Reduced thermal conductivity of TiNiSn/HfNiSn superlattices
Diminution of the thermal conductivity is a crucial aspect in thermoelectric research. We report a systematic and significant reduction of the cross-plane thermal conductivity in a model system consisting of DC sputtered TiNiSn and HfNiSn half-Heusler superlattices. The reduction of $κ$ is measured by the 3$ω$ method and originates from phonon scattering at the internal interfaces. Heat transport in the superlattices is calculated based on Boltzmann transport theory, including a diffusive mismatch model for the phonons at the internal interfaces. Down to superlattice periodicity of 3 nm the phonon spectrum mismatch between the superlattice components quantitatively explains the reduction of $κ$. For very thin individual layers the interface model breaks down and the artificial crystal shows an enhanced $κ$. We also present an enhanced ZT value for all investigated superlattices compared to the single TiNiSn and HfNiSn films.