Thermoelectric performance of layered cobaltate epitaxial films deposited by pulsed laser evaporation

Resumo

Thermoelectric (TE) effect can produce a voltage difference when a charge conductor is subjected to a temperature gradient. From a technological point of view, the use of efficient TE materials has become very promising to generate electricity from thermal solar, geothermal and recovering waste heat from combustion engine. On the other hand, it is fundamentally important because of its direct link with the electronic band structure near Fermi energy. Among different TE materials, some layered cobalt oxides have emerged as very promising because of their large thermopower, high temperature stability. However, some of the potential layered cobaltates show incoherent motion of charge carriers, which makes difficult to understand their TE properties and are reflected in non-conventional sign, magnitude and temperature and doping dependence of thermopower. In this thesis, TE properties of some of the corner shared and edge shared CoO2 layered compounds, like GdBaCo2O5.5 double perovskite and misfit layer BiSrCoO, respectively, were investigated in the form of thin films, which allowed exploring the anisotropic properties in a particular crystallographic direction. This work explores the conditions for the thin film deposition by pulsed laser evaporation, as well as their orientation and microstructure, in order to attain optimum thin film quality and control over the oxygen stoichiometry and epitaxial strain. Thermopower analysis of the thin films indicates that polaron size distribution is an important factor which needs to be taken into account in the classical Heike’s formula to optimize thermopower. This is a different approach than conventional band-structure engineering and therefore, modification of Heike’s formula is proposed.