What are dielectrics and where are they used. Dielectric materials What substances are called dielectrics

Subdivided into 3 groups:

1) petroleum oils;

2) synthetic fluids;

3) vegetable oils.

Liquid dielectrics are used for impregnating high voltage cables, capacitors, for filling transformers, switches and bushings. In addition, they perform the functions of a coolant in transformers, an arc extinguisher in circuit breakers, etc.

Petroleum oils

Petroleum oils are a mixture of paraffinic hydrocarbons (С n Н 2 n + 2) and naphthenic (С n Н 2 n ) rows. They are widely used in electrical engineering as transformer, cable and capacitor oils. Oil, filling gaps and pores inside electrical installations and products, increases the dielectric strength of insulation and improves heat dissipation from products.

Transformer oil obtained from petroleum by distillation. The electrical properties of transformer oil largely depend on the quality of oil purification from impurities, water content and degree of degassing. Dielectric constant of oil 2.2, specific electrical resistance 10 13 Ohm M.

The purpose of transformer oils is to increase the dielectric strength of the insulation; remove heat; promote arcing in oil breakers, improve quality electrical insulation in electrical products: rheostats, paper capacitors, cables with paper insulation, power cables - by pouring and impregnating.

Transformer oil ages during operation, which deteriorates its quality. Oil aging is facilitated by: contact of oil with air, elevated temperatures, contact with metals (Cu, Pb, Fe), exposure to light. To increase the service life, the oil is regenerated by cleaning and removing aging products, adding inhibitors.

Cableand capacitor oils differ from transformer oils in higher purification quality.

Synthetic liquid dielectrics

In some properties, synthetic liquid dielectrics are superior to petroleum insulating oils.

Chlorinated hydrocarbons

Sovol – pentachlorodiphenylС 6 Н 2 Сl 3 - С 6 Н 3 Сl 2 , obtained by chlorination of biphenyl C 12 H 10

С 6 Н 5 - С 6 Н 5 + 5 Сl 2 → С 6 Н 2 Сl 3 - С 6 Н 3 Сl 2 + 5 НСl

Sovolused for impregnation and filling of capacitors. Has a higher dielectric constant compared to petroleum oils. Dielectric constant of sovol 5.0, specific electrical resistance 10 11 ¸ 10 12 Ohm m., sovol is used for impregnating paper power and radio capacitors with increased specific capacity and low operating voltage.

Sovtol - a mixture of owl with trichlorobenzene... Used to isolate explosion-proof transformers.

Organosilicon liquids

The most widespread are polydimethylsiloxane, polydiethylsiloxane, polymethylphenylsiloxane liquids.

Polysiloxane liquids - liquid organosilicon polymers ( polyorganosiloxanes), possess such valuable properties as: high heat resistance, chemical inertness, low hygroscopicity, low pour point, high electrical characteristics in a wide range of frequencies and temperatures.

Liquid polyorganosiloxanes are polymeric compounds with a low degree of polymerization, the molecules of which contain a siloxane group of atoms

where silicon atoms are bonded to organic radicals R: methyl CH 3, ethyl C 2 H 5, phenyl C 6 H 5 ... Molecules of organopolysiloxane liquids can have a linear, linear-branched and cyclic structure.

Liquid polymethylsiloxanes obtained by hydrolysis dimethyldichlorosilane mixed with trimethylchlorosilane .

The resulting liquids are colorless, dissolve in aromatic hydrocarbons, dichloroethane and a number of other organic solvents, do not dissolve in alcohols and acetone. Polymethylsiloxanes are chemically inert, do not have an aggressive effect on metals and do not interact with most organic dielectrics and rubbers. Dielectric constant 2.0¸ 2.8, resistivity 10 12 Ohm m, dielectric strength 12¸ 20 MV / m

Formula polydimethylsiloxane a has the form

– Si(CH 3) 3 - O - [ Si(CH 3) 2 - O] n -Si(CH 3) = O

Liquid organosilicon polymers are used as:

Polydiethylsiloxanes – obtained by hydrolysis diethyldichlorosilane and triethylchlorosilane ... They have a wide boiling range. The structure is expressed by the formula:

Properties depend on the boiling point. The electrical properties are the same as the properties polydimethylsiloxane.

Liquid polymethylphenylsiloxanes have a structure expressed by the formula

Obtained by hydrolysis phenylmethyldichlorosilanes and others. Viscous oil. After processingNaOHthe viscosity increases 3 times. Withstands heating for 1000 hours up to 250 ° С. The electrical properties are the same as the properties polydimethylsiloxane.

At γ - upon irradiation, the viscosity of organosilicon liquids greatly increases, and the dielectric characteristics deteriorate sharply. At a high dose of radiation, liquids turn into rubbery mass, and then into a solid brittle body.

Organofluorine liquids

Organofluorine liquids - C 8 F 16 - non-flammable and explosion-proof, highly heat-resistant(200 ° C), have low hygroscopicity. Their pairs have high electrical strength. The fluids are low viscosity and volatile. They have better heat dissipation than petroleum oils and organosilicon fluids.–) n,

is a non-polar polymer of linear structure. Obtained by polymerizing ethylene gas C 2 H 4 at high pressure (up to 300 MPa), or at low pressure (up to 0.6 MPa). The molecular weight of high-pressure polyethylene is 18,000 - 40,000, low-density polyethylene - 60,000 - 800,000.

Polyethylene molecules have the ability to form areas of the material with an ordered arrangement of chains (crystallites); therefore, polyethylene consists of two phases (crystalline and amorphous), the ratio of which determines its mechanical and thermal properties. Amorphous gives the material elastic properties, and crystalline - rigidity. The amorphous phase has a glass transition temperature of +80 ° C. The crystalline phase has a higher heat resistance.

Aggregates of polyethylene molecules of the crystalline phase are spherulites with an orthorhombic structure. The content of the crystalline phase (up to 90%) in low-pressure polyethylene is higher than in high-pressure polyethylene (up to 60%). Due to its high crystallinity, low-pressure polyethylene has a higher melting point (120 -125 ° C) and a higher tensile strength. The structure of polyethylene is highly dependent on the cooling mode. With its rapid cooling, small spherulites are formed, with slow cooling, large ones. Rapidly cooled polyethylene is more flexible and less hard.

The properties of polyethylene depend on molecular weight, purity, and impurities. Mechanical properties depend on the degree of polymerisation. Polyethylene has great chemical resistance. As an electrical insulating material, it is widely used in the cable industry and in the production of insulated wires.

Currently, the following types of polyethylene and polyethylene products are manufactured:

1. low and high pressure polyethylene - (n.d.) and (h.d.);

2. low pressure polyethylene for the cable industry;

3. low-molecular-weight polyethylene of high or medium pressure;

4. porous polyethylene;

5. special polyethylene hose compound;

6. polyethylene for the production of HF cables;

7. electrically conductive polyethylene for the cable industry;

8. polyethylene filled with soot;

9. chlorosulfonated polyethylene;

10. polyethylene film.

Fluoroplastics

There are several types of fluorocarbon polymers that can be polar or non-polar.

Consider the properties of the product of the polymerization reaction of tetrafluoroethylene gas

(F 2 C = CF 2).

Fluoroplast - 4(polytetrafluoroethylene) is a loose white powder. The molecular structure is

Fluoroplastic molecules have a symmetrical structure. Therefore, fluoroplastic is a non-polar dielectric

Molecular symmetry and high purity provide a high level of electrical performance. Greater bond energy between C and F gives it high cold resistance and heat resistance... Radio components from it can operate from -195 ÷ + 250 ° С. Non-flammable, chemically resistant, non-hygroscopic, hydrophobic, not affected by mold. The specific electrical resistance is 10 15 ¸ 10 18 Ohm m, dielectric constant 1.9¸ 2.2, dielectric strength 20¸ 30 MV / m

Radio parts are made of fluoroplastic powder by cold pressing. Pressed products are sintered in ovens at 360 - 380 ° C. Upon rapid cooling, the products are hardened with high mechanical strength. When cooled slowly, they are not hardened. They are easier to handle, less hard, and have a high level of electrical performance. When the parts are heated to 370 °, they change from the crystalline state to the amorphous state and acquire transparency. Thermal decomposition of the material begins at> 400 °. Wherein toxic fluorine is formed.

The disadvantage of fluoroplastic is its fluidity under mechanical stress. It has low resistance to radiation and is laborious when processed into products. One of the best dielectrics for RF and microwave technology. Produce electrical and radio engineering products in the form of plates, disks, rings, cylinders. They insulate HF cables with a thin film, which are sealed during shrinkage.

Fluoroplastic can be modified using fillers - fiberglass, boron nitride, carbon black, etc., which makes it possible to obtain materials with new properties and improve existing properties.

Classification by molecular structure

Chemical classification

Classification by production method

Physical state classification

Active and passive dielectrics

Determination of dielectric materials

Classification and fields of use of dielectric materials

Dielectrics are substances whose main electrical property is the ability to polarize in an electric field.

Electrical insulating materials are dielectric materials designed to create electrical insulation of live parts of electrical installations.

An insulator is a product made of electrical insulating material, the tasks of which are to fasten and isolate conductors from each other at different potentials (for example, insulators of an overhead power line).

Electrical insulation refers to the electrical insulation system of a particular specific electrical product, made of one or more electrical insulating materials.

Dielectrics used as insulating materials are called passive dielectrics. Currently, the so-called active dielectrics are widely used, the parameters of which can be controlled by changing the electric field strength, temperature, mechanical stresses and other parameters of the factors affecting them.

For example, a capacitor, in which a piezoelectric serves as a dielectric material, changes its linear dimensions under the action of an applied alternating voltage and becomes a generator of ultrasonic vibrations. The capacity of an electric capacitor made of a nonlinear dielectric - ferroelectric varies depending on the strength of the electric field; if such a capacitance is included in an oscillatory LC circuit, then its tuning frequency also changes.

Dielectric materials are classified:

According to the state of aggregation: gaseous, liquid and solid;

By the method of production: natural and synthetic;

By chemical composition: organic and inorganic;

Molecular structure: neutral and polar.

GASEOUS DIELECTRICS

Gaseous dielectrics include: air, nitrogen, hydrogen, carbon dioxide, SF6 gas, freon (freon), argon, neon, helium, etc. They are used in the manufacture of electrical devices (air and SF6 switches, arresters)

The most widely used electrical insulating material is air. Air contains: water vapor and gases: nitrogen (78%), oxygen (20.99%), carbon dioxide (0.03%), hydrogen (0.01%), argon (0.9325%), neon (0 , 0018%), as well as helium, krypton, and xenon, which in total amount to ten-thousandths of a percent.

Important properties of gases are their ability to restore electrical strength, low dielectric constant, high resistivity, virtually no aging, inertness of a number of gases in relation to solid and liquid materials, non-toxicity, their ability to work at low temperatures and high pressure, incombustibility.

LIQUID DIELECTRICS

Liquid dielectrics are designed to remove heat from windings and magnetic circuits in transformers, extinguish the arc in oil circuit breakers, reinforce solid insulation in transformers, oil-filled bushings, capacitors, oil-impregnated and oil-filled cables.

Liquid dielectrics are divided into two groups:

Petroleum oils (transformer, condenser, cable);

Synthetic oils (sovtol, liquid organosilicon and organofluorine compounds).

4.1.7 Areas of use of dielectrics as ETM

Application in the electric power industry:

- linear and substation insulation- this is porcelain, glass and organosilicon rubber in overhead line insulators, porcelain in support and bushing insulators, fiberglass as load-bearing elements, polyethylene, paper in high-voltage bushings, paper, polymers in power cables;

- insulation of electrical appliances- paper, getinax, fiberglass, polymers, mica materials;

- machines, apparatus- paper, cardboard, varnishes, compounds, polymers;

- capacitors of different types- polymer films, paper, oxides, nitrides.

From a practical point of view, in each case of choosing a material for electrical insulation, it is necessary to analyze the working conditions and select the insulation material in accordance with a set of requirements. For orientation, it is advisable to divide the main dielectric materials into groups according to the conditions of use.

1. Heat-resistant electrical insulation. These are primarily products made of mica materials, some of which are capable of operating up to temperatures of 700 ° C. Glasses and materials based on them (glass fabrics, glass mica). Organosilicate and metal phosphate coatings. Ceramic materials such as boron nitride. Compositions of organosilicon with a heat-resistant binder. Of polymers, polyimide and fluoroplastic have high heat resistance.

2. Moisture proof electrical insulation. These materials must be hydrophobic (non-wetting with water) and non-hygroscopic. Fluoroplastic is a striking representative of this class. In principle, hydrophobization is possible by creating protective coatings.

3. Radiation resistant insulation. These are, first of all, inorganic films, ceramics, fiberglass, mica materials, some types of polymers (polyimides, polyethylene).

4. Tropic-resistant insulation. The material must be hydrophobic in order to work in high humidity and temperature conditions. In addition, it must be mold resistant. The best materials: fluoroplastic, some other polymers, the worst - paper, cardboard.

5. Frost-resistant insulation. This requirement is typical mainly for rubbers, because when the temperature drops, all rubbers lose their elasticity. The most frost-resistant is organosilicon rubber with phenyl groups (up to -90 ° C).

6. Insulation for work in vacuum (space, vacuum devices). For these conditions, it is necessary to use vacuum-tight materials. Some specially prepared ceramic materials are suitable, polymers are of little use.

Electrical board used as dielectric spacers, washers, spacers, as insulation of magnetic circuits, groove insulation of rotating machines, etc. Cardboard is usually used after being impregnated with transformer oil. The electric strength of the impregnated cardboard reaches 40-50 kV / mm. Since it is higher than the strength of transformer oil, to increase the electrical strength of transformers, special cardboard barriers are often arranged in the oil environment. Oil barrier insulation usually has a strength of E = 300-400 kV / cm. The disadvantage of cardboard is hygroscopicity, as a result of moisture ingress, the mechanical strength decreases and the electrical strength decreases sharply (4 or more times).

Recently, the production of insulators for overhead lines based on silicone rubber... This material belongs to rubbers, the main property of which is elasticity. This makes it possible to manufacture from rubbers not only insulators, but also flexible cables. Different types of rubbers are used in the energy sector: natural rubbers, butadiene rubbers, styrene butadiene rubbers, ethylene propylene rubbers and organosilicon rubbers.

Electrical porcelain is an artificial mineral formed from clay minerals, feldspar and quartz as a result of heat treatment using ceramic technology. Among its most valuable properties are high resistance to weathering, positive and negative temperatures, to the effects of chemical reagents, high mechanical and electrical strength, low cost of the initial components. This determined the widespread use of porcelain for the production of insulators.

Electrical glass as a material for insulators, it has some advantages over porcelain. In particular, it has a more stable raw material base, a simpler technology that allows for greater automation, the ability to visually inspect defective insulators.

Mica is the basis of a large group of electrical insulating products. The main advantage of mica is its high thermal resistance along with sufficiently high electrical insulating characteristics. Mica is a natural mineral with a complex composition. In electrical engineering, two types of mica are used: muscovite KAl 2 (AlSi 3 O 10) (OH) 2 and phlogopite KMg 3 (AlSi 3 O 10 (OH) 2. High electrical insulating characteristics of mica are due to its unusual structure, namely, layering. can be split into flat plates up to submicron sizes. Destructive stresses at separation of one layer from another layer are about 0.1 MPa, while stretching along the layer - 200-300 MPa. Among other properties of mica, we note a low tg, less than 10 -2; high resistivity, more than 10 12 Ohm · m; sufficiently high dielectric strength, more than 100 kV / mm; heat resistance, melting temperature over 1200 ° C.

Mica is used as electrical insulation, in the form of plucked thin plates, incl. glued together (micanites), and in the form of mica papers, incl. impregnated with various binders (mica or mica plastics). Mica paper is produced using a technology close to that of ordinary paper. The mica is crushed, a pulp is prepared, sheets of paper are rolled out on paper machines.

Mikanites have better mechanical characteristics and moisture resistance, but they are more expensive and less technological. Application - groove and coil insulation of electrical machines.

Slyudinites - sheet materials made from muscovite-based mica paper. Sometimes they are combined with a substrate made of glass cloth (glass mica), or a polymer film (film mica). Papers impregnated with varnish or other binder have better mechanical and electrical characteristics than untreated papers, but their heat resistance is usually lower because it is determined by the properties of the impregnating binder.

Mica plastics - sheet materials made of phlogopite-based mica paper and impregnated with binders. Like mica, they can also be combined with other materials. Compared to mica, they have slightly worse electrophysical characteristics, but are less expensive. Application of mica and mica plastics - insulation of electrical machines, heat-resistant insulation of electrical devices.

The greatest use of gases in the power industry is air. This is due to the low cost, general availability of air, ease of creation, maintenance and repair of air electrical insulating systems, the possibility of visual control. Facilities that use air as electrical insulation - power lines, outdoor switchgear, air circuit breakers, etc.

Of the electronegative gases with high electrical strength, the most widely used SF6 gas.... It got its name from the abbreviation "electric gas". The unique properties of SF6 gas were discovered in Russia, and its use also began in Russia. In the 30s, the famous scientist B.M. Gokhberg investigated the electrical properties of a number of gases and drew attention to some of the properties of sulfur hexafluoride SF6. The dielectric strength at atmospheric pressure and a gap of 1 cm is E = 89 kV / cm. The molecular weight is 146, it is characterized by a very high coefficient of thermal expansion and high density. This is important for power plants in which cooling of any part of the device is carried out, because with a large coefficient of thermal expansion, a convective flow is easily formed that carries away heat. From thermophysical properties: melting point = -50 ° C at 2 atm, boiling point (sublimation) = -63 ° C, which means the possibility of using at low temperatures.

Among other useful properties, we note the following: chemical inertness, non-toxicity, incombustibility, heat resistance (up to 800 ° C), explosion safety, weak decomposition in discharges, low liquefaction temperature. In the absence of impurities, SF6 is completely harmless to humans. However, the decomposition products of SF6 gas as a result of the action of discharges (for example, in an arrester or circuit breaker) are toxic and reactive. The complex of properties of SF6 gas ensured a fairly widespread use of SF6 insulation. In devices, SF6 gas is usually used under a pressure of several atmospheres for greater compactness of power plants, since dielectric strength increases with increasing pressure. On the basis of SF6 insulation, a number of electrical devices have been created and are being operated, including cables, capacitors, switches, compact closed switchgears (closed switchgears).

The most common liquid dielectric in the power industry is transformer oil.

Transformer oil- refined oil fraction obtained during distillation, boiling at temperatures from 300 ° C to 400 ° C. Depending on the origin of oil, they have different properties and these distinctive properties of the feedstock are reflected in the properties of the oil. It has a complex hydrocarbon composition with an average molecular weight of 220-340 a.u. and contains the following main components.

Capacitor and cable oils should be noted among those related to transformer oil in terms of properties and application of liquid dielectrics.

Condenser oils. This term combines a group of different dielectrics used for impregnation of paper-oil and paper-film insulation of capacitors. The most common condenser oil according to GOST 5775-68, they are produced from transformer oil by deeper purification. It differs from conventional oils in greater transparency, lower tan  value (more than ten times). Castor oil of vegetable origin, it is obtained from the seeds of the castor-bean plant. The main area of use is impregnation of paper capacitors for operation in impulse conditions.
The density of castor oil is 0.95-0.97 t / m3, the pour point is from -10 ° C to -18 ° C. Its dielectric constant at 20 ° C is 4.0 - 4.5, and at 90 ° C -  = 3.5 - 4.0; tg  at 20 ° C is 0.01-0.03, and at 100 ° C tg = 0.2-0.8; Ep at 20 ° C is equal to 15-20 MV / m. Castor oil does not dissolve in gasoline, but it does dissolve in ethyl alcohol. Unlike petroleum oils, castor oils do not swell in ordinary rubber. This dielectric belongs to weakly polar liquid dielectrics, its resistivity under normal conditions is 108 - 1010 Ohm · m.

Cable oils are intended for impregnation of paper insulation of power cables. They are also based on petroleum oils. They differ from transformer oil with increased viscosity, increased flash point and reduced dielectric losses. Among the brands of oils, we note MN-4 (low-viscosity, for filling low-pressure cables), S-220 (high-viscosity for filling high-pressure cables), KM-25 (the most viscous).

The second type of liquid dielectrics is hardly combustible and non-combustible liquids. There are a lot of liquid dielectrics with such properties. The most widespread in energy and electrical engineering are chlordiphenyls... In foreign literature, they are called chlorobiphenyls... These are substances that contain a double benzene ring, the so-called. a di (bi) phenyl ring and one or more chlorine atoms attached to it. In Russia, dielectrics of this group are used in the form of mixtures, mainly mixtures of pentachlorobiphenyl with trichlorobiphenyl. The commercial names of some of them are “sovol”, “sovtol”, “calorie-2”.

Dielectric materials are classified according to a number of intraspecific characteristics, which are determined by their main characteristics: electrical, mechanical, physicochemical, thermal.

4.2.1 The electrical characteristics of dielectric materials include:

Specific volumetric electrical resistance ρ, Ohm * m or specific volumetric conductivity σ, S / m;

Specific surface electrical resistance ρ s, Ohm, or specific surface conductivity σ s cm;

Temperature coefficient of specific electrical resistance TK ρ, ˚С -1;

Dielectric constant ε;

Temperature coefficient of dielectric constant ТКε;

Dielectric loss tangent δ;

The dielectric strength of the material is E pr, MV / m.

4.2.2 Thermal performance determines the thermal properties of dielectrics.

Thermal performance includes:

Heat capacity;

Melting temperature;

Softening temperature;

Dropping point;

Heat resistance;

Cold resistance - the ability of dielectrics to withstand low temperatures, while maintaining electrical insulating properties;

Tropic resistance - the resistance of dielectrics to a complex of external influences in a tropical climate (sharp temperature drop, high humidity, solar radiation);

Thermoelasticity;

Flash point of vapors of electrical insulating liquids.

Heat resistance is one of the most important characteristics of dielectrics. In accordance with GOST 21515-76, heat resistance is the ability of a dielectric to withstand high temperatures for a long time for a time comparable to the period of normal operation, without unacceptable deterioration of its properties.

Heat resistance classes. Only seven. They are characterized by the temperature index TI. This is the temperature at which the service life of the material is 20 thousand hours.

4.2.3 Moisture properties of dielectrics

Moisture resistance is the reliability of the insulation operation when it is in an atmosphere of water vapor close to saturation. Moisture resistance is assessed by the change in electrical, mechanical and other physical properties after finding the material in an atmosphere with high and high humidity; by moisture and water permeability; by moisture and water absorption.

Moisture permeability - the ability of a material to pass moisture vapor in the presence of a difference in the relative humidity of the air on both sides of the material.

Moisture absorption - the ability of a material to absorb water during prolonged exposure to a humid atmosphere close to saturation.

Water absorption - the ability of a material to absorb water when it is immersed in water for a long time.

Tropic resistance and tropicalization of equipment - protection of electrical equipment from moisture, mold, rodents.

4.2.4 The mechanical properties of dielectrics are determined by the following characteristics:

Destructive stress under static tension;

Destructive stress in static compression;

Destructive stress in static bending;

Hardness;

Impact strength;

Splitting resistance;

Tear resistance (for flexible materials);

Flexibility in the number of double bends;

Plastoelastic properties.

The mechanical characteristics of dielectrics are determined by the corresponding GOSTs.

4.2.5 Physical and chemical characteristics:

Acid number, which determines the amount of free acids in the dielectric, which worsen the dielectric properties of liquid dielectrics, compounds and varnishes;

Kinematic and conditional viscosity;

Water absorption;

Water resistance;

Moisture resistance;

Arc resistance;

Tracking resistance;

Radiation resistance, etc.

Lecture 1.3.1. Dielectric polarization

Dielectric materials

Dielectrics are substances that can polarize and maintain an electrostatic field. This is a wide class of electrical materials: gaseous, liquid and solid, natural and intellectual, organic, inorganic and organoelement. According to the functions they perform, they are divided into passive and active. Passive dielectrics are used as electrical insulating materials. In active dielectrics (ferroelectrics, piezoelectrics, etc.), electrical properties depend on control signals that can change the characteristics of electrical devices and devices.

According to the electrical structure of molecules, non-polar and polar dielectrics are distinguished. Nonpolar dielectrics are composed of nonpolar (symmetric) molecules in which the centers of positive and negative charges coincide. Polar dielectrics are composed of asymmetric molecules (dipoles). A dipole molecule is characterized by a dipole moment - p.

In the process of operation of electrical devices, the dielectric heats up, since part of the electrical energy in it is dissipated in the form of heat. Dielectric losses are highly dependent on the frequency of the current, especially in polar dielectrics, so they are low-frequency. Non-polar dielectrics are used as high-frequency ones.

The main electrical properties of dielectrics and their characteristics are given in table. 3.

Table 3 - Electrical properties of dielectrics and their characteristics

Polarization is the limited displacement of bound charges or the orientation of dipole molecules in an electric field. Under the influence of the lines of force of the electric field, the charges of the dielectric are displaced in the direction of the acting forces, depending on the magnitude of the intensity. In the absence of an electric field, the charges return to their previous state.

There are two types of polarization: instantaneous polarization, quite elastic, without release of scattering energy, i.e. without heat release, during 10 -15 - 10 -13 s; polarization does not occur instantly, but increases or decreases slowly and is accompanied by energy dissipation in the dielectric, i.e. it heats up - this is a relaxation polarization for a time from 10 -8 to 10 2 s.

The first type includes electronic and ionic polarization.

Electronic polarization (C e, Q e)- elastic displacement and deformation of the electron shells of atoms and ions during 10 -15 s. Such polarization is observed for all types of dielectrics and is not associated with a loss of energy, and the dielectric constant of a substance is numerically equal to the square of the refractive index of light n 2.

Ionic polarization (C u, Q u) characteristic of solids with an ionic structure and is caused by the displacement (vibration) of elastically bound ions in the nodes of the crystal lattice for a time of 10 -13 s. As the temperature rises, the displacement also increases as a result of the weakening of the elastic forces between the ions, and the temperature coefficient of the dielectric constant of ionic dielectrics turns out to be positive.

The second type includes all relaxation polarizations.

Dipole relaxation polarization (C dr, r dr, Q dr) associated with the thermal motion of the dipoles with a polar bond between molecules. The rotation of the dipoles in the direction of the electric field requires overcoming some resistance, the release of energy in the form of heat (r etc). The relaxation time here is of the order of 10 -8 - 10 -6 s - this is the time interval during which the ordering of the dipoles oriented by the electric field after the field is removed will decrease due to the presence of thermal motions 2.7 times from the initial value.

Ion relaxation polarization (C ir, r ir, Q ir) observed in inorganic glasses and in some substances with loose packing of ions. Weakly bound ions of a substance, under the influence of an external electric field, among chaotic thermal motions, receive excess surges in the direction of the field and are displaced along its line of force. After removing the electric field, the orientation of the ions weakens exponentially. Relaxation time, activation energy and natural frequency occurs within 10 -6 - 10 -4 s and is related by the law

where f is the frequency of natural vibrations of particles; v is the activation energy; k is the Boltzmann constant (8.63 10 -5 EV / deg); T is the absolute temperature according to K 0.

Electronic relaxation polarization (C er, r er, Q er) arises due to the excited thermal energies of excess, defective electrons or "holes" in a time of 10 -8 - 10 -6 s. It is characteristic of dielectrics with high refractive indices, a large internal field and electronic conductivity: titanium dioxide with impurities, Ca + 2, Ba + 2, a number of compounds based on metal oxides of variable valence - titanium, niobium, bismuth. With this polarization, there is a high dielectric constant and at negative temperatures there is a maximum in the temperature dependence of e (dielectric constant). e for titanium-containing ceramics decreases with increasing frequency.

Structural polarizations distinguish between:

Migration polarization (C m, r m, Q m) proceeds in solids of an inhomogeneous structure with macroscopic inhomogeneities, layers, interfaces, or the presence of impurities in a time of the order of 10 2 s. This polarization manifests itself at low frequencies and is associated with significant energy dissipation. The reasons for this polarization are conductive and semiconducting inclusions in technical, complex dielectrics, the presence of layers with different conductivity, etc. At the interfaces between the layers in the dielectric and in the electrode layers, the charges of slowly moving ions are accumulated - this is the effect of interlayer or structural high-voltage polarization. For ferroelectrics, a distinction is made between spontaneous or spontaneous polarization, (C cn, r cn, Q cn), when there is a significant dissipation of energy or the release of heat due to domains (separate regions of rotating electron shells) displaced in an electric field, that is, even in the absence of an electric field, there are electric moments in the substance, and at a certain strength of the external field saturation occurs and increase in polarization.

Classification of dielectrics by the type of polarization.

The first group consists of dielectrics with instantaneous electronic and ionic polarizations. The structure of such materials consists of neutral molecules, can be weakly polar and is characteristic of solid crystalline and amorphous materials such as paraffin, sulfur, polystyrene, as well as liquid and gaseous materials such as benzene, hydrogen, etc.

The second group - dielectrics with electronic and dipole-relaxation polarizations - are polar organic liquid, semi-liquid, solid substances such as oil rosin compounds, epoxy resins, cellulose, chlorinated hydrocarbons, etc. materials.

The third group - solid inorganic dielectrics, which are divided into two subgroups, differing in electrical characteristics - a) dielectrics with electronic and dipole relaxation polarizations, such as quartz, mica, rock salt, corundum, rutile; b) dielectrics with electronic and ionic relaxation polarizations are glasses, materials with a glassy phase (porcelain, micallex, etc.) and crystalline dielectrics with loose packing of ions.

The fourth group is dielectrics with electronic and ionic instantaneous and structural polarizations, which is characteristic of many positional, complex, layered and ferroelectric materials.

The dielectric constant can be dispersive.

A number of dielectrics exhibit interesting physical properties.

Links

Virtual Fund of Natural Science and Scientific and Technical Effects "Effective Physics"

Wikimedia Foundation. 2010.

See what "Dielectrics" are in other dictionaries:

DIELECTRICS, substances that poorly conduct electric current (resistivity of the order of 1010 Ohm? M). There are solid, liquid and gaseous dielectrics. An external electric field causes polarization of the dielectric. In some solid ... ... Modern encyclopedia

Dielectrics- DIELECTRICS, substances that poorly conduct electric current (resistivity of the order of 1010 Ohm'm). There are solid, liquid and gaseous dielectrics. An external electric field causes polarization of the dielectric. In some solid ... ... Illustrated Encyclopedic Dictionary

Substances that are poorly conductive of electric current (specific electrical resistance 108 1012 Ohm? Cm). There are solid, liquid and gaseous dielectrics. An external electric field causes polarization of dielectrics. In some solid dielectrics ... ... Big Encyclopedic Dictionary

- (English dielectric, from the Greek dia through, through and English electric electric), substances that poorly conduct electricity. current. The term "D." introduced by Faraday to designate in in, in which electric penetrates. field. D. yavl. all gases (non-ionized), some ... Physical encyclopedia

DIELECTRICS- DIELECTRICS, non-conductors, or insulators of the body, poorly conducting or not conducting electricity at all. Such bodies are, for example. glass, mica, sulfur, paraffin, ebonite, porcelain, etc. For a long time in the study of electricity ... ... Great medical encyclopedia

- (insulators) non-conductive substances. Examples of dielectrics: mica, amber, rubber, sulfur, glass, porcelain, various types of oils, etc. Samoilov KI Marine dictionary. M. L .: State Naval Publishing House of the Union NKVMF ... Marine Dictionary

The name given by Michael Faraday to non-conductive bodies, otherwise, poorly conductive electricity, such as air, glass, various resins, sulfur, etc. Such bodies are also called insulators. Before Faraday's research carried out in the 30s ... ... Encyclopedia of Brockhaus and Efron

DIELECTRICS- Substances that practically do not conduct electric current; are solid, liquid and gaseous. In an external electric field, D. are polarized. They are used to isolate electrical devices, in electrical capacitors, in quantum ... ... Big Polytechnic Encyclopedia

Substances that do not conduct electric current well. The term "D." (from the Greek diá through and English electric electric) was introduced by M. Faraday (See Faraday) to denote substances through which electric fields penetrate. In any substance, ... ... Great Soviet Encyclopedia

Substances, poorly conducting electric current (dielectric conductivity 10 8 10 17 Ohm 1 · cm 1). There are solid, liquid and gaseous dielectrics. An external electric field causes polarization of dielectrics. In some solid ... ... encyclopedic Dictionary

Books

Dielectrics and Waves, A.R. Hippel. The author of the monograph offered to the readers of the monograph, a well-known researcher in the field of dielectrics, American scientist A. Hippel has repeatedly appeared in periodicals and in ...
Effect of laser radiation on polymer materials. Scientific foundations and applied problems. In 2 books. Book 1. Polymer materials. Scientific foundations of laser action on polymer dielectrics, BA Vinogradov, KE Perepelkin, GP Meshcheryakova. The proposed book contains information about the structure and basic thermal and optical properties of polymeric materials, the mechanism of exposure to laser radiation in infrared, visible ...

Dielectrics- these are substances that do not conduct electric current, up to a certain pore. Under certain conditions, conductivity arises in them. These conditions are mechanical, thermal - in general, energy types of influences. Besides dielectrics, substances are also classified into conductors and semiconductors.

How dielectrics differ from conductors and semiconductors

The theoretical difference between these three types of materials can be imagined, and I will do it, in the picture below:

The drawing is beautiful, familiar from school, but you can't really draw something practical from it. However, this graphic masterpiece clearly defines the difference between conductor, semiconductor and dielectric.

And the difference is in the size of the energy barrier between the valence band and the conduction band.

In conductors, electrons are in the valence band, but not all, since the valence band is the outermost boundary. Exactly, it's like with migrants. The conduction zone is empty, but it is glad to guests, since it has a lot of free jobs for them in the form of free energy zones. When exposed to an external electric field, the outermost electrons acquire energy and move to the free levels of the conduction band. We also call this movement electric current.

In dielectrics and conductors, everything is the same, except that there is a “fence” - a forbidden zone. This band is located between the valence and conduction bands. The larger this zone, the more energy is required for electrons to cover this distance. Dielectrics have a larger band than semiconductors. There is even a condition for this: if dE> 3Ev () - then it is a dielectric, in the opposite case, dE

Types and types of dielectrics

The classification of dielectrics is quite extensive. There are liquid, solid and gaseous substances. Then they are divided according to certain criteria. Below is a conditional classification of dielectrics with examples in the form of a list.

gaseous

- polar
- non-polar (air,)

liquid

- polar (water, ammonia)

- liquid crystals

- non-polar (benzene,)

solid

- centrosymmetric

- amorphous

- resins, bitumens (epoxy resin)
- glass
- disordered polymers

- polycrystals

- irregular crystals
- ceramics
- ordered polymers
- sitalls

- single crystals

- molecular
- covalent
- ionic

- displacement paraelectrics
- paraelectrics "order-disorder"

- dipole

- noncentrosymantic

- single crystals

- pyroelectrics

- displacement ferroelectrics
- ferroelectrics "order-disorder"
- linear pyroelectrics

- piezoelectrics

- with hydrogen bonds
- covalent
- ionic

- textures

- electronic defects
- ionic defects
- polar molecules
- macrodipoles
- ferroelectric domains
- crystals in the matrix

If we take liquid and gaseous dielectrics, then the main classification lies in the issue of polarity. The difference is in the symmetry of the molecules. In polar molecules, they are asymmetric, in non-polar ones, they are symmetric. Asymmetric molecules are called dipoles. In polar liquids, the conductivity is so great that they cannot be used as insulating substances. Therefore, for these purposes, non-polar, also transformer oil, is used. And the presence of polar impurities, even in hundredths, significantly lowers the breakdown bar and negatively affects the insulating properties of non-polar dielectrics.

crystals are a cross between liquid and crystal, as the name suggests.

Another popular question about the properties and application of liquid dielectrics will be the following: Is water a dielectric or a conductor? Pure distilled water contains no impurities that could cause current to flow. Pure water can be created in laboratory, industrial conditions. These conditions are complex and difficult to fulfill for the average person. There is an easy way to check if distilled water is conductive.

Create an electrical circuit (current source - wire - water - wire - light bulb - another wire - current source), in which a vessel with distilled water will be one of the sections for the current to flow. When the circuit is turned on, the light will not light up - therefore, the current does not pass. Well, if it lights up, it means water with impurities.

Therefore, any water that we meet: from a tap, in a lake, in a bathroom - will be a conductor due to impurities that create an opportunity for current to flow. Do not swim in a thunderstorm, do not work with wet hands with electricity. Although pure distilled water is a polar dielectric.

For solid dielectrics, the classification mainly lies in the issue of activity and passivity or something. If the properties are constant, then the dielectric is used as an insulating material, that is, it is passive. If the properties change, depending on external influences (heat, pressure), then this dielectric is used for other purposes. Paper is a dielectric, if water is saturated with water, then the current is conducted and it is a conductor, if the paper is saturated with transformer oil, then it is a dielectric.

Foil is a thin metal plate, metal is known to be a conductor. For example, PVC foil is on sale, here the word is foil for clarity, and the word PVC is for understanding the meaning - after all, PVC is a dielectric. Although in Wikipedia - a thin sheet of metal is called a foil.

Amorphous liquids- this is resin, glass, bitumen, and wax. As the temperature rises, this dielectric melts, these are frozen substances - these are wild definitions that characterize only one facet of the truth.

Polycrystals- these are, as it were, accreted crystals, united into one crystal. For example, salt.

Monocrystal- this is a solid crystal, in contrast to the aforementioned polycrystal having a continuous crystal lattice.

Piezoelectrics- dielectrics, in which, under mechanical action (tension-compression), an ionization process occurs. It is used in lighters, detonators, ultrasound examination.

Pyroelectrics- when the temperature changes in these dielectrics, spontaneous polarization occurs. It also occurs under mechanical action, that is, pyroelectrics are also piezoelectrics, but not vice versa. Examples are amber and tourmaline.

Physical properties of dielectrics

To assess the quality and degree of suitability of a dielectric, it is necessary to somehow describe its parameters. If you follow these parameters, you can prevent an accident in time by replacing the element with a new one with acceptable parameters. These parameters are: polarization, electrical conductivity, dielectric strength and dielectric loss. For each of these parameters, there is its own formula and constant value, in comparison with which a conclusion is made about the degree of suitability of the material.

The main electrical properties of dielectrics are polarization (displacement of charges) and electrical conductivity (the ability to conduct electric current). The displacement of the bound charges of a dielectric or their orientation in an electric field is called polarization. This property of dielectric materials is characterized by a relative dielectric constant ε ... When polarized, bound electric charges are formed on the surface of the dielectric.

Depending on the type of dielectric, polarization can be: electronic, ionic, dipole-relaxation, spontaneous. More details about their properties on the infographic below.

Electrical conductivity is understood as the ability of a dielectric to conduct electric current. The current flowing in the dielectric is called the leakage current. The leakage current consists of two components - the absorption current and the through current. Through currents are due to the presence of free charges in the dielectric, the absorption current is due to polarization processes until equilibrium is established in the system.

The amount of electrical conductivity depends on temperature, humidity and the amount of free charge carriers.

With increasing temperature, the electrical conductivity of dielectrics increases, and the resistance decreases.

The dependence on moisture again brings us back to the classification of dielectrics. Indeed, non-polar dielectrics are not wetted with water and they do not care about changing humidity. And in polar dielectrics, with an increase in humidity, the content of ions rises, and the electrical conductivity increases.

Dielectric conductivity consists of surface and bulk conductivities. The concept of specific volumetric conductivity is known, denoted by the letter sigma σ. And the reciprocal is called volume resistivity and is denoted by the letter ro ρ .

A sharp increase in conductivity in a dielectric with increasing voltage can lead to electrical breakdown. And similarly, if the insulation resistance drops, then the insulation is not doing its job and measures must be taken. Insulation resistance consists of surface and volume resistance.

Dielectric losses in dielectrics are understood as current losses inside the dielectric, which are dissipated in the form of heat. To determine this value, enter the parameter tangent delta tgδ... δ - angle, complementary to 90 degrees, the angle between current and voltage in a circuit with a capacitance.

Dielectric losses are: resonance, ionization, electrical conductivity, relaxation. Now let's talk in more detail about each type.

Dielectric strength is the ratio of the breakdown voltage to the distance between the electrodes (or dielectric thickness). This value is determined by the minimum value of the electric field strength at which a breakdown occurs.

Breakdown can be electrical (impact ionization, photoionization), thermal (large dielectric losses, hence a lot of heat, and charring with reflow can occur) and electrochemical (as a result of the formation of mobile ions).

And at the end of the table of dielectrics, how could it be without it.

The table above shows data on dielectric strength, specific volume resistance and relative permittivity for various substances. Also, the tangent of the dielectric loss angle was not bypassed.