Microstructural effects in low loss power ferrites

The effect of additions of CaO, SiO2, SnO2 and Nb2O5 on the microstructure and magnetic properties of Mn–Zn ferrites has been studied. The grain size is determined by the sintering temperature and the concentrations of CaO and SiO2. TEM studies show that Ca and Si are enriched in thin grain boundary layers and grain junctions. Sn is completely incorporated into the bulk of the ferrite while Nb is found to be segregated at amorphous grain boundary layers. Impedance spectroscopy on CaO- and SiO2-doped ferrites with fine-grained microstructure reveals an increased grain boundary resistance. The activation energies for charge transport in the bulk and grain boundaries have been verified to be almost identical; thus it is concluded that the microstructure is not formed as a network of isolated grains, but rather by grains which are not completely separated by a very thin insulating film. The power loss is minimum for a homogeneous and fine-grained microstructure which is mainly controlled by CaO and SiO2 additions and the sintering temperature; Nb2O5 and SnO2 show minor effects.