By: Hui Meng
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There are several kinds of soft magnetic materials. As we have seen in Soft Magnetic Materials II, there are many KPIs for judging a soft magnetic material, and soft magnets may be used for various frequencies, from quasi static to above 1 MHz. As a result, there is no one single kind who can beat all the others in every aspect and for the whole range of frequency. Optimum choice only exists for certain application conditions. Table 1 summarize the most common soft magnetic materials that can be found on the market and their KPIs.
Table 1: Key performance parameters for typical soft magnetic materials of various kinds. (note: the magnetic properties of soft magnetic materials are very sensitive to the production process, therefore, the data in the table are only for rough reference. For the more concrete and practical values, please consult to the sales representative.)
Iron and low carbon steels
Iron and low carbon steels may be the most common and cheapest soft magnetic materials. They have a quite high value of BS ~2.15 T, which is only inferior to the expensive Fe-Co alloys. But their resistivities are rather low, which limits their usage in dynamic applications. Iron and low carbon steels are usually used for static/low frequency applications, such as the core of electromagnet, relays, and some low power motors for which the materials cost is the major concern.
Iron-silicon alloys
Addition of a few of silicon to iron will increase its resistivity notably, therefore, is very beneficial for inhibiting the eddy current loss. Despite of slightly decrease of saturation magnetization and Curie temperature, Fe-Si alloys are widely used in electric machines operating at from 50 Hz to several hundreds Hz. To further reduce the eddy current loss, Fe-Si alloys are often rolled to the form of thin strips. The thickness for the most common Fe-Si alloy is equal to or less than 0.35mm. Depending on the conditions of rolling and heat treatment, Fe-Si alloy can be classified as Grain-Oriented (GO) and Non-Oriented (NO). GO Fe-Si is used for transformers, whereas NO Fe-Si is used for electric motors.
Iron-nickel alloys
Nickel can be added to iron to form uniform solid solutions in a broad composition range of 35 wt. % to 80 wt. % Ni. The alloys with composition near Fe20Ni80 were named as Permalloy (nowadays people tend to call all the iron-nickel alloy with nickel content higher than 35 wt. % as Permalloy). Minor content of other elements such as Mo, Cu, and Cr are usually added to improve the magnetic properties of Permalloy. Processed by delicate composition adjustment and heat treatment, Permalloy can be one of the softest magnetic material in the world, the permeability of which can be as high as 1 200 000. One of the drawbacks of Permalloys is their saturation magnetization, which is only of about 0.8 T, much lower than that of iron and Fe-Si alloys. With decrease of the nickel content, BS will increase firstly, reach its maxima of 1.6T at around nickel content of 48 wt. %, however, the permeability will not be as good as alloys with high nickel content. Iron-nickel alloy is the most versatile magnetic alloy, its magnetic properties can be tuned by adjusting composition, magnetic annealing, and mechanical rolling, etc. Iron-nickel alloy also presents very good formability, which can be rolled down to as thin as 20 microns. As a result, nickel-iron alloys can be found in wide applications such as magnetic field shielding, ground fault interrupter, magnetic sensors, recording head for magnetic tapes, power electronics, etc.
Iron-cobalt alloys
Adding cobalt to iron will increase both the Curie temperature and the BS. For cobalt content in the range of 33 wt. % to 50 wt. %, the BS can be as high as 2.4T. Although not as soft as iron-nickel alloy, iron-cobalt alloys present the highest value of BS among all the other magnetic alloys. To increase the formability, 2 wt. % of vanadium is added to the Fe50Co50 alloy, so that it can be rolled down to as thin as 50 microns. Addition of vanadium can also increase the resistivity of iron-cobalt alloy. Due to the highest BS, iron-cobalt alloys are indispensable for applications where high power to weight ratio is demanding, such as motors and transformers used in spaceborne devices.
Amorphous and nanocrystalline alloys
Amorphous alloys, also frequently called metallic glasses, can be produced by rapid solidification. There is no long-range order for the atoms in amorphous alloys, therefore, the resistivity is usually high, and there is no magneto crystalline anisotropy. Furthermore, amorphous ribbons as thin as around 20 to 30 microns can be easily produced by planar flow casting. All these characters guarantee amorphous alloys to be excellent candidates for soft magnets. According to the compositions, most of the commercially available amorphous soft magnets can be classified as Fe-base, Co-base, and (Fe, Ni)-based. For these three types, the total content of Fe, Co, and Ni is about 75-90 wt.%, the remanent are metalloids and glass forming elements such as Si, B, P, C, and Zr, Nb, Mo, etc. Among these types, Fe-based has the highest BS of about 1.6 T and lowest cost. The iron loss of Fe-based amorphous alloy is only one third of that of Fe-Si steel. If the Fe-Si steel in the power transformers can be replaced by Fe-base amorphous alloy, a huge amount of electric power can be saved, but the materials cost for the latter is higher. Co-based amorphous alloys usually have BS lower than 0.8 T but much higher permeability and near zero value of magnetostriction, which is comparable with the softest permalloy, and can perform even better at higher frequencies due to its higher resistivity. (Fe, Ni)-based amorphous alloys present medium magnetic properties compared with the other two.
Amorphous state is a metastable state. Upon heating above a critical temperature, nucleation and growth of microcrystals take place rapidly. For conventional amorphous soft magnetic alloys, during the crystallization, the size of microcrystals will grow up to several hundreds of nanometers in very short time and degenerate the soft magnetic properties severely. Nevertheless, people found that by addition of certain amount of Nb and Cu to Fe-based amorphous alloy, the crystallization process can be under control and a uniform distribution of nanocrystal with size about 10 nm in the amorphous matrix can be obtained. The magnetic properties of such a Fe-based nanocrystalline alloy are even softer than the corresponding amorphous alloy, i.e., higher permeability and lower coercivity, although the BS is also lower (~1.2 T). The source of the excellent soft magnetic properties for Fe-based nanocrystalline alloys is that both the value of magneto-crystalline anisotropy and magnetostriction can be tuned to near zero. Permalloy and Co-based amorphous alloys can also have near zero value of magneto-crystalline anisotropy and magnetostriction, but the BS of Fe-based nanocrystalline alloys is much higher. Therefore, nanocrystalline alloys may be one of the most promising soft magnetic materials. They are widely used in wireless charger, high frequency inductor, magnetic sensor, electromagnetic shielding, ground fault interrupter, and so on.
Soft magnetic composites
As mentioned before, the thickness of soft magnetic materials plays an important role for reducing eddy current losses, thus the soft magnetic alloys should be made in the form of thin lamination for dynamic uses. If we break down the other two dimensions of the soft magnetic strip, i.e., we use the soft magnetic alloys in the form of powders, then the eddy current losses can be further reduced, and the components made by which can be used at much higher frequencies. To realize such a utilization, the alloy powders are first prepared (in most cases by atomization methods), the particles then should be coated with an insulation layer, after that, the powders are mixed with a tiny amount of lubricant and compressed at an intense pressure of 600-800 MPa to the final shape. Soft magnetic products made by such processes are called Soft Magnetic Composites (SMCs) or powder cores. Another merit of SMCs is that they can be made into various specially shaped cores which are hardly made by the traditional lamination stacking methods, which benefits for novel design of electromagnetic devices. The main drawback of SMCs is that their permeabilities are relatively low. Nowadays the most common SMCs are made by powders of Fe, Fe-Si, Fe-Si-Al, Fe-Ni, amorphous and nanocrystalline alloys, etc.
Soft ferrites
All the soft magnetic materials mentioned above are metals, therefore, eddy current effect cannot be avoided. Soft ferrites are distinctive in that they are ionic compounds and have resistivity several orders of magnitude higher than that of the metallic soft magnetic materials. Therefore, for applications with frequency up to 1 MHz, soft ferrites are the best choices with respect to the energy losses. The main drawback for soft ferrites is that the BS is relatively low. Two kinds of the most common soft ferrites are Mn-Zn ferrites ((Mn, Zn)Fe2O4) and Ni-Zn ferrites ((Ni, Zn)Fe2O4). Mn-Zn ferrites are commonly used below 1 MHz, whereas Ni-Zn ferrites can be used at much higher frequencies, but the BS and permeability for the latter are lower.
To conclude, Soft magnetic materials are sensitive to external magnetic field, this feather make them indispensable for many applications especially in the area of electrical engineering, such as transformers, electric motors, wireless chargers, power electronic devices, etc. For a good soft magnet, its saturation flux density, permeability, resistivity, and Curie temperature should be as high as possible, whereas its coercivity and magnetostriction coefficient should be as low as possible. There is no one single kind of soft magnetic materials that can beat all the others in all the aspects of performance. For choosing the most suitable material, a trade-off between cost, iron loss, saturation flux density, and permeability must be made.
Iron and low carbon steels have excellent saturation flux density, but their resistivities are low, limiting their usage for dynamic application. Various alloying elements can be added into iron to optimize its magnetic performance in certain aspects. Fe-Si alloys have much higher resistivities than pure iron and relatively high saturation flux densities, they are widely used for transformers and electric motors operated at 50/60 Hz and take the biggest part of the whole soft magnetic materials market. Fe-based amorphous alloys perform much better than Fe-Si alloys with respect to the iron losses and can be operated at higher frequencies, but the cost is also higher. Fe-Co alloys present the highest value of saturation flux density. With the same output power/torque, the electric machines made by Fe-Co alloys can have smaller size and less mass. Fe-Ni alloys, Co-based amorphous alloys and Fe-based nanocrystalline alloys are the softest magnetic materials, because both the values of magneto-crystalline anisotropy and magnetostriction coefficient for them can be tuned to near zero simultaneously. Among these, Fe-based nanocrystalline alloys have the highest saturation flux density, they are one kind of the most promising soft magnetic materials. SMCs or powder cores will perform better at higher frequencies than the other metallic soft magnetic materials in the form of thin strip because the particles are separated by insulating layers so the eddy current effect can be inhibited a lot. The drawbacks of SMCs are the low permeability and high hysteresis loss. Soft ferrites have resistivities several orders of magnitude higher than metallic soft magnetic materials, as a result, they are for now the best choice for operating frequencies near or above 1 MHz, but their saturation flux densities are low. Some specialists believe that in some applications soft ferrites may be replaced by SMCs to reduce the size and mass of the high-frequency devices if the processing technology for SMCs can be improved.
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Nickel is a versatile element and will alloy with most metals. Complete solid solubility exists between nickel and copper. Wide solubility ranges between iron, chromium, and nickel make possible many alloy combinations.
Soft magnetic materials are those materials that are easily magnetised and demagnetised. They typically have intrinsic coercivity less than 1000 Am-1. They are used primarily to enhance and/or channel the flux produced by an electric current. The main parameter, often used as a figure of merit for soft magnetic materials, is the relative permeability ( mr, where mr = B/moH), which is a measure of how readily the material responds to the applied magnetic field. The other main parameters of interest are the coercivity, the saturation magnetisation and the electrical conductivity.
The types of applications for soft magnetic materials fall into two main categories: AC and DC. In DC applications the material is magnetised in order to perform an operation and then demagnetised at the conclusion of the operation, e.g. an electromagnet on a crane at a scrap yard will be switched on to attract the scrap steel and then switched off to drop the steel. In AC applications the material will be continuously cycled from being magnetised in one direction to the other, throughout the period of operation, e.g. a power supply transformer. A high permeability will be desirable for each type of application but the significance of the other properties varies.
For DC applications the main consideration for material selection is most likely to be the permeability. This would be the case, for example, in shielding applications where the flux must be channelled through the material. Where the material is used to generate a magnetic field or to create a force then the saturation magnetisation may also be significant.
For AC applications the important consideration is how much energy is lost in the system as the material is cycled around its hysteresis loop. The energy loss can originate from three different sources: 1. hysteresis loss, which is related to the area contained within the hysteresis loop; 2. eddy current loss, which is related to the generation of electric currents in the magnetic material and the associated resistive losses and 3. anomalous loss, which is related to the movement of domain walls within the material. Hysteresis losses can be reduced by the reduction of the intrinsic coercivity, with a consequent reduction in the area contained within the hysteresis loop. Eddy current losses can be reduced by decreasing the electrical conductivity of the material and by laminating the material, which has an influence on overall conductivity and is important because of skin effects at higher frequency. Finally, the anomalous losses can be reduced by having a completely homogeneous material, within which there will be no hindrance to the motion of domain walls.
These alloys are used for transformer cores and are known as electrical steels. In the power industry electrical voltage is almost always AC and at low frequency, 50- 60Hz. At these frequencies eddy currents are generated in the transformer core. Alloying the Fe with Si has a large marked effect on the electric resistivity of the material, with an increase of a factor of 4 for 3wt%Si. Silicon also has the benefit of reducing the magnetostriction (i.e. length change on magnetisation) and the magnetocrystalline anisotropy. In addition, the material is used in the form of laminations, typically 0.3 to 0.7mm thick. The addition of too much silicon makes the material extremely brittle and difficult to produce, giving a practical limitation of 4wt% to the amount of Si that can be added. Recently, a technique has been developed to produce laminations with >6wt% Si, by a SiCl4 chemical vapour deposition treatment to enrich the laminations with Si after forming the laminations. Typically most electrical steels will contain between 3 and 4 wt% Si.
For transformer applications the flux lies predominantly in the length of the laminations and therefore it is desirable to enhance the permeability in this direction. This is achieved by various hot and cold rolling stages to produce textured sheets, known as grain-oriented silicon-steel, with the [ 001] direction in the length of the lamination. The <001> type crystal directions are the easy directions of magnetisation and hence the permeability is greater. Figure 11, shows the anisotropy of Fe and illustrates the two types of texture that can be achieved, which are known as cube-on-edge and cube texture. Note that the cube texture has two < 001> type directions in the plane of the sheet and provides an advantage if E-shaped laminations are to be cut from the sheet.
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(a)
(b)
(c)
Figure 11: (a) The magnetocrystalline
anisotropy of Fe, (b) cube-on-edge texture and (c) cube texture in grain
oriented
silicon steel.
These materials can be produced in the form of a tape by melt-spinning. The alloys consist of iron, nickel and/or cobalt with one or more of the following elements: boron, carbon, phosphorous and silicon. They have extremely low coercivity, an order of magnitude less than standard Fe-Si, and consequently lower hysteresis losses. However, they have relatively low magnetisation and are not suitable for high current applications. They do find a market in low current applications and specialised small devices where they can compete with Ni-Fe.
Instead of casting the alloy onto a rotating wheel to produce tapes it is also possible to squirt a stream of molten alloy into a bath of water or oil to produce amorphous wires of typically 50mm thick. These wires show a very square hysteresis loop with large changes in magnetisation at low field, making them ideal for sensing and switching.
Recently there has been much interest in nano-crystalline material, which is produced by annealing the amorphous material. These alloys can be single phase but are usually comprised of nano-sized grains, in the range 10-50nm, in an amorphous matrix. They have relatively high resistivity, low anisotropy and good mechanical strength.
These alloys, known as permalloy, are extremely versatile and are used over a wide range of compositions, from 30 to 80wt%Ni. Over this composition range the properties vary and the optimum composition must be selected for a particular application. The high Ni content alloys have high permeability; around 50wt%Ni have high saturation magnetisation and low Ni content have a high electrical resistance.
There are special grades of Ni-Fe alloys that have zero magnetostriction and zero magnetic anisotropy, such as mumetal which is produced by a careful heat treatment and minor additions of Cu and Cr. These alloys have extremely high permeable, up to 300000 and intrinsic coercivity as low as 0.4Am-1.
At high frequency metallic soft magnetic materials simply cannot be used due to the eddy current losses. Therefore, soft ferrites, which are ceramic insulators, become the most desirable material. These materials are ferrimagnetic with a cubic crystal structure and the general composition MO.Fe2O3, where M is a transition metal such as nickel, manganese or zinc.
MnZn ferrite, sold commercially as ferroxcube, can be used at frequencies up to 10MHz, for example in telephone signal transmitters and receivers and in switch mode power supplies (also referred to as DC-DC converters). For these type of application the driving force to increase frequency is to allow miniaturisation.
Additionally, part of the family of soft ferrites, are the microwave ferrites, e.g. yttrium iron garnet. These ferrites are used in the frequency range from 100MHz to 500GHz, for waveguides for electromagnetic radiation and in microwave devices such as phase shifters.
AC applications relate to electric circuits and primarily in transformers converting one AC voltage into another. Power transmission is more efficient at high voltage, but more dangerous and less easily used in the home. Therefore, step-up transformers are used to increase the voltage for transmission and step-down transformers are used to reduce the voltage before entering the home.
The smallest type of transformer is a DC-DC converter, also known as a switch mode power supply. These are often mounted on a chip that can be put onto a circuit board. They take a DC voltage input, oscillate the voltage to an AC signal, which then goes through a coil around a toroidal core, a pick-up coil picks up the signal from the core and rectifies it to the output voltage. The relative number of turns on the primary (input) and secondary (output) coils determines the difference in voltage between input and output.
Soft magnetic materials also play an important role in electric motors where they enhance the field produced by the motor windings. In permanent magnet motors they are also used to channel the flux produced by the permanent magnets.
One of the main DC applications is in the field of magnetic shielding. A high permeability magnetic material is used to encapsulate the device that requires shielding. Figure 12 illustrates a simple example of magnetic shield where a tube/sphere of high permeability material channels the magnetic field away from the inside of the tube/sphere. The effectiveness of the shield can be expressed in terms of the shielding factor, S, where S relates to the field outside (Bo) and inside (Bi) the shield by equation 9.
Equ .9
For a sphere, S can be
calculated by equation 10, where mr = relative permeability, d =
thickness of wall and D =
diameter.
Equ .10
It is clear
from equation 10 that the higher the
permeability then the better the material will be at shielding. It is also
apparent that as the shield gets larger then either the permeability of the
material or the wall thickness must increase to compensate.
Soft magnetic materials are also used for electromagnetic pole-pieces, to enhance the fields produced by the magnet. Solenoid switches also rely on soft magnetic materials to activate the switches. Most permanent magnet devices will use soft magnetic materials to channel flux lines or provide a return path for magnetic fields, e.g. MRI body scanners have large permanent magnets with a yoke of soft magnetic material to prevent self demagnetising fields that would reduce the field in the gap of the scanner.
At high frequency metallic soft magnetic materials simply cannot be used due to the eddy current losses. Therefore, soft ferrites, which are ceramic insulators, become the most desirable material. These materials are ferrimagnetic with a cubic crystal structure and the general composition MO.Fe2O3, where M is a transition metal such as nickel, manganese or zinc.
MnZn ferrite, sold commercially as ferroxcube, can be used at frequencies up to 10MHz, for example in telephone signal transmitters and receivers and in switch mode power supplies (also referred to as DC-DC converters). For these type of application the driving force to increase frequency is to allow miniaturisation.
Additionally, part of the family of soft ferrites, are the microwave ferrites, e.g. yttrium iron garnet. These ferrites are used in the frequency range from 100MHz to 500GHz, for waveguides for electromagnetic radiation and in microwave devices such as phase shifters.
Figure 12: Magnetic shielding by a tube/sphere of soft magnetic material.
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