Electrical and magnetic properties of NiZn Ferrite prepared by conventional and solar sintering

The electrical properties of polycrystalline NiZn ferrite, Zn0.44Ni0.38Fe2.18O4, were investigated by impedance spectroscopy over the frequency and temperature ranges, 5 Hz to 2 MHz and 10-600 K and by magnetic permeability measurements at room temperature. Samples were sintered in either conventional or solar furnaces followed by quenching or slow cooling to ambient temperature. Depending on processing conditions, the room-temperature electrical resistivity of conventionally sintered samples varied by seven orders of magnitude, from 5 ohm cm for a sample quenched from 1250 degrees C to 10 Mohm cm for a sample quenched from 400 degrees C. These differences were attributed to variations in oxygen content of the ferrite which decreased with increasing quench temperature. Oxygen deficiency led to mixed valence of Fe in the octahedral B sites of the spinel structure and was responsible for high electronic conductivity with low activation energy at low temperatures in oxygen-deficient samples. By contrast, in oxygen-stoichiometric samples, Fe on the tetrahedral A sites was believed to be divalent and Fe on the octahedral B sites to be entirely trivalent. Electron hopping between A and B sites had much higher activation energy and dominated the conductivity at high temperature for all samples. Samples sintered in the solar furnace were much more conductive than ones that were slow-cooled after conventional sintering and this is attributed to the relatively rapid cooling rate after exposure in the solar furnace, which preserved some of the oxygen deficiency present at high temperature. For the same reason, samples that were slow cooled in N-2 were also much more conductive.The authors thank the regional (Comunidad Autonoma de Madrid) government (MATERYENER3CM S2013/MIT-2753) and the national government (MINECO) (MAT2013-46452-C4-3R) for financial support