What are dielectrics in physics. Dielectric - what is it? Properties of dielectrics

Dielectric constant may have dispersion.

A number of dielectrics exhibit interesting physical properties.

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

Dielectrics- DIELECTRICS, substances that conduct electricity poorly (specific resistance of about 1010 Ohm´m). There are solid, liquid and gaseous dielectrics. An external electric field causes polarization of the dielectric. In some hard... ... Illustrated Encyclopedic Dictionary

Substances that conduct electricity poorly (electrical resistivity 108 1012 Ohm? cm). There are solid, liquid and gaseous dielectrics. An external electric field causes polarization of dielectrics. In some solid dielectrics... ... Big Encyclopedic Dictionary

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

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

- (insulators) substances that do not conduct electric current. Examples of dielectrics: mica, amber, rubber, sulfur, glass, porcelain, various types of oils, etc. Samoilov K.I. Marine Dictionary. M.L.: State Naval Publishing House of the NKVMF Union ... Marine Dictionary

The name given by Michael Faraday to bodies that are non-conducting or, in other words, poorly conducting electricity, such as air, glass, various resins, sulfur, etc. Such bodies are also called insulators. Before Faraday's research in the 1930s... Encyclopedia of Brockhaus and Efron

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

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

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

Books

• Dielectrics and waves, A. R. Hippel. The author of the monograph presented to the attention of readers, a famous researcher in the field of dielectrics, American scientist A. Hippel has repeatedly appeared in periodicals and in…
• Effect of laser radiation on polymer materials. Scientific foundations and applied problems. In 2 books. Book 1. Polymer materials. Scientific foundations of laser action on polymer dielectrics, B. A. Vinogradov, K. E. Perepelkin, G. P. Meshcheryakova. This book contains information about the structure and basic thermal and optical properties of polymer materials, the mechanism of action of laser radiation on them in infrared, visible...

In order to determine what dielectrics are in physics, let us remember that the most important characteristic of a dielectric is polarization. In any substance, free charges move under the influence of an electric field, in this case an electric current appears, and bound charges become polarized. Substances are divided into conductors and dielectrics depending on which charges predominate (free or bound). In dielectrics, polarization occurs mainly under the influence of an external electric field. If you cut a conductor in an electric field, you can separate charges of different signs. This cannot be done with polarization charges of a dielectric. In metallic conductors, free charges can move over long distances, while in dielectrics, positive and negative charges move within a single molecule. In dielectrics, the energy band is completely filled.
If there is no external field, then charges with different signs are distributed uniformly throughout the entire volume of the dielectric. In the presence of an external electric field, the charges entering the molecule are displaced in opposite directions. This displacement manifests itself as the appearance of a charge on the surface of the dielectric when it is placed in an external electric field - this is the phenomenon of polarization.
Polarization depends on the type in the dielectric. Thus, in ionic crystals, polarization arises mainly due to the displacement of ions in an electric field and only slightly due to the deformation of the electron atomic shells. Whereas in diamond, which has a covalent chemical bond, polarization occurs due to the deformation of the electron atomic shells in an electric field.
A dielectric is called polar if its molecules have their own electric dipole moment. In such dielectrics, in the presence of an external electric field, the electric dipole moments are oriented along the field.
The polarization of the dielectric is determined using the polarization vector. This value is equal to the sum of the electric dipole moments of all molecules in a unit volume of the substance. If the dielectric is isotropic, then the equality holds:

where is the electrical constant; — dielectric susceptibility of the substance. The dielectric susceptibility of a substance is related to the dielectric constant as:

where — characterizes the weakening of the external electric field in the dielectric due to the presence of polarization charges. Polar dielectrics have the largest values. So, for water =81.
In some dielectrics, polarization occurs not only in an external electric field, but also under mechanical stress. These dielectrics are called piezoelectrics.
Dielectrics have a much higher electrical resistivity than conductors. It lies in the range: Ohm/cm. Therefore, dielectrics are used to make insulation for electrical devices. An important application of dielectrics is their use in electrical capacitors.

Refers to materials with electrical resistivity ρ< 10 −5 Ом·м, а к диэлектрикам - материалы, у которых ρ >10 8 Ohm m. It should be noted that the resistivity of good conductors can be only 10 −8 Ohm m, and for the best dielectrics it can exceed 10 16 Ohm m. The resistivity of semiconductors, depending on the structure and composition of the materials, as well as on their operating conditions, can vary within the range of 10 −5 -10 8 Ohm m. Metals are good conductors of electric current. Of the 105 chemical elements, only twenty-five are non-metals, and twelve elements can exhibit semiconductor properties. But in addition to elementary substances, there are thousands of chemical compounds, alloys or compositions with the properties of conductors, semiconductors or dielectrics. It is quite difficult to draw a clear boundary between the resistivity values ​​of different classes of materials. For example, many semiconductors behave like insulators at low temperatures. At the same time, dielectrics can exhibit semiconductor properties when heated strongly. The qualitative difference is that for metals the conducting state is ground, and for semiconductors and dielectrics it is excited.

A number of dielectrics exhibit interesting physical properties. These include electrets, piezoelectrics, pyroelectrics, ferroelastics, ferroelectrics, relaxors and ferromagnets.

Usage

When using dielectrics - one of the most extensive classes of electrical materials - the need to use both passive and active properties of these materials was quite clearly defined.

Dielectrics are used not only as insulating materials.

Passive properties of dielectrics

Active properties of dielectrics

Active (controlled) dielectrics are ferroelectrics, piezoelectrics, pyroelectrics, electroluminophores, materials for emitters and shutters in laser technology, electrets, etc.

see also

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See what “Dielectric” is in other dictionaries:

Dielectric... Spelling dictionary-reference book

DIELECTRIC, a material that does not conduct electricity, such as the insulation separating the two conductors in a CAPACITOR. These materials have an indicator called DIELECTRIC CONSTANT, which determines to what extent the material can... ... Scientific and technical encyclopedic dictionary

Pyroelectric, electret, polyisobutylene, polypropylene, insulator, polyethylene terephthalate, polycarbonate, synoxal, polytrifluorochloroethylene, polytetrafluoroethylene, polyarylate Dictionary of Russian synonyms. dielectric noun, number of synonyms: 11 insulator (21) ... Synonym dictionary

dielectric- A substance whose main electrical property is the ability to polarize in an electric field. [GOST R 52002 2003] dielectric Material that does not conduct electric current. Electrical engineering topics, basic... Technical Translator's Guide

DIELECTRIC, dielectric, male. (physical). Dielectric body, substance, e.g. glass. Ushakov's explanatory dictionary. D.N. Ushakov. 1935 1940 … Ushakov's Explanatory Dictionary

DIELECTRIC, huh, husband. (specialist.). A substance that does not conduct electricity well is a non-conductor. | adj. dielectric, oh, oh. Ozhegov's explanatory dictionary. S.I. Ozhegov, N.Yu. Shvedova. 1949 1992 … Ozhegov's Explanatory Dictionary

A substance that weakly conducts electricity. current. D. are: glass, porcelain, mica, marble, rubber, ebonite, dry wood, silk, asbestos, transformer oil, air, etc. are used for insulating live parts, for insulating... ... Technical railway dictionary

Dielectric- a substance whose main electrical property is the ability to polarize in an electric field... Source: ELECTRICAL ENGINEERING. TERMS AND DEFINITIONS OF BASIC CONCEPTS. GOST R 52002 2003 (approved by the Decree of the State Standard of the Russian Federation dated... ... Official terminology

dielectric- dielectric; industry insulator A substance whose main electrical property is the ability to polarize and in which the existence of an electrostatic field is possible ... Polytechnic terminological explanatory dictionary

Dielectric- – a substance whose main electrical property is the ability to polarize in an electric field. [GOST 19880 74] Term heading: Power equipment Encyclopedia headings: Abrasive equipment, Abrasives, Highways... Encyclopedia of terms, definitions and explanations of building materials

Books

• Boundary effects in elements of on-board equipment of spacecraft under the influence of ionizing radiation, Shilobreev Boris Alekseevich, Lazurik Valentin Timofeevich, Yakovlev Mikhail Viktorovich. The basic concepts and methods of computational and experimental determination of the near-boundary distributions of absorbed energy and space charge in structural materials are presented...

Dielectric materials in electronic equipment are separated electrically, while solid materials are separated mechanically by conductors that are under different electrical potentials. They are used for electrical insulation of equipment elements, for storing electric field energy (capacitors), for the manufacture of structural parts, as well as in the form of coatings on the surface of parts, for gluing parts.

Dielectric properties of materials

The main property of a dielectric is not to conduct electric current. THE SPECIFIC VOLUME RESISTANCE of dielectrics is high: from 108 to 1018 Ohm, since there are almost no free electric charge carriers in them. Some conductivity is caused by impurities and structural defects.

There are always more impurities and defects on the surface of any body, therefore, for dielectrics, the concept of surface conductivity and the parameter SURFACE RESISTANCE s are introduced, defined as the resistance measured between two linear conductors 1 m long each, located parallel to each other at a distance of 1 m on the surface of the dielectric . The value of s strongly depends on the method of obtaining (processing) the surface and its condition (dustiness, moisture, etc.). Since surface electrical conductivity usually significantly exceeds volumetric conductivity, measures are taken to reduce it.

A dielectric is an insulator only with respect to direct voltage. In an alternating electric field, current flows through the dielectric due to its polarization.

POLARIZATION is the process of displacement of bound charges over a limited distance under the influence of an external electric field.

The electrons of the atoms are shifted towards the positive pole, the nuclei of the atoms - towards the negative. The same thing happens with ions in ionic crystals, with molecules or sections of molecules with an uneven distribution of charged particles in the volume they occupy. As a result of polarization, its own internal field is formed in the dielectric; its vector is smaller in magnitude and opposite in direction to the external field vector. The electrical capacitance between electrodes with a dielectric is greater than between the same electrodes without a dielectric by a factor of, where is the RELATIVE DIELECTRIC CONTINUITY OF THE DIELECTRIC.

During ELECTRONIC POLARIZATION, under the influence of an external electric field, the electronic shells of the atoms of the substance are deformed. It is characterized by a short (about 10-15 s) settling time and therefore is inertia-free for radio frequencies, does not depend on frequency, weakly depends on temperature and occurs with virtually no losses. Substances with predominantly electronic polarization (weakly polar dielectrics) have a low dielectric constant: from 1.8 to 2.5. This type of polarization is inherent in all substances.

IONIC POLARIZATION occurs in ionic solids, has a settling time of the order of 10-13 s, therefore, practically does not depend on the field frequency, and weakly depends on temperature. The value for most materials with ionic polarization is from 5 to 10.

DIPOLE (ORIENTATION) POLARIZATION manifests itself as the orientation under the influence of a field of polar molecules or groups of atoms. For example, water molecules are polar, in which the hydrogen atoms are located asymmetrically relative to the oxygen atom, or vinyl chloride (polyvinyl chloride monomer) H2C-CHCl. To overcome the interaction of molecules and friction forces, field energy is consumed, which is converted into thermal energy; therefore, dipole polarization is inelastic, relaxation in nature. Due to the large sizes and masses of the dipoles involved in dipole polarization, its inertia is significant and manifests itself in the form of a strong dependence of the dielectric constant and energy losses on frequency.

MIGRATION POLARIZATION is caused by inelastic movements of weakly bound impurity ions over short distances. In terms of consequences (energy loss, frequency dependence), this polarization is similar to dipole.

Energy losses in a dielectric during polarization are estimated by LOSS ANGLE TANGENS tg. A dielectric with losses in an electrical circuit is represented as an equivalent circuit: an ideal capacitor and a loss resistance connected in parallel to it. The angle complements up to 90o the shift angle between current and voltage in the vector diagram of such a two-terminal network. Good (weakly polar) dielectrics have tg10-3, which is slightly dependent on frequency. Poor dielectrics have a tg measured in tenths of unity or even more, strongly dependent on frequency.

Special types are formed by polarization under the influence of mechanical stresses, observed in PIEZOELECTRICS, as well as SPONTANEOUS POLARIZATION in PYROELECTRICS and FERROELECTRICS. Such dielectrics are called ACTIVE and are used in special devices: resonators, filters, piezoelectric generators and transformers, radiation converters, capacitors of large specific capacity, etc.

ELECTRICAL STRENGTH - the ability of a dielectric to maintain high resistivity in high voltage circuits. It is estimated by the breakdown field strength Epr = Upr/d, where Upr is the voltage causing the breakdown, d is the thickness of the dielectric. Dimension Epr - V/m. For different dielectrics, Epr = 10...1000 MV/m, and even for one material this value varies widely depending on the thickness, shape of the electrodes, temperature and a number of other factors. The reason for this is the variety of processes during a breakdown. ELECTRICAL BREAKDOWN is caused by the tunneling transition of electrons into the conduction band from the valence band, from impurity levels or metal electrodes, as well as their avalanche reproduction due to impact ionization in high-intensity fields. ELECTROTHERMAL BREAKDOWN is caused by an exponential increase in the electrical conductivity of the dielectric with increasing temperature. At the same time, the leakage current increases, heating the dielectric even more, a conducting channel is formed in its thickness, the resistance drops sharply, and melting, evaporation, and destruction of the material occur in the thermal impact zone. ELECTROCHEMICAL BREAKDOWN is caused by the phenomena of electrolysis, migration of ions and, as a result, changes in the composition of the material. IONIZATIONAL BREAKDOWN occurs due to partial discharges in a dielectric containing air inclusions. The electrical strength of air is lower, and the field strength in these inclusions is higher than in a dense dielectric. This type of breakdown is typical for porous materials. SURFACE BREAKDOWN (FLASHUP) of a dielectric occurs due to unacceptably large surface currents. With sufficient power of the current source, a surface breakdown develops through the air and turns into an arc. Conditions conducive to this breakdown: cracks, other irregularities and contamination on the surface of the dielectric, humidity, dust, low atmospheric air pressure.

For reliable operation of any electrical device, the operating voltage of its insulation Uwork must be significantly less than the breakdown voltage Ubreak. The ratio Upr/Urab is called the SAFETY FACTOR OF ELECTRICAL INSULATION STRENGTH.

Lecture 1.3.1. Polarization of dielectrics

Dielectric materials

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

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

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

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

Table 3 - Electrical properties of dielectrics and their characteristics

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

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

The first type includes electronic and ionic polarization.

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

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

The second type includes all relaxation polarizations.

Dipole-relaxation polarization (C dr, r dr, Q dr) associated with the thermal movement of dipoles during polar bonds between molecules. Rotating the dipoles in the direction of the electric field requires overcoming some resistance and releasing energy in the form of heat (r dr). The relaxation time here is of the order of 10 -8 – 10 -6 s - this is the period of time during which the ordering of dipoles oriented by the electric field after removing the field will decrease due to the presence of thermal movements by 2.7 times from the initial value.

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

where f is the frequency of natural vibrations of particles; v - activation energy; k – Boltzmann constant (8.63 10 -5 EV/deg); T – absolute temperature according to K0.

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

Structural polarizations distinguish:

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

Classification of dielectrics by type of polarization.

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

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

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

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