Playing it cool - liquid helium-free technologies
Jeremy Good, Director at Cryogenic Ltd in London, UK, explains the science behind liquid helium-free cooling technologies and the benefits they offer to materials development.
At very low temperatures, fundamental characteristics of materials can be observed that offer important insight into their properties. The ability to reach these temperatures together with the production of high magnetic fields has revolutionised materials science for semiconductors and other materials. This has important implications for all kinds of new developments, from increasing the speed of electronics to developing computer memory and other innovative commercial products.
Until recently, liquid helium was required to generate low temperatures and to cool the superconducting magnets used for generating high fields. But advances in cooling technology make it possible to work without liquid helium, which is not only expensive but, in some parts of the world, difficult to obtain. This enables cheaper and easier research into low-temperature physics, which will in turn expand lowtemperature materials research on a global scale.
At temperatures close to absolute zero, the quantum wave nature of particles appears more clearly. This allows the fundamental properties of materials to be studied in greater detail, which is vital when engineering these for new applications. Studies at low temperatures are conducted across the world, both by universities and also by large research centres, such as the Lohe European Synchrotron Radiation Facility in Grenoble, France, and the European Organisation for Nuclear Research (CERN), in Switzerland, which also relies on cryogenics for its superconducting magnets that keep the beam of the Hadron collider accurately in place. Reaching low temperatures has also been essential in furthering understanding of graphene, the discovery of which led Professor Sir Andre Geim and Professor Kostya Novoselov to receive the Nobel Prize in 2010.
At very cold temperatures, the optical, thermal, electric and magnetic properties of all materials undergo significant changes. Temperature has a logarithmic scale, and properties that cannot be studied at 4K (the boiling point of liquid helium) may reveal themselves at 0.3K, which can be achieved by using liquid Helium-3 (He-3), a rare isotope of the element. Similarly, some things that do not manifest at 0.3K can be studied at much lower temperatures of about 10mK, which is around 30 times colder.
All these experimental systems require a holding a temperature of 4K, which traditionally has required the use of liquid helium. Lower temperatures are achieved using a dilution refrigerator in which He-3 is continuously dissolved in liquid He-4. Even lower temperatures are required for some experiments well below 1mK and these are usually achieved with a magnetic refrigerator.
As the temperature is reduced, greater order in space or motion can be observed. For instance, certain metals become superconducting, losing their electrical resistance entirely. Liquid helium itself becomes a superfluid, able to flow through small orifices without dissipating energy. When very low temperatures are reached, matter experiences the loss of thermally induced vibrations and it is possible to observe the effects of interactions at the quantummechanical level.
Fields of cold
There are two ways in which this enhanced quantum mechanical behaviour can be observed – the first is the superconducting quantum interference device (SQUID). There are several metals that are superconducting at a temperature of 4K, including lead, tin and niobium. In these metals, a magnetic field below a critical value is expelled and supercurrents on the surface of the metal shield the interior from magnetic flux (quantised in units of hc/2e, or 2x10-7 gauss cm2 in old units). For metal objects with a hole in the centre, the field will be trapped inside this hole – for example, a piece of niobium sheet with a 1mm2 hole will have discrete flux levels of 2x10-5 gauss. This is because each flux level corresponds to a pair of electrons circulating the hole, whose quantum wave phase must change by an integral value of 2π going around the hole. This goes against previous thinking that quantum mechanics operates at the atomic level – in fact, quantum mechanical waves are continuous and are discrete over dimensions of several millimetres, a phenomenon that would not have been observable without these low-temperature studies.
Being a sensitive amplifier of magnetic and electrical signals, SQUID has several valuable applications. One of these is in metrology, which requires accurate comparison of resistance standards values. Using SQUID standard resistors can be compared to an accuracy of better than 10-9. Another application involves the AC Josephson effect that allows accurate conversion of voltage measurement to frequency measurement, which has led to in a large increase in accuracy of these measurements.
Another remarkable effect comes from the superfluidity of liquid helium. Superfluids can flow without loss and so for a ring-shaped vessel, the fluid can circulate around the ring without any interaction with the walls. Such a device has even been proposed as a means of navigation, however, this would depend on the ability to detect the flow of the liquid without disturbing it, which is not easy. These examples demonstrate the power of low temperatures to reveal new phenomena in physics, as well as allowing the study of materials and, in turn, fabrication of useful devices.
The generation of high magnetic fields is another benefit of studying materials at low temperatures. Up until the 1960s, magnetic fields could only be created with iron electromagnets or expensive copper solenoids. Fields were generally restricted to 2 telsa (T), with volumes of a few centimetres between pole pieces. Once high-field superconductors were discovered, the superconducting magnets were started to be built. Today, fields of 20T can be created, with lower fields being created over large, metre-sized spaces. The MRI, which depends on metre-diameter magnets of between 1–3T, is the best known application of this technology.
In addition to macroscopic quantum phenomena, many studies involving nanoscale structures are carried out at low temperatures. The atomic force microscope allows the study and manipulation of individual atoms, which can be distinguished and their interaction with their neighbours observed. Tiny structures made by photo or electron lithography on the nanoscale have unique electrical and mechanical properties, most of which are best studied using lowtemperature instruments.
The liquid helium story
Liquefaction of helium requires a lot of energy as well as the availability of the gas. In the UK, it costs around £6 to produce a litre of liquid helium, but in Japan this rises to more than £20. Countries with emerging research bases, such as those in Middle East, Nigeria or Brazil, do not have the facilities to produce it and must go down the expensive route of importation.
However, in recent years there has been new hope for making low-temperature physics cheaper, easier and more accessible. The advent of cryogenfree, or dry, systems is slowly eliminating the need for a constant supply of liquid helium, making lowtemperature materials science increasingly accessible to more centres at a lower cost.
Unlike existing systems, cryogen-free systems use mechanical refrigerators that cool to cryogenic temperatures using only electrical power. They use Gifford-McMahon (GM) coolers or pulse tube (PT) coolers known as coldheads, which rely on the compression and expansion of a small quantity of helium gas to create low temperatures and can reach temperatures as low as 2.6K.
Both GM and PT coolers work by repeated expansion of gas, which causes the gas to cool. They have slightly different thermodynamic cycles but in both cases, the gas, which is supplied by an external conventional compressor, expands in volume and its pressure and temperature decrease during the cycle. Finally, the gas returns to the compressor, which completes a closed-loop circulation of gas through the cold head.
Expanding access to cutting-edge physics to new regions that currently cannot access liquid helium, or areas that were being held back by cost implications, can only increase research into materials and the development of useful applications. While this is especially the case in developing countries, where access to liquid helium has been limited, should the price of helium continue to rise, cryogen-free systems could also offer a more cost-effective route for materials researchers across the world.