Reduced thermal conductivity of TiNiSn/HfNiSn superlattices

TL;DR

This study demonstrates a significant reduction in cross-plane thermal conductivity in TiNiSn/HfNiSn superlattices due to phonon scattering at interfaces, leading to improved thermoelectric performance.

Contribution

It provides a systematic analysis of thermal conductivity reduction in superlattices using Boltzmann transport theory and interface models, with experimental validation.

Findings

Thermal conductivity decreases with superlattice periodicity down to 3 nm.

Phonon spectrum mismatch explains the conductivity reduction.

Enhanced ZT values observed in superlattices compared to individual films.

Abstract

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 $\kappa$ is measured by the 3$\omega$ 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 superlattice periodicity of 3 nm the phonon spectrum mismatch between the superlattice components quantitatively explains the reduction of $\kappa$. For very thin individual layers the interface model breaks down and the artificial crystal shows an enhanced $\kappa$. We also present an enhanced ZT value for all investigated superlattices compared to the single TiNiSn and HfNiSn films.