Fig. 1

Results of the effective medium theory (a) the case of a spherical inclusion of a void, i.e. a material with no electrical (σ) or thermal (κ) conductivity in a thermoelectric host (gray); (b) the case of a metallic inclusion with an electrical and thermal conductivity 1200 times that of the host, but no thermopower (α); (c) the case of a void surrounded by a doped layer of thermoelectric with a 100 times higher σ and κ than the host. At filling fractions Φ below the percolation threshold, the thermopower α(Φ) of the effective medium is unchanged. In cases (a) and (b), the effective medium κ(Φ) and σ(Φ) (shown normalized to those of the host) vary with Φ in complementary ways, and the ZT of the composite is unchanged from that of the host. However, in case (c) an increase of ZT can be obtained