S. Hébert, D. Flahaut, D. Pelloquin, C. Martin, A. Maignan

The discovery of a large thermopower coexisting with good metallicity in NaCo₂O₄ has shown the potential applications of oxides as thermoelectric elements. The layered ‘misfit’ cobaltites possess similar CoO₂ layers and were found to be promising thermoelectric materials. Their structure can be described as the stacking of CoO₂ layers with ‘NaCl’-like layers, with the same a and c parameters but two different b parameters. Because of the trigonal symmetry in the CoO₂ planes, the e_g band is splitted into two subbands with light and heavy carriers and the transport and magnetic properties of these oxides can be described using a two-band model. Recent results obtained with new misfit phases will be discussed in this framework.

Another family of cobalt oxides has been investigated. In Ca₃Co₂O₆, ferromagnetic chains distributed on a triangular lattice are antiferromagnetically coupled. The thermopower is very large, close to + 700mV/K at 300K but the resistivity is too large, close to 10 W.cm. Attempts to reduce the resistivity while keeping a large S will be presented.

Finally results showing that in 3D cobaltites derived from LaCoO₃, n type thermoelectric materials can be obtained will be presented. Their high temperature performance will be compared to the ones of n type manganites.