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 work function and acoustic impedance mismatch (AIM) on the TE properties of (1-z)Ge0.87Mn0.05Sb0.08Te/(z)WC composite. In particular, a composite consisting of Mn and Sb co-doped GeTe as a matrix and WC as a dispersed phase is prepared, and its structural and TE properties are investigated. The simultaneous increase in 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. This is further established from the work function measurement using the Kelvin probe force microscopy (KPFM) technique and is also supported by the density functional theory (DFT) calculations. The difference in elastic properties (sound velocity) between Ge0.87Mn0.05Sb0.08Te and WC results in a high AIM that leads to a large interface thermal resistance (Rint) between the phases. The correlation between Rint and the Kapitza radius results in reduced phonon thermal conductivity (\kappa_ph) of the composite and is discussed using the Bruggeman asymmetrical model. The decrease in \kappa_{ph} is further established using phonon dispersion calculations that indicates the decrease in phonon group velocity in the composite. The simultaneous effect of enhanced {\alpha}^2{\sigma} and reduced \kappa_ph results in a maximum figure of merit (zT) of 1.93 at 773K for (1-z)Ge0.87Mn0.05Sb0.08Te/(z)WC composite having z=0.010. This study shows promise to achieve higher zTav across a wide range of composite materials having similar electronic structure and different elastic properties.