Ceramic Magnets

General Information

Ceramic (Ferrite) magnets are sintered permanent magnets composed of Barium or Strontium Ferrite. This class of magnets, aside from good resistance to demagnetization, has the popular advantage of low cost.

Ferrite magnets are very hard and brittle, and require specialized machining techniques. Moreover, they should be machined in an unmagnetized state. We are equipped to machine these materials to specifications.

Anisotropic grades are oriented in the manufacturing direction, and must be magnetized in the direction of orientation. Isotropic grades are not oriented and can be magnetized in any direction, although some degree of greater magnetic strength will be found in the pressing dimension, usually the shortest dimension.

Due to their low cost, Ferrite magnets enjoy a very wide range of applications, from motors and loudspeakers to toys and crafts, and are the most widely used permanent magnets today.

Manufacturing Methods

Pressing and Sintering

Pressing and sintering involves pressing very fine ferrite powder in a die, and then sintering this pressed magnet. All fully dense Ferrite magnets are produced this way. Ferrite magnets can be wet pressed or dry pressed. Wet pressing yields better magnetic properties, but poorer physical tolerances. Generally, the powder is dry for grade 1 or 5 materials, and wet for grade 8 and higher materials. Sintering involves subjecting the material to high temperatures to fuse the pressed powder together, thus creating a solid material. Magnets produced through this process usually need to have some finish machining, otherwise surface finishes and tolerances are not acceptable. Some manufacturers extrude instead of press wet powder slurry and then sinter the material. This is sometimes done for arc segment shapes, where the arc cross-section is extruded in long lengths, sintered, and then cut to length.


Injection Molding

Ferrite powder is mixed into a compound and then injection molded in the same way as plastic. Tooling for this manufacturing process is usually very costly. However, parts produced through this process can have very intricate shapes and tight tolerances. Injection molded ferrite properties are either lower or about the same as grade 1 Ferrite.


Assemblies

Assemblies using metal or other components and magnets can be fabricated by adhering magnets with adhesives to suit a range of environments, by mechanically fastening magnets, or by a combination of these methods. Due to the relatively brittle nature of these magnet materials, press fits are not recommended.


Surface Treatments

The corrosion resistance of Ferrite is considered excellent, and no surface treatments are required. However, Ferrite magnets may have a thin film of fine magnet powder on the surface and for clean, non-contaminated applications, some form of coating may be required.


Machining

Ferrite is brittle, and prone to chipping and cracking. Special machining techniques must be used to machine this material. We are fully equipped to machine these materials to your blueprint specifications.


Magnetizing and Handling

Ferrite magnets require magnetizing fields of about 10 kOe. They can be magnetized with multiple poles on one or both pole surfaces. No special handling precautions are required, except that large blocks of Ferrite magnets are powerful, and care should be taken to ensure that they do not snap towards each other.


Temperature Effects

Up to about 840 F, changes in magnetization are largely reversible, while changes between 840 F and 1800 F are re-magnetizable. For all Ferrite magnets, the degradation of magnetic properties is essentially linear with temperature. At 350 F, about 75% of room temperature magnetization is retained, and at 550 F, about 50% is retained.


MAGNETIC PROPERTIES OF CERAMIC MAGNETS


Grade Maximum Energy
Product (BH)max
Residual
Induction Br
Minimum Intrinsic
Coercivity Hci
Coercivity
Hc
Maximum
Operating Temp
Curie Temp
Tc
Coefficient Induction
20-150 °C a
Coefficient Coercivity
20-150 °C ß
MGOe kG kOe kOe °C °C % / °C % / °C
HF010 1.1 2.3 3.2 2.0 250 450 -0.20 0.35
HF050 3.4 3.8 2.5 2.4 250 450 -0.20 0.35
HF070 2.8 3.4 4.0 3.1 250 450 -0.20 0.35
HF081 3.5 3.85 3.1 2.9 250 450 -0.20 0.35
HF082 3.5 3.8 3.9 3.45 250 450 -0.20 0.35
HF083 4.0 4.1 2.9 2.85 250 450 -0.20 0.35
HF084 3.3 3.7 4.8 3.5 250 450 -0.20 0.35
HF085 3.8 4.0 4.0 3.6 250 450 -0.20 0.35


PHYSICAL PROPERTIES OF CERAMIC MAGNETS


 Curie Temperature  450 - 460°C
 Coefficient of Thermal Expansion  +7.0 - +15.0 x 10-6 °C-1
 Electrical Resistivity  >1010 µO·cm
 Density  4.5 - 5.1 g·cm-3
 Vicker's Hardness  480 - 580 HV
 Young's Modulus  170 kN·mm-2
 Bending Strength  0.05 - 0.09 kN·mm-2
Compressive Strength  1.3 kN·mm-2
Tensile Strength  0.02 - 0.05 kN·mm-2
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