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 $\ensuremath{\kappa}$ is measured by the $3\ensuremath{\omega}$ 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 a superlattice periodicity of 3 nm the phonon spectrum mismatch between the superlattice components quantitatively explains the reduction of $\ensuremath{\kappa}$. For very thin individual layers the interface model breaks down and the artificial crystal shows an enhanced $\ensuremath{\kappa}$.