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Superconductors - Against the grain



WOODSTOCK. For most people the word prompts memories of a
summer concert decades ago, and cool fantasies of sex, drugs and rock 'n'
roll. Not for physicists, though. They remember a gathering of the
American Physical Society in the winter of 1987 which turned so raucous
that it has become known to them as the Woodstock of physics. They
remember a rather different cool fantasy, too: superconductivity.

The transmission of electricity with no losses whatsoever, which is what
superconductivity is, used to involve temperatures that were rather more
than merely cool. Before 1987, it meant chilling things down to within a
few degrees of absolute zero (-273°C), a temperature that can be sustained
only by bathing a superconductor in liquid helium, which is expensive.
What was announced at the meeting was a material that would
superconduct at a temperature as high as —184°C. That, by the standards
of such things, is positively balmy. More to the point, it is above the boiling
point of nitrogen, which means that liquid nitrogen can be relied on as the
necessary coolant. Liquid nitrogen is cheap, so widespread industrial
applications beckoned.

Unfortunately, they still do. But hope springs eternal. In a paper just
published in Nature, Jochen Mannhart of the University of Augsburg,
Germany and his colleagues claim to have come up with a solution to one
of the principal obstacles to commercialisation, which is that although a
“high-temperature” superconductor ( HTS ) can carry current without
losses, it cannot carry very much of it.

The problem is that the new superconductive materials are not flexible
metals but brittle metal oxides. (The one that attracted attention in 1987 was
an oxide of yttrium, barium and copper.) To overcome this brittleness they
are frequently deposited as thin films on other, more flexible, materials. But
such films have a grainy crystalline structure which reduces the number of
electrons that can flow comfortably through them. High-temperature
superconductivity depends on the oxide's crystal structure, and grain
boundaries interrupt that structure.

What Dr Mannhart and his colleagues have found is a way to reduce this
problem. They have discovered, by good old trial and error, that replacing
some of the yttrium atoms at grain boundaries with calcium atoms
increases the current flow from grain to grain. Their new superconductor,
with calcium-doped grain boundaries, can carry up to ten times as much
current as an undoped equivalent—a surprise, considering that calcium
doping throughout the entire crystal structure, rather than merely at the
edges, actually reduces superconductivity.

Yet not everybody is convinced. Indeed, a rival solution, known as the
powder-in-tube method, is now approaching commercialisation. This
involves an oxide of bismuth, strontium, calcium and copper, known as
BSCCO . A powdered form of this material is placed in a tube made of silver
(pricey, but essential because of both its mechanical and its chemical
properties) and the tube is heated. The result, if the conditions are right, is a
material that is superconducting. But the stuff inside the tube is still fairly
brittle, and the flexibility provided by the silver only partially overcomes

The solution, according to Gregory Yurek, the boss of American
Superconductor, the company using this method, draws its inspiration from
fibre-optic cables. Like glass, BSSCO is brittle by nature, but flexible when
made into filaments and packed into bundles. American Superconductor
has been perfecting a way to manufacture such fibres routinely. It plans, in
collaboration with Pirelli, an Italian industrial firm, to turn them into
marketable insulated cables. Dr Yurek says that the two companies are now
shifting from pilot plants to full-scale manufacturing. He expects output to
increase from a few hundred kilometres of HTS cable last year to over
10,000km by 2003. The two firms, in conjunction with Detroit Edison, an
American utility, are about to replace traditional copper wires with HTS wire
for the first time on a public grid.

Dr Yurek remains unimpressed by the advance reported in Nature. He
says that American Superconductor has developed a high-volume coating
process which produces thin films that do not suffer from the
grain-boundary problem—and so do not need calcium doping. What is
more, he thinks that the elaborate vacuum chambers and expensive
equipment used by the Augsburg researchers will prove impossible to scale
up into a commercial plant. But his own favoured process has its problems,
too. BSSCO 's reliance on silver means that it may never be cheap enough
to deal old-fashioned copper wires a knock-out blow. And that could be Dr
Mannhart's silver lining. Physicists have been searching for a
commercially viable “high-temperature superconductor for over a decade.
It is just possible they are getting close

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