BaF₂ post-deposition reaction process for thick YBCO films

Summary: The BaF₂ process is a very attractive fabrication process for YBCO conductors since it allows the deposition and the formation steps of YBCO films to be separately performed, and it has been demonstrated that the films with high critical current densities can be fabricated on the buffered metallic substrates. However, we still face one major challenge for this process to be made as a commercially viable fabrication method. This is the avoidance of non-c-axis-oriented growth of YBCO when the thickness and the growth rates for the films are increased significantly beyond what are currently achieved. Meeting both the thickness and the growth-rate requirements simultaneously is extremely difficult. One possible way to make the rate requirement reduced is to find winding methods of the tapes in such ways that the both-sided tapes can be reacted in a batch process. Although the winding fixture will be more complicated than a simple dram, I believe that there are ways to construct such fixtures if the subatmospheric-pressure processing is incorporated in the design for reacting the wide precursor tapes. Then, we are only faced with the problem of growing the c-axis-oriented thick YBCO films. In order to overcome the latter difficulty, we need a detailed understanding of the YBCO nucleation kinetics in thick films under various atmoshperic conditions. Such an understanding is likely to lead to the development of new or modified precursor-deposition methods and/or reaction-heat treatment procedures to facilitate the growth of the thick YBCO layers without the non-c-axis-oriented YBCO growth.