Ceramic magnets, also known as ferrite magnets, are a widely used type of permanent magnet found in many everyday items. Unlike metallic magnets, they are a composite material, not an alloy. Their widespread use is due to effective magnetic performance and cost-efficient production.
Key Raw Materials
Ceramic magnets are primarily composed of iron oxide, specifically ferric oxide (Fe2O3), commonly known as rust. This iron oxide typically makes up about 80% of the magnet’s composition by weight.
To achieve their magnetic properties, iron oxide is combined with either strontium carbonate (SrCO3) or barium carbonate (BaCO3). Strontium carbonate is more frequently used, leading to magnets known as strontium ferrites. These carbonates react with the iron oxide during manufacturing to form a ferrite compound, such as SrO-6(Fe2O3).
Manufacturing Process
The creation of ceramic magnets transforms raw powders into a hardened magnetic material. The initial stage is calcination, where a finely powdered mixture of iron oxide and strontium or barium carbonate is heated to high temperatures, typically over 1800°F (1000°C). This heating process causes a chemical transformation, forming the ferrite compound that provides the base magnetic properties.
Following calcination, the material undergoes milling. The pre-fired ferrite is ground into an extremely fine powder, often with particles as small as two micrometers or less. This fine milling is important because it promotes consistent magnetization in the final product. Wet milling, where the powder is mixed with water to create a slurry, can also be used.
The powdered material is then formed into the desired shape, usually through pressing. This can be done via dry pressing, where the powder is compacted, or wet pressing, where the slurry is pressed. During pressing, an external magnetic field can be applied to align the particles, creating an anisotropic magnet with improved magnetic properties. If no magnetic field is applied during formation, the resulting isotropic magnet will have weaker magnetic characteristics.
Next, the compacted shapes undergo sintering, a heat treatment similar to firing pottery. The material is heated to high temperatures, typically between 2000°F and 2372°F (1100°C to 1300°C), fusing particles into a dense, hard material. This process significantly increases the magnet’s density and mechanical strength. After sintering, the magnets are slowly cooled.
The final step is magnetization, which imparts permanent magnetic properties. Ceramic magnets are relatively easy to magnetize but require a strong external magnetic field to align their magnetic domains. Due to their hardness and brittleness, any finishing or shaping, such as grinding, is performed using diamond-coated tools after sintering.
Distinctive Properties
Ceramic magnets have several characteristics suitable for various applications. They are hard and brittle, meaning they can chip or break if subjected to impact or rough handling. Their hardness measures around 7 on the Mohs scale.
They resist demagnetization, allowing them to maintain their magnetic strength over time, even in challenging environments. They also exhibit good resistance to corrosion, meaning they do not typically require protective coatings. This inherent corrosion resistance makes them suitable for use in humid or wet conditions. Ceramic magnets are also a cost-effective option compared to other permanent magnets.