New High TC Superconductors Show No Weak-Links Effect, UB Scientists Report

BUFFALO, N.Y. -- The weak-links effect, a major obstacle to the development of superconducting materials for many industrial applications, has been eliminated in new high-quality superconducting films synthesized by University at Buffalo researchers.

Theirs is the first report of superconductors that have transition temperatures (Tc) above 100 degrees Kelvin, but show no weak-links effect.

The new films provide more evidence that it will one day be possible to build liquid-nitrogen-cooled superconducting devices to be used in applications ranging from magnetically levitated trains, to electric motors and generators, to nuclear-magnetic-resonance instruments. Superconductors with transition temperatures below 77 degrees Kelvin only can be cooled with liquid helium, which is too expensive for most industrial uses. Those with Tcs above 77 degrees Kelvin, on the other hand, can use less-expensive liquid nitrogen.

The research, supported by the New York State Energy Development Authority, was reported in a recent issue of Applied Physics Letters.

"We have demonstrated that superconductors that demonstrate very high critical current density at 77 degrees Kelvin in fairly strong magnetic fields can be made," said Jui H. Wang, Ph.D., Einstein professor of science at UB and principal investigator. Z.F. Ren, Ph.D., research scientist, and C.A. Wang, postdoctoral researcher, are co-authors.

Critical current density (Jc) is the maximum current density a superconductor can carry at a given temperature and magnetic field.

Wang explained that bulk superconductors are made of microscopic single-crystalline superconducting grains. Critical current density has historically been limited by poor electrical connections at the boundaries between the tiny grains. These boundaries are called weak links.

Even when there is no magnetic field, Wang explained, weak links significantly decrease a material's Jc, sometimes by as much as a factor of 100. In the presence of an external magnetic field, a prerequisite for most applications, the Jc decreases even further.

"We eliminated the weak-links effect by developing epitaxial films that superconduct," said Wang.

An epitaxial film, he explained, is one whose crystalline lattice is nearly perfectly aligned with the lattice of the substrate on which it is deposited. This near-perfect alignment allows electrons to flow unimpeded between the microscopic grains in the films, even in the presence of a magnetic field.

The UB group has synthesized many nearly identical small pieces of the new film and is now working on synthesizing larger samples. Samples have been requested for research use by Argonne National Laboratory, National Renewable Energy Laboratory and Oak Ridge National Laboratory.

The new films are made of oxides of thallium, bismuth, strontium, barium, calcium and copper.