Facts about the Oak Ridge RABiTS Process for Coated High-Temperature Superconductors
A new superconducting wire developed at the Department of Energy's (DOE) Oak Ridge National Laboratory (ORNL) may pave the way for the future manufacture of practical yttrium- or thallium-based conductors for electric power applications.
ORNL researchers described a new type of high current density, thick-film superconductor on April 10, 1996, at the Materials Research Society meeting in San Francisco.
Oak Ridge researchers have produced a roll-textured, buffered metal, superconducting tape with a critical current density of 300,000 amperes per square centimeter in liquid nitrogen. The higher the current density the greater the amount of electric current that can be transmitted through the wire. Standard household wires typically carry less than 1000 amperes per square centimeter.
Superconductors have virtually no resistance to electric current, offering the possibility of new electric power equipment with improved energy efficiency, smaller size, and lower operating costs than today's devices. These systems could help reduce the U.S. requirements for new power plants, since electricity demand is expected to double by the year 2030.
To produce a superconducting wire sample, the ORNL researchers first developed a process called rolling-assisted biaxial textured substrates, or RABiTS. Substrates provide the underlying foundation for the superconducting wire. RABiTS enables the superconducting materials to have a high degree of grain alignment in all directions, a necessary condition for more efficient current flow through the superconductor. Grains are homogeneous regions within a material in which the atoms are well aligned.
The ORNL team, comprising researchers from the Solid State, Metals and Ceramics, and Chemical and Analytical Sciences divisions, developed substrates for the wire that are chemically compatible with high-temperature superconductors and exhibit sharp biaxial texture. Texture refers to the alignment of the atomic planes within the wire. First, base metal tapes such as nickel are prepared using special rolling and heat treatment procedures at ORNL. No further surface treatments, such as polishing, are used. Next, a buffer layer technology, developed specifically for these textured metals, was used to provide a chemical barrier between the nickel and the superconductor while maintaining the texture. For this, a thin layer of palladium is deposited using electron beam evaporation or sputtering. Metal oxide buffer layers, which include cerium oxide and yttria-stabilized zirconia, are placed on the tapes by pulsed-laser deposition. This is a RABiTS substrate, ready for application of the superconductor.
The high-temperature superconductor yttrium-barium-copper-oxide (YBCO) is then deposited on the conditioned surface by pulsed-laser deposition. The high-current sample was 3 mm wide and 15 mm long. The critical current density was measured across the entire sample geometry.
"These special substrates will enable the next generation of high-temperature superconducting wires to be used in transmission cables, transformers, current limiters, and motors and generators -- anywhere large amounts of electricity are produced, transmitted, or distributed to customers," said Robert Hawsey, director of ORNL's Superconductivity Technology Center. Hawsey said that the wire will likely be found in devices on the utility side of the electric meter in most cases, but not always.
"One of the notable exceptions perhaps will be large, several-thousand horsepower motors used in heavy industry, such as paper or steel mills, or at our nation's electric power generating stations," Hawsey said. "Further development of the Oak Ridge process could lead to many industrial or commercial applications of superconductivity where none presently exist."
A Team Effort
"The key to the success of RABiTS has been the team effort at Oak Ridge," added Hawsey. ORNL leveraged funding from two major branches of the DOE to make this project a success. Full funding for the project was provided jointly by the Office of Energy Efficiency and Renewable Energy, Office of Utility Technologies, and by the DOE Office of Energy Research, Division of Material Science. A unique, three-division team at ORNL combines the strengths of basic and applied scientists, bringing physicists, chemists, and material scientists together to work toward a common objective of wire development. At least twelve staff members are presently participating in this project, according to Hawsey.
Remarking on the intra-laboratory team approach at Oak Ridge, Anthony Schaffhauser, director of Energy Efficiency and Renewable Energy Programs at ORNL said, "This success story in high-temperature superconductivity is a good example of why the continuous feedback model for research and development projects at our nation's laboratories works so well."
High-temperature superconductor study at ORNL began in the late 1980's, almost immediately after the discovery of the ceramic-like, high-temperature superconductors. In late 1986 two IBM scientists announced the discovery of a material that was superconducting at 34 degrees Kelvin, 11 degrees warmer than had ever before been observed. It took just a few more months for scientists in the U.S. and Japan to create new compounds with yet higher superconducting transition temperatures. By March 1987 eight new materials were produced that "superconduct" above 77 Kelvin, the temperature of boiling liquid nitrogen. One of these materials, a compound made of yttrium, barium, copper, and oxygen, or YBCO for short, is superconducting around 92 Kelvin.
RABiTS is Developed
As early as 1990, John Budai of ORNL's Solid State Division found that highly aligned, epitaxial YBCO films could be grown on textured silver foils, single crystals, and even silver buffer layers. Epitaxy means that the orientation of the YBCO film matches that of the underlying substrate. The observed enhancement in critical current density associated with the epitaxy led Budai to suggest a route by which one could fabricate long lengths of highly aligned wires-- growing films on roll-textured metal strips.
Researchers world-wide have been trying to fabricate long lengths of biaxially textured silver strips. However, due to the unique deformation characteristics of silver, in-plane alignment of the silver to within the desired angle of 10 degrees has been a formidable task.
An important step in fabricating long lengths of these near single crystal-like strips was realized by Amit Goyal of the Metals and Ceramics Division. Goyal realized that a noble metal interface for the superconductor could be prepared by the deposition of these metals epitaxially on a base metal surface. Goyal and coworkers demonstrated that, although biaxial texture was difficult to achieve in silver strips, one could develop extremely sharp biaxial textures in other metals, and then deposit on them additional, chemically benign metal layers with epitaxial orientation to the original metal strip. "This is a remarkably simple and versatile process for producing long lengths of near single crystal-like substrates," said Goyal.
Qing He, a student from the University of Tennessee, developed novel methods to put metals and oxides down epitaxially on the base metal strips using sputtering. M. Paranthaman of ORNL's Chemical and Analytical Sciences Division developed various processing conditions for growing metals epitaxially on the base metal strips using electron beam evaporation. Fred List and Patrick Martin of the Metals and Ceramics Division built a home-made evaporator to grow silver on these strips at high growth rates.
The transition from the metal surface to the superconductor presently requires several intermediate materials, called oxide buffer layers, to transfer the alignment while avoiding chemical degradation of the superconductor. A major effort has concentrated on the physical and chemical aspects of putting thin ceramic buffer layers on top of these metals in order to create the best template for the superconductor.
"The oxide buffer layers have been a challenging aspect of the project since day one," said David Norton of ORNL's Solid State Division. Norton, He, and David Christen, also of the Solid State Division, have developed various oxide buffer layer architectures using both pulsed laser deposition and sputtering. These layers maintain the biaxial texture and provide a chemical barrier between the superconductor and the metal tape. This structure, now ready for deposition of the superconductor, is called RABiTS.
There should not be any inherent limitation to the lengths of wires that may be produced using RABiTS. ORNL has focussed on the purity requirements of the starting metal and has learned to produce substrates as thin as 25 micrometers (1 mil), both important issues for the eventual commercialization of wires made from RABiTS). ORNL has also produced the base textured metals in strips as long one meter. These strips are then cut up for further processing, which includes application of the additional layers.
"The RABiTS we can produce have the sharpest rolling-assisted texture I've ever seen," said Don Kroeger, leader of the superconducting materials group in the Metals and Ceramics Division. One RABiTS sample, ready for deposition of the superconductor, was recently measured by Eliot Specht, also of the Metals and Ceramics division. It showed less than 7 degrees variation in in-plane alignment. Also helping analyze the samples using scanning electron microscopy is Dominic Lee, a post-doctoral fellow also working in the division.
ORNL believes that the RABiTS process may be used to make wires from several families of high-temperature superconductors. "Our goal at Oak Ridge is to develop the technology base to enable a practical, high current YBCO or perhaps thallium-based wires using rapid and easily scaled processes," said Hawsey. A patent on the RABiTS has been applied for in the U.S. and certain foreign countries.
High Critical Current Densities Achieved
For the high current density samples, Norton and research assistant Bernd Saffian placed the superconductor YBCO on top of the ORNL substrate with the same laser deposition system. Sharp biaxial texture in the superconducting layer was created. This feature is critical in sustaining useful electric currents if the wire is to be used in high magnetic fields. "The superconducting layer is typically 0.5-2 micrometers thick in these laboratory samples," said Norton. The overall dimensions of the short wires are 3-mm wide and 15-mm long.
John Budai added that the latest superconductor produced using RABiTS is remarkably well-textured. Budai used x-ray diffraction to measure the alignment of the grains (referred to as "delta phi") of each layer of the conductor. The misorientation of the YBCO grains is about 8 degrees.
David Christen and colleague Charlie Klabunde of the Solid State Division measured the superconducting properties of the resulting RABiTS-based superconductor. "The critical current density in a recent sample was 300,000 amperes per square centimeter, measured in self-field at 77 Kelvin," said Christen. "More important for applications, perhaps, was the behavior of this wire in a background magnetic field. When measured in fields as high as 7 Tesla, the superconductor on RABiTS exhibited the kind of strongly-linked current flow I might expect from a deposit of YBCO on a single-crystal substrate," said Christen. One recent sample had a critical current density of 100,000 A/cm2 in a 1 Tesla field at 77 Kelvin, for example, conditions under which the bismuth-based powder-in-tube wires have very poor performance and are thus not useful for most electric power applications.
James Daley, manager for DOE's Superconductivity Systems Program, praised ORNL's efforts. "This research innovation has tremendous significance" for superconducting wires, said Daley. "We now have a path to the goal we've pursued since the 1986 Nobel Prize-winning discovery--a superconducting wire that can be used in motors, generators, and other energy systems while operating at liquid nitrogen temperatures." William Oosterhuis, Chief, Solid State Physics and Materials Chemistry Branch in DOE's Office of Basic Energy Sciences, added, "We are extremely pleased that a combination of fundamental materials science supported by the Office of Basic Energy Sciences, and applied research supported by the Office of Energy Efficiency and Renewable Energy has resulted in the development of materials processing methods for high-temperature superconducting wires which can carry the substantial electric currents needed for practical applications."
Research Implications for Applications
Prototype wires previously developed have been limited to operation at temperatures below 35 Kelvin in any substantial magnetic fields. With this new ORNL-developed technique, wires may be fabricated permitting superconducting machines such as motors, generators, transformers, and current limiters to operate in liquid nitrogen. Use of this mode of operation is cheaper compared to liquid helium or closed-cycle refrigerators needed to cool helium gas. "Even transmission cable technology, with its lower field requirements, may benefit from RABiTS, since higher absolute critical currents in a single tape at 77 Kelvin and lower tape production cost, compared to other HTS wires, may be possible," commented Hawsey.
Liquid nitrogen boils at 77 Kelvin compared to more costly and much colder liquid helium that is traditionally used in superconducting equipment. For example, liquid nitrogen typically costs 20 cents per two-liter bottle versus $10 for the same amount of helium.
Commercialization Opportunities and Further Development
ORNL is presently negotiating with several industrial partners to further develop and commercialize this innovative wire manufacturing process. Technical assistance from ORNL staff members may be available through a cooperative agreement or other formal working arrangements with the Lab. The technology is also available for immediate licensing, according to Larry Dickens, a licensing executive with the Lab. Further information is available by calling Hawsey at (865)574-8057 or Dickens at (865)576-9682 and referring to disclosure number 1640-X.
The Superconductivity Technology Center is one of three national sites funded by DOE's Office of Utility Technologies. The center implements DOE's national program to develop the technology necessary for U.S. industry to proceed to commercialization of high-temperature superconducting electric power products.
ORNL recently initiated cooperative work with the superconductivity center at Los Alamos National Laboratory. Scientists there have refined a different process for aligning the YBCO grains. The labs expect to accelerate progress in understanding the materials science aspects of the substrates and the application of the superconductor through the collaboration. Joint projects with industry and the nation's leading universities researching HTS are also being discussed, according to Hawsey.
Key research at ORNL on high-temperature superconductors is focused on aspects of wire development, including superconducting powder synthesis, new processes for reliable, high performance wire and understanding the relationship between various routes to conductor fabrication and features that enable current to flow. ORNL collaborates with U.S.-based companies in wire development, alternating current applications of HTS, and in coil development for electric power systems.
For more information on ORNL's high-temperature superconductor research, contact:
Dominic Lee, Superconducting Technology Program Manager
Donald M. Kroeger, Manager of Conductor Development
John D. Budai
David P. Norton
Eliot D. Specht
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