A. Sotelo

Growth rate effect on microstructure and thermoelectric properties of melt grown Bi2Ba2Co2Ox textured ceramics

Abstract Abstract Bi2Ba2Co2Ox thermoelectric (TE) ceramics have been grown from the melt using the laser floating zone method at different growth rates (15, 30 and 90 mm h−1). Microstructural analysis has shown an improvement of the grain alignment and a reduction of secondary phases when the growth speed is decreased. These microstructural features have been reflected in the TE performances, with a reduction of the electrical resistivity and, as a consequence, at significant increase in the power factor values, reaching ∼0·15 mW K−2 m−1 at 650°C for the samples grown at 15 mm h−1, which is much higher than the typical values obtained in these materials.

Polymer solution processing of (Bi, Pb)SrCaCuO

Abstract A simple polymer processing route to the 110 K phase (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O 10+δ superconductor has been developed. The influence of the polymer to metal starting ratio, as well as the sintering temperature and time on the quality of the resulting superconducting powders has been studied by XRD, a.c. susceptibility and resistivity measurements. Microstructure of ceramic compacts has been studied by SEM, and qualitatively analysed by EDS for compositional homogeneity. The results indicate that the polymer synthesis route described here may offer a good alternative to the conventional solid state preparation methods towards attaining homogeneous 110 K superconductor powder within a reasonable processing time.

Microstructure of laser floating zone (LFZ) textured (Bi, Pb)SrCaCuO superconductor composites

Abstract Directionally solidified high temperature superconducting (Bi, Pb)SrCaCuO pure ceramics and composites were obtained using a laser floating zone (LFZ) apparatus. The presence of secondary non-superconducting and metallic phases as well as their solidification habit have been analysed. The influence of the LFZ growth conditions and the precursor composition on the microstructure of the final products was studied using optical and electron microscopies.

(Bi,Pb)2Sr2Ca2Cu3O10+δ superconductor composites: Ceramics vs. fibers

Abstract Well characterized (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O 10+δ superconductor powder has been used to prepare superconductor-glass, -metal and -alloy composites through solid state reaction method. A recently developed Laser Floating Zone (LFZ) apparatus has been used to transform the ceramic precursors into oriented fibers. The diamagnetic properties have been studied by a.c. susceptibility. The microstructure of fibers has been studied by SEM and compared with that of the original ceramic precursors. XRD has been used to study phase composition on representative composite samples and fibers. The results indicate some potential for the 2223-Ag composite, which displays improved diamagnetic properties and grain orientation (fiber).

Solution-based synthesis routes to (Bi 1− x Pb x ) 2 Sr 2 Ca 2 Cu 3 O 10+δ

A comparison of several solution syntheses with a solid-state route to the leadcontaining high Tc phase of the Bi-Sr-Ca-Cu-O system has been performed using DSC, TGA, X-ray diffraction, electron microscopy, magnetic ac susceptibility, and critical-current measurements. A novel polymer solution synthesis route is shown to yield an increased percentage of the 2223 high Tc phase when sintering is performed at the low temperature end of this phases stability range. Under the best preparative conditions given in the literature, however, the properties of samples obtained with the different synthetic methods described in the present study are shown to be similar. Samples derived from solution syntheses have been found to contain higher amounts of carbon precipitates, while samples derived from conventional solid-state synthesis contain CuO precipitates, according to energy dispersive spectroscopy analyses.

Stoichiometry variation effect on the superconducting properties of polymer-processed (Bi1-xPbx)2Sr2Ca2Cu3O10 ceramics

Abstract Bismuth family high-temperature superconducting samples with varying initial Pb, Sr and Ca stoichiometries have been prepared using a polymer solution synthesis method. These samples have been characterized by powder X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy and magnetic ac susceptibility. The samples which yield the highest percentage of 2223 phase have in common an excess amount of Pb and a deficiency in Sr. The sample with best intragranular properties has a stoichiometry represented by Bi 1.8 Pb 0.35 Sr 1.87 Ca 2.1 Cu 3 O 10+δ . It is also shown that, in contrast with the intragranular properties, the intergranular properties are irreproducible in almost all the samples studied.

Structural, superconducting and mechanical properties of molybdenum substituted Bi1.8Sr2Ca1.1Cu2.1Oy

The effect of MoO3 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 Bi1.8Sr2MoxCa1.1Cu2.1Oy. Electrical resistivity showed that transition temperature width increased directly with the Mo amount. XRD data have shown that MoO3 addition in the Bi1.8Sr2MoxCa1.1Cu2.1Oy precursor reduces the amount of Bi-2212 phase. In addition, Jc 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.

Floating zone Ag doped (Bi1·6Pb0·4)Sr2CaCu2O8+δ textured rods

Abstract In this work, superconducting samples of (Bi1.6Pb0.4)Sr2CaCu2O8+δ +3 wt.%Ag have been studied and compared with Ag free samples, (Bi1.6Pb0.4)Sr2CaCu2O8+δ , and with Ag and Pb free ones, Bi2Sr2CaCu2O8+δ . It has been found that Pb doped samples present a worse texture when compared with the undoped ones, resulting in a drastic change on the microstructure. The results showed that electrical resistivity at room temperature, critical current as well as flexural strength are decreasing for the Pb doped samples. When Ag is added to these Pb doped samples, all the properties are improved even if they do not reach the same values as the undoped ones. On the other hand, it has been found that Pb doped samples showed E–I curves with very high sharpness values on the zone of the superconducting to normal transition, reaching n values (E∼In) as high as 45 at 65 K, for samples with simultaneous Pb and Ag additions.

Ag/(Bi, Pb)-Sr-Ca-Cu-O superconducting tape processing: Solid state chemistry aspects

Abstract Different preparation methods have been used to obtain starting powders used in the fabrication of composite tapes by the powder-in-tube method. The effect of these distinct starting powders on the superconducting properties of Ag/Bi-Sr-Ca-Cu-O monofilament tapes has been investigated. The changes in the physical properties, including the critical current density at 77 K and ac magnetic susceptibility, and microstructure, using optical and electronic microscopy, have been analyzed in relation to the solid state reactions involved in the Bi 2 Sr 2 CaCu 2 O 8+δ and Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ phase transformations.

New promising Co-free thermoelectric ceramic based on Ba–Fe–oxide

Thermoelectric ceramics are based in a limited number of transition metal oxides (Co, Mn, Ni,…) which produce materials with high thermoelectric performances. Based on previously existing thermoelectric phases, the phase diagram equilibrium can help to design new thermoelectric ceramics based on other transition metals (for example, Fe). In this work, BaFeOx ceramics have been prepared by the classical solid state method using different sintering temperatures. The produced materials have shown promising thermoelectric properties when treatment temperatures are in the perovskite zone domain of the phase equilibrium diagram. In spite of the good values for the Seebeck coefficients, power factor is low due to the high resistivities measured in all cases.