Effect of electrode contact impedance on A.C. electrical properties of a wet hematite sample
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ORIGINAL RESEARCH PAPER
Effect of electrode contact impedance on A.C. electrical properties of a wet hematite sample Mohamed Mahmoud Gomaa • Perparim Alikaj
Received: 17 January 2010 / Accepted: 6 July 2010 / Published online: 17 July 2010 Ó Springer Science+Business Media B.V. 2010
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 M. M. Gomaa (&) National Research Centre, Geophysical Sciences Department, El-Tahrir St., Dokki 12311, Egypt e-mail: [email protected] P. Alikaj Head of Geophysics Section, Department of Earth Sciences at Polytechnic University of Tirana, Tirana, Albania e-mail: [email protected]
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. Keywords Electrode effect Electrical properties Saturation Hematitic sandstone Complex impedance
Introduction The measurement of A.C. electrical properties of humid and saturated rocks (conductivity and dielectric constant) is important in ma
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