Improvement Of Critical Current Density Research Articles

Multi-step process to prepare bulk BSCCO (2223) superconductor with improved transport properties

BSCCO (2223) superconducting powders were prepared by three different techniques (solid state, pyrolysis and sol–gel) and with three different stoichiometries, especially based on variations of Ca/Sr ratio and Cu concentration. Pressureless sintering was performed on cold uniaxially pressed (1 GPa) samples, avoiding the drawbacks connected to hot-pressing; different hot-forging cycles were then applied to attain higher density and orientation factor. It was found that the process phenomenology is strictly linked to the powder starting stoichiometry: when composition is very near to the theoretical (2223), effects of secondary phase extrusion are observed during hot-forging, yielding a purification and inhibition of (2212) formation accompanied by an appreciable increase of J c respect to hot-pressed samples. When a Ca/Sr ratio is considerably >1, recrystallisation of (2223) from the liquid takes place, with a remarkable improvement of critical current density ( J c>10 4 A/cm 2). In this case the non-superconducting secondary phases act as intrinsic oxygen reservoir. Finally, when composition diverges even more from (2223) (Ca/Sr≫1 and Cu excess), the formation of too many precipitates of non-superconducting phases hinders the texturing process and more in general deteriorates the intergranular properties.

Improvement in critical current density under magnetic field in heavily Pb-doped Bi2Sr2CaCu2Ox superconducting tape

A heavily Pb-doped Bi2Sr2CaCu2Ox (Bi-2212) tape has been prepared using an isothermal partial melting method. Width of magnetic hysteresis (ΔM) normalized by ΔM under 0 Oe decreases with increasing magnetic field (H∥c axis) at 20–60 K; this occurs more rapidly in pure Bi-2212 tape (Pb-free tape) than in Pb-doped tape. Irreversibility field under the H∥c axis is elevated by Pb doping at 20–60 K. Moreover, a decrease in transport critical current density (Jc) with increasing H∥c axis is reduced by Pb doping at 20–60 K. In particular, a peak is observed at 2.0 kOe in Jc–H curves at 20 and 30 K. Hole concentration in the Pb-doped tape seems to be close to the optimum, as magnetic superconducting transition starts at 91 K. Therefore, the above results are due to production of new pinning centers induced by applying the magnetic field. It is clear that heavy Pb doping is effective for Bi-2212 polycrystalline tape in producing efficient pinning centers for the H∥c axis as reported in Bi-2212 single crystals.

Improvement of critical current density in YBaCuFO compounds by addition of platinum

Platinum added YBaCuFO compounds with nominal composition YBa 2Cu 3F 0.4O xPt y { y=0, 0.3, 0.5} were prepared by a partial melt process performed at lower temperature than the conventional melt process. Optical micrograph observations showed that Y 2BaCuO 5 (211) particles increase in density and are dispersed finely when increasing the doping of Pt up to 0.3 wt%. For the 0.5 wt% Pt sample, however, the size of the 211 particles became larger than in the samples containing less than 0.3 wt% Pt. The magnetization was measured using a SQUID magnetometer in the temperature range 20–77 K under magnetic fields B e up to 1 T. The magnetization M also increased with increasing quantity of platinum up to 0.3 wt%. The J c values estimated from the width of the magnetization curves were 9.8×10 7 A/m 2 for the 0.3 wt% Pt sample and 5.4×107 A/m 2 for the undoped sample at 77 K and B e=0.8 T. In addition, the whereabouts of the platinum in the quenched samples was investigated by using electron spectroscopy for chemical analysis (ESCA).

Improvement of critical current density and microstructure of Tl-1223 tapes by a two-powder method

Superconducting tapes of (Tl,R)(Ba,Sr)/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ (R=Bi or Pb) were prepared using a mixture of pre-reacted single phase powder and unreacted precursor of identical or different stoichiometry. The effects of mechanical densification and sintering conditions on critical current (I/sub c/) and microstructure of the tapes were investigated. Compared to the tapes made with entirety unreacted powder or pre-reacted powder, the tapes made with a mixture of the two showed a much improved microstructure, an improved self-field I/sub c/ at 77K, as well as a smaller dependence on sintering conditions and better reproducibility of I/sub c/.

Improvement of critical current density and residual resistivity on jelly-roll processed Nb/sub 3/Al superconducting wires

The residual resistivity of copper stabilizer (/spl rho/), is deteriorated by the diffusion of chromium plated on the strand surface during heating of Nb/sub 3/Al strands. To improve this degradation, the copper stabilizer has been located on the center of the cross-section of the strand rather than near the strand surface. As a result, /spl rho/ is improved to one-sixth of previous value achieving 1.6/spl times/10/sup -10/ /spl Omega/m in a laboratory scale strand. The critical current density (J/sub c/) of this strand was 715 A/mm/sup 2/ at 12 T. This technique was applied to prototype strands to be used in a Nb/sub 3/Al Insert Coil developed in ITER R&D programme resulting in a /spl rho/ of 1.5/spl times/10/sup -10/ /spl Omega/m, which meets ITER specification. In addition, the improvement of J/sub c/ was also studied by two-stage reaction including a continuous heating procedure, which resulted in a J/sub c/ of 1130 A/mm/sup 2/ at 12 T and good longitudinal uniformity.

Precipitation from Gd/sub 1+x/Ba/sub 2-x/Cu/sub 3/O/sub 7/ superconductor in low oxygen partial pressure

Precipitation behavior of second phases from Gd/sub 1+x/Ba/sub 2-x/Cu/sub 3/O/sub 7/ solid solution was studied by processing in low oxygen partial pressure. With the decrease of solid solubility in the low oxygen partial pressure, Gd/sub 2/BaCuO/sub 5/ and CuO were precipitated from the solid solution as second phases. This resulted in an increase in the superconducting transition temperature up to that of GdBa/sub 2/Cu/sub 3/O/sub 7/ and an enhancement in the critical current density. The amount and size of the second phases can be controlled by the oxygen partial pressure, processing temperature and the starting composition, thus providing a useful method to introduce flux pinning centers for the improvement of critical current density.

Enhancement of critical current density and strain tolerance in Ag-sheathed BSCCO (2223) tapes by continuous silver reinforcement

We have used a modified powder-in-tube technique in various configurations to demonstrate significant improvement in the transport properties and strain tolerance of Ag-sheathed BSCCO superconducting tapes. In this process, the BSCCO core has been reinforced with a silver rod placed concentrically within the silver tube and then processed into thick tapes by established metal-working techniques. A noticeable improvement in critical current density, , was observed as a result of this modification. This increase is attributed to increased texturing, decrease in thickness of BSCCO layers, increase in the length of the Ag/BSCCO interface, and decrease in the volume fraction of the superconducting core which is less effective in contributing towards the overall critical current of the tape. Further improvements have been made to this configuration by incorporating another ring of superconductor into the core. The improved configurations led to a significant increase in over that of monolithic BSCCO tapes. Critical current densities of at 77 K and self-field have been consistently achieved in thick tapes after 150 h of sintering. Analysis of critical current through various sections of the tape indicates that 80% of the critical current is carried by the centre section of the core within which the Ag rod is located. The bend strain tolerance of the tapes containing a silver core was found to be significantly better than that of the monolithic tapes. At an applied surface bend strain of 1.0%, about 98% of initial was retained in Ag-reinforced tapes, compared with only 54% in monolithic tapes.

Conceptual design of an electrical machine with both low and high T/sub c/ superconductors

The emergence of high T/sub c/ superconducting (HTS) wires with large critical current densities allows the design and construction of many applications, for example, rotating electrical machines. Nevertheless, the current "state of the art" of Bi "powder in tube" wires, the most advanced HTS conductor, is not adequate for an ac operation. We thus propose a fully superconducting synchronous machine with an NbTi stationary 4 K armature and a rotating Bi-2223 field winding at 20 or 40 K. Thanks to the armature cryogenic environment, the cooling of the Bi coils is very simple. The conceptual design of a typical machine is suggested and several electromagnetic designs are presented. The first one deals with a Bi field winding adapted to an existing NbTi armature of a tenth of kYA. Afterwards, large torque motors are theoretically designed comparing the high (at 20 and 40 K) and low T/sub c/ field winding solutions. The results are comparable for NbTi and Bi at 20 K. But with a Bi field winding operating at 40 K, the performances are degraded. Otherwise, at 4 or 20 K fully superconducting motors show high performances in terms of weight, volume, and efficiency. Nevertheless, the Bi field windings require a large amount of material indicating that improvements in critical current density are needed.

Preparation of composite particles of bulk Ba2YCu3O7-X by electrostatic adhesion process

Electrostatic adhesion processes are used in preparing composite aerosol particles with the electrostatic adhesion force acting between electropositively-charged and elec-tronegatively-charged particles. This process enables one to disperse the core and subcomponent particles more uniformly and to control the amounts of subcomponent particles adhered to the surface of individual core particles by adjusting the charging voltage. Using the composite particles prepared by the electrostatic adhesion process as a starting powder for sintering a composite bulk body of the Ba 2 YCu 3 O 7-x oxide superconductor with BaZrO 3 inclusions is produced. BaZrO 3 particles with diameters less than 100 nm are uniformly dispersed in the Ba 2 YCu 3 O 7-x matrix. The composite bulk body shows improvement in critical current density, which is based on the uniform dispersion of fine BaZrO 3 particles as magnetic flux pinning centers. These advantages are attributed to the uniform dispersion of BaZrO 3 over the whole starting powder.

Improvement of critical current density in Eu x Y1?x Ba2Cu3O7?? and Ho y Y1?yBa2Cu3O7?? high temperature superconductors by hot pressing

Improvement of critical current density in Eu x Y1?x Ba2Cu3O7?? and Ho y Y1?yBa2Cu3O7?? high temperature superconductors by hot pressing

Critical current densities and magnetic hysteresis losses in submicron filament bronze-processed Nb/sub 3/Sn wires

Submicron-filament bronze-processed multifilamentary Nb/sub 3/Sn wires with Cu-5at%Sn-X (X: Mn, Ni, Si, Ge, B, P, Zn, Al) ternary alloy matrix and Nb-1at.%Ta alloy cores were fabricated. The cross-sectional structure of wires was the central-Cu-stabilizer type to facilitate, by external Sn diffusion, improvements in critical current density and hysteresis losses. The Nb-1Ta core decreased the hysteresis loss in spite of the increase of the critical current densities. By the addition of Zn, Al, the critical current densities and the hysteresis losses were improved. Although hysteresis losses were not decreased by Sn plating, the critical current densities were apparently improved at relatively low temperature heat treatment.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The evolution of tooling, techniques, and quality control for accelerator dipole magnet cables

The author summarize the results of the Superconducting Super Collider (SSC) R&D program from 1984 to 1990. The SSC R&D program has led to a significant improvement in critical current, piece lengths, and overall manufacturing capability for multifilamentary NbTi superconductors. The requirements for current density, filament size, dimensional control, long lengths, and low current degradation are much more stringent for the present accelerators compared with the earlier Tevatron and HERA accelerators. Also, to achieve higher field strengths with efficient use of the superconductor, the new designs require wider cables with more strands. These requirements have stimulated an active research effort which has led to significant improvements in critical current density and conductor manufacturing. In addition, they have stimulated the development of new cabling techniques, improved tooling, and better measurement techniques. The need to produce over 20 million meters of cable has led to the development of high speed cabling machines and online quality assurance measurements. These new developments are discussed, and areas still requiring improvement are identified. >

Influence of additive elements on the critical current density of Ag sheathed BPSCCO tapes

Abstract Ag sheathed BPSCCO superconducting tapes doped with SnO2, BaCO3 and MgO were prepared by ”powder-in-tube method”. Relation between microstructure change and critical current density has been investigated. Improvement of critical current density for MgO and BaCO3 doped tapes was observed, where fine MgO particles distributed in oxide matrix and dense textured structure were observed, respectively.

Intra- and intergrain critical current in PbMo/sub 6/S/sub 8/ sintered samples

To study the mechanisms limiting the critical current in the superconductor lead-molybdenum-sulfide, powders having a narrow distribution of T/sub c/ and B/sub c2/ have been synthesized. The absence of a broadening in the specific heat jump at the superconducting transition in a magnetic field indicates that the distribution of the critical field over the grains is very narrow. For all the samples, B/sub c2/ lies between 50 and 60 T (at T=0 K), whereas the critical current density displays a sharp decrease at a pseudocritical field which can be half of the calorimetric value of B/sub c2/. This reflects the poor quality of the grain interfaces, which limits the intergrain critical current. On a cold-pressed sample, it was possible to determine the intragrain critical current, which is estimated to be one order of magnitude higher than the best intergrain value. Therefore, an appreciable improvement in critical current density is expected if good superconducting links between grains can be achieved.

Effect of compressive reorientation on theT c andJ c of the superconducting composite Ag x Bi1.7Pb0.3Sr2Ca2Cu3O10

From the technological point of view, the difficulty of brittle mechanical properties and the low critical current density of c~b~per oxide superconductors must be overcome. To improve the mechanical properties, doping with silver has been found to give the most satisfactory results [1]. Improvement of critical current density can be achieved in several ways: by pinning the magnetic flux Well by creating defects [2], or by adding fine particles [3]; or by orienting the high-Jc directions of the microcrystals in the bulk sample in the direction of current flow by zone-melt processing [4], shock compaction [5]. Cold pressing [6] or vibrational alignment [7]. In the present work a series of studies were carried out to determine the optimal silver content in the composite AgxBi~.TPb0.3Sr2Ca2Cu3010, which does not decrease Tc too much but can improve the Jc and ductility of the bulk sample. The silver-ceramic composite was compressed in thez-direction while allowing it to expand freely in the xand y-directions. Both T~ and J~ were increased by adding silver and by compression. Pellets of composition BiL7Pbo3Sr2Ca2Cu3Olo, which was reported as the optimum composition for obtaining pure high-Tc phase in the B i P b S r C a C u O system [8], were prepared by thoroughly mixing Bi203, PbO, SrCO 3, CaCO 3 and CuO, calcining and sintering with intermediate regrinding several times as described previously [8]. The 110 K (onset) and 90-100 K (zero resistance) of the pellets as determined by four-point resistance measurements showed that they consisted of two phases, which is also consistent with the x-ray powder diffraction patterns and with the magnetization data obtained with a superconducting quantum induction device (SQUID). The Bil.TPb0.3Sr2Ca2Cu3010 pellets were ground to a fine powder and mixed with different stoichiometric amounts of Ag20 pressed at 50 MPa into 0.4 g pellets and heated for 30 min at 550 °C to convert all the Ag20 to metallic silver. The resultant pellets of composition Ag~Bil.7Pb0.3Sr2Ca2Cu3010 (x = 0, 4, 8, 12 and 16) were cut into narrow rectangular slices. Each selected small rectangular slice was compressed between the stainless surfaces of a large die at 1.5 GPa for 5 min. Then the irregular edge area was cut off and the slice was compressed again at 4 GPa for 5 min. The compressed as well as the uncompressed samples were annealed at 850 °C for 20 h in air to improve the coupling at the grain boundaries. The temperature dependence of resistance, R(T)/R(280), for a series of uncompressed and compressed samples of AgxBil.7Pb0.3Sr2Ca2Cu3010 ( x = 0 , 8 and 16; x = 4 and 12 not shown) is illustrated in Fig. la and b, respectively• It was found that increasing the silver content from x = 0 to x = 16 decreased the T c of the uncompressed

Improvement of critical current density in a YBa2Cu3Ox ceramic superconductor

Highly oriented YBa2Cu3Ox ceramic samples were prepared a 'mechanical aligning' processing technique which involved growth of large crystallites and vibrational alignment of the crystalline grains. In these samples, crystalline grains were found to be highly oriented and closely packed. A maximum critical current density of 4200 A cm-2 (77 K) was obtained. It was also found that the Jc characteristic was influenced by certain conditions in this process of sample preparation.

Improvement of critical current density in the Bi-Pb-Sr-Ca-Cu-O system through hot isostatic pressing

The effects of hot isostatic pressing (HIPing) on densification and the superconducting properties of the Bi-Pb-Sr-Ca-O system were investigated. A relative density up to 95% and a critical current density greater than 1100 A/cm 2 at 77 K were achieved through HIPing at 650°C for 2 h under 200 MPa argon. Under these conditions, the product gave a value of J c four times that without HIPing. T o was unaffected by HIPing for samples encapsulated with a combination of glass and silver tubes. However, T o was suppressed from 103 K to 86 K for samples encapsulated with stainless steel and silver tubes while J c increased from 260 A/ cm 2 to 1086 A/cm 2 during HIPing, indicating that the weak links were significantly improved. A new minor phase, having a composition of Bi/Pb/Sr/Ca/Cu=0.58/2.8/3.0/2.1/1.1, was observed in HIPed samples but its effect on J c is not clear.

Dependence of critical current density on microstructure in Ag sheathed Ba 2YCu 3O 6+x tapes

This paper considers the dependence of critical current density on microstructure in Ag sheathed Ba 2YCu 3O 6+x tapes. The critical current density increased with decreasing tape thickness when the Ag sheathed tapes were prepared by either pressing or cold rolling techniques. The improvement in critical current density was attributed to structural refinements such as textured grain structure and high increase in density. Pressing was found to be a more effective technique than cold rolling. The dependence of critical current density on tape thickness has been tentatively explained in terms of either surface pinning or grain boundary pinning.

Improvement in critical current density of Bi2Sr2Ca1Cu2Ox tapes synthesized by doctor-blade casting and melt growth

Bi2Sr2Ca1Cu2Ox superconducting tape with an excellent Jc at 4.2 K has been synthesized by a modified doctor-blade process. A green tape prepared by doctor-blade casting is laid on a silver foil and heat treated at various temperatures below and above its partial melting temperature. The Jc of the tape increases dramatically when heat treated in a narrow temperature range just above the melting temperature. A highly oriented layer structure with c-axis alignment is obtained by the growth of aligned Bi2Sr2Ca1Cu2Ox grains in the partially melted state. This oriented microstructure produces an exceptional increase in Jc . Typical Jc ’s of the tape at 4.2 K are 50 000 A/cm2 at 0 T, 22 000 A/cm2 at 10 T, and 17 000 A/cm2 at 23 T. Note the small magnetic field dependence in Jc above 10 T.

Fabrication of Bi-(Pb)-Sr-Ca-Cu-O mono- and multifilamentary superconductors and improvement in critical current density

The Bi-(Pb)-Sr-Ca-Cu-O superconductor has been fabricated into tapes without sheathing and into multifilamentary wires and tapes with Ag sheathing. Metallurgical studies as well as measurements have been made for them. Tape specimens prepared by combination and repetition of cold work (or cold press) and sintering showed critical current density Jc as high as 5×103 A/cm2 at 77 K with good reproducibility and reliability, although these specimens are sufficiently thick (0.5–1.0 mm). (Generally, Jc at zero field is inversely proportional to thickness.) For these tape specimens (with Cu sheath), sintering and Jc measurement were performed after the Cu sheath was removed. X-ray analysis showed that most of these tape specimens consist purely of high-Tc phase (∼107 K). In these specimens, the c axis tends to align well. This c axis alignment as well as compacting effect may be a cause of the enhancement in Jc . On the other hand, the tape specimens cold-worked and sintered only once (not repeated) showed Jc as small as only 102 A/cm2 with the c axis showing no sign of alignment. A 1330-filament Bi-Pb-Sr-Ca-Cu-O wire with a Ag sheath has been successfully fabricated. This multifilamentary wire shows critical temperature Tc of ∼105 K. A multifilamentary tape specimen fabricated by a combination of cold work and sintering showed Jc of 1.05×103 A/cm2 at 77 K.