Thermoelectric properties of Ba-Cu-Si clathrates

Abstract

In the search for cheaper representatives of thermoelectric clathrates, we have investigated the type-I clathrates Ba${}_{8}$Cu${}_{x}$Si${}_{46\ensuremath{-}x}$ ($3.6\ensuremath{\le}x\ensuremath{\le}7$, $x$ $=$ nominal Cu content) in a combined experimental and theoretical study. 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 $\ensuremath{\rho}(T)$ and Seebeck coefficient $S(T)$ measurements between 300 and 800 K reveal the behavior of a degenerate semiconductor for all samples. For $x=5$ and 6, density functional theory calculations are performed to derive the enthalpy of formation and the electronic structure. From the latter, we calculate the electrical resistivity and the Seebeck coefficient 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 $\ensuremath{\rho}(T)$ and $S(T)$ is analyzed. The highest dimensionless thermoelectric figure of merit $ZT$ of 0.28 is reached for a melt-spun sample at $600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C. The relatively low $ZT$ values in this system are attributed to the too-high charge-carrier concentrations.