Applications of nanocrystalline soft magnetic Fe-M-B (M = Zr, Nb) alloys

NanoStruW

Pergamon

Materials. Vol. 8, No. 8. pp. 981-995.1997 Ekevia ScienceLtd (D1998ActaMdallurgia Inc. PrintedintheusA. Allrightsresaved 0965~9773,97$17.00 + .OO

PII SO9659773(98)00034-S

APPLICATIONS OF NANOCRYSTALLINE SOFT MAGNETIC Fe-M-B (M = Zr, Nb) ALLOYS Y. Naitoh’, T. Bitoh’, T. Hatanai’, A. Makino’, A. Inoue* and

T. Masumotd

lCentral Research Laboratory, Alps Electric Co., Ltd., Nagaoka 940, Japan *Institute for Materials Research, Tohoku University, Sendai 980-77, Japan 3Research Institute of Electrical and Magnetic Materials, Sendai 982, Japan (Accepted February 12,1998) Abstract - We have succeeded in the development of new nanocrystalline soft magnetic Fe-M-B (M=Zr, Hf, Nb) alloys with high saturation magneticflux density (B,) above 1.5 Tas well as excellent soft magndtic properties. For example, the alloys show high permeability&) of 16 x 104 at I kHz>high B, of 1.57Tand zero-magnetostriction, simultaneously. In thispaper, we report on the performance of toroidal cores with a gap, common mode choke coils and pulse transformers. The excellent characteristics of the components werefound in conjunction with very low core losses, suJticient thermal stability and low stress sensitivityfor magnetic properties. The nanocrystalllne Fe-M-B based alloys are, therefore, expected to be used for many kinds of magnetic components. 01998 Acta Metallurgica Inc.

1. INTRODUCTION Recently, nanocrystalline soft magnetic alloys consisting of bee nanoscale crystallites have been obtained by crystallizing melt-spun amorphous ribbons (l-5). It has been found that the simultaneous addition of Nb and Cu to Fe-Si-B amorphous alloys enables the controlled crystallization and improves soft magnetic properties (1,2). However, the nanocrystalline Fe-Si-B-Nb-Cu alloys exhibit lower saturation magnetic flux density (B,) (up to 1.3 T) than the Fe-Si-B amorphous alloys. In 1990, we reported that small addition of B to Fe-M (M = Zr, Hf, Nb) amorphous alloys enables the formation of a nanocrystalline structure consisting of a-Fe crystallites with size of 10 to 20 nm. The nanocrystalline Fe-M-B (M = Zr, Hf, Nb) alloys exhibit high BS of 1.5 to 1.7 T as well as excellent soft magnetic properties (3-5). Figure 1 shows the relationship between B, and permeability (cle) at 1 kHz of Fe-M-B (M = Zr, I-If, Nb) based alloys NANOPERMTM and other soft magnetic materials. NANOPEWu alloys possess both high BS and high l&. Figure 2 summarizes expected application fields for NANOPERhW alloys, together with magnetic characteristics which are required for each application. The alloys exhibit high BS, high b, low core loss (W) and zero magnetostriction (A),simultaneously. Furthermore,remanenceratio (Br/B,) can be controlled according to the demands of various applications. In this paper, we 987

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Saturation magnetic flux density, Bs (T) Figure. 1. Relationship between Bs and b at 1 kHz for NANOPERMTM, the nanocrystalline Fe-Si-B-Nb-Cu alloys (6) and conventional soft magnetic materials.

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Figure 2. Magnetic characterizations and application fields for NANOPER WM.

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present some applications of NANOPERMrM alloys, which indicate that they are suitable for core materials in many kinds of magnetic components.

2. EXPERIMENTAL Alloy ingots were prepared by induction melting in an Ar atmosphere. A single-roller melt-spinning method in an Ar atmosphere was used to produce rapidly solidified ribbons 15 mm in width and 15-u) pm in thickness. The as-quenched ribbons were slit and wound into toroidal cores to be used as samples. The samples were annealed in vacuum to prevent oxidation. The annealing treatment was carried out by keeping the samples at 793-923 K for 60-1800 s. The samples were usually annealed without magnetic field.

3. RESULTS AND DISCUSSION 3.1 ToroidalCores withGap Miniaturization is now needed for magnetic components for power electronics. The magnetic energy stored in the components is proportional to the magnetic flux density of the core and the core volume. The miniaturization of the core is limited by the B, value and the core loss of the core material. If the core volume is decreased, the operating flux density should be increased to keep the magnetic energy constant. This causes a large temperature rise of the core because the core loss of the core material increases with increasing flux density. NANOPERMTMexhibits high BS, above 1.5 T, which is comparable to that of the Fe-based soft magnetic amorphous alloys, and low core losses over a wide maximum induction range (4,5). These characteristics indicate that the material is suitable for the core of power electronic components. First, we investigated the toroidal cores with gap, which are used as choke coils for active filters for power supplies. NANOPERICM and amorphous Fe7sSigBts alloy ribbons were wound into toroidal cores 37 mm in outer diameter, 23 mm in inner diameter and 15 mm in height. The cores were encapsulated in epoxy resin and processed by a slicing machine to make a 2 mm air gap. Figure 3 shows the change of the core loss of the cores after annealing, after encapsulation, and after processing the air gap. The core loss after annealing is about 115times as large as that of the Fe-Si-B alloy core. The core loss of the Fe-Si-B alloy core increases extremely after encapsulation in epoxy resin. In the stressed state, the soft magnetic properties of the Fe-Si-B amorphous allloy are inferior due to their large magnetostriction. On the other hand, the NANOPERhVM core exhibits almost the same low core loss value as that of the core before encapsulation because of their zero magnetostriction. The encapsulation in epoxy resin is necessary to make the air gap. It can be said that the small magnetostriction is necessary for obtaining the encapsulated core with low core loss. The core loss increasedin both NANOPEFMTM and Fe-S-B after the air gap processing. One reason is to be short-circuitedfor air gap processing of the layer of the ribbon on the gap cutting side.

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Maximum fluxdensity, B, (T) Figure 4. The change of core loss as a function of maximum flux density for toroidal cores with gap made of NANOPERMTMand Fe&-B amorphous alloy.

Figure 4 shows core losses as a function of maximum flux density (IL) of these cores. The core losses are l/3 to l/5 those of the amorphous Fe-Si-B alloy core. The very low core losses allow the reduction of the core size because the rise in temperature of the core should be small.

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3.2 Common Mode Choke Coils Recently, electromagnetic interference (EMI) considerations are increasingly important as electronic devices become part of our daily lives. This has rapidly increased the required performances for electromagnetic compatibility (EMC) components such as common mode choke coils, which provide protection against incoming and outgoing high frequency noise. They also protect electronic devices against high voltage pulse noise generated by spark discharge, etc. MnZn-ferrite cores have been widely used for common mode choke coils. However, the pulse attenuation characteristics of the choke coils made of MnZn-ferrite is inferior because of their low B, values (typically 0.4 T). The inductance of common mode choke coils is important because higher inductance yields better attenuation characteristics against pulse voltage. If the magnetic flux density of the core is saturated by the magnetic field generated by the input pulse current (magnetic saturation), the choke coil cannot attenuate the input voltage because the inductance of the choke coil is extremely decreased. In order to prevent the magnetic saturation, highb, and low remanence ratio,&/& are necessary. We have developed cores for common mode choke coils with high B, and low B,l&. InordertoobtainalowB,/B,value,wehavetriedmagneticfieldannealingofN~OPE~M. The magnetic field annealing treatment induces a uniaxial anisotropy, the easy axis being parallel to the direction of the magnetic field applied during the heat treatment (7). This allows the shape of the hysteresis curves to be changed. Figure 5 shows thedc B-H curves of toroidal cores annealed under zero magnetic field ( HL = 0) and transverse magnetic field (HI = 160 kA/m). Very low ET/B, value of 0.05 has been obtained by transverse magnetic field annealing.

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0.4’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ 200 250 350 400 300 Temperature, T(K) Figure 6. Normalized inductance of the common mode choke coils made of NANOPERMTM and MnZn-ferriteTM as a function of temperature.

NANOPERMTM ribbons were wound into toroidal cores 22 mm in outer diameter, 14 mm in inner diameter and 6.5 mm in height. Amixture of oxide powders and sodium silicate solution (water glass) was applied to both side of the ribbons to prevent electrical contact between the layers. The cores were annealed under a static transverse magnetic field of 160 kA/ m. Common mode choke coils were made from the NANOPERMTM toroidal core and from a commercial MnZn-ferrite core. The size of the cores was the same, and both had 24 turn coils.

Figure 7. Frequency dependence of noise attenuation for common mode choke coils made of NANOPEWM and MnZn-ferrite.

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Figure 8. Output voltage of the common mode choke coils made of NANOPERMTM and MnZn-ferrite as a function of input pulse voltage when the pulse width is 1 l.ts.

Figure 6 shows the normalized inductance of the common mode choke coils at 100 kHz. NANOPERMTM choke coil exhibits much more stable inductance against temperature change as compared with that of the ferrite choke coil. This result indicates that the NANOPERMTM choke coil shows the stable insertion loss in the wide temperature range. Figure 7 shows the frequency dependence of the noise attenuation of the common mode choke coils made of NANOPERMrM and MnZn-ferrite. The attenuation of the NANOPERMTM choke coil is higher than that of the MnZn-ferrite choke coil in the frequency range 0.3 to 20 MHz, which includes the operating frequency range of normal switched power supplies. The NANOPERM? choke coil is therefore useful for the attenuation of noise generated by switched power supplies. The pulse attenuation characteristics of the common mode choke coils was investigated. Figure 8 shows the output voltage of the choke coils as a function of the input pulse voltage when the pulse width is 1 l.rs. The NANOPER.IvFMchoke coil shows better attenuation due to its low BJB, value. 1.tcan be concluded that transverse field annealed NANOPERMTMcores with high B, and low B,./BIB, are suitable for common mode choke coils. 3.3 ISDN Pulse Transformers Pulse transformers are used for integrated service digital network (ISDN) terminal equipment. The transformers electrically isolate the network circuit from the terminal equipment. Miniaturization is now necessary for the transformer. MnZn-ferrite has been used as the core material for pulse transformers. Because of their low impedance resulting from low be. the reduction in size of the transformer is difficult to achieve. If an increase in impedance is achieved by increase in coil turns, the frequency characteristics of the impedance become inferior, by a

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transformers was 1.5 mm and 7.5 mm for NANOPERMTMand MnZn-ferrite, respectively. Both transformers had 90 primary and 90 secondary turns. Figure 9 shows the frequency dependence of the impedance of the pulse transformers made of NANOPERMTM and MnZn-ferrite. The broken line in Figure 9 shows the impedance required by ITU-T 1.430 standards. The ferrite transformer shows higher impedance around80 kHz, which is the self-resonance frequency of the transformer. However, except for a narrow frequency range around the resonance frequency, almost the same impedance characteristics have been obtained for both transformers, even though the height of the NANOPERMTMcore is l/5 that of the ferrite core. Therefore, smaller transformers can be achieved using NANOPERMTMas the core material because NANOPERMTM has much higher k than ferrite. Figure 10 shows the temperature dependence of inductance at 10 kHz for pulse transformers made of NANOPERMTM and MnZn-ferrite. The inductance value of the ferrite transformer greatly decreases with decreasing temperature and is less than the value of 20 mH required by the ITU-T 1.430 srandards below -10°C. On the other hand, the inductance of the NANOPERMTM transformer is found to exhibit much higher stability against temperature change, due to the high thermal stability of the nanostructure and low stress-sensitivity owing to the zero magnetostriction. 4. CONCLUSION We have succeeded in the development of a new type of nanocrystalline soft magnetic material “NANOPERMT”” with high B, above 1.5 T, as well as excellent soft magnetic properties. The excellent characteristics of a toroidal core with a gap, acommon mode choke coil and apulsetransformer were proved experimentally. The material is expected to be used widely in these magnetic application fields.

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Yoshizawa, Y., Oguma, S. and Yamauchi,K., Journal ofApplied Physics, 1988,64,6044. Herzer, G., IEEE Transactions Magazine, 1990.26, 13997. Suzuki, IL, Kataoka, N., Inoue, A., Makmo, A. and Masumoto, T., Materials Transactions JIM, 1990,31,743. Makino, A., Inoue, A. and Masumoto, T., Materials Transactions JIM, 1995,36,924. Makino, A., Hatanai, T., lnoue, A. and Masumoto, T., Materials Science and Engineering, 1997, A226-22.8,594.

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Yoshizawa, Y., Oguma, S., Hiraki, A. and Yamauchi,K., Hitachi Metals Technical Review, 1989,

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Naitoh, Y., Bitoh, T., Hatanai, T., Makino, A., Inoue, A. and Masumoto, T., Scientific Report of Research Institute, Tohoku University, 1997,A43,161.

5,13.