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Gallium
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Gallium is a chemical element that has the symbol Ga and atomic number 31. Elemental gallium does not occur in nature, but as the Ga (III) salt, in trace amounts in bauxite and zinc ores. A soft silvery metallic poor metal, elemental gallium is a brittle solid at low temperatures. As it liquifies slightly above room temperature, it will melt in the hand. Its melting point is used as a temperature reference point, and from its discovery in 1875 to the semiconductor era, its primary uses were in high-temperature thermometric applications and in preparation of metal alloys with unusual properties of stability, or ease of melting; some being liquid at room temperature (Ga-In eutectic, 75% Ga, 25% In, mp = 15.5°C).
In semiconductors, an important application is in the compounds gallium nitride and gallium arsenide, used most notably in light-emitting diodes (LEDs).
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Encyclopedia
Gallium is a chemical element that has the symbol Ga and atomic number 31. Elemental gallium does not occur in nature, but as the Ga (III) salt, in trace amounts in bauxite and zinc ores. A soft silvery metallic poor metal, elemental gallium is a brittle solid at low temperatures. As it liquifies slightly above room temperature, it will melt in the hand. Its melting point is used as a temperature reference point, and from its discovery in 1875 to the semiconductor era, its primary uses were in high-temperature thermometric applications and in preparation of metal alloys with unusual properties of stability, or ease of melting; some being liquid at room temperature (Ga-In eutectic, 75% Ga, 25% In, mp = 15.5°C).
In semiconductors, an important application is in the compounds gallium nitride and gallium arsenide, used most notably in light-emitting diodes (LEDs). Semiconductor use is now the primary industrial market for gallium, but new uses in alloys and fuel cells continue to be discovered.
Gallium is not known to be essential in biology, but because of the biological handling of gallium’s primary ionic salt Ga(III) as though it were iron(III), gallium ion localizes to and interacts with many processes in the body in which iron(III) is manipulated. As these processes include inflammation, which is present as a marker for many disease states, several gallium salts are used, or are in development, as both pharmaceuticals and radiopharmaceuticals in medicine.
Notable characteristics
Elemental gallium is not found in nature, but it is easily obtained by smelting.
Very pure gallium metal has a brilliant silvery color and its solid metal fractures conchoidally like glass. Gallium metal expands by 3.1 percent when it solidifies, and therefore storage in either glass or metal containers is avoided, due to the possibility of container rupture with freezing. Gallium shares the higher-density liquid state with only a few materials like germanium, bismuth, antimony and water.
Gallium also attacks most other metals by diffusing into their metal lattice. Gallium for example diffuses into the grain boundaries of Al/Zn alloys or steel, making them very brittle. Also, gallium metal easily alloys with many metals, and was used in small quantities in the core of the first atomic bomb to help stabilize the plutonium crystal structure.
The melting point of 302.9146 K (29.7646°C, 85.5763°F) is near room temperature. Gallium's melting point (mp) is one of the formal temperature reference points in the International Temperature Scale of 1990 (ITS-90) established by BIPM.
The triple point of gallium of 302.9166 K (29.7666°C, 85.5799°F), is being used by NIST in preference to gallium's melting point.
Gallium is a metal that will melt in one's hand. This metal has a strong tendency to supercool below its melting point/freezing point. Seeding with a crystal helps to initiate freezing. Gallium is one of the metals (with caesium, rubidium, francium and mercury) which are liquid at or near normal room temperature, and can therefore be used in metal-in-glass high-temperature thermometers. It is also notable for having one of the largest liquid ranges for a metal, and (unlike mercury) for having a low vapor pressure at high temperatures. Unlike mercury, liquid gallium metal wets glass and skin, making it mechanically more difficult to handle (even though it is substantially less toxic and requires far fewer precautions). For this reason as well as the metal contamination problem and freezing-expansion problems noted above, samples of gallium metal are usually supplied in polyethylene packets within other containers.
Gallium does not crystallize in any of the simple crystal structures. The stable phase under normal conditions is orthorhombic with 8 atoms in the conventional unit cell. Each atom has only one nearest neighbor (at a distance of 244 pm) and six other neighbors within additional 39 pm. Many stable and metastable phases are found as function of temperature and pressure.
The bonding between the nearest neighbors is found to be of covalent character, hence Ga2 dimers are seen as the fundamental building blocks of the crystal. The compound with arsenic, gallium arsenide is a semiconductor commonly used in light-emitting diodes.
High-purity gallium is dissolved slowly by mineral acids.
Gallium has no known biological role, although it has been observed to stimulate the metabolism.
History
Gallium (the Latin Gallia means "Gaul," essentially modern France) was discovered spectroscopically by Lecoq de Boisbaudran in 1875 by its characteristic spectrum (two violet lines) in an examination of a zinc blende from the Pyrenees. Before its discovery, most of its properties had been predicted and described by Dmitri Mendeleev (who had called the hypothetical element "eka-aluminium" on the basis of its position in his periodic table). Later, in 1875, Boisbaudran obtained the free metal by electrolysis of its hydroxide in potassium hydroxide solution. He named the element "gallia" after his native land of France. It was later claimed that, in one of those multilingual puns so beloved of men of science in the early 19th century, he had also named gallium after himself, as his name, "Le coq," is the French for "the rooster," and the Latin for "rooster" is "gallus"; however, in an 1877 article Le coq denied this supposition. (The supposition was also noted in Building Blocks of the Universe, a book on the elements by Isaac Asimov.)
Occurrence
Gallium does not exist in free form in nature, nor do any high-gallium minerals exist to serve as a primary source of extraction of the element or its compounds.
Its abundance in the Earth's crust is approximately 16.9 ppm. Gallium is found and extracted as a trace component in bauxite and to a small extent from sphalerite The amount extracted from coal, diaspore and germanite in which gallium is also present is negligible. The United States Geological Survey (USGS) estimates gallium reserves to exceed 1 million tonnes, based on 50 ppm by weight concentration in known reserves of bauxite and zinc ores. Some flue dusts from burning coal have been shown to contain small quantities of gallium, typically less than 1% by weight.
Production
The only two economic sources for gallium are as byproduct of aluminium and zinc production, while the sphalerite for zinc production is the minor source. Most gallium is extracted from the crude aluminium hydroxide solution of the Bayer process for producing alumina and aluminium. A mercury cell electrolysis and hydrolysis of the amalgam with sodium hydroxide leads to sodium gallate. Electrolysis then gives gallium metal. For semiconductor use, further purification is carried out using zone melting, or else single crystal extraction from a melt (Czochralski process). Purities of 99.9999% are routinely achieved and commercially widely available.
An exact number for the world wide production is not available, but it is estimated that in 2007 the production of gallium was 184 tonnes with less than 100 tonnes from mining and the rest from scrap recycling.
Applications
Gallium arsenide (GaAs) and gallium nitride (GaN) used in electronic components represented about 98% of the gallium consumption in the United States. World wide gallium arsenide makes up 95% of the annual global gallium consumption.
Semiconductors
The semiconductor applications are the main reason for the low-cost commercial availability of the extremely high-purity (99.9999+%) metal:
As a component of the semiconductor gallium arsenide, the most common application for gallium is optoelectronic devices (mostly laser diodes and light-emitting diodes.) Smaller amounts of gallium arsenide are use for the manufacture of ultra-high speed logic chips and MOSFETs for low-noise microwave preamplifiers.
Gallium is used as a dopant for the production of solid-state devices such as transistors. However, worldwide the actual quantity used for this purpose is minute, since dopant levels are usually of the order of a few parts per million.
Multijunction photovoltaic cell is used for special application, first developed and deployed for satellite power applications, are made by molecular beam epitaxy or Metalorganic vapour phase epitaxy of thin films of gallium arsenide, indium gallium phosphide or indium gallium arsenide.The Mars Exploration Rovers and several satellites use triple junction gallium arsenide on germanium cells. Gallium is the rarest component of new photovoltaic compounds (such as copper indium gallium selenium sulfide or Cu(In,Ga)(Se,S)2, recently announced by South African researchers) for use in solar panels as a more efficient alternative to crystalline silicon.
Wetting and alloy improvement
- Because gallium wets glass or porcelain, gallium can be used to create brilliant mirrors.
- Gallium readily alloys with most metals, and has been used as a component in low-melting alloys. The plutonium used in nuclear weapon pits is machined by alloying with gallium to stabilize the allotropes of plutonium.
- Gallium added in quantities up to 2% in common solders can aid wetting and flow characteristics.
Liquid alloys
- It has been suggested that a liquid gallium-tin alloy could be used to cool computer chips in place of water. As it conducts heat approximately 65 times better than water it can make a comparable coolant.
- Gallium is used in some high temperature thermometers.
- The liquid gallium-indium-tin alloy galinstan has been used in activating aluminum. Activated aluminum reacts with water generating hydrogen and steam. This reaction is being considered as one of the feasible processes necessary for hydrogen economy.
Biomedical applications
As the free element
- A low temperature liquid eutectic alloy of gallium, indium, and tin, is widely available in medical thermometers (fever thermometers), replacing problematic mercury. This alloy, with the trade name Galinstan (with the "-stan" referring to the tin), has a freezing point of −20°C.
- Much research is being devoted to gallium alloys as substitutes for mercury dental amalgams, but these compounds have yet to see wide acceptance.
As gallium (III) salts
- Gallium nitrate (see Ganite) has been used as an intravenous pharmaceutical to treat hypercalcemia associated with tumor metastatis to bones. Gallium is thought to interfere with osteoclast function. It may be effective when other treatments for maligancy-associated hypercalcemia are not.
- Gallium maltolate is in clinical and preclinical trials as a potential treatment for cancer, infectious disease, and inflammatory disease.
- Research is being conducted to determine whether gallium can be used to fight bacterial infections in people with cystic fibrosis. Gallium is similar in size to iron, an essential nutrient for respiration. When gallium is mistakenly picked up by bacteria such as Pseudomonas, the bacteria's ability to respire is interfered with and the bacteria die. The mechanism behind this is that iron is redox active, which allows for the transfer of electrons during respiration, but gallium is redox inactive.
As radiogallium salts
- Gallium-67 salts such as gallium citrate and gallium nitrate are used as radiopharmaceutical agents in a nuclear medicine imaging procedure commonly referred to as a gallium scan. The form or salt of gallium is not important, since it is the free dissolved gallium ion Ga3+ which is the active radiotracer. For these applications, the radioactive isotope 67Ga is used. The body handles Ga3+ in many ways as though it were iron, and thus it is bound (and concentrates) in areas of inflammation, such as infection, and also areas of rapid cell division. This allows such sites to be imaged by nuclear scan techniques. This use has largely been replaced by fluorodeoxyglucose (FDG) for positron emission tomography, "PET" scan and indium-111 labelled leukocyte scans. However, the localization of gallium in the body has some properties which make it unique in some circumstances from competing modalities using other radioisotopes.
- Gallium-68 has been used as an experimental positron emitting gallium isotope, in a PET scan technique which combines features of the gallium scan and the CT/PET scan.
Other uses
- Magnesium gallate containing impurities (such as Mn2+), is beginning to be used in ultraviolet-activated phosphor powder.
- Neutrino detection. Possibly the largest amount of pure gallium ever collected in a single spot was the GALLEX neutrino detector operated in the early 1990s in an Italian mountain tunnel. The detector contained 12.2 tons of watered gallium-71. Solar neutrinos caused a few atoms of Ga-71 to become radioactive Ge-71, which were detected. The solar neutrino flux deduced was found to have a deficit of 40% from theory. This was not explained until better solar neutrino detectors and theories were constructed (see SNO).
- As a liquid metal ion source for a focused ion beam.
- Gallium when painted on glass or porcelain forms a brilliant mirror
Energy storage
Aluminium is reactive enough to reduce water to hydrogen, being oxidized to aluminium oxide. However, the aluminium oxide forms a protective coat which prevents further reaction. When gallium is alloyed with aluminium, the coat does not form, thus the alloy can potentially provide a solid hydrogen source for transportation purposes, which would be more convenient than a pressurized hydrogen tank. Resmelting the resultant aluminium oxide and gallium mixture to metallic aluminium and gallium and reforming these into electrodes would constitute most of the energy input into the system, while electricity produced by a hydrogen fuel cell could constitute an energy output. The thermodynamic efficiency of the aluminium smelting process is said to be approximately 50 percent. Therefore, at most no more than half the energy that goes into smelting aluminium could be recovered by a fuel cell.
Precautions
While not considered toxic, the data about gallium are inconclusive. Some sources suggest that it may cause dermatitis from prolonged exposure; other tests have not caused a positive reaction. Like most metals, finely divided gallium loses its luster. Powdered gallium appears grey. When gallium is handled with bare hands, the extremely fine dispersion of liquid gallium droplets which results from wetting skin with the metal may appear as a grey skin stain.
See also
External links
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