Two solution synthetic methods, sol-gel and a polymeric route, have been studied in order to obtain Ca 3 Co 4 O 9 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.
Bi 2 Ba 2 Co 2 O x thermoelectric ceramics were textured from the melt using the laser floating zone method, at 5mm/h growth rate. Microstructure has shown a good grain alignment with the growth axis. These microstructural features have been reflected on the thermoelectric performances, with a very important increase on the power factor values, reaching ~0.4mW/K 2 .m at 650ºC, much higher than the typical values obtained in this materials so far.
Ca 3-x Sr x Co 4 O 9 polycrystalline thermoelectric ceramics with small amounts of Sr have been synthesized by the classical solid state method. Microstructural characterizations have shown that all the Sr has been incorporated into the Ca 3 Co 4 O 9 and Ca 3 Co 2 O 6 structures and no Sr-based secondary phases have been produced. Apparent density measurements have shown that samples slightly increase their density until 0.05 Sr content, decreasing for higher contents. Electrical resistivity decreases and Seebeck coefficient slightly raises when Sr content increases until 0.07 Sr addition. The improvement in both parameters leads to higher power factor values than the usually obtained in samples prepared by the conventional solid state routes and close to those obtained in textured materials.
This paper reports a novel composite-based processing route for improving the electrical performance of Ca3Co4O9 thermoelectric (TE) ceramics. The approach involves the addition of metallic Co, acting as a pore filler on oxidation, and considers two simple sintering schemes. The (1-x)Ca3Co4O9/xCo composites (x = 0%, 3%, 6% and 9% vol.) have been prepared through a modified Pechini method, followed by one- and two-stage sintering, to produce low-density (one-stage, 1ST) and high-density (two-stage, 2ST) ceramic samples. Their high-temperature TE properties, namely the electrical conductivity (σ), Seebeck coefficient (α) and power factor (PF), were investigated between 475 and 975 K, in air flow, and related to their respective phase composition, morphology and microstructure. For the 1ST case, the porous samples (56%–61% of ρth) reached maximum PF values of around 210 and 140 μWm−1·K−2 for the 3% and 6% vol. Co-added samples, respectively, being around two and 1.3 times higher than those of the pure Ca3Co4O9 matrix. Although 2ST sintering resulted in rather dense samples (80% of ρth), the efficiency of the proposed approach, in this case, was limited by the complex phase composition of the corresponding ceramics, impeding the electronic transport and resulting in an electrical performance below that measured for the Ca3Co4O9 matrix (224 μWm−1·K−2 at 975K).
Bi 2 Sr 2 Co 1.8 O 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 Bi 2 Sr 2 Co 1.8 O 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.
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