Template Film Development: Ion Beam Assisted Deposition
The ideal substrate for YBCO (Y-Ba-Cu-O ) is a polished single-crystal oxide material with a good lattice match to the superconductor basal plane. For a coated conductor, however, the substrate must be flexible, strong, inexpensive, and available in lengths > 100m. Joining these two disparate sets of requirements is the role of IBAD. The conductor requirements dictate that the substrate be metal, and metal in the appropriate form of long strips will generally be polycrystalline.
An ingenious solution to this problem was devised in the early 1990s. By bombarding a growing film of yttria-stabilized zirconia (YSZ) with argon ions at a few hundred eV, the film gradually acquires biaxial texture, and becomes a reasonable facsimile of a single-crystal oxide substrate. If the IBAD is carried out on thin strips of one of the Ni-Cr superalloys, such as Haynes 242 or Inconel 625, a nearly ideal substrate is created for subsequent deposition of YBCO.
In this photo, Steve Foltyn (left) and Paul Arendt show a one-meter section of their continuously processed YBCO tape. This process demonstrated the feasiblity of making kilometer lengths of YBCO, an important advancement for commercial applications.
Pulsed Laser Deposition
Pulsed Laser Deposition (PLD) has played a key role in coated conductors from the beginning, and today remains the preferred method for producing YBCO coatings, whether for electric power applications or electronic devices. For coating one meter tapes, the only change to a conventional PLD system needed was a means to heat the moving tape to temperatures of 700 to 800°C. This was accomplished by the use of a heated roller, which drives the tape via a stepper motor and heats it conductively, as shown in this photo. An additional change was the use of a rotating and translating target to uniformly remove target material during extended coating runs.