John Budai sets
up an X-ray
diffractometer.
He used this instrument to analyze the structure of substrates and thin films for superconductors. He first demonstrated that rolled pieces of silver could provide sufficient texture to increase the superconductivity of an overlying film of yttrium-barium-copper oxide (YBCO) and proposed that this approach would work for other metals. Photograph
by Tom Cerniglio.
| We found that the critical current densities of Rosa's samples were better than those of YBCO on a polycrystalline substrate. |
 |
| Dave Christen measures the amount of electrical current that can be carried by a YBCO film deposited on a buffered metallic template, or substrate. Photograph by Tom Cerniglio. |
Young took her samples to Dave Christen (also in SSD) for measurements of critical current density. Christen placed the samples in a magnetic field cryostat containing a conventional superconducting magnet and a digital voltmeter. With this apparatus, Christen could chill the sample to 77 K, apply a strong magnetic field to it, and measure the voltage of a direct current passed through the sample. "We found that the critical current densities of Rosa's samples were better than those of YBCO on a polycrystalline substrate (the underlying template in which the various crystalline grains point in many different directions) but worse than those of YBCO on a single-crystal substrate."
Budai then analyzed her samples using X-ray diffraction in which X rays are directed at and scattered by the electron clouds of atoms in a crystal; the scattered X rays are captured as a pattern that yields information about the structure of the crystal. A native of Burlington, Vermont, Budai came to ORNL in 1984 as a Eugene P. Wigner Fellow after earning his Ph.D. degree in physics from Cornell University and working for two years at Bell Labs at Murray Hills, New Jersey. Budai was surprised to find that the YBCO film showed signs of texturing—that is, many crystalline grains mimicked the orientations of the grains in the textured metal, or substrate. He realized that textured growth of YBCO was possible on a rolled piece of metal, such as foils of stockroom silver used by Young. Further investigation in November 1990 showed that YBCO grows epitaxially on single-crystal silver with a much higher degree of alignment than on stockroom silver; this was the first direct proof of epitaxial alignment of YBCO on a metal substrate—the YBCO grains mimicked the alignment of atoms in the silver crystal. (Although YBCO had been found to be epitaxial on many different crystallographic planes of silver, only a certain single plane would result in the desired single orientation of YBCO grains.) It was also shown in February 1991 that a well-aligned YBCO film could be grown on an epitaxial buffer layer of platinum, the first proof that textured growth of YBCO films is possible on metals other than silver.
As a result of these observations, Budai proposed in late 1990 that rolling may cause enough alignment in silver or other metals to result in textured growth of the film and enhance its superconducting properties. Such alignment in rolled metal, which is often considered undesirable, is called biaxial texture. Thus began the development of a new solution to an alignment problem that had thwarted commercial applications of YBCO for several years.
In 1988, Duane Dimos of IBM determined that two crystalline grains of YBCO must be aligned at an angle of less than 10 degrees to achieve high critical current density in overlying superconducting ceramics. The 1990 ORNL critical current measurements and results from subsequent X-ray diffraction and electron microscope studies would confirm that if two crystalline grains are at a high angle at their boundary, electrons won't be able to hop efficiently from one grain to the other, and current will be disrupted. A high population of small-angle grain boundaries was essential for high current densities in YBCO.
Consider crystalline grains to be tiny cross-shaped tubes, tube walls to be grain boundaries, and electrical current to be BBs. If one tube is precisely lined up in front of the other tube, BBs can be shot through both tubes to the next tube. If the second tube is turned slightly at an angle, BBs could still get through, although they might bounce around inside the tube. But if the second tube is turned at a high angle (greater than 10°, lower than 90°), the BBs will crash into the tube wall (or miss the tube completely) rather than travel from tube to tube. Thus, just as more-or-less lined-up tubes are required for BBs to go a long way, small-angle boundaries are essential to efficient electron flow.
Amit Goyal of ORNL's Metals and Ceramics (M&C) Division then began an effort to roll-texture silver. Goyal, a native of northern India, earned his Ph.D. degree in materials science and engineering from the University of Rochester before coming to ORNL in 1990 as a postdoctoral researcher. Goyal, coworker Fred List, and their group leader Don Kroeger liked the idea of using silver as a substrate because it is chemically compatible with YBCO and it is lower in cost than some of the alternatives. However, their experience in melt processing YBCO was marred by the fact that it has a higher melting point than silver. Melt processing, which involves heating a material above its melting point and cooling it under controlled conditions, was perceived at the time to be a promising technique for making conductors. So they thought that alloys of silver-palladium, which have higher melting points than silver, could be used for melt processing. Goyal, who had extensive experience with melt processing YBCO-silver composites, experimented with various substrates and determined that silver-palladium alloys containing 10% palladium could be successfully used for melt processing YBCO. On completion of melt processing, the film contained many large crystals of YBCO several millimeters in size; however, they were all unaligned. The researchers were interested in Budai's observations of epitaxial YBCO on silver because they suggested a possible route to aligning YBCO films if the silver-palladium substrate could be textured. Unfortunately, no evidence of epitaxial growth of YBCO on silver-palladium substrates was ever observed
| Many researchers did not think it possible to build a high-performance wire from a thin film of YBCO. |
Despite the promise held by Budai's initial idea, enthusiasm for this approach remained low at ORNL, as well as at most companies working on superconducting wires. Many researchers did not think it possible to build a high-performance wire from a thin film of YBCO. In fact, ORNL was only able to obtain modest current densities of YBCO on the silver foils. Thin-film–deposited conductors were not believed practical because the best films could be grown on only inflexible, short substrates made from ceramics such as magnesium oxide and strontium titanate. M&C Division researchers, however, were enthusiastic about the progress made in the fabrication of polycrystalline bismuth-based, powder-in-tube tapes and bismuth- and thallium-based thick films respectively. These superconducting tapes are formed by flattening silver tubes packed with powders made of bismuth (or thallium), strontium, calcium, and copper oxides (BSCCO). It was commonly observed by researchers worldwide that long lengths of BSCCO tapes could be made and that each tape carries reasonably high currents, although much smaller than those epitaxial thin films carry. These tapes had commercially useful properties, especially in high-background magnetic fields, but only when chilled by gaseous helium, which is much more expensive than liquid nitrogen used for devices formed from high-temperature YBCO films. The thallium and bismuth-based wires had no biaxial texture, so they were expected to contain numerous high-angle grain boundaries. The question was, were the grain boundaries in these materials different from those in YBCO?
 |
| Eliot Specht analyzes the quality of palladium-nickel substrates for making superconducting tapes using the X-ray diffractometer. He determined that the first palladium layers deposited on rolled nickel were textured like the nickel. Photograph by Tom Cerniglio. |
Yet the idea of a YBCO "deposited conductor" remained alive at ORNL. In the fall of 1992, ORNL co-workers Fred List and David Norton proposed the following "future work" for the Laboratory at a DOE Peer Review meeting: "growth of in-plane textured YBCO films on textured metallic substrates (with) epitaxial growth of buffer layers (if necessary) and YBCO." That these buffer layers are a key to success would soon become apparent as a result of laboratory experiments.
In early 1993, enthusiam at ORNL for a "coated conductor" approach was increased when Goyal learned from a scientific conference that thin films could be used as conductors and reported that meter-long tapes had been made in Japan. This new information inspired him to continue his work on texturing silver because it appeared possible to deposit YBCO films using a thin-film process in long lengths, as opposed to a process like melt processing. The big prize lay in making a biaxially textured conductor out of YBCO because it has the best intrinsic superconducting properties at liquid nitrogen temperatures, permitting a range of applications. So Goyal began experiments to produce a sharp, single-orientation texture in silver but found it difficult. Moreover, additional orientations were present, resulting in many undesirable high-angle boundaries.
Goyal realized that producing a biaxial texture of less than 10 degrees in all directions was most likely in a material that had "cube" texture. However, he found it difficult to produce this texture in silver even though most face-centered cubic (FCC) materials are expected to produce a cube texture when deformed in a certain way. He thought his inability to get the desired texture was due to either small amounts of impurities in the silver or an inadequate rolling procedure. So he decided to conduct rolling experiments with other FCC metals like copper and nickel to learn more about texture development in silver.
While holding a textured nickel tape, Amit Goyal (right) watches technician Ed Hatfield insert a nickel rod in an ORNL rolling machine to make another roll-textured tape using a rolling procedure developed by Goyal. Goyal originally performed studies on texturing silver and silver-palladium alloys for use in superconductors. He suggested that the best approach to making a superconducting wire would be to use base metals rather than silver or silver-based alloys for epitaxial deposition of buffer layers. Photograph
by Tom Cerniglio.
In a typical experiment, ORNL technician Ed Hatfield inserted a copper or nickel rod between two steel rollers turning in opposite directions as they move closer together. After many passes, the rod is rolled into a thin foil and then is heated so that the material recrystallizes, producing the desired orientation. This rolling procedure aligned the copper or nickel grains at angles as low as 7 degrees.
Further experiments indicated to Goyal that such "hot rolling" would be required to texture silver. Hot-rolling experiments on silver at ORNL showed that, although cube texture could be obtained, it was difficult to control "twinning," which results in high-angle grain boundaries. Then Goyal decided to test the highly textured nickel and copper samples that had been lying in his office for months. He thought that, if he could deposit nonreactive noble metals like silver epitaxially on these reactive base metals using a bulk process, he may have a method of producing textured substrates. Goyal used a simple laboratory flash evaporator to deposit silver at room temperature on the textured copper. He found that the silver was epitaxial on the copper but had a slightly different grain orientation. When the silver-copper samples were heated at low temperatures, he found that the grain orientation switched to the cube texture. When the samples were heated at high temperatures, the silver diffused into the copper, forming an alloy without destroying the cube texture. From these experiments, Goyal produced a description of ways to make aligned, buffered substrates, a description that would become a draft of the first ORNL U.S. patent application for the technology. First was a method to obtain biaxially textured laminated surfaces suitable for growth of YBCO by epitaxial deposition of buffer layers on biaxially textured base metals. Second was a method to obtain biaxially textured alloys by diffusing the deposited material into the substrate without destroying the texture. The third was to realize epitaxial layers of the desired orientation by depositing a material onto the surface of a textured base metal and heating it to induce the desired texture. Goyal also proposed that base metals such as copper and nickel might be preferable to silver because of their lower cost and higher strength.
A major problem in using nickel and copper as substrates is that they react easily with oxygen when heated to higher temperatures, forming oxides that destroy the desired texture. Because nickel is more resistant to oxidation than is copper, the researchers eventually abandoned experiments on copper and silver and focused primarily on nickel and silver as the starting template. To minimize oxidation, they decided to use a film deposition technique rather than a bulk processing technique like melt processing to coat the substrate with YBCO.
In 1993, SSD's Dave Norton, working with Budai, began investigating the deposition of YBCO and other oxides on single-crystal silver foils using pulsed-laser deposition (PLD). Norton had been using PLD for superconductor research since 1989 when he came to ORNL as a Wigner Fellow, after obtaining his Ph.D. degree in electrical engineering from Louisiana State University. In PLD, a pulsed laser beam is focused on a target, causing the material to heat rapidly and vaporize, forming an ejecting plasma or plume of ions, atoms, and molecules. The vaporized material then deposits on an adjacent heated substrate, where it forms a crystalline film.
The growth of YBCO on silver presented a challenge. YBCO must be grown on the substrate at 750°C, a temperature at which silver evaporates. When cooled, the substrate and film don't shrink at the same rate. This difference in thermal contraction can lead to cracking of the film.
A rolled nickel substrate for YBCO also has limitations. Nickel atoms have an irritating tendency to swap places with copper atoms in YBCO, ruining the superconductor. A buffer layer was needed to serve as a chemical barrier to the diffusion of nickel atoms to YBCO. Such a layer should also be able to transfer nickel's crystallographic alignment to the YBCO film.
| An effort was begun at ORNL to deposit palladium on rolled nickel to prevent formation of texture-destroying nickel oxide. |
In the fall of 1994, Goyal, program manager Bob Hawsey, and Mariappan Paranthaman, a materials chemist in ORNL's Chemical and Analytical Sciences Division, learned at the Applied Superconductivity Conference in Boston that Russian researchers had epitaxially grown YBCO on layers of magnesium oxide, palladium, and single-crystal nickel on rock salt. They were struck by the fact that the Russians had deposited palladium on single-crystal nickel and that the palladium had helped transfer the alignment to YBCO. An effort was begun at ORNL to deposit palladium on rolled nickel. It was believed that coating nickel with a nonreactive noble metal like palladium or platinum would prevent formation of texture-destroying nickel oxide.
At this time, several vapor deposition processes using vacuum chambers were being used at ORNL for this project, including PLD, electron-beam evaporation (e-beam), and a technique called sputtering. At ORNL, important work in sputtering was being done by Qing He, a University of Tennessee (UT) graduate student in Dave Christen's group.
Qing He, a University of Tenneessee graduate student who has been working in the Solid State Division, used sputtering to show that palladium could be deposited on textured nickel and that cerium oxide could be deposited on palladium-nickel at room temperature. Photograph by Tom Cerniglio.
In the fall of 1994, Qing He began sputter-depositing palladium on roll-textured nickel, based upon information in the Russian work that had used evaporative deposition. After he optimized the sputtering process, He soon was able to deposit epitaxially aligned palladium films on the textured nickel supplied by Goyal, as analyzed by Budai and Eliot Specht, an X-ray diffraction expert in the M&C Division. Shortly thereafter, Qing He also was depositing biaxially textured silver films on the palladium-coated nickel substrates.
| Paranthaman later found a way to use e-beam evaporation to make palladium-nickel samples. Norton pursued the direct deposition of YBCO on silver. |
Paranthaman later found a way to use e-beam evaporation to make palladium-nickel samples. These accomplishments caused great excitement, because they pointed to a route for the deposition of chemically compatible, epitaxial buffer layers on textured nickel substrates. Norton pursued the direct deposition of YBCO and various oxide buffer layers on silver-coated nickel substrates in early 1995, but he encountered the same problems experienced with YBCO on crystalline silver.
Using this pulsed laser deposition system, Dave Norton and Chan Park deposit buffer layers of cerium oxide and yttria-stabilized zirconia and the superconductor YBCO on a rolled nickel substrate for making a high-temperature superconducting tape. Photograph by Tom Cerniglio.
During this time other important factors were recognized from earlier work done in 1993 and 1994. Qing He, who had been depositing oxides on polycrystalline nickel alloys, discovered the importance of surface smoothness in obtaining a single out-of-plane orientation (although no in-plane orientation was ever achieved on these polycrystalline metals). He had found that polished superalloy surfaces yield a much improved buffer layer alignment. Around the same time, Goyal and Paranthaman had observed improved alignment of the thallium based superconductor T11223 deposited on polished random silver surfaces. Christen and He then polished one half of a rolled nickel substrate and followed its progress through all subsequent processing. In characterizing the structure, Specht found that the palladium buffer layer was much better aligned out-of-plane on the polished half.
Clearly, obtaining a smooth, polished surface was essential to getting good thin films. Mechanical and chemical polishing of the nickel provided one way to achieve this goal. However, Goyal thought these methods would be inefficient for polishing kilometer-long conductors. He then tried to get a good surface on nickel by having the rolls polished before rolling the metal. Because very large deformations of the metal are caused by rolling, it was deemed possible that the surface features of the rolls could be replicated on the nickel. After several experiments, Goyal was successful in obtaining a highly smooth surface in rolled nickel, as smooth as that obtained by mechanical and chemical polishing.
These meter-long, well-textured, 125-micrometer-thick nickel tapes are smoother, thinner, and shinier than aluminum foil; in each one you can see your reflection.
More detective work lay ahead for the ORNL team. By May 1995, Christen had acquired a cerium oxide sputtering target and began depositing cerium oxide as a YBCO-compatible buffer layer on palladium-nickel substrates. Although epitaxial cerium oxide could be obtained, the buffer layers had poor structurethey were laced with many cracks, holes, and blisters. The X-ray measurements of Budai and Specht also showed that, during the high-temperature deposition of cerium oxide, atoms in the palladium layer were diffusing into and mixing freely with atoms of the nickel substrate. Electron microscopy observations of the samples by Goyal and by postdoctoral researcher Dominic Lee revealed a rough, coarse, cracked substrate surface.
| Quite by chance, He and Christen found that palladium could be deposited epitaxially on nickel even at room temperature. |
In early 1995, Christen and He conducted high-temperature, high-vacuum annealing studies of as-rolled nickel. They found evidence of gas evolution at the high temperatures at which the buffer layers were being deposited. Better coverage and smoother, blister-free buffer layers resulted from depositions on nickel that had been heat treated in high vacuum at high temperatures. But this approach did not solve the problems of palladium-nickel interdiffusion at the high deposition temperatures. Quite by chance, He and Christen found that palladium could be deposited epitaxially on nickel even at temperatures as low as room temperature, using ideas they had gathered from earlier work of depositing the oxide yttria-stabilized zirconia (YSZ) on polycrystalline superalloys. Even though early attempts had failed, He and Christen realized that it might also be possible to deposit cerium oxide at low temperature, although published work of others had succeeded only at high temperatures. The process worked. Later, detailed studies showed that the optimal deposition temperature was about 300°C, which was adequate to solve the palladium-nickel interdiffusion problems.
In early 1995, the ORNL team thought they had come up with a layered superconductor that could give a respectably high critical current density. Measurements by Charlie Klabunde in Christen's group showed that a layered structure consisting of YBCO, cerium oxide, silver, and palladium on the textured nickel had a critical current density of 80,000 amperes per square centimeter.
At a workshop on thallium-based superconductors in 1995 in Breckenridge, Colorado, Goyal, Hawsey, Kroeger, and Paranthaman felt frustrated. They knew that Hitachi researchers would be announcing their process for roll-texturing silver for use in a thallium-based superconductor. The ORNL researchers felt the "Oak Ridge" process, which also produced biaxially textured silver on which YBCO grows epitaxially, was very promising and should be mentioned at the meeting. But they wanted to give it a distinctive name with a memorable acronym. Because it was the Easter season, Goyal thought of bunnies and came up with the RABiTS acronym for rolling-assisted, biaxially-textured, substrates. The RABiTS process for making textured silver was announced as an important aside during Goyal's invited paper on thallium-based superconductors. A week later, Goyal mentioned the process during a presentation of another paper on grain boundary effects in superconductors at the Materials Research Society (MRS) meeting in San Francisco. The April 29, 1995, issue of Science News reported on ORNL's development of a process of "depositing the superconductor onto a carefully chosen substrate so that the crystal grains fall into alignment." (A patent on RABiTS has been applied for in the United States and certain foreign countries.)
Persistent difficulties in growing oxide buffer layers and YBCO films on silver-coated textured nickel encouraged ORNL researchers to eliminate this noble metal from the structure. Unfortunately, Norton and UT researcher Bernd Saffian discovered a new problem when depositing a film of YBCO on a buffer layer of cerium oxide on a palladium-nickel substrate. The YBCO, although seemingly well aligned, had cracks. The source of these flaws turned out to be cracks in the underlying cerium oxide. The cracking could make cerium oxide less effective in preventing the substrate's nickel atoms from trading places with copper atoms in the YBCO film, as well as introducing electrical discontinuity in the superconductor.
Norton proposed depositing YSZ on the cerium oxide because YSZ had been used successfully with other substrates, such as sapphire and silicon, for the semiconductor industry. He found that YSZ is a crack healer. It either grows across cracks arising in cerium oxide or it is structurally stronger and doesn't crack even if cerium oxide does.
Shortly after Christmas 1995, ORNL researchers had solved the seemingly intractable problems of thermal contraction, volatility, and oxidation in coated superconductors. A workable buffer layer architecture seemed at hand.
From January through March 1996, the ORNL team made superconducting tapes of YBCO on YSZ on cerium oxide on palladium-nickel. Critical current measurements showed that the team's best samples achieved 300,000 amperes per square centimeter.
Because it uses conventional rolling, the RABiTS process was thought to be of great interest to industry. Since ORNL's critical current was considered respectably high, it was decided to announce the details of the RABiTS process at the April 10, 1996, annual meeting of the MRS—and in a news release.
In the meantime, Norton, Saffian, and coworkers were looking for a way to simplify the buffer layer architecture. They wanted to minimize layers to reduce manufacturing costs. They knew that palladium could be eliminated if a way could be found to prevent nickel oxide from forming on the rolled nickel substrate. The key was to sweep out the oxygen in air that leaks into the chamber and reacts with the nickel substrate before it is coated with cerium oxide. Several ORNL researchers independently suggested introducing hydrogen gas into the chamber to reduce nickel oxide. The idea worked well the first time.
After the April 10 announcement, the ORNL team began making palladium-free superconducting tapes. Measurements of short nickel tapes on which the buffer layers and YBCO have been deposited showed that the critical current density of these tapes had doubled—from 300,000 amperes per square centimeter to 710,000 amperes per square centimeter. So in late April, the current-doubling feat was revealed in follow-up news bulletins. In June 1996, ORNL applied for a patent on the simpler buffer layer architecture for RABiTS substrates. Recently, the researchers raised the critical current density of RABiTS tapes to 900,000 amperes per square centimeter.
Work on deposition of oxide buffer layers continues at ORNL. Research is focused on reducing the number of buffer layers between YBCO and the substrate to just one. Using sputtering, Fred List prepared samples of YSZ on cerium oxide (that was grown by Paranthaman using e-beam evaporation) in which the cerium oxide has two different thicknesses in angstroms. X-ray diffraction and scanning electron microscopy analyses showed that cerium oxide cracks if it is 500Å thick but not if it's 50Å thick, suggesting that YSZ might not be needed to prevent cracking if the cerium oxide layer is very thin.
Mariappan Paranthaman uses e-beam evaporation to make a RABiTS substrate 7 centimeters long. He shows the nickel substrates on which buffer layers are deposited by e-beam evaporation. Photograph by Tom Cerniglio.
PLD is the best available technique for growth of oxide layers for research, but industry is more likely to use e-beam evaporation to make coated superconductors because it has experience with this technique and it may be more easily scalable than PLD.
ORNL researchers have optimized e-beam processes for depositing silver on palladium-nickel, cerium oxide on nickel (using cerium metal, not cerium oxide as a target, as in PLD), YSZ on cerium oxide on nickel, magnesium oxide on silver on palladium on nickel, and magnesium oxide on silver on platinum on nickel. In September 1996, Paranthaman made a 7-centimeter long RABiTS tape using only e-beam evaporation to produce the two ultrathin buffer layers of cerium oxide and YSZ.
| What industry may prefer as an alternative for superconducting wire manufacture is a nonvacuum process in which a chemical is spread on a rolled metal that can be heated in a furnace. |
Shara Shoup, a postdoctoral researcher who works with Paranthaman, uses a sol-gel process to chemically coat a substrate with a buffer layer. Chemical methods for making superconducting wire are expected to have wide industrial appeal. Photograph by Tom Cerniglio.
Like PLD, the e-beam process is done in a vacuum. Vacuum processing is expensive. What industry may prefer as an alternative for superconducting wire manufacture is a nonvacuum process in which a chemical is spread on a rolled metal that can be heated in a furnace. The spreading process could be spin casting, dip coating, or spraying. The furnace annealing would cause the organics in the solution to boil off and the rest of the material to crystallize as a film. So, to try to meet this need, Paranthaman, his group leader David Beach, and postdoctoral researcher Shara Shoup have developed sol-gel processes for epitaxially growing buffer layers of lanthanum aluminate, gadolinium aluminate, and barium zirconate deposited on single-crystal strontium titanate. Shoup has shown that sol gel processing can grow epitaxially one buffer layer on a textured metal substrate.
ORNL researchers have also confronted the possibility that nickel's magnetic properties may be a problem. (Nickel's magnetism could distort the shape of magnetic fields needed for accelerators and medical imaging instruments.) The researchers have identified an alternative material that has less magnetism than nickel and can do as good a job as the base metal in a RABiTS substrate.
ORNL researchers have searched continuously for combinations of materials and methods to produce wires and tapes with enhanced superconductivity. During the past three years, silver and palladium played important roles in the development of the wire, but as the work progressed and the needs of industry were considered, other materials appeared more economical. A winning combination of researchers in Oak Ridge and in industry will continue the quest for the winning combination of materials and methods for making the best possible superconducting wire.
| James Daley, team leader for DOE's Superconductivity Systems Program, praised ORNL's efforts in developing a superconducting wire. "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 that can carry the substantial electric currents needed for practical applications." |
DOE: http:/www.eren.doe.gov/superconductivity/
ORNL: http://www.ornl.gov/HTSC/htsc.html
"High Critical Current Density Superconducting Tapes by Epitaxial Deposition of YBa2Cu3Ox Thick Films on Biaxially Textured Metals," A. Goyal et al., Applied Physics Letters, 69 (12), p. 1795, 16 September 1996.
"Epitaxial YBa2Cu3O7 on Biaxially Textured Nickel (001): An Approach to Superconducting Tapes with High Critical Current Density," D. P. Norton et al. Science, Volume 274, p. 755 1 November 1996.
Deposition of Biaxially-oriented Metal and Oxide Buffer-layer Films on Textured Ni Tapes: New Substrates for High-current, High-temperature Super-conductors, Q. He et al, Physica C, vol. 275, p.155, 10 February 1997.
"Growth of Biaxially Textured Buffer Layers on Rolled Ni Substrates by Electron Beam Evaporation," M. Paran-thaman et al., Physica C, Vol. 275, 1997, p. 266, 20 February 1997.
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