Ferrite (magnet)
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Ferrites are electrically non-conductive ferrimagnetic ceramic compound materials, consisting of various mixtures of iron oxides such as Hematite (Fe2O3) or Magnetite (Fe3O4) and the oxides of other metals.
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[edit] Properties
Ferrites that are used in transformer or electromagnet cores contain nickel, zinc or manganese compounds. They have a low coercivity, and are called soft ferrites. Because of their comparatively low losses at high frequencies, they are extensively used in the cores of Switched-Mode Power Supply (SMPS) and RF transformers and inductors.
In contrast, permanent ferrite magnets (or hard ferrites) have a high remanence after magnetization, and are made with iron oxide and barium and strontium oxides. In a magnetically saturated state they conduct magnetic flux very well and have a high magnetic permeability. This enables these so-called ceramic magnets to store stronger magnetic fields than iron.
Today, they are the most commonly used magnets in radios. The maximum magnetic field B is about 0.35 tesla and the magnetic field strength H is about 30 to 160 kiloampere turns per meter (400 to 2000 oersteds). (Hill 2006)
Ferrites are, as many other ceramics, hard and brittle. They may break easily.
[edit] Production
Ferrites are produced by a sintering process. A mixed powder is heated and pressed in a mold.
Permanent magnetic ferrites are formed by a chemical reaction called calcination from the powders of Iron(III) oxide and Barium- or Strontium carbonate. Afterwards the product has to be milled to particles smaller than 2 μm in order to get Weiss domains in the size of one particle. Next the powder is pressed into a shape, dried and sintered. The shaping may be performed in an external magnetic field, in order to achieve a preferred orientation of the particles (anisotropy).
Small and geometrically easy shapes may be produced with dry pressing. However, in such a process small particles may agglomerate and lead to poorer magnetic properties compared to the wet pressing process. As well, direct calcination and sintering without re-milling is possible but leads to poor magnetic properties.
Electromagnets are pre-sintered as well (pre-reaction), milled and pressed. However, the sintering takes place in a specific atmosphere, for instance one with an oxygen shortage). The chemical composition and especially the structure vary strongly between the precursor and the sintered product.
[edit] Uses
Ferrite cores are used in electronic inductors, transformers, and electromagnets where the high electrical resistance of the ferrite leads to very low eddy current losses. They are commonly seen as a lump in a computer cable, called a ferrite bead, which helps to prevent high frequency electrical noise (radio frequency interference) from exiting or entering the equipment.
Early computer memories stored data in the residual magnetic fields of hard ferrite cores, which were assembled into arrays of core memory. Ferrite powders are used in the coatings of magnetic recording tapes. One such type of material is iron (III) oxide.
Ferrite particles are also used as a component of radar-absorbing materials or coatings used in stealth aircraft and in the expensive absorption tiles lining the rooms used for electromagnetic compatibility measurements.
Most common radio magnets, including those used in loudspeakers, are ferrite magnets. Ferrite magnets have largely displaced Alnico magnets in these applications.
It is a common magnetic material for electromagnetic instrument pickups, because of price and relatively high output. However, such pickups lack certain sonic qualities found in other pickups, such as those that use Alnico alloys or more sophisticated magnets.
[edit] External links
- What are the bumps at the end of computer cables?
- {http://electronic-components.globalspec.com/LearnMore/Electrical_Electronic_Components/Electronic_Components/Ferrite_Beads GlobalSpec guide to Ferrite Beads]
[edit] Sources
- Meeldijk, Victor Electronic Components: Selection and Application Guidelines, 1997 Wiley [ISBN 0-471-18972-3]
- Ott, Henry Noise Reduction Techniques in Electronic Systems 1988 Wiley [ISBN 0-471-85068-3]
- Luecke, Gerald and others General Radiotelephone Operator License Plus Radar Endorsement 2004, Master Pub. [ISBN 0-945053-14-2]
- Bartlett, Bruce and others Practical Recording Techniques 2005 Focal Press [ISBN 0-240-80685-9]
- Hill Technical Sales [1]
- Schaller, George E. Ferrite Processing & Effects on Material Performance [2]de:Ferrite
