Research progress on doping modification of Ca₃Co₄O₉ thermoelectric materials: a review

Calcium cobaltate (Ca₃Co₄O₉) is an excellent thermoelectric (TE) material that can realize the mutual conversion between thermal and electrical energy based on the Seebeck effect. Compared with other ceramic TE materials, Ca₃Co₄O₉ has the advantages of good oxidation resistance and high-temperature stability, but its electrical conductivity is low, making its TE properties insufficient. Thus, the figure of merit (ZT) of Ca₃Co₄O₉ materials cannot meet the actual production needs, so it is necessary to modify the Ca₃Co₄O₉ to improve its TE properties. By doping with other elements, the carrier concentration in the Ca₃Co₄O₉ system will increase, and the phonon scattering by impurity atoms will occur, whereas the scattering of electron is rather weak. Thus, the electrical conductivity of the material increases and the thermal conductivity decreases. Therefore, the TE properties of the Ca₃Co₄O₉ material will be improved. This paper summarizes the methods of doping with other elements to improve the TE properties of Ca₃Co₄O₉ materials in recent years, mainly including Ca site doping, Co site doping, and Ca and Co site double doping with various elements, including alkali (earth) metals, transition metals, rare earth metals, and even main group metals and non-metals. Furthermore, the current problems and development trends of the doping modification of Ca₃Co₄O₉ materials are discussed to provide a theoretical basis for the further development and utilization of Ca₃Co₄O₉ TE materials.