Ferrite Magnets - General Information

Ferrites are produced by heating a mixture of powdered precursors pressed into a mold. During the heating process, calcination ofcarbonates occurs:

MCO3 → MO + CO2

The oxides of barium and strontium are typically supplied as their carbonates, BaCO3 or SrCO3. The resulting mixture of oxides undergoes sintering. Sintering is a high temperature process similar to the firing of ceramic ware.

Afterwards, the cooled product is milled to particles smaller than 2 µm, small enough that each particle consists of a single magnetic domain. Next the powder is pressed into a shape, dried, and re-sintered. The shaping may be performed in an external magnetic field, in order to achieve a preferred orientation of the particles (anisotropy).

Small and geometrically easy shapes may be produced with dry pressing. However, in such a process small particles may agglomerate and lead to poorer magnetic properties compared to the wet pressing process. Direct calcination and sintering without re-milling is possible as well but leads to poor magnetic properties.

Most common radio magnets, including those used in loudspeakers, are ferrite magnets. Ferrite magnets have largely displaced Alnico magnets in these applications.

It is a common magnetic material for electromagnetic instrument pickups, because of price and relatively high output. However, such pickups lack certain sonic qualities found in other pickups, such as those that use Alnico alloys or more sophisticated magnets.

A special form of fully dense sintered ferrite magnets is produced using a flexible binder for the ferrite powder. These materials are commonly referred to as "Flexible Magnets".
Key Benefits

Inexpensive yet strong; a good balance between strength and affordability
Does not demagnetize easily
Can be magnetized with multiple poles if necessary.
Does not rust

Key Challenges

Hard and brittle.
Properties degrade linearly with temperature. At 350°F, about 75% of room temperature magnetization is retained, and at 550°F, about 50% is retained.

Quick Facts

Density - 0.180 lbs per cubic inch
Saturation magnetizing field required - about 10kOe
Manufacturing methods - sintering (most common), injection molding, extruding or calendering - the last 3 methods when the material is combined with a carrier. Fully dense sintered Ferrite magnets can be wet or dry pressed - wet pressing leads to higher magnetic properties.
Shapes available - blocks, bars, discs, rings, arc segments, etc.
Grades available - from 0103 to 0404. (First 2 digits represent BHmax, and second two digits represent Intrinsic Coercivity, Hci.)
Sizes - sintered ferrites can be manufactured in blocks up to about 10" x 4" x 1". Extruded Ferrite (otherwise known as Flexible magnets) can be extruded in unlimited lengths. The energy product of Flexible magnets is, however, much lower than that of fully dense, sintered Ferrite magnets.

Surface Treatments

No surface treatments are necessary for Ferrite magnets since they are essentially inert and do not oxidize. However, they can be coated with various epoxy coatings for cleanliness purposes.

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