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Interdisciplinary Research Centre in Superconductivity
www.phy.cam.ac.uk/research/sucon
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In 1911, at Leiden University in the Netherlands, Professor Onnes was cooling
down Mercury with the newly discovered cryogen liquid helium and measuring its
resistance. When the temperature reached 4.15K [-269°C] the electrical resistance
suddenly dropped to zero. After a lot of checking, this result was found to be
correct, and the effect was called superconductivity. Many other superconducting
materials were discovered over the next 82 years but none of them was found to be
superconducting above 23K [-250°C].
Discoveries made recently have raised superconducting temperatures to a much higher
value. Scientists at the University of Houston first synthesised a ceramic compound
containing Yttrium, Barium, Copper and Oxygen which becomes superconducting at 93K
[-180°C]. Its chemical formula is YBa2Cu3O7
although the material sometimes loses
oxygen which also lowers the transition temperature. Fig. 1 shows the sudden
disappearance of the resistivity of YBa2Cu3O7
on cooling the sample. Other
ceramic compounds containing copper also give high transition temperatures.
The latest superconductor HgBa2Ca2Cu3O8+d
discovered in 1993 shows
superconductivity at 160K [-110°C] under pressure.
These newer ceramic superconductors are known as High Temperature Superconductors,
and are superconducting in liquid nitrogen, which is much cheaper than liquid
helium. However they don't carry as big a current, and being ceramics (like a
teacup), they are brittle.
If you pass a current along a normal copper wire, energy will be lost because the
wire has a resistance. If the wire is a power cable this loss is significant. In
fact 1.5% of the power generated in the UK is lost in transmission. This is
significant but the real problem is that if you don't want your wires to melt
you have to dissipate this heat. Superconductors do not have any resistance so
there is no heat to dissipate; this means that you can put much more current in
the same space. This property of superconductors has been exploited in Chicago
and Copenhagen to increase the capacity of cables in the centre of the city,
without having to dig up the road.
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A Magnetic Resonance Imager (MRI)
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To make a strong electromagnet you also need a very large current in a small
space. Therefore, electromagnets are particularly suited to being made from
superconductors.
Superconductors also have the advantage that once you have a current, they don't
use any power. However, superconductors do have disadvantages. You have to cool
them to between -200°C and -269°C, and the high temperature superconductors are
brittle ceramics, which means making wires from them is challenging.
Superconducting magnets are used in MRI scanners, mineral separation machines, and
recently in high power compact electric motors for powering Naval ships.
Superconductors interact with magnetic fields in interesting ways, which allows
them to be used to make very sensitive magnetic sensors, and high frequency
Microwave and Terahertz receivers. They can also be used for very high frequency
electronics and possibly for quantum computing.
Superconductivity will be used much more widely in industry in the future. Some
analysts predict markets of £7 billion per year in 10 years time. It is
essential to combine performance improvements with application technologies,
no matter how efficient the machines and devices making use of superconductivity
are. It is necessary to promote new markets that have requirements that benefit
from high energy density in components of a complete system.
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