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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 significative 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. [Copyright &y& Elsevier]

Abstract: Bi2Sr2Co1.8O x /Ag composites with small amounts of Ag have been synthesized by a sol–gel via nitrates reaction and directionally grown from the melt. Some of the obtained samples were annealed in order to obtain the thermoelectric phase as the major one. As-grown and annealed samples were microstructurally characterized to determine the phases distribution and alignment. Moreover, thermoelectric and mechanical characteristics of annealed samples were determined by the four-probe technique and by three point flexural strength tests, respectively. Scanning electron microscopy revealed that Ag particles appear dispersed among well oriented ceramic grains with large size, providing a plastic flow region which increases the flexural strength for the optimally Ag added samples (1wt.%). The composites electrical resistivity is lower than that of pure Bi2Sr2Co1.8O x while Ag addition does not significantly affect thermopower values. The resistivity reduction leads to power factor improvements of ∼50%, compared with pure samples, for Ag additions of 1wt.% Ag. [Copyright &y& Elsevier]

Abstract: High-performance Ca3Co4O9 thermoelectric ceramic has been prepared from a Ca1−x Co x O/Ca y Co1−y O divorced eutectic structure produced by a directional melt-grown using the laser floating zone technique. This material has been grown at very high solidification rate in order to produce a very fine microstructure to reduce the necessary annealing time to recover the Ca3Co4O9 thermoelectric phase as the major one. As-grown and annealed samples were microstructurally characterized to determine the phases and estimate the extent of Ca3Co4O9 formation with time and related with their thermoelectric performances. The optimum annealing time, 72h, has been determined by the maximum power factor value (about 0.42mWK−2m−1), which is around the best values reported in textured materials (∼0.40mWK−2m−1). This high power factor outcome from the high Ca3Co4O9 phase content, apparent density and Co3+/Co4+ relationship determinations performed in the present work. [Copyright &y& Elsevier]