Growth morphology of large YBCO grains fabricated by seeded peritectic solidification: (I) The seeding process

The growth of large grain YBa2Cu3O7 - delta (YBCO) by peritectic solidification in the presence of a (Sm,Y)Ba2Cu3O7 - delta 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/YBa2Cu3O7 - delta solid solution, which occurs via a reaction between (Sm,Y)(2)Ba2CuO5 and liquid state Ba3Cu5O8, has been observed across this interface at temperatures below the peritectic temperature (T-p) of the seed. The temperature window available for melting the YBCO phase while avoiding full peritectic decomposition of the (Sm,Y)Ba2Cu3O7 - delta seed is maximized for seeds of high Sm content and thickness in excess of 0.2 mm. Finally, the dwell time at temperatures above T-p should be as short as possible if the integrity of the seed is to be maintained throughout the YBCO growth process.