Directional phase formation on melting via peritectic reaction
- PDF / 3,900,757 Bytes
- 14 Pages / 598 x 778 pts Page_size
- 75 Downloads / 248 Views
I.
INTRODUCTION
IN the bulk form, high-temperature type-II ceramic superconductors have low critical transport current densities, which are significantly degraded by the application of even weak magnetic fields. ~1-71In polycrystalline Ba2YCu307-~, this behavior is commonly attributed to the weak flux pinning at intragranular regions and to the weak-link electrical characteristics, generally displayed by high-angle grain boundaries, owing to their short superconductive coherence length and inherent structural disorder, t8,91 The application of several solidification techniques to Ba2YCu3OT+~ has successfully produced melt-textured or "single-crystalline" materials, which are nonweak linked and present better flux pinning characteristics than their polycrystalline counterparts. Current densities in excess of 105 A/cm 2 at 77 K with less pronounced field dependence were recently reported for these materials. I1~ These values are about two orders of magnitude higher than those measured in good polycrystalline samples of Ba2YCu307+~. The isothermal section of the pseudotemary system BaO-Y203-CuO at 850 ~ and a temperature-composition profile containing the superconducting phase Ba2YCu307-8 are depicted in Figure 1. Both sections are from Roth e t al. t~41 and correspond to equilibrium at 0.021 MPa 02. A.M. FIGUEREDO, Research Assistant, M.J. CIMA, Associate Professor, M.C. FLEMINGS, Professor, and J.S. HAGGERTY, Senior Research Scientist, are with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted June 23, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
According to the diagram of Figure l(b), the superconducting compound melts incongruently at 1002 ~ to a semisolid mixture of particles BaYzCuO5 and a liquid poor in yttrium. More precise determinations of the incongruent melting temperature of BazYCu306+8 arrived at a value of 1015 ~ t15] Growth of Ba2YCu306+~ from semisolid mixtures of BaY2CuO5 + L with overall compositions in the range indicated in Figure 1 is a common feature to the majority of melt processing techniques. A previous study t161 has shown that in the solidification of Ba2YCu306+ ~ from semisolid melts, the peritectic reaction occurs by dissolution of the high-temperature phase, BaY2CuOs, in the liquid ahead of the growth interface and subsequent reprecipitation on the Ba2YCu306+~ crystal surface. Solute diffusion in the liquid is the limiting mass transfer process. Solidification rates at which a plane front can be achieved are limited by diffusion distance, hereby, BaY2CuO5 particle size. Thus, coarsening of BaY2CuO5 particles during the time they remain in contact with the liquid can result in reduction of permissible solidification rate. Another problem arises as a result of the pronounced temperature dependence of the liquid's surface tension t17] which will lead to surface tension driven flow of liquid, especially if large temperature gradients are applied. Both the foregoing problems can be minimized by introd
Data Loading...
