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Bi2Ca2Co2 − xCuxOy + 4 wt%Ag (with x between 0 and 0.20) thermoelectric materials were prepared through a sol-gel method via nitrates. Powder XRD patterns showed the presence of thermoelectric phase as the major one, accompanied by metallic silver. SEM observations identified three different secondary phases in the samples, together with an exaggerated grain growth with Cu doping. The microstructural modifications are reflected in an important decrease of electrical resistivity, ∼ 3% when compared to undoped samples, due to the reduction on the number of grain boundaries, and the increase of Co4+ in the conducting layer. The minimum values at 650 ºC were obtained in 0.10 Cu-doped samples, 69 mΩ cm, which are lower than the reported in sintered samples. On the other hand, Seebeck coefficient was only slightly affected and reached the highest values at 650 ºC in 0.15 Cu-doped samples, 215 µV/K, which is practically the same measured in undoped ones, 211 µV/K, and lower than those reported for sintered samples. Consequently, the highest PF at 650 ºC has been determined in 0.10 Cu-doped samples, which is around 50% higher than the obtained in samples prepared through the classical solid-state method, or the undoped ones prepared in this work. [ABSTRACT FROM AUTHOR]

This work explores the effect of exposing the Bi2Ca2Co1.7Ox-textured ceramics at temperatures above working conditions, on mechanical and electrical properties. Microstructural studies have shown a first improvement of microstructure with an important grain growth, followed by the formation of porosity and the appearance of cracks for larger times. These features were reflected on their mechanical and electrical properties. Three point bending tests have revealed an increase of bending strength with the thermal treatment, reaching the maximum at 24 h and decreasing for further treatment time, which is very slight for times larger than 196 h. On the other hand, electrical resistivity is drastically reduced with the thermal treatment when compared to the as-grown samples. Moreover, the samples behavior is modified from semiconducting-like for as-grown samples to metallic-like for the thermally treated ones. Accordingly, Seebeck coefficient is decreased with the thermal treatment, being very similar for all thermally treated samples. As a consequence of the drastic decrease of electrical resistivity, although with lower reduction on the Seebeck coefficient, all thermally treated samples display higher power factor values than the as-grown ones. The highest values at 650 °C (0.29 mW/K2 m) have been obtained in textured samples thermally treated for 48 h, which are comparable to the best values reported in the literature without the use of expensive materials. [ABSTRACT FROM AUTHOR]

Bi2Sr2Ca1Cu1.75Na0.25Oy ceramics have been prepared by the classical solid state reaction method followed by hot-pressing process under several applied pressures and temperatures: 22 and 33 MPa at 750 °C, and 22 MPa at 800 °C. X-ray diffraction data indicate that the major phase in all samples is the Bi2Sr2Ca1Cu1.75Na0.25Oy phase, accompanied by small amounts of Bi2CaO4 and Bi3Sr4CaCu3O4 secondary phases. Microstructural results clearly showed that the best grain orientation has been obtained under 33 MPa at 750 °C. Critical temperatures, TC (onset) and TC (offset), of these samples were significantly increased to 96.6 and 85.6 K, respectively, pointing out to a good intergranular coupling. M-H hysteresis curves for all samples display a very large area due to the microstructural characteristics obtained in these samples. The highest critical current density, calculated from M-H measurements according to Bean's model, has been obtained in samples prepared under 33 MPa at 750 °C (201.5 × 104 A/cm2 at 1.9 T and 15 K), which are much larger than those reported for samples with the same composition prepared through the classical solid state method. [ABSTRACT FROM AUTHOR]

In this study, Bi2Sr2−xNaxCo2Oy (x = 0.0, 0.05, 0.075, 0.10, and 0.15) ceramic powders have been fabricated via the classical ceramic route, followed by a texturing process through the laser floating zone technique. XRD patterns show the thermoelectric phase as the major one. In addition, Na-substitution reduces the amount of secondary phases, when compared to the pure sample. SEM observations point out that grain orientation is significantly improved when Na-content is increased. Na-substitution reduces electrical resistivity from 35 (in pure samples) to 19.6 mΩ cm (in Na = 0.05 ones) at around room temperature, while Seebeck coefficient is, approximately, twice measured in Na-free. On the other hand, thermal conductivity is slightly lower in undoped samples (0.83 W/K m), when compared to the Na-substituted ones (1.10–1.40 W/K m) at room temperature, due to their lower electrical conductivity. Finally, ZT values are higher when the Na-content is increased, reaching 0.022 at around 400 K. [ABSTRACT FROM AUTHOR]

CuCrO2 and Cu0.97AE0.03CrO2 (AE = Mg, Ca, Sr, and Ba) polycrystalline materials have been prepared through the classical solid state route. CuCrO2 is the major phase in all cases, with larger grain sizes when the alkaline earth dopant is larger. Doping also promotes the formation of secondary phases, which appear in the grain boundaries in the Ca, Sr, and Ba doped samples. Density has been drastically improved through doping, reflected in lower electrical resistivities and higher mechanical properties than the measured in undoped samples. In spite of a relatively high decrease of Seebeck coefficient, all doped samples reached higher power factor values than the undoped ones in the whole measured temperature range. The highest power factor has been reached in Mg doped samples, 0.32 mW/K2 m at 800 °C, which is among the highest obtained so far in randomly oriented polycrystalline CuCrO2 materials. [ABSTRACT FROM AUTHOR]

The structural and magnetic properties evolution of Cr-doped BiSrCaCuCrO ceramics (x = 0.0, 0.05, 0.1, and 0.25) have been investigated using XRD, SEM, DC resistivity and M-H measurements. X-ray diffraction studies have shown that all samples are mainly composed of Bi-2212 phase, indicating that Cr can be incorporated in its crystal structure. The microstructure of the highly Cr-doped samples showed that some impurity phases appear, decreasing the conductivity between the grains. T (onset) and T (offset) have been improved for x = 0.05 Cr doping, while 0.25 Cr doped samples are not superconducting. This behavior can be associated to changes in the hole concentration and/or the magnetic nature of Cr. The magnetic properties indicate that low Cr-doping enhances the loop widths. Additionally, the critical current densities, J , of samples have been calculated by using the Bean's critical state model. It has been found that there are important improvements in J values between 0.2 and 0.9 T applied magnetic fields for 0.05 Cr doped samples. [ABSTRACT FROM AUTHOR]

Bi2Ba2Co2Ox thermoelectric materials have been prepared through three different synthesis methods. The corresponding precursors were obtained from a coprecipitation method using oxalic acid, attrition milling, and classical solid state method (used as reference). Microstructural studies of precursors have shown that the ones produced by coprecipitation and attrition milling led to smaller grain sizes than the ones obtained through ball milling. Infrared spectroscopy has demonstrated that a thermal treatment at 450 °C totally decomposes the metallic oxalates, producing a mixture of Bi and Co oxides, and Ba carbonate, the same average composition of the other precursors. After sintering procedure, it has been determined that the small grain size precursors produce high-density samples, reflected in a decrease of electrical resistivity without significant modification of the Seebeck coefficient. As a consequence, power factor values are more than two times higher than the measured in samples prepared by the classical solid state method. Moreover, the highest power factor values are in the order of the best reported in textured materials, demonstrating the critical role of precursors in the final properties of these materials. [ABSTRACT FROM AUTHOR]

BiBaCoO thermoelectric ceramics with Ag additions (0, 1, 3, and 5 wt%) have been successfully textured using the laser floating zone method. Microstructure has shown that samples are composed by thermoelectric grains, together with a high amount of secondary phases, and that 3 wt% Ag addition leads to the best grain orientation. These microstructural benefits have been reflected in the highest thermoelectric performances determined in these samples. It has also been found that annealing procedure leads to a drastic decrease of secondary phases amount and raises the oxygen content in the thermoelectric phase. These modifications are reflected in an important decrease of electrical resistivity, ρ (compared to the as-grown samples) without drastic modification of Seebeck coefficient, S. As a consequence, the highest thermoelectric performances, determined through the power factor, have been reached in the 3 wt% Ag textured and annealed samples in the whole measured temperature range. The maximum PF (=S/ρ) values at 650 °C determined in these samples are very close to the highest reported in single crystals or very low rate LFZ grown BiBaCoO compounds, making them very attractive for practical applications. [ABSTRACT FROM AUTHOR]

The effect of MoO addition on the properties of Bi-2212 superconducting ceramic samples prepared by solid state reaction method has been investigated. Mo content was varied from 0 to 0.25 on a general stoichiometric formula BiSrMoCaCuO. Electrical resistivity showed that transition temperature width increased directly with the Mo amount. XRD data have shown that MoO addition in the BiSrMoCaCuO precursor reduces the amount of Bi-2212 phase. In addition, J values of the samples, calculated from the hysteresis loops using the Bean's model, decreased with increasing Mo substitution. Vickers microhardness measurements show that samples are very sensitive to Mo content and applied load. In addition, various models like Meyer's Law and Young's Modulus equations have been used to better explain the mechanical properties of samples. [ABSTRACT FROM AUTHOR]

Bi2Ba2Co2Ox thermoelectric materials have been prepared through three different synthesis methods. The corresponding precursors were obtained from a coprecipitation method using oxalic acid, attrition milling, and classical solid state method (used as reference). Microstructural studies of precursors have shown that the ones produced by coprecipitation and attrition milling led to smaller grain sizes than the ones obtained through ball milling. Infrared spectroscopy has demonstrated that a thermal treatment at 450 °C totally decomposes the metallic oxalates, producing a mixture of Bi and Co oxides, and Ba carbonate, the same average composition of the other precursors. After sintering procedure, it has been determined that the small grain size precursors produce high-density samples, reflected in a decrease of electrical resistivity without significant modification of the Seebeck coefficient. As a consequence, power factor values are more than two times higher than the measured in samples prepared by the classical solid state method. Moreover, the highest power factor values are in the order of the best reported in textured materials, demonstrating the critical role of precursors in the final properties of these materials. [ABSTRACT FROM AUTHOR]