Growth morphology of large YBCO grains fabricated by seeded peritectic solidification: (I) The seeding process
- PDF / 966,356 Bytes
- 9 Pages / 612 x 792 pts (letter) Page_size
- 62 Downloads / 221 Views
Welcome
MATERIALS RESEARCH
Comments
Help
Growth morphology of large YBCO grains fabricated by seeded peritectic solidification: (I) The seeding process Wai Lo, D. A. Cardwell, and P. D. Hunneyball IRC in Superconductivity, University of Cambridge, Madingley Road, Cambridge CB3 0HE, United Kingdom (Received 23 August 1996; accepted 20 November 1997)
The growth of large grain YBa2 Cu3 O72d (YBCO) by peritectic solidification in the presence of a (Sm,Y)Ba2 Cu3 O72d seed is characterized by the initial seeding process, development of a facet plane around the seed, and finally by continuous nonlocal growth away from the seed. A detailed investigation of the seeding process using electron microscopy, electron probe microanalysis, and thermal analysis techniques is reported here as the first in a series of studies of these key growth features. Results show that the seed partially melts below its nominal melting temperature due to a distribution of yttrium cations across the seed/YBCO interface. The formation of a Sm/YBa2 Cu3 O72d solid solution, which occurs via a reaction between (Sm,Y)2 Ba2 CuO5 and liquid state observed across this interface at temperatures below the peritectic Ba3 Cu5 O8 , has ¢ ° been temperature Tp of the seed. The temperature window available for melting the YBCO phase while avoiding full peritectic decomposition of the (Sm,Y)Ba2 Cu3 O72d seed is maximized for seeds of high Sm content and thickness in excess of 0.2 mm. Finally, the dwell time at temperatures above Tp should be as short as possible if the integrity of the seed is to be maintained throughout the YBCO growth process.
I. INTRODUCTION
The prospect of large, uniform superconducting grains of YBa2 Cu3 O72d (YBCO)1–7 which are able to carry a high critical current density sJc d at liquid nitrogen temperatures (77 K) offers significant potential for the application of this material in a range of engineering devices including magnetic bearings,8–13 flywheel energy storage systems,14 and permanent magnets.15,16 As a result, a number of melt-process techniques have been developed over recent years to fabricate YBCO in large grain form.17,18 Of these, seeded peritectic solidification is a particularly promising process since it enables control of the growth morphology, orientation, and microstructure of the grains. These features dominate the superconducting properties of melt-processed YBCO, and their optimization is essential if high quality samples are to be fabricated reproducibly.5,7,19–21 The peritectic solidification process in YBCO occurs typically according to the following reaction7 : Y2 BaCuO5 sSd 1 Ba3 Cu5 O6.72 sLd 1 0.42O2 ! 2YBa2 Cu3 O6.28 .
(1)
Solid (S) Y2 BaCuO5 (the “211” phase) particles dissolve in the Ba3 Cu5 O6.72 liquid (L) during the above process to provide a source of yttrium cations required for the formation of solid YBa2 Cu3 O6.28 (the “123” phase).22–24 The latter becomes superconducting when subsequently annealed in an oxygen atmosphere.7 The solidification process, and hence the morphological and 2048
Data Loading...
