Neue ergebnisse über den einflüss der sinterbedingungen auf die eigenschaften von Mn-Zn-Ferriten

It is well known that magnetic properties greatly depend on the partial oxygen pressure exerted during sintering of ferrites. This is particularly true for manganese ferrites, since manganese is able to acquire different valences during sintering. The interchange of the ions by diffusion occurs on the surface of the core. Atmospheric conditions must be such that no reception or delivery of gas or metal ions is possible. The first step in controlling these reactions is the nitrogen sintering of Mn-Zn ferrites. According to recent research done for the improvement of quality, it is necessary to supervise first of all the zinc evaporation of the core surface anff also the amount of oxygen in the core. The vapor tension of zinc oxide is substantially higher than that of the other metal ions. These properties are distinctly shown during sintering of ferrites with high permeability. By careful handling during production, using pure oxides and by adjusted sintering methods which keep zinc evaporation to a minimum, we succeeded in raising the permeability of Mn-Zn ferrites to 40 000. The crystallite size of these specimens was 80 μm ; their porosity was low, amounting to approximately one percent. In case zinc evaporation does not remain relatively small, permeabilities are obtained between 15 000 and 25 000 only. The surface film of the core will then contain up to one mol percent zinc oxide less than the core interior. When etching off the detrimental film, which leads to strong stresses in the core interior, one again obtains permeabilities about 35 000. The value of the initial permeability in the sphere of the secondary maximum will be mainly determined by the magnetostrictive and mechanic tensions when crystalline anisotropy diminishes. According to magnetostrictive measurements of high permeable ferrites, may be expected, thus leading to . A similar influence of this surface film with low zinc content also occurs with Mn-Zn ferrites of . By grinding, lapping, or etching off these anomalous films, the curve is altered, particularly in the range of the primary and secondary maxima. An improvement of the hysteresis losses at 25&#- 176;C occurs only with a simultaneous increase of permeability. The increase of the zinc loss on the surface depends on the velocity of flow caused by the supply of N2and on increased sintering temperatures.