Directional solidification by appropriate chemically active single crystal seed: An alternative way of generating large
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Directional solidification by appropriate chemically active single crystal seed: An alternative way of generating large superconducting 123 single domain R. Cloots,a) Fr. Auguste, and A. Rulmont SUPRAS, Chemistry Institute B6, University of Li`ege, Sart-Tilman, B-4000 Li`ege, Belgium
N. Vandewalle and M. Ausloos SUPRAS, Physics Institute B5, University of Li`ege, Sart-Tilman, B-4000 Li`ege, Belgium (Received 25 November 1996; accepted 12 August 1997)
A Dy2 O3 single crystal has been used as a seed for the growth of isothermally melt-textured Dy-123 material. The nucleation-controlled step has been observed to be related to the heterogeneous nucleation of 211 particles at the surface of the dysprosium oxide single crystal. The subsequent growth mode seems to be controlled by a high concentration gradient of dysprosium in the liquid phase. This leads to a directional solidification process of the 123 phase. The size of the 211 particles seems to decrease as the distance from the dysprosium oxide single crystal increases.
The growth of the REBa2 Cu3 O7 (123) phase in the melt-texturing process near the peritectic decomposition temperature is rate limited by diffusion of the rare-earth ion.1,2 On cooling below the peritectic temperature (Tp ), the 123 phase is produced by the reaction between the 211 particles and the liquid phase; the 211 particles are dissolved in the liquid, inducing a rare-earth diffusion from the 211-liquid interface to the 123-liquid interface (or the advancing front). The dissolution of the 211 particles maintains some supersaturation with respect to the 123 composition, leading to the precipitation of the expected 123 phase.3 One interesting aspect of the melt-texturing process is the microstructure of the resulting material: a multinucleation process leads to the formation of multigrains with high angle grain boundaries between each domain. Significant progress has been achieved by modifying the melt-texturing process in order to impose a “directional solidification” of the so-called “liquid plus 211 particles” phase.3 These methods involve a slow cooling process with an imposed “moving” temperature gradient which is used in order to control the specific location where the solidification proceeds. This is generally applied to thin extruded samples.4 The problem in this case is to maintain a-b plane alignment along the sample axis. Moreover, macro- and microcracks develop along grain
a)
Corresponding author, e-mail: [email protected]. J. Mater. Res., Vol. 12, No. 12, Dec 1997
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boundaries making the samples brittle and sensitive to fracture by cleavage along the misaligned a-b planes.5 Another possibility for the synthesis of improved grain alignment 123 is to encourage the growth of large grains by seeded peritectic solidification of the 123 phase.6 The advantage of this method resides in the isothermal conditions used during the process. Moreover, the grain orientation can be contr
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