[en] Thermoelectric performances on cobaltite ceramics can be changed by grain orientation. This can be performed by directional solidification on samples prepared from different synthetic methods. Three synthetic methods, conventional solid state reaction, sol-gel and polymer solution were used for preparation of Bi₂Ca₂Co₁.7O_x powders. The powders have been directionally grown, at a rate of 30 mm/h, by the laser floating zone method. In all the cases, the microstructure shows alternated layers, of Bi₂Ca₂Co₂O_x and a Bi-Ca-O solid solution, with small CoO inclusions. It has been found an improvement on the power factor at 50 degree centigrade for the solution methods with respect to the conventional solid sate sintered samples. (Author)

[en] Bi₁.6Pb₀.4Sr₂Co₁.8O_x thermoelectric ceramics with small Ag additions (0, 1, and 3 wt.%) have been successfully produced by a sol-gel method via nitrates. Microstructure has shown a reduction on the amount of secondary phases and an increase on the bulk density with increasing Ag contents. The microstructural evolution, as a function of Ag content, is confirmed with the electrical resistivity values which show an important decrease for the 3 wt.% Ag samples, leading to maximum power factor values of about 0.025 mW/K2.m at room temperature, which is about two times higher than the obtained for the Ag-free sintered samples. (Author) 40 refs.

[en] Thermo electrical properties of cobaltate ceramics can be tuned up by choosing the adequate synthetic method, cation substitution and subsequent grain orientation. This can be performed preparing the ceramics by a polymer solution method, using Pb partially substituting Bi, and texturing by directionally growing from the melt. In this work, Bi₂-xPb_xCa₂Co₁.7O_y (x 0.0, 0.2, 0.4 and 0.6) ceramics, prepared by a polymer solution method, have been directionally grown, with the Laser Floating Zone (LFZ) technique, at 30 mm/h. In all the cases, the microstructure shows alternated layers with small CoO inclusions. It has been found a very important decrease on the resistivity and, at the same time, on the thermopower. However, the power factor values are improved with Pb addition, reaching power factor values, for samples with 0.4 Pb substitution, as high as two times the values obtained for undoped ones. (Author) 30 refs.

[en] Bi₂Ca₂Co₁.7O_x bulk polycrystalline ceramics were prepared by the solid state method and by directional growth. Moreover, the effect of annealing on the textured materials has been studied. Microstructure has shown randomly oriented grains in the classical sintered materials while in the textured samples those were well oriented with their c-axis nearly perpendicular to the growth direction. Furthermore, the textured-annealed samples showed much lower amount of secondary phases than the as-grown ones. These microstructural changes are reflected on the thermoelectric properties which increase with the grain orientation and with the decrease on the secondary phase content mainly due to the electrical resistivity reduction. As a consequence, a raise on the power factor of about 6 and 9 times, compared with the classically sintered samples, was obtained for the as-grown and textured-annealed ones, respectively. The maximum power factor obtained at 650 degree centigrade in the textured-annealed samples (∼0.31 mW/K²m) is about 50 % higher, at the same temperature, than the obtained in one of the most used methods for texturing this kind of materials, the sinter-forging process. (Author)

[en] Bi₁.6Pb₀.4Sr₂Co₁.8O_x thermoelectric ceramics with small Ag additions (0, 1, and 3 wt.%) have been successfully produced by a sol-gel method via nitrates. Microstructure has shown a reduction on the amount of secondary phases and an increase on the bulk density with increasing Ag contents. The microstructural evolution, as a function of Ag content, is confirmed with the electrical resistivity values which show an important decrease for the 3 wt.% Ag samples, leading to maximum power factor values of about 0.025 mW/K2.m at room temperature, which is about two times higher than the obtained for the Ag-free sintered samples. (Author)

[en] Ca₃-xMg_xCo₄O₉ polycrystalline thermoelectric ceramics with small amounts of Mg have been synthesized by the classical solid state method. Microstructural characterizations have shown that all the Mg has been incorporated into the Ca₃Co₄O₉ structure and no Mg-based secondary phases have been identified. Apparent density measurements have shown that samples do not modify their density until 0.05 Mg content, decreasing for higher contents. Electrical resistivity decreases and Seebeck coefficient slightly raises when Mg content increases until 0.05 Mg 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. (Author)

[en] Several solution synthetic methods, sol-gel and a polymeric route, have been studied in order to obtain Bi₂Sr₂Co₁.8O_x ceramics with improved thermoelectric properties, compared to the classical solid state reaction. The products obtained by these different methods have been compared using DTA-TGA, powder X-ray diffraction, scanning electron microscopy, and thermoelectric characterizations. All the samples obtained by solution synthesis show higher homogeneity and lower content of secondary phases. The main differences in thermo electrical properties are due to the decrease of electrical resistivity in samples obtained by solution methods, compared with the solid state obtained samples. Between them, the decrease is especially high for those samples prepared by the polymer solution method. Therefore, the polymeric solution synthesis route is shown to yield a power factor four times higher than the obtained for the solid state and sol-gel methods at room temperature. (Author) 17 refs.

[en] The flexural strength of 1 wt.% Ag-doped Bi₂Sr₂CaCu₂O_8+δ thin rods textured by a laser heated floating zone was measured as a function of the environmental conditions (air versus water) at room temperature. Loading rates spanning three orders of magnitude (1, 10 and 100 μm/min) were used to explore their susceptibility to the environmental conditions. These mechanical tests were completed with electrical characterization (critical current at 77K and resistivity from 77 to 300 K) of samples submerged in distilled water for different time lengths (0, 12 and 120h). While Bi₂Sr₂CaCu₂O_8+δ has been shown, in previous works, to be unstable during contact with water molecules, the Ag-doped Bi-2212 textured rods tested in this work are very inert to the water environment, with respect to their mechanical and electrical properties, due to the presence of a narrow (∼150 μm) low textured outer ring formed in the growth process.

[en] Textured rods of Bi-2212 based materials with nominal compositions Bi₂Sr₂CaCu₂O_8+δ, Bi₂Sr₂CaCu₂O_8+δ+1 wt% Ag, Bi_1.6Pb_0.4Sr₂CaCu₂O_8+δ, and Bi_1.6Pb_0.4Sr₂CaCu₂O_8+δ+3 wt% Ag were fabricated using a laser floating zone (LFZ) melting method. The electrical, magnetic, and mechanical properties of the resulting rods after annealing were characterized. Pb doping results in the decrease of the transport critical current density, J_c,t (from 4.4 x 10⁷ to 6 x 10⁶ A m^-2 at 65 K and self-field) as well as in the worsening of the mechanical properties, by about 35% compared to the undoped samples. In contrast, Ag doping results in the improvement of both the critical current density and mechanical strength. In this regard we have observed an increase of J_c,t (65 K) from 4.4 x 10⁷ for Bi-2212 to 7.2 x 10⁷ A m^-2 for Bi-2212/Ag and from 6 x 10⁶ for Bi(Pb)-2212 to 8 x 10⁶ A m^-2 for Bi(Pb)-2212/Ag. These described effects are related to the microstructural observations, since Pb doping dramatically reduces the texture while Ag doping improves it. Moreover, for samples with Ag addition, an intergrowth of Bi-2223 inside the Bi-2212 grains is observed, which would explain the improved superconducting properties of these samples.