High-Temperature Superconductivity Fact Sheet — Oak Ridge Simplifies RABiTS™ Fabrication
  

A new substrate for YBa₂Cu₃O_x "coated conductors" developed at the U. S. Department of Energy's Oak Ridge National Laboratory has been produced using common industrial coating equipment. The substrate offers the promise of affordable, second-generation, high-temperature superconducting wires for the emerging multi-billion dollar electric power equipment market.
  

The coated conductor concept uses a metallic nickel foil, two ultra-thin ceramic layers, and a layer of yttrium-based high-temperature superconductor on top. Superconductors can carry large amounts of electric current without losses due to resistance. High-temperature superconductors can perform this feat above the relatively "high" temperature at which liquid nitrogen boils, 77 Kelvin (minus 321 degrees Fahrenheit).

The tape's appeal lies in its simplicity and potential low cost and high speed of fabrication. The ceramic layers on the substrate can be made on equipment that's similar to that used to produce labels on soft drink cans, audio and video tape, and even the liners inside snack food bags. In order to make the substrate work, however, extremely thin oxide layers must be put down uniformly, and in such a way that their crystalline structure mimics almost exactly that of the nickel metal tape.

"ORNL's result represents a major step forward in the development of its new superconducting wire technology," said Bob Hawsey, director of ORNL's Superconductivity Technology Center. The basic idea is called "RABiTS™,," for rolling-assisted, biaxially-textured substrates. The Oak Ridge group recently produced the substrate using a simple buffer layer architecture and a common industrial film growth technique, called electron beam evaporation. The resulting sandwich of materials can then be used to grow high-quality layers of the superconducting materials that actually carry the electric current.
  

Earlier (in 1996), ORNL demonstrated the production of RABiT™ using pulsed laser deposition to grow the buffer layers. High critical current densities were produced using this substrate. Commercial developers were faced with the double challenge of scaling up the way to make the substrate and learning how to grow the superconductor on that substrate, all with limited dollars.

For the last few years, the ability to use the new high-temperature superconductors at the relatively "high" temperatures which their namesake implies (that is, liquid nitrogen's boiling point of 77 Kelvin, or minus 321 degrees Fahrenheit) has been limited. At this temperature, even weak magnetic fields as low as a few thousand gauss can virtually destroy the superconductivity in some superconductors. These are the levels of magnetic field that are present in most electric machines, such as transformers, motors, and generators. The field produced by neighboring wires in a transmission cable can also adversely affect its performance.

Electron beam evaporation technology and RABiTS™ may change all this. What Oak Ridge has done is to produce a new, industrially-scalable template on which the superconductor may be deposited.
  

First, pure nickel is roll-textured and heat treated. Next, extremely thin layers of two ceramic materials are rapidly deposited at Oak Ridge using a laboratory-scale electron beam system. For this, a cerium oxide layer as thin as 100 angstroms is placed "almost instantaneously" on the rolled nickel, followed by a 1400 angstrom layer of yttria-stabilized zirconia. In the lab environment, this layer takes about 20 minutes to grow. The buffer layers produced to date have excellent microstructural characteristics.

The ceramic layers in the RABiTS™ sandwich are remarkably thin. A typical sheet of copier paper is about 500,000 angstroms (0.002 inch) thick. These buffer layers are, therefore, 350 times thinner than a sheet of paper. ORNL staff member M. "Parans" Paranthaman did the electron beam evaporation of the two buffer layers. A paper describing these results is in press (Physica C).

Hawsey said that their recent accomplishment could be just the thing to push superconducting wires into the industrial and electric power sectors of the economy. "While others have been tackling the difficult issue of scaling up superconductor deposition on these kind of substrates, Oak Ridge wanted to see just how simply we could make the RABiT™ substrate that these companies need for good wire properties. We think that the latest results represent a step in the right direction."
  

Early results from growing superconductor on the substrates are encouraging. ORNL staff member David Norton used pulsed laser deposition to grow the superconductor on the newest version of RABiTS™. One sample, a 3-mm (1/8 inch) wide tape, carried 18 amperes of current at 77 Kelvin and 50 amperes at 65 Kelvin, measured across the full tape width. The new tape's performance as a function of temperature and applied magnetic field was similar to that obtained earlier using buffer layers deposited entirely by the laser ablation process. The preparation of additional samples intended to duplicate this result is under way.

ORNL's tape conductor research program is funded by two offices at the U. S. Department of Energy: The Office of Energy Efficiency and Renewable Energy's Office of Utility Technologies, and the Office of Energy Research's Office of Basic Energy Sciences. Dr. James G. Daley, team leader for the DOE Superconductivity Program for Electric Power in the Office of Utility Technologies, commented that "the Oak Ridge approach to substrates for YBCO coated conductors appears to offer attractive cost and performance advantages to wire developers.

For more information contact:
Dominic Lee, ORNL, (865) 241-0775, e-mail leedf@ornl.gov