Ferrites hexaferrites

These requirement.s are fulfilled by the hexagonal ferrites (hexaferrites) BaFei20i9 and SrFe]20m. 

Generally speaking the higher quality hard ferrites are based upon Sr-hexaferrite and those for less demanding applications on the cheaper Ba-hexaferrite. 

Femandes-Rodriges, Morales, Tirado synthesized mechanochemically lithium ferrite and lithium cobaltite from lithium hydroxide and corresponding oxihydroxide 5-FeOOH or CoOOH [37]. Similarly, barium hexaferrite BaFei20i9, including that doped with titanium and cobalt, was obtained from Ba(0H)2-H20 and y-FeOOH [38]. 

Barium hexaferrite. Barium hexaferrite, BaFei20 <<, is used as a ceramic permanent magnet. It is conventionally prepared by firing an appropriate mixture of a-Fe203 with BaCOs at high temperatures (1150-1250°C). To reduce the particle size, thus obtained ferrite should be ground. 

A special feature of hexaferrites is their pronounced magnetic anisotropy. The preferred direction for magnetization is perpendicular to the plane of the plateletshaped hexagonal crystals. By parallelly orienting the platelets in the finished ceramic (anisotropic ferrite) the required remanence of > 4000 G and virtually rectangular hysteresis loops can be obtained. 

Isotropic hexaferrites are manufactured in a similar way to soft ferrites, using natural or waste iron oxide as a raw material. 

A variety of ferrites have been prepared from colloidal suspensions (Fan Matijevic, 1988). Sr hexaferrite was crystallised from a Fe(OH)2 gel in the presence of SrCl2, with KNO3 as a mild oxidising agent at 90 °C. The size of the particles was 0.08-0.51 pm. 

There are two other methods for producing ferrite powders that deserve attention molten-salt synthesis and shock-wave loading. In molten-salt synthesis, Fe203 and the corresponding carbonate, oxide, hydroxide or nitrate needed to produce the ferrite phase are dry blended with a mixture of NaCl and KCl. This reaction mixture, in a Pt crucible, is placed in a furnace at 800-1100 °C for lh the chloride solvent is melted and provides an efficient heating medium for the reaction. After cooling, the solvent is separated from the ferrite by dissolution in water. The product can be finally collected by filtration. Ba and Sr hexaferrite powders have been prepared by this method (Arendt, 1973). Submicron, high-quality crystallites with a low ferrous content were obtained. 

Finishing of sintered anisotropic ferrites involves grinding surfaces to specified dimensions. Since sintered ferrites are hard and brittle, grinding is usually performed with diamond tools. A serious problem in sintering hexaferrites is anisotropic shrinkage. It can be as high as 25% parallel. 

The largest single application of anisotropic hexaferrites is in loudspeakers, Fig. 5.1. The ferrite is an anisotropic ring, axially magnetised. The signal to be transformed to sound, typically from an amplifier, flows through the coil fixed to the end of the speaker cone the interaction... 

Barium hexaferrite. See Barium ferrite Barium hexafluorosilicate Barium hexafluorosilicate (2-). See Barium fluosilicate Barium hydroxide. See Barium hydroxide lime Barium hydroxide lime CAS 17194-00-2 (anhyd.) 12230-71-6 (octahydrate) EINECS/ELINCS 241-234-5 Synonyms Barium dihydroxide Barium hydroxide Caustic baryta Definition Mixt. of barium hydroxide octahydrate and calcium hydroxide Empirical BaH202 Formula Ba(OH)2... 

Pullar, R.C. (2012) Hexagonal ferrites a review of the synthesis, properties and applications of hexaferrite ceramics. Prog. Mater. Scl, 57 (7), 1191-1334. 

Ferrites with the components ratio typical of hexaferrite prepared by co-precipitation and calcined at 700 - 1000°C were characterized by different methods. It has been shown that hexaferrite phase formed at 700°C is amorphous and its crystallization occrrrs at 800°C. Specific srrrface area of the samples calcined at 700°C is 27 - 59 mVg. Temperature-programmed reduction of the samples with H2 proceeds in several steps, Fe(lll) in the sample being partially reduced to Fe. The main components on sample srrrface are in oxidizied state Fe and Mn. Among the samples concerned SrMr (Fei2.xOi9 (0 < x < 2) are the most active catalysts in methane oxidation. 

Ferrites calcined at 700°C are multiphase. Further temperature increase to 800 -1000°C promotes formation of hexaferrite phase. 

Soft ferrites and hard ferrites or hexaferrites are important materials for magnetic uses. Soft ferrites are crystals with a spinel structure whose reference is magnetite Fe304 hard hexaferrites are crystals with a hexagonal structure, whose type is BaFe 2 i9 ... 

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