All materials that exist in nature differ in their electrical properties. Thus, from the whole variety of physical substances, dielectric materials and conductors of electric current are distinguished into separate groups.

What are conductors?

A conductor is such a material, a feature of which is the presence of freely moving charged particles in the composition, which are distributed throughout the substance.

Substances conducting electric current are melts of metals and the metals themselves, undistilled water, salt solution, wet soil, the human body.

Metal is the best conductor of electricity. Also among non-metals there are good conductors, for example, carbon.

All natural conductors of electric current are characterized by two properties:

• resistance indicator;
• conductivity indicator.

Resistance arises due to the fact that electrons in motion experience a collision with atoms and ions, which are a kind of obstacle. That is why conductors are assigned the characteristic of electrical resistance. The reciprocal of resistance is electrical conductivity.

Electrical conductivity is a characteristic (ability) of a physical substance to conduct current. Therefore, the properties of a reliable conductor are low resistance to the flow of moving electrons and, consequently, high electrical conductivity. That is, the best conductor is characterized by a large conductivity index.

For example cable products: copper cable has a higher electrical conductivity compared to aluminum.

What are dielectrics?

Dielectrics are such physical substances in which at low temperatures there are no electric charges. The composition of such substances includes only atoms of a neutral charge and molecules. The charges of a neutral atom are closely connected with each other, therefore they are deprived of the possibility of free movement throughout the substance.

Gas is the best dielectric. Other non-conductive materials are glass, porcelain, ceramics, as well as rubber, cardboard, dry wood, resins and plastics.

Dielectric objects are insulators, the properties of which are mainly dependent on the state of the surrounding atmosphere. For example, at high humidity, some dielectric materials partially lose their properties.

Conductors and dielectrics are widely used in the field of electrical engineering to solve various problems.

For example, all cable and wire products are made of metals, usually copper or aluminum. The sheath of wires and cables is polymer, as well as the plugs of all electrical appliances. Polymers are excellent dielectrics that do not allow the passage of charged particles.

Silver, gold and platinum products are very good conductors. But their negative characteristic, which limits their use, is their very high cost.

Therefore, such substances are used in areas where quality is much more important than the price paid for it (defense industry and space).

Copper and aluminum products are also good conductors, while not having such a high cost. Consequently, the use of copper and aluminum wires is ubiquitous.

Tungsten and molybdenum conductors have less good properties, so they are used mainly in incandescent light bulbs and high temperature heating elements. Poor electrical conductivity can significantly disrupt the operation of the electrical circuit.

Dielectrics also differ in their characteristics and properties. For example, in some dielectric materials there are also free electrical charges, albeit in a small amount. Free charges arise due to thermal vibrations of electrons, i.e. However, an increase in temperature in some cases provokes the detachment of electrons from the nucleus, which reduces the insulating properties of the material. Some insulators are characterized by a large number of "torn off" electrons, which indicates poor insulating properties.

The best dielectric is a complete vacuum, which is very difficult to achieve on planet Earth.

Completely purified water also has high dielectric properties, but such does not even exist in reality. It is worth remembering that the presence of any impurities in the liquid endows it with the properties of a conductor.

The main criterion for the quality of any dielectric material is the degree of compliance with the functions assigned to it in a particular electrical circuit. For example, if the properties of the dielectric are such that current leakage is negligible and does not cause any damage to the operation of the circuit, then the dielectric is reliable.

What is a semiconductor?

An intermediate place between dielectrics and conductors is occupied by semiconductors. The main difference between conductors is the dependence of the degree of electrical conductivity on temperature and the amount of impurities in the composition. Moreover, the material has the characteristics of both a dielectric and a conductor.

With increasing temperature, the electrical conductivity of semiconductors increases, and the degree of resistance decreases. As the temperature decreases, the resistance tends to infinity. That is, when the temperature reaches zero, semiconductors begin to behave like insulators.

The semiconductors are silicon and germanium.

The ability to conduct electric current characterizes the electrical resistance of wood. In general, the impedance of a wood sample placed between two electrodes is defined as the resultant of two resistances: volume and surface. Volume resistance numerically characterizes the obstacle to the passage of current through the thickness of the sample, and the surface resistance determines the obstacle to the passage of current along the surface of the sample. Indicators of electrical resistance are specific volume and surface resistance. The first of these indicators has the dimension of ohm per centimeter (ohm x cm) and is numerically equal to the resistance when current passes through two opposite faces of a 1X1X1 cm cube made of a given material (wood). The second indicator is measured in ohms and is numerically equal to the resistance of a square of any size on the surface of a wood sample when current is applied to the electrodes that limit two opposite sides of this square. The electrical conductivity depends on the type of wood and the direction of current flow. As an illustration of the order of magnitude of volume and surface resistance in table. some data is given.

comparative data on the specific volume and surface resistance of wood

To characterize the electrical conductivity, the volume resistivity is of the greatest importance. The resistance is highly dependent on the moisture content of the wood. As the moisture content of the wood increases, the resistance decreases. A particularly sharp decrease in resistance is observed with an increase in the content of bound moisture from an absolutely dry state to the limit of hygroscopicity. In this case, the specific volume resistance decreases millions of times. A further increase in humidity causes a drop in resistance only tenfold. This is illustrated by the data in Table.

specific volume resistance of wood in a completely dry state

Breed	Specific volume resistance, ohm x cm
	across the fibers	along the fibers
Pine	2.3 x 10 15	1.8 x 10 15
Spruce	7.6 x 10 16	3.8 x 10 16
Ash	3.3 x 10 16	3.8 x 10 15
Hornbeam	8.0 x 10 16	1.3 x 10 15
Maple	6.6 x 10 17	3.3 x 10 17
Birch	5.1 x 10 16	2.3 x 10 16
Alder	1.0 x 10 17	9.6 x 10 15
Linden	1.5 x 10 16	6.4 x 10 15
Aspen	1.7 x 10 16	8.0 x 10 15

influence of humidity on the electrical resistance of wood

The surface resistance of wood also decreases significantly with increasing humidity. An increase in temperature leads to a decrease in the volumetric resistance of wood. Thus, the resistance of false wood wood with an increase in temperature from 22-23 ° to 44-45 ° C (approximately twice) drops by 2.5 times, and beech wood with an increase in temperature from 20-21 ° to 50 ° C - 3 times. At negative temperatures, the volume resistance of wood increases. The specific volume resistance along the fibers of birch samples with a moisture content of 76% at a temperature of 0 ° C was 1.2 x 10 7 ohm cm, and when cooled to a temperature of -24 ° C, it turned out to be 1.02 x 10 8 ohm cm. Impregnation of wood with mineral antiseptics (for example, zinc chloride) reduces the resistivity, while impregnation with creosote has little effect on electrical conductivity. The electrical conductivity of wood is of practical importance when it is used for communication poles, masts of high-voltage transmission lines, handles of power tools, etc. In addition, the electrical moisture meters are based on the dependence of electrical conductivity on wood moisture content.

electric strength of wood

Electrical strength is important when evaluating wood as an electrically insulating material and is characterized by a breakdown voltage in volts per 1 cm of material thickness. The electrical strength of wood is low and depends on the species, humidity, temperature and direction. With increasing humidity and temperature, it decreases; along the fibers it is much lower than across. Data on the electrical strength of wood along and across the fibers are given in table.

electrical strength of wood along and across the fibers

With a moisture content of pine wood of 10%, the following electrical strength was obtained in kilovolts per 1 cm of thickness: along the fibers 16.8; in the radial direction 59.1; in the tangential direction 77.3 (the determination was made on samples 3 mm thick). As you can see, the electrical strength of wood along the fibers is about 3.5 times less than across the fibers; in the radial direction, the strength is less than in the tangential direction, since the core rays reduce the breakdown voltage. Increasing the humidity from 8 to 15% (by a factor of two) reduces the dielectric strength across the fibers by about 3 times (average for beech, birch and alder).

The electrical strength (in kilovolts per 1 cm of thickness) of other materials is as follows: mica 1500, glass 300, bakelite 200, paraffin 150, transformer oil 100, porcelain 100. In order to increase the electrical strength of wood and reduce electrical conductivity when used in the electrical industry as an insulator it is impregnated with drying oil, transformer oil, paraffin, artificial resins; The effectiveness of such impregnation is evident from the following data on birch wood: impregnation with drying oil increases the breakdown voltage along the fibers by 30%, with transformer oil - by 80%, with paraffin - almost twice as compared with the breakdown voltage for air-dry unimpregnated wood.

dielectric properties of wood

The value showing how many times the capacitance of the capacitor increases if the air gap between the plates is replaced by a gasket of the same thickness from a given material is called the dielectric constant of this material. Dielectric constant (dielectric constant) for some materials is given in table.

permittivity of some materials

Material		Wood	The dielectric constant
Air	1,00	Spruce dry: along the fibers	3,06
		in the tangential direction	1,98
Paraffin	2,00
		in the radial direction	1,91
Porcelain	5,73
Mica	7,1-7,7	Beech dry: along the grain	3,18
		in the tangential direction	2,20
Marble	8,34
		in the radial direction	2,40
Water	80,1

Data for wood show a noticeable difference between the dielectric constant along and across the fibers; at the same time, the permittivity across the fibers in the radial and tangential directions differs little. The dielectric constant in a high frequency field depends on the frequency of the current and the moisture content of the wood. With increasing current frequency, the dielectric constant of beech wood along the fibers at a moisture content of 0 to 12% decreases, which is especially noticeable for a moisture content of 12%. With an increase in the moisture content of beech wood, the dielectric constant along the fibers increases, which is especially noticeable at a lower current frequency.

In a high frequency field, the wood heats up; the reason for the heating is the Joule heat loss inside the dielectric, which occurs under the influence of an alternating electromagnetic field. This heating consumes a part of the input energy, the value of which is characterized by the loss tangent.

The loss tangent depends on the direction of the field with respect to the fibers: it is approximately twice as large along the fibers as across the fibers. Across the fibers in the radial and tangential directions, the loss tangent differs little. The dielectric loss tangent, like the dielectric constant, depends on the frequency of the current and the moisture content of the wood. So, for absolutely dry beech wood, the loss tangent along the fibers first increases with increasing frequency, reaches a maximum at a frequency of 10 7 Hz, after which it begins to decrease again. At the same time, at a humidity of 12%, the loss tangent drops sharply with increasing frequency, reaches a minimum at a frequency of 105 Hz, and then increases just as sharply.

maximum loss tangent for dry wood

With an increase in the moisture content of beech wood, the loss tangent along the fibers increases sharply at low (3 x 10 2 Hz) and high (10 9 Hz) frequencies and almost does not change at a frequency of 10 6 -10 7 Hz.

Through a comparative study of the dielectric properties of pine wood and cellulose, lignin and resin obtained from it, it was found that these properties are determined mainly by cellulose. Heating of wood in the field of high frequency currents is used in the processes of drying, impregnation and gluing.

piezoelectric properties of wood

Electric charges appear on the surface of some dielectrics under the action of mechanical stresses. This phenomenon associated with the polarization of the dielectric is called the direct piezoelectric effect. Piezoelectric properties were first discovered in crystals of quartz, tourmaline, Rochelle salt, etc. These materials also have an inverse piezoelectric effect, which consists in the fact that their dimensions change under the influence of an electric field. Plates made of these crystals are widely used as emitters and receivers in ultrasonic technology.

These phenomena are found not only in single crystals, but also in a number of other anisotropic solid materials called piezoelectric textures. Piezoelectric properties have also been found in wood. It was found that the main carrier of piezoelectric properties in wood is its oriented component - cellulose. The intensity of polarization of wood is proportional to the magnitude of mechanical stresses from the applied external forces; the proportionality factor is called the piezoelectric modulus. The quantitative study of the piezoelectric effect, therefore, is reduced to the determination of the values ​​of the piezoelectric moduli. Due to the anisotropy of the mechanical and piezoelectric properties of wood, these indicators depend on the direction of mechanical forces and the polarization vector.

The greatest piezoelectric effect is observed under compressive and tensile loads at an angle of 45° to the fibers. Mechanical stresses directed strictly along or across the fibers do not cause a piezoelectric effect in wood. In table. the values ​​of piezoelectric modules for some rocks are given. The maximum piezoelectric effect is observed in dry wood, with increasing humidity it decreases, and then completely disappears. So, already at a humidity of 6-8%, the magnitude of the piezoelectric effect is very small. With an increase in temperature to 100 ° C, the value of the piezoelectric modulus increases. With a small elastic deformation (high modulus of elasticity) of wood, the piezoelectric modulus decreases. The piezoelectric modulus also depends on a number of other factors; however, the orientation of the cellulose component of wood has the greatest influence on its value.

piezoelectric wood modules

The open phenomenon allows a deeper study of the fine structure of wood. The indicators of the piezoelectric effect can serve as quantitative characteristics of the orientation of the cellulose and therefore are very important for studying the anisotropy of natural wood and new wood materials with properties specified in certain directions.

A dielectric is a material or substance that practically does not transmit electric current. This conductivity is due to the small number of electrons and ions. These particles are formed in a non-conductive material only when high temperature properties are achieved. About what a dielectric is and will be discussed in this article.

Description

Each electronic or radio conductor, semiconductor or charged dielectric passes an electric current through itself, but the peculiarity of the dielectric is that even at a high voltage over 550 V, a small current will flow in it. An electric current in a dielectric is the movement of charged particles in a certain direction (it can be positive or negative).

Types of currents

The electrical conductivity of dielectrics is based on:

• Absorption currents - a current that flows in a dielectric at a constant current until it reaches an equilibrium state, changing direction when it is turned on and energized and when it is turned off. With alternating current, the tension in the dielectric will be present in it all the time while it is in the action of an electric field.
• Electronic electrical conductivity - the movement of electrons under the influence of a field.
• Ionic electrical conductivity - is the movement of ions. It is found in electrolyte solutions - salts, acids, alkalis, as well as in many dielectrics.
• Molionic electrical conductivity is the movement of charged particles called molions. It is found in colloidal systems, emulsions and suspensions. The phenomenon of the movement of molions in an electric field is called electrophoresis.

They are classified according to their state of aggregation and chemical nature. The first are divided into solid, liquid, gaseous and solidifying. By chemical nature, they are divided into organic, inorganic and organoelement materials.

By state of aggregation:

• Electrical conductivity of gases. Gaseous substances have a rather low current conductivity. It can occur in the presence of free charged particles, which appears due to the influence of external and internal, electronic and ionic factors: X-ray radiation and radioactive species, collisions of molecules and charged particles, thermal factors.
• Electrical conductivity of a liquid dielectric. Dependence factors: molecular structure, temperature, impurities, the presence of large charges of electrons and ions. The electrical conductivity of liquid dielectrics largely depends on the presence of moisture and impurities. The conductivity of electricity of polar substances is created even with the help of a liquid with dissociated ions. When comparing polar and non-polar liquids, the former have a clear advantage in conductivity. If the liquid is cleaned of impurities, this will contribute to a decrease in its conductive properties. With an increase in conductivity and its temperature, a decrease in its viscosity occurs, leading to an increase in the mobility of ions.
• solid dielectrics. Their electrical conductivity is determined as the movement of charged dielectric particles and impurities. In strong electric current fields, electrical conductivity is revealed.

Physical properties of dielectrics

When the specific resistance of the material is less than 10-5 Ohm * m, they can be attributed to conductors. If more than 108 Ohm * m - to dielectrics. There are cases when the resistivity will be many times greater than the resistance of the conductor. In the interval 10-5-108 Ohm*m there is a semiconductor. Metallic material is an excellent conductor of electric current.

Of the entire periodic table, only 25 elements belong to non-metals, and 12 of them, possibly, will have semiconductor properties. But, of course, in addition to the substances of the table, there are many more alloys, compositions or chemical compounds with the property of a conductor, semiconductor or dielectric. Based on this, it is difficult to draw a certain line between the values ​​of various substances with their resistances. For example, at a reduced temperature factor, a semiconductor will behave like a dielectric.

Application

The use of non-conductive materials is very extensive, as it is one of the most commonly used classes of electrical components. It became quite clear that they can be used due to their properties in an active and passive form.

In a passive form, the properties of dielectrics are used for use in electrical insulating material.

In active form, they are used in ferroelectrics, as well as in materials for emitters of laser technology.

Basic dielectrics

Common types include:

• Glass.
• Rubber.
• Oil.
• Asphalt.
• Porcelain.
• Quartz.
• Air.
• Diamond.
• Pure water.
• Plastic.

What is a liquid dielectric?

Polarization of this type occurs in the electric current field. Liquid non-conductive substances are used in engineering for pouring or impregnating materials. There are 3 classes of liquid dielectrics:

Petroleum oils are low viscosity and mostly non-polar. They are often used in high-voltage instruments: high-voltage water. is a non-polar dielectric. Cable oil has found application in the impregnation of insulating paper wires with a voltage of up to 40 kV, as well as metal-based coatings with a current of more than 120 kV. Transformer oil has a cleaner structure than capacitor oil. This type of dielectric is widely used in production, despite the high cost compared to analog substances and materials.

What is a synthetic dielectric? Currently, it is banned almost everywhere due to its high toxicity, as it is produced on the basis of chlorinated carbon. A liquid dielectric based on organic silicon is safe and environmentally friendly. This type does not cause metal rust and has the properties of low hygroscopicity. There is a fluidized dielectric containing an organofluorine compound that is particularly popular for its non-combustibility, thermal properties, and oxidative stability.

And the last type is vegetable oils. They are weakly polar dielectrics, these include flaxseed, castor, tung, hemp. Castor oil is highly heated and is used in paper capacitors. The rest of the oils are evaporated. Evaporation in them is not caused by natural evaporation, but by a chemical reaction called polymerization. It is actively used in enamels and paints.

Conclusion

The article discussed in detail what a dielectric is. Various species and their properties have been mentioned. Of course, in order to understand the subtlety of their characteristics, you will have to study the section of physics about them in more depth.

When electricity appeared in our life, few people knew about its properties and parameters, and various materials were used as conductors, it was noticeable that with the same voltage value of the current source, the consumer had a different voltage value. It was clear that this was influenced by the type of material used as a conductor. When scientists took up the issue of studying this problem, they came to the conclusion that electrons are charge carriers in the material. And the ability to conduct electric current is isolated by the presence of free electrons in the material. It was found that some materials have a large number of these electrons, while others do not have them at all. Thus, there are materials that, and some do not have this ability.
Based on the above, all materials were divided into three groups:

• conductors;
• semiconductors;
• dielectrics;

Each of the groups has found wide application in electrical engineering.

conductors

guides are materials that conduct electric current well, they are used for the manufacture of wires, cable products, contact groups, windings, tires, conductive cores and tracks. The vast majority of electrical devices and apparatus is made on the basis of conductive materials. Moreover, I will say that the entire electric power industry could not exist without these substances. The group of conductors includes all metals, some liquids and gases.

It is also worth mentioning that among the conductors there are super conductors, the resistance of which is almost zero, such materials are very rare and expensive. And conductors with high resistance - tungsten, molybdenum, nichrome, etc. Such materials are used to make resistors, heating elements, and lamp coils.

But the lion's share in the electrical field belongs to ordinary conductors: copper, silver, aluminum, steel, various alloys of these metals. These materials have found the widest and greatest application in electrical engineering, especially copper and aluminum, since they are relatively cheap, and their use as conductors of electric current is most appropriate. Even copper is limited in its use, it is used as winding wires, multi-core cables, and more critical devices, copper busbars are even rarer. But aluminum is considered the king among the conductors of electric current, even if it has a higher resistivity than copper, but this is offset by its very low cost and corrosion resistance. It is widely used in power supply, cable products, overhead lines, busbars, general wires, etc.

Semiconductors

Semiconductors, something between conductors and semiconductors. Their main feature is their dependence to conduct electric current from external conditions. The key condition is the presence of various impurities in the material, which just provide the ability to conduct electric current. Also, with a certain arrangement of two semiconductor materials. Based on these materials, at the moment, many semiconductor devices have been produced: LEDs, transistors,semistors, thyristors, stabistors, various microcircuits. There is a whole science devoted to semiconductors and devices based on them: electronic engineering. All computers, mobile devices. What can I say, almost all of our equipment contains semiconductor elements.

Semiconductor materials include: silicon, germanium, graphite, gr aphene, indium, etc.

Dielectrics

Well, the last group of materials is dielectrics Substances that are not capable of conducting electricity. Such materials include: wood, paper, air, oil, ceramics, glass, plastics, polyethylene, polyvinyl chloride, rubber, etc. Dielectrics are widely used due to their properties. They are used as an insulating material. They protect the contact of two current-carrying parts, do not allow a person to directly touch these parts. The role of a dielectric in electrical engineering is no less important than the role of conductors, as they ensure the stable, safe operation of all electrical and electronic devices. All dielectrics have a limit to which they are not able to conduct electric current, it is called breakdown voltage. This is an indicator at which the dielectric begins to pass an electric current, while heat is released and the dielectric itself is destroyed. This value of the breakdown voltage for each dielectric material is different and is given in reference materials. The higher it is, the better, the dielectric is considered more reliable.

The parameter characterizing the ability to conduct electric current is resistivity R , unit [ Ohm ] and conductivity, reciprocal of resistance. The higher this parameter, the worse the material conducts electric current. For conductors, it is from a few tenths to hundreds of ohms. In dielectrics, the resistance reaches tens of millions of ohms.

All three types of materials are widely used in the electric power industry and electrical engineering. They are also closely related to each other.

The value showing how many times the capacitance of the capacitor increases if the air gap between the plates is replaced by a gasket of the same thickness from a given material is called the dielectric constant of this material. Dielectric constant (dielectric constant) for some materials is given in table. 26.

Table 26. Dielectric constant of some materials.

Material	The dielectric constant	Wood	The dielectric constant
		Spruce dry: along the fibers
		in the tangential direction
		in the radial direction
		Beech dry: along the grain
		in the tangential direction
		in the radial direction

Data for wood show a noticeable difference between the dielectric constant along and across the fibers; at the same time, the permittivity across the fibers in the radial and tangential directions differs little. The dielectric constant in a high frequency field depends on the frequency of the current and the moisture content of the wood. With increasing current frequency, the dielectric constant of beech wood along the fibers at a moisture content of 0 to 12% decreases, which is especially noticeable for a moisture content of 12% (Fig. 45). With an increase in the moisture content of beech wood, the dielectric constant along the fibers increases, which is especially noticeable at a lower current frequency.

In a high frequency field, the wood heats up; the reason for the heating is the Joule heat loss inside the dielectric, which occurs under the influence of an alternating electromagnetic field. This heating consumes a part of the input energy, the value of which is characterized by the loss tangent.

The loss tangent depends on the direction of the field with respect to the fibers: it is approximately twice as large along the fibers as across the fibers. Across the fibers in the radial and tangential directions, the loss tangent differs little. The dielectric loss tangent, like the dielectric constant, depends on the frequency of the current and the moisture content of the wood. So, for absolutely dry beech wood, the loss tangent along the fibers first increases with increasing frequency, reaches a maximum at a frequency of 10 7 Hz, after which it begins to decrease again. At the same time, at a humidity of 12%, the loss tangent drops sharply with increasing frequency, reaches a minimum at a frequency of 105 Hz, and then increases just as sharply (Fig. 46).

Table 27. The maximum value of the loss tangent for dry wood.

With an increase in the moisture content of beech wood, the loss tangent along the fibers increases sharply at low (3 x 10 2 Hz) and high (10 9 Hz) frequencies and almost does not change at a frequency of 10 6 -10 7 Hz (see Fig. 46).

Through a comparative study of the dielectric properties of pine wood and cellulose, lignin and resin obtained from it, it was found that these properties are determined mainly by cellulose. Heating of wood in the field of high frequency currents is used in the processes of drying, impregnation and gluing.