THE NATURE OF SOFT FERRITES
Ferrites are dark grey or black ceramic materials. They are very hard, brittle and chemically inert. most modern magnetically soft ferrites have a cubic (spine) structure.
The general composition of such ferrites is MeFe2O4 where Me represents one or several of the divalent transition metals such as manganese (Mn), zinc (Zn), nickel (Ni), cobalt (Co), copper (Cu), iron (Fe) or magnesium (Mg).
The most popular combinations are manganese and zinc (MnZn) or nickel and zinc (NiZn). These compounds exhibit good magnetic properties below a certain temperature, called the Curie Temperature (Tc). They can easily be magnetized and have a rather high intrinsic resistivity. These materials can be used up to very high frequencies without laminating, as is the normal requirement for magnetic metals.
NiZn Ferrites have a very high resistivity and are most suitable for frequencies over 1 MHz, however, MnZn ferrites exhibit higher permeability (ui) and saturation induction levels (Bs) and are suitable up to 3 MHz.
For certain special applications, single crystal ferrites can be produced, but the majority of ferrites are manufactured as polycrystalline ceramics.
The following description of the production process is typical for the manufacture of our range of soft ferrites, which is marketed under the trade name 'Ferroxcube'.
The raw materials used are oxides or carbonates of the constituent metals. The final material grade determines the necessary purity of the raw materials used, which, as a result is reflected in the overall cost.
PROPORTIONS OF THE COMPOSITION
The base materials are weighed into the correct proportions required for the final composition.
The powders are mixed to obtain a uniform distribution of the components.
The mixed oxides are calcined at approximately 1000 degrees C. A solid state reaction takes place between the constituents and, at this stage, a ferrite is already formed.
Pre-sintering is not essential but provides a number of advantages during the remainder of the production process.
MILLING AND GRANULATION
The pre-sintered material is milled to a specific particle size, usually in a slurry with water. A small proportion of organic binder is added, and then the slurry is spray-dried to form granules suitable for the forming process.
Most ferrite parts are formed by pressing. The granules are poured into a suitable die and then compressed. The organic binder acts in a similar way to an adhesive and a so-called 'green' product is formed. It is still very fragile and requires sintering to obtain the final ferrite properties.
For some products, for example, long rods or tubes, the material is mixed into a dough and extruded through a suitable orifice. The final products are cut to the required length before or after sintering.
The 'green' cores are loaded on refractory plates and sintered at a temperature between 1150 degrees C and 1300 degrees C depending on the ferrite grade. A linear shrinkage of up to 20% (50% in volume) takes place. The sintering may take place in tunnel kilns having a fixed temperature and atmosphere distribution or in box kilns where temperature and atmosphere are computer controlled as a function of time. The latter type is more suitable for high grade ferrites which require a very stringent control in conditions.
After sintering, the ferrite core has the required magnetic properties. If can easily be magnetized by an external field (see Fig. 2), exhibiting the well-known hysteresis effect (see Fig. 1). Dimensions are typically within 2% of nominal due to 10-20% shrinkage. IF this tolerance is too large or if some surfaces require a smooth finish (e.g. mating faces between core halves) a grinding operation is necessary. Usually diamond-coated wheels are used. For high permeability materials, very smooth, lapped, mating surfaces are required. If an air-gap is required in the application, it may be provided by centre pole grinding.
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Soft ferrite cores are widely used throughout industry. The number of applications is growing and is virtually limitless.Main application areas are: