Cryogens

Cryogens are substances used to reach and maintain extremely low temperatures. They include liquids and gases that operate at cryogenic temperatures, typically below -150°C (123 K). Cryogens are distinguished by their very low boiling points and by the large amount of energy required to vaporize them, which makes containment and venting essential.

Common cryogenic liquids include liquid nitrogen (-196°C), liquid helium (-269°C), liquid hydrogen (-253°C), liquid oxygen (-183°C), and liquids such as neon and argon. Methane also qualifies (-161°C). The choice depends on cost, availability, and compatibility with materials and processes. Liquids exist at their boiling points and generate cold gases when warmed.

Handling requires specialized equipment: insulated, vented dewars; training; personal protective equipment. The large expansion of cryogens upon warming can cause pressure buildup and explosive hazards; leaks can create oxygen-deficiency atmospheres. Cryogenic burns are common; materials may become brittle at low temperatures and suffer embrittlement. Storage and transport should avoid sealed containers; vent lines must be unobstructed. Adequate ventilation is essential.

Applications span research, medicine, industry, and space. Cryogenics enable superconductivity for MRI and particle accelerators, cryopreservation of biological samples, cryogenic cooling of instruments and detectors, and rocket fuels such as liquid hydrogen and liquid oxygen.