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T-SHIRT technology could make high-temperature superconductors almost as
cheap as chips. Chemists in Austria have developed a way of screen-printing
superconducting films to create complex circuits with zero resistance.

鈥淪creen-printing is a technique that has been abandoned but we鈥檙e trying to
push this technology. We feel it has a lot of potential,鈥� says Gerhard Gritzner
of the Institute for Chemical Technologies of Inorganic Materials at the
University of Linz in Austria, who carried out the work.

In screen-printing, a mask in the shape of the desired pattern is laid over a
substrate, and a liquid or paste forced through so that it leaves an imprint.
But it is not easy to make a paste that will superconduct at the relatively high
temperature of liquid nitrogen (77 kelvin). High-temperature superconductors
contain tiny crystals that conduct only in one plane. In a good superconductor,
the conducting planes have to align and the crystals must make good contact with
each other.

The Austrian team uses crystals of a thallium-based ceramic that
superconducts below 120 K. They mix the crystals with an organic binder to
create a paste that can be forced through a screen onto a substrate of zirconium
dioxide. Then they heat their circuit to align the superconducting planes and
burn off the binder.

Previous attempts to print superconducting circuits failed because burning
off the binder leaves pores in the film that prevent good electrical contact
between crystals. These circuits cannot support current densities of more than a
few thousand amps per square centimetre.

The Austrians succeeded where others failed because of their second step.
After the first round of heating, they compact the circuit with a heavy weight
to make the film less porous, and carefully reheat the sample to improve the
alignment of the crystals. 鈥淲e have reached current densities of 40 000 amps per
square centimetre and everyone is dreaming of 100 000,鈥� says Gritzner.

High-temperature superconducting circuits are already used as antennas to
improve radio reception and as electrical filters in cellphone base stations.
But these circuits are expensive because they are deposited onto a substrate in
a vacuum, similar to the way that silicon chips are produced, and they lack the
volume production that makes chips cheap. Screen-printed superconducting
circuits would be far cheaper because they are easier to make.

Neil Alford, a materials scientist at South Bank University in London, says
that the Austrian work is impressive. 鈥淚f they鈥檝e reached 40 000 amps per square
centimetre, then that鈥檚 a significant advance,鈥� he says. 鈥淏ut I think they will
face stiff competition from the other technologies available to make circuits.鈥�