Microwave Absorption and EMI Shielding Behavior of Nanocomposites Based on Intrinsically Conducting...

8/10/2019 Microwave Absorption and EMI Shielding Behavior of Nanocomposites Based on Intrinsically Conducting Polymers,

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Chapter 3

Microwave Absorption and EMI Shielding

Behavior of Nanocomposites Based onIntrinsically Conducting olymers! "raphene and Carbon Nanotubes

Parveen Saini and Manju Arora

Additional information is available at the end of the chapter

http://dx.doi.org/10. !!"/#$!!%

1. Introduction

Electromagnetic interference (EMI) is an undesirable an d uncontrolled off-shoot of explosivegrowth of el ectronics an d widespread use of t ransient pow er s ources. Conducting polymersnanocomposites represent a novel c lass of materials that possess unique combination ofelectrical, thermal, dielectric, magnetic and/or mechanical properties w hich are u seful forsuppression of el ectromagnetic noises. Now it i s possible to incorporate various dielectric ormagnetic llers w ithin conducting polymer m atrices t o form multifunctional nanocomposites.The rst s ection of t his chap ter gi ves a brief over view of f undamentals o f E MI s hielding &microwave absorpt ion, theoretical aspects o f shielding, governing equations, various t echniquesfor m easurement of shielding effectiveness an d different strategies f or controlling EMI. In thenext section, a com prehensive a ccount of potential materials for handling of EMI are d escribedwith special reference to nanocomposites b ased on intrinsically conducting polymer m atrixlled with conducting [e.g. metals, gr aphite, c arbon back, car bon nanotubes, graphene],dielectr ic ( e.g. BaTiO 3 or T iO 2) or m agnetic ( e.g. -Fe 2O3, Fe 3O4, BaFe 12 O19 ) inclusions.

2. Electromagnetic I nterference ( EMI) shielding

Electromagnetic interference shielding (EMI) is an undesired electromagnetic (EM)induction triggered by extensive u se of alternating cu rrent/Voltage which t ries to producecorresponding induced signals (Voltage and current) in the nearby electronic circuitry,thereby trying to spoil its p erformance. The m utual interference am ong electronic gadgets,

business machines, process equipments, measuring idisturbance or complete breakdown of normal performance of appliances. The EM

& "01" Saini and Arora' licensee (n)ech. )his is an open access chapter distributedunder the terms of the *reative *ommons Attribution +icense,http://creativ e commons.org/licenses/b-/ .0 ' hich permits unrestricted use' distribution'a nd reproduction in an- medium' provided the original or is properl- cited.
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2 2e Po l-mers fo r Special

disturbances across communication channels, automation, and process control may lead toloss of time, energy, resources and also ad versely affect human health. Due t o these reasonsonly, use of mobile phone is restricted inside robotic operation theatres or d uringonboard/ight which may trigger series of electronic failures and a crash in worstcase/scenario. Therefore, some shielding mechanism must be provided to ensureundisturbed functioning of devices even in the presence o f external electromagnetic (EM)noises. For efficient shielding action, shield should possess ei ther m obile ch arge ca rriers(electrons or holes) or el ectric and/or magnetic dipoles w hich interact with the electric(E) and magnetic (H) vectors o f the incident EM radiation. Therefore, in the recent p ast, awide variety of materials (Abbas et al, 2007; Colaneri et al, 1992; Joo & Epstein, 1994; Ott,2009; Paul, 2004; Saini et al , 2009a, 2010, 2011; Schulz et al , 1988; Singh et al , 1999a, 2000b)have been used for E MI shielding with a broad range of el ectrical conductivity (), goodelectromagnetic attributes such as permittivity ( ) or p ermeability () and engineeredgeometries. The d esigning a EMI shielding with a cer tain level of attenuation, meeting a set

of physical cr iteria, m aintaining economics and regulating the involved shieldingmechanism is not a straight forward task and involves complex interplay of intrinsicproperties (, and ) of s hield material and logical selection of ext rinsic parameters.Therefore, to touch the theoretically predicted shielding performance o f a m aterials an d tosatisfy stringent design criteria, elementary knowledge o f shielding theory, set of g overningtheoretical equations, important desi gn parameters an d relevant m easurement technique

becomes a prime prerequisite.

3. Shielding denitions and phenomenon

EMI shield is ess entially a barrier t o regulate the t ransmission of the el ectromagnetic EMwave across its bulk. In power el ectronics, term shield usually refers to an enclosure thatcompletely encloses an electronic p roduct or a portion of t hat product and prevents t he EMemission from an outside sou rce to d eteriorate i ts el ectronic p erformance.

Conversely, it may also be used to prevent an external susceptible (electronic items orliving organisms) from internal emissions of an instruments electronic circuitry.Shielding is the process by which a cert ain level of attenuation is extended using astrategically designed EM shield. The shi elding efficiency is gen erally measured in termsof reduction in magnitude of incident power/eld upon transition across the shield.Mathematically shielding effectiveness ( SE T) can be exp ressed in logarithmic scale as p erexpressions (Saini et al 2009a, 2011):

PT ET HT SE T (dB) =SE +SE +SE =10

log=20

log=20 log

(1)

R A M 10 P I

10 EI

10 H I

where P I (E I or H I) and P T (E T or H T) are t he p ower ( electric or m agnetic eld intensity) ofincident an d transmitted EM waves respectively. As shown in Fig. 1, three differentmechanisms nam ely reection (R), absorption (A) and multiple internal reections (MIRs)contribute towards overall attenuation with SE R, SE A and SE M as corr esponding shieldingeffectiveness components d ue t o reection, absorption and multiple re ections r espectively.


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3.1. Theoretical shielding effectiveness

Before s tarting the sh ielding analysis, it is n ecessary to understand the v arious electromagnetic t erminologies ( Ott, 2009).

#igure $% Dependence of wave impedance on distance from source normalized to /2

ure &% Schematic represen tation of EMI shielding mechanism


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According to the distance r between the radiating source and the observation point, anelectromagnetic radiative region can be divided into three p arts (Fig. 2) relative totalwavelength of the el ectromagnetic wave. The region within the d istance r

<

/2

is th e

near e ld while the distance r> /2

is the far eld. Between the two regions, as the

distance r /2

, is the transition reg ion. For designing a m aterial for particular shielding

application, it i s imperative to have in-depth knowledge of both intrinsic & extrinsicparameters on which shielding effectiveness depend alongwith suitable t heoretical rel ationscorrelating them with reection, absorption and multiple-reection loss components.

3.1.1. Shielding t heory

This section presents the shielding basics based on the transmission line theory(Schelkunoff, 1943) and the plane wave shi elding theory (Schulz et al,1988). Assume auniform plane w ave ch aracteristic by E and H that vary w ithin a p lane only with x direct ionas s howed in Fig. 3. The M axwells cur l equations gi ve:

dE= j H and

dH

= ( + j )E(2)

dx dx

where is t he p ermeability of the m aterial and = o r . o and r are t he p ermeabilitiesof air ( or f ree spa ce ) and shield material r espectively, is t he co nductivity of material in S/m .where is t he p ermittivity of the m aterial and = o r . o and r are t he p ermittivities o fair (or f ree sp ace) and shield materi al resp ectively, =2 f . ( f ) is an gular f requency(linear f requency) in Hz.

#igure 3% Propagation of electromagnetic waves an d its interaction with the shi eld material


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All homogenous materials ar e charac terized by a q uantity known as t he intrinsic impedance:

= (3)

When an electromagnetic wave propagates through the material, the wave impedanceapproaches t he intrinsic i mpedance o f the material. For d ielectric m aterial, the conductivityis ext remely small ( ) and the intrinsic impedance of Eq. (3) becomes:

=

(4)

For a con ductor u sed below optical frequencies de ned by , the intrinsic impedance of Eq. (3) can be w ritten as:

= j = (1 + j ) f

(5)

It is customary t o d ene p ropagation constant ( ) in the m edia su ch that:

= ( + j ) = j ( + j ).

(6)

where is at tenuation constant and is phase constant. A good conductor is a medium forwhich / 1 . Under this condition the Eq. (6) becomes:

= j =(1 + j ) f

(7)

Therefore, we can write = =1/ =

f , where quantity represents skin depth

which i s dened as the d istance r equired by the w ave t o be at tenuated to 1 / e or 37% of i tsoriginal strength. For a d ielectric p lane sh eet / 1 and Eq. (6) becomes:

= 2 = j

(8)

The impedance of a homogenous barrier of thickness t is

Z =Z ( t ) cosh( t)

+ sinh( t) cosh( t) +Z(t) sinh( t)

H (t) =

H (0) cosh( t) +Z(t)sinh( t)

E(t) = Z (t)

E(0) Z(t) cosh( t)+ sinh( t)

j + j


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(9)

(10)

(11)

where Z(0) is the impedance at interface 0 looking into the plane and H (t) is theimpedance at interface t looking into the ri ght of the p lane a t x = t . If Z(t) , reection


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occurs at the boundary x = t . Let Ei and H i are t he i ncident electric an d magnetic elds, Er

and H r the re ected elds, and Et and H t the t ransmitted elds as s hown in Fig. 3. With thecontinuity of the tangential eld component at a bo undary w e can write:

Ei +Er = Et and H i + H r

= H t(12)

The el ectric an d magnetic elds of a p lane w ave are r elated by the intrinsic impedance of themedium

Ei = H i , Er = H r and Et

=Z(l) H i(13)

Solving the ab ove eq uations, the exp ression of reection coefficients can be w ritten as:

E r Z( t ) q = = (14)

EEi

Z(t) + H r Z (t)

q = =

= q

(15)

H H i

+Z(t) E

The cor responding transmission coefficients can b e w ritten as:

E t 2Z(t) p = = =1+q(16)

EEi

+Z(t) E

H t 2 p = = =1+q(17)

H H i

+Z(t) H

When two mismatched interfaces must be considered in the same pl ane, the n et transmissioncoefficients i s t he p roduct of the t ransmission coefficient across t he t wo boundaries i .e.:

p = pE

= p H

= pE

(0) pE

(t) = p H

(0) p H

(t)

Considering the re- reection effect, the t ransmission coefficients acr oss t he p lane a re:

(18)

H (t)T = H (t) H (0). (19) H

H i H (0) H i

E(t) Z(t) H (t) Z(t)TE = iE

= .

Zw

H i

=

T HZw

(20)

where E(0) , E(t) , H (0) and H (t) are th e a ctual values a t interfaces i .e. at x =0 and x = t .Zw is t he impedance o f the i ncident wave. Using equations (9), (10) an d (11) f or t he p lane o f

=


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the thickness 0 and t

H t=

(21) H (0) cosh( t) +Z(t)sinh( t)

( )


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E(t) =E(0)

Z(t)Z(t)cosh( t)+ sinh( t)

(22)

From equations (14) and (15) we m ay w rite:

H t=

H i

2ZwZw

+Z(0)

(23)

SE T =20 log 10 T =20 log 10 p (1

qe2 t )e t (31)

SE =20log p +20

loge t +20log (1 qe 2 t)

(32)

T 1 0 10

10

( )

total shield ing effectiv eness i s:

(30))e2 t t

T = p (1 qe

(29)

TE = T H =

)2k + 1

(k 1)2

q = q H = (

(28)= 4 k

(k + 1)2 H

p =

p

Z(t ) = Zw , taking k = Zw / we can w rite:

(27)+ ) (Z(t ) + )w

(Zw ) (Z(t ) )q H = (Z

w(26)

(Z + ) (Z(t) + ) H

w4Z p =

(25)e2 t )e tE H HT = T = p (1 q

Z(0) is the impedance a t interface x = 0looking into the plane. By substituting (23)

(24)Zw + Z(0)Ei

=E(t ) 2 Z(0)


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SE R SE ASE

M


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Therefore, after car eful comparison of with shield thickness ( t) two situations can bevisualized:

a. When ( t > ): which is val id in our case an d generally occurs at higher f requencieswhere ski n depth becomes m uch less as compared to actual shield thickness i .e. in case

of electrically thick samples. In such regime, attenuations d ue re ection, absorption andmultiple internal sub-phenomenon becomes a straight forward exercise after makinggood conductor ap proximation i.e. T / 1 [or k 1(i.e. Zw ) ].

3.1.2. Reection loss

The reection loss ( SE R) is r elated to the relative i mpedance m ismatch between the shieldssurface and propagating wave. The m agnitude of reection loss under plane w ave (far eldconditions) can be ex pressed as ( Saini et al, 2011):

SE (dB) = 10log

T (34)

R 10 16 o r

where T is t he total conductivity, f is t he frequency in Hz, r is t he relative permeabilityreferred to free sp ace; The above exp ression shows that SE R is a function of the rat io ofconductivity ( T) and permeability ( r) of the shield material i .e. quantity ( T/ r). Further,for a g iven material (i.e. xed T and r) SE R decreases w ith increase i n frequency.

3.1.3. Absorption loss

As shown in Fig. 1, when an electromagnetic wave pass through a medium its amplitudedecreases exp onentially. This d ecay or ab sorption loss occu rs because cu rrents induced inthe medium produce ohmic losses and heating of the material, and E t and H t can beexpressed as Et

=Eie

t

and H t= H

ie

t

(Ott, 2009). Therefore, the magnitude of

absorption term (SE A) in decibel (dB) can be exp ressed by following equation:

t t 1 T r 2SE A (dB) = 20

log 10 e =8.68 = 8.68 t 2 (35) where t is shield thickness in inches an d f is frequency in Hertz. The above exp ressionrevealed that SE A is p roportional to the sq uare r oot of the p roduct of the p ermeability ( r)


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times t he co nductivity ( T) of the shield materi al i .e. quantity ( T. r)1/2 (Saini et al , 2009a,2011). Further, for a given material, absorption loss increases w ith increase i n frequency.Therefore, a good absorbing material should possess high conductivity and highpermeability, and sufficient thickness to achieve t he r equired number of skin depths even atthe l owest frequency of c oncern.

3.1.4. Multiple Internal Reections (MIRs)

If the sh ield is thin, the reected wave from the sec ond boundary i s re-reected from therst boundary and returns to the second boundary to be r eected again and again as shownin Fig. 1. The attenuation due these multiple internal re ections i.e. SE M can bemathematically expressed as ( Ott, 2009, Saini et al, 2011):

SE A =

2t

=

10

SE M 20 log 10 (1e

) 20 log 10 1 10

(36)

Therefore, it can be seen from the ab ove exp ression that SE M is closely rel ated to absorptionloss ( SE A). SE

M

is also important for porous structures and for certain type of lled

composites or for certain design geometries. It can be neglected in the case o f a thickabsorbing shield due high value of SE A so that by the time the wave reaches t he second

boundary, it is of negl A is 10 dB (Saini, etal 2009a, 2011) SE M can be saf ely neglected. Usually SE M is important only when metals ar ethin and are used at very l ow frequencies (i.e. ~kHz range). However, for highly absorbingmaterials or at very h igh frequencies ( ~GHz o r high), conditionand re-reections can be saf ely ignored i.e. SE M 0 .

SE A 10 dB gets sa tised

3.2. Experimental shielding effectiveness

Experimentally, shielding is m easured using instruments cal led network analyzer. Scal arnetwork analyzer (SNA) measures only the amplitude of signals whereas vector net work

analyzer (VNA) measures magnitude as well as phases of various signals. Consequently,SNA can n ot be u sed to m easure c omplex si gnals (e.g. complex p ermittivity or p ermeability)and therefore, despite i ts hi gher cost VNA is t he m ost widely used instrument.

The incident and transmitted waves in a two port VNA (Fig. 4) can be mathematicallyrepresented by complex scat tering parameters (or S-parameters) i.e. S 11 (or S 22 ) and S 12 (orS21 ) respectively which in-turn can be conveniently correlated with reectance (R) andtransmittance ( T) i.e. T = |E T/E I| 2 = |S 12 | 2 = |S 21 | 2, R = |E R/E I| 2 = |S 11 | 2 =|S 22 | 2, givingabsorbance (A) as: A = (1-R-T). When SE A is g reater t han 10 dB, SE M becomes negligible (~-1.0 d B) and can be n eglected (Saini et al, 2011) so t hat SE T can be exp ressed as: SE T = SE R +SE A. In addition, the intensity of the EM wave inside the sh ield after p rimary reection is

based on quantity (1-R), which can (A) to yield effective absorbance {A eff =[(1-R-T)/(1-R)]} so that exp erimental reection andabsorption losses can be ex pressed as ( Hong et al, 2003; Saini et al, 2009a, 2011):


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Micro ave Abso rption and 3M( Shi e lding 4ehavior of2anocomposites 4ased on (ntrin sicall- *onducti ng Pol-mers' 5 raphen e and *arbon 2anotubes $1

SE R = 10log 10 (1 R)(37)

SE = 10log (1 A ) =10log T(38)

A 10 !"" 10 (1 R )Therefore, from the kn owledge of reected and transmitted signals i.e. R and T, VNA can easily compute r eection and absorption loss c omponents of total shielding.

#igure '% A two port VNA (left) and its internal block d iagram (right)


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3.3. Estimation of electromagnetic attributes

The at tenuation of EM radiation by intended shield material is critically dependent on itselectromagnetic attributes like complex permittivity [ * =( ' j " )] and complexpermeability [ * =( ' j " )], and their es timation is of pa ramount importance. Bothcomplex dielectric permittivity and magnetic permeability consists of real part andimaginary p arts as shown in Fig. 5

#igure (% Complex electromagnetic at tributes of a shi eld

Parameter ' or r (real permittivity) r epresen ts t he charge storage (or d ielectric constant)whereas " (imaginary permittivity) is a measure of d ielectric dissipation or losses.Similarly, ' (or r ) and " represents m agnetic storage an d losses r espectively. The extentof losses can be assessed by calculating dielectric/magnetic loss t angent (tan ) (Colaneri

et al, 1992; Joo. et al , 1994; Saini. et al , 2009a, 2011) w hich is the rat io of imaginary andreal permittivity/permeability.

3.3.1. Measurement and conversion techniques

While designing a shield, all t he above parameters must be taken into consideration. T heincident and transmitted travelling waves inside a VNA can be represented by complexscattering param eters ( or S- param eters) i.e. S 11 (or S 22 ) and S 12 (or S 21 ) resp ectively, which are i n-turn closely related to the electromagnetic (EM) attributes ( Nicolson & Ross, 1970; Ott, 2009;Paul, 2004; Weir, 1974). There ar e m any techniques developed for m easuring these S- parameters

like Transmission/Reection method, Open ended coaxial pr obe technique, Free spacetechnique, Resonant cavity method and Parallel plate t echnique (Ott, 2009; Tong, 2009). Amongthese techniques Transmission/Reection method is the most popular as it simultaneouslymeasures of all four S-parameters and gives complex permittivity as well as magneticpermeability by using suitable al gorithms or m odels developed for obtaining the permittivityand permeability from the recorded S-parameters. Table 1 gives an overview of som e theconversion techniques, S-parameters & to and their evaluation capability for output attributes.

Each of the ab ove con version technique h as d ifferent advantages an d limitations. The selectionof the technique depends on several factors such as the measured S-parameters, sample

length, desired output properties, speed of conversion and accuracies i n the co nverted results.Among


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Micro ave Abso rption and 3M( Shi e lding 4ehavior of2anocomposites 4ased on (ntrin sicall- *onducti ng Pol-mers' 5 raphen e and *arbon 2anotubes 1

above-mentioned procedures, Nicholson-Ross-Weir (NRW) technique is the most widely usedregressi ve/iterat ive analysis as it provides d irect calculation of both the p ermittivity ( * )and permeability ( * ) from the i nput S-parameters.

Conversion technique Input S-parameters Output attributes

Nicolson-Ross-Weir (NRW)S11 , S 21 , S 12 and S 22 or andS11 and S 21 (or S 22 and S 12 )

r and r

NIST iterativeS11 , S 21 , S 12 and S 22 or pairS11 and S 21 or S 22 and S 12

r and r = 1

S11 , S 21 , S 12 and S 22 or andNew non-iterative r and r = 1

Short circuit line (SCL) S 11 or S 22 r

)able &% Conversion techniques, input S-parameters & output attributes

3.3.2. Nicholson-Ross-Weir ( NRW) technique

Nicholson-Ross-Weir (NRW) technique (Nicolson & Ross, 1970; weir, 1974) provides d irectcalculation of both the p ermittivity ( * ) and permeability ( * ) from the input S- parameters.It is the most commonly used technique for performing such conversions where themeasurement of reection ( ) and transmission ( T ) coefficient requires al l four ( S 11 , S 21 , S 12 ,S22 ) or a pair (S 11 , S 21 ) of S-parameters of the material under test to be measured. Theprocedure prop osed by NRW method is deduced from the following set of equations:

S11(1 T 2)(1 2T 2)

and S21 = T(1 2)(1 2T 2)

(39)

Once t hese S-parameters ar e ext racted from the n etwork analyzer, simultaneous s olving ofequation set (39) gives t he re ection coefficient as:

=X X2

1(40)

The con dition [|| < 1] is u sed for nding the corr ect root of the q uadratic equ ation so thatparameter X can be exp ressed as:

S2 S2 +1X = 11 21

2S11Therefore, the t ransmission coefficient can be w ritten as:

(41)

S +S

The p ermeability is t hen given by:

= 11 211 (S11 S21

)

(42)T

=


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* = 1 + (43)

(1 ) 1 1 2 2o cwhere o and c are f ree spa ce an d cutoff wavelength respectively and is gi ven by (Tong,2009):

1 * *=

1

1=

1 ln (44)

2 2 2 2L

T

o c Therefore, the p ermittivity can be w ritten as:

2 1 1 1 2 *= ln * 2 2L T (45)

c Equations (44) & (45) have innite number of roots since the imaginary p art of the termln(1/T) is eq ual to i( + 2n) where n = 0, 1, 2, i.e. the integral multiples o f ratioL/ g, where L is sample length and g, is wavelength inside the sam ple. This br ings phaseambiguity and the cor rect value of n can b e d etermined by either of two methods:

a. The an alysis of group delay:

The cal culated group delay for n th solution can b e d etermined from:

2

f r r+ f 2 d ( r r )

cal ,n =L

ddf

r

r

f

c2

1 = 2

2 f2

df

L 1

(46)

c c2 r r 2 2c

The grou p can al so be d irectly measured by network analyzer by measuring the slope of t heplot between phase ( ) of the t ransmission coefficient versus f requency a s:

=1 d

meas (47)2 df

The co rrect root (n=k) should satisfy the co ndition cal ,k meas =0

b. Phase unwrapping method:

By est imating n from g using initial guess values of an d for t he sam ple, we g et:

2

o

c


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(1 )2 2 2 = 1 o + o (48)

r r (1 + )22 2c r c


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where r and r are i nitially guessed permittivity and permeability respectively, ispropagation constant of material, c is vel ocity of light and f is frequency of incident EMradiation.

3.4. Shielding material and design considerations

The caref ul analysis of theoretical shielding expressions revealed that in order to meetdesign requirements and for extending efficient shielding action, shield should possess a

balanced combination of electrical conductivi ) andmagnetic permeability () and physical geometry (Chung, 2001; Joo & Epstein, 1994;Saini, 2009a). Further, as shown in Fig. 1, the primary m echanism of EMI shielding isreection from the front face of the shield, for which the shield must possess mobilecharge carr iers (electrons or holes) that can interact with the electromagnetic elds tocause o hmic (heating) losses i n the sh ield. As a resu lt, the sh ield needs to be electricallyconducting, although only moderate con ductivity (10 -3 to 1.0 S/cm) is sufficient (Olmedoet al, 1997; Saini, et al 2011). The secon dary EMI shielding mechanism is ab sorption forwhich shield should possess electric and/or magnetic dipoles which can interact withthe el ectromagnetic elds in t he radiation.

Metals ar e by far the most common materials for EMI shielding (Ott 2009; Paul 2004; Schulzet al, 1988) owing to their h igh electrical conductivity. In principle, for a h ighly conductingmaterial (e.g. metals l ike Cu, Ag or N i), only conductivity () and magnetic p ermeability ()are important, such that the reection loss (SE R) is dependent upon their ratio (i.e./) whereas t he ab sorption loss ( SE A) is a f unction of their p roduct (i.e. .) [Chung, 2001;

Joo & Epstein, 1994; Ott, 2009, Sainconducting materials p ermittivity ( ) also plays a signicant role (besides and ) indeciding absolute values of SE R and SE A . Such compounds are capable of displayingdynamic dielectric and/or magnetic loss, upon impingement by incident electromagneticwaves (Abbas et al , 2005, 2006; Joo & Epstein, 1994; Olmedo et al, 1997). Nevertheless,metal based compositions are su ffered from problems (Ott, 2009; Paul, 2004; Saini et al,2009a, 2009b) such as high reectivity, corrosi on susceptibility, weight penalty anduneconomic processing. Among other alternatives, carbon based materials (graphite,expanded graphite, carbon black, carbon nanotubes an d graphene) have also been widelyexplored for possible applications in EMI shielding (Chung 2000, 2001, Gupta &Choudhary, 2011; Huang et al, 2006; Joo et al 1999, Makeiff & Huber, 2006; Pandey et al,2009; Saini et al 2007, 2009a, 2009b, 2010, 2011; Singh et al , 2011; Yang, 2005a, 2005b).However, graphite exhibit poor dispersibility and high percolation threshold (Friend,1993; Olmedo, 1997; Saini, 2009a). Similarly, CNTs ar e economically non-viable, difficult toproduce at bulk scale and often require purication, auxiliary treatment andfunctionalization steps (Bal, 2007; O lmedo, 1997; Sai ni, 2009a, 2011). In this regard,intrinsically conducting polymers ( ICPs) w ith tunable electrical con ductivity/dielectricproperties, facile p rocessing and compatibility with other polymeric matrices c an offer anattractive solution over ot her con ducting llers (Chandrasekhar, 1999; Ellis, 1986; Olmedo,

1997; Skotheim, 1986; Trivedi, 1997). Interestingly, d ue to their inherent electricalconductivity and dielectric p roperties, these I CPs can be u sed either as conducting


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ller for various insulating matrices or as an electrically conducting matrix withincorporated conducting/dielectric/magnetic inclusions.

3.4.1. Intrinsically con ducting p olymers (ICPs)Intrinsically conducting polymers (ICPs) combine moderate conductivity, goodcompatibility and ease of processability (as c ompared with carbons) with low density (~ 1.11.3 g/cm 3 compared to metals e.g. ~9.0 g/cm 3 for copper) an d corrosion resistance(compared to metals) (Baeriswyl, 1992; Chandrasekhar, 1999; Ellis, 1986; Freund & Deore,2007; Heeger, 2001a, 2001b; Joo & Epstein, 1994; MacDiarmid, 2001; Nalwa, 1997;Olmedo, 1997 ; Sai ni, 2011, Shirakawa, 2001; Skotheim, 1986; Trivedi, 1997). They possessunique shi elding mechanism of reection plus absorption rather than dominated reectionfor m etals a nd carbons. The ability to regulate their el ectrical c onductivity by controlling

parameters such as oxidation state, doping level, morphology and chemical structure,makes t hem powerful candidate for vari ous techno-commercial applications. Fig. 6 showsthe structure of some of the well known con ducting polymers in their undoped forms.

n

tr#$% Pol'!t&l!$!n

'(% Pol'!t&l!$!

n

Pol& #r# *!$&l!$!

S N

nn n H

Pol& #r# *!$&l!$! $&l!$! Pol&t*(o)*!$! Pol& &rrol!

N

H nPol$ l $!

#igure *% Chemical structures of some un doped con jugated polymers

Since the rst ICP, polyacetylene (PA), was su ccessfully synthesized by Shirakawa et al.(1977) and Chiang et al (1977, 1978a, 1978b) with conductivity as h igh as 10 6 S/cm in dopedform, great interest has been aroused and a ser ies of ICPs such as polyaniline (PANI),polypyrrole (PPY), polythiophene (PTH), poly(p-phenylene-vinylene) (PPV) etc have beendeveloped (Carter et al, 1985; Rahman et al, 1989; Saxman et al, 1985; Snow, 1981; Soga et al,1983; Thomas et al, 1988; Yamamoto et al, 1988). These undoped polymers display poorconducting properties an d lies i n insulating or sem iconducting range (10 10 to 10 5 S/cm)as


20/46

shown in (Fig. 7). However, the controlled doping can transforms the poorly conductingundoped material into a syst em which displays semiconducting or metallic con ductivity(10 -6 to 10 5 S/cm). The predicted theoretical value for highly doped PA is about 2 10 7

S/cm, which is even higher t han that of copper ( Chiang et al, 1978a, 1978b). However, thehighest experimentally recorded conductivity for PA (in highly oriented thin lms form)was greater t han 10 5 S/cm, which is s till the h ighest value that has b een reported for anyconducting polymer t ill date. In contrast, conductivity of other con jugated polymers reachesa maximum value ~10 3 S/cm (Baeriswyl, 1992; Cao et al, 1992, 1995; Chaing et al, 1978a,1978b; Chandrasekhar, 1999; Ellis, 1986; Heeger, 2001a; Nalwa, 1997; MacDiarmid, 2001;Shirakawa, 2001; Skotheim, 1986).

The d isplay of metal like el ectrical and optical properties by the h ighly doped forms of theseICPs ( synthetic p olymers) also en titled them to be called synthetic m etals ( Freund & Deore,2007; Heeger, 2001a, 2001b; Nalwa, 1997; MacDiarmid, 2001; Shirakawa, 2001; Skotheim,1986). The intrinsic conductivity of conjugated polymers in the eld of microwave ( 100 MHz 20 GHz) m akes them a viable shielding material. In particular, dependence of theirconductivity on frequency, has i nspired many sci entic ideas t o ad opt these phenomenon tomicrowave applications (Coleman & Petanck, 1986; Karasz et al, 1985; Natta et al, 1958;Olmedo, 1995, 1997; Saini et al, 2009a, 2011).

#igure +% Conductivity of s ome con jugated polymers i n comparison to typical metals, semiconductors or i nsulators.

The unique properties like tunable conductivity (between insulating and metallic limits),adjustable permittivity/permeability via synthetic means, low density, non-corrosiveness,

nominal cost, facile processi ng (melt or solution), and controllable electromagnetic


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attributes, further st rengthen their can didature a s f uturistic sh ielding material f or v arioustechno-commercial applications. Their u tility can also b e ex tended to high-tech areas l ikespace, defense (military), or navigation/communication control or as a radar absorbingmaterial (RAM) in the stealth technology (Ellis, 1986; Knott et al, 1993; Olmedo, 1997;Nalwa, 1977). Especially, conducting p olymers appear to be on e of the few materials capableof displaying dynamic (switchable) microwave absorption behavior, which are calledintelligent st ealth materials, d ue to the revers ible electrical p roperties of conductingpolymers affected by redox doping/de-doping processes.

A careful comparison of properties o f large n umber of available sh ielding materials r evealedthat no single phase m aterial can take care o f all the aspects of s hield (e.g. absorptioncoefficient, thickness, volume, broadband response) to give desired level of performanceunder d ifferent environments an d applications. Therefore, several attempts h ave a lso beenmade to exploit the worthy property of above m aterials by m aking strategic combinationse.g. admixtures, blends an d composites (Ajayan et al, 2000; Cao et al, 1992, 1995; Chung,2000, 2001; Colaneri et al, 1992; Dhawan, 2003; Gangopadhyay et al, 2001; Grimes, 1994;Gupta & Choudhary, 2011; Huang et al, 2000; In et al, 2010; Joo et al, 1999; Koul et al, 2000;Liang et al, 2009; Pomposo, 1999; Ramanathan et al, 2008; Saini et al, 2009a, 2009b, 2011;Sanjai et al, 1997; Shacklette e t al, 1992; Shi & Liang, 2008; Singh et al, 2011; Stankovich et al ,2006; Taka, 1991; Varrla, 2011; Wang & Jing, 2005; Wessling, 1999; Wojkiewicz et al, 2003;Zhang et al, 2011). Among these op tions, composites based on various organic/inorganicller ( guests) loaded ICP matrices ( hosts) as w ell as I CP (guest) loaded insulating matrices(hosts) have captured maximum attention due to fascinating properties an d wealth of

prevalent applications (Chandrasekhar, 1999; Ellis, 1986; Freund & Deore, 2007; Heeger, 2001a,2001b; MacDiarmid, 2001; Nalwa, 1997; Shirakawa, 2001; Skotheim, 1986). Recently, thediscovery o f various n anomaterials (NMs) and ability to design and tailor t heir el ectrical andelectromagnetic properties has lead to scientic surge to identify the best m aterials forshielding and other ap plications (Ajayan et al, 1994; Alexandre, 2000; Baughman et al, 2002;Geim & Novoselov, 2007; Geim, 2009; Ijima, 1991; Meyer et al, 2007; Moniruzzaman & Winey,2006; Rozenberga & Tenn, 2008; Stankovich et al, 2006, 2007; Thostenson et al, 2005).Especially, nanocomposites h ave a ttracted enormous scientic at tention due to distinguishedset of properties as w ell as p romising applications.

3.4.2. ICP based nanocomposites

Nature ha s the ast onishing ability to form self-organized functional nanomaterials withperfect s tructures a nd unusual p roperties e.g. bacteria, viruses, proteins, cells e tc. w hichordinarily falls in the si ze ran ge of 1-100 nm (1 n m = 10 -9 m). In fact, nature i s con sidered asmaestro nanotechnologist who has created one of the best known nanocomposites such as

bones, hairs, shells, d wood. Therresearches ar e t rying to learn and mimic t he n atural material synthesis p rinciples. However,though high quality bulk composites (e.g. straw reinforced mud, concrete, carbon/glass

ber reinforced polymers) were already realized by researchers, formation ofperfect


22/46

nanocomposite rem ained a biggest scientic cha llenge. In case of nanocomposites, llerspossess n anoscale dimensions (~10 4 times n er than a human hair) and extend ultra-highinterfacial area p er volume to host polymeric matrices. Consequently, marked differences i n

the properties of nanocomposites ar e observed compared to their bu lk counterparts e.g.enhanced strength, better o ptical or el ectrical properties et c. (Ajayan et al, 1994; A lexandre,2000; Choudhary & Gupta, 2011; Mathur et al, 2010; Moniruzzaman & Winey, 2006;Ramasubramaniam, 2003; Rozenberga & Tenn, 2008; Thostenson et al, 2005) even at thelower l oadings. Polymer i s a v ersatile ch oice as a matrix material due t o advantages l ike lowdensity, mechanical exibility, facile processi ng and corrosion resistance. Interest ingly, mostpolymeric matrices possess poor electrical, d ielectric or magnetic properties and aretransparent to electromagnetic radiations (Saini et al, 2009a, 2011). Therefore, most of theelectrical and electromagnetic properties o f the conventional nanocomposites are m ainlycontributed by the n anollers (nature an d concentration) and matrix si mply plays t he r ole o f

holding the ller p articles. In this con sideration, utilization of ICPs as h ost matrix ca n offeran attractive sol ution over con ventional (insulating) polymer ba sed matrices ( Ellis, 1986;Nalwa, 1997; Olmdo, 1995, 1997; Saini et al, 2011) primarily due to microwave non-transparency and design exibility. H owever, t he incorporation of nanollers withinpolymeric matrices i s not a st raightforward task because of the ultrahigh surface area an dagglomeration tendencies. These often resulted in failure to efficiently translate thenanoscopic properties of these llers into macroscopic properties of resultantnanocomposites, thereby inability to utilize their full p otential. H ence, handling anddispersion of na noller i s t he b iggest challenge for n anocomposite sci ence an d technology.

3.4.3. Synthesis of ICP based nanocomposites

i. ICP as ller

As already mentioned in the previous section, inherent el ectric conductivity/dielectricproperties ( i.e. without any added conducting additive e.g. metals, graphite or carbonnanotubes), design exibility and good compatibility with various insulating polymermatrices (e.g. t hermoplastic/thermoset/rubber/elastomer/ber/fabric etc.), I CPs can beused as ller to form composites.

As shown in Fig. 8, such co mposites ar e formed either by solution processing or by melt phasemixing/blending (Pud et al , 2003; Cao et al , 1992, 1995; Colaneri & Shacklette, 1992; Taka,1991; Shacklette et al , 1992; Saini, et al , 2011; Wessling, 1999). In the former case b oth ICP andmatrix polymer are dissolved/dispersed in a common solvent and stirred/sonicated toachieve the nal mixing followed by casting (shaping) and drying/curing. In contrast, melt

blending involves mixing of l(shaping) and cooling/curing. In some cas e e.g. thermosts, ICPs ar e mixed with pre-polymer(resin) by solution blending technique. Finally, cross-linkers ( curing agents) are added andcuring is achi eved by a com bination of heat (not required for room temperature cross-linkers)

and pressure (not required when no volatiles ar e exp elled during curing process).


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#igure ,% Schematic representation of formation of mechanism of ICP matrix based nano-composites by in-situ polymerization ro ute.

Pol-meri6a t ion (n

ure -% Schematic represen tation of steps involved

in the formation of ICP (as conducting ller)

insulating polymer matrix by solution and melt processing techniques

ICP as m atrix polymer u tilization of different ICPs as nanocom posite matrix can be at tributed to advantage s as design exibili ty , g ood l

(*P based nanocompo

Reactor(T, P, Stirring)

Filler (s)Monomer (s) Reaction


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%0 2e Po l-mers fo r Special

The incorporation of various conducting, di electric or magnetic nanoparticles withinconducting polymer matrices can be achieved either by ex-situ physical mixing processes or

by in-situ polymerization (Abbas eFang et al, 2006; Joo et al, 1999; Moniruzzaman & Das, 2010; Pant et al, 2006; Phang et al,2007, 2008; Saini et al, 2007, 2009a, 2009b, 2010; Yang et al, 2010, 2011). However, ex-situmixing leads to poor dispersion of ller particles and failure to overcome theiragglomeration tendencies that r esults in inferior and non-reproducible electrical andelectromagnetic a ttributes. In contrast, the e lectronic p roperties o f such synthetic m etals can

be strictly controllthe p olymerization under the con trolled conditions and in the p resence of specic dopantsand llers (Bredas et al, 1998; Chandrasekhar et al, 2002, Mattosso et al, 1994; Nalwa e t al,1997; Saini et al , 2007, 2009a, 2009b; Savitha et al , 2005; Skotheim, 1986). In a typical r eaction,monomer(s), ller an d dopant or catalyst are cha rged into a su itably designed reactor tomaintain required temperatre ( T), pressure (P) and agitation (stirring) conditions. During

such pre-polymerization process, monomers are generally adsorbed over dispersed nano-ller p articles. The p olymerization was i nitiated by addition of specic i nitiator/oxidant an dallowed to proceed till reaction gets completed leading to formation of ICP basednanocomposite.

3.4.4. Electrical properties of ICP based nanocomposites

As already mentioned and shown in Fig. 1, the primary shielding mechanism is reectionfor which sh ield should possess free charge car riers (electrons/holes) that can interactwith incident EM eld. But the organ ic conjugated polymers are insulators in their

undoped forms e.g. room temperature electrical conductivity ( dc ) of emeraldine base (EB)is ~10 -9 S/cm (Fig. 10, Gupta et al, 2005). However, controlled doping leads toenhancement of conductivity due to formation of charge ca rriers (Fig. 11) i.e. polarons/

bipolarons (Saini et almove under the inuence of external potential and in the Coulmbic eld of counter-ionsdistributed along the chain.

Therefore, increasing dopant concentration leads t o increase i n concentration and mobilityof proto-generated charge car riers resulting in enhancement of conductivity. Furthermore,such a conductivity enhancement in conductivity is strongly dependent on nature andconcentration of dopant and in some case conductivity well exceeds the required limit(Olmedo et al, 1997; Saini et al, 2011) for exh ibiting good shielding effectiveness.

The addition of ICPs particles (guests) as a conducting ller w ithin insulating polymermatrices (hosts) leads to establishment of electrical conductivity (in resultantnanocomposites) due to formation of percolation networks (Colaneri & Shacklette, 1992;Hsieh, 2012; Lakshmi et al, 2009; Shacklette et al, 1992; Taka, 1991; Wessling, 1999). Atpercolation threshold, ICP particles form a 3D conductive network within host m atrix,which can be ea sily estimated by plotting the el ectrical conductivity as a function of thereduced volume fraction of ller (Fig. 9) and performing data tting with a power lawfunction (Saini et al , 2011):


25/46

=o (v vc)

(49)

where is t he electrical conductivity of the composite, o is c haract eristic conductivity, vis t he volume fraction of ller, vc is vo lume f raction at the p ercolation threshold and t is th ecritical exponent. The l og ( ) versus l og ( v vc ) plot (Fig. 12) gives a straight line accordingto eqn. 10. The values o f scaling law parameters i.e. vc and t can be su bsequently obtained

by least-square analysis of host polymer and the ller ar e si milar, mass f raction ( m) becomes same as volume fraction(v) and can be used in above calculations.

3

.

/3

/*

/,

/&$

0d1

0c1

0b1

0e1 0f1 0g10h1

0i1

/&(

/&-

0a1.%. .%$ .%' .%* .%- &%.

2opant Concentration 0M1

#igure &.% Variation of el ectrical con ductivity (ln dc ) of hyd rochloric aci d (HCl) doped Emeraldine base (EB) samples as a function of

M, (e) 0.3 M, (f) 0.5 M, (g) 0.7 M, (h) 0.9 M and (i) 1.0 M .

However, it has been observed that formation of s uch networks an d percolation thresholds(minimum loading level at which rst continuous network of conducting particles isformed) cr itically depend on nature of ICP, its intrinsic conductivity, particle shape,morphology, aspect ratio, its con centration, degree o f dispersion and extent of co mpatibilitywith host matrix.

Nevertheless, at percolation conductivity ( p) remained too low to exhibit any acceptableshielding action and generally higher loadings (>30 wt. %) are required though in mostcases, p is su fficient to extend antistatic act ion. Interestingly, when ICPs are com bined with

other con ducting llers (e.g. Polyaniline with MWCNT, Saini et al, 2011) signicantreduction in percolation threshold, higher con ductivity and better s hielding performance isobserved as c ompared to pristine (unlled) ICPs.

Electrical Conductivity0ln

dc

t


26/46

26 2e Po l-mers fo r Special

In many cases conjugated polymers ar e u sed as m atrix instead of conventional insulatingpolymers. When conducting llers (e.g. metal particles, carbon black, graphite o r CNTs) areincorporated within undoped (poorly conducting) ICP matrices, electrical c onductivityincreases an d follows a typical percolation behavior. In contrast, the loading of aboveconducting llers w ithin microwave non-transparent doped (intrinsically conducting) ICPmatrices l ead to further en hancement (Fig. 13) of electrical conductivity. Such improvementcan be explained on the basis of granular metal/inhomogeneous doping model (Sheng

&

ure &$% Variation of conductivity ( dc ) of PANI-MWCNT nanoller loaded polystyrene solution

Inset shows the percolation and scaling details

ure &&% Protonic acid doping of polyaniline leading to formation of charge carriers polarons (radical

and bipolarons (dications)


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Klafter, 1983) w hich considered that I CPs con sists of highly conducting metallic islandsdispersed within low conductivity amorphous matrix. Therefore, above improvement inconductivity can be at tributed to bridging of these m etallic i slands ( Saini et al, 2009a) by the

metallic ller particles facilitating enhanced inter-particle transport. T he increase inconductivity is strongly dependent on nature, concentration and aspect r atio of llerparticles as well as t ype an d morphology of host ICP matrix.

#igure &'% Correlation between electrical conductivity () and shielding effectiveness ( SE) showing linear d ependences of (a) reection loss (SE R) on log and (b) absorption loss (SE A) on 1/2 .

Nevertheless, the es tablishment and enhancement of electrical conductivity is of paramountimportance because it leads to parallel enhancement o f reection and absorption losscomponents (Fig. 14, Saini et al, 2009a) leading to enhancement of overall shielding

ure &3% Dependence of electrical conductivity of in-si tu polymerized PANI-MWCNT

on MWCNT content


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effectiveness. Interest ingly, absorption loss ( SE A) increases by much larger magnitude (withconductivity) compared to corresponding reection loss (SE R) component. For a non-magnetic m aterial, this can be ex plained on the b asis of logarithmic [i.e. log()] and square

root [ i.e. ()1/2

] conductivity dependence of SE R and SE A respectively as sho wn in Fig. 14.

3.4.5. Dielectric an d magnetic p roperties of ICP based nanocomposites

A secondary mechanism of shielding is absorpt ion for w hich shield should possess electricor m agnetic d ipoles. These d ipoles can interact with transverse el ectric (E) and magnetic ( H)vectors of t he i ncident EM waves t o introduce l osses i nto the sy stem. It is i nteresting to notethat pure (without any external ller loading) conjugated polymers in their u ndoped (base)forms p ossess p oor dielectric an d magnetic properties. However, controlled doping leads tomarked improvement (Fig. 15) in dielectric properties (e.g. d ielectric constant/real-permittivity, dielectric l osses/imaginary-permittivity), although even after d oping magneticproperties ( e.g. real and imaginary m agnetic permeability) remained poor.

$(

$.

&(

&.

(

..%. .%3 .%* .%, &%$ &%( &%- $%&

2opant Concentration 0M1

#igure &(% Dependence o f (a) r eal permittivity or d ielectric co nstant ( ' ) and (b) imaginary permittivity or d ielectric l oss ( " ) on the d opant concentration for acr ylic aci d (AA) dopedemeraldine base (EB) samples

As already discussed, doping of ICPs leads to formation of polarons/bipolarons (Fig. 11)that produces pro nounced polarization/relaxation effects (Olmedo et al, 1995, 1997; Sainiet al , 2008, 2009a, 2011; Stafstrom et al, 1987). Therefore, observed improvement of dielectricproperties w ith doping level can be at tributed formation and increase i n concentration ofabove localized carriers. The correlation between dielectric properties and shieldingresponse f or vari ous ICPs i s p resented in Fig.16 which clearly shows t hat the t otal shieldingefficiency (SE T) increases as t he ab solute v alue o f complex dielectric co nstant increases.

The increase o f both real and imaginary parts of d ielectric permittivity contributes ( Joo &

Epstein, 1994) towards enh ancement of SE T. Furthermore, the com plex d ielectric constant

er mittivity0 4


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dependence of absorption loss (SE A) component (inset of Fig. 16) was found to be muchstronger com pared to that due to re ection loss ( SE R). In some ca ses, especially for h ighlydoped and stretch oriented ICPs; dielectric con stant becomes n egative (Javadi et al, 1989; Joo& Epstein, 1994; Joo et al , 1994; Hsieh et al , 2012; Wang et al , 1991) and ultra-highattenuation is ob served which suggests the p ossibility of ICP based left handed materials(LHMs) or meta-materials.

#igure &+% Loss tangent (tan ) of in-situ synthesized PANI-MWCNT nanocomposites as a f unction ofMWCNT loading

ure &*% Total shielding efficiency (SE T) vs absolute value of complex dielec tric constant [( 2 + 2 )1/2 ]i

conducting polymers. Inset: comparison of reflection () and absorption ( ) shielding efficiency as a function of absolute value

[1994], American Institute of Physics.


30/46

Interestingly, for ICPs, besides doping induced polarization ller induced interfacialpolarization may also contribute towards dielectric properties. For example, whenconducting llers like m etal particles, graphite or car bon nanotubes ar e introduced into ICP

matrices; further improvement of dielectric properties was ob served. Such a polarizationoccurs d ue t o electrical conductivity differences bet ween ICP and metallic llers l eading tocharge l ocalization at interfaces v ia Maxwell-Wagner-Sillars ( Kremer & Schnhals, 2003;Riande & Diaz-Calleja, 2004; Sillars, 1937; Wagner, 1914) interfacial polarizationphenomenon. Such polarization and related relaxation phenomenon contribute towardsenergy storage an d losses. The actual losses can be computed by normalization of theselosses w ith storage terms [ i.e. by ratio of dielectric l osses/ imaginary permittivity ( " ) w ithdielectric co nstant/real p ermittivity ( ' )] to quantity loss t angent (tan ).

In case of in-situ formed MWCNT-polyaniline nanocomposites, improvement of d ielectric

properties l eads to high value of loss t angent (Fig. 17) which further i ncreases ( Saini et al,2009a) with increase in MWCNT loading. However, though for a gi ven thickness, totalshielding is d ominated by absorption, reection loss c omponent bec omes t oo high form theviewpoint s tealth technology. Nevertheless, despite good dielectric properties, magneticproperties of I CPs remained poor to extend any signicant contribution towards EMIregulation. In principle, for hi ghly conducting materials, only conductivity () and magneticpermeability () are i mportant, such that the re ection loss (SE R) is dependent upon theirratio (i.e. /) w hereas the absorption loss ( SE A) i s a function of t heir p roduct ( i.e. .)(Saini et al , 2011). In contrast, for m oderately conducting materials ( e.g. ICPs) permittivity ( )also p lays a s ignicant role (besides and ) in deciding absolute values of SE R and SE A

(Joo & Epstein, 1994). As most ICPs are non magnetic in nature (r

i

0), observedattenuations are mainly governed by and only. Therefore, i t i s expected that a nyimprovement i n magnetic properties w ill lead to denite improvement of absor ption lossalongwith parallel r eduction of reection loss. In addition, the incorporation of highdielectric constant materi als like BaTiO 3, ZnO, TiO 2 etc. within ICP matrices ar e exp ectedto further improve the microwave absorption response. Consequently, in recent years, lotof work has been car ried out to formulate composites of polyaniline with the dielectricor magnetic lled inclusions, either by in-situ polymerization or by ex-situ physical mixingprocesses (Abbas et al, 2005, 2006, 2007, 2008). Such composites possess moderatepolarization or/and magnetization alongwith good microwave conductivity so as tointroduce absorbing properties into the material. They display d ynamic dielectric and/ormagnetic losses, upon impingement by incident electromagnetic waves. Aselectromagnetic wave consists of an pulsating (orthogonal to each other) electric (E) andthe m agnetic (H) elds; therefore, above multi-component composites ar e exp ected to yieldgood attenuation efficiencies, p rimarily due to interaction of conducting/dielectric andconducting/magnetic phases with E and H vectors of the incident EM waves (Fig. 5).Furthermore, most insulating polymer matrices possess poor electrical, dielectric ormagnetic properties and are transparent to radio frequency (RF) or m icrowave (MW)electromagnetic rad iations ( EMRs). Therefore, on ly llers co ntribute towards sh ielding andleakage of radiation from EMR transparent regions t ends t o degrade shi elding effectiveness.However, microwave n on-transparency (Olmedo et al, 1995, 1997; Saini et al, 2011) of ICPscompared to conventional polymers is an added


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advantage a s bot h ller and matrix contribute towards shielding. Moreover, the dominantshielding characteristic o f absorption for ab ove nanocomposites m aterials o ther t han that o freection for m etals r ender I CPs m ore u seful in applications r equiring not only high EMI SE

but also shielding by absor

When ICPs are exp loited as microwave non-transparent matrices, the ad ded dielectric ormagnetic ller particles result in establishment of properties (e.g. d ielectric/magneticcharact er, thermal con ductivity etc.), that are extrinsic to these intrinsically conductingpolymers. Therefore, combination of dielectric or m agnetic nanoparticles w ith conductingpolymer leads to formation of multi-component composite possessing unique combinationof electrical, dielectric an d magnetic properties u seful for suppression of electromagneticnoises and reduction of radar signatures (Abbas et al, 2005, 2007; Chan, 1999;Chandrasekhar, 1999; Cho & Kim, 1999; Dong et al, 2008; Ellis, 1986; Gairola et al , 2010;Huang, 1990; Knott et al, 1993; Kurlyandskaya et al, 2007; Meshram et al, 2004; Nalwa, 1997;Ngoma et al, 1990; Pant et al, 2006; Phang et al, 2007, 2008, 2009, 2010; Xiaoling et al, 2006;Xu et al , 2007; Yang et al , 2009). The incorporation of magnetic llers ( e.g. ferrites l ike -Fe 2O3 or Fe 3O4) within ICP matrices leads to improvement of magnetic properties (Fig. 18)without much loss of conductivity. Such a co mbination is expected to display ad ditionalmagnetic loss l eading to enhanced absorption.

#igure &-% Magnetization of po lyaniline (PANI), Fe 2O3 nanoparticles and nanocomposites formed byin-situ polymerization taking d ifferent weight ratio o f aniline: Fe 2O3 (a) 2: 1, (b) 1 :1 & (c) 1: 2

The magnetization plots (Fig. 18) of polyaniline/-Fe 2O3 composites revealed that pure -Fe 2O3 nanoparticles d isplay pronounced magnetic signatures w ith narrow hysteresis loop.The sat uration magnetization (M s) value of of these particles was found to be 59. 3 em u/g (at5.0 kG) alongwith very sm all retentivity (M r~ 4.3 em u/g) and coercivity (H c~ 83.8 G), whichindicate the super-paramagnetic (SPM) nature of these particles. The SPM character impartsfast r elaxation behaviour and originates due to small s ize of the ferrite particles i.e.approaching towards the single domain limit (Qiao et al, 2009). However, PANI possessesweak ferromagnetic behaviour and with increase i n ferrite con tent, enhancement of M s was


32/46

observed with as p arallel reduction of coercivity (H c). The initial p ermeability ( i) offerromagnetic m aterials can be ex pressed as ( Stonier, 1991):

M 2 =

s

(50)i akH M +b c s

where a and b are two constants d etermined by the m aterial composition, is the magneto-striction constant, is elastic strain parameter of crystal, an d k is a proportionalitycoefficient. The above equation shows that permeability can be enhanced either byenhancing M s or by re ducing H C. In the p resen t system, the incorporation of ferrite w ithinPANI matrix is exp ected to affects t he su rface el ectron density of -Fe 2O3 nanoparticles andhence the sp in-spin or spin-lattice interact ions. The resu lts sh ow that M s value increases(5.94 to 16.4 emu/g) with increasing ferrite content (plots a-c) whereas H c shows a

simultaneous d ecrement (35.7 to 57.8 G). Therefore, It can be seen from the eq n. (50) that both higher M s and lower H c values ar e favorable to the improvement of i value, which in

turn i s exp ected to enhance t he m icrowave ab sorption capability.

In many cases, highly doped ICP particles ar e u sed as c onducting llers (in place of metal orcarbon based materials) for vari ous i nsulating polymer h ost matrices. This n ot on ly leads t oestablishment an d improvement of electrical c onductivity but al so contribute towardsimprovement of both real as w ell as i maginary p ermittivity (Abbas et al, 2005; Colaneri &Shacklette, 1992; Joo et al, 1994; Saini et al, 2011; Shacklette e t al, 1992; Taka, 1991; W essling,1999).

Again the magnetic properties remained poor due to non-magnetic nature of most ICPs.However, when magnetic ller loaded ICPs is use hyb rid ller, improvement in magneticproperties h as al so been observed besides r egular i mprovement of dielectric at tributes. Fo rexample use of PANI-MWCNT hybrid ller within polystyrene m atrix leads t o formation ofcomposites with magnetic properties due to MWCNT core (containing entrappedferromagnetic iron catalyst phase) an d electrical conductivity/dielectric properties d ue toICP and MWCNTs. As the concentration of PANI-MWCNT ller increases, real andimaginary p arts of both permittivity and permeability increases as shown in Fig. 19. Mostimportantly, losses d ue t o reection (SE R) and absorption (SE A) follows the permittivity andpermeability trends an d exhibit corresponding increase. However, SE A was m ore s ensitivetowards el ectromagnetic at tributes com pared to SE R which may b e at tributed to their s quareroot and logarithmic dependences. Furthermore, two most important parameters thatdecide the relative magnitudes of SE R and SE A are microwave con ductivity ( T) and skindepth (). The T can be rel ated to imaginary permittivity (" or i) as ( Saini et al, 2011):

T

= ( ac + dc ) = o " (51)

where ac and dc are frequency dependent (ac) and independent (dc) components of Trespectively, is an gular f requency and o is p ermittivity of free sp ace (8.85 x 10 -12 F/m).Higher the v alue o f T more w ill be re ection for a g iven absorption. Further, the sk in depth() of the sh ield is d ened as d epth of pe netration at which strength of incident EM signal is


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reduced to ~ 37% of its or iginal magnitude. For a g ood conductor (i.e. T >> ), it () can beexpressed in terms of T real permeability ( ' or i ) and as ( Joo. et al, 1994):

1

= 2 2 T ' (52)

Now shallower is the ski n depth, higher will be absorption loss for a given thickness ofmaterial. Fig. 20 shows that as the loading level of PANI-MWCNT ller within PS matrixincrea ses, T increases whereas va lue d ecreases.

#igure &,% Frequency dependence of ( a) dielectric con stant ( ' ) & real permeability ( ' ), (b)dielectri c loss ( " ) & magnetic loss ( " ) of PANI-MWCNT/Polystyrene nanocomposites withincreasi ng loading (10, 20 & 30 weight %) of PANI-MWCNT ller. Dependences of losses due to

absorption (SE A) and reection (SE R) of abo ve co mposites as a f unction of absolute v alue o f (c) complexpermittivity [( 2 + 2 )1/2 ] , (d) complex permeability [( 2 + 2 )1/2 ].

r i r i


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100 2e Pol-mers fo r Special

#igure $&% Frequency an d dopant concentration dependence of total shielding effectiveness (SE T) value of s amples prep ared b y doping of emeraldine ba se ( EB) with different concentrations of acrylic aci d (AA) viz. (a) 0.0 M , (b) 0.05 M, (c) 0.1 M , (d) 0.5 M , (e)1.0 M and (f) 2.0 M

As already discussed doping produces localized defects (polarons/bipolarons) that areresponsible for polarization and electrical c onductivity. With the increase in dopantconcentration, achieved doping level increases l eading to enhancement of polaronic

ure $.% Dependence of (a) T and (b) value of PANI-MWCNT loaded polystyrene composites on the loading of PANI-MWCNT

t previous section we learned about the importance of parameters such as electrical co nductivity and dielectri


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concentration as well as rel ated conductivity/permittivity, which ultimately leads toimprovement of shielding effectiveness (Fig. 21). Nevertheless, though based on microwavedielectric con stant and electrical conductivity values m any speculations were made about

the sh ielding properties of ICPs, the rst direct evidence o f shielding response of ICP basedcomposites alongwith actual shielding effectiveness values was present ed by Taka (1991).He prepared poly(3-octyl thiophene) composites by melt mixing chemically synthesizedpoly(3-octyl thiophene) with PS, PVC, and EVA in and tested for E MI shielding at frequencyrange from 100 kHz to 1GHz. EMI SE of these composites (3 mm thick) increased with thepolymer loadings and -45 dB (from 100 kHz to 10 M Hz) was achieved with high (i.e. 20%)loading in the PVC that was s till lower t han that of a nickel painted sample (-80 dB). Themeasurements showed that P3OT blends behave as pseudo-homogenous metals (PHM). APHM has no intentional holes or slits but lacks homogeneity. The shielding efficiencydepends strongly on the amount of conducting polymer mixed in the blends due toregulation of c onductivity. The a uthors c oncluded that composites w ith 20% or l ess loadingof poly(3-octyl thiophene) were n ot r eadily applicable a s E MI shielding.

#igure $$% EMI shielding effectiveness (at 1.0 GHz f requency) of ICP loaded PVC blends as a function of DC electrical conductivity u nder (a) far eld and (b) near eld regimes

Later, systematic study of EMI shielding behavior of conducting polymer (PANI) basedthermoplastic blends with polyvinyl chloride (PVC) or Nylon was reported (Colaneri &Shacklette, 1992; Shacklette et al, 1992). The EMI SE values of these highly conducting

blends (~0.1-20 S/cm) were measured over a frequency rz also calculated theoretically under both near an d far eld regimes. The results aregraphically presented in Fig. 22 which showed that both near and far led SE followed theDC electrical conductivity and exhibit rapid initial rise followed by slow increment athigher c onductivity. Far eld SE of -70 dB was o btained for t he m elt blend of p olyaniline(again at higher loading level of 30 w t. %) with PVC which a greed well with the theoreticalcalculations as p er expressions d erived by authors. Pomposo et al (1999) have prepared PPY

based conducting hot melt adhesives by mel


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36 2e Pol-mers fo r Special

vinyl acetate (EVA) copolymer and PPY, which was synthesized with oxidant of FeCl 3. Bothnear an d far eld EMI shielding properties of the adhesives were measured at roomtemperature an d found to increase w ith the loading of PPY. Near eld SE in excess of -80 dBwas determined at 1MHz and above -30 dB at 300 MHz, though a decrease with increase offrequency. Far el d SE values of -22, -27 and -30 dB were determined (in the 1 t o 300 MHzfrequency range) for P PY loadings o f 15, 20 and 25% respectively. Similarly, Wessling (1999)prepared highly conductive b lends of PA NI w ith PVC, polymethylmethacrylate (PMMA) orpolyester at Ormecon Chemie, with conductivities of ca. 20 S/ cm and in some cas es up to 100S/cm. These blends exhi bited EMI SE of -40 to -75 d B for bo th near and far el d conditions.However, mechanical properties were not encouraging and demanded considerableimprovement. In addition the higher nec essary thicknesses of 2- 3 mm of t hese blends werefound to be h igher t han technically acceptable t hickness of 0.50.8 m m for p ractical uses.

Naishadham & Kadaba (1991), Naishadham & Chandrasekhar (1998) and Chandrasekhar &Naishadham (1998) reported the cumulative broadband (4-18 GHz) measurements andcomputations of all microwave parameters (e.g. conductivity, absorption, complexpermittivity, shielding and reection) of sulfonate doped PANI. It was found that the totalSE of -35 to -15 dB was obtained with return loss of -5 to -1 dB and nominal absorption of -5 dB for PA NI samples of conductivity 1-7 S/ cm. Authors also d emonstrated that better SEvalue upto -50 dB can be realized by stacking several po lymeric sheets of differentthicknesses or by sandwiching a lossy dielectric bet ween two sh eets of the sam e t hickness.

#igure $3% Thickness dependence of total shielding efficiency ( SE T) of various crosslinked polyaniline(XPANI-ES) samples. Sample I : highly XPANI-ES [3.5 t imes ( ) stretched, parallel (||)], sample I I: intermediate XPANI-ES [3. 5,||), sample I II: highly XPANI-ES (3.5, ), sample IV: highly XPANI-ES (unstretched), and sample V: non-XPANI-ES (12.5, ). Inset: comparison of mw , r, and tan .Reprinted with permission from [J. Joo and A. J. Epstein, Appl. Phys. Lett. 65 (18), 2278-2280, 1994]. Copyright [1994], American Institute o f Physics.

It has b een found that el ectrical conductivity is n ot the so le sci entic cri teria for ex hibitinghigh shielding effectiveness (Joo & Epstein, 1994) and good attenuations were al so extended


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by moderate conductors with good dielectric propeprevious sect ion, it ha s now been established that s hielding effectiveness i ncreases (asshown previously in Fig. 16) with absolute value of complex dielectric permittivity.Furthermore, absorption loss was f ound to be m ore sensi tive towards p ermittivity (inset Fig.16) than corresponding reection loss. Figures 23 and 24 show the microwave SE of pureICPs including PANI and PPY (in thin lm forms) as a f unction of t heir i ntrinsic properties(insets of Fi g. 23 and Fig. 24) such as m icrowave conductivity ( mw ) and dielectric co nstant( r) alongwith its d ependence o n extrinsic p arameters l ike t hickness ( t) and temperature (T).The rol e of parameters like degree of crosslinking and parallel ( || ) or perpendicular () stretch orientation which tends t o affect mw , r or l oss t angent (tan) has b een clear f romthe table d ata (above i nsets).

#igure $'% Thickness dependence of total shielding efficiency ( SE T) of highly con ducting polymers. Sample A : stretched heavily iodine d oped Tsukamoto p olyacetylene (dotted line is ob tained by using approximated and n), sample B: unstretched heavily iodine doped Tsukamoto polyacetylene, sample C: camphor s ulfonic aci d doped polyaniline in m-cresol solvent, sample D : PF 6 doped polypyrrole, and sample E : TsO doped polypyrrole. Inset: comparison of mw , r, and tan . Reprinted with permission from [J. Joo and A. J. Epstein, Appl. Phys. Lett. 65 (18), 2278-2280, 1994]. Copyright [1994], American Institute of P hysics.

It can be c oncluded that SE of PANI and PPY lms show weak temperature dependence.However, pronounced thickness effects were observed with attenuation level of -30 to -90dB depending on thickness and conductivity. Different types of shielding mechanisms i.e.reection, absorption and multiple reections were d iscussed and corresponding theoreticalequations w ere al so p resented. It has b een found that absolute v alue o f tan plays a criticalrole in determining the shi elding effectiveness. When tan >> 1 (e.g. for heavily doped andhighly conducting ICPs or m etals), shielding is solely determined by . However, whentan ~ 1, both and r must be considered when calculating absorption coefficient ( ) andcomplex index of refraction (n) which decide overall shielding effectiveness. Therefore, onecan expect higher shielding efficiency f or m aterials w ith higher and r.


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It has b een observed that magnetic properties al so p lay a vital role in improving shieldingresponse. Kathirgamanathan et al ( 1993) have demonstrated that PPY impregnationmicroporous membranes such as po lyurethane, polyethylene, poly(ethylene terephthalate)

(PET), poly(propylene) etc., showed higher SE (~ -10 to -50 dB) in the 10 k Hz to 1000 MHzfrequency range as c ompared to metal (e.g. Al) based membranes. The au thors po inted outthe h igher r elative magnetic p ermeability ( r>1) due t o the incorporation of paramagnetic Fe(III) during the syn thesis p rocess p rovided extra sh ielding by absorption as com pared withthe r~1 for aluminum. Furthermore, the microscopic orientation of ICPs is expected toimprove SE as showed by the fact that higher SE was exhibited by the PPY impregnatedpolyethylene membranes (-40 to -45 dB) than that of t he impregnated polyurethanemembranes ( -20 to -25 d B), despite t he m uch lowered thickness (1/5 th ) which was due to themore or iented PPY produced in polyethylene t han that in polyurethane.

#igure $(% Frequency d ependence o f losses d ue to reection (SE R) and absorption (SE A) for MWCNTloaded PANI nanocomposites having different loadings of MWCNT relative to aniline monomer vi z.PCNT0 (0.0 wt. %),PCNT5 (5.0 wt. %), PCNT10 (10 wt. %), PCNT20 (20 wt. %) and PCNT25 (25 wt. %).

Synergistic cou pling of llers can give unique co mbination of properties ( Saini et al 2009a)like enhanced conductivity, better dielectric/magnetic traits and improvedprocessability/thermal c onductivity that can not b e achieved by individual llers. T hisultimately gets res ulted in superior shi elding performance ( Fig. 25) so that r eection loss(SE R) increases sl ightly from -8.0 to -12.0 dB whereas ab sorption loss ( SE A) exhibited rapidenhancement from -18.5 to -28.0 dB with the increase in CNT loading. This m ay b e ascribedto increase i n the co nductivity (as w ell as cap acitive coupling effects) of composites leadingto proportional decrease in skin depth which may be helpful in designing thinner EMIshields. The i ncreased conductivity may manifest itself as i ncrease i n both long range chargetransport as w ell as n umber of po ssible re laxation modes, leading to enhanced ohmic losses.

The well-dispersed PANI NPs within insulating epoxy matrix provides continuousconducting networks with higher level of charge delocalization which leads to huge


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negative permittivity (Hsieh et al, 2012) which is a si gnature o f left handed materials ( LHM).The obser ved EMI SE in an electric eld at low frequency (1001000 MHz) range was foundto be - 30 t o -60 dB.

A straight forward solution for handling low conductivity and poor processability (oragglomeration tendency) of ICPs and CNTs respectively is combining these t wo llers incomposites. Saini et al ( 2009a) p repared polyaniline (PANI) coat ed multiwall carbonnanotubes ( MWCNTs) which inherit dielectric an d magnetic at tributes ( ferromagnetism dueto entrapped iron phase) from PANI and MWCNT respectively.

#igure $*% (a) Frequency d ependence of SE T and (b) variation of SE R and SE A with loading of PANI-

MWCNT. Inset shows the theoretical and experimental SE R and SE A value of the composite (PCNT30) in the 12.4 - 18.0 GHz frequency ban d.

This PANI-MWCNT hybrid ller was solution blended with polystyrene (PS) matrix (10-30wt % loading) resulting in absorption dominated total shielding effectiveness ( SE T) o f - 45.7dB (Fig. 26a) in the 12.418.0 GHz range an d at a sam ple thickness of ~2.0 m m. The SE T wasfound to exhibit strong dependence on shield thickness as w ell as loading level of hybridller (PANI-MWCNT).

The enhanced SE T was ascribed to optimization of con ductivity, skin-depth, complexpermittivity and permeability leading to nominal reection and high absorption (Fig. 26b).

A good agreement between t heoretical and experimental shielding measurements (inset ofFig. 26b & Fig. 27) was al so observed. Besides, role of highly reecting planes of PANI-MWCNTs separated with less conducting matrix regions was also explained to introducemultiple re ections r esulting in enhancement of absorption loss.

The above s tudies suggests that ICPs based n anocomposites may give SE value as high as -70 to -80 dB depending on nature of ICP, its loading level and presence of co-llers.However, high loadings (>30-40%) are required which leads to phase segregation andextreme disturbance of physical properties of host matrices and consequently poormechanical properties in most cases. Nevertheless, combination of st rategies l ike thin

lm/membrane technologies, porous structures, negative permittivity materials (orleft


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handed materials), multilayered structures and hybrid llers based on broad range of ICP-ller combination are exp ected to provide an effective solution to realize a lightweight,mechanically strong, processable and economically viable shielding material suitable for

commercial and defence sectors.

T #ig ure $+% Variation of (a) SE R as function of log 10 and (b) SE A as function of ( T ')2

'

for PANI-

MWCNT lled polystyrene composites

Recently, nanoscale materials based on 2-D graphene sheet s have at tracted much a ttentionrecently due to unusual properties ( Geim, 2009; Geim & Novoselov, 2007; Meyer et al, 2007) .Like CNTs, here ag ain it is exp ected that the u se of grap hene, with large asp ect ratio andhigh conductivity would provide a h igh EMI SE. Although many studies (In et al, 2010;Liang et al, 2009; Ramanathan et al, 2008; Stankovich et al, 2006; Varrla et al, 2011, Zhang etal, 2011) about the EMI shielding properties of graphene loaded insulating polymer matrixcomposite sy stems ar e available, Basavaraja et al (2011) presented the rst EMI shieldingresults on ICP/oxidized graphene based nanocomposites i.e. Polyaniline/gold-nanoparticles/graphene-oxide (PANI-GNP-GO) based composites in the 2.0-12.0 GHzfrequency range. According to authors, the SE values observed for GO and PANI-GNP andPANI-GNP-GO composites were in the ranges -20 to -33 dB, -45 to -69 dB and -90 to -120 dBrespectively. However, considering the fact that GO is a p oor conductor, conclusion fromthe p resented thickness dep endences of above com posites and from our own experience, theresults seem to be f ar f rom realistic. Nevertheless, the graphene n anocomposites res earch isstill at very ea rly stage o f evolution especially from the v iew point of EMI shielding materialdevelopment.

For many applications e. g. radar ab sorbers or s tealth technology, the sam ple sh ould reectas low energy as p ossible. However, conducting ller loaded composites gi ves si gnicantreection (primary shielding mechanism) alongwith absorption which is secondary EMIshielding mechanism. For reduction of reection loss an d signicant absorption of the

1


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radiation, the sh ield should have electric an d/or m agnetic d ipoles w hich interact with theelectromagnetic elds in the incident radiation. Therefore, numerous at tempts have also

been made to introduce dielectri 3, TiO 2 etc.) or m agnetic ( -Fe 2O3, Fe 3O4, BaFe 12 O19etc.) materials w ithin various ICP matrices as lled inclusions ( Abbas et al, 2005, 2007; Chan,1999; Chandrasekhar, 1999; Cho & Kim, 1999; Dong et al, 2008; Ellis, 1986; Gairola et al, 2010;Huang, 1990; Knott et al, 1993; Kurlyandskaya et al, 2007; Meshram et al, 2004; Nalwa, 1997;Ngoma et al, 1990; Pant et al, 2006; Phang et al, 2007, 2008, 2009, 2010; Xiaoling et al, 2006;Xu et al, 2007; Yang et al, 2010, 2011). It has b een observe d that thickness i s an extrinsicparameter that can be ad justed to reg ulate the shi elding offered by a sh ielding with givenpermittivity or permeability which can be tuned by nature and concentration of ller. Anoptimized dielectric p articulates lled composite sam ple b ased on BaTiO 3 and polyaniline i npolyurethane matrix (Abbas et al 2005) exhibited a maximum reection loss of -15 dB (>99%power absorption) at 10 GHz with a bandwidth of 3.0 GHz for a 2.98 mm thick sample.Again the role of thickness and dielectric attributes to modulate absorption wasdemonstrated by theoretical calculations an d experimental results. Similarly, they have alsoprepared polyaniline-BaTiO 3-carbon based com posites (Abbas et al 2006) with maximumreection loss of -25 dB (2.5 mm thick sample) at 11.2 GHz and bandwidth of 2.7 GHz. Manyattempts w ere al so made to introduce m agnetic losses i nto the sys tem for exam ple, Yang etal (2009) produced PANI-Fe 3O4 composites w ith reection loss of -2 dB at 14.6 GHz for 3mm thick sample. Gairola and coworkers (2010) prepared PANI with Mn 0.2 Ni 0.4 Zn 0.4 Fe 2O4ferrite nanocomposites by mechanical blending with absorption loss of -49.2 dB in the 8.2-12.4 GHz r ange. Dong et al (2008) synthesized PANI-Ni core shel l composites w ith reectionloss of less than -10 dB in the 4.218 GHz range. Phang and coworkers (2009) formulatedPANI-HA based nanocomposites containing TiO 2 and Fe 3O4 nanoparticles a s d ielectric ller

and magnetic ller, respectively. The resultant composites show good microwaveabsorption response w ith attenuation of -48.9 dB. Phang et al (2007, 2008) produced PANInanocomposites con taining combination of dielectric (TiO 2) and conducting (CNTs) llerspossessing m oderate con ductivity and dielectric prop erty w ith maximum reection loss of -31 dB (for PANI-TiO 2) at 10 GHz and -21.7 dB (PANI-TiO 2-CNT) at 6GHz. In abovecomposites, use of conducting llers such as CNTs is expected to improve thermalconductivity (e.g. 0.19 W/mK for PANI and 0.3-0.6 W/mK for PANI-CNT composites)

besides extending enhanced shielding performance. Such an provement conductivity is b enecial for f ast dissipation of heat w hich is ge nerated due t o interaction ofshield with high frequency (GHz r ange) microwave r adiations.

4. Conclusions

Although, much w ork h as been d one to introduce electrical conductivity in various p olymermatrices but high percolation threshold and lower aspect ratios o f ICPs compared to metalsor carbon based llers remained a ch allenging issue. Therefore, considerable work is stillneeded to improve further the SE as well as mechanical properties of conducting polymer

based composites. Synthesis of hybrid lconducting polymers, carbon based materials an d dielectric/magnetic n anoparticles seemto be a possible so lution. Nevertheless, in the light of current scenario it may be st ated

that


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there i s a l ot be d one t o attain a sh ielding material that can satisfy all the t echno commercialspecication and maintain the p rocess economics at the same time.

Author d etailsParveen Saini and Manju AroraNational Physical Laboratory, New Delhi, India

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Bredas, J.L., Cornil, K., Meyers, F. and Beljonne, D.1998, Handbook of Conducting Polymers,Ed.: Skotheim, T.A., Elsenbaumer, R.L., Reynolds, J.R. and Dekker, M. Inc., New York.Cao, Y., Smith, P. and Heeger, A.J. 1992, Synth. Met., 48, 91.Cao, Y., Qiu, J., Smith, P. 1995, Synth. Met. , 69, 187.Carter, G.M., Thakur, M.K., Chen, Y.J. and Hryniewicz, J.V. 1985, Appl. Phys. Lett ., 47, 457.Chan, H.L.W., Cheung, M.C. and Choy, C.L. 1999, Ferroelec. , 224, 113.Chandrasekhar, P. 1999, Conducting Polymers: fundamental and Applications, A

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