Pks Matchemphys

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Materials Chemistry and Physics 113 (2009) 919926

Contents lists available atScienceDirect

Materials Chemistry and Physics

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m a t c h e m p h y s

PolyanilineMWCNT nanocomposites for microwave absorptionand EMI shielding

Parveen Saini a, Veena Choudhary b, B.P. Singh c, R.B. Mathur c, S.K. Dhawan a,

a Polymeric & Soft Materials Section, National Physical Laboratory, New Delhi 110012, Indiab Centre for Polymer Science & Engineering, Indian Institute of Technology, New Delhi 110016, Indiac Carbon Technology Unit, National Physical Laboratory, New Delhi 110012, India

a r t i c l e i n f o

Article history:

Received 15 July 2008

Received in revised form 12 August 2008

Accepted 14 August 2008

Keywords:

Composite materials

Fourier transform infrared spectroscopy

(FTIR)

Thermogravimetric analysis (TGA)

Electrical conductivity

a b s t r a c t

Highly conducting polyaniline (PANI)multi-walled carbon nanotube (MWCNT) nanocomposites were

prepared by in situ polymerization. The FTIR and XRD show systematic shifting of the characteristic

bands and peaks of PANI, with the increase in MWCNT phase, suggesting significant interaction between

the phases. The SEM and TEM pictures show thick and uniform coating of PANI over surface of individual

MWCNT. Based on observed morphological features in SEM, the probable formation mechanism of these

composites has been proposed. The electrical conductivity of PANIMWCNT composite (19.7S cm1) was

even better than MWCNT (19.1S cm1) or PANI (2.0 S cm1). This can be ascribed to the synergistic effect

of two complementing phases (i.e. PANI and MWCNT). The absorption dominated total shielding effec-

tiveness (SE) of27.5 to 39.2dB of these composites indicates the usefulness of these materials for

microwave shielding in the Ku-band (12.418.0GHz). These PANI coated MWCNTs with large aspect ratio

are also proposed as hybrid conductive fillers in various thermoplastic matrices, for making structurally

strong microwave shields.

2008 Elsevier B.V. All rights reserved.

1. Introduction

The proliferation of electronics and instrumentation in com-

mercial, industrial, healthcare and defense sectors has led to a

novel kind of pollution known as electromagnetic interference[1].

This is a serious issue caused by the interference effects of current

induced by electric and magnetic fields, emanating from nearby

wide range of electrical circuitry[2].The interference among busi-

ness machines, process equipments, consumer products and other

instruments may lead to disturbance of usual performance or even

complete malfunction. The disturbances across communication

channels, automation and process control may lead to loss of valu-able time, energy, resources, money or even precious human life.

Therefore some kind of shielding mechanism must be provided to

guard the concerned article from spurious electromagnetic noises

or pollution. Metals are most common materials for EMI shielding

[3,4].But they suffered from the disadvantages like high density,

susceptibility to corrosion, complex and uneconomic processing.

Further, metals mainly reflect the radiation and cannot be used

Corresponding author. Tel.: +91 11 45609401; fax: +91 11 25726938.

E-mail address: [email protected](S.K. Dhawan).

in applications where absorption is prime requisite, e.g. in stealth

technology[5,6].

In the last four decades, conductingpolymers (CPs) have gained

a special status owing to wealth of applications [715].The EMI

shielding and microwave absorption properties of these poly-

mers can be explained in terms of electrical conductivity and

presence of bound/localized charges (polarons/bipolarons) lead-

ing to strong polarization and relaxation effects [16,17]. Polyaniline

(PANI) has special status among other conducting polymers due

to its non-redox doping, good environmental stability and eco-

nomic feasibility. The properties can be further tuned by controlled

polymerization conditions and using substituted anilines, specific

comonomers, dopants and fillers[1822].PANI has low inherent

specific strength and requires dispersion in some binding matrix

to form composites for any commercially useful product. However,

percolation threshold tends to be high due to low compatibilities,

phase segregated morphology and low aspect ratio of the conduct-

ing polymer particles. Therefore, high concentration of conducting

polymers is required in matrix for acceptable electrical properties

which often affect the mechanical properties of resultant compos-

ites.

Since their discovery [23], the exceptional mechanical, elec-

trical and thermal properties of carbon nanotube (CNT) [24,25]

made them potential candidate for high-tech applications [2630].

0254-0584/$ see front matter 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.matchemphys.2008.08.065
http://www.sciencedirect.com/science/journal/02540584http://www.elsevier.com/locate/matchemphysmailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_2/dx.doi.org/10.1016/j.matchemphys.2008.08.065http://localhost/var/www/apps/conversion/tmp/scratch_2/dx.doi.org/10.1016/j.matchemphys.2008.08.065mailto:[email protected]://www.elsevier.com/locate/matchemphyshttp://www.sciencedirect.com/science/journal/02540584


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920 P. Saini et al. / Materials Chemistry and Physics 113 (2009) 919926

However, poor dispersion and lack of interfacial adhesion of CNTs

possess serious obstaclesto theirfurther development.To solve the

above problems, functionalization routes have been developed[31]

thatoften manifested themselves in some deterioration of inherent

properties of pristine CNTs[32].

To combine the good properties of nanotubes and CPs, several

attempts have been made to introduce CNTs into the matrices of

CPs. These nanocomposites have shown promising response forfuturistic applications like battery materials [33], optoelectronic

devices [34,35],sensors,[36,37], actuators[38]and electrorheol-

ogy[39].Recently, PANICNT composites have been proposed for

EMI shielding applications [40] but only fewbrief reports are avail-

able on their microwave absorption characteristics[41,42]. In this

paper, we report the potential of PANI coated MWCNT as possible

microwave absorber. The nanoporous coating of PANI may also acts

as functional handle providing good dispersibility and processabil-

ity of MWCNTs due to better interaction with host matrix. These

PANI coated MWCNTs mayact as hybridconductive fillerfor making

composites with insulating matrices.

2. Experimental details

2.1. Materials

Aniline (Loba Chemie, India) was freshly distilled before use. MWCNTs were

synthesized in laboratory by chemical vapor deposition (CVD) route. Hydrochloric

acid (HCl, Merck,India) andammoniumpresulfate (APS, Merck,India) were used as

received. Aqueous solutions were prepared from the double distilled water having

specific resistivity of 106 Ohm-cm.

2.1.1. Preparation of MWCNT

MWCNTs have been synthesized by thermal decomposition of toluene in

presence of iron catalyst obtained from organometallic ferrocene. In the present

experimental setupa singlezone electric furnace hasbeen used in theplaceof con-

ventional two zone furnace. This mixture was injected into a quartz reactor (o.d.

42 mm) that was kept at a constant temperature of 750 C. The details of the exper-

imental set up are given elsewhere[43].The purity of these tubes as determined

from TGA was 88%.

2.1.2. Synthesis of PANI

The doped PANI was prepared by free radical chemical oxidative polymeriza-

tion, through direct route using HCl as dopant [44]. In a typical reaction, 0.1 mol

of aniline was dissolved in aqueous solution of 1.0 M HCl. The above mixture was

stirred continuously and the polymerization was initiated by the drop wise addi-

tion of ammonium persulfate (0.1 mol, (NH4 )2S2O8 in 100 ml distilled water). The

temperature of the mixture was maintained at 00.1C throughout the course of

reaction (6 h). The polymer has been produced directly in the doped state as fine

slurry of darkgreen particles.The reaction mixture was filtered and washed repeat-

edly with distilled water until filtrate becomes colorless. The wet polymer cake so

obtained wasdried at 60 C under dynamic vacuum. Thedriedmass wascrushed to

obtain the powder of the doped PANI (emeraldine salt) designated as PCNT0.

2.1.3. Synthesis of composites

The composites were prepared in similar fashion by keeping the amounts of

both HCl and aniline same as in preparation of pure PANI (PCNT0) and taking the

additional component, i.e. MWCNT. The required weight of MWCNT was calculated

from the desired percentage of MWCNT relative to aniline monomer, i.e. 5, 10, 20

and, 25% for PCNT-5, PCNT-10, PCNT-20 and PCNT-25, respectively. The polymer-ization was initiated by drop wise addition of ammonium persulfate and allowed

to propagate for 6 h. After complete polymerization the polymer was isolated from

the dark green precipitate by filtration. The wet cake so obtained was dried under

dynamic vacuumand crushed toform thepowder. Thecontrol MWCNTsample was

also prepared and designated as PCNT100.

2.2. Measurements

Forthe conductivity measurements, pellets of length 13mm, width7 mm,thick-

ness 1.5 mm were prepared using hydraulic press under pressure of 100kg cm2.

Thesilver contacts were applied andthe currentvoltage (IV) measurements were

takenin four-probe configuration usingKeithley 220 Programmable Current Source

and181Nanovoltmeter.The thermalstabilitywasmeasuredunderinertatmosphere

of nitrogen usingthermogravimetricanalyzer (Mettler Toledo TGA/SDTA851e).The

material is heated from 25 to 700 C at a constant heating rate of 10 Cmin1. The

weight loss was continuously monitored as a function of temperature (or time).

The infrared spectra of powdered samples were taken at resolution of 4.0 cm1

in

4000400cm1 range, using NICOLET 5700 FTIR spectrophotometer and diamond

ATR accessory. TheXRD spectra ofpowderedsamplesweretaken in1070 range at

a scan rate of 0.1 degreemin1 using D8 Advance Bruker AXS X-ray diffractometer.

Morphologies were observed using SEM (Leo 440, UK) and TEM (Phillips, CM-12).

The EMI shielding measurements were taken on pressed rectangular pellets (2 mm

thick) placed inside the homemade sample holder. The holder matches the internal

dimensions of Ku-band (12.418.0 GHz) waveguide placed between the two ports

of Vector Network A nalyzer (VNA E8263BAgilent Technologies).

3. Results and discussion

3.1. Mechanism of composite formation

On the basis of basic concepts of polymerization and cataly-

sis [45] and well supported by SEM/TEM measurements of our

samples, we have proposed the probable formation mechanism

of the PANIMWCNT composites depicted schematically inFig. 1.

The MWCNT being electron acceptor and aniline being electron

donor form a kind of weak charge transfer complex[46].The for-

mation of complex is facilitated by the lone pair of electrons on

the amine nitrogen of aniline monomer. HCl acts as dopant and

it also complexes with aniline facilitating its solubilization and

dispersion. Therefore, the reaction system is heterogeneous with

presence of adequate aniline on the surface of MWCNT and theremaining aniline monomer in the solution phase. On addition

of oxidant, polymerization proceeds both in solution (bulk poly-

merization) and on the surface of MWCNTs. Like most other large

surface area nanophase materials, MWCNTs also act as polymer-

ization catalyst. This increases the generation of cation radicals on

the surface of MWCNT which brings about the surface polymeriza-

tion. However, in the solution phase polymerization proceeds in

uncatalyzed fashion and highly agglomerated polymer was formed

which can be seen in the SEM. Therefore, polymerization proceeds

at much faster rate on the surface of MWCNTs than in solution

phase. As the polymerization of aniline takes place on the surface

of MWCNTs, the additional anilinium radical cations are constantly

attached to the surface of MWCNT from the reaction medium.

Therefore, effective rate of deposition of PANI on MWCNTs is muchfaster thanbulk polymerization in solutionphase. As theproportion

of MWCNT increases the surface polymerization becomes more

important than bulk polymerization. Therefore, at certain critical

amount of MWCNT the polymerization exclusively takes place on

the surface leading to uniform coating of PANI over MWCNT (see

SEM and TEM).

3.2. FTIR spectra

Fig. 2 shows the FTIR spectra of PANI and PANIMWCNT

nanocomposites. The bandaround 798cm1 isduetotheoutofthe

plane CHbending vibrations. The bands near 1560and 1480cm1

are characteristic stretching bands of nitrogen quinoid (N Q N)

and benzenoid (NBN). These are due to the conducting stateof the polymer. The 1290 and 1240 cm1 bands are assigned to

the bending vibrations of NH and asymmetric CN stretching

modes of polaron structure of PANI, respectively[47].The promi-

nent absorption band around 1120 cm1 (CN stretching) is due to

the charge delocalization over the polymeric backbone[44].With

increase in MWCNT phase, a slight shift was observed in the posi-

tionof themain characteristicbands of dopedPANI, which indicates

interactionof MWCNT withPANI matrix. Theincreasein therelative

intensity of bands around 1480 and 1240 cm1 may be attributed

to the PANI attached to MWCNT[48].The 1120 cm1 band of PANI

becomes broader and shifted to 1106 cm1 in PCNT25. This can

be ascribed to the charge transfer interactions between the -

conjugated surfaces of MWCNT and quinoid moiety of PANIHCl

[45,49,50].


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P. Saini et al. / Materials Chemistry and Physics 113 (2009) 919926 921

Fig. 1. Proposed mechanism for the formation of PANIMWCNT nanocomposites.

3.3. XRD studies

Fig. 3 shows the XRD patterns of pure MWCNT, PANI and

PANIMWCNT nanocomposites. The pure MWCNT (PCNT100)

shows a sharp peak centered on 2value of 26 which corresponds

to the (0 0 2) planes of MWCNT. The peaks around 43

are due to

the(1 1 0) and(1 0 0) graphitic planesplus small amountof catalyst

particle encapsulated inside the walls of the MWCNTs[51].

The characteristicpeaks of the dopedPANI(PCNT0) areobserved

around 2values of 15, 20, 25, 30 corresponding to (01 1), (0 2 0),(20 0)and (02 2)reflectionsof emeraldinesaltform [52]. Thecom-

posites show the characteristic peaks of both PANI and MWCNT


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Fig. 2. FTIR spectra of PANIMWCNT nanocomposites with differentnanotubecon-

tent.

without any additional bands indicating absence of covalent inter-

actions between the phases [42]. As the MWCNT content increases,

the relative intensity of characteristic bands of PANI decreaseswhereas bands of MWCNT become more prominent. The slight

shifting in the peak positions may be ascribed to charge transfer

interactions between PANI and MWCNTs leading to variations in

chain packing and configurations.

Fig. 3. XRD scans of PANI, MWCNT and PANIMWCNT nanocomposites.

Fig. 4. Variation of room temperature electrical conductivity of PANIMWCNT

nanocomposites with different loading levels of MWCNT.

3.4. Electrical conductivity

The room temperature currentvoltage (IV) characteristicswere measured and resistance values were obtained from the slope

of these plots. The electrical conductivities of the pellets can be

calculated by considering the sample dimensions as:

=L

RA (1)

whereListhelengthofthepellet, RisresistanceandAiscross-

sectional area of the pellets normal to direction of current flow.

The conductivity (Fig. 4)was found to exhibit continuous increase

with the increase in the MWCNT content. The high conductivities

of these composites are due to micrometer long MWCNTs as core

and PANI coating as shell (seeFig. 5SEM images).

Measurement of electrical conductivity of terphenyl and quater-

phenyl films revealed that organic nanocrystallites play interfacenano trapping levels effectively interacting with phonon sub-

systems [53]. These states may be principal for the achievement

of conductivities varying in the large range of parameters. Intro-

duction of MWCNT to PANI enhances the electrical properties by

facilitating the charge transfer processes between the two com-

ponents [54]. Due to their highly conducting nature as well as

high aspect ratio, the nanotubes can act as interconnecting bridge

between the various conducting grains of the polyaniline, which

are coated over individual MWCNTs. This increases the coherence

or coupling between the chains andleads to enhancement of inter-

chain transport. Further, the PANIHCl coating is likely to facilitate

the intertube charge transport by reducing the interfacial contact

and tunneling resistances. This may be explained on the basis of

cushioning effect of softer polyaniline coating over tubes whichdeforms easily during the pellet formation improving the surface

contacts of coated tubes. This synergistic effect of two comple-

menting phases (i.e. PANI and MWCNT) leads to conductivity of

19.7Scm1 in case of PCNT25, which is even better than bulk

conductivity of either phase alone, i.e. control MWCNT (PCNT100,

19.1 S cm1) or pure PANI (PCNT0, 2.0 S cm1). The lower conduc-

tivity of bulk MWCNT pellet may be due to the fact that pressed

pellets of uncoated MWCNTs contain highly entangled tubes (SEM

Fig. 5f) which even after pelletizationrepresents poorly packed sys-

tem [41]. Therefore, the reduced intertubular charge transport was

responsible for observedlow bulkconductivities.The uniform coat-

ingof PANIreduces the disorder/voids in composites and improves

intertubular charge transport leading to enhanced electrical prop-

erties.


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Fig. 5. SEM images of (a) PCNT0, (b) PCNT5, (c) PCNT10, (d) PCNT20, (e) PCNT25 and (f) PCNT100. Inset shows TEM of PCNT25.

3.5. Morphological details

Fig. 5 shows theSEM images of MWCNT, PANI andPANIMWCNTnanocomposites. These micrographs show that PCNT0 (Fig. 5a)

exits as highly agglomerated globular particles whereas as grown

MWCNTs (PCNT100) are entangled tubules (Fig. 5f) with diame-

ter in the range of 1060 nm and their lengths ranging in several

microns.

The small size of the nanotubes having high specific surface

area provides large number of sorption sites to aniline monomer

which can polymerize to form coating over the nanotubes. At very

low concentration of MWCNTs, PANI coated tubes exist as globular

agglomerates (Fig. 5b). This may be attributed to the large propor-

tion of bulk/solution polymerized PANI (existing in agglomerated

form) as compared to aniline polymerized over MWCNT surface.

However, with the increase in MWCNT content, there is systematic

change in morphology from highly aggregated globules (Fig. 5b)

towards uniformly coated tubules (Fig. 5e). Therefore, at certain

critical concentration of MWCNT (in our case achieved in PCNT25)

the polymerization takes place exclusively on surface of MWCNTwith minimal bulk polymerization and agglomeration effects. TEM

of pure MWCNT[43] clearly shows that tubes are multiwalled with

outer diameter in the range of 1060 nm and their lengths ranging

in several microns. The TEM of the PCNT25 (inset Fig. 5e) shows

the presence of thick and uniform coating of PANI over surface of

MWCNT. The SEM of PCNT25 also revealed the nanoporous nature

of coatings, which can be used for selective incorporation of other

nanoparticles.

3.6. Thermogravimetric analysis (TGA)

Fig.6 shows thethermograms (TG)of thepure MWCNT, PANI and

PANIMWCNT nanocomposites. These TG plots show that weight

loss occurs in several systematic steps each corresponds to the loss


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Fig. 6. Thermogravimetric (TG) traces of PANI, MWCNT and PANIMWCNT

nanocomposites.

of particular species. However, there is always some overlap in

weight lossrangeswith no sharp transition between differentsteps.

The pure nanotubes (MWCNT) have excellent thermal stability upto 700 C and weight loss was only 0.5%.

In composites, the first step loss (25110 C) may be attributed

to the loss of sorbed water molecules. The second loss step

(110185 C) involves the loss of dopant in the form of HCl gas. The

evolvedHCl acts as catalyst for the degradation reaction and rate of

degradation increases with increase in the PANI proportion in the

composites. The third loss step(185300C) involves the loss of low

molecular weight fragments, cross linking of chains and onset of

degradation of polymeric backbone. As evident from the TG traces,

the increasing amount of MWCNTs does not have much influence

on the decomposition temperature of composites. The fourth loss

step (300470 C) can be ascribed to the degradation of polymeric

backbone into heavier fragments. The final loss step (470700 C)

corresponds to the complete breakdown of the polymeric back-bone as well as heavier fragments into still smaller fractions and

gaseous byproducts. The char residues (remaining at 700 C) are

mainly thermally stable inert materials like MWCNTs, iron catalyst

residues and the carbonized polymeric fragments. As the amount

of MWCNT increases, the correspondingchar residue also increases

indicating increased incorporation of nanotubes inside the PANI

matrix.

3.7. Shielding effectiveness (SE)

EMI shielding is defined as the attenuation of the propagat-

ing electromagnetic waves produced by the shielding material. As

shown in Fig. 7b, the shieldingis a directconsequence of reflection,

absorption and multiple internal reflection losses at the existing

interfaces, suffered by incident electromagnetic (EM) waves. EMI

SE can be expressed as[5558]:

EMI SET = 10 log PIPT

= 20 log

EIET

= 20 log HIHT

(dB) (2)where PI (EI)and PT (ET) are the power (electricfield) ofincidentand

transmitted EM waves, respectively. For a single layer of shielding

material, the total EMI SETobtained from Eq. (2) is describedas the

sum of the contribution due to reflection (SER), absorption (SEA),

and multiple reflections (SEM) as the following[59,60]:

SET = SER+ SEA + SEM (dB) (3)

SER= 20 log

(1 + n2)

4n

(dB) (4)

SEA = 20 Im(k)d log e (dB) (5)

SEM = 20 log

1 (1 n2)

(1 + n)2 exp(2ikd)

(dB) (6)

Here, n is the refractive index of shielding material and Im(k) isthe

imaginary part of wave vector in the shielding material.

The S11 (or S22) and S12 (or S21) are the scattering parame-

ters (S-parameters) of the two-port vector network analyzer (VNA)

system and are shown schematically inFig. 7a. They represent the

reflectionand transmissioncoefficients,respectively.The transmit-

tance (T), reflectance (R), and absorbance (A) through the shielding

material can be described as below:

T=

ETEI

2

= |S12|2(= |S12|

2) (7)

R =

EREI

2

= |S11|2(= |S22|

2) (8)

A = 1 R T (9)

The SEM is a correction term whose value may be positive,

negative or zero. The effect of multiple reflections between both

interfaces of the material is negligible when SEA 10dB [61].

Fig. 7. (a) Schematic of the VNA used for the measurement of shielding effectiveness in Ku-band (12.418.0GHz), (b) interaction of electromagnetic waves with shield

material.


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Fig. 8. Shielding effectiveness of PANI and PANIMWCNT nanocomposites in Ku-

band (12.418.0 GHz).

Therefore, the relative intensity of the effectively incident EM

wave inside the materials after reflection is based on (1R).

Therefore, the effective absorbance (Aeff) can be described asAeff= (1RT)/(1R) with respect to the power of the effectively

incidentEM wave inside the shielding material. It is convenientthat

reflectance and effective absorbance are expressed as the form of

10log(1R) and 10 log(1Aeff) in decibel (dB), respectively which

provide the SERand SEAas follows:

SER= 10 log(1 R) dB (10)

SEA = 10 log(1 Aeff) = 10 log

T

(1 R)

dB (11)

In the case of non-negligible SEM, the earlier relations are no

longer valid and further analysis of theS-parameters is required.

The average value of shielding effectiveness (SE) of the com-

posites has been measured in the Ku-band (12.418.0 GHz range)

on the pressed rectangular pellets (2 mm thick) to suit the internaldimensions of Ku-band waveguides.

For a plane wave radiation the far field reflection loss can be

expressed as[61]:

SER(dB) = 108+ log

f

(12)

where an d are conductivity and permeability of the

medium, respectively and f is the frequency of radiation. Fornon-

magnetic material, permeability can be taken as unity (i.e. =1).Fig. 8 (upper plots) shows that reflection loss increases slightly

from 8.0 to 12.0dB with the increase in MWCNT content which

Fig. 10. Variation of loss tangent with increase in loading level of MWCNT.

may be ascribed to increase in the conductivity of composites. The

plots of SER(dB) versus log (Fig. 9a) are straight lines revealing

the applicability of Eq.(12).Further, the skin depth which can be

described as the distance over which the amplitude of a trav-

eling plane wave decreases by a factor e1

, can be expressed as[1]:

= (f)1/2 (13)

where is skin depth and other symbols have usual meanings as

defined above. Eq.(13)reveals that increasing conductivity leads

to corresponding decrease in skin depth which may be helpful in

designing thinner EMI shields.

The attenuation of wave by absorption is related to skin depth

and thickness of the shield by[1]:

SEA(dB) = K

t

= Kt(f)1/2 (14)

whereKisaconstant,tisthicknessoftheshieldandfisthefre-

quency of radiation. The absorption loss(Fig.8, lowerplots) exhibitsrapid enhancement from18.5 to28.0dB with theincreased CNT

loading. This may be explained in terms of increase in conduc-

tivity as well as capacitive coupling effects. The plot of SER (dB)

versus ()1/2 (Fig. 9b) shows linear dependence confirming thevalidity of Eq.(14).The increased conductivity may manifest itself

as increase in both long range charge transportas well as numberof

possible relaxation modes, leading to enhanced ohmic losses[42]

in the proposed electromagnetic shielding material. Loss tangent

(tan = /) is a good measure of the microwave absorption [41].Materials with tan > 1 are considered as lossy materials havingstrong absorption characteristics. The increase in the CNT content

Fig. 9. Plots of (a) |SER| versus log() and (b) |SEA| versus ()1/2

.


8/8


shows proportionate enhancement of the loss tangent (Fig. 10)

leading to increase in absorption loss. The absorption dominated

total shielding effectiveness in range of27.5 to39.2dB indicates

that these materials could be utilized effectively for the shielding

purposes in the Ku-band (12.418.0 GHz).

4. Conclusions

Highly conducting PANIMWCNT composites were prepared by

in situ polymerization. The FTIR and XRD show systematic shifting

in the positionsof characteristic bands and peaks of PANI. This sug-

gests significant interactions between the MWCNT and PANI. The

SEMand TEMpictures show thick anduniformcoatingof PANI over

surface of individual MWCNTs. Based on observed morphological

features, we have suggested the probable formation mechanism

of these composites. At very low concentration of MWCNTs, PANI

coated tubes exist as globular agglomerates (PCNT5). However, at

certain critical concentration of MWCNT (in PCNT25) the poly-

merization takes place exclusively on surface of MWCNT. The

high electrical conductivity of 19.7 S cm1 in PCNT25 (even bet-

ter than bulk conductivity of control MWCNT pellet 19.1 S cm1)

has been ascribed to the synergistic effect of two complementing

phases (PANI and MWCNT). The TGA studies indicate that increas-

ing amount of MWCNTs does not have any effect on the thermal

decompositiontemperature. The shielding measurementsrevealed

thatreflection lossincreases slightlyfrom8.0to12.0 dB whereas

absorption loss exhibitsrapid enhancement from18.5 to28.0dB

with the increased CNT loading. The absorption dominated total

shielding effectiveness in range of27.5 to 39.2 dB indicates that

these materials could be utilized effectively for the shielding pur-

poses in the Ku-band (12.418.0GHz). These PANI coated MWCNTs

withlargeaspect ratio are alsoproposedas hybrid conductive fillers

in various thermoplastic matrices for making structurally strong

microwave shields.

Acknowledgements

Authors wish to thank Director NPL for his keen interest in the

work and for providing necessary research facilities. We are also

thankful to Mr. K.N. Sood for recording the SEM micrographs and

Dr. S.K. Halder for XRD patterns. Authors would like to thank Mr.

Vinod Khanna of IIT Delhi for TEM measurements.

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