Lecture
The value of a substance's relative permittivity is determined by its degree of polarizability and is therefore primarily determined by the mechanisms of polarization. However, the value of ε also depends to a large extent on the state of aggregation of the substance, since the density, viscosity, and isotropy of the substance change substantially during transitions from one state to another.
Permittivity of gases. Gaseous substances are characterized by very low densities due to the large distances between molecules. Because of this, the polarization of all gases is insignificant, and their relative permittivity is close to unity. The permittivity of different gases is greater the larger the gas molecule.
For air under normal conditions, the relative permittivity is 1.0006.
Permittivity of liquid dielectrics. Liquid dielectrics can consist of nonpolar or polar molecules.
The relative permittivity of nonpolar liquids is determined by electronic polarization, and is therefore small, close to the value of the square of the refractive index of light, ε ≈ n2, and usually does not exceed 2.5.
The temperature dependence of the permittivity of a nonpolar liquid is related to the decrease in the number of molecules per unit volume, i.e., to the decrease in density.
The polarization of liquids containing dipolar molecules is determined simultaneously by electronic and dipole-relaxation components. Such liquids have a higher permittivity the greater the value of the electric moment of the dipoles and the greater the number of molecules per unit volume.
The temperature dependence of permittivity in the case of polar liquids has a more complex character than in the case of nonpolar liquids. Frequency has a significant effect on the value of ε of a polar liquid. As long as the frequency is small enough that the dipoles have time to follow the change in the field, ε is large and close to the value determined at constant voltage. When the frequency becomes so high that the molecules no longer have time to follow the change in the field, the permittivity decreases and its value approaches the value due to electronic polarization.
Permittivity of solid dielectrics. The permittivity of solids can take on a wide variety of numerical values, reflecting the diversity of structural features of the solid dielectric. All types of polarization are possible in solids. The lowest permittivity values are found in solid dielectrics consisting of nonpolar molecules and possessing only electronic polarization.
Solid dielectrics that are ionic crystals with a dense packing of particles have both electronic and ionic polarization and have permittivity values lying within wide limits. The temperature coefficient of permittivity of ionic crystals is positive in most cases, because as temperature rises, not only does the density of the substance decrease, but the displacement of ions also increases; and the influence of this factor on the value of ε is stronger than the effect of the change in density. An exception is formed by crystals containing titanium ions – rutile (TiO2) and some ceramic materials based on it, which have a negative temperature coefficient of permittivity. The negative sign of the temperature coefficient of these materials is due to the specific interaction of the electron shells of the titanium and oxygen ions. The specifics of this interaction are that, as temperature rises and the overlap of the electron shells of the interacting ions weakens, the elastic bond between ions in the TiO2 lattice does not decrease but, on the contrary, increases. Correspondingly, the displacement of ions under the action of the field becomes more difficult.
The permittivity of various inorganic glasses, which are close in structure to amorphous dielectrics, lies within relatively narrow limits – roughly from 4 to 20; and the temperature coefficient of permittivity of glasses is generally positive.
Polar organic dielectrics, as noted, exhibit dipole-relaxation polarization in the solid state. The permittivity of these materials depends to a large extent on temperature and the frequency of the applied voltage, following the same patterns as ε of dipolar liquids.
Comments