Ferrite applications

This group of soft-ferrite applications is based on their ability to transform ac signals of small amplitude into substantially large variations of magnetic flux. The fact that at low fields the initial permeability is a linear function of the field explains the name these devices are also known as small-signal applications. There are other materials, such as metallic alloys (see Chapter 6), which possess permeability values considerably higher than the typical values of ferrites however, as frequency increases, conductivity losses prevent efficient use of metallic materials. 

Iron oxide is also used for nonpigmentary applications, eg, ferrites (qv) and foundry sands making total world demand for iron oxide close to 1 X 10 t. The principal worldwide producers of iron oxide pigments are Bayer AG (ca 300,000 t/yr worldwide), and Harcros Pigments Inc., a subsidiary of Harrisons Crosfield PLC. In the United States, Bayer produces the Bayferrox line of iron oxide pigments in New Martinsville (see Iron compounds). 

AU other carbon steel, low-intermediate, and high-aUoy steels, ferritic steels Base metal Deposited weld metal and heat-affected zone (See Note 1) 2. Except when conchtions conform to Note 2, the material shall be heat-treated to control its microstructure by a method appropriate to the material as outlined in the specification applicable to the product form and then impact-tested. (See Note 1.) Deposited weld metal and heat-affected zone shall be impact-tested. 

In dc magnetic applications, an air-gap is usually required somewhere along the magnetic path of the core. In ferrite cores, the gap is placed in the center-leg of the core. The flux leaves one end of the core and flows towards the opposing end. The flux, though, repels itself and causes the flux lines to bulge out away from the centerline of the core. The presence of an air-gap creates an area... 

Selecting the core material is the first issue to be addressed. All core materials are alloys based on ferrite. The major factor in a material s worthiness is its loss at the frequency of operation and the flux density of the application. A good place to start is with the materials the core manufacturer s themselves recommend for PWM switching power supplies and those that are commonly used by the designers in the field (see Table D-f). 

Using one of the core materials listed in Table D-1, the designer can feel reasonably confident that he or she has made the best choice for a ferrite. Mopermalloy is a ferrite alloy that has nonmagnetic molybdenum mixed with it. The molybdenum acts as a distributed air-gap within the material, which makes the material excellent for dc biased or unipolar applications. Unfortunately, it is only available in toroid core styles, and it typically used for output filter chokes. 

The product crystals find industrial application as a component raw material for optical glass, fibreglass, Braun tubes, electric condensers, barium ferrite, etc. Needles shaped crystals are obtained at high pH, while pillar-shaped crystals are formed at neutral pH. The formation of carboxyl ions is via hydroxy ions at high pH, but at neutral pH it may accompany the production of hydrogen sulphide, as... 

Major uses of the ferritic steels have been on motor vehicles as trim and in domestic equipment such as cutlery and hollow ware, but use has also been made in refrigerators, washing machines and on sinks and similar fittings. Some types would no doubt find much wider application in the chemical field and other fields where their superior corrosion resistance would be a considerable advantage if it was not for the fact that the austenitic types have advantages (sometimes considerable) in fabrication. However, the availability of the low interstitial weldable types and the super ferritics is increasing in scope. 

Cathodic protection applications in fresh water include use of ferrite-coated niobium , and the more usual platinum-coated niobium . Platinised niobium anodes have been used in seawater, underground and in deep wells " and niobium connectors have been used for joining current leads Excellent service has been reported in open-seawater, where anodic potentials of up to 120V are not deleterious, but crevice corrosion can occur at 20 to 40V due to local surface damage, impurities such as copper and iron, and under deposits or in mud ... 

The ferritic steel 430S17 has enhanced oxidation resistance and finds some applications in sheet form, but its strength at elevated temperature is low. The higher chromium (20-30%) ferritic types show excellent oxidation resistance, but have poor elevated-temperature strength and, being difficult to produce and fabricate, are not used in large quantity. Cast versions of 27-30% Cr are quite widely used, especially where oxidation resistance, coupled with abrasion resistance, is required when high carbon contents are utilised. Such alloys are brittle. 

The ferritic steels rapidly lose strength at elevated temperature as shown in Fig. 7.26 and are of little value for load-bearing applications. 

Ferritic type. Welding produces a brittle deposit and a brittle heat-affected zone caused by the very large grain size that is produced. The problem may be reduced in severity by the use of austenitic fillers and/ or the application of pre- and post-weld heat treatments the latter is a serious limitation when large welded structures are involved. 

Sintered and sprayed ceramic anodes have been developed for cathodic protection applications. The ceramic anodes are composed of a group of materials classified as ferrites with iron oxide as the principal component. The electrochemical properties of divalent metal oxide ferrites in the composition range 0- lA/O-0-9Fe2O3 where M represents a divalent metal, e.g. Mg, Zn, Mn, Co or Ni, have been examined by Wakabayashi and Akoi" . They found that nickel ferrite exhibited the lowest consumption rate in 3% NaCl (of 1 56 g A y at 500 Am and that an increase in the NiO content to 40mol 7o, i.e. O NiO-O-bFejO, reduced the dissolution rate to 0-4gA y at the expense of an increase in the material resistivity from 0-02 to 0-3 ohm cm. 

Product yields may also be determined by magnetic measurements, as in the formation of ferrites [340], where kinetic data were obtained at reaction temperature. Quantitative applications of Mossbauer spectroscopy have also been described [326]. 

ASTM A 890/A 890M-99(2003) Standard Specification for Castings, Iron-Chromium-Nickel-Molybdenum Corrosion-Resistant, Duplex (Austenitic/Ferritic) for General Application (contains the major duplex grades)... 

The synthesis [33] of magnesium ferrite (MgFe2C>4) from Fe203 and MgO for catalytic applications has been reported by the use of ultrasound of 20 kHz and power 100 W/cm2. Short range ordering of the alloy of Ni-(0.2-10.3) atomic percent in Cr solid could be affected by 17.5 Hz sound wave at liquid helium temperature (40 K) [34]. 

In this chapter the technological development in cathode materials, particularly the advances being made in the material s composition, fabrication, microstructure optimization, electrocatalytic activity, and stability of perovskite-based cathodes will be reviewed. The emphasis will be on the defect structure, conductivity, thermal expansion coefficient, and electrocatalytic activity of the extensively studied man-ganite-, cobaltite-, and ferrite-based perovskites. Alterative mixed ionic and electronic conducting perovskite-related oxides are discussed in relation to their potential application as cathodes for ITSOFCs. The interfacial reaction and compatibility of the perovskite-based cathode materials with electrolyte and metallic interconnect is also examined. Finally the degradation and performance stability of cathodes under SOFC operating conditions are described. 

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