Frequency-dependent electrical mixing law behavior in spherical particle composites

The electrical properties of cement-based composites with mono-size conductive (steel) or insulating (glass) spherical inclusions were investigated by combined 2-point impedance spectroscopy and 4-point dc resistance measurements. The matrix was ordinary Portland cement (w/c = 0.4; cured for 7 days). Particle loading was varied over an extended range to as high as 42% volume fraction. The steel particle composites behaved like the glass particle composites at dc and low ac frequencies; conductivity decreased with increasing particle loading. Under ac excitation, however, the steel particles were rendered conductive; conductivity increased dramatically with particle loading. The results were analyzed in terms of various mixing laws and effective media theories and the proposed "frequency-switchable coating model," which accounts for the unusual frequency-dependent behavior of the steel particle composites.