The dielectric loss characteristics of a chlorinated diphenyl

Abstract

Considerable attention has been devoted of recent years to the development of synthetic insulating material devoid of the uncertainties and limitation of naturally occurring dielectrics and their modifications (such as cellulose, rubber, oil, and wax compound). Into this category falls a range of non-inflammable chlorinated diphenyl compounds known in this country as Permitol. The members of this range consist of a double benzene ring as basis with which chlorine atoms have been combined. By controlling the degree of chlorination, it is possible to produce compounds ranging at room temperatures from crystalline solids to liquids of relatively low viscosity. The present paper deals with a study of the dielectric behaviour over the frequency range 50 to 10 7 cycles per second and at temperatures extending from — 20 to + 80° C of a sample of chlorinated diphenyl classed as suitable for condenser impregnation. Its viscosity at 60° C is that of a light transformer oil, but with temperature decrease the viscosity increases very rapidly until the material becomes a glassy 8olid about — 10° C. One of the most desirable characteristics of a material to be used for this purpose is a high dielectric constant (permittivity) allied to freedom from dielectric loss. A high permittivity is usuallv associated, however, with the presence of polar molecules in the material, and these, according to the Debye theory of dielectric absorption, must be responsible for dielectric loss in alternating electric fields under suitable conditions of temperature and frequency. For a dilute solution of polar molecules in a non-polar solvent, the power factor passes through a maximum at a frequency given by f = k t/8π 2 a 3 η, where k is Boltzmann's constant, T is the absolute temperature, a is the radius of an equivalent sphere representing the dipole molecule or group, and η is the viscosity opposing molecular orientation. The power factor becomes zero at both zero and infinite frequency. At sufficiently low frequency the polar molecules orient freely in sympathy with the electric field variations ; the power factor is therefore low and the dielectric con­stant high; but at high frequencies the sympathetic orientation of the polar molecules is prevented by their viscous environment, their contribution to the total dielectric polarization becomes negligible, and the power factor and dielectric constant are together low. similar power factor and permit­tivity changes result on variation of the temperature, and with it the viscosity at con8tant frequency. It is evident that maximum dielectric constant and low dielectric loss for a given polar material can exist simultaneously by a suitable choice of the operating conditions of tempera­ture and frequency.