Nanostructured complex cobalt oxides as potential materials for solar thermoelectric power generators

Thermoelectrically active and stable perovskite-type materials e.g. La1-xCaxCoO3 (0 < x < 0.4), La1-xCaxCo0.99Ti0.01O3 (0 < x < 0.2), and Ca3Co4O9 have been successfully synthesized using selected precursor compounds. This soft chemistry route allows the control of the elemental composition from 1% level to 50% substitution. Accordingly the Co valency and therefore the transport properties of the product phases can be controlled. It has been found that the pH of the precursor solution results in a pronounced influence on the morphology of the products. The thermoelectric values measured of the nanostructured "misfit cobaltite Ca3Co4O9" shows a Seebeck coefficient of S-300K similar to + 123 mu VK-1, and a resistivity of p similar to 1.9 m Omega cm at room temperature, which is comparable to the reported value for single crystals. Ca- and Ti-substituted LaCoO3 reveal thermopower values in the range from S-300K similar to + 70 to + 180 mu VK-1. The electrical conductivity of the nanostructured compounds is high in spite of the fact that the grain boundary influence is increasing. The Seebeck coefficient values of the products are positive in the whole temperature range indicating p-type conduction.