An article about dielectrics. This article brings together materials from a variety of electrical engineering tutorials and books. The molecular structure, electrical moment of dielectrics are described. A dielectric is a substance whose main electrical property is the ability to polarize in an electric field.
A characteristic feature of dielectrics is the presence of strongly coupled positive and negative charges in the molecules that make up the substance. Of the existing types of bonding for dielectrics used in electrical and radio engineering, the most typical are covalent non-polar, covalent polar or homeopolar, ionic or heteropolar, donor-acceptor. The forces of connection determine not only the structure and basic properties of a substance, but also the presence in it of chaotically or orderly oriented electrical moments in micro- or macroscopic volumes of a substance.
The electric moment appears in a system of two electric charges of equal magnitude and opposite in sign ± q, located at a certain distance l from each other, and is determined by the ratio = ql.
Such a system of charges is usually called a dipole, and a molecule formed by this system of charges is called a dipole.
Covalent bondarises when atoms combine into molecules, as a result of which valence electrons are socialized and the outer electron shell is supplemented to a stable state.
Molecules with a covalent non-polar bond arise when atoms of the same name, such as H2, O2, Cl2, C, S, Si, etc. are combined. and have a symmetrical structure. As a result of the coincidence of the centers of positive and negative charges, the electric moment of the molecule is zero, the molecule is non-polar and the substance (dielectric) is non-polar.
If molecules with a covalent bond are formed from dissimilar atoms due to the sharing of pairs of valence electrons, for example, H2O, CH4, CH3Cl, etc., then the absence or presence of an electric moment will depend on the mutual arrangement of atoms relative to each other. With a symmetric arrangement of atoms and, therefore, coincidence of centers of charges, the molecule will be non-polar. With an asymmetric arrangement due to the displacement of the centers of charges at a certain distance, an electric moment arises, the molecule is called polar and the substance (dielectric) is polar. Structural models of non-polar and polar molecules are shown in the figure below.
Regardless of whether it is a polar or non-polar dielectric, the presence of an electric moment in molecules leads to the appearance of an intrinsic electric field in each microscopic volume of a substance. With a chaotic orientation of the electrical moments of molecules due to their mutual compensation, the total electric field in the dielectric is zero. If the electric moments of molecules are oriented predominantly in one direction, then the electric field arises in the entire volume of the substance.
This phenomenon is observed in substances with spontaneous (spontaneous) polarization, in particular, in ferroelectrics.
Ionic and donor-acceptor bondsarise when a substance is formed from unlike atoms. In this case, the atom of one chemical element gives up, and the other attaches or captures an electron. As a result, two ions are formed, between which an electrical moment arises.
Thus, according to the structure of molecules, dielectrics can be divided into three groups:
- non-polar dielectrics, the electric moment of the molecules of which is equal to zero;
- polar dielectrics, the electric moment of the molecules of which is nonzero;
- ionic dielectrics, in which an electric moment occurs between the ions of the chemical elements that make up the substance.
The presence of electrical moments in dielectrics, regardless of the reasons for their occurrence, determines their main property - the ability to polarize in an electric field.