Cryogen

A cryogen is a substance used to produce, maintain, or store materials at extremely low temperatures. In practical terms, cryogenic applications typically involve fluids that boil at temperatures below about 120 kelvin (−153°C) at standard pressure, though some gases are stored as cryogenic liquids at even lower temperatures. Cryogens can also include gases that are liquefied for cooling.

Common cryogens include liquid nitrogen (boiling point 77 kelvin, −196°C), liquid helium (4 kelvin, −269°C), liquid hydrogen (20 kelvin, −253°C), liquid argon (87 kelvin, −186°C), and liquid oxygen (90 kelvin, −183°C). These substances are stored in specialized vacuum-insulated dewars to minimize heat input and boil-off. Cryogens are used for cooling superconducting magnets (such as MRI machines and particle accelerators), research in low-temperature physics, medical cryopreservation, space technology, and industrial gas liquefaction and separation.

Handling requires caution: contact with skin or eyes can cause severe cold burns; many cryogens expand significantly as they warm, creating pressure in containers. They can displace oxygen in poorly ventilated spaces, posing asphyxiation risks; some are flammable (e.g., liquid hydrogen) and others can cause rapid pressure buildup if sealed. Equipment must be designed to tolerate cryogenic temperatures and venting is common.