Silicon - Properties
Silicon is a chemical element of atomic number 14 and symbol Si. This chemical element belongs to group IV A of the periodic table. J.J.Berzelius 1824 isolated it by potassium reduction of silicon tetrafluoride (SiF4).
Silicon is a very uniquely used component in photovoltaic panels because of its semiconductor properties. This means that its physical and chemical properties are very favorable to propitiate the photovoltaic effect. The photovoltaic effect is the effect that allows to transform the energy of the photons present in the sunlight in the movement of electrons, and therefore, electric energy.
Origin of silicon
Silicon is, after oxygen, the most abundant element of the earth's crust, of which it constitutes 26% by weight. It consists of a mixture of three natural, stable isotopes with masses 28 (92.21%), 29 (4.70%) and 30 (3.09%), which determine an atomic weight of 28,086. Five artificial radioisotopes of the element are also known, with masses ranging from 25 to 32.
Silicon is widespread in nature, and its presence in the sun, stars and meteorites has been detected. It does not occur native and its most important types are the various varieties of silica (SiO2) and silicates.
Obtaining of silicon
The silicon is obtained by several methods according to the purpose of the element. The most important commercial method is the reduction of silica with carbon in an electric furnace. The preparation of high purity silicon (99.7%) is achieved by conversion of impure silicon into volatile tetrachloride (SiCl4), purification of the latter by distillation and subsequent reduction with zinc.
The silicon for use in electronics is reached by zonal purification, to remove boron, aluminum, phosphorus, gallium, arsenic, indium and antimony, and later obtaining a monocrystal by the method of Czochralski, which consists of submerging a monocrystal of small Dimensions in a liquid silicon bath heated to the melting temperature and slowly withdrawn to cause monocrystal growth.
Normally, doping of the single crystal is performed simultaneously.
Aspect and chemical properties of silicon
The elemental crystalline silicon is grayish with metallic luster, very hard, with very high melting and boiling points, and is an intrinsic semiconductor. The amorphous form of the element occurs in brown, conductive powders of electricity, which can easily be melted and vaporized.
As for the chemical properties, silicon presents very remarkable differences with the carbon, first element of group IV A, to have a much smaller electronegativity, that carries notable variations in the polarity of certain bonds, a tendency to very attenuated chaining (There are no chains of more than six silicon atoms), a maximum coordination of six, due to the existence of empty d orbitals with appropriate energy, and the absence of multiple bond formation with itself or with any other element.
The most important chemical characteristic of silicon is its tendency to combine with oxygen to form discrete or polymeric structures in which each silicon atom is surrounded by four oxygen atoms. Given the high energy of the Si-O bond (89.3 kcal / mol), these structures are highly stable.
Silicon is essentially non-metallic and its reactivity depends mainly on the degree of division. In compact form, it is coated in air from a surface layer of oxide, while finely divided is easily ignited in the air. It is attacked by hot hydrogen chloride, with formation of tetrachloride (SiCl4) and hydrogen release, and by cold hydrogen fluoride, with formation of hexafluorosilicic acid (H2SiF6), and is not attacked by the other acids. It dissolves in strong bases, forming silicates and releasing hydrogen (Si + 2KOH + H2O → K2SiO3 + 2H2), and combines directly with the halogens and, in hot, with other non-metals (silicur).
Given the electronic structure of silicon (3s23p2), it acts, analogously to carbon, practically exclusively with valence 4. Its compounds are essentially covalent and many enjoy considerable economic importance. Among the most important are the hydrides (silane) and the alkylated and halogenated derivatives thereof, which, by hydrolysis, lead to the silanols, structural constituents of the silicones; Halides such as tetrafluoride (SiF4), which forms an acid adduct and hexacoordinat with hydrogen fluoride (H2SiF6), and tetrachloride (SiCl4), used in the preparation of the element; Silicon dioxide and its derivatives; Silicurs, analogous to carbides; And binary compounds with electronegative elements (carbide, nitride), very hard and with refractory properties.
Silicon can be introduced into the structure of a whole range of organic compounds (silylate), which confers very interesting properties. From the biological point of view, silicon has in some cases an important role, and is the basic material for the construction of the cell wall of various algae (chrysophytes, diatoms).
Silicon macromolecules similar to fullerenes of carbon were discovered in 1990. It was an unexpected discovery as it was believed that stable silicon fullerenes could not be made. The problem of the instability of closed silicon networks was solved by placing a central metallic atom, namely tungsten.
One of the most stable forms obtained has a set of twelve silicon atoms that form a cage around a tungsten atom. This configuration has the advantage that it isolates the chemical effects of the central atom, so it can be very useful in the capture of atoms or in catalysis.
Continued breathing of dust of siliceous nature causes humans a serious lung disease, silicosis.
Applications of silicon
Silicon finds application in elementary form in electronics for the manufacture of semiconductor devices and photoelectric cells, and in metallurgy for the preparation of special steels (ferrosilicon) and other alloys and in the preparation of various compounds. In composite form (silicates, silica, silicones), it enjoys a very general use. The crystalline elemental silicon is grayish with metallic luster, very hard, with very high melting and boiling points, and is an intrinsic semiconductor. The amorphous form of the element occurs in brown, conductive powders of & rsquo; Electricity, which can be easily melted and vaporized.
Last review: November 8, 2016Back