Chemical Stability of the Bi(Pb)-Sr-Ca-Cu-O Superconductors

The changes in volume of superconducting high-T _c (2223) phase of the high-temperature superconductors (HTSC) with nominal composition Bi_3.2Pb_0.8Sr₄Ca₅Cu₇O _x are studied by magnetic susceptibility during soaking of material in water and more aggressive aqueous solutions. The apparent volumes of the 2223 superconducting phase after long-term (168 hr) soaking in H₂O, NaCl_(aq), and NaOH_(aq) decreased to 22%, 17%, and 57%, respectively. The atomic absorption analysis of resulting solutions showed high amounts of Sr²⁺ and Ca²⁺ in H₂O and NaCl_(aq) and lower amounts of Sr²⁺ and Ca²⁺ in NaOH_(aq). The highest content of Pb²⁺ and Cu²⁺ was detected in NaOH_(aq).     

Formation of high-Tc phase in Sn-incorporated (Bi,Pb)-Sr-Ca-Cu-O superconducting system

Superconductor Bi₂Pb_{1 − x}Sn_xSr₂Ca₂Cu₃O_y (x = 0.0, 0.05, 0.08, 0.1, 0.2 and 0.3) samples were prepared by the conventional solid state reaction and analyzed by XRD, SEM, TGA and DSC and their resistivities were measured by standard ac four-probe method. When a small quantity of Sn (x < 0.1 ) was added, it was found from the SEM and XRD results that high-T_c phase occurred preferentially and the critical temperature was elevated. With the results of XRD, EDX, DSC and TG, it was concluded that a small amount of Sn promotes the formation of liquid phase, which makes the weak link of grain boundary better. When a large amount (x ≥ 0.1) of Sn was added to the (Bi,Pb) 2223 system, however, the solid state reactions became more important than liquid phase-assisted reactions. Thus, the reaction rate decreased and there is no longer a significant role of tin on the formation of 2223 phase in that region (0.1 ≤ x ≤ 0.3).     

Preparation and characterization of glassy and superconducting (Bi,Pb, Sb)-Sr-Ca-Cu-O systems

Superconducting compounds with nominal compositions Bi_1.7Pb_0.3Sr_1.6Ca₂ Cu_3.4O _x and Bi_1.9Sb_0.1Sr₂Ca₂Cu₃O _y have been synthesized by ceramic and glass routes and characterized by X-ray diffraction, electrical resistivity and dielectric constant measurements. The zero-electrical resistance temperatures are about 70K. The dielectric constant of the glasses at room temperature is around 30.     

Pinning strength in Ag-sheathed Bi(Pb)-Sr-Ca-Cu-O superconductor tapes studied from current-electric field characteristics

The relationship between the exponent n of the power law in the current-electric field (I-E) curve and pinning properties of Ag-sheathed Bi(Pb)-Sr-Ca-Cu-O tapes prepared by three different thermo-mechanical treatments was investigated. Two methods are used to determine the pinning strength of the tapes. The first method is comparing the rate of decrease of the transport current density (J_c). The second method is comparing the peak of the pinning force density of the tapes. It was found that for the same sample, the rate of decrease of n in applied fields shows a similar profile with the rate of decrease of the tapes pinning strength. Hence, the n value which can be obtained directly from the I-E curve, can be used as an alternative method to study the pinning properties of superconductor.     

The effect of tin dopant on the phase formation and superconductivity in Bi(Pb)-Sr-Ca-Cu-O

The influence of SnO dopant on the superconductivity and the phase formation of the Bi(Pb)-Sr-Ca-Cu-O system has been studied using differential thermal analysis/thermogravimetric analysis, X-ray diffraction, a.c. susceptibility and resistance measurements. It was found that Sn²⁺ can be dissolved in the 110 K phase, but reduces the formation of this phase. Resistance measurements show 107 K zero resistance can be obtained for a short-time sintered sample (60 h), but that theT _c(0) was found to decrease to 90 and 75 K after the specimens were sintered for 90 and 140 h, respectively. The a.c. susceptibility measurements show that no superconducting phase other than 110 K phase exists in the specimens sintered for longer times and the signal for the 110 K phase became weaker after the specimen had been sintered for 140 h, indicating a decrease in the volume fraction of this phase.     

Transport characteristics related with microstructure of (Bi,Pb)-Sr-Ca-Cu-O superconductor prepared by the sol-gel method

Differences in the transport characteristics of Bi₂Sr₂Ca₂Cu₃O _y (high-T _c phase) superconducting ceramics produced by two-step heat-treatment (calcination and reheating at various temperatures) of metal acetate-derived gels are discussed in relation to their microstructure. Variation of the volume fraction increase rate of the high-T _c phase, which depends on the calcination temperature, results in much difference in the transport characteristics of the resultant samples. Superconducting bodies possessing a higher T _c(end) and critical current density can be obtained at a lower volume fraction increase-rate of the high-T _c phase. It is considered that the origin of the above results is an improvement of the weak-link structure.     

Time-resolved neutron diffraction study of the superconducting phase formation process in the Bi(Pb)-Sr-Ca-Cu-O system

The results of real-time neutron diffraction measurements during the superconducting phase formation process in the Bi(Pb)-Sr-Ca-Cu-O system are reported. A Sr-Ca-Cu-O type precursor, with the same stoichiometry as the 2223 phase, was used as starting material, and the temperature range favorable to the formation of the 2223 phase was investigated. The diffraction patterns were processed by a multiphase Rietveld refinement. The formation and decomposition of the 2201 and 2212 phases were directly observed. Experimental evidence on the existence of a partially melted phase in the range 855–860°C, involved in the formation of the 2223 phase, is discussed.     

Primary Phase Field of the Pb-Doped 2223 High-Tc Superconductor in the (Bi,Pb)-Sr-Ca-Cu-O System

Both liquidus and subsolidus phase equilibrium data are of central importance for applications of high temperature superconductors in the (Bi, Pb)-Sr-Ca-Cu-O system, including material synthesis, melt processing and single crystal growth. The subsolidus equilibria of the 110 K high-T_c Pb-doped 2223 ([Bi, Pb], Sr, Ca, Cu) phase and the location of the primary phase field (crystallization field) have been determined in this study. For the quantitative determination of liquidus data, a wicking technique was developed to capture the melt for quantitative microchemical analysis. A total of 29 five-phase volumes that include the 2223 phase as a component was obtained. The initial melt compositions of these volumes range from a mole fraction of 7.3 % to 28.0 % for Bi, 11.3 % to 27.8 % for Sr, 1.2 % to 19.4 % for Pb, 9.8 % to 30.8 % for Ca, and 17.1 % to 47.0 % for Cu. Based on these data, the crystallization field for the 2223 phase was constructed using the convex hull technique. A section of this “volume” was obtained by holding two components of the composition at the median value, allowing projection on the other three axes to show the extent of the field.     

Rapid formation of the high-T c phase in (Bi,Pb)-Sr-Ca-Cu-O superconductor via the sol-gel method

Bi-Pb-Sr-Ca-Cu-O powder was synthesized by the oxalate gel method. A sample with the composition of Bi_1.7Pb_0.4Sr_1.6Ca_2.4Cu_3.6O_y was used in this study. After pyrolysis of the gel precursor at 500 °C for 5 h, the resulting powder was calcined at 850 °C for another 5 h. The black powder was pressed into pellets and sintered at 852 °C for 5 h. The high-T _c phase was formed more easily in the sample with excess calcium and copper than in the theoretical composition. (Bi,Pb)₂Sr₂Ca₂Cu₃O_y (above 90%) was prepared as above within a relatively short time. Characterization of (Bi,Pb)₂Sr₂Ca₂Cu₃O_y superconductor by X-ray diffraction, scanning electron microscopy, electron probe microanalysis, resistivity measurement and magnetic measurement, is reported.     

Variation of structural and superconducting properties with initial stoichiometry variation in (Bi,Pb)-Sr-Ca-Cu-O bulk superconductor

The effect of the initial stoichiometry variation (variation in Bi, Pb, Sr, Ca and Cu) on various structural and superconducting properties of Pb-doped Bi-based superconductor has been studied. The sample having the initial stoichiometry of (Bi_1.8Pb_0.4)Sr₂Ca_2.2Cu₃O₁₀ was found to give best structural and superconducting properties. The most important factors affectingJ _c in the samples were the type of impurity phases present in the sample, their amount and distribution in the superconducting matrix. Cu-rich stoichiometry enhanced the formation of 110 K phase, but large amounts of undesirable impurities present in the samples decreasedJ _c. The formation of 110 K phase and texturing of (001) planes in a sample depended on the period of heat treatment and stoichiometry.     

Formation and properties of copper film on Tl-(Pb,Bi)-Sr-Ca-Cu-O superconductor by electrodeposition from aqueous solution

A new process of electrodeposition in saturated cupric sulphate aqueous solution was successfully developed for the formation of copper film on a high-T_c, three Cu-O layered Tl-(Bi, Pb)-Sr-Ca-Cu-O superconductor substrate surface for the first time. Scanning electron microscopy and electron micro-probe analysis were used to investigate the morphology of the substrate surface and the composition of the copper superconductor interface. After the electrodeposition process, no evident changes in the temperature dependence of electrical resistivity were found by four-point probe measurement. The difference of magnetic properties before and after electrodeposition was investigated from magnetization measurements. Almost no degradation of the bulk properties was observed from the susceptibility data. The copper-superconductor contact was confirmed to show Ohmic behaviour by two-pointI–V characterization at liquid nitrogen boiling temperature.