Difference between revisions of "Ferrite (magnet)" - New World Encyclopedia
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[[Image:Ceramic magnets.jpg|thumb|A stack of ferrite magnets.]] | [[Image:Ceramic magnets.jpg|thumb|A stack of ferrite magnets.]] | ||
| − | '''Ferrites''' are a class of [[chemical compound]]s | + | '''Ferrites''' are a class of [[ferrimagnetism|ferrimagnetic]] ceramic [[chemical compound]]s consisting of mixtures of various metal oxides, usually including [[iron]] oxides. Their general [[chemical formula]] may be written as AB<sub>2</sub>O<sub>4</sub>, where A and B represent different metal [[cation]]s. These materials are used in applications ranging from magnetic components in microelectronics. |
| − | + | == Properties == | |
| − | + | Ferrites are electrically non-conductive ferrimagnetic ceramics. They are usually mixtures of [[iron oxide]]s, such as [[Hematite|hematite]] (Fe<sub>2</sub>O<sub>3</sub>) or [[Magnetite|magnetite]] (Fe<sub>3</sub>O<sub>4</sub>), and [[oxide]]s of other metals. Like most other ceramics, they are hard and [[brittle]]. In terms of their magnetic properties, they are often classified as "soft" or "hard," referring to low or high [[coercivity]] of their magnetism, respectively. | |
| − | Ferrites are | ||
===Soft ferrites=== | ===Soft ferrites=== | ||
| − | Ferrites that are used in [[transformer]] or [[electromagnetic]] [[Magnetic core|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|Switched-Mode Power Supply (SMPS)]] and [[ | + | Ferrites that are used in [[transformer]] or [[electromagnetic]] [[Magnetic core|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|Switched-Mode Power Supply (SMPS)]] and [[radio frequency]] (RF) transformers and [[inductor]]s. A common ferrite, abbreviated "MnZn," is composed of the oxides of manganese and zinc. |
===Hard ferrites=== | ===Hard ferrites=== | ||
In contrast, permanent ferrite magnets (or "hard ferrites"), which have a high [[remanence]] after magnetization, are composed of iron and [[barium]] or [[strontium]] oxides. In a magnetically [[Saturation (magnetic)|saturated]] state they conduct [[magnetic flux]] well and have a high magnetic [[permeability (electromagnetism)|permeability]]. This enables these so-called ''ceramic magnets'' to store stronger [[magnetic field]]s than iron itself. They are the most commonly used magnets in radios. The maximum magnetic field ''B'' is about 0.35 [[tesla (unit)|tesla]] and the magnetic field strength ''H'' is about 30 to 160 kiloampere turns per meter (400 to 2000 [[oersted]]s) (Hill 2006). | In contrast, permanent ferrite magnets (or "hard ferrites"), which have a high [[remanence]] after magnetization, are composed of iron and [[barium]] or [[strontium]] oxides. In a magnetically [[Saturation (magnetic)|saturated]] state they conduct [[magnetic flux]] well and have a high magnetic [[permeability (electromagnetism)|permeability]]. This enables these so-called ''ceramic magnets'' to store stronger [[magnetic field]]s than iron itself. They are the most commonly used magnets in radios. The maximum magnetic field ''B'' is about 0.35 [[tesla (unit)|tesla]] and the magnetic field strength ''H'' is about 30 to 160 kiloampere turns per meter (400 to 2000 [[oersted]]s) (Hill 2006). | ||
| + | |||
| + | === Crystal structure === | ||
| + | Ferrites are a class of [[spinel]]s. They adopt a crystal motif consisting of cubic close-packed (FCC) oxides (O<sup>2-</sup>) with A cations occupying one-eighth of the octahedral holes and B cations occupying half of the octahedral holes. The magnetic material known as "ZnFe" has the formula ZnFe<sub>2</sub>O<sub>4</sub>, with Fe<sup>3+</sup> occupying the octahedral sites and half of the tetrahedral sites. The remaining tetrahedral sites in this spinel are occupied by Zn<sup>2+</sup>.<ref>D.F. Shriver, P.W. Atkins, T.L. Overton, J.P. Rourke, M.T. Weller, and F.A. Armstrong. 2006. ''Inorganic Chemistry''. New York, NY: W.H. Freeman. ISBN 0716748789.</ref> | ||
== Production == | == Production == | ||
| Line 41: | Line 43: | ||
* [[Iron]] | * [[Iron]] | ||
* [[Magnetism]] | * [[Magnetism]] | ||
| + | * [[Nickel]] | ||
| + | * [[Zinc]] | ||
== Notes == | == Notes == | ||
Revision as of 01:56, 28 October 2008
Ferrites are a class of ferrimagnetic ceramic chemical compounds consisting of mixtures of various metal oxides, usually including iron oxides. Their general chemical formula may be written as AB2O4, where A and B represent different metal cations. These materials are used in applications ranging from magnetic components in microelectronics.
Properties
Ferrites are electrically non-conductive ferrimagnetic ceramics. They are usually mixtures of iron oxides, such as hematite (Fe2O3) or magnetite (Fe3O4), and oxides of other metals. Like most other ceramics, they are hard and brittle. In terms of their magnetic properties, they are often classified as "soft" or "hard," referring to low or high coercivity of their magnetism, respectively.
Soft ferrites
Ferrites that are used in transformer or electromagnetic 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 radio frequency (RF) transformers and inductors. A common ferrite, abbreviated "MnZn," is composed of the oxides of manganese and zinc.
Hard ferrites
In contrast, permanent ferrite magnets (or "hard ferrites"), which have a high remanence after magnetization, are composed of iron and barium or strontium oxides. In a magnetically saturated state they conduct magnetic flux well and have a high magnetic permeability. This enables these so-called ceramic magnets to store stronger magnetic fields than iron itself. 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).
Crystal structure
Ferrites are a class of spinels. They adopt a crystal motif consisting of cubic close-packed (FCC) oxides (O2-) with A cations occupying one-eighth of the octahedral holes and B cations occupying half of the octahedral holes. The magnetic material known as "ZnFe" has the formula ZnFe2O4, with Fe3+ occupying the octahedral sites and half of the tetrahedral sites. The remaining tetrahedral sites in this spinel are occupied by Zn2+.[1]
Production
Ferrites are produced by heating an intimate mixture of powdered precursors and then heated and pressed in a mold. During the heating process, calcination of carbonates occurs:
- MCO3 → MO + CO2
The oxides of barium and strontium are typically supplied as their carbonates, BaCO3 or SrCO3. The resulting mixture of oxides undergoes sintering. Afterwards the cooled product is milled to particles smaller than two μm in order to produce Weiss domains in the size of one particle. Next the powder is pressed into a shape, dried, and re-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. Direct calcination and sintering without re-milling is possible as well 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.
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.
See also
Notes
- ↑ D.F. Shriver, P.W. Atkins, T.L. Overton, J.P. Rourke, M.T. Weller, and F.A. Armstrong. 2006. Inorganic Chemistry. New York, NY: W.H. Freeman. ISBN 0716748789.
ReferencesISBN links support NWE through referral fees
- Amorphous Magnetic Cores For High Frequency Electronics Hill Technical Sales. Retrieved October 17, 2008.
- Bartlett, Bruce and others. 2005. Practical Recording Techniques. Oxford, UK: Focal Press. ISBN 0240806859
- Luecke, Gerald and others. 2004. General Radiotelephone Operator License Plus Radar Endorsement. Richardson, TX: Master Pub. ISBN 0945053142
- Meeldijk, Victor. 1997. Electronic Components: Selection and Application Guidelines. Hoboken, NJ: Wiley. ISBN 0471189723
- Ott, Henry. 1988. Noise Reduction Techniques in Electronic Systems. Hoboken, NJ: Wiley. ISBN 0471850683
- Schaller, George E. Ferrite Processing & Effects on Material Performance Retrieved October 17, 2008.
External links
- What are the bumps at the end of computer cables? Retrieved October 17, 2008.
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