The next generation of superconducting wires, which could operate efficiently at the high temperatures needed to make applications such as levitating trains feasible, has been created by researchers.

For 20 years, researchers have worked to develop the perfect high-temperature superconducting wires to replace today's copper-based power grid. But the secret, it now seems, is to build flawed ones. The key may be to position non-conducting nanodots at strategic points within the wire.

Electrical current flowing through superconducting materials experiences virtually no resistance, enabling wires of the material to carry high current loads very efficiently. However, such a powerful current will disrupt itself because it produces a strong, fluctuating magnetic field.

By depositing lines of 10-nanometre-wide, non-conducting dots of barium zirconate at fixed distances along the wire, researchers at Oak Ridge National Laboratory, Tennessee, US, have found a way to disrupt current flow in just the right way to tone down these fluctuations.

Slow progress

"The potential applications for high-temperature superconducting materials are so significant that the people who discovered them were awarded Nobel prizes the year after their announcement," says Amit Goyal, one of the wire's developers. "But we still don't have a physical understanding of how they work, so getting anywhere near these applications has taken two decades. Now we may see some steps forward soon."

The wires, made of yttrium barium copper oxide (YBCO), will first be worked into lightweight and powerful rotating machinery such as generators and motors, says Venkat Selvamanickam, head of materials research at Superpower Inc. The company's contractual agreement with Oak Ridge means it will probably be given first refusal to exploit the technology.

However, Selvamanickam says, the "killer app" for these wires will be as the infrastructure of tomorrow's electrical grid. A typical modern electrical grid based on copper wires can ferry current along with just under 90% efficiency. A grid based on an infrastructure of high-temperature superconducting wires could be more than 97% efficient.

Right now, that is too small a change to merit the cost of installation, says Paul Grant, one of the patent holders of YBCO and a consultant for the US government's department of energy.

Widespread blackouts

However, large areas of high power consumption are starting to become problematic for current grids, as was shown by the widespread blackout in Canada and the US in August 2003. Superconducting wires play a large role in building stable grid structures, not only as transmission cables but also as components in transformer stations and other maintenance equipment, Grant says.

"This change, in the end, is going to be moved by governmental policy," he says. "So it may be politics now that's slowing things down more than technology."

Not that the remaining technological challenges are insignificant. The method Oak Ridge used to produce the wires only spins out lengths measured in inches, rather than miles. There are two techniques for producing longer wires, but it is unclear how they could accommodate the inclusion of nanodots.

Also, the term "high temperature" is somewhat of a misnomer. Such superconducting materials, in this case YBCO, need to be cooled to around -200°C (conventional superconductors only work at near absolute zero temperatures, closer to -273°C). This means that a system that circulates liquid nitrogen along the wires would need to accompany any installation.

Fortunately liquid nitrogen is inexpensive, which means that superconducting wire applications such as levitating trains would become economically feasible.

Journal reference: Science (vol 311, p 1911)