Dielectric constant

The dielectric constant K is a measure of a material’s ability to store electric charge. In scalar form the defining relations are as follows: . . . D = εE, . . . where D is the electric displacement measured in C/m2, ε is the electric permittivity in F/m, and E is electric field in V/m. The dielectric constant K is the relative permittivity: . . . K = ε/ε0, . . . where ε0 = 8.85 × 10−12 F/m is the permittivity of free space. The electric displacement D is equal to the sum of the charges stored on the electrode plus those originating from the polarization, P [C/m2] . . . D = ε0 E + P. . . . In this chapter we discuss the tensor nature of the dielectric constant, how it is represented geometrically, and some typical structure–property relationships. Dielectric constants range over about four orders of magnitude in insulator materials. Because of their low density, gases have dielectric constants only slightly larger than one. At one atmosphere, the dielectric constant of air is 1.0006. Most common ceramics and polymers have dielectric constants in the range between 2 and 10. Polyethylene is 2.3 and silica glass is 3.8. These are low-density dielectrics with substantially covalent bonding. More ionic materials like NaCl and Al2O3 have slightly higher K values in the 6–10 range. High K materials like water (K ∽ 80) and BaTiO3 (K∽1000) have special polarization mechanisms involving rotating dipoles or ferroelectric phase transformations. A schematic view of the principal types of polarization mechanisms is illustrated in Fig. 9.1. The electronic component of polarization arising from field-induced changes in the electron cloud around each atom is found in all matter. The ionic contribution is also common and is associated with the relative motions of cations and anions in an electric field. Orientational polarizability arises from the rotation of molecular dipoles in the field. These motions are common in organic substances. Many materials also contain mobile charge carriers in the form of ions or electrons that can migrate under applied fields.