Liquid helium 351583 226159037 2008-07-17T02:23:24Z TomyDuby 2575906 /* Further Reading */ fixed the Dirk vad Delft reference details '''[[Helium]]''' exists in '''[[liquid]] form''' only at very low [[temperature]]s. The [[boiling point]] and [[critical point (thermodynamics)|critical point]] depend on the [[isotope]] of 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 <ref>[http://www-safety.deas.harvard.edu/services/helium.html Liquid Helium], accessed 2007-09-12</ref> Helium-4 was first liquefied on [[July 10]], [[1908]] by [[Kamerlingh Onnes]]. Liquid helium-4 is used as a [[cryogenic]] [[refrigerant]]; it is produced commercially for use in [[superconducting magnet]]s such as those used in [[Magnetic resonance imaging|MRI]] or [[Nuclear magnetic resonance|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 [[atom]]s. The [[interatomic force]]s are weak in the first place because helium is a [[noble gas]]. But the interatomic attraction is reduced even further by [[quantum]] effects, which are important in helium because of its low [[atomic mass]]. The [[zero point energy]] of the liquid is less if the atoms are less confined by their neighbors; thus the liquid can lower its [[ground state]] energy 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 [[superfluid]] phase (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 separation]] into 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 [[enthalpy]] by 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]]. {| class="wikitable" ! Properties of Liquid Helium !! [[Helium-4]] !! [[Helium-3]] |- | [[Critical temperature]] || 5.2 K || 3.3 K |- | [[Boiling point]] at 1 atm || 4.2 K || 3.2 K |- | Minimum [[melting]] [[pressure]] || 25 atm || 29 atm at 0.3 K |- | [[Superfluid]] transition temperature at saturated [[vapor pressure]] || 2.17 K || 1 mK in zero [[magnetic field]] |} ==See also== * [[Liquid nitrogen]] * [[Liquid hydrogen]] * [[Superfluid]] * [[Liquid air]] * [[Cryogen]] * [[Liquid oxygen]] ==Further Reading== *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. ==References== <references/> ==External links== *[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] [[Category:Noble gases|Helium, liquid]] [[Category:Coolants]] [[Category:Refrigerants]] [[Category:Cryogenics]] [[fr:Hélium liquide]] [[pl:Ciekły hel]] [[ru:Жидкий гелий]]