There are several kinds of soft magnetic materials. As we have seen in Inside 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 1Key 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.
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.
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.
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.
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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