The angular dependence of critical current densities in bulk magnetically melt textured YBaCuO in magnetic fields up to 20 teslas

Bulk magnetically melt textured YBaCuO at 77 K shows critical current densities of up to 19000 A/cm² in 20 teslas when the field is parallel to the(a, b) planes. The angular dependence ofJc is shown to support a recent interpretation of the high superconducting transport currents measured above the irreversibility line in terms of intrinsic surface superconductivity. The behavior when the field is applied very close to the(a, b) planes is shown to follow a two dimensional model meaning that the maximum current density is determined by thec-axis component of the applied field. The size and density of 211 inclusions are probably the main factors which determines the maximum critical current density.     

The influence of neutron irradiation on (B0.65C0.35)Ba1.4Sr0.6Ca2Cu3Oz superconducting phase: The role of the grain edge

Using the transport and magnetization measurements, the influence of neutron irradiation at a fluence of 5 × 10¹⁷ n cm⁻² on (B_0.65C_0.35)Ba_1.4Sr_0.6Ca₂Cu₃O _z has been investigated. The neutron irradiation was found to decrease critical temperature and transport critical current density, increase the residual and normal state resistivity, and improve the intragranular critical current density with 1.6 × 10⁵ A/cm² (at 77.3 K and in the applied field up to 160 kA m) and ΔM _irr/ΔM _nonirr ratio (up to factor of 3) at highest field used for investigation. The field dependence of this ratio, which is below the unity at very low field but higher than 1 at high fields, correlated with the shape of the hysteretic loops as well as with the change of the transport parameters after irradiation suggests the role of the irradiation-induced effects on the grain edges. We discuss these effects in the framework of the Bean-Livingstone surface barriers and geometrical barriers.     

The Influence of Neutron Irradiation on (B0.65C0.35)Ba1.4Sr0.6Ca2Cu3Oz Superconducting Phase: The Role of the Grain Edge

Using the transport and magnetization measurements, the influence of neutron irradiation at a fluence of 5 × 10¹⁷ n cm⁻² on (B_0.65C_0.35)Ba_1.4Sr_0.6Ca₂Cu₃O _z has been investigated. The neutron irradiation was found to decrease critical temperature and transport critical current density, increase the residual and normal state resistivity, and improve the intragranular critical current density with 1.6 × 10⁵ A/cm² (at 77.3 K and in the applied field up to 160 kA m) and ΔM _irr/ΔM _nonirr ratio (up to factor of 3) at highest field used for investigation. The field dependence of this ratio, which is below the unity at very low field but higher than 1 at high fields, correlated with the shape of the hysteretic loops as well as with the change of the transport parameters after irradiation suggests the role of the irradiation-induced effects on the grain edges. We discuss these effects in the framework of the Bean-Livingstone surface barriers and geometrical barriers.     

On the influence of magnetic field processing on the texture,phase assemblage and properties of low aspect ratio Bi2Sr2CaCu2Ox/AgMg wire

Abstract

Bi₂ Sr₂ CaCu₂ O_x/AgMg conductors are potentially important for many applications up to 20 K, including magnets for cryogen-free magnetic resonance imaging and high field nuclear magnetic resonance research. One promising approach to increased critical current density is partial-melt processing in the presence of a magnetic field which has been shown to enhance c-axis texturing of wide, thin tape conductors. Here, we report on low aspect ratio rectangular conductors processed in an 8 T magnetic field. The magnetic field is applied during different stages of the heat treatment process. The conductors are electrically characterized using four-point critical current measurements as a function of magnetic field and magnetic field orientation relative to the conductor. The superconductive transition and magnetization hysteresis are measured using a SQUID magnetometer. The microstructures are characterized using scanning electron microscopy and energy dispersive spectroscopy and analyzed using digital image processing. It is found that the presence of a magnetic field during split melt processing enhances the electrical transport and magnetic behavior, but that the anisotropy is not consistently affected. The magnetic field also affects development of interfilamentary Bi2212 bridges, and that this depends on the initial shape of the Bi2212 filament. At least two behaviors are identified; one impacts the oxide phase assemblage and the other impacts textured growth.     

Effects of Lu Doping on the Magnetic Behavior of YBCO Superconductors Prepared by MPMG Method

We report on magnetic behavior of Y _1?xLu _xBa ₂ Ca ₃ O _7?δ (x= 0, 0.5) samples synthesized by a modified melt powder melt growth (MPMG) method at three different sintering temperatures (920, 930, 940 ^°C). Isothermal magnetization measurements were done as a function of magnetic field at 20 and 77 K upon zero-field cooling (ZFC) process. The critical current densities as a function of magnetic field have been estimated from M–H hysteresis loop. Partial Lu substitution for Y on YBCO superconductors improved significantly the bulk critical current density, which is confirmed by field-cooled magnetization and AC susceptibility measurements. Magnetic relaxation measurement has been carried out in remanent magnetization regime. AC susceptibility measurements have been performed as a function of temperature to determine the transition temperature.     

The effect of LiOH addition to the superconducting properties of YBa2Cu3O7-x

The effect of LiOH addition to YBa₂Cu₃O_7-x is presented. The crystal structure remains orthorhombic, bat the orthorhombic splitting decreases from 0.0171 at 2 at.% Li to 0.0158 at 50 at. / Li. The critical temperature exhibits a decrease from 91.18 to 83.79 K in the same range of lithium concentrations. From the magnetization curves were obtained the lower critical fields and the intragranular critical current density. The latter exhibits a negative power law,j _c∞ B^-α with α≈ 1/2.V-I characteristics are typical for grained superconducting materials:Vα(I-I_c)_n(B,t). The transport critical current dependence on temperature and magnetic field is also presented.     

On the influence of magnetic field processing on the texture,phase assemblage and properties of low aspect ratio Bi2Sr2CaCu2Ox/AgMg wire

Bi₂Sr₂CaCu₂O_x/AgMg conductors are potentially important for many applications up to 20 K, including magnets for cryogen-free magnetic resonance imaging and high field nuclear magnetic resonance research. One promising approach to increased critical current density is partial-melt processing in the presence of a magnetic field which has been shown to enhance c-axis texturing of wide, thin tape conductors. Here, we report on low aspect ratio rectangular conductors processed in an 8 T magnetic field. The magnetic field is applied during different stages of the heat treatment process. The conductors are electrically characterized using four-point critical current measurements as a function of magnetic field and magnetic field orientation relative to the conductor. The superconductive transition and magnetization hysteresis are measured using a SQUID magnetometer. The microstructures are characterized using scanning electron microscopy and energy dispersive spectroscopy and analyzed using digital image processing. It is found that the presence of a magnetic field during split melt processing enhances the electrical transport and magnetic behavior, but that the anisotropy is not consistently affected. The magnetic field also affects development of interfilamentary Bi2212 bridges, and that this depends on the initial shape of the Bi2212 filament. At least two behaviors are identified; one impacts the oxide phase assemblage and the other impacts textured growth.     

Magnetic Field Angular Dependence of Magnetization Loss in ReBCO Superconducting Tapes

In the applications of high-temperature superconductors (HTS), the HTS tapes are usually exposed to the external magnetic field with different orientations. The critical current and AC loss are affected by both the field amplitude and field angles due to the anisotropy of HTS tapes. In this work, we first introduce the experimental system to measure the magnetization loss in HTS tapes based on the calibration-free method. Then, we present the magnetization loss results in 4.8-mm-wide AMSC wire, 4-mm-wide SuperPower wire, 4-mm-wide SuNam wire, and 10-mm-wide Fujikura wire in a perpendicular applied field at 77 K. The field amplitude is up to 100 mT, and the frequency varies from 44.2 to 87.1 Hz. We also present the magnetization loss in AMSC wire, SuperPower wire, SuNam wire, and Fujikura wire at different field angles. The field angle varies from 10 to 90 ^° in 10 ^° steps. The loss reduction with the decreasing of the field angle shows the anisotropic property of HTS tapes. We finally plot the measured magnetization loss of the samples as a function of the magnetic field amplitude normalized by the field angle to verify an empirical formula.     

Investigations on the Magnetization Switching Dynamics in Spin Valve Multilayers Under Periodic Applied Magnetic Field

Magnetization switching dynamics in a spin valve nanopillar, induced by spin transfer torque in the presence of a periodic applied field is investigated by solving the Landau–Lifshitz–Gilbert–Slonczewski equation. Under steady state conditions, the switching of magnetization occurs in the system, above a threshold current density value J _c. A general expression for the critical current density is derived and it is shown that this further reduces when there is magnetic interface anisotropy present in the free layer of the spin valve. We also investigated the chaotic behavior of the free layer magnetization vector in a periodically varying applied magnetic field, in the presence of a constant DC magnetic field and spin current. Further, it is found that in the presence of a nonzero interfacial anisotropy, chaotic behavior is observed even at much smaller values of the spin current and DC applied field.     

Voltage Relaxation and Its Influence on Critical Current Measurements

Based on the collective creep model, we numerically studied evolution of electric field and current density in superconductors and its influence on transport measurements of critical current. It is shown that many experimental facts, such as the dependence of V-I curves on sweeping rate of applied current and voltage relaxation are the results of this evolution. The simulation results are confirmed by electric transport measurements on Ag-sheathed Bi_2–x Pb _x Sr₂Ca₂Cu₃O _y tapes. Discussions on influences of the voltage relaxation on electric transport measurements including superconducting critical current are made.     

Superconducting Performance of Ex-situ SiC Doped MgB2 Monofilamentary Tapes

The superconducting performance of the ex-situ SiC doped MgB₂ monofilamentary tapes are reported. Polycrystalline powders of MgB₂ doped with 5 and 10 wt% SiC were synthesized by a conventional solid-state reaction route and characterized for their superconducting performances. It was found that the superconducting parameters viz., upper critical field (H _c2), irreversibility field (H _irr) and critical current density (J _c) were improved significantly with SiC addition. It was also found that relatively lower synthesis temperature resulted in further improved superconducting parameters in comparison to higher synthesis temperature. Thus, synthesized powders are used for the fabrication of ex-situ powder-in-tube (PIT) monofilamentary tapes. The superconducting performance in terms of critical current density (J _c), being determined from both magnetization (J _cm) and transport (J _ct) measurements, was improved significantly. In particular, the SiC doped MgB₂ tapes (fabricated using 700 °C heat treated bulk powder) exhibited the transport J _ct of nearly 10⁴ A/cm² under applied fields of as high as 7 Tesla. Further, it was found that the J _ct anisotropy decreases significantly for SiC doped tapes. Disorder due to substitution of C at B site being created from broken SiC and the presence of nano SiC respectively in SiC added ex-situ MgB₂ tapes was responsible for decreased anisotropy and improved J _c(H) performance.     

Transport ac loss of a superconducting cylinder with field dependent critical current density

The transport ac loss per cycle per unit length of a hard superconducting cylinder is calculated from the critical state model assuming a Kim-type and an exponential field dependent critical current density. Without such dependence, the results are consistent with Norris’ equations for an ellipse bar, in which the critical current density is assumed not to depend on the flux density. Based on Norris’ equations, the expressions of the loss are derived for a finite length cylinder. It is shown that the field dependence decreases and increases the loss at low and high ac currents, respectively, and the effects of the parameter p on the loss are related to the magnetization process. Compared to Norris’ prediction, the results for the Kim and exponential model show the same trend with respect to the external transport current.     

The Influence of BaZrO3 on the Magnetic Response of (Bi,Pb) : 2223 High-Temperature Superconductors

DC magnetization and AC complex susceptibility measurements on (Bi,Pb) : 2223 high-temperature superconductors impurified with various amounts of BaZrO₃ are presented. The results are discussed in the frame of the critical state model, and the values of the inter- and intragranular critical current density as well as of the field for full penetration are estimated. The values of the intergranular critical current density are consistent with those obtained from transport measurements. The intragranular critical current density and the field for full penetration have similar values from both DC magnetization and AC susceptibility measurements. It was shown that, in the (Bi,Pb) : 2223 system, BaZrO₃ impurification changes only the properties of the intergrain matrix, while the superconducting properties of the grains are not modified.     

“Peak effect” in the magnetic field dependence of the transport critical-current density in high-T c superconducting ceramics

The maximum observed in the magnetic field dependence of the transport critical-current density of high-T _c superconducting ceramics was analyzed. Transport critical current and magnetization measurements performed on Bi(Pb)-Sr-Ca-Cu-O bulk sintered samples allowed us to conclude that such a maximum may result from the influence of demagnetization effects at low applied field values and an increase of the intergrannular pinning at higher fields.     

Magnetic field dependence of shielding current density in Y-Ba-Cu-O rings at 77 K

A method for contactless measurement of the shielding critical current density and its dependence on the external magnetic field is described and analyzed. The obtained values are compared with those measured resistively on two different samples. It is shown that the shielding critical current densityJ _cs and the intergranular transport current densityJ _cr are identical if the measurement conditions are similar. A degradation ofJ _cs measured in the external field with AC ripple has been observed.     

High TC Superconductors Give New Impulse to Magnetic Levitation

When the new YBaCuO superconductors were developed about 18 years ago, the critical maximum current density, at a magnetic field of 1.0 T and a temperature of 77 K, was still quite low. Roughly 8 years ago, it became possible to raise the critical current density to 10⁶ A/cm² at 1.0 T and 77 K. This opened up possibilities for application in self‐stabilizing superconducting magnetic bearings and also in maglev trains capable of movement along a track. An important criterion in the high current‐carrying capacity of YBaCuO superconductors is the excellent texture of melt‐textured YBaCuO moldings, which must also be free of microcracks. Narrow distributions of the a‐, b‐, and c‐axis orientations, in the absence of isotropic fractions, allow almost the entire mirror flux in the superconductor to be induced by permanent magnets.     

Improved microstructural features in screen printed NdBCO thick films on YSZ substrates

An investigation of the effect of using Ba-rich precursors, Nd_0.9Ba_2.1Cu₃O _y + 15 wt% Nd_3.6Ba_2.4Cu_1.8O_z, on the melt growth, microstructure and superconducting properties of NdBCO screen-printed thick films on YSZ substrates has been carried out. The presence of unreacted powder (BaCuO₂) in the precursor increased the amount of liquid and resulted in the formation of a large grain size. However, the addition of Nd₄₂₂ was effective in stabilising the liquid viscosity at the processing temperature and reducing the film/substrate interaction. As a consequence, high transition temperatures and sharp superconducting transitions were obtained. It has been found that the cooling rate affects the texture orientation, grain size and quality in addition to the interaction between the film and the substrate. The magnetic flux dynamics have been investigated using the Magneto-Optical (MO) technique at various low temperatures and different external applied magnetic fields. The magnetization critical current density was 4.1 × 10⁴ A/cm² at 5 K and zero field.     

Studies on magnetization and microwave properties of gamma-ray-irradiated Sm-Ba-Cu-O superconductors

Studies on the effect of gamma-ray irradiation on critical transition temperature, transport and magnetic critical current density (at 4.2 K), high-field magnetization (at 4.2 K), and microwave-induced d.c. voltage (inverse a.c. Josephson effect) have been performed on SmBa₂Cu₃O_7– ceramic superconducting samples prepared by the coprecipitation technique. Gamma-ray irradiation of the samples was carried out using a⁶⁰Co source of 10³ Ci strength for several hundreds of hours; the dose received by the samples was 80 K rad/h.-Ray irradiation was found to have no effect on its structural modification and on the critical transition temperature. However, transport and magnetic critical current density are increased. Irradiation also caused a significant increase in the high-field magnetic hysteresis, which is presumably connected with the creation of radiation-induced mobile defects. An appreciable decrease in the microwave-induced d.c. voltage at 77 K was also observed after irradiation, which suggests that the mobile defects are clustered at the major defect region and thus reduce the total number of weak links. Enhancement of transport and magnetic critical current density may be due to the stronger pinning of flux lines at the-ray-induced defect site in SBCO ceramic superconductors.     

Upper Critical Magnetic Field for a Modified Maltifiamental Nb3A1Wire

We measured the upper critical field Μ₀H_c2 for a modified Nb_3A.l sample above 10 K and evaluated the values using a different criterion. The values from theI ^c measurement, converting from the ac susceptibility and direct judging from the transition point on the magnetization curve, were close and larger than that extrapolated from the magnetization basedon the theory of Kramer and the critical state model.