Thermoelectric properties of layer-structured (ZnO)/sub m/In/sub 2/O/sub 3/ (m=integer) improved by elemental substitution

We have already proposed that homologous compounds of (ZnO)/sub m/In/sub 2/O/sub 3/ with layer structures can become candidate materials for high-temperature thermoelectric conversion due to their low thermal conductivity and fairly high electron mobility. Crystal structures can be modified by the substitution of either divalent or trivalent metal ions for Zn or In ions, respectively: Substitution of those smaller than Zn or larger than In in ionic size gave rise to the shrinkage of c-axis and the elongation of a-axis of a hexagonal unit cell. An optimum amount of substitution increased electron mobility and hence Z values coupled with lowered thermal conductivity which was possibly caused by suitable modification of the electronic structure associated with distortion of the crystal structure. For instance, substitution of 3% Y for In realized about one order of magnitude increase in Z up to /spl sim/1.3/spl times/10/sup -1/ K/sup -1/ at 1200 K. However, substitution of small trivalent ions all deteriorated the properties.