Growth of melt-textured Nd-123 by hot seeding under reduced oxygen partial pressure
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The growth of large, melt-textured Nd1+xBa2−xCu3O6+␦ (Nd-123) crystals has been achieved by hot seeding and isothermal solidification under a 1% oxygen in nitrogen atmosphere. These crystals, which exhibit a sharp, faceted growth interface, were grown epitaxially from a small Nd-123 single crystal seed placed on the sample surface at elevated temperature. The growth length of the melt-processed crystal was directly proportional to the isothermal holding time (approximately 17 h), as is observed for the growth of YBa2Cu3O7−␦ (Y-123). The variation of growth rate with undercooling for this material was linear, however, in contrast to the parabolic dependence observed for Y-123 crystals grown in air. The growth rate of Nd-123 under reduced oxygen was consequently lower than that of Nd-123 and Y-123 grown in air at relatively high values of undercooling. Evaluation of the experimental data against a solidification models suggested that the interface kinetics are responsible, at least in part, for the observed growth features in hot-seeded Nd-123 crystals. This was attributed to the difference in oxygen partial pressure under the respective growth atmospheres, rather than to the species of rare-earth element in the compound.
I. INTRODUCTION
Superconducting Nd1+xBa2−xCu3O6+␦ (Nd-123) offers significant potential for engineering applications due to its relatively high critical temperature, Tc , high critical current density in magnetic field, J c , and highirreversibility field, Hirrev , compared with YBa2Cu3O7−␦ (Y-123).1 Nd-123, however, generally forms a solid solution within the Nd-123 lattice in which Nd ions substitute onto Ba ion sites to yield a superconducting phase with reduced Tc. This reduction in Tc increases with increasing substitution until x ≈ 0.4, at which point the material becomes nonsuperconducting.2 It is important, therefore, to control the degree of substitution, x, to within 0 艋 x 艋 0.02 if the optimum superconducting properties of this compound are to be obtained. Two techniques that suppress the formation of the solid solution have been reported to date. These are the use of (i) a Ba-enriched initial composition for material processed under air3 and (ii) Nd-123 processed under low-oxygen partial pressure atmosphere (pO2).4 Nd-123 single crystals with a sharp Tc of 96 K containing a uniform distribution of a small amount of Nd → Ba substitution have been fabricated from a a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 4, Apr 2001
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Ba-rich composition under air by a top-seeded solution growth (TSSG) technique.5 Microsegregation of the solid-solution phase was identified clearly in the matrix crystal,6 however, for bulk crystals containing nonsuperconducting Nd4Ba2Cu2O10 (Nd-422) phase particles (i.e., the stable NdBCO phase at high temperature). In contrast to the growth of single crystals, the presence of Nd-422 particles at the growth front significantly inhibits liquid diffusion du
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