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

The misfit-layered cobalt oxide [Ca₂CoO_3-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 Co₃OM₄ impurities or a change of the defects, i.e., the relative Co content in the [Ca₂CoO_3-δ][CoO₂]_1.62 phase. We observe that increasing the relative Co content in the [Ca₂CoO_3-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 Co₃OM₄ impurities leads to a reduction of ZT when the volume fraction of the Co₃OM₄ phase is increased from 1% to 3%. Hence, the best figure of merit is expected close to the upper phase boundary of the [Ca₂CoO_3-δ][CoO₂]_1.62 phase. [ABSTRACT FROM AUTHOR]