Applied Surface Science 255 (2009) 9381–9385

Surface modification of M-Ba-ferrite powders by polyaniline: Towards improvingmicrowave electromagnetic response

Xin Tang *, Yuanguang Yang

School of Petroleum Enginering, Southwest Petroleum University, 8#, Xindu Road, Xindu District, Chengdu 610500, PR China

A R T I C L E I N F O

Article history:

Received 19 September 2008

Received in revised form 14 July 2009

Accepted 16 July 2009

Available online 23 July 2009

Keywords:

Polyaniline

M-Ba-ferrite

Composite powders

Microwave absorption

A B S T R A C T

A composite of polyaniline (PANI)-coated M-type hexagonal barium ferrite (M-Ba-ferrite) powder was

prepared by an in situ polymerization of an aniline monomer in the presence of M-Ba-ferrite particles.

The obtained composite was characterized by Fourier transform infrared spectra (FT-IR), X-ray

diffraction (XRD) and transmission electron microscopy (TEM). The structure and microwave response

properties were investigated. The continuous coverage of polyaniline has been produced on the platelet

M-Ba-ferrite particle surface, and a core–shell structure has been formed. The results show that the

coverage of polyaniline has a great influence on microwave response of M-Ba-ferrite particles. A

polyaniline thin layer formed on the surface of a barium ferrite particle changes the character of

frequency dispersion of microwave absorption. The results indicate the existence of an interaction at the

interface of polyaniline macromolecule and barium ferrite particle, which influences the physical and

chemical properties of the composite. The interaction and interfacial polarization are seen as important

factors contributing to the influence on microwave response of the PANI-coated ferrite composite

powders.

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1. Introduction

There are intensive interests in shielding against electromag-netic radiation in commercial, military, scientific electronic devicesand communication instruments. Usually, conducting materialsare widely used to shield electromagnetic wave. However, thereflecting wave from conducting materials may cause unimagin-able bad aftereffects. Recently, electromagnetic wave absorbingmaterials have attracted considerable attention because they canabsorb radiation energy from microwave generated from anelectric source. So they are becoming promising candidates forreplacement of conducting metal materials. Extensive research hasbeen carried out to develop high efficient electromagnetic waveabsorbing materials [1,2]. As is well-known, the complexpermittivity (er = e0 � je00) and permeability (mr = m0 � jm00) deter-mine absorbing characteristic of materials. Many researches arebeing carried out to synthesize composite absorbing materials andinvestigate its frequency dispersion characteristic of microwaveabsorbing property [3]. Polymer/magnetic composites with uniquephysical properties have attracted more and more attention.Because they can not only combine the merits of conductingpolymers and magnetic particles but also reveal the novel physical

* Corresponding author. Tel.: +86 28 83032901; fax: +86 28 83032901.

E-mail address: tungqin@126.com (X. Tang).

0169-4332/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2009.07.040

and chemical properties which do not exist in the constituentphases. Earlier researchers have reported preparation and staticmagnetic properties of polymer/magnetic composites. For exam-ple, Yang et al. reported an incorporation of metal oxide particlesinto polypyrrole by microemulsion polymerization method inaqueous solution [4]. Murillo et al. reported CoFe2O4–PPynanocomposites made by a chemical method [5]. Deng et al.studied the synthesis of Fe3O4–PANI particles [6]. Li et al. reportedmagnetic behavior of the polyaniline–barium ferrite composites[7,8]. Zhang et al. also prepared PANI–NSA–Fe3O4 composites by atemplate-free approach [9]. Modifying the properties of onematerial by coating it with another type of material has become apopular approach widely documented in the literature [10–14].However, less attention has been paid to the synthesis andinvestigation of the microwave absorbing property of surfacemodified M-Ba-ferrite particles by polyaniline.

In order to obtain a material with improved electromagneticproperties in the frequency of 2–12 GHz, our group focused onsynthesizing a new type of core–shell (magnetic-polymer)composites which are composed of polyaniline and M-typehexagonal barium ferrite (M-Ba-ferrite) by an in situ depositiontechnique. The M-Ba-ferrite is chosen as a magnetic core because ithas high saturation magnetization, high anisotropy field, andexcellent magnetic properties which make it potential goodmagnetic loss materials in the GHz range [15,16]. However,negligible (poor) dielectric loss, that is to say, a very low complex

X. Tang, Y. Yang / Applied Surface Science 255 (2009) 9381–93859382

permittivity, confined its application. In addition, polyaniline wasselected due to its advantages such as thermal stability at hightemperature, low density, controllable conductivity and reason-ably facile processibility. Polyaniline also has a wide application inthe stealth technology of aircraft, electromagnetic interferenceshielding, microwave dark room [17,18]. Polyaniline/M-Ba-ferritecomposites combine the merits of polyaniline macromolecule andinorganic magnetic barium ferrite. Furthermore, we are able totailor the materials according to the properties required in eachapplication. These are the factors motivating us to work on thesynthesis of polyaniline (PANI) in the presence of M-Ba-ferriteparticles.

The main aim of the present study is to synthesize PANI-coatedM-Ba-ferrite composite powders and investigate microwaveabsorption frequency dependencies of pure M-Ba-ferrite particlesand PANI-coated M-Ba-ferrite composite particles in the 2–12 GHzfrequency range. The origin of their electrical and magneticproperties is also discussed on the basis of the structuralcharacterizations including Fourier transform infrared (FT-IR)spectrum and transmission electron microscopy.

2. Experiment details

2.1. Coating of barium ferrite particles with polyaniline

Aniline (AR grade, Shanghai Chemical Works, China), ammo-nium peroxydisulfate ((NH4)2S2O8) (AR grade), hydrochloric acid(HCl) (AR grade), water (de-ionized water) were used as startingmaterials. M-type hexagonal barium ferrite powders were madeby using a wet chemical route as described in our previous report[19]. The polyaniline-coated M-Ba-ferrite composite particles wereprepared according to the following steps. First, 0.93 g of anilinewas dissolved in 300 ml of distilled water containing 10 ml of 1 Mconcentrated HCl aqueous solution. The mixed solution was pre-cooled at 0 8C in ice bath. Second, 2.5 g of the M-Ba-ferrite powderswere added to the above pre-cooled solution with continuousstirring. Third, 11.5 g of 1 M aqueous solution of ammoniumperoxydisulfate was slowly added into the above reaction mixture.During the synthesis, the mixture was stirred at about 400–500 rpm for 16 h and the temperature was kept at 0 8C. Finally,PANI-coated particles were recovered from the solution bymagnetic separation, and then washed repeatedly with a largeamount of de-ionized water and ethanol until the washings werecolorless. The wet PANI-coated particles were then dried in avacuum oven at 80 8C for 24 h. To remove any trace of water, thedrying temperature was stringently controlled. A schematicdiagram of the preparation process was shown in Scheme 1.

Additionally, for comparison, PANI was also prepared asfollows: 1.86 g of aniline was injected to 20.24 ml of 1 M HCl.22 g of 1 M aqueous solution of the ammonium peroxydisulfatewas dropped into the solution with constant magnetic stirring. Thepolymerization was processed for 16 h at 0 8C. The product was

Scheme 1. Schematic representation of (a) platelet M-Ba-ferrite particle, (b) M-Ba-fe

composite.

filtered, washed and dried at the same condition to obtain fine darkgreen powders. The volume fractions of PANI in the PANI/bariumferrite composites were 0, 3, 6, 10 and 15%, and the correspondingsamples were marked with PF0, PF1, PF2, PF3 and PF4, respectively.

2.2. Characterization

The FT-IR spectra of the PANI, M-Ba-ferrite, and PANI-coated M-Ba-ferrite composites were recorded on FT-IR spectrometer(Equinox 55, Bruker Analytische Messtechnik GmbH). The spectrawere collected from 4000 to 400 cm�1, with a 4 cm�1 resolutionover 20 scans, and measurements were performed in thetransmission model in KBr pellets. The morphology of the particleswas observed by TEM on a JEM-2010 Electron Microscope. X-raypowder diffraction patterns of the samples were recorded by X-raydiffractometer (Bruker AXS D8 Advance Diffractometer) using thewavelength of 1.54051 A of Cu Ka radiation. A network analyzer(Agilent Technologies, E8363A) was employed to determine thevalues of the complex permittivity (er = e0 � je00) and permeability(mr = m0 � jm00) at the frequency range of 2–12 GHz by usingcoaxial reflection/transmission technique. For this, the sampleswere prepared with PANI/barium ferrite powders loading in epoxyresin, in which the weight percentage of PANI/barium ferritepowders was 70%, and toroidal shaped samples of 3.0 mm innerdiameter, 7.0 mm outer diameter and 3–4 mm length wereprepared. The test samples of toroidal shape were tightly insertedinto the standard coaxial line, the measured values of the complexpermittivity (er = e0 � je00) and permeability (mr = m0 � jm00) wereused to determine microwave absorption [20].

3. Results and discussion

The TEM images of the uncoated M-Ba-ferrite and PANI-coatedM-Ba-ferrite composite particles are given in Fig. 1. Due to PANIcoating on the M-Ba-ferrite particle, different surface morphologiesbetween the PANI-coated M-Ba-ferrite composite and uncoatedparticles are clear. Fig. 1(a) reveals that the M-Ba-ferrite particles areplatelet, and aggregated due to magnetic attraction between theparticles. After coating with PANI, it is clearly seen that thecontinuous coverage of a polyaniline has been produced on theplatelet M-Ba-ferrite surfaces in Fig. 1(b). In comparison with theuncoated M-Ba-ferrite particles, the PANI formed on the surface ofM-Ba-ferrite particles leads to the increase of the particle size.

FT-IR spectra are used to characterize particle structures. Fig. 2shows the FT-IR spectra of the PANI, M-Ba-ferrite and PANI-coatedM-Ba-ferrite composite particles. The main characteristic bands ofpolyaniline (curve b in Fig. 2) are assigned as follows: the band at3462 cm�1 is attributable to N–H stretching mode, C55N and C55Cstretching mode for the quinoid and benzenoid rings occur at 1582and 1495 cm�1, the bands at about 1306 and 1236 cm�1

correspond to N–H bending and asymmetric C–N stretching modefor benzenoid ring respectively [21]. Curve c in Fig. 2 shows the

rrite particle dispersed in aniline–HCl solution and (c) PANI-coated M-Ba-ferrite

Fig. 1. (a) TEM image of M-Ba-ferrite powder and (b) TEM image of PANI-coated M-Ba-ferrite composite.

Fig. 2. FI-IR spectra of (a) M-Ba-ferrite, (b) PANI and (c) PANI-coated M-Ba-ferrite

composite.

X. Tang, Y. Yang / Applied Surface Science 255 (2009) 9381–9385 9383

main characteristic bands of polyaniline-coated M-Ba-ferritecomposite particles. As can be seen, the FT-IR spectrum of thePANI-coated M-Ba-ferrite composite particles (curve c in Fig. 2) isalmost identical to that of PANI, besides the bands at 580 and430 cm�1 attributed to the stretching vibration of M-Ba-ferrite(curve a in Fig. 2). The IR absorption bands of solids in the 100–1000 cm�1 range are usually assigned to vibrations of ions in thecrystal lattices. In ferrites, the metal ions situated in two differentsub-lattice, designated tetrahedral and octahedral according to thegeometrical configuration of oxygen nearest neighbors. Theabsorption bands around 580 and 430 cm�1 are attributed tothe stretching vibration of tetrahedral and octahedral groupcomplexes of ferrites, respectively [22]. Comparing the FT-IRspectrum of composite particles (Fig. 2(c)) with that of the PANI(Fig. 2(b)), some peaks shifted to lower wave-numbers. It is notruled out that the internal magnetic field of the ferrite may affectthe polymerization of aniline, and modify the structure andproperties of the PANI. Thus, we suppose these differences indicatethe existence of a chemical bond between polyaniline macro-molecule and magnetic particles in the composites. Besides thedifference of the peak location in the spectra of Fig. 2, the strengthalso has some difference. The intensity of the peak near to1147 cm�1 from aromatic amine in the spectrum of the compositereduced in comparison with that of the pure polyaniline. Thesedifferences in the IR spectra can be explained on the basis of theconstrained growth model of the PANI grown in the presence of theM-Ba-ferrite shown in Fig. 3. Other authors [5] studied N1selemental X-ray photoelectron spectroscopy spectra of polypyrrole(PPy) and PPy–Fe3O4 composite materials, they showed that the Nspectrum of PPy was only one state, however, that of PPy–Fe3O4

composite were two states, one being N of the PPy backbone andthe other the N interacting with an Fe atom. In this research, theaniline monomer initially absorbed on the M-Ba-ferrite particle,the polymerization process took place on the surface of these oxideparticles when (NH4)2S2O8 was added to the reaction system. The

Fig. 3. The schematic plan of bonding model fo

polymerization process leads to adhesion of the polyaniline to theM-Ba-ferrite particle and the constrained growth around theseparticles, and it is just this kind of adsorption and constrainedmotion of the polymer chains that leads to the difference in the IRspectra.

Fig. 4 shows the XRD spectra of the PANI (curve a) and PANI-coated M-Ba-ferrite composite particles (curve b). Fig. 4(a) revealsthat the PANI is an amorphous nature, and also has a certain degreeof crystallinity, the weight fraction of amorphous materials isabout 82% in our polyaniline sample by the XRD phase analysissoftware. A diffuse broad peak around 208, which may be assignedto the scattering from polyaniline chains at inter-planar spacing, it

r the PANI-coated M-Ba-ferrite composite.

Fig. 4. X-ray diffraction patterns of (a) PANI and (b) PANI-coated M-Ba-ferrite

composite.Fig. 5. Reflection loss as a function of frequency for PF–resin composite at same

thickness for PF composite at a fixed volume fraction (PF0 = 0%, PF1 = 3%, PF2 = 6%,

PF3 = 10% and PF4 = 15%).

Fig. 6. Reflection loss as a function of frequency for the sample PF3 at different

thickness.

X. Tang, Y. Yang / Applied Surface Science 255 (2009) 9381–93859384

is consistent with the results obtained by other research groups[23]. The PANI-coated M-Ba-ferrite composite particles show highdegree of crystalline order due to presence of a high amount of M-type barium ferrite in the composite, and there is no other ironoxide phase which is present as impurity. The amorphous bump forPANI did not show in the XRD pattern of PANI-coated M-Ba-ferritecomposite particles (curve b). The reason is as follows, when the(NH4)2S2O8 is added to the reaction system, polymerizationprocess initially occurs on the surface of the M-Ba-ferrite particles,due to the restrictive effect of the surface of the M-Ba-ferriteparticles, the crystalline behavior of polyaniline is hampered. Thus,the degree of crystallinity of polyaniline decreases and thediffraction peak disappears in the composites.

According to transmission line theory, the reflection loss ofelectromagnetic radiation, RL (dB), under normal wave incidenceat the surface of a single-layer material backed by a perfectconductor can be defined by [20,24]:

RL ¼ 20 logZin � Z0

Zin þ Z0

� �(1)

where Z0 is the characteristic impedance of free space:

Z0 ¼ffiffiffiffiffiffiffim0

e0

r(2)

Zin is the input impedance at free space and material interface:

Zin ¼ Z0

ffiffiffiffiffiffimr

er

r� �tanh j

2p fd

c

� �ð ffiffiffiffiffiffiffiffiffiffimrerp Þ

� �(3)

where mr(mr = m0 � jm00) and er(er = e0 � je00) are the complex relativepermeability and permittivity, respectively, of the compositemedium, c is the velocity of electromagnetic waves in free space,f is the frequency of microwaves, and d is the thickness of theabsorber. Thus, the surface reflection loss of an absorber is a functionof six characteristic parameters, viz., e0, e00, m0, m00, f and d. Themicrowave absorbing properties of the powder–resin compositeshave been calculated from a computer simulation using Eqs. (1)–(3),assuming d = 2.0 mm. The calculated reflection losses of compositesas a function of frequency for all samples are shown in Fig. 5. Asshown in Fig. 5, the samples all show the maximum reflection loss inthe higher frequency. When the volume fraction of PANI is 15%, thevalue of microwave reflection loss is small, which may be due to

deteriorating the permeability when the volume fraction of PANIexceeds a critical value. The reflection loss maximum is equivalent tothe occurrence of minimal reflection of the microwave power for theparticular thickness. The difference on reflection loss maximum as afunction of the samples is associated with the magneto-crystallineanisotropy of synthesized materials. In the present study, theinterfacial polarization results from the heterogeneous structure offerrite comprising low-conductivity grains separated by higher-resistivity grain boundaries as proposed by Koops [25]. Compositematerials, in which magnetic nanoparticles are coated withconducting polymer layers, introduce an additional interfaces andmore polarization charges on the surface of the particles, this makesthe microwave response behavior more complex. The presence ofconducting layer on the surface of ferrites, which changed theboundary conditions for the microwave field at the interfacebetween the ferrite particle and polymer, can influence theresonance frequency. Fig. 6 shows the frequency dependence ofthe reflection loss of the PF3 composites at sample thickness of 2.0,2.5, 3.0, 3.5, 4.0 and 4.5 mm. It is clearly demonstrated that the

X. Tang, Y. Yang / Applied Surface Science 255 (2009) 9381–9385 9385

intensity and frequency of reflection loss maximum for the sampledepends on the material’s thickness. The dip of the maximumreflection shifts to a lower frequency with increase in the thickness.

4. Conclusions

In this study, the M-type hexagonal barium ferrite particleshave been coated with the polyaniline (PANI) by an in situdeposition technique. It was found that the continuous coverage ofthe PANI has been produced on the platelet hexagonal bariumferrite particle surface, and a core–shell structure has been formed.The results show that the coverage of polyaniline has a greatinfluence on microwave response of M-Ba-ferrite particles. Theformation of polyaniline thin layer on the surface of M-Ba-ferriteparticle changes the character of the frequency dispersion of themicrowave absorbing. The result indicates the existence of aninteraction at the interface of polyaniline macromolecule andbarium ferrite particle, which influences the composite’s physicaland chemical properties. The samples all show the maximumreflection loss in the higher frequency. When the volume fractionof PANI is 15%, the value of microwave reflection loss is small.

Acknowledgement

This work was supported by the research foundation ofSouthwest Petroleum University (SWPU).

References

[1] M. Matsumoto, Y.J. Miyata, Appl. Phys. 79 (1996) 5486–5488.[2] S.M. Abbas, A.K. Dixit, R. Chatterjee, T.C. Goel, Mater. Sci. Eng. B 123 (2005) 167–

171.[3] R. Sharma, R.C. Agarwala, V. Agarwala, J. Alloys Compd. 467 (2009) 357–365.[4] X. Yang, L. Xu, S.C. Ng, Nanotechnology 14 (2003) 624–629.[5] N. Murillo, E. Ochoteco, Y. Alesanco, Nanotechnology 15 (2004) S322–S327.[6] J.G. Deng, X.B. Deng, W.C. Zhang, Polymer 43 (2002) 2179–2184.[7] Y.X. Li, H.W. Zhang, et al. Chin. J. Chem. Phys. 20 (2007) 739–742.[8] Y.X. Li, Y.L. Liu, H.W. Zhang, et al. Chem. J. Chin. Univ. 29 (2008) 640–644.[9] Z.M. Zhang, M.X. Wan, Synth. Met. 132 (2003) 205–212.

[10] B.M. Vogel, D.M. DeLongchamp, C.M. Mahoney, et al. Appl. Surf. Sci. 254 (2008)1789–1796.

[11] A.S. Komolov, P.J. Moller, J. Mortensen, et al. Appl. Surf. Sci. 253 (2007) 7376–7380.

[12] O.K. Park, Y.S. Kang, Colloids Surf. A 257–258 (2005) 261–265.[13] A. Arenillas, F. Rubiera, J.B. Parra, et al. Appl. Surf. Sci. 252 (2005) 619–624.[14] R. Sharma, R.C. Agarwala, V. Agarwala, Appl. Surf. Sci. 252 (2006) 8487–8493.[15] R.C. Pullar, M.D. Taylor, A.K. Bhattacharya, J. Eur. Ceram. Soc. 22 (2002) 2039–

2045.[16] S. Sugimoto, S. Kondo, K. Okayama, IEEE Trans. Magn. 35 (1999) 3154–3156.[17] P. Chandrasekhar, K. Naishadham, Synth. Met. 105 (1999) 115–120.[18] G.R. Pedro, Adv. Mater. 13 (2001) 163–174.[19] X. Tang, B.Y. Zhao, K.A. Hu, J. Mater. Sci. 41 (2006) 3867–3871.[20] S.S. Kim, S.B. Jo, K.I. Gueon, K.K. Choi, J.M. Kim, K.S. Chun, IEEE Trans. Magn. 27

(1991) 5462–5464.[21] D.C. Trivedi, in: H.S. Nalva (Ed.), Handbook of Organic Conductive Molecules and

Polymers, Wiley, New York, 1997, p. 537.[22] R.D. Waldron, Phys. Rev. 99 (1955) 1727–1735.[23] Y. Xia, J.M. Wiesinger, A.G. Macdiarmid, Chem. Mater. 7 (1995) 443–445.[24] Y. Natio, K. Suetake, IEEE Trans. Microw. Theory Technol. 19 (1971) 65–72.[25] C.G. Koops, Phys. Rev. 83 (1951) 121–124.