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Silicon

Silicon

What is silicon?

Silicon is a chemical element of atomic number 14 and symbol Si. This chemical element belongs to group IV A of the periodic table. JJ Berzelius 1824 isolated it by reduction with potassium of silicon tetrafluoride (SiF4).

The crystalline elemental silicon is grayish in color with a 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 powders, conductive of electricity, which can be easily melted and vaporized.

Silicon is a component widely used in photovoltaic panels due to its semiconductor properties. This means that its physical and chemical properties are very favorable to promote the photovoltaic effect. The photovoltaic effect is the effect that allows to transform the energy of the photons present in the solar light in movement of electrons, and therefore, electrical energy.

Monocrystalline silicon

Monocrystalline silicon is the base material of the electronics industry. It consists of a single crystal of silicon, which has a continuous crystalline network, without interruption throughout the solid.

Monocrystalline silicon is perhaps the most important technological material of the last half century, because its availability at industrially acceptable costs has been and is essential for the development of electronic microcircuits in which based on the current electronic and computer revolution.

Monocrystalline silicon is also used in the production of high-performance photovoltaic cells. Solar cells, unlike microcircuits, can, however, tolerate, within certain limits, small structural imperfections.

Polycrystalline silicon

Polycrystalline or polysilicon silicon is a material made of misaligned (polycrystalline) silicon glass. It occupies an intermediate position between amorphous silicon and monocrystalline silicon.

Polycrystalline silicon often replaces aluminum for the production of metal parts within semiconductor electronic devices due to the improved mechanical strength of the integrated circuit production process. It is also used for the realization of capacitors in an integrated environment. However, the capacities of these capacitors are very low.

For electronic applications, polycrystalline silicon can be obtained with less sophisticated and less expensive techniques than those required for silicon deposition. Polycrystalline silicon can also be obtained during silicon manufacturing processes. Polycrystalline silicon has an impurity level of 1 part per billion or less.

Origin of silicon

Silicon is, after oxygen, the most abundant element in the earth's crust, of which it constitutes 26% by weight. It is constituted by a mixture of three stable natural 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 widely spread in nature, and its presence in the Sun, stars and meteorites has been detected.

Obtaining silicon

The obtaining of silicon is done 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 silicon preparation of high purity (99.7%) is obtained by transformation of impure silicon into the tetrachloride (SiCl4), volatile, purification of this by distillation and subsequent reduction with zinc.

Silicon for use in electronics is obtained by zonal purification, to eliminate boron, aluminum, phosphorus, gallium, arsenic, indium and antimony. Subsequently a monocrystal is obtained by the Czochralski method. This method consists of immersing a small crystal in a liquid silicon bath heated to the melting temperature and slowly removing it to cause the growth of the monocrystal. Normally, doping of the monocrystal is carried out simultaneously.

Chemical properties of silicon

Silicon is a semiconductor elementRegarding the chemical properties, silicon presents very notable differences with carbon, the first element of group IV A. Unlike carbon, silicon has a much smaller electronegativity, which entails notable variations in the polarity of certain bonds, a tendency to chaining very attenuated (no chains of more than six silicon atoms), a maximum coordination of six, thanks to the existence of empty d orbitals with appropriate energy. Also, silicon lacks multiple link formation with itself or with any other element.

The most important chemical characteristic of silicon is its tendency to combine with oxygen to form polymeric or discrete 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 enjoy great stability.

Silicon is essentially non-metallic and its reactivity depends above all on the degree of division. In compact form, it is coated in the air with a superficial layer of oxide, while finely divided it is easily ignited in the air. Silicon can be attacked by hot hydrogen chloride, with the formation of tetrachloride and hydrogen liberation, and by cold hydrogen fluoride, with the formation of hexafluorosilicic acid. On the other hand, this material can not be attacked by the other acids. It dissolves in strong bases, with formation of silicates and release of 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, it acts, analogously to carbon, practically exclusively with valence 4. Its compounds are essentially covalent and many enjoy a remarkable economic importance.

Silicon can be introduced into the structure of a whole range of organic compounds, to which it 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.

In 1990, silicon macromolecules similar to carbon fullerenes were discovered. It was an unexpected discovery since it was believed that stable silicon fullerenes could not be manufactured. The problem of the instability of the closed silicon networks was solved by placing a central metal atom, namely tungsten.

One of the most stable forms achieved 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.

The continuous breathing of dust of a siliceous nature causes humans a serious lung disease, silicosis.

Applications of silicon

Silicon can be applied in electronics, for the manufacture of semiconductor devices and photoelectric cells. In metallurgy it is used for the preparation of special steels (ferrosilicon) and other alloys and in the preparation of various compounds.

In compound form (silicates, silica, silicones), silicon is used in a very general way.

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Last review: June 10, 2019