| Liquid Helium |
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Helium-4 was first liquified in 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 MRI or NMR . It is liquified using the Hampson-Linde Cycle . The temperatures required to liquify helium are low because of the weakness of the attraction between helium Atoms . 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 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 Entropy 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 . REFERENCES
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