A comparison of soft magnetic com posites designed from different ferrom agnetic pow ders and phenolic resins☆

M agdalena Streckova*,Radovan BuresM aria FaberovaLubom ir Medvecky Jan Fuzer ,Peter Kollar

1 Institute of Materials Research,Slovak Academy of Sciences,Watsonova 47,043 53 Ko?ice,Slovak Republic

2 Institu te of Physics,Faculty of Science P.J.?afárik University,Park Angelinum 9,040 01,Ko?ice,Slovak Republic

Keyw ords:Com posite materials Core-shell particles Fou rier transform in frared(FTIR)spectroscopy Pow der metallurgy Sol-gel method

ABSTRACT Softm agnetic com posites(SMCs)were prepared from three different ferrom agnetic pow der particles:iron pow der ASC100.29,sphericalFeSiparticlesand vitroperm(Fe73Cu1Nb3Si16B7)flakes.Tw o typesofhybrid organic-inorganic phenolic resinsm odified with eithersilicananoparticlesorboronwere used to design a thin insulating layerperfectly covering the ferrom agnetic particles.Fourier transform infrared(FTIR)spectrometry confirm ed an incorporation of silica or boron into the polym er matrix,which manifesteditself through an im proved thermal stability of the hybrid resins verified by therm ogravimetric-differential scanning calorimetry(TG-DSC)analysis.The core-shell particlesp repared from the ferrom agnetic pow der particlesand them odified hybrid resinswere further com pacted to the cylind rical and toroidal shapes for the mechanical,electrical and magnetic testing.A uniform distribution of the resin between the ferrom agnetic particles was evidenced by scanning electron microscope(SEM)analysis,which was also reflectedin a rather high value of the electrical resistivity.A low porosity and extraordinary high values of mechanical hardness andflexural strength were foundin SMC consisting of the iron pow der and phenolic resinm odified with boron.The coercivefieldsoftheprepared sam pleswere com parablewith the comm ercialSMCs.

1.Introduction

Softm agnetic com posites(SMCs)are ferrom agnetic particles coated by a thin electroinsu lating layer and then pressedin to the desired shape by the pow der metallu rgy(PM)methods.PM technology is a comm on econom ic process for a mass production of near-net-shape parts.PM techniqueshavebeen successfully usedin order to prepare SMCsm ostly by com paction of the pure iron pow der particles,which are spatially separated from each other by insu lating(dielectric)m aterial[1-3].The aim of PM technologies is to ach ieve SMCs with a high enough density and sufficien tly stable mechanical properties,whereas an insulating layer between magnetic pow der particles shou ld ensure a high electrical resistivity minim izing the overall magnetic losses[4].In comparison with lam inated soft magnetic materials,SMCs prepared by PM methods have un ique magnetic properties such as the threedim ensional isotropic ferrom agnetic behavio r,low eddy cu rren t loss,as w ell as relatively low er total core loss at medium and high frequencies[5,6].The disadvantagesof thesem aterialsare in a low density and high inhom ogeneity,which are responsib le for a low er flexu ral and mechanical strength.Th is fact rep resen ts serious prob lem s for the hand ling and transportof the insu latedm aterialcom ponen tsand lim its their application in high-speed motors[7].The insu lation coating may have organic or inorganic character[8-11].The advantages of organic binders lie in the sm art preparation of com posites,high density offinal green com pacts and thermally undem anding curing process[12].How ever,it has been demonstratedin ou r previous works that the therm osetting resins of hybrid organic-inorgan ic character are quite superior with respect to the pure organic binders because the chemical incorporation of app rop riate inorganic additives into polym eric structure substantially im proves mechanical properties of the final com posites[13,14].The chemical modification of phenolic resin by silica or boron generally im proves the thermal stability,m echanical and flexu ral strength of the hybrid organic-inorganic coating[15,16].Moreover,the dim ensional and shape stability after thermal treatm ent of the final com posite composed of ferrom agneticparticlesand hybrid resin isensured by the convenien t set up of productions parameters and addition of fillers[17].Differen t kinds of com posites with various com binations of used fillers and additives have found their application in microw ave absorption due to their outstanding physicochemical properties[18-21].

The main goal of this work was to prepare SMCs with op tim al mechan ical and magnetic properties for technological applications at medium frequencies.Fe,FeSiand vitroperm(Fe73Cu1Nb3Si16B7)pow ders were used asbase ferrom agneticm aterials for the com posite preparation.The phenol-form aldehyde resin modified with either silica or boron was synthesized through the sol-gel process,which was subsequently used aselectroinsu lating spacerbetween the ferrom agnetic particles.The thermal degradation of both synthesized resins was detected by TG analysis.The chemicalem bedding ofsilicaorborateesters into the polym erm atrix was analyzed by FTIR spectrometry.The morphology and microstructure wereobserved by SEM and opticalMicroscope(OM).In addition,theelectric resistivity and coercivity field of the prepared sam ples were investigatedin order to prove that they all belong to the fam ily of SMCs.

2.Experimental

2.1.Material

The iron pow der(ASC 100.29,H?gan?s)with the size fraction in the range from 45 to 212 μm,FeSi with 3%of Si(H?gan?s)and Vitroperm(Fe73Cu1Nb3Si16B7)m illed pow der from stripe(Vacuum schm elze Gm bH&Co.KG)was used as the base ferrom agnetic material for the preparation of microcom posites.Pheno l(P,99%,Ald rich),form aldehyde(F,37%aq.,Ald rich),amm on ia(NH3,26%aq.,Ald rich),tetraethy lo rthosilicate(TEOS,99%,M erck),3-glycidoxyp ropyltrimethoxysilane(GLYMO,98%Ald rich)and boric acid(H3BO399.5%Lachem a)wereused for the synthesisofpure organic and hybrid organ ic-inorganic resins.Tetrahyd rofu ran(THF,99.9%,Ald rich)and ethano l(abso lu te,Fisher)were used as so lven ts.The base properties of three types of ferrom agnetic pow der particles and hybrid pheno lic resins are show n in Table 1.

Tab le 1 Properties of original pow ders and used hybrid resins

2.2.Composite preparation

The chemical syn thesis of pu re pheno l-form aldehyde resin(PFR)and hybrid pheno l-form aldehyde resin(PFRGT)m odified with silica using GLYMO and TEOS were carried out according to the procedu re thorough ly describedin our previous work[12].The new hybrid resin PFRB modified with boron was syn thesized by the analogous so l-gel process with excep tion that H3BO3was used as a source of boron additionally incorporatedin to the polym erm atrix.The initialm olar reaction ratio of Ph/F/NH3/H3BO3was 1/1.5/0.35/0.1.The chemical com position of resins was analyzed by FTIR spectrometry(Shim adzu,IRAffinity,KBr pellets 1 mg sam ple+300 mg KBr).The comparison of thermal degradation of the prepared resins was detected by TG-DSC analysis(METTLER 2000 C).

The hybrid resins were dissolved app roxim ately in 10 ml of solvents and the predetermined am ount of soft magnetic pow ders was mixedin this solution after com plete evaporation of solven t from the suspension.PFR and PFRB are soluble in ethanol,while PFRGT is so luble just in the THF.In this w ay,the core-shell pow ders Fe/PFRB,Fe/PFRGT,FeSi/PFRGT,vitroperm/PFRGT and vitroperm/PFRGT pre-annealed at 500°C were prepared.The com position of the final products is given in Table1.In order to prepare thefinalMicrocom posites,the coated pow der was pressed at 800 MPa into the required shapes for mechanical,electrical and magnetic testing.The prepared sam ples were cured under the am bient pressure up to 200°C.

2.3.Composite testing

The microstructure and morphology of all sam ples were examined by the scanning electron microscope(SEM)(JEOLJSM-7000F)and op tical microscope(OM)(Olym pus GX 71).The density of each prepared sam ple was determined by the helium pycnometer(AccuPyc II 1340,Microm eritics).The average diameters and surfaceareaofused pow der werem easured by granulometer(Malvern Mastersizer 2000)equipped with statistical softw are 5.60.Vickers hardness test HV10[STN-EN-ISO 6507-1(42 0374),MPIF 43]and electrical resistivity were measured on the cylind rical shaped sam ples with dim ensions of 10×3 mm(d×h)determined by Teraohmmeter-Picoam permeter Sefelec M 1501P.Theflexu ral strength transverse rup ture strength[TRS)(STN(42-0891-4),MPIF41]was detected on prism-shaped sam ples of dim ensions 5×4×2 mm(w×h×l).M agnetic measurements were recorded on the toroid-shaped sam ples with the outer diameter of 24 mm,the inner diameter of 17 mm and the heigh t of 2 mm.The direct cu rren t(DC)hysteresis loops at maxim um induction(Bm)of 0.1 T were measured by fluxmeter based hysteresisgraph.

3.Results and Discussion

3.1.FTIR and TG characterization of hybrid resins

The chemical structure of PFR and PFRGT as derived from FTIR and NMR analysis are illustratedin Fig.1(a)and(b)(see Ref.[13]for more details on chemical aspects of the relevan t polym eric structure).On the other hand,two typical bridging modes as show n in Fig.1(c)and(d)cou ld be expectedin the syn thesized PFRB resin according to the previously pub lished literature[24].

FTIRspectra for the native PFR,the hybrid PFRGTand PFRB resinsare depictedin Fig.2.The recorded FTIR spectrum for PFR gives evidence of the condensation reaction between phenol and form aldehyde as it has been already discussedin detail in ou r previous works and work by Po ljansek and Krajnc[13,25].FTIR spectrum of PFRGT dem onstrates reaction between resol-type phenolic resin and GLYMO.This spectrum displays noticeab le increase of two signals 2942 and 2881 cm-1,which reveals a presence of stretching vibration of-CH2-alkane arising from GLYMO.Moreover,the signal at 1458 cm-1(-CH2-methylene bridge)in the PFR spectrum was dividedin to two peaks in the spectrum of PFRGT:1492 cm-1(C-H aliphatic)and 1462 cm-1,which confirm the chemical linkage between phenolic resin and silica.The chemical bonding of Si to polym er matrix is also eviden t from the presence of symmetric stretching peak Si-O-Si and a new covalen t bond Si-O-C,which nearly overlap and give the signal at 1112 cm-1.The band at461 cm-1rep resen ts the deform ation vibration ofSi-O-Si,which is also clearly visible in the spectrum of PFRGT.The FTIR spectrum for the resole-type resin modified with boric acid was reported by Gao and co-workers[26].According to this work,the apparen t change in the PFRB spectrum in comparison with PFR spectrum is observab le in the range 1180 cm-1-900 cm-1.This qualitative change in the FTIR spectrum can be attributed to the chemical reaction of the methylol hyd roxyl group(1200 cm-1)with boric acid,which yields arom atic borate esters as displayedin Fig.1(c)and(d).

Fig.1.The typical chemical structures of(a)native PFR resin,(b)hybrid PFRGT resin modified by chemically bonded of SiO2,(c),(d)hybrid PFRB resin modified by boron.

Next,TG analysis was perform edin order to provide in form ation about thermal stability of synthesized resins.It can be seen from Fig.3 that pure PFR exhibits a sharp transition at 175°C,which can be attribu ted to a rapid evolution of water and other volatile by-p roducts(the solid-black line).Such a rapid crosslinking of polym er causes undesirable creation of cracks and foam ing of PFR resin on the sam ple surface[13,17].Con trary to this,a slow er and more gradual release of water and other vo latile by-p roducts can be observed for the hybrid PFRGT and PFRB resins,which is high ly dem anding from technological point of view owing to a reduction of mechanical im perfections low ering the shape and dim ensional stability of the prepared resins.The slow est release of water and other volatile by-p roducts is found for PFRB resin(dot-b lue line)without any sharp transitions,whereas this hyb rid resin sim u ltaneously exhibits the low est charge yield un til 700°C.Hence,it fo llow s that the boron-con taining phenolic resin PFRB has much higher heat oxidation resistance than the comm on pheno lic resin because O-B-O linkage has a higher oxidative resistance than the C-O-C ether linkage.

Fig.2.FTIR spectra of PFR,PFRGT and PFRB resins in uncured form.

3.2.Morphology and microstructure

Fig.3.TG curves of the native PFR,the hybrid PFRGT and PFRB resins.

It is w orthwhile to rem ark that three ferrom agnetic pow der materials used as core in the final core-shell microcom posites differ signi fican tly in their morphology as evidenced by SEM im ages depictedin Fig.4(a),(b),(c).The Fe(ASC 100.29)pow der with the typical rugged morphology[Fig.4(a)]along with the ideal spherical FeSi pow der[Fig.4(b)]was ob tained from H?g?nes Corporation[23].The very sharp and rectangu lar flakes of vitroperm were obtained after the milling of stripe distributed by Vacuum schm elze Gm bH&Co.KG[27].The macrostructure of three selected final microcom posites:(a)Fe/PFRB(Sam ple A),(b)FeSi/PFRGT(Sam ple C),and(c)vitroperm/PFRGT(Sam ple E)is show n in Fig.5(a),(b),(c).It is w orthy to notice that any im perfections like cracks,foam ing of the resins or exfo liation of the particles from the surface are not observed.M oreover,the shape and dim ensional stability of the final com posites is maintained even after the thermal treatm en t.Con trary to this,pheno lic resin without any modification causesa shape and dim ensionalinstability of thefinalsamples on the account of the foam ing of resin on the surface and evolution of vo latile by-p roducts during a crosslinking process of the polym eric resin[13].The PFRGT and PFRB resins can be therefore considered as very convenien t insu lating materials for a preparation of the coreshell microcom posites from the techno logical view poin t.A more detailed SEM im age on the microstructure of:(a)Fe/PFRB(Sam ple A),(b)FeSi/PFRGT(Sam ple C)and(c)vitroperm/PFRGT(Sam ple E)fractured surface isdepictedin Figs.6 and 7.The iron pow der covered by PFRB resin has a high tendency to hold together in a very tigh t arrangementwithoutany significant porosity[Fig.6(a)].The PFRBpolym er melts du ring the heat treatm en t,and consequently,it com pletely fills em pty space between Fe particles.SEM im age of the fractured surface in Fig.6(b)show s the uncovered FeSi particle due to the exfo liation of PFRGT resin after breaking of the FeSi/PFRGT sam ple and the callosity of resin after falling outofone sphericalFeSiparticle.The picture clearly confi rm s a precise coating of the surface of spherical FeSi particles by the hybrid PFRGT resin.How ever,it shou ld be mentioned that one generallyfinds in the microcom posites invo lving the spherical FeSi particles a higher porosity due to a w orse filling of the spacer of spherical pow der.The very in teresting building-wall arrangement is detectedin the microcom posites invo lving vitroperm[Fig.6(c)].The vitroperm flakes have a high tendency to slide on the resin,whereas the final structure is rem iniscent of building wall with vitroperm flakes serving as bricks and PFRGT resin asm ortar.How ever,it can be seen from Fig.7(c)that the relatively sharp edges of vitroperm flakes are responsib le for a creation of pores and regions filled by higher am ount of the PFRGT resin.

Fig.4.SEM im ages of the morpho logy of comm ercial ferrom agnetic base materials:(a)Fe(ASC 100.29),(b)FeSi spherical pow der,(c)vitroperm flakes.

Fig.5.SEM im ages displaying the macrostructure of the fractured surface of the prism-shaped sam ples designed from original pow der particles show n in Fig.4(a),(b),(c)and hyb rid polym eric resins:(a)ASC/PFRB(Sam ple A),(b)FeSi/PFRGT(Sam ple C),(c)vitroperm/PFRGT(Sam ple E).

Fig.6.SEM im ages displaying the detailed microstructure of the fractured surface of the sam ples:(a)ASC/PFRB(Sam ple A),(b)FeSi/PFRGT(Sam ple C),(c)vitroperm/PFRGT(Sam ple E).

Fig.7.OM im ages displaying the microstructure of the polished surface and distribution of resins between particles in the sam ples:(a)ASC/PFRB(Sam ple A),(b)FeSi/PFRGT(Sam ple C),(c)vitroperm/PFRGT(Sam ple E).

3.3.Material properties

The com position of thefinalcuredMicrocom positesis listedin Table 2 together with mechanical properties(hardness,flexural strength,density),electric resistivity and coercive field.Note that the higher am ount of PFRGT resin(4%)is added to the Sam ples C,D,E and F because of dif ficu lties connected with the com paction of spherical(FeSi)and flakeshaped(vitroperm)particles.How ever,the highest value of mechanical and transverse rup ture strength is measured for the Sam ple A,where the final microcom posite is designed from the Fe pow der coated by the low er am oun t of PFRB resin(3%).It is tem pting to con jecture that the extrem ely high mechanical strength of the Sam ple A originates from a stronger crosslinking of the boron-contain ing resin in comparison with the polym eric resin incorporating silica.The low value of porosity of the Sam ple A is also consistent with the highest density of this sam ple.In comparison with theother sam ples,the Sam ples Eand Fexhibit substantially low er mechanical strength,which can be attributed to a w orse ability of vitroperm flakes to create com pact arrangement in the bu lk.Moreover,the com posites including the vitroperm are brittle and susceptible to exfoliation,the transverse rup ture strength of the Sam plesEand F cou ld notbe thereforem easured.Itcan beeasily understood from Table1 that the prepared microcom posite materials indeed belong to the fam ily of SMCs with the coercive force in the range 0.06-0.2 kA·m-1.The very uniform and hom ogeneous distribution of PFRB and PFRGT coating on the surface of ferrom agnetic pow der particles is also evidenced by a rather high value of the specific electric resistivity reaching the values up to 108μΩ·m.

To bring a deeper understanding in to material properties and their relation to the underlying com position,the properties of the individual sam ples are com pared according to the base ferrom agnetic particles used.only sm all differences in SEM im ages are visib le between com posites Fe/PFRBand Fe/PFRGT[Fig.8(a),(b)].Both sam ples include chemically modified resins with app rox im ate ly the sam e density prio r to cu ring,and hence,the distribu tion of polym eric resin around the Fe(ASC 100.29)particles is quite similar.How ever,the extraordinary high mechanical hardness of the sam ple based on PFRB resin can be with a high certainty related to a special molecular structure of the polym eric network arising out du ring the crosslinking process.The precise structure and mo lecu lar ordering of the prepared hyb rid o rgan icinorgan ic resin willbe sub jectofour future investigation.A comparison between the com posites involving the sam e individualcom ponents but different sizes of ferrom agnetic FeSi pow ders are show n in Fig.9(a),(b)and(c).An enorm ous increase in the specific electric resistivity of the sam ple FeSi(355μm)/PFRGT(the Sam ple D),which isseveralorders of magnitude greater than that of other com parab le sam ples,can be connected to the largestdiameter ofsphericalFeSi particles.Asa matter of fact,the sphericalnatureof the large FeSiparticlesallow sonly a sm all contact between surfaces of neighboring spherical particles,whereas the residual space can be filled with the dielectric resin that gives rise to a subsequent increase of the specific electric resistivity.The difference between thesam plesvitroperm/PFRGT(the Sam ple E)and pre-annealed vitroperm/PFRGT(the Sam ple F)is depictedin Fig.10(a)and(b).Vitroperm was pre-annealedin order to create nanocrystalline centers in the originally am orphous particles,which shou ldim prove in general the overall magnetic properties.It is quite eviden t from Tab le 1 that the density and coercive force of both vitroperm-based sam ples rem ain nearly the sam e,bu t the mechanical hardness and electric resistivity is much low er for the sam ple containing the pre-annealed vitroperm.A deterioration of the mechanical hardness is attributable to a disordering of the building arrangement[see Fig.10(a)and(b)],which has been already men tionedin Section 3.2.

3.4.Magnetic properties

DC magnetic properties of the sam ples are illustrated by hysteresis loops measured at Bm=0.1 T.A slope of the hysteresis loop determining the perm eability[28]is the highest for the sam ple Fe/PFRGT(the Sam ple B)(Fig.11),but the coercive fieldis relatively large quite similarly as forFe/PFRB(the Sam ple A).As usual,the perm eability is low er for the sam ples with sm aller base ferrom agnetic particles.A similar situation concerns with the perm eability of the Sam ples C and D,which involve spherical FeSi particles with the diameter of 150 μm and 355 μm(see Fig.12).The low est coercive field and hysteresis losses(p roportional to the areaofhysteresis loop)is detected for the Sam ples Eand Fcontaining Vitroperm due to its excellent soft magnetic properties[29].On the other hand,the pores between the flaky-shaped Vitroperm are responsible for the low er density,which consequently reduces also the perm eability(see Fig.13).The pre-annealing at500°Ccauses the rise of the brittleness of this material[30],leading to a reduction of the particle size after com paction.

Tab le 2 Com position of final microcom posites andits mechanical properties,densities and coercive fields

Fig.8.SEM im ages displaying the microstructure of the fractured surface of the sam ples(a)Fe(ASC)/PFRB(Sam ple A)and(b)Fe(ASC)/PFRGT(Sam ple B).

Fig.9.SEM and OM im agesdisplaying themicrostructure of the fractured surfaceand distribution ofm odified resins in the polishedMicrocom posites(a)FeSi(150μm)/PFRGT(Sam ple C),(b)FeSi(355 μm)/PFRGT(Sam ple D),(c)FeSi(150 μm)/PFRGT(Sam ple C),(d)FeSi(355 μm)/PFRGT(Sam ple D).

4.Conclusions

Fig.10.SEM im ages displaying the microstructure of the fractured surface of the sam ples:(a)vitroperm/PFRGT(Sam ple E),(b)pre-annealed vitroperm/PFRGT(Sam ple F).

The presentarticle dealtwith the design andm aterialp ropertiesofa novel class of SMCs,which were prepared from the base ferrom agnetic pow der particles insu lated by the phenolic resins modified with either silica or boron.The chemical incorporation of the inorganic fillers to the phenol-form aldehyde polym eric structurewasverified by FTIRanalysis.The therm ogravimetric analysis confirm ed a very slow and gradualevolution of water and other volatile by-p roducts from the hybrid organicinorganic resin du ring the cu ring,and w hat was necessary condition from the technological poin t of viewin order to main tain of the shape and dim ensional stability of the final microcom posite sam ples.It was demonstrated that an incorporation of boron in to the polym er matrix caused a considerab le increase in the mechanical strength of the Fe/PFRB com posite(nearly three times)in comparison with the SMCs containing the pheno lic resin modified with silica.The similar effect was also observed after mechanical tests of transverse rup ture strength.SEM analysis of the fractured surface proved the uniform distribution of the insu lating coating on the surface of base ferrom agnetic particles,and w hat is them ost im portant requirement in order to achieve desired magnetic properties.The rather high specific electric resistivity show ed an independen t confirm ation of the su fficien t covering of the surface of ferrom agnetic particles by the insu lating po lym er.The DC magnetic propertiesofSMCswere strongly influenced by notonlym agnetic properties of ferrom agnetic com ponen t including particle size,but also the density of the com posite depending on the properties and am ount of isolation matrix and pores con ten t.

Fig.11.DChysteresis loops of Fe/PFRB(Sam ple A)and Fe/PFRGT(Sam ple B)at B m=0.1 T.

Fig.12.DC hysteresis loops of FeSi(150 μm)/PFRGT(Sam ple C)and FeSi(355 μm)/PFRGT(Sam ple D)at B m=0.1 T.

Fig.13.DC hysteresis loops of vitroperm/PFRGT and pre-annealed vitroperm/PFRGT at B m=0.1 T.