Ferrites are electrically bad or non-conductive ferrimagnetic ceramic materials from the iron oxide hematite (Fe2O3 ), rarely of magnetite (Fe3O4 ) and other metal oxides. Depending on the composition ferrites are magnetically hard or magnetically soft.
A distinction is made between soft magnetic and hard magnetic ferrites. Soft magnetic ferrites have the lowest possible coercivity hard magnetic ferrites as high as possible. Whether a magnetic material is rather soft or hard magnetic, can be determined by its hysteresis curve. For soft magnetic ferrites as light as possible (re) magnetization is sought, which corresponds to a narrow hysteresis curve.
Ferrites are, like all ceramic materials, hard and brittle and therefore prone to breakage.
Soft magnetic ferrites
Soft magnetic ferrites are used in electrical as ferrite cores in transformers in switching power supplies and coils. In the unsaturated case, a high magnetic permeability (permeability) is possible.
Since they are hardly electrically conductive and therefore almost no eddy current losses occur, they are also suitable for high frequencies up to several megahertz. Special suitable for microwave ferrites consist of spinels and garnets.
The usual soft magnetic Ferritmaterialen are:
- Manganese - zinc ferrite ( MnZn ) in the composition MnaZn ( 1-a) Fe2O4
- Nickel - zinc ferrite ( NiZn ) in the composition NiaZn ( 1-a) Fe2O4
MnZn NiZn has towards a higher permeability and higher saturation magnetization. Electrical conductivity of less than NiZn, MnZn, NiZn therefore is suitable for higher frequencies.
Hard magnetic ferrites
Hard magnetic ferrites are used as cost-effective permanent magnet. In contrast to the rare earth magnet, they have a markedly lower magnetic energy density, that is a smaller coercive force and remanent flux density smaller.
The usual hard magnetic Ferritmaterialen are:
- Strontium ferrites in the composition SrFe12O19
- Barium ferrites in the composition BaFe12O19
- Cobalt ferrites in the composition CoFe2O4
Barium ferrites are relatively robust and insensitive to humidity.
Ferrites are usually prepared in a sintering process. Hard magnetic ferrites are produced by a chemical reaction, the calcination of the raw materials iron (III ) oxide and barium carbonate or strontium carbonate. This process is outdated as a " pre-sintering " refers to. The reaction product as possible finely ground ( single-domain, and White counties, particle size 1-2 microns ) must be formed into pellets, dried and sintered. The forming of the compacts can be done in an external magnetic field, wherein the grains ( preferably single-domain particles ) are placed in a preferred orientation ( anisotropy ).
For small, simple geometric shapes can also be the so-called " dry pressing " are used for shaping workpieces; here is the strong tendency to ( re) agglomeration of very small particles ( 1-2 microns ) the cause " wet" for most poorer magnetic characteristics compared to the pressed parts. Direct shaped from the raw material compacts can indeed performs calcined and sintered, the magnetic characteristics of products prepared in this way are very bad.
Soft magnetic ferrites are also pre-sintered ( formation reaction ), milled and pressed. However, (for example, lack of oxygen), the subsequent sintering takes place in a specially adapted atmospheres. The chemical composition and especially the structure of pre-sintering and sinter product differ greatly.
Areas of application
Application find magnetic materials based on ferrites particularly in electrical engineering.
Soft magnetic ferrites:
- To change the line properties, see coil-loaded line
- In high-frequency (HF ) and the antenna construction, such as cores in baluns and circulators in diplexers, as well as variable attenuators and absorbers.
- Pulse transformer and signal transducer for high frequencies, such as baluns
- Magnetic heads in tape recorders ( erase head ), video recorders, computer hard drives and floppy drives
- For sealing of microwave ovens (ferrite absorbs the emitted electromagnetic waves from the cooking chamber, thus preventing the emission to the outside)
- Stealth technology to disguise (ferrite absorbs radar waves)
Depending on the application a wide range of designs are produced:
Ring cores, rod cores, so-called bobbin cores, pot cores, E and U cores (in combination with the same or with I- cores ). The letter identification is carried out in accordance with the form. Much of the mass ferrites is now manufactured in Asia. For several years, tremendous production capacity will be built in China.
Hard magnetic ferrites:
- Magnetizable coating on audio and video tapes ( here but not vitrified bonded )
- Core memory in computers with threaded onto copper wires magnet rings (now obsolete)
- Permanent magnets of all kinds, such as magnet segments in permanent magnet excited electric motors in speakers
- Composites are (compounds) of Hartferritpulvern and thermal or thermosets, either the injection-molded, extruded, or calendered. In most injection-molded workpieces from Hartferritcompound after injection into the mold an external magnetic field is applied in order to improve the magnetic characteristics. In calendered, and in some cases also extruded compound orientation is ideally held mechanically, the platelet shape of the grains is exploited by hard ferrite. Hard ferrite powder for injection-moldable compounds, however, are said to have round grains as possible, because they can turn in the alignment in the external magnetic field in the very viscous ( viscous ) plastic matrix still should.