Optimization of Y 2 BaCuO 5 phase morphology for the growth of large bulk YBCO grains
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YBCO samples were prepared using different types of precursor powders: quenched, or YBa 2 Cu 3 0 7 x and Y 2 BaCu0 5 commercial powders, with and without Pt addition. Two stoichiometries, corresponding to 0 and 40% molar excess of the Y 2 BaCu05 phase, were adopted. The behavior of the Y 2 BaCu0 5 phase through thermal treatments, typical of melt processes, was observed by quenching the samples at various stages. YBa 2 Cu 3 0 7 _ x grain growth kinetics was enhanced and liquid phase losses were limited by optimizing the morphology of the Y 2 BaCu0 5 phase in the partially melted state, with the simultaneous presence of round particles =£ 1 /mm and needle-like particles with very high aspect ratio. The optimized processing conditions were adopted, together with a seeding technique, to grow YBCO samples. At 77 K, a magnetic levitation force of 2.5 kg was measured for a sample with 18 mm diameter, and magnetization critical current densities over 104 A/cm 2 were reached at 1 T.
I. INTRODUCTION It is well known what fundamental a role is played by the insulating phase Y 2 BaCuO 5 (Y-211) in bulk YBCO samples prepared following melt processes such as MTG,1 QMG,2 and MPMG. 3 Its importance is connected both to the peritectic solidification mechanism of the YBa 2 Cu 3 0 7 - x (Y-123) phase and to the electric transport properties of the final material. Various studies4"8 have demonstrated that the peritectic recombination of the Y-123 phase takes place by the simultaneous dissolution of Y-211 phase in the B a Cu-rich liquid with consequent Y supply, and the direct precipitation of the solid Y-123 phase from the liquid itself; cm-size oriented domains can thus be formed. In the absence of a temperature gradient, two main factors can influence Y-123 growth kinetics5'7: the rate of dissolution of Y-211 particles, depending critically in their size and shape, and the degree of undercooling with respect to the peritectic temperature. Small Y-211 particles with high surface/volume ratio and high surface curvature are likely to undergo a faster dissolution. The presence in the partially melted state of an excess of Y-211 phase, with proper shape and dimension, has a positive effect on the final microstructure. A considerable enlargement of grains is guaranteed, due to a more continuous growth, with reduction of porosity and cracks, a complete consumption of the Ba-Cu-rich liquid phase, preventing the formation of large insulating inclusions, especially at grain boundaries.9
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On leave from CISE S.p.A., 12081 1-20134 Milano, Italy. J. Mater. Res., Vol. 10, No. 9, Sep 1995 http://journals.cambridge.org
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The improved microstructure of YBCO samples can itself be at the basis of an increase in critical current density Jc, which is so critical an achievement in view of near future applications. Moreover it has been argued that, even though /xm-size trapped Y-211 inclusions are too large to be effective pinning centers, Y-211/ Y-123 interfaces10, as well as the crystal defects associated with them, such as
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