Y.C. Guo

Addition of nanometer SiC in the silver-sheathed Bi2223 superconducting tapes

Abstract Nanometer SiC particles were added to (Bi,Pb)2Sr2Ca2Cu3Ox (Bi2223) precursor powders and the powders were transformed into silver-sheathed superconducting tapes by the standard powder-in-tube (PIT) technique. The influence of SiC on the melting temperature, high-Tc (Bi2223) phase formation, microstructure and transport property of the tapes was studied by means of DTA, XRD, SEM/EDX, electrical and magnetic measurements. The results showed that SiC addition lowered the melting temperature of the superconductor powders and consequently, decreased the optimum sintering temperature of the tapes. However, the addition did not affect the Bi2223 phase formation and critical temperature (Tc) of the tapes up to 1.00 wt.% of SiC. Most importantly, the addition of a small amount of SiC (0.15 wt.%) improved the critical current (Ic) and Ic behavior in magnetic field as a result of the enhancement in density, grain alignment, grain connectivity and flux pinning of the tapes.

Current limiting effect of residual Bi2Sr2CuO6 in silver-sheathed (Bi,Pb)2Sr2Ca2Cu3O10 superconductors

Abstract A controversial issue in the study of Bi-based superconductors is the role of Bi2Sr2CaCu2O8 in determining the properties of silver-sheathed (Bi,Pb)2Sr2Ca2Cu3O10 (Bi2223) tapes. In this study, we prepared a series of Bi2223/Ag tapes with different Bi2Sr2CaCu2O8 (Bi2212) and Bi2Sr2CuO6 (Bi2201) ratios and investigated its effect on electrical and magnetic properties. It was found that residual Bi2201, instead of Bi2212, was the limiting factor for the critical current (Ic). Elimination of Bi2201 using a two-step sintering process in the final thermal cycle, or by cooling the tapes slowly, increased Ic by a factor of 2. The flux pinning and irreversibility field behavior were also improved by removing Bi2201.

Introduction of pinning centres into Bi - (Pb) - Sr - Ca - Cu - O superconductors

High critical current density is essential for most large-scale applications of high-temperature superconductors (HTSs). In addition to the weak link problem, weak flux pinning is a major cause of the rapid decline of with magnetic field at high temperatures. Through intensive research in the past 10 years the weak pinning problem has been partially overcome and has reached a level approaching the requirement for some commercial applications. In this article, a number of techniques by which effective pinning centres can be introduced to improve flux pinning in Bi - (Pb) - Sr - Ca - Cu - O HTSs are reviewed. These include surface pinning through a spiral growth, fine precipitates produced through phase transformation or phase formation - decomposition, inclusions through chemical doping or addition, cascades and columnar defects created by fast-neutron and heavy ion beam irradiation and various defects induced by mechanical deformation. In particular, the results of Bi-2223 formation - decomposition and cold deformation versus hot deformation have a significant implication for HTS wire fabrication. Comparative studies with comprehensive electrical, magnetic and microstructural characterizations have been carried out to assess the effectiveness of these techniques.

Preparation of Ag - Bi-2223 tape by controlling the phase evolution prior to sintering

The critical current densities and microstructures of tapes prepared using various pre-sintering thermomechanical treatments were compared. The phase composition prior to sintering was found to have a strong effect on the rate of 2223 phase growth and texture and, consequently, final transport properties. The difference in optimum requisite tape sintering processes when using powder with the same stoichiometry from different powder processes or even using identical powder obtained from the same source is due to variations in the content and composition of the secondary phases in the calcinated powder and/or the different phase transformations that occur due to different thermomechanical treatments prior to the sintering procedure. The optimum phase composition before sintering is a mixture of the main phase (textured mature Pb-2212 and Pb free 2212) and a small amount of secondary phases consisting of SrCaCuO and with small particle size. BiSrCuO, CuO and CaCuO are all detrimental to final tape performance characteristics and should be minimized in precursor powder. The optimum phase composition can be prepared by appropriate pre-sintering preparation and thermomechanical procedures. The particle size of the oxide in the Ag sheath can be decreased to a large extent by the deformation process. A critical current density of and a high (1 T, 77 K)/ of 23.4% has been achieved reproducibly using short sintering times and one intermediate pressing procedure. The reproducibility is a direct consequence of controlling the phase composition of the oxide core prior to sintering.

Magnetic field properties of Bi-2223/Ag HTS coil at 77 K

Abstract High T c superconducting (HTS) (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10+ x Ag clad multifilament wire, is used to prepare a HTS coil. Magnetic field behaviour of the HTS coil is studied at 77 K operation. Experiments carried out include magnetic field measurements of the HTS coil, DC magnetic field, AC magnetic field, and critical current ampere-turns when used as a magnetic core bias winding. The magnetic field distribution of the HTS coil is also investigated for use at 77 K with the anisotropic HTS wire. The experimental results and analysis provide basic information for design and operation of a HTS coil made from the present HTS (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10+ x Ag clad wire.

Improvement of grain connectivity in Bi2223/Ag tapes by reducing Bi2201 phase at grain boundaries

Abstract The critical current of Bi2223/Ag tapes with different secondary phase content was measured in an applied magnetic field at 77 K. The phase content was altered using different cooling rates after the final sintering, or by using an additional sintering stage at a lower temperature. The difference in strong link grain connectivity in the tapes was obtained by extracting strong link critical current ( I c0 s ) from the I c vs. H curves. Flux pinning was monitored via the field of the peak of pinning force density. Secondary phase content was assessed using X-ray diffraction. SEM analysis was used to study the secondary phase distribution and microstructure in the tapes. Normally prepared tapes showed an initial increase of normalized I c0 s with J c0 . For J c0 >20 kA/cm 2 , however, normalized I c0 s saturated at about 35% [J. Horvat, W.G. Wang, R. Bhasale, Y.C. Guo, H.K. Liu, S.X. Dou, Physica C 275 (1997) 327], while tapes with low Bi2201 content did not show this saturation of normalized I c0 s . This resulted in a substantial improvement of field dependence of I c over conventional PIT tapes. For the field direction parallel to the tape plane, 28% of I c0 was retained at 1 T. SEM analysis revealed that the Bi2201 was located between the grains. It is suggested that by removing Bi2201 from low angle tilt boundaries, a higher number of strong links was formed in the tapes.

Critical currents in Ag/Bi-2223 superconducting tapes: grain connectivity and flux pinning

Abstract Critical current ( I c ) was measured for a large number of different Ag/Bi-2223 tapes at 77 K, with field H perpendicular to the tape plane. I c vs. H shows weak and strong link regimes. Critical current through strong links (for H > 100 mT) was described by: I c s ( H ) = I c0 s exp(−( H / H 0 ) a + 1 ). It was shown that I c0 s / I c0 is correlated with critical current density ( J c0 ) and it can be used as a reliable parameter for determining the quality of the links between the grains. A method for obtaining accurate values of I c0 s / I c0 was presented. I c0 s / I c0 increases with J c0 and attains a constant value of about 0.36 for J c0 > 20 kA/cm 2 . Similarly, normalized critical current for a parallel field of 1 T ( I c (1 T)/ I c0 ) increases with J c0 and reaches a constant value of about 0.2 for J c0 > 20 kA/cm 2 .

Effect on the phase formation of Bi-2223 in some Ag-alloy sheathed PIT tapes

Powder-in-tube mono- and multifilamentary tapes with sheaths of Ag, AgCu0.02, Ag(AgCu0.02), AgAl0.25, Ag(AgAl0.25), AgNi0.25Mg0.25, AgTi0.25Mg0.25, and AgTi0.25 alloys have been fabricated and their physico-chemical properties and effect on the phase formation of Bi-2223 determined. Alloying was found to have a significant effect on phase formation of the Bi-2223 phase formation linked to the alloying elements reactivity to oxygen Cu<Ni<Al<Mg<Ti. Multifilamentary tapes, composed of an inner Ag sheath and an outer Ag alloy sheath showed no such effects. The resistivity and mechanical properties of the various Ag alloys used as sheathing materials are also reported on and their possible use in improving the performance of PIT tapes.

Formation of weak and strong links in Ag/Bi-2223 superconducting tapes

Abstract Ag-sheathed Bi-2223 tapes were prepared by the oxide-powder-in-tube method. Multifilamentary and monofilamentary tapes were sintered with intermediate uniaxial cold pressing. The critical current ( I c ) of the tapes was measured at each step of the sintering, before the pressing. Measurements of I c were performed with the four-probe dc method, with the 1 μV/cm criterion. The samples were submerged in liquid nitrogen and a magnetic field H applied perpendicular to the tape plane. The critical current through the strong links in zero field ( I c0 s ) was distinguished from the total I c using the field dependence of I c . I c0 s was used as a measure of the population of strong links in the samples. It was found that strong links are mainly formed in the late sintering stages. I c 0 s I c 0 increases with sintering time, showing that grain alignment and phase purity are more important for the formation of strong than weak links. Therefore, the preparation of the samples requiring a large I c0 s has to involve large sintering times. Mechanical deformation after Bi-2223 was fully formed is detrimental for the formation of strong links. An increase of I c0 s by a factor 2.6 was obtained with prolonged sintering if the samples are not pressed in the late sintering stages.

Bi-2223 bar current leads fabricated by the combination of cold isostatic pressing and hot-pressing

Abstract A new method using a combination of cold isostatic pressing (CIP) and hot-pressing (HP) was applied to fabricate Bi-2223 bar current leads. The critical current density (Jc) achieved by this method reached as high as 1000 A/cm2 at 77 K and self-generated magnetic field. This value of Jc presented here is much higher than the best Jc reported for rod current leads which is 570 A/cm2 achieved by CIP technique [Y. Yamada, Bi-based bulk current leads and their applications, in: H. Meada, K. Togano (Eds.), Bismuth-based High-temperature Superconductors, 1996, p. 277.]. The phases and microstructures were analysed by XRD and SEM. The texture and weak link were studied by pole figures and AC susceptibility, respectively. The results show that the grain connectivity, matrix density and texture of the samples were improved significantly by this method.