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ferrite core[′fe‚rīt ′kȯr]
a magnetic flux bar made of ferrite. The ferrites used exhibit a very low specific electric conductivity; thus, for all practical purposes eddy currents do not appear during magnetic reversal, which eliminates energy losses. As a result, ferrite cores can be used in electronic equipment operating in the radio-frequency range. The primary areas of application for ferrite cores are radio engineering, automation, telemechanics, and computer technology.
Production technology for the manufacture of ferrite cores is based on the techniques of powder metallurgy. Cores of a required shape are compressed from a mixture of powders, and the material is sintered at a temperature of 850°–1500°C in air and then slowly cooled for several hours. The magnetic and dielectric properties of a ferrite core depend on the composition of the mixture, the percentage content of the base components, and the process of heat treatment used. By altering these factors, it is possible to obtain ferrite cores with various required properties, for example, with a high initial magnetic permeability, for use in high-frequency and pulse transformers, or with a rectangular magnetic hysteresis loop, for use in memories.
The techniques of powder metallurgy make it possible to manufacture ferrite cores in various shapes (U-shaped, E-shaped, ring-shaped, and pot cores and complex configurations with several openings on one or several planes) and in a variety of sizes (from tenths of a millimeter to several centimeters). Ring cores that exhibit rectangular hysteresis loops are the most common type. After magnetization and removal of the magnetized field, they can preserve indefinitely one of two possible stable magnetic states, corresponding to the two values of residual magnetic induction (+ Br and –Br). This property makes them suitable for use as elements in memories and logic devices, for example, diode and transistor ferrite storage cells.
Magnetic reversal of the ferrite core, that is, the shift from one magnetic state to another, is caused by the magnetic field of an electric current in the windings of the ferrite core. The time required for magnetic reversal depends on the amplitude and shape of the pulse current, the coercive force, the rectangularity of the hysteresis loop, and the geometric dimensions of the core; it ranges from tenths of a microsecond to several microseconds. Ring ferrite cores with nonrectangular hysteresis loops are primarily used in pulse transformers and high-frequency choke coils.
REFERENCESPirogov, A. I., and Iu. M. Shamaev. Magnitnye serdechniki dlia ustroistv avtomatiki i vychislitel’noi tekhniki, 3rd ed. Moscow, 1973.
Bardizh, V. V. Magnitnye elementy tsifrovykh vychislitel’nykh mashin, 2nd ed. Moscow, 1974.
A. V. GUSEV