Soft Magnetic Materials: Applications

Soft Magnetic Materials: Applications High-performance soft magnetic materials are characterized primarily by their high permeability and low losses which lead to, for example, a rather low coercive force Hc. A high permeability level requires easy rotation of magnetization as well as easy domain wall motion resulting in the demand for low crystalline anisotropy energy K and low magnetostriction λs. Applications may "also require high saturation induction Bs or low eddy current losses at high frequencies requiring a high electrical resistivity. These different requirements limit the alloy selection and lead to material optimization depending on the respective application. Table 1 gives an overview of the basic material properties governing the magnetic behavior of the groups of alloys envisaged in this section (for a more detailed overview on basic material properties see Pfeifer and Radeloff 1980). Besides these basic magnetic properties a high degree of material purity is crucial with respect to soft magnetic properties. With crystalline alloys this is achieved best by vacuum induction melting. A final annealing treatment yields recrystallization and grain growth and thus enables low coercivity levels as Hc usually behaves inversely proportional to the grain size. A dry hydrogen atmosphere additionally leads to a further purification with respect to residual elements. In the case of residual sulfur, for example, this occurs via the formation of H S. After having performed the #

final annealing treatment stresses have to be avoided, as mechanical stress severely degrades magnetic performance. The final annealing treatment may also be used to form a certain recrystallization texture. In 50%-NiFealloys, for example, a high degree of cold work and a final annealing at not too high temperatures yields a very perfect cube-texture. The crystalline anisotropy in this case is responsible for the rather rectangular B–H-loop of such materials. During the 1980s and 1990s conventional crystalline materials were supplemented by amorphous and nanocrystalline alloys which take another approach. Due to their extremely small grain size or even vanishing grain microstructure there is no crystalline anisotropy, that is, K l 0. Additionally there is " no domain wall pinning from grain boundaries. Magnetostriction λs remains an important quantity governing the magnetic properties of these alloys. Depending on the respective application materials may be selected according to their basic magnetic parameters. High permeability\low loss applications at low frequencies are best met by NiFe-alloys due to the rather low crystalline anisotropy energy in this alloy system. Applications requiring high induction levels corresponding, e.g., to high forces in actuator systems, require the use of CoFe-alloys. Mass market applications, where costs are the most decisive factor, are governed by steels and SiFe-alloys and in case of required corrosion resistance by CrFe-steels. As eddy

Table 1 Basic material properties of high performance metallic soft magnetic materials.

Pure Fe (R Fe12) 3%-SiFe 13–17%CrFeb FeSiAl (Sendust) MnO . ZnO . (Fe O ) . c !$ !# # $ !& NiFe alloys Ni40Fe Ni50Fe Ni80Fe(CuMo) CoFe alloys 50%-CoFe 17%-CoFee Amorphous alloys FeSiB Co Fe (Si.B) (! & #& Nanocrystalline alloys Fe Cu Nb Si B f $ "' B( f Fe($Cu"(Nb,Zr) )' " ' (

Bs (T)

Tc (mC)

ρ (µΩ m)

K (J m"−$)

λs (ppm)

Hc (A m−")

µi (10$)a

µmax (10$)a

2.15 2.0 1.3–1.8 0.95 0.45

770 750

47 000 38 000

k9 8

12 20 50–200 2

2 1

500 120–220

0.1 0.4 0.55–0.8 0.90 200 000

40 20 "3 100

100

1

1.45 1.55 0.7–0.8

310 450 360–400

0.6 0.45 0.60

1,000 800 0d

25 25 0.5

2.35 2.22

950 920

0.4 0.4

8000 40 000

1.6 0.5–1.0

400 200–400

1.3 1.4

1.3 1.6

600

1.1 0.6

40 15

6 4 1

9 [200] 12 [200] 300 [70]

75 [200] 150 450 [70]

70 25

70 150

1

15 4

0 0

25–30 0.2

10 1

10 [25] 200 [25]

1 1

0.5 0.5

1 2

100 [25] 30 [25]

a Initial permeability µi and maximum permeability µmax are given as a static permeability or as 50 Hz dynamic permeability (in this case the tape thickness in µm is given additionally in square brackets). b Corrosion resistance is the main issue with this group of alloys; they may also be alloyed with additional elements like Mo—the typical range of properties is therefore given. c Data added for comparison. d Strong temperature dependence of K (Pfeifer and Radeloff 1980). e CoFeCrMoV-alloy. f Flat loop due to magnetic field annealing. "

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Soft Magnetic Materials: Applications current losses are proportional to d #\ρ amorphous or nanocrystalline alloys offer a superior solution for high frequency\low loss applications due to their low tape thickness of the order of 25 µm and their vanishing crystalline anisotropy energy. In this field cobalt-rich amorphous alloys offer very high permeability due to their very low magnetostriction close to zero. Iron-rich alloys on the other hand offer higher induction levels, their permeability level being limited due to their high magnetostriction. More recently developed nanocrystalline alloys offer high induction levels combined with a high permeability due to their vanishing anisotropy energy and magnetostriction (Herzer 1997, see Amorphous and Nanocrystalline Materials). Metallic soft magnetic materials cover a broad range of applications starting from d.c. conditions such as shielding of static magnetic fields up to frequencies of about 1 MHz (see Ferrites). Beyond 1 MHz ferrites as the classical high frequency material are mostly used. Amorphous and nanocrystalline alloys, however, have bridged part of the frequency gap between metallic materials and ferrites serving applications around 10–100 kHz, which has traditionally been reserved for ferrites. There are four major areas for application presented: (i) high-permeability applications; (ii) high-B applications; (iii) high-frequency applications; and (iv) sensor applications. In the following the most important fields of application of high-performance soft magnetic alloys will be discussed in more detail. We will not cover applications in electrical motors or 50\60 Hz power line transformers served with FeSi steel nor the specific applications of ferrites, since these topics are covered elsewhere (see Ferrites; Steels, Silicon Iron-based: Magnetic Properties)

1. High-permeability Applications 1.1 Cores for Ground Fault Circuit Breakers Ground fault circuit breakers for human protection must interrupt electric power for fault currents as low as 30 mA, both for sinusoidal currents as well as for pulsating currents. Due to this the transformer core for low fault currents without electronic amplification the transformer core must be very sensitive requiring highest material permeability µ. The ever increasing demand for sensitivity and additionally cost savings by reducing the core weight has led to the development of 80%-NiFe-(Cu,Mo)alloys with extremely high initial permeabilities of up to more than 300 000 at the operating frequency of 50 Hz. To achieve an improved sensitivity for nonsinusoidal currents a flat hysteresis loop is required which can be achieved via an appropriate magnetic field tempering. Recently developed nanocrystalline alloys based on rapid quenching technology offer 2

superior properties as a flat loop can be obtained with higher permeability and even more importantly with higher saturation induction.

1.2 Cores for Current Transformation—Electronic Watt-hour Meters, Current Sensors, Modem Transformers, etc. Common to all these applications is the demand for a flat, linear B–H-loop. In most cases this is achieved by laminating high-permeability alloys. The demagnetization effects introduced by the air gap between the individual laminations or by an air gap produced by well-defined grinding operations give rise to a flat loop. A necessary prerequisite is that the permeability µ is large compared to the ratio of iron path length lFe to air gap length lag(µ  lFe\lag). In low-permeability applications with rather large air gaps this can be achieved using grain oriented SiFe- or 40 to 50%NiFe-alloys. High-permeability applications like, for example, modem transformers or current sensors, require the use of 80%-NiFe(Cu,Mo)-alloys. Generating a flat loop by utilizing an appropriate magnetic field tempering treatment of tape wound cores may be a different approach. Indeed, cores made from amorphous or nanocrystalline alloys have been introduced for the application of electronic watt-hour metering (Ferch 1998).

1.3 Magnetic Shielding Some applications require a low level of residual magnetic field or a shielding against external disturbances. Obviously this can best be met by highpermeability 80%-NiFe alloys like umetal. Typical examples are shieldings for CRT-monitors, scientific instruments, or cables. Biomagnetic measurements require magnetically shielded rooms. The shielding of such rooms may consist of several layers of, e.g., umetal, in order to achieve d.c. magnetic field shielding factors of the order of 10% (Harakawa et al. 1996, Mager 1981, Erne! et al. 1981). A d.c.-shielding factor of 75 000 could be achieved in a large shielded room applying a total of seven layers of umetal (Bork et al. 2000). Obviously such applications require a material with very high initial permeability. There are also a lot of other shielding applications with less stringent demands. For example cross-coil instruments are shielded by small cups made from 36–45%NiFe-alloys.

1.4 Parts for SensitiŠe Relays and Stepper Motors Relays for ground fault interrupters have to be very sensitive, their switching power typically being in the

Soft Magnetic Materials: Applications range of 100 µW or even lower. The relay parts have to exhibit a high dynamic permeability and must show a sufficient corrosion resistance in a humid climate test. Their functional surfaces must be ground to a flatness of the order of 1 µm to yield a sufficiently small air gap in the magnetic circuit. The wear resistance of the functional surfaces must be sufficient to enable at least a few thousand switching operations without changing the switching power by more than p20%. The compromise between required performance and cost has resulted in the widespread use of especially highgrade 50%-NiFe-alloys for this kind of application. Similar requirements are valid for stepper motors of quartz watches. High permeability, low hysteresis losses, and a rather high saturation induction led to the selection of 50%-NiFe for the core material. The stator on the other hand must exhibit a lower saturation induction in order to attain saturation and to have flux penetration into the air for rotation of the permanent magnetic rotor part. Up to now the stator of quartz watches therefore has been produced from 80%-NiFe-alloys exhibiting a saturation induction of about 0.75 T. A similar induction level can be obtained by diluting 36–38%-NiFe- alloys with, e.g., chromium. A chromium content of 6–8% adjusts the saturation induction to the required level of 0.75 T (Behnke and Radeloff 1994). Although maximum permeability is lower compared to 80%-NiFe-alloys it is by far sufficient for this kind of application. Due to the reduced material costs associated with the lower nickel content such alloys are becoming more and more widespread. As a variant of 50%-NiFe-alloys, a precipitation hardening by titanium and niobium is possible. These elements form Ni (Nb,Ti). As the size of these pre$ cipitates is much smaller than the Bloch wall thickness, the magnetic performance is not degraded too much by such additions. The precipitation of Ni (Nb,Ti) enables to achieve a Vickers hardness of about$ 250 for the annealed state compared to a level of 100 for binary NiFe-alloys. As coercivity can be maintained below 10 A m−" such alloys may find usage in relays and in other applications to improve wear resistance.

1.5 Temperature Compensation To compensate for the temperature dependence of magnetic properties in magnet systems like electricity meters 30%-NiFe-alloys are used. At this level range of nickel content the Curie temperature Tc strongly depends on the nickel content. By properly controlling the nickel content different alloys with Tc in the range 30–120 mC are produced. Further appropriate treatment of these alloys yields a rather straight temperature dependence of the induction. These alloys are used as a parallel shunt in permanent magnet systems

enabling to keep the flux in the air gap of the system constant at different temperatures.

2. High-B Applications 2.1 Transformers and Generators CoFe-alloys offer the highest induction levels of all known soft magnetic alloys. Where weight is a limitation, e.g., in the case of aircraft generators, and a high power density is required alloys based on 50%-CoFe are used. These are usually alloyed with vanadium or tantalum in order to slow down the ordering process and to be able to deform the material after an appropriate quenching treatment from above the ordering temperature (Kawahara 1983). To achieve highest induction levels and lowest coercivity such an alloy has to be very pure, the amount of impurities being strictly limited. In case of mechanical requirements a trade off between magnetic performance and mechanical strength has to be realized. By performing the final annealing treatment at lower temperatures an improved yield and tensile strength can be obtained at the expense of a slightly degraded magnetic performance. Much higher yield strengths can be achieved by additional alloying with small amounts of niobium (Ackermann et al. 1972). With niobium the formation of Nb Fe Lavesphase limits grain growth and improves yield# strength (Masteller et al. 1996) according to the Hall–Petch relation. Consequently the reduced grain size means a higher coercivity and also slightly reduced induction level. Therefore alloys have to be selected depending on the individual magnetic and mechanical requirements of the envisaged application.

2.2 Electromagnetic Actuators for Dot Printers, ValŠes, etc. Actuators usually need high forces and, since the generated force is proportional to B#, SiFe- or CoFealloys may be used. By far the majority of magnetic actuators are manufactured from simple magnetic steels or SiFe-alloys. Highest forces on the other hand are obtained by using 50%-CoFe. A compromise between cost and magnetic performance has been the development of CoFe-alloys with lower cobalt content. By alloying 17%CoFe with chromium, molybdenum, and vanadium an alloy has been developed with saturation induction well above 2.1 T (Vaerst and Theuss 1999). The alloying yields a rather high specific resistance. Both high induction and high specific resistance are necessary for successful application of this alloy in modern diesel injection systems. Figure 1 shows a comparison of B(H ) data for different types of high saturation induction alloys. 3

B(T)

Soft Magnetic Materials: Applications

H(A cm–1)

Figure 1 Static B(H)-curve of different high saturation induction alloys.

3. High-frequency Applications 3.1 Switched Mode Power Supplies Depending on the output power, switched mode power supplies (SMPS) operate in the frequency range from about 10 kHz up to the MHz range. SMPS contain various magnetic components such as common mode radio frequency interference (RFI) chokes as filters on the input side, power transformers, magnetic amplifiers (magamps) for independent regulation of several output voltages, storage chokes, and spike killers to suppress output voltage noise. Introduced to the SMPS-market in the mid-1980s, cores for magnetic amplifiers are now mainly produced from zero magnetostrictive amorphous cobalt-based alloys. Magnetic amplifiers have a rapidly growing market in particular for power supplies in personal

computers and servers with a market volume exceeding 50 million amorphous cores a year. The use of nanocrystalline alloys has been suggested for magamps. Common mode RFI chokes serve to attenuate circuit interference. The attenuation properties are determined by the impedance curve across the interference spectrum. For the lower frequency range of 0.01–1 MHz impedance and insertion loss will be determined by the inductance L. This requires material permeabilities µi as high as possible. Figure 2 shows the magnetic permeability µ versus frequency for various high-permeability core materials such as highpermeability ferrite, amorphous cobalt-based alloys (VITROVAC 6025F), and nanocrystalline alloys (VITROPERM 800F). Further advantages of nanocrystalline alloys result from the lower hysteresis losses, the high saturation induction Bs, and the excellent thermal stability (Ferch et al. 1998). Due to new power semiconductors such as IGBT, SMPS with higher clock rates have been established in the upper power range. In these circuits the power transformer is a key component affecting device volume and efficiency of the SMPS. The main material requirements are low cores losses pFe, high saturation flux density Bs, and high temperature stability. Figure 3 gives a comparison of core losses (B l 0.1 T) in the relevant frequency range of 10–100 kHz. Again the nanocrystalline material turns out to be the preferred choice, exhibiting exceptionally low losses, constant pulse permeability, better temperature stability, and safer operation leading to reduced device volumes, reduced weight, and very high transmittable power of up to 500 kW with a single transformer. There is a wide spectrum of applications for such SMPS-kW-transformers (Ferch 1997).

Permebility, |l|

3.2 Telecommunications: ISDNjxDSL

Frequency, f (kHz)

Figure 2 Magnetic permeability as function of frequency for different high permeability core materials.

4

Over the 1990s a new application area has emerged in digital telecommunication with an increasing demand for interface transformers. These transformers are used to connect the network termination to the consumer terminal equipment like phone, fax, PC, and others. To achieve fidelity of pulse transmission each side of the interface requires two signal transformers incorporating galvanic separation. The transformers must meet very stringent technical requirements such as a compact design and fulfilling the impedance pulse mask (i.e., high permeability µi between 0.01 and 1 MHz even under d.c.-premagnetization). Depending on the d.c.-premagnetization conditions various amorphous cobalt alloys or nanocrystalline alloys are used. Amorphous and nanocrystalline cores have gained a share of more than 30% of the total world market for ISDN transformers. Due to their versatile magnetic properties these

Core losses, PFe (W kg–1)

Soft Magnetic Materials: Applications

Frequency, f (kHz)

Figure 3 Core losses as function of frequency for different high permeability materials.

materials are again a preferred choice for future telecommunication applications such as HDSL, VDSL, and ADSL. Broadband transformers and filters for these technologies using amorphous or nanocrystalline cores have already been introduced into the market.

4. Sensor Applications Soft magnetic materials are the key functional element in a wide variety of sensors to measure magnetic fields, electric currents, displacements, and mechanical stresses. Since the 1980s a quite unique application of magnetic sensors has emerged in the field of electronic article surveillance (EAS) to protect merchandise by the attachment of a small magnetic label. An electronic detection system is installed at the checkout or exit of a store to detect the movement of such a label. If merchandise is removed from the store with a nondeactivated label an alarm will sound. Most common in the market is an acoustomagnetic system operating with electromagnetic pulses at a frequency of about 60 kHz. The key element of this magnetoelastic sensor label is a thin strip made of an amorphous magnetostrictive alloy (O’Handley 1993). The length of this strip is selected to give mechanical resonance just at this frequency.

5. Conclusions Depending on the requirements of the envisaged applications the basic magnetic material properties like crystalline anisotropy, magnetostriction, saturation induction, resistivity, etc., determine the alloy selection. Traditional high-performance soft magnetic

alloys based on the NiFe and CoFe alloy system are and will be used successfully in a huge variety of applications. Development in this field concentrates on improving the alloy performance even further or tailoring alloy properties according to specific customer and application needs. Since the 1980s the successful implementation of amorphous and nanocrystalline soft magnetic alloys has broadened the range of applications significantly. These alloys offer unique properties and extend the application frequency range of metallic soft magnetic alloys up to the MHz-range. Due to their high resistivity and inherently low tape thickness of about 25 µm soft magnetic components with exceptionally low losses could be realized. Development in this field is very rapid and these materials are finding ever more applications. The story of high performance metallic soft magnetic materials will continue.

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Copyright ' 2001 Elsevier Science Ltd. All rights reserved. No part of this publication may be reproduced, stored in any retrieval system or transmitted in any form or by any means : electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. Encyclopedia of Materials : Science and Technology ISBN: 0-08-0431526 pp. 8679–8684 6