M. A. Madre

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.

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.

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.

Long-term high-temperature stability of directionally grown [Bi2Ba2O4]p[CoO2] rods

[Bi2Ba2O4]p[CoO2] thermoelectric ceramics have been successfully grown from the melt using the laser floating zone method, followed by a thermal treatment at 700 °C under air between 0 and 1532 h. The microstructural, thermoelectric, and mechanical properties were evaluated as a function of the thermal treatment length. Microstructure has shown that as-grown samples are composed of thermoelectric grains, together with a relatively high amount of secondary phases. Thermal treatment decreased the number and amount of secondary phases, producing nearly single-phase samples after 384 h. Consequently, the thermoelectric properties evaluated through the power factor showed a slight increase with the thermal treatment length, mainly due to the decrease of electrical resistivity, while the Seebeck coefficient was nearly unchanged. On the other hand, flexural strength was practically constant after 24 h thermal treatment.

Thermoelectric properties in Ca3Co4−xMnxOy ceramics

Abstract Ca3Co4−xMnxOy polycrystalline thermoelectric ceramics with small amounts of Mn have been prepared by the classical solid state method. X-ray diffraction data have shown that Ca3Co4O9 is the major phase, with small amounts of the Ca3Co2O6 one. Moreover, they show that the Mn has been incorporated into these two phases. Electrical resistivity decreases, compared with the values for undoped samples, with Mn content until a minimum for the 0·03 doped ones, increasing for higher Mn substitution. Seebeck coefficient does not change in all the measured temperature range, independently of Mn content. The improvement in electrical resistivity leads ∼30% higher power factor values for the 0·03 Mn doped samples than that obtained in the undoped ones. The maximum power factor at 800°C, ∼0·28 mW K−2 m−1, is close to that obtained in much higher density samples, clearly indicating the good thermoelectric properties of these samples.

Dynamic fatigue behaviour of Ag-doped Bi-2212 textured thin rods

The flexural strength of 1 wt.% Ag-doped Bi2Sr2CaCu2O8+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 Bi2Sr2CaCu2O8+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.

Preparation of high performance Bi2Sr2Co1.8Ox thermoelectric materials from nanosized precursors

ABSTRACT Bi2Sr2Co1.8Ox thermoelectric (TE) materials were prepared by three different synthesis methods producing nanosized precursors: coprecipitation (with ammonium carbonate or oxalic acid) and attrition milling, which were compared with those obtained by the classical solid state method. Microstructure has shown that precursors produced by coprecipitation and attrition milling methods produced nanometric precursors much smaller than the typical sizes produced by the solid state route. The TE properties are in agreement with the microstructural features, leading to lower resistivity in all the samples, compared with the solid state ones, while Seebeck coefficient is practically unchanged in all cases. As a consequence, maximum power factor values of around four times higher than those obtained in the classical solid state method have been determined. Moreover, the highest power factor value at 650°C is higher than the best results obtained in as-grown textured materials produced by the laser floating zone technique.

Effect of Yb substitution in Bi-2212 ceramics prepared by laser floating zone technique

In the present work, the influence of Yb substitution for Bi on the structural, electrical and magnetic properties of textured Bi-2212 ceramics has been investigated. X-ray diffraction studies indicated that the major peaks correspond to the Bi-2212 phase. The SEM images clearly show that there is an increase in the number of secondary phases with raising Yb content while all samples maintain similar grain morphology. Magnetic analysis of all samples has been done by magnetic hysteresis measurements, indicating that an enhancement in the M-H loops is obtained by a low Yb doping. The critical current density of samples was calculated from the M-H curves by using Bean’s critical state model, indicating that sample with 0.05 Yb substitution possesses the highest Jc values.

Fast preparation route to high-performances textured Sr-doped Ca 3 Co 4 O 9 thermoelectric materials through precursor powder modification

This work presents a short and very efficient method to produce high performance textured Ca3Co4O9 thermoelectric materials through initial powders modification. Microstructure has shown good grain orientation, and low porosity while slightly lower grain sizes were obtained in samples prepared from attrition milled powders. All samples show the high density of around 96% of the theoretical value. These similar characteristics are reflected in, approximately, the same electrical resistivity and Seebeck coefficient values for both types of samples. However, in spite of similar power factor (PF) at low temperatures, it is slightly higher at high temperature for the attrition milled samples. On the other hand, the processing time reduction (from 38 to 2 h) when using attrition milled precursors, leads to lower mechanical properties in these samples. All these data clearly point out to the similar characteristics of both kinds of samples, with a drastic processing time decrease when using attrition milled precursors, which is of the main economic importance when considering their industrial production.