Exploring thermoelectric performance of Ca₃Co₄O_9+δ ceramics via chemical electroless plating with Cu

Ca₃Co₄O_9+δ ceramic is a potential p-type thermoelectric material for high temperature applications, however, its thermoelectric performance is limited by the strongly interrelated thermoelectric parameters. In this work, we develop an effective method to simultaneously tune the electrical and thermal transport properties of Ca₃Co₄O_9+δ. We firstly synthesize Ca₃Co₄O_9+δ powders and subsequently obtain surface modified Cu/Ca₃Co₄O_9+δ through a well-controlled electroless plating technique followed by a fast Spark Plasma Sintering process. The detailed characterizations of microstructures and chemical compositions of Cu/Ca₃Co₄O_9+δ samples suggest that the plated Cu has been doped into the Ca₃Co₄O_9+δ after sintering. The doped Cu substitutes Ca ions, creates local oxygen deficiency and increases the ratios of Co³⁺ and Co⁴⁺, which reduces the carrier concentration and induces Ca-missed regions, thus, reducing the electrical and thermal conductivity. This study indicates that introducing metallic Cu phase by chemical electroless technique is a promising method to tune the thermoelectric properties of Ca₃Co₄O_9+δ-based ceramics by altering the chemical composition and microstructures.