Nanostructured half-Heusler superlattices as a model system for thermoelectric materials
Paulina Komar (Institut für Physik)

The thermoelectric figure of merit provides information about the heat to electricity conversion efficiency. It is expressed by the dimensionless figure of merit ZT=S2σTκ-1 containing only material dependent parameters (S - Seebeck coefficient, σ - electrical conductivity, κ - thermal conductivity) and the average operation temperature T. The goal of our work is to reduce κ by phonon scattering at the superlattice (SL) interfaces and, therefore, enhance ZT.
For symmetric SLs (TiNiSn:HfNiSn ratio=1 and variable SL period) we observe a very good agreement between the cross-plane thermal conductivity measured by the 3ω method and a calculation based on Boltzmann transport theory, including a diffusive mismatch model for the phonons at the internal interfaces, down to a SL period of 3 nm. At the SL period of 3 nm a crossover between the particle- and the wave-like transport of phonons takes place and, therefore, κ increases for decreasing periods [1, 2].
The κ of non-symmetric SLs (constant period thickness = 6 unit cells, and variable material ratio (TiNiSn)n:(HfNiSn)6-n, with 0 ≤ n ≤ 6), shows a strong dependence on the material content achieving a minimum value for n=3. The measured κ can be well modeled using non-symmetric strain relaxation applied to the model of the series of thermal resistances. Additionally, a systematic improvement of the in-plane Seebeck coefficient in the non-symmetric sample series is observed [3].

References
[1] P. Hołuj, C. Euler, B. Balke, U. Kolb, G. Fiedler, M. M. Müller, T. Jaeger, E. Chávez Angel, P. Kratzer, and G. Jakob, Phys. Rev. B 2015, 92, 125436.
[2] P. Komar, T. Jaeger, C. Euler, E. Chávez Ángel, U. Kolb, M. M. Müller, B. Balke, M. H. Aguirre, S. Populoh, A. Weidenkaff, and Gerhard Jakob, Phys. Status Solidi A 2016, 213, 732–738.
[3] P. Komar, E. Chávez-Ángel, C. Euler, B. Balke, U. Kolb, M. M. Müller, H. J. Kleebe, G. H. Fecher, and G. Jakob, accepted to APL Materials