Fabrication of p- and n-type Thermoelectric Cobalt Oxides through the Powder-In-Tube Method

We report the synthesis and the physical characterization of p- and n-type thermoelectric oxides, i.e. respectively Na_xCoO ₂ with x nominally close to 0.75 and LaNi_1-xCo_xO₃ with x nominally equal to 0.3, materials that can be used to build unicouples and therefore thermoelectric modules. In order to take advantage of the anisotropy of the Na_xCoO₂ crystal structure and of the resistivity, the powder-in-tube method usually used to fabricate superconducting tapes has been employed for the first time to produce thermoelectric wires and tapes, also based on the similarity between the thermoelectric cobalt oxides and the superconducting cuprates. Connectivity was improved through drawing, while texturing was induced in tapes through flat-rolling. This made it possible to significantly reduce the resistivity value, thus enhancing the power factor and the figure of merit of such oxides. Various cold working procedures have been exploited: this method is a fast, reliable way to produce TE legs on a large scale and it has been applied also to the production of the n-type LaNi_1-xCo_xO₃