Abstract
We have proposed that homologous compounds of (ZnO)m In2O3 with layer structures can become candidate materials for high-temperature thermoelectric conversion due to their low thermal conductivity and high electron mobility. Crystal structures can be modified by the isoelectronic substitution of either divalent or trivalent metal ions for Zn or In ions, respectively. Substitution of Mg2+, Co2+, and Y3+ gave rise to shrinkage of the c axis and elongation of the a axis of a hexagonal unit cell. Rietveld structure refinement indicated that Mg2+ and Co2+ ions occupy both 3a and 6c sites, while Y3+ ions occupy only 3a sites. An optimum amount of substitution of these cations increased electron mobility and hence thermoelectric efficiency Z=σα2/κ (σ=electrical conductivity, α=Seebeck coefficient, κ=thermal conductivity). Z values coupled with lowered thermal conductivity, which was possibly caused by suitable modification of the electronic structure, were associated with distortion of the crystal structure. For instance, the figure of merit of (ZnO)5(In0.97 Y0.03)2O3 was Z=1.3×10−4 K−1.
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