For certain applications of soft or non-retentive materials, special alloys and other materials have been developed which, after proper fabrication and heat treatment, have superior properties in certain ranges of magnetisation. Several of these alloys will be described.
1. Nickel-Iron Alloys:
i. Nickel:
Nickel alloyed with iron in various proportions produces a series of alloys with a wide range of magnetic properties.
With 30% nickel, the alloy is practically nonmagnetic, and has a resistivity of 86 μΩcm.
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With 78% nickel the alloy properly heat treated has a very high permeability. These effects are shown in Fig. 8.22 and Fig. 8.23. Many variations of this series have been developed for special purposes.
ii. Permalloy:
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Permalloy is a term applied to a number of nickel-iron alloys developed by the Bell Laboratories, each specified by a prefix number indicating the nickel content. This term is usually associated with the 78.5% nickel-iron alloys, the important properties of which are high permeability and low hysteresis loss in relatively low magnetising fields.
These properties are obtained by a unique heat treatment consisting of a high-temperature anneal, preferably in hydrogen, with slow cooling followed by rapid cooling from about 625°C.
The alloy is very sensitive to mechanical strain; so it is desirable to heat-treat the alloy in its final form.
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The addition of 3.8% chromium or molybdenum increases the resistivity from 16 to 65 and 55 μΩ-cm, respectively, without seriously impairing the magnetic quality. In fact, low- density permeabilities are better with these additions.
These alloys have found their principal application as a material for the continuous loading of submarine cables and in loading coils for land lines.
By special long-time high temperature treatments, maximum permeability values greater than 1 million have been obtained. The double treatment required by the 78% permalloy is most effective when the strip is thin say under mils. For greater thicknesses, the quick cooling from 625°C is not uniform throughout the section, and loss of quality results.
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By the use of special techniques of cold reduction and annealing the 48% nickel-iron alloy develops directional properties resulting in high permeability and a square hysteresis loop in the rolling direction. For optimum properties, these materials are rapidly cooled after a 2-hour anneal in pure hydrogen at 1100°C. They are generally used in wound cores of thin tape for applications such as phase transformers and magnetic amplifiers.
iv. Iron-Nickel-Copper-Chromium:
The addition of copper and chromium to high nickel-iron alloys has the effect of raising the permeability at low flux density. For optimum properties they are annealed after cutting and forming for 4 hours at 1000°C in pure hydrogen and cooled slowly.
Important applications are as magnetic shielding for instruments and electronic equipment and as cores in magnetic amplifiers.
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v. Constant-Permeability Alloys:
These alloys having a moderate permeability which is quite constant over a considerable range of flux densities are desirable for use in circuits in which waveform distortion must be kept at a minimum. Isoperm and copernik are two alloys of this type. They are nickel-iron alloys containing 40 to 55% nickel which have been severely cold-worked.
Permnivar is the name given to a series of cobalt-nickel-iron alloys (for example, 50% nickel, 25% cobalt, 25% iron) which exhibit this characteristic of constant permeability over a low density range. When magnetised to higher flux densities, they give a double loop constricted at the origin so as to give no measurable remanence or coercive force. The characteristics of the alloys in this group vary greatly with the chemical content and heat treatment.
It is an alloy of 67% nickel, 28% copper, and 5% other metals. It is slightly magnetic below 95°C.
The addition of cobalt to iron has the effect of raising the saturation intensity of iron upto about 36% cobalt (Fe2Co). This alloy is useful for pole pieces of electromagnets and for any application where high magnetic intensity is desired. It is workable hot but quite brittle cold.
Hyperco contains approximately 1/3 Co, 2/3 Fe, plus 1 to 2% “added element”. It is available as hot-rolled sheet, cold-rolled strip, plates, and forgings. The 50% cobalt-iron alloy Permendur has a high permeability and, with 2% vanadium added, can be cold-rolled.
viii. Iron-Silicon Aluminium Alloys:
Aluminium in small percentages, usually under 0.5% is a valuable addition to the iron- silicon alloy. Its principal function appears to be as a deoxidiser. Certain compositions have very low magnetostriction and anisotropy, high initial permeability, and high electrical resistivity.
An alloy of 9.6% silicon and 6% aluminium with iron has better low-flux-density properties than the Permalloys. However, poor ductility has limited these alloys to D.C. applications in cast configurations or in insulated pressed-powder cores to high-frequency uses. These alloys are commonly known as sendust.
ix. Temperature-Sensitive Alloys:
In as much as the Curie point of metal may be moved up or down the temperature scale by the addition of other elements, it is possible to select alloys which lose their ferromagnetism at almost any desired temperature upto 1115°C, the change point in cobalt.
Iron-base alloys are ordinarily used to obtain the highest possible permeability at points below the Curie temperature. Nickel, manganese, chromium and silicon are the most effective alloy elements for this purpose, and most alloys made for temperature control application, such as instruments, reactors, and transformers, use one or more of these.
These are ferromagnetically composed of “nonmagnetic” elements. Copper, manganese and aluminium are frequently used as the alloying elements. The saturation induction is about one-third that of pure iron.
2. High-Frequency Materials:
Applications:
The ideal core materials for small reactors and transformers employed in communication equipment should possess the following characteristics:
(i) Constant permeability.
(ii) Small hysteresis loss.
(iii) Small eddy-current loss within the range of small magnetizing forces and over the wide range of frequencies met in such applications.
At the higher frequencies the control of eddy-currents becomes of primary importance, not only to reduce losses but also to minimise skin effect produced by eddy-current shielding. This is accomplished by the use of high-permeability alloys in the form of wound cores of thin tape, by the use of compressed powdered iron-alloy cores, or by sintered ferrites. Comparison of these materials is given in Table 8.1.
Thin electrical steels are insulated 3% iron-silicon alloys designed for applications involving frequencies of 400 to 20,000 Hz and for pulse components. They are made in strip 0.025 to 0.175 mm thick for wound cores, with high effective permeability and low losses at relatively high flux densities.
Nickel-alloy of high permeability is used in thicknesses of 0.0025 mm to 0.025 mm for wound cores designed for the frequency range 100 to 100,000 c/s. Commonly used alloys for this purpose are the Permalloy, MoPermalloy, Deltamax and Supermalloy, the thickness being chosen to provide the desired permeability at the application frequency.
Fig. 8.24 shows the effect of tape thickness (mm) on the initial permeability of these types of materials (see Table 8.1).
Ferrite cores are moulded from a mixture of metallic oxide powders such that certain iron atoms in the cubic crystal of magnetite (ferrous ferrite) are replaced by other metal atoms, such as Mn and Zn, to form manganese zinc ferrite, or by Ni and Zn to form nickel zinc ferrite.
They resemble ceramic materials in production processes and physical properties. The D.C. resistivities correspond to those of semiconductors, being at least 1 million times those of metals. Magnetic permeabilities may be as high as 5,000 and apparent dielectric constants in excess of 100,000. The Curie point is quite low, however, in the range of 100 to 300°C.
Ferrite cores provide design advantages over strip and power cores for such uses as filter cores upto 200 kc, as deflection transformers and yokes and in antenna rods, pulse transformers, delay lines and wave guide elements. (See Table 8.1, Figs. 8.25 and 8.26).
C. Powdered-Iron Cores:
Iron powders having a grain size of the order of 10 microns are manufactured by a chemical process and coated to a thickness of 1 micron with a special insulating material. The powder is then mixed with phenol resin binder, compressed at high pressure, and baked. The product is a chemically stable magnetic body containing 90% pure iron by weight; it can be worked mechanically in the same manner as soft metals.
Compressed powdered alloy has also been developed for applications similar to those of compressed powdered iron and has superseded the latter for certain purposes, such as loading coils for long telephone-cable circuits. The superior magnetic properties permit the use of smaller cores, with considerable saving in the overall dimensions of a coil of given inductance.
It is necessary, however, to apply the proper heat-treatment in order to develop the desired properties of Permalloy Compressed powdered molybdenum Permalloy, having lower hysteresis losses and higher resistivity than the 78 or 81% nickel-iron alloy, is considered the best of this class of alloys. It contains about 2% molybdenum, 81% nickel, and 17% iron. Much of the quality of all these powder cores depends upon the use of specially developed particle insulation which will withstand the desired pressure and temperature and be of minimum thickness.
3. Permanent-Magnet (Retentive) Materials:
Although it is possible to make permanent magnets of almost any kind of steel that is capable of being hardened by heat treatment, it is best to use materials especially produced for this purpose. Before the development of the special magnet steels, magnets were generally made of plain high-carbon tool steel. This type of steel is relatively inexpensive, but its magnetic properties are greatly inferior to those of the special steels.
Permanent-magnet materials may be grouped in five classes as follows:
I. Precipitation-hardened alloys.
II. Quench-hardened alloys.
III. Ceramic.
IV. Iron-powder compacts.
V. Work-hardened materials.
Fig. 8.27 shows typical demagnetisation curves for several permanent magnet materials.
I. Precipitation-Hardening Alloys:
a. Alnico Magnet Alloys:
Alnico magnet alloys have the highest energy per unit of cost or volume of any permanent-magnet material commercially available. They are usually characterised by a higher coercivity, a higher energy, and a lower retentivity than the magnet-steel types. They are formed only by casting or sintering, are relatively weak and brittle and cannot be readily machined except by grinding.
b. Cobalt-Molybdenum-Iron:
It is known as Remalloy or Comal. It is a cast and hot rolled magnet j material, preferably containing 12% Co and 17% Mo, of precipitation-hardening type. After quenching in air or oil at 1200 to 1300°C, it can be formed and machined and is then aged for 1 hour at 650 to 700°C, after which it is not sensitive to further ageing.
c. Cunife:
It is a copper-nickel-iron alloy that is malleable, ductile, and machinable, even in their age-hardened form. It has directional properties and should be magnetised in the direction in which it was drawn. In small sizes, Cunife has a tensile strength of approximately 8 kN/cm2. Remanence decreases markedly at elevated temperatures, about 50% at 325°C, and is non-magnetic above 400°C.
d. Cunico:
It is a copper-nickel-cobalt alloy which is ductile prior to its final heat treatment but cannot be readily machined thereafter. Magnets can be made from rods, strips, and wire and can also be cast. The magnetic properties are independent of the direction of cold working or heat treatment.
e. Vicalloy:
It is the trade name for permanent-magnet alloys of iron, cobalt and vanadium. Vicalloy I contains 9.5% vanadium and has a much higher energy product, which is obtained by heavy cold working. It is therefore, strongly directional, having its best properties in the direction of cold working. It is aged at 600°C, after which it no longer ductile. It is used in tape form for magnetic recording.
f. Silmanal:
It is a ternary alloy of silver, manganese, and aluminium, having the highest intrinsic coercive force of any of the magnet materials. It will withstand severe demagnetising effects. The alloy is ductile enough to be drawn to a fine wire and can be machined as readily as soft steel.
Magnets are made in a wide variety of shapes from wire swaged rods, or rolled steels. Care must be taken not to heat the material above 200°C, after which it is no longer ductile. It is used in tape form for magnetic recording.
1. Carbon Magnet Steel:
The coercivity and retentivity of quenched carbon steel increase with the carbon content up to the eutectoid point, or about 0.85% of carbon; with still higher carbon content the retentivity decreases.
2. Tungsten Magnet Steel:
It contains approximately 5 to 6% tungsten, 0.60 to 0.80% carbon, and about 0.50% manganese. There are two general types, viz., oil hardening and water hardening. In designs subject to breakage in water quenching, the oil-hardening type should be used.
3. Chrome Magnet Steels:
These contain approximately 2.5 to 6% chromium and about the same proportions of carbon and manganese as tungsten magnet steel. They are nearly as efficient magnetically as tungsten steel and less expensive.
4. Cobalt-Chrome Magnet Steel:
It is an alloy steel containing about 11% cobalt and 9% chromium. It has the advantage of being readily machinable under production conditions. Magnetically it is not quite so efficient as cobalt steel but is less expensive.
5. Cobalt Magnet Steel:
It is also known as K.S. Steel, contains approximately 36% cobalt, 4% tungsten, and 6% chromium. It forms and punches well when hot but is not well suited for magnets requiring considerable machining, although it can be drilled. Because of its high cobalt content it is expensive.
6. Platinum-Iron Alloys:
These alloys containing 60 to 90% platinum develop high values of coercive force depending on heat-treatment. Highest values are obtained by quenching from 1200°C. The Curie points for these alloys are high (near 1100°C), making them of value in high-temperature applications. The preferred alloy contains about 78% platinum.
7. Platinum-Cobalt Alloys:
These alloys have the highest energy product of any of the alloys with noble metals. The high coercivity is attained by quenching at 1200°C, after which they may be machined to shape, followed by ageing at 700°C. Machining after ageing is not possible because of their hardness.
III. Ceramic Magnet Materials:
Commercial developments in permanent magnet materials which are increasing in importance each year are the barium ferrite ceramic permanent-magnet materials. These are chemical compounds with mechanical characteristics similar to those of other ceramics.
These materials are hard and brittle and have a lower density than metals and extremely high electrical resistivity ingredients are barium carbonate and iron oxide. The materials in powdered form are compressed in dies under high pressure to the required shape. This compacted material is then sintered at a high temperature.
This process produces a material which has its demagnetization curve practically a straight line. Further improvements in ceramic materials have resulted in a highly oriented barium-iron oxide whose magnetic properties, on weight basis, are almost equal to those of Alnico V. At right angles to the direction of grain orientation, however, this material exhibits negligible permanent-magnet properties and has relative permeability of only approximately 1.0.
IV. Powder Magnets:
Although pure iron is usually regarded as a high-permeability, or “magnetically soft”, material, yet theory has predicted and experiments have proved that compacts of pure iron powders may produce very good permanent magnets. Powder magnets have been produced of iron and iron alloys (such as 70% iron and 30% cobalt) with particle size of about 10-5 cm diameter.
The permanent-magnet properties result from the discrete small particles of a single phase instead of from the presence of two or more phases, as in most other metallic permanent- magnet material. Further experimental work with particle size and shape and processes of manufacture has produced, in the laboratory, magnets with energy products comparable with those of Alnico V, and theoretical considerations predict even higher values.
Manganese-bismuth permanent magnets also belong to this group. This material is an anisotropic aggregate of crystals of the intermetallic compound manganese bismuthide (MnBi) and is al product of powder metallurgy. Manganese bismuthide is prepared from the chemical action between molten bismuth and powdered manganese when heated to approximately 700°C in an inert atmosphere or argon or helium. Cooling is accomplished in such a manner as to produce crystallization of the compound.
Powder metallurgy has also produced sintered Alnico magnets. These magnets have greater mechanical strength and more uniform magnetic properties than the cast variety, at the expense of a slight decrease in the magnetic properties.
Magnet materials prepared from metal oxides such as cobalt ferrites and Vectolite have been made and used for many years; however, they have been practically superseded by the barium ferrites.
Several ordinarily “nonmagnetic” alloys of iron may become ferromagnetic after cold working owing to a phase change in the material. Stainless steel (18% chromium, 8% nickel) is “nonmagnetic” at room temperature after being rapidly cooled from 1200°C in the usual process of manufacture.
However, if it is hardened by cold working such as drawing through a reducing die, it may develop properties such that it makes an acceptable permanent-magnet material at room temperature. If this work-hardened alloy is then reheated to a high temperature and cooled slowly, it regains its original nonmagnetic condition at room temperature. Another alloy that shows this property contains 45% iron, 15% Ni and 40% Cu.