Microwave dielectric properties of magnetite colloidal particles in magnetic fluids

Measurements performed over the frequency range 700 MHz to 3 GHz, of the complex dielectric permittivity of (i) a kerosene-based magnetic fluid sample with magnetite particles stabilized with oleic acid, (ii) oleic acid and (iii) kerosene, are presented. The biphasic dielectric model describing the dielectric behaviour of magnetic fluids was redefined based on the experimental results of Rocchiccioli-Deltcheff et al. According to this proposed new model, the magnetic fluids can be considered as being biphasic systems in which one phase consists of surfacted colloidal particles electrically charged by adsorbed or chemisorbed ions and the other phase consists of the carrier liquid. Using the Sillars theory for heterogeneous dielectrics, in the biphasic approximation, we have determined the dielectric permittivity, electric conductivity and shape factor of the colloidal magnetite particles within the investigated magnetic fluid, in the microwave range. The values obtained for the colloidal magnetite particles are the shape factor n = 2.27, the real part of the dielectric permittivity epsilon'(1) = 36.8, and the effective conductivity sigma(1) (eff) = 1.5 Omega(-1) m(-1). The differences between the experimental results and the values of conductivity and permittivity of bulk magnetite are explained in terms of the proposed biphasic model.