A transmission electron microscopy study of the crystallinity and secondary phase formation in melt-processed YBa 2 Cu 3
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A transmission electron microscopy study of the crystallinity and secondary phase formation in melt-processed YBa2 Cu3 O72d Y. Yan, D. A. Cardwell, and A. M. Campbell IRC in Superconductivity, University of Cambridge, Madingley Road, Cambridge, CB3 0HE, United Kingdom
W. M. Stobbsa) Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, United Kingdom (Received 3 August 1994; accepted 13 September 1996)
The microstructure of large grain melt-processed YBa2 Cu3 O72d containing 10 molar % excess Y2 BaCuO5 prepared and oxygenated under atmospheric pressure has been investigated by transmission electron microscopy (TEM) and optical microscopy. These materials always contain parallel structural and microscopic platelet-like features in the crystallographic a-b plane of a few microns spacing which have been variously described as grain boundaries or microcracks. We have observed such features, which clearly influence the flow of current in melt-processed YBCO, to consist of copper deficient, impurity phase material which can be either amorphous or crystalline in nature. A variety of defects have been observed by high-resolution electron microscopy (HREM) in the vicinity of these platelet boundaries, including double and triple CuO layer stacking faults, which may constitute effective flux pinning sites.
I. INTRODUCTION
The presence of grain boundaries in bulk forms of the high temperature (high-Tc ) superconductor YBa2 Cu3 O72d (YBCO) greatly reduces its ability to carry an electric current and enhances its sensitivity to the application of a magnetic field.1 However, the advent of melt-processing techniques for the fabrication of large grain YBCO over recent years has yielded bulk samples which exhibit significantly increased intraand intergrain critical current densities compared with their sintered counterparts.2–4 Critical current densities deduced from magnetic hysteresis measurements of 30,000 A cm22 at 77 K and 1 T are now measured routinely in melt-processed YBCO2 and have been observed to flow over the entire length scale of the grain (typically one centimeter).5 Grains of melt-processed YBa2 Cu3 O72d (the 123 phase) characteristically contain a fine dispersion of unconnected Y2 BaCuO5 particles (the 211 phase) which act as flux-pinning centers and are responsible, at least in part, for the observed magnitude of Jc .6 The combination of a high Jc and an increased length scale over which current flows greatly increase the flux-trapping ability of melt-processed YBCO in comparison with sintered material. This is extremely important for permanent magnet-type applications, such
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J. Mater. Res., Vol. 11, No. 12, Dec 1996
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as magnetic bearings, and underlines the potential of this material for practical uses. In this paper we present results of a transmission electron microscopy (TEM) study of the microstructure of melt-processed YBCO conta
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