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 the electric current, substances are divided into groups:
- dielectrics;
- semiconductors;
- conductors.
Metals conduct excellent current - their electrical conductivity reaches 106-108 (Ohm · m) -1 .
And dielectric materials are not able to conduct electric current, so they are used as insulators. They do not have free charge carriers; they differ in the dipole structure of molecules.
Semiconductors are solid materials having 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 offset from the center. The molecules of such substances in their electrical parameters are similar to a rigid dipole having its own dipole moment. As polar dielectrics can lead water, ammonia, hydrogen chloride.
Nonpolar dielectrics differ in 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 an insignificant number of free electrons in their molecules. When the charges are displaced inside the substance over a certain period of time, a gradual establishment of the equilibrium position is observed, which is the reason for the appearance of current. The conductivity of dielectrics exists at the time of switching off and on voltage. Technical samples of insulators have the maximum number of free charges, therefore, small through currents appear in them.
The conductivity of dielectrics in the case of a constant voltage value is calculated by the through current. This process involves the release and neutralization of the existing charges on the electrodes. In the case of an 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 and material resistance.
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 resistivity and surface resistivity are used.
The total conductivity is summarized from these two values; its value depends on the humidity of the medium and the temperature of the surrounding air. In the case of continuous operation under voltage, a decrease in the through current passing through the liquid and solid insulators is observed.
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 low electrical conductivity if the field strength takes minimum values. The occurrence of current in gaseous substances is possible only in those cases when free electrons or charged ions are present in them.
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 collisions of gas ions, as well as during thermal exposure, ultraviolet or X-ray action, the 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 in 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 do not have time to recombine, therefore, they are discharged at the electrodes. With a subsequent increase in voltage, the current does not increase, it is called the 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 structural features of the liquid molecules. In non-polar solvents there are dissociated impurities, including moisture. In polar molecules, the conductivity of an electric current is also explained by the process of decay 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 unreality of the complete removal of impurities from such a dielectric, problems arise in the production of liquids with negligible current conductivity.
All types of insulation involve the search for options to reduce the conductivity of dielectrics. For example, remove impurities, adjust the temperature indicator. 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.
Conductivity of solids
It is explained by the movement of not only the ions of the insulator itself, but also the charged particles of impurities contained inside the solid material. As it passes through the solid insulator, partial removal of impurities occurs, which gradually affects the current conductivity. Given the structural features of the crystal lattice, the movement of charged particles is due to fluctuations in thermal motion.
At low temperatures, the movement of positive and negative ions of impurities. Such types of isolation are characteristic of substances with a molecular and atomic crystalline structure.
For anisotropic crystals, the conductivity varies depending on its axes. For example, in quartz in a direction parallel to the main axis, it exceeds 1000 times the perpendicular position.
In solid porous dielectrics, where there is practically no moisture, a slight increase in electrical resistance leads to an increase in their electrical resistance. For substances that contain water-soluble impurities, there is a significant decrease in volume resistance due to changes in humidity.
Dielectric polarization
This phenomenon is associated with a change in the position of the insulator particles in space, which leads to the acquisition by each macroscopic volume of the dielectric of a certain electric (induced) moment.
There is a polarization that occurs under the influence of an external field. A spontaneous variant of polarization is also distinguished, which appears even in the absence of an external field.
The relative dielectric constant is characterized by:
- capacitor capacitance with this dielectric;
- its magnitude in a vacuum.
This process is accompanied by the occurrence of bound charges on the surface of the dielectric, which reduce the intensity inside the substance.
In the case of complete absence of an external field, a single 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
With electron polarization, a shift occurs under the influence of the external field of the electron shells of the atom. In the ionic version, a displacement of the lattice sites is observed. Dipole polarization is characterized by losses due to overcoming internal friction and coupling forces. The structural variant of polarization is considered the slowest process, it is characterized by the orientation of inhomogeneous macroscopic impurities.
Conclusion
Electrical insulating materials are substances that make it possible to obtain reliable insulation of certain components of electrical equipment under certain electrical potentials. Compared to current conductors, many insulators have significantly greater 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 it is these insulators that are used in a variety of electrical devices.
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, we can distinguish an insignificant dependence of the strength on stress and temperature, thickness.