CRYOGENICS

production and application of low-temperature phenomena. The cryogenic temperature range has been defined as from -150 C (-238 F) to absolute zero (-273 C or -460 F), the temperature at which molecular motion comes as close as theoretically possible to ceasing completely. Cryogenic temperatures are usually described in the absolute or Kelvin scale, in which absolute zero is written as 0 K, without a degree sign. Conversion from the Celsius to the Kelvin scale can be done by adding 273 to the Celsius scale. Cryogenic temperatures are considerably lower than those encountered in ordinary physical processes. At these extreme conditions, such properties of materials as strength, thermal conductivity, ductility, and electrical resistance are altered in ways of both theoretical and commercial importance. Because heat is created by the random motion of molecules, materials at cryogenic temperatures are as close to a static and highly ordered state as is possible. Cryogenics had its beginning in 1877, the year that oxygen was first cooled to the point at which it became a liquid (-183 C, 90 K). Since then, the theoretical development of cryogenics has been connected to the growth in capability of refrigeration systems. In 1895, when it had become possible to reach temperatures as low as 40 K, air was liquefied and separated into its major components; in 1908 helium was liquefied (4.2 K). Three years later, the propensity of many supercooled metals to lose all resistance to electricitythe phenomenon known as superconductivitywas discovered. By the 1920s and 1930s temperatures close to absolute zero were reached, and by 1960 laboratories could produce temperatures of 0.000001 K, a millionth of a degree Kelvin above absolute zero. Temperatures below 3 K are primarily used for laboratory work, particularly research into the properties of helium. Helium liquefies at 4.2 K, becoming what is known as helium I. At 2.19 K, however, it abruptly becomes helium II, a liquid with such low viscosity that it can literally crawl up the side of a glass and flow through microscopic holes too small to permit the passage of ordinary liquids, including helium I. (Helium I and helium II are, of course, chemically identical.) This property is known as superfluidity. The most important commercial application of cryogenic gas liquefaction techniques is the storage and transportation of liquefied natural gas (LNG), a mixture largely composed of methane, ethane, and other combustible gases. Natural gas is liquefied at 110 K, causing it to contract to 1/600th of its volume at room temperature and making it sufficiently compact for swift transport in specially insulated tankers. Very low temperatures are also used for preserving food simply and inexpensively. Produce is placed in a sealed tank and sprayed with liquid nitrogen. The nitrogen immediately vaporizes, absorbing the heat content of the produce. In cryomedicine a low-temperature scalpel or probe can be used to freeze unhealthy tissue. The resulting dead cells are then removed through normal bodily processes. The advantage to this method is that freezing the tissue rather than cutting it produces less bleeding. A scalpel cooled by liquid nitrogen is used in cryosurgery; it has proved successful in removing tonsils, hemorrhoids, warts, cataracts, and some tumours. In addition, thousands of patients have been treated for Parkinson's disease by freezing the small areas of the brain believed to be responsible for the problem. The application of cryogenics has also extended to space vehicles. In 1981 the U.S. space shuttle Columbia was launched with the aid of liquid hydrogen/liquid oxygen propellants. Of the special properties of materials cooled to extreme temperatures superconductivity is the most important. Its chief application has been in the construction of superconducting electromagnets for particle accelerators. These large research facilities require such powerful magnetic fields that conventional electromagnets could be melted by the currents required to generate the fields. Liquid helium cools the cable through which the currents flow to about 4 K, allowing much stronger currents to flow without generating heat by resistance.