ernzerhof functional, developed for solids (/PBEsol) as implemented in Quantum ESPRESSO (QE) and associated codes (EPW, Boltztrap, ShengBTE), was used. We evaluated the energy of the UN to be lower for ferromagnetic ordering than non-magnetic. We found, using QE code, that the lattice constant calculated using PBEsol functional and norm-conserving pseudopotentials is slightly larger for ferromagnetic UN (0.497 nm) than non-magnetic UN (0.489 nm) and they agree with experiment. We noted a significant contribution from the optical phonons to the lattice thermal conductivity, which was previously observed experimentally for urania. The phonons' calculated contribution to the thermal conductivity, which decreases with temperature, is smaller at room temperature (7.20 Wm−1K−1) than evaluated from the correlation recommended for urania (8.79 Wm−1K−1). However, urania's thermal conductivity deteriorates faster with temperature; therefore it becomes lower than that calculated for UN for temperatures higher than 490 K. The total thermal conductivity, evaluated here, leads to the total thermal conductivity of UN being overestimated below 1000 K. Therefore, further investigations are needed to evaluate the effect of the interaction of magnetic moments on uranium with phonons and electrons. Furthermore, the electronic thermal conductivity can only be performed for non-magnetic UN, using EPW and Boltztrap codes. The results are dependent on a selection of electronic carriers but show good qualitative agreement with experiment at higher temperatures. The calculated electrical resistivity of the UN at low temperature is much lower than measured, but is similar to the experimentally measured behaviour of non-magnetic ThN. On the other hand, Ziman's model predicts two orders of magnitude lower resistivity than presently calculated, which is in strong disagreement with the measured resistivity of UN, but compares well with the resistivity of aluminum.