The electrical conductivity of dielectrics is an important physical characteristic. Information about it allows you to identify areas of application of materials.
Terms
According to the conductivity of electric current, substances are divided into groups:
- dielectrics;
- semiconductors;
- conductors.
Metals are excellent current conductors - their electrical conductivity reaches 106-108 (Ohm m)-1.
And dielectric materials are not capable of conducting electricity, so they are used as insulators. They do not have free charge carriers, differ in the dipole structure of molecules.
Semiconductors are solid materials with intermediate conductivity values.
Classification
All dielectric materials are divided into polar and non-polar types. In polar insulators, the centers of positive and negative charges are off-center. The molecules of such substances are similar in their electrical parameters to a rigid dipole, which has its own dipole moment. Water can be used as polar dielectrics.ammonia, hydrogen chloride.
Non-polar dielectrics are distinguished by the coincidence of the centers of positive and negative charges. They are similar in electrical characteristics to an elastic dipole. Examples of such insulators are hydrogen, oxygen, carbon tetrachloride.
Electrical conductivity
The electrical conductivity of dielectrics is explained by the presence of a small number of free electrons in their molecules. With the displacement of charges inside the substance over a certain period of time, a gradual establishment of an equilibrium position is observed, which is the reason for the appearance of a current. The electrical conductivity of dielectrics exists at the moment of switching off and on the voltage. Technical samples of insulators have the maximum number of free charges, therefore, insignificant through currents appear in them.
The electrical conductivity of dielectrics in the case of a constant voltage value is calculated from the through current. This process involves the release and neutralization of the existing charges on the electrodes. In the case of alternating voltage, the value of active conductivity is affected not only by the through current, but also by the active components of the polarization currents.
The electrical properties of dielectrics depend on the current density, the resistance of the material.
Solid Dielectrics
The electrical conductivity of solid dielectrics is divided into bulk and surface. To compare these parameters for different materials, the values of volume specific and surface specific are used.resistance.
Full conductivity is the sum of these two values, its value depends on the humidity of the environment and the ambient temperature. In the case of continuous operation under voltage, there is a decrease in the through current passing through liquid and solid insulators.
And in the case of an increase in current after a certain period of time, we can talk about the fact that irreversible processes will occur inside the substance, leading to destruction (breakdown of the dielectric).
Features of the gaseous state
Gaseous dielectrics have negligible electrical conductivity if the field strength takes on minimum values. The occurrence of a current in gaseous substances is possible only in those cases when they contain free electrons or charged ions.
Gaseous dielectrics are high-quality insulators, therefore they are used in modern electronics in large volumes. Ionization in such substances is caused by external factors.
Due to the collisions of gas ions, as well as under thermal exposure, ultraviolet or X-ray exposure, the process of formation of neutral molecules (recombination) is also observed. Thanks to this process, the increase in the number of ions in the gas is limited, a certain concentration of charged particles is established after a short time period after exposure to an external ionization source.
In the process of increasing the voltage applied to the gas, the movement of ions to the electrodes increases. They are nothave time to recombine, so they are discharged at the electrodes. With a subsequent increase in voltage, the current does not increase, it is called saturation current.
Considering non-polar dielectrics, we note that air is a perfect insulator.
Liquid dielectrics
The electrical conductivity of liquid dielectrics is explained by the peculiarities of the structure of liquid molecules. Nonpolar solvents contain dissociated impurities, including moisture. In polar molecules, the conductivity of the electric current is also explained by the process of disintegration into ions of the liquid itself.
In this state of aggregation, the current is also caused by the movement of colloidal particles. Due to the impossibility of completely removing impurities from such a dielectric, problems arise in obtaining liquids with low current conductivity.
All types of insulation involve the search for options to reduce the specific conductivity of dielectrics. For example, impurities are removed, the temperature indicator is adjusted. An increase in temperature causes a decrease in viscosity, an increase in the mobility of ions, and an increase in the degree of thermal dissociation. These factors affect the conductivity of dielectric materials.
Electrical conductivity of solids
It is explained by the movement of not only the ions of the insulator itself, but also charged particles of impurities contained inside the solid material. As it passes through the solid insulator, a partial removal of impurities occurs, which graduallyaffects the conduction. Taking into account the structural features of the crystal lattice, the movement of charged particles is due to fluctuations in thermal motion.
At low temperatures, positive and negative impurity ions move. Such types of isolation are typical for substances with a molecular and atomic crystal structure.
For anisotropic crystals, the value of specific conductivity varies depending on its axes. For example, in quartz in the direction parallel to the main axis, it exceeds the perpendicular position by 1000 times.
In solid porous dielectrics, where there is practically no moisture, a slight increase in electrical resistance leads to an increase in their electrical resistance. Substances containing water-soluble impurities show a significant decrease in volume resistance due to changes in humidity.
Polarization of dielectrics
This phenomenon is associated with a change in the position of the particles of the insulator in space, which leads to the acquisition of some electrical (induced) moment by each macroscopic volume of the dielectric.
There is a polarization that occurs under the influence of an external field. They also distinguish a spontaneous version of polarization that appears even in the absence of an external field.
The relative permittivity is characterized by:
- capacitance of a capacitor with this dielectric;
- its magnitude in a vacuum.
This process is accompanied by the appearance ofthe surface of the dielectric of bound charges, which reduce the amount of tension inside the substance.
In the case of the complete absence of an external field, a separate element of the dielectric volume does not have an electric moment, since the sum of all charges is zero and there is a coincidence of negative and positive charges in space.
Polarization options
During electron polarization, a shift occurs under the influence of an external field of the electron shells of the atom. In the ionic variant, a shift of the lattice sites is observed. Dipole polarization is characterized by losses to overcome internal friction and bonding forces. The structural version of polarization is considered the slowest process, it is characterized by the orientation of inhomogeneous macroscopic impurities.
Conclusion
Electrical insulating materials are substances that allow you to obtain reliable insulation of some components of electrical equipment under certain electrical potentials. Compared to current conductors, numerous insulators have a significantly higher electrical resistance. They are able to create strong electric fields and accumulate additional energy. It is this property of insulators that is used in modern capacitors.
Depending on the chemical composition, they are divided into natural and synthetic materials. The second group is the most numerous, therefore these insulators are used in a variety of electrical appliances.
Depending on the technological characteristics, structure, composition, film, ceramic, wax, mineral insulators are isolated.
When the breakdown voltage is reached, a breakdown is observed, leading to a sharp increase in the magnitude of the electric current. Among the characteristic features of such a phenomenon, one can single out a slight dependence of strength on stress and temperature, thickness.
