Anomalously large lattice thermal conductivity in metallic tungsten carbide and its origin in the electronic structure

Abstract

Usually, the thermal conductivity is predominantly contributed by electrons in metals. In this work, by using first-principles calculations we find that in tungsten carbide (WC) the phonon-contributed thermal conductivities (κph) are 131 and 158 Wm−1K−1 along the a and c axes, respectively, three times as much as the electronic contribution (κe). In isotopically pure samples, κph can be further increased to 204 and 249 Wm−1K−1 along the a and c axes, respectively, which is comparable to the κe of Al. The anomalously large κph is attributed to the weak phonon-phonon and electron-phonon scattering, both of which have their origin in the electronic structure of the group-VI carbides. The Fermi energy falls within the pseudogap between the bonding and antibonding states, suggesting stronger interatomic bonding and weaker electron-phonon scattering than in group-IV and V carbides. The unique combination of strong interatomic bonding and large atomic mass of W results in a large acoustic-optical gap in the phonon dispersion, suppressing phonon-phonon scattering. In contrast, in another group-VI carbide, MoC, also with strong interatomic bonding, the smaller atomic mass of Mo increases the acoustic phonon frequencies and reduces the acoustic-optical gap. Furthermore, electron-phonon scattering, though not very strong in absolute magnitude, also plays an important role in phonon scattering, leading to a weak temperature dependence of κph in WC. The large thermal conductivity, persisting at high temperatures, facilitates the use of this material in applications such as cutting tools.