The effects of composition deviations on the microstructure and thermoelectric performance of Ca3Co4O9 thin film

Abstract

This work investigated composition deviations in Ca3Co4O9 (CCO) films during rf-sputtering process and their effects on the properties of post-annealed CCO films. The negative oxygen ions produced during sputtering resulted in a great reduction in deposition rate and preferential re-sputtering of calcium atoms from the substrate due to their lighter weight compared to cobalt. Experiment results showed that the single phase CCO film exhibited the lowest electrical resistivity of ~ 4 mΩ cm at room temperature and a high power factor of ~ 1.1 mW/mK2 at 973 K. Compared with the single phase CCO sample, the resistivity of the Co-rich sample increased slightly and the Seebeck coefficient increased by ~ 20% at high temperature, thereby leading to a high power factor. The upturn of the Seebeck coefficient behaviours in Co-rich samples from 700 K is most likely associated with the strain induced by the mismatch of thermal expansion coefficients (CTE) between CCO and Co3O4 grains.