Dielectric Behavior of the Film Formed on Mica Cleaved in Moist Air

Water adsorbed on a freshly peeled mica crystal causes the loss tangent, D, to increase by 1 to 2 orders of magnitude. The nature of the film is investigated as a function of relative humidity by the measurement of D for the frequency range 100 to 50,000 c/s with a capacitor comprising concentric, parallel, circular electrodes of different diameter on opposite sides of the dielectric sheet. This geometry is then analyzed as consisting of the two regions of that within the plates where the electric field, E, is normal to the dielectric plane and that at the edge where tangential E exists. The first is considered in terms of an equivalent circuit for a two layered dielectric. The second is considered in terms of transmission line concepts. It is predicted and verified that the adsorbed film causes the first component to vary as 1/t and the second as √t where t is the thickness of the crystal. Numerical solutions are used to derive the behavior of D, R, and C of the adsorbed film itself. D for the normal direction is ~0.4 and follows a frequency dependency like fresh snow. Resistivities normal to and parallel to cleavage are considerably different while C is much less than expected for a surface film. It is suggested that the surface film is not continuous but instead is localized in patches.