Microstructure and properties of Cu-rich 123. Part I: Copper at the grain boundaries
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C. H. Lin and J. A. Eades Science and Technology Center for Superconductivity, Materials Research Laboratory, University of Illinois at Champaign-Urbana, 104 South Goodwin, Urbana, Illinois 61801
A. Sodonis, W. Wolbach, J. M. Chabala, and R. Levi-Setti Science and Technology Center for Superconductivity, The Enrico Fermi Institute and Department of Physics, The University of Chicago, Chicago, Illinois 60637 (Received 2 August 1991; accepted 8 November 1991)
A range of copper-rich bulk YBa2Cu3+A:07+>, superconductors have been prepared by mixing excess copper oxide in the initial material and characterized for their magnetic properties and microstructure. The microstructure of the materials exhibits a high density of planar defects at the grain boundaries and a grain boundary amorphous phase. There is a small increase in the magnetic Jc at 4.5 K but a decrease compared to a conventional material at 77 K, and there is no correlation of the magnetic Jc with the twin boundary density. The change in Jc can be understood if the grain boundary pinning is strong at 4.5 K but weak at 77 K. Data obtained using a very wide range of different microstructure characterization techniques clearly indicate the dangers of relying on only one technique to obtain a full picture of the material.
I. INTRODUCTION One of the more important issues in the new high temperature superconductors, both technologically and scientifically, is flux pinning; to be a useful material, flux pinning in some form is required for high critical currents. Very early it became apparent that thin films could carry high currents1"3 and, presumably, contained adequate flux pinning sites. Although there has been recent progress with bulk superconductors, notably melt textured materials,4"6 bulk materials have yet to achieve critical currents as high as those in thin films. What in the thin films is responsible for the high current carrying properties? For instance, is it planar defects,7 twin boundaries,8'9 point defects,10"13 or is it surface pinning?14"16 There have recently been two sets of experiments that have suggested that planar defects may lead to higher critical currents, namely magnetic measurements of decomposed YBa 2 Cu 4 0 8 1 7 and of shock treated YBa 2 Cu 3 0 7 . 18 In this paper we report the results of a controlled comparison of copper-rich bulk YBa2Cu3+A:07+>, superconductors prepared with excess copper in the initial material. Although there is a small enhancement of the flux trapping at 4 K, the materials are in fact inferior to conventional YBa 2 Cu 3 0 7 at 77 K. There is definitely no correlation of the magnetic Jc with the twin boundary density. In a later paper,19 we will present results using copper oxide dissolved in nitric 572 http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 3, Mar 1992 Downloaded: 16 Mar 2015
acid and added to 123 powder where precipitates are formed throughout the material and flux pinning does occur. These results indicate that the reaction kinetics of the added copper is critical to determining the
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