Critical Current Decreases Research Articles

The effect of magnetic field orientation on the critical current of HTS conductor and coils

The critical current of short samples of HTS multifilamentary conductor and ring-shaped coils has been measured at helium temperatures with varying magnetic field orientation with respect to the conductor. The samples and coil conductor consist of a multifilamentary composite of BSCCO-2223 filaments in a silver matrix. Short conductor samples were tested in a variable temperature system with up to 8 T background field using a sample rotational system. Ring-shaped coils made from the sample type of conductor were exposed to a large background field at liquid helium temperatures and critical current was measured with the ring located at various axial positions within the bore. As the ring moves closer to the end of the magnet, the measured critical current decreases, even though the magnitude of the field to which the ring is exposed decreases. This decrease in J/sub c/ is due to the strong anisotropy of the superconductor and is consistent with short sample measurements.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Irreversible properties of micrometer-thick, superconducting MoGe/Ge multilayers as a function of anisotropy.

Micrometer-thick, superconducting MoGe/Ge multilayers were prepared with various anisotropies. As the anisotropy increases, the irreversibility line shifts to lower fields and temperatures, and the critical current decreases and becomes more strongly field and temperature dependent. The data experimentally demonstrate that the anisotropy is a major factor in determining irreversible properties of high-temperature superconductors.

Localization effects on the critical current of a superconductor-normal-metal-superconductor junction.

Interaction effects associated with Anderson localization are confirmed for the critical current in a superconductor---normal-metal---superconductor junction using $p$-type InAs as the normal metal. In the weak localization regime and with decreasing temperature the critical current shows a saturated temperature dependence which agrees well with a theoretical prediction. In the strong localization regime the critical current decreases with lowering temperature.

Magnetic field dependence of the critical current of a layered superconductor

We have developed a theory of the effect of a pinned ordered lattice of anisotropic vortices on the critical current J c of a layered superconductor. The external field H ext is taken to be such that vortices lie in the plane of the layers. In this case J c is determined by the interlayer Josephson coupling and depends on the distribution of the Josephson phase between adjacent superconducting layers. In view of the anisotropic field penetration we find that the usual Fraunhofer oscillations are absent, and that the critical current decreases as 1− H ext/H 0 , with the magnetic field scale H 0 determined by the anisotropy of the material. Our results provide a possible explanation for some of the interesting transport properties recently observed in high- T c cuprates.

(001) grain boundaries and critical currents

Magnetization measurements and high resolution electron microscopy (HREM) have been carried out on the series Y1-xCaxBa2-xLaxCu3O7- delta to determine the correlation between critical current and (001) grain boundaries. Substituting Ca for Y reduces the fraction of (001) grain boundaries and increases the fraction of grain boundaries other than (001). The critical current increases linearly with the fraction of clean grain boundaries other than the (001) boundaries. It is concluded that the (001) grain boundaries in YBa2Cu3O7 act as weak links. It is argued that the atomic structure of the interface is responsible. In addition it is observed that the intragrain critical current decreases with increase of x.

Multifilamentary Bi-2223 composite tapes made by a metallic precursor route

A process based on metallic precursors has been developed for manufacturing high filament count oxide superconductor-silver composite tapes with critical current densities of up to 7.5 kA/cm/sup 2/ at 77 K in zero field. A 30-cm prototype multistrand conductor made of these tapes has a critical current of 240 A at 77 K over a 9 cm gauge length, with an average critical current density of 6 kA/cm/sup 2/. The mechanical properties of tapes made from metallic precursors containing up to 10000 Bi-2223 superconducting oxide filaments were investigated. Critical tensile strains average 0.6%, and bend tests show negligible dropoff in current density up to a 0.70% surface strain. The critical current decrease beyond the 0.70% surface bend strain follows a simple model based on extensive filament damage beyond the critical tensile strain. Increased flow stresses of the composite tapes, compared to similarly processed silver, indicate considerable strengthening of the composite by the oxide filaments. >

Orientational disorder and critical current enhancement in high-Tc, superconductors

Abstract The influence of fast neutron irradiation, with fluences 1.5 to 3.7 × 1017n/cm2, on the structural and magnetic properties of superconducting 90 K, 70 K and 60 K phases of sintered YBaCuO has been investigated. Extensive X-ray diffraction (XRD) analyses, using both (⊖-2⊖), and rocking curve techniques, have been used to investigate the nature of various types of disorder as a consequence of neutron irradiation. The findings from the X-ray data are found to be consistent with the dc magnetization data, especially in regard to the increase of the intragranular critical current and decrease of the transport critical current. A major infuence of irradiation appears to be a significant change of the oxygen position in the Ba-plane

Magnetic properties of oxygen-depleted YBa2Cu3O7-y single crystals.

Magnetic properties and flux pinning of oxygen-deficient ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{y}}$ single crystals have been studied. The superconducting transition temperature and inductive critical current density decrease as the level of oxygen depletion increases. No obvious enhancement in the pinning strength was found in the oxygen-depleted ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{y}}$ system for the applied field along the c axis. We attribute this absence of enhanced pinning to the mismatch of the oxygen vacancy size with the superconducting coherence length over the entire superconducting regime.

High-Tc substrate step junctions operated at 77 K

The authors have developed a simple technique which permits fabrication of Josephson junctions in high-Tc epitaxial thin films. The junctions demonstrate both DC and AC Josephson effects in LN temperatures. The critical current decreases linearly with external microwave current, and is proportional to (Tc-T)32/.

Ar Ion Implantation Induced Tc Degradation of YBa2Cu3O7-x Thin Films

The Ar ion implantation induced changes of structure and transport properties of YBa2Cu3O7-x thin films were studied as a function of fluence. Both the zero resistance temperature Tc and the critical current density Jc decrease rapidly with fluence, however, the transition temperature onset almost remains constant at low fluences. At dose of 1.2 × 1013 Ar/cm2, the sample has a metal to semiconductor phase transition. When the fluence further increases, a crystalline to amorphous phase transition occurs, and the sample becomes insulator.

Flux creep limited current in YBa 2 Cu 3 O 7-† ceramics under applied fields

Abstract We present measurements of the field dependence of critical currents in YBa 2 Cu 2 O 7-† ceramics prepared by the sintering and by the sol-gel reaction methods. For high quality samples we find a region in magnetic fields where the transport critical current decreases like H −n with n=0.52±0.06. This behavior is qualitatively similar to that observed in single grains, interpreted as indicative of flux creep rather than Josephson limited critical current.

Dynamical properties of superconducting arrays.

We calculate the current-voltage characteristics of arrays of resistively shunted Josephson junctions (RSJ's), with and without a transverse magnetic field. In zero field, finite-temperature effects are included via an effective temperature-dependent current noise in each shunt resistance. In an ordered array at zero field, the calculated correlation length and order-parameter susceptibility are consistent with a Kosterlitz-Thouless transition, and the zero-temperature time-dependent voltage V(t) above the critical current resembles that of a single RSJ. For finite transverse magnetic fields at T=0 the calculated critical currents depend on sample size, and differ somewhat from those predicted by purely static calculations. At fields smaller than about one flux quantum per lattice, the critical current decreases quadratically with field. We find no evidence for vanishing critical current in an irrational field, at least for the small finite samples we consider. When the applied current is larger than its critical value, there are regions of both periodic and aperiodic V(t), with discrete and quasicontinuous power spectra, respectively. Under some conditions, we also see hysteretic behavior in the IV characteristics. When the applied field is very small, V(t) sometimes shows a regular sequence of spikes of equal amplitude, interrupted occasionally by aperiodic bursts.

Nb/sub 3/Sn critical-current measurements using tubular fiberglass-epoxy mandrels

A systematic study of the effect of sample mounting techniques on the superconducting critical-current measurement was made. A seemingly small change in mandrel geometry can result in a 40% change in the measured critical current of a Nb/sub 3/Sn sample at 12 T. This is a result of a change in the conductor prestrain at 4 K caused by variation in thermal contraction between thick-and thin-walled fiberglass-epoxy composite (G-10) tubes. An approximate measure of the vibrations in thermal contraction (from room to liquid-nitrogen temperature) indicate a 0.2% greater contraction for the thick-walled tube. This difference, combined with strain sensitivity measurements, is consistent with the observed decrease in critical current.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effects of irradiation — fast neutrons and implantation on sintered Y [sbnd]Ba [sbnd]Cu [sbnd]O superconductors

Effects of irradiation damage by fast neutrons, and nitrogen implantation on magnetic and electrical properties of Y Ba Cu O materials have been studied. The samples were subjected to a fluence of 2 × 10 17 and 1 × 10 17 neutrons / cm 2. In the implantation studies dosages of 10 15 and 10 16ions/ cm 2 of N + were used. Here we will report on the detailed changes in the magnetic hysteresis loop due to irradiation. It is found that the transport inter-grain critical current decreases with irradiation, while the critical current inside the grains is enhanced.

Nb metallurgical transformations occurring in the microbridge area of an rf SQUID during its critical current adjustment

We describe the metallurgical transformations of Nb thin-film Dayem microbridges related to the adjustment of their critical current at operating temperature (4.2 K). The critical current is reduced to a few tens of microamperes when the microbridge is submitted to proper current pulses. Subsequent Transmission Electron Microscopy (TEM) observations show an increase of the mean Nb grain size in the microbridge area. Nb contamination by the silicon substrate is also observed, Both critical temperature and critical current decreases are attributed to these pnenomena.

Mechanism of Dezincification and the Inhibitive Efficiency of some Azoles for the Corrosion of 70/30 Brass in Sulphuric Acid

A study has been made of the mechanism of dezincijication of α-brass and the effect of azoles as inhibitors for the corrosion of 70/30 brass in sulphuric acid using the weight loss method, solution analysis and electrochemical techniques. The inhibitive efficiency of azoles for the corrosion of α-brass has been found to decrease in the order: 2-mercaptobenzothiazole > benzotriazole > benzimidazole. The critical current density and corrosion current decrease in the same order as the increase in inhibitive efficiency of the compounds. Potentiostatic polarisation studies have shown that both cathode and anode are polarised indicating a mixed type of inhibition.

Influence of chemical composition and temperature on the passivation of some stainless steels and nickel-based alloys in sulphuric acid

The critical current densities for passivation of a number of industrial stainless steels and nickel-based alloys have been measured in 20% H2SO4 at temperatures from 30 to 103° C (boiling). The critical current decreases — and then the aptitude for passivation increases — as the chromium and molybdenum contents of the alloys are increased; molybdenum was about three times as effective as chromium. Increasing the temperature extends the range of active dissolution of the alloys and markedly increases the critical current density for passivation. The effect is least severe for the alloys of high Cr and Mo content, but even for these, spontaneous passivation does not take place above 50° C. The logarithm of the active current density varies linearly with the reciprocal of absolute temperature over limited ranges. Apparent activation energies have been measured and proposals for the rate-determining factors in the mechanisms are outlined on the basis of these measurements.

Mechanisms for critical-current degradation in NbTi and Nb&amp;lt;inf&amp;gt;3&amp;lt;/inf&amp;gt;Sn multifilamentary wires

Critical currents of NbTi and Nb 3 Sn multifilamentary wires have been studied in magnetic fields to 9 T as a function of mechanical load applied at 4 K. Degradation of the critical current in NbTi is limited to about 30% with the effect becoming large only at strains above ∼ 1%. The change in critical current with strain is much larger in Nb 3 Sn, commencing at strains of 0.1 to 0.3%. For both superconductors, the first 20 to 30% decrease in critical current is almost totally reversible. A number of possible explanations of the observed degradation are considered, including filament breakage, heat generation by mechanical creep, degradation of the stabilizing matrix, and defect formation in the superconductor itself. Results of experiments to test the source of degradation are reported. Evidence for microcrack damage in the Nb 3 Sn reaction layer has been found and it is suggested that defect size variations on the order of a coherence length (∼ 5 nm) can account for the reversibility of the degradation as well as low-strain enhancement effects.

Influence of low temperature neutron irradiation on the superconducting properties of V&amp;lt;inf&amp;gt;3&amp;lt;/inf&amp;gt;Ga multifilamentary wires

The influence of low temperature neutron irradiation on multifilamentary wires of V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Ga has been investigated. The critical field H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C2</inf> was measured as a function of temperature in the range of fields 0 to 8 Teslas before, after irradiation and after annealing at room temperature ; so was the critical current I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</inf> (H,T) on the temperature range 4.2 K to T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</inf> . The low field part of the curve H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C2</inf> (T) measured in all cases allowed the determination of the Maki parameter and an extrapolation of this curve was done using the Werthamer formula. A decrease of critical current was found, amounting to about 30 % at 4.2 K for the largest irradition dose, 3×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">18</sup> n/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The samples were sensitive to room temperature annealing. A plot of the pinning force versus reduced field shows that the samples do not obey a scaling law, except near T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</inf> .

The effect of shock deformation on the anodic polarization behavior of polycrystalline nickel in 1N sulphuric acid

The results of anodic potentiostatic polarization of shocked nickel polycrystals in 1N H 2SO 4 are reported. With increasing shock pressure, passivation and Flade potentials shift in the cathodic direction, the critical current density decreases, and the passive current density decreases. These effects are discussed in terms of substructural changes accompanying shock loading.