cryogenics


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low-temperature physics

low-temperature physics, science concerned with the production and maintenance of temperatures much below normal, down to almost absolute zero, and with various phenomena that occur only at such temperatures. The temperature scale used in low-temperature physics is the Kelvin temperature scale, or absolute temperature scale, which is based on the behavior of an idealized gas (see gas laws; kinetic-molecular theory of gases). Low-temperature physics is also known as cryogenics, from the Greek meaning “producing cold.” Low temperatures are achieved by removing energy from a substance. This may be done in various ways. The simplest way to cool a substance is to bring it into contact with another substance that is already at a low temperature. Ordinary ice, dry ice (solid carbon dioxide), and liquid air may be used successively to cool a substance down to about 80K (about −190℃). The heat is removed by conduction, passing from the substance to be cooled to the colder substance in contact with it. If the colder substance is a liquefied gas (see liquefaction), considerable heat can be removed as the liquid reverts to its gaseous state, since it will absorb its latent heat of vaporization during the transition. Various liquefied gases can be used in this manner to cool a substance to as low as 4.2K, the boiling point of liquid helium. If the vapor over the liquid helium is continually pumped away, even lower temperatures, down to less than 1K, can be achieved because more helium must evaporate to maintain the proper vapor pressure of the liquid helium. Most processes used to reduce the temperature below this level involve the heat energy that is associated with magnetization (see magnetism). Successive magnetization and demagnetization under the proper combination of conditions can lower the temperature to only about a millionth of a degree above absolute zero. Reaching such low temperatures becomes increasingly difficult, as each temperature drop requires finding some kind of energy within the substance and then devising a means of removing this energy. Moreover, according to the third law of thermodynamics, it is theoretically impossible to reduce a substance to absolute zero by any finite number of processes. Superconductivity and superfluidity have traditionally been thought of as phenomena that occur only at temperatures near absolute zero, but by the late 1980s several materials that exhibit superconductivity at temperatures exceeding 100K had been found. Superconductivity is the vanishing of all electrical resistance in certain substances when they reach a transition temperature that varies from one substance to another; this effect can be used to produce powerful superconducting magnets. Superfluidity occurs in liquid helium and leads to the tendency of liquid helium to flow over the sides of any container it is placed in without being stopped by friction or gravity.

Bibliography

See A. C. Helden, The Coldest Spot on Earth (1989).

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Cryogenics

The science and technology of phenomena and processes at low temperatures, defined arbitrarily as below 150 K (-190°F). Phenomena that occur at cryogenic temperatures include liquefaction and solidification of ambient gases; loss of ductility and embrittlement of some structural materials such as carbon steel; increase in the thermal conductivity to a maximum value, followed by a decrease as the temperature is lowered further, of relatively pure metals, ionic compounds, and crystalline dielectrics (diamond, sapphire, solidified gases, and so forth); decrease in the thermal conductivity of metal alloys and plastics; decrease in the electrical resistance of relatively pure metals; decrease in the heat capacity of solids; decrease in thermal noise and disorder of matter; and appearance of quantum effects such as superconductivity and superfluidity. See Electrical resistivity, Specific heat, Superconductivity, Superfluidity, Thermal conduction in solids

Low-temperature environments are maintained with cryogens (liquefied gases) or with cryogenic refrigerators. The temperature afforded by a cryogen ranges from its triple point to slightly below its critical point. Commonly used cryogens are liquid helium-4 (down to 1 K), liquid hydrogen, and liquid nitrogen. Less commonly used because of their expense are liquid helium-3 (down to 0.3 K) and neon. The pressure maintained over a particular cryogen controls its temperature. Heat input—both the thermal load and the heat leak due to imperfect insulation—boils away the cryogen, which must be replenished. See Liquid helium, Thermodynamic processes

A variety of techniques are available for prolonged refrigeration. Down to about 1.5 K, refrigeration cycles involve compression and expansion of appropriately chosen gases. At lower temperatures, liquid and solids serve as refrigerants. Adiabatic demagnetization of paramagnetic ions in solid salts is used in magnetic refrigerators to provide temperatures from around 4 K down to 0.003 K. Nuclear spin demagnetization of copper can achieve 5 × 10-8 K. Helium-3/helium-4 dilution refrigerators are frequently used for cooling at temperatures between 0.3 and 0.002 K, and adiabatic compression of helium-3 (Pomeranchuk cooling) can create temperatures down to 0.001 K. See Adiabatic demagnetization

Both the latent heat of vaporization and the sensible heat of the gas (heat content of the gas) must be removed to liquefy a gas. Of the total heat that must be removed to liquefy the gas, the latent heat is only 1.3% for helium and 46% for nitrogen. Consequently, an efficient liquefier must supply refrigeration over the entire temperature range between ambient and the liquefaction point, not just at the liquefaction temperature. The Collins-Claude refrigeration cycle forms the basis (with a multitude of variations) of most modern cryogenic liquefiers. Gas is compressed isothermally and cooled in a counterflow heat exchanger by the colder return stream of low-pressure gas. During this cooling, a fraction of the high-pressure stream (equal to the rate of liquefaction) is split off and cooled by the removal of work (energy) in expansion engines or turbines. This arrangement provides the cooling for the removal of the sensible heat. At the end of the counterflow cooling, the remaining high-pressure stream is expanded in either a Joule-Thomson valve or a wet expander to give the liquid product and the return stream of saturated vapor. See Liquefaction of gases

The work input required to produce refrigeration is commonly given in terms of watts of input power per watt of cooling, that is, W/W. Cooling with a refrigerator is more efficient (that is, requires a lower W/W) than cooling with evaporating liquid supplied from a Dewar because the refrigerator does not discard the cooling available in the boil-off gas.

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.

cryogenics

[‚krī·ə′jen·iks]
(physics)
The production and maintenance of very low temperatures, and the study of phenomena at these temperatures.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

Cryogenics

The science and technology of phenomena and processes at low temperatures, defined arbitrarily as below 150 K (-190°F). Phenomena that occur at cryogenic temperatures include liquefaction and solidification of ambient gases; loss of ductility and embrittlement of some structural materials such as carbon steel; increase in the thermal conductivity to a maximum value, followed by a decrease as the temperature is lowered further, of relatively pure metals, ionic compounds, and crystalline dielectrics (diamond, sapphire, solidified gases, and so forth); decrease in the thermal conductivity of metal alloys and plastics; decrease in the electrical resistance of relatively pure metals; decrease in the heat capacity of solids; decrease in thermal noise and disorder of matter; and appearance of quantum effects such as superconductivity and superfluidity.

Low-temperature environments are maintained with cryogens (liquefied gases) or with cryogenic refrigerators. The temperature afforded by a cryogen ranges from its triple point to slightly below its critical point. Commonly used cryogens are liquid helium-4 (down to 1 K), liquid hydrogen, and liquid nitrogen. Less commonly used because of their expense are liquid helium-3 (down to 0.3 K) and neon. The pressure maintained over a particular cryogen controls its temperature. Heat input—both the thermal load and the heat leak due to imperfect insulation—boils away the cryogen, which must be replenished. See Thermodynamic processes

A variety of techniques are available for prolonged refrigeration. Down to about 1.5 K, refrigeration cycles involve compression and expansion of appropriately chosen gases. At lower temperatures, liquid and solids serve as refrigerants. Adiabatic demagnetization of paramagnetic ions in solid salts is used in magnetic refrigerators to provide temperatures from around 4 K down to 0.003 K. Nuclear spin demagnetization of copper can achieve 5 × 10-8 K. Helium-3/helium-4 dilution refrigerators are frequently used for cooling at temperatures between 0.3 and 0.002 K, and adiabatic compression of helium-3 (Pomeranchuk cooling) can create temperatures down to 0.001 K.

Both the latent heat of vaporization and the sensible heat of the gas (heat content of the gas) must be removed to liquefy a gas. Of the total heat that must be removed to liquefy the gas, the latent heat is only 1.3% for helium and 46% for nitrogen. Consequently, an efficient liquefier must supply refrigeration over the entire temperature range between ambient and the liquefaction point, not just at the liquefaction temperature. The Collins-Claude refrigeration cycle forms the basis (with a multitude of variations) of most modern cryogenic liquefiers. Gas is compressed isothermally and cooled in a counterflow heat exchanger by the colder return stream of low-pressure gas. During this cooling, a fraction of the high-pressure stream (equal to the rate of liquefaction) is split off and cooled by the removal of work (energy) in expansion engines or turbines. This arrangement provides the cooling for the removal of the sensible heat. At the end of the counterflow cooling, the remaining high-pressure stream is expanded in either a Joule-Thomson valve or a wet expander to give the liquid product and the return stream of saturated vapor. See Liquefaction of gases

The work input required to produce refrigeration is commonly given in terms of watts of input power per watt of cooling, that is, W/W. Cooling with a refrigerator is more efficient (that is, requires a lower W/W) than cooling with evaporating liquid supplied from a Dewar because the refrigerator does not discard the cooling available in the boil-off gas. See Refrigeration, Refrigeration cycle, Thermodynamic cycle

McGraw-Hill Concise Encyclopedia of Engineering. © 2002 by The McGraw-Hill Companies, Inc.

cryogenics

the branch of physics concerned with the production of very low temperatures and the phenomena occurring at these temperatures
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005

cryogenics

Using materials that operate at very cold temperatures. See superconductor.
Copyright © 1981-2019 by The Computer Language Company Inc. All Rights reserved. THIS DEFINITION IS FOR PERSONAL USE ONLY. All other reproduction is strictly prohibited without permission from the publisher.
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