Intrinsic critical current of Ag-clad (Bi,Pb)2Sr2Ca2Cu3O z tapes

True zero-field critical current densityJ _c of a well-characterized BPSCCO/Ag tape has been determined by means of high-resolution ac susceptibility in the temperature range 77–110 K. The resultant values (30,000 A/cm² at 77 K) agree well with the transportJ _c of the same tape. Because of a very thin BPSCCO, the coreJ _c determined from the imaginary part of the ac susceptibility is nearly the same as the zero field one. AllJ _c 's follow the same (1-T/T _c )ⁿ withn=1.45 dependence.J _c shows an approximateH ^?0.5 field dependence over the explored temperature range. Accordingly, the variations ofJ _c withT andH seem to be determined by the flux creep.     

The influence of columnar defects on the critical current density in (Bi,Pb)2Sr2Ca2Cu3O10 + δAg tapes

An excellent tool to enhance the critical current density in superconductors is the controlled introduction of defects by irradiation damage. Irradiation with 2.65 GeV Au ions provides the generation of amorphous line defects. Polycrystalline (Bi, Pb)₂Sr₂Ca₂Cu₃O_{10 + δ} Ag tapes processed by the powder-in-tube technique were irradiated up to doses of 3.5× 10¹¹ Au ions/cm² at a temperature of 100 K. On the one hand, our irradiation experiments were performed to investigate the degree ofj _c enhancement and, on the other hand, to examine the flux pinning mechanism in (Bi, Pb)₂Sr₂Ca₂Cu₃O_{10 + δ} tapes. Characterization of the samples was performed by magnetization measurements using a vibrating sample magnetometer (VSM) up to 13 T in a temperature range between 5 and 100 K. Further, we have measured the relaxation of the magnetic moment before and after irradiation to determine the influence of columnar defects on the activation energyU. A comparison of the irradiation-induced effects with previous results obtained on Bi₂Sr₂CaCu₂O_{8 + δ} irradiation experiments is included.     

Field and temperature dependence of the current-voltage characteristics of Bi2Sr2Ca2Cu3O10/Ag multifilamentary tapes

The current-voltage characteristics of Bi₂Sr₂Ca₂Cu₃O₁₀/Ag multifilamentary tapes were measured at different temperatures close to the critical temperature and in different applied magnetic fields up to 1000 Oe. The data were interpreted in terms of thermally activated flux creep by using an intermediate phenomenological model that takes into account the collective pinning. Temperature and field dependences of the pinning potential and of the collective pinning exponent were determined through numerical analysis.     

Texturing and texture-induced intergranular critical state anisotropy of superconducting (Bi,Pb)2Sr2Ca2Cu3Ox ceramics

Different techniques of texturing superconducting (Bi,Pb)₂Sr₂Ca₂Cu₃O_x ceramics, including magnetic alignment, cold-die pressing, unidirectional hot pressing and a combination of these techniques, have been systematically examined with respect to the final microstructures and superconducting properties of the ceramics. The intergranular critical state of the textured ceramics was found to be anisotropic, due to a high degree of grain alignment in these materials. All the intergranular critical states in these materials could be described by the Bean model, although the temperature dependence of the average intergranular pinning force densities were different. The microstructures of the samples were characterized using scanning electron microscopy and bulk density measurement. The critical states of the textured materials were studied using a.c. magnetic susceptometry.     

Sc Addition to (Bi,Pb)2Sr2Ca2Cu3Ox Freeze-Dried Ceramic

In a freeze-dried nitrate precursor powder with cation composition Bi:Pb:Sr:Ca:Cu = 1.7:0.3:2.0:2.5:3.5, Sc₂O₃ was added. The stoichiometry for the mixed precursor powder was Bi:Pb:Sr:Sc:Ca:Cu = 1.7:0.3:2.0:0.25:2.5:3.5. The two nitrate precursor powders with and without Sc were thermally decomposed, pressed, and sintered in the same conditions. They show different behavior, as revealed by thermal analysis, X-ray diffraction (XRD), Scanning electron microscopy (SEM), and χ′(T) measurements. Addition of Sc slows down synthesis processes, including 2223-phase during reactive sintering. After 70 h of sintering, 43% of 2223-phase formed in the pellet with Sc, whereas in the Sc-free pellet, 51% of the 2223-phase was attained. Moreover, addition of Sc leads to a certain morphology, resulting in lower superconducting characteristics.     

The effect of the number of Ag layers on some physical properties of Bi1.8Pb0.4Ca2.2Sr2Cu3Ox

Bi_1.8Pb_0.4Ca_2.2Sr₂Cu₃O_x/Ag multilayered samples with 2, 4 and 8 layers are prepared by powder in tube (PIT) method and the effect of multilayering with Ag on the superconducting and the microstructural properties of Bi_1.8Pb_0.4Ca_2.2Sr₂Cu₃O_x is studied. The investigations consist of XRD, SEM, EDS, magnetoresistivity, and I–V measurements. In the magnetoresistivity measurements, magnetic fields up to 0.5 T are applied both parallel and perpendicular to the current flow direction. The superconducting and microstructure properties of the samples are enhanced as the number of Ag layers are increased. The samples are composed of a highly oriented Bi-2223 phase in the regions near the Ag layers. The formation of the dense oriented structure is near the interface between oxide and the Ag layer. T_c and J_c values are enhanced from 107 to 109 K and from 240 to 600 A/cm² by increasing the number of Ag layers, respectively. The results suggest that Ag plays an important role in the improvement of superconducting properties. The possible mechanisms of the enhancement of superconducting properties due to Ag layering are discussed.     

Effect of platinum on the critical current density of fluorine-doped YBa2Cu3Ox superconductors

The critical current density, Jc, in fluorine-doped YBa2Cu3Ox bulk superconductors has been measured at several temperatures. The measured Jc value is 108 A m-2 at 77 K and 0.5 T. A sample with a larger critical current density can be produced by the addition of a small amount of platinum during the sintering process. The added platinum has the effect of evenly distributing Y2BaCuO5 particles and voids in the matrix in comparison with the undoped sample. These distributions of Y2BaCuO5 particles and voids are reflected in an increase in the critical current density. However, these distributions are not directly related to the observed peak effect in the critical current density. If the fluorine-doped YBa2Cu3Ox superconductor consists of the matrix and a phase with a Ginzburg–Landau parameter, that is slightly different from that of the matrix, then the occurrence of the peak effect can be explained.     

Zero resistance of quenched Bi1.5Pb0.4Sb0.1Sr2Ca2Cu3Ox at 140 K

A glass-ceramic of nominal composition Bi_1.5Pb_0.4Sb_0.1Sr₂Ca₂Cu₃O_x was prepared by the melt-quenching method. The subsequent heat treatment of the sample at 860±5°C showedT _c(zero) at 94 K. Exposure of the sample for 4 days in air improved itsT _c(zero) to 140 K. In another similarly heat-treated sample, a two-step transition in the p-Tcurve at 140 K and at 1 1 1 K, was observed. The superconducting transition appeared to be of a percolative nature.     

HighT c of liquid-quenched Bi1.6Pb0.4Sr2Ga2Cu3Ox

AlthoughT _c cannot be found for a liquid-quenched Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_x glassy sample, a highT _c is found after annealing for 24 h at 1100 K. The maximum offset temperature of the superconducting transition is 113.3 K at 2.2 × 10^–2mAmm^–2. The maximumT _{c off} is larger than that (the maximumT _{c off} is 103.4 K at 2.0 × 10^–2 mAmm^–2) of sintered specimens before liquid quenching.     

Microstructure controls and their effects on the properties of Bi2Sr2Ca1Cu2Ox superconductors

This paper reports our recent progresses in the development of Bi₂Sr₂Ca₁Cu₂O _x /Ag tape conductors for the applications of magnetic field generation in liquid helium or around 20 K, using a refrigerator. We have carried out extensive work to optimize the processing parameters, investigating the relationship between the microstructure and transportJ _c. We have found that the partial melting in oxygen atmosphere is effective to have large transportJ _c with good reproducibility. The pre-annealing and intermediate rolling (PAIR) process has been successfully applied to the multilayer conductors to improve the grain alignment and transportJ _c. TheJ _c of 5×10⁵A/cm² at 4·2 K and 10 T has been achieved, which is the highest value reported so far. Two magnets fabricated by using different types of Bi-2212/Ag conductors were tested. One is a magnet designed as an insert magnet for a 18 T-class large bore Nb-Ti/Nb₃Sn superconducting magnet. The conductor of this magnet was multifilamentary tape processed by powder-in-tube method. TheI _c was 98 A in the backup field of 18 T, which generated the self field of 1·79 T. A large pancake coil was fabricated with multilayer conductor and tested under the operation of cryocooler system. The coil was stably operated up to theJ _c of the coil at the temperatures below 30 K.     

Thermodynamic study of high-temperature superconductors YBa2Cu3Ox and Bi2Sr2CaCu2Oy

S. V. Starodubtsev Physicotechnical Institute, Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Inzhenerno-fizicheskii Zhurnal, Vol. 62, No. 1, pp. 5–9, January, 1992.     

Anisotropic plastic and elastic deformation in highly textured superconducting (Bi,Pb)2 Sr2 Ca2 Cu3Ox ceramics

A highly textured (Bi, Pb)₂Sr₂Ca₂Cu₃O_x superconductor has been prepared and the hardness, deformation and microcracking of the material explored using Vickers indentation. The present study has investigated the hardness anisotropy of the highly textured material on cross-sections of the specimen which have been chosen to give specific low-index orientations. The hardness was found to decrease as the indentation rotated from 0 ° (a diagonal of the indent parallel to the direction of the texturing) to 45 ° (the angle between a diagonal of the indent and the direction of the texturing). The square shape of the indent also changed with rotation from 0 ° to 45 °. The hardness anisotropy has been attributed to the influence of elastic and plastic anisotropy in the polycrystalline materials. In addition, a weakness in the grain-boundary bonding results in the movement of adjacent grains by a sliding mechanism which dominated the indentation anisotropy effect. The presence of microcracking in the grain and at the grain boundary was found to affect the hardness anisotropy.     

Effect of composition on critical current density of Bi2212/Ag round wires

We have fabricated Bi2212/Ag round wires using three kinds of precursor to study the effect of a narrow variation of composition. Slightly different compositions - Bi_2.17Sr_1.94Ca_0.89Cu_2.0O_x(N13), Bi_2.15Sr_1.94Ca_0.89Cu_2.0O_x(N14), and Bi_2.17Sr_1.98Ca_0.89Cu_2.0O_x(N15) - were used and Sr/Ca ratio of them were 2.18, 2.18, and 2.22, respectively. The Ag ratios of the wires were 2.7-2.8 and average filament diameter was 19-21 μm. DTA analysis of the wire showed the peritectic temperature of three wires was very similar value of the range of 880-881 °C. The best engineering critical current density (J_e) of three wires at 4.2 K and 0 T was 414-448 A/mm² at the maximum process temperature range of 884-892 °C. The n-value of N14 showed 13.6, whereas other two wires showed lower n-value, estimating the existence of micro-cracks. Although Bi2212/Ag round wires fabricated by three kinds of composition showed similar J_e value, n-value was quite different. It is likely that the fabrication process such as the drawing as well as the composition of precursor will affect on J_e of Bi2212/Ag round wire.     

The influence of the first heat-treatment on the critical current density of (Bi,Pb)-2223/Ag superconducting tapes

Optimization heat-treatments have been performed on multi-filament Bi-2223/Ag superconducting tapes under 1 bar total gas pressure, the oxygen partial pressure being 8.5%. In a first heat-treatment (HT1), the tapes were sintered within 822-838 °C for 1-50 h. After intermediate deformation, all the samples underwent the second heat-treatment (HT2) at 825 °C and 830 °C for 20 h followed by a thermal sliding procedure. The relative content of the phases present in HT1 samples was measured by XRD. It was found that the Bi-2212 phase content after HT1 strongly influences the values of J_c after HT2. There is a correlation between the amount of Bi-2212 phase after HT1 and the final J_c values after HT2. A maximum of J_c was found for a ratio of 0.15 between Bi-2212 and Bi-2223.     

Phase formation and distribution of (Bi2−xPbx)Sr2Ca2Cu3O10+δ in silver-sheathed composite tapes

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.