Local Electrical Behavior, Crystallography, and Chemistry of Grain Boundaries in Mn-Zn Ferrites

Abstract

This investigation is devoted to grain boundaries, since, for high-frequency applications, eddy-current losses can be minimized by selectively increasing the resistivity of grain boundaries. Two specific commercial Mn-Zn ferrites were considered. The first one, selected to be a reference sample, exhibited good permeability, whereas the second one was chosen to promote a strong segregation of calcium on the grain boundaries via the addition of CaO and SiO2. To explain the behavior of this material chemical, crystallographical and electrical properties were collated. Initially, maps of potential barriers were recorded on a scale representative of the bulk microstructure by voltage contrast via scanning electron microscopy at 1 keV. In addition to the potential maps, the local electrical potential was measured quantitatively via local probing. It was evidenced that there was no percolation path through weak barriers at the grain boundaries. To gain information on very small voltage drops, in particular within grains, the microelectrode technique was used on the same areas. It showed a very different behavior between the two samples for the intragranular resistivity, as well as for interfacial barriers. Then, the crystallochemistry of the grain boundaries was investigated via analytical conventional transmission electron microscopy and scanning transmission electron microscopy techniques, and the specific segregation of calcium and the depletion of Fe2+ versus Fe3+ ions at general grain boundaries (GBs) was evidenced. The chemistry and microstructure of the grain boundaries were related to their crystallography. By local electrical probing via transmission electron microscopy, the interfacial barriers were directly related to the type of grain boundary. It was shown that high electrical barriers correspond to high-energy GBs, where there are no orientation relationships between adjacent grains; on the other hand, low-energy GBs, which present orientation relationships and/or dense interfacial planes, are characterized by low barriers.