Nitrogen is a liquid under −195.8 °C (77.3 K).
This is a diagram of an infrared space telescope, that needs a cold mirror and instruments. One instrument needs to be even colder, and it has a cryocooler. The instrument is in region 1 and its cryocooler is in region 3 in a warmer region of the spacecraft (see MIRI (Mid-Infrared Instrument) or James Webb Space Telescope).
A medium-sized dewar is being filled with liquid nitrogen by a larger cryogenic storage tank.

In physics, cryogenics is the production and behaviour of materials at very low temperatures.

The 13th International 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 the conventional refrigeration.[1][2][3][4] This is a logical dividing line, since the normal boiling points of the so-called permanent gases (such as helium, hydrogen, neon, nitrogen, oxygen, and normal air) lie below 120 K, while the Freon refrigerants, hydrocarbons, and other common refrigerants have boiling points above 120 K.[5][6]

Discovery of superconducting 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 to Heike Kamerlingh Onnes on July 10, 1908. The discovery came after the ability 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 the Kelvin or Rankine temperature scale, both of which measure from absolute zero, rather than more usual scales such as Celsius which measures from the freezing point of water at sea level[9][10] or Fahrenheit which measures from the freezing point of a particular brine solution at sea level.[11][12]

Definitions and distinctions

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 of biology involving the study of the effects of low temperatures on organisms (most often for the purpose of achieving cryopreservation). Other applications include Lyophilization (freeze-drying) of pharmaceutical[13] components and medicine.
Cryoconservation of animal genetic resources
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 of stem cells at birth. They may be used to study the generation of cell lines or for stem-cell therapy.[14]
The branch of surgery applying cryogenic temperatures to destroy and kill tissue, e.g. cancer cells. Commonly referred to as Cryoablation.[15]
The study of electronic phenomena at cryogenic temperatures. Examples include superconductivity and variable-range hopping.
Cryopreserving humans and animals with the intention of future revival. "Cryogenics" is sometimes erroneously used to mean "Cryonics" in popular culture and the press.[16]


The word cryogenics stems from Greek κρύος (cryos) – "cold" + γενής (genis) – "generating".

Cryogenic fluids

Cryogenic fluids with their boiling point in Kelvin[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

Industrial applications

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Catalogue image of a cryogenic valve
Cryogenic valves in situ, with condensed atmospheric humidity

Further information: Low-temperature technology timeline

Liquefied gases, such as liquid nitrogen and liquid 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 the lowest attainable temperatures to be reached.

These liquids may be stored in Dewar 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 liquefied hydrogen. Thermos bottles are smaller vacuum 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 on LNG piers to transfer liquefied natural gas from LNG carriers to LNG storage tanks, as are cryogenic valves.

Cryogenic processing

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 of cryogenic hardening, the commercial cryogenic processing industry was founded in 1966 by Bill and Ed Busch. With a background in the heat treating industry, the Busch brothers founded a company in Detroit 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 using cryogenic tempering instead of heat treating.[citation needed] This evolved in the late 1990s into the treatment of other parts.

Cryogens, such as liquid nitrogen, are further used for specialty chilling and freezing applications. Some chemical reactions, like those used to produce the active ingredients for the popular statin drugs, must occur at low temperatures of approximately −100 °C (−148 °F). Special cryogenic chemical reactors are used to remove reaction heat and provide a low temperature environment. The freezing of foods and biotechnology products, like vaccines, requires nitrogen in blast freezing or immersion freezing systems. Certain soft or elastic materials become hard and brittle at very low temperatures, which makes cryogenic milling (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.


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Another use of cryogenics is cryogenic fuels for rockets with liquid hydrogen as the most widely used example. Liquid oxygen (LOX) is even more widely used but as an oxidizer, not a fuel. NASA's workhorse Space Shuttle used cryogenic hydrogen/oxygen propellant as its primary means of getting into orbit. LOX is also widely used with RP-1 kerosene, a non-cryogenic hydrocarbon, such as in the rockets built for the Soviet space program by Sergei Korolev.

Russian aircraft manufacturer Tupolev developed a version of its popular design Tu-154 with a cryogenic fuel system, known as the Tu-155. The plane uses a fuel referred to as liquefied natural gas or LNG, and made its first flight in 1989.[20]

Other applications

Astronomical instruments on the Very Large Telescope are equipped with continuous-flow cooling systems.[21]

Some applications of cryogenics:

Cryogenic gases delivery truck at a supermarket, Ypsilanti, Michigan


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Cryogenic cooling of devices and material is usually achieved via the use of liquid nitrogen, liquid helium, or a mechanical cryocooler (which uses high-pressure helium lines). Gifford-McMahon cryocoolers, pulse tube cryocoolers and Stirling 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 the magnetocaloric effect.


There are various cryogenic detectors which are used to detect particles.

For cryogenic temperature measurement down to 30 K, Pt100 sensors, a resistance temperature detector (RTD), are used. For temperatures lower than 30 K, it is necessary to use a silicon diode for accuracy.

See also


  1. ^ International Dictionary of Refrigeration, Archived 2019-10-01 at the Wayback Machine
  2. ^ ASHRAE Terminology,
  3. ^ "Cryogenics is usually defined as the science and technology dealing with temperatures less than about 120 K [4,5], although this review does not adhere to a strict 120 K definition." K.D. Timmerhaus, R. Reed. Cryogenic Engineering: Fifty Years of Progress. Springer Science+Business Media LLC (2007), chapter: 1.2 The Beginning of Cryogenics, p. 7
  4. ^ "About Cryogenics". In terms of the Kelvin scale the cryogenic region is often considered to be that below approximately 120 K (-153 C).
  6. ^ "PROPANE at Pubchem".
  7. ^ J. M. Nash, 1991, "Vortex Expansion Devices for High Temperature Cryogenics", Proc. of the 26th Intersociety Energy Conversion Engineering Conference, Vol. 4, pp. 521–525.
  8. ^ Radebaugh, R. (2007), Timmerhaus, Klaus D.; Reed, Richard P. (eds.), "Historical Summary of Cryogenic Activity Prior to 1950", Cryogenic Engineering, International Cryogenics Monograph Series, New York, NY: Springer, pp. 3–27,, doi:10.1007/0-387-46896-x_1, ISBN 978-0-387-46896-9
  9. ^ Celsius, Anders (1742) "Observationer om twänne beständiga grader på en thermometer" (Observations about two stable degrees on a thermometer), Kungliga Svenska Vetenskapsakademiens Handlingar (Proceedings of the Royal Swedish Academy of Sciences), 3: 171–180 and Fig. 1.
  10. ^ Don Rittner; Ronald A. Bailey (2005): Encyclopedia of Chemistry. Facts On File, Manhattan, New York City. p. 43.
  11. ^ Fahrenheit temperature scale, Encyclopædia Britannica Online. 25 September 2015
  12. ^ "Fahrenheit: Facts, History & Conversion Formulas". Live Science. Retrieved 2018-02-09.
  13. ^ Evans, Nicole. "What is Cryobiology?". Retrieved 2023-11-27.
  14. ^ Hunt, Charles (April 3, 2011). "Cryopreservation of Human Stem Cells for Clinical Application: A Review". Transfusion Medicine and Hemotherapy. 38 (2): 107–123. doi:10.1159/000326623. PMC 3088734. PMID 21566712.
  15. ^ "Cryosurgery to Treat Cancer". NCI. June 21, 2021. Retrieved 2023-11-27.
  16. ^ "Cryonics is NOT the Same as Cryogenics". Cryogenic Society of America. Archived from the original on 2 December 2018. Retrieved 5 March 2013.
  18. ^ Thermal, Timmy. "Cryogenic Labels". MidcomData. Retrieved 11 August 2014.
  19. ^ Gantz, Carroll (2015). Refrigeration: A History. Jefferson, North Carolina: McFarland & Company, Inc. p. 227. ISBN 978-0-7864-7687-9.
  20. ^ "Tu-155 / Tu-156". Retrieved 2023-11-27.
  21. ^ "ESO Signs Technology Transfer Licence Agreement for Cooling System". Retrieved 11 June 2015.
  22. ^ "Pfizer–BioNTech COVID-19 Vaccine Vaccination Storage & Dry Ice Safety Handling". Pfizer-BioNTech. Archived from the original on 24 January 2021. Retrieved 17 December 2020.

Further reading