The effect of annealing oxygen concentration in the transformation of CaxCoO2 to thermoelectric Ca3Co4O9

Abstract

Calcium cobalt oxide thin films were deposited using radio-frequency (RF) sputtering on single crystal c-axis sapphire substrates. The as-deposited calcium cobaltite crystallized in the form of Ca_xCoO₂, which has a slight calcium deficiency compared to the annealed Ca₃Co₄O₉ phase. When the films have a slight calcium deficiency, transformations to the Ca₃Co₄O₉ phase proceed with Co₃O₄ as a by-product. It was discovered that this transformation can take place with a relatively low oxygen concentration in the annealing gas, as low as 5% in this study. However, optimum thermoelectric and electrical properties were achieved in the films annealed with an oxygen concentration of about 20%. There was no significant change in such properties when the films were annealed at higher oxygen concentrations. This suggests that the optimum defect concentration induced via the transformation occurs with an oxygen concentration of about 20% in the annealing atmosphere.