Electret effect in cement
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The electret effect was observed for the first time in cement-based materials. It is potentially useful for rendering multifunctionality to a concrete structure. The cement is in the form of a cement paste, with no admixture other than a water reducing agent. It is prepared conventionally and has not been subjected to poling. The electret effect is associated with an inherent voltage (negative, with magnitude up to 0.115 V), which was observed in this work in as-prepared cement. That this is indeed an electret is confirmed by the observed effect of a current (negative, with magnitude 10-20 μA, with a current density range of 6.25 x 10 -3 A/m 2 to 12.5 x 10 -3 A/m 2 ) pulse (22 s), which causes the voltage to be even more negative, and subsequently change from being negative to being positive and finally become stable at a value (positive, up to 0.04 V) that is comparable in magnitude to the inherent voltage. The change in sign is a classical indication of the electret effect. A higher current magnitude gives a stronger electret effect. The electret formation after the application of a current pulse involves the well-known mechanism of skin-core formation, with the voltage of concern corresponding to that of the skin. The use of surface electrical contacts rather than embedded contacts is necessary for probing the skin. Excessive polarization, as in the case of a long time of current application (22 min instead of 22 s), impedes skin-core formation, thus making the electret formation not possible.