GROWTH-MECHANISM OF BI2SR2CACU2OX SUPERCONDUCTING WHISKERS

Bi2Sr2CaCu2Ox (2212) whiskers are grown by annealing a melt-quenched g lass plate in a stream of O2 gas. Other than the appropriate annealing temperature and starting composition, there are three important facto rs for the growth of the whiskers. The first is the glass precursor an d the second is the presence of Al in the glass precursor; although th e 2212 whiskers contain no Al, the addition of Al to the glass precurs or (approximately 10 at%) is found to greatly enhance the whisker grow th. The third factor is a steady stream of O2 gas (> 100 ml/min) durin g annealing. The growth site of the whisker is the bottom part. Using the results of a microstructural and compositional analysis of the cry stallized glass substrate, a model of the growth mechanism of the 2212 whiskers is devised. According to this model, the Al-added glass prec ursor forms the Bi-Sr-Ca-Al-O complex oxide distributed near the surfa ce, which acts as a ''rigid skeleton'' to support the bottom part of t he 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 equilib rated with the 2212 phase and melts at the annealing temperature. The whisker growth takes place through continuous precipitation of the 221 2 phase from the melt at the base end of the whisker. This model can b e called a self-supporting micro-top-seeding mechanism.