An Analysis of Mn-Zn Ferrite Microstructure by Impedance Spectroscopy, STEM and EDS Characterisations

AC resistivity measurement results on Mn-Zn sintered ferrite were analyzed in the 0.1-500 MHz range. From electrical point of view, the material could be represented by an equivalent circuit of parallel resistance-capacitance cells connected in series corresponding to the contributions from bulk grains in one hand, and grain boundary layers in the other hand. The experimental resistivity curves were fitted with the model. The as obtained parameters give information on dielectric properties and conductivity of both bulk grains and boundary layers. For the studied material, it appears that the resistivity at low frequencies is increased 27 times due to the boundary layers effects. STEM (Scanning Transmission Electron Microscopy) and EDS (Energy Dispersion Spectrometry) characterization where performed in order to detect impurities at a grain boundary layer which can explain those wide differences between bulk grains and boundary layers electrical properties. It appears that the two components have close chemical compositions, but some calcium impurities segregate at the boundary which increases dramatically the resistivity of these layers. Furthermore, the bulk grains show relative permittivity around 350 at low frequency which is much smaller than the one measured for the whole material which is in the 50,000-100,000 range. This giant-dielectric behaviour can be explained by an internal barrier layer (IBLC) at the grain boundaries. At last, the components of classical eddy current losses including losses due to ohmic effects and (true) dielectric losses on both bulk grain and boundary layers are distinguished.