The effect of very high barium content in the precursor on the properties of GdBCO single grain bulk superconductors
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N. Hari Babu Interdisciplinary Research Centre (IRC) in Superconductivity and Department of Engineering, University of Cambridge, Cambridge CB3 0HE, United Kingdom; and Brunel Centre for Advanced Solidification Technology (BCAST), Brunel University, West London UB8 3PH, United Kingdom
Kazumasa Iida The Leibniz Institute for Solid State and Materials Research Dresden (IFW)-Dresden, 01069 Dresden, Germany
David A. Cardwell Interdisciplinary Research Centre (IRC) in Superconductivity and Department of Engineering, University of Cambridge, Cambridge CB3 0HE, United Kingdom (Received 16 January 2008; accepted 3 July 2008)
A new melt processing route for the fabrication of large grain Gd-Ba-Cu-O (GdBCO) bulk superconductors has been developed based on the use of novel GdBa4Cu3O8 d (Gd-143) and GdBa6Cu3O10 d (Gd-163) precursor compositions. The new processing route enables the fabrication of large single grains from precursor powders that contain high concentrations of Ba. The superconducting properties and microstructures of GdBCO single grains with extra Ba fabricated via this new processing route are reported. Most importantly, the possible formation of a new form of Gd1+xBa2 xCu3O7 d solid solution (Gd-123ss) with x < 0 in single grains fabricated from the Ba-rich precursor are discussed for the first time based on the superconducting, chemical, and structural properties of the large GdBCO grains. I. INTRODUCTION
Large grain Gd-Ba-Cu-O (GdBCO) bulk superconductors have significant potential for practical applications due to their enhanced flux trapping potential at 77 K compared to other (RE)BCO materials (where RE is a rare earth element or Yttrium). However, single GdBCO grains fabricated in air tend to form a Gd1+xBa2 xCu3O7 d (where x > 0) type solid solution phase (Gd-123ss), since the ionic radii of Gd and Ba elements are relatively similar.1 Unfortunately, the effect of the formation of such a solid solution is to impair the superconducting properties of the sample, with higher levels of substitution, x, giving increasingly reduced performance. In attempts to overcome this problem for bulk GdBCO superconductors fabricated in air, Ba rich compounds such as BaO2, BaCuO2, and BaCO32–6 have been added to GdBCO precursor compositions to suppress the Gd/Ba substitution process. We reported previously that the addition of small amounts of BaO2 (e.g., 1 wt%) to the precursor suppresses Gd/Ba substitution in this material significantly, and yields good a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0032
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J. Mater. Res., Vol. 24, No. 1, Jan 2009
http://journals.cambridge.org
Downloaded: 19 Mar 2015
superconducting properties in fully processed samples.7 The addition of over 4 wt% BaO2, however, causes a deterioration of the superconducting properties and generates fine, globular second phase inclusions in the sample microstructure. It was not clear whether these inclusions were Gd2BaCuO5 (Gd-211) or a new, unidentified phase. Furthermore, the growth rate of the sup
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