1. Soft magnetic ferrite
At present, soft magnetic ferrite has the largest variety and the most widely used. According to different uses, there are all kinds of ferrites with composition and consistent performance, such as in the ultra-high frequency range, magnetic lead type hexagonal ferrite can be selected, in the audio, medium frequency, high frequency range, commonly used spinel type Mn-Zn ferrite, Ni-Zn ferrite, Li -- Zn ferrite, etc.
The production of soft magnetic ferrite requires high purity of raw materials and accurate ratio. The results show that when the content of Fe2O3 in Mn-Zn ferrite is between 53 and 635%, the magnetostrictive coefficient of ferrite is close to O, which is suitable for different frequencies and has good characteristics. In addition, the permeability and magnetic induction increase with the increase of ZnO content. When ZnO content exceeds 25%, the permeability decreases. In order to improve magnetic properties and promote sintering, admixtures can be introduced into the formula. If a small amount of WO3 is added, it will help grain growth and improve permeability. Adding CaO, SiO2 or CaO, Ba2O3 and CeO2 can improve the resistivity and reduce the loss. Adding CaO can decrease the sintering temperature and increase the density. Adding a small amount of P2O5 can also reduce the sintering temperature.
The content of Fe2O3 in Ni - Zn ferro - oxygen should be strictly controlled, and the content of NiO should be increased. Generally speaking, the higher the frequency requirement, the higher the content of NiO, and the lower the content of ZnO. In order to reduce sintering temperature and improve magnetic properties, admixtures such as CaO, CuO, CoO and MnO can be introduced. The formulation of a Ni-Zn soft magnetic ferrite suitable for high frequency is as follows: Fe2O3 50 ~ 70%ZnO 5 ~ 40% NiO 5 ~ 40%.
Soft magnetic ferrite forming, general color with; Dry pressing, extrusion, grouting or hot die casting as well as isostatic pressing, hot pressing and explosive methods. In order to reduce the shrinkage of the product firing, make the reaction sufficient, and improve the internal performance of ferrite, it is generally necessary to preburn first, some for low temperature preburning, some for high temperature preburning, and some even require two preburning. The sintering of ferrite is generally carried out in an oxidation atmosphere of 1150 ~ 1350℃ by using silicon carbon rod electric kiln. In the firing of soft ferrite with high permeability, the cooling rate must be controlled to cool it slowly so as to degrade the internal stress of ferrite.
