Liquid helium

(cryogenics) The state of helium which exists at atmospheric pressure at temperatures below -268.95°C (4.2 K), and for temperatures near absolute zero at pressures up to about 25 atmospheres (2.53 megapascals); has two phases, helium I and helium II.

Helium boils at a substantially lower temperature, 4.2 K (−452°F or −269°C), than any other substance; and below 2.172 K (−455.76°F) the liquid exhibits the extraordinary properties of superfluidity, notably the ability to flow through narrow channels with complete absence of friction. In addition to the common isotope of atomic weight 4, helium has a rare isotope of atomic weight 3 with a normal boiling point of 3.2 K (−454°F) and a superfluid transition at a very much lower temperature near 0.001 K. Both forms of helium remain in a liquid state at absolute zero. All of these characteristics are due to the weakness of the attractive force between two helium atoms and to the small atomic mass, which according to the laws of quantum mechanics makes the atoms difficult to localize.

At 4.2 K (−452°F) liquid ⁴He is colorless and of low refractive index (n = 1.024), with a density of 0.125 g/cm³ (0.125 times that of water). The latent heat of vaporization, 5 cal/g (21 J/g), is very small, and so care must be taken to reduce the heat input by conduction and radiation into the storage container. The classical container consists of two vacuum-insulated vessels of silvered glass (Dewar flask) or metal, with the inner vessel containing the liquid helium immersed in a larger outer vessel filled with liquid nitrogen. Modern superinsulated Dewars are able to dispense with the liquid nitrogen.

The phase diagram of ⁴He (see illustration) shows several remarkable characteristics. Helium remains a liquid down to absolute zero unless a pressure greater than 2.53 megapascals (25.0 atm or 367 lb/in.²) is applied. A more subtle feature is a transition between two different liquid phases. This λ-transition is so named because the specific heat has a singularity resembling the Greek letter lambda. There is no latent heat; such a transition is called second-order. The high-temperature liquid phase, called helium I, is a rather ordinary liquid. The λ-transition at 2.172 K or −455.76°F (at vapor pressure) marks the onset of superfluidity, which is the characteristic property of the low-temperature phase, helium II. See also Helium; Superfluidity.

Phase diagram for ⁴He. The critical point is at T_c = 5.20 K (−450.3°F), P_c = 229 kPa (2.26 atm or 33.2 lb/in.²).

Liquid helium in a cup.

Helium exists in liquid form only at extremely low temperatures. The boiling point and critical point depend on the isotope of the helium; see the table below for values. The density of liquid helium-4 at its boiling point and 1 atm is approximately 0.125 g/mL ^[1]

Helium-4 was first liquefied on 10 July 1908 by Dutch physicist Heike Kamerlingh Onnes.^[2] 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 liquefied using the Hampson-Linde cycle.^[3]

The temperatures required to liquefy 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.^[4]

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).^[4]

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 millikelvin above absolute zero.^[5][6]

Properties of Liquid Helium	Helium-4	Helium-3
Critical temperature[4]	5.2 K	3.3 K
Boiling point at 1 atm [4]	4.2 K	3.2 K
Minimum melting pressure[7]	25 atm	29 atm at 0.3 K
Superfluid transition temperature at saturated vapor pressure	2.17 K [8]	1 m K in zero magnetic field [9]

Liquid helium (in a vacuum bottle) at 4.2 K and 1 atmosphere, 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 stops completely.

Superfluid phase at temperature below 2.17 K (no boiling).

See also

• Expansion ratio
• Liquid nitrogen
• Liquid hydrogen
• Superfluid
• Liquid air
• Cryogen
• Liquid oxygen

References

1. ^ Liquid Helium, accessed 2007-09-12
2. ^ Wilks, p. 7
3. ^ Daniel V. Schroeder (2000). An Introduction to Thermal Physics. Addison Wesley Longman. p. 141. ISBN 0201380277. 
4. ^ ^{a b c d} Wilks, p. 1.
5. ^ D. O. Edwards, D. F. Brewer, P. Seligman, M. Skertic, and M. Yaqub (1965). "Solubility of He3 in Liquid He4 at 0°K". Phys. Rev. Lett. 15: 773. doi:10.1103/PhysRevLett.15.773. 
6. ^ Wilks, p. 244.
7. ^ Wilks, pp. 474-478.
8. ^ Wilks, p. 289.
9. ^ Dieter Vollhart and Peter Wölfle (1990). The Superfluid Phases of Helium 3. Taylor and Francis. p. 3. 

• J. Wilks (1967). The Properties of Liquid and Solid Helium. 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.

External links

• He-3 and He-4 phase diagrams, etc.
• Helium-3 phase diagram, etc.
• Onnes's liquifaction of helium
• Polypedilum vanderplanki

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