Silicon

Silicon (from Latin: flint) is a metalloid chemical element, atomic number 14 and located in group 14 of the table of the elements with symbol Si. It is the second most abundant element in the earth's crust (25.7% by weight) after oxygen. It occurs in amorphous and crystallized form; The first is a brownish powder, more active than the crystalline variant, which occurs in blue-gray octahedrons with a metallic luster.

Features

Silicon powder.

Silicon polycristal.

Olivino.

Its properties are intermediate between those of carbon and germanium. In crystalline form it is very hard and sparingly soluble and has a metallic luster and grayish color. Although it is a relatively inert element and resists the action of most acids, it reacts with halogens and dilute alkalis. Silicon transmits more than 95% of the wavelengths of infrared radiation.

It is prepared as a yellow-brown powder or grayish-black crystals. It is made by heating silica, or silicon dioxide (SiO₂), with a reducing agent, such as carbon or magnesium, in an electric furnace. Crystalline silicon has a hardness of 7, enough to scratch metal. glass, hardness 5 to 7. Silicon has a melting point of 1.411 °C, a boiling point of 2.355 °C, and a relative density of 2.33(g/ml). Its atomic mass is 28.086 u (atomic mass unit).

It dissolves in hydrofluoric acid to form the gas silicon tetrafluoride, SiF₄ (see fluorine), and is attacked by nitric, hydrochloric, and sulfuric acids, although the silicon dioxide formed inhibits the reaction. It also dissolves in sodium hydroxide, forming sodium silicate and hydrogen gas. At ordinary temperatures silicon is not attacked by air, but at elevated temperatures it reacts with oxygen forming a layer of silica that prevents the reaction from continuing. At high temperatures it also reacts with nitrogen and chlorine to form silicon nitride and silicon chloride, respectively.

Silicon makes up 28% of the earth's crust. It does not exist in a free state, but is found in the form of silicon dioxide and complex silicates. Silicon-containing minerals make up about 40% of all common minerals, including more than 90% of the minerals that make up volcanic rocks. The mineral quartz, its varieties (carnelian, chrysoprase, onyx, flint and jasper) and the minerals cristobalite and tridymite are the crystalline forms of silicon existing in nature. Silicon dioxide is the main component of sand. Silicates (specifically those of aluminium, calcium and magnesium) are the main components of clays, soil and rocks, in the form of feldspars, amphiboles, pyroxenes, micas and zeolites, and of semi-precious stones such as olivine, garnet, zircon, topaz and tourmaline.

Silicon as a biochemical base

Its shared characteristics with carbon, such as being in the same 14 family, not being a metal itself, being able to build compounds similar to enzymes (zeolites), other long compounds with oxygen (silicones), and having the same four basic bonds, gives it a certain opportunity to become the basis of living beings, even if it is not on Earth, in a hypothetical biochemistry.

Applications

It is used in alloys, in the decantation of silicones, in the technical ceramics industry and, because it is a very abundant semiconductor material, it is of special interest in the electronics and microelectronics industry as a basic material for the creation of wafers or chips that can be implanted into transistors, solar cells, and a wide variety of electronic circuits. Silicon is a vital element in many industries. Silicon dioxide (sand and clay) is an important constituent of concrete and bricks, and is used in the production of portland cement. Due to its semiconductor properties it is used in the manufacture of transistors, solar cells and all kinds of semiconductor devices; For this reason, the region of California in which numerous companies in the electronics and information sector are concentrated is known as Silicon Valley. Potential applications of silicene, which is an allotrope of silicon that forms a two-dimensional lattice similar to graphene, are also being studied. Other important uses of silicon are:

• As a refractory material, it is used in ceramics, glazed and enameled.
• As a fertilizing element in the form of primary mineral rich in silicon, for agriculture.
• As an element of alloy in casts.
• Glass manufacture for windows and insulation.
• Silicon carbide is one of the most important abrasives.
• It is used in lasers to obtain a light with a wavelength of 456 nm.
• Silicone is used in medicine in breast implants and contact lenses.

Metallurgical Industry

It is used in the steel industry as a component of silicon-steel alloys. To make steel, molten steel is deoxidized by adding small amounts of silicon; ordinary steel contains less than 0.30% silicon. Silicon steel, which contains 2.5 to 4% silicon, is used to make electrical transformer cores, as the alloy has low hysteresis (see Magnetism). There is a steel alloy, duriron, which contains 15% silicon and is hard, brittle and resistant to corrosion; Durirón is used in industrial equipment that is in contact with corrosive chemicals. Silicon is also used in copper alloys, such as bronze and brass.

Semiconductors

Silicon is a semiconductor; their resistivity to electric current at room temperature varies between that of metals and that of insulators. Silicon's conductivity can be controlled by adding small amounts of impurities called dopants. The ability to control the electrical properties of silicon and its abundance in nature have made possible the development and application of transistors and integrated circuits used in the electronics industry.

Aerospace Industry

The applications of silicon in the aerospace industry specializes in seeking a significant improvement in the electronic circuits they use, which are more resistant to gamma rays, this was done by covering these devices with about 20 centimeters of lead, but this it made them very heavy and it would be too expensive for the satellites to carry them.

Argentina

A group of Argentine researchers seeks to create a satellite memory capable of storing data that can withstand radiation, sudden changes in temperature and low pressure, which is why these nanosatellites from the Satellogic company are placed in orbit. This project was carried out from the saturation in Moore's Law where these Silicon memories are doubling their capacity every year, and consequently these are increasingly limited, according to Carlos Acha.

Photovoltaic Industry

Argentina

In 1976, Argentina began its activities in the field of solar energy through the GES (Grupo Energía Solar), resulting in a solid knowledge and command of the technology for converting solar energy into electricity. In the early 1980s, a comprehensive search for information on the development status of the global photovoltaic industry was conducted. Since 1992, the event has focused on the design, simulation, development and characterization of crystalline silicon solar cells. This allowed obtaining equipment with efficiencies higher than 17% in 1997. For terrestrial applications, GES promotes and participates in the development of national standards for solar energy collection systems by the Argentine Institute of Standardization. In 1999, several prototypes were tested and calibrated at the National Meteorological Service, two of which were deployed at weather stations in the provinces of Chaco and Corrientes. The property, where the construction of the integrated solar silicon plant will be carried out in ingots, wafers and crystalline cells and photovoltaic solar modules with an annual production of 71 MW, is located in the department of Pocito, at Calle Maurín and Calle 6, Province of San Juan.

Silica and Silicates

Silica and silicates are used in the manufacture of glass, varnishes, enamels, cement, and porcelain, and have important individual applications. Fused silica, which is a glass made by melting quartz or hydrolyzing silicon tetrachloride, is characterized by a low coefficient of expansion and high resistance to most chemicals. Silica gel is a colorless, porous and amorphous substance; It is prepared by removing part of the water from a gelatinous precipitate of silicic acid, SiO₂•H₂O, which is obtained by adding hydrochloric acid to a solution of sodium silicate. Silica gel absorbs water and other substances and is used as a drying and bleaching agent.

Sodium silicate (Na₂SiO₃), also called glass, is an important synthetic silicate, an amorphous solid, colorless and soluble in water, which melts at 1088°C. It is obtained by reacting silica (sand) and sodium carbonate at high temperature, or by heating sand with concentrated sodium hydroxide under high pressure. The aqueous solution of sodium silicate is used to preserve eggs; as a substitute for glue or glue to make boxes and other containers; to join artificial gems; as a fireproofing agent, and as a filler and binder in soaps and cleaners. Another important silicon compound is carborundum, a compound of silicon and carbon that is used as an abrasive.

Silicon monoxide, SiO, is used to protect materials by coating them so that the outer surface is oxidized to dioxide, SiO₂. These layers also apply to interference filters.

It was first identified by Antoine Lavoisier in 1787.

Abundance and obtaining

Measured by weight, silicon makes up more than a quarter of the earth's crust and is the second most abundant element after oxygen. Silicon is not in the native state; sand, quartz, amethyst, agate, flint, opal and jasper are some of the minerals in which oxide appears, while forming silicates it is found, among others, in granite, feldspar, clay, hornblende and mica. It is also found in meteorites.

Physical methods of metallurgical silicon purification

These methods are based on the greater solubility of the impurities in the liquid silicon, so that it is concentrated in the last solidified areas. The first method, used to a limited extent to build radar components during World War II, involves grinding the silicon so that impurities accumulate on the grain surfaces; dissolving these partially with acid gave a purer powder. Zone fusion, the first method used on an industrial scale, consists of melting one end of a silicon bar and slowly moving the heat source along the bar so that the silicon solidifies with a higher purity by dragging the molten zone much of the impurities. The process can be repeated as many times as necessary until the desired purity is achieved, then it is enough to cut the final end where the impurities have accumulated.

Chemical methods of purifying metallurgical silicon

Silicon purification processes. Schematic diagram of the conventional Siemens process and the alternative fluidized bed reactor process (FBR).

Chemical methods, currently used, act on a silicon compound that is easier to purify, decomposing it after purification to obtain silicon. Commonly used compounds are trichlorosilane (HSiCl₃), silicon tetrachloride (SiCl₄) and silane (SiH₄).

In the Siemens process, high-purity silicon rods are exposed at 1150 °C to trichlorosilane, a gas that decomposes, depositing additional silicon on the rod according to the following reaction:

2 HSiCl₃ → Yes + 2 HCl + SiCl₄

Silicon produced by this and similar methods is called polycrystalline silicon and typically has an impurity fraction of 0.001 ppm or less.

The Dupont method consists of reacting silicon tetrachloride at 950 °C with very pure zinc vapors:

SiCl₄ + 2 Zn → Yes + 2 ZnCl₂

This method is fraught with difficulties (zinc chloride, a by-product of the reaction, solidifies and clogs the lines), so it was eventually abandoned in favor of the Siemens process.

Once the ultrapure silicon is obtained, it is necessary to obtain a single crystal, for which the Czochralski process is used.

Solar-grade silicon: state-of-the-art technology

The different SoG-Si production alternatives are presented below. All of them have been collected and presented since 2004 at the Silicon Solar Conferences. These conferences are organized annually by Photon International magazine in Munich, following growing concerns about polysilicon shortages. So far, none of these alternatives have made it to production, though some are close.^{[citation needed]}

Fluidized Bed Reactor

Wacker Chemie, Hemlock and Solar Grade Silicon propose a fluidized bed reactor. This consists of a quartz tube into which trichlorosilane (Wacker, Hemlock) or silane (SGS) is introduced at the bottom, together with hydrogen. The gas passes through a bed of silicon particles on which deposition occurs, thus giving particles of larger size. Reached a certain size, the particles are too heavy and fall to the ground and can be removed. This process not only uses a much lower amount of energy than Siemens, but can also be carried out continuously.

Tube Reactor

Joint Solar Silicon GmbH & Co. KG (JSSI) presents a reactor similar to Siemens, whose differences are: a.) the silicon is deposited in a hollow silicon cylinder instead of rods; b.) Silane is used instead of trichlorosilane, and therefore the process temperature can be limited to 800 °C.

Vapor to Liquid Reservoir

Tokuyama Corporation proposes its VLD (Vapor to Liquid Deposition) process. In a reactor, a tube of graphite is heated to 1500 °C, above the melting point of silicon. Trichlorosilane and hydrogen are fed at the top. Silicon is deposited on the graphite walls in liquid form. Therefore, it drips onto the reactor floor, where it solidifies into pellets and can be collected. The higher energy cost compared to the Siemens reactor compensates for the 10 times higher deposition speed.

Reduction with Zn

Chisso Corporation and the Japanese government are investigating a process based on the reduction of silicon tetrachloride (SiCl4) with zinc (Zn) vapor. Zinc silicon chloride is formed. This alternative was discarded in the 1980s by Bayer AG as traces of residual metals could not be removed. Chisso ensures that its metallic impurities are at an acceptable level.

Metallurgical Alternatives

Great efforts have also been put into achieving SoG-Si by avoiding the energetically expensive step of using trichlorosilane, silane or tetrachlorosilane, and subsequent deposit at Siemens or the like.

Elkem purifies mg-Si in three relatively simple refining steps, pyrometallurgical, hydrometallurgical, and cleaning, consuming only 20-25% of the energy used in the Siemens route. Together with the University of Konstanz, they have achieved cell efficiencies just half a point below commercial cells.

Apollon Solar SAS and the French national research laboratory CNRS purify Mg-Si with a plasma. Solar cells with 11.7% efficiency have been achieved.

Another metallurgical alternative is to produce mg-Si with quartz and carbon black so pure that further refining is not necessary. There are two parallel works: one is from the Kazakh National Technical University in Alma Ata, Kazakhstan. The other is the SOLSILC project, funded by the European Commission. Solar cells made from this material have achieved relatively low momentum efficiencies. 28 percent of this material no longer exists.

Isotopes

Silicon has nine isotopes, with a mass number between 25 and 33. The most abundant isotope is Si-28 with an abundance of 92.23%, Si-29 has an abundance of 4.67% and Si-30 has an abundance of 3.1 %. All of them are stable, the rest of the isotopes having a very small proportion. Si-32 is a radioactive isotope that comes from the decay of argon. Its half-life time is approximately 132 years. It undergoes a beta decay that transforms it into P-32 (which has a half-life of 14.28 days).

Precautions

Inhalation of crystalline silica dust can cause silicosis.