Electrical properties of iron oxide-polyethylene glycol humidity-sensitive elements

Films of α-Fe2O3 particles in a polyethylene glycol matrix exhibit electrical properties which are strong functions of relative humidity. Electrical impedance measurements over the frequency range 10−3–105 Hz were used to establish equivalent circuits for the samples including the electrodes. The variation of the equivalent-circuit elements with electrode geometry can be used to give a simple physical interpretation to the equivalent-circuit elements. We conclude that the humidity response is associated with the bulk resistance of the glycol. This resistance is dependent upon ion mobility which in turn depends on the viscosity. Viscosity is, of course, a function of water content. The resistance is in series with the electrode capacitance due to dipole double layers. By a suitable choice of electrode geometry, the impedance of the series capacitance will be negligible at a convenient operating frequency.