  A numerical study of thermoelectric nanocomposites is presented.
Thermoelectric properties as a function of average grain size or nanoparticle
density are examined by simulating the measurements as they would be done
experimentally. In accordance with previous theory and experimental results, we
find that the Seebeck coefficient, power factor and figure of merit, zT, can be
increased by nanostructuring when energy barriers exist around the grain
boundaries or embedded nanoparticles. When we simulate the performance of a
thermoelectric cooler with the same material, however, we find that the maximum
temperature difference is much less than expected from the given zT. The same
nanocomposite without electrically active grain boundaries has a lower measured
zT but a higher maximum cooling temperature. The physical reason for these
results is explained. The results illustrate the limitations of zT as a figure
of merit for nanocomposites with electrically active grain boundaries and
suggest that the ability to enhance the electrical performance of
thermoelectric materials by nanostructuring is limited.
