Ferrite
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Ferrite may refer to: (1) ferromagnetic ceramic materials, used in magnetic applications; (2) iron or iron alloys with a body centred cubic crystal structure.
(1) Ferrites are ferromagnetic ceramic materials, compounds of iron, boron and barium or strontium or molybdenum. Ferrites have a high magnetic permeability, which allows them to store stronger magnetic fields than iron. Ferrites are often produced as powder, which can be sintered into solid cores. 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. Early computer memories stored data in the residual magnetic fields of 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 used in stealth aircrafts and in the expensive absorption tiles lining the rooms used for electromagnetic compatibility measurements.
(2) In Materials Science, "ferrite" (alpha-iron) is iron, or a solid solution with iron as the main constituent, with a body centred cubic crystal structure. In pure iron, ferrite is stable below 910°C. Above this temperature the face-centered cubic form of iron, austenite (gamma-iron) is stable. Above 1390°C, up to the melting point at 1534°C, the body-centred cubic crystal structure is again the more stable form (delta-iron).
Only a very small amount of carbon can be dissolved in ferrite; the maximum solubility is about 0.02wt% at 723°C. This is because carbon dissolves in iron interstitially, with the carbon atoms being about twice the diameter of the interstitial "holes", so that each carbon atom is surrounded by a strong local strain field. Hence the enthalpy of mixing is positive (unfavourable), but the contribution of entropy to the free energy of solution stabilises the structure for low carbon content. 723°C also is the minimum temperature at which iron-carbon austenite (0.8 wt% C) is stable; at this temperature there is a eutectoid reaction between ferrite, austenite and cementite.
Most "mild" steels (plain carbon steels with up to about 0.2 wt% C) consist mostly of ferrite, with increasing amounts of pearlite (a fine lamellar structure of ferrite and cementite) as the carbon content is increased.
External link
- Ferrite FAQ (http://www.fair-rite.com/FaqFer.htm)de:Ferrit