Synthesis of conducting ferromagnetic nanocomposite with improved microwave absorption properties

Materials Chemistry and Physics 119 (2010) 201–207

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Synthesis of conducting ferromagnetic nanocomposite with improved microwave absorption properties Kuldeep Singh a , Anil Ohlan a , A.K. Bakhshi a,b , S.K. Dhawan a,∗ a b

Polymeric & Soft Materials Section, National Physical Laboratory, New Delhi 110 012, India Department of Chemistry, University of Delhi, Delhi 110007, India

a r t i c l e

i n f o

Article history: Received 29 December 2008 Received in revised form 14 July 2009 Accepted 25 August 2009 Keywords: Composite materials Polymers Nanostructures Electron microscopy Electrical characterization

a b s t r a c t The present paper reports the synthesis of polyaromatic amine–ferromagnetic composite with nanosize TiO2 (∼70–90 nm) and ␥-Fe2 O3 (∼10–15 nm) particles via in situ emulsion polymerization. Magnetic and conductivity studies demonstrate that the conducting ferromagnetic composite possesses saturation magnetization (MS ) value of 26.9 emu g−1 and conductivity of the order of 0.46 S cm−1 , which are measured by vibrating sample magnetometer and four-probe technique, respectively. It is observed that the presence of the nanosized ␥-Fe2 O3 in the polyaniline–TiO2 matrix affects the electromagnetic shielding property of the composite. Polyaniline–TiO2 –␥-Fe2 O3 nanocomposite has shown better shielding effectiveness due to absorption (SEA ∼ 45 dB) than the polyaniline-␥-Fe2 O3 (SEA ∼ 8.8 dB) and polyaniline–TiO2 (SEA ∼ 22.4 dB) nanocomposite. The polymer composites were further characterized by high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) technique. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Conducting polymer and their composites have attracted the attention of material researchers extensively, because of their wide spread applications in organic light emitting diodes (OLED) [1,2], polymer solar cells [3,4], antistatic coatings [5,6], and electromagnetic interference (EMI) shielding [7]. With the development and wide use of electric and telecommunication equipment in electric and electronics industries, some problems regarding EMI have been faced because it degrades the life time, efficiency, and also affect the safety operation of many electronic devices. In order to avoid such problems, all electronic equipments must be shielded against electromagnetic aggressions. Composites based on polymers, like hexagonal-ferrite/polymer, metal/polymer composites, and single wall carbon nanotube–epoxy composites [8–11] have been reported by many research groups for this purpose. The nanocomposites of conducting polymer like polyaniline, polypyrrole, and polyethylene dioxythiophene with different ferrites like ␥-Fe2 O3 , Fe3 O4 , ferrites of Mn and Ni have been prepared by different methods [12–15]. These nanocomposites of conducting polymer have expended their horizon and now finding their application in electromagnetic interference (EMI) shielding technology [16–19]. Generally, metals or ferrites are used to shield the electronic devices but now a days intrinsic conducting polymers (ICPs) are attracting attention for being used as an EMI shielding material

∗ Corresponding author. Tel.: +91 11 45609401; fax: +91 11 25726938. E-mail address: [email protected] (S.K. Dhawan). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.08.060

in the form of paints or coatings. Polyaniline possesses the unique structural, physical, and electrochemical properties. The molecular structure of polyaniline is composed of alternatively reduced (–B–NH–B–NH–) and oxidized (–B–N Q N–) units where B and Q denote C6 H4 rings in the benzenoid and quinoid states, respectively. Polyaniline exists in different forms namely leucoemeraldine, pernigraniline, emeraldine and conductive emeraldine salt. These forms refer to different oxidation states of polyaniline ([(–B–NH–B–NH–)y (–B–N Q N–)1−y ]x ) where y = 1, 0.5, and 0, respectively. This shows that oxidant play a vital role in controlling the structure and electric properties which can be tailored according to the application. Earlier studies have been made on the development of nanocomposites having both conducting and ferromagnetic properties, by the incorporation of ferrite in the polymer matrix using in situ or ex situ polymerization [20,21], but the values obtained for shielding effectiveness are not enough. It was assumed that the shielding effectiveness value of 30 dB would be enough to protect the electronic equipment for the commercial applications. But this value does not fulfill the military requirement for which the shielding effectiveness of the material should be in the range of 80–100 dB. In this paper we demonstrate an approach for creating multicomponent polyaniline–ferromagnetic composite with nanosize TiO2 (∼70–90 nm) and ␥-Fe2 O3 (∼10–15 nm) particles via in situ emulsion polymerization using dodecyl benzene sulfonic acid (DBSA) as dopant which also acts as a surfactant. Here, we have prepared two different compositions by taking aniline, TiO2 , and ␥-Fe2 O3 in the 1:1:1 (PTF11) and 1:1:2 (PTF12) weight ratios and the results of these composites were compared

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Scheme 1. Schematic representation of synthesis of TiO2 and ␥-Fe2 O3 ferromagnetic conducting nanocomposite.

with the polyaniline–TiO2 (PT11) and polyaniline–␥-Fe2 O3 (PF12) composites. From the observed results, it was concluded that multicomponent nanocomposite of polyaniline has better EMI shielding properties than the previously prepared polyaniline–␥-Fe2 O3 and polyaniline–TiO2 nanocomposite. TiO2 was chosen as one of the component because it has high dielectric constant and well known for its use in pigments. 2. Experimental

331) and a cryo cooler from 300 to 70 K. For measurements, compressed pellets of powder sample were created at 5 ton in a rectangular die (13 mm × 7 mm) and four contacts were made on each end using silver paste. Electromagnetic shielding, dielectric and permeability measurements were carried out on an Agilent E8362B Vector Network Analyzer in a microwave range of 12.4–18 GHz (Ku-band). Powder samples were compressed in the form of rectangular pellets (2 mm thick) and inserted in 15.8 mm × 7.9 mm × 6 mm copper sample holder connected between the wave-guide flanges of network analyzer. Full twoport calibration is performed along with the sample holder to neglect any loss and power redistribution due to sample holder.

2.1. Materials

3. Results and discussion Aniline (An), ammonium peroxydisulfate (NH4 )2 S2 O8 (APS), isopropyl alcohol, FeCl3 ·6H2 O, FeCl2 ·4H2 O, TiO2 , and aqueous ammonia solution were purchased from Merck, India. The aniline monomer was purified by distillation in vacuum before use. The other chemicals were of reagent grade and used as received. 2.2. Preparations of the polyaniline ferromagnetic composites The nanosize ␥-Fe2 O3 particles were synthesized by the co-precipitation of Fe2+ and Fe3+ ions in aqueous medium by ammonium hydroxide [22], while the TiO2 was used after ball milling it for 6 h using high energy planetary ball mill (PM400, Retch) in tungsten carbide jars. The formation of ␥-Fe2 O3 and TiO2 phase was confirmed by X-ray diffractometer. The resulting nanosize ␥-Fe2 O3 along with TiO2 nanoparticles is homogenized in 0.3 M aqueous solution of DBSA to form a whitish brown emulsion solution. Appropriate amount of aniline (0.1 M) was added to above solution and again homogenized for 2–3 to form micelles of aniline with ␥-Fe2 O3 and TiO2 . The micelles so formed are polymerized below 0 ◦ C through chemical oxidization polymerization by (NH4 )2 S2 O8 (0.1 M). The product so obtained was demulsified by treating with equal amount of isopropyl alcohol. The precipitates were filtered out and washed with alcohols and dried at 60–65 ◦ C. Different formulations of polymer composite having different weight ratio of monomer to ferrite/TiO2 , An:␥-Fe2 O3 :TiO2 ; 1:1:1 (PTF11); 1:1:2 (PTF12), were synthesized in DBSA medium to check the effect of ferrite constituents on the properties. Beside this, polyaniline–TiO2 (PT11) composites having monomer to TiO2 weight ratio of 1:1, polyaniline–Fe2 O3 (PF12) with monomer to Fe2 O3 weight ratio of 1:2 and pure polyaniline doped with DBSA (PD13) were also synthesized for comparative study. 2.3. Characterization The particle size and the morphology of TiO2 , ␥-Fe2 O3 , and polymer composites were examined using transmission electron microscopy (Phillips CM-12). The TEM samples were prepared by dispersing the powder in iso-propanol using sonification and placing small drops of the suspension on carbon coated copper grids. HRTEM was carried out on Technai G20-stwin (200 kV) with point resolution of 1.44 Å and line resolution of 2.32 Å. The presence of TiO2 and ␥-Fe2 O3 in the polymer composite was confirmed by X-ray diffraction (XRD) studies, carried out on D8 Advance X-ray diffractometer (Bruker) using Cu K␣ radiation ( = 1.540598 Å) in scattering range (2) of 10–70◦ with a scan rate of 0.02◦ s−1 and slit width of 0.1 mm. The magnetic measurements of the samples were performed using the vibrating sample magnetometer (VSM) model 7304 Lakeshore Cryotronics Inc., USA, with a maximum magnetic field of 1.2 T and the sample is filled in Perspex sample holder which is vibrated horizontally with the frequency of 76 Hz. The conductivity of polyaniline composites was measured by four-probe technique using Keithley programmable current source (model 6221) and nanovoltmeter (model 2182A) attached to a digital temperature controller (Lakeshore

For the formation of PTF composite, emulsion polymerization (oil in water type) has been carried out in which the droplet of aniline (oil) emulsified with surfactant DBSA containing TiO2 and ␥-Fe2 O3 particles, in a continuous phase of water. A large amount of surfactant lead to the formation of micelle which in aqueous solution forms a roughly spherical or globular aggregate with the hydrophilic “head” regions in contact with surrounding solvent, sequestering the hydrophobic tail regions in the micelle center. Generally, in micellar solution there are the chances of formation of macroscopic particles that can be prevented by adding the steric stabilizers like poly (vinyl alcohol), poly (N-vinylpyrrolidone), and cellulose ethers, but in the present system the bulky surfactant dodecyl benzene sulfonic acid itself act to prevent the formation of the macroscopic precipitation [23]. When monomer aniline is added to the micellar solution containing nanoparticles, it diffuses through water to micelles. The addition of oxidant APS to the solution leads to the oxidative polymerization where aniline is oxidized and form anilinium radical cations which subsequently combine with another unit to form neutral dimer. The further oxidation of this dimer leads to form a trimer and finally to polymer containing nanoparticles of TiO2 and ␥-Fe2 O3 embedded in between the polymer chains as shown in Scheme 1.

3.1. Electromagnetic shielding The EMI shielding effectiveness (SE) of a material is defined as the ratio of transmitted power to incident power and given by SE (dB) = −10 log

P  T

PO

(1)

where PT and PO are the transmitted and incident electromagnetic power, respectively. For a shielding material, total SE = SER + SEA + SEM , where, SER is due to reflection, SEA is due to absorption and SEM is due to multiple reflections.

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In two-port network, S-parameter S11 (S22 ), S21 (S12 ) represents the reflection and the transmission coefficients given as

 2      ET   2  2  = S21 = S12 , E

T =

(2)

I

 2      ER   2  2  = S11 = S22 , E

R=

(3)

I

and Absorption coefficient (A) = 1 − R − T

(4)

Here, it is noted that absorption coefficient is given with respect to the power of the incident EM wave. If the effect of multiple reflection between both interfaces of the material is negligible, the relative intensity of the effectively incident EM wave inside the materials after reflection is based on the quantity as 1 − R. Therefore, the effective absorbance (Aeff ) can be described as Aeff = (1 − R − T)/(1 − R) with respect to the power of the effectively incident EM wave inside the shielding material. It is convenient to express the reflectance and effective absorbance in the form of −10 log(1 − R) and −10 log(1 − Aeff ) in decibel (dB), respectively [24], which give SER and SEA as SER = −10 log(1 − R) and SEA = −10 log(1 − Aeff ) = −10 log

T 1−R

(5)

For the material the skin depth (ı) is the distance up to which the intensity the of the electromagnetic wave decrease to 1/e of its original strength. The skin depth is related with the  attenuation 2/ωAC constant (ˇ) of the wave propagation vector ı = 1/ˇ = with the approximations that   ωε. As ı ∝ ω−1/2 , therefore, at low frequencies for the electrically thin samples (d  ı) the shielding effectiveness of the sample is describe as



SE (dB) = 20 log 1 +

1 ZO d 2



(6)

where  is the AC conductivity, ZO is free space impedance and d is the sample thickness. Whereas for the higher frequencies, sample thickness (electrically thick samples) is sufficiently greater than skin depth and EMI shielding effectiveness for the plane electromagnetic wave [25] is given as SE (dB) = SER (dB) + SEA (dB), SER (dB) ≈ 10 log



AC 16ωεO r

(7)

 ,

(8)

and SEA (dB) = 20 ·

d · log e ı

(9)

where  AC depends upon the dielectric properties [26] ( AC = ωε0 ε ) of the material, ω is the angular frequency (ω = 2f), εO is the free space permittivity and r is the relative magnetic permeability of the sample. In Eq. (7), the first term is related to the reflection of the EM wave and contributes as the shielding effectiveness due to reflection. The second term expresses the loss due to the absorption of the wave when it passes trough the shielding material. In microwave range, the contribution of the second part becomes more as compared to the reflection term. 3.2. X-ray diffraction studies Fig. 1 shows the X-ray diffraction patterns of TiO2 , ␥-Fe2 O3 and composites of polyaniline with TiO2 and ␥-Fe2 O3 . The main peaks for TiO2 were observed at 2 value 25.283 (d = 3.520 Å), 37.784 (d = 2.379 Å), 38.530 (d = 2.335 Å), 48.032 (d = 1.893 Å), 53.874 (d = 1.700 Å), 55.025 (d = 1.667 Å), and 62.660 (d = 1.481 Å) corresponding to (1 0 1), (0 0 4), (1 1 2), (2 0 0), (1 0 5), (2 1 1), and (2 0 4)

Fig. 1. XRD plots of (a) TiO2 , (b) PT11, (c) PTF11, (d) PTF12, (e) PF12, and (f) ␥-Fe2 O3 .

reflections (curve a) while for ␥-Fe2 O3 main peaks are observed at 2 value of 30.281 (d = 2.949 Å), 35.699 (d = 2.513 Å), 43.435 (d = 2.081 Å), 53.805 (d = 1.702 Å), 57.437 (d = 1.603 Å), 63.0460 (d = 1.473 Å) corresponding to the (2 0 6), (1 1 9), (0 0 12), (2 2 12), (1 1 15), and (4 4 1) reflections, respectively (curve f). All the observed peaks were matched with the standard XRD pattern of TiO2 (Powder Diffraction File, JCPDS No. 84-1285) and ␥-Fe2 O3 (Powder Diffraction File, JCPDS No. 39-1346). The peaks of ␥-Fe2 O3 were found in all the compositions of PTF composites, which designate the presence of ferrite particles in the polymer matrix while the increase in intensity of peaks demonstrate the increase in the ratio of iron oxide. The distinguish sharp peaks were observed for the TiO2 nanoparticles as compared to the iron oxide because of its larger crystallite size, calculated using Scherer’s formula D=

k ˇ cos 

(10)

where  is the X-ray wavelength, k the shape factor, D the crystallite size for the individual peak of the crystal in angstroms,  the Bragg angle in degrees and ˇ is the line broadening measured by halfheight in radians. The value of k is assigned as 0.89, which depends on several factors, including the Miller index of the reflecting plane and the shape of the crystal. The crystallite size of ␥-Fe2 O3 particles was calculated by Eq. (10) is estimated to be 10.3 nm for pure ␥Fe2 O3 while it was 13.7 nm in PTF12. The crystallite size of TiO2 was 36.6 nm for pure TiO2 sample and 29.6 nm in PTF12 nanocomposite. The presence of peaks of TiO2 and ␥-Fe2 O3 shows the formation of composite having separate phase of both the compound properly dispersed in the polymer matrix.

3.3. TEM and HRTEM analysis TEM images of ␥-Fe2 O3 , TiO2 , and polyaniline composite (PTF12) were shown in Fig. 2. Well-dispersed spherical particles of ␥-Fe2 O3 and TiO2 particles were observed. The particle size of ␥-Fe2 O3 was estimated to be 10–15 nm while TiO2 has slightly larger particles of 70–90 nm (Fig. 2a and b). When these nanoparticles were incorporated in the polymer matrix, they show the agglomerated morphology (Fig. 2c). The dispersion of ␥-Fe2 O3 and TiO2 particles in the polymer matrix is confirmed by the HRTEM image (Fig. 2d). The lattice fringes of ␥-Fe2 O3 with lattice spacing 0.25 nm corresponding to (1 1 9) plane and 0.35 nm corresponding to (1 0 1) plane for TiO2 were matched with the XRD pattern of the polymer composite.

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Fig. 2. TEM images of (a) ␥-Fe2 O3 , (b) TiO2 , (c) PTF12, and (d) HRTEM results of PTF12, showing the well-dispersed nanoparticles of ␥-Fe2 O3 and TiO2 in the polymer matrix.

3.4. Conductivity measurements The temperature dependent DC conductivity ( DC ) of the PT11 and PTF12 composites having different weight ratio of ferric oxide contents was measured in temperature ranging from 30 to 300 K. The variation of log  DC as function of T−1/4 was plotted in Fig. 3, whereas inset shows the log  DC vs. 1000/T plot. It shows that the conductivity tends to saturate at lower temperature. To check the effect of nanoparticles on conductivity the room temperature conductivity measurement of polyaniline and its composite was performed. It is observed that with the addition of nanoparticles of Fe2 O3 ( ∼ 10−9 S cm−1 ) and TiO2 ( ∼ 10−11 S cm−1 ) in the polymer matrix the conductivity of the polyaniline doped with DBSA decreases from 2.2 to 0.46 S cm−1 for the composite. The expected decrease in conductivity is due to the incorporation of insulating nanoparticles in the polymer matrix which hinder the conduction path. Several models were established in order to explain the conductivity variations of conducting polymers but it is observed that the conductivity studies are best explained by VRH (Variable range hopping) model which follows Mott’s equation [27–29]

  1/  TO

(T ) = O exp −

T

(11)

where  O and TO are constants and exponent is the dimensionality factor having values 2, 3, 4 for 1-dimension, 2-dimension and 3-dimension conduction mechanism. In order to calculate the exponent , log  DC vs. T−1/4 is plotted which yields a straight line for the temperature range of 70–300 K. To satisfy the Mott’s equation, activation energy [30,31],



EA =

−∂ln  ∂(1/KB T )



(12)

of the samples was calculated from the slope of log  DC vs. 1000/T plot. Eq. (12) can be correlated with the with Mott’s equation (11) by the following expression EA = mKB TO

 T  −1 O

T

(13)

it is evident from expression (13) that a plot of log EA vs. log T should give a straight line of slope −( − 1). The straight line corresponding to = 1/4 indicate that the variable range hopping mechanism of type T−1/4 explains the conduction mechanism in the polymer composite PT11 and PTF12. For 3D conduction mechanism, the values of Mott characteristic temperature (TO ) and  O (conductivity at T = ∞) are given by TO =

16˛3 [kB N(EF )]

(14)

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205

Fig. 4. Variation of SEA and SER for the samples PTF12, PTF11, PT11, and PF12 with frequency in 12.4–18 GHz range.

conduction mechanism wherein the charge transport occurs by phonon aided hopping or by thermally stimulated jumps between the localized sites. Conductivity in the polymer composite is due to semi-quinone radical cations formed by H-bonding between neighboring polymers. The new states are generated between the valence and conduction bands by the doping and are responsible for conduction and leads to the variation in activation energy. The overlapping of ␲-delocalized wave orbital of aniline ring with the d-orbital of metal ion in polymer composite forms the charge trans-

Fig. 3. Temperature dependence of log  as function of T−1/4 for the samples PT11 (a) and PTF12 (b) in the temperature range from 300 to 70 K whereas the inset figure shows the log  variation vs. 1000/T.

O = e2 R2 ph N(EF ) where R=



9 {8˛KB TN(EF )}

(15)

1/4 (16)

is the average hopping distance, ˛−1 is the localization length, N(EF ) is the density of states at the Fermi level, and ph is the phonon frequency (∼1013 Hz). The average hopping energy W can be estimated by knowing the average hopping distance R and the density of states at the Fermi level N(EF ) by the following relation W=

3 4R3N(EF )

(17)

The values of various Mott’s parameters T0 , N(EF ), R, and W for the composite PT11 and PTF12 are given in Table 1 which are calculated by using above Eqs. (11)–(17). Our results are consistent with the Mott’s requirement that ˛R  1 and W  KB T for conductivity by hopping to distant sites [32,33]. The conductivity data fits for the 3D-VRH model with = 4 having the linearity factor of 0.9996 for PT11 and 0.9997 for PTF12 composite from 300 to 70 K. Thus it was concluded that 3D-VRH model is suitable for explaining the Table 1 Linearity factor and various Mott’s parameters for the samples PT11 and PTF12. Sample name

Linearity T−1/4

T0 (K)

N(EF ) (cm−3 eV−1 )

R (Å)

W (meV)

(˛R)

PT11 PTF12

0.9997 0.9996

1.43 × 106 9.3 × 105

1.76 × 1018 2.7 × 1018

194 174

18.6 16.6

4.62 4.15

Fig. 5. Dielectric constant (a) and dielectric loss (b) behavior with the variation of frequency for PT11 (), PTF11 (䊉), PTF12 (), and PF12 ().

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Fig. 6. Variation of real part of magnetic permeability (a) of PT11 (), PTF11 (䊉), PTF12 (), and PF12 () with frequency while figure (b) shows the change in magnetic moment per unit mass with applied field for Fe2 O3 (), PTF11 (䊉), and PTF12 ().

Fig. 7. Dependence of SEA as function of ( AC )1/2 while the inset shows the change in skin depth (ı) with the increase in frequency (a) and (b) shows the dependence of SER as a function of log  AC while the inset shows the variation of  AC with the increase in frequency for the sample PTF11.

fer complex site which act as the localized states from where the hoping of charge carrier take places. Below 70 K it was observed that conductivity data deviates from the linear behavior because in low temperature region charge conduction is mainly dominated by the thermally stimulated tunneling through the localized sites as reported earlier for the other conjugated polymers [34–36].

polaron/bipolaron and other bound charges, which leads to high value of ε and ε . With the increase in frequency, the dipoles present in the system cannot reorient themselves along with the applied electric field and as a result dielectric constant decreases. The main characteristic feature of TiO2 is that it has high dielectric constant with dominant dipolar polarization and the associated relaxation phenomenon constitutes the loss mechanism [37]. With the addition of ␥-Fe2 O3 and TiO2 in polyaniline matrix, significant increase in real and imaginary part of complex permittivity was observed. The higher values of dielectric constant and loss is owing to more interfacial polarization due to the presence of insulating ␥-Fe2 O3 particles and high dielectric TiO2 particles consequently leading to more shielding effectiveness due to absorption. Fig. 6a shows the variation of real part of magnetic permeability ( ) with frequency while change in the induced magnetization with the applied field is shown in Fig. 6b. The magnetic permeability of all the samples decreases with the increase in frequency. The superior permeability was observed for higher percentage of iron oxide in the polymer matrix. The PTF12 nanocomposite has relative permeability value of 5.4, which decrease to 1.1 for the PTF11. The saturation magnetization (MS ) value of the ␥-Fe2 O3 was found to 67 emu g−1 at an external field of 6 kOe having small value of coercivity and negligible retentivity with no hysteresis loop, indicating the super paramagnetic nature. When these nanoparticles were incorporated in the polyaniline matrix in weight ratio of 1:1 (PTF11), the saturation magnetization (MS ) value was found to be 11.7 emu g−1 . However, on changing the weight composition

3.5. Electromagnetic shielding, dielectric and permeability studies Fig. 4 shows the variation of the SE with frequency in the 12.4–18 GHz range. It has been observed that conducting ferromagnetic composite of polyaniline with Fe2 O3 and TiO2 have shielding effectiveness (SE) mainly due to absorption. From the experimental measurement, the shielding effectiveness due to absorption (SEA ) was found to be 8.8, 22, 35, and 45 dB for PF12, PT11, PTF11, and PTF12 samples, respectively, while the shielding effectiveness due to reflection (SER ) was nominal and contributed very little. The higher value of SEA of PTF composites were mainly due to combined effect of TiO2 and Fe2 O3 . The electromagnetic absorption behavior of material depends on complex permittivity (εr = ε − jε ) and permeability (r =  − j ). The real (ε ) and imaginary (ε ) part of complex permittivity vs. frequency are shown in Fig. 5. The real part (ε ) is mainly associated with the amount of polarization occurring in the material and the imaginary part (ε ) is related with the dissipation of energy. In all the samples, ε is found to decrease with frequency. In polyaniline strong polarization occurs due to the presence of

K. Singh et al. / Materials Chemistry and Physics 119 (2010) 201–207

of An/␥-Fe2 O3 to 1:2 (PTF112), the MS value was increased from 11.7 to 26.9 emu g−1 , keeping the external applied field at 6 kOe. MS value increases due to high poly-dispersivity of the ␥-Fe2 O3 in polyaniline matrix. The surface area, number of dangling bonded atoms and unsaturated coordination on the surface of polymer matrix are all enhanced. These variations lead to the interface polarization and multiple scattering, which is useful for the absorption of large number of microwaves [38]. Fig. 7a (inset) shows the variation of  AC with the frequency for the sample PTF11, calculated from the dielectric measurements ( AC = ωε0 ε ). To relate  AC with the shielding parameters of the material, SER is plotted against log  AC (Fig. 7a). Therefore, higher value of conductivity is required for high shielding effectiveness due to reflection. For the absorption part, the skin depth of the 2/ωAC and its samples was calculated using the relation, ı = variation with frequency is shown in Fig. 7b (inset). From the plot, it was observed that skin depth decreases with frequency, which demonstrates that mainly surface conduction exists at the higher frequencies. The dependence of skin depth on the conductivity and magnetic permeability reveal that, for highly conducting and magnetic material, the skin depth is very small. From Eq. (9), better SEA can be achieved from the highly conducting and magnetic materials. The dependence of SEA on ( AC )1/2 is shown in Fig. 7b. 4. Conclusion In conclusion, polyaniline–TiO2 –␥-Fe2 O3 nanocomposite has been successfully synthesized using microemulsion method. Addition of TiO2 (dielectric filler) and ␥-Fe2 O3 (magnetic filler) in the conducting matrix proposed new kind of composite materials having better microwave absorption properties (SEA ∼ 45 dB) as compared to conventional materials. Magnetic and conductivity studies demonstrate that the conducting ferromagnetic composite possesses moderate saturation magnetization (MS ) value of 26.9 emu g−1 and conductivity of the order of 0.46 S cm−1 . The microwave absorption property of the composites strongly depends on the intrinsic properties of ␥-Fe2 O3 and TiO2 nanoparticles in the polymer matrix. The higher EMI shielding is mainly arising due to combined effect of the ␥-Fe2 O3 and TiO2 . The incorporation of TiO2 and ␥-Fe2 O3 nanoparticles leads to more interfacial dipolar polarization and higher anisotropic energy due to nanosize that consequently contributed to the high values of shielding effectiveness. The dependence of SEA on magnetic permeability and AC conductivity shows that better absorption value can be obtained for material with moderate conductivity and magnetization. The high EMI SE was dominated by absorption rather than reflection. Electromagnetic shielding properties are also dependent on the amount of ␥-Fe2 O3 and increases with ␥-Fe2 O3 content in polymer matrix. Acknowledgments Authors wish to thank Director N.P.L for his keen interest in the work. The authors also thank Dr. S.K. Halder and Dr. Rashmi for

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