pom

LvPR28,s, H

Accepted 3 March 2012Available online 10 March 2012

sit

samples from more than 4.0 to about 0.55 g cm3. The porous structures remarkably decreased the per-

culture is usually abandoned or burnt in air, not only wasted theresources in corncob, but also polluted the environment. Therefore,the utilization of corncob is still an important issue for the treat-ment of agricultural wastes.

cated porous carbon/Co nanocomposites using a solgel method[28], which present enhanced EM wave absorption with low den-sity. Porous Fe3O4/Fe/SiO2 nanorods were prepared using a seriesof heat treatment, showing excellent EM wave absorption proper-ties [29].

However, these methods usually use activated carbon as tem-plate or require rigorous reagents and procedures, which increasedthe preparation cost of porous magnetic materials. In this study,we prepared novel low-density porous magnetic (including Feand FeNi) nanocomposites using corncob powders as template.

Corresponding authors. Tel.: +86 571 88206399; fax: +86 571 88208890(X.G. Chen), tel.: +86 571 87984550; fax: +86 571 88208890 (Y. Ye).

E-mail addresses: chenxg83@zju.edu.cn (X.-G. Chen), gsyeying@zju.edu.cn

Composites Science and Technology 72 (2012) 908914

Contents lists available at

Composites Science

ev(Y. Ye).Corn is one of the major crops in the world, sharing equalimportance with rice and wheat. Corncob is the agriculturalbyproduct of corn produced by peeling off the corn kernels. About800 million tons of corn is produced per annum all over the world,which giving about 150 million tons of corncob [1]. The majorcompositions of corncob include cellulose, hemicellulose, lignin,and so on [2]. Therefore, the researches on the utilization of corn-cob focused on the extracting of organic compounds and prepara-tion of absorbers [3,4]. For instance, high concentration ethanol [5]and xylose [6] can be produced from pre-treated corncob. Likeother herb husks, corncob can also be activated by acid treatment[7] or calcination [8,9], the resulting products activated carbon ex-hibit strong adsorption capacities. Nevertheless, corncob in agri-

from EM wave irradiation or to protect the military equipmentfrom being detected by radar waves [10,11]. Ideal EMwave absorb-ers should exhibit strong absorption and broad bandwidth for EMwaves on the basis of thin matching thickness and low density[11,12]. The requirements of strong absorption and broad band-width can be achieved by traditional magnetic or magnetic/carbonabsorbers such as ferrite [1317], carbonyl iron [1820], carbonnanotube nanocomposites [2123], and so on. However, decreas-ing the matching thickness and density is still a challenge for theEM wave absorbers. Various methods have been applied to prepareEM wave absorbers with wide-band strong absorption and lowdensity, among which preparation of porous magnetic nanocom-posites is an efcient way [2427]. For example, Liu et al. fabri-Keywords:A. Functional compositesD. X-ray diffraction (XRD)B. Magnetic propertiesA. NanocompositesE. Heat treatment

1. Introduction0266-3538/$ - see front matter 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.compscitech.2012.03.001mittivity (e) and permeability (l) and enhanced the impendence matching between the absorber and air.The porous magnetic nanocomposites exhibit enhanced absorption for EM waves at thin matching thick-ness. The optimum thickness is only 1.01.4 mm, with bandwidth of RL < 5 dB of about 8 GHz, coveringthe half X-band and the whole Ku-band. The areal density of magnetic absorbers at this study is onlyabout 0.71.0 kg m2 at thickness of 1.01.4 mm, much lower than the reported values of other magneticabsorbers. Due to the strong absorption at low density and thin matching thickness, the porous magneticnanocomposites prepared using corncob powders as template are promising light-weight EM waveabsorbers.

2012 Elsevier Ltd. All rights reserved.

Electromagnetic (EM) wave absorbers are widely used indomestic and military elds, either to protect the human beingReceived 30 August 2011Received in revised form 1 March 2012

netic (EM) wave absorbers. In this study, we prepared novel porous magnetic nanocomposites usingcorncob powders as template. The presence of corncob will signicantly decrease the bulk density ofPreparation of porous magnetic nanocomtemplate and their applications for electr

Xue-Gang Chen a,, Ji-Peng Cheng b, Shuang-ShuangaDepartment of Ocean Science and Engineering, Zhejiang University, Hangzhou 310058,bDepartment of Materials Science and Engineering, Zhejiang University, Hangzhou 3100cDepartment of Nonmetallic Research, Zhejiang Institute of Geology & Mineral Resourced Second Institute of Oceanography, SOA, Hangzhou 310012, PR China

a r t i c l e i n f o

Article history:

a b s t r a c t

Strong absorption, low den

journal homepage: www.elsll rights reserved.osites using corncob powders asagnetic wave absorption

c, Ping-Ping Zhang d, Shu-Ting Liu a, Ying Ye a,ChinaPR Chinaangzhou 310007, PR China

y, and thin matching thickness are important parameters for electromag-

SciVerse ScienceDirect

and Technology

ier .com/ locate /compsci tech

3.3. Complex permittivity and permeability

Relative complex permittivity (e = e0 je00) and complex perme-ability (l = l0 jl00) are essential parameters for the EM waveabsorbers. e0 and l0 are the dielectric constant and real magneticpermeability of absorber, while the imaginary part suggests thedielectric or magnetic loss for EM waves [34]. As shown in Fig. 3,the e0 of porous Fe and porous FeNi decrease slightly with theincreasing of frequency. On the contrast, the e00 of porous Fe andporous FeNi increase with frequency. The e of magnetic particlesis usually predominated by various polarizations, and orientationalpolarization is the most promising mechanism in this studyaccording to the curves [3537]. The e0 and e00 of porous FeNi arerelative higher than that of porous Fe and exhibit a peak at fre-quency of 1314 GHz, which may be attributed to the dielectricresonance effect [38]. The dielectric loss tangents (tande = e00/e0)of samples were calculated from the measured e0 and e00. The tandeof porous Fe decreases with frequency at 213 GHz and increases

ce anWe examined the complex permittivity (e) and permeability (l),and calculated the EM wave absorption of the porous magneticnanocomposites as a function of thickness. The use of corncobpowders is found to be essential for the EM wave absorption ofporous magnetic nanocomposites which showing enhanced reec-tion loss (RL) and absorption bands at thin matching thickness.

2. Materials and methods

2.1. Materials

Corncob powders were obtained from Hangzhou, China, andwas washed thoroughly by water and dried at 60 C before usage.Ferric nitrate (Fe(NO3)39H2O, AR), Nickel Nitrate (Ni(NO3)26H2O,AR), Citric acid (C6H8O7, AR), and Dimethylformamide (DMF) werepurchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai,China. Polyamic acid (PAA) was obtained from Guangcheng plasticCo., Ltd., Changzhou, China. All reagents are used without furtherpurication.

2.2. Preparation of porous magnetic nanocomposites

In a typical procedure, 20 g ferric nitrate (11.6 g ferric nitrateand 8.4 g nickel nitrate for the preparation of porous FeNi nano-composite) and 20 g citric acid were dissolved in 50 mL deionizedwater under continuous stirring. A certain amount (20 g) corncobpowders were added into the mixture. After stirred vigorously,the product was dried in a temperature-controlled oven at 80 Cfor 1 h and then heated at 140 C for 1 h. The obtained sampleswere calcined at 500 C for 2 h under air and then reduced at450 C for 30 min under mixed atmosphere of 200 mL min1 H2and 500 mL min1 N2. After cooled to room temperature andcoated by polyimide (PI, prepared from cyclo-dehydration of PAAusing DMF as solvent) [30,31], porous Fe (or porous FeNi) nano-composite was prepared.

In order to investigate the effects of corncob powders on thestructure, morphology, and EM characteristics of magnetic sam-ples, we prepared FeNi alloy without using corncob powders astemplate, while other conditions are kept constant.

2.3. Characterizations

The phase purity and crystal structure of the samples weredetermined by a D/max 2550 X-ray diffractometer (XRD) (Rigaku,Japan) with Cu Ka radiation (k = 0.15406 nm) at a scan rate of0.02 s1. The operation voltage and current were maintained at36 kV and 34 mA, respectively. The surface morphologies and ele-mental compositions of the samples were studied by an S-4800scanning electron microscope (SEM) and energy disperse spectros-copy (EDS) (Hitachi, Japan) at accelerating voltage of 5.0 kV. Thecomplex permittivity and permeability were determined by aHP8720ES vector network analyzer at EM wave frequency of 218 GHz and thickness of 2 mm. The lling rate of the samples is60% with parafn as substrate.

3. Results and discussion

3.1. XRD patterns

Fig. 1 shows the XRD patterns of the samples. The porous Fenanocomposite is mainly composed by Fe (Iron, JCPDF # 06-0696) and Fe3O4 (magnetite, JCPDF # 72-2303). The relative inten-

X.-G. Chen et al. / Composites Sciensity and sharply shape of the corresponding peaks indicate thatboth Fe and Fe3O4 are well crystallized, and the content of Fe ismuch higher than that of Fe3O4. Both porous FeNi and FeNi alloythat prepared without using corncob exhibit similar diffractionpatterns, which can be indexed to well-crystallized FeNi (Taenite,JCPDF # 47-1417) with characteristic peaks of 111, 200, and 220.EDS results show that all three samples exhibit 1823 atomic per-cent of carbon, which may be derived from the carbonization of cit-ric acid and corncob powders.

3.2. SEM characterizations

The SEM images of the samples are shown in Fig. 2. When usingcorncob powders as template, the obtained Fe and FeNi nanocom-posites exhibit similar porous structures. The porous nanocompos-ites maintained the initial plate-like structures of corncobpowders. The diameter of Fe and FeNi particles in the nanocompos-ites is about 100 nm, forming pores with diameter of 20200 nm.When without using corncob powders as template, however, theresulting FeNi particles are melted together, without showingany pores. The diameter of a single FeNi alloy is about 200500 nm, growing according to the VLS mechanism [32]. It is indi-cated that during the heat treatment, corncob will be decomposedto carbon and releasing numerous gases, which separated the mag-netic particles and construct porous structures. The magneticnanocomposites will benet from the porous structures, becausethe presence of pores will signicantly decrease the bulk densityof samples and enhance their EM wave absorption via scatteringeffect [33].

Fig. 1. XRD patterns of (a) porous Fe nanocomposite, (b) porous FeNi nanocom-posite, and (c) FeNi alloy prepared without using corncob powders.

d Technology 72 (2012) 908914 909signicantly thereafter. The tande of porous FeNi increases withfrequency and shows a peak at about 14 GHz. According to thetande values, the dielectric loss of porous Fe and porous FeNi

e an910 X.-G. Chen et al. / Composites Sciencincreases with frequency and porous Fe present higher dielectricloss for EM waves at 213 GHz.

The l0 of porous magnetic nanocomposite decreases slightlywith the increasing of frequency. However, the l00 and magneticloss tangent (tandm = l00/l0) increases with frequency rstly andexhibit maximums at frequency of about 412 GHz. The magneticloss types include hysteresis loss, eddy current effect, natural res-onance, and so on [39]. According to the l values, the magnetic lossof porous Fe and FeNi nanocomposites is manly induced by eddycurrent loss and natural resonance effect. Both l00 and tandm valuesof porous FeNi are higher than that of porous Fe, indicating thatporous FeNi exhibit higher magnetic loss for EM waves.

When without using corncob powders as template, however,the as-prepared FeNi alloy shows much higher e, l, and loss tan-gent than that of porous FeNi (Fig. 4), which may be ascribed tothe melting and adhesion of FeNi particles. The conductive net-work formed by melting and adhesion will signicantly improvethe conductivity, and therefore increased the e, l, and loss tangent.Both e and l of FeNi alloy decrease with the increasing frequencyand exhibit some peaks. The peaks of tandm appear at frequency of6.2 GHz, 9.2 GHz, and 12.5 GHz, which are the natural resonancepeaks of FeNi alloy. The C value with formula of C = l00(l0)2 f1,which was suggested by Wu et al. [40] indicates that the magneticloss of FeNi alloy is predominated by natural resonance effect.

3.4. EM wave absorption properties

According to the transmission line theory, the RL values of sin-gle-layer EMwave absorbers can be evaluated from the measured eand l using the following equations [41]:

Fig. 2. SEM images of (a and b) porous Fe nanocomposite, (c and d) porous FeNi nanod Technology 72 (2012) 908914Zin Z0le

rtanh j

2pfdc

le

p 1

RLdB 20 log Zin Z0Zin Z0

2

where Zin and Z0 are the impedance of absorber and air, respec-tively. e and l are the complex permittivity and permeability of ab-sorber, respectively. f is the frequency of EMwave. d is the thicknessof the absorber. c is the velocity of light. Fig. 5 shows the calculatedthree-dimensional and color-lling patterns of RL values for porousFe and porous FeNi prepared using corncob powders as template.Both porous Fe and porous FeNi absorbers present weak absorption(RL < 5 dB) for EM waves at thickness of less than 1 mm. With theincreasing of thickness, the absorption band with maximum RLmove to lower frequencies. The maximum RL values of porous Feand porous FeNi absorbers are 37.9 dB and 40.8 dB, respectively.Both porous Fe and porous FeNi absorbers show low RL (2.5 dB to10 dB) but wide-band absorption for EM waves at high frequency(818 GHz) and high thickness (410 mm). To obtain the optimumthickness and frequency of porous magnetic absorbers, we analyzedthe variations of bandwidth of RL as a function of thickness. Asshown in Fig. 6a and b, the bandwidth of RL < 10 dB (B10) for bothporous Fe and porous FeNi is 0 at thickness of less than 0.7 mm. TheB10 values increase dramatically with thickness from 0.7 mm to1.3 mm and decrease thereafter. The maximum B10 values for por-ous Fe and porous FeNi absorbers are 3.6 GHz at 1.3 mm and2.8 GHz at 1.0 mm, respectively. The variation of bandwidth ofRL < 5 dB (B5) for porous FeNi is similar to the B10 values. TheB5 value of porous FeNi absorber increases with thickness from

composite, and (e and f) FeNi alloy prepared without using corncob as template.

Fig. 3. Frequency dependence of complex permittivity, permeability, and loss tangent for porous Fe (hollow patterned lines) and porous FeNi (solid patterned lines) preparedusing corncob powders as template. The lling rate is 60% using parafn as substrate.

Fig. 4. Frequency dependence of electromagnetic parameters for FeNi alloy prepared without using corncob powders as template. The lling rate is 60% using parafn assubstrate.

X.-G. Chen et al. / Composites Science and Technology 72 (2012) 908914 911

e an912 X.-G. Chen et al. / Composites Scienc0 GHz at thickness of 0.10.6 mm to 8 GHz at 1.1 mm. It starts todecrease thereafter and maintains at about 1 GHz at thickness of7.710 mm. The B5 values of porous Fe absorber, however, exhibitthree peaks with the increasing of thickness. The maximum B5 va-lue of porous Fe absorber is 6.8 GHz at 1.4 mm. All B5 values of por-ous Fe absorber are greater than 3 GHz at thickness of 1.210 mm.

Fig. 6c shows the maximum RL of porous Fe and porous FeNiabsorbers as a function of thickness. We can conclude that porousFeNi exhibits higher maximum RL value (40.8 dB), but porous Feabsorber shows wide range thickness of RL < 20 dB. According tothe higher B5, B10, and maximum RL values, it is suggested thatporous Fe is more suitable for the application in EM wave absorb-

Fig. 5. Three-dimensional and color-lling patterns of RL for (a and b) porous Fe and (c a(For interpretation of the references to color in this gure legend, the reader is referred

Fig. 6. Effects of thickness on the bandwidth of (a) RL < 5 dB and (b) RL < 10 dB, (c) mlines), (d) frequency dependence of RL for porous Fe (hollow patterned lines) and porousd Technology 72 (2012) 908914ers. The optimum thickness for porous Fe and porous FeNi absorb-ers are 1.31.4 mm and 1.01.2 mm, respectively. At the optimumthickness, as shown in Fig. 6d, the optimum EM wave frequencyshould be 1018 GHz, showing RL values of less than 5 dB.

When without using corncob powders as template, as shown inFig. 7, the resulting FeNi alloy exhibit much lower RL values thanthat of porous FeNi absorber. Although the e, l, and loss tangentof FeNi alloy are much higher than that of porous FeNi, the extre-mely poor impendence matching caused by the huge difference be-tween e and l prevented the EM waves from entering into theabsorber. As a result, the maximum RL for EM waves at 218 GHz with thickness of 0.110 mm is only about 5 dB.

nd d) porous FeNi nanocomposites at lling rate of 60% using parafn as substrate.to the web version of this article.)

aximum RL for porous Fe (hollow patterned lines) and porous FeNi (solid patternedFeNi (solid patterned lines) at optimum thickness.

f RL

sorb

5 dB

ce anConsequently, the use of corncob powders is essential for theEM wave absorption of magnetic absorbers (Table 1). First, porousstructures will be fabricated when using corncob powders as tem-

Fig. 7. (a) Frequency dependence of RL at 2 mm and (b) three-dimensional pattern oparafn as substrate.

Table 1Comparisons between magnetic absorbers of this study and other porous EM wave ab

Samples Bulk density(g cm3)

Thickness(mm)

Bandwidth of RL < (GHz)

Porous Fe usingcorncob

0.53 0.05 1.4 6.82.0 4.65.3 4.6

Porous FeNi usingcorncob

0.56 0.06 1.1 8.02.0 3.62.5 2.6

FeNi alloy 4.1 0.2 2.0 Porous cement/EPS

[26]1.108 20 /

Porous carbon ber[27]

1.28 2.9 /X.-G. Chen et al. / Composites Scienplate, which will signicantly decrease the bulk density of thematerials and absorbers. The density is of great importance tothe EM wave absorbers especially in military elds. The bulk den-sities of porous Fe and porous FeNi are only 0.53 0.05 g cm3 and0.56 0.06 g cm3, respectively, which are much lower than that ofFeNi alloy (4.1 0.2 g cm3) prepared without using corncob pow-ders. When applied as EM wave absorbers, the calculated arealdensities of porous Fe and porous FeNi absorbers at optimumthickness are 0.95 kg m2 and 0.77 kg m2, respectively, muchlower than the reported values of magnetic EM wave absorbers[42]. Second, the porous magnetic nanocomposites prepared usingcorncob powders exhibit much higher EM wave absorption be-cause of their improved impendence matching and scattering ef-fect of porous materials. The bandwidth of RL < 5 dB, bandwidthof RL < 10 dB and maximum RL of porous Fe and porous FeNiabsorbers are much higher than that of the counterpart. Further-more, we compared our results with other porous EM waveabsorbers. As shown in Table 1, although the bandwidths of porouscarbon ber and porous cement composite are larger than that ofthis study, their bulk densities and matching thicknesses are muchhigher either. Therefore, this study provides an important methodto fabricate porous magnetic EM wave absorbers with low density,strong absorption, and thin matching thickness.

4. Conclusions

In summary, we have fabricated novel porous magnetic nano-composites using corncob powders as template in this study. Theuse of corncob powders is found to be essential for the propertiesof magnetic nanocomposites. First of all, the presence of corncobsignicantly decreased the bulk density of the samples, whichfacilitated the applications for EM wave absorbers. The second,the porous structures decreased both e and l values of the samplesand the impendence matching between the absorber and air wasenhanced as a result. The porous magnetic nanocomposites exhibit

for FeNi alloy prepared without using corncob powders. The lling rate is 60% with

ers.

Bandwidth of RL < 10 dB(GHz)

Maximum RL(dB)

Areal density of absorber(kg m2)

3.0 21.86 0.952.2 21.01 1.360.8 37.85 3.602.6 12.29 0.771.6 18.82 1.391.2 40.80 1.74 5.20 5.646.2 15.27 /

10.9 15.53 /

d Technology 72 (2012) 908914 913much stronger absorption for EM waves than the FeNi alloy thatprepared without using corncob powders. The matching thicknessof the porous magnetic absorbers is only 1.01.4 mm, with arealdensity of only about 0.71.0 kg m2. The absorption bands ofRL < 5 dB covers half X-band and the whole Ku-band. This studynot only fabricated novel porous magnetic nanocomposites withstrong EM wave absorption at low density and thin matchingthickness, but also utilized the agricultural waste corncob. Theprepared porous magnetic nanocomposites are potential light-weight EM wave absorber with bright future.

Acknowledgement

This research is supported by the Fundamental Research Fundsfor the Central Universities, Peoples Republic of China.

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914 X.-G. Chen et al. / Composites Science and Technology 72 (2012) 908914

Preparation of porous magnetic nanocomposites using corncob powders as template and their applications for electromagnetic wave absorption1 Introduction2 Materials and methods2.1 Materials2.2 Preparation of porous magnetic nanocomposites2.3 Characterizations

3 Results and discussion3.1 XRD patterns3.2 SEM characterizations3.3 Complex permittivity and permeability3.4 EM wave absorption properties

4 ConclusionsAcknowledgementReferences