Electric response of cells of ferrofluids to ac external electric field: dependence on the concentration of magnetic particles and on the electrodes

The electric response of samples of kerosene containing magnetic particles to an ac electric field is experimentally investigated by means of the impedance spectroscopy technique in the frequency range 3mHz, 25 kHz. The samples have different thicknesses, and contain different concentrations of magnetic particles. The case where the thickness and the concentration of the sample are fixed, but the electrodes are different has been also investigated. From the analysis of the spectra of the real and imaginary parts of the electric impedance of the cell, in series representation, we derive that the sample is well described by an electrical circuit formed by the series of two parallel resistance-capacitance. One of the resistance-capacitance element is charged to describe the bulk properties of the sample, whereas the other to take into account the properties of the interface electrode-solution. By means of the best fit procedure the bulk and interface resistances and capacitances are determined. From the data relevant to the bulk lumped elements, the dielectric constant of the kerosene and the diffusion coefficient of the magnetic particles in kerosene are determined. From the analysis of the values of the resistance and capacitance of the interface we have obtained the thickness of the interface layer and the conductivity of the electrode. In particular, the thickness of the surface layer depends on the bulk concentration of magnetic particles in the same manner as the length of Debye depends on the ionic concentration. From this experimental result it derives that the ionic contamination seems due to the presence of the magnetic particles. According to our experimental data, for frequency larger than few hundreds of Hz, the electric response of the sample is independent of the properties of the electrodes. The analysis of the experimental data at dielectric level is also reported, and the characteristics parameters of the model of Debye with dc conductivity determined in terms of the lumped elements appearing in our Maxwell-Wagner model.