Verifying Solution Component Concentration by Measuring Frequency Dependent Conduction of Electrically Charged Species

A conductive layer in a film or a sheet is an effective method for preventing static problems during production operations and in customer applications. Additionally, conductive layers form active electrodes in MEMS devices and traces in printed electronic circuits that are now produced in continuous roll-to-roll manufacturing processes. The conductive layer is formed by coating a solution or ink onto the moving substrate and subsequently drying the liquid to form the solid layer. The coated solution contains many components including particles or large organic molecules that provide conductivity in the dried layer. The conductivity of the layer must be controlled to satisfy product requirements. Variation in the concentration of the conductive species in solution is common source of variability. Verification of the concentration is proposed using a conductivity sensor immersed in the solution and measuring electrical admittance as a function of frequency. For sufficient sensitivity, the conductive species must be the largest, electrically mobile species in the solution as is the case for the formulation of many conductive layers. The decrease in admittance with increasing frequency verifies the concentration. For satisfactory measurement sensitivity, the concentration of the mobile species must be sufficiently high so that conduction due to this species can be detected. And, the relaxation time tau_s, which is unique to a specific species must be large compared with the charge relaxation time tau_e of the solution so that the decrease in conduction due to the mobile species will be evident.