Hard Ferrite (Ceramic) Magnets
Hard Ferrite (Ceramic) magnet is manufactured from oxide materials using powder metallurgical process. Hard ferrite magnet is most widely used because of its low cost, high-energy, good electric insulation and excellent resistance to demagnetization. The most common type of hard ferrite magnet is anisotropic strontium, anisotropic barium and isotropic barium magnet.
Hard Ferrite (Ceramic) magnet has the following advantages: high coercive force, high electric resistance, long-time stability, and economical price.
Magnetic Properties of Ceramic Magnets
|Value (min/typical in our factory)|
Manufacture and Handling of Ceramic Magnets
Ceramic magnets (also known as ferrite magnets) were developed in the 1960s as a low cost alternative to metallic magnets. They are composed of iron oxide and strontium carbonate. While their hard, brittle quality and low energy exclude them from some applications, ceramic magnets have won wide acceptance due to their corrosion and demagnetization resistance, and low price per pound. Ferrite represents more than 75 per cent of world magnet consumption (by weight). It is the first choice for most types of DC motors, magnetic separators, magnetic resonance imaging and automotive sensors.
Ceramic Magnet Manufacturing Methods
Ceramic magnets are manufactured using powder technology techniques. The primary raw material – ferrite – is made by using iron oxide and strontium carbonate. These materials are mixed together and then elevated in temperature to 1800-2000 degrees F. At this temperature they undergo a chemical conversion and the resulting material is ferrite.
The ferrite material is then reduced to very small particle size by wet milling. The milled powder is then either dried (for dry pressed material) or injected into a die (in wet slurry form) in a large hydraulic press. The die is non-magnetic steel with carbide liners. The die cavities are the shape of the part to be pressed.
Essen Magnetics mostly used the sintered magnet process in our manufacturing facilities with the trialled and tested production method.
The wet powder (slurry) is then compacted in the presence of a magnetic field. The water allows the flat ferrite particle to more easily align itself in the magnetic field. Most of the water is removed during the compaction process. The remaining water is evaporated during the initial stages of the sintering process. The sintering takes place at 2000 degrees F. approximately. After sintering the material is fully dense and ready to finish grinding to customer specifications. As the material is very hard and brittle, all of the grindings of ceramic magnets is done using diamond wheels.
Grades and Properties of Ceramic Magnets
Soft ferrites that are used in transformer or electromagnetic cores contain nickel, zinc, and/or manganese compounds. They have a low coercivity and are called soft ferrites. The low coercivity means the material’s magnetization can easily reverse direction without dissipating much energy (hysteresis losses), while the material’s high resistivity prevents eddy currents in the core, another source of energy loss.
Semi-hard ferrites are in between soft and hard magnetic material and are usually classified as a semi-hard material. It is mainly used for its magnetostrictive applications like sensors and actuators thanks to its high saturation magnetostriction. Moreover, its magnetostrictive properties can be tuned by inducing a magnetic uniaxial anisotropy. This can be done by magnetic annealing, magnetic field-assisted compaction, or reaction under uniaxial pressure. This last solution has the advantage to be ultra-fast (20 min) thanks to the use of spark plasma sintering. The induced magnetic anisotropy in cobalt ferrite is also beneficial to enhance the magnetoelectric effect in the composite.
Hard ferrites, in contrast, permanent ferrite magnets are made of hard ferrites, which have a high coercivity and high remanence after magnetization. Iron oxide and barium or strontium carbonate are used in the manufacturing of hard ferrite magnets. The high coercivity means the materials are very resistant to becoming demagnetized, an essential characteristic for a permanent magnet. They also have high magnetic permeability.
The most common hard ferrites are:
Strontium ferrite used in small electric motors, micro-wave devices, recording media, magneto-optic media, telecommunication and electronic industry.
Strontium hexaferrite is well known for its high coercivity due to its magneto cry saline anisotropy. It has been widely used in industrial applications as permanent magnets and, because they can be powdered and formed easily, they are finding their applications into micro and nano-types systems such as biomarkers, bio diagnostics and biosensors.
Barium ferrite, a common material for permanent magnet applications. Barium ferrites are robust ceramics that are generally stable to moisture and corrosion-resistant.
Ceramic magnets offer several noteworthy benefits, one of which being a low cost. Statistics show roughly three-quarters of all magnets produced globally consist of ceramic magnets. As a result, they typically cost less than other magnets, such as rare-earth magnets.
Aside from their low cost, ceramic magnets also offer a superior level of corrosion resistance when compared to their rare-earth counterparts. In fact, rare-earth magnets are often plated or coated with a protective exterior to inhibit corrosion. Ceramic magnets don’t require the use of a protective exterior since they are naturally protected against corrosion.