2
  He  
4.002602
Helium

Name: Helium
Symbol: He
Atomic Number: 2
Atomic Weight: 4.002602
Family: Noble Gases
CAS RN: 7440-59-7
Description: An unreactive, colorless, and odor less gas.
State (25C): Gas
Oxidation states: 0

Molar Volume: 31.8 cm3/mole
Valence Electrons: 1s2

Boiling Point:  4.365K, -268.785C, -451.813F
Melting Point:
1.1K, -272.05C, -458F
Electrons Energy Level: 2
Isotopes: 2 + 2 Stable
Heat of Vaporization: 0.0845 kJ/mol
Heat of Fusion: 5.23 kJ/mol
Density: 0.1785 g/L @ 273K & 1atm
Specific Heat: 5.193 J/gK
Atomic Radius: 0.49
Ionic Radius: unknown
Electronegativity: N/A (Pauling); 5.5 (Allrod Rochow)
Helium (from Greek: helios, meaning "sun") is a colorless, odorless, tasteless chemical element, the least reactive of the nearly inert noble gas elements.  Its boiling and melting points are the lowest among the elements; except in extreme conditions, it exists only as a gas.   At temperatures near absolute zero and standard pressure, Helium exists as a superfluid, a nearly frictionless phase of matter with unusual properties.

After hydrogen, Helium is the second lightest element and also the second most abundant element in the universe, (hydrogen being number one), created during big bang nucleosynthesis and to a lesser extent from nuclear fusion of Hydrogen in stars.  First detected in 1868 by French astronomer Pierre Janssen as an unknown yellow spectral line signature in the light of a solar eclipse.  Sir Norman Lockyer, an English astronomer, realized that this line, with a wavelength of 587.49 nanometers, could not be produced by any element known at the time. It was hypothesized that a new element on the sun was responsible for this mysterious yellow emission. This unknown element was named helium by Lockyer.

The hunt to find helium on earth ended in 1895.  Sir William Ramsay, a Scottish chemist, conducted an experiment with a mineral containing uranium called clevite. He exposed the clevite to mineral acids and collected the gases that were produced.  He then sent a sample of these gases to two scientists, Lockyer and Sir William Crookes, who were able to identify the helium within.  Two Swedish chemists, Nils Langlet and Per Theodor Cleve, independently found helium in clevite at about the same time as Ramsay.  It is primarily a product of the radioactive decay of much heavier elements, which emit Helium nuclei called alpha particles.  Its presence in natural gas, the only place it is found in significant amounts, was identified in 1905.  Commercially it is extracted at low temperatures from natural gas by fractional distillation.  Natural gas from different wells on different continents varies greatly in Helium gas content.

Because no helium compounds are known, this family of gases was once thought to be inert. In 1962 the first noble gas compound was prepared with xenon.  However, helium only occurs in uncombined form and must either be extracted from the atmosphere by liquefaction of air or separated from deposits of natural gas. It is thought that some terrestrial helium is produced by alpha decay of radioactive isotopes beneath the crust.

2
He
4.002
10
Ne
20.17
18
Ar
39.94
36
Kr
83.80
54
Xe
131.3
86
Rn
222.0
118
Uuo
293.0

1s2

History

Scientific Discoveries

Evidence of Helium was first detected on August 18, 1868 as a bright yellow line with a wavelength of 587.49 nanometres in the spectrum of the chromosphere of the Sun, by French astronomer Pierre Janssen during a total solar eclipse in Guntur, India.  This line was initially assumed to be Sodium.  On October 20 of the same year, English astronomer Norman Lockyer observed a yellow line in the solar spectrum, which he named the D3 line, for it was near the known D1 and D2 lines of Sodium, and concluded that it was caused by an element in the Sun unknown on Earth.  He and English chemist Edward Frankland named the element with the Greek word for the Sun, (helios).

On March 26, 1895, British chemist William Ramsey isolated helium on Earth by treating the mineral Cleveite with mineral acids.  Ramsay was looking for Argon but, after separating Nitrogen and Oxygen from the gas liberated by Sulfuric Acid, H2SO4, noticed a bright-yellow line that matched the D3 line observed in the spectrum of the Sun.  These samples were identified as Helium by Lockyer and British physicist William Crookes.  It was independently isolated from Cleveite the same year by chemists Per Teodor Cleve and Abraham Langlet in Uppsala, Sweden, who collected enough of the gas to accurately determine its atomic weight.  Helium was also isolated by the American geochemist William Francis Hillebrand prior to Ramsay's discovery when he noticed unusual spectral lines while testing a sample of the mineral Uraninite.  Hillebrand, however, attributed the lines to Nitrogen.  His letter of congratulations to Ramsay offers an interesting case of discovery and near-discovery in science. 

In 1907, Ernest Rutherford and Thomas Royds demonstrated that an alpha particle is a Helium nucleus.  In 1908, Helium was first liquefied by Dutch physicist Heike Kamerlingh Onnes by cooling the gas to less than 1oKelvin.  He tried to solidify it by further reducing the temperature but failed because Helium does not have a triple point temperature where the solid, liquid, and gas phases are at equilibrium. It was first solidified in 1926 by his student Willem Hendrik Keeson by subjecting helium to 25 atmospheres of pressure.

In 1938, Russian physicist Pyotr Leonidovich Kapitsa discovered that Helium-4 has almost no viscosity at temperatures near absolute zero, a phenomenon now called superfluidity.  In 1972, the same phenomenon was observed in Helium-3 by American physicists Douglas D. Osheroff, David M. Lee, and Robert C. Richardson.

History of Extraction and Use

After an oil drilling operation in 1903 in Dexter, Kansas produced a gas geyser that would not burn, Kansas state geologist Erasmus Haworth collected samples of the escaping gas and took them back to the University of Kansas at Lawrence where, with the help of chemists Hamilton Cady and David McFarland, he discovered that the gas contained, by volume, 72% Nitrogen, 15% Methane—insufficient to make the gas combustible, 1% Hydrogen, and 12% of an unidentifiable gas.  With further analysis, Cady and McFarland discovered that 1.84% of the gas sample was helium.  Far from being a rare element, Helium was present in vast quantities under the American Great Plains, available for extraction from natural gas.

This put the United States in an excellent position to become the world's leading supplier of Helium.  Following a suggestion by Sir Richard Threlfall, the United States Navy sponsored three small experimental Helium production plants during World War I.   The The goal was to supply barrage balloons with the non-flammable lifting gas.   A total of 200,000 cubic feet (5700 m3) of 92% Helium was produced in the program even though only a few cubic feet (less than 100 liters) of the gas had previously been obtained.  Some of this gas was used in the world's first Helium-filled airship, the U.S. Navy's C-7, which flew its maiden voyage from Hampton Roads, Virginia to Bolling Field in Washington, D.C. on December 1, 1921.

Although the extraction process, using low-temperature gas liquefaction, was not developed in time to be significant during World War I, production continued. Helium was primarily used as a lifting gas in lighter-than-air craft. This use increased demand during World War II, as well as demands for shielded arc welding.  Helium was also vital in the atomic bomb Manhattan Project.

The government of the United States set up the National Helium Reserve in 1925 at Amarillo Texas, with the goal of supplying military airships in time of war and commercial airships in peacetime. Helium use following World War II was depressed but the reserve was expanded in the 1950s to ensure a supply liquid helium as a coolant to create oxygen/hydrogen rocket fuel (among other uses) during the Space Race and Cold War.   Helium use in the United States in 1965 was more than eight times the peak wartime consumption.

After the "Helium Acts Amendments of 1960" (Public Law 86–777), the U.S. Bureau of Mines arranged for five private plants to recover Helium from natural gas.   For this Helium Conservation program, the Bureau built a 425-mile (684 km) pipeline from Bushton, Kansas to connect those plants with the government's partially depleted Cliffside gas field, near Amarillo, Texas.  This Helium-Nitrogen mixture was injected and stored in the Cliffside gas field until needed, when it then was further purified.

By 1995, a billion cubic metres of the gas had been collected and the reserve was US$1.4 billion in debt, prompting the Congress of the United States in 1996 to phase out the reserve.  The resulting "Helium Privatization Act of 1996" (Public Law 104–273) directed the United States Department of the Interior to start liquidating the reserve by 2005. 

Helium produced before 1945 was about 98% pure (2% Nitrogen), which was adequate for airships.  In 1945 a small amount of 99.9% Helium was produced for welding use.   By 1949 commercial quantities of Grade A 99.995% Helium were available.

For many years the United States produced over 90% of commercially usable Helium in the world.  Extraction plants created in Canada, Poland, Russia, and other nations produced the remaining Helium.  In the mid 1990's, A new plant in Arzew, Algeria producing 600mmcf came on stream, with enough production to cover all of Europe's demand.   Subsequently, in 2004 -2006 two additional plants, one in Ras Laffen,Qatar and the other in Skikda, Algeria were built, but as of early 2007, Ras Laffen is functioning at 50%, and Skikda has yet to start up. Algeria quickly became the second leading producer of Helium. Through this time, both Helium consumption and the costs of producing Helium increased.

Helium is commercially recovered from natural gas deposits, mostly from Texas, Oklahoma and Kansas. 

Gas and Plasma Phases

Helium is a colorless, odorless, and non-toxic gas.  It is the least reactive member of the noble gas elements, and thus also the least reactive of all elements; it is inert and monatomic in virtually all conditions.  Due to Helium's relatively low molar (molecular) mass, in the gas phase it has a thermal conductivity, specific heat, and sound conduction velocity that are all greater than any gas, except Hydrogen.  For similar reasons, and also due to the small size of its molecules, Helium's diffusion rate through solids is three times that of air and around 65% that of Hydrogen.

Helium is less water soluble than any other gas known, and Helium's index of refraction is closer to unity than any other gas. Helium has a negative Joule-Thomson coefficient at normal ambient temperatures, meaning it heats up when allowed to freely expand.  Only below its Joule-Thomson inversion temperature (of about 40oK at 1 atmosphere) does it cool upon free expansion. Once precooled below this temperature, Helium can be liquefied through expansion cooling.

Helium is chemically unreactive under all normal conditions due to its valence of zero.   It is an electrical insulator unless ionized.  As with the other noble gases, Helium has metastable energy levels that allow it to remain ionized in an electrical discharge with a voltage below its ionization potential.  Helium can form unstable compounds with Tungsten, Iodine, Fluorine, Sulfur and Phosphorus when it is subjected to an electric glow discharge, through electron bombardment or is otherwise a plasma.   HeNe, HgHe10, WHe2 and the molecular ions He2+, He2++, HeH+, and HeD+ have been created this way.  This technique has also allowed the production of the neutral molecule He2, which has a large number of band systems, and HgHe, which is apparently only held together by polarization forces.  Theoretically, other compounds, like Helium Fluorohydride (HHeF), may also be possible.

Helium has been put inside the hollow Carbon cage molecules (the fullerenes) by heating under high pressure of the gas.  The neutral molecules formed are stable up to high temperatures. When chemical derivatives of these fullerenes are formed, the Helium stays inside.  If Helium-3 is used, it can be readily observed by Helium NMR spectroscopy.   Many fullerenes containing Helium-3 have been reported.  These substances fit the definition of compounds in the Handbook of Chemistry and Physics. They are the first stable neutral Helium compounds to be formed.

Throughout the universe, Helium is found mostly in a plasma state whose properties are quite different from atomic Helium.  In a plasma, Helium's electrons and protons are not bound together, resulting in very high electrical conductivity, even when the gas is only partially ionized.  The charged particles are highly influenced by magnetic and electric fields. For example, in the solar wind together with ionized Hydrogen, they interact with the Earth's magnetosphere giving rise to Birkeland currents and the auroa borealis.

Solid and Liquid Phases

Helium solidifies only under great pressure. The resulting colorless, almost invisible solid is highly compressible; applying pressure in the laboratory can decrease its volume by more than 30%.  With a bulk modulus on the order of 5107 Pa it is 50 times more compressible than water. Unlike any other element, Helium will fail to solidify and remain a liquid down to absolute zero at normal pressures.  This is a direct effect of quantum mechanics: specifically, the zero point energy of the system is too high to allow freezing. Solid Helium requires a temperature of 1–1.5K (about -272C or -457F) and about 26 standard atmospheres (2.6 MPa) of pressure.  It is often hard to distinguish solid from liquid helium since the refractive index of the two phases are nearly the same. The solid has a sharp melting point and has a crystalline structure.

Helium I State

Below its boiling point of 4.22 kelvin and above the lambda point of 2.1768 kelvin, the isotope Helium-4 exists in a normal colorless liquid state, called Helium I.   Like other cryogenic liquids, helium I boils when heat is added to it.  It also contracts when its temperature is lowered until it reaches the lambda point, when it stops boiling and suddenly expands. The rate of expansion decreases below the lambda point until about 1K is reached; at which point expansion completely stops and Helium I starts to contract again.

Helium I has a gas-like index of refraction of 1.026 which makes its surface so hard to see that floats of Styrofoam are often used to show where the surface is.  This colorless liquid has a very low viscosity and a density 1/8th that of water, which is only 1/4th the value expected from classical physics.  Quantum mechanics is needed to explain this property and thus both types of liquid helium are called quantum fluids, meaning they display atomic properties on a macroscopic scale. This is probably due to its boiling point being so close to absolute zero, which prevents random molecular motion (heat) from masking the atomic properties.

Helium II State 

Liquid helium below its lambda point begins to exhibit very unusual characteristics, in a state called helium II. Boiling of helium II is not possible due to its high thermal conductivity; heat input instead causes evaporation of the liquid directly to gas. The isotope helium-3 also has a superfluid phase, but only at much lower temperatures; as a result, less is known about such properties in the isotope helium-3.

Helium II is a superfluid, a quantum-mechanical state of matter with strange properties. For example, when it flows through even capillaries of 10-7 to 10-8 m width it has no measurable viscosity.  However, when measurements were done between two moving discs, a viscosity comparable to that of gaseous helium was observed. Current theory explains this using the two-fluid model for Helium II. In this model, liquid helium below the lambda point is viewed as containing a proportion of helium atoms in a ground state, which are superfluid and flow with exactly zero viscosity, and a proportion of helium atoms in an excited state, which behave more like an ordinary fluid.

Helium II also exhibits a "creeping" effect. When a surface extends past the level of helium II, the helium II moves along the surface, seemingly against the force of gravity.  Helium II will escape from a vessel that is not sealed by creeping along the sides until it reaches a warmer region where it evaporates.  It moves in a 30 nm thick film regardless of surface material. This film is called a Rollin film and is named after the man who first characterized this trait, Bernard V. Rollin.  As a result of this creeping behavior and helium II's ability to leak rapidly through tiny openings, it is very difficult to confine liquid helium. Unless the container is carefully constructed, the helium II will creep along the surfaces and through valves until it reaches somewhere warmer, where it will evaporate.

In the fountain effect, a chamber is constructed which is connected to a reservoir of helium II by a sintered disc through which superfluid helium leaks easily but through which non-superfluid helium cannot pass. If the interior of the container is heated, the superfluid helium changes to non-superfluid helium. In order to maintain the equilibrium fraction of superfluid helium, superfluid helium leaks through and increases the pressure, causing liquid to fountain out of the container. 

The thermal conductivity of helium II is greater than that of any other known substance, a million times that of helium I and several hundred times that of copper.   This is because heat conduction occurs by an exceptional quantum-mechanical mechanism.  Most materials that conduct heat well have a valence band of free electrons which serve to transfer the heat. Helium II has no such valence band but nevertheless conducts heat well.  The flow of heat is governed by equations that are similar to the wave equation used to characterize sound propagation in air.  So when heat is introduced, it will move at 20 meters per second at 1.8 K through helium II as waves in a phenomenon called second sound.

Occurrence

Helium is the second most abundant element in the known Universe after hydrogen and constitutes 23% of the elemental mass of the universe. It is concentrated in stars, where it is formed from hydrogen by the nuclear fusion of proton-proton chain reaction and CNO cycle.  According to the Big Bang model of the early development of the universe, the vast majority of helium was formed during Big Bang nucleosynthesis, from one to three minutes after the Big Bang. As such, measurements of its abundance contribute to cosmological models.

In the Earth's atmosphere, the concentration of helium by volume is only 5.2 parts per million (about 0.0005%), largely because most helium in the Earth's atmosphere escapes into space due to its inertness and low mass. In the Earth's heterosphere, a part of the upper atmosphere, helium and other lighter gases are the most abundant elements.

Nearly all helium on Earth is a result of radioactive decay.  The decay product is primarily found in minerals of uranium and thorium, including cleveites, pitchblende, carnotite, monazite and beryl, because they emit alpha particles, which consist of helium nuclei (He2+) to which electrons readily combine. In this way an estimated 3.4 litres of helium per year are generated per cubic kilometer of the Earth's crust. In the Earth's crust, the concentration of helium is 8 parts per billion. In seawater, the concentration is only 4 parts per trillion. There are also small amounts in mineral springs, volcanic gas, and meteoric iron. The greatest concentrations on the planet are in natural gas, from which most commercial helium is derived.

Applications

Helium is used for many purposes that require some of its unique properties, such as its low boiling point, low density, low solubility, high thermal conductivity, or inertness.  Pressurized helium is commercially available in large quantities.

Modern Extraction

For large-scale use, helium is extracted by fractional distillation from natural gas, which contains up to 7% helium.  Since helium has a lower boiling point than any other element, low temperature and high pressure are used to liquefy nearly all the other gases (mostly nitrogen and methane).  The resulting crude helium gas is purified by successive exposures to lowering temperatures, in which almost all of the remaining nitrogen and other gases are precipitated out of the gaseous mixture.  Activated charcoal is used as a final purification step, usually resulting in 99.995% pure, Grade-A, helium.  The principal impurity in Grade-A helium is neon.

As of 2004, over one hundred and fifty million cubic metres of helium were extracted from natural gas or withdrawn from helium reserves, annually, with approximately 84% of production from the United States, 10% from Algeria, and most of the remainder from Canada, China, Poland, Qatar, and Russia. In the United States, most helium is produced in Kansas and Texas.

Diffusion of crude natural gas through special semi-permeable membranes and other barriers is another method to recover and purify helium. Helium can be synthesized by bombardment of lithium or boron with high-velocity protons, but this is not an economically viable method of production.

Isotopes

Although there are eight known isotopes of helium, only helium-3 and helium-4 are stable.  In the Earth's atmosphere, there is one He-3 atom for every million He-4 atoms.  However, helium is unusual in that its isotopic abundance varies greatly depending on its origin. In the interstellar medium, the proportion of He-3 is around a hundred times higher.  Rocks from the Earth's crust have isotope ratios varying by as much as a factor of ten; this is used in geology to study the origin of such rocks.

The most common isotope, helium-4, is produced on Earth by alpha decay of heavier radioactive elements; the alpha particles that emerge are fully ionized helium-4 nuclei. Helium-4 is an unusually stable nucleus because its nucleons are arranged into complete shells.  It was also formed in enormous quantities during Big Bang nucleosynthesis.

Equal mixtures of liquid helium-3 and helium-4 below 0.8oK will separate into two immiscible phases due to their dissimilarity (they follow different quantum statistics: helium-4 atoms are bosons while helium-3 atoms are fermions).  Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures of a few millikelvins. There is only a trace amount of helium-3 on Earth, primarily present since the formation of the Earth, although some falls to Earth trapped in cosmic dust.   Trace amounts are also produced by the beta decay of tritium.  In stars, however, helium-3 is more abundant, a product of nuclear fusion.  Extraplanetary material, such as lunar and asteroid regolith, have trace amounts of helium-3 from being bombarded by solar winds.

The different formation processes of the two stable isotopes of helium produce the differing isotope abundances.  These differing isotope abundances can be used to investigate the origin of rocks and the composition of the Earth's mantle.

It is possible to produce exotic helium isotopes, which rapidly decay into other substances. The shortest-lived isotope is helium-5 with a half-life of 7.610-22 second. Helium-6 decays by emitting a beta particle and has a half life of 0.8 second. Helium-7 also emits a beta particle as well as a gamma ray.  Helium-7 and helium-8 are hyperfragments that are created in certain nuclear reactions.

Isotope Atomic Mass Half-Life
He3 3.016 Stable
He4 4.0026 Stable
He5 5.0122  
He6 6.0189 806.7 ms
He7 7.028  
He8 8.0339 119 ms
He9 9.0438  
He10 10.0524  

Precautions 

The voice of a person who has inhaled helium temporarily sounds high-pitched. This is because the speed of sound in helium is nearly three times greater than in air. Because the fundamental frequency of a gas-filled cavity is proportional to the speed of sound in the gas, when helium is inhaled there is a corresponding increase in the resonant frequencies of the vocal tract.

40px-Skull_and_crossbones.svg.jpg (1420 bytes) Although the vocal effect of inhaling helium may be amusing, it can be dangerous if done to excess since helium is a simple asphyxiant, thus it displaces oxygen needed for normal respiration. 

Death by asphyxiation will result within minutes if pure helium is breathed continuously.  In mammals (with the notable exceptions of seals and many burrowing animals) the breathing reflex is triggered by excess of carbon dioxide rather than lack of oxygen, so asphyxiation by helium progresses without the victim experiencing air hunger.  Inhaling helium directly from pressurized cylinders is extremely dangerous as the high flow rate can result in barotrauma, fatally rupturing lung tissue.   Helium may also cause lung collapse.

Neutral helium at standard conditions is non-toxic, plays no biological role and is found in trace amounts in human blood.  At high pressures, a mixture of helium and oxygen (heliox) can lead to high pressure nervous syndrome; however, increasing the proportion of nitrogen can alleviate the problem.

Containers of helium gas at 5 to 10 K should be handled as if they have liquid helium inside due to the rapid and significant thermal expansion that occurs when helium gas at less than 10 K is warmed to room temperature.


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Helium Data

 

Atomic Structure

Atomic Radius (): 0.49
Atomic Volume cm3/mol : unknown
Covalent Radius: 0.93
Crystal Structure: Hexagonal
Ionic Radius: unknown

Chemical Properties

Electrochemical Equivalents: unknown
Electron Work Function: unknown
Electronegativity: N/A (Pauling); 5.5 (Allrod Rochow)
Heat of Fusion: 5.23 kJ/mol
Incompatibilities: unknown
First Ionization Potential: 24.587
Second Ionization Potential: 54.416
Third Ionization Potential: unknown
Valence Electron Potential: unknown
Ionization Energy (eV): 24.587 eV

Physical Properties

Atomic Mass Average: 4.002602
Boiling Point: 4.365K, -268.785C, -451.813F
Melting Point: 1.1K, -272.05C, -458F
Heat of Vaporization: 0.0845 kJ/mol
Coefficient of Lineal Thermal Expansion/K-1: N/A
Electrical Conductivity: unknown
Thermal Conductivity: 0.00152 W/cmK
Density: 0.1785 g/L @ 273K & 1atm
Enthalpy of Atomization: unknown
Enthalpy of Fusion: unknown
Enthalpy of Vaporization: 0.083 kJ/mole
Flammability Class: Non-flammable gas
Molar Volume: 31.8 cm3/mole
Optical Refractive Index: 1.000035 (gas) 1.028 (liquid)
Relative Gas Density (Air=1): unknown
Specific Heat: 5.193 J/gK
Vapor Pressure: unknown
Estimated Crustal Abundance: 810-3 milligrams per kilogram
Estimated Oceanic Abundance: 710-6 milligrams per liter


(Gr. helios, the sun). Evidence of the existence of helium was first obtained by Janssen during the solar eclipse of 1868 when he detected a new line in the solar spectrum; Lockyer and Frankland suggested the name helium for the new element; in 1895 Ramsay discovered helium in the uranium mineral clevite, and it was independently discovered in cleveite by the Swedish chemists Cleve and Langlet about the same time. Rutherford and Royds in 1907 demonstrated that alpha particles are helium nuclei. Except for hydrogen, helium is the most abundant element found throught the universe. Helium is extracted from natural gas: all natural gas contains at least trace quantities of helium. It has been detected spectroscopically in great abundance, especially in the hotter stars, and it is an important component in both the proton-proton reaction and the carbon cycle, which account for the energy of the sun and stars. The fusion of hydrogen into helium provides the energy of the hydrogen bomb. The helium content of the atmosphere is about 1 part in 200,000. While it is present in various radioactive minerals as a decay product, the bulk of the Free World's supply is obtained from wells in Texas, Oklahoma, and Kansas. The only known helium extraction plants, outside the United States, in 1984 were in Eastern Europe (Poland), the U.S.S.R., and a little in India. The cost of helium fell from $2500/ft3 in 1915 to 1.5 cents/ft3 in 1940. The U.S. Bureau of Mines has set the price of Grade A helium at $37.50/1000 ft3 in 1986. Helium has the lowest melting point of any element and has found wide use in cryrogenic research as its boiling point is close to absolute zero. Its use in the study of superconductivity is vital. Using liquid helium, Kurti and co-workers and others, have succeeded in obtaining temperatures of a few microkelvins by the adiabatic demagnetization of copper nuclei. Seven isotopes of helium are known. Liquid helium (He4) exists in two forms: He4I and He4II, wiht a sharp transition point at 2.174K. He4I (above this temperature) is a normal liquid, but He4II (below it) is unlike any other known substance. It expands on cooling; its conductivity for heat is enormous; and neither its heat conduction nor viscosity obeys normal rules. It has other peculiar properties. Helium is the only liquid that cannot be solidified by lowering the temperature. It remains liquid down to absolute zero at ordinary pressures, but it can be readily be solidified by increasing the pressure. Solid 3He and 4He are unusual in that both can be changed in volume by more than 30% by application of pressure. The specific heat of helium gas is unusually high. The density of helium vapor at the normal boiling point is also very high, with the vapor expanding greatly when heated to room temperature. Containers filled with helium gas at 5 to 10 K should be treated as though they contained liquid helium due to the large increase in pressure resulting from warming the gas to room temperature. While helium normally has a 0 valence, it seems to have a weak tendency to combine with certain other elements. Means of preparing helium diflouride have been studied, and species such as HeNe and the molecular ions He+ and He++ have been investigated. Helium is widely used as an inert gas shield for arc welding; as a protective gas in growing silicon and germanium crystals, and in titatium and zirconium production; as a cooling medium for nuclear reactors, and as a gas for supersonic wind tunnels. A mixture of helium and oxygen is used as an artificial atmosphere for divers and others working under pressure. Different ratios of He/O2 are used for different depths at which the diver is operating. Helium is extensively used for filling balloons as it is a much safer gas than hydrogen. One of the recent largest uses for helium has been for pressuring liquid fuel rockets. A Saturn booster such as used on the Apollo lunar missions required about 13 million ft3 of helium for a firing, plus more for checkouts. Liquid helium's use in magnetic resonance imaging (MRI) continues to increase as the medical profession accepts and develops new uses for the equipment. This equipment is providing accurate diagnoses of problems where exploratory surgery has previously been required to determine problems. Another medical application that is being developed uses MRE to determine by blood analysis whether a patient has any form of cancer. Lifting gas applications are increasing. Various companies in addition to Goodyear, are now using "blimps" for advertising. The Navy and Air Force are investigating the use of airships to provide early warning systems to detect low-flying cruise missiles. The Drug Enforcement Agency is using radar-equipped blimps to detect drug smugglers along the southern border of the U.S. In addition, NASA is currently using helium-filled balloons to sample the atmosphere in Antarctica to determine what is depleting the ozone layer that protects Earth from harmful U.V. radiation. Research on and development of materials which become superconductive at temperatures well above the boiling point of helium could have a major impact on the demand for helium. Less costly refrigerants having boiling points considerably higher could replace the present need to cool such superconductive materials to the boiling point of helium.

Source: CRC Handbook of Chemistry and Physics, 1913-1995. David R. Lide, Editor in Chief. Author: C.R. Hammond