Magnetics Business & Technology - Summer 2014 - (Page 8)
FEATURE ARTICLE
Moving Beyond Helium in Magnetics Research
By Jeremy Good, Managing Director | Cryogenic Ltd.
Liquid helium has been a cornerstone of magnetics research for more than a century. The
discovery of superconductivity in 1911 was
only made possible by the first successful liquefaction of the element, a transition requiring
temperatures below 4K. Ever since then, liquid
helium has been used to cool materials to close
to absolute zero, a temperature range where exciting electronic and
magnetic properties can be observed.
Despite being the second most abundant element in the Universe,
however, helium is in short supply on Earth, an issue of increasingly
pressing concern to magnetics researchers the world over. Unless reliable alternatives can be developed and implemented in the near
future, a global helium shortfall could kick in by 2016 with dramatic
consequences for price and availability.
Why Helium Matters
At very cold temperatures the optical, thermal, electric and magnetic properties of all materials undergo significant changes. The
logarithmic nature of the temperature scale means properties that
cannot be studied at 4K (the boiling point of liquid helium) may reveal themselves at 0.3K, a temperature which can be achieved by using the rarer isotope Helium-3 (He-3). Similarly, some properties not
manifest at 0.3K can be studied at much lower temperatures of say
10 millikelvin.
Any experimental system built to examine these properties requires a stable temperature of 4K and traditionally this has involved
using liquid helium. Lower temperatures are then achieved using a
Dilution Refrigerator in which He-3 is continuously dissolved in liquid
He-4. Even lower temperatures well below 1 millikelvin can also be
achieved with the aid of a magnetic refrigerator.
An important phenomenon observable at these low temperatures
is the generation of high magnetic fields. Up until the 1960s magnetic fields could only be created with iron electromagnets or very
expensive copper solenoids. Fields were generally restricted to 2 Tesla
across pole pieces a few centimeters apart. The discovery of highfield superconductors paved the way for the construction of superconducting magnets, and today fields of 20 Tesla can be created over
spaces spanning several meters.
Magnetics researchers the world over have come to rely on liquid
helium for their experiments. Raphaël Hermann, Group leader at the
Jülich Centre for Neutron Science, Germany, and Invited Professor
at the University of Liège, Belgium, for many years used such a 'wet'
cryogenic system to determine the magnetic properties of iron and
cobalt nanoparticles. When provided with an alternative system that
relied on mechanical cooling instead, he was able to save thousands
of dollars a month that would otherwise have been wasted through
the unavoidable boiling of liquid helium.
Other exciting practical magnetics applications arising from low temperature research include use in high-energy particle accelerators, improved wind turbine and increased memory storage on hard disks.
A Diminishing Resource
Helium is produced as a significant byproduct of natural gas extraction, and is used in a number of industrial applications as well as in
scientific instruments and superconducting magnets.
Once helium is released into the air it is lost forever, and few natural
gas wells are in a position to produce more helium in an economically
viable way. So we are reliant on our limited existing supplies.
The Federal Helium Reserve in Amarillo, which provides 42 percent
of America's helium and 35 percent of the world's, is a major source
8
Magnetics Business & Technology * Summer 2014
of the gas. The US House of Representatives' decision in September
to delay the closing of the reserve is therefore good news for the
community. But it is a reserve that is not being replenished and is projected to last no more than another 25 years. And as supply dwindles,
the price will rise rapidly.
In addition to decreasing supplies and rising prices, liquefaction of
helium also requires a great deal of energy. In the UK a liter of liquid
helium costs more than $10 to produce, but in Japan, a world leader
in low temperature research, this rises to more than $30.
For regions with emerging research bases, such as the Middle East,
Nigeria and Brazil, liquid helium is an even more expensive commodity. As they do not have the facilities to produce liquid helium they
must import it, an operation that takes time and money and often
leaves it boiling away in customs for weeks.
The challenges do not end there. Superconducting magnets, which
use liquid helium need to be regularly topped up, a complicated process during which the magnets run the risk of quenching, a process
in which the liquid helium surrounding the magnet rapidly boils off.
Systems using liquid helium require large areas with suitable ventilation facilities and a technician on hand who is trained in cryogenics.
So, while the days of liquid helium aren't quite over, it is clearly
becoming less and less attractive.
Where Next?
One attempted solution has been to try to recover the gas as it
boils off, but this can be costly and, except in the most advanced
systems such as the LHC at CERN, users struggle to capture and then
reliquify 100 percent of the gas.
Having spent a lifetime in cryogenics, I'm convinced the future lies
in 'cryogen-free' or 'dry' mechanical systems.
Unlike existing systems, cryogen-free technology uses mechanical refrigerators consisting of a compressor and cold head package.
These cool to cryogenic temperatures using only electrical power.
They are heat engines and use the Gifford-McMahon (GM) or Pulse
Tube (PT) thermodynamic cycles to provide cooling, both of which
involve repeated compression and expansion of a small quantity of
helium gas to generate low temperatures.
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Table of Contents for the Digital Edition of Magnetics Business & Technology - Summer 2014
Editor's Choice
Moving Beyond Helium in Magnetics Research
Advances in Ferrite-Integrated On-Chip Inductors Using Aerosol Deposition
A Fresh Look at Design of Buck and Boost Inductors for SMPS Converters
Research & Development
Handheld Three-Axis Magnetometers
Magnetics, Materials & Assemblies
Software & Design
Test & Measurement
Industry News
Marketplace / Advertising Index
Spontaneous Thoughts: A Different Paradigm
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