(259e) Thermal Conductivity of Tungsten: Effects of Plasma-Related Structural Defects from Molecular-Dynamics Simulation

Authors: 
Hu, L., University of Massachusetts Amherst
Wirth, B. D., University of Tennessee, Knoxville
Maroudas, D., University of Massachusetts, Amherst
The high thermal conductivity of tungsten (175 W/m·K at room temperature) is one of the main reasons for its consideration as a plasma-facing material (PFM) in nuclear fusion reactors. However, the exposure of PFMs to large fluxes of low-energy helium (He) irradiation from the plasma causes significant microstructural changes in the PFM’s near-surface region. Structural defects such as He nanobubbles due to plasma exposure are expected to have significant impact on tungsten’s thermal conductivity in the near-surface region, as a result of the stronger electron and phonon scattering mechanisms by the plasma-related structural defects.

In this presentation, we report on the effects of plasma-related structural defects on the lattice, i.e., phonon, thermal conductivity of tungsten based on an atomic-scale computational analysis. We have conducted a systematic study of thermal transport using non-equilibrium molecular-dynamics (NEMD) simulations and computed the lattice thermal conductivities of tungsten single crystals and tungsten single crystals containing nanoscale-size pores or voids and He nanobubbles as a function of void/bubble size and gas pressure in the He bubbles. The calculated lattice thermal conductivities of tungsten single crystals range from 15.0 to 15.6 W/m·K along different heat flux directions, which is, as expected for a metal, just under 10% of the overall thermal conductivity of tungsten. Heat fluxes along the [100], [110], [111], and [211] crystallographic directions have been examined, and the weak dependence of the lattice thermal conductivity on the heat flux direction implies a weak anisotropy in heat conduction in tungsten. The presence of nanoscale voids in the crystal causes a significant reduction in its lattice thermal conductivity, which decreases with increasing void size. Filling the voids with He to form He nanobubbles and increasing the bubble pressure leads to further reduction, up to ~80%, of the tungsten’s lattice thermal conductivity, while the anisotropy in heat conduction remains weak for tungsten single crystals containing nanoscale size voids and He nanobubbles throughout the pressure range examined.

In addition, we have calculated the pressure distribution in the crystalline region that surrounds the He nanobubbles and analyzed the phonon transport and scattering mechanisms that mediate heat conduction in tungsten. We have found that the significant reduction of tungsten’s lattice thermal conductivity in the region that contains He nanobubbles is due to phonon scattering from the nanobubbles, as well as the deformation of the lattice around the nanobubbles and the formation of lattice imperfections in these regions at higher bubble pressure.