Dielectric material. The main characteristics of dielectric materials
Classification on the structure of molecules
Classification by chemical composition
Classification by method of receipt
Classification by aggregative state
Active and passive dielectrics
Determination of dielectric materials
Classification and field of use of dielectric materials
Dielectrics are called substances, the main electrical property of which is the ability to polarize in the electric field.
Electrical insulating materials are called dielectric materials designed to create electrical insulation of current-carrying parts of electrical installations.
The insulator is a product of an electrical insulating material, the tasks of which are fastening and isolation from each other conductors under different potentials (for example, air power transmission insulators).
Electrical insulation is an electrical insulating system of a specific specific electrical product, made of one or more electrical insulating materials.
Dielectrics used as electrically insulating materials are called passive dielectrics. Currently, the so-called, active dielectrics, parameters of which can be adjusted, changing the electric field strength, temperature, mechanical stresses, and other parameters affecting them factors.
For example, a condenser, the dielectric material in which the piezoelectric is served, under the action of the applied alternating voltage, changes its linear dimensions and becomes an ultrasonic oscillation generator. The capacity of the electrical capacitor, made of nonlinear dielectric - ferroelectric, varies depending on the electric field strength; If such a container is included in the oscillatory LC circuit, its setting frequency changes.
Dielectric materials classified:
By aggregate state: gaseous, liquid and solid;
By method of obtaining: Natural and synthetic;
By chemical composition: organic and inorganic;
On the structure of molecules: neutral and polar.
Gaseous dielectrics
Dielectric gaseous dielectrics include: air, nitrogen, hydrogen, carbon dioxide, Elegaz, chladone (freon), argon, neon, helium, etc. They are used in the manufacture of electrical apparatuses (air and email switches, arresters)
The most widely as an electrically insulating material uses air. Air contains: pairs of water 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 terms of amounts amount to ten thousand dollars percent.
Important properties of gases are their ability to restore electrical strength, low dielectric constant, high resistance value, practically no aging, inertness of a number of gases with respect to solid and liquid materials, non-toxicity, the ability to operate at low temperatures and high pressure, non-causing.
Liquid dielectrics
Liquid dielectrics are designed to remove heat from windings and magnetic lines in transformers, arc suction in oil switches, enhancing solid isolation in transformers, oil-filled inputs, capacitors, oil-plated and oil-filled cables.
Liquid dielectrics are divided into two groups:
Oil oils (transformer, condenser, cable);
Synthetic oils (Sovopol, liquid silicone and fluoroorganic compounds).
4.1.7 Areas of using dielectrics as ETM
Application in the electric power industry:
- linear and substation isolation - It is porcelain, glass and silicone tires in suspended insulators VL, porcelain in support and passing insulators, fiberglasss as carrier elements, polyethylene, paper in high-voltage inputs, paper, polymers in power cables;
- Isolation of electrical appliances - paper, getinax, fiberglass, polymers, salivary materials;
- machines, devices - paper, cardboard, varnishes, compounds, polymers;
- condensers of different types- polymeric films, paper, oxides, nitrides.
From a practical point of view, in each case, the choice of electrical insulation material should analyze the working conditions and choose the insulation material in accordance with the Complex Complex. For orientation, it is advisable to divide the main dielectric materials into groups under the conditions of application.
1. Heating-resistant electrical insulation. This is primarily the products from mica materials, some of which are capable of working to 700 ° C. Glasses and materials based on them (fiberglass, fiberglass). Organosilicate and metal phosphate coatings. Ceramic materials, in particular boron nitride. Compositions from silicone with heat-resistant binder. Polyimide, fluoroplast possess high heating resistance of polymers.
2. Moisture-resistant electrical insulation. These materials should be hydrophobic (low water) and non-hygroscopic. A bright representative This class is fluoroplastic. In principle, hydrophobization is possible by creating protective coatings.
3. Radiation resistant insulation. This is, first of all, inorganic films, ceramics, fibercstolitical, Slobinite materials, some types of polymers (polyimides, polyethylene).
4. Tropic resistant insulation.The material must be hydrophobic to work in high humidity and temperature. In addition, it must be resistant against mold fungi. Best materials: fluoroplastic, some other polymers, worst - paper, cardboard.
5. Frost-resistant insulation. This requirement is characteristic, mainly for rubber, because With a decrease in temperature, all rubber losing elasticity. The most frost-resistant silicone tires with phenyl groups (up to -90 ° C).
6. Isolation to work in vacuum (space, vacuum instruments). For these conditions, vacuum-dense materials must be used. Some specially cooked ceramic materials are suitable, polymers are unsuitable.
Electrotechnical Cardboard Used as dielectric distancing pads, washers, struts, as isolation of magnetic pipes, groove insulation of rotating machines, etc. Cardboard is usually used after impregnation with transformer oil. The electrical strength of 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 barriers from cardboard are often suitable in the oil environment. Oil-worker insulation usually has the strength e \u003d 300-400 kV / cm. The disadvantage of cardboard is hygroscopicity, the mechanical strength decreases as a result of moisture and, electrical strength (in 4 or more times) decreases dramatically.
IN lately The production of insulators for VL based on silicon Rubber. This material refers to rubbers, the main property of which is elasticity. This allows not only insulators, but also flexible cables from rubbers. In the energy sector, different types of rubbers are used: natural rubber, butadiene, styrene butadiene, ethylene propylene and silicone.
Electrotechnical porcelain It is an artificial mineral formed from clay minerals, wild spheres and quartz as a result of heat treatment on ceramic technology. Its most valuable properties includes high resistance to weathering, positive and negative temperatures, to the effects of chemical reagents, high mechanical and electrical strength, low cost of source components. This has determined the widespread use of porcelain for the production of insulators.
Electrotechnical glass As a material for insulators, has some advantages over porcelain. In particular, he has a more stable raw material base, it's easier to technology that allows for greater automation, the possibility of visual control of faulty insulators.
Micait is the basis of a large group of electrical insulating products. The main advantage of mica is high heat resistance along with sufficiently high electrical insulation characteristics. Mica is a natural mineral of complex composition. In the electrical engineering, two types of mica are used: Cal 2 muscovit ()) 2 and Floogopit KMG 3 (ALSI 3 O 10 (OH) 2. High electrical insulating characteristics of mica are obliged to its unusual structure, namely - lamination. Slyudyamy plates It is possible to split into flat plates up to submicron sizes. Destructive voltages when the separation of one layer from the other layer is about 0.1 MPa, whereas when stretching along the layer - 200-300 MPa. From other mica properties, we note the low TG less than 10 -2; High resistivity, more than 10 12 ohms · m; sufficiently high electrical strength, more than 100 kV / mm; heat resistance, melting point of more than 1200 ° C.
Mica is used as electrical insulation, as in the form of cigrous thin plates, incl. glued together (mikanites) and in the form of sali papers, incl. impregnated with various binders (soludinates or mica). Slyuda paper is made by technology close to the technology of ordinary paper. The mica grinds, prepare a pulp, on paper machines roll over the sheets of paper.
Mikanits. They have the best mechanical characteristics and moisture resistance, but they are more expensive and less technologically. Application - Passion and vitk insulation of electrical machines.
Slyudinites - Sheet materials made of musculite-based mica paper. Sometimes they are combined with a fiberglass substrate (Flossomudinitis), or a polymer film (film compound). Paper impregnated with varnish, or other binders, possess the best mechanical and electrophysical characteristics than untropy paper, but their heat resistance is usually lower, because It is determined by the properties of the impregnating binder.
Slyudoplasts - Sheet materials made of flogopite-based mica paper and impregnated with binders. Like Slobinites, they are also combined with other materials. Compared to miclowities, they have several worst electrophysical characteristics, but have a smaller cost. The use of miclowitis and mica carriers - insulation of electrical machines, heating-resistant insulation of electrical appliances.
The greatest use of gas in power engineering has air. This is due to the cheap, accessibility of air, ease of creation, maintenance and repair of air insulating systems, the possibility of visual control. Objects in which air is used as electrical insulation - power lines, open distribution devices, air switches, etc.
From electronegative gases with high electrical strength, I found the greatest application elegas SF6.. He received its name from the reduction of "Electric Gas". The unique properties of Eleginaz were opened in Russia, its use also began in Russia. In the 30s, the famous scientist B.M. Gokhberg explored the electrical properties of a number of gases and drew attention to some properties of sixfluoride sulfur SF6. The electrical strength at atmospheric pressure and the gap 1 cm is E \u003d 89 kV / cm. The molecular weight is 146, characteristic is a very large thermal expansion coefficient and high density. This is important for energy plants in which the cooling of any parts of the device is carried out, since With a large thermal expansion coefficient, a convective stream carrying heat is easily formed. From heat physical properties: Melting point \u003d -50 ° C at 2 atm, boiling point (sublimation) \u003d -63 ° C, which means the possibility of use at low temperatures.
From others useful properties We note the following: chemical inertia, non-toxicity, non-care, heat resistance (up to 800 ° C), explosion safety, weak decomposition in discharges, low liquefaction temperature. In the absence of impurities, Elegaz is completely harmless to humans. However, email decomposition products as a result of the discharges (for example, in the discharge or switch) are toxic and chemically active. The complex of the properties of Elegaz provided a fairly widespread use of Eleginase isolation. Elegase devices are usually used under pressure in a few atmospheres for greater compactness of power plants, because Electrical strength increases with increasing pressure. Based on the Eleginazic isolation, a number of electrical dispensers are created and operated, cables, capacitors, switches, compact CRC (closed switchgear).
The most common liquid dielectric in the energy sector is transformer oil.
Transformer oil - Purified oil fraction obtained by distillation, boiling at a temperature of from 300 ° C to 400 ° C. Depending on the origin of oil, they have different properties and these distinguishing properties of the raw materials are reflected on the properties of the oil. It has a complex hydrocarbon composition with an average weight of molecules 220-340 A.E., and contains the following main components.
From related transformer oil according to the properties and use of liquid dielectrics, it is worth noting the condenser and cable oils.
Condenser oils. Under this term combined a group of various dielectrics used for the impregnation of paper-oil and paper-film isolation of capacitors. The most common condenser oil According to GOST 5775-68, produce from transformer oil by deeper cleaning. It differs from ordinary oils greater transparency, a smaller value of TG (more than ten times). Castor oil Vegetable origin, it is obtained from mite seeds. The main area of \u200b\u200buse is the impregnation of paper capacitors for operation in impulse conditions.
The density of castor oil 0.95-0.97 t / m3, the temperature of frozen from -10 ° C to -18 ° C. Its dielectric constant at 20 ° C is 4.0 - 4.5, and at 90 ° C - \u003d 3.5 - 4.0; TG at 20 ° C is 0.01-0.03, and at 100 ° C TG \u003d 0.2-0.8; EPR at 20 ° C is 15-20 mV / m. Castor oil does not dissolve in gasoline, but dissolves in ethyl alcohol. Unlike oil oils, castor does not cause swelling of ordinary rubber. This dielectric refers to weaklyolar liquid dielectrics, its resistivity under normal conditions is 108 - 1010 OM · m.
Cable oils Designed for impregnation of paper insulation of power cables. The basis of them is also oil oils. From transformer oil is distinguished by increased viscosity, increased flash point and reduced dielectric losses. From the brands of oils, we note MN-4 (low-viscous, to fill low pressure cables), C-220 (high viscosion for filling high pressure cables), km-25 (most viscous).
The second type of liquid dielectrics is difficult and non-combustible fluids. Liquid dielectrics with such properties are quite a lot. The greatest distribution in power engineering and electrical engineering received chlordiphenyl. IN foreign literature they're called chlorbiphenyls. These are substances that have a double benzene ring, so-called. 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 a mixture of pentachlorodiphenyl with trichlorodiphenyl. The commercial names of some of them are "Council", "Sovvol", "Caloriya-2".
Dielectric materials are classified for a number of intraspecific signs, which are determined by their main characteristics: electrical, mechanical, physico-chemical, thermal.
4.2.1 The electrical characteristics of dielectric materials include:
Specific voluminous electrical resistance ρ, OM * M or specific volume conductivity σ, cm / m;
Specific surface electrical resistance ρ s, Ohm, or specific surface conductivity σ s cm;
Temperature coefficient of specific electrical resistance TC ρ, ˚С -1;
Dielectric permeability ε;
Temperature coefficient of dielectric constant TKε;
Tangent angle of dielectric losses Δ;
Electrical strength of material E PR, MV / M.
4.2.2 Thermal characteristics determine the thermal properties of dielectrics.
The thermal characteristics include:
Heat capacity;
Melting temperature;
Softening temperature;
Capple temperature;
Heat resistance;
Heating resistance;
The cold resistance is the ability of dielectrics to resist low temperatures, while maintaining electrical insulating properties;
Tropic resistance - resistance of dielectrics to a complex of external influences in a tropical climate (sharp temperature difference, high humidity, solar radiation);
Thermoelastic;
Flash temperature of the vapor of electrical insulating fluids.
Heating resistance is one of the most important characteristics of dielectrics. In accordance with GOST 21515-76, heating resistance is the ability of a dielectric to exist for a long time of an increased temperature over the time comparable to a period of normal operation, without an invalid deterioration of its properties.
Heating-resistance classes. Just seven. Characterized by the temperature index of TI. This temperature at which the service life of the material is 20 thousand hours.
4.2.3 Welding properties of dielectrics
Moisture resistance is the reliability of insulation operation when it is located in the atmosphere of a water vapor close to saturation. Moisture resistance is assessed by changing electrical, mechanical and other physical properties after finding material in an atmosphere with increased and high humidity; by moisture and water permeability; By moisture and water absorption.
Moisture permeability - the ability of the material to skip moisture pairs in the presence of the difference between relative air humidity on both sides of the material.
Moisture-absorptionability - the ability of the material to sorbit water during long-term depression in a humid atmosphere close to saturation state.
Water abscess - the ability of the material to sorbit water with long immersion in water.
Tropic resistance and tropicalization of equipment - protection of electrical equipment from moisture, mold, rodents.
4.2.4 Mechanical properties of dielectrics define the following characteristics:
Destructive stress with static tension;
Destructive tension during static compression;
Destructive tension during static bend;
Hardness;
Shock viscosity;
Splitting resistance;
Resistance to durability (for flexible materials);
Flexibility in the number of double beggars;
Platestroistic properties.
Mechanical characteristics of dielectrics define the corresponding GOSTS.
4.2.5 Physico-chemical characteristics:
Acid number determining the amount of free acids in the dielectric, worsening the dielectric properties of liquid dielectrics, compounds and varnishes;
Kinematic and conditional viscosity;
Water absorption;
Water resistance;
Moisture resistance;
Dug resistance;
Tracking capacity;
Radio durability, etc.
Are divided into 3 groups:
1) oil oils;
2) synthetic liquids;
3) vegetable oils.
Liquid dielectrics are used to impregnating high voltage cables, capacitors, for filling transformers, switches and inputs. In addition, they perform the functions of the coolant in transformers, the dug-shaker in the switches, etc.
Oil oils
Oil oils represent a mixture of paraffin hydrocarbons (With n 2 n + 2) and naphthene (with n 2 n ) Rows. They are widely used in electrical engineering as transformer, cable and condenser oils. Oil, filling the gaps and pores inside electrical installations and products, increases the electrical strength of the insulation and improves the heat sink from the products.
Transformer oil get out of oil by distillation. The electrical properties of transformer oil are largely dependent on the quality of oil purification from impurities, the content of water in it and the degree of aduggation. Dielectric permeability of oil 2.2, specific electrical resistance 10 13 ohms · M..
The purpose of transformer oils is to increase the electrical strength of isolation; Distill heat; Promote dugwashing in oil switches, improve quality electric insulation In electrical products: risostats, paper capacitors, paper insulation cables, power cables - by fill and impregnation.
Transformer oil during operation aggravates, which worsens its quality. Aging oil promotes: contact oil with air, elevated temperatures, contact with metals (Cu, PB, FE), the impact of light. To increase the service life, oil is regenerated by cleaning and removing aging products, adding inhibitors.
Cableand condenser Oils differ from transformer more high quality Cleaning.
Synthetic liquid dielectrics
Synthetic liquid dielectrics for some properties exceed oil electrical insulating oils.
Chlorinated hydrocarbons
Sobol – pentachlordiphenylC 6 H 2 SL 3 - C 6 H 3 SL 2 obtained during chlorination of diphenylFrom 12 H 10
C 6 H 5 - C 6 H 5 + 5 CL 2 → C 6 H 2 CL 3 - C 6 H 3 SL 2 + 5 HCL
Sobol It is used for impregnation and fill capacitors. It has a more high dielectric constant compared to oil oils. Dielectric permeability of the Council of 5.0, specific electrical resistance 10 11 ¸ 10 12 ohms · m. Primirms the Council for the impregnation of paper power and radioDesters with increased specific capacity and low operating voltage.
Sovvol. - a mixture of owl with trichlorobenzene. Used to insulate explosion-proof transformers.
Silicon fluids
Have the greatest distribution polydimethylsiloxanes, polydiethylsiloxanes, polymethylphenylsiloxanes liquids.
Polysiloxane fluids - liquid silicone polymers ( polyorganosiloxanes), have such valuable properties as: high heating resistance, chemical inertness, low hygroscopicity, low sprouting temperature, high electrical characteristics in a wide range of frequencies and temperatures.
Liquid polyorganosiloxanes are polymer compounds with a low degree of polymerization, the molecules of which contain a siloxian grouping of atoms
,
where silicon atoms are associated with organic radicalsR: methyl CH 3, ethyl C 2 H 5, phenyl C 6 H 5 . Molecules of polyorganosiloxane fluids may have a linear, linear-branched and cyclic structure.
Liquid polymethylsiloxanes obtained at hydrolysis dimethydichlorsilana In the mixture of C. trimethylchlorsilane .
The resulting liquids are colorless, dissolved in aromatic hydrocarbons, dichloroethane and a number of other organic solvents, are not dissolved in alcohols and acetone. Polymethylsiloxanes Chemically inert, do not have aggressive action to metals and do not interact with most organic dielectrics and rubber. Dielectric constant 2.0.¸ 2.8, specific electrical resistance 10 12 Ohm · M., Electric strength 12¸ 20 mV / m
Formula polydimethylsiloxanebut Has appearance
– SI(CH 3) 3 - O - [ SI(CH 3) 2 - O] n -SI(CH 3) \u003d O
Liquid silicone polymers are used as:
Polydyethylsiloxanes – obtained at hydrolysis diethyldihlorsilana and triethylchlorsilana . Have a wide boiling temperature range. The structure is expressed by the formula:
Properties depend on the boiling point. Electrical properties coincide with properties polydimethylsiloxane.
Liquid polymethylphenylsiloxanes have a structure expressed by the formula
Get hydrolysis phenylmethyldihlorsilanov and other oil viscous. After processingNAON Viscosity rises 3 times. Withstand heating for 1000 hours to 250 ° C. Electrical properties coincide with properties polydimethylsiloxane.
For γ - irradiation The viscosity of silicone liquids increases greatly, and the dielectric characteristics deteriorate sharply. With a large dose of radiation fluid transfect in rubber-like Mass, and then into a solid fragile body.
Fluorogenic fluids
Fluorogenic fluids - From 8 F 16 - nonsense and explosion-proof, high championship (200 ° C), have low hygroscopicity. Pair them have high electrical strength. Liquids have a low viscosity, bats. They have the best heat sink than oil oils and silicone liquids.–) n.,
it is a non-polar polymer of a linear structure. It turns out to polymerization of ethylene gasC 2N 4 At high pressure (up to 300 MPa), or at low (up to 0.6 MPa). The molecular weight of high pressure polyethylene - 18000 - 40000, low - 60000 - 800000.
Polyethylene molecules have the ability to form areas of material with an ordered arrangement of chains (crystallites), so polyethylene consists of two phases (crystalline and amorphous), the ratio of which determines its mechanical and thermal properties. The amorphous gives the material elastic properties, and crystalline is rigidity. The amorphous phase has a glass transition temperature of +80 ° C. The crystalline phase has a higher heated resistance.
The aggregates of the crystalline phase polyethylene molecules are spherolites 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 the high crystallinity of low pressure polyethylene, has a higher melting point (120-125 ° C) and higher tensile strength. The structure of polyethylene largely depends on the cooling mode. With its rapid cooling, small spherolites are formed, with slow cooling - large. Fast cooled polyethylene is characterized by large flexibility and less hardness.
The properties of polyethylene depend on molecular weight, purity, extraneous impurities. Mechanical properties depend on the degree of polymerization. Polyethylene has large chemical resistance. As an electrical insulating material 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 - (ND) and (V.D.);
2. low pressure polyethylene for the cable industry;
3. low molecular weight polyethylene high or medium pressure;
4. porous polyethylene;
5. polyethylene special hose plastic;
6. polyethylene for the production of RF cable;
7. electrically conductive polyethylene for the cable industry;
8. polyethylene filled with soot;
9. chlorosulfied polyethylene;
10. polyethylene film.
Fluoroplasts
There are several types of fluorocarbon polymers that can be polar and non-polar.
Consider the properties of the Tetrafluoroethylene Gas Polymerization Reaction Product
(F 2 C \u003d CF 2).
Fluoroplast - 4. (Polytetrafluoroethylene) - loose white powder. The structure of molecules has the form
Fluoroplast molecules have a symmetrical structure. Therefore, fluoroplast is a non-polar dielectric
The symmetry of the molecule and high purity provide high level electrical characteristics. Big bond energy betweenC and F. gives him high cold resistance and heating resistance. Radio petals from it can work from-195 ÷ + 250 ° C. Non-flamm, chemically racks, nongigroscopic, has a hydrophobicity, is not amazed by mold. Specific electrical resistance is 10 15 ¸ 10 18 Ohm · M., dielectric constant 1.9¸ 2.2, electric strength 20¸ 30 mV / m
Radio methods are made from fluoroplastic powder with cold pressing. Pressing products sach out in the furnaces at 360 - 380 ° C. With quick cooling, the product is obtained by hardened with high mechanical strength. With slow cooling - non-perk. They are easier handled, less hard, have a high level of electrical characteristics. When the parts are heated to 370 ° from the crystalline state, they are moving into amorphous and acquire transparency. Thermal decomposition of the material begins at\u003e 400 °. Whereina toxic fluorine is formed.
The lack of fluoroplast is its turnover under the action of mechanical load. It has low resistance to radiation and laborious time consuming in the product. One of the best dielectrics for equipment HF and microwave. Made electro - and radio engineering products in the form of plates, disks, rings, cylinders. Isolate RF cables thin film, sealing with shrinkage.
Fluoroplast can be modified by applying fillers - fiberglass, boron nitride, soot, etc., which makes it possible to receive materials with new properties and improve the available properties.
Dielectrics - These are substances that do not conduct an electric current to a certain pore. Under certain conditions, the conductivity in them is born. These conditions are mechanical, thermal - in general, energy types of influences. In addition to dielectrics, substances are also classified for conductors and semiconductors.
What is the difference between dielectrics from conductors and semiconductors
The theoretical difference between these three types of materials can be represented, and I will do it, in the figure below:
The drawing is a beautiful, familiar with a school bench, but something practical from it will not be very expensive. However, in this graphic masterpiece, the difference between the conductor, a semiconductor and a dielectric is clearly defined.
And the difference is the magnitude of the energy barrier between the valence zone and the conduction zone.
In conductors, electrons are in the valence zone, but not all, since the valence zone is the most external border. For sure, it's like migrants. The conduction zone is empty, but glad to guests, since it is full of free jobs for them in the form of free energy zones. When exposed to an external electric field, extreme electrons acquire energy and move into free levels of conduction zone. This movement we also call the electric shock.
In dielectrics and conductors, everything is similar, except for the fact that there is a "fence" - a prohibited zone. This zone is located between the valence and conductivity zone. The more this zone, the greater the energy is required to overcome the electrons of this distance. Dielectrics have the magnitude of the zone more than in semiconductors. There is even a condition: if DE\u003e 3EV () is a dielectric, in the reverse case of DE
Types and types of dielectrics
The classification of dielectrics is satisfied extensive. There are liquid, solid and gaseous substances. Next, they are divided according to certain signs. Below is the 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
- - Centrosimmenitrical
- - amorphous
- - resins, bitumens (epoxy resin)
- - Glasses
- - Disordered polymers
- - polycrystals
- - irregular crystals
- - Ceramics
- - Ordered polymers
- - Satalles
- - Monocrystals
- - Molecular
- - Covalent
- - ionic
- - Paraelectric displacement
- - Paraelectric "order-mess"
- - dipole
- - non-centrosmenzyrical
- - Monocrystals
- - Pyroelectrics
- - ferroelectric displacement
- - ferroelectric "order-mess"
- - linear pyroelectrics
- - Piezoelectrics
- - with hydrogen bonds
- - Covalent
- - ionic
- - Texture
- - electronic defects
- - ion defects
- - polar molecules
- - Macrodipole
- - ferroelectric domains
- - crystals in the matrix
If you take liquid and gaseous dielectrics, then the main classification lies in the question of polarity. The difference in symmetry molecules. In polar molecules are asymmetrical, in non-polar - symmetrical. Asymmetric molecules are called dipoles. In polar fluids, the conductivity is so large that they cannot be used as insulating substances. Therefore, for these purposes is used non-polar, also transformer oil. And the presence of polar impurities even in hundredths significantly reduces the breakdown bar and negatively affects the insulating properties of non-polar dielectrics.
crystals are among the average between liquid and crystal, as follows from the name.
Another popular question about the properties and use of liquid dielectrics will be the following: water - dielectric or conductor? In pure distilled water there are no impurities that could cause current flowing. Clean water You can create in laboratory, industrial conditions. These conditions are complex and difficult to fulfill ordinary person. There is an easy way to check whether distilled water current current.
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 current flow. When you turn on the scheme to work, the bulb will not light up - hence the current does not pass. Well, if it turns, it means water with impurities.
Therefore, any water that we meet: from the crane, in the lake, in the bathroom - it will be a conductor due to impurities that create an opportunity for current leakage. Do not bathe in a thunderstorm, do not work with wet hands with electricity. Although pure distilled water is a polar dielectric.
For solid dielectrics, the classification basically lies in the question of activity and passivity. If the properties are constant, then the dielectric is used as an insulating material, that is, it is passive. If properties change, depending on external influences (heat, pressure), then this dielectric is used for other purposes. Paper is a dielectric if water is impregnated with water - then the current is carried out and it is conducted if the paper is impregnated with transformer oil - then this is a dielectric.
Foil is called a thin metal plate, metal - as it is known is the conductor. For sale, for example, PVC foil is available, here the word foil for clarity, and the word PVC is to understand the meaning - because PVC is a dielectric. Although in Wikipedia - the foil is called a thin sheet of metal.
Amorphous liquids - It is a resin, glass, and bitumen, and wax. With increasing temperature, this dielectric melts, these are frozen substances - these are wild definitions that characterize only one line of truth.
Polycrystals - This, as if hitting the crystals, combined into one crystal. For example, salt.
Monocrystall - This is a solid crystal, in contrast to the aforementioned polycrystal having a continuous crystal lattice.
Piezoelectrics - Dielectrics, in which, with mechanical exposure (spraying), the 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 during mechanical exposure, that is, pyroelectrics are also piezoelectrics, but not vice versa. Examples serve as amber and tourmaline.
Physical properties of dielectrics
To evaluate the quality and degree of suitability of the dielectric, it is necessary to somehow describe its parameters. If you monitor these parameters, you can prevent an accident in time by replacing the item to a new one with valid parameters. These parameters are: polarization, electrical conductivity, electrical strength and dielectric losses. For each of these parameters, there is its own formula and a constant value, in comparison with which the material suitability is concluded.
The main electrical properties of dielectrics are polarization (charge displacement) and electrical conductivity (the ability to conduct an electric current) The displacement of the associated charges of the dielectric or their orientation in the electrical field is called polarization. This property of dielectric materials is characterized by relative dielectric constant ε . With polarization on the surface of the dielectric, connected electrical charges are formed.
Depending on the type of dielectric, polarization can be: electronic, ionic, dipole-relaxation, spontaneous. In more detail about their properties in infographics below.
Under the electrical conductivity understand the ability of the dielectric to carry out an electric current. The current flowing in the dielectric is called leakage current. The leakage current consists of two components - the current of the absorption and current through. Through currents are due to the availability of free charges in the dielectric, the absorption current - polarization processes until the equilibrium is established in the system.
The magnitude of the electrical conductivity depends on the temperature, humidity and the number of free charge carriers.
With increasing temperature, the electrical conductivity of dielectrics increases, and the resistance drops.
The dependence on the humidity returns us to the classification of dielectrics. After all, non-polar dielectrics are not wetted by water and there is no case for changing humidity. And at the polar dielectrics, with increasing moisture, the content of ions increases, and the electrical conductivity increases.
The conductivity of the dielectric consists of surface and volumetric conductivity. The concept of specific volume conductivity is known, indicated by the letter of the sigma σ. And the reverse value is the specific volume resistance and is indicated by the RO letter ρ .
A sharp increase in conduction in a dielectric with an increase in voltage can lead to an electric breakdown. And in the same way, if the insulation resistance drops, it means that the insulation does not cope with its task and measures must be applied. The insulation resistance consists of surface and volumetric resistance.
Dielectric losses in dielectrics understand the loss of current inside the dielectric, which are dissipated in the form of heat. To determine this value, the Tangent Delta parameter is introduced tGΔ.. δ is an angle that complements up to 90 degrees, the angle between the current and the voltage in the circuit with the container.
Dielectric losses are: resonant, ionization, electrical conductivity, relaxation. Now let's talk about each type.
Electrical strength is the ratio of the punching voltage by the distance between the electrodes (or the thickness of the dielectric). This value is determined by the minimum magnitude of the electric field strength at which the breakdown will occur.
The breakdown can be electric (shock ionization, photoionization), thermal (large dielectric losses, therefore a lot of heat, and melting charring can occur) and electrochemical (as a result of the formation of moving ions).
And at the end of the Dielectric Table, how without it.
The table above shows data on electrical strength, specific volume resistance and relative dielectric constant for various substances. Also, the tangent of the angle of dielectric losses was not bypass.
Recent articles
Most popular
All liquid and solids in the nature of the operation on them of the electrostatic field are divided into conductors, semiconductors and dielectrics.
Dielectrics (insulators)- Substances that do not spend poorly or do no electric current. Dielectricians include air, some gases, glass, plastics, various resins, many types of rubber.
If you put neutral bodies in an electric field from materials such as glass, ebonite, you can observe their attraction as a positively charged and negatively charged bodies, but significantly weaker. However, when dividing such bodies in the electric field, their parts are neutral, like the whole body as a whole.
Hence, there are no free electrically charged particles in such bodies, capable of moving in the body under the action of an external electric field. Substances that do not contain free electrically charged particles are called dielectrics or insulators.
The attraction of uncharged bodies from dielectrics to charged bodies is due to their ability to polarization.
Polarization- the displacement of the bound electrical charges inside atoms, molecules or inside the crystals under the action of an external electric field. Simplest an example of polarization - The effect of an external electric field on a neutral atom. In the external electric field, the force acting on a negatively charged shell is directed oppositely, which acts on a positive core. Under the action of these forces, the electron shell slightly shifts relative to the kernel and deformed. The atom remains generally neutral, but the centers of a positive and negative charge in it no longer coincide. Such an atom can be viewed as a system of two equal dot charges of the opposite sign, which is called dipolem. 
If you place a dielectric plate between two metal plates with the charges of the opposite sign, all dipoles in the dielectric under the action of an external electric field are converted positive charges to a negative plate and negative charges to a positively charged plate. The dielectric plate remains generally neutral,but its surfaces are covered opposing by the sign of the associated charges.
In the electric field, polarization charges on the dielectric surface create an electric field oppositely sent by an external electric field. As a result, the electric field strength in the dielectric decreases, but not to become zero.
The ratio of the voltage module E 0 of the electric field in vacuum to the voltage module E of the electric field in a homogeneous dielectric is called dielectric constant ɛ Substance:
ɛ \u003d e 0 / e
When the interaction of two point electrical charges in the medium with dielectric constant ɛ as a result of a decrease in the field strength in ɛ times, the Coulomb force also decreases at once:
F e \u003d k (Q 1 · Q 2 / ɛr 2)
Dielectrics are able to relax an external electric field. This property is applied in capacitors.
Condencators- These are electrical appliances for the accumulation of electrical charges. The simplest condenser consists of two parallel metal plates, separated by a dielectric layer. When reporting plates equal in the module and opposite by the sign of charges + Q and -q Between the plates, the electric field with tension is created. E.. Outside the plates, the effect of electric fields directed oppositely charged plates is mutually compensated, the field strength is zero. Voltage U. between the plates are directly proportional to the charge on one plate, so the charge ratio q. To tension U.
C \u003d Q / U
is a constant capacitor for any values \u200b\u200bof charge q.This attitude FROMcalled the power capacity of the condenser.
Have questions? Do not know what dielectrics are?
To get a tutor help - register.
The first lesson is free!
the site, with full or partial copying of the material reference to the original source is required.
Dielectrics, substances, poorly conductive electric current. The term "dielectric" was introduced by M. Faraday to designate substances in which the electrostatic field penetrates. When placed in an electrical field of any substance, electrons and atomic kernels are experiencing force from this field. As a result, part of the charges moved to the electrical current. The remaining charges are redistributed so that the "centers of gravity" of positive and negative charges are shifted relative to each other. IN last case Speak about polarization of the substance. Depending on which of these two processes (polarization or electrical conductivity) prevails, substances are divided into dielectrics (all non-ionized gases, some liquids and solid bodies) and conductors (metals, electrolytes, plasma).
The electrical conductivity of dielectrics in comparison with metals is very small. Specific electrical resistance of dielectrics 10 8 -10 17 Ohm · cm, metals - 10 -6 -10 -4 ohm · cm.
The quantitative difference in the electrical conductivity of dielectrics and metals Classical physics tried to explain the fact that in metals there are free electrons, while in dielectrics all electrons are connected (belong to individual atoms) and the electric field does not take off, but only slightly shifts them.
Quantum theory solid body Explains the difference in the electrical properties of metals and dielectrics by various distribution of electrons by energy levels. In dielectrics top filled with electrons The energy level coincides with upper border One of the permitted zones (in metals it lies inside the resolved zone), and the nearest free levels are separated from the forbidden zone filled with a prohibited zone, to overcome which electrons cannot under the action of non-too strong electric fields (see the zone theory). The electric field effect is reduced to the redistribution of electron density, which leads to the polarization of the dielectric.
Polarization of dielectrics. Mechanisms of polarization of dielectrics depend on the nature of the chemical bond, i.e. the distribution of electron density in dielectrics. In ionic crystals (for example, NaCl), polarization is the result of the ion shift relative to each other (ion polarization), as well as the deformation of the electronic shells of individual ions (electron polarization), i.e. the sum of ionic and electron polarizations. In covalent bonding crystals (for example, diamond), where electron density is evenly distributed between atoms, polarization is mainly due to the displacement of electrons exercising chemical communications. In so-called polar dielectrics (for example, solid H 2 S) groups of atoms are electric dipoles that are focused chaotic in the absence of an electric field, and in the field acquire a predominant orientation. Such orientation polarization is typical for many liquids and gases. A similar polarization mechanism is associated with "Croskom" under the action of an electrical field of individual ions from some equilibrium positions in the grille to others. Especially often such a mechanism is observed in substances with hydrogen bond (for example, ice), where hydrogen atoms have several equilibrium positions.
Polarization of dielectrics is characterized by a polarization vector P, which is an electric dipole moment of a unit of a dielectric:
where P i is the dipole moments of particles (atoms, ions, molecules), n is the number of particles per unit of volume. The vector p depends on the voltage of the electric field E. in weak fields ρ \u003d ε 0 κε. The proportionality coefficient κ is called dielectric susceptibility. Often instead of vector P use vector of electrical induction (1)
where ε is a dielectric permeability, ε 0 - electrical constant. The values \u200b\u200bof κ and ε are the main characteristics of the dielectric. In anisotropic dielectrics (for example, in non-comic crystals), the direction P is determined not only by the direction of the field E, but also the direction of the axis of the symmetry of the crystal. Therefore, the vector p will range different angles with vector E, depending on the orientation, with respect to the axes of the symmetry of the crystal. In this case, the vector D is determined by the vector E with the help of not one value of ε, and several (in general, six), forming a dielectric constant tensor.
Dielectrics in a variable field. If the E field changes in time T, the polarization of the dielectric does not have time to follow it, since the shifts of charges cannot occur instantly. Since any variable field can be represented as a set of fields varying by harmonic law, it is sufficient to study the behavior of the dielectric in the E \u003d E 0 field, where ω is the frequency of the variable field, e 0 is the amplitude of the field strength. Under the action of this field, D and P will depend harmonically and with the same frequency. However, the difference between the phases Δ appears between the oscillations P and E, which is caused by the lag of polarization p from the field E. Harmonic law can be represented in the complex form E \u003d E 0 E iωt, then d \u003d d 0 e iωt, and D 0 \u003d ε (Ω) Ε 0. The dielectric constant in this case is a complex value: ε (Ω) \u003d ε '+ iε' ', ε' and ε '' depend on the frequency of the variable electrical field Ω. Absolute value
determines the amplitude of the oscillation D, and the ratio ε '/ ε "\u003d TGΔ is the phase difference between the oscillations D and E. The value Δ is called the angle of dielectric losses. In a constant electric field ω \u003d 0, ε" \u003d 0, ε' \u003d ε.
In variables of electric fields of high frequencies, the properties of the dielectric are characterized by the refractive indices of N and the absorption K (instead of ε 'and ε "). The first equal to relation Distribution rates electromagnetic waves in dielectric and in vacuum. The absorption indicator K characterizes the attenuation of electromagnetic waves in the dielectric. The values \u200b\u200bof n, k, ε 'and ε are connected by relation (2)
Polarization of dielectrics in the absence of an electric field. In a number of solid dielectrics (pyroelectrics, ferroelectrics, piezoelectrics, electrically plates), polarization may exist without an electric field, i.e. may be caused by other reasons. Thus, in pyroelectric charges there are so asymmetrically, the centers of gravity charges of the opposite sign do not coincide, i.e. the dielectric is spontaneously polarized. However, polarization in pyroelectrics is manifested only when the temperature is changed when the electrical charges compensate for the polarization do not have time to restructure. A variety of pyroelectrics are ferroelectrics, spontaneous polarization of which can be significantly changed under the influence of external influences (temperature, electric field). In piezoelectrics, polarization occurs during crystal deformation, which is associated with the peculiarities of their crystal structure. Polarization in the absence of a field may also be observed in some substances of the type of resin and braid, called electrically.
The electrical conductivity of dielectrics is small, but always different from zero. Movable charge carriers in dielectrics can be electrons and ions. Under normal conditions, the electronic conductivity of dielectrics is small compared to ionic. Ionic conductivity can be due to moving both own ions and impurity. The possibility of moving ions by crystal is associated with the presence of defects in crystals. If, for example, there is a vacancy in the crystal, then under the action of the field the neighboring ion may take it, in the newly formed vacancy can go through the next ion, etc. As a result, the vacancy movement occurs, which leads to the transfer of charge through the entire crystal. The movement of ions occurs as a result of their robes on interstices. With increasing temperature, ionic conductivity increases. A noticeable contribution to the electrical conductivity of the dielectric can be made surface conductivity (see surface phenomena).
Sample dielectrics. The density of the electric current J through the dielectric is proportional to the voltage of the electric field E (Ohm law): J \u003d ς, where ς is the electrical conductivity of the dielectric. However, in sufficiently strong fields, the current increases faster than according to the law of Ohm. With a certain critical value, the electric dispersion of the dielectric occurs. The magnitude of the EF is called the electric durability of the dielectric. With a breakdown, almost all current flows through the narrow channel (see the current lacing). In this channel j, it reaches large values, which can lead to the destruction of the dielectric: the through hole or the dielectric is adjusted via the channel. Channel can flow chemical reactions; For example, carbon is precipitated in organic dielectrics, in ionic crystals - metal (channel metallization), etc., always those present in the dielectric inhomogeneity are preferably promoted, since in places of inhomogeneities, the field E can increase locally.
In solid dielectrics distinguish thermal and electric trifles. With heat, the amount of heat released in the dielectric is growing with heat, and, therefore, the dielectric temperature, which leads to an increase in the number of charge carriers N and reduce the specific electrical resistance ρ. With an electric sample with an increase in the field, the generation of charge carriers under the action of the field and ρ also decreases.
The electrical strength of liquid dielectrics to a strong depends on the purity of the liquid. The presence of impurities and contaminants significantly reduces E PR. For pure homogeneous liquid dielectrics E PR is close to firm dielectrics. The breakdown in gas is associated with impact ionization and manifests itself in the form of an electric discharge.
Nonlinear properties of dielectrics. The linear dependence P \u003d ε 0 is valid only for field E, significantly smaller intracrystalline fields E CR (E CB of about 10 8 V / cm). Because E Pr.<< Е кр, то в большинстве диэлектриков не удаётся наблюдать нелинейную зависимость Р(Е) в постоянном электрическом поле. Исключение составляют сегнетоэлектрики, в которых в сегнетоэлектрической области и вблизи точек фазовых переходов наблюдается сильная нелинейная зависимость Р(Е). При высоких частотах электрическая прочность диэлектрика повышается, поэтому нелинейные свойства любых диэлектриков проявляются в ВЧ-полях больших амплитуд. В частности, в луче лазера могут быть созданы электрические поля напряжённостью порядка 10 8 В/см, в которых становятся существенными нелинейные свойства диэлектрика, что позволяет осуществить преобразование частоты света, самофокусировку света и другие нелинейные эффекты (смотри Нелинейная оптика).
Application of dielectrics. Dielectrics are mainly used as electrical insulating materials. Piezoelectrics are used to convert mechanical signals (displacements, deformations, sound oscillations) to electrical and vice versa (see the piezoelectric converter); Pyroelectrics - as thermal detectors of various radiation, especially IR radiation; Segroesoelectrics, being also piezoelectrics and pyroelectrics, are used, in addition to condenser materials (due to high dielectric permeability), as well as nonlinear elements and memory elements in a variety of devices. Most optical materials are dielectrics.
Lit.: Frielich Dielectric Theory. M., 1960; Hippel A. R. Dielectrics and Waves. M., 1960; Feynman R., Leighton R., Sands M. Fainman lectures in physics. M., 1966. Vol. 5: Electricity and magnetism; Kalashnikov S. G. Electricity. 5th ed. M., 1985.
A. P. Leangyuk, D. G. Sannikov.
