Influence of the sintering conditions on the densification of manganese-zinc ferrites

In order to prepare reproducibly high permeability manganese-zinc ferrites, it is necessary to carefully control the sintering schedule for obtaining both a homogeneous chemical composition and the desired microstructure. The latter should consist of big pore-free crystals, the remaining porosity being a few number of big pores situated at the grain boundaries. In our theoretical analysis of the densification, a volume diffusion model is used taking into account the geometrical parameters of the average crystal in thermodynamical equilibrium in the polycrystalline material. In the intermediate stage of sintering, the grain growth and densification are correlated, the densification rate being approximately inversely proportional to the grain volume. Sintering experiments have been carried out under various conditions of temperature and duration, the atmosphere composition being adjusted for maintaining the same stoichiometry. These results permit one to define isoporosity curves. It is then possible to compute the activation energies and Q1responsible for grain growth and for densification. The values found are kcal/mole and kcal/mole, in agreement with the previous measurements of Paulus [5] and Ogawa [3]. The experimental results show that both processes are controlled by the diffusion energy of oxygen anions which are the slowest diffusing species. The variation of the porosity versus time follows a law, being roughly equal to 0.5 in good agreement with our predicted model. The experimental results also confirm that the use of a more oxidizing atmosphere increases the grain growth rate but decreases the densification rate. A sintering schedule leading to very high permeability ferrites is determined taking into account these results. The microstructure is obtained during a first soak at high temperature under oxidizing atmosphere. This leads to big crystals and open porosity while preventing zinc losses. This first treatment is followed by a second soak under a more reducing atmosphere leading to a high density ferrite and an homogeneous chemical composition possessing a ferrous iron content so as to obtain a maximum permeability at room temperature.