Inphysics,cryogenics is the production and behaviour of materials at very lowtemperatures.
The 13thInternational Institute of Refrigeration's (IIR) International Congress of Refrigeration (held in Washington, DC in 1971) endorsed a universal definition of "cryogenics" and "cryogenic" by accepting a threshold of 120 K (−153 °C) to distinguish these terms from conventional refrigeration.[1][2][3][4] This is a logical dividing line, since the normalboiling points of the so-called permanentgases (such ashelium,hydrogen,neon,nitrogen,oxygen, and normalair) lie below 120 K, while theFreon refrigerants,hydrocarbons, and other common refrigerants have boiling points above 120 K.[5][6]
Discovery ofsuperconducting materials with critical temperatures significantly above the boiling point of nitrogen has provided new interest in reliable, low-cost methods of producing high-temperature cryogenic refrigeration. The term "high temperature cryogenic" describes temperatures ranging from above the boiling point of liquid nitrogen, −195.79 °C (77.36 K; −320.42 °F), up to −50 °C (223 K; −58 °F).[7] The discovery of superconductive properties is first attributed toHeike Kamerlingh Onnes on 10 July 1908, after they were able to reach a temperature of 2 K. These first superconductive properties were observed in mercury at a temperature of 4.2 K.[8]
Cryogenicists use theKelvin orRankine temperature scale, both of which measure fromabsolute zero, rather than more usual scales such asCelsius which measures from the freezing point of water at sea level[9][10] orFahrenheit which measures from the freezing point of a particular brine solution at sea level.[11][12]
The branches of engineering that involve the study of very low temperatures (ultra low temperature i.e. below 123 K), how to produce them, and how materials behave at those temperatures.
The branch ofbiology involving the study of the effects of low temperatures onorganisms (most often for the purpose of achievingcryopreservation). Other applications include Lyophilization (freeze-drying) of pharmaceutical[13] components and medicine.
The conservation of genetic material with the intention of conserving a breed. The conservation of genetic material is not limited to non-humans. Many services provide genetic storage or the preservation ofstem cells at birth. They may be used to study the generation of cell lines or forstem-cell therapy.[14]
The branch of surgery applying cryogenic temperatures to destroy and kill tissue, e.g. cancer cells. Commonly referred to asCryoablation.[15]
The study of electronic phenomena at cryogenic temperatures. Examples includesuperconductivity andvariable-range hopping.
Cryopreserving humans and animals with the intention of future revival. "Cryogenics" is sometimes erroneously used to mean "Cryonics" inpopular culture and the press.[16]
The wordcryogenics stems fromGreekκρύος (cryos) – "cold" +γενής (genis) – "generating".
Cryogenic fluids with their boiling point inKelvin[17] and degree Celsius.
| Fluid | Boiling point (K) | Boiling point (°C) |
|---|---|---|
| Helium-3 | 3.19 | −269.96 |
| Helium-4 | 4.214 | −268.936 |
| Hydrogen | 20.27 | −252.88 |
| Neon | 27.09 | −246.06 |
| Nitrogen | 77.09 | −196.06 |
| Air | 78.8 | −194.35 |
| Fluorine | 85.24 | −187.91 |
| Argon | 87.24 | −185.91 |
| Oxygen | 90.18 | −182.97 |
| Methane | 111.7 | −161.45 |
| Krypton | 119.93 | −153.415 |
Liquefied gases, such asliquid nitrogen andliquid helium, are used in many cryogenic applications. Liquid nitrogen is the most commonly used element in cryogenics and is legally purchasable around the world. Liquid helium is also commonly used and allows for thelowest attainable temperatures to be reached.
These liquids may be stored inDewar flasks, which are double-walled containers with a high vacuum between the walls to reduce heat transfer into the liquid. Typical laboratory Dewar flasks are spherical, made of glass and protected in a metal outer container. Dewar flasks for extremely cold liquids such as liquid helium have another double-walled container filled with liquid nitrogen. Dewar flasks are named after their inventor,James Dewar, the man who first liquefiedhydrogen.Thermos bottles are smallervacuum flasks fitted in a protective casing.
Cryogenic barcode labels are used to mark Dewar flasks containing these liquids, and will not frost over down to −195 degrees Celsius.[18]
Cryogenic transfer pumps are the pumps used onLNG piers to transferliquefied natural gas fromLNG carriers toLNG storage tanks, as are cryogenic valves.
The field of cryogenics advanced during World War II when scientists found that metals frozen to low temperatures showed more resistance to wear. Based on this theory ofcryogenic hardening, the commercialcryogenic processing industry was founded in 1966 by Bill and Ed Busch. With a background in theheat treating industry, the Busch brothers founded a company inDetroit called CryoTech in 1966.[19] Busch originally experimented with the possibility of increasing the life of metal tools to anywhere between 200% and 400% of the original life expectancy usingcryogenic tempering instead ofheat treating.[20] This evolved in the late 1990s into the treatment of other parts.
Cryogens, such as liquidnitrogen, are further used for specialty chilling and freezing applications. Some chemical reactions, like those used to produce the active ingredients for the popularstatin drugs, must occur at low temperatures of approximately −100 °C (−148 °F). Special cryogenicchemical reactors are used to remove reaction heat and provide a low temperature environment. The freezing of foods and biotechnology products, likevaccines, requires nitrogen in blast freezing or immersion freezing systems. Certain soft or elastic materials become hard andbrittle at very low temperatures, which makes cryogenicmilling (cryomilling) an option for some materials that cannot easily be milled at higher temperatures.
Cryogenic processing is not a substitute for heat treatment, but rather an extension of the heating–quenching–tempering cycle. Normally, when an item is quenched, the final temperature is ambient. The only reason for this is that most heat treaters do not have cooling equipment. There is nothing metallurgically significant about ambient temperature. The cryogenic process continues this action from ambient temperature down to −320 °F (140 °R; 78 K; −196 °C).In most instances the cryogenic cycle is followed by a heat tempering procedure. As all alloys do not have the same chemical constituents, the tempering procedure varies according to the material's chemical composition, thermal history and/or a tool's particular service application.
The entire process takes 3–4 days.
Another use of cryogenics iscryogenic fuels for rockets withliquid hydrogen as the most widely used example, withliquid methane starting to become more prevalent in recent years.Liquid oxygen (LOX) is even more widely used but as anoxidizer, not a fuel.NASA's workhorseSpace Shuttle used cryogenic hydrogen/oxygen propellant as its primary means of getting intoorbit. LOX is also widely used withRP-1 kerosene, a non-cryogenic hydrocarbon, such as in the rockets built for theSoviet space program bySergei Korolev.
Russian aircraft manufacturerTupolev developed a version of its popular designTu-154 with a cryogenic fuel system, known as theTu-155. The plane uses a fuel referred to asliquefied natural gas or LNG, and made its first flight in 1989.[21]
Some applications of cryogenics:
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Cryogenic cooling of devices and material is usually achieved via the use ofliquid nitrogen,liquid helium, or amechanical cryocooler (which uses high-pressure helium lines).Gifford-McMahon cryocoolers,pulse tube cryocoolers andStirling cryocoolers are in wide use with selection based on required base temperature and cooling capacity. The most recent development in cryogenics is the use of magnets as regenerators as well as refrigerators. These devices work on the principle known as themagnetocaloric effect.
There are variouscryogenic detectors which are used to detect particles.
For cryogenic temperature measurement down to 30 K, Pt100 sensors, aresistance temperature detector (RTD), are used. For temperatures lower than 30 K, it is necessary to use asilicon diode for accuracy.
In terms of the Kelvin scale the cryogenic region is often considered to be that below approximately 120 K (−153 C).
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