On thermal conductivity of amorphous niobium monoxide

Thermal conductivity of NbO is studied experimentally and theoretically. Sputtered NbO samples, slightly overstoichiometric in oxygen, are x-ray amorphous with a combined NbO and NbO2 short-range order, which is consistent with the quantum mechanical data. Upon annealing, NbO2 crystallites form in the amorphous matrix due to an energetic preference of 44 meV atom(-1) over NbO counterparts. The measured thermal conductivity at room temperature using time-domain thermoreflectance is 6.9 W m(-1) K-1, followed by a continuous increase due to electronic contributions, and it yields a total increase of 40 W m(-1) K-1 at 650 K partly due to crystallisation. Phonon-phonon scattering can be induced by introducing Ni0.75Ta0.25O layers in NbO. These multilayers exhibit a thermal conductivity of 5.4 W m(-1) K-1 at room temperature and a monotonic drop upon increasing temperature. It is thus feasible to modulate the high-temperature thermal conductivity of amorphous NbO by an order of magnitude. Hence, amorphous NbO may be of interest for thermoelectric devices, sensors and thermal insulation applications.