  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.
