The discovery of superconductivity at 39 K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. This compound has twice the transition temperature of Nb3Sn and four times that of Nb-Ti alloy, and the vital prerequisite of strongly linked current flow has already been demonstrated. One possible drawback, however, is that the magnetic field at which superconductivity is destroyed is modest. Furthermore, the field which limits the range of practical applications-the irreversibility field H*(T)-is approximately 7 T at liquid helium temperature (4.2 K), significantly lower than about 10 T for Nb-Ti (ref. 6) and approximately 20 T for Nb3Sn (ref. 7). Here we show that MgB2 thin films that are alloyed with oxygen can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding an H* value at 4.2 K greater than 14 T. In addition, very high critical current densities at 4.2 K are achieved: 1 MA cm-2 at 1 T and 105 A cm-2 at 10 T. These results demonstrate that MgB2 has potential for high-field superconducting applications.

KEY WORDS:weak links, bicrystals, EELS, YBa2CU307_,S, grain boundary structure A BSTRACT Grain-boundary weak links limit the critical current density of poly crystalline high-Tc superconductors. Themes that have evolved from research on these grain boundaries include (a) an apparent general and dramatic distinction between the behavior of low-angle and high-angle grain boundaries, (b) electrical heterogeneity along as well as among boundaries, (c) indications that the macroscopic current path may or may not cross certain high-angle grain boundaries, (d) a key role for TEM EELS in studies of the electronic structure near grain boundaries, and (e) evidence that current models for grain boundary structure pertain to the high-Tc superconductors and may eventually help us to understand their electromagnetic character. Results regarding each of these topics are reviewed in this paper. 0084-6600/95/0801-0193$05.00 193 Annu. Rev. Mater. Sci. 1995.25:193-222. Downloaded from www.annualreviews.org by University of Massachusetts -Amherst on 09/27/12. For personal use only. Quick links to online content Further ANNUAL REVIEWS Annu. Rev. Mater. Sci. 1995.25:193-222. Downloaded from www.annualreviews.org by University of Massachusetts -Amherst on 09/27/12. For personal use only. (HIGH-Tel SUPERCONDUCTOR GRAIN BOUNDARIES [> EPITAXY OF DISSIMILAR MATERIALS, C. J. Palmstrnm 389 V11 Annu. Rev. Mater. Sci. 1995.25:193-222. Downloaded from www.annualreviews.org by University of Massachusetts -Amherst on 09/27/12. For personal use only. Quick links to online content Further ANNUAL REVIEWS

Extended voltage-current (V -I) characteristics of ͓001͔ tilt YBa 2 Cu 3 O 7Ϫx bicrystal films having misorientation angles, , of 3°to 20°showed a substantial and progressive change with , when measured in large fields. Transmission electron microscopy of the 10°bicrystal showed the grain boundary to contain edge dislocations separated by channels of relatively undisturbed lattice. A large drop in the intergrain irreversibility field, H*, occurred between 10°and 15°, and the characteristics became qualitatively different by 20°. Both the microscopy and the electromagnetic data support a heterogeneous description of the grain boundary, consisting of strongly coupled channels that close at a misorientation angle of around 15°. © 1996 American Institute of Physics. ͓S0003-6951͑96͒03630-3͔It is well established that the critical current density, J c , of ͓001͔ tilt thin film bicrystals of high temperature superconductors is strongly dependent on the misorientation angle, , of the grain boundary ͑GB͒. 1-4 Dimos et al. 1 proposed that the cores of primary grain boundary dislocations ͑PGBD͒ subdivide low angle grain boundaries. Chisholm and Pennycook 5 additionally proposed that the strain fields of the PGBDs extend the effective size of the dislocation cores. Both models postulate that the supercurrent is restricted to channels of good lattice between the PGBDs. The effective size of the PGBD cores should then determine the magnitude of J b /J c ͑the ratio of the inter-to intragranular critical current density͒, and the critical misorientation angle at which strong coupling properties disappear. Measurements on different kinds of samples suggest that this critical angle can vary over a wide angular range of 5°-20°. 1,4,6,7 Since this range covers a PGBD separation of 4.5-0.6 nm, a more explicit test of the above models is highly desirable. 8 This issue has regained importance because of renewed activity on making conductors with a strong biaxial texture from YBa 2 Cu 3 O 7Ϫx ͑YBCO͒, using an ion beam assisted deposition process. [9][10][11] Since such conductors would be useful for the production of strong fields, provided that the spread in the a/b grain axis misalignment is controlled, knowledge of the extended high field voltage current (V -I) characteristics of YBCO bicrystals of varying misorientations is of both fundamental and applied interest.This letter describes the first systematic measurements of the V -I curves of ͓001͔ tilt YBCO bicrystal thin films with misorientation angles in the low-to-high angle crossover regime of 3°-20°in large magnetic fields. Prior measurements of single crystal thin film 12,13 and bulk 14 high temperature superconductors have established that the irreversibility field, H*, above which J c tends to zero, can be experimentally defined by the change in sign of the curvature of the electric field-current density, E -J, characteristic plotted on a double logarithmic scale. In single crystal YBCO films, this transition occurs near 4 T at 77 K. 12 To test the channel models described...

No abstract