Wires find path of least resistance
TWENTY YEARS ago this month, two researchers discovered a class of materials that sparked dreams of electricity grids that would transmit power without any losses and trains that would levitate along friction-free tracks.
Researchers now think they have overcome one of the key obstacles that have stopped these visions becoming a reality. They have shown how to fashion the high-temperature superconductors discovered in 1986 into the wires and cables that engineers need.
High-temperature superconductors, usually made of copper oxides, conduct electricity without any resistance at temperatures around the point at which nitrogen becomes liquid: -196 degrees C.
Making them work
That might not sound very high, but other super conducting materials require much lower temperatures. In fact, relatively cheap cooling systems are sufficient to make high-temperature superconductors work.
If these materials could be turned into wires, they could transmit energy without heating up, making them very efficient power cables. In theory, they should also be able to carry much higher currents than metal wires of the same size. But the materials are brittle and have to be plastered on to more ductile ones to create wires.
The resulting cables also have to work in the presence of strong magnetic fields — in a transformer or generator, for example. Researchers have been struggling for years to do all that.
Civilian and military uses
Now Amit Goyal and colleagues at Oak Ridge National Laboratory in Tennessee, U.S., have taken a step forward. In Science, they report wires made of a common high-temperature superconductor containing the metals barium and yttrium (YBCO) plastered on to a strip of flexible metal: The wires meet the electrical current requirements for many civilian and military applications. Magnetic fields disrupt high-temperature superconductors because lines of magnetic flux, like those visible in iron filings scattered around a magnet, create mini vortices in the electrical current.
These microscopic tornadoes wander through the material producing a kind of drag on the current, which creates electrical resistance. And that means the superconductivity is no longer perfect. To eliminate this drag, the flux lines and vortices need to be `pinned,' or fixed to particular points in the material. Various methods of pinning have been demonstrated over the past few years, with different degrees of success.
For example, two years ago, a team at Los Alamos National Laboratory in New Mexico reported that lacing YBCO films with nanometre-wide rods of ceramic barium zirconate created crack-like imperfections in the superconductor that acted as suitable pins for the vortices.
The stubby nanoparticles enabled the wires to maintain their superconductivity when placed in a magnetic field.
Goyal and his colleagues have brought super conducting wires up to industry standards by making films of YBCO with tiny `nanodots' of barium zirconate threaded right through the material.
Act as great pins
To make the wires, they blast a mixture of YBCO and barium zirconate powder with intense laser pulses, creating a vapour that settles on to a metal strip.
Nanoparticles of barium zirconate tend to align themselves into columns within the YBCO, and these act as great pins.
PHILIP BALL
Nature News Service
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