Nobody completely understands superconductors. So fathom how James S. Schilling, Ph.D., led a team that makes the phenomenon work better.

Schilling, a professor of physics in Arts and Sciences at Washington University in St. Louis, collaborated with recent doctoral graduate Takahiro Tomita and researchers at Argonne (Ill.) National Laboratory to determine whether one region in superconductors, called grain boundaries (GB), are oxygen deficient. Such oxygen deficiency impairs superconductor performance.

Their paper, titled "Enhancement of the Critical Current Density of YBa2Cu3Ox Superconductors under Hydrostatic Pressure," is reported in the Feb. 24 issue of the highly regarded journal Physical Review Letters.

A superconductor is a solid material that conducts electricity without resistance when it is cooled to certain subzero temperatures. Because there is no resistance, current uniquely travels through superconductors without losing energy.

Their study involves the newer, so-called "high-temperature" ceramic superconductors. They superconduct at less frigid temperatures than other superconductors, eventhough still in the subzero realm.

The superconducting material used in this study was a ceramic compound consisting of millions of microscopic crystals (grains). The WUSTL/Argonne team specifically developed a technique to determine whether a desired maximum number of possible sites are filled with oxygen in the GB, which surrounds every crystalline grain. The GB is a region of misfit between the grains and commonly is only a few atoms wide.

The study used the most widely employed ceramic superconductor, known as YBCO. YBCO (or YBa2Cu3Ox) simply represents its "yttrium-barium-copper-oxide" content.

Fully oxygenated
Full oxygenation is essential for the manufacture of reliable ceramic superconductors. Maximizing oxygen in the GB helps maximize critical current density (Jc), or the maximum current that a superconductor can carry. In the subatomic world of superconductors, unrestricted current flow must be the outcome.

"Even in the best superconductors," Schilling noted, "GBs limit their ability to carry the high electric currents mandatory for applications in electric power grids or to generate enormous magnetic fields. To enhance the current carrying capacity, it is essential to bathe the grain boundaries in as much oxygen as possible. Unfortunately, it is very difficult to determine how much oxygen is really present in the GB.

"We have developed a method which allows one to estimate this, called pressure-induced oxygen relaxation."

Boyd W. Veal, Ph.D., an Argonne physicist and a co-author of their paper, said the technique "could tremendously ease the superconductor manufacturing problem. There is hope that these discoveries can make (superconductor) materials more accessible for practical applications."

Until now, science had determined how to check ceramic superconductors' crystalline structures - but not their GBs - to ensure all potential oxygen sites were filled. It also was known that full oxygenation is essential. The researchers note in the paper, "Even when the bulk material is fully oxygenated, the GBs are likely oxygen deficient."

"This is the most applied thing we've ever done," Schilling said of his WUSTL research. "But we've done a huge amount of work in the past on oxygen ordering; that was in the (superconductor crystalline structure) bulk itself - not in the grain boundary."



Source: Washington University in St.Louis - School of Medicine

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