ORAL SESSION 3C4: Grain Boundary and Critical Current

Wednesday, Feb. 23, 10:00 a.m. – 12:00 p.m., Room 302A (GRB)

Chairs: H.W. Weber (Atomic Inst. of Austrian Universities), W. Lo (TCSUH/U Houston)

3C4.1 The Properties of Low Angle Grain Boundaries in High-Temperature Superconductors

D. Larbalestier 1,3, S. Babcock 1,2, G. Daniels 1,2, M. Feldmann 1,3, A. Gurevich 1, A. Polyanskii 1, and J. Reeves 1,2. 1 Department of Materials Science and Engineering, University of Wisconsin, Madison WI 53706, USA. 2 Applied Superconductivity Center, University of Wisconsin, Madison WI 53706, USA. 3 Department of Physics, University of Wisconsin, Madison WI 53706 USA.

Presenting Author: D. Larbalestier

It is well-known that there is a transition in transport properties as the misorientation angle of grain boundaries in YBCO increases. At low misorientation angles, grain boundaries are strongly coupled, becoming weak links at higher misorientation angles, then suffering a strong reduction in critical current density, especially in high magnetic fields. In spite of the great importance of this transition, the exact angle at which the transition occurs is not well-known and varies from sample to sample. We have been making a detailed study of the misorientation angle dependence of this transition in bicrystals of YBCO and in YBCO coated conductors made on polycrystalline substrates. Magneto optical studies show when the GB becomes a barrier to current flow, both in bicrystals and in coated conductors. MO and transport studies show that there is a threshold, which varies from ~4-7°, at which the boundary starts to become a barrier to current flow. These phenomena are being started by transport measurements, by magneto optical imaging, and by electron backscatter Kikuchi pattern analysis. We will present an overview of our most recent results.

*Work on coated conductors is carried out in collaboration with colleagues at Air Force Research Laboratory and Oak Ridge National Laboratory.

3C4.2 Doping Induced Enhancement of the Critical Currents of Grain Boundaries in High-Tc Superconductors

J. Mannhart, H. Bielefeldt, B. Goetz, H. Hilgenkamp, A. Schmehl, C.W. Schneider, and R.R. Schulz, University of Augsburg, Institute of Physics, Augsburg D86135, Germany

Presenting Author: J. Mannhart

The transport properties of interfaces in high-Tc compounds differ systematically from those involving conventional superconductors. Over the years, the electronic properties of the interfaces have been studied in great detail by numerous groups, but a comprehensive understanding of the mechanisms controlling their properties is still in development. In the presentation we point out the important influences of the d-wave symmetry component of the order parameter and of space charge layers associated with band bending at the interface [1]. The space charge layers can be altered to optimize the properties of interfaces for a given application. This is demonstratetd by enhancing the critical current densities and reducing the normal state resistivities of grain boundaries in high-Tc superconductors to unprecedented values by appropriately doping the superconductor [2].

[1] H. Hilgenkamp and J. Mannhart, Appl. Phys. Lett. 73, 265 (1998).

[2] A. Schmehl et al., Europhys. Lett. 47, 110 (1999).

3C4.3 Grain Boundary Dissipation in High-Tc Superconductors

Kenneth E. Gray, Dean J. Miller, Michael B. Field, Peter Berghuis, and Dong Ho Kim, Material Science Division, Argonne National Laboratory, Argonne, IL 60439

Presenting Author: K.E. Gray

Thin-film and bulk bicrystal grain boundaries (GB) in YBa2Cu3O7 exhibit a strong dependence of critical current density, Jc, on misorientation angle. Remarkably, in microscopically more perfect bulk GBs, Jc is 30x smaller than in thin film GBs. We review an explanation of this zero-field data, which is based on the pinning of Josephson vortices by the meandering thin-film GBs.

In a field, there is evidence that Jc of GBs does not drop as quickly with field as expected by simple Josephson junction models. The long-wavelength pinning potential due to meandering is less effective at high fields, but Gurevich and Cooley (GC) proposed a new mechanism for an enhanced GB Jc arising from pinned Abrikosov vortices in the banks of a GB which present a static, quasiperiodic pinning potential for GB vortices. We find a peak in Jc and an unusual hysteresis which give considerable support to the GC concept. In low fields, the GBs exhibit a larger Jc for field cooling, which is opposite to the usual hysteresis but agrees with GC due to the larger Abrikosov vortex density in the banks. Magnetization data on the same sample are consistent including the onset of irreversibility.

3C4.4 Grain Boundaries in Bulk YBCO

Kamel Salama, Texas Center for Superconductivity, University of Houston, Houston, TX 77204-5932, USA

Presenting Author: Kamel Salama

The problem of grain boundaries in high temperature superconductors is very important in determining the ability of these materials to carry supercurrent. The success of fabricating polycrystalline bulk YBCO with high-angle grain boundaries carrying high critical currents initiated intense microstructural studies of these boundaries in order to determine mechanisms involved. TEM studies has been performed on grain boundaries with known Jc, followed by simulation of their atomic and electron structures as well as calculations of their superconducting order parameter. Using a model developed to correlate grain boundary microstructure and superconducting properties, we have shown that coupling behavior of grain boundaries does not only depend on misorientation characteristics. The grain boundary plane as well as the atomic ordering in the vicinity of grain boundary were also found to play an important role in the current carrying capabilities of these grain boundaries. In this presentation, TEM studies and theoretical calculations will be presented and show close correlations between experiment and theory for high angle misorientation grain boundaries.

3C4.5 Electromagnetic and Microstructural Properties of Bulk Bicrystal Grain Boundaries in High Tc Superconductors

Qiang Li, Y. Tsay, Y. Zhu, and M. Suenaga, Brookhaven National Lab, Upton, NY 11973 USA

Presenting Author: Q. Li

We shall present systematic studies of the electromagnetic and microstructural properties of bulk Bi-2212 bicrystals having synthetic [001] twist boundaries or naturally grown [100] tilt boundaries. Both conventional and high resolution TEM images showed that the synthetic [001] twist bicrystals have microstructurally well-defined uniform grain boundary plane located at double BiO layer, while the grain boundary facets were generally observed in naturally-grown tilt bicrystals.

We found that [001] twist boundaries in as-prepared bicrystals, regardless of twist angles, can conduct virtually the same amount of critical currents as constitute single crystals. In these twist bicrystals, the dissipation at grain boundary seems to be dominated by the extremely weak pinning of the single crystal grains. We also irradiated some of the [001] twist bicrystals with 2.2 Gev Au-ions, and found that the twist boundaries carry significantly less critical current than the irradiated single crystals in magnetic field. In those irradiated twist bicrystals, the dissipation at grain boundaries seems to be dominated by the break down of the intrinsic Josephson coupling across the BiO layers.

For naturally-grown [100] tilt boundaries, we found a general Tc depression at the grain boundary, which is likely induced by the change of the local oxygen content and strain field. The behavior of strong coupling was observed in several high angle tilt grain boundaries below the grain boundary Tc. However, the coupling-strength does not seem to uniquely depend on the misorientation angle. The detailed microstructure near the grain-boundary seems to play more important role in the electromagnetic properties of grain boundaries than what we had anticipated.

3C4.6 Uranium Doping and Thermal Neutron Irradiation to Enhance Flux Pinning with Reduced Radioactivity in Bi-2223 Tapes

S.X. Dou 1, Y.C. Guo 1, D. Marinaro 1, D. Milliken 1, J. Boldeman 1, R. Weinstein 2, A. Gandini 2, R. Sawh 2, Y. Ren 2, S. Tönies 3, C. Klein 3 and H. Weber 3. 1 Institute of Superconducting and Electronic Materials, University of Wollongong, NSW 2522, Australia. 2 Institute for Beam Particle Dynamics, University of Houston, TX 77204-5506. 3 Atominstitut der Oester. Universities, A-1020 Vienna, Austria.

Presenting Author: S.X. Dou

Chemistry of U-doping in Ag/Bi-2223 tapes at levels of 0.15% to 2.0% of 98% 235UO3.5 has been studied. It was found that uranium oxide reacted with superconductor to form compounds such as USrxCa2-xOy. UO3.5 acted as sintering aid, lowering the melting temperature. The optimal sintering temperature decreased with increasing uranium-doping level. Tc of the U-doped tape was slightly depressed with increasing U-doping level. Jc of the U-doped tape decreased with increasing UO3.5 content and it reduced to 50% of undoped tape at a doping level of 2.0% pure UO3. The degradation in Jc due to U-doping was minimised by using UCa2Oy to replace pure uranium oxide. Doping with the uranium compound also improved the homogeneity of U-distribution.

The correlation between the enhancement of Jc due to irradiation and U-doping level at the optimal neutron fluency will be presented. For example, the Jc increased by a factor of 60 at 0.7T in H½ ½ c for 0.15% U-doped tape at a neutron fluency of 4x1018/m2 whereas the Jc increased by a factor of 250 at 0.7T in H½ ½ c for 0.6% U-doped tape at neutron fluency of 3x1018/m2. Further, the irreversibility field increased, anisotropy decreased and required neutron fluency reduced with increasing U-doping level. TEM examination revealed that fission tracks generated through thermal neutron irradiation were randomly distributed in the oxide core, which act as effective pinning sites, responsible for the improvements in flux pinning and anisotropy.

3C4.7 Pinning Centers From Uranium Fission in Various HTS

Roy Weinstein 1, Ravi Sawh 1, Yanru Ren 1, Alberto Gandini 1, Masato Murakami 2, Tadashi Mochida 2, Noriko Chikumoto 2, Naomichi Sakai 2, Gernot Krabbes 3, and Wofgang Bieger 3. 1 Texas Center for Superconductivity, University of Houston, Houston, TX 77204-5506, USA. 2 International Superconductivity Technology Center, SRL, 12-16-25 Shibaura, Minato-ku, Tokyo 105, Japan. 3 Institute für Festkörper- und Werkstofforschung, P.O. Box 27 00 16, D-01171 Dresden, Germany.

Presenting Author: R. Weinstein

The insertion of uranium to HTS precursor powders prior to texturing, and irradiation with thermal neutrons after texturing is called U/n processing. Damage caused by the resulting uranium fission fragments has been extensively studied for pinning center behavior in Y123, and is under intensive study in Ag-BiSCCO tape. In both HTS systems results for Jc, anisotropy, etc. are excellent. We consider the chemical and physical conditions under which U/n processing is successful, including conditions on spacing, S, of deposits of U compounds, and conditions on neutron absorption by the HTS. In particular, if d =length of quasi-columnar fission fragment defects, we find S<2d ~5.4µm is needed for the best U/n results. Application of these conditions is considered in Y123, Ag-BiSCCO tape, Nd123, Sm123 and Gd123. Studies in progress on Nd123 and Sm123 are reported.