A new model for the formation of high-T c phase in superconductive (Bi, Pb)2Sr2Ca2Cu3O x glass-ceramics

The formation mechanism of the high-T _c phase through the glass-ceramic route and the role of Pb on the formation of this phase have been investigated. It was found that a new compound with the chemical composition Pb₂Sr_{3 – x} Ca _x CuO _y (x = 1.8) precipitates at around 550C. This phase is stable up to 800C, where it begins to decompose, and at 850 C it completely disappears. It was found that some part of the released Pb diffuses into the 2212 phase leading to the formation of Pb-containing 2212 phase, (Bi, Pb)₂Sr₂CaCu₂O _x . On the other hand, an endothermic peak, probably arising from the melting of (Bi, Pb)₂Sr₂CaCu₂O _x phase or melting at grain boundaries containing Pb²⁺, was observed at 856C only in Pb-containing samples that were heat treated. The liquid phase attributed to the endothermic peak may enhance the formation of high-T _c phase (2223 phase). The growth kinetics for the high-T _c phase were analysed using the Johnson-Mehl-Avrami equation; the results indicate that the growth of the high-T _c phase is controlled by a diffusion process and the activation energy for its formation in the initial stage (shorter than 96 h) is 576 ± 45 kJ mol^–1.     

Synthesis and characterization of 110 K superconducting phase in Bi(Pb)-Sr-Ca-Cu-oxide

The temperatures and sequence of formation of superconducting phases within the composition Bi_1.6Pb_0.4Sr₂Ca₂Cu₄O _x were determined using simultaneous differential thermal analysis (DTA) and thermogravimetric analysis (TGA), X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). A single phase high-T _c ceramic was obtained by a solid state reaction using predetermined firing conditions, although a transmission electron microscopy (TEM) study showed a small amount of glass phase in the grain boundary tripoint regions. The unit cell of the high-T _c phase was refined asa=0.5413 nm,b=0.5414 nm,c=3.715 nm. The melting temperature of the high-T _c phase is in the region of 852–862°C. The effect of lead was believed to lower the temperature of formation of the high-T _c phase. Lead was also found to evaporate from the matrix during and after high-T _c phase formation, whereas bismuth was found to be stable in both the low-T _c phase and high-T _c phase compounds.     

Superconducting Properties of B<Subscript>2</Subscript>O<Subscript>3</Subscript>-Added (Bi,Pb)-2223 HTSs Prepared on Alumina Plates

Nominally pure and B₂O₃-added (Bi,Pb)-2223 HTS samples were synthesized in air on alumina plates. The influence of boron-doping as well as annealing conditions on the high-T _c 2223 phase evolution was studied using X-ray diffraction (XRD), resistivity and AC susceptibility measurements. The B₂O₃-added samples with starting composition Bi_1.7Pb_0.3Ca₂Sr₂Cu₃B _x O _y (x=0.05 and 0.5) reveal significant enhancement of 2223 phase formation compared to the undoped sample. Higher-level boron doping x=1.5 leads to the degradation of high-T _c 2223 phase.     

Effect of Nb addition on magnetic,structural and superconducting properties of (Bi,Pb)-2223 superconductors

In this work, the effects of Nb₂O₅ addition with different ratios on the structural and magnetic properties of Bi_1.7?xPb_0.3Nb_xSr₂Ca₂Cu₃O_y (x = 0.00–0.20) superconducting samples were investigated. (Bi, Pb)-2223 superconducting samples were prepared by conventional solid-state reaction method. The phase formation, phase fraction and lattice parameters were determined from X-ray powder diffraction (XRD) measurements, the microstructure, surface morphology analyses of the samples were carried out using scanning electron microscope (SEM). Additionally, ac susceptibility measurements were done in order to determine the critical current density (J_c) and hole concentration (p) of the samples. AC susceptibility measurements were done at various ac fields (ranging from 20 to 160 A/m) to understand the effect of Nb addition on magnetic properties of Bi_1.7?xPb_0.3Nb_xSr₂Ca₂Cu₃O_y superconductor. Critical onset (T _c ^on ) and loss peak temperatures (T_p) were estimated from the ac susceptibility curves. It was observed from ac susceptibility measurements that the critical onset temperatures decreased from about 108–98 K with increasing Nb addition (x = 0.00–0.20). The imaginary part of susceptibility was used to calculate the intergranular critical current density (J_c) by means of the Bean’s model. X-ray diffraction analysis revealed that the samples consisted of a mixture of Bi-2223 and Bi-2212 phases as the major constituents and non-superconducting phase Ca₂PbO₄ as the minor. It was also shown from XRD measurements that volume fraction of high-T_c phase decreases with increasing Nb addition up to x = 0.20. The sample with Nb addition of x = 0.20 showed the highest volume fraction of Bi-2223 phase (86 %). When Nb addition was increased, the surface morphology and grain connectivity are found to degrade, the grain sizes decrease and porosity of the samples were observed to increase from SEM images except the sample with x = 0.20 Nb addition.     

Enhancement of (Bi,Pb)-2223 HTS Formation by Boron-Doping

Influence of boron-doping on the superconducting properties of (Bi,Pb)-2223 HTS ceramics prepared in an alumina crucibles has been investigated. X-ray diffraction, resistivity, and AC susceptibility measurements were performed on the undoped and boron-doped compounds. Obtained results have shown that B₂O₃ addition in the Bi_1.7Pb_0.3Ca₂Sr₂Cu₃O _y precursor enhances the formation of high-T _c phase. The boron-doped samples with starting composition Bi_1.7Pb_0.3Ca₂Sr₂Cu₃B _x O _y (x=0.05 and 0.5) reveal significant improvement in the zero resistivity temperature compared to the undoped sample (from 72 K up to 100 K). Boron-doping level x=1.5 results in a substantial degradation of the (Bi,Pb)-2223 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.     

Effect of BN-Added Precursors on Phase Formation and Transport Properties of (Bi,Pb)-2223 HTS

(Bi,Pb)-2223 HTSs (high temperature superconductors) were synthesized from nominally pure (reference) and BN-added Bi_1.7Pb_0.3Ca₂Sr₂Cu₃O _y (BN) _x precursors (x=0,0.10,0.15, and 0.20) by the solid state reaction method using alumina crucibles. The influence of boron nitride addition on the phase formation kinetics and transport properties of (Bi,Pb)-2223 HTSs was studied using X-ray diffraction (XRD), resistivity and critical current density measurements. BN-added compounds reveal a significant enhancement in both the high-T _c 2223 phase formation and critical current density compared to the reference specimen.     

Effect of Bi/Pb ratio and annealing temperature onTc and formation of high-Tc phase in Bi-Pb-Sr-Ca-Cu-O superconductor

We studied the effect of Bi/Pb ratio and annealing temperature onT _c and formation of the high-T _c ; phase in Bi-Pb-Sr-Ca-Cu-O superconductor by the three-step reaction process. The optimum Bi/Pb ratio is about 1.80.3 and the optimum annealing temperature is about 845–855°C. It is found that a variate high-T _c phase existed at the higher annealing temperature. The zero-resistance temperature of the variate high-T _c phase decreased when the annealing temperature increased, although the phase is isostructural with the 110 K phase.     

Monitoring phase formation of Pb-substituted Bi-Sr-Ca-Cu-O superconducting samples at different preparative stages using Raman spectroscopy and X-ray diffraction

Raman spectroscopy and X-ray diffraction were used to follow structure and phase formation at various stages in order to study the effect of Pb substitution in (Bi_1–x Pb _x Sr₃Ca₃Cu₄O _y samples. We found that major reactions involving Bi₂O₃ occurred at around 700°C and that reactions with Ca, Cu, and Pb started at a lower temperature. The amount of Ca₂PbO₄ formed increases as a function of lead concentration and annealing temperature (up to 800°C), but excess lead substitution (50%) destroys the superconductivity. The high-temperature superconducting phase (2223) is only observed in the 15% and 20% leaded samples. These two samples exhibit a higher amount of Ca₂PbO₄ during intermediate processing stages, which suggests that the presence of the Ca₂PbO₄ is important for the formation of the high-T _c phase. It was found that theT _c increases with lead concentration (up to 20% Pb).     

The Effect of Gd Concentration on the Physical and Magnetic Properties of Bi1.7Pb0.3-xGdxSr2Ca3Cu4O12+y Superconductors

Bi–Pb–Gd–Sr–Ca–Cu–O bulk samples with nominal composition Bi_1.7Pb _0.3-xGd_xSr₂Ca₃Cu₄O_12+y (x=0.01, 0.05, 0.075, 0.10) were prepared by the melt-quenching method. The effects of different Gd doping on the structure have been investigated by electrical resistance, scanning electron micrographs, XRD, magnetization and magnetic hysteresis loop measurements. The magnetization measurements have been carried out as a function of magnetic field for fields up to 5 kOe at temperatures well below the zero resistance temperatures of the annealed samples. It has been found that the high-T_c superconducting phase, (2 2 2 3), is formed in the sample A with concentration x = 0.01, annealed at 840°C for 120 h. However, with increasing Gd³⁺ doping for Pb²⁺ the (2 2 2 3) phase gradually transforms into the (2 2 1 2) phase. The magnitudes of magnetization and initial susceptibility, | M | and | dM/dH|, and the hysteresis loop areas decrease with increasing Gd concentration x and/or temperature T. The fast decreases in | M|, | dM/dH |, and the hysteresis loop areas related to the superconducting volume, with increasing x and/or T seem to imply an existence of flux pinning centres in our samples. In order to support this implication the critical current densities J_c, of the samples, have been estimated at two fixed temperatures, 9 and 30 K. Our data have indicated that J_c decreases with increasing temperature and/or Gd concentration, as expected.     

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).     

Structural and Physical Properties of Nd Substituted Bismuth Cuprates Bi1.7 Pb0.3−x Nd x Sr2Ca3Cu4O12+y

BiPb-2234 bulk samples with nominal composition of the compound Bi_1.7Pb_0.3−x Nd _x Sr₂Ca₃Cu₄O_12+y (BSCCO) (0.025≤x≤0.10) have been prepared by the melt-quenching method. The effects of Nd substitution on the BSCCO system have been investigated by electrical resistance (R–T), scanning electron microscopy (SEM), X-ray diffraction (XRD) and magnetic hysteresis measurements. It has been the BSCCO (2212) low-T _c phase is formed for all the substitution levels, together with the BSCCO (2223) high-T _c phase. The results obtained suggest that with increasing Nd³⁺ doping for Pb²⁺ the (2223) phase existing in undoped BSCCO gradually transforms into the (2212) phase and hence all of the samples have a mixed phase formation. The R–T result of the samples show two-step resistance transition; first transition occurs at 100 K and second in an interval of 80–90 K, depending on the Nd concentration. We have found that the magnetization decreases with increasing temperature in agreement with the general characteristic of the high-T _c materials. The samples exhibit weak field dependence particularly after 2 T and changes on the magnetic hysteresis, M–H curve rather small compared to the conventional superconducting materials. The maximum critical current density, J _c, value was calculated to be 8.51×10⁵ at 4.2 K and J _c decreases with increasing temperature and the substitution level.      

Effect of Bi/Pb ratio and annealing temperature onT c and formation of high-T c phase in Bi-Pb-Sr-Ca-Cu-O superconductor

We studied the effect of Bi/Pb ratio and annealing temperature onT _c and formation of the high-T _c ; phase in Bi-Pb-Sr-Ca-Cu-O superconductor by the three-step reaction process. The optimum Bi/Pb ratio is about 1.8∶0.3 and the optimum annealing temperature is about 845–855°C. It is found that a variate high-T _c phase existed at the higher annealing temperature. The zero-resistance temperature of the variate high-T _c phase decreased when the annealing temperature increased, although the phase is isostructural with the 110 K phase.     

Preparation of superconducting-grade copper from commercial grade copper salt

Fine samples with nominal composition of Bi_1·6Pb_0·4Sr₂Ca₂Cu₃O _x have been produced by solid state method using various purity grades of starting copper oxide powder. Studies onT _c and high-T _c volume fraction measurements of these samples revealed that the samples produced using CuO powders obtained in laboratory after double purification of the commercially available copper salts have higherT _c (104·46 K) and increased percentage of highT _c volume fraction (58%) compared to even the samples prepared from Aldrich grade (99·99%) CuO. A simple and cost-effective chemical route for the purification of CuO from commercially available copper salts has been outlined.     

Evolution of 2223 phase in the Bi2Pb0.6Sr2Ca2Cu3.1Oy system

The role of cationic ratios (Sr/Ca and Bi/Pb) in the evolution of high-T _c phase (2223) in samples prepared under different sintering conditions, starting with a composition of Bi₂Pb_0.6Sr₂Ca₂Cu_3.1O_y, has been investigated by employing energy-dispersive X-ray analysis, scanning electron microscopy and X-ray diffraction. As manifested by the observed decrease in Sr/Ca ratio, an increase in sintering temperature from 822 to 852 °C increases the disorder in Sr-O and Ca layers. The observed increase in the volume fraction of 2223 phase and the contraction inc-axis parameter have been explained on the basis of the observed decrease in Sr/Ca ratio. It thus appears that the disorder caused by the intersubstitution of Ca and Sr in SrO and Ca layers and partial replacement of Bi by Pb in the structure promote the evolution and growth of 2223 phase.     

Influence of sintering time and quenching on the formation of highT c phase

Influence of sintering time and quenching in Bi_{2 −x} Pb _x Ca₂Sr₂Cu₃O _y (x=0.0, 0.1, 0.2, 0.25, 0.3 and 0.4) samples have been studied by resistance and XRD measurements. In samples sintered at 850°C for 4 days,T _c(0) increases with Pb concentration.T _c(0) increased from 81 K forx=0.0 to 109 K inx=0.30 sample and then decreased. Increasing the sintering time to 10 days decreased theT _c Quenching further decreased theT _c(0). From X-ray diffraction patterns, the intensity peaks of low and highT _c phases have been measured. The addition of Pb promotes highT _c-phase. Sintering time, slow cooling and rapid quenching studies show that there is an optimum sintering time and cooling rate to produce a highT _c-phase.     

Spin Injection in High-T c Cuprates: An STM Spectroscopy Study

To study how the high-T _c order parameter (OP) evolves under the injection of spin-polarized quasiparticles, STM spectroscopy has been performed on superconductor/ferromagnet thin-film heterostructures comprising YBa₂Cu₃O_7–x (YBCO) and La_0.7Ca_0.3MnO₃ (LCMO) at 4.2 K. Quasiparticle-tunneling and Andreev-reflection characteristics measured on the YBCO under spin-injection from the LCMO were analyzed with the d-wave Blonder-Tinkham-Klapwijk theory, to reveal a spectral evolution which provides direct evidence for dynamic magnetic pair-breaking. The spectral analysis also shows the d-wave OP to remain time-reversal invariant as it is suppressed by the spin-injection. These results are discussed in terms of the general search for quantum-critical points in the high-T _c phase diagram.     

Stabilization of the high temperature phase in Bi-Ca-Sr-Cu-O system

Since the discovery of Bi-containing high-T _c superconductors, difficulties in obtaining monophase samples have been known. The study of phase formation in the system Bi₂Sr_(3−x)Ca _x Cu₂O _y , where 0.5<x<2.5 and conditions for raising HT phase (110 K) content by loading a superstoichiometric excess of Ca and Cu ions into the initial composition as well as by a substitution of lead for a part of bismuth is the object of the present paper.     

Synthesis and Physical Properties of the Hg-Based Superconductors with the 1212 Structure: Hg0.7 V0.3Sr2Ca1-xYxCu2O6+gd

A partial substitution of vanadium at the Hg site helps to stabilize the Sr analogue of the Hg-1212 phase. Simultaneous replacement of Ca by a rare earth ion is found necessary to form single-phase materials. Crystalline and nearly single-phase compounds were synthesized for x≥0.4 in the system Hg_0.7V_0.3Sr₂Ca{i−x}Y _x Cu₂O_6+δ. The lattice parameters show an increase in thea axis and a significant decrease in thec axis with increase inx. A maximumT _c of 85K was obtained for x= 0.4.T _c decreases with increase in Y concentration. However, all the compositions in the single-phase region show a broad superconducting transition. Annealing in vacuum and re-annealing in oxygen helps to improve the diamagnetic volume fraction, but the nature of the transition remained unaffected. Studies on the magnetizationhysteresis loops suggest a poor flux pinning nature of the compounds. Although the compounds have a large decrease in anisotropy along thec axis, partial occupancy of Y³⁺ in the Ca²⁺ layer adjacent to the CuO₂ conduction plane is detrimental to the flux pinning behavior. This study suggests that the presenceof defects in the proximity of the conduction plane in the high-T _c copper oxide system has an adverse effect on the superconducting properties.     

Role of Pb substitution and a study of synthesizing procedure for Bi-based superconductors

Bulk superconductors of the (Bi_1–x Pb _x )₂Sr₂Ca₂Cu₃O _y system have been synthesized by changing the Bi/Pb ratio. The effect of Pb substitution onT _c has been studied by standard d.c. resistivity measurements. An appropriate thermal procedure and time for the preparation of the 110 K phase has also been studied at length. The experiments indicate that the best results are obtained forx=0.2 and that a slow cooling process is necessary for a better control of the thermal process. Indexed X-ray diffraction patterns indicate the lattice parameters of low- and high-T _c phases asa _L=0.54004 nm,b _L=0.5445 nm,c _L=3.084 nm anda _H=0.5483 nm,b _H=0.5339 nm,c _H=3.772 nm, respectively. The observed superconducting behaviour is stable on thermal cycling between 77 and 300 K.