Effect of a powder compaction process on the thermoelectric properties of Bi2Sr2Co1.8O ceramics (original) (raw)
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Journal of Solid State Chemistry, 2010
Several solution synthetic methods, sol-gel and a polymeric route, have been studied in order to obtain Bi 2 Sr 2 Co 1.8 O 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 thermoelectrical 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.
Processing effects on the thermoelectric properties of Bi2Ca2Co1.7Ox ceramics
2014
Bi 2 Ca 2 Co 1.7 O 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 they were well oriented with their c-axis nearly perpendicular to the growth direction. Furthermore, the 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 phases 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 annealed samples, respectively. The maximum power factor obtained at 650 ºC in the annealed samples (~ 0.31 mW/K 2 m) is about 50 % higher than the obtained in sinter-forged textured materials at the same temperature.
Effect of annealing on the thermoelectric properties of directionally grown Bi2Sr2Co1.8Ox ceramics
Ceramics International, 2012
The effect of annealing on directionally solidified Bi 2 Sr 2 Co 1.8 O x ceramic rods has been studied for different times up to 1008 h. Microstructure has shown five different phases in the as-grown materials which have been reduced to two major ones after 1008 h thermal treatment, accompanied by an important grain growth. These microstructural changes are reflected on the mechanical properties which are higher than for the as-grown materials in all cases. Moreover, they also produce an important decrease on the resistivity and increase of thermopower, leading to a raise on the power factor on thermally treated samples, about two times, compared to the as-grown samples.
Materials Research Bulletin, 2011
Thermoelectric performances of misfit cobaltites can be controlled by grain orientation and/or by cation substitution. Both processes have been simultaneously performed by directional solidification at 0.03 m/h, using the laser floating zone technique, of Bi 2-x Pb x Sr 2 Co 1.8 O y (with x = 0.0, 0.2, 0.4 and 0.6). The microstructure has shown two different main phases as a function of Pb content, a Co-poor phase for low Pb content (0.0 and 0.2) and a Co-rich one for higher Pb substitution. These microstructural changes are reflected on the thermoelectric properties leading to an important decrease on the resistivity and increase of thermopower for samples with 0.4Pb substitution. Both improvements lead to power factor values higher than usual in textured misfit cobaltites.
Thermoelectric Properties of Dual Doped Bi2Sr2Co2Oy-Based Ceramics
Journal of Electronic Materials, 2019
We have investigated the crystal structure and high temperature thermoelectric properties of polycrystalline Bi 2-2x Na 2x Sr 2 Co 2Àx W x O y (0 £ x £ 0.075) samples. Powder x-ray diffraction data show that all samples are phase pure consisting of misfit-layered structure of alternately stacked hexagonal CoO 2 and double rock-salt BiSrO 2 layers. It is found that dual doping of Na and W in Bi 2 Sr 2 Co 2 O y system is fairly effective in improving the thermoelectric properties owing to simultaneous decrease of electrical resistivity (q) and thermal conductivity (j) of samples. All samples exhibit a large Seebeck coefficient (S), which seems not to be affected by the level of doping. As a result, a very high power factor (PF) of 2.82 9 10 À4 W/m K 2 has been obtained for x = 0.025 sample at 1000 K. The corresponding dimensionless figure of merit (zT) for this sample has been determined to be 0.35 at 1000 K, which is $ 2.2 times higher than zT value of the pristine sample providing a promising class of material for high-temperature thermoelectric applications.
Bi-Sr-Co-O thermoelectrics prepared by sol-gel methods with modified gel decomposition
IOP Conference Series: Materials Science and Engineering, 2012
We prepared misfit Bi 2 Sr 2 Co 1.8 O x (Bi-222) phase as a member of a Bi-Sr-Co-O family. Two water based sol-gel methods were chosen with regard to the presence of a strongly hydrolysing Bi 3+ ion -chelating route combining EDTA (ethylenediaminetetraacetic acid) and TEA (triethanolamine) and, secondly, water soluble polymer method using PEI (polyethylenimine). We focused on the influence of gel decomposition process on the grain size of precursor and, consequently, on the bulk density of the final samples. We tested decomposition in N 2 atmosphere followed by a treatment in pure oxygen. The precursors decomposed in "N 2 /O 2 " regime were mainly composed of Bi 2 O 3 , SrCO 3 and cobalt oxides with the grain size of approximately 50-100 nm. The powders arising from gel decomposition in air contained the desired Bi-Sr-Co-O oxide as the major phase independently of the chosen sol-gel method. The final sintered samples were almost single-phase with traces of the other pseudoternary phase Bi 2 Sr 2 CoO x independently of the decomposition atmosphere. For comparison, samples were also prepared by solid state reaction. The sol-gel prepared samples were always of higher bulk density with larger grains, moreover partly microstructurally ordered. These facts were also reflected in transport thermoelectric measurements.
SN Applied Sciences, 2021
Bi2Sr2−xCsxCo2Oy materials with 0 ≤ x ≤ 0.15, have been fabricated via the classical ceramic technique. XRD results have indicated that undoped and Cs-substituted samples are composed of Bi2Sr2Co2Oy phase as the major one. Microstructural studies have demonstrated the formation of a liquid phase, which allows a drastic grain growth. This factor is responsible for a drastic improvement of relative density, reaching about 95% of the theoretical one for 0.125 Cs content. On the other hand, electrical resistivity has been reduced up to 14 mΩ cm at 650 °C for 0.125 Cs content, around 40% lower than the obtained in undoped samples. As a consequence, Seebeck coefficient has been decreased due to the raise in charge carrier concentration. The highest power factor at 650 °C (0.21 mW/K2 m) has been found for 0.125 Cs substituted sample, about 40% larger than the obtained in undoped samples, and very similar to the notified in single crystals (0.26 mW/K2 m). Magnetisation with respect to tempe...
Thermoelectric sintered glass-ceramics with a Bi2Sr2Co2O x phase
Applied Physics A, 2015
Glass-ceramic materials containing Bi 2 Sr 2 Co 2 O x crystals with plate-like structures are prepared by a melting process and a subsequent sintering step after manual quenching. The chemical starting compositions of the samples are Bi 2 Sr 2 Co 2 O x (BSC222), Bi 1.8 Sr 2 Co 2 O x (Bi1.8) and Bi 2 Sr 2 Co 1.7 O x (Co1.7). All three samples are p-type conductors. The electric properties of Seebeck coefficient S and electrical resistivity q show only a slight dependence on chemical composition. The Seebeck values increase with increasing temperature, and at T = 873 K, they reach S = 180, 176 and 167 lV/K, respectively. The electrical resistivity slightly decreases with temperature for two samples and increases for the Co1.7 sample. The thermal conductivity for all measured samples at this temperature is around j = 0.8 W/(m K). The figure of merit ZT increases with temperature for all samples. The materials reach a ZT value of 0.03 at T = 873 K.
Boletín de la Sociedad Española de Cerámica y Vidrio, 2019
Bi 2 Sr 2−x K x Co 2 O y (x = 0.0, 0.025, 0.050, 0.075, 0.100, 0.125, and 0.15) ceramic materials have been fabricated using a classical ceramic process, followed by texturing through directional solidification. SEM analysis of samples, before and after annealing has shown that grains were aligned along the growth direction. While as-grown samples have several phases, besides the thermoelectric one, the annealed ones show much higher thermoelectric phase content. In addition, K-substitution increases the thermoelectric phase content and enhances grain orientation. These improvements decrease the electrical resistivity without significant changes in Seebeck coefficient. The maximum power factor value for 0.10K-substituted annealed samples has been found as 0.20 mW/K 2 m. This is superior to the highest reported for hot-pressed materials. However, all samples have similar magnetic properties.
Structure, non-stoichiometry and thermodynamic properties of Bi1.85Sr2Co1.85O7.7−δ ceramics
Thermochimica Acta, 2014
The structure and oxygen non-stoichiometry of misfit layered cobaltite Bi1.85Sr2Co1.85O 7.7−δ was determined by Rietveld analysis and by thermogravimetric measurements. The heat capacity and enthalpy increments of pseudoternary oxide Bi1.85Sr2Co1.85O 7.7−δ was measured by the relaxation time method (PPMS) from 2 K to 256 K, by differential scanning calorimetry (DSC) from 258 K to 355 K and by the drop calorimetry from 573 K to 1153 K. Above room temperature the temperature dependence of the molar heat capacity in the form Cpm = (305.8 + 0.07325·T -4702536·T −2 ) J K −1 mol −1 was derived by the least-squares method from the experimental data. The heat capacity was analyzed in terms of a combined Debye -Einstein model. The molar entropy S • m (298.15) = 317.7 J mol −1 K −1 was evaluated from the low temperature heat capacity measurements.