The Dielectric Properties of Glass and their Structural Interpretation

JSGT 1949 V33 T220-T238

Our knowledge of the atomic structure of simple glasses has been advanced sufficiently to be used as a basis for interpreting their properties. The dielectric properties of glasses are determined by electronic and atomic polarisation processes which are not different from those encountered in crystalline solids. In addition, however, glasses show a phenomenon which formally may be described as “orientation polarisation.” This phenomenon is the result of some of the glass constituents being mobile even at room temperature and the fact that the open structure of glasses offers alternative positions for these ions. The removal of a sodium ion from its original position into a neighbouring “hole” produces a domain which has the electrical properties of a dipole. In external force fields these dislocations of sodium ions do not remain random, but are directed by the field. As a result, such a glass becomes anisotropic and may be described as containing oriented dipoles. The number of these units, the height of the energy barriers separating the different potential wells and the thermal energy available for overcoming these potential barriers are the factors which determine the dielectric properties of glasses. The response of a glass in an external electrical field affects the average inter-nuclear distance. As a result, the volume of a glass changes with the strength of the field applied (electrostriction). The additional motion of atoms which result from their assuming new equilibrium positions in the electrical fields is responsible for the power loss, i.e. the change of electrical energy into heat. There exists a parallelism between the response of a glass to external electrical and to external mechanical fields. It is emphasised that previous attempts to treat these phenomena on the basis of mathematical equations led to convenient ways of describing them in another language but not to an explanation.

W. A. Weyl