Effects of Nd123/MgO thin film and MgO single-crystal seeds in isothermal solidification of YBaCuO/Ag
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The seeding effects of (001) Nd123/MgO thin films and MgO single crystals were studied in isothermal solidification of YBa2Cu3Oy composite with the additions of 40 mol% Y2BaCuO5, 10 wt% Ag, and 0.5 wt% Pt. Seeding with the Nd123/MgO thin film resulted in single-domain growth of Y123 crystal with a stable growth along the 〈100〉 direction, while seeding with MgO single crystal produced multidomain growth in which the dominant growth facet is rotated 45° about (100) plane of MgO. Multidomain growth in MgO seeded sample was suppressed by decreasing undercooling degree. The effects of undercooling degree and seed size on multidomain growth are discussed in view of classical nucleation and growth theory.
I. INTRODUCTION
Melt-processed REBa2Cu3Oy (RE123, RE: Y or rareearth element) superconducting bulk materials have significant potential for a variety of engineering applications such as magnetic levitation devices and trapped flux magnets. This potential strongly depends on the improvement of critical current density (Jc), current loop size, and mechanical strength.1–6 The excess nonsuperconducting Y2BaCuO5 (Y211) particle inclusions in the melt-processed Y123 matrix have been found to be effective in hindering liquid-phase loss, improving growth stability, and enhancing flux pinning as well as Jc due to the increasing of the microdefects at Y123–Y211 interfaces. In particular, when small amounts of platinum (Pt) are added to Y123–Y211 precursor, Y211 particle inclusions will become smaller and more uniform.1,4–6 As a result, Jc is further improved. Silver (Ag) addition to Y123 is widely used to enhance mechanical properties without degrading superconducting properties.7–9 In addition, silver addition can decrease the decomposition temperature of Y123/Ag system down to 970 °C in air. Recently, Y. Nakamura et al.7,8 studied the Y211–Y123– Ag quasi-ternary phase relation instead of Y211–Y123 quasi-binary phase diagram and concluded that Y123 phase was solidified by monotectoperitectic reaction: Y211 + L1 (Ba–Cu-rich liquid) ⳱ Y123 + L2 (Ag-rich liquid). On the other hand, current loop size is correlated directly with the texture domain size. Top-seeding techniques are widely used to obtain large-sized single-
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J. Mater. Res., Vol. 15, No. 8, Aug 2000 Downloaded: 24 Mar 2015
domain RE123 bulk materials.4–6,10,11 A seed crystal can be introduced at the top of a sample prior to a heating cycle (commonly called cold seeding) or after a melting step (commonly called hot seeding). For cold seeding, it is essential to prepare a seed crystal with a decomposition temperature higher than that of the superconductor, whereas for the hot seeding, it is not necessary because the seeding procedure is feasible at an undercooling. Melt-grown or flux-grown Nd(Sm)123 and even Y123 itself are commonly believed to be good seeds for Y123 crystal growth.10,11 A successful seed should not only ensure singledomain growth but also control
