Cryogenic Information Library
Helium - Cryogenic Fluids

Helium (He) boils at -452.1°F (-268.93°C or 4.2 K).  Helium does not freeze at atmospheric pressure; solid helium only forms at pressures above 20 times atmospheric.  Liquid helium, because of its low boiling point, is used in many cryogenic systems when temperatures below the boiling point of nitrogen are needed.  A convenient way to cool many kinds of apparatus, such as the super-conducting magnet in an MRI, is to submerge them in liquid helium or liquid nitrogen. Helium, the second most abundant element in the universe, was discovered on the sun before it was found on the earth.  Pierre-Jules-César Janssen, a French astronomer, noticed a yellow line in the sun's spectrum while studying a total solar eclipse in 1868.  Sir Norman Lockyer, an English astronomer, realized that this line, with a wavelength of 587.49 nanometers, could not be produced by any element known at the time.  It was hypothesized that a new element on the sun was responsible for this mysterious yellow emission.  This unknown element was named helium by Lockyer. The hunt to find helium on earth ended in 1895.  Sir William Ramsay, a Scottish chemist, conducted an experiment with a mineral containing uranium called clevite.  He exposed the clevite to mineral acids and collected the gases that were produced.  He then sent a sample of these gases to two scientists, Lockyer and Sir William Crookes, who were able to identify the helium within.  Two Swedish chemists, Nils Langlet and Per Theodor Cleve, independently found helium in clevite at about the same time as Ramsay. Helium makes up about 0.0005% of the earth's atmosphere.  This trace amount of helium is not gravitationally bound to the earth and is constantly lost to space.  The earth's atmospheric helium is replaced by the decay of radioactive elements in the earth's crust.  Alpha decay, one type of radioactive decay, produces particles called alpha particles.  An alpha particle can become a helium atom once it captures two electrons from its surroundings.  This newly formed helium can eventually work its way to the atmosphere through cracks in the crust. Helium is commercially recovered from natural gas deposits, mostly from Texas, Oklahoma and Kansas.  Helium gas is used to inflate blimps, scientific balloons and party balloons.  It is used as an inert shield for arc welding, to pressurize the fuel tanks of liquid fueled rockets and in supersonic wind tunnels.  Helium is combined with oxygen to create a nitrogen-free atmosphere for deep-sea divers so that they will not suffer from a condition known as nitrogen narcosis.  Liquid helium is an important cryogenic material and is used to study superconductivity and to create super conductive magnets.  The Department of Energy's Jefferson Lab uses large amounts of liquid helium to operate its super conductive electron accelerator. Helium is an inert gas and does not easily combine with other elements.  There are no known compounds that contain helium, although attempts are being made to produce helium diflouride.

Atomic Number 2 Atomic Weight 4.0026 Melting Point .95 K Boiling Point 4.22 K Heat of Vaporization 20.73 kj/kg K Specific Heat 4.56 kj/kg K Viscosity 3.57 kg/m-s X 106 Thermal Conductivity 27.2 mW/m-k Dielectric Constant 1.0492 Critical Temperature 5.201 K Critical Pressure .227 MPa Density .0001785 grams/cc Phase at Room Temperature Gas

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