Thermoelectric properties of Ba-Cu-Si clathrates

TL;DR

This study investigates the thermoelectric properties of Ba-Cu-Si type-I clathrates through experimental measurements and density functional theory calculations, revealing their metallic behavior and a maximum ZT of 0.28, limited by high charge carrier concentrations.

Contribution

It combines experimental synthesis and characterization with theoretical modeling to analyze the thermoelectric performance of Ba-Cu-Si clathrates, providing new insights into their electronic structure and transport properties.

Findings

All samples exhibit metal-like electrical behavior.

Maximum ZT of 0.28 achieved at 600°C for melt-spun sample.

High charge carrier concentrations limit thermoelectric efficiency.

Abstract

Thermoelectric properties of the type-I clathrates Ba$_8$Cu$_x$Si$_{46-x}$ ($3.6 \leq x \leq 7$, $x$ = nominal Cu content) are investigated both experimentally and theoretically. The polycrystalline samples are prepared either by melting, ball milling and hot pressing or by melt spinning, hand milling and hot pressing techniques. Temperature-dependent electrical resistivity, $\rho(T)$, and the Seebeck coefficient, $S(T)$, measurements reveal metal-like behavior for all samples. For $x = 5$ and 6, density functional theory calculations are performed for deriving the enthalpy of formation and the electronic structure which is exploited for the calculation of Seebeck coefficients and conductivity within Boltzmann's transport theory. For simulating the properties of doped clathrates the rigid band model is applied. On the basis of the density functional theory results the experimentally observed compositional dependence of $\rho(T)$ and $S(T)$ of the whole sample series is analyzed. The highest dimensionless thermoelectric figure of merit $ZT$ of 0.28 is reached for a melt-spun sample at $600^{\circ}$C. The relatively low $ZT$ values in this system are attributed to the too high charge carrier concentrations.