Helium exists in liquid form only at very low temperatures. The boiling point and critical point depend on the isotope of the helium; see the table below for values.
Helium-4 was first liquified in 1908 by Kamerlingh Onnes. Liquid helium-4 is commercially important today because of its use as a cryogenic refrigerants for conventional superconducting magnets such as those used in MRI, NMR, quantum dots, and amorphous solid waters. 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 forces 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; it does not solidify at any pressure below the melting pressure (see table below). 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. 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.
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