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 SiO 2 . 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 Fe 2؉ versus Fe 3؉ 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.

Relation Between Firing Conditions Grain Boundary Structure and Magnetic Properties in Polycrystalline MnZn-Ferrites

“…However, the great majority of the studies in the scientific literature concentrate on the effect of additives on the grain boundaries [1,8,9] or on the relation between grain boundary nature and magnetic properties [10,11]. In the patented literature the correlation between additives and magnetic properties is done directly [e.g.…”

Geometry and Electrical Properties of Grain Boundaries in Manganese Zinc Ferrite Ceramics

“…The microcontact impedance spectroscopy with a welldefined electrode geometry used in this work allows us to distinguish bulk and GB contributions and to obtain specific local resistivities of the materials from the bulk contribution [13][14][15][16] and, thus, has distinct advantages compared with microcontact dc techniques. 4,[17][18][19] For a strongly resistive GB of a bigrain, microelectrodes on top of each grain act almost as two conventional extended electrodes on opposite sides and, therefore, allow a normalization of the measured GB resistance to the GB area. 13 However, when individual boundaries in the grain assembly, rather than in a bigrain, are probed, the current can detour through adjacent, less-blocking grain boundaries; therefore, the measured resistance or voltage of a single probed boundary is, in principle, influenced by the whole grain assembly in a complicated way.…”

“…Laval and co-workers [2][3][4] demonstrated the inhomogeneity of GB resistivity of a MnZn ferrite using four-probe microelectrode measurements and a voltage contrast map recorded using scanning electron microscopy (SEM). Transmission electron microscopy (TEM) microanalysis revealed a large variation in the oxidation degree that depended on the GB structure, and local electrical probing of GBs using TEM demonstrated a direct relationship between the electrical properties and structural aspects of GBs.…”

“…Thus, the purpose of the present work is to further clarify the relationship between GB structure and GB property in the MnZn ferrite system in an experimental approach much in common with Laval et al 4 Compared with the work of Laval et al, microstructure and probe size have been scaled up, which allows for local probing of electrical properties of GBs using optical microscopy and impedance spectroscopy. Grain misorientations (in terms of rotation angle-axis pairs, three DOF) are determined ex situ using electron backscatter diffraction (EBSD) in the SEM.…”

Direct correlation between ferrite microstructure and electrical resistivity

“…Grain boundary segregations of CaO and SiO 2 tend to result in the formation of an insulating layer, which can lead to an increase in the electrical resistivity at the grain boundary. [6][7][8][9] Other additives, such as Bi 2 O 3 , can promote grain growth and increase magnetic permeability. 10 However, it has been reported that applied stress has a significant effect upon the electrical properties of ferrites, especially on permeability and power loss.…”