Liquid Helium

The chemical element helium exists in a liquid form only at the extremely low temperature of -269 degrees Celsius (about 4 Kelvin or -452.2 degrees Fahrenheit). Its boiling point and critical point depend on which isotope of helium is present: the common isotope helium-4 or the rare isotope helium-3. These are the only two stable isotopes of helium. See the table below for the values of these physical quantities. The density of liquid helium-4 at its boiling point and a pressure of one atmosphere (100 kilopascals) is about 125 grams per liter.

Helium was first liquefied on July 10, 1908, by the Dutch physicist Heike Kamerlingh Onnes in Holland. At that time, helium-3 was unknown because the mass spectrometer had not been invented yet. In more recent decades, some liquid helium has been used as a cryogenic refrigerant, and liquid helium is produced commercially for use in superconducting magnets such as those used in magnetic resonance imaging (MRI), nuclear magnetic resonance (NMR), and experiments in physics. Helium can be liquefied only by using the exotic Hampson-Linde cycle, and not by simpler methods.

The temperature required to produce liquid helium is low because of the weakness of the attractions between the helium atoms. These interatomic forces in helium are weak to begin with because helium is a noble gas, but the interatomic attractions are reduced even more by the effects of quantum mechanics. These are significant in helium because of its low atomic mass of about four atomic mass units. The zero point energy of liquid helium is less if its atoms are less confined by their neighbors. Hence in liquid helium, its ground state energy can decrease by a naturally-occurring increase in its average interatomic distance. However at greater distances, the effects of the interatomic forces in helium are even weaker.

Because of the very weak interatomic forces in helium, this element would remain a liquid at atmospheric pressure all the way from its liquefaction point down to absolute zero (an unreachably-low temperature). Liquid helium solidifies only under very low temperatures and great pressures. At temperatures well below their liquefaction points, both helium-4 and helium-3 undergo transitions to superfluids. (See the table below.)

Liquid helium-4 and the rare helium-3 are not completely miscible at temperatures below 0.9 kelvin at their saturated vapor pressure. Below this temperature, a mixture of the two isotopes undergoes a phase separation into a normal fluid (mostly helium-3) that floats on a denser superfluid consisting mostly of helium-4. (This phase separation happens because the overall mass of liquid helium can reduce its thermodynamic enthalpy by separating.)

At extremely low temperatures, the superfluid phase, rich in helium-4, can contain up to 6% of helium-3 in solution. This makes possible the small-scale use of the dilution refrigerator, which is capable of reaching temperatures of a few millikelvins.

• Liquid helium (in a vacuum bottle) at 4.2 K and 1 atm, boiling slowly.

• Lambda point transition: as the liquid is cooled down through 2.17 K, the boiling suddenly becomes violent for a moment, and then it stops, apparently completely. In the superfluid state, the thermal conductivity is extremely large. This causes heat in the body of the liquid to be transferred to its surface so quickly that vaporization takes place only at the free surface of the liquid. Thus, there are no gas bubbles in the body of the liquid.

• Superfluid phase at temperature below 2.17 K (no visible boiling, see caption of picture to the left).