Finite element modeling of resistive surface layers by micro‐contact impedance spectroscopy

Abstract

Micro-contact impedance spectroscopy (MCIS) is potentially a powerful tool for the exploration of resistive surface layers on top of a conductive bulk or substrate material. MCIS employs micro-contacts in contrast to conventional IS where macroscopic electrodes are used. To extract the conductivity of each region accurately using MCIS requires the data to be corrected for geometry. Using finite element modeling on a system where the resistivity of the surface layer is at least a factor of ten greater than the bulk/substrate, we show how current flows through the two layers using two typical micro-contact configurations. This allows us to establish if and what is the most accurate and reliable method for extracting conductivity values for both regions. For a top circular micro-contact and a full bottom counter electrode, the surface layer conductivity (σ s ) can be accurately extracted using a spreading resistance equation if the thickness is ~10 times the micro-contact radius; however, bulk conductivity (σ b ) values can not be accurately determined. If the contact radius is 10 times the thickness of the resistive surface, a geometrical factor using the microcontact area provides accurate σ s values. In this case, a spreading resistance equation also provides a good approximation for σ b . For two top circular micro-contacts on thin resistive surface layers, the MCIS response from the surface layer is independent of the contact separation; however, the bulk response is dependent on the contact separation and at small separations contact interference occurs. As a consequence,

Evaluation of electrode-sample contact impedance under different curing humidity conditions during measurement of AC impedance of cement-based materials

“…This point needs to be further investigated. According to the discussion in "Conductive theory of two-point and four-point measurement of ACIS" section, polarization resistance can be negligible for a four-point measurement; however, voltage electrodes need to be precast and thus the extended impedance arises due to current spreading within the specimen in the vicinity of the contact as the electrode size is smaller than the sample section area, these effects were strictly geometric and ohmic and were in no way associated with electrochemical reactions or polarization effect 10,48,49 , additionally, interface impedance truly exists. Therefore, it is necessary to evaluate the effect of the contact impedance on the tested ACIS from the four-point measurement.…”