Ceramic hard ferrite magnets

Synthetic ferrites also exhibit ferrimagnetism. Ferrites are magnetic ceramic materials which are made by mixing iron (III) oxide with, for instance, powdered oxides and carbonates and subsequently compressing and sintering them. They are produced in large quantities and in many different shapes. We distinguish soft and hard ferrites. 

Magnetic ceramics represent an important fraction of the magnetic industry in the US, an estimated 40% of the total hard magnetic materials market value is dominated by ferrites, and in spite of the continuous development of new materials, ferrite consumption is still growing. In soft material applications, ferrites participate with an estimated 20% of the market value. In 1990, the estimated world production was 159 500 metric tons of soft ferrites, and 431 100 metric tons of hard ferrites (Ruthner, 1989). In addition to the versatility of ferrites, there are two essential factors which explain this success the low electrical conductivity, and the low production cost. The market value of ferrites ( 3/kg) is very low compared with other electroceramics 33/kg for varistors, 330/kg for thermistors and 1100/kg for ceramic capacitors (Cantagrel, 1986). 

Kools, F. (1991) Hard magnetic ferrites. In Concise Encyclopedia of Advanced Ceramic Materials, R.J. Brook (ed.), Pergamon Press, Oxford, pp. 200-6. 

Natural and synthetic iron oxides not possessing pigment properties are used as raw materials in the production of hard and soft ferrites (see Section 5.5.5.2, Electro- and Magneto-Ceramics) for radio, television and telephone technology, for adhesive magnets, for rotors in dynamos, for low-loss magnetic layers, for DC-motors, for transformer cores, for electronic calculators and high frequency furnaces. This amounts to an annual worldwide production of more than 300 10 t/a. 

The reader probably knows already that real motors have more coils and poles, and more complex commutators. There is more than one pulse of attraction, and sometimes the current is reversed to also cause repulsion. The coil almost always has a small soft iron core, and the permanent magnet is a large piece of hard steel or ferrite (iron oxide and barium oxide ceramic). Because the permanent magnet is heavier than the coil, it is usually the stationary part, not the way it is shown here. However, this diagram communicates the main ideas. 

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