Thermoelectrical properties and the microstructure of [Ca2CoO3]xCoO2 thin films fabricated on the various substrates using the pulsed laser deposition

Abstract

We have fabricated c-axis oriented thin films of the oxide thermoelectric material Ca3Co4O9 (CCO) on MgO (100), SrTiO3(100) (STO), YSZ polycrystal and YSZ(100) substrates by pulsed laser deposition. The resistivity of CCO thin films on SrTiO3 and YSZ substrates hardly varied with increasing temperature from 300 to 700 K. On the other hand, the resistivity of the films on MgO was relatively high, because of an impurity phase grown on the surface and the change of lattice constant at high temperature. From the microstructure of the CCO films on STO observed by transmission electron microscopy (TEM) techniques, a main structure of cobaltite [Ca2CoO3]xCoO2 has been found with identical a, b1 and c parameters: a = 0.48 nm, b = 0.45 nm and c = 1.07 nm. The composition ratios of the main phase and secondary phase in the CCO films were Ca:Co = 0.98:1.0 and Ca:Co = 0.39:1.0, respectively.