Polyaniline–antimony oxide composites for effective broadband EMI shielding

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Iranian Polymer Journal ISSN 1026-1265Volume 22Number 7 Iran Polym J (2013) 22:473-480DOI 10.1007/s13726-013-0149-z

Polyaniline–antimony oxide composites foreffective broadband EMI shielding

Muhammad Faisal & Syed Khasim

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ORIGINAL PAPER

Polyaniline–antimony oxide composites for effective broadbandEMI shielding

Muhammad Faisal • Syed Khasim

Received: 16 October 2012 / Accepted: 10 April 2013 / Published online: 25 April 2013

� Iran Polymer and Petrochemical Institute 2013

Abstract Conducting polyaniline (PAni)–antimony tri-

oxide (Sb2O3) composites with different weight percent-

ages (wt%) of Sb2O3 in PAni have been synthesized by

in situ chemical oxidative polymerization. The composites

were structurally and morphologically characterized by

X-ray diffraction (XRD) and scanning electron microscopy

(SEM). Measurements of electromagnetic interference

(EMI) shielding, complex permittivity and microwave

absorbing as well as reflecting properties of the composites

were carried out in the frequency range of 8–18 GHz,

encompassing the microwave X and Ku bands of practical

relevance. All the computations are based on microwave

scattering parameters measured by transmission line

waveguide technique. It is observed that the presence of

Sb2O3 in the PAni matrix affects the electromagnetic

shielding and dielectric properties of the composites at

microwave frequencies. The composites have shown better

shielding effectiveness (SE) in both the X (SE in the range

-18 to -21 dB) and Ku (-17.5 to -20.5 dB) bands. e0 and

e00 values of the PAni–Sb2O3 composites are in the range of

64–37 and 63–30, respectively, in the frequency range of

8–18 GHz. Dielectric measurements indicated the decrease

in dielectric constant with the increase in wt% of Sb2O3.

The results obtained for the reflection and absorption

coefficients indicated that PAni–Sb2O3 composites exhibit

better electromagnetic energy absorption throughout the X

and Ku bands. The results indicated that PAni–Sb2O3

composites can be used as potential microwave absorption

and shielding materials.

Keywords Polyaniline � Sb2O3 composites � Scanning

electron microscopy � EMI � Broadband � Shielding

Introduction

With rapid increase of electromagnetic pollution due to the

growing proliferation of electronics and instrumentation,

the study of materials that can prevent electromagnetic

interference (EMI) has greater relevance. EMI occurs when

electrical devices receive electromagnetic radiation that is

being emitted by electronic or electric devices such as

wireless computers, microwaves, radios, mobile phones,

etc. These electromagnetic radiation byproducts can

degrade the life time, efficiency and also affect the safety

operation of many electronic devices, therefore, EMI

shielding materials are essential to suppress these electro-

magnetic aggressions to safer limits. A good shielding

material should prevent both incoming and outgoing EMIs.

More precisely, an EMI shield in electronic equipments

controls the excessive self-emission of electromagnetic

waves and also ensures the undisturbed functioning of the

device in presence of external electromagnetic fields [1].

Due to their light weight, versatility, low cost, tunable

conductivity and processability, conducting polymer com-

posites are attractive materials for EMI shielding applica-

tions. Composites based on conducting polyaniline and

polypyrrole, like polyaniline–thermoplastic [2, 3], polyan-

iline–inorganic oxide [4] composites, polypyrrole–ferrite

[5], polypyrrole–thermoplastic composites and polyaniline

nanofibers composites [6, 7] have been reported by many

research groups for this purpose.

M. Faisal � S. Khasim

Department of Physics, PES Institute of Technology,

Bangalore South Campus, 560100 Bangalore, India

S. Khasim (&)

Department of Physics, University of Tabuk, 71491 Tabuk,

Kingdom of Saudi Arabia

e-mail: [email protected]

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Petrochemical Institute 123

Iran Polym J (2013) 22:473–480

DOI 10.1007/s13726-013-0149-z

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Conducting polymer/inorganic solid composites have

been the subject of considerable interest because of the

unique combination of organic and inorganic components

at the molecular level. This includes composites of con-

ducting polymer with Ce(OH)3-PrO3/montmorillonite [8],

TiO2 [9], clay [10], hydrogen zeolites [11], Fe2O3 [12], etc.

These composites are expected to improve or complement

the electrical, electromagnetic, chemical and structural

properties over their single components, to achieve maxi-

mum efficiency required on the different processes taking

place in specific applications. Recently, many interesting

research works focusing on the conducting polymer–inor-

ganic dispersant composite are being carried out to achieve

the novel functional materials with characteristics of both

conducting polymer and the inorganic particles. Of the

conducting polymers under study, polyaniline (PAni) gains

prime importance owing to its relatively easy and cost

effective synthesis route, high yield, and tunable electrical

and optical properties [13–15]. Antimony trioxide is very

attractive material due to its unique physical and optical

properties. It exhibits crystalline structure with polymorph

nature as the structural specialty. Sb2O3 particles are also

characterized by high dielectric and coercive force prop-

erties. They have wide variety of potential applications in

optoelectronic and photoelectric devices [16, 17]. The

demand for composite materials possessing a combination

of wide range of desirable properties is increasing day by

day for instance in EMI shielding and microwave absorb-

ing applications. Hence, the incorporation of Sb2O3 parti-

cles into the conducting PAni matrix is expected to form a

heterogeneous hybrid system exhibiting novel electro-

magnetic and dielectric functionalities.

While electrical conductivity of polyaniline–inorganic

oxide composites have been well documented in the literature

[18–20], very limited studies exist on EMI shielding prop-

erties of polyaniline–inorganic oxide-based composites [4,

21, 22]. To our knowledge, there has been no report yet about

composite containing Sb2O3 particles coated with polyaniline

for the broadband EMI shielding. EMI shielding effective-

ness (SE) of a material depends not only on its conductivity

but also its dielectric properties. In response to the need for

broadband microwave shielding materials, the present work

deals with the broadband EMI shielding properties such as

EMI SE, microwave absorptivity as well as reflectivity and

dielectric properties of synthesized PAni–Sb2O3 composites

in the X and Ku bands, relevant for practical applications. In

this work, we have focused to correlate the EMI shielding

with the dielectric properties in PAni–Sb2O3 composites with

better EMI SE values compared to our previous work using

polyaniline–stannous oxide composites [4]. The study illus-

trates the use of novel PAni–Sb2O3 composite materials

that provide microwave absorption/shielding over a wide

bandwidth (covering X and Ku band frequencies).

Experimental

Materials

All the chemicals used were of research grade. Aniline and

Sb2O3 were purchased from Sigma-Aldrich, India.

Ammonium peroxydisulfate (NH4)2S2O8 (APS) and

hydrochloric acid (HCl) were purchased from S.D. Fine

Chemicals, India. The aniline monomer was purified by

distillation in vacuum before use and other chemicals were

used as received.

Synthesis and processing of PAni–Sb2O3 composites

The synthesis of polyaniline was carried out by chemical

oxidative polymerization of 0.5 M solution of double dis-

tilled aniline in 1 M HCl. Ammonium per sulfate (APS)

was used as an oxidant. The yield in terms of weight per-

centage of the composites was confirmed by the trial syn-

thesis of PAni [12]. The weight percentages of the

dispersant Sb2O3 (10, 20, 30, 40, and 50 wt%) in PAni

were calculated from the desired percentage of fixed PAni

yield. The composites were synthesized by the suspension

of Sb2O3 in the selected wt% into the polymerization

mixture of aniline as we have reported earlier [12]. The

oxidant was added slowly to the reaction mixture and the

polymerization was allowed to propagate for 6 h at con-

trolled temperature of 0–5 �C. After completion of the

polymerization, the composite (PSB1–PAni with 10 wt%

Sb2O3, PSB2–PAni with 20 wt% Sb2O3, PSB3–PAni with

30 wt% Sb2O3, PSB4–PAni with 40 wt% Sb2O3, PSB5–

PAni with 50 wt% Sb2O3) precipitates were filtered and

washed with de-ionized water and acetone. The wet com-

posites thus obtained were dried in hot air oven at 50 �C for

8 h to achieve constant weight and were ground into fine

powder.

The fine powders of synthesized PAni–Sb2O3 compos-

ites were compressed in the form of rectangular pellets of

standard X and Ku band rectangular waveguide dimensions

(X-band waveguide standard: WR-90 with inside dimen-

sions 2.286 cm 9 1.016 cm and Ku-band waveguide

standard: WR-62 with inside dimensions 1.579 cm 9

0.789 cm) using homemade die under a pressure of 9 tons

in a table-top hydraulic press at room temperature. The

quantity of the composite sample and pressing time were

optimized and maintained for X-band as well as Ku-band

samples.

Measurements

Structural characterization of PAni–Sb2O3 composites

were carried out by X-ray powder diffraction at ambient

temperature. A Rigaku (model: RU-300, Japan) X-ray

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diffractometer with Cu-Ka (1.54 A) radiation was used,

and the patterns were recorded in the range 2h = 3�–80�.

Surface morphologies of PAni, Sb2O3, and PAni–Sb2O3

composites were investigated by scanning electron

microscopy (SEM) using Philips ESEM (model XL30, The

Netherlands). The EMI shielding measurements were car-

ried out on the pressed rectangular pellets of the compos-

ites by waveguide transmission line technique [4, 23, 24],

with the rectangular pellets of PAni–Sb2O3 composites fit

exactly into the X- and Ku-band rectangular waveguide

sample holder coupled to an HP vector network analyzer

(model-8510C, 45 MHz–26.56 GHz, USA). Transverse

electromagnetic waves in the frequency range from 8 to

18 GHz were incident onto the loaded samples. The scat-

tering parameters (S-parameters) of the samples that cor-

respond to the reflection (S11) and transmission (S21) of the

incident electromagnetic waves were measured and were

used to investigate the microwave absorption and EMI

shielding properties of the composites in the X and Ku

bands. A full two port calibration was performed before the

measurement of S-parameters to remove errors due to

source match, load match, directivity, isolation, and fre-

quency response in both the transmission and reflection

modes.

Results and discussion

XRD analysis

Figure 1 shows the X-ray diffraction (XRD) patterns of

PAni, Sb2O3 and PAni–Sb2O3 (30 wt%) composites. The

XRD pattern of pure PAni (Fig. 1a) shows the

characteristic broad diffraction peaks at 2h = 6�, 20�, and

25� indicate the semi-crystalline regions dominated by

amorphous nature of PAni [12]. The XRD pattern of Sb2O3

in Fig. 1b shows the characteristic peaks at 2h = 19�, 25�,

27� correspond to crystalline Sb2O3 having orthorhombic

structure (JCPDS 5-0534) [25]. Figure 1c shows the XRD

pattern of the composites with 30 wt% of Sb2O3 in PAni.

The pattern retained the characteristic diffraction peaks of

Sb2O3 located at about 2h = 19�, 25�, 27�, 28.5�, 32�, 34�,

43�, 47�, 50�, and 54�, which indicate that the Sb2O3 has

retained its structure even though dispersed in PAni matrix

with characteristic diffraction peaks of Sb2O3 became

slightly weaker, which reveals the effect of PAni on Sb2O3

during the composite formation.

Morphological characterization

Figure 2a, b, c shows the SEM micrographs of pure PAni,

Sb2O3, and PAni–Sb2O3 composite (30 wt%), which reveal

the difference in surface morphology before and after the

formation of the composite. The pristine PAni has irregular

morphology with granular agglomeration. This globular

morphology is the most typical form for PAni prepared in

acidic aqueous media [26]. It is observed from the SEM of

Sb2O3 (Fig. 2b) that, the Sb2O3 exhibited agglomeration of

the flaky strips which is due to dipole interaction between

the Sb2O3 particles. It has been established that the mor-

phology of the dispersed phase has a greater impact on the

electromagnetic properties of the composites [27, 28]. The

micrograph of composite (Fig. 2c) shows that PAni

deposited over Sb2O3 particles forms a cloudy agglomer-

ation. This change in morphology of Sb2O3 which is

deposited over by PAni is mainly responsible for the

improved characteristic features of composite. These het-

erogeneous structures of the conducting composites might

induce attenuation of electromagnetic waves from various

microwave sources by the mechanism of absorption and/or

reflection.

EMI shielding studies

The EMI SE of a material is the attenuation of propagating

electromagnetic waves produced by the shielding material.

EMI SE can be expressed as the ratio of the transmitted

radiation energy (Pt) to the incident energy (Pi) [29, 30].

EMI SE dBð Þ ¼ �10 log Pt=Pið Þ ð1Þ

The values of EMI SE measured both in the X and Ku

bands for pure PAni and PAni–Sb2O3 composites are

shown in Fig. 3a, b. Figure 3a indicates that, compared

to EMI SE of pure PAni which is in the range of -13 to

-14 dB, the EMI SE in the PAni–Sb2O3 composites

increased with increasing content of Sb2O3 in both X andFig. 1 XRD spectra of a PAni, b Sb2O3, and c PAni–Sb2O3 (30 wt%-

PSB3) composite

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Ku bands. In X-band, PAni–Sb2O3 composites exhibit EMI

SE in the range -17.9 to -20.8 dB. It is observed in

Fig. 3b that, the SE of the composites in the Ku band

shows a similar trend as in the X band. The SE of pure

PAni in the range of -13 to -14.5 dB in Ku band

enhanced to high attenuation levels of -17.4 to -20.5 dB

in PAni–Sb2O3 composites. In both X and Ku bands, the

EMI SE increased with increasing concentration of Sb2O3

in PAni up to 30 wt% and reduces for higher loadings (40

and 50 wt%) of Sb2O3 in PAni. Such a behavior indicates

the non-homogeneity in the composites due to strained

polymer chain clusters in the presence of increased

concentration of Sb2O3 particles beyond 30 wt% in PAni

matrix. The variation of SE in these composites in X- and

Ku-band frequencies follows percolation characteristics

with a percolation threshold at 30 wt% loading of Sb2O3

in PAni. In both the X and Ku bands, PAni with

30 wt% concentration of Sb2O3 shows the highest EMI

SE, indicating the promising combination of dielectric

properties and heterogeneity which might facilitate

interaction of the incident microwaves with the charge

carriers at the composite interfaces. Thus, the EMI SE of

the composites in 8–18 GHz represents the key influence of

Sb2O3 content in PAni towards the microwave shielding.

Also, the shielding properties of PAni–Sb2O3 composites

in both the X and Ku bands meet the required value of EMI

SE for commercial applications (-20 dB) [31].

Microwave absorption analysis

Using the scattering parameters correspond to reflection, S11

and transmission, S21 of vector network analyzer, reflection

coefficient, R and transmission coefficient, T can be

expressed as R = |S11|2 and T = |S21|2. Applying electro-

magnetic wave power balance equation, A ? R ? T = 1,

the absorption coefficient, A (A = 1-R-T) can be evalu-

ated [32].

Figure 4a, b shows the absorption and reflection coef-

ficients at X- and Ku-band frequencies, respectively. The

values of absorption and reflection coefficients indicate that

the observed SE is a result of dominant electromagnetic

wave absorption process in both the X and Ku bands. The

dissipative effects of charge carriers (polarons/bipolarons)

and multiple relaxation processes of the magnetic and

Fig. 2 SEM photographs of a PAni, b Sb2O3, and c PAni–Sb2O3 (30 wt%) composite

Fig. 3 EMI shielding effectiveness for PAni–Sb2O3 composites: a in

the frequency range of 8–12 GHz and b in the frequency range of

12–18 GHz

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electric dipoles in the PAni–Sb2O3 composite interfaces

were effective in microwave shielding by absorption. The

reflection and absorption coefficients show stabilized val-

ues in the X band (Fig. 4a) with minimal fluctuations in the

Ku band (Fig. 4b). From Fig. 4a, b, it can be seen that the

absorption coefficient increased up to 30 wt% loading of

Sb2O3 in PAni and a shift occurred for higher wt% PAni–

Sb2O3 composites (i.e., PSB4 and PSB5) indicating the

percolation threshold at 30 wt% of Sb2O3 in PAni. The

observed variation in the microwave absorption of the

composites with different wt% of the Sb2O3 dispersant

shows similar trend as that of EMI SE of the PAni–Sb2O3

composites. These results show the compatibility between

the PAni and inorganic Sb2O3 over the observed percola-

tion threshold at 30 wt% of Sb2O3 in PAni.

Dielectric properties

The EMI shielding and microwave absorbing properties of a

material depends on its complex permittivity (e = e0-je00).The real (e0) and imaginary (e00) relative permittivity val-

ues are related to the stored energy and dissipated energy

within the composite material, respectively [33]. Accord-

ing to the theory of permittivity, when a material comes

under the influence of an electromagnetic field, the electric

field induces two types of electrical currents within the

material, the conduction and displacement currents [34,

35]. The conduction current is assumed to arise due to the

charge carriers and provide electric loss (dielectric loss,

e00) inside the material. The displacement current arises

due to bound charges, i.e., polarization and gives real part

of permittivity (dielectric constant, e0). PAni exhibits

unusually high polarizability and high dielectric constant,

which extends into microwave region [36]. The complex

permittivity of PAni–Sb2O3 composites shown in Figs. 5a,

b and 6a, b was obtained through scattering parameters S11

and S21 using Nicholson–Ross–Weir method [37, 38]. The

presence of Sb2O3 in the PAni matrix resulted in the

formation of more interfaces with space charge accumu-

lation and associated relaxation phenomenon. The

increased anisotropy of PAni–Sb2O3 composites leads to

various polarization mechanisms due to the interaction

between electric dipoles. The PAni–Sb2O3 interface in

these composites makes the electric relaxation behavior

more complex and brings about changes in the internal

electric field through dipole interactions. This results in

the enhanced dielectric response and microwave absorp-

tion behavior of the composites, followed by various

effects like anomalous variation of the complex permit-

tivity, shift of EMI SE, considerable decrease in complex

permittivity at higher frequencies, etc. It has been found

that, in composite materials the relaxation of dipole

oscillations is strongly affected by structural disorders and

heterogeneity [39]. The composites exhibited high values

of e0 and e00 in both the X and Ku bands, in X band, the e0

values are in the range of 64–54 and the dielectric loss

values are in the range of 63–45. The e0 and e00 of the

composites in the Ku-band frequencies show variation in

the range of 47–37 and 39–30, respectively. Such a shift in

e0 and e00 in the X and Ku bands of these composites can

be attributed to the unsaturated interface polarization and

multiple scattering of Sb2O3 particles. The complex per-

mittivity values have increased with increasing Sb2O3

content from 10 to 30 wt%. In these composites, both the

dielectric constant and loss decrease with the increase in

frequency. This is a result of different polarization

mechanisms to seize as a function of frequency and hence,

the decrease in dielectric constant [40]. The observed

frequency dispersion effect in PAni–Sb2O3 composites isFig. 4 Reflection and absorption coefficients of PAni–Sb2O3 com-

posites at a X-band frequencies and b Ku-band frequencies

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important as this feature is only observed in wave-

absorbing materials with wide absorption bands [41].

These dielectric characterizations are in accordance with

the measurement of the EMI SE of the composites shown

in Fig. 3. The variation of complex permittivity of PAni–

Sb2O3 composites also indicates percolation behavior

similar to the exhibited shielding properties of the com-

posites. The percolation threshold corresponds to 30 wt%

loading of Sb2O3 (PSB3) in PAni matrix and increase in

the concentration of Sb2O3 particles (40 and 50 wt%

composites) resulted larger amount of Sb2O3 unable to

form the conducting network deteriorating the dielectric

properties. The absorption dominant EMI SE of the PAni–

Sb2O3 composites can be ascribed to the observed

dielectric properties.

Conclusion

The materials with high shielding efficiency in broadband

(X and Ku bands) are of great importance and of growing

demand. EMI shielding properties of the synthesized

PAni–Sb2O3 composites were investigated in a wide range

of frequencies encompassing the microwave X and Ku

bands. Systematic analyses on the broadband EMI shield-

ing and dielectric parameters of interest for practical

applications were carried out. The experimental results

carried out in this work indicate that the composites exhibit

good electromagnetic absorption performance over the

broadband range. The presences of Sb2O3 particles in the

polyaniline matrix induced morphological modifications

and variable dipole relaxation towards enhanced shielding

Fig. 5 Behavior of real part of permittivity of PAni–Sb2O3 compos-

ites at a X-band frequencies and b Ku-band frequencies Fig. 6 Variation of imaginary part of permittivity of PAni–Sb2O3

composites at a X-band frequencies and b Ku-band frequencies

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properties. The composites exhibit percolation behavior

with maximum EMI SE observed for 30 wt% loading of

Sb2O3. Due to the easy and low-cost preparation routes, the

PAni–Sb2O3 composite has a promising potential for broad

band EMI shielding and microwave absorption.

Acknowledgments The Authors would like to thank the manage-

ment of PES Institute of Technology-Bangalore South Campus for

their support and encouragement towards carrying out this work.

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