I.

Introduction

Cryogenics, study and use of materials at very low temperatures. The National Institute of Standards and Technology (NIST) has suggested that the term cryogenics be applied to all temperatures below -150°C (-238°F or 123° above absolute zero on the Kelvin scale). Some scientists regard the normal boiling point of oxygen (-183°C or -297°F) as the upper limit. Cryogenic temperatures can approach, but can never reach, absolute zero, the theoretical absence of all thermal energy (heat) at zero degrees on the Kelvin scale (0 K), equivalent to -273.15°C (-459.67°F). The term cryogenics comes from a combination of Greek words meaning “frost producing.”

Cryogenic temperatures are achieved by a number of methods under laboratory or industrial conditions, often applied in a sequence. The earliest cooling methods used either the rapid evaporation of volatile liquids or the expansion of gases confined initially at pressures of 150 to 200 atmospheres. (One atmosphere is a unit of pressure based on normal atmospheric pressure measured at sea level.) Devices used to chill materials to cryogenic temperatures are called cryogenic refrigerators or “cryocoolers.” Other techniques employed for specialized scientific research include using laser beams or radio waves to remove more and more thermal energy from a substance, often when it is confined in a magnetic field. Scientists have cooled substances to temperatures as low as a few hundred billionths of a degree Kelvin above absolute zero.