Preparation of Ferrites under Hot Pressing and Their Magnetic Properties

Abstract

This paper describes the sintering process of ferrites under hot pressing and their magnetic properties. Bulk density ρ and grain growth aspect were investigated as a function of the maximum firing temperature Tm and the hot pressing pressure at Tm (Ph) in the case of Ni0.3 Zn0.7 Fe2O4 and Cu0.4 Zn0.6 Fe2O4 and compared hot pressed specimen with the specimen obtained by conventioned firing method. From ρ-Tm curves of CuZn and NiZn ferrite, we can see that the specimen prepared by the conventional method shows anomalous expansion in the sintering process. However, in the case of hot pressing, no expansion phenomenon was observed and the densification was considerably promoted. As the result, we could obtain the specimen with same density as the conventionally sintered one under hot pressing at lower temperature by 200°C than the ordinary firing. The effect of hot pressing on grain growth was examined by means of an electron microscope. The result shows that hot pressing greatly obstructs grain growth. The magnetic properties of the hot pressed ferrite differed from that of the ordinary fired ferrite. Especially, in the NiZn ferrite, the decrease of initial permeability μ and altered temperature characteristic of μ were the result of hot pressing. And, in the hot pressing case, lower maximum flux density Bs and higher coercive force Hc were also obtained as compared with that prepared by the conventional method. Measurement on the ferromagnetic resonance was carried out on NiZn specimen at 6790Mc. From this result we can see that the static magnetic field required for resonance Hc decreases and the magnitude of line width ΔH increases considerablly by hot pressing.