Effect of Extrusion on the Critical Current Density of Bi1.4Pb0.6Sr2Ca2Cu3Ox Superconducting Wires

The effect of extrusion on improving the critical current density ( J _c) of Bi_1.4Pb_0.6Sr₂Ca₂Cu₃O _x superconducting wires is investigated. Calcined powders (Bi_1.4Pb_0.6Sr₂Ca₂Cu₃O _x ) are first mixed with a forming aid—a thermoplastic polymer (polyethylene)—for workability, and then extruded, using a capillary rheometer, to form wires 2 mm in diameter. The J _c value, measured by the four-probe method in liquid nitrogen at 77 K, is improved substantially by the following process: (1) the superconducting precursors are extruded at high viscosity with a forming aid, to align the platelike particles unidirectionally; (2) the forming aid alone is carefully burned out, without destroying the extruded configuration; and (3) the extruded wires are annealed at 850°C in air for more than 96 h.     

Superconducting Properties of Barium-Doped (Bi,Pb)2Sr2Ca2Cu3Ox Glass-Ceramics

The Ba-doped superconducting (Bi,Pb)₂Sr_{2- x} Ba _x Ca₂Cu₃O _y and (Bi,Pb)₂Sr₂Ca_{2- x} Ba _x Cu₃O _y (0 ≦ x ≦ 1.0) were prepared by using a melt-quenching method, and the effect of Ba additions on the glass-forming ability and the crystalline phase was examined. The glass-forming ability was not improved by substitution of Ba for Sr or Ca, and particularly BaPbO₃ as well as CaO was observed in the melt-quenched sample of (Bi,Pb)₂SrBaCa₂Cu₃O _y . BaPbO₃ crystals were precipitated in all glass-ceramics with Ba substituted for Sr or Ca. The partial substitution of Ba substituted for Sr was effective for the formation of the high- T _c phase, and (Bi,Pb)₂Sr_1.4Ba_0.6Ca₂Cu₃O _y glass-ceramics obtained by annealing at 830°C for 100 h exhibited superconductivity with a T _c of 103 K, although BaPbO₃ and the low- T _c phase were still largely present.     

Stability of Bi2Sr2Ca1Cu2O8 ±δ Thick Films at Elevated Oxygen Pressures and Temperatures

Bi₂Sr₂Ca₂Cu₂O_8±δ-type compound thick films were exposed to oxygen-argon-gas mixtures (1% to 20% oxygen gas) at elevated pressures (up to 207 MPa) and temperatures (500° to 940°C) for times ranging from 5 to 96 h. At a sufficiently high oxygen fugacity and temperature, Bi₂Sr₂Ca₁Cu₂O_8±δ decomposed via a solid-state reaction. Room-temperature X-ray diffractometry and electron probe microanalysis of decomposed films revealed the presence of Bi₂(Sr,Ca)₂-Cu₁O_6±θ ro-type compound, Bi₂Sr₂,Ca₁O_8±δ-type compound, and CuO. Bi₂Sr₂Ca₁Cu₂O_8±δ decomposition was accompanied by a modest weight gain, which was consistent with an oxidation reaction. The solid-state decomposition reaction could be reversed by heat treatment of decomposed films at 860°C in pure, flowing oxygen at ambient pressure.     

Phase Separation and Crystallization of BiSrCaCu2Al0.5Ox Glass

Al₂O₃ addition to the melt of a BiSrCaCu₂O _x composition was found by TEM observation to cause the liquid-liquid phase separation of the melt-quenched glass, and to result in preferential precipitation of superconducting Bi₂Sr₂Ca₁Cu₂O _x crystals from the melt during the cooling process.     

Quantitative Electron Microscopy of (Bi,Pb)2Sr2Ca2Cu3O10+δ/Ag Multifilament Tapes During Initial Stages of Annealing

The microstructural and compositional evolution during initial annealing of a superconducting (Bi,Pb)₂Sr₂Ca₂Cu₃O_10+δ/Ag tape is studied using quantitative transmission electron microscopy. Special attention is devoted to the occurrence of Pb-rich liquids, which are crucial for the Bi₂Sr₂CaCu₂O_8+δ to (Bi,Pb)₂Sr₂Ca₂Cu₃O_10+δ transformation. Ca and/or Pb-rich (Bi,Pb)₂Sr₂CaCu₂O_8+δ grains dissolve into a liquid, which reacts with Ca-rich phases to increase the liquid's Ca-content. This leads to (Bi,Pb)₂Sr₂Ca₂Cu₃O_10+δ formation. Apparently, a Ca/Sr ratio of around 1 is sufficient to keep (Bi,Pb)₂Sr₂Ca₂Cu₃O_10+δ nucleation going. It is confirmed that Ag particles are transported from the Ag-sheath into the oxide core by the liquid and not by mechanical treatment of the tape.     

Fundamental Material Aspects Underlying the Preparation of High-Temperature Superconducting (Bi,Pb)2+xSr2Ca2Cu3O10+d Ceramics

Phase equilibria of the quasi-quinary system Bi₃O₃-PbO-SrO-CaO-CuO were studied between 650° and 900°C in air with emphasis on the high-temperature superconducting phase (Bi,Pb)_{2+ x} Sr₂Ca₂Cu₃O_{10+ d} (2223). The 2223 phase lies in equilibrium with a number of nonsuperconducting phases and also with the superconducting phase (Bi,Pb)₂Sr₂CaCu₂O_{8+ d} (2212). The single-phase region was found to be very limited. The Pb solubility of the 2223 phase is strongly temperature dependent. The phase relations are very sensitive to variations of the cation concentration and temperature. This effect significantly influences the preparation of 2223 ceramics.     

Effect of High-Temperature, High-Oxygen-Fugacity Annealing on the Stability of the (Bi,Pb)2Sr2Ca2Cu3O10 ±δ-type Compound

The stability of the (Bi,Pb)₂Sr₂Ca₂Cu₃O_10±δ-type compound has been evaluated under conditions of elevated temperature (500°-860°C) and elevated oxygen fugacity (i.e., in O₂/Ar gas mixtures containing ≤120% O₂, at total pressures of 5207 MPa). At sufficiently high oxygen fugacities and temperatures, the (Bi,Pb)₂Sr₂Ca₂Cu₃O_10±δ-type compound transformed into a mixture of a strontium-rich (Bi,Pb)₁-(Sr,Ca,Cu)₂O_y-type compound, a calcium-rich (Bi,Pb)₂-(Sr,Ca,Cu)₂O_y-type compound, CuO, and a small amount of (Sr,Ca)O. The decomposition of the (Bi,Pb)₂Sr₂Ca₂-Cu₃O_10±δ-type compound was accompanied by a 2%-3% weight gain, which was consistent with an oxidation reaction. The conditions of oxygen fugacity and temperature leading to decomposition, and the resulting decomposition products, are compared for the (Bi,Pb)₂Sr₂Ca₂Cu₂O_10±δ-type and Bi₂Sr₂Ca₁Cu₂O_8±Ψ-type compounds.     

Retrograde Densification of Calcined and Glass Precursors of Approximate (Bi,Pb)2Sr2Ca2Cu3O10 Stoichiometry

Retrograde densification of pelletized calcines and glasses having an approximate (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ starting stoichiometry and sintered at ∼850°C can be described by first-order rate equations. Retrograde densification in the calcine precursors was largely due to the development of open pores, and was approximately proportional to the concentration of the (Bi,Pb)₂Sr₂CaCu₃O₁₀ phase. In the glasses, retrograde densification is mainly caused by porosity accompanying the growth of a needlelike Sr─Ca─Cu─O phase, together with (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ and (Bi,Pb)₂Sr₂CaCu₂O₈.     

Enhancement of Tc(0) by Substitution of Gallium in the Bismuth-Based High-Tc Superconducting Material

We report the enhancement of the zero resistivity T _c(0) by 5.5 K i.e. from 104 to 109.5 K by substitution of gallium 1.34% of copper in the bismuth 2223 compound. A series of Ga-containing compounds Bi₂Pb_0.4Sr₂Ca₂Cu_{3− x} Ga _x O _y ( x =0.00, 0.02, 0.04, 0.06, and 0.08) are synthesized by the solid-state reaction method. The samples are characterized by measurements of their dc electrical resistivity and ac magnetic susceptibility and by the powder X-ray diffraction analysis. It is noted that the high- T _c (2223) phase increases from 57.55% in an undoped sample to 92.99% in samples containing a low concentration of gallium i.e. x ≤0.04.     

Characterization of a Large-Scale Nondoped YBa2Cu3O7−x Superconductor Prepared by Plastic Forming Without High-Pressure Molding

A slurry containing YBa₂Cu₃O_{7− x} particles and a fine YBa₂Cu₃(OH) _x colloid solution was prepared, and a large-scale bulk YBa₂Cu₃O_{7− x} superconductor (about 50 mm × 35 mm × 2 mm) was produced by plastic forming without high-pressure molding. The samples molded from the slurry were dried and then fired at 1223 K in air. X-ray diffraction data indicated that the samples had the characteristic orthorhombic YBa₂Cu₃O_{7− x} structure. Measurements of electrical resistance were carried out between 300 and 50 K by the standard four-probe DC electrical measurement. The samples began superconducting at an onset temperature around 92 K, and the full-transition temperature (critical temperature) ( T _c) was 88.7±1.4 K. The critical current density ( J _c) measured at 77 K was about 440 A/cm², the value of J _c was improved by the heat treatment under an oxygen atmosphere, and J _c=1.6 × 10³ A/cm² was observed. Under the magnetic field (B=1 T), the sample held its superconductivity, and demonstrated that this method can be used to produce the magnetic shielding used in magnetic resonance imaging diagnosis.     

Solid-Solution Ranges of the n= 2 and n= 3 Superconducting Phases in Bi2(SrxCa1 −x)n+1CunOy and the Effect on Tc

The cation solubility limits of the n = 2 and n = 3 superconducting phases in the Bi₂(Sr _x Ca_{1 − x} ) _{n +1}Cu _n O _y system were established along tie lines with compatible phases via electron probe microanalysis on bi- (or poly-) phasic samples prepared at 860°C. Pb additions (15 mol% of the Bi content) were used to facilitate formation of the n = 3 phase. In each case football-shaped volumes in composition space were established as the solubility limits which bordered on the nominal compositions 2212 or 2223 (Bi:Sr:Ca:Cu) with the long axis parallel to the Sr-Ca side of the quaternary (i.e., Sr-to-Ca intersolubility) but also extending toward Bi and Cu. This means that, for the most part, the superconducting phases are alkaline-earth deficient relative to the ideal 2212 and 2223 compositions. The Pb content in the 2223 phase is typically 10% of the Bi content. T _c variations could be correlated with variations in Sr or (Sr + Bi) content and the length of the c -axis parameter.     

30 kbar Phase Equilibria of the La2O3–SrO–CuO System at 950°C

Phase equilibria of the La₂O₃–SrO–CuO system have been determined at 950°C at 30 kbar (3 GPa). Stable phases at the apexes of the ternary phase diagram are CuO, La₂O₃, and SrO. Stable intermediate phases are La₂, CuO₄ and La₂Cu₂O₅ in the LaO_1.5–CuO binary and Sr₂CuO₃, SrCuO₂, and Sr₁₄Cu₂₄O₄₁ in the CuO–SrO binary. The La_{2– x} Sr _x -CuO_4–δ solid solution is stable for 0.00 is ≤ x ≤ 1.29, the La_{2– x} Sr_{1+ x} Cu₂O_6+δ solid solution is stable for 0.03 ≤ x ≤0.20, the La_{2– x} Sr _x Cu₂O_5–δ solid solution is stable for 0.00 ≤ x ≤1.08, and the La _x Sr_{14– x} Cu₂₄O₄₁ solid solution is stable for 0.00 ≤ x ≤ 6.15. The 30 kbar phase diagram differs from the 1 atm (0.1 MPa) and 10 kbar (1 GPa) results principally in the absence of La_{1– x} Sr_{2+ x} Cu₂O_5.5+δ as a stable phase and the extended range of the La_{2– x} Sr _x Cu₂O_5–δ solid solution at 30 kbar.     

Microstructure, Piezoelectric, and Ferroelectric Properties of Bi2O3-Added (K0.5Na0.5)NbO3 Lead-Free Ceramics

Lead-free piezoelectric (K_0.5Na_0.5)NbO₃– x wt% Bi₂O₃ ceramics have been synthesized by an ordinary sintering technique. The addition of Bi₂O₃ increases the melting point of the system and improves the sintering temperature of (K_0.5Na_0.5)NbO₃ ceramics. All samples show a pure perovskite phase with a typical orthorhombic symmetry when the Bi₂O₃ content <0.7 wt%. The phase transition temperature of orthorhombic–tetragonal ( T _{O − T} ) and tetragonal–cubic ( T _C) slightly decreased when a small amount of Bi₂O₃ was added. The remnant polarization P _r increased and the coercive field E _c decreased with increasing addition of Bi₂O₃. The piezoelectric properties of (K_0.5Na_0.5)NbO₃ ceramics increased when a small amount of Bi₂O₃ was added. The optimum piezoelectric properties are d ₃₃=140 pC/N, k _p=0.46, Q _m=167, and T _C=410°C for (K_0.5Na_0.5)NbO₃–0.5 wt% Bi₂O₃ ceramics.     

Phase Regions and Kinetics of Formation of (Bi,Pb)2Sr2Ca2Cu3Ox in Ag-Sheathed Tape

Ag-sheathed (Bi,Pb)₂Sr₂Ca₂Cu₃O, (2223) tapes were made by the oxide-powder-in-tube method. Tapes were heat-treated isothermally at several different temperatures in 7.5% O₂/Ar, then quenched into oil to retain the phase assemblages at the reaction temperatures. 2223 formed between ∼810° and ∼837°C. The Avrami equation was applied to describe the kinetics of 2223 formation from a mixture of Bi₂Sr₂CaCu₂O _x and nonsuperconducting phases, mainly Ca₂PbO₄ and CuO. The calculated Avrami exponent, n ∼ 1, indicated that the kinetics in this system could be described as a diffusion-controlled, two-dimensional nucleation and growth process. The apparent activation energy for forming 2223 was ∼2900 kJ/mol from ∼817° to ∼825°C and ∼890 kj/mol from ∼825° to ∼837°C. A temperature-time-transformation diagram was constructed based on the kinetic data; it describes the transformational behavior of this particular system.     

Preparation and Characterization of a Large-Scale YBa2Cu3O7−x Superconductor Prepared by Plastic Forming without a High-Pressure Molding: Effect of Polyvinyl Alcohol (PVA) Addition on the Superconducting Properties

The preparation of large-scale YBa₂Cu₃O_{7− x} superconductor samples was investigated. This method is based on plastic forming using a slurry consisting of YBa₂Cu₃O_{7− x} particles and a sol solution made up of multimetallic hydroxide particles (YBa₂Cu₃(OH) _x colloidal particles) and poly(vinyl) alcohol (PVA). The effects of adding PVA on the product, the crystallinity, and the superconducting properties of the sample were investigated. It was found that PVA acted as a protective colloid in the sol solution and stabilized YBa₂Cu₃(OH) _x colloidal particles, and that the role of PVA changed from a thickener to a flocculant during drying so that the formability/workability of the green sheet sample was improved and large samples (about 80 mm × 80 mm × 3 mm) without large cracks were obtained after firing. The samples became superconducting at 91.5±0.5 K ( T _con) and the full transition temperature ( T _coff) was 88.5±1.5 K. The critical current density ( J _c) of the sample prepared from the slurry containing 1 wt% PVA was 713±150 A/cm² at 77 K. This J _c value was improved to 2300 A/cm² by heat treatment at 773 K under an oxygen atmosphere.     

Preparation and Structural Investigation of Bi16(Sr,Ca)14O38

Single-crystal X-ray diffraction was used to investigate the structure of the ternary oxide of approximate formula Bi₁₄(Sr,Ca)₁₂O₃₃. The crystal examined was found to have a composition Bi₂Sr_0.68Ca_1.07O_4.75 or Bi₁₆Sr_5.44Ca_8.56O₃₈ as a result of refinement. It crystallizes in the monoclinic space group (C2/m), with cell parameters a = 21.764 (4) A, b = 4.3850 (13)° A, c = 12.905 (3) A, β= 102.72 (2)°, and V 1201.2 (5) A³. Strontium and calcium ions were found to substitute for each other. The structure consists of a network of alkaline-earth oxide polyhedra with broad channels parallel to the b axis, in which the bismuth ions reside. Two out of four crystallographically distinct Bi ions form infinite Bi—O zigzag ribbons of edge-linked Bi—O units, while each of the other two form oxygen corner-shared Bi—O chains along b . The lone pair electrons of the Bi ions point toward each other in the channels.     

Microstructure–Dielectric Properties Relationship in Ba0.6Sr0.4TiO3–Mg2SiO4–Al2O3 Composite Ceramics

0.60Ba_0.6Sr_0.4TiO₃(BST)–(0.40− x )Mg₂SiO₄(MS)– x Al₂O₃ ( x =0, 0.5, 3, 5wt%) composite ceramics exhibit excellent characteristics suitable for tunable device applications. With increasing amount of Al, the dielectric peak can be quantitatively broadened and suppressed; the "phase transition temperature" T _c or ( T _m) shifts to a lower temperature. Meanwhile, the tunability is still high in a wider temperature range. Far from T _c, pyroelectric effects are observed by using the Byer and Roundy technology and Slim polarization hysteresis loops are observed under high ac dielectric field at 10Hz. These proved the existence of spontaneous polarization in certain possible orientations in a broad temperature range above T _c in the paraelectric medium and reveal why 0.60BST–(0.40− x )MS– x Al₂O₃ have such remarkable dielectric nonlinearity.     

Crystal Chemistry and Phase Equilibrium Studies of the BaO(BaCO3)–½R2O3–CuOx Systems in Air: VI, R = Neodymium

Crystal chemistry and subsolidus phase equilibrium studies of the Ba-Nd-Cu-O system near the CuO and Nd₂O₃ corners have been carried cut at 950°C in air. Two solid-solution series have been identified in the Ba-Nd-Cu-O system. The first series involves the high- T _c superconductor phase, and has the formula Ba_2–xNd_1+xCu₃O_6+z, where × < ≅ 0.7. At the ideal compound stoichiometry of Ba₂NdCu₃O_6+z, the transformation from the high- T _c orthorhombic to tetragonal phase occurs at 550°–575°C in air. This temperature varies as a function of composition, and at x ≅ 0.2 to 0.3 it occurs at 950°C. The second solid solution is the non-superconducting "brown phase" represented by Ba_2+2x-Nd_4–2xCu_2–xO_10–2z 0 ≤ x ≤ 0.1. Preliminary phase diagrams of the BaO–Nd₂O₃ and Nd₂O₃–CuO_x systems are also presented. Standard X-ray diffraction patterns of BaNd₂–CuO₅ and (Nd_1.9Ca_0.1)CuO_4–z are provided.     

Intrinsic Kinetics of Carbon Dioxide Degradation of YBa2Cu3O7-x

The intrinsic kinetics, unaffected by diffusional and masstransfer effects, of the CO₂ degradation of superconducting particles have been determined using a nonisothermal technique. Below 900°C, the carbonization of YBa₂Cu₃O_{7- x} leads to formation of BaCO₃, Y₂Cu₂O₅, CuO, and Cu₂O. A further increase in temperature results in formation of BaCuO₂ from BaCO₃ and CuO. The carbonization rate shows the 1.5th-order dependence on the amount of unreacted YBa₂Cu₃O_{7- x} for the temperature range of 550° to 815°C. The activation energy of carbonization was determined to be 95.1 kJ · mol⁻¹.     

Phase Diagram Studies in the System Tl2O3–BaO–CaO–CuO–Ag

The phase relations within the system Tl₂O₃–BaO–CaO–CuO including Ag have been studied with emphasis on the high-temperature superconducting phases TlBa₂Ca₂Cu₃O_8.5 (1223 phase), Tl₂Ba₂Ca₂Cu₃O₁₀ (2223 phase), TlBa₂CaCu₂O_6.5 (1212 phase), and Tl₂Ba₂CaCu₂O₈ (2212 phase) at 890°C in an oxygen atmosphere. 1223 has been found to be in equilibrium with a liquid phase that is Tl poor. 2223 and 2212 exhibit varying Tl/(Ba + Ca) ratios. The three-phase field 1223 + 2223 + 2212 has been identified. The results of this study emphasize that multiphase samples can be prepared which consist of three superconducting phases, each exhibiting a critical temperature of 100 K or above.