At present, there are the most varieties of soft ferrite and the most widely used. According to different applications, there are various types of ferrites with the same composition and performance. For example, in the ultra-high frequency range, the magnetoplumbite type hexagonal ferrite can be selected. -Zn ferrite, Ni-Zn ferrite, Li-Zn ferrite, etc.
The production of soft ferrite requires high purity of raw materials and precise proportioning. Experiments have proved that: in the formula composition of Mn-Zn ferrite, when the content of Fe2O3 is 53-635%, the magnetostriction coefficient of ferrite is close to O, which is suitable for different frequencies and has good characteristics. In addition, as the content of ZnO increases, the permeability and magnetic induction increase. If the ZnO content exceeds 25%, the magnetic permeability drops instead. In order to improve the magnetic properties and promote sintering, additives can be introduced into the formulation. If adding a small amount of WO3, it will help the grain growth and improve the magnetic permeability; adding CaO, SiO2 or CaO, Ba2O3 and CeO2 can increase the resistivity and reduce the loss; adding CaO can reduce the sintering temperature and increase the density; adding a small amount of P2O5 can also Lower the firing temperature.
In Ni-Zn ferrite, the content of Fe2O3 should be strictly controlled, and the content of NiO should be increased accordingly. Generally speaking, the higher the frequency of use, the higher the content of NiO and the lower the content of ZnO. In order to reduce the sintering temperature and improve the magnetic properties, small amounts of additives such as CaO, CuO, CoO and MnO can be introduced. A formula range of Ni-Zn soft ferrite suitable for high frequency is: Fe2O3 50～70% ZnO 5～40% NiO 5～40%.
The molding of soft ferrite is generally used in color; dry pressing, extrusion, grouting or hot die casting, and isostatic pressing, hot pressing and explosive methods. In order to reduce the sintering shrinkage of the product, make the reaction sufficient, and at the same time improve the intrinsic properties of the ferrite, it is generally necessary to perform pre-sintering first, some low-temperature pre-sintering, some high-temperature pre-sintering, and some even require two pre-sintering. The firing of ferrite is generally carried out in an electric kiln with silicon carbide rods in an oxidizing atmosphere at 1150 to 1350°C. When sintering soft ferrite with high magnetic permeability, the cooling rate must be controlled so that it is cooled slowly to degrade the internal stress of the ferrite.