RECENT INVESTIGATIONS OF Sr-Ca-Co-O THERMOELECTRIC MATERIALS

摘要

Three low-dimension cobaltites in the Sr-Ca-Co-O system have been studied for their structure and thermoelectric properties. Using x-ray pole figure construction technique, a Ca_3Co_4O_9 thin film showed excellent fiber texture but no ab in-plane texture. The fiber texture is mainly responsible for the excellent thermoelectric properties. Our study of a high-throughput combinatorial thin film (synthesized using Pulsed Laser Deposition) with a custom-built power factor screening tool demonstrated that the La substitution in the Ca-site of Ca_3Co_4O_9 increases Seebeck coefficient and resistivity due to decrease of hole concentration. The Sr-substitution on the Ca-site increases the carrier mobility, therefore lower resistivity of the film. The homologous compounds Ca_(n+2)Co_nCo'O_(3n+3) and Sr_(n+2)Co_nCo'O_(3n+3) consist of parallel 1-dimensional cobalt oxide chains that are built by successive alternating face-sharing CoO_6 trigonal prism and 'n' units of CoO_6 octahedra along the c-axis. However, due to the high resistivity of these compounds, they will unlikely yield ZT values comparable to that of Ca_3Co_4O_9.