J. D. Hettinger

Flux pinning in hot isostatically pressed Bi2Sr2CaCu2Ox

Magnetic hysteresis data were taken from 4.2 to 35 K on Bi2Sr2CaCu2Ox samples that were hot isostatically pressed at 105 MPa in an inert atmosphere at 825 °C. One set of samples was pressed for only 15 min while the other was pressed for 120 min. The samples pressed for 15 min contained a high density of dislocations and planar faults, while the samples pressed for 120 min contained fewer dislocations and faults, with most dislocations present within subgrain boundaries. The samples with the complex dislocation/planar fault structures exhibited substantially larger hysteresis loops, suggesting enhanced flux pinning.

Electrical transport properties of [001] tilt bicrystal grain boundaries in YBa2Cu3O7

The zero‐field electrical transport properties of 24° [001] tilt bicrystal grain boundaries in YBa2Cu3O7 were found to be in excellent agreement with the Ambegaokar–Halperin model over an extended range of currents and voltages. This model gives a firm basis for characterizing and comparing boundaries, and provides two independent measures of the critical current, which were proportional to (1−T/Tc)2 close to the transition temperature Tc.

Brick‐wall structure in polycrystalline TlBa2Ca2Cu3Ox thick films with high critical currents

Microstructural studies of TlBa2Ca2Cu3Ox(Tl‐1223) thick films that exhibit high critical current densities (Jc) for nonepitaxial polycrystalline materials show that these films possess a ‘‘brick‐wall’’ structure that may be partly responsible for high current densities. The magnetic field dependence of Jc is similar to that reported for Bi‐Sr‐Ca‐Cu‐O materials that exhibit this structure. Structural analyses indicate a high degree of c‐axis alignment but little in‐plane texture, suggesting that high‐angle [001] tilt boundaries are prevalent. Scanning and transmission electron microscopy reveal that the microstructure consists of overlapping layers reminiscent of the brick‐wall structure. These results suggest that high critical current densities may be achieved by percolative transport through this structure.

Effective electric field in dc magnetization measurements: Comparing magnetization to transport critical currents

We report values of critical current (Jc) for Tl2Ba2CaCu2Ox(Tl‐2212) epitaxial thin films in magnetic fields up to 5 T and at temperatures of 5, 40, and 77 K determined from dc magnetization and using a standard transport technique. Although we verify the applicability of Bean’s model [Rev. Mod. Phys. 36, 31 (1964)], these values only agree at small applied magnetic fields and at low temperatures. Our results indicate that the differences observed in Jc’s determined from the two techniques may be understood by considering only the field‐dependent, nonlinear E‐J characteristic and the ‘‘effective’’ electric‐field criterion used in determining Jc.

Properties of Tl-1223 superconducting thick films fabricated from precursor inks using a thallium vapor transport process

Using a two-zone thallium vapor transport furnace and a thick film ink process, we have successfully made superconducting films 10 to 50 /spl mu/m thick on a variety of substrates. Ba/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ and Bi/sub 0.22/Sr/sub 1.6/Ba/sub 0.4/Ca/sub 2/Cu/sub 3/O/sub x/ precursor films with and without Ag additions were made by mixing powders in an organic vehicle, painting a substrate and burning the vehicle off. Films were converted to the superconducting phase by passing an O/sub 2/ carrier gas over a Tl/sub 2/O/sub 3/ source and then the sample. TBCCO 1223 films generally form over a narrow sample temperature range near 860/spl deg/C, whereas TBSBCCO films form 1213, 1223 or a mixture of these phases depending on sample annealing temperature and O/sub 2/ partial pressure. TBCCO T/sub c/s average 104 K with zero-field J/sub c/ (77 K) /spl ap/3500 A/cm/sup 2/ while TBSBCCO T/sub c/s are higher at 110 K with J/sub c//spl ap/6800 A/cm2. Both compounds show weak-link behavior in a magnetic field.<<ETX>>

Pinning effect on critical dynamics in YBa2Cu3O7−δ films with inherent random disorder and with columnar defects

Abstract The in-plane longitudinal resistivities of YBa 2 Cu 3 O 7− δ (YBCO) films before and after Sn ion irradiation of a matching field 1 T have been measured as a function of magnetic field H (≤6 T) and temperature T . The extracted fluctuation part of the conductivity σ xx * ( T , H ) of the unirradiated sample with inherent random disorder exhibits 3D- XY scaling behavior that reveals dynamic critical exponent z =1.86±0.1 and ν ≈0.669. The inherent random disorder increases the dynamic critical exponent z to 1.86±0.1 from the value of 1.5 that is for detwinned YBCO single crystal. After the irradiation, the fluctuation conductivity enhanced by the strong pinning of vortex shows deviation from the scaling behavior for the unirradiated one, suggesting that the critical dynamics is strongly perturbed by the strong pinning due to columnar defects.

Composition and phase development of epitaxial superconducting Tl0.78Bi0.22Sr1.6Ba0.4Ca2Cu3O9−δ thin films by laser ablation and post annealing

Abstract For the formation of epitaxial superconducting Tl 0.78 Bi 0.22 Sr 1.6 Ba 0.4 Ca 2 Cu 3 O 9−δ ((Tl,Bi)-l223) thin films, the dependence of phase development on the chemical state of the source materials, deposition distance from source to substrate, and O 2 pressure during deposition by laser ablation and post annealing was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). It was found necessary to have thallium in the precursor film and that a reacted source target is better than the unreacted source for obtaining (Tl,Bi)-1223 phase-pure films. Better (Tl,Bi)-1223 films were obtained at a deposition distance of 44 mm from source to substrate than at a distance of 61 mm over a range of O2 pressure from 5.7 to 31.3 mTorr The best films prepared under the optimal conditions exhibited transport J c close to 3 × 10 6 A/cm 2 at 77 K and zero field.

Resistive measurement of the temperature dependence of the penetration depth of Nb in Nb/AlOx/Nb Josephson junctions

The temperature dependence of the penetration depth of Nb films was determined from resistive transitions of Nb/AlOx/Nb Josephson junctions in a constant magnetic field applied parallel to the junction planes. Distinct resistance peaks were observed as temperature decreases and those peaks were found to appear when the total flux threading the junction equals an integral multiple of the flux quantum. From this condition, the penetration depth at those peak positions has been determined. The temperature dependence was well described by either the dirty local limit or the two‐fluid model. This method can be useful for a highly fluctuating system such as high‐temperature superconductors.

Enhanced superconducting properties in Bi/sub 2/Sr/sub 2/Ca/sub 1/Cu/sub 2/O/sub y/ by thermal and mechanical processing

The effect of processing on transition temperature, grain boundary coupling, and flux pinning has been examined for Bi/sub 2/Sr/sub 2/Ca/sub 1/Cu/sub 2/O/sub y/. Enhancement of critical temperature based on composition can be achieved by control of crystallization and subsequent annealing processes while thermomechanical processing may be used to modify weak link and flux pinning behavior. The microstructural basis for these changes is related to the composition of the superconducting phase and the presence of defects associated with deformation processing. The implications of these results for conductor development are related to the selection of alloy composition for optimum transition temperature and controlled thermomechanical processing which yields a uniform defect structure.<<ETX>>

Identification of grain boundary effects in current-voltage curves of polycrystalline high-Tc superconductors

Abstract Dissipation and critical current densities in high- T c superconductors can depend on both intragranular flux creep and intergranular weak links. Separating these effects in polycrystals can be a formidable task. We present current-voltage data which can in some cases identify weak-link , and verify it by using ion irradiation to greatly diminish the contribution from flux creep. We conclude that the occurrence of significant weak links in series with so-called strongly-coupled current paths may be more common than previously deduced.