Effect of vacancy defects on thermal transport properties of tungsten nitride compounds on divertor surface in ITER

Abstract

In tokamak, tungsten nitrides (WNx) layers that form on the divertor surface are byproducts of the nitrogen seeding system. The impact of their thermal transport properties is an important issue as they will be subjected to continuous high heat flow during operation. Leveraging density functional theory calculations along with the Kubo-Greenwood method, we investigate how vacancy defects influence the electrical conductivity and thermal conductivity of h-W2N1, β-W1N1, and h-W2N3 compounds, respectively. Our findings suggest that both nitrogen vacancy and tungsten vacancy defects can suppress the electrical and thermal conductivities of β-W1N1 to some extent. The electrical and thermal conductivity of h-W2N1 compound decrease in the presence of W vacancy but are insensitive to N vacancy. Conversely, for h-W2N3, both types of vacancy defects can enhance its electrical and thermal conductivities. Furthermore, we reveal that the fluctuation in the electrical conductivity of the three WNx compounds correlates with the changes in the mean free path of electrons and the density of states at the Fermi energy level induced by the vacancies in each system. The insights gleaned from our findings are beneficial for assessing and comprehending the thermal conductivity performance of WNx layers on the divertor surface.