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.

Mo behavior in Fe-rich Mn-Zn ferrites during sintering process was studied by adding MoO3 or MnMoO4 as additive. The influences of the additive amounts and treatment temperatures on microstructure, density, porosity as well as magnetic properties were investigated. It is found that formation of MnMoO4 in the grain boundary plays an important role. Saturation flux density (Bs) at 373 K as high as about 513mT could be obtained when the equivalent Mo content is 133ppm. Based on the experimental results, a new hypothesis is proposed to explain why proper Mo addition is essential to obtain ferrites of good qualities.

Possible chemical states of molybdenum in Mn-Zn ferrite at high temperatures were investigated by experiments and thermodynamic calculations. It is found that solid or liquid MnMoO4(s,l) is the main Mo-containing phase during the sintering process of Mn-Zn ferrite up to about 1623 K, even if MoO3is employed as the additive. When temperature is higher than the melting point of MnMoO4, Mo in Mn-Zn ferrite tends to evaporate in form of Mo3O9(g) from thermodynamic point of view.