Magnets hard ferrite
The ceramic material for inexpensive permanent magnets.
Due to its low cost, the magnet made of hard ferrite is the most commonly used magnet today. Its main component is the inexpensive iron oxide, to which alkaline earth oxides such as barium oxide or strontium oxide are added. At home, these magnets are often found on the back of refrigerator magnets.
Hard ferrite magnets are a special form in which a flexible binder is added to the ferrite powder. The magnetic powder is thus particularly densely packed. These materials are usually referred to as “flexible magnets”.
Features/Advantages
- Cost-effective
- Good chemical resistance
- corrosion resistant
Magnet production
Magnets made of hard ferrite can be manufactured in various ways: Sintering, injection molding, extrusion (extruding) or calendering.
During sintering, the material can be pressed wet or dry. Wet pressing provides higher magnetic values, but results in poorer physical tolerances. Sintering allows between 96% and 99% of the theoretically possible density. However, this pressing process usually requires reworking.
Magnet shapes
In the case of pressed magnets, all shapes that can be produced using pressing technology can be realized. Cuboids, cylinders, rings, segments, discs and other molded parts are available. Holes, recesses, grooves, etc. can also be made, provided they run parallel to the pressing direction.
Application areas
- Electric, servo, DC, synchronous and linear motors
- Central rotary and spur rotary couplings
- Hysteresis and eddy current brakes
- Sensors and actuators
- Loudspeakers
- Magnetic therapy
- Toys
- Promotional items
- Adhesive applications
Temperature behavior
Although the maximum possible operating temperature for hard ferrite magnets is approx. 250 °C, it depends on the position of the operating point. As with all magnetic materials, the magnetic properties of hard ferrite magnets are dependent on temperature. The changes are reversible and thus, within the operating temperature range, the original values are largely regained after one temperature cycle. However, depending on the design and application, minor permanent changes may occur initially. They can be anticipated by artificial ageing.
Chemical and mechanical properties
The corrosion behavior of hard ferrite magnets is excellent. Surface treatments are not necessary as they are resistant to moisture, weak acids, alkalis, salts, solvents and lubricants. For special applications, where cleanliness is important, for example, or for unusual designs, the magnets can also be coated.
Due to the high brittleness of hard ferrite magnets, impact stress must be avoided when screwing or reinforcing. When completing with different materials, the different expansion coefficients must be taken into account. Magnetized magnets, for example, attract iron filings and dust. Therefore, where possible, it is recommended to magnetize the magnets only after mounting.
| Material1) | Preferred- position |
Max. magnet. Energy density (BH) max [kJ/m 3 ] > |
Remanence flux density B r [mT] > |
Coercive- field strength H cB [kA/m] > |
Coercive- field strength H cJ [kA/m] > |
Rel. perm. Permeability µ p [-] > |
Operating temp.2) T max [°C] |
Temp. coefficient TK(B R ) [%/K] |
Temp.coefficient coefficient TK(H cJ ) [%/K] |
|---|