General information about electrical materials. Materials used in electrical machines
Topic #1
ELECTRICAL MATERIALS, CLASSIFICATION, MAIN PROPERTIES.
The materials widely used in radio electronic equipment have various names: electrical materials, radio engineering materials, electronic engineering materials. However, there is no fundamental difference between these materials. Despite the differences in names, they are all used for the manufacture of parts or components and devices of electrical, radio engineering, microelectronic, computer equipment. Nevertheless, all materials in the technical field of interest to us must have a well-defined set of properties due to which they find a specific application.
The unifying principle of all electrical materials is a set of their properties in relation to the electromagnetic field. When interacting with an electromagnetic field, electrical and magnetic properties are manifested. This allows us to define the concept of "electrotechnical materials" and classify them.
Electro (radio) technical materials (ETM) are materials that are characterized by certain properties in relation to the electromagnetic field and are used in technology, taking into account these properties.
According to the main electrical property of substances - electrical conductivity - electrical materials are divided into three groups: conductors, semiconductors and dielectrics.
According to the magnetic properties, substances are divided into five groups: diamagnets, paramagnets, ferromagnets, antiferromagnets and ferrimagnets.
Each of these groups is in turn subdivided into subgroups. quantitative parameters characterizing their main properties. This allows us to present the classification of radio materials in the form of a generalized scheme (Fig. 1.1).
For practical use it is necessary that in quantitative terms, electrical or magnetic properties are sufficiently pronounced, and mechanical, technological and other characteristics meet certain requirements. Therefore, not all of the listed groups are equally widely used in technology.
1.2. PHYSICO-CHEMICAL NATURE OF MATERIALS
All materials that exist in nature, regardless of their state of aggregation (gaseous, liquid, solid) are built from atoms of more than 100 chemical elements. Any substance (material) consists of a huge number of electrically charged particles - electrons and atomic nuclei of chemical elements, which determine its properties.
There are methods for simplified analysis of the properties of materials that allow using some of the macroscopic characteristics obtained experimentally. In this case, the most significant features of the interaction between electrons and nuclei of chemical elements that form a substance are taken into account integrally or automatically.
One of these methods is the analysis of the chemical bonds of the elements of a substance. Naturally, the differences in the types of substances are due to the difference in the nature of the distribution of electrons in atoms and molecules, and especially in the nature of the distribution of the valence electrons and ionic atomic cores that are most distant from the nucleus. By comparing the arrangement of atoms in the structure of a substance, the electronic configuration of these atoms, the type of chemical bond between them, one can answer a number of important questions about the macroscopic properties of a material, such as electrical conductivity, magnetization ability, density, hardness, plasticity, melting point, etc.
Most important in this approach to the analysis of the properties of materials is the question of the bonding forces that hold the atoms together. These forces are almost entirely the forces of electrostatic interaction between electrons and atomic nuclei. The role of forces of magnetic origin is very insignificant, and gravitational forces, due to the small values of the masses of interacting particles, can be neglected. The existence of stable bonds between the atoms of matter suggests that the total energy E V p particles in volume V substances in the form of the sum of the kinetic E to and potential U n E V p= N (E V k + U V n) less than the total energy of the same number of particles outside the volume, i.e. in a free state E c p \u003d N (E c k + U c n). The difference between these energies E s p – E V p= E St is called the chemical bond energy, or connection energy.
It has been experimentally established that the electrophysical and mechanical properties substance or material is determined by the nature of the bond and the quantitative value of the bond energy E St.
According to the nature of the interaction between the particles that make up the substance, six types of chemical bonds are distinguished:
Covalent non-polar;
Covalent polar, or homeopolar;
Ionic, or heteropolar;
Donor-acceptor;
metal;
Intermolecular.
Covalent non-polar bond arises when atoms of the same name are combined into molecules, for example, H 2, O 2, Cl 2, N 2, diamond, sulfur, Si, Ge, etc. In this case, the socialization of valence electrons occurs, which leads to the addition of the outer electron shell to a stable state. Molecules with a covalent nonpolar bond have a symmetrical structure, i.e. the centers of positive and negative charges coincide. As a result, the electric moment of the molecule is equal to zero, i.e. the molecule is non-polar or neutral.
It should be recalled that an electric moment other than zero is characteristic of dipole molecules. They are a system of two electric charges of the same magnitude and opposite in sign. q, located at some distance I from each other. For such a system of charges or a molecule, the electric or dipole moment μ= ql.
Covalent non-polar bond is characteristic of dielectrics and semiconductors.
Covalent polar (homeopolar, or pair-electronic) bond arises when combining dissimilar atoms, for example, H 2 O, CH 4, CH 3 C1, CC1 4, etc. In this case, the socialization of pairs of valence electrons and the addition of the outer shell to a stable state also take place. However, each bond has a dipole moment. However, the molecule as a whole can be neutral or polar (Fig. 1.2).
Homeopolar compounds can be dielectrics (polymeric organic materials) and semiconductors.
Ionic (heteropolar) bond occurs during the formation of a molecule by elements located at the end (VII group) and the beginning (I group) of the table D.I. Mendeleev, for example NaCl. In this case, the valence electron of the metal, weakly bound to the atom, passes to the halogen atom, completing its orbit to a stable state (8 electrons). As a result, two ions are formed, between which electrostatic attraction forces act.
The ionic forces of interaction are quite large, so substances with an ionic bond have relatively high mechanical strength, melting and evaporation temperatures. Ionic bonding is characteristic of dielectrics.
Donor-acceptor bond in essence, it is a kind of ionic bond and occurs when a material is formed by elements of various groups of the table D.I. Mendeleev, for example compounds A III B V - GaAs, etc.; compounds A III B V - ZnS, CdTe, etc. In such compounds, an atom of one element, called a donor, donates an electron to another atom, called an acceptor. As a result, a donor-acceptor chemical bond appears, which is quite strong. Materials with such a bond can be dielectrics and semiconductors.
metal connection arises between atoms in metals and is a consequence of the socialization of all valence electrons that form an electron gas and compensate for the charge of the ions of the crystal lattice. Due to the interaction of electron gas and ions, a metallic bond is formed. Shared electrons are weakly bound to atomic cores and, from the energy point of view, are free. Therefore, even at very weak external electric fields, a high electrical conductivity of metals is manifested.
Intermolecular, or residual, bond characteristic of substances of organic origin, such as paraffin. It occurs between the molecules of the substance and is weak, due to which such materials have a low melting point and mechanical characteristics, indicating the fragility of the molecular structure of the substance.
It should be noted that, usually, atoms in a solid are not bound by any of the types of bonds considered. Therefore, it is more convenient to consider and evaluate the properties of substances and materials based on them by analyzing the energy spectrum of the electrons of the atoms that make up the substance.
FEDERAL AGENCY FOR EDUCATION
State educational institution
higher professional education
Nizhny Novgorod State University named after N.I. Lobachevsky
The fourth faculty of distance learning
By discipline: "Materials Science"
On the topic: "Electrical materials and their properties"
Completed by: 3rd year student,
group 4-43EU16/1
R.V. Belov
G. Nizhny Novgorod 2011
1. Introduction
2. Conducting materials
3. Electrical insulating materials
4. Electrical insulating varnishes and enamels
5. Electrical insulating compounds
6. Unimpregnated fibrous electrical insulating materials
7. Electrical insulating varnished fabrics (varnished fabrics)
8. Plastics
9. Laminated electrical insulating plastics
10. Wound electrical insulating products
11. Mineral electrical insulating materials
12. Mica electrical insulating materials
13. Mica electrical insulating materials
14. Mica plastic electrical insulating materials
15. Electroceramic materials and glasses
16. Magnetic materials
17. Electrical sheet steel
18. Permalloys
19. Magnetically hard materials
20. Ferrites
21. Semiconductor materials and products
22. Electric carbon products (brushes for electric machines)
1. Introduction
Electrical materials are a set of conductive, electrically insulating, magnetic and semiconductor materials designed to work in electric and magnetic fields. This also includes the main electrical products: insulators, capacitors, wires and some semiconductor elements. Electrical materials in modern electrical engineering occupy one of the main places. Everyone knows that the reliability of the operation of electrical machines, apparatus and electrical installations mainly depends on the quality and correct selection of appropriate electrical materials. An analysis of accidents in electrical machines and apparatus shows that most of them occur as a result of failure of electrical insulation, consisting of electrical insulating materials.
Magnetic materials are of no less importance for electrical engineering. Energy losses and dimensions of electrical machines and transformers are determined by the properties of magnetic materials. A fairly significant place in electrical engineering is occupied by semiconductor materials, or semiconductors. As a result of the development and study of this group of materials, various new devices have been created that make it possible to successfully solve some problems of electrical engineering.
With a rational choice of electrical insulating, magnetic and other materials, it is possible to create reliable electrical equipment with small dimensions and weight. But for the realization of these qualities, knowledge of the properties of all groups of electrical materials is necessary.
2. Conductor Materials
This group of materials includes metals and their alloys. pure metals have low resistivity. The exception is mercury, which has a rather high resistivity. Alloys also have high resistivity. Pure metals are used in the manufacture of winding and mounting wires, cables, etc. Conductor alloys in the form of wire and tapes are used in rheostats, potentiometers, additional resistances, etc.
In the subgroup of alloys with high resistivity, a group of heat-resistant conductor materials that are resistant to oxidation at high temperatures is distinguished. Heat-resistant, or heat-resistant, conductive alloys are used in electric heaters and rheostats. In addition to low resistivity, pure metals have good ductility, i.e., they can be drawn into thin wire, into strips, and rolled into foil with a thickness of less than 0.01 mm. Metal alloys have less plasticity, but are more elastic and mechanically stable. A characteristic feature of all metallic conductor materials is their electronic electrical conductivity. The resistivity of all metallic conductors increases with increasing temperature, and also as a result of mechanical processing, which causes permanent deformation in the metal.
Rolling or drawing is used when it is necessary to obtain conductor materials with increased mechanical strength, for example, in the manufacture of wires for overhead lines, trolley wires, etc. To return the deformed metal conductors to their previous resistivity value, they are subjected to heat treatment - annealing without oxygen.
3. electrical insulating materials
Electrical insulating materials, or dielectrics, are such materials with which insulation is carried out, that is, they prevent the leakage of electric current between any conductive parts that are under different electrical potentials. Dielectrics have very high electrical resistance. According to the chemical composition, dielectrics are divided into organic and inorganic. The main element in the molecules of all organic dielectrics is carbon. There is no carbon in inorganic dielectrics. Inorganic dielectrics (mica, ceramics, etc.) have the highest heat resistance.
According to the method of preparation, natural (natural) and synthetic dielectrics are distinguished. Synthetic dielectrics can be created with a given set of electrical and physical and chemical properties Therefore, they are widely used in electrical engineering.
According to the structure of the molecules, dielectrics are divided into non-polar (neutral) and polar. Neutral dielectrics consist of electrically neutral atoms and molecules, which, before exposure to them electric field do not have electrical properties. Neutral dielectrics are: polyethylene, fluoroplast-4, etc. Among the neutrals, ionic crystalline dielectrics (mica, quartz, etc.) are distinguished, in which each pair of ions constitutes an electrically neutral particle. Ions are located at the nodes of the crystal lattice. Each ion is in oscillatory thermal motion near the center of equilibrium - a node of the crystal lattice. Polar, or dipole, dielectrics consist of polar dipole molecules. The latter, due to the asymmetry of their structure, have an initial electric moment even before the action of the electric field force on them. Polar dielectrics include bakelite, polyvinyl chloride, etc. Compared to neutral dielectrics, polar dielectrics have higher dielectric constants, as well as slightly increased conductivity.
According to the state of aggregation, dielectrics are gaseous, liquid and solid. The largest is the group of solid dielectrics. The electrical properties of electrical insulating materials are evaluated using quantities called electrical characteristics. These include: specific volume resistance, specific surface resistance, dielectric constant, temperature coefficient of dielectric constant, dielectric loss tangent and dielectric strength of the material.
Specific volume resistance is a value that makes it possible to estimate the electrical resistance of a material when a direct current flows through it. The reciprocal of the specific volume resistance is called the specific volume conductivity. Specific surface resistance - a value that allows you to evaluate the electrical resistance of the material when a direct current flows along its surface between the electrodes. The reciprocal of the specific surface resistance is called the specific surface conductivity.
The temperature coefficient of electrical resistivity is a value that determines the change in the resistivity of a material with a change in its temperature. With increasing temperature, the electrical resistance of all dielectrics decreases, therefore, their temperature coefficient of resistivity has a negative sign. Dielectric constant - a value that allows you to evaluate the ability of a material to create an electrical capacitance. The relative permittivity is included in the value of the absolute permittivity. The temperature coefficient of the dielectric constant is a value that makes it possible to evaluate the nature of the change in the dielectric constant, and, consequently, the capacitance of the insulation with a change in temperature. The dielectric loss tangent is a value that determines the power loss in a dielectric operating at alternating voltage.
Electrical strength - a value that allows you to evaluate the ability of a dielectric to resist destruction by its electrical voltage. The mechanical strength of electrical insulating and other materials is evaluated using the following characteristics: tensile strength of the material, tensile elongation, compressive strength of the material, static bending strength of the material, specific impact strength, splitting resistance.
The physicochemical characteristics of dielectrics include: acid number, viscosity, water absorption. The acid number is the number of milligrams of caustic potassium required to neutralize the free acids contained in 1 g of dielectric. The acid number is determined for liquid dielectrics, compounds and varnishes. This value makes it possible to estimate the amount of free acids in the dielectric, and hence the degree of their effect on organic materials. The presence of free acids degrades the electrical insulating properties of dielectrics. Viscosity, or the coefficient of internal friction, makes it possible to evaluate the fluidity of electrically insulating liquids (oils, varnishes, etc.). Viscosity can be kinematic and conditional. Water absorption is the amount of water absorbed by the dielectric after it has been in distilled water for a day at a temperature of 20 ° C and above. The water absorption value indicates the porosity of the material and the presence of water-soluble substances in it. With an increase in this indicator, the electrical insulating properties of dielectrics deteriorate.
The thermal characteristics of dielectrics include: melting point, softening point, dropping point, vapor flash point, heat resistance of plastics, thermoelasticity (heat resistance) of varnishes, heat resistance, frost resistance.
Film electrical insulating materials made from polymers have received wide application in electrical engineering. These include films and tapes. Films are produced with a thickness of 5-250 microns, and tapes - 0.2-3.0 mm. High-polymer films and tapes are characterized by high flexibility, mechanical strength and good electrical insulating properties. Polystyrene films are produced with a thickness of 20-100 microns and a width of 8-250 mm. The thickness of polyethylene films is usually 30-200 microns, and the width is 230-1500 mm. Films from fluoroplast-4 are made with a thickness of 5-40 microns and a width of 10-200 mm. Also, non-oriented and oriented films are produced from this material. Oriented PTFE films have the highest mechanical and electrical characteristics.
Polyethylene terephthalate (lavsan) films are produced with a thickness of 25-100 microns and a width of 50-650 mm. PVC films are made from vinyl plastic and plasticized polyvinyl chloride. Films made of vinyl plastic have greater mechanical strength, but less flexibility. Films from vinyl plastic have a thickness of 100 microns or more, and films from plasticized polyvinylchloride - 20-200 microns. Cellulose triacetate (triacetate) films are made unplasticized (rigid), blue-colored, slightly plasticized (colorless) and plasticized (blue-colored). The latter are highly flexible. Triacetate films are produced in thicknesses of 25, 40 and 70 microns and a width of 500 mm. Plenkoelektrokarton - flexible electrical insulating material, consisting of insulating cardboard, pasted over on one side with Mylar film. Film-electrocardboard on lavsan film has a thickness of 0.27 and 0.32 mm. It is produced in rolls 500 mm wide. Film asbestos cardboard is a flexible electrical insulating material consisting of a lavsan film 50 microns thick, glued on both sides with asbestos paper 0.12 mm thick. Film asbestos cardboard is produced in sheets of 400 x 400 mm (at least) with a thickness of 0.3 mm.
4. Electrical insulating varnishes and enamels
Varnishes are solutions of film-forming substances: resins, bitumen, drying oils, cellulose ethers or compositions of these materials in organic solvents. In the process of drying the lacquer, solvents evaporate from it, and physico-chemical processes occur in the lacquer base, leading to the formation of a lacquer film. According to their purpose, electrical insulating varnishes are divided into impregnating, coating and adhesive.
Impregnating varnishes are used to impregnate the windings of electrical machines and apparatuses in order to fix their turns, increase the thermal conductivity of the windings and increase their moisture resistance. Coating varnishes allow you to create protective moisture-resistant, oil-resistant and other coatings on the surface of windings or plastic and other insulating parts. Adhesive varnishes are intended for gluing mica leaves with each other or with paper and fabrics in order to obtain mica electrical insulating materials (micanite, mica tape, etc.).
Enamels are varnishes with pigments introduced into them - inorganic fillers (zinc oxide, titanium dioxide, red iron, etc.). Pigments are introduced to increase the hardness, mechanical strength, moisture resistance, blow resistance and other properties of enamel films. Enamels are classified as covering materials.
According to the method of drying, varnishes and enamels of hot (furnace) and cold (air) drying are distinguished. The former require a high temperature for their curing - from 80 to 200 ° C, and the latter dry at room temperature. Varnishes and stoving enamels, as a rule, have higher dielectric, mechanical and other properties. In order to improve the characteristics of air-drying varnishes and enamels, as well as to speed up curing, they are sometimes dried at elevated temperatures - from 40 to 80 ° C.
The main groups of varnishes have the following features. Oil varnishes form, after drying, flexible elastic films of yellow color, resistant to moisture and heated mineral oil. In terms of heat resistance, the films of these varnishes belong to class A. In oil varnishes, scarce linseed and tung oils are used, so they are replaced by varnishes based on synthetic resins, which are more resistant to thermal aging.
Oil-bitumen varnishes form flexible black films, resistant to moisture, but easily soluble in mineral oils (transformer and lubricating). In terms of heat resistance, these varnishes belong to class A (105 ° C). Glyphthalic and oil-glyphthalic lacquers and enamels are characterized by good adhesion to mica, papers, fabrics and plastics. The films of these varnishes have increased heat resistance (class B). They are resistant to heated mineral oil, but require hot drying at temperatures of 120-130 ° C. Pure glyphthalic varnishes based on unmodified glyphthalic resins form hard, inflexible films used in the production of hard mica insulation (hard micanites). Oil-glyptal varnishes, after drying, give flexible elastic films of yellow color.
Silicone varnishes and enamels are characterized by high heat resistance and can work for a long time at 180-200 ° C, so they are used in combination with fiberglass and mica insulation. In addition, the films have high moisture resistance and resistance to electrical sparks.
Varnishes and enamels based on PVC and perchlorovinyl resins are resistant to water, heated oils, acidic and alkaline chemicals, therefore they are used as coating varnishes and enamels to protect windings, as well as metal parts from corrosion. Attention should be paid to the weak adhesion of PVC and perchlorovinyl varnishes and enamels to metals. The latter are first covered with a layer of soil, and then with varnish or enamel based on polyvinyl chloride resins. Drying of these varnishes and enamels is carried out at 20, as well as at 50-60 ° C. The disadvantages of this kind of coatings include their low operating temperature, which is 60-70 ° C.
Varnishes and enamels based epoxy resins are characterized by high adhesive ability and slightly increased heat resistance (up to 130 ° C). Varnishes based on alkyd and phenolic resins (phenol-alkyd varnishes) have good drying properties in thick layers and form elastic films that can work for a long time at temperatures of 120-130 ° C. The films of these varnishes are moisture and oil resistant.
Water-based varnishes are stable emulsions of varnish bases in tap water. Lacquer bases are made from synthetic resins, as well as from drying oils and their mixtures. Water-based varnishes are fire and explosion-proof, because they do not contain flammable organic solvents. Due to their low viscosity, these varnishes have a good impregnating ability. They are used for impregnation of fixed and moving windings of electrical machines and devices that operate for a long time at temperatures up to 105 ° C.
5. Electrical insulating compounds
Compounds are insulating compounds that are liquid at the time of use and then harden. Compounds do not contain solvents. According to their purpose, these compositions are divided into impregnating and filling. The first of them is used for impregnating the windings of electrical machines and apparatuses, the second - for filling cavities in cable boxes, as well as in electrical machines and devices for the purpose of sealing.
Compounds are thermosetting (not softening after curing) and thermoplastic (softening with subsequent heating). Compounds based on epoxy, polyester and some other resins can be attributed to thermosets. Thermoplastic compounds include compounds based on bitumen, waxy dielectrics and thermoplastic polymers (polystyrene, polyisobutylene, etc.). Impregnation and potting compounds based on bitumen in terms of heat resistance belong to class A (105 ° C), and some to class Y (up to 90 ° C). Epoxy and organosilicon compounds have the highest heat resistance.
MBK compounds are made on the basis of methacrylic esters and are used as impregnating and filling compounds. After hardening at 70-100°C (and with special hardeners at 20°C) they are thermosetting substances that can be used in the temperature range from -55 to +105°C.
6. Unimpregnated fibrous electrical insulating materials
This group includes sheet and roll materials consisting of fibers of organic and inorganic origin. Fibrous materials of organic origin (paper, cardboard, fiber and fabric) are obtained from plant fibers of wood, cotton and natural silk. The normal moisture content of insulating cardboards, paper and fibers ranges from 6 to 10%. Fibrous organic materials based on synthetic fibers (nylon) have a moisture content of 3 to 5%. Approximately the same humidity is observed in materials obtained on the basis of inorganic fibers (asbestos, fiberglass). Characteristic features inorganic fibrous materials are their incombustibility and high heat resistance (class C). These valuable properties in most cases are reduced when these materials are impregnated with varnishes.
Insulating paper is usually made from wood pulp. The mica paper used in the production of mica tapes has the highest porosity. Electric cardboard is made from wood pulp or from a mixture of cotton fibers and wood (sulfate) pulp fibers taken in various proportions. An increase in the content of cotton fibers reduces the hygroscopicity and shrinkage of the cardboard. Electric cardboard designed to work in air has a denser structure compared to cardboard designed to work in oil. Cardboard with a thickness of 0.1-0.8 mm is produced in rolls, and cardboard with a thickness of 1 mm or more is produced in sheets of various sizes. Fiber is a monolithic material obtained by pressing sheets of paper, pre-treated with a heated solution of zinc chloride and washed in water. The fiber lends itself to all types of mechanical processing and molding after soaking its blanks in hot water.
Leteroid- thin sheet and roll fiber used for the manufacture different kind electrical insulating gaskets, washers and fittings.
Asbestos papers, cardboards and tapes are made from chrysotile asbestos fibers, which have the greatest elasticity and the ability to twist into threads. All asbestos materials are resistant to alkalis, but are easily destroyed by acids.
Electrically insulating glass tapes and fabrics are produced from glass threads obtained from alkali-free or low-alkali glasses. The advantage of glass fibers over vegetable and asbestos fibers is their smooth surface, which reduces the absorption of moisture from the air. The heat resistance of glass fabrics and tapes is higher than asbestos ones.
7. Electrical insulating varnished fabrics (varnished fabrics)
Varnished fabrics are flexible materials consisting of fabric impregnated with varnish or some kind of electrical insulating compound. The impregnating varnish or composition after curing forms a flexible film, which provides good electrical insulating properties of the varnished fabric. Depending on the fabric base, varnished fabrics are divided into cotton, silk, nylon and glass (fiberglass).
Oil, oil-bitumen, escapon and organosilicon varnishes, as well as organosilicon enamels, solutions of organosilicon rubbers, etc. are used as impregnating compositions for varnished fabrics. Silk and nylon varnished fabrics have the greatest extensibility and flexibility. They can operate at temperatures up to 105°C (class A). All cotton varnished fabrics belong to the same class of heat resistance.
The main areas of application of varnished fabrics are: electrical machines, apparatus and low voltage devices. Varnished fabrics are used for flexible coil and slot insulation, as well as various electrical insulating gaskets.
8. Plastics
Plastic masses (plastics) are called solid materials, which at a certain stage of manufacture acquire plastic properties and in this state, products of a given shape can be obtained from them. These materials are composite substances consisting of a binder, fillers, dyes, plasticizers and other components. The starting materials for the production of plastic products are pressing powders and pressing materials. In terms of heat resistance, plastics are thermosetting and thermoplastic.
9. Laminated electrical insulating plastics
Laminated plastics - materials consisting of alternating layers of sheet filler (paper or fabric) and a binder. The most important of the layered electrically insulating plastics are getinaks, textolite and fiberglass. They consist of sheet fillers arranged in layers, and bakelite, epoxy, silicone resins and their compositions are used as a binder.
As fillers, special grades of impregnating paper (in getinax), cotton fabrics (in textolite) and alkali-free glass fabrics (in fiberglass) are used. The listed fillers are first impregnated with bakelite or silicone varnishes, dried and cut into sheets. certain size. Prepared sheet fillers are collected in packages of a given thickness and subjected to hot pressing, during which individual sheets are firmly connected to each other with the help of resins.
Getinaks and textolite are resistant to mineral oils, therefore they are widely used in oil-filled electrical appliances and transformers. The cheapest laminate is wood-laminated plastic (delta wood). It is obtained by hot pressing thin sheets of birch veneer, pre-impregnated with bakelite resins. Delta wood is used for the manufacture of power structural and electrical insulating parts operating in oil. To work on outdoors this material needs careful protection from moisture.
Asbestos textolite is a layered electrically insulating plastic obtained by hot pressing sheets of asbestos fabric, pre-impregnated with bakelite resin. It is produced in the form of shaped products, as well as in the form of sheets and plates with a thickness of 6 to 60 mm. Asbogetinax is a laminated plastic obtained by hot pressing sheets of asbestos paper containing 20% sulfate cellulose or asbestos paper without cellulose, impregnated with an epoxy-phenol-formaldehyde binder.
Of the considered layered electrical insulating materials, fiberglass laminates based on organosilicon and epoxy binders have the highest heat resistance, the best electrical and mechanical characteristics, increased moisture resistance and resistance to fungal mold.
10. Wound electrical insulating products
Wound electrical insulating products are solid tubes and cylinders made by winding any fibrous materials on metal round rods, pre-impregnated with a binder. As fibrous materials, special grades of winding or impregnating papers are used, as well as cotton fabrics and fiberglass. Binders are bakelite, epoxy, organosilicon and other resins.
The wound electrical insulating products, together with the metal rods on which they are wound, are dried at high temperature. For the purpose of hygroscopicity of the wound products, they are varnished. Each layer of varnish is dried in an oven. Solid textolite rods can also be classified as wound products, because they are also obtained by winding blanks from a textile filler impregnated with bakelite varnish. After that, the blanks are subjected to hot pressing in steel molds. Wound electrical insulating products are used in transformers with air and oil insulation, in air and oil circuit breakers, various electrical appliances and electrical equipment units.
11. Mineral electrical insulating materials
Mineral electrical insulating materials include rocks: mica, marble, slate, soapstone and basalt. This group also includes materials obtained from Portland cement and asbestos (asbestos cement and asboplast). This whole group of inorganic dielectrics is characterized by high resistance to electric arc and has sufficiently high mechanical characteristics. Mineral dielectrics (except mica and basalt) can be machined, except for threading.
Electrical insulating products from marble, slate and soapstone are obtained in the form of boards for panels and electrical insulating bases for knife switches and low voltage switches. Exactly the same products from fused basalt can only be obtained by casting into molds. In order for basalt products to have the necessary mechanical and electrical characteristics, they are subjected to heat treatment in order to form a crystalline phase in the material.
Electrical insulating products made of asbestos cement and asboplast are boards, bases, partitions and arc chutes. For the manufacture of such products, a mixture consisting of Portland cement and asbestos fiber is used. Asboplast products are obtained by cold pressing from a mass to which 15% of a plastic substance (kaolin or molding clay) is added. This achieves a greater fluidity of the initial pressing mass, which makes it possible to obtain electrical insulating products of a complex profile from asboplast.
The main disadvantage of many mineral dielectrics (with the exception of mica) is the low level of their electrical characteristics, caused by a large number of pores and the presence of iron oxides. This phenomenon allows the use of mineral dielectrics only in low voltage devices.
In most cases, all mineral dielectrics, except for mica and basalt, are impregnated with paraffin, bitumen, styrene, bakelite resins, etc. before use. The greatest effect is achieved by impregnating already machined mineral dielectrics (panels, partitions, chambers, etc.).
Marble and products from it do not tolerate sudden changes in temperature and crack. Slate, basalt, soapstone, mica and asbestos cement are more resistant to sudden changes in temperature.
12. Mica electrical insulating materials
These materials consist of mica sheets glued together with some kind of resin or adhesive varnish. The glued mica materials include micanites, mikafolium and mica tapes. Glued mica materials are mainly used for insulating windings of high voltage electrical machines (generators, electric motors), as well as for insulating low voltage machines and machines operating under difficult conditions.
Micanites are hard or flexible sheet materials obtained by gluing plucked mica leaves with shellac, glyptal, organosilicon and other resins or varnishes based on these resins.
The main types of micanites- collector, gasket, molding and flexible. Collector and gasket micanites belong to the group of solid micanites, which, after gluing mica, are pressed at elevated specific pressures and heating. These micanites have less shrinkage in thickness and greater density. Moldable and flexible micanites have a looser structure and lower density.
collector micanite- this is a solid sheet material made from mica sheets glued together with shellac or glyptal resins or varnishes based on these resins. To ensure mechanical strength when working in the collectors of electrical machines, no more than 4% of the adhesive is introduced into these micanites.
Gasket micanite is a solid sheet material made from plucked mica sheets glued together with shellac or glyptal resins or varnishes based on them. After gluing, the sheets of cushioning micanite are pressed. This material contains 75-95% mica and 25-5% adhesive.
Molding micanite- a solid sheet material made from leaves of plucked mica glued together with shellac, glyphthalic or silicone resins or varnishes based on them. After gluing, the molding micanite sheets are pressed at a temperature of 140-150°C.
Flexible micanite is a sheet material that is flexible at room temperature. It is made from plucked mica leaves glued together with oil-bitumen, oil-glyphthalic or organosilicon varnish (without desiccant), forming flexible films.
Some types of flexible micanite are glued on both sides with mica paper to increase mechanical strength. Flexible glass micanite is a sheet material that is flexible at room temperature. This is a kind of flexible micanite, characterized by increased mechanical strength and increased resistance to heat. This material is made from leaves of plucked mica glued together with organosilicon or oil-glyptal varnishes, forming flexible heat-resistant films. Sheets of flexible glass micanite are pasted over on both or one side with alkali-free fiberglass.
Mikafoly- this is a rolled or sheet electrical insulating material molded in a heated state. It consists of one or more, more often two or three, layers of mica sheets glued together and with a sheet of paper 0.05 mm thick, or with fiberglass, or with a fiberglass mesh. Shellac, glyptal, polyester or organosilicon are used as adhesive varnishes.
Micalenta is a rolled electrical insulating material, flexible at room temperature. It consists of one layer of plucked mica leaves glued together and pasted over on one or both sides with thin mica paper, fiberglass or fiberglass. Oil-bitumen, oil-glyphthalic, organosilicon and rubber solutions are used as adhesive varnishes.
Mikashelk- rolled electrical insulating material, flexible at room temperature. Mikashelk is one of the varieties of mica tape, but with increased mechanical tensile strength. It consists of a single layer of plucked mica leaves glued together and pasted over on one side with a cloth of natural silk, and on the other with mica paper. As adhesive varnishes, oil-glyphthalic or oil-bitumen varnishes were used, forming flexible films.
Mikapolotno- roll or sheet electrical insulating material, flexible at room temperature. The mica canvas consists of several layers of plucked mica glued together and pasted over on both sides with cotton fabric (percale) or mica paper on one side and cloth on the other.
Micalex is a mica plastic made by pressing a mixture of powdered mica and glass. After pressing, the products are subjected to heat treatment (drying). Mikalex is produced in the form of plates and rods, as well as in the form of electrical insulating products (panels, bases for switches, air capacitors, etc.). When pressing mycalex products, metal parts can be added to them. These products lend themselves to all types of mechanical processing.
13. Mica electrical insulating materials
In the development of natural mica and in the manufacture of electrical insulating materials based on plucked mica, a large amount of waste remains. Their utilization makes it possible to obtain new electrical insulating materials - mica. Such materials are made from mica paper, pre-treated with some kind of adhesive (resins, varnishes). Solid or flexible mica electrical insulating materials are obtained from mica paper by gluing with adhesive varnishes or resins and subsequent hot pressing. Adhesive resins can be introduced directly into the liquid mica mass - mica suspension. Among the most important mica materials, the following should be mentioned.
Collector slyudinite- solid sheet material, calibrated in thickness. It is obtained by hot pressing sheets of mica paper treated with shellac varnish. Collector mica is produced in sheets ranging in size from 215 x 400 mm to 400 x 600 mm.
Slyudinite gasket- solid sheet material obtained by hot pressing sheets of mica paper impregnated with adhesive varnishes. Gasket mica is produced in sheets 200 x 400 mm in size. Solid gaskets and washers are made from it for electrical machines and apparatuses with normal and increased overheating.
Glass mica molding- solid sheet material in a cold state and flexible - in a heated state. It is obtained by gluing mica paper with fiberglass substrates. Molding heat-resistant glass mica is a solid sheet material molded in a heated state. It is made by gluing sheets of mica paper with fiberglass using a heat-resistant silicone varnish. It is available in sheets of 250 x 350 mm or more. This material has increased mechanical tensile strength.
Slyudinite flexible- sheet material, flexible at room temperature. It is obtained by gluing sheets of mica paper, followed by hot pressing. Polyester or organosilicon varnish is used as a binder. Most types of flexible mica are pasted over with fiberglass on one or both sides. Flexible (heat-resistant) glass mica is a sheet material that is flexible at room temperature. It is produced by gluing one or more sheets of mica paper with fiberglass or glass mesh using silicone varnishes. After gluing, the material is subjected to hot pressing. It is covered with fiberglass on one or both sides in order to increase mechanical strength.
Sludinitofolium- roll or sheet material, flexible when heated, obtained by gluing one or more sheets of mica paper with telephone paper 0.05 mm thick, used as a flexible substrate. The scope of this material is the same as that of micafolium based on plucked mica. Sludinitofolium is produced in rolls 320-400 mm wide.
Mica tape- rolled heat-resistant material, flexible at room temperature, consisting of mica paper, glued on one or both sides with fiberglass mesh or fiberglass. Mica tapes are produced mainly in rolls with a width of 15, 20, 23, 25, 30 and 35 mm, less often in rolls.
Glass Boom Mica Tape- rolled, cold-flexible material consisting of mica paper, fiberglass and mica paper, glued and impregnated with epoxy-polyester varnish. From the surface, the tape is covered with a sticky layer of the compound. It is produced in rollers with a width of 15, 20, 23, 30, 35 mm.
Glass mica electric cardboard- sheet material, flexible at room temperature. It is obtained by gluing mica paper, electric cardboard and fiberglass with varnish. Is issued in sheets of 500 x 650 mm in size.
14. Mica plastic electrical insulating materials
All micaceous materials are produced by gluing and pressing sheets of micaceous paper. The latter is obtained from non-industrial mica waste as a result of mechanical crushing of particles by an elastic wave. Compared to mica, mica-plastic materials have greater mechanical strength, but are less homogeneous, since they consist of larger particles than mica. The most important mica-plastic insulating materials are as follows.
Mica collector- solid sheet material, calibrated in thickness. It is obtained by hot pressing sheets of mica-plastic paper, previously coated with a layer of adhesive. Is issued in sheets of 215 x 465 mm in size.
Mica cushioning- solid sheet material made by hot pressing sheets of micaceous paper coated with a layer of binder. Is issued in sheets of 520 x 850 mm in size.
Micaceous molding- pressed sheet material, hard when cold and capable of being formed when heated. Available in sheets ranging in size from 200 x 400 mm to 520 x 820 mm.
Mica flexible- pressed sheet material, flexible at room temperature. Available in sheets ranging in size from 200 x 400 mm to 520 x 820 mm.
Glass mica flexible- pressed sheet material, flexible at room temperature, consisting of several layers of mica-plastic paper, pasted over on one side with fiberglass, and on the other with fiberglass mesh or on both sides with glass mesh. Available in sheets ranging in size from 250 x 500 mm to 500 x 850 mm.
Mica plastofolium- roll or sheet material, flexible and formed in a heated state, obtained by gluing several sheets of micaceous paper and pasted on one side with telephone paper or without it.
Mica tape- flexible at room temperature roll material, consisting of mica paper, pasted over with mica paper on both sides. This material is available in rolls 12, 15, 17, 24, 30 and 34 mm wide.
Heat-resistant glass mica tape- a flexible material at room temperature, consisting of a single layer of mica-plastic paper, pasted over on one or both sides with fiberglass or glass mesh using a silicone varnish. The material is produced in rollers 15, 20, 25, 30 and 35 mm wide.
15. Electroceramic materials and glasses
Electroceramic materials are artificial solids resulting from heat treatment(firing) of the initial ceramic masses, consisting of various minerals (clay, talc, etc.) and other substances, taken in a certain ratio. Various electroceramic products are obtained from ceramic masses: insulators, capacitors, etc.
In the process of high-temperature firing of these products, complex physical and chemical processes occur between the particles of the initial substances with the formation of new substances of a crystalline and glassy structure.
Electroceramic materials are divided into 3 groups: materials from which insulators are made (insulator ceramics), materials from which capacitors are made (capacitor ceramics), and ferroceramic materials with anomalously high values of dielectric constant and piezoelectric effect. The latter have been used in radio engineering. All electroceramic materials are distinguished by high heat resistance, weather resistance, resistance to electric sparks and arcs, and have good electrical insulating properties and sufficiently high mechanical strength.
Along with electroceramic materials, many types of insulators are made of glass. Low-alkaline and alkaline glass is used for the production of insulators. Most types of high voltage insulators are made from tempered glass. Tempered glass insulators are superior in mechanical strength to porcelain insulators.
16. Magnetic materials
The quantities by which the magnetic properties of materials are evaluated are called magnetic characteristics. These include: absolute magnetic permeability, relative magnetic permeability, temperature coefficient of magnetic permeability, maximum magnetic field energy, etc. All magnetic materials are divided into two main groups: magnetically soft and magnetically hard.
Magnetically soft materials are distinguished by low hysteresis losses (magnetic hysteresis is the lag of the body's magnetization from the external magnetizing field). They have relatively large values of magnetic permeability, low coercive force and relatively high saturation induction. These materials are used for the manufacture of magnetic cores of transformers, electrical machines and devices, magnetic screens and other devices that require magnetization with low energy losses.
Magnetically hard materials are characterized by large hysteresis losses, i.e., they have a large coercive force and a large residual induction. These materials, being magnetized, can store the received magnetic energy for a long time, i.e., they become sources of a constant magnetic field. Hard magnetic materials are used to make permanent magnets.
According to their basis, magnetic materials are divided into metallic, non-metallic and magnetodielectrics. Metallic magnetically soft materials include: pure (electrolytic) iron, sheet electrical steel, iron-armco, permalloy (iron-nickel alloys), etc. Metallic magnetically hard materials include: alloyed steels, special alloys based on iron, aluminum and nickel and alloying components (cobalt, silicon, etc.). Ferrites are non-metallic magnetic materials. These are materials obtained from a powder mixture of oxides of certain metals and iron oxide. Pressed ferrite products (cores, rings, etc.) are fired at a temperature of 1300-1500 ° C. Ferrites are magnetically soft and magnetically hard.
Magnetodielectrics are composite materials consisting of 70-80% powdered magnetic material and 30-20% organic high-polymer dielectric. Ferrites and magnetodielectrics differ from metallic magnetic materials in their high volume resistivity, which sharply reduces eddy current losses. This allows the use of these materials in high frequency technology. In addition, ferrites have the stability of their magnetic characteristics over a wide frequency range.
17. Electrical sheet steel
Electrical steel is a magnetically soft material. To improve the magnetic characteristics, silicon is added to it, which increases the resistivity of steel, which leads to a decrease in eddy current losses. Such steel is produced in the form of sheets with a thickness of 0.1; 0.2; 0.35; 0.5; 1.0 mm, width from 240 to 1000 mm and length from 720 to 2000 mm.
18. permalloys
These materials are iron-nickel alloys with nickel content from 36 to 80%. To improve certain characteristics of permalloys, chromium, molybdenum, copper, etc. are added to their composition. Characteristic features of all permalloys are their easy magnetization in weak magnetic fields and increased values of electrical resistivity.
permalloys- ductile alloys, easily rolled into sheets and strips up to 0.02 mm thick or less. Due to the increased resistivity and stability of the magnetic characteristics, permalloys can be used up to frequencies of 200-500 kHz. Permalloys are very sensitive to deformations that cause deterioration of their original magnetic characteristics. The restoration of the initial level of the magnetic characteristics of the deformed permalloy parts is achieved by their heat treatment according to a strictly developed regime.
19. Magnetic hard materials
magnetic semiconductor electrically insulating electrical
Magnetically hard materials have large values of coercive force and high residual induction, and consequently, large values of magnetic energy. Hard magnetic materials include:
alloys hardened to martensite (steels alloyed with chromium, tungsten or cobalt);
iron-nickel-aluminum non-forging precipitation hardening alloys (alni, alnico, etc.);
malleable alloys based on iron, cobalt and vanadium (vikkaloy) or based on iron, cobalt, molybdenum (comol);
alloys with a very high coercive force based on noble metals (platinum - iron; silver - manganese - aluminum, etc.);
· ceramic-metal non-forged materials obtained by pressing powdered components with subsequent firing of pressed products (magnets);
magnetically hard ferrites;
· metal-plastic non-forged materials obtained from pressing powders consisting of particles of a magnetically hard material and a binder (synthetic resin);
magneto-elastic materials (magnetoelasts), consisting of a powder of a magnetically hard material and an elastic binder (rubber, rubber).
The residual induction of metal-plastic and magneto-elastic magnets is 20-30% less compared to cast magnets from the same hard magnetic materials (alni, alnico, etc.).
20. Ferrites
Ferrites are non-metallic magnetic materials made from a mixture of specially selected metal oxides with iron oxide. The name of the ferrite is determined by the name of the divalent metal, the oxide of which is part of the ferrite. So, if zinc oxide is included in the ferrite, then the ferrite is called zinc; if manganese oxide is added to the composition of the material - manganese.
In technology, complex (mixed) ferrites are used, which have higher values of magnetic characteristics and greater resistivity compared to simple ferrites. Examples of complex ferrites are nickel-zinc, manganese-zinc, etc.
All ferrites are polycrystalline substances obtained from metal oxides as a result of sintering powders of various oxides at temperatures of 1100-1300 ° C. Ferrites can only be processed with an abrasive tool. They are magnetic semiconductors. This allows them to be used in high frequency magnetic fields, since their eddy current losses are negligible.
21. Semiconductor materials and products
Semiconductors include a large number of materials that differ from each other internal structure, chemical composition and electrical properties. According to the chemical composition, crystalline semiconductor materials are divided into 4 groups:
1) materials consisting of atoms of one element: germanium, silicon, selenium, phosphorus, boron, indium, gallium, etc.;
2) materials consisting of metal oxides: copper oxide, zinc oxide, cadmium oxide, titanium dioxide, etc.;
3) materials based on compounds of atoms of the third and fifth groups of the Mendeleev's system of elements, denoted by the general formula and called antimonides. This group includes compounds of antimony with indium, with gallium, etc., compounds of atoms of the second and sixth groups, as well as compounds of atoms of the fourth group;
4) semiconductor materials of organic origin, such as polycyclic aromatic compounds: anthracene, naphthalene, etc.
According to the crystal structure, semiconductor materials are divided into 2 groups: single-crystal and polycrystalline semiconductors. The first group includes materials obtained in the form of large single crystals (single crystals). Among them are germanium, silicon, from which plates are cut for rectifiers and other semiconductor devices.
The second group of materials are semiconductors, which consist of many small crystals soldered together. Polycrystalline semiconductors are: selenium, silicon carbide, etc.
In terms of volume resistivity, semiconductors occupy an intermediate position between conductors and dielectrics. Some of them dramatically reduce electrical resistance when exposed to high voltage. This phenomenon has found application in valve arresters for the protection of power lines. Other semiconductors dramatically reduce their resistance when exposed to light. This is used in photocells and photoresistors. A common property for semiconductors is that they have both electron and hole conduction.
22. Electric carbon products (brushes for electric machines)
This kind of products include brushes for electric machines, electrodes for arc furnaces, contact parts, etc. Electric coal products are made by pressing from the initial powdery masses, followed by firing.
The initial powdery masses are composed of a mixture of carbonaceous materials (graphite, soot, coke, anthracite, etc.), binders and plasticizers (coal and synthetic tars, pitches, etc.). In some powdered masses there is no binder.
Brushes for electric machines are graphite, carbon-graphite, electrographite, metal-graphite. Graphite brushes are made from natural graphite without a binder (soft grades) and with a binder (hard grades). Graphite brushes are soft and cause little noise during operation. Carbon-graphite brushes are made from graphite with the addition of other carbonaceous materials (coke, soot), with the introduction of binders. The brushes obtained after heat treatment are coated with a thin layer of copper (in an electrolytic bath). Carbon-graphite brushes have increased mechanical strength, hardness and low wear during operation.
Electrographite brushes are made from graphite and other carbonaceous materials (coke, soot), with the introduction of binders. After the first firing, the brushes are subjected to graphitization, i.e., annealing at a temperature of 2500-2800 ° C. Electrographitized brushes have increased mechanical strength, resistance to jerky load changes and are used at high circumferential speeds. Metal-graphite brushes are made from a mixture of graphite and copper powders. Some of them introduce powders of lead, tin or silver. These brushes feature low resistivity, high current densities and low transient voltage drops.
Lecture 1
Introduction. Subject and content of the course. Classification of electrical materials by properties and applications. The role of electrical materials in the development of energy.
Every specialist working in the field of electrical and radio engineering should know list of basic electrical, magnetic, mechanical and other characteristics possessed by dielectric, semiconductor, conductive, magnetic and structural materials. In the manufacture and repair of radio and electrical equipment, parts and assemblies are required that are made of materials of certain classes and have specific electrical and magnetic characteristics, and for load-bearing parts - mechanical characteristics. Knowing for each class of materials a list of these characteristics, it is necessary to know the units of their measurement and the order of magnitude, as well as how (and why) these characteristics change under the influence of temperature, magnitude and frequency of stress, mechanical load, etc.
I want to please you, you already have a certain amount of knowledge in this course. So, for example, when buying clothes, shoes and other goods, you choose them based not only on their shape, size and operating conditions (in winter or summer, in rainy or humid weather, etc.), but also on the characteristics of the materials from which they are made - color, thermal conductivity, resistance to water, sunlight, etc.
Materials science is a science that studies the composition, structure, properties of materials, the behavior of materials under various influences: thermal, electrical, magnetic, etc., as well as a combination of these influences.
Electrotechnical materials science is a branch of materials science that deals with materials for electrical engineering and energy, i.e. materials with specific properties necessary for the design, production and operation of electrical equipment.
In this course "Materials Science", Part II, the following will be considered:
ü Structure and properties of metallic and non-metallic electrical materials;
ü We will consider in detail the features of polarization, electrical conductivity, dielectric losses and breakdown of dielectrics, we will study the process of electrical aging of insulation;
ü New electrical materials will be studied: active dielectrics, conductors, superconductors used in modern devices.
ü The physics of phenomena occurring in dielectrics, semiconductors, conductors and magnetic materials in an electric or magnetic field will be considered;
For a better understanding of the material being studied, a multi-projector will be used in lectures, some information will be presented in the form of handouts.
The course includes 6 hours of lectures and laboratory classes, 36 hours of independent work. At the end - a test.
To study the course, you will need the following literature:
1. N.P. Bogoroditsky. Electrotechnical materials: Textbook for universities / N.P. Bogoroditsky, V.V. Pasynkov, B.M. Tareev. – L.: Energoatomizdat., 1985.
2. Kolesov S.N. Materials Science and Technology of Structural Materials: Textbook for High Schools / S.N. Kolesov, I.S. Kolesov. - 2nd ed., revised. and additional - M.: Higher. school, 2007. - 535 p.: ill.
3. Pasynkov V.V. Materials of electronic technology: Textbook for universities / V.V. Pasynkov, V.S. Sorokin. - M .: Higher. school, 2003.
4. Novikov L.I. Guidelines for laboratory exercises No. 1, 2, 3, 4: Guidelines / L.I. Novikov. - Kirov, ed. VyatGU, 2007.
5. Novikov L.I. Guidelines for laboratory exercises No. 6: Guidelines / L.I. Novikov. - Kirov, ed. VyatGU, 2007.
The role of materials in the development of electrical and radio engineering
Contemporary electrical equipment is a complex device with a large number of various parts, the manufacture of which requires a wide range of various electrical and structural materials with well-defined electrical, mechanical and chemical properties that depend on their chemical composition and structure, as well as the intensity of external energy exposure (electric field strength and frequency, temperature, pressure, etc.). Without knowing the basic properties of ETMs, without understanding the physical processes that occur in ETMs when they are placed in an electric or magnetic field, without understanding the relationship of these processes with the chemical composition and structure of the material, it is impossible to design and manufacture electrical equipment, it is impossible to correctly operate it. Therefore, the main task of the science of materials science is:
1. The study of the main physical processes occurring in materials when exposed to electric, magnetic or thermal fields and mechanical stress;
2. Studying the dependence of electrical, mechanical and other properties of materials on their chemical composition and structure;
3. Description of the properties and familiarity with the materials most commonly used in production electrical equipment.
Classification of electrical materials by properties and applications
To begin with, let's note what a material is.
Material- an object with a certain composition, structure and properties, designed to perform certain functions.
Materials vary in:
1. Aggregate state:
a. Solid;
b. liquid;
c. gaseous;
d. Plasma (a state of ionized gas in which the concentration of positive and negative charges are equal).
2. Functions performed. The functions that materials perform are varied:
a. Ensuring the flow of current - conductive materials;
b. Preservation of a certain shape under mechanical loads (KM);
c. Providing insulation - dielectric materials;
d. transformation electrical energy in thermal - resistive materials.
Typically, the material performs several functions. For example, a dielectric necessarily experiences some kind of mechanical load, that is, it is a structural material.
Classification of substances by electrical properties:
During manufacture and under various operating conditions radio electronic equipment ETM is affected by electric and magnetic fields (separately and jointly). Based on their behavior in an electric field, these materials are subdivided into conductor, semiconductor, and dielectric materials.
The classification of ETMs according to electrical properties is based on the concepts of the band theory of the electrical conductivity of solids, the essence of which is as follows.
It is well known that in the vast majority of solids, electric current is due to the movement of electrons. Such electrons are called conduction electrons. They appear in the outer regions of the atom, remote from the nucleus. These regions form valence bands in the solid. In order for an electric current to arise, electrons must climb from the valence band higher on the energy scale and go to the conduction band, while overcoming the zone of forbidden energy values, or the band gap. If all three mentioned bands are placed along the energy axis, then the lower energy band will be the valence band, followed by the band gap and then the band with the highest energy - the conduction band.
Both the valence band and the conduction band are a very dense packing of many discrete energy levels available to electrons - energy "isolines". These levels are located so close to each other that they practically merge into a continuous band, which is called the energy zone. On the contrary, there are no energy levels accessible to an electron in the band gap, and electrons cannot be there. So, in order for an electric current to arise, it is necessary that electrons from the valence band jump over the band gap and fall into the conduction band.
As is known from the school physics course, substances, depending on how they conduct electric current, can be divided into metals, semiconductors and dielectrics. From the point of view of the band theory, metals are solids in which the band gap is absent, instead of it there is a strong overlap of the valence and conducting bands. It turns out that the electrons in the metal do not need to spend energy to overcome the band gap, and therefore, under external influence - in an electric field - they easily pass into the conduction band. From this it is easy to understand why metals are good conductors.
In dielectrics, the band gap is much larger than the thermal energy of electrons even at room temperature, which means that the vast majority of potential current carriers cannot jump into the conduction band - they do not have enough energy. The band gap can be overcome only at very strong fields (then an electrical breakdown of the dielectric is observed) or at very high temperatures.
And finally, if the band gap is comparable to the energy of the thermal motion of electrons, then we have a semiconductor. Increasing the temperature exponentially increases the number of electrons jumping through the band gap into the conduction band.
If W is equal to or close to zero, then electrons can go to free levels due to their own thermal energy and increase the conductivity of the substance. Substances with such a structure of energy bands belong to conductors. Conductive materials are used to conduct electric current. Typically, conductors include substances with electrical resistivity ρ< 10 -5 Ом×м. Типичными проводниками являются металлы.
If the band gap exceeds a few electron volts (1 eV is the energy of an electron obtained by moving between two points of an electric field with a potential difference of 1 V), then significant energy is required to transfer electrons from the valence band to the conduction band. Such substances are dielectrics. Dielectrics have a high electrical resistivity and have the ability to block the flow of current. Dielectric materials include substances with electrical resistivity p > 107 Ohm m. Due to their high electrical resistivity, they are used as electrical insulating materials.
If the band gap is 0.1 ... 0.3 eV, then electrons easily pass from the valence band to the conduction band due to external energy. Substances with controlled conductivity belong to semiconductors. The electrical resistivity of semiconductors is 10 -6 ... 10 9 Ohm×m. Semiconductor materials have conductivity, which can be used to control voltage, temperature, illumination, etc.
Depending on the structure and external conditions materials can move from one class to another. For example, solid and liquid metals are conductors, and metal vapors are dielectrics; semiconductors germanium and silicon, which are typical under normal conditions, become conductors when exposed to high hydrostatic pressures; carbon in the diamond modification is a dielectric, and in the graphite modification it is a conductor.
Dielectric has the ability to polarize under the action of an applied electric field and are divided into:
1. Passive dielectrics. Apply:
a. To create electrical insulation of conductive parts. They prevent the passage of current in other, undesirable ways and are electrical insulating materials.
b. In capacitors to create a certain electrical capacitance.
2. Active dielectrics. Used for the manufacture of active elements electrical circuits. They serve to generate, amplify, convert an electrical signal.
Semiconductor in terms of electrical conductivity, they occupy an intermediate position between dielectrics and conductors. Their characteristic feature is the significant dependence of electrical conductivity on the intensity of external energy impact: electric field strength, temperature, illumination, wavelength of incident light, pressure, etc.
conductors divided into 4 subclasses:
1. High conductivity materials. They are used where it is necessary that the current flow with minimal losses. Such materials include metals: Cu, Al, Fe, Ag, Au, Pt and alloys based on them. They are used to make wires, cables, conductive parts of electrical installations.
2. Superconductors - materials in which, at temperatures below a certain critical Tcr, the resistance to electric current becomes equal to 0.
3. Cryoconductors are highly conductive materials operating at cryogenic temperatures (the boiling point of liquid nitrogen is 195 o C).
4. Conducting materials of high resistance - metal alloys that form solid solutions.
Magnetic- materials designed to work in a magnetic field with direct interaction with this field. These include ferromagnets and ferrites. The intrinsic magnetic field is hundreds and thousands of times greater than the external magnetic field that causes it. They are capable of being strongly magnetized even in weak fields, and some of them remain magnetized even after the removal of the external magnetic field. The most widely used magnetic materials in technology are Fe, Co, Ni.
The most fragile form of communication - molecular bond(van der Waals connection). Such a bond exists in some substances between molecules with covalent intramolecular bonds.
Intermolecular attraction is due to the coordinated movement of valence electrons in neighboring molecules. At any moment in time, the electrons are as far apart as possible from each other and as close as possible to positive charges. In this case, the forces of attraction of valence electrons by the positively charged cores of neighboring molecules turn out to be stronger than the forces of mutual repulsion of the electrons of the outer orbits. The Van der Waals bond is observed between the molecules of certain substances (for example, paraffin) that have a low melting point, indicating the fragility of their crystal lattice.
The main process characteristic of any dielectric, which occurs when an electric voltage is applied to it, is polarization -- limited displacement of bound charges or orientation of dipole molecules.
For brevity, dipole-relaxation polarization is called dipole. It differs from electronic and ionic polarization in that it is associated with the thermal motion of particles. Dipole molecules in chaotic thermal motion are partially oriented under the action of the field, which is the cause of polarization.
Dipole polarization is possible if molecular forces do not prevent dipoles from orienting themselves along the field. As the temperature increases, the molecular forces weaken, the viscosity of the substance decreases, which should enhance the dipole polarization, but at the same time, the energy of the thermal motion of the molecules increases, which reduces the orienting effect of the field. Therefore, with increasing temperature, the dipole polarization first increases (until the weakening of molecular forces has a stronger effect than the increase in chaotic thermal motion), and then, when the chaotic motion becomes more intense, the dipole polarization begins to fall with increasing temperature.
Turning dipoles in the direction of the field in a viscous medium requires overcoming some resistance, and therefore dipole polarization is associated with energy losses.
The permittivity of solids depends on structural features solid dielectric. All types of polarization are possible in solids. For solid nonpolar dielectrics, the same regularities are characteristic as for nonpolar liquids and gases. This is confirmed by dependence ? r (t) for paraffin. During the transition of paraffin from a solid state to a liquid state (melting point of about +54°C), a sharp decrease in the dielectric constant occurs due to a decrease in the density of the substance.
Gaseous substances are characterized by low densities. Therefore, the permittivity of all gases is negligible and close to unity. If the gas molecules are polar, then the polarization can be dipole, however, for polar gases, the electronic polarization is of primary importance.
The polarization of liquids containing dipole molecules is determined by the electron and dipole polarizations. The greater the electric moment of the dipoles and the number of molecules per unit volume, the greater the dielectric permittivity of liquid dielectrics. The permittivity of liquid polar dielectrics varies from 3 to 5.5.
Solid dielectrics, which are ionic crystals with close packing of particles, have electronic and ionic polarizations and have a dielectric constant that varies over a wide range. For inorganic glasses (quasi-amorphous dielectrics), the permittivity varies from 4 to 20. Solid dielectrics, which are ionic crystals with a loose packing of particles, in addition to electronic and ionic polarization, have ion-relaxation polarization and are characterized by a low value of dielectric permittivity. For example ? r rock salt has a value of 6, corundum 10, rutile 110, and calcium titanate 150. (All values ? r are given for a temperature of 20 °C.)
Polar organic dielectrics exhibit dipole-relaxation polarization in the solid state. Such dielectrics include cellulose and products of its processing, polar polymers. Dipole-relaxation polarization is also observed in ice. The permittivity of these materials depends to a large extent on temperature and on the frequency of the applied voltage, following the same patterns as are observed for polar liquids.
It can be noted that the permittivity of ice changes dramatically with temperature and frequency. At low frequencies and temperatures close to 0°C, ice, like water, has ? r ~ 80, however, with decreasing temperature ? r falls rapidly and reaches 2.85.
The dielectric constant of complex dielectrics, which are a mechanical mixture of two components with different dielectric constants, is determined, in the first approximation, on the basis of the logarithmic mixing law.
Current in gases can only occur if there are ions or free electrons in them. Ionization of neutral gas molecules occurs either under the action of external factors, or due to collisions of charged particles with molecules.
The electrical conductivity of liquid dielectrics is closely related to the structure of liquid molecules. In nonpolar liquids, electrical conductivity depends on the presence of dissociated impurities, including moisture. In polar liquids, the electrical conductivity is determined not only by impurities, but sometimes by the dissociation of the molecules of the liquid itself. The current in a liquid can be due to both the movement of ions and the movement of relatively large charged colloidal particles.
The electrical conductivity of solids is determined by the movement of both the ions of the dielectric itself and the ions of random impurities, and in some materials it can be caused by the presence of free electrons. Electronic electrical conductivity is most noticeable in strong electric fields.
In dielectrics with an atomic or molecular lattice, electrical conductivity is associated only with the presence of impurities, their specific conductivity is very small.
In the SI system, volume resistivity ?v equal to the volume resistance of a cube with an edge of 1 m, mentally cut out of the material under study (if the current passes through the cube, from one of its faces to the opposite), multiplied by 1 m.
For a flat sample of material in a uniform field, the volume resistivity (ohm-meter) is calculated by the formula
R-- sample volume resistance, Ohm;
S - electrode area, m 2 ;
h-- sample thickness, m.
Specific volume conductivity? measured in siemens per meter
Dielectric losses (dielectric losses) are the power dissipated in a dielectric when an electric field is applied to it and causing heating of the dielectric. Losses in dielectrics are observed both at alternating voltage and at constant voltage, since a through current is detected in the material due to conductivity.
At a constant voltage, there is no periodic polarization. The quality of the material is characterized by the values of specific volume and surface resistance. With an alternating voltage, it is necessary to use some other characteristic of the quality of the material, since in this case, in addition to the through current, there are additional causes that cause losses in the dielectric.
Dielectric losses in an electrically insulating material can be characterized by power dissipation per unit volume, or specific losses; more often, to assess the ability of a dielectric to dissipate power in an electric field, the dielectric loss angle, as well as the tangent of this angle, is used.
Unacceptably large dielectric losses in the electrical insulating material cause strong heating of the product made from it and can lead to its thermal destruction. Even if the voltage applied to the dielectric is not large enough to cause unacceptable overheating due to dielectric losses, then in this case, large dielectric losses can cause significant harm, increasing, for example, the active resistance of the oscillatory circuit in which this dielectric is used and, consequently, the damping value.
Rubber and paper are organic dielectrics of molecular structure with polar molecules. These substances, due to their inherent dipole-relaxation polarization, have large losses. Loss tangent tg? ~ 0.03, for particulate rubber up to 0.25.
Glasses, inorganic quasi-amorphous substances of ionic structure, which are complex systems of various oxides. Dielectric losses in such substances are associated with the phenomenon of polarization and electrical conductivity. The electrical properties are highly dependent on their composition. For quartz glass, the loss tangent is tg?~0.0002.
Foams are materials with a cellular structure in which gaseous fillers are isolated from each other and from environment thin layers polymer binder. Epoxy resin-based foams have a loss tangent tg? ~ 0.025 - 0.035. Foam plastics based on expanded polystyrene tg? ~ 0.0004.
Thus, less electrical loss is to be expected from glass.
The dielectric, being in an electric field, loses the properties of an electrically insulating material if the field strength exceeds a certain critical value. This phenomenon is called dielectric breakdown or violation of its electrical strength. The voltage at which breakdown of a dielectric occurs is called breakdown voltage, and the corresponding value of the field strength -- dielectric strength.
The breakdown voltage is denoted U np and is usually measured in kilovolts. The electrical strength is determined by the breakdown voltage related to the thickness of the dielectric at the breakdown point:
Where h-- dielectric thickness
Convenient for practical purposes, the numerical values of the electrical strength of dielectrics are obtained if the breakdown voltage is expressed in kilovolts, and the thickness of the dielectric is in millimeters. Then the electrical strength will be in kilovolts per millimeter. To save numerical values and switch to SI units, you can use the unit MV/m:
Liquid dielectrics have a higher electrical strength than gases under normal conditions. Extremely pure liquids are extremely difficult to obtain. Permanent impurities in liquid dielectrics are water, gases and solid particles. The presence of impurities determines mainly the phenomenon of breakdown of liquid dielectrics and causes great difficulties in creating an exact theory of the breakdown of these substances.
The theory of electrical breakdown can be applied to liquids that are maximally purified from impurities. At high electric field strengths, electrons can be ejected from metal electrodes and, as in gases, the molecules of the liquid itself can be destroyed due to impacts with charged particles. In this case, the increased electrical strength of a liquid dielectric compared to a gaseous one is due to a much shorter electron mean free path. The breakdown of liquids containing gas inclusions is explained by local overheating of the liquid due to the energy released in relatively easily ionized gas bubbles, which leads to the formation of a gas channel between the electrodes. Water in the form of individual small droplets in transformer oil at normal temperature significantly reduces E etc. Under the influence of a long electric field, spherical water droplets of a strongly dipole liquid become polarized, acquire the shape of ellipsoids and, being attracted to each other by opposite ends, create chains with increased conductivity between the electrodes, along which an electrical breakdown occurs.
Fired porcelain has a density of 2.3-2.5 Mg/m 3 . Ultimate strength in compression 400-700 MPa, in tension 45-70 MPa, in bending 80-150 MPa. From which it can be seen that the mechanical strength of porcelain is higher when working in compression.
The protective properties of various materials to high-energy corpuscular and wave radiation can be conveniently characterized by the concept of a tenfold attenuation layer, i.e. the thickness of the layer of matter, after passing through, the intensity of the radiation is attenuated ten times. This characteristic greatly simplifies the calculation of protection elements. For example, to weaken by 100 times, it is necessary to take the thickness of the protective substance equal to two layers of tenfold weakening. Obviously, P tenfold attenuation layers will reduce the radiation intensity by a factor of 10n.
The absorption of quantum energy by a substance depends on the density of this substance. Of these substances, lead has the highest density. To absorb 1 MeV quantum radiation, the thickness of lead should be ~ 30 mm, steel ~ 50 mm, concrete ~ 200 mm, water 400 mm. Thus, lead has the smallest thickness of the tenfold attenuation layer.
The most important practically used solid conductor materials in electrical engineering are metals and their alloys. High-conductivity metals with resistivity stand out from them? at normal temperature, not more than 0.05 μΩ * m, and high-resistance alloys having a resistivity? at normal temperature not less than 0.3 μΩ * m. High conductivity metals are used for wires, conductors of cables, windings of electrical machines. Such metals include copper (0.017 μΩ * m), Silver (0.016 μΩ * m) Aluminum (0.028 μΩ * m)
Metals and alloys of high resistance are used for the manufacture of resistors for electric heaters, filaments of incandescent lamps. High-resistance metals and alloys include Manganin (0.42-0.48 µOhm * m), Constantan (0.48-0.52 µOhm * m), Chrome-nickel alloys (1.1-1.2 µOhm * m), Chrome-aluminum (1.2-1.5 µOhm * m), Mercury, Lead, Tungsten.
In 1911, the Dutch physicist H. Kamerliig-Onnes investigated the electrical conductivity of metals at very low temperatures approaching absolute zero. He found that when cooled to a temperature approximately equal to the temperature of helium liquefaction, the resistance of a ring of frozen mercury suddenly, in a sharp jump, drops to an extremely small, unmeasurable value. Such a phenomenon, i.e. the presence of a substance of almost infinite specific conductivity, was called superconductivity. Temperature T WITH , when cooled to which the substance passes into a superconducting state, - superconducting transition temperature. Substances that pass into the superconducting state superconductors.
The phenomenon of superconductivity is due to the fact that an electric current, once induced in a superconducting circuit, will circulate for a long time (for years) along this circuit without a noticeable decrease in its strength, and, moreover, without any energy supply from the outside.
At present, 35 superconducting metals and more than a thousand superconducting alloys and chemical compounds of various elements are already known. At the same time, many substances, including those with very small values? at normal temperature, metals like silver, copper, gold, platinum and others, at the lowest temperatures reached at the present time (about a millikelvin) could not be transferred to the superconducting state.
Semiconductors used in practice can be divided into simple semiconductors (their main composition is formed by atoms of one chemical element) and complex semiconductor compositions, the main composition of which is formed by atoms of two or more chemical elements. Also currently being studied vitreous And liquid semiconductors. Simple semiconductors are: Boron, Silicon, Germanium, Phosphorus, Arsenic, Selenium, Sulfur, Tellurium, Iodine. complex semiconductors are compounds of elements of various groups of the periodic table, corresponding to general formulas A IV B, IV (for example, SiC), A III B V (InSb, GaAs, GaP), A II B IV (CdS, ZnSe), as well as some oxides (CU 2 O). TO semiconductor compositions materials with a semi-conductive or conductive phase of silicon carbide and graphite bonded by a ceramic or other bond can be attributed.
In modern technology, silicon, germanium, and partly selenium, which are used for the manufacture of diodes, triodes, and other semiconductor devices, have gained particular importance.
Thermistors (thermistors) are made in the form of rods, plates or tablets using ceramic technology. The resistance and other properties of thermistors depend not only on the composition, but also on the grain size, on technological process manufacturing: pressure during pressing (if the semiconductor is taken in the form of a powder) and firing temperatures. Thermistors are used for measurement, temperature control and thermal compensation, for voltage stabilization, limiting pulsed starting currents, measuring the thermal conductivity of liquids, as non-contact rheostats and current time relays.
From semiconductor ceramics with a Curie point, thermistors are made, which differ from all other thermistors in that they have not a negative, but a very large positive temperature coefficient of resistance (over + 20% / K) in a narrow temperature range (about 10 ° C). These thermistors are called posistors. They are made in the form of discs of small thickness and are intended for temperature control and regulation, use in fire alarm systems, protecting engines from overheating, limiting currents, measuring the flow of liquids and gases.
Semiconductor oxides are used mainly for the manufacture of thermistors with a large negative temperature coefficient of resistivity [--(Z-4)% / K].
For storage devices of computer technology, ferrites are used that have a rectangular hysteresis loop. The main parameter of products of this type is the squareness coefficient of the hysteresis loop K p which is the ratio of the residual induction W t to the maximum induction B max
Kp \u003d W / Vmax
For the manufacture of transformer cores, soft magnetic materials are used in the form of a set of thin sheets isolated from each other. This design of the transformer core can significantly reduce eddy current losses (Foucault currents).
Hard magnetic materials are used mainly for the manufacture of permanent magnets.
According to the composition, state and method of obtaining hard magnetic materials are divided into:
1) alloyed martensitic steels,
2) cast hard magnetic alloys,
3) powder magnets,
4) hard magnetic ferrites,
5) plastically deformable alloys,
6) magnetic tapes.
The characteristics of materials for permanent magnets are the coercive force, residual induction and the maximum energy given off by the magnet to the external space. The magnetic permeability of materials for permanent magnets is lower than that of soft magnetic materials, and the higher the coercive force, the lower the magnetic permeability.
The most simple and affordable material for the manufacture of permanent magnets are alloyed martensitic steels. They are alloyed with tungsten, chromium, molybdenum, cobalt additives. The value of W max for martensitic steels is 1--4 kJ/m 3 . The magnetic properties of such steels are guaranteed for martensitic steels after a heat treatment specific to each steel grade and a five-hour structural stabilization in boiling water. Martensitic steels began to be used for the production of permanent magnets before all other materials. Currently, they are of limited use due to their low magnetic properties, but they are not completely abandoned, since they are cheap and can be machined on metal-cutting machines.
For work in high-frequency installations, the most suitable material is magnetically hard ferrite (barium ferrite). Unlike soft magnetic ferrites, it has not a cubic, but a hexagonal crystal lattice with uniaxial anisotropy. Barium ferrite magnets have a coercive force of up to 240 kA/m, but in terms of residual induction of 0.38 T and stored magnetic energy of 12.4 kJ/m 3 they are inferior to Alni system alloys. The specific resistance of barium ferrite is 10 4 - 10 7 Ohm * m, i.e. millions of times higher than the resistivity of cast metal hard magnetic alloys.
Metal-plastic magnets (with rather low magnetic properties) have a high electrical resistance and, consequently, a small magnetic loss tangent, which also allows them to be used in equipment with an alternating magnetic field of increased frequency.
The use of electrical materials as components of electrical power, electrical and radio-electronic equipment. Electrical materials are a combination of conductive, electrical insulating, magnetic and semiconductor materials designed to work in electric and magnetic fields. This also includes the main electrical products: insulators, capacitors, wires and some semiconductor elements. Electrical materials in modern electrical engineering occupy one of the main places. Everyone knows that the reliability of the operation of electrical machines, apparatus and electrical installations mainly depends on the quality and correct selection of appropriate electrical materials. An analysis of accidents in electrical machines and apparatus shows that most of them occur as a result of failure of electrical insulation, consisting of electrical insulating materials. Magnetic materials are no less important for electrical engineering. Energy losses and dimensions of electrical machines and transformers are determined by the properties of magnetic materials. A fairly significant place in electrical engineering is occupied by semiconductor materials, or semiconductors. As a result of the development and study of this group of materials, various new devices have been created that make it possible to successfully solve some problems of electrical engineering. With a rational choice of electrical insulating, magnetic and other materials, it is possible to create electrical equipment that is reliable in operation with small dimensions and weight. But to realize these qualities, knowledge of the properties of all groups of electrical materials is required. Conductor materials This group of materials includes metals and their alloys. Pure metals have low resistivity. The exception is mercury, which has a rather high resistivity. Alloys also have high resistivity. Pure metals are used in the manufacture of winding and mounting wires, cables, etc. Conductor alloys in the form of wires and tapes are used in rheostats, potentiometers, additional resistances, etc. Electrical insulating materials Signal filtering against the background of interference. Tasks and methods of filtering An electric filter is a passive four-terminal network that transmits electrical signals of a certain frequency band without significant attenuation or with amplification, and oscillations outside this frequency band with great attenuation. Such devices are used to isolate useful signals against the background of interference. Electrical insulating materials, or dielectrics, are such materials with which insulation is carried out, that is, they prevent the leakage of electric current between any conductive parts that are under different electrical potentials. Dielectrics have very high electrical resistance. According to the chemical composition, dielectrics are divided into: organic and inorganic. The main element in the molecules of all organic dielectrics is carbon. There is no carbon in inorganic dielectrics. Inorganic dielectrics (mica, ceramics, etc.) have the highest heat resistance. According to the method of preparation, natural (natural) and synthetic dielectrics are distinguished. Synthetic dielectrics can be created with a given set of electrical and physico-chemical properties, therefore they are widely used in electrical engineering. Electrical insulating varnishes and enamels Varnishes are solutions of film-forming substances: resins, bitumens, drying oils, cellulose ethers or compositions of these materials in organic solvents. In the process of drying the lacquer, solvents evaporate from it, and physico-chemical processes occur in the lacquer base, leading to the formation of a lacquer film. According to their purpose, electrical insulating varnishes are divided into impregnating, coating and adhesive varnishes. Impregnating varnishes are used to impregnate the windings of electrical machines and apparatus in order to fix their turns, increase the thermal conductivity of the windings and increase their moisture resistance. Coating varnishes allow you to create protective moisture-resistant, oil-resistant and other coatings on the surface of windings or plastic and other insulating parts. Adhesive varnishes are intended for gluing mica leaves with each other or with paper and fabrics in order to obtain mica electrical insulating materials (micanites, mica tape, etc.). Electrical insulating compounds Compounds are insulating compounds that are liquid at the time of use and then harden. Compounds do not contain solvents. According to their purpose, these compositions are divided into impregnating and filling. The first of them is used for impregnating the windings of electrical machines and apparatuses, the second - for filling cavities in cable boxes, as well as in electrical machines and devices for the purpose of sealing. The advantage of glass fibers over vegetable and asbestos fibers is their smooth surface, which reduces the absorption of moisture from the air. The heat resistance of glass fabrics and tapes is higher than asbestos ones. Electrical insulating varnished fabrics (varnished fabrics) The main areas of application for varnished fabrics are: electrical machines, low-voltage apparatus and devices. Varnished fabrics are used for flexible coil and groove insulation, as well as various electrical insulating gaskets. Plastic masses Plastic masses (plastics) are solid materials that acquire plastic properties at a certain stage of manufacture and in this state products of a given shape can be obtained from them. These materials are composite substances consisting of a binder, fillers, dyes, plasticizers and other components. The starting materials for the production of plastic products are pressing powders and pressing materials. By heat resistance, plastics are thermosetting and thermoplastic. ov iron. This phenomenon allows the use of mineral dielectrics only in low voltage devices. Mica electrical insulating materials These materials consist of mica sheets glued together with some kind of resin or adhesive varnish. The glued mica materials include micanites, mikafolium and mica tapes. Glued mica materials are mainly used for insulating the windings of high voltage electrical machines (generators, electric motors), as well as for insulating low voltage machines and machines operating in harsh conditions. Their utilization makes it possible to obtain new electrical insulating materials - mica. Such materials are made from mica paper, pre-treated with some kind of adhesive (resins, varnishes). Solid or flexible mica electrical insulating materials are obtained from mica paper by gluing with adhesive varnishes or resins and subsequent hot pressing. Electroceramic materials and glasses Electroceramic materials are divided into 3 groups: materials from which insulators are made (insulator ceramics), materials from which capacitors are made (capacitor ceramics), and ferroceramic materials with abnormally high values of dielectric constant and piezoelectric effect. The latter have been used in radio engineering. Magnetic materialsSoft magnetic materials are used for the manufacture of magnetic cores of transformers, electrical machines and devices, magnetic screens and other devices where magnetization with low energy losses is required. Hard magnetic materials are used for the manufacture of permanent magnets. Kozlov Yu.S. MATERIALS SCIENCE. - M .: "Agar", St. Petersburg, "Lan", 1999.
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