2802 IEEE TRANSACTIONS ON MAGNETICS, VOL. 42, NO. 10, OCTOBER 2006

Effects of the Addition of Permalloy Powder on theHigh-Frequency Magnetic Properties of Fe-Based

Amorphous Powder CoresYoon B. Kim and K. Y. Kim

Advanced Metals Research Center, Korea Institute of Science and Technology, Seoul 136-791, Korea

FeSiB amorphous powder was produced by gas atomization, and subsequently toroidal-shaped FeSiB amorphous powder cores wereprepared by cold pressing, using permalloy powder as an additive. The characteristics of gas-atomized FeSiB amorphous powder andhigh-frequency magnetic properties of the cores were investigated. FeSiB amorphous powder, exhibiting good soft magnetic propertieswith a saturation magnetization of 125 emu/g and a coercivity of 0.4 Oe, was successfully produced by gas atomization in the particle sizerange below 75 m. The addition of permalloy powder was effective for compaction of FeSiB amorphous powder cores by cold pressing.Permalloy powder increases the effective permeability of the cores but deteriorates the high-frequency dependence of the permeabilityat high content.

Index Terms—Cold pressing, core loss, FeSiB amorphous powder core, gas atomization, high-frequency characteristics.

I. INTRODUCTION

THE technological application of Fe-based amorphous al-loys is considerably limited, because the shape of prac-

tical Fe-based soft magnetic amorphous alloys has been limitedto thin ribbon and wire due to the high cooling rate for the for-mation of an amorphous phase. Recently, great effort has beendevoted to fabricating Fe-based soft magnetic amorphous coresby a powder metallurgical process using various consolidationtechniques [1]–[4]. In an Fe-based amorphous alloy with a largesupercooled liquid region, which is defined by the differencebetween glass transition temperature and crystallizationtemperature , amorphous powder can be easily transformedinto bulk form by consolidation using the significant viscousflow of supercooled liquid. However, in an Fe-based amorphousalloy with a narrow or without a supercooled liquid region, itis difficult to obtain the bulk amorphous alloy due to the largeelastic limit and lack of plasticity, which are typical characteris-tics of the amorphous alloy. Therefore, the increase of packingdensity is an important issue for the fabrication of soft magneticamorphous powder cores by cold pressing. In this paper, FeSiBamorphous powder cores with an addition of permalloy powderwere prepared by cold pressing. The effects of the permalloypowder on the compaction and high-frequency magnetic prop-erties were reported.

II. EXPERIMENTAL PROCEDURE

FeSiB amorphous powder with composition of Fe SiB was fabricated by a gas atomization technique using amaster alloy prepared by vacuum induction melting. An argonpressure of 45 bars and a guide tube with a hole diameter of3.0 mm were used during the atomization process. The charac-teristics of the gas-atomized powder were verified by an X-raydiffractometer (XRD) using CuK radiation, a vibrating sample

Digital Object Identifier 10.1109/TMAG.2006.879884

Fig. 1. XRD patterns of gas-atomized powder with different particle sizerange.

magnetometer (VSM), and differential scanning calorimetry(DSC), respectively. The powder morphology was observedby a scanning electron microscope (SEM). Toroidal-shapedFeSiB amorphous powder cores (outer diameter mm,inner diameter mm, thickness mm) with the ad-dition of different amounts of permalloy (50Ni50Fe) powderwere prepared by compaction under a pressure of 18 ton/cmat room temperature. Silicon resin of 1 wt.% was used as aninsulator between FeSiB amorphous powders. The compactedcores were annealed at 743 K for 1 h in N atmosphere toreduce the internal stress caused by compaction. The effectivepermeability and the dc-bias property of the cores weremeasured using an impedance analyzer. The core loss wasmeasured by a – analyzer.

III. RESULTS AND DISCUSSION

The formation of an amorphous phase of the FeSiB gas-at-omized powder was confirmed by XRD. The XRD patterns ofthe gas-atomized powder with different particle size ranges areshown in Fig. 1. As can be seen in Fig. 1, a fully amorphous

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KIM AND KIM: EFFECTS OF ADDITION OF PERMALLOY POWDER ON HIGH-FREQUENCY MAGNETIC PROPERTIES 2803

Fig. 2. DSC heating curve of FeSiB amorphous powder with particle sizesmaller than 75 �m.

Fig. 3. SEM micrographs of (a) FeSiB amorphous powder and (b) permalloypowder.

phase of the gas-atomized powder without any crystallinityformed in the particle size range below 75 m. Further increasein the particle size of the gas-atomized powder resulted in thecoexistence of amorphous and crystalline phases. Therefore,FeSiB amorphous powder with a particle size smaller than75 m was used for subsequent compaction in this paper. Thecrystallization behavior of the gas-atomized FeSiB amorphouspowder was examined by DSC measurement. Fig. 2 shows theDSC heating curve of the FeSiB amorphous powder with aparticle size smaller than 75 m at a constant heating rate of40 K/min. The amorphous FeSiB powder smaller than 75 mshows an onset temperature of crystallization of 873.3 K.Annealing of the FeSiB amorphous cores was carried out at743 K to avoid crystallization.

The SEM morphology of the gas-atomized FeSiB amorphouspowder with a particle size smaller than 75 m is shown inFig. 3(a). No appreciable contrast revealing the formation of acrystalline phase is seen on the surface of the FeSiB amorphouspowder. Most of the gas-atomized FeSiB amorphous powdersare in spherical form, and this can be suitable for the consol-idation. The density of the FeSiB amorphous alloy measuredwith melt spun ribbon was 7.18 g/cm . The hysteresis curve (notshown) of the gas-atomized FeSiB amorphous powders with aparticle size smaller than 75 m measured by VSM exhibitsa saturation magnetization of 125 emu/g and a coercivity of0.4 Oe. Here, it can be emphasized that FeSiB amorphous pow-ders with good soft magnetic properties can be successfully pro-duced by a high-pressure gas atomization technique. Permalloy

Fig. 4. Frequency dependence effective permeability for FeSiB amorphouspowder cores with addition of permalloy powder.

Fig. 5. Cross-sectional SEM micrographs of FeSiB amorphous powder core(a) without and (b) with addition of permalloy powder.

powder used in this paper has a particle size smaller than 50 m[see Fig. 3(b)].

Fig. 4 shows the frequency dependence of the effective per-meability of the FeSiB amorphous powder cores with theaddition of permalloy powder. Since voids and nonmagnetic ma-terial, including insulators, exist in the powder core, the effectivepermeability is used in this paper. The FeSiB amorphous powdercore without permalloy powder shows stable permeability of 25to 10 MHz, indicating a low permeability in that range of fre-quency. The low permeability of the FeSiB amorphous powdercore without permalloy powder addition may be attributed tothe low packing density of the core. As can be seen in Fig. 5(a),voids and pores were observed in the FeSiB amorphous powdercore without the permalloy powder addition.

2804 IEEE TRANSACTIONS ON MAGNETICS, VOL. 42, NO. 10, OCTOBER 2006

Fig. 6. Frequency dependence of core loss for FeSiB amorphous powder coreswith addition of permalloy powder.

The core with an addition of 50 wt.% permalloy powdershows stable permeability of about 45 to 1 MHz. The valueof permeability of the cores increases with an increase of thepermalloy powder content. According to the cross-sectionalSEM micrograph [see Fig. 5(b)] of the FeSiB amorphouspowder core with 50 % permalloy powder, the permalloy pow-ders were plastically deformed by compaction under a pressureof 18 ton/cm . These plastically deformed permalloy powdersfilled the empty space between FeSiB amorphous powders,which can be also effective in improving the permeability ofthe FeSiB amorphous powder cores. However, the addition ofthe permalloy powder deteriorates the frequency dependenceof permeability at a higher content of permalloy powder.

Fig. 6 shows the frequency dependence of the core loss of theFeSiB amorphous powder cores with an addition of permalloypowder. The FeSiB amorphous powder core without permalloypowder shows a core loss of about 480 mW/cm at 50 kHzfor T. The core loss has a tendency to increase asthe amount of permalloy powder increases. The core with anaddition of 50 wt.% permalloy powder shows a core loss of915 mW/cm at 50 kHz for T. Core loss consistsof hysteresis loss, eddy-current loss, and residual loss. Amongthese, eddy-current loss is known to be dominant at high fre-quency and can be effectively reduced by appropriate insulationof the magnet powder. The increase of core loss with an addi-tion of permalloy powder is considered to be due to the increaseof the eddy-current loss caused by interparticle contact. With anincrease of permalloy powder content, the electrical contact be-tween noninsulated permalloy powder increases, which resultsin the increase of eddy-current loss.

The dc-bias field dependence of the percent permeability,which is defined by the percentage of the permeability uponthe dc-bias field to the permeability in the no dc-bias field,for FeSiB amorphous powder cores at 100 kHz with differentpermalloy powder content is shown in Fig. 7. The FeSiB amor-

Fig. 7. Variation of percentage of permeability with dc-bias field for FeSiBamorphous powder cores with addition of permalloy powder.

phous powder core without permalloy powder shows superiordc-bias properties higher than 90 % permeability at H Oe.The addition of permalloy powder has a tendency to deterioratethe dc-bias properties of the FeSiB amorphous powder cores,but the FeSiB amorphous powder core with an addition of 50%permalloy powder still shows superior dc-bias properties ofabout 85% permeability at H Oe. This implies that FeSiBamorphous powder cores were not easily saturated under lowapplied field. The FeSiB amorphous powder cores preparedby this route can hence provide a potential alternative forhigh-frequency applications.

ACKNOWLEDGMENT

This work was supported in part by the Korea Institute of Sci-ence and Technology of the Institutional R&D Program and inpart by the ATC Program of the Ministry of Commerce, Industryand Energy, Korea.

REFERENCES

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Manuscript received February 27, 2006 (e-mail: ybkim@ kist.re.kr).