Cryogenics
The term cryogenics comes from the Greek word kruos , for "frost." The subject deals with the properties of matter at very low temperatures, close to absolute zero (-273° C or 0° K) as well as with the equipment and processes by which such temperatures can be attained. The science of cryogenics is closely related to the commercial process of refrigeration, and progress in one field is often the result of progress in the other.
The origin of cryogenic research is often association with the first liquefaction of oxygen, nitrogen, and carbon monoxide gas by the French physicist, Louis Paul Cailletet, in 1877. Cailletet's work was closely paralleled by that of the Swiss chemist, Raoul Pierre Pictet. Cailletet and Pictet--as well as all early cryogenic researchers--used the Joule-Thomson effect to attain low temperatures. The first step in using the Joule-Thomson effect is to cool and compress a gas as much as possible. The gas is then allowed to expand quickly as it escapes through a needle valve. Rapid expansion cools the gas even more, lowering its temperature below its boiling point. Pictet used a variation of this technique to liquefy oxygen. He used the Joule-Thomson effect with a series of gases, each with a lower boiling point, until he was able to reduce the temperature of oxygen below its boiling point. The German chemist, Carl von Linde, adopted a similar approach in constructing the first commercial refrigeration system in 1876.
An important advance in early cryogenics research was the development of an efficient system for storing liquefied gases. That system was invented by the Scottish chemist James Dewar in 1891. The Dewar flask is well known today as the vacuum flask used to keep liquids hot or cold. The flask consists of a double-walled bottle containing a vacuum between the two silvered walls. The vacuum prevents loss or entry of heat by conduction or convection and the silver coating reduces heat transfer by reflection. With this technology, every gas had been successfully liquified by the early 1900s except for hydrogen and helium. Then, between 1906 and 1908, the Dutch physicist, Heike Kamerlingh Onnes achieved success, first with hydrogen, then with helium.
Further cryogenic research demanded the development of new cooling techniques. In the mid-1920s, the method of adiabatic demagnetization was proposed independently by Peter Debye and W. F. Giauque. In this process, the material to be cooled is placed in contact with a paramagnetic field and with liquid helium and then subjected to a strong magnetic field. The liquid helium is then removed and the magnetic field reduced to zero. At that point, the temperature in the test material may drop as low as 10-2°-10-3° K. Yet a third process-- nuclear adiabatic demagnetization--has been used to reduced temperatures even further, to about 2 x 10-7° K.
Materials tend to exhibit unusual properties at very low temperatures. Kamerlingh Onnes found, for example, that helium loses its viscosity near absolute zero. It can flow up the sides of a container and through openings that it is unable to penetrate as a gas. This form of helium became known as helium-II.
Perhaps best known and commercially most significant of all low-temperature phenomena is superconductivity, the tendency of a material to lose all electrical resistance at low temperatures. Originally observed at temperatures no greater than a few degrees kelvin, superconductivity has now been reported at temperatures as high as 100° K. The properties of superfluidity and superconductivity have now been explained by the Bardeen-Cooper-Schrieffer (BCS) theory.
A number of commercial applications for cryogenics have been suggested. Visionaries claim that this science may one day be used to preserve humans with incurable diseases until the day when a cure is found for those diseases. On a more practical level, freezing with liquid nitrogen is now a standard method of food preservation. In addition, cryogenics has become an important industrial process used to substantially increase the wear resistance of cast iron and low carbon steels. The gear making industry uses cryogenic processes to improve dimensional stability, increase surface hardness, and improve wear properties.
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