1 Basic Concepts of Polarization
Under the action of the electric field, the dielectric phenomenon of positive and negative charges making small displacements and generating a dipole moment in the direction of the electric field, or inducing a bound charge on the surface of the dielectric is called dielectric polarization.
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2 Basic types of polarization
The capacitance of a parallel plate capacitor in a vacuum is that if a solid dielectric is inserted between the parallel plates, the capacitance of this capacitor becomes e,co,er, and the relative permittivity of this dielectric, also known as the permittivity, is greater than 1. The increase in capacitance is due to the polarization of the dielectric.
There are 5 basic forms of dielectric polarization: electron displacement polarization, ionic displacement polarization, steering polarization, space charge polarization, and interfacial polarization of sandwich dielectrics.
(1) Electronic displacement polarization
The process by which the electrons in an atom, molecule, or ion in a dielectric are displaced relative to the nucleus by the action of an external electric field, resulting in a dipole moment in the direction of the electric field, is called electron displacement polarization.
This polarization is characterized by the presence of all dielectrics. Due to the small mass of the electrons, the establishment polarization time is extremely short, 10-15 10-14s. The degree of polarization depends on the electric field strength e, independent of the power supply frequency f, and has little to do with temperature, because the temperature is not enough to cause a change in the energy state of the electrons inside the particle. This polarization is elastic, there is no energy loss, and the polarization phenomenon disappears immediately after the external electric field is removed.
(2) Ion displacement polarization
In a dielectric formed by ions, the action of the external electric field causes the positive and negative ions to produce a small displacement, and the dipole moment in the direction of the electric field is averaged, and this form of polarization is called ion displacement polarization.
This polarized form is present in dielectrics with ionic structure. The time to establish this polarization is extremely short, 10-13 10-12s, and the degree of polarization is independent of the supply frequency f. However, as the temperature increases, the ionic displacement polarization increases slightly, i.e., r generally has a positive temperature coefficient. Ions are more susceptible to polarization due to the increase in the distance between ions and the decrease in the inter-ion forces. Ion displacement polarization is also elastic with no energy loss. By removing the external electric field, the polarization phenomenon disappears immediately.
Turning to polarization
Steering polarization is also known as dipole relaxation polarization. In polar dielectrics, the positive and negative charge centers in the molecule do not coincide, and as far as a single molecule is concerned, it already has a dipole moment, which is called a polar molecule. In the absence of an external electric field, the polar molecules are in a state of thermal motion, and do not have a dipole moment on average. Under the action of the external electric field, the orientation probability of polar molecules in the direction of the electric field increases, and the dipole moment in the direction of the electric field is averaged to the outside, which is called the directional polarization.
Steering polarization is present in polar dielectrics. The dipole steering polarization is inelastic, the steering needs to overcome the interaction between them and do work, and the energy consumed cannot be recovered when recovering. The polarization time is longer, 10-6 10-2s;The degree of polarization is related to the frequency of the power supply f, and when the frequency is higher, the polar molecules do not have time to turn with the change of the electric field, so that the degree of polarization decreases.
The relationship between steering polarization and temperature is complex, and the degree of steering polarization increases first and then decreases with increasing temperature. In the low temperature section, the molecular connection between the solid and liquid dielectric is tight, and it is difficult to turn and polarize. As the temperature increases, the degree of polarization increases. However, when the temperature is higher, the molecular thermal motion intensifies, which hinders the orientation of the dipole along the direction of the electric field, and the degree of polarization decreases.
In ionic dielectrics with a non-compact structure, there is ionic relaxation polarization, which has similar characteristics to dipole relaxation polarization and can be classified as steering polarization.
(4) Space charge polarization
The above three polarizations are formed by the elastic displacement or steering of the charged particles, while the spatial charge polarization is quite different from the above three polarizations and is formed by the movement of the charged particles.
The free positive and negative ions in the dielectric move under the action of the electric field, change the distribution, and form a space charge near the electrode or inside the dielectric, so this form of polarization is called space charge polarization.
This form of polarization is present in inhomogeneous dielectrics with energy loss, the conductance of high-voltage insulating dielectrics is usually very small, the polarization takes a long time to establish, and polarization of this property can only occur at low frequencies.
(5) Interfacial polarization of the sandwich dielectric
In the actual electrical equipment, many of them are multi-layer dielectric insulation structures, and the simplest double-layer dielectric model is used to analyze the dielectric interface polarization. In Figure 4-1, the initial voltage ratio of the two layers of dielectric at the moment of closing is determined by the capacitance, and the voltage ratio at steady state is determined by the conductance: t=0, u1 u2=c2 c1;t, u1 u2=g2 g1.
If C2 C1=G2 G1, then the surface charge of the bilayer dielectric is not redistributed and the initial voltage ratio is equal to the steady-state voltage ratio. However, in practice, it is difficult to meet the above conditions, and the charge needs to be redistributed, so that the charge will accumulate at the interface of the two layers of dielectrics, so it is called interfacial polarization of the sandwich dielectric. The accumulation of charges at the sandwich interface is accomplished by the dielectric conductance g, which has similar properties to space charge polarization.
3 Dielectric constant of a dielectric
In a vacuum, there are relational formulas.
d=ε0e
where e is the electric field vector, v mm;d is the flux density vector, c. d and e are in the same direction, and the proportional constant 0 is the dielectric constant of the vacuum, and its value is about 8 854x10-12f m in the dielectric, then there is a relation.
d=εe
where d and e are still in the same direction, the proportionality constant is the dielectric constant of the dielectric, and r is the relative permittivity.
It should be said that the dielectric is not constant, not only changes with temperature and frequency, but even divided into real and imaginary parts in in-depth research, but under normal circumstances, only the real part of is used, so it is called the dielectric constant in electrical termsThe r of the real part is called the relative permittivity, which is greater than 1 and has no dimension and unit.
(1) Dielectric constant of a gas dielectric
The distance between the gas molecules is large, the density is small, the degree of polarization of the gas is very small, the relative permittivity of all gases is close to 1, and Table 1 lists the relative permittivity values of several gases.
Table 1 Relative permittivity of some gases (at 20, 1 atm).
Note: 1atm (standard atmosphere) 101325x105 pa。
The dielectric constant of the gas decreases slightly with temperature and increases slightly with pressure, but the change is small.
(2) The dielectric constant of a liquid dielectric
Non-polar and weakly polar dielectrics. There are many liquid dielectrics that belong to this category, such as petroleum, benzene, carbon tetrachloride, silicone oil, etc. Their relative permittivity is not large, and its value does not exceed 28。The relative permittivity is similar to the relationship between temperature and the number of molecules per unit volume as opposed to temperature.
Polar dielectric. The relative permittivity of this type of dielectric is large, with a value between 3 80, and the r value that can be used as an insulating dielectric is 3 6. This kind of liquid dielectric is used as a capacitor impregnating agent, which can increase the specific capacitance of the capacitor, but usually the loss is large, castor oil and several synthetic liquid dielectrics have practical applications. The relative permittivity as a function of temperature and frequency is shown in Figure 1.
Fig.1 Relative permittivity of chlorinated biphenyls as a function of temperature
(3) The dielectric constant of a solid dielectric
Non-polar and weakly polar solid dielectrics. There are many types of such solid dielectrics, such as polyethylene, polypropylene, PTFE, polystyrene, mica, paraffin, asbestos, sulfur, inorganic glass, etc. Among them, mica, asbestos, etc. are crystalline ionic structures;Inorganic glass is an amorphous ionic structure. This type of dielectric has only electronic polarization and ionic polarization, and the dielectric constant is not large, usually 20～2.7。The relative permittivity is also close to the relationship between the number of molecules per unit volume and temperature.
Polar solid dielectric. Solid dielectrics that fall into this category include resins, fibers, rubber, shellac, plexiglass, polyvinyl chloride, and polyester. The relative permittivity of this type of dielectric is larger, generally 3 6, and may be greater. The relative permittivity is similar to that of a polar liquid dielectric as it is related to temperature and frequency.
Depending on the characteristics of the steering polarization, the tendency of the dielectric constant to change with temperature and frequency can be explained.
Ionic dielectrics. Such solid dielectrics include ceramics, mica, etc., and their relative permittivity r is generally 5 8.
4 Discuss the significance of polarization
(1) Select insulation
When actually choosing an insulation, the dielectric constant r should be taken into account in addition to the electrical strength. For capacitors, if you want to have a larger capacitance under the same volume conditions, you should choose a larger dielectric. For cables, to reduce the capacitive current, choose a smaller dielectric.
(2) Reasonable coordination of multilayer dielectrics
For multilayer dielectrics, under AC and impulse voltage, the voltage distribution of each layer is inversely proportional to it, and attention should be paid to the selection of electric field distribution of each layer of dielectric.
and (3) the theoretical basis for studying dielectric loss
Dielectric loss is related to polarization form, and it is necessary to understand the influence of different polarization types on dielectric loss.
(4) The theoretical basis of the insulation test
It provides a theoretical basis for the establishment of preventive test items for electrical equipment.