Gallium: important but rare metal for a wide range of applications
Gallium is a silvery-white metal that is resistant to air, is extremely soft and can have a slightly bluish shine. Its properties make it suitable for a wide range of applications. In the periodic table it has the element symbol Ga. It is found in the 4th period - here it is the third element of group 13 (third main group or boron group). Gallium is extremely rare in nature. It practically does not occur here at all as an element (i.e.: independently), but almost always as part of a compound with aluminium, zinc or germanium ores. Gallium has a content of 19 ppm in the continental earth's crust: it is thus on a par with lead and lithium.
Properties of gallium
Gallium has a very low melting point. It is only 29.76 degrees Celsius. Theoretically, you can even liquefy the metal in your hand. In fact, however, this usually happens in dilute acids, which leads to hydrogen evolution - or simply in hot water. Gallium is not soluble in cold water or with concentrated nitric acid. In these cases, passivation (i.e. spontaneous formation of a non-metallic protective layer) occurs.
After melting, gallium remains liquid for some time even when it is cooled down. Gallium crystals are formed during the cooling process. The slower the cooling process, the cleaner the crystals. One point is significant here: the metal does not crystallise in a form that is otherwise often found, but in an orthorhombic structure. This is a right-angled system with three 90 degree angles. However, no axes are of the same length. For gallium, this structure is the most stable.
Toxiologically, it should be noted that the metal is corrosive to the skin and mucous membranes. This is due to the low melting point, which causes it to liquefy at body temperature. There are no toxicological values for the metal in solid form. Overall, however, the danger to humans when handling gallium is low: only the eyes are seriously threatened. After wetting with the liquefied metal, it is difficult to clean the cornea. Gallium is also found in dental alloys. If used in this way, it can lead to allergic reactions.
Other properties:
- boiling point: 2400 degrees (burns under high pressure in pure oxygen)
- mohs hardness: 1.5
- density: Density: 5.904 g-cm-3
- molar volume: 11.80 - 10-6 m3/mol
- heat of fusion: 5.59 kJ/mol
- heat of vaporisation: 256 kJ/mol
Possible uses of gallium
Most gallium is used for the production of III-V compound semiconductors, which consist of the substance gallium arsenide. These are used, for example, in optoelectronics (light-emitting diodes, laser diodes, e.g. for Blu-Ray data carriers). They are also found in solar cells and in high-frequency technology components (e.g. in HF transistors).
Because gallium is liquid over a wide temperature range, the metal is also suitable for thermometers, which can reach up to 1200 degrees Celsius. As an alloy, gallium is mainly used with iron, yttrium, lithium, magnesium and gadolinium. In such cases, the aim is to produce magnetic materials. An exception is the alloy with plutonium: in this case it is about the construction of a nuclear bomb.
Another possible use of gallium alloys is as a substitute for toxic mercury. This is because the metal is liquid at room temperature and otherwise has similar properties to the toxic substance. Certain gallium alloys are also used in automatic fire extinguishing systems as barrier devices. Here, too, the low melting point helps: if the room temperature rises, the extinguishing system reacts in a fail-safe manner because it is a chemical process. The alloy is necessary to raise the melting point of the device somewhat so that the system does not simply react on days that are too warm.
Extraction of gallium
The metal is extracted from bauxite during aluminium production. The whole process is carried out according to the Bayer process. The gallium automatically separates from the extracted sodium aluminate on an iron cathode that has been coated with mercury. The yields, however, are low: a maximum of 60 grams of gallium are extracted per tonne of bauxite. This is then purified. This process succeeds through electrolytic refining. Afterwards, the single crystals are produced for further processing.
History of the metal
Gallium has two "fathers": Dmitri Mendeleev and Paul Émile Lecoq de Boisbaudran. In 1871, the former predicted the existence of a previously undiscovered metal, which he called eka-aluminium. He used a periodic table that he had developed himself. His French colleague knew nothing of this prediction, but nevertheless managed to prove it four years later in Paris. In the emission spectrum of zinc blende ore dissolved in acid and mixed with metallic zinc, he was able to show the existence of two violet spectral lines. These belonged to a still unknown metal, he concluded.
The naming of the metal probably also goes back to de Boisbaudran. There are two theories about this. The Frenchman is at the centre of both. Firstly, he could have named the new element after the Roman name of his homeland: So in this case, Gaul has become gallium. Secondly, the Latin word gallus could have been the inspiration. This means "cock". In such a case, the name would be an allusion to his middle name "Lecoq" - written separately, "le Coq" means "the cock". Gallium would then be named after himself.
