Texturing Crystal Growth in Vertical Thermal Gradients
- PDF / 1,007,235 Bytes
- 4 Pages / 420.48 x 639 pts Page_size
- 45 Downloads / 161 Views
TEXTURING CRYSTAL GROWTH IN VERTICAL THERMAL GRADIENTS N. X. Tan+, A. J. Bourdillon+ and N. Savvides++ + Department of Materials Science and Engineering, State University of New York at Stony Brook, Stony Brook, NY 11794 ++ CSIRO Division of Applied Physics, NSW 2070, Australia ABSTRACT ceramic melting in incongruently growth Texture superconductor materials, such as YBa 2 Ct O7 _x can be combined with zone refinement in thermal gradients to increase critical current densities. In gravity aided texture growth (GATEG), a vertical thermal gradient is used with downward motion of the partially molten specimen (relative to the furnace) so that the molten The oxidation features phase recrystallizes with grain growth. of Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O can be explained by Madelung potentials. INTRODUCTION superconductors so far In all high critical temperature (Tc) discovered, such as YBa2Cu 7_. (123), electrical conduction is The grain bounlary weak links can result in low anisotropic [1]. However, many applications of high critical current density, Jc. TC [21 require high Jc and so there has been considerable interest in developing superconducting material with aligned According to the phase grains and clean grain boundaries. diagram (3), at around 1010 °C (123) transforms, by incongruent At just below this melting, into liquid and solid Y2 BaCuO (2111. can occur preferentially in the temperature, growth of (123) i.e. in the plane of maximum (001) crystallographic plane, If a negative temperature gradient lies parallel to conduction. this plane, elongated grain growth along the gradient results in In this partial melting regime the specimen can be texture. kept rigid, but molten components tend to run away from the bulk Here we describe how the resulting in changes of composition. molten components can be controlled by gravity to run into the of crystal growth where not only is stoichiometry region preserved but second phases are excluded by zone refinement. There are special difficulties in texturing coils by GATEG. EXPERIMENTAL APPARATUS AND DISCUSSION Powders of (123) and of Bi 2 Sr CaCtOIn (2223) were prepared by normal ceramic processing routes [ ]. Trhe grain size was in were firstly heat treated the range 1-10um. Specimens of (123) tube furnace for comparison with in a conventional horizontal were treated in a other specimens of (123) and of (2223) which vertical tube furnace. Microstructural studies (51 on specimens sintered in a horizontal tube furnace show that Perfect stoichiometry was not retained: second phases of (211) and of CuO were observed The material had partial crystal together with majority (123). alignment with high density except for the presence of a few Resolidified melt was found to be gathered at the large bubbles. Mat. Res. Soc. Symp. Proc. Vol. 169. @1990 Materials Research Society
284
base
of the
material, where some reaction
observed. Secondly, (123) avoid problems of
with the crucible was
To was sintered in a vertical tube furnace. heatmelt segregation the partial-melt
treatment wa
Data Loading...
