Crystallite size dependence of thermoelectric performance of CuCrO2

The layered delafossite CuCrO2 has attracted attention as a promising thermoelectric material because its electrical conductivity can be greatly increased by doping. Here we study the effect of crystallite size and morphology on the thermal conductivity, Seebeck coefficient and electrical resistivity, all important factors for thermoelectric performance. We have synthesized polycrystalline CuCrO2 by three routes (solid state reaction, sol-gel method and hydrothermal synthesis), leading to samples with distinctly different particle sizes and morphologies. The smallest crystallites with a quasi-hexagonal shape of dimensions similar to 20 nm can be obtained by hydrothermal synthesis. These samples have the lowest thermal conductivity but their high resistivity dominates and has a detrimental effect on the thermoelectric figure of merit, ZT. Samples prepared by the sol-gel method exhibit relatively low thermal conductivity and resistivity and consequently the best ZT. We discuss the possibility of enhancing ZT further in this type of material.