The effects of sintering temperature on preparation, resistivity, and thermopower of c-axis oriented Ca 3Co 3.95Fe 0.05O 9 + δ films fabricated using sol–gel spin coating method

Abstract

We report fabrication of the c-axis oriented Ca 3Co 3.95Fe 0.05O 9 + δ films by a simple sol–gel spin coating method. The films prepared in the temperature range of 650–700 °C show nonmetallic temperature dependence of resistivity in the whole investigated temperature range, whereas the films prepared in the temperature range of 750–775 °C show metallic temperature dependence in the high temperature regime. Sintering the films at higher temperature leads to larger grains, lower resistivity and smaller thermoelectric power. This can be explained in the framework of the barrier theory and confirmed by the higher hole carrier concentration from the Hall measurements according to Seto's derivation between the carrier concentration and the barrier height. The temperature dependence of resistivity resembles that of the in-plane single crystal of Ca 3Co 4O 9 + δ in terms of the Fermi-liquid behavior. The effects of lower sintering temperature on the transport coefficient A and Fermi-liquid scale T⁎ of Fe-substituted cobaltite films seems to be similar to applying hydrostatic pressure on a single crystal of Ca 3Co 4O 9 + δ . The temperature dependence of resistivity in the nonmetallic region follows the variable-range hopping conduction in the form of T − 1/3 , due to the 2-dimensional character of the films.