Silicon chemical properties. Silicon. Properties of silicon. Silicon Applications

One of the most common elements in nature is silicium, or silicon. Such a wide distribution indicates the importance and significance of this substance. This was quickly understood and learned by people who learned how to properly use silicon for their purposes. Its use is based on special properties, which we will discuss further.

Silicon - chemical element

If we characterize a given element by position in the periodic table, we can identify the following important points:

Serial number - 14.
The period is the third small one.
Group - IV.
The subgroup is the main one.
The structure of the outer electron shell is expressed by the formula 3s 2 3p 2.
The element silicon is represented by the chemical symbol Si, which is pronounced "silicium".
The oxidation states it exhibits are: -4; +2; +4.
The valency of the atom is IV.
The atomic mass of silicon is 28.086.
In nature, there are three stable isotopes of this element with mass numbers 28, 29 and 30.

Thus, from a chemical point of view, the silicon atom is a fairly studied element; many of its different properties have been described.

History of discovery

Since various compounds of the element in question are very popular and abundant in nature, since ancient times people have used and known about the properties of many of them. Pure silicon for a long time remained beyond human knowledge in chemistry.

The most popular compounds used in everyday life and industry by peoples of ancient cultures (Egyptians, Romans, Chinese, Russians, Persians and others) were precious and ornamental stones based on silicon oxide. These include:

opal;
rhinestone;
topaz;
chrysoprase;
onyx;
chalcedony and others.

It has also been customary to use quartz in construction since ancient times. However, elemental silicon itself remained undiscovered until the 19th century, although many scientists tried in vain to isolate it from different connections, using catalysts, high temperatures, and even electric current. These are such bright minds as:

Karl Scheele;
Gay-Lussac;
Thenar;
Humphry Davy;
Antoine Lavoisier.

Successfully obtain silicon in pure form Jens Jacobs Berzelius succeeded in 1823. To do this, he conducted an experiment on fusing vapors of silicon fluoride and potassium metal. As a result, I obtained an amorphous modification of the element in question. The same scientists proposed a Latin name for the discovered atom.

A little later, in 1855, another scientist - Sainte-Clair-Deville - managed to synthesize another allotropic variety - crystalline silicon. Since then, knowledge about this element and its properties began to expand very quickly. People realized that it has unique features that can be very intelligently used to meet their own needs. Therefore, today one of the most popular elements in electronics and technology is silicon. Its use only expands its boundaries every year.

The Russian name for the atom was given by the scientist Hess in 1831. This is what has stuck to this day.

In terms of abundance in nature, silicon ranks second after oxygen. Its percentage in comparison with other atoms in the composition earth's crust- 29.5%. Additionally, carbon and silicon are two special elements that can form chains by bonding with each other. That is why more than 400 different natural minerals are known for the latter, in which it is found in the lithosphere, hydrosphere and biomass.

Where exactly is silicon found?

In deep layers of soil.
In rocks, deposits and massifs.
At the bottom of bodies of water, especially seas and oceans.
In plants and marine life of the animal kingdom.
In the human body and terrestrial animals.

We can identify several of the most common minerals and rocks, which contain large quantities silicon is present. Their chemistry is such that the mass content of the pure element in them reaches 75%. However, the specific figure depends on the type of material. So, rocks and minerals containing silicon:

feldspars;
mica;
amphiboles;
opals;
chalcedony;
silicates;
sandstones;
aluminosilicates;
clays and others.

Accumulating in the shells and exoskeletons of marine animals, silicon eventually forms powerful silica deposits at the bottom of water bodies. This is one of the natural sources of this element.

In addition, it was found that silicon can exist in its pure native form - in the form of crystals. But such deposits are very rare.

Physical properties of silicon

If we characterize the element in question according to the set physical and chemical properties, then first of all it is necessary to designate the physical parameters. Here are a few main ones:

It exists in the form of two allotropic modifications - amorphous and crystalline, which differ in all properties.
The crystal lattice is very similar to that of diamond, because carbon and silicon are practically the same in this regard. However, the distance between the atoms is different (silicon is larger), so diamond is much harder and stronger. Lattice type - cubic face-centered.
The substance is very fragile, high temperatures becomes plastic.
The melting point is 1415˚C.
Boiling point - 3250˚С.
The density of the substance is 2.33 g/cm3.
The color of the compound is silver-gray, with a characteristic metallic luster.
It has good semiconductor properties, which can vary with the addition of certain agents.
Insoluble in water, organic solvents and acids.
Specifically soluble in alkalis.

The identified physical properties of silicon allow people to manipulate it and use it to create various products. For example, the use of pure silicon in electronics is based on the properties of semiconductivity.

Chemical properties

The chemical properties of silicon are very dependent on the reaction conditions. If we talk about standard parameters, then we need to indicate very low activity. Both crystalline and amorphous silicon are very inert. They do not interact with strong oxidizing agents (except fluorine) or with strong reducing agents.

This is due to the fact that an oxide film of SiO 2 is instantly formed on the surface of the substance, which prevents further interactions. It can be formed under the influence of water, air, and vapor.

If you change the standard conditions and heat silicon to a temperature above 400˚C, then its chemical activity will greatly increase. In this case, it will react with:

oxygen;
all types of halogens;
hydrogen.

With a further increase in temperature, the formation of products by interaction with boron, nitrogen and carbon is possible. Carborundum - SiC - is of particular importance, as it is a good abrasive material.

Also Chemical properties silicon are clearly visible in reactions with metals. In relation to them, it is an oxidizing agent, which is why the products are called silicides. Similar compounds are known for:

alkaline;
alkaline earth;
transition metals.

The compound obtained by fusing iron and silicon has unusual properties. It is called ferrosilicon ceramics and is successfully used in industry.

Silicon does not interact with complex substances, therefore, of all their varieties, it can dissolve only in:

aqua regia (a mixture of nitric and hydrochloric acids);
caustic alkalis.

In this case, the temperature of the solution must be at least 60˚C. All this once again confirms physical basis substances - a diamond-like stable crystal lattice, giving it strength and inertness.

Methods of obtaining

Obtaining silicon in its pure form is a fairly costly process economically. In addition, due to its properties, any method gives only a 90-99% pure product, while impurities in the form of metals and carbon remain all the same. Therefore, simply obtaining the substance is not enough. It should also be thoroughly cleaned of foreign elements.

In general, silicon production is carried out in two main ways:

From white sand, which is pure silicon oxide SiO 2 . When it is calcined with active metals (most often magnesium), a free element is formed in the form of an amorphous modification. The purity of this method is high, the product is obtained with a 99.9 percent yield.
A more widespread method on an industrial scale is the sintering of molten sand with coke in specialized thermal kilns. This method was developed by the Russian scientist N. N. Beketov.

Further processing involves subjecting the products to purification methods. For this purpose, acids or halogens (chlorine, fluorine) are used.

Amorphous silicon

The characterization of silicon will be incomplete if each of its allotropic modifications is not considered separately. The first of them is amorphous. In this state, the substance we are considering is a brownish-brown powder, finely dispersed. It has a high degree of hygroscopicity and exhibits fairly high chemical activity when heated. Under standard conditions, it is able to interact only with the strongest oxidizing agent - fluorine.

It is not entirely correct to call amorphous silicon a type of crystalline silicon. Its lattice shows that this substance is only a form of finely dispersed silicon, existing in the form of crystals. Therefore, as such, these modifications are one and the same compound.

However, their properties differ, which is why it is customary to talk about allotropy. Amorphous silicon itself has a high light absorption capacity. In addition, under certain conditions, this indicator is several times higher than that of the crystalline form. Therefore, it is used for technical purposes. In this form (powder), the compound is easily applied to any surface, be it plastic or glass. This is why amorphous silicon is so convenient to use. Application based on different sizes.

Although batteries of this type wear out quite quickly, which is associated with the abrasion of a thin film of the substance, their use and demand are only growing. After all, even in a short service life solar panels based on amorphous silicon can provide energy to entire enterprises. In addition, the production of such a substance is waste-free, which makes it very economical.

This modification is obtained by reducing compounds with active metals, for example, sodium or magnesium.

Crystalline silicon

Silver-gray shiny modification of the element in question. This form is the most common and most in demand. This is explained by the set of qualitative properties that this substance possesses.

The characteristics of silicon with a crystal lattice include the classification of its types, since there are several of them:

Electronic quality - the purest and highest quality. This type is used in electronics to create particularly sensitive devices.
Sunny quality. The name itself determines the area of use. It is also silicon of fairly high purity, the use of which is necessary to create high-quality and long-lasting solar cells. Photoelectric converters created on the basis of a crystalline structure are of higher quality and wear-resistant than those created using an amorphous modification by sputtering onto various types of substrates.
Technical silicon. This variety includes those samples of the substance that contain about 98% of the pure element. Everything else goes to various kinds of impurities:

aluminum;
chlorine;
carbon;
phosphorus and others.

The last type of the substance in question is used to obtain polycrystals of silicon. For this purpose, recrystallization processes are carried out. As a result, in terms of purity, products are obtained that can be classified as solar and electronic quality.

By its nature, polysilicon is an intermediate product between the amorphous and crystalline modifications. This option is easier to work with, it is better processed and cleaned with fluorine and chlorine.

The resulting products can be classified as follows:

multisilicon;
monocrystalline;
profiled crystals;
silicon scrap;
technical silicon;
production waste in the form of fragments and scraps of matter.

Each of them finds application in industry and is fully used by humans. Therefore, those that touch silicon are considered non-waste. This significantly reduces its economic cost without affecting quality.

Using pure silicon

Industrial silicon production is quite well established, and its scale is quite large. This is due to the fact that this element, both pure and in the form of various compounds, is widespread and in demand in various branches of science and technology.

Where is crystalline and amorphous silicon used in its pure form?

In metallurgy, as an alloying additive capable of changing the properties of metals and their alloys. Thus, it is used in the smelting of steel and cast iron.
Different types of substances are used to make a purer version - polysilicon.
Silicon compounds are a whole chemical industry, which has gained particular popularity today. Organosilicon materials are used in medicine, in the manufacture of dishes, tools and much more.
Manufacturing of various solar panels. This method of obtaining energy is one of the most promising in the future. Environmentally friendly, economically beneficial and wear-resistant are the main advantages of this type of electricity generation.
Silicon has been used for lighters for a very long time. Even in ancient times, people used flint to produce a spark when lighting a fire. This principle is the basis for the production of various types of lighters. Today there are types in which flint is replaced by an alloy of a certain composition, which gives an even faster result (sparking).
Electronics and solar energy.
Manufacturing of mirrors in gas laser devices.

Thus, pure silicon has a lot of advantageous and special properties that allow it to be used to create important and necessary products.

Application of silicon compounds

In addition to the simple substance, various silicon compounds are also used, and very widely. There is a whole industry called silicate. It is based on the use various substances, which contain this amazing element. What are these compounds and what is produced from them?

Quartz, or river sand - SiO 2. Used to make construction and decorative materials such as cement and glass. Everyone knows where these materials are used. No construction can be completed without these components, which confirms the importance of silicon compounds.
Silicate ceramics, which includes materials such as earthenware, porcelain, brick and products based on them. These components are used in medicine, in the manufacture of dishes, decorative jewelry, household items, in construction and other everyday areas of human activity.
- silicones, silica gels, silicone oils.
Silicate glue - used as stationery, in pyrotechnics and construction.

Silicon, the price of which varies on the world market, but does not cross from top to bottom the mark of 100 Russian rubles per kilogram (per crystalline), is a sought-after and valuable substance. Naturally, compounds of this element are also widespread and applicable.

Biological role of silicon

From the point of view of its importance for the body, silicon is important. Its content and distribution in tissues is as follows:

0.002% - muscle;
0.000017% - bone;
blood - 3.9 mg/l.

About one gram of silicon must be ingested every day, otherwise diseases will begin to develop. None of them are mortally dangerous, but prolonged silicon starvation leads to:

hair loss;
emergence acne and acne;
fragility and brittleness of bones;
easy capillary permeability;
fatigue and headaches;
the appearance of numerous bruises and bruises.

For plants, silicon is an important microelement necessary for normal growth and development. Experiments on animals have shown that those individuals that consume sufficient amounts of silicon on a daily basis grow better.

Chemistry preparation for cancer and DPA
Comprehensive edition

PART AND

GENERAL CHEMISTRY

CHEMISTRY OF ELEMENTS

CARBON. SILICIA

Applications of carbon and silicon

Application of carbon

Carbon is one of the most sought after minerals on our planet. Carbon is primarily used as a fuel for the energy industry. The annual production of hard coal in the world is about 550 million tons. In addition to using coal as a coolant, a considerable amount of it is processed into coke, which is necessary for extracting various metals. For every ton of iron obtained as a result of the blast furnace process, 0.9 tons of coke are consumed. Activated carbon is used in medicine for poisoning and in gas masks.

Graphite is used in large quantities to make pencils. The addition of graphite to steel increases its hardness and abrasion resistance. This steel is used, for example, for the production of pistons, crankshafts and some other mechanisms. The ability of the graphite structure to exfoliate allows it to be used as a highly effective lubricant at very high temperatures (about +2500 °C).

Graphite has another very important property - it is an effective moderator of thermal neutrons. This property is used in nuclear reactors. Recently, plastics have begun to be used, to which graphite is added as a filler. The properties of such materials make it possible to use them for the production of many important devices and mechanisms.

Diamonds are used as good hard material for the production of mechanisms such as grinding wheels, glass cutters, drilling rigs and other devices requiring high hardness. Beautifully cut diamonds are used as expensive jewelry, which are called diamonds.

Fullerenes were discovered relatively recently (in 1985), so they have not yet found any practical application, but scientists are already conducting research on creating information carriers of huge capacity. Nanotubes are already used in various nanotechnologies, such as administering drugs using a nanohead, making nanocomputers, and much more.

Silicon Applications

Silicon is a good semiconductor. It is used to make various semiconductor devices such as diodes, transistors, microcircuits and microprocessors. All modern microcomputers use processors based on silicon chips. Silicon is used to make solar panels that can convert solar energy into electricity. In addition, silicon is used as an alloying component for the production of high-quality alloy steels.

Slide 2

Being in nature.

Among the many chemical elements, without which the existence of life on Earth is impossible, carbon is the main one. More than 99% of the carbon in the atmosphere is contained in the form of carbon dioxide. About 97% of the carbon in the oceans exists in dissolved form (), and in the lithosphere - in the form of minerals. Elemental carbon is present in the atmosphere in small quantities in the form of graphite and diamond, and in the soil in the form of charcoal.

Slide 3

Position in PSHE. General characteristics of elements of the carbon subgroup.

The main subgroup of group IV of D.I. Mendeleev’s periodic table is formed by five elements - carbon, silicon, germanium, tin and lead. Due to the fact that from carbon to lead the radius of the atom increases, the sizes of the atoms increase, the ability to attach electrons, and, consequently, the non-metallic properties will weaken, and the ease of giving up electrons will increase.

Slide 4

Electronic engineering

In the normal state, elements of this subgroup exhibit a valence equal to 2. Upon transition to an excited state, accompanied by the transition of one of the s - electrons of the outer layer to a free cell of the p - sublevel of the same level, all electrons of the outer layer become unpaired and the valence increases to 4.

Slide 5

Production methods: laboratory and industrial.

Carbon Incomplete combustion of methane: CH4 + O2 = C + 2H2O Carbon monoxide (II) In industry: Carbon monoxide (II) is produced in special furnaces called gas generators as a result of two sequential reactions. In the lower part of the gas generator, where there is enough oxygen, complete combustion of coal occurs and carbon monoxide (IV) is formed: C + O2 = CO2 + 402 kJ.

Slide 6

As carbon monoxide (IV) moves from bottom to top, it comes into contact with hot coal: CO2 + C = CO – 175 kJ. The resulting gas consists of free nitrogen and carbon (II) monoxide. This mixture is called generator gas. In gas generators, water vapor is sometimes blown through hot coal: C + H2O = CO + H2 – Q, “CO + H2” - water gas. In the laboratory: Acting on formic acid with concentrated sulfuric acid, which binds water: HCOOH  H2O + CO.

Slide 7

Carbon monoxide (IV) In industry: By-product of lime production: CaCO3 CaO + CO2. In the laboratory: When acids interact with chalk or marble: CaCO3 + 2HCl  CaCl2 + CO2+ H2O. Carbides Carbides are produced by calcining metals or their oxides with coal.

Slide 8

Carbonic acid is prepared by dissolving carbon monoxide (IV) in water. Since carbonic acid is a very weak compound, this reaction is reversible: CO2 + H2O H2CO3. Silicon In industry: When heating a mixture of sand and coal: 2C + SiO2Si + 2CO. In the laboratory: When a mixture of pure sand interacts with magnesium powder: 2Mg + SiO2  2MgO + Si.

Slide 9

Silicic acid is obtained by the action of acids on solutions of its salts. At the same time, it precipitates in the form of a gelatinous precipitate: Na2SiO3 + HCl  2NaCl + H2SiO3 2H+ + SiO32- H2SiO3

Slide 10

Allotropic modifications of carbon.

Carbon exists in three allotropic modifications: diamond, graphite and carbyne.

Slide 11

Graphite.

Soft graphite has a layered structure. Opaque, gray with a metallic sheen. It conducts electricity quite well due to the presence of mobile electrons. Slippery to the touch. One of the softest among solids. Fig.2 Model of graphite lattice.

Slide 12

Diamond.

Diamond is the hardest natural substance. Diamond crystals are highly valued both as a technical material and as a precious decoration. A well-polished diamond is a diamond. Refracting the rays of light, it sparkles with pure, bright colors of the rainbow. The largest diamond ever found weighs 602 g, has a length of 11 cm, a width of 5 cm, and a height of 6 cm. This diamond was found in 1905 and is named “Callian”. Fig. 1 Diamond lattice model.

Slide 13

Carbyne and Mirror Carbon.

Carbyne is a deep black powder interspersed with larger particles. Carbyne is the most thermodynamically stable form of elemental carbon. Mirror carbon has a layered structure. One of the most important features mirror carbon (except for hardness, resistance to high temperatures, etc.) - its biological compatibility with living tissues.

Slide 14

Chemical properties.

Alkalis convert silicon into silicic acid salts with the release of hydrogen: Si + 2KOH + H2O = K2Si03 + 2H2 Carbon and silicon react with water only at high temperatures: C + H2O ¬ CO + H2 Si + 3H2O = H2SiO3 + 2H2 Carbon, unlike silicon interacts directly with hydrogen: C + 2H2 = CH4

Slide 15

Carbides.

Compounds of carbon with metals and other elements that are electropositive relative to carbon are called carbides. When aluminum carbide interacts with water, methane is formed Al4C3 + 12H2O = 4Al (OH)3 + 3CH4 When calcium carbide interacts with water, acetylene is formed: CaC2 + 2H2O = Ca (OH)2 + C2H2

A brief comparative description of the elements carbon and silicon is presented in Table 6.

Table 6

Comparative characteristics carbon and silicon

Comparison criteria	Carbon – C	Silicon – Si
position in the periodic table of chemical elements	, 2nd period, IV group, main subgroup	, 3rd period, IV group, main subgroup
electron configuration of atoms
valence possibilities	II – in a stationary state IV – in an excited state
possible oxidation states	, , , , ,	,
higher oxide	, acidic	, acidic
higher hydroxide	– weak unstable acid	() or – weak acid, has a polymer structure
hydrogen connection	– methane (hydrocarbon)	– silane, unstable

Carbon. The carbon element is characterized by allotropy. Carbon exists in the form of the following simple substances: diamond, graphite, carbyne, fullerene, of which only graphite is thermodynamically stable. Coal and soot can be considered as amorphous varieties of graphite.

Graphite is refractory, slightly volatile, chemically inert at ordinary temperatures, and is an opaque, soft substance that weakly conducts current. The structure of graphite is layered.

Alamaz is an extremely hard, chemically inert (up to 900 °C) substance, does not conduct current and conducts heat poorly. The structure of diamond is tetrahedral (each atom in a tetrahedron is surrounded by four atoms, etc.). Therefore, diamond is the simplest polymer, the macromolecule of which consists of only carbon atoms.

Carbin has linear structure( – carbyne, polyine) or ( – carbyne, polyene). It is a black powder and has semiconductor properties. Under the influence of light, the electrical conductivity of carbyne increases, and at temperature carbyne turns into graphite. Chemically more active than graphite. Synthesized in the early 60s of the 20th century, it was later discovered in some meteorites.

Fullerene is an allotropic modification of carbon formed by molecules having a “football” type structure. Molecules and other fullerenes were synthesized. All fullerenes are closed structures of carbon atoms in a hybrid state. The unhybridized bond electrons are delocalized as in aromatic compounds. Fullerene crystals are of the molecular type.

Silicon. Silicon is not characterized by bonds; it is not typical to exist in a hybrid state. Therefore, there is only one stable allotropic modification of silicon, crystal cell which is like the lattice of a diamond. Silicon is hard (on the Mohs scale, hardness is 7), refractory ( ), a very fragile substance of dark gray color with a metallic sheen under standard conditions - a semiconductor. Chemical activity depends on the size of the crystals (large crystalline ones are less active than amorphous ones).

The reactivity of carbon depends on the allotropic modification. Carbon in the form of diamond and graphite is quite inert, resistant to acids and alkalis, which makes it possible to make crucibles, electrodes, etc. from graphite. Carbon exhibits higher reactivity in the form of coal and soot.

Crystalline silicon is quite inert; in amorphous form it is more active.

The main types of reactions reflecting the chemical properties of carbon and silicon are given in Table 7.

Table 7

Basic chemical properties of carbon and silicon

reaction with	carbon	reaction with	silicon
simple substances	oxygen		oxygen
halogens		halogens
gray		carbon
hydrogen		hydrogen	does not react
metals		metals
complex substances	metal oxides		alkalis
water vapor		acids	does not react
acids

Cementing materials

Cementing materials – mineral or organic building materials used for the manufacture of concrete, fastening individual elements of building structures, waterproofing, etc..

Mineral binders(MVM)– finely ground powdery materials (cements, gypsum, lime, etc.), which form when mixed with water (in in some cases– with solutions of salts, acids, alkalis) a plastic, workable mass that hardens into a durable stone-like body and binds particles of solid aggregates and reinforcement into a monolithic whole.

Hardening of MVM occurs due to dissolution processes, the formation of a supersaturated solution and colloidal mass; the latter partially or completely crystallizes.

MVM classification:

1. hydraulic binding materials:

When mixed with water (mixing), they harden and continue to maintain or increase their strength in water. These include various cements and hydraulic lime. When hydraulic lime hardens, CaO interacts with water and carbon dioxide air and crystallization of the resulting product. They are used in the construction of above-ground, underground and hydraulic structures exposed to constant exposure to water.

2. air binders:

When mixed with water, they harden and retain their strength only in air. These include aerated lime, gypsum-anhydrite and magnesia aerated binders.

3. acid-resistant binders:

They consist mainly of acid-resistant cement containing a finely ground mixture of quartz sand and; They are usually locked aqueous solutions sodium or potassium silicate, they retain their strength for a long time when exposed to acids. During hardening, a reaction occurs. Used for the production of acid-resistant putties, mortars and concrete in the construction of chemical plants.

4. Autoclave hardening binders:

They consist of calc-siliceous and calc-nepheline binders (lime, quartz sand, nepheline sludge) and harden when processed in an autoclave (6-10 hours, steam pressure 0.9-1.3 MPa). These also include sandy Portland cements and other binders based on lime, ash and low-active sludge. Used in the production of silicate concrete products (blocks, sand-lime bricks, etc.).

5. Phosphate binders:

Consist of special cements; they are sealed with phosphoric acid to form a plastic mass that gradually hardens into a monolithic body and retains its strength at temperatures above 1000 °C. Usually titanophosphate, zinc phosphate, aluminophosphate and other cements are used. Used for the manufacture of refractory lining mass and sealants for high-temperature protection of metal parts and structures in the production of refractory concrete, etc.

Organic binders(OBM)– substances of organic origin that are capable of transitioning from a plastic state to a solid or low-plasticity state as a result of polymerization or polycondensation.

Compared to MVM, they are less brittle and have greater tensile strength. These include products formed during oil refining (asphalt, bitumen), products of thermal decomposition of wood (tar), as well as synthetic thermosetting polyester, epoxy, phenol-formaldehyde resins. Used in the construction of roads, bridges, floors production premises, rolled roofing materials, asphalt polymer concrete, etc.

In binary compounds of silicon with carbon, each silicon atom is directly bonded to four neighboring carbon atoms located at the vertices of a tetrahedron, the center of which is the silicon atom. At the same time, each carbon atom is in turn connected to four neighboring silicon atoms located at the vertices of a tetrahedron, the center of which is a carbon atom. This mutual arrangement of silicon and carbon atoms is based on the silicon-carbon bond Si - C- and forms a dense and very strong crystalline structure.

Currently, only two binary compounds of silicon and carbon are known. This is a very rare moissanite mineral found in nature, which does not yet have practical application, and artificially produced carborundum SiC, which is sometimes called silund, refrax, carbofrax, cristolan, etc.

In laboratory practice and in technology, carborundum is obtained by reducing silica with carbon according to the reaction equation

SiO 2 + 3C = 2СО + SiC

In addition to finely ground quartz or pure quartz line and coke, table salt and coke are added to the mixture to produce carborundum. sawdust. Sawdust loosens the charge during firing, and salt, reacting with ferrous and aluminum impurities, transforms them into volatile chlorides FeCl 3 and AlCl 3, which are removed from the reaction zone at 1000-1200 ° C. In fact, the reaction between silica and coke begins already at 1150 ° C, but proceeds extremely slowly. As the temperature rises to 1220° C, its speed increases. In the temperature range from 1220 to 1340 ° C it becomes exothermic and proceeds violently. As a result of the reaction, a mixture is first formed consisting of tiny crystals and an amorphous variety of carborundum. With an increase in temperature to 1800-2000 ° C, the mixture recrystallizes and turns into well-developed, tabular-shaped, rarely colorless, often colored green, gray and even black with a diamond shine and iridescent hexagonal crystals, containing about 98-99.5% carborundum. The process of obtaining carborundum from the charge is carried out in electric furnaces burning at 2000-2200 ° C. To obtain chemically pure carborundum, the product obtained by firing the charge is treated with an alkali, which dissolves unreacted silica.

Crystalline carborundum is a very hard substance; its hardness is 9. The ohmic resistance of polycrystalline carborundum decreases with increasing temperature and at 1500 0 C becomes insignificant.

In air at temperatures above 1000 0 C, carborundum begins to oxidize, first slowly, and then vigorously with an increase in temperature above 1700 ° C. In this case, silica and carbon monoxide are formed:

2SiC + ZO 2 = 2SiO 2 + 2CO

The silicon dioxide formed on the surface of the carborundum is a protective film that somewhat slows down the further oxidation of the carborundum. In an environment of water vapor, the oxidation of carborundum under the same conditions proceeds more vigorously.

Mineral acids, with the exception of orthophosphoric acid, do not affect carborundum; chlorine at 100 ° C decomposes it according to the reaction equation

SiC + 2Cl 2 = SiCl 4 + C

and at 1000° C, instead of carbon, CC1 4 is released:

SiC + 4C1 2 = SiCl + CC1 4

Molten metals, reacting with carborundum, form the corresponding silicides:

SiC + Fe =FeSl + C

At temperatures above 810° C, carborundum reduces alkaline earth metal oxides to metal; above 1000° C, it reduces iron (III) oxide Fe 2 O 3 and above 1300-1370° C, iron (II) oxide FeO, nickel (II) oxide NiO and manganese oxide MnO.

Molten caustic alkalis and their carbonates in the presence of atmospheric oxygen completely decompose carborundum with the formation of the corresponding silicates:

SiC + 2KOH + 2O 2 = K 2 SiO 3 + H 2 O + CO 2

SiC + Na 2 CO 3 + 2O 2 = Na 2 SiO 3 + 2CO 2

Carborundum can also react with sodium peroxide, lead (II) oxide and phosphoric acid.

Due to the fact that carborundum has high hardness, it is widely used as abrasive powders for grinding metal, as well as for the manufacture of carborundum abrasive wheels, whetstones and sanding paper. Electrical conductivity carborundum at high temperatures makes it possible to use it as the main material in the manufacture of so-called silit rods, which are resistance elements in electric furnaces. For this purpose, a mixture of carborundum and silicon is mixed with glycerin or other organic cementing substance and rods are formed from the resulting mass, which are fired at 1400-1500 ° C in an atmosphere of carbon monoxide or in a nitrogen atmosphere. During firing, the cementing organic substance decomposes, the released carbon, combining with silicon, turns it into carborundum and gives the rods the required strength.

Special fireproof crucibles are made from carborundum
for melting metals produced by hot pressing
carborundum at 2500° C under a pressure of 42-70 MPa. Also known
We have refractories made from mixtures of carborundum and nitrides
boron, steatite, molybdenum-containing bonds and other substances
creatures.

SILICON HYDRIDES, OR SILANES

Hydrogen compounds of silicon are usually called silicon hydrides, or silanes. Like saturated hydrocarbons, silicon hydrides form a homologous series in which the silicon atoms are connected to each other by a single bond

Si-Si -Si -Si -Si- etc.

The simplest.representative

of this homologous series is monosilane, or simply silane, SiH 4, the molecular structure of which is similar to the structure of methane, followed by

disilane H 3 Si-SiH 3, which is similar in molecular structure to ethane, then trisilane H 3 Si-SiH 2 -SiH 3,

tetrasilane H 3 Si-SiH 2 -SiH 2 -SiH 3,

pentasilane H 3 Si-SiH 2 -SiH 2 -SiH 2 ^--SiH 3 and the last of the obtained silanes of this homologous series

hexasilane H 3 Si-SiH 2 -SiH 2 -SiH 2 -SiH 2 -SiH 3. Silanes do not occur in nature in their pure form. They are obtained artificially:

1. Decomposition of metal silicides with acids or alkalis according to the reaction equation

Mg 2 Si+ 4HCI = 2MgCl 2 + SiH 4

this produces a mixture of silanes, which is then separated by fractional distillation at very high temperatures. low temperatures.

2. Reduction of halogenosilanes with lithium hydride or lithium aluminum hydride:

SiCl 4 + 4 LiH = 4LiCl + SiH 4

This method of producing siles was first described in 1947.

3. Reduction of halogenosilanes with hydrogen. The reaction takes place at 300 - 400 ° C in reaction tubes filled with a contact mixture containing silicon, copper metal and 1 - 2% aluminum halides as catalysts.

Despite the similarities in molecular structure sitanes and saturated hydrocarbons, their physical properties are different.

Compared to hydrocarbons, silanes are less stable. The most stable of them is monosilane SiH4, which decomposes into silicon and hydrogen only at red heat. Other silanes with a high silicon content form lower derivatives at much lower temperatures. For example, disilane Si 2 H 6 gives silane and a solid polymer at 300 ° C, and hexasilane Si 6 H 14 decomposes slowly even at normal temperatures. When in contact with oxygen, silanes easily oxidize, and some of them, for example monosilane SiH 4, spontaneously ignite at -180 ° C. Silanes easily hydrolyze into silicon dioxide and hydrogen:

SiH 4 + 2H 2 0 = SiO 2 + 4H 2

In higher silanes this process occurs with splitting

bonds - Si - Si - Si - between silicon atoms. For example, three

silane Si 3 H 8 gives three molecules of SiO 2 and ten molecules of hydrogen gas:

H 3 Si - SiH 2 - SiH 3 + 6H 3 O = 3SiO 2 + 10H 2

In the presence of caustic alkalis, hydrolysis of silanes results in the formation of silicate of the corresponding alkali metal and hydrogen:

SiH 4 + 2NaOH + H 2 0 = Na 2 Si0 3 + 4H 2

SILICON HALIDES

Binary silicon compounds also include halogenosilanes. Like silicon hydrides - silanes - they form a homologous series chemical compounds, in which the halide atoms are directly connected to silicon atoms connected to each other by single bonds

etc. in chains of appropriate length. Due to this similarity, halogenosilanes can be considered as products of the replacement of hydrogen in silanes with the corresponding halogen. In this case, replacement can be complete or incomplete. In the latter case, halogen derivatives of silanes are obtained. The highest halogenosilane known to date is considered to be chlorosilane Si 25 Cl 52. Halogenosilanes and their halogen derivatives do not occur in nature in pure form and can only be obtained artificially.

1. Direct combination of elemental silicon with halogens. For example, SiCl 4 is obtained from ferrosilicon containing from 35 to 50% silicon, treating it at 350-500 ° C with dry chlorine. In this case, SiCl 4 is obtained as the main product in a mixture with other more complex halogenosilanes Si 2 C1 6, Si 3 Cl 8, etc. according to the reaction equation

Si + 2Cl 2 = SiCl 4

The same compound can be obtained by chlorinating a mixture of silica and coke at high temperatures. The reaction proceeds according to the scheme

SiO 2 + 2C=Si +2CO

Si + 2C1 2 = SiС1 4

SiO 2 + 2C + 2Cl 2 = 2CO + SiCl 4

Tetrabromosilane is obtained by bromination of elemental silicon at red heat with bromine vapor:

Si + 2Br 2 = SiBr 4

or a mixture of silica and coke:

SiO 2 + 2C = Si+2CO

Si + 2Br 3 = SiBi 4

SiO 2 + 2C + 2Br 2 = 2CO + SiBr 4

In this case, simultaneously with tetrasilanes, the formation of silanes of higher degrees is possible. For example, when chlorinating magnesium silicide, 80% SiCI 4, 20% SiCl 6 and 0.5-1% Si 3 Cl 8 are obtained; when chlorinating calcium silicide, the composition of the reaction products is expressed as follows: 65% SiC1 4; 30% Si 2 Cl 6 ; 4% Si 3 Cl 8 .

2. Halogenation of silanes with hydrogen halides in the presence of AlBr 3 catalysts at temperatures above 100° C. The reaction proceeds according to the scheme

SiH 4 + HBr = SiH 3 Br + H 2

SiH 4 + 2HBr = SiH 2 Br 2 + 2H 2

3. Halogenation of silanes with chloroform in the presence of AlCl 3 catalysts:

Si 3 H 8 + 4СН1 3 = Si 3 H 4 Cl 4 + 4СН 2 С1 3

Si 3 H 8 + 5CHCl 3 = Si 3 H 3 C1 5 + 5CH 2 C1 2

4. Silicon tetrafluoride is obtained by treating silica with hydrofluoric acid:

SiO 2 + 4HF= SiF 4 + 2H 2 0

5. Some polyhalosilanes can be prepared from the simplest halogenosilanes by halogenating them with the appropriate halide. For example, tetraiodosilane in a sealed tube at 200-300 ° C, reacting with silver, releases hexaiododisilane according to

Iodosilanes can be obtained by reacting iodine with silanes in a medium carbon tetrachloride or chloroform, as well as V the presence of an AlI 3 catalyst during the interaction of silane with hydrogen iodide

Halogenosilanes are less durable than structurally similar halogenated hydrocarbons. They easily hydrolyze, forming silica gel and hydrohalic acid:

SiCl 4 + 2H 2 O = Si0 2 + 4HCl

The simplest representatives of halogenosilanes are SiF 4 , SiCl 4 , SiBr 4 and SiI 4 . Of these, tetrafluorosilane and tetrachlorosilane are mainly used in technology. Tetrafluorosilane SiF 4 is a colorless gas with a pungent odor, fumes in air, and hydrolyzes into hydrosilicic acid and silica gel. SiF 4 is obtained by the action of hydrofluoric acid on silica according to the reaction equation

SiO 2 + 4HF = SlF 4 + 2H 2 0

For industrial production. SiF 4 uses fluorspar CaF 2, silica SiO 2 and sulfuric acid H 2 SO 4. The reaction occurs in two phases:

2CaF 2 + 2H 3 SO 4 = 2CaSO 4 + 4HF

SiO 2 + 4HF = 2H 2 O + SiF 4

2CaF 2 + 2H 2 S0 4 + SiO 2 = 2CaSO 4 + 2H 2 O + SiF 4

The gaseous state and volatility of tetrafluorosilane is used for etching sodium-lime silicate glasses with hydrogen fluoride. When hydrogen fluoride reacts with glass, tetrafluorosilane, calcium fluoride, sodium fluoride and water are formed. Tetrafluorosilane, evaporating, releases new deeper layers of glass for reaction with hydrogen fluoride. At the site of the reaction, CaF 2 and NaF remain, which dissolve in water and thereby free up access for hydrogen fluoride for further penetration to the freshly exposed glass surface. The etched surface can be matte or transparent. Matte etching is obtained by the action of gaseous hydrogen fluoride on glass, transparent - by etching with aqueous solutions of hydrofluoric acid. If you pass tetrafluorosilane into water, you get H 2 SiF 6 and silica in the form of a gel:

3SiF 4 + 2H 2 O = 2H 2 SiF 6 + Si0 2

Hydrofluorosilicic acid is a strong dibasic acid; it is not obtained in a free state; upon evaporation it decomposes into SiF 4 and 2HF, which volatilize; with caustic alkalis forms acidic and normal salts:

H 2 SlF 6 + 2NaOH.= Na 2 SiF 6 + 2H 2 O

with an excess of alkalis gives alkali metal fluoride, silica and water:

H 2 SiF 6 + 6NaOH = 6NaF + SiO 2 + 4H 2 O

The silica released in this reaction reacts with caustic
fog and leads to the formation of silicate:

SiO 2 + 2NaOH = Na 2 SiO 3 +H 2 O

Salts of hydrofluorosilicic acid are called silicofluorides or fluates. Currently known silicofluorides are Na, H, Rb, Cs, NH 4, Cu, Ag, Hg, Mg, Ca, Sr, Ba, Cd, Zn, Mn, Ni, Co, Al, Fe, Cr, Pb and etc.

In technology, for various purposes, sodium silicofluorides Na 2 SiF 6, magnesium MgSiF 6 * 6HgO, zinc ZnSiF 6 * 6H 2 O, aluminum Al 2 (SiF 6) 3, lead PbSiF 6, barium BaSiF 6, etc. are used. Silico fluorides have antiseptic and sealing properties; at the same time they are fire retardants. Because of this, they are used to impregnate wood to prevent premature decay and protect it from ignition during fires. Artificial and natural stones for construction purposes are also impregnated with silicofluoride to compact them. The essence of impregnation is that a solution of silicofluorides, penetrating into the pores and cracks of the stone, reacts with calcium carbonate and some other compounds and forms insoluble salts that are deposited in the pores and seal them. This significantly increases the stone's resistance to weathering. Materials that do not contain calcium carbonate at all or contain little of it are pre-treated with avanfluates, i.e. substances containing dissolved calcium salts, alkali metal silicates and other substances capable of forming insoluble precipitates with fluates. Silicofluorides of magnesium, zinc and aluminum are used as fluates. The fluting process can be represented as follows:

MgSiF 6 + 2CaCO 3 = MgF 2 + 2CaF 2 + SiO 2 + 2CO 2

ZnSiF 6 + ZCaС0 3 = 3CaF 6 + ZnCO 3 + SiO 2 + 2CO 2

Al 2 (SiF 6) 3 + 6CaCO 3 =. 2A1F 3 + 6CaF 2 + 3SiO 2 + 6CO 2

Silicofluorides of alkali metals are obtained by reacting hydrofluorosilicic acid with solutions of salts of these metals:

2NaCl + H 2 SiF 6 = Na 2 SlF 6 + 2HC1

These are gelatinous precipitates, soluble in water and practically insoluble in absolute alcohol. Therefore they are used in quantitative analysis when determining silica by volumetric method. For technical purposes, sodium silicofluoride is used, obtained in the form of a white powder as a by-product in the production of superphosphate. From a mixture of Na 2 SiF 6 and Al 2 About 3 at 800° C, cryolite 3NaF٠AlF 3 is formed, which is widely used in the production of dental cements and is a good opacifier both in glassmaking and in the manufacture of opaque glazes and enamels.

Sodium silicofluoride, as one of the components, is introduced into the composition of chemically resistant putties produced on liquid glass:

Na 2 SiF 6 + 2Na 2 SiO 3 = 6NaF + 3SiO 2

The silica released by this reaction gives the hardened putty chemical resistance. At the same time, Na 2 SiF 6 is a hardening accelerator. Sodium silicofluoride is also introduced as a mineralizer into raw mixtures in the production of cements.

Tetrachlorosilane SiCl 4 is a colorless, fuming in air, easily hydrolyzed liquid obtained by chlorinating carborundum or ferrosilicon by acting on silanes at elevated temperatures

Tetrachlorosilane is the main starting product for the production of many organosilicon compounds.

Tetrabromosilane SiBr 4 is a colorless liquid that fumes in air, easily hydrolyzes into SiO 2 and HBr, obtained at a red-hot temperature when bromine vapor is passed over hot elemental silicon.

Tetraiodosilane SiI 4 - white crystalline substance, obtained by passing a mixture of iodine vapor and carbon dioxide over hot elemental silicon.

Silicon borides and nitrides

Silicon borides are compounds of silicon and boron. Currently, two silicon borons are known: silicon triboride B 3 Si and silicon hexaboride B 6 Si. These are extremely hard, chemically resistant and fire-resistant substances. They are obtained by fusing electric current finely ground mixture consisting of 5 wt. parts of elemental silicon and 1 wt. h. boron. The cured mass is cleaned with molten potassium carbonate. G. M. Samsonov and V. P. Latyshev obtained silicon triboride by hot pressing at 1600-1800 0 C.

Silicon triboride with pl. 2.52 g/cm 3 forms black plates -
fine structure rhombic crystals, translucent
in a thin layer in yellow-brown tones. Silicon hexaboride with pl.
2.47 g/cm 3 is obtained in the form of opaque opaque grains
fork shape.

Silicon borides melt at about 2000° C, but oxidize very slowly even at high temperatures. This makes it possible to use them as special refractories. The hardness of silicon borides is very high, and in this respect they are close to carborundum.

Silicon compounds with nitrogen are called silicon nitrides. The following nitrides are known: Si 3 N 4, Si 2 N 3 and SIN. Silicon nitrides are obtained by calcining elemental silicon in an atmosphere of pure nitrogen in the temperature range from 1300 to 1500 ° C. Normal silicon nitride Si 3 N 4 can be obtained from a mixture of silica with coke, calcined in an atmosphere of pure nitrogen at 1400-1500 ° C:

6С + 3Si0 2 + 2N 3 ͢ Si 3 N 4 + 6CO

Si 3 N 4 is a grayish-white fireproof and acid-resistant powder that volatilizes only above 1900° C. Silicon nitride hydrolyzes to release silica and ammonia:

Si 3 N 4 + 6H 2 O = 3SiO 2 + 4NH 3

Concentrated sulfuric acid when heated, it slowly decomposes Si 3 N 4, and diluted hydrofluorosilicic acid decomposes it more energetically.

Silicon nitride of the composition Si 2 N 3 is also obtained by the action of nitrogen at high temperatures on elemental silicon or on carbonitrogen silicon C 2 Si 2 N + N 2 = 2C + Si2N 3 .

In addition to binary compounds of silicon with nitrogen, many other more complex compounds are currently known, which are based on the direct bond of silicon atoms with nitrogen atoms, for example: 1) aminosilanes SiH 3 NH 2, SiH 2 (NH 2) 2, SiH(NH 2) 3, Si(NH 2) 4; 2) silylamines NH 2 (SiH 3), NH(SiH 3) 2, N(SiH 3) 3; 3) nitrogen-containing silicon compounds of a more complex composition.

GENERAL VIEWS