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

Abstract

In Tokomak, tungsten nitrides (WNx) films that form on the surface of the divertor are a byproduct of the nitrogen seeding system. The impact of their thermal transport properties is an important issue. 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, respectively. Our findings suggest that both nitrogen vacancy and tungsten vacancy defects can suppress the electrical and thermal conductivities of h-W2N1 and β-W1N1 to some extent, with the tungsten vacancy having a more considerable effect than the nitrogen 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 density of states at the fermi energy level induced by the vacancies for each system. The insights gleaned from our findings are beneficial for the assessment and comprehension of the thermal conductivity performance of WNx films on the divertor surface.