Si - Silicon

Properties

Silicon when in ultra-pure crystal form appears as a blue-grey metallic substance, but this form is rarely found within nature and silicon usually appears in nature within dusts, planetoids, planets and meteorites in various forms of silicon dioxide (silica), such as quartz and as other silicates (amethyst, opal and jasper), or silicate minerals such as the feldspar group occurring in clays, sand, sandstone and granite.  It is, after oxygen, the second most abundant element within the Earth’s crust making up 25.7% by mass.

Silicon belongs to Group 14 of the Periodic Table, along with another very important element in nature, carbon. Silicon is the most similar element to its neighbour, carbon, in that it can form many complex molecular chains, giving rise to the speculation in science fiction that if we came across extra-terrestrial life, then silicon is the most likely element from which life forms could be based, apart from carbon, which is the building block of life on this planet.  Silicon is a very useful material, as it has the ability to form semi-conducting crystals giving it many uses, which will become clear. Another interesting fact is that silicon in its elementary form has the same crystal structure as diamond.

History

Silica in the form of sharp flints have been used for thousands of years, and were among the first tools ever made by humans.  In the form of sand, silica was used by ancient civilisations to make glass.  Not until 1800 did Humphry Davy discover that silica was in fact a compound and not an element, but Davy was unable to reduce it to its constituent elements using electrolysis. Other scientists tried to isolate silicon, but failed and therefore credit of the discovery of silicon goes to a Swede, Jöns Jacob Berzelius, who in 1824 obtained silicon by heating potassium fluorosilicate with potassium metal, giving him a product which was contaminated with potassium silicide. In order to eliminate this contamination, he stirred the substance with water, with which it reacts, giving him relatively pure silicon powder, which he called silicium. This name was later changed to silicon, as it was argued to be a non-metallic element, which should therefore be named to accord with similar non-metals such as Boron and Carbon.

Sources

Silicon production involves different processes to obtain varying levels of purity.  For standard metallurgical grade silicon which is 98-99.5% pure, high-purity silica is reacted with wood or charcoal in an electric arc furnace.  Purer > 99.9% Si can be extracted directly from silica or other silicon compounds, by a process of molten salt electrolysis.  Polycrystalline silicon, which has impurity levels at less than 10-9, is produced via the Siemens process.  World production is estimated by the USGS at 5,700,000 mt per year (including all forms of silicon).  For silicon metal alone, world production stands at a fraction of the figure above, at 1,750,000 mt per year, where China produces 900,000 mt per year of silicon metal, Russia 500,000 mt per year, with other countries producing in the region of 800,000 mt per year.

Uses

Possibly due to the abundance of silicon, it has a variety of applications and comes in many forms.  The newest application with perhaps the greatest potential is polycrystalline silicon, in the form of wafers, used as semiconductors for thin-film solar cells and photovoltaics, for which 4N5-8N purity is needed. This type of silicon also has prospective growth within the general field of electronics, as semiconductors and power transistors in integrated circuits/ microchips (9N).

As silicon forms many compounds, it is a principal constituent of natural stone, glasses, concrete and cements.  55% of the world’s consumption of silicon is for aluminium-silicon (AlSi) alloys for cast parts used in the automotive industry.  Silicon is also a vital constituent in electrical steel, as well as an additive to molten cast iron to form ferro-silicon (FeSi).  Finally silicon is used to form silicones, comprising 40% of the world’s production, which are used to make breast implants among many other applications.

The chemical industry consumes 45% of silicon and the aluminium industry, 52%, while the solar industry is currently only consuming 3%.  China currently consumes 250,000 mt per year,  which is estimated to grow to 700,000 mt per year.

Silicon can be substituted in certain application by other elements, such as gallium-arsenide (GeAs) and germanium (Ge) in semiconductors and infrared applications.