Low-frequency Impedance Spectroscopy Research Articles

Characterization of surface corrosion of reinforcing steel in cement paste by low frequency impedance spectroscopy

Surface corrosion of reinforcing steel embedded in cementitious materials containing chloride ion is described by characteristic change of impedance spectra. Phase angle shift, surface impedance and capacitive responses, recorded at very low frequency (100Hz-10mHz), are demonstrated to be very sensitive to surface corrosion processes. Information that is difficult to obtain by traditional dc techniques is provided. It was found that longer immersion of specimens in chloride ion solution results in a more significant decrease of the phase angle. A modified equivalent circuit model considering surface corrosion of the steel-cement interface was constructed to simulate the steel surface oxidation behavior. Circuit parameters can be used to explain the experimental results in a qualitative way. The low frequency impedance technique appears to be a very useful non-destructive method for assessing reinforcing steel corrosion.

Ionic conductivity in dehydrated zeolites

The ionic conductivity of pressed pellets of dehydrated synthetic offretite, cancrinite and zeolite A, with various alkali metal ions, was determined by low-frequency impedance spectroscopy. Experiments were carried out in the frequency range 10 Hz–10 MHz at temperatures from 100–600°C. The conduction activation energies range between 55 kJ mol−1 (Na-zeolite A) and 108 kJ mol−1 (Li-cancrinite). The best conductivity value obtained was that of Na-zeolite A with 2.9×10−3Ω−1cm−1 at 600°C.

The space charge capacitance of non-degenerate semiconductors with shallow and deep impurity levels

The formation of a space charge layer in non-degenerate n-type semiconductors with both shallow and deep impurity levels in the forbidden zone is described theoretically utilizing the exact expression for the charge density in the depletion layer. The impedance of n-type semiconductors with shallow and deep impurities can be modelled with an equivalent circuit comprising two capacitances in parallel. These two capacitances represent the space charge due to the response of electrons in the conduction band, and the response of the deep donors, respectively. At high frequencies only the former response contributes to the differential space charge capacitance, whereas at low frequencies the former as well as the latter response contributes. The dependence of both capacitances on dc bias approximately follows the Mott-Schottky behaviour in certain potential intervals and from the C −2 vs. V plots the shallow as well as the deep donor density together with the position of the deep donors in the forbidden zone can be derived. Numerical simulations illustrate the behaviour of the capacitances as a function of the dc bias and provide insight into the accuracy of low-frequency impedance spectroscopy.