New technique for fabricating superconducting wire has promise

October 10, 1992

By Jeffery Kahn, [email protected]

LBL scientists have developed a process for fabricating superconducting wire tapes that are able to carry the large electrical currents necessary 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 they developed, Energy and Environment Division researchers Rick Russo and Paul Berdahl and graduate student Ron Reade have created small samples of the new materials. These superconductors carry hundreds of times the electrical current of conventional copper wire and 10 times the current of the next best superconducting wires. An electrical coil wound with the new wire tapes could produce much higher magnetic fields than a conventional iron-core electromagnet. A technical account describing the new process was published in the Nov. 2 Applied Physics Letters.

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. More powerful magnets would find use in medical and scientific equipment, including magnetic resonance imaging machines and particle accelerators.

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.

Previous 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 LBL process 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 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 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.

Russo, Berdahl, and Reade's new 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. Consequently, the scientists 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.

The researchers say one major challenge remains before thin film-based wire tapes can be commercialized--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 such as the BSCCO process. Japanese researchers are continuing to develop the wire tape technology.