Effect of particle size on the preparation and microwave absorption properties of FeSiAl magnetically soft alloy hollow microspheres

Xu-dong Cai,Xiao-jun Jiang,Wei Xie,Jing-yang Mu,De-fei Yin

63983 Troops,Wuxi,214035,China

Keywords:FeSiAl magnetically soft alloy Hollow microspheres Particle size Microwave absorption properties Low frequency

ABSTRACT FeSiAl magnetically soft alloy hollow microspheres(MSAHMs)were prepared by self-reactive quenching technology based on Fe+Si+Al+KNO3reactive systems,in order to obtain absorbents with light weight,low frequency and high efficiency.Firstly,twice-balling adhesive precursor method was used to obtain FeSiAl magnetically soft alloy agglomerate powders.Then agglomerate powders with the mesh number of 150-240,240-325 and 325-400 were sprayed through the flame field into the quenching water.At last,FeSiAl MSAHMs with coarse(average at 86.97μm),medium(average at 52.16μm)and fine particles(average at 31.80μm)were got.Effect of particle size on the phases and microwave absorption properties in low frequency band was studied by XRD and vector network analyzer.The results show that,Fe3Si0?7Al0.3and Fe3Si0?5Al0.5appear in the phase components of FeSiAl MSAHMs,which is important to improve the microwave absorption properties in low frequency.In addition,the real part(ε′)and imaginary part(ε′′)of complex permittivity,the real part(μ′)and imaginary part(μ′′)of complex permeability of FeSiAl MSAHMs all present the trend of fine particles＞medium particles＞coarse particles.The microwave absorption properties in low frequency are improved with the increasing of particle size,and the absorption peak moves to lower frequency range.The properties of fine particles are the best.Their matching thickness of samples is at 5 mm,and the minimum reflectivity is-43 dB at this thickness.The absorption frequency band lower than-10 dB is 4.6-7.6 GHz with a bandwidth of 3 GHz.

1.Introduction

As the electromagnetic field environment of battle field gets worse,safety performance and viability of weapon systems are meeting serious challenge.In order to improve this condition,invisible technology is greatly developed,which is the high-tech product of original camouflage technology.Absorbents is an important application during the invisible technologies.Especially,the development of radar absorbents in 0.5-18GHz has great significance on the improving the viability of weapon systems in complex electromagnetic field environment.However,the investigation on the absorbents in high frequency band of 8-18GHz are becoming mature[1],and the study on low frequency band(＜8 GHz)is relatively scarce.This makes it difficult for electromagnetic shielding materials with large wavelength to meet the need of military and civility.

Characteristics of high saturation magnetiation,FeSiAl alloy has great microwave absorption properties in low frequency[2].Previous investigations[3]of mine have shown that FeSiAl MSAHMs with uniform particle size distribution prepared by self-reactive quenching technology based on Fe+Si+Al+KNO3reactive system.The results show that,the density of original FeSiAl alloy is decreased by 38%,and the microwave absorption properties are improved.The minimum reflectivity of the absorbent sample is-22.1dB when the thickness is at 5mm,and the corresponding frequency is 7.0 GHz.The effective absorption frequency band lower than-10 dB is 5.3-8.5 GHz,with the bandwidth of 3.2GHz.

Effect of particle size on the permittivity,permeability and microwave absorption properties of magnetically soft alloy is significant[4].Several investigations have been conducted in recent years.Influence of particle size on the microwave absorption properties of FeCrSiAl alloy was studied by Wang et al.[5].The results show that the permittivity and permeability increases with the increasing of particle size,and the absorption peak moves to lower frequency range.When the average particle size is at 48μm,the minimum reflectivity of the absorbent sample is-6.3dB when the thickness is at 1 mm,and the corresponding frequency is 3.3 GHz.The effective absorption frequency band lower than-5 dB is 2.2-4.1 GHz,with the bandwidth of 1.9 GHz.In addition,Liu et al.[6]also found that decreaing the equivalent particle size is benifical for improve the microwave absorption properties of FeSi alloy.

In this paper,twice-balling adhesive precursor method was used to obtain FeSiAl magnetically soft alloy agglomerate powders.And then agglomerate powders with the mesh number of 150-240,240-325 and 325-400 obtained by Taylor's classification method were sprayed through the flame field.Self-reactive quenching technology was used to prepare FeSiAl MSAHMs with coarse,medium and fine particles.Effect of particle size on the phases and microwave absorption properties in low frequency band was studied.As a result,this paper is used to develop a method of decreasing the density of FeSiAl alloy materials,and adjust the microwave absorption properties of FeSiAl alloy by controlling particle size.This will provide a feasibility scheme for the development of absorbents in low frequency.

2.Experiments

2.1.Materials and methods

Analytical reagent raw materials of Fe powders,Si powders,Al powders,KNO3powders,sucrose(precursor of C,20 wt%,in order to obtain agglomerate powders with fine fluidity)and epoxy resin(bonding agent,10wt%,used to bonding reactive units)were selected.Related information is shown in Table 1.Twice-balling adhesive precursor method was used to obtain FeSiAl magnetically soft alloy agglomerate powders.The preparation technology was as follows.

Firstly,Fe powders,Si powders and Al powders were put into ND7-1L planetary ball mill with the stoichiometry ratio shown in Eq.(1).Sucrose was added into the ball mill to be grinded for 6 h so that particles could be sufficiently grinded.Secondly,the mixtures were transformed to wet batch ball mill,and anhydrous ethyl alcohol was taken as the medium sphere to be grinded for 6 h.After that,the mixtures were taken out and put into an agitator,and epoxy resin was added into the agitator to be stirred for 2 h.Finally,all the mixtures were dried and carbonized at 150oC in 101-2A horizontal drying cabinet until no smoke released,and then they were comminuted by the FW177 disintegrator.Then agglomerate powders with the mesh number of 150-240,240-325 and 325-400 were obtained by Taylor screening.Fig.1 illustrates the SEM images of agglomerate powders with the mesh number of 240-325.It can be concluded that the agglomerate powders have obvious arris edges and uniform particle size distributions.And it can be found that the agglomerate powders have fine fluidity in the experiment.However,previous investigations showed that the agglomerate powders prepared by adhesive precursor method did not have obvious arris edges and fine fluidity[7].This could be ascribed to the low hardness of FeSiAl alloy particles.Twice-balling adhesive precursor method used in this paper effectively solve the problem.In this experiment,epoxy resin was used to cooperatewith sucrose to enlarge the contact area of the components in the agglomerate powders and to increase the bonding strength of reactive components.It can be transformed to CO,CO2and H2O in SHS reaction due to the high temperature.In addition,KNO3is used as the oxygen source of SHS reactions.The reaction activity is increased and the initial temperature of SHS reaction is decreased.As a result,SHS reactions are completed finished and FeSiAl phase can obtained easily.

Table 1 Raw materials and chemical composition used in the experiments.

2.2.Mechanism of self-reactive quenching technology

Fig.2 shows the preparation diagram of FeSiAl MSAHMs.As shown in the figure,the agglomerate powders are sprayed into the oxyacetylene flame field(about 3500 K)through a CP-D type highenergy flame-spraying gun,and the temperature of the materials increases gradually.When reaching the ignition temperature,the SHS reactions of Fe+Si+Al+KNO3occur promptly and the temperature of reactive system exceeds the melt point of products.So droplets are generated.Simultaneously,large volumes of gases are produced,which results in the droplets with hollow structures.Then the droplets are quenched quickly into distilled water.As the gas cannot escape from the droplets,FeSiAl MSAHMs are obtained immediately after the quenching products are dried and filtered.In the experiment,the flame field is used to ignite the SHS reaction.And once the reactions occur,they can be selfsustaining based on the heat released by themselves and the flame field is not required.

In this experiment,O2is selected as the protective gas and it provides an oxygen-enriched atmosphere,which are benefical to obtain hollow structures.In addition,the rate of O2and acetylene is set as 1.4.The quenching distance was set to 500mm,which guarantee the high balling rates of FeSiAl MSAHMs.

2.3.Characterization of the samples

The morphology of the quenching products was detected by scanning electron microscope(SEM QUANTA FEG-250;FEI,Syracuse,NY).The phase composition of the quenching products was studied using X-ray diffraction(XRD BRUKER D2 PHASER,AXS,Berlin,Germany),with Cu Ka radiation(k=0.15418 nm).Scanning step is 0.02o,and scanning degree is from 10oto 80o.Particle size distribution was measured using the Beckman Coulter LS 13320 laser particle size analyzer(testing from 0.04μm to 2000μm).The density of FeSiAl MSAHMs was determined through Archimedes method.

The absorbing composite was prepared by molding and curing the mixture of FeSiAl MSAHMs and paraf fin.Paraf fin was used as a polymer matrix due to its good flexibility and wave-transparent property.The mix ratio of FeSiAl MSAHMs-to-paraf fin was 3:2 by weight.The testing specimens have a toroidal shape with both thickness being 2.5 mm,and outer and inner diameters being 7.0 mm and 3.0 mm,respectively.The ε′, ε′′, μ′and μ′′versus frequency were measured by coaxial reflection/transmission method using vector network analyzer(Agilent-N5242A;Agilent,Los Angeles,CA)in 1-18 GHz range.The absorbing characteristics can be represented as the reflection loss(R.L.),as shown in Eqs.(2)and(3).Where Zinis the normalized input impedance relating to the impedance in free space, εr= ε′-jε′′, μr= μ′-jμ′′are the complex relative permeability and permittivity of the material,respectively,d is the thickness of the absorber,and C and f are the velocity of light and the frequency of microwave in free space,respectively.The impedance matching condition is given by Zin=1 to represent the perfect absorbing properties.The impedance matching condition is determined by the combinations of six parameters ε′,ε′′,μ′,μ′′,f and d.Also,knowing εrand μr,the R.L.value(in dB)versus frequency can be evaluated using metlab 8.0 at a specified thickness.

3.Results and discussion

3.1.Effect of particle size on the microstructures of FeSiAl MSAHMs

Fig.3(a)-(c)illustrate the SEM images of FeSiAl MSAHMs prepared by agglomerate powders with the mesh number of 150-240,240-325 and 325-400,respectively.From the figures,it is indicated the FeSiAl MSAHMs prepared by agglomerate powders at 150-240 mesh has the maximum particle size.That at 325-400 mesh has the minimum particle size.According to the particle size,FeSiAl MSAHMs prepared by 150-240 mesh,240-325 mesh and 325-400 mesh are defined as coarse,medium and fine microspheres.It also can be concluded that three kinds of microspheres,including closed type microspheres,opening type microspheres and several irregular particles.The formation of irregular particles is mainly composed of two reasons.One is that agglomerate powders with large particle size enter when Taylor screening.The other is that,when the agglomerate powders fly in the flame field,particles at the edge of flame have not completed melt and then enter quenching medium.

Beckman Coulter LS 13320 laser particle size analyzer was used to measure the particle size distribution of FeSiAl MSAHMs,and the result was shown in Fig.4.From Fig.4,it is found that the average particle size of coarse,medium and fine microspheres are 86.97μm,52.16μm and 31.80μm,respectively.In addition,the specific surface area increases gradually with the decreasing of particle size.

Fig.5 shows XRD patterns of FeSiAl MSAHMs with different particle sizes.From the figure,it can be concluded that the main phase components of coarse spheres are constituted of Fe3Si0?7Al0.3,Fe3Si0?5Al0.5,Fe0?9Si0.1,Fe and Si.Those of medium spheres are constituted of Fe3Si0?7Al0.3,Fe3Si0?5Al0.5,Fe0?9Si0.1and Fe.And those of fine spheres are Fe3Si0?7Al0.3,Fe3Si0?5Al0.5,Fe0?9Si0.1,Fe3Si and Fe.In addition,diffraction peaks of Si can be found in XRD patterns of coarse spheres,while no diffraction pattern of Si appears in XRD patterns of medium and fine spheres.It can be concluded that Si has melt in Fe,and solid solution ofα-Fe(Si,Al)with bcc structure has been formed.The diffraction peaks of phases except Si are almost coincide with each other,and each diffraction peak can be distinguished only by magnification of XRD patterns.Different from mechanical alloying,self-reactive quenching technology has complex chemical reactions and extremely short reactive time.So there are unreacted phases in the products,including Fe and Si.

3.2.Effect of KNO3on microwave absorption properties of FeSiAl MSAHMs

The frequency dependence of complex permittivity and permeability for FeSiAl MSAHMs with different particle sizes is shown in Fig.6.Fig.6(a)and(b)show comparative charts of permittivity(ε=ε′-jε′′).It is shown that the real part of permittivity(ε′)and the imaginary part(ε′′)have the same changing trend,that is fine spheres＞medium spheres＞coarse spheres.The dielectric properties of fine spheres are better than those of other two spheres,which may be ascribed to the following aspects.

On one hand,FeSiAl MSAHMs are dispersed in paraf fin,and large amount of hollow microspheres/paraf fin interfaces appears.The smaller particle size is,the larger specific surface area is(also proved by the data of Fig.4),the larger area of hollow microspheres/paraf fin interface is,and the stronger space-charge polarization effect is[8].On the other hand,the volume fraction of FeSiAl MSAHMs in the composites gradually increases with the decreasing of particle size.According to effective medium theory[9,10],the smaller particle size of hollow microspheres is,the better dielectric properties are.Moreover,the smaller particle size of hollow microspheres is,the more free electrons are.The stronger effect of charge accumulation on the interfaces is,the bigger electric conductivity is,and the larger permittivity is[11].

Fig.6(c)and(d)show comparative charts of permeability(μ=μ′-jμ′′).It can be found that the real part of permeability(μ′)decreases with the increasing of frequency,so does the imaginary part of permeability(μ′′).The value ofμ′has the changing trend of fine spheres＞medium spheres＞coarse spheres when the frequency is lower than 11GHz.The value ofμ′′shows the changing trend of finespheres＞medium spheres in 0.5-3 GHz and 9.2-12 GHz,while there is little difference of μ′′between fine spheres and medium spheres in other frequency bands.The value ofμ′′for coarse spheres is smaller than that of fine spheres in all frequency bands.According to Fig.6(c)and(d),it can be concluded that permeability has an increasing trend with the decreasing of particle size.Investigations show that the effect of particle size on permeability of FeSiAl MSAHMs relates to eddy current loss,which is useless[4].And the decreasing of particle size is useful for FeSiAl MSAHMs to suppress eddy current loss.As a result,FeSiAl MSAHMs with small particle size have high permeability.In addition,with the decreasing of particle size,the area of hollow microspheres/paraf fin interface is getting larger,the volume fraction of FeSiAl MSAHMs in the composites is getting higher,and the coupling effect of magnetic moment among adjacent spheres is getting stronger[12].Magnetic moment is composed of orbital magnetic moment and spin magnetic moment.Orbital magnetic moment is the magnetic moment of electrons orbiting from the nucleus,and spin magnetic moment is the magnetic moment of electrons rotation.So orbital magnetic moment and spin magnetic moment are getting stronger with the decreasing of particle size.As the magnetic properties of the material result from the magnetic moment of atoms,magnetic properties of FeSiAl MSAHMs with small particle size are better than those with big particle size.Meanwhile,spheres with small particle size can effectively fill the void among the particles,the magnetic bridge effect is strengthened[13],and permeability is increased.

The calculated reflectivity curve of FeSiAl MSAHMs with different particle size is shown in Fig.7.Fig.7(a),(b)and(c)are the reflectivity curves of coarse,medium and fine spheres,respectively.As the figure shows,In fact,the absorption peak moves to low frequency with the increasing of thickness.This is because that when the thickness of absorbent is equals the odd times of 1/4 wavelength,there will be interference resonance between electromagnetic waves and the absorbent.When the electromagnetic waves reflected from the upper surface and the lower surface superimpose upon each other,and interference appears.So the energy of electromagnetic waves is strongly weakened.And the absorption peaks appear one by one with the increasing of the thickness.The best matching thicknesses of coarse spheres and medium spheres are both lager than 7mm,while best matching thicknesse of fine spheres is 5mm.At this thickness,the minimum reflectivity is-43 dB,and the effective absorption frequency band less than-10dB is 4.6-7.6 GHz,with the bandwidth of 3GHz.Fine spheres also have excellent microwave absoption properties when the thickness is at 2 mm.The effective absorption frequency band less than-10dB is 9.0-16 GHz in the whole frequency range,and that is 3 GHz in＜12 GHz proportional to the frequency.And there are two peaks in the whole frequency range,which is called“double absorption peak”.

In order to meet the need for novel absorbents which are“thin,light,wide and strong”,the thickness of the absorbents should not be too large.The best matching thicknesse of fine spheres is 5 mm,so 5 mm is selected as the standard thickness to investigate the effect of particle size on the microwave absorption properties.thickness.The reflectivity curves of FeSiAl MSAHMs at 5mm with different particle size are shown in Fig.8.As Fig.8 shows,with the increasing of particle size,the minimum reflectivity decreases,the microwave absorption peak moves to lower frequency range,and the effective absorption frequency bandwidth less than-10 dB gets larger.The parameters of microwave absorption properties are shown in Table 2.According to the table,the microwave absorption properties in low frequency increase with the decreasing of particle size.This may be attributed to the following reasons.

Firstly,excellent permittivity and permeability will improve the microwave absorption properties of the absorbents.Especially,ε′′and μ′′with high values will increase the dielectric loss and magnetic loss.FeSiAl MSAHMs with little particle size have a higher value of ε′′and μ′′in low frequency,as shown in Fig.6.As a result,the microwave absorption properties in low frequency increase with the decreasing of particle size.

Secondly,Si can be seen in the phase components of coarse spheres,which has no microwave absorption properties.For medium and fine spheres,Si has melt in Fe,and solid solution ofα-Fe(Si,Al)with bcc structure has been formed.Fe3Si0.5Al0.5and Fe3Si0?7Al0.3appear in phase components,which have magnetocrystalline anisotropy.This effect will generate an anisotropy field in the materials,and the magnetic moment will have a precession with damping around this field.When the frequency of incident alternating magnetic field equals the frequency of effective field of

Thirdly,the smaller particle size is,the stronger scattering effect of the spheres is.De fining the interface of air and the absorbent as x=0,when electromagnetic wave enter the absorbent in the direction of x=0,energy of electromagnetic wave at x=0 can be calculated by Eq.(4)as follows[14].

here,I0is the energy of the incident wave,n is quantity of absorbent particles per unit volume,andσscis scattering interface of absorbent particles.When the particle size of FeSiAl MSAHMs decreases,the value of nσscincreases.As a result,more energy of electromagnetic wave sis absorbed due to the decreasing of particle size,and the microwave absorption properties are improved.

4.Conclusions

FeSiAl magnetically soft alloy hollow microspheres(MSAHMs)were prepared by self-reactive quenching technology based on Fe+Si+Al+KNO3reactive systems.Firstly,twice-balling adhesive precursor method was used to obtain FeSiAl magnetically soft alloy agglomerate powders.Then agglomerate powders with the mesh number of 150-240,240-325 and 325-400 were sprayed through the flame field into the quenching water.At last,FeSiAl MSAHMs with coarse(average at 86.97μm),medium(average at 52.16μm)and fine particles(average at 31.80μm)were got.With the decreasing of particle size,Fe3Si0?5Al0.5and Fe3Si0?7Al0.3appear in phase components,which have magnetocrystalline anisotropy.Space-charge polarization effect gets stronger,the volume fraction of FeSiAl MSAHMs in the composites gradually increases,and the conductivity gets larger.These changes make permittivity has the trend of increasing.Eddy current loss is suppressed,and magnetic bridge effect is strengthened.The two changes also make permeability has the trend of increasing.

With the decreasing of particle size,the soft magnetism absorbing phases including Fe3Si0?5Al0.5and Fe3Si0?7Al0.3appear,complex permittivity and permeability increase,and the scattering effect of the spheres gets stronger.These changes make the microwave absorption properties of FeSiAl MSAHMs increases,and those of fine spheres are the best.The best matching thicknesse of fine spheres is 5 mm.At this thickness,the minimum reflectivity is-43 dB,and the effective absorption frequency band less than -10 dB is4.6-7.6 GHz,with the bandwidth of 3 GHz.These fine spheres sufficiently meet the need for novel absorbents which are“thin,light,wide and strong”.Decreasing of particle size is beneficial for improving the magnetic absorption effects and microwave absorption properties of FeSiAl MSAHMs.magneto crystalline anisotropy,the resonance will happen.In the action of the natural resonance,FeSiAl MSAHMs absorb electromagnetic waves dramatically.As a result,the microwave absorption properties of fine and medium spheres are stronger than those of coarse spheres.

Table 2 Microwave absorption properties of FeSiAl hollow microspheres samples with different particle sizes at 5 mm.

Acknowledgements

Authors would acknowledge financial support of National Natural Science Fund of China(No.51172282)and Hebei Natural Science Fund of China(No.E2015506011).Authors also wish to thank Xuliang Lv for valuable experimental measurement.