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Journal of the European Ceramic Society, Volume 32, Issue 10, August 2012, Pages 2415-2422
A. Sotelo, G. Constantinescu, Sh. Rasekh, M.A. Torres, J.C. Diez, M.A. Madre
Instituto de Ciencia de Materiales de Aragon (CSIC-Universidad de Zaragoza), Mª de Luna 3, 50018 Zaragoza, Spain
Departamento de Ingenieria de Diseno y Fabricacion, Universidad de Zaragoza, Mª de Luna 3, 50018 Zaragoza, Spain
Abstract
Two solution synthetic methods, sol–gel and a polymeric route, have been studied in order to obtain Ca3Co4O9 misfit compounds with improved thermoelectric properties, compared to the classical solid state reaction. A comparison among the final products obtained by these different methods has been performed using DTA, TGA, FTIR, X-ray diffraction, scanning electron microscopy, and thermoelectric characterizations. All the samples obtained by solution synthesis show a very significant reduction on the secondary phases content. As a consequence, an important decrease on the electrical resistivity values is produced, compared to the solid state prepared samples, leading to a relatively important power factor raise.
Additional Information:
Nowadays, thermoelectric oxide ceramic materials are usually prepared via the classical solid state synthesis methods. As a consequence of the typical drawbacks of this kind of processes, these materials are far away from their top performances. In order to raise their properties different approaches have been done, some of them are dealing with the grain orientation, as hot forging, templated grain growth, spark plasma sintering or directional solidification from the melt, where Constantinescu et al. have reached very important improvements in the Bi2Ba2Co2Ox system [Scripta Mater. 68, 75 (2013)]. On the other hand, directional growth performed on the Ca3Co4O9 thermoelectric ceramic has produced a divorced eutectic microstructure, in agreement with the binary CaO-CoO phase diagram. These materials, when properly annealed lead to very dense bulk ceramics with increased thermoelectric performances, as reported by Madre et al. [J. Eur. Ceram. Soc. (2013) DOI: (2013), 10.1016/j.jeurceramsoc.2013.01.029].
The figure shows the improvements obtained on the thermoelectric performances of Ca3Co4O9 thermoelectric ceramics when they are processed by solution methods, sol-gel via nitrates (=) and polymer matrix method (¨) described by Sotelo et al. [J. Eur. Ceram. Soc. 32, 2415 (2012)]. In the figure the data obtained for the directionally grown samples are also presented (p). As it can be clearly seen, in all cases a very important improvement is achieved, when compared with the samples prepared by the classical solid state route (<). All these data show that the development of new synthesis methods and adequate processing techniques can lead to attractive thermoelectric performances for practical green energy generation applications.
