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Solid State Sciences 37 (2014) 40e46

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Solid State Sciences

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Synthesis, characterization and microwave absorption properties ofpolyaniline/Sm-doped strontium ferrite nanocomposite

Juhua Luo a, *, Yang Xu a, Duoduo Gao b

a School of Materials Engineering, Yancheng Institute of Technology, Yancheng 224051, Chinab School of Material Science and Engineering, Changzhou University, Changzhou 213164, China

a r t i c l e i n f o

Article history:Received 21 April 2014Received in revised form4 August 2014Accepted 13 August 2014Available online 24 August 2014

Keywords:Microwave absorption propertiesPolyanilineSm-doped strontium ferriteNanocomposite

* Corresponding author. Tel.: þ86 515 88298867; fE-mail address: luojuhua@163.com (J. Luo).

http://dx.doi.org/10.1016/j.solidstatesciences.2014.08.01293-2558/© 2014 Elsevier Masson SAS. All rights re

a b s t r a c t

Sm-doped strontium ferrite nanopowders (SrSm0.3Fe11.7O19) and their composites of polyaniline (PANI)/SrSm0.3Fe11.7O19 with 10 wt% and 20 wt% ferrite were prepared by a solegel method and an in-situpolymerization process, respectively. The structure, magnetic properties and microwave absorptionproperties of the samples were characterized by means of X-ray diffraction (XRD), Fourier transforminfrared spectra (FT-IR), transmission electron microscope (TEM), vibrating sample magnetometer (VSM)and vector network analyzer, respectively. The particle size of SrSm0.3Fe11.7O19 was about 35 nm by usingXRD. The ferrite successfully packed by PANI. PANI/SrSm0.3Fe11.7O19 possessed the best absorptionproperty with the optimum matching thickness of 3 mm in the frequency of 2e18 GHz. The value of themaximum reflection loss (RL) were �26.0 dB at 14.2 GHz with the 6.5 GHz bandwidth and �24.0 dB at13.8 GHz with the 7.9 GHz bandwidth for the samples with 10 wt% and 20 wt% ferrite, respectively.

© 2014 Elsevier Masson SAS. All rights reserved.

1. Introduction

In recent decades, electromagnetic waves in the GHz range arebeing increasingly used in wireless communication tools, local areanetworks, healthcare and defense sectors and other communica-tion equipment. Unfortunately, the increasing usage of electro-magnetic wave devices results in serious electromagneticinterference (EMI) [1]. To solve the EMI problem, considerable in-terest has been attracted to electromagnetic wave absorber withstrong absorption, wide absorption bandwidths, low density, andgood thermal stability. The conventional hexagonal ferrites such asstrontium ferrites do not function well in microwave range due tothe high anisotropy field of HA. Many researchers have reportedthat the HA of hexagonal ferrite can be changed by using rare earth(RE) ion substituting Fe3þ such as La3þ [2e4], Pr3þ [3], Eu3þ [5],Nd3þ [6], etc [7e9], resulting in not only a shift in resonance fre-quency but also a remarkable improvement of magnetic loss in theGHz frequency. Because the RE ions have unpaired 4f electrons, theoccurrence of 4f-3d couplings of the angular momentum whichimprove the electromagnetic properties. Moreover, 4f shell of rareearth ions is shielded by 5s25p6 and almost not affected by thepotential field of surrounding ions leading to the enhancement of

ax: þ86 515 88298249.

07served.

the coupling [3,4]. However the single magnetic loss can not bringabout ideal consequence of the microwave absorption because thatit can only absorb electromagnetic waves generated by magneticsources while it does not work to electromagnetic waves producedby an electric source, and the high density of the hexagonal ferrite isa big problem limiting its application. A good way to overcomethese problems is a combination of the strontium ferrite with theconductive polymer. Compared with the traditional ferrite mate-rials, conducting polymers such as polyaniline (PANI) own advan-tages of low density and high complex permittivity values [6],which not only improves the dielectric loss of ferrite/conductivepolymer composite but also reduces its density. The composites ofPANI wrapped ferrite provide a promising further of absorptionmaterial which can absorb the microwave both generated byelectric and magnetic source.

Recently, considerable efforts have been made towards thedevelopment of preparation conducting polymer/ferrite composite.S.P. Gairola et al. prepared polyaniline (PANI) nanocomposite withMn0.2Ni0.2Zn0.4Fe2O4 ferrite by mechanical blending, and they re-ported the composite of 2.5 nm thickness with 20%wt PANI showedstrong microwave absorption in 8e12 GHz (X-band) [10]. Tzu-HaoTing et al. synthesized PANI/BaFe12O9 composite by in situ poly-merization at different aniline/Ba ferrite weigh ration (Ani/Baferrite ¼ 1:2, 1:1 and 2:1). Their results indicated that the micro-wave absorption properties can bemodulated simply by controllingthe content of PANI on the samples for the required frequency

J. Luo et al. / Solid State Sciences 37 (2014) 40e46 41

bands [11]. Azadeh Tadjarodi et al. reported an extension inchemical free surfactant methods to synthesize the PANI nano-composites with the magnetic core of Ba0.69Cr0.17Cd0.07Zn0.7-Fe12O19. As a result of this research, the products showed themaximum reflection loss of �16 dB with 2.1 mm thickness of thecomposites [12]. Xin Tang et al. also studies the PANI-coatedM-typeBaFe12O19 ferrite composites, and they found that the interactionand interfacial polarization were seen as important factorscontributing to the influence on microwave absorption of thecomposite [13]. PANI/BaFe12O19/Ni0.8Zn0.2Fe2O4 nanocompositewas prepared by Ying Huang et al., and the maximum reflectionloss is�19.7 dB at 7.3 GHz [14]. Liangchao Li et al. prepared the Sm-substituted LiNi ferrite (Sm:Li ¼ 0.8:1) by a novel rheological phasereaction method, and LiNi0.5Sm0.08Fe1.92O4/PANI nanocompositewas synthesized by an in situ polymerization in aqueous in thepresence of LiNi0.5Sm0.08Fe1.92O4 [15]. Their group also reported thecoreeshell structure of Zn0.6Cu0.4Cr0.5Fe1.46Sm0.04O4/PANI com-posite which the nanosized Zn0.6Cu0.4Cr0.5Fe1.46Sm0.04O4 ferritedoped with Sm as magnetic core and PANI as conducting shell.When the content of Zn0.6Cu0.4Cr0.5Fe1.46Sm0.04O4 in compositeapproximately 20 wt%, the maximum reflection loss of �22.46 dBappeared at approximately 14.7 GHz [16]. While the composite ofpolyaniline/Sm-doped strontium ferrite nanoparticles has beenreported rarely.

In our previous work, we have reveals the RE-doped strontiumferrite (SrRExFe12�xO19, 0 � x � 0.5) obtains enhancing magneticproperties, and when x ¼ 0.3, the ferrite behaves the best magneticproperties without changing the phase of hexagonal structure ofstrontium ferrite [17]. Therefore, in this paper, we motivated tosynthesize Sm-doped strontium ferrite nanopowder(SrSm0.3Fe11.7O19) by solegel method, and PANI/SrSm0.3Fe11.7O19nanocomposite was prepared by in-situ polymerization method,respectively. The influences of PANI on the structure and thebehavior of magnetic and absorption properties of SrSm0.3Fe11.7O19at room temperature are discussed.

2. Experimental

2.1. Preparation of the samples

2.1.1. MaterialsIron nitrate (Fe(NO3) 3$9H2O, 98.5% purity), strontium nitrate

(Sr(NO3)2, 99.5% purity), aniline (An, 99.5% purity), hydrochloricacid (HCl, 36%~38% purity) and ammonium persulfate (APS, 98.0%purity) were all provided by the Sinopharm Chemical Reagent Co.,Ltd. Citric acid (C6H8O7, 99.5% purity), and ammonia solution(NH3$H2O, 25% purity) were received from Jiangsu TongshengChemical Regent Co., Ltd. Samarium nitrate (Sm(NO3)3$6H2O, 99.9%purity) was purchased from Aldrich. All of the chemical reagentswere used without further purification.

2.1.2. Preparation of Sm-doped strontium ferrite nanoparticlesStoichiometric amounts of Sm(NO3)3, Fe(NO3)3∙9H2O and

Sr(NO3)2 were dissolved in aminimum amount of deionized H2O bystirring at 40 �C with Fe/Sr ratio of 10.5. Citric acid was then addedto the mixture solution to chelate these ions. The molar ratio ofcitric acid to metal ions of Sr2þ and Fe3þ was 1.5:1. Ammonia wasadded to adjust the pH value to 7. The clear solution was slowlyevaporated at 70 �C under constant stirring, forming a viscous gel.By increasing the temperature up to 200 �C, the gel precursors werecombusted to form loose powders. Finally, the obtained powderwas calcined in air at 900 �C for 2 h in a muffle oven. TheSrSm0.3Fe11.7O19 particles were thus obtained.

2.1.3. Preparation of PANI/SrSm0.3Fe11.7O19 nanocomposite1 ml aniline monomer and SrSm0.3Fe11.7O19 (account for 10 wt%

and 20 wt% of aniline quantity) were added in 35 ml hydrochloricacid solution (0.1 mol L�1) and dispersed by ultrasonic wave for30 min. 2.49 g of ammonium persulfate was a dissolved in 15 mlhydrochloric acid solution (1 mol L�1). The ammonium persulfatesolution was then slowly added dropwise to the above mixturesolution with vigorous stirring. The polymerization was carried outfor 12 h. The compositeswere obtained by filtering andwashing thereaction mixture with deionized water and ethanol and dried un-der vacuum at 60 �C for 24 h. PANI/SrSm0.3Fe11.7O19 nanocompositewas thus synthesized.

2.2. Characterization

The resulting powder was characterized by X-ray powderdiffraction (XRD) using a diffractometer (RIGAKU, model D/max)with CuKa radiation of wavelength l ¼ 0.15418 nm. Its morphologywas studied with a transmission electron microscope (JEOL, modelJEM 2001). Fourier transform infrared spectroscopy (FT-IR) for theprepared samples were carried out using the infrared spectro-photometer (NICOLET, model NEXUS 670) in the range from 2500to 400 cm�1 with a resolution of 1 cm�1. Magnetization measure-ments were taken at room temperature (293 K) using a vibratingsample magnetometer (LDJ, model 9600-1). The complex permit-tivity (εr ¼ ε

0�jε00) and permeability (mr ¼ m

0�jm00) of the samples

were measured by a microwave vector network analyzer (AGILENT,model N5244A) in the frequency range 2e18 GHz by using coaxialreflection/transmission technique (where the ε

0, ε

00, m

0and m

00is

measured and the dielectric loss angle tangent (tan dε ¼ ε

00/ε

0) and

the magnetic loss angle tangent (tan dm ¼ m00/m

0) were calculated by

the measured parameters.). The samples for vector networkanalyzer were pressed to be toroidal samples with OD 7 mm, ID3.04mm and height about 3mm according to themass ration 1:1 ofbetween paraffin and PANI/SrSm0.3Fe11.7O19 nanocomposite. Mi-crowave absorption properties were evaluated by the reflection loss(RL), which was derived from the following formulas [18]:

zin ¼ z0

ffiffiffiffiffimrεr

rtanh

�j2pfdc

ffiffiffiffiffiffiffiffiffimrεr

p �(1)

RL ¼ 20 log����zin � z0zin þ z0

���� (2)

where f is the frequency of incident electromagnetic wave, d is theabsorber thickness, c is the velocity of light, Z0 is the impedance offree space, and Zin is the input impedance of absorber. The bestabsorbing properties is described by the impedance matchingcondition when Z0 ¼ Zin. The �10 dB absorbing bandwidth meansthat the frequency bandwidth can achieve 90% of reflection loss.

3. Results and discussion

3.1. Phase structure and composition analysis

3.1.1. PolymerizationIt is known to us that the surface charge of metal oxide is pos-

itive below the pH of the point of zero charge (PZC), while it be-comes negative above PZC. Since the surface of magnetite has PZCof pH¼ 6 [19], it is positive charged in 0.1 mol L�1 hydrochloric acidsolutionwhich the value of pH is above 6. Therefore, cl� is absorbedand compensates the positive charge on ferrite. In this approach,aniline monomers are converted to cationic anilinium ions in acidicconditions. Thus, the electrostatic interactions occur between

J. Luo et al. / Solid State Sciences 37 (2014) 40e4642

anions absorbed on ferrite surface and cationic anilinium ions. Theaniline monomers are polymerized by ammonium persulfate asan initiator agent on the ferrite surface at room temperature. Aschematic diagram of the polymerization process was shown inScheme 1.

Fig. 1. XRD patterns of samples: (a) PANI, (b) SrSm0.3Fe11.7O19, (c) PANI/SrSm0.3Fe11.7O19 (10 wt% ferrite), and (d) PANI/SrSm0.3Fe11.7O19 (20 wt% ferrite).

3.1.2. XRD analysisFig. 1 shows the XRD patterns of the synthesized samples. Fig. 1a

of PANI shows amorphous nature in partially crystalline state withtwo diffraction peak at 20.41 and 25.61 due to the densely packedphenyl rings those exhibit an extensive interchain p-p orbitaloverlap [16,20,21]. Fig. 1b shows the XRD pattern ofSrSm0.3Fe11.7O19. The characteristic diffraction peaks ofSrSm0.3Fe11.7O19 were at 30.2�, 32.1�, 34.1�, 37.0�, 40.3�, 42.3�, etc[22,23]. And these peaks could be deduced the sample ofSrSm0.3Fe11.7O19 ferrite was M-type SrFe12O19 which was a memberof the space group P63/mmc (PDF card #33-1340). The crystallitesize of SrSm0.3Fe11.7O19 sample is calculated from the intensereflection peak of (110), (107), (114) using Scherrer's equation (i.e.,D ¼ 0.89l/(bcos q)), where l is the wavelength of the X-ray radia-tion, q is the diffraction angle and b is the full width at halfmaximum (FWHM), and were found to be 35.094(7) nm, 34.976(3)nm, and 34.929(5) nm, respectively. Furthermore, the averagecrystallite size was about 35 nm, which is calculated from them ofthree intense reflection peak. The value of the lattice constantsfrom the general structure analysis system (GSAS) software suitewere found to be a ¼ 5.88312 (5) Å and c ¼ 23.18427 (3) Å,respectively [24]. Fig. 1c and d shows the XRD patterns of PANI/SrSm0.3Fe11.7O19 nanocomposite. It can be observed that charac-teristic diffraction peaks of PANI and SrSm0.3Fe11.7O19 appeared innanocomposite with somewhat change in the intensity, whichconfirms the presence of PANI and SrSm0.3Fe11.7O19 in the PANI/SrSm0.3Fe11.7O19 nanocomposites. The peak intensity ofSrSm0.3Fe11.7O19 is weaker than that of pure ferrite and decreaseswith increasing of PANI content, which reveal that PANI coatinglayer has an effect on the peak intensity of SrSm0.3Fe11.7O19 ferrite

Scheme 1. The simple synthesis procedure mech

[16]. In further observation, it can be found that the diffraction peakshift clearly towards lower 2q from sample a to c. The result is ingood agreement with the result reported earlier [25,26]. Theshifting peaks indicate that the unit cell of SrSm0.3Fe11.7O19 isincreased due to the absorption of PANI molecular chains on thesurface of SrSm0.3Fe11.7O19 nanoparticles and SrSm0.3Fe11.7O19nanoparticles are incorporating into the PANI polymer matrix.

3.1.3. FT-IR analysisFig. 2 shows the FT-IR spectra of the PANI and PANI/

SrSm0.3Fe11.7O19 nanocomposite. The FT-IR spectrum of PANI/SrSm0.3Fe11.7O19 composite (Fig. 2b, c) is almost identical to that ofPANI (Fig. 2a). The characteristic absorption peaks of PANI at 1635，1567 cm�1 are attributed to the C]N stretching vibration of

anism of PANI coating with SrSm0.3Fe11.7O19.

Fig. 2. FT-TR spectra of the samples: (a) PANI, (b) PANI/SrSm0.3Fe11.7O19 (10 wt% ferrite)and (c) PANI/SrSm0.3Fe11.7O19 (20 wt% ferrite).

J. Luo et al. / Solid State Sciences 37 (2014) 40e46 43

quinoid and the C]C stretching mode of benzenoid rings, respec-tively. The peak at 1417 cm�1 is corresponding to the stretchingmode of NeQeN where Q represents the benzenoid ring [27]. Thebands at about 1344 and 1295 cm�1 correspond to NeH bendingand asymmetric CeN stretching mode for benzeniod ring, respec-tively [13]. The peak at 1123 cm�1 is attributed to aromatic CeH in-plane bending mode [16]. And these peaks are seen again in PANI/SrSm0.3Fe11.7O19 composite, but all of the peaks are red shifted.Moreover, with the increase in the PANI content, the intensity ofpeak at 470 and 580 cm�1 which corresponds to SrSm0.3Fe11.7O19ferrite is greatly diminished [13]. These results reveal that ferritesare packed successfully by PANI, and there is an interaction be-tween SrSm0.3Fe11.7O19 nanoparticles and PANI chains. This inter-action is caused by sep interaction between ferrite and PANI,which include (1) the p molecular orbital of PANI overlaps theempty d-orbital of Fe3þ to form the s-bond where metal ions playsa role of the electron pair acceptor; (2) the p* molecular orbital ofPANI overlaps the d-orbital of Fe3þ to form thep-bond, inwhich theFe3þ is the electron pair donor [28]. In addition, the hydrogenbonding interaction between PANI and the oxygen atoms on theferrite surface occurs in the composites, which make ferrite parti-cles be embedded into polymer chain [29] which is agree with XRDresults. .

Fig. 3. TEM micrographs of the samples: (a) SrSm0.3Fe11.7O19 and (b) PANI/SrSm0.3Fe11.7O19.

3.1.4. TEM analysisFig. 3a shows the micrograph of SrSm0.3Fe11.7O19. The hexagonal

structure of ferrite can be clearly observed and aggregate due to themagnetic dipole interaction between ferrite particles [13]. Theaverage particle size induced from the TEM micrograph was in therange 50e100 nm. In addition the selected area electron diffraction(SAED) pattern in inset of Fig. 3a indicates that ferrite particle ishighly crystalline with M-type ferrite. And the size discrepancybetween XRD and TEM is mainly attributed to the differentapproach of the crystal and accuracy of the Scherrer's equation areaffected by many factors such as diffraction line width, defects andsurface tension, so the Scherrer's formula may induce some errorsin measuring the absolute of the crystallite size [30]. TEM image ofFig. 3b indicates that the ferrite particles are embedded in the PANImatrix. The increase in the size of nanoparticles in the compositeshows that the surface of ferrite nanoparticles has interaction withPANI molecular chains, which is also supported by FT-IR analysis.

3.2. Magnetic properties

Fig. 4 shows hysteresis loops of the synthesized samples whichwere carried out by VSM at room temperature with maximumapplied field of 12,000 Oe. The variation of saturation magnetiza-tion (Ms) and coercivity (Hc) of as-prepared sample is shown inTable 1. As can be seen from Fig. 4, after SrSm0.3Fe11.7O19 packedwith PANI, the saturation magnetization dropped dramaticallywhile coercivity value declined slightly compared withSrSm0.3Fe11.7O19.

The coercivity force is involved in many factors, such as gainshape, components, magnetic anisotropy and magnetic scalability,etc. In the polymerization process, the coating PANI developed onthe surface of ferrite particles and crystal boundary lead to interfaceinteraction which will reduce the surface anisotropy of ferrite par-ticles. In addition, there is charge transfer from PANI to ferrite onferrite surface, altering ferrite surface charge density, effectingmechanism of electron spin system, weakening the domain wallmovement resistance [14,31]. Taking the two sides into consider-ation, the coercivity value drops mildly owing to the reduction of thesurface anisotropy and the weaker domain wall movement resis-tance. Otherwise, the saturation magnetization of the compositebecomes much smaller than those ferrite nanoparticles, which is notonly determined by the ferrite content of the composite but also thenon-magnetic PANI. Because on one hand, saturation magnetization

Fig. 4. Hysteresis loops of the samples: (a) SrSm0.3Fe11.7O19, (b) PANI/SrSm0.3Fe11.7O19

(10 wt% ferrite), and (c) PANI/SrSm0.3Fe11.7O19 (20 wt% ferrite).Fig. 5. The dielectric loss tangent of the samples as a function of frequency: (a)SrSm0.3Fe11.7O19, (b) PANI/SrSm0.3Fe11.7O19 (10 wt% ferrite), and (c) PANI/SrSm0.3Fe11.7O19 (20 wt% ferrite).

J. Luo et al. / Solid State Sciences 37 (2014) 40e4644

of the composite is decided by the volume fraction of ferrite, whichcan be calculate from Ms ¼ jms (Ms is the saturation magnetizationof composite, j is the volume fraction of magnetic particle,ms is thesaturation magnetization of a single magnetic particle). On the otherhand, with the increasing percentage of PANI, the interaction be-tween magnetic grains has been suppressed [14].

3.3. Electromagnetic parameter

It is known that the dielectric loss angle tangent (tan dε¼ ε

00/ε

0)

and the magnetic loss angle tangent (tan dm ¼ m00/m

0) are vital pa-

rameters charactering the electromagnetic wave loss.Fig. 5 shows the dielectric loss tangent spectra of the synthe-

sized samples. The dielectric loss tangent of SrSm0.3Fe11.7O19 isalmost to be zero, which means strontium ferrite is a kind of non-dielectric absorbing wave materials. The dielectric loss tangent ofthe composites is apparently much higher than theSrSm0.3Fe11.7O19. The result implies that the addition of PANI has animportant impact on the improvement of dielectric properties. Thepolarons produced by oxidized PAIN main chain can form theconductive current by changing their position in PANI molecule onthe effect of electromagnetic field. The former conductive current inPANI molecule turns into eddy accounting to oscillation of elec-tromagnetic field, and the eddy can convert electric energy intothermal energy so as to consume the electromagnetic wave [32].

When the frequency is 2 GHz, the reflection peak of curve b andc is 0.55 and 0.49, respectively. In the frequency range of11e18 GHz, curve b reaches the maximum 0.65 in 16.54 GHz, andthe curve c owns a peak of 0.39 in 16.96 GHz. Clearly, dielectric lossof the composite increases rapidly because of the increasing con-tent of PANI which owns high dielectric loss.

Fig. 6 shows the magnetic loss tangent spectra of the synthe-sized samples. The curves show that all of the samples have lowmagnetic loss compared with their dielectric loss at 2e18 GHz. Inthe frequency range of 11e18 GHz, appears an overall upward trend

Table 1The room temperature magnetic parameters of as-prepared samples.

Sample Ms/emu g�1 Hc/Oe

SrSm0.3Fe11.7O19 58.48 7168.0PANI/SrSm0.3Fe11.7O19 (10 wt% ferrite) 5.61 6938.5PANI/SrSm0.3Fe11.7O19 (20 wt% ferrite) 10.81 6979.5

of the magnetic loss angle tangent regardless the negative sign. Itcan be explained that the magnetic loss is not only originated fromthe spin-rotation resonance of SrSm0.3Fe11.7O19 but also enhancedby the interfacial polarization and relaxation effects betweenpolymer chain and magnetic particles. It is interesting to observethe negative magnetic loss of the composites in 8e18 GHz whichmust be caused by the negative value of m

00because of the positive

value of ε00(according to electromagnetic wave theory). As we all

know, the magnetic behavior may be modulated by the dielectricbehavior. The negative value of m

00can be explained by the LRC

equivalent circuit model where L, R and C are inductance, resistanceand capacitance respectively. According to Maxwell equations, amagnetic field can be induced by an electric field and be radiatedout. So the negative magnetic loss of the composites in 8e18 GHz isattributed to the capacitance C lead or lag behind the angle of90�than the inductance L [33].

3.4. Microwave absorption property

Fig. 7 shows the reflection loss of the synthesized composites atsample thickness of 2.0, 3.0, 4.0 mm. As shown in Fig. 7, the

Fig. 6. The magnetic loss tangent of the samples as a function of frequency: (a)SrSm0.3Fe11.7O19, (b) PANI/SrSm0.3Fe11.7O19 (10 wt% ferrite), and (c) PANI/SrSm0.3Fe11.7O19 (20 wt% ferrite).

Fig. 7. The reflectance curves of the samples: (a) SrSm0.3Fe11.7O19, (b) PANI/SrSm0.3Fe11.7O19 (10 wt% ferrite), and (c) PANI/SrSm0.3Fe11.7O19 (20 wt% ferrite).

Table 2Microwave absorption properties of the samples.

Sample Thickness(mm)

Absorptionpeak (GHz)

Peakvalue (dB)

Bandwidth(<�10 dB) (GHz)

PANI/SrSm0.3Fe11.7O19

(10 wt% ferrite)2.0 17.5 �11.0 1.03.0 14.2 �26.0 6.54.0 10.1 �44.0 4.7

PANI/SrSm0.3Fe11.7O19

(20 wt% ferrite)2.0 17.1 �10.5 1.23.0 13.8 �24.0 7.94.0 9.1 �21.8 4.3

J. Luo et al. / Solid State Sciences 37 (2014) 40e46 45

remarkable enhancement of microwave absorption property of thecomposite in whole frequency of 2e18 GHz was observed. It isexplained that the microwave absorption mechanism of the com-posite mainly depends on the dielectric loss in the frequency rangeof 2e9 GHz, while in the frequency range of 9e18 GHz, it mainlydepends on magnetic loss. The dielectric loss of the composite isimproved by the coating effect of PANI and the forced vibrationincreasing of the atoms and molecules in the alternating electricfield. When the microwave gets to 9e18 GHz, it occurs spin mo-ments, the magnetic domain wall forced harmonic vibration [27],and interface polarization relaxation between polymer and mag-netic particles which both enhance the magnetic loss [16].

Fig. 7 also showed that with the increasing matching thicknessd, absorption peaks move to low frequency. Fig. 7b and c shows thatwhen matching thickness d increased from 2 mm to 4 mm, thereflectivity loss value shift negatively. This phenomenon is causedby electromagnetic wave dimensional resonance during to coatinglayer. Therefore, the microwave absorption properties can bemodulated simply by manipulating the thickness of the as-prepared PANI/SrSm0.3Fe11.7O19 composite for application indifferent frequency bands.

However, the peak at about 17.5 GHz does not show on the RLgraph, which appears both on graph of the dielectric loss tangentand magnetic loss tangent. According to electromagnetic theory,the good impedance match condition is the balance of dielectricloss match and magnetic loss. It means that the maximal absorp-tion is simply due to the same value of magnetic loss and dielectricloss regardless of the existence of the peak of dielectric loss andmagnetic loss or not [34]. However, at 17.5 GHz, the high dielectricloss and the low magnetic loss are harmful to the impedancematch. Therefore, the absorption peak at about 17.5 GHzdisappears.

Compared with pure SrFe12O19 nanoparticles [35] which onlyachieves the maximum RL of �3.0 dB without any strong absorp-tion in 2e18 GHz, the Sm-doped ferrite indicate a stronger micro-wave absorption. Also, the sharp and strong absorption peaksappear at 13.2 and 14.56 GHz respectively and the maximum RLis �5.34 dB. Furthermore, the composites of PANI/SrSm0.3Fe11.7O19possess the best absorption property with the optimum matchingthickness of 3 mm. The microwave absorption date of Fig. 7b and cis summarized in Table 2. The value of the maximum RLare�26.0 dB at 14.2 GHz with the 6.5 GHz bandwidth and�24.0 dBat 13.8 GHz with the 7.9 GHz bandwidth for the samples with 10 wt% and 20 wt% ferrite, respectively.

4. Conclusion

Nanosized Sm-doped strontium ferrites packed by PANI weresuccessfully prepared through in situ polymerization process.Compared with SrSm0.3Fe11.7O19, the saturation magnetization ofPANI/SrSm0.3Fe11.7O19 was decreased markedly with the increasingcontent of the PANI, while coercivity value was slightly declined.The composite possessed the best absorption property with the

J. Luo et al. / Solid State Sciences 37 (2014) 40e4646

optimum matching thickness of 3 mm in the frequency of2e18 GHz. The value of themaximumRLwere�26.0 dB at 14.2 GHzwith the 6.5 GHz bandwidth and �24.0 dB at 13.8 GHz with the7.9 GHz bandwidth for the samples with 10 wt% and 20 wt% ferrite,respectively.

Acknowledgments

The author would like to thank Key Laboratory for Ecological-Environment Materials and Key Laboratory for Advanced Technol-ogy in Environmental Protection of Jiangsu Province of China andQing Lan project for financial support for this research.

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