Gallium

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Gallium is a chemical element in the periodic table that has the symbol Ga and atomic number 31. A rare, soft silvery metallic poor metal, gallium is a brittle solid at low temperatures but liquefies slightly above room temperature and indeed will melt in the hand. It occurs in trace amounts in bauxite and zinc ores. An important application is in the compound gallium arsenide, used as a semiconductor, most notably in light-emitting diodes (LEDs).

1 Notable characteristics
2 Applications
3 History
4 Occurrence
5 Precautions
- 5.1 Air travel
6 See also
7 References
8 External links

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Notable characteristics

Very pure gallium has a stunning silvery color and its solid metal fractures conchoidally like glass. Gallium metal expands by 3.1 percent when it solidifies, and therefore should not be stored in either glass or metal containers, as they may break as the metal solidifies. Gallium also corrodes most other metals by diffusing into their metal lattice. It is very important to keep gallium away from any type of metal containers such as steel or aluminum.

Gallium is one of the metals (with caesium, 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.

This metal has a strong tendency to supercool below its melting point, thus necessitating seeding in order to solidify. High-purity gallium is attacked slowly by mineral acids. The melting point temperature is very low, T = 30 °C, and the density is higher in the liquid state than in the crystalline state (like water, but unlike most materials).

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.

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Applications

Analog integrated circuits are the most common application for gallium, with optoelectronic devices (mostly laser diodes and light-emitting diodes) as the second largest end use.
Other uses include:

Since it wets glass or porcelain, gallium can be used to create brilliant mirrors.
Used widely as a dopant to dope semiconductors and produce solid-state devices like transistors.
Gallium readily alloys with most metals, and has been used as a component in low-melting alloys. The plutonium pits of nuclear weapons employ an alloy with gallium to stabilize the allotropes of plutonium. Much research is being devoted to gallium alloys as substitutes for mercury dental amalgams, but such compounds have yet to see wide acceptance.
Gallium added in quantities up to 2% in common solders can aid wetting and flow characteristics.
Gallium is used in some high temperature thermometers, and a eutectic alloy of gallium, indium, and tin is widely available in fever thermometers, replacing mercury. This alloy, with the trade name Galinstan, has a freezing point of −20 °C.
Magnesium gallate containing impurities (such as Mn⁺²), is beginning to be used in ultraviolet-activated phosphor powder.
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 makes a considerably better coolant. [1]
Gallium salts like gallium citrate and gallium nitrate are used as radiocontrast agents in medical imaging. For these applications, a radioactive isotope such as ⁶⁷Ga is used.
Gallium is the rarest component of new photovoltaic compounds (such as copper indium gallium selenium sulphide or Cu(In,Ga)(Se,S)2, recently announced by South African researchers) for use in solar panels as an alternative to crystalline silicon, which is currently in short supply.

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History

Gallium (Latin Gallia meaning Gaul; also gallus, meaning "rooster") 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 called the hypothetical element eka-aluminium) on the basis of its position in his periodic table. Later, in 1875, Boisbaudran obtained the free metal through the electrolysis of its hydroxide in KOH solution. He named the element "gallia" after his native land of France and, in one of those multilingual puns so beloved of men of science of the early 19th century, after himself, as 'Lecoq' = the rooster, and Latin for rooster is "gallus".

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Occurrence

Gallium does not exist in pure form in nature, nor are gallium compounds a primary source of extraction. It is rather found and extracted as a trace component in bauxite, coal, diaspore, germanite, and sphalerite. The USGS estimates gallium reserves 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 as much as 1.5 percent gallium.

Most of the gallium is extracted from the crude aluminium hydroxide solution of the Bayer process. A mercury cell electrolysis and hydrolysis of the amalgam with sodium hydroxide leads to the sodium gallate. Electrolysis then gives pure gallium, for semiconductors further purification is done similar to silicon like zone melting and with the Czochralski process.

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Precautions

While not considered toxic, the data about gallium is inconclusive. Some sources suggest that it may cause dermatitis from prolonged exposure; other tests have not caused a positive reaction. It will however stain your skin if you hold it in your bare hands.

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Air travel

Since aluminum and gallium form a low-melting eutectic mixture, air transport of gallium poses a potential risk that the gallium could melt and leak from its container, possibly contacting the aluminum structure of the aircraft. If this were to occur, the gallium could attack the aluminum and weaken it, possibly with catastrophic results. For this reason, air transport regulations for gallium metal place stringent requirements on the packaging and maximum permissible quantities of the gallium.

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