Characterization of dielectric powders by a new defined form factor

Abstract

Capacitance measurements of oxide powders in the frequency range of 1 kHz to 10 MHz have been performed in order to deduce a form factor which is related to the morphology of the particles and their arrangement in the powder. Using a capacitor measuring setup the frequency independent real part of the complex dielectric constant was measured in the range of 1 kHz to 2 MHz in dependence on the volume fraction. The interpretation follows the models derived for inhomogeneous dielectrics, which allow to calculate a morphology dependent parameter provided that the permittivity of the solid is known. A new defined form dependent weighting factor introduced in Bruggeman’s model of geometrical averaging between lamellar and spherical constituents is shown to supply the better approximation to measuring data than the aspect ratio of ellipsoids determining the depolarization factor. The results are verified for aggregated and partially stepwise ground powders of Mg 2TiO 4, MgAl 2O 4, SrTiO 3, CaTiO 3, Al 2O 3, and ZnO and related to other particle shape parameters derived from SEM studies indicating very clearly the suitability to describe the morphology of powders by this new form factor. The frequently applied Lichtenecker rule does not allow to consider particle morphology. For comparison the specific surface area BET data and the d 50 values from grain size distribution measurements of the powders are also reported. The applicability of the method for checking the reproducibility of different lots of capacitor raw materials is shown for two COG powders.