Journal of Magnetism and Magnetic Materials 332 (2013) 15–20

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Journal of Magnetism and Magnetic Materials

0304-88

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n Corr

E-m

jun_cai@

journal homepage: www.elsevier.com/locate/jmmm

Microwave characteristics of low density flaky magnetic particles

Zhang Wenqiang a,b, Zhang Deyuan a, Cai Jun a,n

a Bionic and Micro/Nano/Bio Manufacturing Technology Research Center, Beihang University, Beijing 100191, PR Chinab College of Engineering, China Agricultural University, Beijing 100083, PR China

a r t i c l e i n f o

Article history:

Received 3 September 2011

Received in revised form

9 September 2012Available online 30 November 2012

Keywords:

Diatomite

Flaky magnetic particle

Electromagnetic wave absorbing

a-Fe

Chemical vapor deposition

53/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.jmmm.2012.11.021

esponding author. Tel./fax: þ86 10 82316603

ail addresses: zwqzwqzwqzwq@126.com (Z. W

buaa.edu.cn (C. Jun).

a b s t r a c t

Diatomite coated with thin Fe films were obtained by the Chemical Vapor Deposition process. The

resultant Fe-coated flaky diatomite particles had low densities (2.7–4.0 g/cm3) and high saturation

magnetization (93–157 emu/g). Annealing treatment led to grain growth and an increased saturation

magnetization. The high frequency properties of the composites consisting of Fe-coated flaky diatomite

particles and wax were investigated. The permittivity and permeability increased with increasing flaky

magnetic particles content in the composite and increasing the Fe weight percentage of the particles.

The reflection loss of the composite was found dependent on the absorber material thickness, wax:flaky

magnetic particles ratios, the Fe content, as well as the annealing treatment. At a thickness of 1 mm, the

composite records a minimum reflection loss of �18 dB at 6 GHz.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

With rapid development of wireless communication technology,in order to reduce the threat to the environment, human health, andmilitary security caused by electromagnetic wave (EM-wave), theelectromagnetic absorbing material is emerged [1,2]. Fine magneticmaterials, such as powders of iron [3] and ferrites [4], embedded inan insulating matrix are widely used as microwave absorbers.However, the conventional absorptive materials have difficulties inincreasing the permeability in gigahertz region because of Snoek’slimit for ferrites [5] or eddy current loss for magnetic metals [6].Also, those materials are quite heavy, which restricts their useful-ness in applications requiring light weight mass [7,8]. The use ofmetal thin films coated on microspheres or micro-organisms of lowdensity may be one of the ways to overcome these problems [9].

The flake is considered to be one of the best shapes of EM-wave absorbing particles, which would improve their electro-magnetic parameters [10] and increase the strength of EM-waveabsorbing materials. Currently, the flake electromagnetic waveabsorbing particles are generally obtained by milling. However,no study has ever been carried out on the fabrication of disk flakeshape iron. The diatomite mineral is formed by Coscinodiscus sp.,which fixes and concentrates silicon [11]. The main chemicalcomposition of flaky diatomite particles is SiO2. It is a good basematerial for synthesizing the EM-wave absorbing materials for its

ll rights reserved.

.

enqiang),

low density, chemical stability, disk flake shape and rich pore-structure.

In the present investigation, we propose Fe metal thin film asthe coating layer on the flaky diatomite particles and we haveconducted a preliminary work on the processing and properties ofthese Fe-coated flaky diatomite particles. The microstructure,static magnetic properties, and dynamic electric and magneticproperties of Fe-coated flaky diatomite particles of low densityhave been reported in this paper.

2. Experimental section

2.1. Materials

Commercially available diatomite C292 were obtained fromCelite Corporation. The flaky diatomite particles are mainly com-posed of SiO2 (70–90%), with a diameter range of 20–60 mm,thickness range of 2–10 mm and bulk density of 0.4–0.9 g/cm3.

2.2. Preparation of flaky magnetic particles

Coating the flaky diatomite was conducted by the ChemicalVapor Deposition (CVD) process. The preparation was started byadding diatomite to the reactor with N2 atmosphere, followed byheating the reactor to 300 1C and keeping the temperature constant.Then the diatomite was stirred at 100 r/min, and Fe(CO)5 steam wasadded into the reactor until the reaction is completed.

After completion of the reaction, cool down the sample in thereactor to room temperature, and remove it for use.

Z. Wenqiang et al. / Journal of Magnetism and Magnetic Materials 332 (2013) 15–2016

Put some sample into the muffle furnace with N2 atmosphere,and annealed it at different temperatures (550 1C, 700 1C) for 1 h.After that, cool down the sample in the furnace to room tem-perature, and remove it for use.

2.3. Characterization

The phase structure analysis of products was identified (within2y range of 20–851) using a X-ray diffractometer (XRD, RigakuD/max-3) utilizing Cu-Ka X-radiation of wavelength 1.5418 A.A scanning electron microscopy (SEM, CS3400) equipped with anenergy dispersion X-ray spectroscopy (EDS, INCA X-sight 7421)was used to analyze the surface morphology and the sizedistribution of the flaky particles, while EDS analysis was doneto clarify their chemical makeup. Their magnetic properties werethen evaluated on a vibrating sample magnetometer (VSM, JDM-13), and the field reached up to 1.5�104 Oe. A vector networkanalyzer was used to measure the microwave parameters of thesamples over the frequency range of 2–18 GHz. The cylindricaltoroidal measurement samples were composed of the developedferrite flakes randomly dispersed in wax with various weightratios (40 wt%, 50 wt%, and 55 wt%). The sample had an innerdiameter of 3 mm, an outer diameter of 7 mm and a thickness of2 mm. Microwave absorption properties were evaluated by thereflection loss (RL), which was derived from the followingformulae [12]: where Zin is the input impedance of absorber,Z0 is the impedance of air and c the velocity of light.

RL¼ 20logZin�Z0ð Þ

ZinþZ0ð Þ

�������� ð1Þ

Zin ¼ Z0

ffiffiffiffiffimr

er

rtanh j

2pf d

c

ffiffiffiffiffiffiffiffiffimrer

p ��ð2Þ

Fig. 1. SEM images of: (a) flaky diatomite particles, (b) Fe-coated flaky particles (pla

particles (plated for 3 h).

3. Results and discussion

Fig. 1 shows the SEM images of flaky diatomite particles andFe-coated flaky diatomite particles by the CVD process in variouscoating time (1 h, 2 h, and 3 h). Fig. 1(a) shows the appearance offlaky diatomite particles without coating, whose surfaces are verysmooth. It could obviously be seen that the diatomite possesses aporous arrays structure and a disk flake shape. Fig. 1(b)–(d) showsthe appearance of Fe-coated flaky diatomite particles, demon-strating that all of the flakes are coated with Fe nanoparticles. Thesurfaces of flakes are very coarse, and their porous arraysdisappear. It indicates that Fe nanoparticles continuously depositon the surfaces of flakes. It was found that with the reaction timeincreases, the Fe film thickness and grain size becomes larger andlarger. In current investigation, we could coat the flaky diatomiteparticles with various Fe film thicknesses by controlling thecoating time.

The EDS patterns of flaky diatomite particles and flaky diato-mite particles with Fe coating are presented in Fig. 2. It is clearthat the flaky diatomite particles consist of Si and O elements(Fig. 2(a)). As to flaky diatomite particles with Fe coating(Fig. 2(b)), elements Fe emerge apart from the elements of flakediatomite particles (Si and O elements) due to Fe nanoparticlescoating flaky diatomite particles. The X-ray diffraction (XRD)pattern for as-plated Fe-coated flaky diatomite particles (platedfor 1 h) is shown as curve (a) in Fig. 3, indicating an amorphous ornanocrystalline structure. The strong background intensity andunclear peak in the XRD spectrum result from the amorphoussubstrate (flaky diatomite particles) due to the rather thin coatinglayer. To investigate the effect of crystallization on the properties,part of as-plated diatomite particles was annealed in the mufflefurnace with N2 atmosphere at different temperatures (550 1C,700 1C) for 1 h. Fig. 3(b) and (c) shows the XRD patterns ofannealed flaky particles at 550 1C and 700 1C, respectively. FromFig. 3(a) well crystallized structure has been demonstrated.

ted for 1 h), (c) Fe-coated flaky particles (plated for 2 h), and (d) Fe-coated flaky

Fig. 2. EDX spectra of flaky diatomite particles before (a) and after (b) coated

(plated for 1 h).

Fig. 3. XRD patterns of: (a) Fe-coated flaky diatomite particles (plated for 1 h),

(b) annealed the flaky magnetic particles at 500 1C and (c) annealed the flaky

magnetic particles at 700 1C.

Fig. 4. Hysteresis loops for selected Fe-coated flaky diatomite particles (a) before

and after annealing and (b) with various plated time.

Z. Wenqiang et al. / Journal of Magnetism and Magnetic Materials 332 (2013) 15–20 17

As shown in Fig. 3, the composition of the diatomite coating wasa-Fe because the measured values of crystal face distance wereconsistent with the theoretical values of a-Fe (JCPDS No.06-0696). An apparent grain growth after annealing has beenobserved. For as-plated film, the grain size is around 15 nm, butfor annealed film, it is 20–80 nm.

The hysteresis loops for selected Fe-coated diatomite under alarge applied field are shown in Fig. 4. A low coercivity was obtainedin all samples. Fig. 4(a) shows that the saturation magnetization (Ms)is greatly increased after annealing, also indicating that a crystal-lization process happened. Fig. 4(b) shows that, as expected, Ms

increases with the increasing plated time due to the increase of Fecontent in the flaky particles.

Table 1 lists the Fe weight percentage, microsphere density,Ms, and Hc for experimental Fe-plated flaky particles, where Ms

and Hc were obtained from the hysteresis loop, and Fe weightpercentage were estimated from the EDS average value. Thedensity of the flaky particles was tested by the pycnometermethod. It is shown that the flaky particles’ densities are in therange of 2.7–4.0 g/cm3. For the Fe plated for 3 h, the Ms of theflaky particles are up to 117.88 and 157.52 emu/g for as-platedand annealed statuses (700 1C), respectively. It is also noticed thatthe Ms value does not increase linearly with the Fe weightpercentage, indicating that for the microspheres with very thinfilm, the Fe layer coating is not uniform and fully covered. Thecoercivities for as-plated and annealed Fe films are different. Withthe annealed temperature and the Fe weight percentage increase,the coercivities decline rapidly.

For application as EM-wave absorber, the flaky magneticparticles must be mixed with wax to produce bonded composites.Fig. 5(a) and (b) shows the real permittivity (e0) and imaginarypermittivity (e00), respectively, for the composites comprising waxand as-plated flaky magnetic particles with Fe-plated for 1 h. Theflaky magnetic particles weight ratio in the composite varies from40 wt% to 55 wt%. As expected, the values of e0 and e00 increasewith increasing flaky particles content in the composites. e0 ¼39

Table 1The Fe weight percentage, density r, magnetization Ms, and coercivity Hc for Fe-

coated flaky diatomite particles.

No. Fe (wt%) r(g/cm3)

Status Ms

(emu/g)

Hc (Oe)

1 (plated

for 1 h)

45 2.7 As plated 93.2886 309.7878

Annealed at 550 1C 110.9987 190.7807

Annealed at 700 1C 129.5248 109.7603

2 (plated

for 2 h)

56 3.3 As plated 97.55984 371.5454

Annealed at 550 1C 114.5999 165.0561

Annealed at 700 1C 130.5641 108.2125

3 (plated

for 3 h)

63 4.0 As plated 117.8124 296.0774

Annealed at 550 1C 143.0026 146.7861

Annealed at 700 1C 157.9083 99.1631

Fig. 5. Complex permittivity spectra for wax–flaky magnetic particles composites

composite with various weight ratios (plated for 1 h).

Fig. 6. Complex permeability spectra for wax–flaky magnetic particles composites

composite with various weight ratios (plated for 1 h).

Z. Wenqiang et al. / Journal of Magnetism and Magnetic Materials 332 (2013) 15–2018

and e00 ¼35 can be obtained in the composite with high flakymagnetic particles content. The high value of dielectric constant(e0 up to 39) is attributed to the metallic behavior of Fe film. Thespace charge polarization between adjacent conductive particles(separated by insulating wax) gives rise to a high value ofdielectric constant. The loss due to conduction (represented bye00) is observed to be 0–35, increasing with flaky particles contentin the composite.

Fig. 6(a) and (b) shows the frequency dispersion of complexpermeability of the composites containing various as-plated flaky

particles with Fe plated for 1 h. Similar to the permittivity, the m0and m00 increase with increasing flaky particles content in thecomposites. Though the values of m0 and m00 are just beyond 1 and0, due to relatively low concentration of Fe in the whole composites,the permeabilities at resonance peak are up to m0 ¼1.8 and m00 ¼0.7for certain flaky particles content.

The Fe weight percentage of flaky particles dependent permit-tivity and permeability for the composites with 50 wt% are shownin Fig. 7. Increasing Fe weight percentage leads to increasingvalues of permittivity and permeability, especially for the e0 ande00, which is obviously due to the increasing metal content in thecomposites. Similar results were also obtained in the compositewith different ratios.

Fig. 8 shows the effect of annealing on the permittivity andpermeability for the composite with 55 wt% and plated for 3 h.Annealing significantly increases the real and imaginary permit-tivities. The reason should be related to the disappearance ofamorphous phase in the annealed sample. The effect of annealingon the permeability is more complicated. With the annealedtemperature rose, m0 increased and m00 decreased, respectively.It would be due to the amorphous disappearance and thenanocrystallites growth with the temperature rose.

Fig. 9 shows the calculated reflection loss as a function offrequency for the selected Fe-coated flaky diatomite (plated for1 h) composites with various sample thickness (50 wt%) (a) and the

Fig. 7. Complex permittivity spectra and complex permeability spectra for wax–

flaky magnetic particles composites with various plated time. Fig. 8. Complex permittivity spectra and complex permeability spectra for wax–

flaky magnetic particles composites comprising as-plated and annealed Fe-coated

flaky diatomite particles.

Z. Wenqiang et al. / Journal of Magnetism and Magnetic Materials 332 (2013) 15–20 19

composites comprising various Fe-coated flaky diatomite (b). Thecalculations use the actual values of e and m obtained from themeasurements. As shown in Fig. 9(a), the reflection loss is found tobe dependent sensitively on the thickness of the absorber. For theas-plated Fe-coated flaky diatomite, the maximum attenuation ofthe incident wave is predicted for a thickness of 2.0 mm for thecomposite. The minimal reflection of the flaky magnetic particlescomposites is firstly decreased with an increase in the thickness ofthe absorber (from 0 mm to 2 mm); then it increases with thethickness of absorber; for a d¼2.0 mm, a RL¼�40 dB wasobtained. The peak of the RL is moving to lower frequency andis getting narrower when the sample thickness increases. It issuggested that the reflection loss is related to a matching thick-ness. Fig. 9(b) shows the RL for the composites consisting ofvarious types of flaky particles. All samples have a thickness of1.0 mm. The composite (55 wt%, plated for 3 h) records a mini-mum reflection loss of �18 dB at 6 GHz.The minimal reflectionloss decreases with increasing flaky particle percentage in thecomposite and the decreasing of Fe weight percentage in flakyparticles. In addition, the minimal reflection loss is increased byannealing the microspheres. Our results on reflection loss are

better than that for the Fe microspheres reported in Ref. [13]. Theresults also indicate that the EM-wave absorbing properties can beadjusted by modifying the composition of composite and flakyparticle as well as the plated time. It proved that the flakymagnetic particles can make thin, light weight and high perfor-mance microwave absorbers at lower frequency.

4. Conclusions

Fe magnetic metals were coated on the flaky diatomiteparticles using the CVD method. The flaky magnetic particleshad very low densities (2.7–4.0 g/cm3) and relatively high satura-tion magnetization (93 emu/g). Annealing treatment led to a fullycrystallized structure and increased grain size, which increasedthe saturation magnetization of the flaky particles. The highfrequency characteristics of the composites consisting of Fe-plated flaky particles and wax have been investigated. Thepermittivity and permeability of the composites increased withthe flaky particles content and the Fe weight percentages of flaky

Fig. 9. Frequency dependence of the reflection loss of the wax–flaky magnetic

particles composites: (a) with 50 wt% (plated for 1 h) and various sample

thicknesses d and (b) with d¼1.0 mm and various weight ratios and various

plated time.

Z. Wenqiang et al. / Journal of Magnetism and Magnetic Materials 332 (2013) 15–2020

particles. Annealing also affected the microwave properties. Thereflection loss of the composites not only depends on the weight

ratio of flaky particles in the composites, but also depends on themicrostructure and weight percentages of the Fe thin films on theflaky particles. The properties of these low density Fe-coated flakydiatomite particles were comparable to those of conventionalferrite powder materials. The calculations show that the flakemagnetic particles have potential applications as thin, lightweight and high performance EM-wave absorbers. This researchalso broadens applications of diatomite and provides a newpreparation method for light weight iron.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (Grant no. 50805005), the National ‘‘863’’Project of China (Grant no. 2009AA043804), the Foundation forthe Author of National Excellent Doctoral Dissertation of PR China(Grant no. 2007B32), and the Innovation Foundation of BUAA forPhD Graduates. The authors would like to thank Song Honghai,Fan Xuechou from Beihang University for SEM and XRD test of thesamples.

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