Liquid helium
From Wikipedia, the free encyclopedia
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 on July 10, 1908 by 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 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; 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 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.
Nearly all of the world's helium supply is found within a 250-mile radius of Amarillo, Texas. A byproduct of billions of years of decay, helium is distilled from natural gas that has accumulated in the presence of radioactive uranium and thorium deposits. If it's not extracted during the natural gas refining process, helium simply soars off when the gas is burned, unrecoverable.
At our current rate of consumption, this resource will likely be empty in 10 to 25 years, and the Earth will be virtually helium-free by the end of the 21st century.
| 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 |
[edit] References
- The Properties of Liquid and Solid Helium, J. Wilks (1987). Oxford: Clarendon Press. ISBN 0-19-851245-7.
[edit] External links
- Watch liquid helium being poured out of a dewar flask and [1]
- Phase diagram, etc.
- Onnes's liquification of helium
