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Ferrites are a class of chemical compounds with the formula AB2O4, where A and B represent various metal cations, usually including iron. These ceramic materials are used in applications ranging from magnetic components in microelectronics.

Properties
Ferrites are usually non-conductive ferrimagnetic ceramic compounds derived from iron oxides such as hematite (Fe2O3) or magnetite (Fe3O4) as well as oxides of other metals. Ferrites are, like most other ceramics, hard and brittle. In terms of the magnetic properties, ferrites are often classified as "soft" and "hard" which refers to their low or high coercitivity 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 RF transformers and inductors. A common ferrite, abbreviated "MnZn," is composed of the oxides of manganese and zinc.

Production
Ferrites are produced by heating an intimate mixture of powdered precursors are 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 2 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.

Small and geometrically easy shapes may be produced with dry pressing.

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.

Characteristic Symbol Unit R5K R7K R10K R13K Initial permeability μi - 5500±25% 7000±25% 10000±25% 13000±25% Relative loss factor x10-6 ≤15(100kHz) ≤7(10kHz) ≤7(10kHz) ≤7(10kHz) Effective saturation magnetic flux density Bs mT 420(800A/m) 400(800A/m) 400(1194A/m) 360(1194A/m) Effective retentivity Br mT 150 90 90 100 Effective coercivity HcB A/m 8 10.4 7.2 4.4 Temperature Factor of permeability 1 x10-6 0~1.5 0~2 -0.5~1.5 -0.5~1.5 oC 20~60 20~55 20~70 20~70 Disaccommodation Factor DF x10-6 ≤3 ≤3 ≤2 ≤2 Curie Temperature Tc oC >140 >125 >120 >110 Resistivity p Ω.m 0.3 0.3 0.15 0.15 Density d g/m3 4.9 4.9 4.9 4.9

Aksa magnet Arge Referance Catolog
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