New Superconductors Carry Record Electrical Currents

January 22, 1993

Contact: Jeffery Kahn,

Technical background information

Lawrence Berkeley Laboratory scientists have developed a process for fabricating superconducting wire tapes that are able to carry the large electrical currents requisite for commercial usage.

High temperature superconductors were discovered in 1986 but up until now, researchers have had difficulty producing wires with good superconducting properties. Consequently, superconducting wire has yet to make its commercial debut.

Using the new fabrication process, LBL researchers have created small samples of the new materials. These superconductors carry hundreds of times the electrical current of conventional copper wire and ten times the current of the next best superconducting wires. Moreover, an electrical coil wound with the new wire tapes could produce much higher magnetic fields than a conventional iron-core electromagnet.

These two performance characteristics -- greater current density and superconducting magnets with higher magnetic fields -- are vital achievements toward the eventual manufacture of a new generation of motors and generators. Superconducting motors and generators would be lighter, smaller, higher-powered, and more energy-efficient and would find uses everywhere from the factory floor to power plants. More powerful magnets would find use in medical and scientific equipment including magnetic resonance imaging machines and particle accelerators.

The techniques for fabricating these high performance materials were developed by LBL scientists Rick Russo and Paul Berdahl, and UC Berkeley graduate student Ron Reade. A technical account of their results was published November 2, 1993 in Applied Physics Letters.

Since 1988, Russo, Berdahl, and Reade have focused on a very high-risk goal, attempting to create high-current films on a metal substrate (a metal foundation layer). Up until now, the only high-current films available were those deposited on single crystals. Unlike a superconductor-coated metal substrate, single crystals are limited in length and cannot serve as wires.

Scientists have devised many different approaches to depositing superconducting films on metal substrates. But no matter the method, the results have been disappointing in terms of the current-carrying capacity of the wire tape.

Researchers believe the key to improving the current density of wire tapes is to control how the millions of individual crystallites of superconducting material are oriented when they are deposited as a film. Films, particularly those derived from the yttrium family of superconducting materials, have not performed well because of the random orientation of their crystallites. Ordered randomly, like cornflakes in a box, the crystallites are weakly linked, which impedes the flow of electrons.

Prior research has shown that the weak link problem can be solved by aligning all the crystallites so that they have the same orientation. However, actually producing a film like this has been an elusive goal.

The new process developed by LBL researchers begins with a high-strength nickel alloy substrate. Before the superconducting layer (an yttrium-barium-copper oxide compound) is deposited, a special zirconia buffer layer is grown on the substrate. This buffer and the process by which it is deposited is the key to the success of the LBL process. Uniquely, the buffer becomes a template, dictating the orientation of the individual crystallites of superconductor when this next layer is grown.

Pulsed lasers are used to create the two layers. Pulses of laser light vaporize the buffer and superconducting compounds, which are grown in turn onto the nickel substrate.

Researchers control the crystal orientation of the zirconia buffer layer through a process called ion-assisted growth. As the buffer layer is being grown, it is bombarded by an ion beam from an oblique angle. Only the buffer crystallites with the correct orientation to the incoming ion stream survive; those that are deposited broadside to the stream are eroded away. The net result is a buffer template that makes possible a superconducting film with unprecedented current densities.

Despite its promise, the new LBL process remains an underdog in its bid to become the first commercial technology for fabricating superconducting wire. Hundreds of researchers around the world have been working on another promising technique, the powder-in-tube method for making tapes.

In this process, superconducting powder (bismuth, strontium, calcium, copper oxide, collectively known as BSCCO) is pressed into silver tubes and drawn out to make wire. Compared to laser deposition, BSCCO fabrication has the advantage of being a relatively conventional form of wire manufacturing.

The new LBL wire tapes, however, have distinct performance advantages. Their current densities (at the temperature of liquid nitrogen) have been measured at 10 times higher than the BSCCO. Additionally, the current densities drop rapidly in the BSCCO wire as the magnetic field increases. LBL researchers consequently believe their new wire tapes are a likely candidate for use in high-performance roles which require higher currents, higher magnetic fields, and/or higher temperatures.

LBL researchers say one major challenge remains before thin film-based wire tapes can be commercialized. Next, they say, the new fabrication techniques must be refined to make large amounts of wire tape cheaply. The LBL program that produced the wire tapes ended in September as the federal research effort narrowed its focus, concentrating on the development of more familiar wire manufacturing technologies like the BSCCO process. Japanese researchers are continuing to develop the wire tape technology.

Lawrence Berkeley Laboratory is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.

Technical Background Information

Lawrence Berkeley Laboratory scientists have proved the technical feasibility of high-current superconducting tapes, which are made by deposition of thin films on metal substrates.

The new films can carry 600,000 amperes per square centimeter at the temperature of liquid nitrogen, 77 kelvin (K). Thus a thin metal tape one centimeter wide with one-half micrometer films on both sides is capable of carrying 60 amperes. A composite conductor fabricated from such tapes, with a cross section of 1 centimeter by 1 millimeter could carry about 6,000 amperes. At lower temperatures, the current carrying capacity is even higher.

High-current capacity superconducting thin films operating at 77 K have been available for several years. However, they have formerly been available only on single crystal substrates, which cannot be used for wires. Thus, more practical substrates such as metals are required.

The new LBL process uses high-strength nickel alloy substrates. The key to the process is a special buffer layer of YSZ (yttria-stabilized zirconia), which is deposited by pulsed laser deposition. As the buffer layer is grown, it is bombarded by an ion beam from an oblique angle. This special deposition process causes the YSZ layer to form crystallites which all have nearly the same orientation. Thus the buffer layer resembles a single crystal. The superconducting film of YBCO (yttrium-barium-copper oxide) is then deposited on the oriented buffer layer, using the standard pulsed laser deposition process.

An important potential application of the new film process is to fabricate conductors for magnet coils. Such coils are necessary for high temperature (77 K) superconducting generators and motors to become practical. They also would be useful for magnetic resonance imaging, magnetically levitated trains, microwave power generators, particle accelerators, and any other applications which benefit from high magnetic fields.

The performance of the new films in magnetic fields is the subject of further study, but it is already clear that the properties are rather good. For example, in a magnetic field of 4,000 gauss, the films can carry 1,000,000 amperes per square centimeter at 20 K (80,000 at 77 K).

One competing technology is the powder-in-tube method for making BSCCO (bismuth-strontium-calcium-copper oxide) tapes. In this process, superconductor powder is pressed into silver tubes and drawn out to make wire. This and related processes are not yet commercial but seem likely to be used in the future for magnets and other applications, especially for use at the very low temperature of liquid helium (4 K). However, the new YBCO thin-film-based tapes are expected to succeed in high-performance applications which require access to higher current densities, higher magnetic fields, or higher temperatures than currently achievable with the BSSCO process.

In addition to uses in magnets, motors, generators, and superconducting solenoid energy-storage devices, the new thin film process also has potential uses in electronics.

Most of the electronics applications for high-temperature superconductors currently under development rely on films deposited on single-crystal substrates. However, even in electronics, single crystal substrates (which are limited in terms of size) may not do the job. For instance, the new thin film process may prove useful in multi-chip modules where many semiconductor chips are to be linked with superconducting interconnects.

The new LBL process also offers the potential to deposit high-temperature superconducting films on amorphous substrates and onto other materials for which the usual epitaxial methods do not work.