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Researchers track cause of energy loss in superconducting

High-temperature superconducting materials have almost limitless potential but are often less "super" in real performance, since they lose as much as 95 percent of the current running through them.

A University of Wisconsin-Madison experiment has found a surprising contributor to this energy sink, by pinpointing tiny defects that clog electrical flow through the wires.

The study, published in the April 30 issue of the journal Nature, provides promising evidence that one of high-temperature superconductivity's biggest obstacles can be overcome. The answer lies in devising new manufacturing methods to eliminate the flaws.

"What is absolutely critical to high-temperature superconductivity's future is making better, more efficient conductors which have improved current flow," said David C. Larbalestier, senior author of the study and director of UW-Madison's Applied Superconductivity Center (ASC).

David C. Larbalestier

David C. Larbalestier (large image)

Larbalestier noted that today's materials have a critical-current density - or total volume of current that reaches its destination - of only about one-fourth to one-tenth of their potential.

"Unchecked damage during the production of these materials is a major barrier to current flow," he added. "This experiment provides a clear map of what to do next to improve high-temperature superconductivity." Superconducting materials have the ability to conduct electricity with no loss of energy. It was first demonstrated more than 80 years ago that some materials cooled to almost absolute zero will lose all resistance to electricity. Absolute zero is zero degrees Kelvin or minus 460 degrees Fahrenheit.

Since 1986, the field has been energized by a flurry of discoveries of materials that superconduct at higher temperatures - a full 100 degrees "warmer." But the problem has been getting these materials to conduct energy efficiently across long wires.

While the race to achieve superconductivity at higher temperatures grabs most of the popular attention, Larbalestier said superconducting temperatures are high enough now to be commercially useful. The greatest hurdle remains improving the current density by manufacturing more efficient materials.

Larbalestier said superconducting current has a tendency to percolate through materials, rather than sailing through unimpeded, resulting in huge losses of energy. "One of the tricks of the technology has been to explainand understand this problem of percolation," Larbalestier said.

The Applied Superconductivity Center is in a unique position to study the problem by using a novel technology called magneto-optical imaging. Developed and patented by ASC scientist Anatolii Polyanskii, the device allows researchers to literally create a visual image of current flow and barriers through microscopic filaments of superconducting material.

Larbalestier said the research found that much of the energy loss comes from two sources. Some current is blocked by grain boundaries made when the material is crystallized. But an even larger "limiting factor" are the man-made defects introduced by the manufacturing process.

The research team tested one of the best samples of superconducting "tape" on the market, made by American Superconductor Corporation in Massachusetts. The tape is about 2 millimeters thick, and filled with 85 filaments only a few microns thick. Electric current passes through this honeycomb of tiny wires.

ASC scientist Cai Xue-Yu tested individual filaments extracted from these tapes and revealed cracks that were once invisible to scientists. Larbalestier said that cracks are difficult to eliminate completely from manufacturing, but this research points directly to new fabrication techniques to reduce the problem.

The current state of the technology is "marvelous but crude," he said, and its potential is enormous. Efficient superconducting wires could replace copper wire and provide 10 times the energy density of copper. The advances could solve problems associated with power industry deregulation, by bringing more efficient power cables into city centers and placing transformers inside buildings instead of outside in power centers.