- Liquid helium
Heliumexists in liquidform only at very low temperatures. The boiling pointand critical point depend on the isotopeof the helium; see the table below for values. The density of liquid helium at its boiling point and 1 atm is approximately 0.125 g/mL [ [http://www-safety.deas.harvard.edu/services/helium.html Liquid Helium] , accessed 2007-09-12]
Helium-4 was first liquefied on
July 10, 1908by Dutch physicist Heike Kamerlingh Onnes. Liquid helium-4 is used as a cryogenic refrigerant; it is produced commercially for use in superconducting magnets such as those used in MRI or NMR. It is liquefied using the Hampson-Linde cycle.
The temperatures required to liquefy helium are low because of the weakness of the attraction between helium
atoms. The interatomic forces are weak in the first place because helium is a noble gas. But the interatomic attraction is reduced even further by quantumeffects, which are important in helium because of its low atomic mass. The zero point energyof the liquid is less if the atoms are less confined by their neighbors; thus the liquid can lower its ground stateenergy by increasing the interatomic distance. But at this greater distance, the effect of interatomic forces is even weaker.
Because of the weak interatomic forces, helium remains liquid down to
absolute zero; helium solidifies only under great pressure. At sufficiently low temperature, both helium-3 and helium-4 undergo a transition to a superfluidphase (see table below).
Liquid helium-3 and helium-4 are not completely miscible below 0.9 K at the saturated
vapor pressure. Below this temperature a mixture of the two isotopes undergoes phase separationinto a lighter normal fluid that is mostly helium-3, and a denser superfluid that is mostly helium-4. (This occurs because the system can lower its enthalpyby separating.) At low temperatures, the helium-4 rich phase may contain up to 6% of helium-3 in solution, which makes possible the existence of the dilution refrigerator, capable of reaching temperatures of a few mK above absolute zero.
*The Properties of Liquid and Solid Helium, J. Wilks (1987). Oxford: Clarendon Press. ISBN 0-19-851245-7.
*Freezing Physics: Heike Kamerlingh Onnes and the Quest for Cold , Van Delft Dirk (2007). Edita - The Publishing House Of The Royal Netherlands Academy of Arts and Sciences. ISBN 9789069845197.
* [http://boojum.hut.fi/research/theory/he3.html Phase diagram, etc.]
* [http://hyperphysics.phy-astr.gsu.edu/hbase/lhel.html Onnes's liquifaction of helium]
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