GROWTH-MECHANISM OF BI2SR2CACU2OX SUPERCONDUCTING WHISKERS

Bi2Sr2CaCu2Ox (2212) whiskers are grown by annealing a melt-quenched glass plate in a stream of O2 gas. Other than the appropriate annealing temperature and starting composition, there are three important factors for the growth of the whiskers. The first is the glass precursor and the second is the presence of Al in the glass precursor; although the 2212 whiskers contain no Al, the addition of Al to the glass precursor (approximately 10 at%) is found to greatly enhance the whisker growth. The third factor is a steady stream of O2 gas (> 100 ml/min) during annealing. The growth site of the whisker is the bottom part. Using the results of a microstructural and compositional analysis of the crystallized glass substrate, a model of the growth mechanism of the 2212 whiskers is devised. According to this model, the Al-added glass precursor forms the Bi-Sr-Ca-Al-O complex oxide distributed near the surface, which acts as a ''rigid skeleton'' to support the bottom part of the whisker, which itself is surrounded by a thin layer of the Bi-rich phase and fits into the ''microcrucible'' of the Bi-Sr-Ca-Al-O phase. The Bi-rich phase has the same composition as the liquid phase equilibrated with the 2212 phase and melts at the annealing temperature. The whisker growth takes place through continuous precipitation of the 2212 phase from the melt at the base end of the whisker. This model can be called a self-supporting micro-top-seeding mechanism.