Takuya Aoki

Calcium Behaviors in MnZn Ferrite at Different Temperatures

MnZn ferrites have been widely used as magnetic core materials. It is well known that Ca addition is effective to obtain homogeneous microstructure of fine grains and highly resistive grain boundaries. However, the behaviors of calcium as one of the main additives at different temperature ranges during sintering process are not completely understood yet. In this study, the influence of CaCO3 content and sintering temperature on the microstructure was investigated in 1473-1623 K. It was found that there existed a critical temperature around 1550 K. The grain size decreased with the increase of Ca content when the sintering temperature was lower than the critical temperature, but completely opposite result was observed at higher temperatures range. Possible mechanisms were discussed. When the sintering temperature was lower than the critical temperature, Ca content greatly affected the grain boundary mobility and dominated the grain growth. At higher temperatures, however, formation of liquid phases might be the main cause for the grain growth.

Influence of Al2O3 Addition on the Temperature Dependence of Initial Permeability of NiCuZn Ferrites

The effect of Al2O3 addition on the temperature dependence of the initial permeability and microstructure of NiCuZn ferrites is investigated. The temperature dependence of initial permeability decreases with an increase in Al2O3 addition. EPMA analysis reveals that Al2O3 dissolves in the NiCuZn ferrite grains but is not uniformly dispersed. Separation of the initial permeability into two components, one due to spin rotation and the other due to domain wall motion, reveals that with an increase in the Al2O3 addition, the temperature dependence of the domain wall motion component decreases, while that of the spin rotation component shows no notable change. This indicates that the decrease in the temperature dependence of initial permeability may be attributed to the influence of Al2O3 on the domain wall motion.