The effect of microstructure on the initial permeability of Ni-Zn ferrite

Abstract

We have studied the sintering behavior, microstructure, and permeability of the ferrite Ni0.50Zn0.50Fe2O4 sintered at temperatures from 1000 °C to 1400 °C for various dwell times. The shrinkage rate is maximum at 1140 °C; hence samples sintered at T ≥ 1100 °C exhibit densities larger than 97%. Moderate grain growth is observed up to 1250 °C; however, if the density exceeds a threshold of 98% enhanced grain growth sets in. All samples are single-phase spinels with the lattice constant slightly increasing with sintering temperature. At T ≥ 1300 °C significant ZnO evaporation sets in and the corresponding change in ferrite composition is indicated by mass loss, EDX-analysis, and Seebeck coefficient measurements. The complex permeability spectra show systematic changes with sintering temperature and dwell time. For samples sintered at 1100 °C, the permeability increases from µ’=133 for a dwell time of 30 min to µ’=318 after 24 h sintering. The variation of the permeability is discussed as function of density and grain size. The non-magnetic grain boundary model is applied to interpret the grain size dependence of the permeability.