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Liquid helium is a physical state of helium at very low temperatures at standard atmospheric pressures. Liquid helium may show superfluidity.
At standard pressure, the chemical element helium exists in a liquid form only at the extremely low temperature of '269 °C ('452.20 °F; 4.15 K). 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 (101.3 kilopascals) is about 125 g/L (0.125 g/ml), or about one-eighth the density of liquid water.[1]
Liquefaction
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Helium was first liquefied on July 10, , by the Dutch physicist Heike Kamerlingh Onnes at the University of Leiden in the Netherlands. At that time, helium-3 was unknown because the mass spectrometer had not yet been invented. In more recent decades, liquid helium has been used as a cryogenic refrigerant (which is used in cryocoolers), and liquid helium is produced commercially for use in superconducting magnets such as those used in magnetic resonance imaging (MRI), nuclear magnetic resonance (NMR), magnetoencephalography (MEG), and experiments in physics, such as low temperature Mössbauer spectroscopy. The Large Hadron Collider contains superconducting magnets that are cooled with 120 tonnes of liquid helium.[3]
Liquified helium-3
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A helium-3 atom is a fermion and at very low temperatures, they form two-atom Cooper pairs which are bosonic and condense into a superfluid. These Cooper pairs are substantially larger than the interatomic separation.
Characteristics
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Phase diagram of helium-4 Liquid helium 3 and 4 isotopes in phase diagram, showing the demixing zone.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, the element remains a liquid at atmospheric pressure all the way from its liquefaction point down to absolute zero. At temperatures below their liquefaction points, both helium-4 and helium-3 undergo transitions to superfluids. (See the table below.) Liquid helium can be solidified only under very low temperatures and high pressures.[5]
Liquid helium-4 and the rare helium-3 are not completely miscible.[6] Below 0.9 kelvin at their saturated vapor pressure, 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.[7] 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% helium-3 in solution. This makes the small-scale use of the dilution refrigerator possible, which is capable of reaching temperatures of a few millikelvins.[6]
Superfluid helium-4 has substantially different properties from ordinary liquid helium.
History
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In , Kamerlingh-Onnes succeeded in liquifying a small quantity of helium. In , he provided advice to the Canadian physicist John Cunningham McLennan, who was the first to produce quantities of liquid helium almost on demand.[9]
In Einstein reported that the liquid helium could help in creating an atomic bomb.
Important early work on the characteristics of liquid helium was done by the Soviet physicist Lev Landau, later extended by the American physicist Richard Feynman.
In , Vignos and Fairbank reported the existence of a different phase of solid helium-4, designated the gamma-phase. It exists for a narrow range of pressure between 1.45 and 1.78 K.[10]
Data
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Gallery
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Liquid helium (in a vacuum bottle) at 4.2 K ('268.95 °C) and 1 bar (15 psi) boiling slowly.
Lambda point transition: as the liquid is cooled down through 2.17 K ('270.98 °C), the boiling suddenly becomes violent for a moment.
Superfluid phase at temperature below 2.17 K ('270.98 °C). In this state, the thermal conductivity is extremely high. 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.
The liquid helium is in the superfluid phase. A thin invisible film creeps up the inside wall of the bowl and down on the outside. A drop forms. It will fall off into the liquid helium below. This will repeat until the cup is empty'provided the liquid remains superfluid.
See also
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References
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A cryogenic storage dewar (or simply dewar) is a specialised type of vacuum flask used for storing cryogens (such as liquid nitrogen or liquid helium), whose boiling points are much lower than room temperature. It is named after inventor James Dewar, who developed it for his own work. They are commonly used in low-temperature physics and chemistry.
Cryogenic storage dewars can range widely in size and may take several different forms, including open buckets, flasks with loose-fitting stoppers, and self-pressurising tanks. All dewars have walls constructed from two or more layers, with a high vacuum maintained between the layers. This provides very good thermal insulation between the interior and exterior of the dewar, which reduces the rate at which the contents boil away. Precautions are taken in the design of dewars to safely manage the gas which is released as the liquid slowly boils.
Design
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The simplest dewars allow the gas to escape either through an open top or past a loose-fitting stopper. More sophisticated dewars trap the gas above the liquid, and hold it at high pressure. This increases the boiling point of the liquid, allowing it to be stored for extended periods. Excessive vapour pressure is released automatically through safety valves. Dewars are also designed to be resistant to any sort of puncture to preserve the contents, as cryogens are costly to produce, and some (like helium) are in limited global supply.
The method of decanting liquid from a dewar depends upon its design. Simple dewars may be tilted, to pour liquid from the neck. Self-pressurising designs use the pressure of the gas in the top of the dewar to force the liquid upward through a pipe leading to the neck.
Safety
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Cryogens present several safety hazards, and their storage vessels are designed to reduce the associated risk.
Firstly, no dewar can provide perfect thermal insulation and the cryogenic liquid slowly boils away, which yields an enormous quantity of gas. This is known as the liquid nitrogen evaporation rate. In dewars with an open top, the gas simply escapes into the surrounding area. However, very high pressures can build up inside sealed dewars, and precautions are taken to minimise the risk of explosion. One or more pressure-relief valves allow gas to vent away from the dewar whenever the pressure becomes excessive. In an incident in at Texas A&M University, the pressure-relief devices of a tank of liquid nitrogen were sealed with brass plugs. As a result, the tank failed catastrophically and exploded.[3]
Secondly, if a dewar is left open to the air for extended periods, atmospheric chemicals can condense or freeze on contact with the cryogenic material. This can introduce contaminants. If these materials freeze, for example, water vapor becoming ice, they can block the openings, leading to pressure buildup and the risk of an explosion.
Thirdly, the gas escaping from a dewar can gradually displace the oxygen from the air in the surrounding area, which presents an asphyxiation hazard. Users are trained to store dewars only in a well-ventilated area, and before transporting dewars in an elevator, the excess gas pressure is vented away and the dewars are sent unaccompanied to their destination.[4]
References
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