electrical insulating materials. Scheme, description

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electrical insulating materials. Encyclopedia of radio electronics and electrical engineering

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Electrical insulating materials, or dielectrics, are such materials with which insulation is carried out, i.e., 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 physicochemical 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 do not have electrical properties until an electric field is applied to them. 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, tropical 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.

Author: Smirnova L.N.

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