Synthesis of In2-xGexO3 nanopowders for thermoelectric applications

Bulk ceramics In2-xGexO3 have been synthesized in air by using citrate gel process. Nanoparticles
of less than 20 nm have been synthesized through an accurate control of the processing
parameters. X-ray diffraction and scanning electron microscopy studies confirmed that the
solubility limit of Ge in In2O3 (xℓ) is very small and that additions of more than about 0.5 at.% Ge
lead to the presence of In2Ge2O7 inclusions. Thanks to a high interdispersion of metal ions and
homogeneity in elemental composition of the nanopowders obtained by citrate gel process,
well-dispersed In2Ge2O7 secondary phases can be formed in the Ge-doped In2O3 matrix. An abrupt increase in the electrical conductivity and in the carrier concentration with x is observed in the monophasic region (x , xℓ), whereas in the biphasic region (x . xℓ), these values do not vary
significantly. Similarly, the thermopower |S| value is correlated to this variation decreasing as x
increases for x , xℓ. Above the solubility limit, the decrease in the lattice thermal conductivity is
shown to be dependent on the presence of well-dispersed In2Ge207 secondary phases. The
dimensionless figure of merit value is increased up to 0.3, thanks to electron doping and phonon
scattering.