  Engineering materials to include nanoscale porosity or other nanoscale
structures has become a well-established strategy for enhancing the
thermoelectric performance of dielectrics. However, the approach is only
considered beneficial for materials where the intrinsic phonon mean-free path
is much longer than that of the charge carriers. As such, the approach would
not be expected to provide significant performance gains in polycrystalline
semiconducting alloys, such as SixGe1-x, where mass disorder and grains provide
strong phonon scattering. In this manuscript, we demonstrate that the addition
of nanoscale porosity to even ultrafine-grained Si0.8Ge0.2 may be worthwhile.
The semiclassical Boltzmann transport equation was used to model electrical and
phonon transport in polycrystalline Si0.8Ge0.2 containing prismatic pores
perpendicular to the transport current. The models are free of tuning
parameters and were validated against experimental data. The models reveal that
a combination of pores and grain boundaries suppresses phonon conductivity to a
magnitude comparable with the electronic thermal conductivity. In this regime,
ZT can be further enhanced by reducing carrier concentration to the electrical
and electronic thermal conductivity and simultaneously increasing thermopower.
Although increases in ZT are modest, the optimal carrier concentration is
significantly lowered, meaning semiconductors need not be so strongly
supersaturated with dopants.
