Chapter 15 - Dielectric properties

Abstract

Electric permittivity or the dielectric constant of a material is defined by the ratio of the electric displacement and electric field vectors. The temperature and frequency dependence of alternating current (ac) conduction in glass is usually given by an augmented Jonscher's power law comprising at least four terms. The first of these is the thermally activated, frequency-independent dc conductivity at low frequency and high temperature. The second term, also thermally activated, is the universal power law applicable to nearly all disordered solids in an intermediate frequency range, up to several MHz. The third, weakly T-dependent term with linear frequency dependence, is ascribed to the cooperative vibration of a small group of atoms in a manner similar to jellyfish. The fourth term arises from mean squared displacement vibrational relaxation, applies to very high frequencies. The dielectric constant of glasses is generally in the range of 4–11 at 1MHz and 20°C. Fused silica has the lowest dielectric constant of about 3.8. The dielectric constant decreases slightly by the addition of B2O3, is almost unaffected by the addition of TiO2, and increases considerably by the addition of alkalis, alkaline earths, and other network-modifying oxides. Typical values of ɛ′ for soda lime glasses are 7–10, whereas those for sodium borosilicates are around 4.5–8.