Polymer Nanocomposites for Electromagnetic Interference Shielding

Review Journal of Nanoscience and Nanotechnology

Copyright © 2018 American Scientific Publishers All rights reserved Printed in the United States of America

Vol. 18, 7641–7669, 2018 www.aspbs.com/jnn

Polymer Nanocomposites for Electromagnetic Interference Shielding: A Review Sayan Ganguly, Poushali Bhawal, Revathy Ravindren, and Narayan Chandra Das∗ Rubber Technology Centre, Indian Institute of Technology, Kharagpur 721302, India This review corresponds to particulate conducting material assisted polymer composites bearing a unique aspect of self-sensing application of electromagnetic interference (EMI) shielding or radiation shielding. High performance and cost-effective materials in various forms of particulate composites have been developed for such applications to inhibit unwanted radiation. With a minimum concentration of conducting particles either inorganic or organic or an effective combination of both, viz. the hybrid inorganic-organic system enhances the radiation shielding features with ultralow percolation concentration (approximately 0.005–0.1 vol%), superior electrical conductivity (up to 106 S/m). This article consists of carbonaceous particulate materials embedded nanocomposites, conducting polymer assisted nanocomposites and rubber matrix based nanocomposites to achieve the suitable radiation shielding phenomenon. The dislodging of stratified carbonaceous materials like graphene, into the polymer matrix helps the composite better mechanical strength as well as better electrical conductivity in relatively easy fabrication methods. The electromagnetic interference shielding IP:in46.243.173.199 On: Thu,in02 2018 09:57:09 effectiveness in dB unit the X-band also reported theAug article to tally the particulate segregated Copyright:inAmerican Scientific conducting polymer nanocomposites the different aspects.Publishers The hypothesis behind those formuDelivered by Ingenta lations effectively establishes the efficient development of conducting nanocomposites for shielding of radiation pollution in the X-band.

Keywords: Conductive Polymer, Polymer Nanocomposites, Particulate Segregated Conducting Polymer, Carbon Materials, Carbonaceous Particulate Materials, Self-Sensing, Radiation Shielding.

CONTENTS 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Shielding Phenomenon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Modeling and Structural Elucidation of Shielding Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Theory of EMI Shielding . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Shielding Mechanisms for Lossy Materials . . . . . . . . . . . . 3. Preparation Techniques of Polymer Nanocomposites . . . . . . . . . 3.1. In Situ Polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Solution Compounding . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Melt Blending . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Recent Developments in Carbonaceous Reinforcements . . . . . . 4.1. Generic Stratified Carbonaceous Materials Assisted Polar Matrix Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Conducting Polymer Based EMI Shielding Materials . . . . 4.3. Shape Memory Polymer Based Radiation Shielding Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Conclusion and Future Perspective . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ∗

Author to whom correspondence should be addressed.

J. Nanosci. Nanotechnol. 2018, Vol. 18, No. 11

1. INTRODUCTION 7641 7643 7644 7645 7646 7648 7648 7648 7649 7649 7649 7652 7661 7663 7663 7663

Polymer based composites were first invented in 1960, which was a new archetype for the materials. Dispersion of strong, highly stiff fibers in the polymer matrix results the high-performance, lightweight composites.1 2 Today we are standing in a new era of polymer nanocomposites, with the promise of strong, durable, multi-functional materials with minimum nanofiller contents. Nanocomposites are the most interesting emerging materials in this century. Because of their nanometer sizes, filler dispersed nanocomposites exhibit markedly improved properties when compared to the pure polymers or their traditional composites. Advancements in these disciplines depend largely on the ability to synthesize nanoparticles of various materials, sizes and shapes, as well as to assemble them efficiently into complex architectures.3 4 Electromagnetic interference (EMI) shielding is the most detrimental by-product of explosive growth of electronics, telecommunications instrumentation and widespread use of transient power sources.

1533-4880/2018/18/7641/029

doi:10.1166/jnn.2018.15828

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Polymer Nanocomposites for Electromagnetic Interference Shielding: A Review

Electromagnetic radiation has harmful effect, which not only tries to disturb the normal operation of appliances but may also adversely affect human health. Over the past two decades a continuous effects have been done to reduce EMI using a number of variety of materials including metals, carbon based materials, conducting polymers, dielectric/magnetic materials.5–15 Metal based materials always show better EMI shielding effectiveness but their physical features limits their applications with respect to polymers.

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Polymers are lightweight, corrosion resistant, flexible, cost effective and offer easy fabrication.16 17 They also have unparalleled combination of electrical, thermal, dielectric, magnetic and physico-mechanical belongings which are desirable for mitigating the electromagnetic noises. Therefore, numerous efforts have also been attained to apply the good properties of aforementioned materials. Herein we discussed a brief overview of fundamentals of EMI and microwave absorption. Theoretically the shielding has also

Sayan Ganguly obtained his B.Sc. degree in Chemistry (honours) in 2009 at Ramakrishna Mission Vidyamandira, Belur Math, University of Calcutta; and then his post-graduation BTech in 2012 (University of Calcutta). He obtained M.Tech. degree in polymer science and technology in 2014 from the University of Calcutta. Thereafter he joined to Professor Narayan Chandra Das research group in Indian Institute of Technology (IIT), Kharagpur, India. His primary research interests include superabsorbent hydrogels including their fluorescence, nanomaterials and biomedical applications.

Poushali Bhawal completed B.Sc. in Chemistry from Bidhan Nagar College, University of Calcutta in 2006. She received B.Tech. degree (2009) in Polymer Science and Technology from Calcutta University. She worked as an Assistant Manager of Quality Control department of Phillips Carbon Black Limited from 2009 to 2011. She pursued M.Tech. degree in Rubber Technology at the Rubber Technology Centre of IIT Kharagpur in 2013 IP: 46.243.173.199 On: Thu, Aug 2018 09:57:09 where he is currently working as 02 a research fellow. Copyright: American Scientific Publishers Delivered by Ingenta

Revathy Ravindren is her pursuing Ph.D. degree at Rubber Technology Centre of Indian Institute of Technology (IIT), Kharagpur under the supervision of Professor Narayan Chandra Das. She received B.Tech. degree in Polymer Science and Engineering from Cochin University of Science and Technology in Kerala. She obtained M.Tech. degree in Polymer Science and Technology from Mahatma Gandhi University, Kerala. Her research interest includes conductive polymer composites for EMI shielding applications.

Narayan Chandra Das received his B.Sc. degree in Chemistry (honours) from Bagnan College, University of Calcutta in 1992 and B.Tech. degree in Polymer Science and Technology from Calcutta University (1996). Subsequently, he completed his M.Tech. degree (1998) in Rubber Technology and Ph.D. (2002) in polymer science from Indian Institute of Technology (IIT), Kharagpur, India. He carried out his post-doctoral research at Hiroshima University in Japan and Michigan Technological University in USA. He worked as a Research Associate at SUNY Binghamton in USA. He also worked as a Research Professor at Indiana University, Bloomington, USA. Currently, he is an Associate Professor at Rubber Technology Centre of IIT Kharagpur. His research interests include nanomaterials, thermoplastics elastomers, polymer nanocomposites, rheology and processing of rubber, drug delivery, biomaterials, carbon dots, membrane for water purification, food packaging, SAXS and SANS. 7642

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been discussed associated with governing equations, several methods for evaluation of shielding effectiveness (SE).

2. SHIELDING PHENOMENON An electromagnetic interference (EMI) shielding material is a material that attenuates radiated electromagnetic energy. EM radiation can be divided into two subclasses; one is near field and another one is far field regions. In near field, the EM signal can be predominately an electric or magnetic vector depending upon the nature of the incident radiation. In case of far field, plane waves exist in which the electric and magnetic vectors lie in an equal ratio and they are in phase and orthogonal to each other. The plane radiation (far field) is the most concern in measuring shielding effectiveness (SE). EMI is specified as specious voltage and current stimulated in the electronics Figure 1. Schematic representation of EMI shielding mechanism. circuit by means of any external sources. The main activity of an EMI shielding is to produce a blockage made of electrically conductive materials that attenuates radiated or are generally done by electroplating. Electroless plating or conducted EM energy through reflections and absorption. vacuum deposition is commonly used for shielding.21 22 The mechanism of shielding effectiveness (SE) is shown in The coating is done on bulk materials, fibers or particles. Figure 1, where the incident energy is attenuated by shieldCoatings tend to suffer from their poor wear or scratch ing material through reflections, like direct (R) and mulresistance. tiple (B) reflections or 2nd reflection, and absorption (A) The reflection loss (SER basically depends on the basis (Fig. 1). SE is typically a measurement of an attenuation of of relative mismatch between the incident wave and the the EM signal after a shield is inserted. Therefore, attenuasurface impedance of the shield. The reflection losses can tion is a measure of the reductionIP: of 46.243.173.199 intensity of EM field. estimated by calculating On: Thu,be02 Aug 2018 09:57:09 shielding effectiveness for inciCopyright: American Publishers Here we are showing the various electromagnetic termi- Scientific dent electric fields. The magnitude of reflection loss for the Delivered bythree Ingenta nologies which are helpful to understand the mechanism principle fields can be given by following equation.16 of electromagnetic interference. The EM radiation blocking efficiency of a shield is meaSER = C + 10 log10 r /r 1/f n r m (2) sured in terms of a logarithmic quantity called shielding where, → relative conductivity of copper, f → effectiveness that can be expressed as:10 18–20 frequency in Hz, → relative permeability of free space, SET dB = SER + SEA + SEM r → distance fromsource to shielding in meter, C, n and m are constants for calculating reflection losses = 10 log10 Pt /Pi = 20 log10 Et /Ei (in dB) for plane waves, electricfields, and magnetic fields, (1) = 20 log10 Ht /Hi respectively. So the reflection loss (SER for a plane wave radiation where Pi (Ei or Hi and Pt (Et or Ht are the power (eleccan be expressed as: tric or magnetic field intensity) of incident and transmitted EM waves respectively. In actual practice, three different SER = −10 log 10/16 r (3) phenomena named reflection (SER , absorption (SEA and The above expression are showing that SER is a function multiple reflections (SEM contribute towards SET , which of the ratio of conductivity () and permeability () of are shown in the above Figure 1. Primarily the electroshield material i.e., quantity (/). magnetic interference shielding is depending on the reflecAnother method of EMI shielding is absorption mechation mechanism. To continue the reflection the shield must nism. The presence of electric and magnetic dipoles in the have some mobile charge carriers (electrons and holes) shield can help in absorption of the radiation by the shield. which can interact with the electromagnetic field in the These dipoles interact with the electromagnetic fields in radiation. So the shield should be electrically conducting. the radiation. Materials with high value of the dielectric The conductivity should not be very high, since electriconstant, e.g., BaTiO3 provide the electric dipoles and the cal conductivity is not the criterion of shielding. A typical magnetic dipoles may be provided by Fe3 O4 or other havvolume resistivity of 1 cm is needed. Metals are generally used as the conducting material due to the presence of ing a high value of the magnetic permeability.23 The magfree electrons which can help in reflection. Metal coatings netic permeability enhances by reducing the number of J. Nanosci. Nanotechnol. 18, 7641–7669, 2018

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magnetic domain walls through the use of a multilayer of magnetic films.24 25 Absorption loss (SEA is a function of the product r r , which is physical characteristics of shield and independent of the type of source field. Therefore, the term SEA is same for all three waves. When an electromagnetic wave pass through a medium its amplitude decreases exponentially due to induction of currents in the medium which produce ohmic losses and heating of the material. The distance required by the wave to be attenuated to 37% is defined as the skin depth ( ). Therefore, the magnitude of absorption term (SEA in decibel (dB) can be expressed by following equation: (4) SEA = 334t fr r

of electronic gadgets necessitate alteration of metal by conducting polymers and composites in order to the perspective of anticorrosion, lasting time span, ease of fabrication, ease to replacement, surprisingly low specific gravity with respect to other metals and very good compatibility after incorporation to develop a particulate conducting nanocomposites.26–29 Carbon fiber laminate woven materials are commonly used for shielding purposes.30 31 Composite dielectric materials that baring indiscriminately distributed conductive filler addition, including carbon fibers, at a small concentrations can be applied for engulfing EM waves at radio frequency (RF) and microwave frequencies (MF).32 33 To evaluate electromagnetic features of composite systems, it is crucial to investigate the EM variables of a host material (either matrix or dispersed syswhere t is shield thickness in inches and f is frequency tem or base) and conducting particulate segregated strucin Hertz. The parameters r and r represent permeability tural systems. There are many efficient media theories and conductivity respectively relative to copper. From the (EMT) providing homogenization of composite media.34–39 above equation it is clear that reflection loss minifies with The Maxwell Garnett (MG) model is the simplest and raising frequency, whereas the absorption loss goes proporthe most well-known model to use. This model is effectional to the frequency. Silver, copper, gold and aluminum tively applicable for multiphase systems, and grants freare excellent for reflection because of their high conductivquency features being represented in the figure of rational ity. Superpermalloy and Mumetal are superior for absorpfractional functions convenient to incorporate in numerition due to their high magnetic permeability. Other than cal time domain EM codes.29 Relative permittivity of the reflection and absorption, another important mechanism of matrix phase and foreign particulate matters as conducting shielding is multiple reflections. This involves the reflecfiller can be complex functions of frequency. The Maxwell tions at various surfaces or interfaces in the shield. The Garnett multiphase formula for composites having particpresence of a large surface area or interface area in the IP: 46.243.173.199 02 Aug 2018 09:57:09 conducting inclusions below the percolation threshshield helps in multiple reflection. Porous or foamOn: are Thu,ulate Copyright: American Scientific Publishers old is good example of shielding materials having a large surface Delivered by Ingenta n 3 area. An example of a shield with a large interface area b 1 vi i − b eff = b + is a filler loaded composite material. The multiple reflec3 i=1 + Nik i − b k=1 b tion loss can be neglected when the inter-distance between n 3 the reflecting surfaces or interfaces is large with respect b 1 v − b 1− to the skin depth. The losses are usually conveyed in dB. 3 i=1 i i + Nik i − b k=1 b The sum of all three losses is called total shielding effec(6) tiveness (SET in dB). In case of thin shielding we can’t neglect the reflection where b is the relative permittivity of the dielectric matrix off of the first boundary, whereas in case of thick shieldphase; i is the relative permittivity of the i-th type of paring this can be neglected due to high absorption loss. The ticulate inclusions; vi is the volume fraction occupied by loss due to multiple reflection can be expressed by the the conducting filler or inclusions of the i-th type; Nik is 10 following equation. the depolarization factor of the i-th type of foreign inclusions, and the indices k = 1, 2, 3 correspond to x, y, and SEM = 20 log10 1 − e−2t/ (5) z Cartesian coordinates.40 41 whenever SEA is ≥10 dB, SEM can be safely neglected. In case of composite materials assisted with the conIn practical calculation, SEM can also be neglected for ductive inclusions around the percolation threshold, the electric fields and plane waves. effective permittivity can be incurred from the McLachlan’s effective medium theory.42 McLachlan’s equation is 2.1. Modeling and Structural Elucidation of suitable for the tracing out the effective parameters of the Shielding Composites mixture close to or above the percolation threshold is Electromagnetic shielding is vehemently practiced to slen1/s 1/s 1/s 1/s vi i − eff 1 − vi b − eff derize emissions or improve in susceptibility or last+ =0 1/s 1/s ing power of electronic gadgets. External coverings of 1/s 1/t i + 1 − c / c eff b + 1 − c / c eff (7) electronic sophisticated machineries are made of highlywhere s and t stand for the process exponents. The ratio conducting metal to attain the needed shielding. Nevof these exponents (s/t influences the balance of the real ertheless, demands to cut down the size and weight 7644


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and imaginary part of the permittivity around the percolation threshold pc .43 Different polymer materials, such as Teflon or other thermoplasts, can be used as the base or host material for design of radiation shields for microwave frequencies (range 100 MHz–10 GHz). Teflon is almost non-dispersive in the frequency range of interest, and its loss factor can be neglected.44

Table I. Constants for predicting SER under plane wave, magnetic and electric fields.57 Types of field

C

n

M

Electric Plane wave Magnetic wave

322 168 146

3 1 −1

2 0 −2

enhanced by connectivity via the conducting particulate 2.2. Theory of EMI Shielding materials. Metals are by far the most common materials for The mechanism of electromagnetic interference (EMI) EMI shielding but metals have several disadvantages viz. shielding is a combination of reflection and adsorption corrosion problem, high specific gravity which comprises in a consecutive way by such a material, which poswith high mass to volume ratio. They works principally sesses sufficient conductivity. As electromagnetic radiaby reflection on account of the free mobile electrons in tion, especially that at high frequencies (i.e., radio waves, them.8 53 54 waves emanating from cellular phones), generally interThe reflection loss (SER is the phenomena related to the feres with electronic devices (computers). EMI shielding comparative mismatch between the incident wave and the of both electronics and the radiation source is demanded surface impedance of the shield material. The figuring of and is progressively necessitated by almost all governreflection losses modified by considering SE for incident ments across the whole world. The significance of EMI electric fields as a big problem to handle generated from shielding have high requirement on the dependability of electromagnetic waves. The magnitude of reflection loss electronic gadgets.18 45–52 On the basis of the reflection can be given by following generalized expressions.55 56 and absorption of electromagnetic waves the mechanism of shielding is normally divided into three typical phe1 r SE = C + 10 log · (8) R 10 nomenological views. r f m r m On the verge of the starting of shielding effiwhere is relative conductivity; is the relative permeciency analysis, it is essential to understand the various ability referred to free space; f is the frequency in Hz; electromagnetic terminologies. If the distance between the IP: 46.243.173.199 On: Thu,r 02 Aug 2018 09:57:09 is the inter distance between source and shield in meter. radiating source and the observation Copyright: point is x, an elec- Scientific American Publishers The values of constants C, n and m are listed in Table I tromagnetic radiation affected region normally split into by Ingenta Delivered for computing reflection losses (in dB) for electric fields, three parts relative to the entire wavelength of electroplane waves, and magnetic fields, respectively. magnetic wave. The region in the range where r < /2 Hence the reflection loss (SER for a plane wave radiais the named as near field while the distance r > /2 tion it can be conveyed as: is assigned as the far field. To design a shielding mate rial for exceptional shielding application, it is in attention (9) = −10 log SE both intrinsic and extrinsic parameters on which shielding R 10 16 r effectiveness depend and several theoretical and empirical relations associated with the above parameters with SER is a function of the ratio of DC conductivity (DC contributing reflection, absorption and multiple reflection to permeability ( of shield composite material i.e., the components which are discussed here. quantity (DC /). 2.2.1. Primary Mechanism of EMI Shielding: Reflection The basic mechanism of EMI shielding is commonly reflection. In case of reflection of the incident radiation by the material, the primary thing is that its mobile charge carriers either in form of electrons or in form of holes to interact with the electromagnetic fields in the radiation when exposed. As a result, the shield inclines to be electrically conducting, but it is noteworthy that very high conductibility is not mandatory issue. Electrical conductivity is not the technological criterion for EMI shielding as per various hypothesizes; as conduction demands a continuous connectivity in the conduction path (percolation in the cause of a composite material containing a conductive filler), whereas shielding does not follow that rule. Although shielding does not necessitate connectivity, it is J. Nanosci. Nanotechnol. 18, 7641–7669, 2018

2.2.2. Secondary Mechanisms: Absorption Absorption is attributed as the secondary mechanism of EMI shielding. Substantial absorption of the unwanted radiation by the shield, the shield should have electric and/or magnetic dipoles as their primary requirements, which interact with the incident electromagnetic propagating wave fronts in the radiation. The electric dipoles may be rendered by some materials having a high value of the dielectric constant and the magnetic dipoles may be allowed for a high value of the magnetic permeability, which may be heightened by cutting the number of magnetic boundary by using multilayer magnetic films.58 The absorption loss is a function of the dot product of r and r i.e., r r , whereas the reflection loss is a function of the ratio r /r , where r is the electrical conductivity 7645

Polymer Nanocomposites for Electromagnetic Interference Shielding: A Review Table II. Electrical conductivity relative to copper (r and relative magnetic permeability (r of selected materials.60 Materials Silver Copper Gold Brass Bronze Tin Lead Nickel Stainless steel (430) Mumetal (at 1 kHz) Superpermalloy (at 1 kHz)

r

r

r r

r /r

105 1 07 061 018 015 008 02 002 003 003

1 1 1 1 1 1 1 100 500 20,000 100,000

1.05 1 0.7 0.61 0.18 0.15 0.08 20 10 600 3,000

1.05 1 0.7 0.61 0.18 0.15 0.08 2 × 10−3 4 × 10−5 15 × 0−6 3 × 10−7

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This expression unveiled that SEA can be assumed as directly proportional to the square root of product of permeability () times conductivity () of shield material i.e., quantity ( · 1/2 . Hence, a good absorbing material should own high conductivity and high permeability and adequate thickness to achieve the required number of skin depths at the lowest frequency of pertain.

2.2.3. Multiple Reflections Besides reflection and absorption phenomena of shielding mechanisms, another mechanism of shielding is multiple reflections, which is a basically consecutive reflection at respective surfaces/interfaces in the shield. Such mechanism commands the presence of a reasonably large surface area or interacting interface area in the shield. Porous with respect to metallic copper and r is the relative magtype materials or foam having extremely enormous internetic permeability. Table II depicts the factors stated above acting surface area is also used in shielding applicafor assorted materials. Silver, copper, gold, and aluminum tion. The loss due to multiple reflections can be omitted show excellent reflection behavior, due to their high intrinafter assuming the distance between the reflecting sursic conductivity. Superpermalloy and mumetal are studfaces/interfaces is prominent compared to the skin depth ied with their excellent absorption, due to their enormous of the shielding materials. The losses, whether due to magnetic permeability. The reflection loss is inversely proreflection, absorption, or multiple reflections, are usually portional with the frequency, whereas the absorption loss expressed in decibel (dB). The absorption loss is generfollows the directly proportional to the working frequency. ally directly proportional to the thickness of the shield. As shown in Figure 1, at the time of penetrating of elecElectromagnetic radiation at higher frequencies infiltrates tromagnetic wave through a medium it shows exponential the near surface region of an electrical conductor which decay pathways. This decay or absorption loss happens is termed as the skin effect effectively depends on critical because currents induced in the medium give rise to Ohmic IP: 46.243.173.199 On: Thu,skin 02 Aug depth2018 of the09:57:09 shielding materials. The applied electric losses and subsequent heating of the Copyright: material andAmerican Et and Scientific Publishers 59 field of an incident plane wave penetrating a conductor Ht can be expressed as Delivered by Ingenta drops exponentially with increasing depth into the conductor. The depth at which the field drops to 1/e of the initial Et = Ei e−t/

(10) value is called the skin depth ( ), which is given by and 1 (11) Ht = Hi e−t/

(15)

= √ f It is also reported that the distance demanded by the where f = frequency, = magnetic permeability = 0 r , wave to be attenuated to 1/e or 37% is specified as the r = relative magnetic permeability, 0 = 4 × 10−7 H/m, skin depth ( ) which can be expressed as: and = electrical conductivity in −1 m−1 . 1 The factor SEM can be mathematically conveyed as76 (12)

= √ f (16) SEM = 20 log10 1 − e−2t/

Therefore, the magnitude of absorption term (SEA ) Thus, when SEA is ≥10 dB, SEM can be safely expressed in decibel (dB) can be conveyed by following neglected. In practical calculation, SEM can also be generalized equation: neglected for electric fields and plane waves. The expres (13) SEA = 334t f r r sions (1) to (5) depict the correlation between theoretical shielding and electromagnetic attributes (permeability and where t denotes the shield thickness in inches and f is permittivity) and revealed that for any given sample, SER the frequency in Hertz (Hz). The parameters ‘r ’ and ‘r ’ value decreases whereas SEA increases with increase in interpret conductivity and permeability respectively comfrequency. parative to metallic copper. The rearrangement of the equations stated above gives following expression for SEA in 2.3. Shielding Mechanisms for Lossy Materials decibel (dB): In general the ongoing development and modification t t of shielding effectiveness equations are based on some SEA = −20 log10 e = −868 = −868t f assumptions because normally SE is calculated by assum

(14) ing the material as a highly conductive one but in case of 7646


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rubber polymer composites it is not up to the metallic conductivity domains. Although such type of conductive materials bearing moderate conductivities, they can even render substantial degrees of shielding of electromagnetic waves. Several composite materials with moderate conductivities are not reproducible with the presumptions used to derive those equations related to the lossy materials like rubber composites. Investigating of the shielding mechanisms for a moderately lossy material can be workable for analyzing these composite materials. The equation already evoked by Pozar where he explains and elucidated the mathematical derivations to achieve the calculation to measure shielding effectiveness particularly for the lossy materials i.e., rubber which are already terminologically named as ‘captive liquid’ due to its inherent Brownian motion and slippage of the rubber long chains. Here in this review paper we referred some mathematical implementations to establish the shielding effectiveness for lossy materials. Here the equations presented are normally divided into two primary components: one is reflection loss and another one is the absorption loss. In case of linear and isotropic lossy type materials the alternating current (AC) electrical wave propagation has two basic components: (a) the free electron/hole transport obeying the complex conductivity which is justified by the equation,

The inherent impedance of a material, , is,

= − j + /

(22)

Where is the magnetic permeability if the composite material. For a viscoelastic lossy material viz. rubbery system, the expression related to the attenuation constant, as a function of the electrical, magnetic losses and dielectric losses which are reported elsewhere by Paul.9 The propagation constant, can be expressed as = j + j

(23)

If the Eq. (23) is combined with the complex permittivity, we get 2 = j + j − j = j + + j (24) The propagation constant is also defined in terms of its complex components, i.e., the attenuation, , and phase constant, ; 2 = j + j = + j2

(25)

After expansion of Equation number (25), we get + j2 = 2 + j2 − 2 = 2 − 2 + j2

(26)

Or, = − j (17) 2 IP: 46.243.173.199 On: Thu, 02 Aug −2018 2 +09:57:09 j2 = j + − 2 (27) Copyright: American And (b) the bound electron dielectric displacement implied Scientific Publishers Delivered Now it can be compared in simultaneous Equation numin the permittivity which is expressed in Equation num- by Ingenta ber (27), we can get, ber (18), (18) = − j (28) 2 = + Where and are the real and imaginary part of the complex conductivity respectively and j = −11/2 .61 The real component is colligated to the phase of polarization and the as an imaginary component is the result of the losses consorted with the dielectric damping i.e., the bound electrons in the dipoles experience fluctuation of electric field at an angular frequency designated as . Thus, the current density, J , determined by an electric field, E, will be J = j − j − j + E (19) However, the imaginary conductivity, , is usually negligible at frequencies beneath 300 GHz, so it can be assumed that (20) = DC = Hence it can be resulted as,

E J = j − j +

(21)

Where the term + / linearly depends on the total effective conductivity.62 J. Nanosci. Nanotechnol. 18, 7641–7669, 2018

And

2 − 2 = −2

(29)

Now by solving for from Eq. (28), + = 2

(30)

If the value of is substituted in Equation number (29), the result will be, 2 + 2 − = −2 2 Or,

+ + − 2 4

2

2

2 =0

(30a)

Now in order to solve the equation of (30a) for 2 , quadratic formula will be, 2 −2 + 2 2 + 4 + /2 2 = 2 Or, −2 + 2 2 + + 2 2 = 2 7647


Or, 2 = −

2 =

nanofillers (here graphitic filler) in the monomer or oligomer during polymerization. As a result of which there will be a chance of stronger interaction between the reinforcing filler and the polymeric matrix. Thus, composites prepared by the in situ polymerization technique should exhibit better mechanical properties than those made by the solution compounding or melt blending techniques. The main disadvantage of this technique is the consumption of lot of electrical energy to disperse the filler in the polymer matrix, so this method is not fruitful for the mass production.

2 1 + 2 2 + + 2 2 2

Or,

⎡

⎣ +1 + 2 2

1+

/ +

2

Hence the value of will be, ⎤ ⎡ 2 / + ⎣ = 1+ − 1⎦ 2

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⎤ ⎦

3.2. Solution Compounding This method is based on a solvent system, where the polymer is dissolved in a solvent and filler is dispersed in the resulting solution. This allows the fillers to be finely 3. PREPARATION TECHNIQUES OF dispersed in the polymer matrix.63–70 Here shear force POLYMER NANOCOMPOSITES induced in the polymer-filler solution is much lower than Nanocomposites can be prepared by the proper dispersion that applied induced in the kneaded polymer melt. Preof the nano-fillers in the polymeric matrix. Generally three dispersion of nanofillers in the solution is achieved by an blending methods are used to synthesize nano-composites; external force such as ultrasonic waves. With the aid of they are (i) in situ polymerization, (ii) solution compoundthis ultrasonic wave the polymer-filler can be easily loaded ing and (iii) melt blending. All these methods have some with very low viscosity than a polymer melt, which has advantages and some disadvantages. an advantage over the melt blending method. After mixing the solvent is removed by evaporation and the matrix is 3.1. In Situ Polymerization molded to give the appropriate shape. Here surface modIn situ polymerization is the most efficient method among ification of the filler can also be done without drying. IP: 46.243.173.199 On: Thu, 02 Aug 2018 09:57:09 the other three methods. In this method we added the The electrical conducting composites prepared by solution Copyright: American Scientific Publishers Delivered by Ingenta (31)

Figure 2. Diagrammatic illustration of in situ reduction of GO in presence of EMA. Reprinted with permission from [81], P. Bhawal, et al., Superior electromagnetic interference shielding effectiveness and electro-mechanical properties of EMA-IRGO nanocomposites through the in-situ reduction of GO from melt blended EMA-GO composites. Composites Part B 134, 46 (2018). © 2018, Elsevier.

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compounding method has low percolation threshold. However, there is a major problem which restricts this method for large scale of production,71–78 e.g., solvent hazards and removal of solvent from the composites and also the limitation of use of polymer matrix.77

also the frequency of the radiation. It is also hypothesized that with increasing multilayer graphene content enhances the average electrical conductivity invariably in thickness with the desirable optimum flexibility.85 In case of polar polymer systems another work is proposed by Zhang et al. about immobilization of pristine graphene layers by PET via melt compounding at 285 C using a Brabender mixer 3.3. Melt Blending at 50 rpm/min for 4 min followed by compression moldMelt blending is the most promising and practical method ing. The thermally triggered exfoliation of GO plays a to be used in industry. Melt blending is a direct, cost significant role when incorporated into PET as a “shieldeffective and environmental friendly process. Here solvents ing enrichment” material. Unadulterated graphene itself are not required and traditional mixing devices such as does not occur naturally, but bulk and solution processextruder, internal mixer, and two-roll mill are used.77–80 able functionalized graphene (SPFG) can now be preIn the case of melt blending, the percolation threshold pared through rigorous chemical sniping of graphite intact is comparatively high than the in situ polymerization or crystals to introduce oxygen containing defects in the solution compounding. Unlike the studies involving ISP graphite stack, followed by complete dislodging of sheets and SC techniques, here, a fine balance has often been of atomic-thickness through either thermal or mechanistruck between mechanical and electrical properties, which cal treatments.86 87 The graphene/epoxy composites were could lead to faster realization into commercial products. developed using an in situ operation by Jiajie Liang et al. The comparative report of the three processing methodoloIn this very work graphite oxide (GO), prepared by the gies cannot be prepared, because exactly similar polymeric 71 modified Hummers method88 from graphite as raw matenanocomposites have not often been studied. Another rial, was first entirely exfoliated to SPFG sheets in H2 O newly adopted fabrication method also has been reported via sonication to form a well dispersed of SPFG/H2 O by Bhawal et al. where GO has been used as a filler which solution. Hydrazine treated GO which is well known as turned into RGO during high temperature melt mixing. reduced GO (RGO) is then incorporated into epoxy sysThis was a plausible approach to prepare in situ reduced tems with hardener followed by immense stirring in order graphene oxide (IRGO). This process has the uniqueness 81 to homogenization. These composites indicate a low infilof single strep preparation and commercial production. tration threshold of 0.52 vol%. The highest EMI SE of IP: 46.243.173.199 On: Thu, 02 Aug 2018 09:57:09 The in situ reduction method of graphene oxide in presCopyright: American Scientific Publishers the composites containing 15 wt% SPFG was measured ence of EMA has been showed in Figure 2. Delivered byatIngenta 21 dB in the X-band. The eye catching deservingness alleviated here is that these sort of EMI shielding 4. RECENT DEVELOPMENTS IN materials where epoxy resins acts as thermosetting binder CARBONACEOUS REINFORCEMENTS matrix has a great commercial importance due to its low 4.1. Generic Stratified Carbonaceous Materials cost. Similarly when epoxy is used to make such effecAssisted Polar Matrix Systems tive EMI shielding materials PET also draws the attention Electromagnetic interference (EMI) shielding is veheto be a good thermoplastic system where graphite sheets mently needed for defending electronics, debarring unauare incorporated in the PET to enlighten the EMI shieldthorized surveillance and dissuading electromagnetic ing behaviors. Melt processable PET has another merit forms of sleuthing. The malfunction of electronics can for industry related end product cultivation that here GO be speculative. Consequently, EMI shielding colligates to can be reduced in situ at the time of processing PET. electromagnetic pulse (EMP) protection, which is needed It has more effective “greenness” because this is just therin case of a high-altitude nuclear explosion.82 83 Being mally triggered reduction process without using of any a stratified assembly it highly recommended by the foreign reducing agent. Here high electrical conductivity researchers that graphene has an utmost importance as is achieved at a value of 211 S/m with a low percolation a conducting phase.84 But the main problem which is threshold of 0.47 vol.%.89 Primitively, carbon fibers were assessed that due to its powder like appearance it is quite constructed via graphitization of PAN or petroleum asphalt problematic to fabricate in a film flexible like manner. but the more low-priced vapor grown process from hydroHence multilayer graphene/polymer assembly in a film like carbon has become the most crucial preparation technique. architecture with good mechanical flexibility was fabriThe morphology, structure and dimensional architecture cated electromagnetic interference (EMI) shielding. Song greatly depend on the catalyst and the source of carbon. et al. exemplified by their proposed pathways that multiNickel, copper, iron and their alloys are the main catalayered graphene concurs a vehement importance for EMI lysts used while the sources of carbon generally involve propane, ethylene and acetylene. The morphology of these shielding materials where EVA played the binding matrix. vapor grown carbon nanofibers (VGCNF) length can be The layer assembly can be represented as wax/PVA/EVAsuperior to 50 mm giving rise to aspect ratios around multilayer graphene/PVA/wax where sandwich architec250 and 2000.90 91 ture which is not only dependent on graphene content but J. Nanosci. Nanotechnol. 18, 7641–7669, 2018

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Shielding properties of composites.

Filler [Ref.]

Matrix

Filler loading

HOPC [92] HOPC [93] SWCNTs [94] SWCNTs [95] MWCNTs [96] Fe/MWCNT [97] MWCNTs/Fe [98] MWCNTs/Co [98] MWCNTs/Ni [99] RGO [100] RGO [100] RGO [101] RGO [101] RGO/Ni [101] RGO/Fe3 O4 [102] Carbon coated Ni [103] Fe3 O4 /Al2 O3 /carbon Nanocoils [104] Porous carbon/Co [105] In situ RGO [81]

Wax Wax PU PU Varnish Epoxy Epoxy Epoxy Epoxy PEO PEO NBR NBR Wax Wax Wax Wax Wax EMA

1 wt% (lowest) 5 wt% 2 wt% (lowest) 5 wt% 8 wt% (lowest) 20% 60% 60% 60% 1 wt% (lowest) 5 wt% 2 wt% (lowest) 10 wt% 60 wt% 40 wt% 50 wt% 25 wt% 30 wt% 5 wt%

Corresponding peak in reflection coefficient (dB)

Thickness (mm)

Frequency range (GHz) (RL < 10 dB)

Effective bandwidth (GHz) (2.04–3.47 >2.35–3.51 >1.83–3.07 6.0–8.5 12.5–18.0 – 8.0–12.0 3.0–4.0, 12.0–14.0 4.5–6.5 11.2–15.5 10.5–14.0 3.5–5.0 8.2-12.4

2.0 4.5 – 3.0 4.8 16.0 >1.4 >1.2 >1.2 2.5 5.5 – 4.0 3.0 2.0 4.3 3.5 1.5 –

Rapid progresses in carbon-based fillers have already enabled a new and more hopeful platform in the exploitation of electromagnetic attenuation composites. Suitable alignment of fillers in composites with particular

architecture and morphologies has been widely pursued to achieve high performance based on taking advantage of unique filler characteristics. In another work, fewlayer graphene (FLG), obtained from direct exfoliation of graphite, was fabricated into paraffin wax to prepare composites to investigate their electromagnetic IP: 46.243.173.199 On: Thu,FLG/wax 02 Aug 2018 09:57:09 interference (EMI) shielding performance followed by Copyright: American Scientific Publishers Delivered bytheir Ingenta alignments in the matrix of paraffin wax. The dislodged FLG/wax samples have shown much rectified EMI performance compared to the commercial graphite/wax ones which are already been used as radiation shielding systems (Fig. 3). For advance improvement of EMI shielding performance, a new technique of split–press–merge approaches were implemented to align the FLG fillers to achieve anisotropic features in the plane perpendicular to the pressing direction. Much enhanced EMI shielding performance coupled with an improvement in absorption and reflection was ascertained in the post-alignment FLG/wax composites (Fig. 4). An average interparticle mean distance model colligated with bettered electrically conducting interconnection and elaborated effective reflection regions with respect to enhanced reflection efficiency were hashed out. The results suggest a platform and promising opportunities for preparing high-performance EMI shielding composites. The figures of the composite development techniques are depicted here. With the assistance of a paraffin wax matrix, based on its expandable feature, the strategy to arrange the embedded FLG nanosheets by multiple pressing is quite interesting, specifically for achieving anisotropic characteristics Figure 3. Schematic illustration of split–press–merge alignment to take advantage of two-dimensional graphene with its approaches. Reprinted with permission from [106], W.-L. Song, et al., exceptional large aspect ratios in EMI shielding composAlignment of graphene sheets in wax composites for electromagite formulation. The array of those intercalated particunetic interference shielding improvement. Nanotechnology 24, 115708 (2013). © 2013, IOP Publishing. late matters is visualized by electron microscopic image 7650


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Figure 4. (a–c) Schematic illustrations of the interparticle distance (IPD) model for random distribution (a) and aligned distribution (c) of FLG in composites; effective reflection regions (Seff1 before alignment (d) and effective reflection regions (Seff2 after alignment (e) for investigated units. Reprinted with permission from [106], W.-L. Song, et al., Alignment of graphene sheets in wax composites for electromagnetic interference shielding improvement. Nanotechnology 24, 115708 (2013). © 2013, IOP Publishing.

IP: 46.243.173.199 On: Thu, 02 Aug 2018 09:57:09 Copyright: American Publishers data with effective particle length (Figs. 4(a–c)). The Scientific high electrical conductivity (up to 16.8 S/m) and EMI Delivered by Ingenta cross sectional SEM images of unaligned as well aligned shielding efficiency (SE) (up to 34 dB), which is an topfiller nanocomposites are depicted there also (Fig. 5). The drawer performance at such a thin thickness (∼1 mm).110 detailed absorption behavior and radiation shielding charBesides the thermoplastic EMI shielded materials innoacteristics have been have been showed in Figure 6. vation there are a lots of works on the ground of therAnother work in the field of radiation shielding composmoset matrices other than epoxy composites. The worth ites development launched on graphene-based nanosheets knowing one of them is novolak type resin composites. fabricated by integrating solution processable functionalIn the above stated work compression molded sheets conized graphene into an epoxy matrix, and their electromagsisting of phenolic resin filled with a mixture of reduced netic interference (EMI) shielding studies were studied. grapheme oxide (RGO), -Fe2 O3 and carbon fibers have Here EMI shielding effectiveness was examined over a been developed accompanied with its electrical conductivfrequency range of 8.2–12.4 GHz (X-band), and shieldity in the range 0.48–171.21 S/cm. The microwave absorping efficiency was obtained 21 dB for 15 wt% (8.8 vol.%) tion belongingness of the sheets have been analyzed in the loading, indicating that they may be used as lightweight range of 8.2–12.4 GHz (X-Band). The maximum shield107 effective EMI shielding materials with respect to others. ing effectiveness (SE) detected is around 45 dB, which strongly depends on dielectric loss and volume fraction In the region of X-band, shielding can be enhanced by of -Fe2 O3 in RGO matrix. Addition of nanoparticles of another work by Joshi et al. where thin multi-walled carbon nanotubes were repositioned by applying rigorous -Fe2 O3 (magnetic filler) in the conducting matrix conoxidation. This sort of chemical exhaustion to MWCNT tributed a new kind of composite material having improved makes graphene nanoribbons which is an effective shieldmicrowave absorption properties (approximately 35 dB) ing constituent. Here polyvinyl alcohol (PVA) was used as which strongly depend upon volume fraction of -Fe2 O3 host material which housed the nano-ribbon type architecin RGO matrix. Therefore, the high value of EMI SE is tural segregation into it.108 109 In case of achieving the EMI predominated by absorption rather than reflection.111 GO shielding properties a interesting layer-by-layer classical assisted EMI shielding materials cultivation is a common assembly is attributed where water bourne polyurethane practice now a days, but it is customary to say that with (WPU)/GO composites plays a vehement role as shieldgreen synthesis methods it is quite attractive to environing materials where the developed composite exhibited a mental issues. In 2011 Xin Bai et al. developed a green J. Nanosci. Nanotechnol. 18, 7641–7669, 2018

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detrimental effects for lump sum production.114 The conductivity of those RGO was synthesized by various methods are referred here from their findings. 4.2. Conducting Polymer Based EMI Shielding Materials 4.2.1. Polyaniline (PANI) Based EMI Shielding Materials Another development of high-performance shielding materials against electromagnetic pollution by conducting polymer based nanocomposite and Fe3 O4 decorated GO have new importance in the shield material formulation. This work leads to heightened solid state charge transformation between the polyaniline (PANI) and graphene decorated with Fe3 O4 , and contributes to the higher microwave absorption. This nanocomposite demonstrates strong microwave absorption properties over 12.4–18 GHz having a SE value of 26 dB at 15 GHz with a reflection loss of 6 dB.115 Conducting polymers acquires a major role in the field of EMI shielding materials where the pioneer was polyaniline (PANI) based conducting composites (Fig. 7). We already met some novel developments based on this easy made conducting polymer. Now we come to venture from the Figure 5. Cross-sectional SEM images of the unaligned (a) and postwide references another class of conducting PANI based alignment (b) FLG/wax composites, looking at the fracture plane perpendicular to the pressing direction. Reprinted with permission from [106], MWCNT loaded nanocomposite through in situ proceW.-L. Song, et al., Alignment of graphene sheets in wax composites dure. The shielding efficiency measurements disclosed that 46.243.173.199 On: Thu, 02 Aug 2018 09:57:09 for electromagnetic interference shieldingIP: improvement. Nanotechnology reflection loss enhances marginally from −8.0 to −12.0 dB 24, 115708 (2013). © 2013, IOP Publishing. Copyright: American Scientific Publishers Delivered bywhereas Ingentaabsorption loss exhibits rapid enhancement from −18.5 to −28.0 dB with the increased CNT loading. The method for the development of GO/PEO based composabsorption predominated total shielding effectiveness in ite where GO is reduced in situ with l-ascorbic acid. range of −27.5 to −39.2 dB suggests that these conAs significant parameters for microwave electromagnetic ducting composites could be applied effectively for the properties, the real part ( ) and imaginary part ( ) of shielding purposes in the Ku-band (12.4–18.0 GHz). These complex permittivity of PEO based composites were meaPANI/MWCNT composites with large aspect ratio are also sured in the frequency range of 2–18 GHz. The composites aimed as hybrid composite conductive fillers in several showed high and than PEO. In X-band that was frethermoplastic matrices for constructing structurally strong quently studied, PEO/chemically reduced graphene commicrowave shields.11 The inorganic filler loaded PANI posite (2.6 vol.%) had value almost 8.0 and about 3, based nanocomposites are also played a major role in while PEO had ranged from 3.1 to 3.0 and about 0.4. this context. It is also present to synthesize Mn–Zn ferIt is reported that PEO/chemically reduced graphene comrite (MZF)–Ni–Mn–Zn–ferrite (NiMZF)/(PANI) particles, posite achieved more prominent permittivity than most where Mn–Zn ferrite is the magnetic core, and PANI attains carbon nanotube/polymer composites, while high was the concentric conducting outer layer of those inorganic desirable for electrical loss.112–114 It is seen in another particles. The commercial ferrite and Ni–ferrite particles experiment that deoxygenating of GO by solvo-thermal are principally used, and the PANI–ferrite particles with method developed in 2010 by Dubin et al. was much supea hybrid structure are prepared via an oxidative electrorior than others more commonly known reducing procechemical polymerization of aniline in an aqueous solution, dures. That paper authenticity established that this process which contains well-dispersed ferrite particles.116 When all is more green rather using other reducing agents and as works are on synthetic polymers there also some effective well as it is more acceptable due to its stabilization effects try on biomaterials based PANI composites. Conductive in colloidal atmosphere with an outstanding dispersion. polyaniline/bacterial cellulose (PANI/BC) nanocomposite The resulting solvo-thermally reduced GO can also be dismembranes by in situ oxidative polymerization of aniline persed into in a variety of polar organic solvents for potengrab quite attractive area for shielding materials develtial applications in solar cells or polymer nanocomposites, opment. These types of bacterial cellulose based films where traces of hydrazine as a reducing agents may cause shows a good thermal stability and additionally good 7652


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IP: 46.243.173.199 On: Thu, 02 Aug 2018 09:57:09 Copyright: American Scientific Publishers Delivered by Ingenta

Figure 6. Absorption (a), reflection (b), transmission (c) coefficients, respectively, and shielding effectiveness (d) and corresponding increments or decrements in the post-alignment FLG/wax (-1-) composites based on unaligned FLG/wax (- -) by the waveguide method. Reprinted with permission from [106], W.-L. Song, et al., Alignment of graphene sheets in wax composites for electromagnetic interference shielding improvement. Nanotechnology 24, 115708 (2013). © 2013, IOP Publishing.

Table IV. Electrical conductivities of solvothermally reduced graphite oxide (RGO) paper samples prepared by various methods and their comparison with hydrazine-RGO samples.114

Sample description

Drying conditions

Conductivity (S/m)

Solvothermal RGO

Air-dried Annealed at 250 C Annealed at 500 C Annealed at 1000 C Air-dried Annealed at 1000 C Air-dried

3.74 × 102 1.38 × 103 5.33 × 103 5.73 × 104 8.28 × 103 6.67 × 104 1.0 × 101

Air-dried

Insulator

Hydrazine RGO GO boiled in H2 O for 24 h GO


tensile strength property at a range of 95.7 MPa. In addition, the conductivity of the composite PANI/BC membrane increases linearly above 200 C.117 Core–shell type structure is quite attractive due to its binary behaviors of electrical as well as magnetic. Conductive polyaniline (PANI)–manganese ferrite (MnFe2 O4 nanocomposites with core–shell structure were also synthesized by in situ polymerization with a negligible reflection loss of −15.3 dB was observed at 10.4 GHz with at thickness of 1.4 mm of nanocomposite film.118 The PANI based polar binder matrix is also came to picture by Das et al. in 2005 where electrical conductivity as well as electromagnetic interference (EMI) shielding effectiveness (SE) at 7653


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Table V. List of the atomic composition of solvothermal reduced graphite oxide (RGO) and hydrazine RGO as measured by X-ray photoelectron spectroscopy (XPS).114 Description Solvothermal RGO Solvothermal RGO Hydrazine RGO Hydrazine RGO GO boiled in water for 24 h GO

Drying conditions

C (atomic %)

O (atomic %)

N (atomic %)

C/O ratio

Air-dried Annealed at 1000 C Air-dried Annealed at 1000 C Air-dried Air-dried

80.4 83.2 76.0 84.5 75.4 69.3

15.6 13.8 21.0 13.3 21.1 29.3

4.0 3.0 3.0 2.2 0.5 1.1

5.15 6.03 3.62 6.36 3.12 2.34

microwave (200–2000 MHz) and X-band (8–12 GHz) frequency range of polyaniline (PANI)/ethylene vinyl acetate (EVA) composites were studied. Melt mixing in Brabender Plasticorder at 50 C contributes PANI/EVA based composite with electrical resistivity at an order of 1.1 × 103 -cm which infers the descending behavior with increment of doped PANI in the EVA matric. For exploring more conducting behavior PANI can be doped by hexanoic acid which is also a prime step for enhancement of shielding properties. Wai and his coworkers fabricated a dielectric filler (here used TiO2 loaded material where CNT also shows a prominent role for enriching the shielding effectiveness in microwave pollution. Organic acid doped PANI/TiO2 /SWCNT composite with 20% SWNT displays the most promising microwave absorption property (99.2% absorption).119 The PANI also boosted by

5-lithium isophthalic acid (LiSIPA) as dopant which is also enriched by colloidal graphite for better dispersion in the matrix. This type of composite shows more captivating behavior like thermal stability (300 C) and electrical conductivity in the range of 67.4 S/cm at 17.4% colloidal graphite content. The maximum effectiveness value of −39.7 dB was reported by Saini et al. in their respective publication.120 Blends of polystyrene with polyaniline (PANI) coated multi-walled carbon nanotubes (MWCNTs) were projected which acquire dielectric and magnetic attributes from PANI and MWCNT respectively. These blends show absorption dominated total shielding effectiveness of −45.7 dB (>99.99% attenuation) in the 12.4–18.0 GHz range, suggesting their usefulness for making efficient microwave absorbers. The enhanced shielding effectiveness was attributed to optimization of


Figure 7. Schematic representation of the formation of PGF composites and polaronic and bipolaronic solid state charge transformation in PGF composites. Reprinted with permission from [115], K. Singh, et al., Nanostructured graphene/Fe3 O4 incorporated polyaniline as a high performance shield against electromagnetic pollution. Nanoscale 5, 2411 (2013). © 2013, Royal Society of Chemistry.

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conductivity, skin-depth measurements, complex permittivity and permeability.121 Yuhua and coworkers manifested how camphor sulphonic acid doped PANI/acrylate composite is utilized for EMI shielding materials through in situ polymerization.122 High proliferation of EMI shielding Hybrid organic/inorganic electromagnetic absorbing materials on polyaniline (PANI) and magnetite (Fe3 O4 fillers dispersed in epoxy resin matrix were successfully prepared by Belkacem et al. He confirmed the semi-crystallinity of the PANI-doped PTSA and a high Figure 8. A schematic for the RF plasma-polymerization set-up. crystalline phase of magnetite in the composite which elucidated that after 20% loading of the PANI in the the utmost importance to prepare PANI. RF plasma polycomposite the reflection coefficient became −11 dB at 18 GHz whereas the composite with a thickness around merization is employed for preparing PANI thin films. 1 mm bearing 15% of PANI and 10% of Fe3 O4 (magPlasma polymerization is an inexpensive tool for fabnite) leads to a minimum coefficient reflection, almost of ricating organic thin films. This technique results in −42 dB which is more than 99.99% of the microwave homogeneous, highly cross-linked and thermally stable absorption.123 In another one-pot experimentation gold polymer thin films. The method of plasma polymerizananoparticles embedded PANI shows also the high conduction employs AC/RF/DC and pulsed techniques. Among tivity which is also used for shielding purposes.91 Cotton these, RF plasma polymerization needs special mention was also be used for in situ PANI/cotton polymer comsince it yields conjugate structures of the polymers, which posite which also acts as a potential shielding agent. This are reckoned to be essential for making conducting thin composite also behaves like anti-flammable EMI shielding films (Fig. 8). Former investigations on PANI, using ac coating.124 The explosive growth of electronics has proplasma polymerization technique, indicate that low dielecduced electromagnetic interference (EMI) as most undesirtric constant.128 129 Dielectric constant and ac conductivity able outgrowths. PANI coated MWCNT also occupied an were measured in the frequency range 100 Hz–1 MHz and interesting field of shield materials preparation. These type in the temperature range 300–373 K. The dielectric perof functionalization is also improves the shielding propIP: 46.243.173.199 On: Thu,mittivity 02 Augin2018 09:57:09 the high-frequency range is substantially low American Publishers 329 × 10−2 S/cm Scientific erty with conductivity 499 × 10−3 toCopyright: and this type of low dielectric material is a potential canIngenta thermo- bydidate reported.125 Makeiff128 hypothesized conductive Delivered as intermetallic dielectrics in microelectronics and plastic composites bearing both multi-walled carbon nanoshielding purposes. The dielectric constant dwells between tubes and polyaniline doped with para-toluene sulfonic 7.52 and 1.38 for the total frequency range for which the acid (PTS) were articulated using two different methexperiment was accomplished (300–373 K). The dielecods. In the first method, PANI-PTS-coated MWNTs were tric constant lies between 1.45 and 1.20 at room tempersynthesized and processed into an insulating matrix. The ature for the entire frequency, which is considerably low second method involved mechanical mixing of separate (Fig. 9).130 synthetic PANI-PTS and non-coated MWNT solids into an insulating matrix. Microwave absorption measurements at 4.2.2. Polypyrrole (PPy) Based EMI Shielding Materials X-band frequencies (8–12 GHz) indicated that the former Polypyrrole has a potential conducting application as composites are poor absorbers, while the latter are good intrinsically conducting polymer (ICP) which can be absorbers and showed stronger absorption than composused as a conducting phase in many conducting ites containing only PANI-PTS or MWNT. The dc confilms,11 113–115 117 118 131 132 radiation shielding,105 133–149 ductivity (dc of composites carrying doped PANI and hot melt adhesives150 151 since a long days ago. MWNTs were in the range of 0.1–0.99 S/cm.126 Spinel reinforced PANI composite is another material which can 4.2.3. Polythiophene (PT) Based EMI also performed in shielding purpose. The PANI/spinel Shielding Materials (Co05 Zn05 Fe2 O4 nanocomposite was successfully develA new class of polymer from the family of conjuoped by an in situ polymerization by Ma et al. which gated -structure material has become an exclusive shows reflection loss of the nanocomposite is higher than materials achieving weather resistant, lightweight, excepthat of the unfilled PANI. The maximum reflection loss of tional electron transport, high surface area besides their the PANI/Co05 Zn05 Fe2 O4 nanocomposite is about 39.9 dB extraordinary mechanical properties. PT polymers can be at 22.4 GHz with bandwidth of 5 GHz in a sample thickexploited as electrical conductors, recording materials, ness of 2.0 mm.127 nonlinear optical devices, polymer light-emitting diodes Thermally stable materials with low dielectric constant (PLED), electrochromic or smart windows, photoresists, (k < 39) are being a challenge to develop such shieldantistatic coatings, sensors, batteries, electromagnetic ing efficient material where RF plasma mechanism has J. Nanosci. Nanotechnol. 18, 7641–7669, 2018

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Figure 9. Frequency dependence of (a) dielectric parameters ( ) (b) dielectric loss ( ) (c) permeability (u ) and (d) magnetic loss (u ) for the PANI and PANI/Co05 Zn05 Fe2 O4 nanocomposites. Reprinted with permission from [130], R. Ma, et al., Preparation, characterization and microwave absorption properties of polyaniline/CoO · 5ZnO · 5Fe2 O4 nanocomposite. Mater. Res. Bull. 45, 1064 (2010). © 2010, Elsevier.

shielding materials, artificial noses and muscles, solar cells, electrodes, microwave absorbing materials, new types of memory devices, transistors, nanoswitches, optical modulators and valves, imaging materials, polymer electronic interconnects, and nanoelectronic and optical devices.153–155 Regioregularity is an important aspect of preparing thiophene related conducting polymer assembly, that is why substituted thiophenes have a great importance for preparing head to tail assembly of substituted polythiophenes. SWNT assisted conducting PT composites (SWNT/PT) have been synthesized successfully with the in situ chemical oxidative polymerization method of the thiophene monomer onto SWNTs. The characterization of the molecular structure has indicated that thiophene molecules are adsorbed onto and then polymerized on the surface of SWNTs, and SWNTs have been used as the core in the formation of hybrid SWNT/PT composites. The conductivity through SWNT–PT is as large as 0.41 S/cm, which is much 7656

larger than the usual value of bulk PTh films (167 × 10−6 S/cm). The SWNT–PT composites show amended thermogravimetric stability in comparison with the PT homopolymer.155 Polyelectrolyte multilayers arresting electrochemically act dynamic polythiophenes (PT) have been constructed on ITO-glass substrates using the stratified layer-by-layer adsorption deposition proficiency. In a work layers of poly(cyclopentadithienyl-alkylsulfonate), poly(cyclopentadithienyl-alkylammonium), and bis(carboxyhexyl) sexithiophene were deposited with non-electroactive layers of polyallylamine and polystyrenesulfonate which is a major improvement in the conducting polymer synthesis which can be used as electroactive actuation and sensor applications with effective development of mobile conjugated -electron array. Hosseini et al. contextualized that polythiophene nanofibers coated MnFe2 O4 /Fe3 O4 core–shell nanoparticles were synthesized via co-precipitation and in situ polymerization exhibited an enhancement of electronic J. Nanosci. Nanotechnol. 18, 7641–7669, 2018

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Table VI. Polypyrrole based EMI shielding composites.

of this is that by this solution cast method we can make shielding materials with suitable shape. SoluComposite [Ref.] EMI SE (dB) tion casting method was also played an important role Porous crosslinked PS/PPy [133] – to develop conducting composite from an insulation PET fabric/PPy [134] 36 polymer matrix. Polystyrene (PS) was used as binder Ni/PPy [135] – in poly(3-butylthiophene) whisker which was reported Mn-Zn ferrite/PPy [137] – Electrochemically synthesized PPy [137] – elsewhere.157 10 wt% poly(3-butylthiophene) loading PTS doped PPy [138, 139] 40 achieved conducting pathways in the polymer matrix Nylon-6/PPy [140] 40 and the PS composite wholly turns into a conducting Ag-Pd coated fabric/PPy [141] 80 one. The upcoming important member of substituted PPy coated Polyester fabric [142] 35.61 PP/MMT/PPy [141] regioregular polythiophene with head-to-tail arrange28.9 ZnFe2 O4 /PPy [142] ments is Poly(3,4-ethylenedioxythiophene) i.e., PEDOT Iron oxide/PPy [143] 10.10 which has attention-getting architecture after formation PANI coated cotton fabric [144] 6 of -Fe2 O3 composite. The ensuing conducting ferrimagPPy coated MWCNT [146] 28.6 netic composite depicted microwave absorption loss of 3.3 PPy/Al2 O3 [147] SiC/PPy [148] – 18.7–22.8 dB in the frequency range of 12.4–18 GHz.158 Polyurethane/PPy [148] – For improvement of metal oxide nano particle to polyAmino functionalized fly ash/PPy [149] – mer phase one novel achievement was to improve the – CoFe2 O4 /PPy [152] electronic behavior of poly(3-thiophene acetic acid) after incorporating the magnetite filler into it.159 The acetic acid group in the “3” position of a thiophene molecule mobility in the composite. The microwave absorption enables the regioregular nature of the poly(3-thiophene properties of the nanocomposites were investigated in the acetic acid) which is quite “easy to process” to overfrequency range 8.0–12.0 GH where saturation magnecome the solubility in the solution blending process.160 tization and coercivity reduced from 60.76 emu/g and A fascinating aspect to improve the conducting features 99.10 Oe for MnFe2 O4 to 0.77 emu/g and 33.5 Oe, respecof a composite with inorganic or organometallic functiontively for MnFe2 O4 /Fe3 O4 /PT (Fig. 10).156 In another alities also carries an emerging field in the conducting experiment insulating polymer is blended with conIP: 46.243.173.199 On: Thu, 02 Aug 2018 09:57:09 nanocomposite community. The functionalization with ducting polymer via solution blending in same solvent. Copyright: American Scientific Publishers groups is particularly attractive due to their Poly(3-butylthiophene) and polystyrene was Delivered solution byorgano-borane Ingenta electron-starving nature, which can be exploited in the blended in ortho-dichlorobenzene and casted to evaporate solvent. The composite film did not show effecsensing of nucleophiles and may lead to unusual electronic tive enhancement of conductivity but the importance devices. Silylated polymer prepared by Stille condensation

Figure 10. Schematic of the procedure for the formation of the core–shell structure for MnFe2 O4 /Fe3 O4 /PT nanocomposites. Reprinted with permission from [156], S. H. Hosseini, et al., Magnetic, conductive, and microwave absorption properties of polythiophene nanofibers layered on MnFe2 O4 /Fe3 O4 core–shell structures. Mater. Sci. Semicond. Proc. 24, 272 (2014). © 2014, Elsevier.


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Figure 11. Schematic representation of (a) polymerization of EDOT containing the barium ferrite nanoparticles using APS as oxidant and (b) the interaction of the microwave with the polymer composite resulting in its attenuation due to the scattering with the nanoparticles. Reprinted with permission from [163], A. Ohlan, et al., Microwave absorption behavior of core–shell structured poly(3,4-ethylenedioxy thiophene)-barium ferrite nanocomposites. ACS Appl. Mater. Interfaces 2, 927 (2010). © 2010, American Chemical Society.

reaction developed a regioregular structure of the polymer doped barium ferrite/poly(3-methylthiophene) composite to increase the thermal stability as well as conductivity backbone which seems to be a more electrically mobile (Fig. 12).165 phase. In order to accomplishing high conductivity in IP: 46.243.173.199 On: Thu, 02 Aug 2018 09:57:09 polymer composites the metal oxide of molybdenum likely Copyright: American Scientific Publishers MoO3 encapsulated by polythiophene/fullerene Delivered polymer by4.2.4. Rubber Matrix Based Conducting Ingenta composite exploited in the respective paper of Voroshazi Polymer Composites et al.161 Besides these experiments it is worth mentioning Fast emergence of electrical and electronics devices, which that water solubilizing of the synthesized polythiophene emit electromagnetic energy in the identical frequency by self-assembling complex with a glycopolymer which bands used by other users in commercial aspects, it arrives was reported elsewhere.162 Texturisation of polythiophene essentially to limit and shield electronic equipment against phase by a nanorod like fashion which acted a seeding all generators of interference due to all these electromagmaterial for surface fastening of a glycopolymer named netic energies. Before understanding to the EMI shielding poly(N -p-vinylbenzyl-D-lactonamide) (PVLA) exhibiting of rubber based conductive composites, a few attributes its cylindrical morphology. Hence in this publication on conductivity should be revealed. In general rubbers are author proposed that PT acquitted like molecular nanowire insulators. But it can be conductive after incorporation with the self-assembled structure in the hydrophobic core of some conductive filler into its matrix. These conducof PVLA. Core–shell type architecture for conducting tive fillers generically found as metal powders,166–168 metal composites is common practice to improve better shieldfibers,169 carbon blacks170–172 and graphite fibers.169 173 ing. In that context Ohlan et al. reported that barium These fillers are normally dispersed in rubber matrix just ferrite/PEDOT composite has major importance in this above a threshold concentration to fabricate a conductive area. Author also denoted complex permittivity, permerubber composite.174 175 The main applications of the rubability, and microwave absorption belongingness of core ber based conductive composites generally depend upon shell type poly(3,4-ethylenedioxythiophene) (PEDOT) their volume electrical resistivity. Figure 12 gives a diananocomposite with barium ferrite in the 12.4–18 GHz grammatic overview of the conductivity and filler disperfrequency range (Fig. 11).163 The high radiation absorpsion in rubber composites. tion properties primarily result from the high dielectric In the primitive time of shielding composite preparaand magnetic losses in the composites and depend on the tion polychloroprene rubber was filled with carbon fiber concentration of barium ferrite in the polymer composite to increase the shielding effectiveness of the composhaving shielding effectiveness of 22.5 dB at 15 GHz ites. The SEM micrograph picture established the effecwith minimal reflection loss of 2 dB.164 This type of tive dispersion of carbon fiber in the rubber matrix. The SE increases with increasing fiber concentration in the barium/polythiophene composite was modified by La 7658


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Figure 12. Preparation of core–shell nanocomposite by BaLax Fe12−x O19 /poly(m-toluidine). Reprinted with permission from [165], Y. Xie, et al., La-doped Barium-ferrite/Poly-m-toluidine Composites: Preparation, characterization and properties. Curr. Nanosci. 10, 427 (2014). © 2014, Bentham Science Publishers.

composite. In this work author also reported that in the makes the composite more cost effective as well as lighter frequency range of 100 to 1000 MHz, variation of SE with that the high filled one. That paper also revealed that the frequency is not systematic, but in the frequency range composites containing SCF are technically useful mate8–12 GHz SE increment is directly proportional to frerial (SE ≥20 dB) in X-band region frequency range i.e., quency increment. Crosslink density is also another inde8–12 GHz parallel to that at microwave frequency range pendent parameter which can be tuned precisely to get viz. 100–2000 MHz.180 181 In other terms Das et al. also effective shielding property. It is seen that higher crossdesigned EVA/EPDM blend composite enriched with SCF linking density in the composite vulcanized by the convenplays a role of not only reinforcement but also their outtional method produces higher electrical conductivity and standing shielding performance.182 higher EMI shielding effectiveness in composite compared There is an immense interest in the design of addresswith an identical fiber loaded thermo vulcanized.176 In this able functional conducting blends for use in shielding IP: 46.243.173.199 On: Thu, 02 Aug 2018 09:57:09 applications. As a matter of fact, there are a number of context it is very common practice to Copyright: incorporate American conduc- Scientific Publishers tive filler in the matrix by means of mixing in a desirable Delivered byproblems Ingenta involving the preparation of conducting blend such as weak interfacial adhesion, poor compatibility manner so that the conductivity as well as the resulting among two polymer phases, and high concentration of shielding property shows isotropically through the rubber the conductive filler, which has to be deliberated and matrix. Thus, conductive carbon black in one of the most assured to find a polymer blend with well-suitable propercommonly nurtured conductive filler which occupies a big ties. Mainly, two different approaches have been declared, region of filler shielding composites. Vulcan XC-72 was one is to use the organic and/or inorganic filler as an interan enormously used conductive black which is reliable facial conducting phase to the polymer system and the for a rubber matrix turned into solely conductive comother is to insert nanoscale filler as conducting charges. posite. Recently poly(ethylene-co-methyl acrylate) based A new category of functional conductive butyl rubber (IIR) rubbery carbon black composites were reported by Bhwal assisted with different loadings of low-density polyethyand her co-workers. They used Vulcan XC72 conductive lene (PE) was developed by roll mixing at a specific rotor carbon clusters to enhance the electrical conductivity in speed. It is quite rigorous to execute the thermodynamlow percolation threshold (