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Permeability and electromagnetic-interference characteristics of FeSiAl alloy

flakespolymer composite

Shigeyoshi Yoshida, Mitsuharu Sato, Eishu Sugawara, and Yutaka Shimada

Citation: Journal of Applied Physics 85, 4636 (1999); doi: 10.1063/1.370432

View online: http://dx.doi.org/10.1063/1.370432

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Permeability and electromagnetic-interference characteristicsof FeSiAl alloy flakespolymer composite

Shigeyoshi Yoshida,a) Mitsuharu Sato, Eishu Sugawara, and Yutaka Shimadab)

Tokin Corporation, R & D Center, 298 Shibokuchi, Takatsu-ku, Kawasaki, Kanagawa 213-0023, Japan

A magnetic sheet made of FeSiAl flakes was found to have excellent permeability andelectromagnetic-interferences characteristics in a quasimicrowave band. The flakes prepared byattrition were embedded in a polymer with their planes lying parallel to the sheet plane. This sheet

exhibits a peculiar dual dispersion in permeability. The Curie temperature of the flakes increaseswith increasing specific surface area of the flakes. This suggests the composition of the flakeschanges to excess Fe by oxidation of Si and Al. This deviation from the Sendust composition bringsan increase of a magnetoelastic effect and the dual frequency dispersion seems to be associated withstress caused by attrition. On the basis of these results, flakes with more desirable permeabilitycharacteristics were produced from raw powder with less Fe content. Consequently, we couldfabricate a magnetic composite that possesses superior noise suppressing effect in aquasi-microwave band. 1999 American Institute of Physics. S0021-89799942308-4

I. INTRODUCTION

In the high frequency bands, where wavelengths of elec-trical signals are comparable with dimensions of electric cir-cuits, connections of circuit elements have to be looked uponas distributed constant circuits. This tends to cause imped-ance mismatching in electrical junctions resulting in circuitoscillation and microwave radiation. To suppress radiationfrom these distributed circuits, a material with a large high-frequency loss placed in the vicinity of the lines is effective.That is expected to damp the oscillation by inductive cou-pling with the lines, or by absorbing radiation.1

The excellent high frequency permeability characteris-tics of sputtered multilayer magnetic films patterned into nar-row stripes have been studied.2 In our experiments, the flakes

are thinner than the skin depth and a large aspect ratio andelectrical isolation with dielectric matrix such as polymer,are expected to give rise to analogous high frequency char-acteristics.

This article describes the permeability characteristics ofFeSiAl alloy flakespolymer composites and its noisesuppression effect in a near field mode.

II. EXPERIMENTAL PROCEDURE

The FeSiAl flakes that have a thickness close to theskin depth and a large aspect ratio, can be produced by me-chanical attrition of FeSiAl powder. Attrition of water

atomized FeSiAl alloy powder 9.7 wt% Si5.4 wt%Albal Fe comprising teardrop particle shapes of 14 mmean diameter was carried out in a hydrocarbon solvent byusing an attritor. Magnetic slurry containing the flakes andpolymer was coated repeatedly by using doctor-blade, andthe flakes were oriented with their planes parallel to that ofthe layer by shearing stress generated in coating.

Permeability measurements were carried out for toroidalsamples of833 mmt by 1 turn coil method using an

impedance-material analyzer (HP-4291A16454A). Curietemperatures were estimated by extrapolation of the decreas-ing temperature curve of saturation magnetization measuredwith a magnetic balance. Noise suppressing effects of 100mm square sheet samples were measured by a near fieldmode measurement system1 that assumes use of the sheets insmall size electric devices.

III. RESULTS AND DISCUSSION

Permeability characteristics of FeSiAl flakespolymer composites are shown in Fig. 1a as a function offrequency. As shown in Fig. 1b, permeability (max) of

the composite made of FeSiAl flakes is obviously supe-rior to that of a rubber-ferrite or composite made of as-atomized FeSiAl powder. The composite made of flakesattrited for 7.5 h shows maximum of about 16, thereafter,it decreases with increasing attrition time. Mechanical distor-tion associated with the attrition process or change of thecrystalline phase in FeSiAl are the subjects to be studiedhere.

The ultimate products of a polymer composite exhibitmagnetic resonance in a few 10 MHz range, and moreover,decrease of permeability in 100 MHz range which seemsalso to be caused by a magnetic resonance. As a result,imaginary part of permeability () shows twin peak dis-

persion and is high over a wide frequency range. As aresult, the material has a remarkable feature of noise sup-pressing in high frequency electromagnetic-interferencesEMIcountermeasures.

Mechanisms for this peculiar frequency characteristicsmay be any of: 1 appearance of a new crystalline phasewith different magnetic anisotropy, 2 resonance by localmagnetic anisotropy caused by local mechanical distortion,3

3 L C resonance due to the metallic/dielectric multilayerstructure,4 or 4domain wall resonance. Validity of1canbe studied by measuring the temperature dependence of the

aElectronic mail: yosida-s@po.iijnet.or.jpbAlso with: Tohaku University, Res. Inst. for Sci. Measurements, Aoba-ku,

Sendai, Miyagi 980-0812, Japan.

JOURNAL OF APPLIED PHYSICS VOLUME 85, NUMBER 8 15 APRIL 1999

46360021-8979/99/85(8)/4636/3/$15.00 1999 American Institute of Physics

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saturation magnetization (Ms). Examples of temperature de-pendence ofMsare shown in Fig. 2. In Fig. 2a, as-attritedflakes exhibit an irreversible change on temperature depen-dence of Ms . But as shown in Fig. 2b, no remarkablechange was observed between the increasing and decreasingrun for as-atomized powder and after annealing of flakes.This suggests that the irreversible change for flakes by longerattrition is due to relaxation of residual distortion. The mo-notonous temperature dependence of Ms suggests that theflakes after annealing are single phase. However, we noticethat the Curie temperature Tc of the flakes attrited 20 h be-comes higher by 80 centigrade in comparison with Tcof theas atomized powder. Attrition time dependency of Tc isshown in Fig. 3, where Tc increases proportionally with in-creasing attrition time. This suggests that the composition ofFeSiAl flakes tends to acquire excess Fe by attrition.From the fact that specific surface area of the alloy powder isalso proportional to the attrition time, the increase of Tc

seems to occur by selective oxidation of Si and/or Al at thesurface of the flakes.This suggests that a magneto-elastic effect plays a role in

determining the resonance frequency and the frequency char-acteristics of FeSiAl flakes, because the magnetostrictionconstant s changes sensitively as the composition of origi-nal FeSiAl alloy changes.

To verify the composition change in the flakes after at-trition, scanning Auger electron microscope SAManalysisand x-ray photoelectron spectroscopyXPSwere performedfor the bulk FeSiAl alloy samples which were heat treatedfor 50C24 h. From the depth profiles of the bulk FeSiAl alloy obtained by SAM, the oxidation depth is esti-

mated to be 30 . Considering this result, semiquantitativeanalysis by XPS was carried out for the surface sputtered to50 depth. Table I shows the results. For the surface at thedepth of 20 s sputtering, a large quantity of oxygen close to

FIG. 1. Frequency dependence of permeability of the composite materials.a Change of the permeability with the attrition time. b Example of arubber-ferrite.

FIG. 2. Increasing and decreasing temperature runs of normalized saturationmagnetization Ms(T)/Ms(25 C) of FeSiAl alloy powder. aAs-attritedflakes20 h attrition.bAs-atomized powder and after annealed flakes20h attrition.

FIG. 3. Curie temperature Tc of FeSiAl alloy flakes as a function ofattrition time.

TABLE I. Contents of C, O, Si, Al, and Fe of FeSiAl 9.89 wt % Si5.59 wt % Albal Fealloy surface.

Etching time s C O Si Al Fe at. %

0 35 43 6 6 1020 49 13 14 2460 1 11 8 14 66

4637J. Appl. Phys., Vol. 85, No. 8, 15 April 1999 Yoshida et al.

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50 at. % exists. Table II shows normalized quantities for thethree elements that constitute the original alloy. Si and Al arecondensed at the surface of bulk alloy by about 3 and 5times, respectively, of the original alloy composition.

Moreover, from the bonding-state analysis by XPS, atthe depth of 2060 s sputtering, most of Si and Al are inoxidized state. Therefore, in the surface layer of FeSiAlbulk alloy, Si and Al seem to be relatively condensed incomparison with composition of original alloy, and they ex-ist as oxides.

To get more quantitative information on the compositionchange of original alloy accompanying surface oxidation,

nine kinds of FeSiAl alloy samples that have differentcompositions were prepared, and quantitative analysis by in-ductively coupled plasma ICP and Tc measurement wereperformed. As a result, Tc of FeSiAl alloys with compo-sitions near that of Sendust are sensitively dependent on Fecontents, and it is estimated that Tc increases by 4050 Cwith an increase of 1% of Fe.

Thus, we conclude that the increase of Tc of flakes ac-companying progress of attrition is due to a shift of the com-position to excess Fe.

Therefore, one of the dominant factors that determinethe permeability characteristics of the attrited FeSiAlflakes is a magnetoelastic effect caused by composition

change of the alloy powder. Further quantitative study on therelation between these factors and frequency characteristicsof permeability is in progress.

By speculation of the composition change caused by in-crease of the powder surface area, we can fabricate a com-posite with lower sand consequently a large magnetic lossin a lower frequency range. Raw powder with composition

of poorer Fe 9.8 wt % Si6 wt % Albal Fe than that ofSendust was prepared. The permeability characteristics of thecomposite made of poorer Fe flakes is shown in Fig. 4, su-perior permeability in a quasi-microwave band was obtainedas expected.

Noise suppressing effects in near field were investigated.A result is shown in Fig. 5 for the sample made of flakes

attrited 7.5 h and the sample made of poor Fe flakes and arubber-ferrite. In the comparison of transmission attenuationshown in Fig. 5a, the composite with poor Fe flakes showslarge attenuation over the whole measurement band. Further-more, the poor Fe sample exhibits obvious superiority incoupling attenuation normalized to air Fig. 5b.

IV. CONCLUSION

The FeSiAl polymer composite exhibits superior per-meability characteristics at high frequencies. A Tc increaseof the flakes with progress of attrition suggests excess Fewithin the flakes and, consequently, a magnetoelastic effectseems to give a large influence to permeability. By preparingraw compositions with poorer Fe than that of Sendust, asheet with excellent permeability characteristics and EMIsuppressing effect in a quasimicrowave band is available.

1 S. Yoshida, M. Sato, and Y. Sato, Abstract of 97 EMC Symposium, 4-1-11997.

2 S. Tanabe, H. Ohji, K. Inoue, and T. Ozeki, Proc. Perp. Mag. Rec., 91,197 1991.

3 Y. Shimada, M. Shimoda, and O. Kitakami, Jpn. J. Appl. Phys., Part 1 34,47861995.

4 B. C. Webb, M. E. Re, M. A. Russak, and C. V. Jahnes, J. Appl. Phys. 68,42901990.

FIG. 4. Frequency dependence of permeability of the composite sheet madeof poor Fe raw material.

FIG. 5. Near field noise suppression effect. Sheet thickness: 0.5 mm. 1Flakes with less Fe content. 2 Flakes with Sendust composition. 3Rubber-ferrite.

TABLE II. Contents of Si, Al, and Fe of FeSiAl 9.89 wt % Si5.59wt % Albal Fealloy surface.

Etching time s Si Al Fe at. %

0 27 27 4520 25 27 4760 9 16 75

4638 J. Appl. Phys., Vol. 85, No. 8, 15 April 1999 Yoshida et al.

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