Cryogenics is the study of the properties of matter at very low temperatures. (The word is derived from the Greek kryos, meaning “icy cold.”) The highest temperature dealt with by cryogenics is about -100 degrees C (-148 degrees F), and the lowest is the absolute zero temperature of -273.15 degrees C (-459.67 degrees F). It is a fundamental theorem of thermodynamics that absolute zero can be approached but can never actually be reached. In cryogenic studies the standard practice is to express temperatures in the lowest range by using the Kelvin scale, with Celsius-size degrees but with absolute zero at 0 K.
The study of cryogenics began about 1877, when Swiss physicist Raoul Pictet and French engineer Louis Cailletet separately liquefied oxygen for the first time. Cailletet accidentally achieved this in his laboratory through an adiabatic expansion process, whereas Pictet used the cooling produced by the Joule-Thomson effect.
In 1883 a third cooling mechanism, known as cascading, was pioneered by Karol Olszewski and Zygmunt von Wroblewski in Poland, who were able to produce quantities of liquid oxygen by this method. The liquefaction of oxygen, which occurs at 90 K, was soon followed by the liquefaction of nitrogen at 77 K. In 1898 a British professor, James Dewar, who succeeded in liquefying hydrogen gas, achieved a major advance in cryogenics. Liquid hydrogen, which boils at 20 K, presented a severe handling and storage problem.
Dewar solved this problem by devising a double-walled vacuum storage vessel now known as the Dewar flask. Even the near-perfect vacuum of the Dewar flask, however, is not impervious to radiation heat transfer, although it prevents thermal conduction and convection. Other insulation schemes, involving expanded foam materials and layered radiation shields, have been developed. These methods are commonly employed in cryostats for storing and transporting liquefied gas.
Dutch physicist Heike Kamerlingh Onnes first liquefied helium, the most difficult gas to liquefy, in 1908. Two isotopes of liquid helium, (4)He and (3)He, boil at temperatures of 4.2 K and 3.2 K, respectively. With no substances more difficult than helium to be liquefied, further attempts at chilling involve special heat-absorption techniques, such as the demagnetization of paramagnetic salts. Such methods have produced temperatures down to about 40 millionths of a degree Kelvin above absolute zero, using sodium gas.
Superconductivity and Superfluidity
It had been known for many years that the electrical resistivity of metals decreases with falling temperatures. Nevertheless, many were surprised when Kamerlingh Onnes discovered in 1911 that the resistance of pure mercury undergoes a sharp drop just below liquid helium temperatures and then vanishes. The same phenomenon of superconductivity was subsequently observed in other metals. The enormous potential value of electricity flow when resistive losses are zero is demonstrated by the superconducting electromagnets used in particle accelerator and fusion energy research as well as in magnetic resonance imaging devices in hospitals.
Other fields of application include electronics, where high-speed cryogenic computer memories and communication devices are in various stages of research and development. The cost of attaining very low temperatures, however, must be weighed against the advantages obtained. Thus great interest has centered on materials that can achieve superconductivity at somewhat higher temperatures, since the first discovery of such materials in 1987.
Closely related to superconductivity is the phenomenon of Superfluidity observed with liquid helium. Helium cooled below 2.2 K becomes an elusive superfluid that runs uphill, escapes many solid containers, and generally defies the ordinary principles of fluid mechanics. Such spectacular effects occur because the superfluid displays essentially none of the viscous tendencies characteristic of common fluids. The explanation of these effects depends on quantum physics, rather than classical physics, and remains under intensive study.
Large-scale cryogenic operations are currently being used to transport energy in the form of liquefied natural gas. The processing, handling, and preservation of food by cryogenic means is a major industry, providing both frozen and freeze-dried foodstuffs. Other applications of modern cryogenics are in chemical synthesis and catalysis, gas separation, metals fabrication, and miscellaneous uses ranging from fire fighting to the drilling of oil wells.
Liquid hydrogen plays a role in high-energy physics studies and, along with liquid oxygen, powers rocket engines for space research. Present laboratory studies are directed toward characterizing material properties at cryogenic temperatures.
Related research in solid-state physics has shown a superconductivity effect in Organo-metallic compounds. Other research examines the phenomena of cryogenic heat transfer and boiling and explores the potential of low-temperature magnetohydrodynamics (MHD).