Reduced thermal conductivity of TiNiSn/HfNiSn superlattices

Paulina Hołuj,Gerhard Jakob,Tino Jaeger,Emigdio Chávez Angel,Ute Kolb,Christoph Euler,Gregor Fiedler,Peter Kratzer,Benjamin Balke,Mathis M Müller

doi:10.1103/physrevb.92.125436

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.

Highlights

The ability to convert a temperature difference to electricity and the possibility of both heating and cooling are very valuable properties of thermoelectric materials
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
Every bright layer of the stack corresponds to HfNiSn, whereas dark layers demonstrate the presence of TiNiSn

Summary

INTRODUCTION

The ability to convert a temperature difference to electricity and the possibility of both heating and cooling are very valuable properties of thermoelectric materials. Promising approaches include the introduction of grain boundaries by melt spinning or ball milling processes and further spark plasma sintering [2], phase separation during the solidification of bulk materials [3,4,5,6], or the thin film and superlattice (SL) approach [7,8] The latter was investigated by Venkatasubramanian et al for the Bi2Te3/Sb2Te3 SL system and resulted in the highest ever reported ZT value of 2.4 at room temperature [9,10]. They should better be considered not as layers of material 1 and material 2 separated by interfaces, but rather as an artificial tailor-made new material with a large crystallographic unit cell

EXPERIMENTAL DETAILS

Structure of the superlattices

Thermal conductivity

Power factor

CONCLUSIONS