Gallium

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).

Contents

1 Notable characteristics

2 Applications

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. Gallium also corrodes most other metals by diffusing into their metal lattice.

Gallium is one of the four metals (with caesium, mercury, and rubidium) which are liquid at or near normal room temperature, and can therefore be used in 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).

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 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 use 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]

History

Gallium (Latin Gallia meaning "France"; also gallus, meaning "cock") was discovered spectroscopically by Lecoq de Boisbaudran in 1875 by its characteristic spectrum (two violet lines) in an examination of a zinc blend 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 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".

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 1.5 percent gallium.

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.

References

Los Alamos National Laboratory – Gallium