Synergistic effect of workfunction and acoustic impedance mismatch for improved thermoelectric performance in GeTe/WC composite

The preparation of composite materials is promising for concurrent optimization of electrical and thermal transport properties to realize an improved thermoelectric (TE) performance. We report the effect of percolation network and acoustic impedance mismatch (AIM) on TE properties of (1-z)Ge$_{0.87}$Mn$_{0.05}$Sb$_{0.08}$Te/(z)WC composite. In particular, a composite consisting of Mn and Sb co-doped GeTe as a matrix and WC (tungsten carbide) as a dispersed phase is prepared, and its structural and TE properties are investigated. The simultaneous increase in the electrical conductivity (\sigma) and Seebeck coefficient (\alpha) with WC volume fraction (z) results in an enhanced power factor (\alpha^2\sigma) in the composite. The rise in \sigma is attributed to increased carrier mobility in the composite, which is further analyzed from the work function measurement using the Kelvin probe force microscopy technique. The difference in elastic properties (sound velocity) between Ge$_{0.87}$Mn$_{0.05}$Sb$_{0.08}$Te and WC results in a high AIM that leads to a large interface thermal resistance (R_{int}) between the phases. The correlation between R_{int} and the Kapitza radius results in reduced phonon thermal conductivity (\kappa_{ph}) of the composite and is discussed using the Bruggeman asymmetrical model. The simultaneous effect of improvement in \alpha^2\sigma and reduction in \kappa_{ph} results in a maximum figure of merit (zT) of 1.93 at 773 K for (1-z)Ge$_{0.87}$Mn$_{0.05}$Sb$_{0.08}$Te/(z)WC composite for z=0.010, with an average zT (zT_{av}) of 1.02 for a temperature difference ({\Delta}T) of 473 K. This study shows promise to achieve higher zTav across a wide range of composite materials having similar electronic structure and different elastic properties.