Transient Dielectric Response of Conduction on Humid Mica

In a study aimed at elucidating the physical nature of the widely observed low-frequency dispersion (LFD) in carrier-dominated dielectrics, we report a study of the time dependence of surface conduction on humid mica in relation to the ambient humidity and the amplitude of the step-function voltage. This system shows clear LFD behavior in the frequency domain and is known to be nonlinear with voltage. Surface conduction along the cleavage planes on humid mica is reported to have a time dependence fully consistent with LFD, but while the charging current is proportional to the applied voltage less an internal EMF of approximately 1 V, the discharge current is voltage-independent. The behavior is shown to be fully consistent with a model involving a secondary battery, probably arising at the mica-aluminum contact, and an internal humidity-dependent surface resistance. The results clearly confirm the validity of the earlier assumption that the LFD phenomena are related to electrochemical interactions and cannot be reconciled with normal electrostatic charge and energy storage. The model explains the enormously high low-frequency capacitances observed in many LFD systems, sometimes exceeding 100 F/m2.