Current Status and Prospects of Soft Magnetic Metal Powder Core Materials

Published: 2023-06-06 Views: 次

By chemical composition, soft magnetic materials fall into two categories: metallic soft magnetic materials and ferrite soft magnetic materials. Before ferrites emerged, soft magnetic materials were metals and their alloys—pure iron, permalloy, Fe-Si alloys, Fe-Si-Al alloys, and so on. These offer high permeability (μ) and high saturation flux density (Bs), but their low resistivity (about 10⁻⁶–10⁻⁹ Ω·cm) makes them unusable at high frequencies, where eddy-current losses rise with frequency. If iron powder is uniformly mixed with an insulating medium and pressed into a core, the resulting resistivity—thanks to the insulator—is far higher than that of the metal or alloy, greatly reducing high-frequency eddy-current losses; moreover, the core's high Bs allows use at high power. Metal powder cores are precisely this: a new class of soft magnetic material with well-balanced overall performance, made by pressing powder of metallic or alloy soft magnetic material through a special process. They retain some of the excellent characteristics of both metallic and ferrite soft magnetic materials while overcoming certain of their drawbacks to the greatest extent possible, giving metal powder cores low permeability but good linearity, relatively high saturation flux density, and a wide operating frequency range—of great significance as electronic products move toward higher precision, greater sensitivity, larger capacity, and miniaturization. Because the raw material was iron powder, these cores were originally called iron powder cores.

As science and technology advanced, ferrites—including soft ferrites—appeared in the 1940s. Their greatest advantage is high permeability and high resistivity (10²–10⁶ Ω·cm) at high frequencies, so they once replaced iron powder cores; but their Bs is far lower than that of metals, so they can only be used at low power. Iron powder cores then revealed their own unique role, filling precisely the gap between metallic soft magnetic materials and ferrites: in operating frequency from low to high, the order is metal—powder core—ferrite; in power handling from low to high, it is ferrite—powder core—metal.

Because applications demanded it, metal alloy powders other than iron powder came into use, so "iron powder core" was no longer an adequate name. Japanese magnetics circles call them pressed powder cores; in China they are commonly called powder cores. By performance, today's powder cores fall into several types: low-loss, high-Bs, low-cost, and low-loss/high-Bs combinations. By composition and application, metal powder cores still dominate; although amorphous and nanocrystalline amorphous powder cores have appeared, they still belong to the metal family. The commonly used types remain carbonyl iron, Fe-Si-Al, and permalloy; among the faster-growing ones are Fe-based and Co-based amorphous alloy powder cores.

2 History of Metal Powder Cores

2.1 Overseas

    Metal powder cores have a long history. As early as 1921, C. W. Elmen and colleagues at Western Electric in the U.S. first succeeded in pressing electrolytic iron powder into cores for telephone loading coils, though these cores had high losses and low Q. In 1923 they invented high-μ permalloy, and in 1927 produced the first permalloy powder cores. In 1940, F. J. Given and colleagues at Bell Labs developed molybdenum permalloy powder cores; with 2–4% molybdenum added, these cores offered high permeability and resistivity, good time stability, a low temperature coefficient, and low loss, and attracted much attention. Meanwhile, Siemens in Germany developed carbonyl iron powder cores in 1935, and Yamamoto and colleagues at the Institute for Materials Research, Tohoku University, in Japan developed and commercialized Fe-Si-Al (Sendust) powder cores.

    After molybdenum permalloy powder cores were developed in 1940, their processing technology was continuously refined and matured; production became standardized and seriesized, and commercial manufacturing was achieved. At the time, U.S. output of such cores alone reached 20 million pieces a year. Besides Western Electric, Butler Magnetic Materials and others in the U.S. also produced Mo-permalloy powder cores. In addition, Standard Telephones and Cables in the UK and relevant manufacturers in the former Soviet Union produced powder cores, and they were widely used in the 1950s. After World War II, the rapid development of ferrites replaced powder cores in general applications. But because ferrites' low Bs limits them to low power, in the early 1980s Japan developed a new core (the HP core) for high-frequency rectifier power supply chokes; its raw materials are inexpensive, and its performance rivals Mo-permalloy cores. At the same time, the U.S. developed amorphous alloy powders; in 1984, D. Raybould of Allied Corporation pressed amorphous powder cores from Fe79B16Si5 (Allied's grade, Metglas 2605S3) amorphous powder, injecting new vitality into the redevelopment and application of powder cores.

Few companies overseas produce powder cores; the main ones are:

MAGNETICS (U.S.): mainly high flux, Sendust, and MPP; does not make iron powder cores; output and technical level are world-leading. CSC (Korea): products similar to MAGNETICS. ARNOLD (U.S.): products similar to MAGNETICS. DONGBU (Korea): products similar to MAGNETICS, at a lower technical level. MMG (UK): specializes in Fe-Si alloy powder cores. MICROMETALS (U.S.): specializes in iron powder cores. Russia produces Fe-Ni-Mo products of very high technical level, with overall specifications exceeding MAGNETICS', but only for dedicated domestic use.

2.2 Domestic (China)

    In the past decade, China's metal powder cores have also developed rapidly. In particular, with the rapid growth of green energy, many Chinese companies have begun R&D on metal powder cores; product quality and production scale have greatly improved, advancing China's metal powder core sector and narrowing the gap with international levels.

China's R&D on metal powder cores started relatively late. Around the mid-1960s, the Central Iron and Steel Research Institute (CISRI) under the Ministry of Metallurgy, the Shanghai Iron and Steel Research Institute, the Wuhan Iron and Steel Research Institute, and Beijing No. 3 Radio Factory (Factory 798) successively began research on Mo-permalloy powder cores. Extensive experimental research was carried out on powder preparation, insulating binder coating, pressing, and core heat treatment, establishing a production process with Chinese characteristics. For example, instead of the overseas flake-rolling method, powder was made by grinding cast ingots, and the resulting cores matched the performance of American flake-powder cores. While continuously improving mechanical crushing, atomization powder-making technology was also developed. By adding a small amount of negative-temperature-coefficient compensating alloy powder to the master alloy powder, Mo-permalloy powder cores with a low temperature coefficient were developed. China was the first in the world to develop medium-Ni (50Ni) powder cores with high saturation flux density; it also developed high-Q (>80) powder cores for aircraft altimeter resonant filters, high-stability constant-permeability powder cores for switching power supplies, high-frequency powder cores for deflection-ring radio interference suppression filters, FeNi50 powder cores for high current and high frequency, and Ni80Mo4 high-frequency powder cores made from atomized powder. In 1985, China took the lead in establishing a national standard for Ni-Fe powder cores (GBn251-85). The standard lists nine grades (classified by effective permeability) of powder cores in two alloys, Ni81Mo2 and Ni50Fe50, and aligns as closely as possible with international standards—showing that China's powder core production had embarked on the path of seriesization and standardization.

    The Shanghai Iron and Steel Research Institute was the first in China to apply pattern recognition and regression analysis to study the influence of process parameters on powder core performance, and used the results to guide production, obtaining powder cores with high permeability, high quality factor, and high stability whose performance reached the advanced level of American samples. CISRI developed high-constant-permeability, high-Q (Q≥700), low-temperature-coefficient cores for reverberation suppression filters and double-tuned bandpass filters. The performance and specifications of these Mo-permalloy cores have reached the level of foreign products, and amorphous powder cores were also developed. The Shanghai Iron and Steel Research Institute developed the high-frequency powder core Fe47Ni29V2Si8B14; over the wide range of 1 kHz–1 MHz, its inductance varies by only 0.77%, its Q at 100 kHz is 8 times that of Mo-permalloy powder cores, and its constant-permeability upper limit has reached 7960 A/m, approaching advanced international levels.

At the end of the last century, Taiwanese businesses entered this industry and set up plants in mainland China, mainly Zhuhai Keda, Dongguan Jiacheng, and Jiuci.

As iron powder cores entered an era of thin margins, and given their relatively high loss and temperature rise, the 21st century brought higher requirements for high frequency, high power, and EMC in inverter circuits, along with demand from photovoltaics and wind power. Combined with a deeper understanding of metal powder core materials, alloy powder cores drew attention from many manufacturers, and their industrialization has developed rapidly in recent years.

At present, there are estimated to be more than forty metal powder core material manufacturers of various sizes in China, mainly concentrated in South and East China. The quality of ordinary grades is basically on par with foreign levels, and product standards are all based on Micrometals' products. Only five are listed companies: Hengdian Group DMEGC Magnetics, Beijing Qixing Flight Electronics, Advanced Technology & Materials (AT&M), Tiantong Holdings, and Jien Nickel. Those with notable technical advantages include Beijing Qixing Flight Electronics, Hengdian DMEGC, Zhejiang Keda Magnetoelectricity, and the earlier Wuhan Haoyuan Magnetic Materials; some of their products have reached international standards. New entrants have also appeared, such as Shenzhen POCO, Dongmu Keda, Anhui Shouwen (invested by Shougang), Hangzhou Bofeng, Deqing Aozhen, Jiangxi Dayou, Foshan Zhongyan, Yiyang Xinjiyuan, Jiangxi Dayou Technology, Qingdao Yunlu New Energy, and Huzhou Tianjing Electronics. According to incomplete statistics, at the beginning of this century China's metal powder core production was a few hundred tons per year, of which over 95% was iron powder cores. Since then, production has grown at roughly 20% per year. However, domestically produced metal powder cores account for only a small part of market demand; the vast majority still relies on imports. Beijing Qixing Flight Electronics produces high flux, Sendust, MPP, and iron powder cores—a complete range—with some materials at world-leading levels, sold across most of the domestic market. At the start of the new century, Beijing Qixing Flight Electronics successfully developed a new high-permeability 125 Sendust material and put it into volume production, reaching advanced international levels. In particular, in 2006, by optimizing the production process, it further raised the technical level of its Sendust series; its power loss is 10%–30% lower than that of equivalent products from MAGNETICS, the world's most advanced, putting it at the international forefront. Hengdian Group DMEGC Magnetics can mass-produce products such as MPP DM572160 and Sendust DS572125. Zhejiang Keda Magnetoelectricity (KDM) began R&D and large-scale production of iron powder cores right from its founding in September 2000; after more than 10 years of development, product performance and output have greatly improved. The company now has a full alloy powder core product line—Sendust Cores, Si-Fe Cores, High Flux Cores, MPP Cores, and Neu Flux Cores—with annual output of 4,000 tons and performance comparable to Micrometals, giving it strong market competitiveness. The company has developed a high-temperature-resistant iron powder core (HTC200 Iron Powder Core), currently available in -75 and -76 materials, capable of withstanding 200°C—a product with an excellent price-performance ratio.

At the start of the new century, Beijing Qixing Flight Electronics and the Standardization Institute of the Ministry of Information Industry jointly drafted the industry general specification SJ20829-2002 "General Specification for Metal Powder Cores," laying the foundation for the development of metal powder cores.

2.3 New Developments in Metal Powder Cores

    To meet the needs of magnetic devices moving toward miniaturization, intelligence, high integration, and SMT, inductive devices must have excellent dynamic performance and low AC loss at high frequencies. Especially in modern communications equipment, inductors must have high and constant permeability over a wide frequency range—that is, high frequency, wide constant permeability, and high Q. To this end, in recent years researchers around the world have done extensive R&D on metal powder cores, amorphous, ultrafine-crystalline, nanocrystalline, and composite powder cores, with considerable progress. For example, as early as the mid-1980s, D. Raybould and colleagues made amorphous powder cores by simple pressing of Metglas 2605-S3 (Fe79B16Si5) amorphous powder, achieving μ=30 at 10 kHz and Bm=0.1 T, and μ as high as 60 with alternating pressing; in the late 1980s, the Shanghai Iron and Steel Research Institute first mechanically pressed Fe47Ni29V2Si8B14 powder into amorphous powder cores whose Q at 100 kHz was 8 times that of 80NiFeMo. Thanks to materials advances, ultrafine-crystalline powder cores were then developed. Good progress has been made in preparing and pulverizing amorphous and ultrafine-crystalline powders, powder coating and pressing, and controlling the grain size and crystallization fraction of ultrafine-crystalline powder. Roba Ellen Ultrafine Powder Company in the U.S. developed 50NiFe, 50CoFe, FeSiAl, and FeSi fine powders smaller than 1 μm; Sugayg and colleagues in Japan used oxide films as insulating layers and added small amounts of Si and Al to pure iron to make 10–20 μm fine powders, providing the prerequisite for ultrafine powder cores.

At present, few manufacturers can truly mass-produce and market amorphous powder cores; the technology is mainly held by Japan and Korea—Hitachi Metals and Furukawa (FAA series) in Japan, and AMOSENSE (APH series) in Korea. AMOSENSE's APH series in particular has been widely adopted, reportedly reaching 2KK per month, and the company has also launched the APM series of nanocrystalline powder cores, overcoming the poor strength and noise problems of earlier amorphous powder cores while further reducing loss to MPP standards. In recent years, some domestic manufacturers such as Beijing AT&M and Jiangxi Dayou Technology have also made attempts in this area, but no true volume marketing has been seen. Zhejiang Keda Magnetoelectricity (KDM) successfully developed a new-generation nanocrystalline powder core product—Nanodust—in November 2011. This material features low loss, high Bs, and low cost. It currently has two series, KAH and KAM, both now in volume production.

Although amorphous and ultrafine-crystalline powder cores have excellent magnetic properties, their temperature stability is inferior to that of metal powder cores, and their manufacturing process is still relatively complex; most companies cannot yet mass-produce them, and they remain in an early stage of development. Although many problems remain to be solved, their prospects are self-evident.

3 Process Technology for Metal Powder Cores

The general process flow for metal powder core production is as follows:
Alloy melting → powder making → powder classification → powder treatment → batching → insulation coating → pressing → sintering and treatment → performance testing → impregnation → drying → coating → acceptance → packaging and warehousing

In metal powder core production, two methods are commonly used to make powder. One is atomization, producing spherical or near-spherical powder. The main process factors to control here are the alloy's melt flow rate, the atomizing medium and its pressure, and the cooling medium. The other method uses a hammer mill. This produces flake-shaped powder. It is carried out under inert gas protection, with the inert gas flow carrying the ground powder into a collection bin.

The purpose of powder treatment is to further purify the alloy powder, make the alloy's crystalline state as perfect as possible or restore to some degree the grains damaged during powder making, and eliminate stresses generated during powder making.

Batching and insulation are critical steps in preparing metal powder cores. Based on the performance requirements of the core, the batching process is first determined. The composition and ratio of the alloy powder, the particle size distribution and composition of the alloy powder, and the ratios of each component must all be correctly selected and controlled. Insulation means uniformly coating the surface of each metal powder particle with a layer of insulating film.

The insulated magnetic powder is then press-formed into cores; the pressed cores must have specified dimensions, density, and strength.

Sintering is the most critical step in powder core production and the one most sensitive to process factors. It can be carried out in air or under hydrogen or inert gas protection.

The production process for metal powder cores is relatively complex, with many process factors that directly or indirectly affect core performance—some even decisively. Therefore, in R&D and production, the influence of each process factor must be strictly and correctly controlled. Particular attention must be paid to controlling the main factors; by reasonably controlling the various process conditions in powder core production, cores with optimal performance can be produced. This is also the most critical part of powder core production.

4 Properties and Classification of Metal Powder Cores

4.1 Classification of Metal Powder Cores

Metal powder cores mainly comprise six material categories: iron powder cores, Sendust (Fe-Si-Al) cores, 50Ni-Fe high flux cores, MPP (Fe-Ni-Mo) cores, Fe-Si cores, and amorphous/nanocrystalline soft magnetic powder cores. Except for iron powder cores, the others are generally called alloy powder cores.

4.2 Properties of Metal Powder Cores

The magnetic properties of several common metal powder cores are shown in Table 1.

Table 1  Magnetic properties of several common metal powder cores

Note: DC bias characteristics and loss are measured on materials with permeability 60.

Table 1 shows the following properties and features of metal powder cores:

(1) High saturation flux density (magnetic induction). Metal powder cores have a saturation flux density of 0.75–1.6 T, versus about 0.4 T for ferrite cores, so metal powder cores can be used in high-power signal electronic equipment.

(2) Good magnetic stability. Metal powder cores have good frequency, temperature, and time stability. Metal powder cores with μi of 125 have good frequency stability from 100 to 200 kHz; cores with μi of 60 work from 300 to 500 kHz; above 500 kHz, cores with μi of 26 or below are used. The temperature coefficient of permeability is less than 500×10⁻⁶/℃, and in demanding cases can be as low as below 100×10⁻⁶/℃, giving high reliability in harsh environments. At the same time, metal powder cores undergo special aging treatment during production, giving them good performance stability.

(3) Relatively high initial permeability. MPP cores in particular can have μi as high as 550.

(4) Metal powder cores have a unique structure: the magnetic flux paths are confined within the material, with no electromagnetic signal leakage, giving excellent EMC characteristics.

(5) Simple manufacturing process—no high-temperature sintering required, precise dimensions, energy- and labor-saving.

Among alloy powder cores, Sendust cores are the most commonly used. Sendust raw materials contain no precious metals, so the cost is lower than that of nickel-containing high flux and MPP cores. With relatively low price, good high-frequency magnetic properties, good temperature stability, wide constant permeability, low loss, near-zero magnetostriction, and low cost, they are ideal for power factor correction circuits and unidirectional driver applications such as pulse transformers and flyback transformers, and are the best choice for eliminating audible-frequency noise in line noise filters and inductors—hence their particularly rapid growth. Sendust currently has global sales of about US$200 million.

Among other alloy powder cores, high flux cores, with their excellent properties, are very common in high-end applications. Their main feature is a saturation flux density of about 15,000 Gs, the highest DC bias capability among powder cores; they are mainly used in line filters, AC inductors, output inductors, and power factor correction circuits, and are often used in DC circuits with high DC bias, high DC current, and low AC current.

5 Applications of Metal Powder Cores

In terms of operating frequency and power, metal powder cores sit between metals and ferrites, filling the gap between them. They are therefore mainly used as main transformers and high-frequency chokes in switching power supplies, and also in high-power stage and studio lighting dimmers, home appliances, industrial equipment, office equipment, and more, for current limiting, voltage transformation, and matching. Their high Bs (1.5 T) means DC output rectifier filter chokes are not easily saturated under high current, and they have good constant-permeability characteristics—that is, DC magnetic fields have little effect on μ. Their special magnetic properties give them advantages in many applications that other materials cannot match, and today this material has become an important part of soft magnetic materials.

Metal powder core materials are widely used in power equipment and high-performance electronic functional devices. Filters, inductors, transformers, and chokes made from metal powder cores are used in various EMC systems and electronic products. In addition, metal powder cores are widely used in many important industrial fields, including aircraft manufacturing, shipbuilding, automotive manufacturing, computers, air conditioners, color TVs and other home appliances, and automatic door control. In national defense and cutting-edge technology, permalloy-based powder cores, though expensive, are widely used as energy storage components because of their excellent overall magnetic properties. In recent years, with the depletion of traditional energy sources and growing environmental awareness, green energy has received increasing attention. Green energy mainly refers to wind, hydro, and solar power; apart from solar, which is widely distributed globally, other green energy sources are limited by conditions and cannot be widely promoted. Solar photovoltaic power generation is therefore considered the most effective and promising solution to the energy crisis. A PV system mainly consists of PV cells, a boost circuit, and an inverter. Fe-Si powder cores, with excellent magnetic properties such as high permeability, low hysteresis loss, high saturation flux density, and outstanding DC bias characteristics, are widely used in inverters as isolation transformers and high-energy-storage power inductors.

5.1 Applications in Switching Power Supplies

    Switching power supplies are widely used in modern electronic information technology and equipment such as computers, electronic instruments, and communication systems, and have become important and key components. Because they are light, thin, small, and efficient, they have gradually replaced conventional continuous (analog) power supplies. In their development, both related electronic technologies and new power materials and components have been developed, driving switching power supply technology to grow at double-digit rates toward high frequency, low loss, high reliability, high current, high power, and interference immunity. A switching power supply may use as many as six transformers and chokes, such as input differential-mode filter inductors, power factor correction inductors (>500 W), and DC output rectifier filter chokes.

Powder cores are key components of high-frequency, high-current, high-power inductive devices. They are widely used in switching power supplies for DC output rectifier filter chokes, and even more widely in TDK's SF coils for dimmers, home appliances, and office equipment to suppress noise in power conversion input/output devices, with a good price-performance ratio. At present, the widely used types are mainly made of carbonyl iron, iron powder, Fe-Ni, and Fe-Ni-Mo. In China, the transformers, chokes, and inductors used in designing and building various switching power supplies are basically all Fe-Ni and Fe-Ni-Mo pressed powder cores. Because their Ni content is as high as 50%–80% and Mo is also a rare metal, costs and prices are high, limiting their application and failing to meet the needs of rapidly developing electronic information technology.

5.2 Applications at High Power

      ① Filters  A certain radio station used a π-section filter with a μi=10, φ7.8×3.98×3.2 (mm) carbonyl