Growth of thick YBa2Cu3O7 layers via a barium fluoride process

The ex situ process has emerged as a popular technology for growing epitaxial YBa2Cu3O7 (YBCO) for the second generation of coated conductors. The process combines low cost, ease of scale-up, the possibility of achieving high critical currents in relatively thick YBCO layers, as well as high tolerances to substrate misalignment. All of these factors explain why the process is increasingly becoming adapted by manufacturing companies for the production of YBCO wires. Studies of the barium fluoride variant of the ex situ process have offered rare insights into the structure, chemistry and pinning in ultra-thick, 5 mu m, ex situ YBCO layers. The transition to thick YBCO layers is viewed as one of the key ingredients of the commercial success of the YBCO wire technology, which makes these studies especially relevant. This review is intended to be a comprehensive introduction into the history, physics and chemistry of barium fluoride processing, discussed concomitantly with recent results and trends in this field. Furthermore, comparisons are made between YBCO films made by ex situ and in situ deposition processes.