Soft magnetic material evaluation standard decryption

- Nov 07, 2017 -

In general, when we first learned about magnetic components, we learned that a soft magnetic material refers to a magnetic material that has a small coercivity and is easy to magnetize. Made of soft magnetic materials mainly electromagnetic components, is one of the main components of the power supply.

Electromagnetic components on the choice of soft magnetic materials, according to different use characteristics, have different requirements. But one thing in common is that it requires a low loss of soft magnetic material. Regardless of the kind of electromagnetic components selected soft magnetic materials, the loss as a major indicator. The loss of soft magnetic materials including eddy current loss, hysteresis loss and residual loss, in addition to the material's resistivity, width and thickness of the material itself parameters, but also with the magnetic flux changes in the speed of electromagnetic components, that is, with the work The frequency f and the work flux Bm product. Therefore, the parameter representing the loss of soft magnetic material - loss per unit weight (or volume) P, shall indicate the working frequency f and the working magnetic flux density Bm, generally written as PBm / f. Bm units with T or 0.1T (kGs), f units with Hz or kHz. In more than 1MHz high frequency, due to the limitations of the test power supply, not directly measured loss, and measured permeability μ, but also indicate the test frequency f and the magnetic field strength H. Transformer is the first important electromagnetic component in power supply, which is of special concern for the loss of soft magnetic materials. Therefore, low loss is the main pursuit of the development of soft magnetic materials.

This article introduces the main soft magnetic materials used in the power supply, and some analysis.

Silicon steel

Silicon steel is the earliest use of power soft magnetic material, it has good stability, environmental adaptability, high magnetic flux density, low cost, suitable for mass production, and small differences in performance between batches in the frequency and frequency range The largest use of soft magnetic materials. Now silicon steel, after nearly 80 years of development, performance has been greatly improved, the scope of its use has been extended to 20kHz above, up to 200kHz ~ 325kHz. Therefore, silicon steel can no longer be excluded from the soft magnetic materials used for high frequency power supply.

Since the 1920s, silicon steel has been produced by the hot rolling process. From the 50's onwards, gradually shifted to cold-rolled production process. After heat treatment, the non-orientation of the silicon steel grains from the non-directional arrangement is changed into oriented silicon steel with the grain orientation. In the 1960s, the highly-integrated oriented HI-B texture technology was invented. The saturation magnetic flux density Bs increased and the loss decreased. The typical parameters of the currently produced 0.30mm thick HI-B textured oriented silicon steel are 2.03T for Bs, 0.60W / kg for P1.3T / 50Hz and 1.02W / kg for P1.7T / 50Hz. China's production of silicon steel is only the general orientation of cold-rolled silicon steel, individual can reach HI-B texture oriented silicon steel level.

Reduce the thickness of silicon steel, eddy current loss can be reduced. The earliest production of silicon steel strip thickness of 0.50mm, after the gradual decline to the 50's 0.35mm, has now dropped to 0.30mm and 0.23mm. From 0.30mm down to 0.23mm, the loss of oriented silicon steel P1.7T / 50Hz can drop 0.15W / kg. Thinner silicon steel strip thickness has been reduced to 0.10mm, 0.08mm, 0.05mm. However, as strip thickness decreases, the saturation flux density decreases. In order to overcome this shortcoming, a new recrystallization process was developed three times in the late 1980s, and the thickness of the ultra-thin ribbon silicon steel manufactured can reach 0.005 mm (5 μm). From the data listed in Table 1, it can be seen that silicon steel with thicknesses of 81 μm and 32 μm has a significantly lower loss than that of 300 μm-oriented silicon steel, approaching and exceeding those of a 20 μm to 40 μm thick iron-based amorphous alloy strip heat-treated by a magnetic field Level. However, this ultra-thin silicon steel with complex processes, high cost. China's production of 0.10mm thick silicon steel prices have been 20μm ~ 40μm thick iron-based amorphous alloy strip more than 2 times, not to mention ultra-thin silicon steel below 0.08mm, and only in the pursuit of small size loss is also a small power transformer And reactor before use.

Increasing the silicon content in the silicon steel can decrease the iron loss, has the best characteristics when the silicon content increases to 6.5%, the magnetostriction approaches zero, the magnetic permeability is higher than the non-oriented 3% silicon steel, and the loss is small. However, as the silicon content increases, the elongation decreases sharply. Therefore, the use of rolling production of silicon steel strip silicon content of 3.5% or less. The successful development of the early 90's chemical vapor deposition (CVD) production of 6.5% silicon steel manufacturing process, with 3% silicon steel strip as the raw material, heated to 1200 ℃ and SiCl4 gas reaction to form a high silicon layer, the gradual and even diffusion To the strip center, resulting in 6.5% silicon steel strip. Now able to produce 0.50mm ~ 0.05mm thick 6.5% silicon steel strip, the maximum width of 640mm. Its main features and 3% oriented silicon steel, non-oriented silicon steel in Table 2. The table also lists the data for Fe-based amorphous alloys and Mn-Zn ferrites. It can be seen from the table that the loss of power frequency and intermediate frequency electromagnetic components (50Hz ~ 20kHz) made of 6.5% silicon steel is smaller than that of 3% silicon steel and the audible noise is low due to the small magnetostriction coefficient λs. This is particularly important where noise reduction and environmental protection are important. What deserves to be happy is our country trial-and-succeeds this kind of low-loss low-noise 6.5% silicon steel strip.

Of particular interest is the use of chemical deposition to control the silicon content of the surface and center of the silicon steel strip, resulting in special properties such as high and medium frequency ultra-low loss silicon steel and low remanence silicon steel. High and medium frequency ultra-low loss silicon steel surface high silicon content, high permeability, magnetic flux concentration, the eddy current is also concentrated in the surface (plus the skin effect). However, the surface layer of silicon content higher than the central layer, a gradient distribution. This gradient distributed high content silicon steel (NKSuperHF) has lower losses than the uniformly distributed high content silicon steel (see Table 3) and can be used in power transformers and reactors below 20 kHz. Low remanence silicon steel is also obtained by controlling the silicon content of the surface layer and the center layer (brand NKSuperBR), Br is 0.35T, and Br of oriented silicon steel is 1.28T, so that ΔB from 0.4T can be increased to 1.2T, can be used for power supply In the one-way excitation pulse transformers and switching power transformers. Hope that our metallurgical workers in the successful trial of 6.5% based on the silicon steel, the early trial of this gradient distribution of silicon steel.

Further improvements to silicon steel have entered the domain range. Take the magnetic domain refinement process, you can make the loss P1.5T / 50Hz than the original 0.1W / kg. The magnetic domain refinement process includes mechanical scoring, pulsed laser irradiation, direct laser irradiation, electric spark grinding, plasma irradiation, serrated roll groove, electrolytic etching into a groove and the like. Japan uses magnetic domain refinement silicon steel manufacturing energy-saving power transformers, compared with China's use of oriented silicon steel S9-type power transformers no-load loss lower than 35%. If the transformer and DC high-power power rectifier transformer with silicon grain refinement of silicon steel, its energy-saving effect will be no less energy-efficient power transformers, it is noteworthy.

Soft ferrite

Since the 1940s, the soft ferrite has been widely used in medium and high frequency electromagnetic components due to its high resistivity, easy mass production, various shaped cores, stable performance and low cost. The use of soft magnetic materials, especially in household appliances occupy the absolute dominance. Due to the processing of large ferrite is not easy, and easy to break, so the use of power is limited. Due to the low saturation flux density, soft-ferrites are seldom used in power frequencies and in the intermediate frequencies below 1 kHz.

Absolute dominance. Due to the processing of large ferrite is not easy, and easy to break, so the use of power is limited. Due to the low saturation flux density, soft-ferrites are seldom used in power frequencies and in the intermediate frequencies below 1 kHz.

If the ferrite resistivity is considered to be high, it is concluded that the ferrite loss is lower than other soft magnetic materials in the intermediate frequency and high frequency ranges. Soft ferrite and other soft magnetic materials, its loss including hysteresis loss, eddy current loss and residual loss of three parts. The hysteresis loss and eddy current loss are related to the product of the working flux density Bm and the operating frequency f. When f rises, the loss does not increase rapidly and the Bm decreases accordingly. The eddy current loss is inversely proportional to the resistivity ρ, but ρ also varies with the operating frequency f. Below a certain limit operating frequency, ρ is relatively high; f exceeds the limit operating frequency, ρ drops sharply; and then ρ is essentially the same, but the values are quite low. The residual loss depends on the motion and resonance of the magnetic domain walls and can not be ignored.

A detailed study was made on the loss mechanism of a Mn-Zn ferrite with addition of CaO and SiO2 below 10 MHz, and detailed measurements and analyzes were carried out. At fBm of 25000kHzT, when f is less than 1.1MHz, the loss is determined by the hysteresis loss, inversely proportional to f, and gradually decreasing as f increases, reaching the lowest point at 1.1MHz with a power loss of 60kW / m3 At 0.06 W / cm3). More than 1.1MHz to 3MHz, the loss depends on the residual loss, rising rapidly with f. At more than 3MHz, the loss is determined by the eddy current loss, but then ρ has been quite low, the power loss at a high level of 200kW / m3, basically unchanged. The best working frequency of this Mn-Zn ferrite is about 1MHz, the limit operating frequency is about 3MHz.

IEC has released the classification standard for ferrite for power transformers. China has also released the same industry standards and classified PW1, PW2 and PW3 according to the operating frequency, the limit operating frequency, the working flux density and the loss at 100 ℃. PW4 and PW5 five categories (see Table 4). PW2 is equivalent to the first generation of high frequency soft ferrite developed in the 1970s. PW3 is equivalent to the second generation of high frequency soft ferrite developed in the early 1980s, such as Japan's TDK's PC30, China's R2KG, RM2KB2, R2KH. PW4 is equivalent to the third generation of high-frequency soft ferrite developed in the late 1980s, such as Japan's TDK's PC40, China's R2KB1, RM2KB3. PW5 is equivalent to the fourth generation of high-frequency soft ferrite developed since the mid-1990s. For example, Japan's TDK's PC50 and China's trial version of R1.4K have been successfully used in 750kHz switching power supplies. Most of China's ferrite magnets for power transformers are at PW3 and PW4. At the same time, the soft ferrite produced in China with high permeability inductors, μi is still below 1 × 104, while most foreign products are higher than 1 × 104.

Soft ferrite performance and temperature-related, so given its performance parameters must be marked when the temperature value. For example, there is a saturation magnetic flux density Bs of Mn-Zn ferrite that is 70% at 25 ° C at 100 ° C.

Meanwhile, the magnetostrictive coefficient of the soft ferrite is relatively large, and the electromagnetic element operating in the audio frequency range of 10 Hz to 20 kHz has a relatively large audible noise. Even for medium and high frequencies over 20 kHz, there is audible noise if there is an audio-frequency oscillating carrier.

High Permeability Alloy (Permalloy)

High permeability magnetic alloy is the initial permeability and maximum permeability of iron-nickel alloy, most of the trade name is called "Permalloy." In addition to high permeability, permalloy loss is relatively low, especially the environment is better, stable performance, although the price is expensive, but still used in the more stringent conditions of the power supply.

High permeability alloy category

Permalloy is the main type of iron-nickel alloy, nickel (35% ~ 85%), iron and added molybdenum, copper, tungsten and other components. In the 1940s has been basically stereotypes, to the 70's and 80's extensive use of the formation of dozens of models, generally based on how much to classify the nickel content. Nickel content of 30% to 50% for the low-nickel alloy, such as China's 1J30, 1J34, 1J50, 1J51 and so on. Nickel content between 65% ~ 85% for high nickel alloys, such as China's 1J66, 1J79, 1J80, 1J88 and so on. According to the needs of the power, has developed a variety of permalloy strip. A hysteresis loop is rectangular, non-rectangular, linear (constant permeability) material. Can be rolled into 0.20mm to 0.005mm (5μm) thickness of the various specifications. The permalloy of 0.20mm thickness is generally used in 50Hz, 0.005mm thick permalloy for 500kHz ~ 1MHz, covering the entire frequency range of power frequency, intermediate frequency to high frequency, and has long exceeded the old concept that can only be used below 20kHz .

Like silicon steel and soft ferrite, permalloy also develops rapidly in the past decade. One is the use of low nickel content of iron-nickel alloy elements such as chromium added to make it to achieve high permeability of the magnetic properties of nickel, thereby reducing costs. It has been reported Ni38Cr8Fe alloy, H = 0.4A / m magnetic permeability of 100000 ~ 300000, close to the high level of nickel alloy. Even more prominent are the recent years both at home and abroad have introduced a high initial permeability of 200000 ~ 300000, the maximum permeability 350,000 ~ 500000 Permalloy products. Another is to break permalloy ribbon manufacturing process, rolled into 0.01mm ~ 0.005mm thick strip, to expand the frequency range of applications. 0.005mm thick Ni80Mo5 permalloy ultrathin strip, loss of 0.126W / g at 500kHz at Bm of 0.1T, 0.392W / g at 1MHz, 6.79W / g at 5MHz, 23.1W / g. Can be used for power transformers above 1MHz.

Amorphous and microcrystalline alloys

Since the late 1960s and later, various amorphous alloy soft magnetic materials made by rapid solidification technology and various microcrystalline materials produced by re-annealing crystallization technology have been developed and become the research and development direction of contemporary soft magnetic materials for electromagnetic components .

Amorphous alloys do not form crystalline grain lattices and form an alloy like glass, hence the trade name "Metallic Glass." Now amorphous alloy soft magnetic material has three basic types:

(1) Iron-based amorphous alloy, the main component of iron boron boron, high saturation flux density, low frequency and IF loss, the price is cheap for the power frequency and IF power supply.

(2) cobalt-based amorphous alloy, the main component of cobalt iron silicon boron, high permeability, medium and high frequency loss is low, the price is expensive, mainly used in the field of high-frequency.

(3) Fe-Ni-based amorphous alloy, high initial permeability, up to 106, low loss under low frequency, can be used for detecting electromagnetic components in the power supply and leakage transformer.

Amorphous alloys can also be made rectangular, non-rectangular and linear hysteresis loop. Amorphous alloy strip thickness is generally 20μm ~ 40μm, can be made of 150μm ~ 250μm thick strip can also be made of 18μm ~ 3.5μm thick strip, can also be made of less than 1μm film. The application of amorphous alloy covers all kinds of electromagnetic components in the power supply from low frequency to high frequency and is the most promising soft magnetic material in the future.

In order to overcome the disadvantages of cobalt-based alloy with low saturation flux density and high price, Hitachi Company developed a microcrystalline alloy under the trade name "Finement" in 1988, adding a trace amount of copper and niobium to the iron-based amorphous alloy After proper heat treatment, it is partially crystallized to obtain a microcrystalline alloy having a grain size in the range of microns to nanometers. Also known as nanocrystals, the grain size is in the nanometer range. After using a similar process to create a variety of micro-alloy. Such as FeMB and FeZrNbCu microcrystalline alloys, under the trade designation "Nanoperm."

Amorphous and microcrystalline alloys have developed rapidly in the last decade, not only in materials and processes, but also in applications.

Iron-based amorphous alloy is mainly used in low-frequency electromagnetic components. It has achieved good results in the application of power distribution transformers, becoming now the largest production of amorphous alloys. Can be rectified to the power transformer, filter reactors and other electromagnetic components to expand. FeMB (M is Zr, Hf, Ta) and FeZrNbBCu microcrystalline alloy (Nanoperm alloy) developed in 1990 not only have low frequency loss, but also have high saturation magnetic density and small magnetostriction coefficient, which are used in power frequency electromagnetic components Soft magnetic materials in the performance of the ideal, in the field of low-frequency silicon steel and iron can replace the field, in the high-frequency areas can replace cobalt-based amorphous alloy and iron-nickel high permeability alloy. The FeCoZrBCu amorphous alloy (Hitperm) was developed in 1998 and the saturation magnetic flux density Bs is as high as 2.0T. It can replace FeCoV high permeability magnetic alloy and is the latest progress of soft magnetic materials for low frequency electromagnetic components. The performance of the amorphous and microcrystalline alloys used in the low-frequency fields described above is shown in Table 5. Table 5 Nanoperm alloy which also lists the loss in the high-frequency areas.

The preferred amorphous and microcrystalline alloys in the mid and high frequency fields are cobalt-based amorphous alloys and iron-based microcrystalline alloys. General (18 ~ 25) μm thick strip for thin bands of 100kHz, less than 18μm thick for 500kHz to 1MHz. Cobalt-based amorphous alloy 20μm thick ribbon, P0.2T / 100kHz only 30W / kg. We now see the best reported 3.8μm thick chromochip CoFeCrSiB amorphous alloy ribbon, P0.1T / 1MHz 140W / kg, P0.1T / 10MHz 1022W / kg, μe (1MHz) of 1 × 104. Table 6 lists the medium and high frequency with amorphous and micro-crystalline alloy performance, with the exception of Nanoperm-type micro-alloy has been listed in Table 5, is reported in the previous reports to see the lowest loss of medium and high frequency soft magnetic material.

The reported DC performance reported so far for the above soft magnetic materials is shown in Table 7. It can be seen that all kinds of soft magnetic materials have their own advantages, have their own can show the overall quality of applications, and in the low frequency, intermediate frequency and high frequency applications in the field of fierce competition, which also promoted a variety of Soft magnetic materials to move forward.

(1) The magnetic powder core belongs to a kind of core structure. A composite core made of several types of materials is not a soft magnetic material, so this article does not introduce it. Due to limited information collected, the soft magnetic materials presented in this paper belong to high dimensional (3D) materials, including three-dimensional core structure materials with a height of centimeter level and planar core structure materials with a height of millimeter (low height). Low-dimensional materials include films of two-dimensional materials, filaments of one-dimensional materials, and microparticles of zero-dimensional materials. Since one-dimensional nanoscale scales are called nanomaterials, most (except partial films) are classified as nanomaterials, and their physical properties are quite different from those of conventional non-nanomaterials. Nanomaterials now including soft magnetic nanomaterials are becoming a hot research topic in materials science. Examples of their applications in power sources are available and will be introduced later.

(2) All kinds of soft magnetic materials have their own advantages and disadvantages, have their own fields of application. Ideal soft magnetic material is only a pursuit of the goal.

(3) The choice of a soft magnetic material should be based on the requirements of the power supply, electromagnetic components should have the performance and conditions of use, that is, based on the actual operating frequency and magnetic flux density, considering the performance and price factors to decide. For the electromagnetic components used in power frequency and IF, more soft magnetic materials are available, and price is a major factor. For the electromagnetic components used in the high frequency field, there are few available soft magnetic materials, the price is not the main factor, but the weight, the volume and the loss are the main factors.

(4) There are many factors affecting the price of soft magnetic materials, not only based on the composition of the material, but also consider the complexity of the process in order to ultimately determine its cost. For example: 0.35mm ~ 0.30mm thick silicon steel strip than the price of iron-based amorphous alloy strip, but 0.10mm thick silicon steel strip has been higher than the price of iron-based amorphous strip, not to mention 0.025mm ~ 0.005 mm thick ultra-thin silicon steel strip. Another example: 20μm ~ 40μm thick cobalt-based amorphous alloy and microcrystalline alloy strip than the same thickness permalloy strip low price, but below 18μm thin strip and ultra-thin strip, Permalloy can be more Rolling, relatively easy, and cobalt-based amorphous alloy and microcrystalline alloy to spray out a uniform ribbon is quite difficult, so the price is higher than permalloy. For another example: soft ferrite due to mass production and low cost, in the field of high frequency accounted for the vast majority. Soft ferrite is now used in the trendy low-profile planar transformers and planar inductors. However, in recent years the rise of thin-film soft magnetic materials, not only easy to mass production, low cost, more importantly, good performance, the electromagnetic components to achieve a higher working frequency, more light and thin, higher cost performance. Therefore, the future of high-frequency electromagnetic components in the field of thin film transformers and thin film inductors world. This future is not a decade, a decade, but a few years. Nowadays the advantages of thin-film electromagnetic components have been used in mobile phones (cell phones) and personal computers in large-scale use. And because the semiconductor integrated circuit processing equipment can be easily changed to the production of thin film electromagnetic components processing equipment, the conditions are basically mature. More importantly, the market is pursuing the idea of low prices and high performance, so that such a replacement is only a question of time sooner or later.

(5) Due to the continuous upgrading of technology, we can not judge the soft magnetic materials of some kind at the previous level of cognition. We must constantly improve our understanding of developing soft magnetic materials, constantly receive new information and keep up The pace of technological development, the development of the use of updated soft magnetic materials electromagnetic components.

Article from NdFeB Industry Network

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