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A colorless, odorless inert gaseous element occurring in natural gas and with radioactive ores. It is used as a component of artificial atmospheres and laser media, as a refrigerant, as a lifting gas for balloons, and as a superfluid in cryogenic research. Atomic number 2; atomic weight 4.0026; boiling point −268.9°C; density at 0°C 0.1785 gram per liter.
[From Greek hēlios, sun (so called because its existence was deduced from the solar spectrum).]
Background
Helium is one of the basic chemical elements. In its natural state, helium is a colorless gas known for its low density and low chemical reactivity. It is probably best known as a non-flammable substitute for hydrogen to provide the lift in blimps and balloons. Because it is chemically inert, it is also used as a gas shield in robotic arc welding and as a non-reactive atmosphere for growing silicon and germanium crystals used to make electronic semiconductor devices. Liquid helium is often used to provide the extremely low temperatures required in certain medical and scientific applications, including superconduction research.
Although helium is one of the most abundant elements in the universe, most of it exists outside of Earth's atmosphere. Helium wasn't discovered until 1868, when French astronomer Pierre Janssen and English astronomer Sir Joseph Lockyer were independently studying an eclipse of the Sun. Using spectrometers, which separate light into different bands of color depending on the elements present, they both observed a band of yellow light that could not be identified with any known element. News of their findings reached the scientific world on the same day, and both men are generally credited with the discovery. Lockyer suggested the name helium for the new element, derived from the Greek word helios for the sun.
In 1895, English chemist Sir William Ramsay found that cleveite, a uranium mineral, contained helium. Swedish chemists P.T. Cleve and Nils Langlet made a similar discovery at about the same time. This was the first time helium had been identified on Earth. In 1905, natural gas taken from a well near Dexter, Kansas, was found to contain as much as 2% helium. Tests of other natural gas sources around the world yielded widely varying concentrations of helium, with the highest concentrations being found in the United States.
During the early 1900s, the development of lighter-than-air blimps and dirigibles relied almost entirely on hydrogen to provide lift, even though it was highly flammable. During World War I, the United States government realized that non-flammable helium was superior to hydrogen and declared it a critical war material. Production was tightly controlled, and exports were curtailed. In 1925, the United States passed the first Helium Conservation Act which prohibited the sale of helium to nongovernmental users. It wasn't until 1937, when the hydrogen-filled dirigible Hindenburg exploded while landing at Lakehurst, New Jersey, that the restrictions were lifted and helium replaced hydrogen for commercial lighter-than-air ships.
During World War II, helium became a critical war material again. One of its more unusual uses was to inflate the tires on long-range bomber aircraft. The lighter weight of helium allowed the plane to carry 154 lb (70 kg) of extra fuel for an extended range.
After the war, demand for helium grew so rapidly that the government imposed the Helium Act Amendments in 1960 to purchase and store the gas for future use. By 1971, the demand had leveled off and the helium storage program was canceled. A few years later, the government started storing helium again. As of 1993, there were about 35 billion cubic feet (1.0 billion cubic meters) of helium in government storage.
Today, the majority of the helium-bearing natural gas sources are within the United States. Canada, Poland, and a few other countries also have significant sources.
Raw Materials
Helium is generated underground by the radioactive decay of heavy elements such as uranium and thorium. Part of the radiation from these elements consists of alpha particles, which form the nuclei of helium atoms. Some of this helium finds its way to the surface and enters the atmosphere, where it quickly rises and escapes into space. The rest becomes trapped under impermeable layers of rock and mixes with the natural gases that form there. The amount of helium found in various natural gas deposits varies from almost zero to as high as 4% by volume. Only about one-tenth of the working natural gas fields have economically viable concentrations of helium greater than 0.4%.
Helium can also be produced by liquefying air and separating the component gases. The production costs for this method are high, and the amount of helium contained in air is very low. Although this method is often used to produce other gases, like nitrogen and oxygen, it is rarely used to produce helium.
The Manufacturing
Process
Helium is usually produced as a byproduct of natural gas processing. Natural gas contains methane and other hydrocarbons, which are the principal sources of heat energy when natural gas is burned. Most natural gas deposits also contain smaller quantities of nitrogen, water vapor, carbon dioxide, helium, and other non-combustible materials, which lower the potential heat energy of the gas. In order to produce natural gas with an acceptable level of heat energy, these impurities must be removed. This process is called upgrading.
There are several methods used to upgrade natural gas. When the gas contains more than about 0.4% helium by volume, a cryogenic distillation method is often used in order to recover the helium content. Once the helium has been separated from the natural gas, it undergoes further refining to bring it to 99.99+% purity for commercial use.
Here is a typical sequence of operations for extracting and processing helium.
Pretreating
Because this method utilizes an extremely cold cryogenic section as part of the process, all impurities that might solidify—such as water vapor, carbon dioxide, and certain heavy hydrocarbons—must first be removed from the natural gas in a pretreatment process to prevent them from plugging the cryogenic piping.
Separating
Natural gas is separated into its major components through a distillation process known as fractional distillation. Sometimes this name is shortened to fractionation, and the vertical structures used to perform this separation are called fractionating columns. In the fractional distillation process, the nitrogen and methane are separated in two stages, leaving a mixture of gases containing a high percentage of helium. At each stage the level of concentration, or fraction, of each component is increased until the separation is complete. In the natural gas industry, this process is sometimes called nitrogen rejection, since its primary function is to remove excess quantities of nitrogen from the natural gas.
Purifying
Crude helium must be further purified to remove most of the other materials. This is usually a multi-stage process involving several different separation methods depending on the purity of the crude helium and the intended application of the final product.
Distributing
Helium is distributed either as a gas at normal temperatures or as a liquid at very low temperatures. Gaseous helium is distributed in forged steel or aluminum alloy cylinders at pressures in the range of 900-6,000 psi (6-41 MPa or 60-410 atm). Bulk quantities of liquid helium are distributed in insulated containers with capacities up to about 14,800 gallons (56,000 liters).
Quality Control
The Compressed Gas Association establishes grading standards for helium based on the amount and type of impurities present. Commercial helium grades start with grade M, which is 99.995% pure and contains limited quantities of water, methane, oxygen, nitrogen, argon, neon, and hydrogen. Other higher grades include grade N, grade P, and grade G. Grade G is 99.9999% pure. Periodic sampling and analysis of the final product ensures that the standards of purity are being met.
The Future
In 1996, the United States government proposed that the government-funded storage program for helium be halted. This has many scientists worried. They point out that helium is essentially a waste product of natural gas processing, and without a government storage facility, most of the helium will simply be vented into the atmosphere, where it will escape into space and be lost forever. Some scientists predict that if this happens, the known reserves of helium on Earth may be depleted by the year 2015.
Where to Learn More
Books
Brady, George S., Henry R. Clauser, and John A. Vaccari. Materials Handbook, 14th Edition. McGraw-Hill, 1997.
Heiserman, David L. Exploring Chemical Elements and Their Compounds. TAB Books, 1992.
Kroschwitz, Jacqueline I., executive editor, and Mary Howe-Grant, editor. Encyclopedia of Chemical Technology, 4th edition. John Wiley and Sons, Inc., 1993.
Stwertka, Albert. A Guide to the Elements. Oxford University Press, 1996.
Periodicals
Powell, Corey S. "No Light Matter." Scientific American (March 1996): 28, 30.
Other
http://www.intercorr.com/periodic/2.htm (This website contains a summary of the history, sources, properties, and uses of helium.)
[Article by: Chris Cavette]
A gaseous chemical element, He, atomic number 2 and atomic weight 4.0026. Helium is one of the noble gases in group 18 of the periodic table. It is the second lightest element. The world's chief source of helium is a group of natural gas fields in the United States. See also Inert gases; Periodic table.
Helium is a colorless, odorless, and tasteless gas. It has the lowest solubility in water of any known gas. It is the least reactive element and forms essentially no chemical compounds. The density and the viscosity of helium vapor is very low. Thermal conductivity and heat content are exceptionally high. Helium can be liquefied, but its condensation temperature is the lowest of any known substance. The properties of helium are given in the table.
Property | Value |
|---|---|
Atomic number | 2 |
Atomic weight | 4.0026 |
Melting point* at 25.2 atm pressure | −272.1°C (1.1 K) |
−271.37°C (1.78 K) | |
Triple point = λ-point (helium gas, helium I, helium II) | −270.96°C (2.19 K) |
Boiling point at 1 atm pressure | −268.94°C (4.22 K) |
Gas density at 0°C and 1 atm pressure, g/liter | 0.17847 |
Liquid density at its boiling point, g/ml | 0.1249 |
Solubility in water at 20°C, ml helium (STP)/1000 g water at 1 atm partial pressure of helium | 8.61 |
*The melting point varies with the pressure.
Helium was first used as a lifting gas in balloons and dirigibles. This use continues for high-altitude research and for weather balloons. The principal use of helium is in inert gas–shielded arc welding. The greatest potential for helium use continues to emerge from extreme-low-temperature applications. Helium is the only refrigerant capable of reaching temperatures below 14 K (−434°F). The chief value of ultralow temperature is the development of the state of superconductivity, in which there is virtually zero resistance to the flow of electricity. Other helium applications include use as a pressurizing gas in liquid-fueled rockets, in helium-oxygen breathing mixtures for divers, as a working fluid in gas-cooled nuclear reactors, and as a carrier gas for chemical analysis by gas chromatography.
Terrestrial helium is believed to be formed in natural radioactive decay of heavy elements. Most of this helium migrates to the surface and enters the atmosphere. The atmospheric concentration of helium (5.25 parts per million at sea level) could be expected to be higher. However, its low molecular weight permits helium to escape into space from the upper atmosphere at a rate roughly equal to its formation. Natural gases contain helium at concentrations higher than in the atmosphere.
Helium is an element with a closed electronic shell, a large ionization potential, and a low polarizability, which makes it a very unlikely candidate to form chemical bonds. However, solid helium compounds have been found to form at high pressure, one with nitrogen [He(N2)11] and one with neon [Ne(He)2]. These compounds belong to a class known as van der Waals compounds. See also Intermolecular forces.
Other helium compounds have also been observed in a clathrate hydrate, He(H2O)6+δ, and helium has been detected inside the carbon molecule buckminsterfullerene (C60), forming HeC60. Mixtures of helium and other components prevail under conditions of high pressure in the outer planets of the solar system and their satellites. Therefore, it is believed that helium compounds are important in the modeling of the interiors of such celestial bodies. The formation of helium compounds at high pressures illustrates that under such conditions different chemical behavior occurs compared to that observed under ambient conditions. See also Chemical bonding; Clathrate compounds; Fullerene.
Helium-3 is a rare stable isotope of helium was discovered by L. W. Alvarez and R. Cornog in 1939. Its concentration in nature is so low, approximately one part per hundred million in well helium, that it was 1951 before sufficient quantities of pure gas became available for experimentation. The gas was then, and continues to be, obtained as a by-product from the decay of tritium, the heavy isotope of hydrogen. Tritium is produced in a nuclear reactor from the reaction between lithium and a neutron.
The 3He nucleus is composed of two protons and one neutron, one fewer than for 4He; as a consequence, 3He is a fermion whereas 4He is a boson. The two isotopes are the exemplars of Fermi-Dirac and Bose-Einstein systems, respectively. It is principally for this reason that helium, an apparently featureless chemical element, has been studied intensively. See also
A colorless, odorless, tasteless gas; one of the inert gaseous elements. Symbol, He; atomic number, 2; atomic weight, 4.003. Used in medicine as a diluent for other gases.
For more information on helium, visit Britannica.com.
Spectroscopic evidence for the presence of helium in the sun was first obtained during a solar eclipse in 1868. A bright yellow emission line was observed and was later shown to correspond to no known element; the new element was named by J. N. Lockyer and E. Frankland from helios [Gr.,=sun]. Helium was isolated (1895) from a sample of the uranium mineral cleveite by Sir William Ramsay.
Properties and Isotopes
Helium is less dense than any other known gas except hydrogen and is about one seventh as dense as air. Extremely unreactive, it is an inert gas in Group 18 of the periodic table. Natural helium is a mixture of two stable isotopes, helium-3 and helium-4. In helium obtained from natural gas about one atom in 10 million is helium-3. The unstable isotopes helium-5, helium-6, and helium-8 have been synthesized. The alpha particles that are emitted from certain radioactive substances are identical to helium-4 nuclei (two protons and two neutrons).
Helium-4 is unusual in that it forms two different kinds of liquids. When it is cooled below 4.22K (its boiling point at atmospheric pressure) it condenses to liquid helium-I, which behaves as an ordinary liquid. When liquid helium-I is cooled below about 2.18K (at atmospheric pressure), liquid helium-II is formed. Liquid helium-II has a number of unusual properties. It is sometimes called a superfluid because it has extremely low viscosity. It also has extremely high heat conductivity and expands on cooling. It cannot be contained in an open beaker since a thin film of it creeps up the side, over the lip, and flows down the outside. The study of these phenomena is a part of low-temperature physics. When helium-3 is liquefied and cooled it does not exhibit the properties of liquid helium-II; this difference in properties between helium-3 and helium-4 can be explained in terms of quantum mechanics.
Natural Occurrence and Preparation
Helium is rare and costly. Wells in Texas (where the Federal Helium Reserve was established in 1925 near Amarillo), Oklahoma, and Kansas are the principal world source. Crude helium is separated by liquefying the other gases present in the natural gas; it is then either further purified or stored for later purification and use. Some helium is extracted directly from the atmosphere; the gas is also found in certain uranium minerals and in some mineral waters, but not in economic quantities. It has been estimated that helium makes up only about 0.000001% of the combined weight of the earth's atmosphere and crust; it is most concentrated in the exosphere, which is the outermost region of the atmosphere, 600–1500 mi (960–2400 km) above the earth's surface. Helium is abundant in outer space; it makes up about 23% of the mass of the visible universe. It is the end product of energy-releasing fusion processes in stars (see interstellar matter).
Uses
Helium's noncombustibility and buoyancy (second only to hydrogen) make it the most suitable gas for balloons and other lighter-than-air craft. A mixture of helium and oxygen is often supplied as a breathing mixture for deep-sea divers and caisson workers and is used in decompression chambers; because helium is less soluble in human blood than nitrogen, its use reduces the risk of caisson disease, or the “bends.” Helium can also be used wherever an unreactive atmosphere is needed, e.g., in electric arc welding, in growing crystals of silicon and germanium for semiconductors, and in refining titanium and zirconium metals. It is also used to pressurize the fuel tanks of liquid-fueled rockets. Liquid helium is essential for many low temperature applications (see low-temperature physics).
A chemical element, usually found in the form of a gas, in which two electrons are in orbit, and the nucleus consists of two protons and two neutrons. Its symbol is He.
A chemical element, atomic number 2, atomic weight 4.003, symbol He.
Helium is a chemically inert element that is odorless, tasteless and noncombustible. Because of its low density it is easily moved through the air passages and therefore requires little effort in breathing on the part of the patient who is in respiratory distress. Although helium itself has no chemical therapeutic value, when combined with oxygen it facilitates the delivery of this gas to the lungs (see helium–oxygen therapy).
An element with atomic number 2; symbol: He. It is the second most common element in the Sun and outer planets, but rare on the rocky planets.
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| General | |||||||||||||||||||||||||
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| Name, symbol, number | helium, He, 2 | ||||||||||||||||||||||||
| Chemical series | noble gases | ||||||||||||||||||||||||
| Group, period, block | 18, 1, s | ||||||||||||||||||||||||
| Appearance | colorless |
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| Standard atomic weight | 4.002602(2) g·mol−1 | ||||||||||||||||||||||||
| Electron configuration | 1s2 | ||||||||||||||||||||||||
| Electrons per shell | 2 | ||||||||||||||||||||||||
| Physical properties | |||||||||||||||||||||||||
| Phase | gas | ||||||||||||||||||||||||
| Density | (0 °C, 101.325 kPa) 0.1786 g/L |
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| Melting point | (at 2.5 MPa) 0.95 K (−272.2 °C, −458.0 ° |
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| Boiling point | 4.22 K (−268.93 °C, −452.07 ° |
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| Critical point | 5.19 K, 0.227 MPa | ||||||||||||||||||||||||
| Heat of fusion | 0.0138 kJ·mol−1 | ||||||||||||||||||||||||
| Heat of vaporization | 0.0829 kJ·mol−1 | ||||||||||||||||||||||||
| Heat capacity | (25 °C) 20.786 J·mol−1·K−1 | ||||||||||||||||||||||||
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| Atomic properties | |||||||||||||||||||||||||
| Crystal structure | hexagonal close-packed | ||||||||||||||||||||||||
| Electronegativity | no data (Pauling scale) | ||||||||||||||||||||||||
| Ionization energies | 1st: 2372.3 kJ/mol | ||||||||||||||||||||||||
| 2nd: 5250.5 kJ/mol | |||||||||||||||||||||||||
| Atomic radius (calc.) | 31 pm | ||||||||||||||||||||||||
| Covalent radius | 32 pm | ||||||||||||||||||||||||
| Van der Waals radius | 140 pm | ||||||||||||||||||||||||
| Miscellaneous | |||||||||||||||||||||||||
| Thermal conductivity | (300 K) 151.3 m W·m−1·K−1 | ||||||||||||||||||||||||
| CAS registry number | 7440-59-7 | ||||||||||||||||||||||||
| Selected isotopes | |||||||||||||||||||||||||
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| References | |||||||||||||||||||||||||
Helium (He) is a colorless, odorless, tasteless, non-toxic, inert monatomic chemical element that heads the noble gas series in the periodic table and whose atomic number is 2. Its boiling and melting points are the lowest among the elements and it exists only as a gas except in extreme conditions. Extreme conditions are also needed to create the small handful of helium compounds, that are all unstable at standard temperature and pressure. It has a second, rare, stable isotope which is called helium-3. The behavior of liquid helium-4's two fluid phases, helium I and helium II, is important to researchers studying quantum mechanics (in particular the phenomenon of superfluidity) and to those looking at the effects that temperatures near absolute zero have on matter (such as superconductivity).
Helium is the second most abundant and second lightest element in the universe and was one of the elements created in the Big Bang. In the modern universe almost all new helium is created as a result of the nuclear fusion of hydrogen in stars. On Earth it is created by the radioactive decay of much heavier elements (alpha particles are helium nuclei). After its creation, part of it is trapped with natural gas in concentrations up to 7% by volume. It is extracted from the natural gas by a low temperature separation process called fractional distillation.
In 1868 the French astronomer Pierre Janssen first detected helium as an unknown yellow spectral line signature in light from a solar eclipse. Since then large reserves of helium have been found in the natural gas fields of the United States, which is by far the largest supplier of the gas. It is used in cryogenics, in deep-sea breathing systems, to cool superconducting magnets, in helium dating, for inflating balloons, for providing lift in airships and as a protective gas for many industrial uses (such as arc welding and growing silicon wafers). A much less serious use is to temporarily change the timbre and quality of one's voice by inhaling a small volume of the gas (see precautions section below).
Helium 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.[1]
Helium is less water soluble than any other gas known, and helium's index of refraction is closer to unity than that of 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 40 K 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++, [[Hydrohelium(1+) ion|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.[1] 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 aurora.
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%.[2] With a bulk modulus on the order of 5×107 Pa[3] 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 1–1.5 K (about −272 °C or −457 °F) and about 25 bar (2.5 MPa) of pressure.[4] 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.
Solid helium has a density of 0.214 ±0.006 g/ml (1.15 K, 66 atm) with a mean isothermal compressibility of the solid at 1.15 K between the solidus and 66 atm of 0.0031 ±0.0008/atm. Also, no difference in density was noted between 1.8 K and 1.5 K. This data projects that T=0 solid helium under 25 bar of pressure (the minimum required to freeze helium) has a density of 0.187 ±0.009 g/ml.[5]
The boiling point of helium is 4.22 kelvin (-269
oC). The first scientist to obtain liquid helium was the Dutchman Heike
Kamerlingh Onnes. This achievement made the discovery of superconductivity
possible. Above the lambda point of 2.1768 kelvin, the
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.[6] 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.[6] 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.[6]
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.[7]
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
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.[11]
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.[8]
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. Helium is commercially available in either liquid or gaseous form. As a liquid, it can be supplied in small containers called dewars which hold up to 1,000 liters of helium, or in large ISO containers which have nominal capacities as large as 11,000 gallons (41,637 liters). In gaseous form, small quantities of helium are supplied in high pressure cylinders holding up to 300 standard cubic feet, while large quantities of high pressure gas are supplied in tube trailers which have capacities of up to 180,000 standard cubic feet.
As a lighter than air gas, airships and balloons are inflated with helium for lift. In airships, helium is preferred over hydrogen because it is not flammable and has 92.64% of the buoyancy (or lifting power) of the alternative hydrogen (see calculation.)
Due to its low solubility in water, the major part of human blood, air mixtures of helium with oxygen and nitrogen (Trimix), with oxygen only (Heliox), with common air (heliair), and with hydrogen and oxygen (hydreliox), are used in deep-sea breathing systems to reduce the high-pressure risk of nitrogen narcosis, decompression sickness, and oxygen toxicity.
Liquid helium can be used as a cryogenic material, and is used to cool certain metals to produce superconductivity, such as in superconducting magnets use in magnetic resonance imaging and NMR spectroscopy.
Due to its inertness, helium is used as a shielding gas in arc welding processes on materials that are contaminated easily by air. It is especially useful in overhead welding, because it is lighter than air and thus floats, whereas other shielding gases sink. It is also used as a protective gas in growing silicon and germanium crystals, in titanium and zirconium production, in gas chromatography, and as an atmosphere for protecting historical documents. This property also makes it useful in supersonic wind tunnels. In rocketry, helium is used as an ullage medium to displace fuel and oxidizers in storage tanks and to condense hydrogen and oxygen to make rocket fuel. It is also used to purge fuel and oxidizer from ground support equipment prior to launch and to pre-cool liquid hydrogen in space vehicles. For example, the Saturn V booster used in the Apollo program needed about 13 million cubic feet (370,000 m³) of helium to launch.[2]
Apart from its inertness, helium has high thermal conductivity, neutron transparency, and does not form radioactive isotopes under reactor conditions, so it is used as a coolant in some nuclear reactors, such as pebble-bed reactors. The high thermal conductivity and sound velocity of helium is also desirable in thermoacoustic refrigeration. The inertness of helium adds to the environmental advantage of this technology over conventional refrigeration systems which may contribute to ozone depleting and global warming effects.
Other applications include:
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,[13] 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, ἥλιος (helios)[14]
On 26 March 1895 British chemist William Ramsay 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, noticed a bright-yellow line that matched the D3 line observed in the spectrum of the Sun.[15][16][17][18] [15] 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.[19] 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.[20]
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 one kelvin. 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 Keesom 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.
After an oil drilling operation in 1903 in Dexter, Kansas, USA 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.[21] With further analysis, Cady and McFarland discovered that 1.84% of the gas sample was helium.[22] 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
