Ferrite

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ferrite

[′fe‚rīt] (inorganic chemistry) An unstable compound of a strong base and ferric oxide which exists in alkaline solution, such as NaFeO₂. (metallurgy) Iron that has not combined with carbon in pig iron or steel. (petrology) Grains or scales of unidentifiable, generally transparent amorphous iron oxide in the matrix of a porphyritic rock. (solid-state physics) Any ferrimagnetic material having high electrical resistivity which has a spinel crystal structure and the chemical formula XFe₂O₄, where X represents any divalent metal ion whose size is such that it will fit into the crystal structure.

Ferrite

an iron-alloy structure that is a solid solution of carbon and alloying elements in α-iron. Ferrite’s crystal lattice is body-centered cubic. The solubility of carbon in the solution is 0.02–0.03 percent (by weight) at 723°C and 10^–6–10^–7 percent at room temperature. The solubility of the alloying elements may be very significant, even unlimited.

In most cases, alloying strengthens ferrite. Unalloyed ferrite is relatively soft and ductile, and it is strongly ferromagnetic at temperatures up to 768°–770°C. The microstructure, grain size, and substructure of ferrite depend on the conditions under which the solid solution is formed when the γ → α change occurs. With slight supercooling, polyhedral grains that are nearly equiaxed are formed; with greater supercooling and the presence of alloying elements (Cr, Mn, Ni), ferrite is formed through a martensitic transformation. The ferrite thus formed is also hardened by the transformation. The enlargement of austenite grains often leads upon cooling to the formation of Widmannstaetten ferrite, especially in cast and overheated steels. The separation of hypoeutec-toid ferrite occurs primarily at the boundaries of austenite grains. At temperatures above 1390°C, a solid solution of carbon in δ-iron is formed in iron-carbon alloys, a solution whose crystal lattice is also body-centered cubic. The solubility of carbon in δ-iron is 0.1 percent. This phase may be regarded as high-temperature ferrite.

REFERENCES

Bochvar, A. A. Metallovedenie, 5th ed. Moscow, 1956.
Bunin, K. P., and A. A. Baranov. Metallografiia. Moscow, 1970.

R. I. ENTIN

Ferrite

any of the chemical compounds that ferric oxide (Fe₂O₃) forms with oxides of other metals. Many ferrites combine high magnetization with semiconductor or dielectric properties, a fact explaining their widespread use as magnetic materials in radio engineering, radio electronics, and computer technology.

Ferrites contain oxygen anions (O^2–), which make up the frame of the crystal lattice; Fe³⁺ cations, which have a radius smaller than that of the O^2– anions, are positioned in the spaces between the oxygen ions. These spaces are also occupied by cations Of other metals (Me^k+), which are of various radii and valences k. The coulomb (electrostatic) interaction between the cations and anions leads to a particular crystal lattice and a particular arrangement of cations within the lattice. As a result of the ordered arrangement of the Fe³⁺ and Me^k+ cations, ferrites exhibit ferrimagnetism and typically have fairly high magnetization and high Curie points. Distinctions are made between ferrites of spinel structure, ferrites of garnet structure, orthoferrites, and hexagonal ferrites.

Ferrites of spinel structure have the general formula MeFe₂O₄, where Me can represent Ni²⁺, Co²⁺, Fe²⁺, Mn²⁺, Mg²⁺, Li¹⁺, and Cu²⁺. The unit cell of these ferrites is a cube formed by eight molecules of MeOFe₂O₃ and consisting of 32 O^2– anions, among which there are 64 tetrahedral (A) sites and 32 octahedral (B) sites partially occupied by Fe³⁺ and Me²⁺ cations (Fig. 1). Depending on the ions that occupy sites A and B and the order in which the sites are occupied, a distinction is made between straight spinel ferrites (nonmagnetic spinel ferrites) and reversed spinel ferrites (ferrimagnetic spinel ferrites). In the latter type, one-half of the Fe³⁺ ions are in the tetrahedral sites, with the remaining one-half, along with the Me²⁺ ions, in the octahedral sites. The magnetization M_A, of the octahedral sublattice is greater than the magnetization M_B of the tetrahedral sublattice, a difference giving rise to ferrimagnetism.

Ferrites of garnet (cubic) structure are made up of rare-earth elements R³⁺ (Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Sm³⁺, Eu³⁺) and yttrium Y³⁺ and have the general formula R₃Fe₅O₁₂. The unit cell of garnet ferrites contains eight R₃Fe₅O₁₂ molecules; there are 96 O^2– ions, 24 R³⁺ ions, and 40 Fe³⁺ ions. These ferrites have three types of sites for the cations. Most of the Fe³⁺ ions occupy tetrahedral sites (d); a smaller number occupy octahedral sites (a), and the R³⁺ ions occupy dodecahedral sites (c). The relative magnitudes and directions of the magnetizations of the cations occupying sites d, a, and c are shown in Figure 2.

Figure 1. Crystal structure of spinel ferrites: (1) schematic representation of the unit cell of the spinel structure (divided into octants); (2) arrangement of ions in adjacent (shaded and white) octants, with the white circles representing O^2– ions and the black circles representing metal ions in octahedral and tetrahedral sites; (3) metal ion in a tetrahedral site; and (4) metal ion in an octahedral site

Orthoferrites are ferrites with an orthorhombic crystal structure. They are formed by rare-earth elements or yttrium according to the general formula RFeO₃. Orthoferrites are isostructural with perovskite. In comparison with garnet ferrites, they have low magnetization because of their noncollinear antiferromagnetism (weak ferromagnetism); orthoferrites exhibit ferrimagnetism only at very low temperatures, of the order of a few degrees K and below.

Hexagonal ferrites have the general formula MeO(Fe₂O₃), where Me represents ions of Ba, Sr, or Pb. The unit cell of the crystal lattice consists of 38 O^2– anions, 24 Fe³⁺ cations, and two Me²⁺ cations (Ba²⁺, Sr²⁺, or Pb²⁺). The cell is constructed of two spinel blocks separated by Pb²⁺, Ba²⁺, or Sr²⁺ ions, O^2– ions, and Fe³⁺ ions. If ferric oxide and barium oxide are sintered with the appropriate amounts of the metals Mn, Cr, Co, Ni, and Zn, a series of ferrimagnetic oxide materials may be obtained.

Certain hexagonal ferrites possess high coercive force; these ferrites are used in the production of permanent magnets. Ferrites of garnet structure containing yttrium, certain hexagonal ferrites, and most ferrites of spinel structure are used as soft-magnetic materials.

Figure 2. Schematic representation of the magnitudes and directions of the magnetization vectors of the cations forming magnetic sublattices d, a, and c in ferrites having the garnet structure

With the introduction of admixtures and the creation of a nonstoichiometric composition (with variations in composition with respect to both the cations and oxygen), the electrical resistance of ferrites is altered within a wide range. Ferrites are not used in semiconductor technology owing to the low mobility of the current carriers. The synthesis of polycrystalline ferrites is carried out through the methods of ceramic technology. Items of the required form are molded from a mixture of the starting oxides, which are then sintered at temperatures between 900°C and 1500°C in the air or in a special gas medium.

Single crystals of ferrites are grown by the Czochralski and Verneuil methods.

REFERENCES

Rabkin, L. I., S. A. Soskin, and B. Sh. Epshtein. Ferrity: Stroenie, svoistva, tekhnologiia proizvodstva. Leningrad, 1968.
Smit, J., and H. Wijn. Ferrity. Moscow, 1962. (Translated from English.)
Gurevich, A. G. Magnitnyi rezonans v ferritakh i antiferro-magnetikakh. Moscow, 1973.

K. P. BELOV

ferrite

fer·rite

ferrite

fer•rite

Ferrite

ferrite

Ferrite

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Ferrite

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ferrite

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noun a solid solution in which alpha iron is the solvent

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