Silicon in free form was isolated in 1811 by J. Gay-Lussac and L. Tenard by passing vapors of silicon fluoride over metallic potassium, but it was not described by them as an element. The Swedish chemist J. Berzelius in 1823 gave a description of the silicon obtained by him by treating the potassium salt K 2 SiF 6 with potassium metal at high temperature. The new element was given the name "silicon" (from the Latin silex - flint). The Russian name "silicon" was introduced in 1834 by the Russian chemist German Ivanovich Hess. Translated from other Greek. krhmnoz- "cliff, mountain".
Being in nature, getting:
In nature, silicon is found in the form of dioxide and silicates of various compositions. Natural silicon dioxide occurs mainly in the form of quartz, although other minerals exist - cristobalite, tridymite, kitite, cousite. Amorphous silica is found in diatom deposits at the bottom of the seas and oceans - these deposits were formed from SiO 2, which was part of diatoms and some ciliates.
Free silicon can be obtained by calcining fine white sand with magnesium, which is almost pure silicon oxide in chemical composition, SiO 2 +2Mg=2MgO+Si. Industrial grade silicon is obtained by reducing the SiO 2 melt with coke at a temperature of about 1800°C in arc furnaces. The purity of silicon obtained in this way can reach 99.9% (the main impurities are carbon, metals).
Physical properties:
Amorphous silicon has the form of a brown powder, the density of which is 2.0 g/cm 3 . Crystalline silicon - a dark gray, shiny crystalline substance, brittle and very hard, crystallizes in the diamond lattice. It is a typical semiconductor (conducts electricity better than a rubber-type insulator, and worse than a conductor - copper). Silicon is brittle, only when heated above 800 °C does it become plastic. Interestingly, silicon is transparent to infrared radiation starting at a wavelength of 1.1 micrometers.
Chemical properties:
Chemically, silicon is inactive. At room temperature, it reacts only with gaseous fluorine to form volatile silicon tetrafluoride SiF 4 . When heated to a temperature of 400-500 ° C, silicon reacts with oxygen to form dioxide, with chlorine, bromine and iodine - to form the corresponding easily volatile tetrahalides SiHal 4 . At a temperature of about 1000°C, silicon reacts with nitrogen to form nitride Si 3 N 4 , with boron - thermally and chemically stable borides SiB 3 , SiB 6 and SiB 12 . Silicon does not directly react with hydrogen.
For silicon etching, a mixture of hydrofluoric and nitric acids is most widely used.
Silicon dissolves in hot alkali solutions: Si + 2KOH + H 2 O = K 2 SiO 3 + 2H 2
Silicon is characterized by compounds with an oxidation state of +4 or -4.
The most important connections:
Silicon dioxide, SiO 2- (silicic anhydride), colorless. crist. substance, refractory (1720 C), with high hardness. Acid oxide, chemically inactive, interacts with hydrofluoric acid and alkali solutions, forming in the latter case salts of silicic acids - silicates. Silicates are also formed when silicon oxide is fused with alkalis, basic oxides, and some salts.
SiO 2 + 4NaOH = Na 4 SiO 4 + 2H 2 O; SiO 2 + CaO \u003d CaSiO 3;
Na 2 CO 3 + CaCO 3 + 6SiO 2 = Na 2 CaSi 6 O 14 + 2CO 2 (mixed sodium calcium silicate, glass)
Silicic acids- weak, insoluble, formed by adding acid to a silicate solution in the form of a gel (gelatinous substance). H 4 SiO 4 (orthosilicon) and H 2 SiO 3 (metasilicon, or silicon) exist only in solution and irreversibly turn into SiO 2 when heated and dried. The resulting solid porous product - silica gel, has a developed surface and is used as a gas adsorbent, desiccant, catalyst and catalyst carrier.
silicates- salts of silicic acids for the most part (except for sodium and potassium silicates) are insoluble in water. Soluble silicates in solution undergo strong hydrolysis.
Hydrogen compounds- analogues of hydrocarbons, silanes, compounds in which silicon atoms are connected by a single bond, Silenes if the silicon atoms are double bonded. Like hydrocarbons, these compounds form chains and rings. All silanes can ignite spontaneously, form explosive mixtures with air and easily react with water: SiH 4 + 2H 2 O \u003d SiO 2 + 4H 2
Silicon tetrafluoride SiF 4, a gas with an unpleasant odor, poisonous, formed by the action of hydrofluoric (hydrofluoric) acid on silicon and many of its compounds, including glass:
Na 2 SiO 3 + 6HF = 2NaF + SiF 4 + 3H 2 O
Reacts with water to form silica and hexafluorosilicon(H 2 SiF 6) acids:
3SiF 4 + 3H 2 O \u003d 2H 2 SiF 6 + H 2 SiO 2
H 2 SiF 6 is close in strength to sulfuric acid, salts are fluorosilicates.
Application:
Silicon finds the greatest use in the production of alloys for giving strength to aluminum, copper and magnesium and for the production of ferrosilicides, which are important in the production of steels and semiconductor technology. Silicon crystals are used in solar cells and semiconductor devices - transistors and diodes. Silicon also serves as a raw material for the production of organosilicon compounds, or siloxanes, obtained in the form of oils, lubricants, plastics and synthetic rubbers. Inorganic silicon compounds are used in ceramic and glass technology, as an insulating material and piezocrystals.
For some organisms, silicon is an important biogenic element. It is part of the supporting structures in plants and skeletal structures in animals. In large quantities, silicon is concentrated by marine organisms - diatoms, radiolarians, sponges. Large amounts of silicon are concentrated in horsetails and cereals, primarily in the Bamboo and Rice subfamilies, including common rice. Human muscle tissue contains (1-2) 10 -2% silicon, bone tissue - 17 10 -4%, blood - 3.9 mg / l. With food, up to 1 g of silicon enters the human body daily.
Antonov S.M., Tomilin K.G.
KhF Tyumen State University, 571 groups.
Sources: Silicon. Wikipedia; Silicon in the Online Encyclopedia "Krugosvet" , ;
Silicon site
Silicon (Si) - stands in period 3, group IV of the main subgroup of the periodic system. Physical properties: silicon exists in two modifications: amorphous and crystalline. Amorphous silicon is a brown powder with a density of 2.33 g/cm3, which dissolves in metal melts. Crystalline silicon is dark gray crystals with a steel luster, hard and brittle, with a density of 2.4 g/cm3. Silicon consists of three isotopes: Si (28), Si (29), Si (30).
Chemical properties: electronic configuration: 1s22s22p63 s23p2 . Silicon is a non-metal. At the external energy level, silicon has 4 electrons, which determines its oxidation states: +4, -4, -2. Valence - 2, 4. Amorphous silicon has a greater reactivity than crystalline. Under normal conditions, it interacts with fluorine: Si + 2F2 = SiF4. At 1000 °C, Si reacts with non-metals: with CL2, N2, C, S.
Of the acids, silicon interacts only with a mixture of nitric and hydrofluoric acids:
With respect to metals, it behaves differently: it dissolves well in molten Zn, Al, Sn, Pb, but does not react with them; with other melts of metals - with Mg, Cu, Fe, silicon interacts with the formation of silicides: Si + 2Mg = Mg2Si. Silicon burns in oxygen: Si + O2 = SiO2 (sand).
Silicon dioxide or silica- stable connection Si, is widely distributed in nature. It reacts with its fusion with alkalis, basic oxides, forming salts of silicic acid - silicates. Receipt: in industry, pure silicon is obtained by reduction of silicon dioxide with coke in electric furnaces: SiO2 + 2С = Si + 2СO?.
In the laboratory, silicon is obtained by calcining white sand with magnesium or aluminum:
SiO2 + 2Mg = 2MgO + Si.
3SiO2 + 4Al = Al2O3 + 3Si.
Silicon forms acids: H2 SiO3 - meta-silicic acid; H2 Si2O5 is two metasilicic acid.
Finding in nature: quartz mineral - SiO2. Quartz crystals have the shape of a hexagonal prism, colorless and transparent, called rock crystal. Amethyst - rock crystal, dyed purple with impurities; smoky topaz is painted brownish; agate and jasper are crystalline varieties of quartz. Amorphous silica is less common and exists in the form of the mineral opal, SiO2 nH2O. Diatomaceous earth, tripolite or kieselguhr (diatomaceous earth) are earthy forms of amorphous silicon.
42. The concept of colloidal solutions
Colloidal solutions– highly dispersed two-phase systems consisting of a dispersion medium and a dispersed phase. Particle sizes are intermediate between true solutions, suspensions and emulsions. At colloidal particles molecular or ionic composition.
There are three types of internal structure of primary particles.
1. Suspensoids (or irreversible colloids)– heterogeneous systems, the properties of which can be determined by a developed interfacial surface. Compared to suspensions, they are more highly dispersed. They cannot exist for a long time without a dispersion stabilizer. They are called irreversible colloids due to the fact that their precipitation after evaporation again does not form sols. Their concentration is low - 0.1%. They differ slightly from the viscosity of the dispersed medium.
Suspensoids can be obtained:
1) dispersion methods (grinding large bodies);
2) condensation methods (obtaining insoluble compounds by means of exchange reactions, hydrolysis, etc.).
The spontaneous decrease in dispersion in suspensoids depends on the free surface energy. To obtain a long-lasting suspension, conditions are necessary for its stabilization.
Stable disperse systems:
1) dispersion medium;
2) dispersed phase;
3) stabilizer of the disperse system.
The stabilizer can be ionic, molecular, but most often high-molecular.
Protective colloids- macromolecular compounds that are added for stabilization (proteins, peptides, polyvinyl alcohol, etc.).
2. Associative (or micellar colloids) - semi-colloids arising at a sufficient concentration of molecules consisting of hydrocarbon radicals (amphiphilic molecules) of low molecular weight substances during their association into aggregates of molecules (micelles). Micelles formed in aqueous solutions detergents(soaps), organic dyes.
3. Molecular colloids (reversible or lyophilic colloids) - natural and synthetic high molecular weight substances. Their molecules have the size of colloidal particles (macromolecules).
Dilute solutions of colloids of macromolecular compounds are homogeneous solutions. When strongly diluted, these solutions obey the laws of dilute solutions.
Non-polar macromolecules dissolve in hydrocarbons, polar ones - in polar solvents.
Reversible colloids- substances, the dry residue of which, when a new portion of the solvent is added, again goes into solution.
silicates. Among them, aluminosilicates are the most common (it is clear that these silicates contain aluminum). Aluminosilicates include granite, various types of clays, and mica. An aluminium-free silicate is, for example, asbestos.
SiO2 oxide is essential for plant and animal life. It gives strength to the stems of plants and the protective covers of animals. Fish scales, insect shells, butterfly wings, bird feathers and animal fur are strong because they contain silica.
3) Rhinestone
Rock crystal is a colorless, transparent, usually chemically pure, almost free of impurities, a kind of low-temperature modification of quartz - SiO2, which crystallizes in a trigonal system. It occurs in the form of single or prismatic-hexagonal crystals collected in druses, sometimes weighing a ton or more.
Quartz is one of the most common minerals in the earth's crust, a rock-forming mineral of most igneous and metamorphic rocks. Chemical formula: SiO2.
Varieties of quartz: colorless, rose quartz, "hairy", carnelian, agate, "tiger's eye", polished pebbles.
5) Carnelian Formula - SiO2, a kind of chalcedony. Chemical composition - SiO2 content - 90-99%; Fe2O3, Al2O3, MgO, CaO, H2O impurities are noted. Carnelian, like agate, is an aggregate of essentially chalcedonic composition with a complex structure.
Jasper is an opaque variety of quartz - silicon dioxide SiO2 - with a fibrous structure that includes a wide variety of minerals: garnets, hematite, pyrite, etc. Therefore, jasper is distinguished by a great variety of its color, including all tones except pure blue.
7) Amethyst
Amethysts are purple or reddish quartz crystals, which are silicon dioxide and belong to the trigonal system.
Opal is an amorphous variety of quartz SiO2 with a variable water content (6-10%). Opal's chemical name is silicon dioxide polyhydrate. The main advantage of opal is the ability to emit successively different rays under the influence of sunlight, to cause a varied play of colors. Three types of opal are known: black opal, which has a very dark blue color with "flashes" of colors; orange-red fire opal and white opal.
7) Citrine The name of the stone, derived from the word citreus - "lemon", indicates the yellow tint of this variety of quartz, which is given to citrine by impurities of ferric iron. Citrine is good for concentration, concentration.
Jade is a translucent mineral of white and green color. From a mineralogical point of view, jade is a silica compound.
9) Agate is a kind of translucent quartz. Chemical formula: SiO2.
Application of silicon compounds:
Silicon is used in the silicate industry:
Natural silicon compounds - sand (SiO2) and silicates are used for the production of ceramics, glass and cement.
Silicate glue is widely known, used in construction as a desiccant, and in pyrotechnics and in everyday life for gluing paper.
Silicone oils and silicones, materials based on organosilicon compounds, have become widespread.
54) Physical and chemical bases of corrosion of concrete and mineral materials.
Concrete corrosion is the main enemy of all mineral building materials and structures (concrete, reinforced concrete, brick, asbestos cement, silicate, foam concrete and aerated concrete blocks). The most serious problem is the influence of the atmospheric-chemical factor - the impact of aggressive atmospheric substances (carbonates, sulfates, chlorides), as well as frequent freeze-thaw cycles.
Mineral-based building materials are capillary-porous. As a result of aggressive atmospheric action, crystals are formed inside the porous structure, the growth of which leads to the appearance of cracks. As a result of exposure to water, salts and carbon dioxide - corrosion of concrete and destruction of building structures.
Protection of mineral surfaces is a global task in the design, construction and operation of any facilities. It is relevant for all types of buildings, structures and structures used in modern construction.
Silicon (Si) is the second element of the main (A) subgroup of the 4th group of the Periodic Table, established by Dmitry Ivanovich Mendeleev. Silicon is very common in nature, so it ranks second (after oxygen) in abundance. So, without silicon and its compounds, the Earth's crust would not exist, which consists of compounds of this chemical element by more than a quarter. What are the features of silicon? What are the formulas of its compounds and their uses? What are the most important substances containing silicon? Let's try to figure it out.
Silicon element and its properties
Silicon exists in nature in several allotropic modifications - the most common are silicon in crystalline form and amorphous silicon. Let's consider each of these modifications separately.
Crystalline silicon
Silicon in this modification is a dark gray rather hard and brittle substance with a steel sheen. Such silicon is a semiconductor; his useful property is that, unlike metals, its electrical conductivity increases with increasing temperature. The melting point of such silicon is 1415 °C. In addition, crystalline silicon is not able to dissolve in water and various acids.
The use of silicon and its compounds in the crystalline modification is incredibly diverse. For example, crystalline silicon is part of solar panels installed on spacecraft and rooftops. Silicon is a semiconductor and is capable of converting solar energy into electrical energy.
In addition to solar cells, crystalline silicon is used to create many electronic devices and silicon steels.
Amorphous silicon
Amorphous silicon is a brown/dark brown powder with a diamond-like structure. Unlike crystalline silicon, this allotropic modification of the element does not have a strictly ordered crystal lattice. Although amorphous silicon melts at approximately 1400°C, it is much more reactive than crystalline silicon. Amorphous silicon does not conduct current and has a density of about 2 g/cm³.
Such silicon is most often used in the food industry and in the manufacture of medicines.
Chemical properties of silicon
- The main chemical property of silicon is combustion in oxygen, which results in the formation of an extremely common compound - silicon oxide:
Si + O2 → SiO2 (at temperature).
- When heated, silicon, as a non-metal, forms compounds with various metals. Such compounds are called silicides. For example:
2Ca + Si → Ca2Si (at temperature).
- Silicides, in turn, decompose without difficulty with the help of water or some acids. As a result of this reaction, a special hydrogen compound of silicon is formed - silane gas (SiH4):
Mg2Si + 4HCl → 2MgCl2 + SiH4.
- Silicon is also able to interact with fluorine (under normal conditions):
Si + 2F2 → SiF4.
- And when heated, silicon interacts with other non-metals:
Si + 2Cl2 → SiCl4 (400–600°).
3Si + 2N2 → Si3N4 (1000°).
Si + C → SiC (2000°).
- Also, silicon, interacting with alkalis and water, forms salts called silicates, and hydrogen gas:
Si + 2KOH + H2O → K2SiO3 + H2.
However, we will analyze most of the chemical properties of this element by considering silicon and its compounds, since they are the main substances on which the use and interaction of silicon with other chemical elements is based. So, what are the most common silicon compounds?
Silicon compounds
Earlier we found out what element silicon is and what properties it has. Now consider the formulas of silicon compounds.
With the participation of silicon, a huge number of different compounds are formed. The first place in terms of prevalence is occupied by oxygen compounds of silicon. This category includes SiO2 and insoluble silicic acid.
The acidic residue of silicic acid forms various silicates (eg CaSiO3 or Al2O3 SiO2). In such salts and the compounds of silicon with oxygen presented above, the element has a typical oxidation state of +4.
Silicon salts are also quite common - silicides (Mg2Si, NaSi, CoSi) and silicon compounds with hydrogen (for example, silane gas). Silane is known to ignite spontaneously in air with a blinding flash, and silicides are easily decomposed with both water and various acids.
Let us consider in more detail silicon and its compounds, which are considered the most common.
Silica
Another name for this oxide is silica. It is a solid and refractory substance that does not dissolve in water and acids and has an atomic crystal lattice. In nature, silicon oxide forms such minerals and gems like quartz, amethyst, opal, agate, chalcedony, jasper, flint and some others.
It should be noted that it is from silicon primitive people made their own tools of labor and hunting. Flint laid the foundation for the so-called stone age due to its ubiquitous availability and ability to form sharp cutting edges when chipped.
It is silicon oxide that makes the stems of plants such as reeds, reeds and horsetails, sedge leaves and grass stems strong. The protective outer coverings of some animals also contain silica.
In addition, it is the basis of silicate adhesive, which creates silicone sealant and silicone rubber.
Chemical properties of silicon oxide
Silicon dioxide interacts with a huge number of chemical elements - both metals and non-metals. For example:
- At high temperatures, silica interacts with alkalis, forming salts:
SiO2 + 2KOH → K2SiO3 + H2O (at temperature).
- As a typical acidic oxide, this compound gives silicates as a result of interaction with oxides of various metals:
SiO2 + CaO → CaSiO3 (at temperature).
- Or with carbonate salts:
SiO2 + K2CO3 → K2SiO3 + CO2 (at temperature).
- One of the most important chemical properties of silicon dioxide is the ability to obtain pure silicon from it. This can be done in two ways - by reacting dioxide with magnesium or carbon:
SiO2 + 2Mg → 2MgO + Si (at temperature).
SiO2 + 2C → Si + 2CO (at temperature)
Silicic acid
Silicic acid is very weak. It is insoluble in water and during reactions forms a gelatinous precipitate, which is sometimes able to fill the entire volume of the solution. When this mixture dries, you can see the formed silica gel, which is used as an adsorbent (absorbent of other substances).
The most accessible and common way to obtain silicic acid can be expressed using the formula:
K2SiO3 + 2HCl → 2KCl + H2SiO3↓.
Silicides
Considering silicon and its compounds, it is very important to say about such salts as silicides. Silicon forms such compounds with metals, acquiring, as a rule, an oxidation state of -4. However, metals such as mercury, zinc, beryllium, gold and silver are not able to interact with silicon and form silicides.
The most common silicides are Mg2Si, Ca2Si, NaSi and some others.
silicates
Compounds such as silicates are the second most common after silicon dioxide. Salts-silicates are considered quite complex substances, as they have a complex structure, and they are also part of most minerals and rocks.
The most common silicates in nature - aluminosilicates - include granite, micas, various types of clays. Another well-known silicate is asbestos, from which fire-resistant fabrics are made.
Application of silicon
First of all, silicon is used to obtain semiconductor materials and acid-resistant alloys. Silicon carbide (SiC) is often used for sharpening machine tools and polishing precious stones.
Molten quartz is used to make stable and strong quartz dishes.
Silicon compounds underlie the production of glass and cement.
Glasses differ from each other in composition, in which silicon is necessarily present. For example, in addition to window glass, there are refractory, crystal, quartz, colored, photochromic, optical, mirror and other glasses.
When cement is mixed with water, a special substance is formed - a cement mortar, from which a building material such as concrete is subsequently obtained.
The silicate industry is engaged in the production of these substances. In addition to glass and cement, the silicate industry produces bricks, porcelain, earthenware, and various articles made from them.
Conclusion
So, we found out that silicon is the most important chemical element, widespread in nature. Silicon is used in construction and artistic activity and is also indispensable for living organisms. Many substances, from simple glass to the most valuable porcelain, contain silicon and its compounds.
The study of chemistry allows you to get to know the world around us and understand that not everything around, even the most magnificent and expensive, is as mysterious and mysterious as it might seem. We wish you success in scientific knowledge and the study of such an excellent science as chemistry!
SILICON
(Silicium), Si - chem. element of group IV of the periodic system of elements; at. n. 14, at. m. 28.086. Crystalline silicon is a dark gray substance with a resinous sheen. In most compounds, it exhibits oxidation states - 4, +2 and +4. Natural silicon consists of stable isotopes 28Si (92.28%), 29Si (4.67%) and 30Si (3.05%). Radioactive 27Si, 31Si, and 32Si have been obtained with half-lives of 4.5 sec, 2.62 h, and 700 years, respectively. K. was first identified in 1811 by the French. chemist and physicist J. L. Gay-Lussac and fr. chemist L. J. Tenar, but identified only in 1823 by the Swede, chemist and mineralogist J. J. Berzelius.
In terms of prevalence in the earth's crust (27.6%), silicon is the second (after oxygen) element. Located preim. in the form of silica Si02 and other oxygen-containing substances (silicates, aluminosilicates, etc.). Under normal conditions, a stable semiconductor modification of diamond is formed, which is distinguished by a face-centered cubic structure of the diamond type, with a period a = 5.4307 A. The interatomic distance is 2.35 A. The density is 2.328 g/cm. At high pressure (120-150 kbar) it transforms into denser semiconductor and metallic modifications. The metallic modification is a superconductor with a transition temperature of 6.7 K. With increasing pressure, the melting point decreases from 1415 ± 3 ° C at a pressure of 1 bar to 810 ° C at a pressure of 15 104 bar (the triple point of coexistence of semiconductor, metallic and liquid K. ). During melting, an increase in the coordination number and metallization of interatomic bonds occur. Amorphous silicon is close to liquid in character of the short-range order, which corresponds to a strongly distorted body-centered cubic structure. Debye t-ra is close to 645 K. Coeff. temperature linear expansion changes with a change in t-ry according to an extreme law, below t-ry 100 K it becomes negative, reaching a minimum (-0.77 10 -6) deg -1 at t-re 80 K; at t-re 310 K it is equal to 2.33 10 -6 deg -1, and at t-re 1273 K -4.8 10 deg -1. Melting heat 11.9 kcal/g-atom; tboil 3520 K.
The heat of sublimation and evaporation at the melting point is 110 and 98.1 kcal/g-atom, respectively. The thermal and electrical conductivity of silicon depend on the purity and perfection of the crystals. With the growth of t-ry coefficient. The thermal conductivity of pure K. first increases (up to 8.4 cal/cm X X sec deg at t-re 35 K), and then decreases, reaching 0.36 and 0.06 cal/cm sec deg at t-re, respectively 300 and 1200 K. The enthalpy, entropy, and heat capacity of K. under standard conditions are respectively 770 cal/g-atom, 4.51 and 4.83 cal/g-atom—deg. Silicon is diamagnetic, the magnetic susceptibility of solid (-1.1 10 -7 emu/g) and liquid (-0.8 10 -7 emu/g). Silicon weakly depends on t-ry. The surface energy, density, and kinematic viscosity of liquid K. at the melting point are 737 erg/cm2, 2.55 g/cm3, and 3 x 10 m2/sec. Crystalline silicon is a typical semiconductor with a band gap of 1.15 eV at a temperature of 0 K and 1.08 eV at a temperature of 300 K. At room temperature, the concentration of intrinsic charge carriers is close to 1.4 10 10 cm - 3, the effective mobility of electrons and holes is 1450 and 480 cm 2 /v sec, respectively, and the electrical resistivity is 2.5 105 ohm cm. With increasing t-ry, they change exponentially.
The electrical properties of silicon depend on the nature and concentration of impurities, as well as on the perfection of the crystal. Usually, to obtain semiconductor K. with p- and n-type conductivity, it is doped with elements IIIb (boron, aluminum, gallium) and Vb (phosphorus, arsenic, antimony, bismuth) subgroups, creating a set of acceptor and donor levels, respectively, located near the boundaries of the zones . For doping, other elements are also used (for example,) that form so-called. deep levels, to-rye determine the capture and recombination of charge carriers. This makes it possible to obtain materials with a high electr. resistance (1010 ohm cm at t-re 80 K) and a short duration of existence of minority charge carriers, which is important for increasing the performance of various devices. Coeff. the thermoelectric power of silicon essentially depends on the temperature and the content of impurities, increasing with increasing electrical resistance (at p \u003d 0.6 ohm - cm, a \u003d 103 microvolt / deg). The dielectric constant of silicon (from 11 to 15) weakly depends on the composition and perfection of single crystals. The patterns of optical absorption of silicon strongly change with a change in its purity, concentration and nature of structural defects, as well as the wavelength.
The limit of indirect absorption of electromagnetic oscillations is close to 1.09 eV, of direct absorption - to 3.3 eV. In the visible region of the spectrum, the parameters of the complex refractive index (n - ik) depend very significantly on the state of the surface and the presence of impurities. For especially pure K. (atλ \u003d 5461 A and t-re 293 K) n \u003d 4.056 and k \u003d 0.028. The electron work function is close to 4.8 eV. Silicon is fragile. Its hardness (t-ra 300 K) according to Mohs - 7; HB = 240; HV y \u003d 103; And \u003d 1250 kgf / mm2; modulus of norms, elasticity (polycrystal) 10 890 kgf/mm2. The tensile strength depends on the perfection of the crystal: for bending from 7 to 14, for compression from 49 to 56 kgf/mm2; coefficient compressibility 0.325 1066 cm2/kg.
At room temperature, silicon practically does not interact with gaseous (excluding) and solid reagents, except for alkalis. At elevated t-re actively interacts with metals and non-metals. In particular, it forms SiC carbide (at t-re above 1600 K), Si3N4 nitride (at t-re above 1300 K), SiP phosphide (at t-re above 1200 K) and arsenides Si As, SiAS2 (at t-re above 1000 K). It reacts with oxygen at t-re above 700 K, forming Si02 dioxide, with halogens - SiF4 fluoride (at t-re above 300 K), SiCl4 chloride (at t-re above 500 K), SiBr4 bromide (at t-re 700 K) and nodide SiI4 (at t-re 1000 K). Intensively reacts with many others. metals, forming solid solutions of substitution in them or chemical. compounds - silicides. The concentration ranges of homogeneity of solid solutions depend on the nature of the solvent (eg, in germanium from 0 to 100%, in iron up to 15%, in alpha zirconium less than 0.1%).
There are much fewer metals and non-metals in hard flint and it is usually retrograde. At the same time, the limiting contents of impurities that create shallow levels in crystals reach a maximum ( 2 10 18 , 10 19 , 2 10 19 , 1021. area t-r from 1400 to 1600 K. Impurities with deep levels are noticeably less soluble (from 1015 for selenium and 5 10 16 for iron to 7 10 17 for nickel and 10 18 cm-3 for copper). In the liquid state, silicon mixes indefinitely with all metals, often with a very large release of heat. Pure silicon is prepared from a technical product of 99% Si and 0.03% Fe, Al and Co each, obtained by reducing quartz with carbon in electric furnaces. First, impurities are washed out of it (with a mixture of hydrochloric and sulfuric, and then hydrofluoric and sulfuric), after which the resulting product (99.98%) is treated with chlorine. Synthesized purified by distillation.
Semiconductor silicon is obtained by reducing chloride SiCl4 (or SiHCl3) with hydrogen or by thermal decomposition of SiH4 hydride. The final purification and growth of single crystals is carried out by a crucibleless zone smooth or by the Czochralski method, obtaining especially pure ingots (impurity content up to 1010-1013 cm-3) cp > 10 3 ohm cm. Depending on the purpose of K. in the process of preparing chlorides or during growth single crystals, dosed amounts of the necessary impurities are introduced into them. This is how cylindrical ingots are prepared with a diameter of 2-4 and a length of 3-10 cm. For special. targets also produce larger single crystals. Technical silicon, and especially it with iron, is used as steel deoxidizers and reducing agents, as well as alloying additives. Highly pure samples of single-crystal K., doped with various elements, are used as the basis for various low-current (in particular, thermoelectric, radio, lighting, and phototechnical) and high-current (rectifiers, converters) devices.
Silicon or silicon
Silicon is a non-metal, its atoms have 4 electrons at the outer energy level. It can donate them, showing the oxidation state + 4, and attach electrons, showing the oxidation state - 4. However, silicon's ability to accept electrons is much less than that of carbon. Silicon atoms have a larger radius than carbon atoms.
Finding silicon in nature
Silicon is very common in nature. it accounts for over 26% of the mass of the earth's crust. In terms of prevalence, it ranks second (after oxygen). Unlike carbon, C does not occur in the free state in nature. It is part of various chemical compounds, mainly of various modifications of silicon oxide (IV) and salts of silicic acids (silicates).
Getting silicon
In industry, silicon of technical purity (95 - 98%) is obtained by restoring SiO 2 coke in electric furnaces during calcination:
SiO 2 + 2C \u003d Si + 2CO
SiO 2 + 2Mg \u003d Si + 2MgO
In this way, an amorphous brown silicon powder with impurities is obtained. By recrystallization from molten metals (Zn, Al), it can be transferred to a crystalline state.
For semiconductor technology, silicon is very high purity obtained by reduction at 1000°C silicon tetrachloride SiCl 4 pairs of zinc:
SiCl 4 + 2Zn \u003d Si + 2ZnCl 2
and purifying it after that by special methods.
Physical and chemical properties of silicon
Pure crystalline silicon is brittle and hard, scratches. Like diamond, it has a cubic crystal lattice with a covalent bond type. Its melting point is 1423 °C. Under normal conditions, silicon is an inactive element, it combines only with fluorine, but when heated, it enters into various chemical reactions.
It is used as a valuable material in semiconductor technology. Compared to other semiconductors, it has a significant resistance to acids and the ability to maintain a high electrical resistance up to 300°C. Technical silicon and ferrosilicon are also used in metallurgy for the production of heat-resistant, acid-resistant and tool steels, cast irons and many other alloys.
With metals, silicon forms chemical compounds called silicides; when heated with magnesium, magnesium silicide is formed:
Si + 2Mg = Mg 2 Si
Metal silicides resemble carbides in structure and properties, so metal-like silicides, as well as metal-like carbides, are distinguished by high hardness, high melting point, and good electrical conductivity.
When a mixture of sand and coke is calcined in electric furnaces, silicon compounds with carbon are formed - silicon carbide, or carborundum:
SiO2 + 3C = SiC + 2CO
Carborundum is a refractory, colorless solid, valuable as an abrasive and heat-resistant material. Carborundum, like, has an atomic crystal lattice. In its pure state it is an insulator, but in the presence of impurities it becomes a semiconductor.
silicon as well as , forms two oxides: silicon oxide (II) SiO and silicon oxide (IV) SiO 2 . Silicon oxide (IV) is a solid refractory substance, widely distributed in nature in a free state. This chemically stable substance interacts only with fluorine and gaseous hydrogen fluoride or hydrofluoric acid:
SiO 2 + 2F 2 \u003d SiF 4 + O 2
SiO 2 + 4HF \u003d SiF 4 + 2H 2 O
The given direction of reactions is explained by the fact that silicon has a high affinity for fluorine. In addition, silicon tetrafluoride is a volatile substance.
In technology, transparent SiO 2 used for the manufacture of stable refractory quartz glass, which transmits ultraviolet rays well, has a large expansion coefficient, and therefore withstands significant instantaneous temperature changes. Amorphous modification of silicon oxide (II) tripoli - has a large porosity. It is used as a heat and sound insulator, for the production of dynamite (explosive carrier) and so on. Silicon (IV) oxide in the form of ordinary sand is one of the main building materials. It is used in the production of fire-resistant and acid-resistant materials, glass, as a flux in metallurgy and so on.
Comparable molecular formulas, chemical and physical properties carbon monoxide (IV) and silicon oxide (IV), it is easy to see that the properties of these chemically similar compounds are different. This is due to the fact that silicon (IV) oxide does not simply consist of SiO molecules 2 , but from their associates, in which silicon atoms are interconnected by oxygen atoms. Silicon (IV) oxide (SiO 2 )n . Its image on the plane is:
¦ ¦ ¦
o o o
¦ ¦ ¦
¦ ¦ ¦
o o o
¦ ¦ ¦
— O — Si — O — Si — O — Si — O —
¦ ¦ ¦
o o o
¦ ¦ ¦
Silicon atoms are located in the center of the tetrahedron, and oxygen atoms are located at its corners. Si-O bonds are very strong, which explains the high hardness of silicon oxide (IV).
According to its chemical properties, silicon (IV) oxide is an acidic oxide. It does not react directly with water, therefore, silicic acid can only be obtained indirectly, by acting on salts of silicic acid with hydrochloric or sulfuric acids.
