No abstract

Thoria (ThO2) has lately gained attention due to its potential for use as a nuclear fuel. From a physics standpoint, ThO2 is an actinide-bearing material with no 5f electrons and is thus ideally suited as a baseline material for future studies of the physical properties of actinide systems with correlated electrons. Current investigations of ThO2 as a nuclear fuel focus on the influence of radiation-induced lattice defects on its thermal properties, especially the conductivity. This work presents a first investigation of the impact of point defect disorder on phonon thermal conductivity of ThO2 by solving the Boltzmann transport equation within the single-mode relaxation time approximation. The relaxation times of intrinsic, three-phonon scattering are calculated by a rigorous sampling of k-points within the irreducible Brillouin zone of the face-centered cubic crystal structure. The effect of point defects on the thermal conductivity of ThO2 is predicted using the classic model by Klemens for phonon relaxation times that result from the change in mass and induced lattice strain associated with point defects. Within this model, the change in force constants and atomic radii are computed using input from an atomistic model of ThO2. The defects considered are uranium substitution at a thorium site, oxygen vacancies and interstitials, and thorium vacancies and interstitials. The results show that the conductivity of ThO2 is highly sensitive to intrinsic point defects and less sensitive to U substitution on the cation sublattice.

To develop practically useful systems for ultra-high-density information recording with densities above terabits/cm 2 , it is necessary to simultaneously achieve high thermal stability at room temperature and high recording rates. One method that has been proposed to reach this goal is heat-assisted magnetization reversal (HAMR). In this method, the magnetic orientation is assigned to a high-coercivity material by temporarily reducing the coercivity during the writing process through localized heating. Here we present kinetic Monte Carlo simulations of a model of HAMR for ultrathin films, in which the temperature in the central part of the film is momentarily increased above the critical temperature, for example by a laser pulse. We observe that the speed-up achieved by this method, relative to the switching time at a constant, subcritical temperature, is optimal for an intermediate strength of the writing field. This effect is explained using the theory of nucleation-induced magnetization switching in finite systems. Our results should be particularly relevant to recording media with strong perpendicular anisotropy, such as ultrathin Co/Pt or Co/Pd multilayers.

The Monte Carlo (MC) method is applied to solve the Boltzmann transport equation for phonons in uranium dioxide single crystals, with the objective of understanding thermal transport in this material at the mesoscale. The overall solution scheme tracks the phonon density as it evolves in space and time due to phonon drift and phonon–phonon scattering by normal and Umklapp processes. Unlike most previous works on solving the Boltzmann transport equation for phonons by the MC technique, our scheme for calculating phonon lifetime, based on normal and Umklapp scattering, eliminates the need for using many fitting parameters. Instead, the Grüneisen parameter, which is a well-characterized material property, is the only parameter of the problem. The results elucidate the simulation domain size over which the computed conductivity is size dependent; this helps to determine the minimum domain size required to simulate bulk thermal transport and hence calculate the thermal conductivity. The latter is computed over the temperature range 300–1000 K. The computed conductivity values are in good agreement with the previously published experimental and molecular dynamics simulation results.

We develop a theoretical model for thermal conductivity of α-U that combines density functional theory calculations and the coupled electron–phonon Boltzmann transport equation. The model incorporates both electron and phonon contributions to thermal conductivity and achieves good agreement with experimental data over a wide temperature range. The dominant scattering mechanism governing thermal transport in α-U at different temperatures is examined. By including phonon–defect and electron–defect scatterings in the model, we study the effect of point defects including U-vacancy, U-interstitial, and Zr-substitution on the thermal conductivity of α-U. The degradation of anisotropic thermal conductivity due to point defects as a function of defect concentration, defect type, and temperature is reported. This model provides insights into the impact of defects on both phonon and electron thermal transport. It will promote the fundamental understanding of thermal transport in α-U and provide a ground for investigation of coupled electron–phonon transport in metallic materials.