Transport Critical Current Research Articles

Distribution of the transport critical current in Ag-(Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ reinforced tape-form conductors

The spatial distribution of the transport critical current, I/sub c/, in the reinforced-tape-form, silver-clad (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ conductor manufactured by cold multistep rolling has been established by the in-situ measurements as a function of the thickness of the ceramic core. The measurements show that current is almost uniformly distributed through the whole cross-section of the core in the wire, almost independently of the initial I/sub c/ value of the tape. The effect of densification, texturing, and surface diffusion on the formation of the intergrain connectivity along the a-b plane of the core, at the silver-ceramic interface, and in the core has been investigated and is discussed in detail. The I/sub c/ anisotropy measurements and a study of magnetically aligned and randomly oriented samples lead to a functional relationship between the crystallographic orientation, morphology and connectivity of the grains in the tape. The variation of the hole concentration near the grain boundary region of the 2223 phase has been investigated by transmission electron energy loss spectroscopy.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Critical current measurements on Ag/Bi-Pb-Sr-Ca-Cu-O composite coils as a function of temperature and external magnetic field

Transport critical currents have been measured on two coils of high-temperature superconducting (HTSC) tape as a function of temperature and external magnetic field. The sample coil windings have inside and outside diameters of roughly 25 mm and 40 mm, respectively, and a length of 50 mm. They contain about 300 turns of filamentary Bi-Pb-Sr-Ca-Cu-O 2223 HTSC material sheathed in Ag to form a 0.18-mm by 2.54-mm tape, with a total length of about 30 m. Critical current results are reported for temperatures between 4.2 K and 90 K, in magnetic fields ranging up to 5.5 T. >

Transport critical currents and grain boundary peak links in bulk YBa2Cu3Ox

Abstract We summarize our previous studies on transport critical current density and present transport and electron microscopy data that provide significant physical insights into grain boundary weak-links and transport properties. By changing the sintering parameters and liquid processing methods, the transport Jc in the plateau regime is considerably increased. We conclude that transport Jc characteristics under a magnetic field are determined mainly by the portions of strongly coupled regions at grain boundaries. Thus, the transport Jc may not necessarily be connected with the gross orientation of the grain boundaries. The magnitude of the transport Jc under a magnetic field is proportional to the total area of the strongly coupled regions at the grain boundaries.

Transport critical currents, flux dynamics and a.c. magnetization of YBa 2Cu 3O 7- δ and Bi 2Sr 2CaCu 2O 8 single crystal rings

The hysteretic magnetic properties of ring-shaped single crystals of YBa 2Cu 3O 7−δ and Bi 2Sr 2CaCu 2O 8 have been investigated using a VSM for long time scale measurements and an electronic integration magnetometer for a.c. hysteresis measurements at frequencies from 10 Hz to 1 kHz. Measurements of the bulk critical current and its temperature, field and materials processing dependence can be made without the need for attaching leads or having to subdivide the crystal to investigate the scaling dependence of the magnetization on sample size. A strong frequency dependence of the magnetic hysteresis is observed, which is related to the non-linear V( I) characteristics of the ring. Such characteristics result in flux creep at low frequencies and strongly frequency dependent magnetic hysteresis at high frequencies and sweep amplitudes. Flux penetration into the single crystal ring samples is determined by an effective time constant involving the ring inductance and the non-linear V( I) characteristics. Computer models are used to determine the time dependence of flux penetration into a superconducting slab following a linear field ramp and into a superconducting ring in an a.c. field using illustrative E( J) characteristics. Computed results reproduce the qualitative features of our experimentally derived a.c. magnetization measurements.

Orientation and thermal cycling effects on the critical currents of high Tc superconducting composites

Transport critical currents of short lengths of high T c superconducting composite tapes have been measured as a function of magnetic field applied parallel to and perpendicular to the tape surface. Anisotropy factors have been calculated from the results on a number of samples, differing only in their final annealing temperatures, and show great similarity for samples whose critical current densities vary widely. We explain these results in terms of the different microstructures of the samples. We discuss n values and present some data on Ag magnetoresistance. Mechanisms leading to a degradation in J c as a result of thermal cycling are described along with possible solutions.

TEM study of epitaxial YBa 2Cu 3O 7−x thin films deposited on GaAs with MgO buffer layers

Epitaxial, c-oriented YBa 2Cu 3O 7− x (YBCO) thin films yielding large critical transport currents ( j c > 1.2×10 6 A/cm 2 at 77 K) were deposited on GaAs using MgO buffer layers. The MgO was deposited by electron gun evaporation at a substrate temperature of 460°C whereas the YBa 2Cu 3O 7− x was deposited at 620°C by reactive thermal coevaporation. The low deposition temperatures which can be applied for the thermal co-evaporation deposition to obtain high-quality YBCO thin films, are particularly important for the deposition on GaAs substrates. High-resolution transmission electron microscopy was used to image the various interfaces and to investigate the quality of the deposited films. In spite of a significant mismatch between MgO and GaAs ( a MgO=0.4213 nm and a GaAs=0.565 nm) the MgO film grows in single orientation with a 〉100〈 MgO∥〉100〈 GaAs orientational relationship. the GaAsMgO interface was very smooth and had a roughness of 1–2 nm. The MgO buffer layer was significantly strained and contained planar defects parallel to the (100) plane. The MgOYBa 2Cu 3O 7− x interface was surprisingly smooth and planar. In the c-oriented film, stacking faults, dislocations and a-oriented areas a few nm in size were found.

Hysteretic time dependence of the critical intergrain transport current in sintered YBa 2Cu 3O 7

In our experiments we have found that the intergrain critical transport current I c in sintered, non-oriented high- T c superconductors shows a hysteretic time relaxation behavior. As a function of prior magnetic field cycling the I c-relaxation rate ∂ c/∂ t be positive or negative with different magnitudes and can also be made to vanish. Not only I c but the whole E-I-characteristics is shifting with time. We explain these results qualitatively in the frame of a magnetic field superposition model at the grain boundaries, with our earlier measurements on the relaxation of the intragrain magnetization. This could also be of importance for superconducting electronic applications.

Yttrium oxide inclusions in YBa 2Cu 3O x thin films : Enhanced flux pinning and relation to copper oxide surface particles

Semi-coherent Y 2O 3 (yttria) inclusions are identified in high quality epitaxial YBa 2Cu 3O x (YBCO) thin films by using transmission electron microscopy and energy dispersive X-ray analyses. Magnetization measurements show enhanced flux pinning in films containing an increasing number density of inclusions. The inclusions are incorporated into the films without disturbing the YBCO structure appreciably. The number density is ≈ 10 17 cm -3 in films having transport critical currents of 4x10 6 A cm -2 at 77 K. Nucleation of the yttria inclusions is suggested to occur spontaneously during film growth. The abundance of yttria inclusions is connected to the occurrence of large Cu-rich surface particles in films that have optimum superconducting properties.

Resistance changes in ion-irradiated Bi-Pb-Sr-Ca-Cu-O (BPSCCO) superconductors

In situ measurements of the resistance changes in BPSCCO bulk superconductor were carried out using 100 MeV oxygen ion irradiation at room temperature with fluence in the range 1.5*1012 to 3.5*1015 ions cm-2. The irradiation caused degradation of the zero-resistance temperature (Tc0) and increase in the room-temperature resistance (R) at fluence exceeding 3*1012 ions cm-2 up to a maximum of 3.5*1015 ions cm-2. At fluence less than 3*1012 ions cm-2, however, resistance decreased without changing Tc0, indicating improvement in the transport critical current. It is shown that the presence of grain boundaries plays a major role in these phenomena, which are due to mobile irradiation-induced defects. A possible mechanism for bringing about the observed changes is discussed.

Transport critical currents and flux pinning in melt-grown YBa 2Cu 3O 7−δ

The critical current and flux pinning properties of melt grown YBa 2Cu 3O 7−δ bulk material were investigated by transport measurements and V-I characteristics. A J c of 16 000 A cm −2 in zero field and 3000 A cm −2 in 1 T at 77 K was obtained using a criterion of 3.3 μV cm −1. No features of granularity could be detected as J c showed no hysteresis with increasing or decreasing field. By considering a distribution of activation energy U 0 together with the correlation volume of flux bundles V c and the pinning range r p, the measured V-I characteristics in the flux creep regime were found to be consistent with that of thermally activated flux flow (TAFF). The temperature and field dependence of the derived V c r p points to a collective pinning mechanism by point defects. The depinning current J c, one to tow orders of magnitude higher than the transport J c, was also obtained by using the U 0 and V c r p deduced from the V-I characteristics.

Electromagnetic granularity, critical current density, and low- Tc phase formation at the grain boundaries in (Bi,Pb) 2Sr 2Ca 2Cu 3O χ silver-sheathed tapes

Transport critical currents are correlated with DC magnetization, AC susceptibility and high resolution electron microscopy in progressively heat treated Ag-clad (Bi,Pb) 2Sr 2Ca 2Cu 3O χ (2223) tapes. Both the zero field cooled magnetization shielding curves and the real component of the AC susceptibilities show two diamagnetic transitions. One transition is found to occur at 109 K and is the signature of the 2223 phase. A second, much larger transition occurs over a range of temperatures between 75 and 105 K. The transport critical current densities correlate directly with the lower magnetic transition temperature. High resolution electron microscopy reveals a few layers of Bi 2Sr 2CaCu 2O χ (2212) phase clustered at the numerous (001) twist grain boundaries. These residual layers require a long reaction for their elimination and directly control the critical current while they are present.

Formation of layered microstructure in the Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O superconductors

The layered grain microstructure is essential for overcoming the weak link problem and ensuring high transport critical currents in the cuprate superconductors. In this paper we discuss the processing and the mechanisms for layer information in Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O. In melt-processed Y-Ba-Cu-O, sympathetic nucleation on previously nucleated YBa 2Cu 3O 7−δ plates during solidification appears to be dominant mechanism for the formation of parallel plate-shaped grains. In the Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O ribbons, the interface reaction between the superconductor layer and the silvers substrate seems to be the main mechanism for the c-axis texturing of the layered grains. The drastically different critical current behavior in the c-axis textured Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O ribbons is discussed in terms of possible differences in the nature of the twist and tilt grain boundaries.

New 100 K TlSr2(Ca,Cr)Cu2O7 superconducting films

New TlSr2(Ca,Cr)Cu2O7 superconducting films on MgO(100) substrates have been prepared and characterized. The superconducting films were fabricated via a two-step process. Precursor SrCaCrCuO films were first deposited by spray pyrolysis at temperature of 350–400 °C, and thallium was then incorporated by heating the precursor films between unfired TlSrCaCrCuO bulk pellets at 880–900 °C in an oxygen atmosphere. The films were then cooled down to room temperature in an oxygen or argon atmosphere. The superconducting films were single 1212 phase with c-axis perpendicular to the surface of the MgO(100) substrates. Onset temperatures up to 110 K and zero-resistance temperatures up to 100 K were achieved. The critical transport currents (Jc) of the films were about 7×103 A/cm2 at 77 K.

Microstructural analysis of high critical current density Ag-clad BiSrCaCuO (2:2:1:2) tapes

The critical current density ( J c) values of variously processed Ag-clad Bi 2Sr 2Ca 1Cu 2O x tapes are related to their microstructures as observed by TEM, SEM, and X-ray pole figure analysis. The coupled microstructure- J c results confirm the general importance of a highly c-axis aligned microstructure for high critical current density. However, TEM experiments revealed a “colony” microstructure in all sample in which groups of several grains with nearly perfect c-axis alignment (colonies) are formed but where the c-axes of adjacent colonies are not parallel. SEM imaging of etched cross-sections showed that important features of this colony misalignment are not accurately reflected by stand alone X-ray pole-figure analysis or TEM imaging. Near-perfect alignment occurs within a few microns of the Ag cladding for all processing methods. However, the processing details control the macroscopic grain alignment within the bulk of the superconductor. In particular, second-phase particles disrupt the alignment of macroscopic colonies. These misalignments may control the transport critical current, yet they are not clearly evidenced by either macroscopic pole-figure analysis or by very local TEM examination.

CRITICAL TRANSPORT CURRENT IN HARD SUPERCONDUCTORS IN ROTATING MAGNETIC FIELD

The problem of critical transport current in hard superconductors in a rotating magnetic field is considered on the basis of the double critical state model for the macroscopic and local approach. The obtained relations predict an angular dependence of the transport current for both the flux flow and flux cutting zone. The influence of an accurate distribution of the magnetic induction module and its phase angle on the critical transport current is investigated. Suggestions concerning the application of the model to high T c materials are included.

Crystal chemistry of the texture formation in superconductive YBa2Cu3O7−δ ceramics

Classical sintering of YBa2Cu3O7−δ always leads to poor critical transport currents whatever the powder process and the thermal cycle are. Best results are obtained by synthesis of textured ceramics. Fabrication of YBa2Cu3O7−δ with well-oriented grain structure by the melt textured growth process was performed in order to understand the mechanism that controls the texture formation. The evolution of the microstructure was studied by quenching samples at different stages of the MTG cycle. The importance in the texture formation process of the presintering temperature, of the high temperature plateau, of the thermal gradient and its fluctuations, of the presence of second phase inclusions (essentially 211 phase), and of the substrate and its reactivity with Y–Ba–Cu–O is shown.

Transport critical currents in field-oriented Y1Ba2Cu3Oy polycrystals

Field-orientedY1Ba2Cu3Oy samples with various values of the transport critical current density, Jc, have been prepared with various heat treatments. The Jc increased rapidly with an increase in relative sintered density of the oriented samples. The field-oriented samples with higher Jc had grains of larger sizes and better connections between the neighboring grains than the low Jc samples. The maximum values of Jc at a magnetic field of 7 T were 370 A/cm2 at 4.2 K and 24 A/cm2 at 77 K. The hysteresis in the Jc-H curves at 4.2 K decreased with an increase in Jc both for an applied field H∥ab plane and for H∥c axis. The critical currents of the samples with higher Jc were less degraded by aging in the air for 1 year than those of the low Jc samples. The relation between the field-cooled Jc-H curves and the zero-field-cooled ones for an oriented sample with low Jc is qualitatively interpreted by the effect of the intragrain currents on the intergrain fields.

Transport critical currents in granular high temperature superconductors

The transport critical current I c of more than 100 samples of high temperature superconductor bulk material was measured as a function of temperature using a contactless magnetic induction method. All samples (mainly YBa 2Cu 3O 7−δ, but also Tl-Ba-Ca-Cu-O and (Bi, Pb)-Sr-Ca-Cu-O) show a temperature dependence of I c which can be described by I c( T) = I c, 0(1 − T/ T c) m with m ≈ 0.9 for O < T < 0.9 T c. For temperatures closer to T c we find evidence for m ≈ 1.5. Systematic investigations of a number of YBa 2Cu 3O 7−δ samples with different cross-sectional area ( A = 0.15 … 2.71 × 1.8 mm 2) prepared under identical conditions show that I c is proportional to A only for small cross-sectional areas. Furthermore, we find a maximum of I c as a function of starting material powder size and a weak dependence on the uniaxial pressure applied for sample preparation.

Critical transport currents and microstructure in textured and untextured (Bi,Pb)2Sr2Ca2Cu3Ox wires

The powder-in-tube method has been used to produce Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3Ox wires. Their mechanical density is reduced considerably with sintering time, because of the anisotropic growth of the Bi high-Tc phase. The wires were doped with silver and textured by melting and rolling to improve the mechanical density and thereby the critical current density. The superconducting properties, the phase growth and the microstructure were investigated by rho (T,B) jc(B,T), DTA, as well as XRD, SEM and EDX measurements. Whereas Ag doping and melt texturing results in a decomposition of the high-Tc phase, mechanically textured samples prepared by cold rolling exhibit an increased critical current density up to a factor 10 in zero magnetic field, with an improved dependence of the critical current density on magnetic fields. From a plot of ln rho vs. T-1 activation energies, Uo, for thermally activated flux creep could be estimated to lie between 200 and 20 meV for magnetic fields from 0.005 up to 10 T. A power law of Uo approximately B- alpha with alpha =0.277(30) was found.

Critical current density in Ag-sheathed wires of YBaCuO

Transport critical currents in Ag-sheathed wires of YBCO have been studied. At zero field the characteristic weak-link properties were masked by the self-field of the transport current. From a scaling law with respect to temperature and diameter the self-field effects could be separated, allowing the weak-link parameters to be determined. The observed improvement of the weak-link properties with decreasing wire diameter is due to the reduction of the junction length and the enhancement of the number of junctions. For a wire of 0.25 mm diameter a maximum Josephson current density of 2.7 × 10 4 A cm −2 at 4.2 K has been obtained.