Thermoelectric properties of [Ca2CoO3−δ][CoO2]1,62 as a function of Co/Ca defects and Co3O4 inclusions

Abstract

The misfit-layered cobalt oxide [Ca2-wCoO3-δ][CoO2]1.62 is a promising material for high-temperature thermoelectric heat recovery. Here, we show that discrepancies in the numerously reported thermoelectric performances can result from Co3O4 impurities or a change of the defects, i.e., the relative Co content in the [Ca2-wCoO3-δ][CoO2]1.62 phase. We observe that increasing the relative Co content in the [Ca2-wCoO3-δ][CoO2]1.62 phase leads to a larger figure of merit ZT. We attribute this increase of ZT to additional p-type charge carriers introduced by Ca vacancies and the resulting reduction of the electrical resistivity. For Co/Ca ratios above the miscibility limit, the increase in thermal conductivity due to the formation of Co3O4 impurities leads to a reduction of ZT when the volume fraction of the Co3O4 phase is increased from 1% to 3%. Hence, the best figure of merit is expected close to the upper phase boundary of the [Ca2-wCoO3-δ][CoO2]1.62 phase.