Journal of Alloys and Compounds 581 (2013) 293–297

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Journal of Alloys and Compounds

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Effect of heat treatment on magnetic properties of iron-based softmagnetic composites with Al2O3 insulation coating produced by sol–gelmethod

0925-8388/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jallcom.2013.07.008

⇑ Corresponding author. Tel.: +98 9171046058.E-mail address: maryamyaghtin@gmail.com (M. Yaghtin).

Maryam Yaghtin ⇑, Amir Hossein Taghvaei, Babak Hashemi, Kamal JanghorbanDepartment of Materials Science and Engineering, School of Engineering, Shiraz University, Shiraz, Iran

a r t i c l e i n f o

Article history:Received 23 May 2013Received in revised form 28 June 2013Accepted 1 July 2013Available online 18 July 2013

Keywords:Composite materialsSol–gel processMagnetic measurementsX-ray diffraction

a b s t r a c t

In this study, influences of the annealing process on the magnetic properties of new soft magnetic com-posite (SMC) materials with alumina insulator coating were investigated. Iron powders were coated withalumina by the sol–gel process at room temperature. The results of energy dispersive X-ray spectroscopy(EDS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and density measurementsshowed that the iron powders were uniformly coated by a thin layer of alumina coating with high ther-mal stability. Magnetic measurements indicated that the annealing treatment increased the permeabilityof the composites at low and medium frequency ranges. It was shown that the annealed compositesexhibited noticeably higher frequency stability of the magnetic permeability compared to the heat trea-ted pure iron compacts at the same annealing temperature. The results of the loss separation implied thatthe heat treatment suppressed the hysteresis loss coefficient while it increased the eddy current losscoefficient of the SMCs. The annealed SMCs showed a lower eddy current loss and higher hysteresis losscoefficients compared to the relaxed pure iron compacts due to the preservation of the alumina coatingafter heat treatment.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Recent developments in the powder metallurgy techniqueshave made soft magnetic composites (SMCs) interesting for theelectromagnetic applications. These materials have very low eddycurrent loss and relatively low total core loss at medium to highfrequencies [1]. Consequently, approaching the optimum electro-magnetic loss characteristics is one of the principal goals in theSMC designs [1–4]. The relatively high hysteresis loss of the SMCsat low frequencies is an important problem and should be consid-ered in fabrication of these materials. The hysteresis loss resultsfrom the internal stresses introduced during the compaction pro-cess, which leads to some cold work in the ferromagnetic particlesand therefore increases the density of defects, especially disloca-tions. This fact hinders the movement of the magnetic domainwalls and consequently increases the hysteresis loss which is thedominant part of the total core loss at low and medium frequencyranges. Heat treatment at high temperatures and warm compac-tion are two effective methods to suppress the residual stresses,imperfections and therefore decrease the hysteresis loss of the

SMCs [2,4,5]. The reduced inner distortion of the particles by heattreatment could significantly decrease the coercive force and en-hance the magnetic permeability [2]. On the other hand, high tem-perature annealing can destroy the insulating layer and increasethe eddy current loss especially at high frequencies. Accordingly,the insulating material should have a high thermal stability specif-ically between 823 and 1048 K range, which is the temperaturerange for stress relieving of pure iron [4]. Most of the organic coat-ings could be decomposed or burned at these temperatures. As aresult, the inorganic coatings with high thermal resistance can beused to keep the insulation intact upon the heat treatment andconsequently minimize the eddy current loss. Iron phosphate asa well-known insulating material has been widely used due to itsgood adhesiveness to iron and high electrical resistivity [2,5]. Nev-ertheless, it has been shown in several researches that the ironphosphate exhibits a low thermal stability and decomposes duringthe heat treatment through the diffusion of oxygen and phospho-rous from the coating to the iron substrate [2,5].

In this research, new kind of SMCs with alumina insulatinglayer with high thermal stability were produced by sol–gel process.In addition, the effect of heat treatment at different temperatureson their microstructure and magnetic properties was investigated.Furthermore, different components of the core loss factor were cal-culated for the as-prepared and annealed SMCs.

294 M. Yaghtin et al. / Journal of Alloys and Compounds 581 (2013) 293–297

2. Experimental procedure

The sol–gel method at room temperature was used for coating the iron powderswith Al2O3 insulating layer. Iron powders with an average particle size of 10 lmwere supplied by Merck. Aluminum isopropoxide (AIP, Merck), acetylacetone (AcAc,Merck), nitric acid (Merck) and absolute ethanol were used as the precursor, stabi-lizer, catalyst and solvent, respectively. Alumina sol was prepared by dissolving8 wt.% aluminum isopropoxide in 100 ml ethanol. The hydrolysis of the alkoxidesis very fast, leading to the formation fine particles in the sol. This effect can be sup-pressed by using the stabilizer in order to form a clear solution [6]. The mixture ofalumina sol and stabilizer was mixed with a magnetic stirrer for 1 hour. The ironpowders (�12.4 g) were then dispersed in the freshly prepared solution using aspiral mixer for 5 min. Then a mixture of demineralized water and ethanol (1:5(V/V)) was added dropwise to the suspension with vigorous stirring over a periodof 15 min. The molar ratio of water to the alkoxide for the hydrolysis was 2:1. Nitricacid was used to adjust the pH value around 4. The mixture was stirred for 1 h afterthe addition of water and ethanol mixture. Subsequently, the suspension wasstanding for 1 h before separation and washing with ethanol. After five cycles ofseparation/washing/redispersion with ethanol, the coated powders were dried at348 K in air. For the calcination process, the coated powders were encapsulatedin a quartz tube under nitrogen atmosphere and heated at 773 K for 3 h.

The alumina insulated powders were pressed at 800 MPa into cylindrical shapewith diameter of 10 mm and height of 20 mm. The compaction of the powders wasperformed using graphite as a die wall lubricant. Finally, the prepared compositeswere annealed in air at 673 K and 873 K for 30 min.

The coated powders were characterized by scanning electron microscopy (SEM,HITACHI-SU 70) coupled with energy dispersive X-ray spectrometer (EDS), X-raydiffraction (XRD, Shimadzu 6000 using Cu Ka radiation) and Fourier transforminfrared spectroscopy (FTIR, Shimadzu 8000). To investigate the thermal stabilityof the alumina insulating layer after heat treatment, compositional maps of alumi-num and oxygen from the cross section of a sample annealed at 873 K were pre-pared EDS analysis. The magnetic properties of the compacts were measured byan inductance/ capacitance/resistance meter (LCR meter, KC-605, KDK Co. Ltd.) atroom temperature. The magnetic permeability was calculated from L/L0, where Land L0 are the inductance of the solenoid with and without core, respectively [7].Density of the samples was measured three times using the principle of Archimedesand the average value was reported.

3. Result and discussion

The EDS analysis of the iron powders after the coating process isshown in Fig. 1. As the figure shows, the presence of iron, alumi-num and oxygen peaks indicates the formation of the aluminacoating on the surface of iron particles. In addition, the comparisonbetween the intensity of the aluminum and oxygen peaks with thatof iron, confirms that the produced coating is thin.

In order to study the uniformity of the insulation coating, theEDS mapping of iron, aluminum and oxygen at the cross sectionof the composites was carried out (Fig. 2). As can be observed,the aluminum and oxygen elements are distributed homoge-neously along the particles boundary. This result confirms that a

Fig. 1. EDS analysis of the alumina coated iron powders.

uniform coverage of the iron powders with alumina insulatinglayer is obtained by the sol–gel method.

Fig. 3 represents the XRD pattern of the alumina coated pow-ders. According to JCPDS Card No. 11-0661, formation of the Braggpeak of low intensity near two Theta of 68� can be assigned to the(300) plane of the a-alumina coating [8]. The low intensity of thispeak indicates that the iron powders are coated with a thin layer ofalumina insulation, as previously confirmed by the EDS analysis. Inaddition, the produced composites exhibit a density of 6.64 g/cm3

which is slightly lower than that of pure iron compacts (6.85 g/cm3), demonstrating that the thin alumina insulation has a negligi-ble influence on compressibility of the iron powders.

Furthermore, the formation of an alumina insulating layer onthe iron particles surface can be confirmed by the FTIR analysis.The spectra of the coated powders before and after the calcinationprocess are shown in Fig. 4(a and b), respectively. The broadabsorption band around 3430 cm�1 is due to sol–gel products de-rived from aluminum alkoxide hydrolysis and the hydroxyl groupspresent on the surface of pseudo-boehmite and boehmite [9–11].The bands at 3020 cm�1 and 2930 cm�1 are assigned to the asym-metric and symmetric stretching vibrations of CH3 and CH2 groups[12]. The absorption peak around 1484 cm�1 corresponds to thebending mode of CH2 groups. The bands at 1638 cm�1 and1445 cm�1 are due to the asymmetric COO and symmetric COOstretching vibrations, respectively [13]. The bands around1100 cm�1 are related to the stretching modes of AlO4 tetrahedra,whereas those near 680 cm�1 are assigned to the stretching vibra-tions of AlO6 octahedra [12,14]. After calcination at 773 K, thebroad band of the hydroxyl groups vanished and the new band cor-responding to the stretching vibration of the Al@O bond appearedat 1350 cm�1 [15]. The bands found near 803 cm-1 and 619 cm-1

are attributed to the AlAOAAl and AlAO bonds, respectively [16].The observed effects imply that the large thermal conversion ofthe aluminum hydroxide to the alumina happened after calcina-tion. Hence, according to the FTIR analysis, the sol–gel method suc-cessfully produced an alumina insulating layer on the surface ofiron particles.

Fig. 5 shows the variations of the magnetic permeability of theas-prepared and annealed samples versus frequency. As can beseen, both the coating process and heat treatment have noticeableeffects on the magnetic permeability. At low frequencies, additionof the alumina insulation slightly decreases the magnetic perme-ability of the composites compared to that of compacts made bythe uncoated powders. The alumina coating as a non-magneticphase could act as the distributed air gap and enhance the internalstray field and consequently decreases the magnetic permeabilityat low frequencies [5]. On the other hand, the composite samplesexhibit a higher permeability at higher frequencies (see Fig. 5)due to the reduction of the demagnetizing field corresponding tothe eddy currents. Hence, addition of the alumina insulation layercould enhance the electrical resistivity and decrease the eddy cur-rent loss. Fig. 5 shows also the influence of the annealing on themagnetic permeability of the compacts made by pure iron andcoated powders. As can be inferred, annealing increases the mag-netic permeability of the composites at low frequency range. Inaddition, the low frequency permeability of the composites in-creases with increasing the annealing temperature. During thecompaction of the powders, plastic deformation increases theresidual stress, dislocation density and other defects in the parti-cles microstructure. Such imperfections as the main sources ofthe domain wall pinning could hinder the movement of the mag-netic domain walls and consequently decreases the permeability.Heat treatment at high enough temperatures can provide thelow-volume fraction of defects, reduce the dislocation density, re-lief the residual stresses and result in the increasing of the mag-netic permeability [2]. In contrast, according to Fig. 5, the

Fig. 2. The SEM image of the cross section of the composites (a) and the corresponding EDS maps of iron (b) and aluminum (c) and oxygen (d).

Fig. 3. XRD pattern of the iron powders after the coating process.

Fig. 4. FTIR spectra of the alumina coated powders: (a) before calcination and (b)after calcination at 773 K for 3 h.

Fig. 5. Variations of the magnetic permeability versus frequency for the as-prepared and annealed compacts.

M. Yaghtin et al. / Journal of Alloys and Compounds 581 (2013) 293–297 295

annealed SMCs represent a lower magnetic permeability at higherfrequencies and a faster rate of permeability reduction is seen forthe higher annealing temperature. Annealing can significantly de-cline the electrical resistivity through decreasing the distortion,dislocation density and residual stresses in the iron particles andtherefore increases the eddy current loss [5,17]. Hence, the an-nealed SMCs show a lower magnetic permeability at higher fre-quencies. As Fig. 5 shows, the annealed SMCs indicate drasticallya higher magnetic permeability almost at the whole frequencyrange compared to the annealed pure iron compacts. This fact orig-inates from the lower eddy current loss of the annealed SMCs as aresult of the stability of the alumina insulation after the annealingwhich can be furthermore confirmed by the EDS analysis.

Fig. 6. The SEM image of the composites after annealing at 873 K for 30 min (a) and the X-ray maps of aluminum (b) and oxygen (c) at the cross section.

Fig. 7. Total loss factors of the as-prepared and annealed samples at differentfrequencies.

296 M. Yaghtin et al. / Journal of Alloys and Compounds 581 (2013) 293–297

Fig. 6 represents the EDS maps of aluminum and oxygen at thecross section of the composites after annealing at 873 K for 30 min.A homogenous distribution of the aluminum and oxygen elementsalong the particles boundaries shows that the continuous aluminainsulating layer was not degraded upon annealing. This effectproves that the new insulation coating has significantly a higherthermal stability compared to the other conventional coatings suchas iron phosphate insulation [8].

The magnetic losses of a core can be separated into three typesincluding hysteresis, eddy current and residual losses. For a circuitwith a ferromagnetic core, the total loss resistance (Rs) can be con-sidered as a combination of winding (Rc), hysteresis (Rh), eddy cur-rent (Re) and residual (Rr) components according to the seriesequivalent circuit [3]. Hence, the total loss tangent can be formu-lated as follows [7]:

tan dt ¼Rc

xLþ Rh

xLþ Re

xLþ Rr

xLð1Þ

The loss tangent due to the winding resistance is:

tan ddc ¼qclw109

xAwFwFqAl¼ k1

fð2Þ

where qc is the copper resistivity, lw is the mean turn length, Aw isthe cross sectional area of the winding space in the coil former, Fw isthe copper space factor of the winding, Al is the inductance factor inlH for 1 turn, f is the frequency and k1 is the winding loss coeffi-cient [7].

The hysteresis loss tangent is usually calculated by the Rayleighmodel at a weak magnetic field as follows:

tan dh ¼4mb̂

3pl0l2a¼ gbb̂ ¼ k2 ð3Þ

where m is the Rayleigh coefficient, l0 is the permeability of vac-uum, la is the amplitude permeability at a low applied field andk2 is the hysteresis loss coefficient [7,18]. Hence, for a constantmaximum induction b̂, the hysteresis loss factor is a constant inde-pendent of the frequency.

The loss factor corresponding to the eddy currents without con-sideration of the magnetic domain structure is:

tan de ¼l0lD2x

2qb¼ k3f ð4Þ

where D is core diameter, b is the shape factor, k3 is the eddy cur-rent loss coefficient and q is the specific electrical resistivity [7].As the equation indicates, the eddy current loss factor has a linearrelationship by frequency with the slope corresponding to the eddycurrent loss coefficient.

The residual or excess contribution of the total loss factor can bedetermined by the extrapolation of the maximum induction tozero and subtraction of the eddy current loss from the total coreloss [3]. The excess loss has different contributions depending on

the working frequency. At high frequency range, the main contri-butions to the excess loss factor arise from the ferromagnetic res-onance and domain wall resonance [3]. However, at lowerfrequencies, the main contribution of the excess loss originatesfrom the micro eddy currents produced at the domain walls whichcould be neglected compared to the eddy current loss factor (Eq.(4)) [7].

The total loss factor is obtained by adding the mentioned com-ponents as [7]:

tan dtot ¼ tan ddc þ tan dh þ tan de þ tan dr ð5Þ

or

tan dtot ¼k1

fþ k2 þ k3f þ k4ðf Þ ð6Þ

Fig. 7 shows the variations of the total loss factor of the as-pre-pared and annealed samples versus frequency. It is obviously seenthat the total loss factor suddenly decreases to a minimum value atlow frequencies and then increases gradually by increasing the fre-quency. At low frequencies, the winding loss which is proportionalto reciprocal of the frequency is dominant (Eq. (6)). However, thispart of the total loss factor is the same for all samples because ofthe same winding parameters used in all of the measurements.At higher frequencies, the total loss is dominated by one or moreof the other components.

Fig. 8 depicts the variations of core loss factors of the as-pre-pared and heat treated compacts with frequency. To calculate thecore loss factor, the winding loss factor which is prevailing at verylow frequencies can be subtracted from the total loss factor.According to Fig. 8, the core loss factor exhibits nearly a linear rela-

Fig. 8. The variations of the core loss factor with frequency for the as-prepared andannealed samples.

M. Yaghtin et al. / Journal of Alloys and Compounds 581 (2013) 293–297 297

tion with frequency for all samples. Regarding to this fact and thelinear relationship of the eddy current loss factor with frequency(Eq. (6)), the slope of each line in Fig. 8 may indicate the eddy cur-rent loss coefficient (k3). In addition, by extrapolation of the men-tioned lines to zero frequency, the hysteresis part of the core losscan be determined.

Table 1 lists the loss coefficients of the as-prepared and an-nealed samples at different temperatures. According to Table 1,the hysteresis loss coefficient is lower for the annealed SMCs andits value decreases with increase of the annealing temperature.The suppression of the hysteresis loss coefficient results from thestress relieving and decreasing the dislocation density by theannealing process [5]. From Table 1, the annealed pure iron com-pacts exhibit a lower hysteresis loss coefficient compared to theSMCs annealed at the same temperature. The existence of the alu-mina insulation even after annealing at 873 K (see Fig. 6) as a non-magnetic phase increases the internal stray field and consequentlythe hysteresis loss coefficient. The results are in a good agreementwith the permeability variations at very low frequencies (Fig. 5). Ascan be seen, the pure iron compact after annealing at 873 K has thehighest permeability besides its lowest hysteresis loss coefficient.In contrast, the annealing process increases the core loss factor athigher frequencies (Fig. 8) and the core loss factor enhancement in-creases with increase of the annealing temperature. At high fre-quencies, the core loss factor can be described by the eddycurrent loss factor which has an inverse relation with electricalresistivity (Eq. (4)). The increase of the core loss by annealing athigh frequencies results in increasing the eddy current loss

Table 1The hysteresis and eddy current loss coefficients of the as-prepared and annealedcompacts at different annealing temperatures.

SMCs Hysteresis losscoefficient, K2

Eddy current losscoefficient, K3

Pure Fe, annealed at873 K

0.0012 4.16 � 10�6

Coated Fe, withoutannealing

0.0026 1.95 � 10�6

Coated Fe, annealed at673 K

0.0019 3.47 � 10�6

Coated Fe, annealed at873 K

0.0016 3.66 � 10�6

coefficient according to Table 1. As explained before, the annealingtreatment can decrease the particles distortion produced duringthe compaction step and consequently decrease the electricalresistivity. This effect leads to increasing the eddy loss coefficientof the SMCs after the heat treatment. Table 1 shows that the an-nealed SMCs exhibit a lower eddy current loss coefficient in com-parison with the heat treated pure iron compacts. Thisadditionally proves that the alumina insulation remains intactupon the annealing (see Fig. 6) and contributes effectively todecreasing the eddy current loss factor of the composites.

4. Conclusion

In this work, effect of heat treatment on the microstructure andmagnetic properties of the soft magnetic composites (SMCs) withthe alumina insulation was investigated. Furthermore, differentcomponents of the magnetic loss factor were calculated by a lossseparation method for the as-prepared and annealed samples.The following conclusions could be drawn:

1. Coating of the iron particles was carried out successfully by thesol–gel method. The EDS, XRD, and FTIR spectra proved that theiron particles were uniformly covered with alumina insulation.

2. Results of the FTIR and EDS analyses showed that alumina insu-lation had a high thermal stability and the SMCs produced withthis insulation could be annealed at high temperatures withoutany considerable degradation of the insulating layer.

3. Annealing treatment increased the permeability of the compos-ites at low and medium frequency ranges. The annealed com-posites exhibited significantly higher frequency stability of themagnetic permeability compared to the heat treated pure ironcompacts.

4. The results of the loss separation indicated that the annealingtreatment decreased the hysteresis loss coefficient while itenhanced the eddy current loss coefficient of the SMCs.

5. The preservation of the alumina insulation upon the heat treat-ment resulted in lower eddy current loss and higher hysteresisloss coefficients of the annealed SMCs in comparison with theheat treated pure iron compacts.

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