Critical current in Ag/BPSCCO tapes using low purity materials

Silver-clad Bi_1·7Pb_0·4Sr_1·8Ca₂Cu_3·5O _x (BPSCCO) tapes have been fabricated using low purity (98–99%) starting materials and following the powder-in-tube technique. MaximumJ _c values of 6·14 × 10³ A·cm⁻² at 77 K and 1·4 × 10⁵ A·cm⁻² at 4·2 K have been obtained in tapes subjected to the process of intermediate rolling and sintering. The bulk superconducting material used for the tape-fabrication contains both 2223 and 2212 phases in the ratio 60:40. A pure phase material and the optimization of the sintering parameters are expected to yield much higherJ _c values at 77 K. It is possible that the copper-rich phase(s) and/or a small amount of iron impurity (60 ppm) present in CuO might be acting as flux pinning sites and could be responsible for highJ _c values.     

Transport properties of long monofilamentary Bi(2223) tapes

Summary Monofilamentary Bi(2223) tapes withJ _c(77 K, 0 tesla) up to 30 000 A/cm² have been prepared by cold rolling using the powder-in-tube method. An optimization of the precursor powders has led to a higher phase purity after the reaction heat treatment. The deformation process has been optimized in order to increase the oxide density and to reduce sausaging effects on the oxide thickness. The transport properties of these tapes have been studied in a wide range of temperature (4.2K-T _c) and magnetic fields (up to 28 tesla). The critical-current values at 77 K fields of 0.5 T and 1 T parallel to the tape surface are 10 000 A/cm² and 5400 A/cm², respectively. At 4.2 K theJ _c value decreases from 1.6·10⁵ A/cm² at 0 T to 6·10⁴ A/cm² at 15 T. At fields higher than 15 tesla a very low field dependence ofJ _c has been found, regardless of the tape orientation. Transport properties have also been studied by cutting small sections of the tape in order to investigate the local critical-current distribution. It has been found that, even in rolled tapes of good quality (J _c (77 K, 0 T)>20000 A/cm²), theJ _c distribution is homogeneous: the critical current density increases gradually from the centre of the tape to the sides, the latter exhibiting much higherJ _c (46000 A/cm²) than in the centre (18000 A/cm²). Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

Further studies on Ag/BPSCCO tapes using low purity materials

Transmission electron microscopic (TEM) studies are reported on Ag-clad Bi_1.7 Pb_0.4Sr_1.8Ca₂Cu_3.5O _x tapes prepared by using low purity (98–99%) commercial grade materials. The self-fieldJ _c values of these tapes viz. 6.14 × 10³ A.cm⁻² at 77 K and 1.4 × 10⁵ A.cm⁻² at 4.2 K, reported in an earlier publication, were significantly higher than the correspondingJ _c values in tapes prepared with high purity (99.99%) materials. The TEM pictures on the low purity core material of the tapes reveal the presence of stacking faults and the intergrowth of the 2212 and 2223 phases which could be acting as flux pinning sites and responsible for enhancedJ _c values. These defects can perhaps be traced back to the presence of 60 ppm iron in the low purity CuO as revealed by atomic absorption analysis reported earlier.     

Electrical and magnetic properties of hot extruded Bi-2223/Ag concentric tapes

Summary In this work we report on the anisotropic physical properties of silver-sheathed Bi-2223 tapes fabricated by means of hot extrusion and repeated pressing and sintering processes. The obtained Bi-2223/Ag short tapes, having critical current densitiesJ _c of 20–30 kA/cm² at 77 K, 0 T, were measured in external magnetic fields up to 0.5T applied in two different orientations (i.e. μ₀H‖(a,b)-planes and μ₀H ⊥(a,b)-planes). The magnetic characterizations were performed in a wide range of temperatures and magnetic fields to study the first magnetization curve of tapes evaluating the lower critical fields μ₀H_c1⊥ab and ⊥₀H_c1#x2016;ab and their dependences on temperature. TheJ _c values at different fields in the temperature range 4.6–90 K, calculated from the magnetization data by the critical state model, are also presented. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

The transition width and critical current density measurements on slow-cooled YBa2Cu3O7−x superconductor

It is shown that by adopting a very gradual programmed cooling procedure in oxygen environment one can obtain a sharp transition (ΔT _c⋍1 K) in YBa₂Cu₃O_7−x while retaining the high-T _c value (⋍105 K) of samples prepared by a semi-wet route. This is attributed to a maximum occupancy of oxygen at 0 1/2 0 sites and a near-perfect ordering of vacancies at 1/2 0 0 sites in the orthorhombic unit cell which maximizes the availability of conduction paths in the form of continuous CuO₄ chains. Critical current densities (J _c) of 204 A cm⁻² are obtained for bulk samples at 77 K. It is suggested that the intergrain coupling is weak and thus limits the transportJ _c-values.     

Critical current densities of high pressure oxygen sputtered thin films of YBa2Cu3O7−x by non-resonant rf absorption method

The critical current densities (J _c) have been measured at 77K in high pressure oxygen sputtered thin films of YBa₂Cu₃O_7−x superconductor using the non-resonant rf absorption technique. High values ofJ _c (∼ 10⁵ A/cm²) are observed in these relatively large area (∼ 1·2 cm²) films.     

Critical current in silver clad Y-Ba-Cu-O superconducting wires

Silver clad wires of highT _c superconductor Y₁Ba₂Cu₃O_7−x have been fabricated through the powder metallurgy technique. The reacted wires show a midpointT _c of 84K. A critical current density of 26·4 A cm⁻² (77K, 0T) is obtained in these wires. The wires, however, turn complete normal only at a current density of 280 A cm⁻². The reasons for low critical current density obtained in these wires are discussed.     

MgB2 coated superconducting tapes with high critical current densities fabricated by hybrid physical-chemical vapor deposition

The MgB₂ coated superconducting tapes have been fabricated on textured Cu (0 0 1) and polycrystalline Hastelloy tapes using coated conductor technique, which has been developed for the second generation high temperature superconducting wires. The MgB₂/Cu tapes were fabricated over a wide temperature range of 460-520 °C by using hybrid physical-chemical vapor deposition (HPCVD) technique. The tapes exhibited the critical temperatures (T_c) ranging between 36 and 38 K with superconducting transition width (ΔT_c) of about 0.3-0.6 K. The highest critical current density (J_c) of 1.34 × 10⁵ A/cm² at 5 K under 3 T is obtained for the MgB₂/Cu tape grown at 460 °C. To further improve the flux pinning property of MgB₂ tapes, SiC is coated as an impurity layer on the Cu tape. In contrast to pure MgB₂/Cu tapes, the MgB₂ on SiC-coated Cu tapes exhibited opposite trend in the dependence of J_c with growth temperature. The improved flux pinning by the additional defects created by SiC-impurity layer along with the MgB₂ grain boundaries lead to strong improvement in J_c for the MgB₂/SiC/Cu tapes. The MgB₂/Hastelloy superconducting tapes fabricated at a temperature of 520 °C showed the critical temperatures ranging between 38.5 and 39.6 K. We obtained much higher J_c values over the wide field range for MgB₂/Hastelloy tapes than the previously reported data on other metallic substrates, such as Cu, SS, and Nb. The J_c values of J_c(20 K, 0 T) ∼5.8 × 10⁶ A/cm² and J_c(20 K, 1.5 T) ∼2.4 × 10⁵ A/cm² is obtained for the 2-μm-thick MgB₂/Hastelloy tape. This paper will review the merits of coated conductor approach along with the HPCVD technique to fabricate MgB₂ conductors with high T_c and J_c values which are useful for large scale applications.     

Synthesis and superconducting properties of Nd0.33Eu0.08Gd0.58Ba2Cu3O z materials

We have studied the effect of the ratio of different rare-earth element on the superconducting properties and phase formation of (Nd_0.33Eu_0.08Gd_0.58)Ba₂Cu₃O _z ceramic. Bulk NEG samples were prepared using the solid-state reaction process. The superconducting transition for Nd_0.33Eu_0.08Gd_0.58Ba₂Cu₃O _z is T _c ≈90 K and the value of the critical current density (J _c ) is 13.9 A/cm² at 77 K under zero magnetic fields. This value is twice as high when compared with the (J _c ) value of YBCO systems (J _c = 7.31 A/cm²). The obtained bulk sample was used for the production of superconducting Ag-sheathed tapes by OPIT method including hot rolling. The critical current density of the obtained tape (337 A/cm²) is one order higher than the one of the bulk sample.      

Electrical and magnetic characterization of hot-extruded BSCCO tape samples

Summary Ag/BSCCO-2223 phase tapes were prepared with a new plastic deformation cycle. Starting from the hot extrusion of composite billets, hundreds of metres of tape 0.15×3 mm were obtained by drawing and rolling the extruded bars. The cross-section, monofilamentary with a central Ag insert, is quite regular along all the length. Samples from 30 cm to 100 cm in length were heat-treated at different temperatures between 830°C and 480°C in Ar+10% O₂ atmosphere. Electrical-transport properties were tested at 77K.J _c of 3000 A/cm² were usually achieved after first heat treatment. The superconducting properties were also tested by magnetic measurements using a vibrating sample magnetometer. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

MgB2 coated conductors directly grown on flexible metallic Hastelloy tapes by hybrid physical–chemical vapor deposition

MgB₂ coated conductors (CCs), which can avoid the low packing density problem of powder-in-tube (PIT) processed wires, can be a realistic solution for practical engineering applications. Here we report on the superior superconducting properties of MgB₂ CCs grown directly on the flexible metallic Hastelloy tapes without any buffer layer at various deposition temperatures from 520 to 600 °C by using hybrid physical–chemical vapor deposition (HPCVD) technique. The superconducting transition temperatures (T_c) are in the range of 38.5–39.4 K, comparable to bulk samples and high quality thin films. Clear (101) and (002) reflection peaks of MgB₂ are observed in the X-ray diffraction patterns without any indication of chemical reaction between MgB₂ and Hastelloy tapes. From scanning electron microscopy, it was found that connection between MgB₂ grains and voids strongly depend on the growth temperature. A systematic increase in the flux pinning force density and thereby the critical current density with decreasing growth temperature was observed for the MgB₂ CCs. The critical current density (J_c) of J_c(5 K, 0 T) ～10⁷ A/cm² and J_c(5 K, 2.5 T) ～10⁵ A/cm² has been obtained for the sample fabricated at a low growth temperature of 520 °C. The enhanced J_c (H) behavior can be understood on the basis of the variation in the microstructure of MgB₂ CCs with growth temperature.     

Effect of mechanical deformation on the critical current in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">b</Emphasis></Subscript> superconducting films

It is shown that local mechanical bending of YBa₂Cu₃O_7−b Ag-coated superconducting films deposited on flexible metal substrates in the temperature interval 77–300 K may increase the critical density J _c of the transport current to values as high as 10⁶ A/cm² or even higher at 77 K. Also, bending decreases the voltage criterion. This means a rise in the intergranular conductivity and, accordingly, a reduction of Joule losses.     

Effect of processing temperature on the superconducting properties of acetone doped MgB2 tapes

Correlation of phase formation, critical transition temperature T_c, microstructure, and critical current density J_c with sintering temperature has been studied for acetone doped MgB₂/Fe tapes. Sintering was performed at 600–850 °C for 1 h in a flowing Ar atmosphere. High boron substitution by carbon was obtained with increasing the sintering temperature; however, the acetone doped samples synthesized at 800 °C contain large size MgB₂ grains and more MgO impurities. Incomplete reaction for the acetone doped samples heated at 600 °C result in bad intergrain connectivity. At 4.2 K, the best J_c value was achieved in the acetone doped sample sintered at 700 °C, which reached 24,000 A/cm² at 10 T and 10,000 A/cm² at 12 T, respectively. Our results indicate that the small grain size and less impurity were also important for the improvement of J_c–B properties besides the substitutions of B by C.     

Transport critical-current and voltage-current characteristics of Bi(2223) silver-clamped thick films

The transport critical current properties of Bi(2223) silver-clamped thick films are studied by the measurement of its dependence on magnetic field and temperature close to T _c. It is found that the transport critical current follows a power law J _c(1–T/T _c)^3/2 for the sample with J _c>2.0×10⁴ A/cm² (77K, zero field) and that J _c(H) is basically reversible for increasing and decreasing magnetic field. After the transport current exceeds the critical current, the voltage-current (V-I) characteristics show a flux-creep-like behaviour until they smoothly join the flux-flow state. From the measurement of V-I curves, the pinning property of the sample may be estimated using the flux-creep mode. The flux-flow resistance is found to have a nonlinear magnetic field dependence.     

YBa2Cu3O7−δ /CeO2 heterostructures on sapphire R-plane

(001)YBa₂Cu₃O_7−δ epitaxial films were prepared by laser ablation on surface. A thin (001)CeO₂ or (111)CeO₂ buffer layer was deposited between the substrate and the superconductor film to reduce their chemical interaction. In the initial stages of CeO₂ buffer formation, its orientation depended strongly on the intensity of cerium ion interaction with oxygen. Epitaxial growth of (001)YBa₂Cu₃O_7−δ films was achieved both on and . The T _c temperature of epitaxial (001)YBa₂Cu₃O_7−δ films was within 88–90 K, and the current J _c at 77 K was in excess of 10⁶ A/cm⁻². Fiz. Tverd. Tela (St. Petersburg) 40, 205–208 (February 1998)     

Large transport critical currents of powder-in-tube Sr0.6K0.4Fe2As2/Ag superconducting wires and tapes

We report the achievement of transport critical currents in Sr_0.6K_0.4Fe₂As₂ wires and tapes with a T_c = 34 K. The wires and tapes were fabricated through an in situ powder-in-tube process. Silver was used as a chemical addition as well as a sheath material. All the wire and tape samples have shown the ability to transport superconducting current. Critical current density J_c was enhanced upon silver addition, and at 4.2 K, a largest J_c of ～1200 A/cm² (I_c = 9 A) was achieved for 20% silver added tapes, which is the highest in iron-based wires and tapes so far. The J_c is almost field independent between 1 T and 10 T, exhibiting a strong vortex pinning. Such a high transport critical current density is attributed to the weak reaction between the silver sheath and the superconducting core, as well as an improved connectivity between grains. We also identify a weak-link behavior from the apparent drop of J_c at low fields and a hysteretic phenomenon. Finally, we found that compared to Fe, Ta and Nb tubes, Ag was the best sheath material for the fabrication of high-performance 122 type pnictide wires and tapes.     

Properties of Nb3Ge tapes for high field magnets

A laboratory process for long Nb₃Ge tapes fabrication by chemical vapor deposition (CVD) has been set up. The Nb₃Ge tapes which were fabricated offer the possibility of high current and high field operation at 4.2 K since the values of critical current densities, J_c, measured in high magnetic fields at 20T and 4.2K exceed 5 × 10⁴ A cm^?2 which is the generally accepted criterion for producing a superconducting magnet.     

Modeling a dimer state in CuGeO3 in the two-dimensional anisotropic Heisenberg model with alternated exchange interaction

The two-dimensional Heisenberg spin-1/2 model with alternated exchange interaction along the c axis and an anisotropic distribution of the exchange interaction in the lattice, J _b/J _c=0.1, is examined. A quantum Monte Carlo method is used to calculate the phase diagrams of the antiferromagnet, the dimer state in a plane, the value of the alternation δ of the exchange interaction, and the anisotropy Δ=1−J ^xy/J ^z of the exchange interaction, Δ∼δ ^0.58(6). The following characteristics are calculated for Δ=0.25: the dependence of the temperature of the dimer-state-paramagnet transition on the alternation of the exchange interaction, T _c(δ)=0.55(4)(δ−0.082(6))^0.50(3), the singlet-triplet energy gap, and the dependence of the magnetization on the external field for some values of δ. The value of the exchange interaction, J _c=127 K, the alternation of the exchange interaction, δ=0.11J _c, and the correlation radius along the c axis, ξ _c≈28c, are determined. Finally, it is found that the temperature dependence of the susceptibility and the specific heat are in good agreement with the experimental data. Zh. éksp. Teor. Fiz. 112, 2184–2197 (December 1997)     

High current densities at 20T in long (20m) tapes of CVD-prepared Nb3Ge

Critical current densities J_c as a function of magnetic field have been measured on tapes of chemically vapor deposited (CVD) Nb₃Ge that are up to 20m long. Values of J_c exceed 5 × 10⁴ A/cm² at 20T. Such high critical current densities are encouraging for use of this material in producing a 20T superconducting magnet.