Formation of the high-Tc phase in lead-doped Bi-Sr-Ca-Cu-O superconductors

The characteristics of Bi-Sr-Ca-Cu-O superconductors have been investigated as a function of the amount of Pb addition. The volume fraction of the high-T _c phase increases with increasing Pb addition up to x=0.3 in (Bi_1–xPb_x)₂Sr₂Ca₂Cu₃O_y and above that it decreases. The grain size is increased and density is decreased with increasing Pb addition, due to two-dimensional grain growth which results from formation of the high-T _c phase. The specimen with x=0.3 has a transition temperature of 104 K and high magnetic susceptibility due to the fact that most of the volume fraction is of the high-T _c phase. Pb addition cause formation of Ca₂PbO₄ as a secondary phase and it produces a partially melted liquid phase below the sintering temperature, which acts as a flux and promotes formation of the high-T _c phase.     

Doping of Bi-Sr-Ca-Cu-O superconductors with Pb,Sn and Ag

Basic and substituted superconductors in the homologous series Bi₂Sr₂Ca _n−1Cu _n O_2n+4+y , Bi_2−x/2Sr_2−x/2L _x Ca _n−1Cu _n O_2n+4+y and Bi₂Sr_2−x L _x Ca _n−1Cu _n O_2n+4+y (L=Sn, Pb or Ag,x=0−0.4,n=2, 3 or 4) have been synthesized. All the prepared ceramic samples show superconducting behaviour with zero resistance atT _c=70 to 85 K. The compounds withn=3 or 4 showed onset temperature around 115K indicating involvement of a disproportionate solid-state reaction and formation of a two-phase system. The phase involving tin or lead oxides showed similar superconducting properties. Final rapid quenching of samples contributed to preservation of the high temperature equilibria with higher solubility of tin oxide in the quaternary system Bi-Sr-Ca-Cu-O. Silver was not soluble but precipitated in a colloidal form at interfaces between the crystalline grains.     

Effect of addition of barium,tin and silver on Bi-Sr-Ca-Cu-O ceramic superconductors

The effect of addition of barium, tin and silver, at the expense of bismuth, on the transition temperature and structure of the Bi-Sr-Ca-Cu-O system has been studied. A comparison of the results with that of lead added samples indicates that ability of lead in enhancing the superconducting phase formation is unique. It is observed that none of the above additives replaces Bi³⁺ ions in the lattice structure.     

Synthesis and property evaluation of Y-Ba-Cu-O superconductors

Fine powders of yttrium, barium and copper oxide were synthesized by two different chemical precipitation methods using a spray atomizer. The average particle size in the sintered ceramic was in the range 1–2.5 microns and the superconducting transition temperature of different powders was in the range 60–90 K. With few exceptions, it was found that recalcination and annealing in oxygen will not improve the superconducting transition temperature.     

Synthesis of ceramic superconductors under low oxygen pressure

We have developed a process for synthesizing orthorhombic YBa₂Cu₃O _x (123) superconducting powders by calcination of the precursor powder under reduced total oxygen pressure. Because a single calcination at 800°C for 4 h in flowing oxygen at a pressure of 2 mm Hg results in essentially phase-pure material, total calcination times have been drastically reduced. At liquid nitrogen temperature, sintered pellets made from this powder have critical current densities of ∼1000 A/cm² in zero applied magnetic field.     

Properties of Bi-Sr-Ca-Cu-O highT c superconductor

The effect of sintering in air and annealing in oxygen flow at various temperatures and times on the superconducting transition of Bi₂Sr₂Ca₁Cu₂O₈ ceramic was studied. Samples sintered at 1138 K for 24 h in air and slowly-cooled to room temperature are single-phase with the highest zero resistivity temperature of 88 K. XRD pattern showed that the single-phase was characteristic of tetragonal structure witha=0.5398 nm andc=3.0620 nm. Both a.c. susceptibility and resistivity measurements give similar values for the onset (100 K) of superconducting transition.     

Doping of Bi-Sr-Ca-Cu-O Compounds with Mn

Formation of the superconducting phases Bi-2212 and Bi-2223 has been studied for samples with nominal composition Bi_aPb_bMn_cSr₄Ca₅Cu₇O _x (a + b + c = 4). The influence of the stoichiometric replacement of Bi and/or Pb by Mn on the electric and magnetic properties of the Bi-based superconductors has been studied. The partial substitution of Mn for Bi in the samples without Pb did not promote the growth of the Bi-2223 phase. The samples with double substitution Pb-Mn for Bi revealed a large amount of the Bi-2223 phase.     

Role of dopants in Bi-Sr-Ca-Cu-O system

The role of Al and Ag dopants in the nominal composition, 4336 of Bi-Sr-Ca-Cu oxide has been studied, using a new chemical phase analysis method known as differential dissolution besides XRD and resistivity and magnetic susceptibility measurements. Our investigation reveals that (a) it is relatively easy to realize high-T _c phase (2223) in both Al- and Ag-doped samples with the same nominal composition and heat treatment history (b) both samples consist of superconducting phases with more or less the same basic composition and equal amounts, (c) 2112 phase may be a superconductor and (d) superconducting properties with Al and Ag dopants are different.     

Synthesis and characterization of YBaCuO superconductors from solutions in liquid ammonia

YBa₂Cu₃O_7–x (123) superconducting powders have been advantageously prepared from liquid ammonia solutions of nitrate, acetate, and perchlorate precursors. The synthetic techniques used include solution pyrolysis, freeze drying, and chemical solidification by reaction with a Lewis acid. Metal stoichiometry was determined by gravimetric analysis and atomic absorption spectroscopy. Phase characterization was accomplished by powder X-ray diffraction which revealed at least 98% pure 123 for some samples. Resistivity and magnetic susceptibility measurements confirm the superconducting properties of the products with critical temperatures around 90 K. Finally, chemical and superconducting properties are discussed as a function of process parameters.     

Structural analysis of high-T c cuprate superconductors

We have shown in this paper that members of the Tl-Ba-Ca-Cu-O and Bi-Sr-Ca-Cu-O superconducting families fabricated so far are built up of superconductor-semiconductor arrays. Four general homologous series are written down for these two families. The basic fundamental block in each series is itself a superconductor. We suggest that the Y-Ba-Cu-O family has a similar characteristic if YBaCu₂O_5-d is a superconductor.     

Studies on Bi-Sr-Ca-Cu-O glasses and superconducting glass ceramics

Bi-Sr-Ca-Cu-O glasses and glass ceramics of various compositions were synthesised. The glass transition temperature varies from 396 to 422°C depending on the glass composition. The bulk glass ceramics of 4334, 4336, 2223 and 4246 compositions show superconductivity when the corresponding glass samples were heat-treated in air at 820°C for 3, 9, 12 and 24 h respectively. X-ray diffraction studies show that the superconducting phase present in all these compositions is Bi₂Sr₂Ca₁Cu₂O _x . The 4334 glass ceramic is almost a single-phase material with a preferred orientation such that thec axis is normal to the sample surface. The 2223 glass ceramic has a higherT _c (onset) than the other three compositions indicating the presence of highT _c phase (110K) also. ESR studies on the glass samples indicate the existence of Cu²⁺. The effect of heat treatment on ESR shows that the intensity of resonance decreases with increase in heat-treatment duration. This effect is more pronounced for the 4334 and 2223 compositions. The advantages of synthesizing superconducting materials by glass route are discussed in view of practical applications.     

Bi-Sr-Ca-Cu-O 系氧化物的超导电性

1 引言为了寻找性能更好的氧化物超导体,人们对 Y-Ba-Cu-O 系氧化物作了多种元素替代。我们制备了 Bi-Sr-Ca-Cu-O 系氧化物,测量了交、直流磁化率和电阻与温度的关系。     

Environmental contamination of amorphous Bi-Sr-Ca-Cu-O fibers

A study of the environmental contamination of amorphous YBa₂Cu₃O₇- fibers has been undertaken in order to determine the best method to handle these materials during the fabrication of superconducting wire. The fibers often need to be handled in organic solvents as part of the cleaning and manipulating process. In organics that are free of water, the fibers retain their mechanical strength with little carbon contamination or other ill effects. Water, however, causes premature crystallization and destroys the mechanical strength of the fibers.     

AC susceptibility studies on Bi-Sr-Ca-Cu-O system

We report the low field a.c. susceptibility data χ′(T, H _rms), χ″(T, H _rms) for the newly prepared superconducting system Bi _x Sr₄Ca₂Cu₄O _y (x=0·5, 1·0 and 1·5) containing Bi at concentrations lower than that in the conventional Bi-based system. Our experimental results are discussed in the light of existing theories.     

Growth of highT c Bi-Sr-Ca-Cu-O thick films

Thick films of Bi-Sr-Ca-Cu-O were deposited on (100) MgO substrates by screen-printing technique with the starting composition 1112. To attain the superconducting state, the films were subjected to two-step heat-treatment. R-T and XRD have been studied for films annealed at different durations of the second step. InitiallyT _c (R=0) increased from 77 to 103 K as the annealing duration was increased after whichT _c decreased. Kinetics of the growth of highT _c phase is discussed in the light of our results.     

Laser fabrication of Pb doped Bi-Sr-Ca-Cu-O superconductor

It has been shown that the addition of Pb enhances the formation of 110 K (2 2 2 3) phase in Bi-Sr-Ca-Cu-O system due to kinetic reasons. With the premise that a local temperature spike, exceeding the melting temperature of the compound, might enhance the nucleation and growth of the 110 K phase, we have conducted laser calcination experiments of Pb doped Bi-Sr-Ca-Cu-O compound by using a high energy Nd: YAG laser. It is found that laser calcination of this material indeed enhances kinetics of formation of the 110 K phase. Nearly 100% 2223 phase has been obtained in laser processed samples in much shorter sintering time compared with the conventional sintering.