Solid phases
Helium remains liquid down to absolute zero at atmospheric
pressure, but it freezes at high pressure. Solid helium requires a temperature
of 1–1.5 K (about −272 °C or −457 °F) at about 25 bar (2.5 MPa) of pressure. It
is often hard to distinguish solid from liquid helium since the refractive
index of the two phases is nearly the same. The solid has a sharp melting point
and has a crystalline structure, but it is highly compressible; applying
pressure in a laboratory can decrease its volume by more than 30%. With a bulk
modulus of about 27 MPa, it is ~100 times more compressible than water. Solid
helium has a density of 0.214±0.006 g/cm3 at 1.15 K and 66 atm; the projected
density at 0 K and 25 bar (2.5 MPa) is 0.187±0.009 g/cm3. At higher
temperatures, helium will solidify with sufficient pressure. At room
temperature, this requires about 114,000 atm.
Helium-4 and helium-3 both form several crystalline solid
phases, all requiring at least 25 bars. They both form an α phase, which has a
hexagonal close-packed (hcp) crystal structure, a β phase, which is
face-centered cubic (fcc), and a γ phase, which is body-centered cubic (bcc).
Isotopes
There are nine known isotopes of helium, of which two,
helium-3 and helium-4, are stable. In the Earth's atmosphere, there are 3 He has
for every million that are 4 He atoms. Unlike most elements, helium's isotopic
abundance varies greatly by origin, due to the different formation processes.
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.
Helium-3 is present on Earth only in trace amounts. Most of
it has been present since Earth's formation, though some falls to Earth trapped
in cosmic dust. Trace amounts are also produced by the beta decay of tritium.
Rocks from the Earth's crust have isotope ratios varying by as much as a factor
of ten, and these ratios can be used to investigate the origin of rocks and the
composition of the Earth's mantle. 3 He is much more abundant in stars as a
product of nuclear fusion. Thus, in the interstellar medium, the proportion of
3 He to 4 He
He is about 100 times higher than on Earth. Extraplanetary
material, such as lunar and asteroid regolith, has trace amounts of helium-3
from being bombarded by solar winds. The Moon's surface contains helium-3 at
concentrations on the order of 10 ppb, much higher than the approximately 5 ppt
found in the Earth's atmosphere. Several people, starting with Gerald Kulcinski
in 1986, have proposed exploring the Moon, mining lunar regolith, and using the
helium-3 for fusion.
Liquid helium-4 can be cooled to about 1 K (−272.15 °C;
−457.87 °F) using evaporative cooling in a 1-K pot. Similar cooling of
helium-3, which has a lower boiling point, can achieve about 0.2 kelvin in a
helium-3 refrigerator. Equal mixtures of liquid 3 He and 4
He below 0.8 K separates 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 use this
immiscibility to achieve temperatures of a few millikelvins.
It is possible to produce exotic helium isotopes, which
rapidly decay into other substances. The shortest-lived heavy helium isotope is
the unbound helium-10 with a half-life of 2.6(4) ×10−22 s. Helium-6 decays by
emitting a beta particle and has a half-life of 0.8 seconds. Helium-7 and
helium-8 are created in certain nuclear reactions. Helium-6 and helium-8 are
known to exhibit a nuclear halo.
Properties
Table of thermal and physical properties of helium gas at
atmospheric pressure:
Temperature (K) Density
(kg/m^3) Specific heat (kJ/kg °C) Dynamic viscosity (kg/m s) Kinematic viscosity (m^2/s) Thermal conductivity (W/m °C) Thermal diffusivity (m^2/s) Prandtl number
Compounds
Structure of the helium hydride ion, HHe+
Structure of the suspected fluoroheliate anion, OHeF−
Helium has a valence of zero and is chemically unreactive
under all normal conditions. 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, known as excimers,
with tungsten, iodine, fluorine, sulfur, and phosphorus when it is subjected to
a glow discharge, to electron bombardment, or reduced to plasma by other means.
The molecular compounds HeNe, HgHe10, and WHe2, and the molecular ions He+ 2,
He2+ 2, HeH+, and HeD+ have been created this way. HeH+ is also stable in its
ground state but is extremely reactive—it is the strongest Brønsted acid known
and therefore can exist only in isolation, as it will protonate any molecule or
counteranion it contacts. This technique has also produced the neutral molecule
He2, which has a large number of band systems, and HgHe, which is held together
only by polarization forces.
Van der Waals compounds of helium can also be formed with
cryogenic helium gas and atoms of some other substance, such as LiHe and He2.
Theoretically, other true compounds may be possible, such as
helium fluorohydride (HHeF), which would be analogous to HArF, discovered in
2000. Calculations show that two new compounds containing a helium-oxygen bond
could be stable. Two new molecular species, predicted using theory, CsFHeO and
N(CH3)4FHeO, are derivatives of a metastable FHeO− anion first theorized in
2005 by a group from Taiwan.
Helium atoms have been inserted into the hollow carbon cage
molecules (the fullerenes) by heating under high pressure. The endohedral
fullerene molecules formed are stable at 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 nuclear magnetic
resonance spectroscopy. Many fullerenes containing helium-3 have been reported.
Although the helium atoms are not attached by covalent or ionic bonds, these
substances have distinct properties and a definite composition, like all
stoichiometric chemical compounds.
Under high pressures, helium can form compounds with various
other elements. Helium-nitrogen clathrate (He(N2)11) crystals have been grown
at room temperature at pressures ca. 10 GPa in a diamond anvil cell. The
insulating electride Na2He is thermodynamically stable at pressures above 113
GPa. It has a fluorite structure.
Occurrence and production
Natural abundance
Although it is rare on Earth, helium is the second most
abundant element in the known Universe, constituting 23% of its baryonic mass.
Only hydrogen is more abundant. The vast majority of helium was formed by Big
Bang nucleosynthesis one to three minutes after the Big Bang. As such,
measurements of its abundance contribute to cosmological models. In stars, it
is formed by the nuclear fusion of hydrogen in proton–proton chain reactions
and the CNO cycle, part of stellar nucleosynthesis.
In the Earth's atmosphere, the concentration of helium by
volume is only 5.2 parts per million. The concentration is low and fairly
constant despite the continuous production of new helium because most helium in
the Earth's atmosphere escapes into space by several processes. In the Earth's
heterosphere, a part of the upper atmosphere, helium and hydrogen is the most
abundant elements.
Most helium on Earth is a result of radioactive decay.
Helium is found in large amounts in minerals of uranium and thorium, including
uraninite and its varieties cleveite and pitchblende, carnotite and monazite (a
group name; "monazite" usually refers to monazite-(Ce)),
because they emit alpha particles (helium nuclei, He2+) to which electrons
immediately combine as soon as the particle is stopped by the rock. In this way,
an estimated 3000 metric tons of helium are generated per year throughout the
lithosphere. 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. Because
helium is trapped in the subsurface under conditions that also trap natural
gas, the greatest natural concentrations of helium on the planet are found in
natural gas, from which most commercial helium is extracted. The concentration
varies in a broad range from a few ppm to more than 7% in a small gas field in
San Juan County, New Mexico.
As of 2021, the world's helium reserves were estimated at 31
billion cubic meters, with a third of that being in Qatar. In 2015 and 2016,
additional probable reserves were announced to be under the Rocky Mountains in
North America and in the East African Rift.
The Bureau of Land Management (BLM) has proposed an October
2024 plan for managing natural resources in western Colorado. The plan involves
closing 543,000 acres to oil and gas leasing while keeping 692,300 acres open.
Among the open areas, 165,700 acres have been identified as suitable for helium
recovery. The United States possesses an estimated 306 billion cubic feet of
recoverable helium, sufficient to meet current consumption rates of 2.15
billion cubic feet per year for approximately 150 years.
Modern extraction and distribution
Extracting helium from air is not economical. For
large-scale use, helium is extracted by fractional distillation from natural
gas, which can contain as much as 7% helium. Since helium has a lower boiling
point than any other element, low temperatures 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. In a final production step, most of the helium that
is produced is liquefied via a cryogenic process. This is necessary for
applications requiring liquid helium and also allows helium suppliers to reduce
the cost of long-distance transportation, as the largest liquid helium
containers have more than five times the capacity of the largest gaseous helium
tube trailers.
In 2008, approximately 169 million standard cubic meters
(SCM) of helium were extracted from natural gas or withdrawn from helium
reserves, with approximately 78% from the United States, 10% from Algeria, and
most of the remainder from Russia, Poland, and Qatar. By 2013, increases in
helium production in Qatar (under the company Qatargas managed by Air Liquide)
had increased Qatar's fraction of world helium production to 25%, making it the
second largest exporter after the United States. An estimated 54 billion cubic
feet (1.5×109 m3) deposit of helium was found in Tanzania in 2016. A
large-scale helium plant was opened in Ningxia, China, in 2020.
In the United States, most helium is extracted from the
natural gas of the Hugoton and nearby gas fields in Kansas, Oklahoma, and the
Panhandle Field in Texas. Much of this gas was once sent by pipeline to the
National Helium Reserve, but since 2005, this reserve has been depleted and
sold off, and it is expected to be largely depleted by 2021 under the October
2013 Responsible Helium Administration and Stewardship Act (H.R. 527). The
helium fields of the western United States are emerging as an alternate source
of helium supply, particularly those of the "Four Corners"
region (the states of Arizona, Colorado, New Mexico, and Utah).
Diffusion of crude natural gas through special semipermeable
membranes and other barriers is another method to recover and purify helium. In
1996, the U.S. had proven helium reserves in such gas well complexes of about
147 billion standard cubic feet (4.2 billion SCM). At rates of use at that time
(72 million SCM per year in the U.S.; see pie chart below), this would have
been enough helium for about 58 years of U.S. use, and less than this (perhaps
80% of the time) at world use rates, although factors in saving and processing
impact effective reserve numbers.
Helium is commercially available in either liquid or gaseous
form. As a liquid, it can be supplied in small insulated containers called
dewars, which hold as much as 1,000 liters of helium, or in large ISO
containers, which have nominal capacities as large as 42 m3 (around 11,000 U.S.
gallons). In gaseous form, small quantities of helium are supplied in
high-pressure cylinders holding as much as 8 m3 (approximately 282 standard
cubic feet), while large quantities of high-pressure gas are supplied in tube
trailers, which have capacities of as much as 4,860 m3 (approx. 172,000
standard cubic feet).
Conservation advocates
According to helium conservationists like Nobel laureate
physicist Robert Coleman Richardson, writing in 2010, the free market price of
helium has contributed to "wasteful" usage (e.g., for helium
balloons). Prices in the 2000s had been lowered by the decision of the U.S.
Congress to sell off the country's large helium stockpile by 2015. According to
Richardson, the price needed to be multiplied by 20 to eliminate the excessive
wasting of helium. In the 2012 Nuttall et al. paper titled "Stop
squandering helium", it was also proposed to create an International
Helium Agency that would build a sustainable market for "this precious
commodity".
Applications
The largest single use of liquid helium is to cool the
superconducting magnets in modern MRI scanners.
Estimated 2014 U.S. fractional helium use by category. Total
use is 34 million cubic meters.
Cryogenics (32.0%)
Pressurizing and purging (18.0%)
Welding (13.0%)
Controlled atmospheres (18.0%)
Leak detection (4.00%)
Breathing mixtures (2.00%)
Other (13.0%)
While balloons are perhaps the best-known use of helium,
they are a minor part of all helium use. 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. Of the 2014
world helium total production of about 32 million kg (180 million standard
cubic meters) helium per year, the largest use (about 32% of the total in 2014)
is in cryogenic applications, most of which involve cooling the superconducting
magnets in medical MRI scanners and NMR spectrometers. Other major uses were
pressurizing and purging systems, welding, maintenance of controlled
atmospheres, and leak detection. Other uses by category were relatively minor
fractions.
Controlled atmospheres
Helium is used as a protective gas in growing silicon and
germanium crystals, in titanium and zirconium production, and in gas
chromatography because it is inert. Because of its inertness, thermally and
calorically perfect nature, high speed of sound, and high value of the heat
capacity ratio, it is also useful in supersonic wind tunnels and impulse
facilities.
Gas tungsten arc welding
Helium is used as a shielding gas in arc welding processes
on materials that are contaminated and weakened by air or nitrogen at welding
temperatures. Several inert shielding gases are used in gas tungsten arc
welding, but helium is used instead of cheaper argon, especially for welding
materials that have higher heat conductivity, like aluminium or copper.
Minor uses
Industrial leak detection
One industrial application for helium is leak detection.
Because helium diffuses through solids three times faster than air, it is used
as a tracer gas to detect leaks in high-vacuum equipment (such as cryogenic
tanks) and high-pressure containers. The tested object is placed in a chamber,
which is then evacuated and filled with helium. The helium that escapes through
the leaks is detected by a sensitive device (helium mass spectrometer), even at
the leak rates as small as 10−9 mbar·L/s (10−10 Pa·m3/s). The measurement
procedure is normally automatic and is called the helium integral test. A
simpler procedure is to fill the tested object with helium and to manually
search for leaks with a hand-held device.
Helium leaks through cracks should not be confused with gas
permeation through a bulk material. While helium has documented permeation
constants (thus a calculable permeation rate) through glasses, ceramics, and
synthetic materials, inert gases such as helium will not permeate most bulk
metals.
Flight
The Good Year Blimp
Because of its low density and incombustibility, helium is
the gas of choice to fill airships such as the Goodyear blimp.
Because it is lighter than air, airships and balloons are
inflated with helium for lift. While hydrogen gas is more buoyant and escapes through
a membrane at a lower rate, helium has the advantage of being non-flammable and
indeed fire-retardant. Another minor use is in rocketry, where helium is used
as an ullage medium to backfill rocket propellant tanks in flight and to
condense hydrogen and oxygen to make rocket fuel. It is also used to purge fuel
and oxidizer from ground support equipment before launch and to pre-cool liquid
hydrogen in space vehicles. For example, the Saturn V rocket used in the Apollo
program needed about 370,000 cubic meters (13 million cubic feet) of helium to
launch.
Minor commercial and recreational uses
Helium as a breathing gas has no narcotic properties, so
helium mixtures such as trimix, heliox, and heliair are used for deep diving to
reduce the effects of narcosis, which worsen with increasing depth. As pressure
increases with depth, the density of the breathing gas also increases, and the
low molecular weight of helium is found to considerably reduce the effort of
breathing by lowering the density of the mixture. This reduces the Reynolds
number of the flow, leading to a reduction of turbulent flow and an increase in
laminar flow, which requires less breathing. At depths below 150 meters (490
ft), divers breathing helium-oxygen mixtures begin to experience tremors and a
decrease in psychomotor function, symptoms of high-pressure nervous syndrome.
This effect may be countered to some extent by adding an amount of narcotic gas
such as hydrogen or nitrogen to a helium–oxygen mixture.
Helium–neon lasers, a type of low-powered gas laser
producing a red beam, had various practical applications, which included
barcode readers and laser pointers, before they were almost universally
replaced by cheaper diode lasers.
For its inertness and high thermal conductivity, neutron
transparency, and because it does not form radioactive isotopes under reactor
conditions, helium is used as a heat-transfer medium in some gas-cooled nuclear
reactors.
Helium, mixed with a heavier gas such as xenon, is useful
for thermoacoustic refrigeration due to the resulting high heat capacity ratio
and low Prandtl number. The inertness of helium has environmental advantages
over conventional refrigeration systems, which contribute to ozone depletion or
global warming.
Helium is also used in some hard disk drives.
Scientific uses
The use of helium reduces the distorting effects of
temperature variations in the space between lenses in some telescopes due to
its extremely low index of refraction. This method is especially used in solar
telescopes, where a vacuum-tight telescope tube would be too heavy.
Helium is a commonly used carrier gas for gas
chromatography.
The age of rocks and minerals that contain uranium and
thorium can be estimated by measuring the level of helium with a process known
as helium dating.
Helium at low temperatures is used in cryogenics and in
certain cryogenic applications. As examples of applications, liquid helium is
used to cool certain metals to the extremely low temperatures required for
superconductivity, such as in superconducting magnets for magnetic resonance
imaging. The Large Hadron Collider at CERN uses 96 metric tons of liquid helium
to maintain the temperature at 1.9 K (- −271.25 °C; −456.25 °F).
Medical uses
Helium was approved for medical use in the United States in
April 2020 for humans and animals.
As a contaminant
While chemically inert, helium contamination impairs the
operation of microelectromechanical systems (MEMS) such that iPhones may fail.
Inhalation and safety
Effects
Neutral helium at standard conditions is non-toxic, plays no
biological role, and is found in trace amounts in human blood.
Effect of helium on a human voice
The speed of sound in helium is nearly three times the speed
of sound in air. Because the natural resonance frequency of a gas-filled cavity
is proportional to the speed of sound in the gas, when helium is inhaled, a
corresponding increase occurs in the resonant frequencies of the vocal tract,
which is the amplifier of vocal sound. This increase in the resonant frequency
of the amplifier (the vocal tract) gives increased amplification to the
high-frequency components of the sound wave produced by the direct vibration of
the vocal folds, compared to the case when the voice box is filled with air.
When a person speaks after inhaling helium gas, the muscles that control the
voice box still move in the same way as when the voice box is filled with air;
therefore, the fundamental frequency (sometimes called pitch) produced by
direct vibration of the vocal folds does not change. However, the
high-frequency-preferred amplification causes a change in timbre of the
amplified sound, resulting in a reedy, duck-like vocal quality. The opposite
effect, lowering resonant frequencies, can be obtained by inhaling a dense gas
such as sulfur hexafluoride or xenon.
Hazards
Inhaling helium can be dangerous if done to excess, since
helium is a simple asphyxiant and so displaces oxygen needed for normal
respiration. Fatalities have been recorded, including a youth who suffocated in
Vancouver in 2003 and two adults who suffocated in South Florida in 2006. In
1998, an Australian girl from Victoria fell unconscious and temporarily turned
blue after inhaling the entire contents of a party balloon. Inhaling helium
directly from pressurized cylinders or even balloon filling valves is extremely
dangerous, as high flow rate and pressure can result in barotrauma, fatally
rupturing lung tissue.
Death caused by helium is rare. The first media-recorded
case was that of a 15-year-old girl from Texas who died in 1998 from helium
inhalation at a friend's party; the exact type of helium death is unidentified.
In the United States, only two fatalities were reported
between 2000 and 2004, including a man who died in North Carolina of barotrauma
in 2002. A youth asphyxiated in Vancouver during 2003, and a 27-year-old man in
Australia had an embolism after breathing from a cylinder in 2000. Since then,
two adults were asphyxiated in South Florida in 2006, and there were cases in
2009 and 2010, one of whom was a Californian youth who was found with a bag
over his head, attached to a helium tank, and another teenager in Northern
Ireland died of asphyxiation. At Eagle Point, Oregon, a teenage girl died in
2012 from barotrauma at a party. A girl from Michigan died from hypoxia later
in the year.
On February 4, 2015, it was revealed that, during the
recording of their main TV show on January 28, a 12-year-old member (name
withheld) of Japanese all-girl singing group 3B Junior suffered from air
embolism, losing consciousness and falling into a coma as a result of air
bubbles blocking the flow of blood to the brain after inhaling huge quantities
of helium as part of a game. The incident was not made public until a week
later. The staff of TV Asahi held an emergency press conference to communicate
that the member had been taken to the hospital and is showing signs of
rehabilitation, such as moving eyes and limbs, but her consciousness has not
yet sufficiently recovered. Police have launched an investigation due to a
neglect of safety measures.
The safety issues for cryogenic helium are similar to those
of liquid nitrogen; its extremely low temperatures can result in cold burns,
and the liquid-to-gas expansion ratio can cause explosions if no
pressure-relief devices are installed. Containers of helium gas at 5 to 10 K
should be handled as if they contain liquid helium due to the rapid and
significant thermal expansion that occurs when helium gas at less than 10 K is
warmed to room temperature.
At high pressures (more than about 20 atm or two MPa), a
mixture of helium and oxygen (heliox) can lead to high-pressure nervous
syndrome, a sort of reverse-anesthetic effect; adding a small amount of
nitrogen to the mixture can alleviate the problem.
Notes
A few authors dispute
the placement of helium in the noble gas column, preferring to place it above
beryllium with the alkaline earth metals. They do so on the grounds of helium's
1s2 electron configuration, which is analogous to the ns2 valence
configurations of the alkaline earth metals, and point to some specific trends
that are more regular if helium is placed in group 2. These tend to relate to
kainosymmetry and the first-row anomaly: the first orbital of any type is
unusually small, since, unlike its higher analogues, it does not experience
interelectronic repulsion from a smaller orbital of the same type. Because of
this trend in the sizes of orbitals, a large difference in atomic radii between
the first and second members of each main group is seen in groups 1 and 13–17:
it exists between neon and argon, and between helium and beryllium, but not
between helium and neon. This similarly affects the noble gases' boiling points
and solubilities in water, where helium is too close to neon, and the large
difference characteristic between the first two elements of a group appears
only between neon and argon. Moving helium to group 2 makes this trend
consistent in groups 2 and 18 as well, by making helium the first group 2
element and neon the first group 18 element: both exhibit the characteristic
properties of a kainosymmetric first element of a group. However, the
classification of helium with the other noble gases remains near-universal, as
its extraordinary inertness is extremely close to that of the other light noble
gases neon and argon.
https://en.wikipedia.org/wiki/Helium
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