Effect of electrode contact impedance on A.C. electrical properties of a wet hematite sample |
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| Authors: | Mohamed Mahmoud Gomaa Perparim Alikaj |
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| Institution: | (1) National Research Centre, Geophysical Sciences Department, El-Tahrir St., Dokki, 12311, Egypt;(2) Head of Geophysics Section, Department of Earth Sciences at Polytechnic University of Tirana, Tirana, Albania |
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| Abstract: | Electrode polarization is a major problem in the determination of dielectric properties of samples, particularly at low frequencies.
Understanding of these interfacial phenomena is essential in order to measure correctly the electrical properties of a sample
of interest. This paper presents a comparative study of the effect of electrode contact impedance on A.C. electrical properties
of a partially and fully saturated hematitic sandstone sample. The electrical properties of the sample were first measured
using stainless steel electrodes with high contact impedance, and measured again with a four terminal Cu–CuSO4 electrode of low contact impedance. Complex impedance measurements at room temperature (~16°C) were performed in the frequency
range from 1 Hz to 100 kHz. Measured electrical spectra vary strongly with the electrode type. The difference in the electrical
properties between the two electrode types (stainless steel and Cu–CuSO4) may be attributed to the surface contact impedance between the sample and the electrode. Experimental data indicate that
the electrical properties vary strongly with water saturation. The dielectric constant decreases with frequency and increases
with saturation up to a certain saturation limit then decreases. Charge transport can occur either through the bulk of the
solid matrix (hematite or sand) or along the grain boundaries of aggregates (water). When soil minerals are exposed to water,
exchangeable ions go into solution. Most of the ionic or covalent bonded rock forming minerals such as quartz, mica, and feldspars
are nonconductors. When the surfaces of these minerals come into contact with liquid water, electrolytes are formed and ionic
drift associated with the electrical field causes electrical conduction. The anomalous dielectric properties of partially
saturated rocks can be interpreted using percolation theory. This theory predicts that when the conductive fraction (water)
increases, clustering of conductive inclusions develops, and the thickness of insulating gaps between conductive clusters
decreases, causing a large increment in the capacitance of the sample. Further increases in the conductive component causes
the shunting of insulating capacitive gaps. |
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