Nucleation of YBa2Cu3O7 from precursor films using the barium fluoride process

Coated-conductor applications of YBCO require YBCO layers with high J_c and a thickness of several micrometers. The barium fluoride process offers a convenient way of depositing crack-free fluorinated precursor layers up to 5 microns thick. However, converting thick precursor layers into c-axis-oriented YBCO films is challenging due to extensive nucleation of random grains. In this paper we address this problem with both a theoretical and experimental analysis of YBCO nucleation. We utilized optical polarization contrast to observe YBCO nuclei imbedded in the precursor matrix. We observed that the nuclei density strongly depends on processing parameters, with the oxygen partial pressure being one of the strongest factors. During ex-situ processing the nuclei merge and form grains and the nuclei density is, therefore, directly related to the grain size of YBCO film. It is desirable to have a small grain structure, since large grains do not connect well and the J_c of such a film is low. However, attempts to increase the nuclei density also generates more randomly oriented grains. Therefore the optimization of ex-situ processing is essentially finding a balance between c-axis grain density and random grain content.