Sintering of Ferrites

Iron-based sintering materials have been extensively used for producing structural and mechanical parts in different industries. It is because of their flexibility of manufacturing, good balance between ductility and tensile strength, low cost and high performance, good magnetic properties and corrosion resistance.

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Although a number of iron-based alloys can be sintered, in this article we will only focus on sintering of ferrite-based materials. 

Ferrite composition with superior sintering properties

Ferrite composition containing Ni—Cu—Zn has superior sintering properties at low temperature.

It shows high permeability and low dielectric constant. A superior frequency characteristic of permeability and strength is measured when a nonmagnetic substance material is added in a predetermined ratio to Ni—Cu—Zn based ferrite.

The nonmagnetic substance material can be represented by a general formula: a(bZnO.cCuO).SiO2. Mixing ratio of the magnetic material and the low dielectric constant nonmagnetic material is 80 wt %:20 wt % to 10 wt %:90 wt %.

Ultrafine strontium ferrite sintering powder

Molten salt assisted sintering powder of strontium ferrite can be prepared by mixing 85%-87% of Fe2O3 with the purity of greater than 99% and 13%-15% of SrCO3 with the purity of greater than 99%. Then an industrial pure alumina ingredient with the amount of 0.1%-0.3% of the total feed material is added to the mixture. After that, a pre-sintered powder is prepared by ball milling, sieving and calcining of the mixture for 1-2 h at a low temperature of 900°C.

Taking a sodium salt as a dispersing agent, the ferrite pre-sintered material is mixed with anhydrous sodium sulfate. Then, mechanical ball milling is again carried out for 1h, where the mass ratio of the pre-sintered material to the sodium salt does not exceed 1:5.

The mixture is sintered for 2h in an air atmosphere, at the heating-up rate of 10/min and at the temperature of 1000-1250°C, followed by dissolving the sulphates and drying the powder at 100°C to obtain an ultrafine strontium ferrite powder.

Using microwave sintering to make ultrathin ferrite sheets

Ultra thin ferrite sheets can be made in a two step process. 

In the first step, the ferrite raw cook is made by mixing 100 parts of ferromagnetic oxide powders, tackiness agent 5-25 part and softening agent 1-10 part to form a formation slip, slip is coated on plastic film or steel band. The dry rear ferrite raw cook that forms a thickness of 50-500 μm.

In the second step, ferrite raw cook is placed on a load bearing board where microwave sintering is carried out. Then, the obtained finished product is naturally cooled. The microwave frequency of described microwave sintering is 2.45GHZ.

During microwave sintering, ferrite raw cook is heated to 400-450°C with the low fire of microwave power 200-300W, followed by heating with the low fire insulation 4-5 hours of microwave power 150-250W. With the height of microwave power 1100-1300W, ferrite raw cook is heated to 900-1000°C again. Finally, the ferrite raw cook is incubated 1-3 hours with the moderate heat of microwave power 500-700W.

Sintered ferrites for electronic parts

The process of manufacturing a sintered ferrite body is by adding a binder to ferrite powder, followed by a step of molding, then a removal of binder followed by sintering.

Ferrite powder has a spinelization ratio of 10-60% with the addition of binder in a range of 1.3-0.02S% by weight. The total amount of ferrite powder and binder is 100% by weight. The oxygen concentration in the atmosphere from the binder-removing step to the completion of the sintering step is 0.1% or less by volume.

This process produces a sintered ferrite body with a high maximum magnetic flux density, which is particularly high at as high a temperature as 100°C. Thus, the sintered ferrite body is suitable for making electronic parts that need to withstand large current and high temperature, e.g., choke coils.

Efficient manufacturing of ferrite particles

Ferrite particles can be efficiently manufactured by crushing and granulating ferrite raw materials followed by sintering.

The sintering of ferrite particles is done in a positive pressure reducing atmosphere. The pressure inside the rotary furnace is around 10 Pa. The rotary furnace is also equipped with a mechanism for removing adhered matter. The sintering temperature is maintained between 800 to 1,180°C.

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