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Review of electromagnetic interference shielding materials fabricated by iron ingredients

Vineeta Shukla *

Nuclear Condensed Matter Physics Laboratory, Department of Physics, Indian Institute of Technology, Kharagpur-721302, India. E-mail: vineeta@phy.iitkgp.ernet.in; Tel: +91 9026690597

Received 21st February 2019 , Accepted 1st April 2019

First published on 1st April 2019

Iron (Fe) and its counterparts, such as Fe 2 O 3 , Fe 3 O 4 , carbonyl iron and FeO, have attracted the attention of researchers during the past few years due to their bio-compatibility, bio-degradability and diverse applications in the field of medicines, electronics and energy; including water treatment, catalysis and electromagnetic wave interference shielding etc. In this review paper, we aimed to explore iron based materials for the prevention of electromagnetic interference (EMI) by means of both reflection and absorption processes, including the standard methods of synthesis of Fe-based materials along with the determination of EMI performance. It is customary that a proper combination of two dielectric-losses, i.e. electrical and magnetic losses, give excellent microwave absorption properties. Therefore, we focused on the different strategies of preparation of these iron based composites with dielectric carbon materials, polymers etc. Additionally, we explained their positive and negative aspects.

Vineeta Shukla Vineeta Shukla was born in Uttar Pradesh, India, and received degrees from the University of C. S. J. M. Kanpur: Master of Philosophy in Physics (2012); Post-Graduation in Physics (2010); and Graduation in Physics and Mathematics (2007). She received two gold medals, the Sir C. V. Raman Commemoration Medal and the Kailashpat Singhania Medal for attaining the first position in the University. Currently, she is pursuing a PhD from the Indian Institute of Technology, Kharagpur, West Bengal, under the supervision of Dr Sanjeev Kumar Srivastava. Her research work includes the characterization and energy applications of carbon, polymer, metal and alloy materials.

1 Introduction

1.1 Electromagnetic interference (EMI) pollution

etc. , because when these EM waves interfere with the input signal of the electronic devices, they create a noise that is known as electromagnetic interference (EMI) pollution. In general, EMI pollution could be considered as an undesirable outcome of modern engineering that has become grievous to human health, causing many diseases, e.g. headaches, sleeping disorders and trepidation. In communication devices ( e.g. cell phones, computers, bluetooth devices, laptops), commercial appliances ( i.e. microwave ovens, the design of microwave circuits) and the automotive industries ( i.e. integrated electrical circuits), EMI pollution deteriorates the durability and proper functioning of electronic equipment. Therefore, this new kind of pollution has become a serious worldwide problem and its mitigation could be achieved only by use of EMI shielding materials. i.e. magnetic materials. Accordingly, metallic conductors suffer a lack of flexibility, heaviness, and high costs. Meanwhile, ferromagnetic or ferrimagnetic materials have an intrinsic cut-off frequency, usually below the low GHz range, that hinders their use in EMI shielding over a broad GHz range. From this we concluded that, at present, we need to explore broadband shielding materials, those do not only work in the MHz range, but also neutralize EM waves in the GHz range. Most importantly, a lot of effort has been made in this direction; unfortunately to obtain simultaneously minimum reflection with a view to maximum absorption is still challenging task for practical applications. In recent years, electromagnetic (EM) wave radiation in the gigahertz (GHz) range has been regarded as an alarming danger for commercial appliances, biological systems, high quality information technology and defense safety technologies,, because when these EM waves interfere with the input signal of the electronic devices, they create a noise that is known as electromagnetic interference (EMI) pollution. In general, EMI pollution could be considered as an undesirable outcome of modern engineering that has become grievous to human health, causing many diseases,headaches, sleeping disorders and trepidation. In communication devices (cell phones, computers, bluetooth devices, laptops), commercial appliances (microwave ovens, the design of microwave circuits) and the automotive industries (integrated electrical circuits), EMI pollution deteriorates the durability and proper functioning of electronic equipment. Therefore, this new kind of pollution has become a serious worldwide problem and its mitigation could be achieved only by use of EMI shielding materials. 1 EMI shielding is defined in terms of the reflection and/or absorption of electromagnetic radiation by a material that acts as a barrier against the penetration of the radiation passing through the shielding materials. These materials prevent the transmission of EM radiation by reflection and/or absorption of the electromagnetic radiation or by suppressing the EM signals so that EM waves do not affect the functioning and durability of electronic equipment. In general, conductive materials like metals, owing to their high reflectivity, are widely used to isolate spaces or equipment from surrounding EM waves. This reflection shielding is based on the principle of the Faraday cage, in which inside the cage, space is completely impervious to external electric fields. On the other hand, absorption shielding is related to permeable materialsmagnetic materials. Accordingly, metallic conductors suffer a lack of flexibility, heaviness, and high costs. Meanwhile, ferromagnetic or ferrimagnetic materials have an intrinsic cut-off frequency, usually below the low GHz range, that hinders their use in EMI shielding over a broad GHz range. From this we concluded that, at present, we need to explore broadband shielding materials, those do not only work in the MHz range, but also neutralize EM waves in the GHz range. Most importantly, a lot of effort has been made in this direction; unfortunately to obtain simultaneously minimum reflection with a view to maximum absorption is still challenging task for practical applications. 2–6

2 Scope of review

Up to date, iron (Fe) based composites have been extensively studied and are most desirable composites in various applications. Ten years of data on Fe-containing composites, collected by Scopus and shown in Fig. 1(a and b) , show how the demand for Fe composites has increased year-by-year in several fields of research such as materials science, engineering and many others. It is expected that this review article will benefit ongoing research pertaining to iron nanostructures in the field of EMI shielding, since reviews play a crucial role in continuing interest on current aspects of research in every academic field. Therefore, this review mainly focuses on the development of high performance EMI shielding materials, considering iron as one of the important ingredients.

Fig. 1 (a) and (b) Scopus database (09/12/2018) for iron based research articles.

2.1 Mechanisms of shielding

2.1.1 Shielding efficiency in terms of reflection/absorption. T ) could be defined as parameter that measures how well a material impedes the EM energy of a certain frequency when passing through it. P I ) is reflected ( P R ), while a certain part is absorbed and dissipated in form of energy, and the remaining part is transmitted ( P T ) through the shielding material. Therefore, three different processes namely reflection, absorption and multiple internal reflections contribute to the whole attenuation, corresponding to shielding effectiveness SE R , SE A and SE M , respectively. (1) Here P , E and H refer to power and electric and magnetic field intensities while subscripts I, R and T represent the incident, reflected and transmitted components, respectively. Thus, SE R refers to net reflection and SE A represents shielding due to absorption. Note that contributions from secondary reflections (output interface) in M can be neglected ( SE T = SE R + SE A . (2) Shielding efficiency (SE) could be defined as parameter that measures how well a material impedes the EM energy of a certain frequency when passing through it. Fig. 2a represents the possible interactions of EM waves with materials. When the EM waves fall on the front-face of the material then a certain part of the incident power () is reflected (), while a certain part is absorbed and dissipated in form of energy, and the remaining part is transmitted () through the shielding material. Therefore, three different processes namely reflection, absorption and multiple internal reflections contribute to the whole attenuation, corresponding to shielding effectiveness SE, SEand SE, respectively.Hereandrefer to power and electric and magnetic field intensities while subscripts I, R and T represent the incident, reflected and transmitted components, respectively. Thus, SErefers to net reflection and SErepresents shielding due to absorption. Note that contributions from secondary reflections (output interface) in Fig. 2a and b occur in finite-dimensional media but in thicker slabs SEcan be neglected ( Fig. 2c ). Then, the equation takes the form

Fig. 2 (a) Schematic diagram of incident, reflected and transmitted power and electro-magnetic field intensities when an EM wave is incident on a 3D material; (b and c) sources of reflection in a thin sample (input and output interfaces, R in and R out ) and in a thick sample; (d–g) multiple reflections in the case of a porous structure, a hollow structure, a multiple shell structure and a solid sphere.

2.1.2 Reflection loss (SE R ). R ) is related to the relative impedance mismatching between the surface of the shielding material and the EM waves. The magnitude of the reflection loss can be given by (3) where σ is the total conductivity, f is the frequency, and μ is the relative permeability. It can be seen that SE R is a function of the ratio of conductivity ( σ ) and permeability ( μ ) of the material i.e. SE R ∝ ( σ / μ ). Thus, for a constant σ and μ , SE R decreases with frequency. Therefore, materials must have mobile charge carriers (electrons or holes) for reflection of the EM radiation. The primary mechanism of EMI shielding is reflection. Reflection loss (SE) is related to the relative impedance mismatching between the surface of the shielding material and the EM waves. The magnitude of the reflection loss can be given bywhereis the total conductivity,is the frequency, andis the relative permeability. It can be seen that SEis a function of the ratio of conductivity () and permeability () of the materialSE∝ (). Thus, for a constantand, SEdecreases with frequency. Therefore, materials must have mobile charge carriers (electrons or holes) for reflection of the EM radiation.

2.1.3 Absorption loss (SE A ). A ) in decibels (dB) can be written as: (4) where d and α are the thickness and attenuation constant of the slab, respectively. The attenuation constant defines the extent at which the intensity of an EM wave is reduced when it passes through the material. It is clear that SE A depends on conductivity ( σ ), permeability ( μ ) and sample thickness ( d ). Such a dependency of SE R /SE A on μ and σ indicates that in magnetic conducting metals, shielding is dominated by absorption rather than reflection. Moreover (5) where λ 0 is the wavelength in vacuum and n is the refractive index, which is given by ( εμ ) 1/2 ; in the case of nonmagnetic materials μ = 1. Hence, (6) A secondary mechanism of EMI shielding is absorption. As we know from the plane wave theory, the amplitude of the EM wave decreases exponentially inside the material as it passes through it. Thus, absorption loss results from ohmic losses and heating of the material due to the currents induced in the medium. For conductive materials, absorption loss (SE) in decibels (dB) can be written as:whereandare the thickness and attenuation constant of the slab, respectively. The attenuation constant defines the extent at which the intensity of an EM wave is reduced when it passes through the material. It is clear that SEdepends on conductivity (), permeability () and sample thickness (). Such a dependency of SE/SEonandindicates that in magnetic conducting metals, shielding is dominated by absorption rather than reflection. Moreoverwhereis the wavelength in vacuum andis the refractive index, which is given by (; in the case of nonmagnetic materials= 1. Hence, It is clear from eqn (5) that high permittivity is particularly crucial for the enhancement of SE A , as well S R .

2.1.4 Multiple reflection (SE M ). i.e. EM waves reflect from the second boundary, come back to first boundary and are re-reflected from the first to second boundary, and so on, as shown in (7) where δ is the skin depth, defined as the thickness below the outer surface at which the incident field is attenuated to 1/ e of its initial value, given by δ = ( f π σμ ) −1/2 . (8) For thinner materials, radiation is trapped between two boundaries due to multiple reflection,EM waves reflect from the second boundary, come back to first boundary and are re-reflected from the first to second boundary, and so on, as shown in Fig. 2a whereis the skin depth, defined as the thickness below the outer surface at which the incident field is attenuated to 1/of its initial value, given by SE M depends on d and is closely related to absorption. Hence, multiple reflection plays an important role for porous structures and some definite geometries. For more visualization, Fig. 2d–g shows trapping/scattering of EM radiation by porous, hollow, multi-shell and solid structures. In this structure, a large surface area and a big vacant space excluding the solid structure gives more active sites for scattering and multiple reflection of electromagnetic waves. The hollow/porous structure shows unique properties, e.g., high surface area, disciplinable internal structures, low density and complimentary permeability that can fulfil the quest for improving EMI performance. These multiple reflections (SE M ) can be neglected when the thickness of the shielding materials is greater than the penetration depth (δ) or when SE A is more than 10 dB because in thick shielding materials (high SE A ) the EM wave hits at the second boundary with negligible amplitude so SE M can be neglected.

2.1.5 Perspective to minimize reflection. R , depends solely on σ / μ , while SE A ( dσμ ) also depends on the sample thickness. Such dependency of SE R /SE A on μ and σ indicates that in non-magnetic materials shielding is mainly governed by reflection, while in magnetic conducting metals shielding is dominated by absorption rather than reflection. This situation is quite different for composite materials in which heterogeneous micro structures show the great variations in the local fields due to these nano/micro extent which works as polarization sites. These sites create the lag of the displacement current relative to conduction current. Further, matrix and filler inclusions both have different electro-magnetic properties. In these conditions permittivity and permeability can be replaced by effective permittivity ( ε = ε ′ + i ε ′′) and permeability ( μ = μ ′ + j μ ′′), respectively: ε = ε ′ + i ε ′′ (9) where i is an imaginary number and ε are complex numbers. In the above equation, ε ′ denotes the electric energy storage capacity, while ε ′′ is related to dielectric losses. Similarly, permeability is given by μ = μ ′ + j μ ′′ (10) where j is an imaginary number. In case of magnetic systems, μ ′ denotes the magnetic energy storage while the imaginary part relates to ohmic losses similar to an electrical system. In complex permeability the μ ′ and μ ′′ of the materials are directly related to the energy density and magnetic loss power stored in the magnetic system. Therefore, these possess a complex dependency on the geometry, size, conductivity and volume fraction of each constituent. For several applications such as radar (to reduce the radar cross section) and military applications ( e.g. hiding military devices), the essential requirement is to adjust the effective permittivity and permeability to certain values by which reflection can be minimized. Therefore, a prerequisite of conductive EMI shielding composites is to limit reflection and enhance absorption for effective EMI shielding materials. This is possible only when we minimize the mismatch of impedance between free space and shielding materials. According to the transmission line theory, intrinsic surface impedance in relation to complex permittivity and permeability for a given medium can be written as, (11) It is clear that reflection, SE, depends solely on, while SE) also depends on the sample thickness. Such dependency of SE/SEonandindicates that in non-magnetic materials shielding is mainly governed by reflection, while in magnetic conducting metals shielding is dominated by absorption rather than reflection. This situation is quite different for composite materials in which heterogeneous micro structures show the great variations in the local fields due to these nano/micro extent which works as polarization sites. These sites create the lag of the displacement current relative to conduction current. Further, matrix and filler inclusions both have different electro-magnetic properties. In these conditions permittivity and permeability can be replaced by effective permittivity (′ + i′′) and permeability (′ + j′′), respectively:where i is an imaginary number andare complex numbers. In the above equation,′ denotes the electric energy storage capacity, while′′ is related to dielectric losses. Similarly, permeability is given bywhere j is an imaginary number. In case of magnetic systems,′ denotes the magnetic energy storage while the imaginary part relates to ohmic losses similar to an electrical system. In complex permeability the′ and′′ of the materials are directly related to the energy density and magnetic loss power stored in the magnetic system. Therefore, these possess a complex dependency on the geometry, size, conductivity and volume fraction of each constituent. For several applications such as radar (to reduce the radar cross section) and military applications (hiding military devices), the essential requirement is to adjust the effective permittivity and permeability to certain values by which reflection can be minimized. Therefore, a prerequisite of conductive EMI shielding composites is to limit reflection and enhance absorption for effective EMI shielding materials. This is possible only when we minimize the mismatch of impedance between free space and shielding materials. According to the transmission line theory, intrinsic surface impedance in relation to complex permittivity and permeability for a given medium can be written as, The microwave absorption properties of the materials in terms of reflection loss could be given by (12) The maximum absorption of microwaves means that a minimum reflection loss (RL min ) occurs when the impedance of the composite and free space is matched. The ideal impedance matching conditions are when Z in = Z 0 = 377 Ω. Here Z 0 is the impedance of air, and Z in is the input impedance of the absorber. The above condition is fulfilled at a specific matching thickness (t m ) and matching frequency (f m ). An ideal EM absorption should make the effective width as broad as possible, which can be controlled by the 1/4 wavelength equation:5 (13) where n is the refractive index and c is the velocity of light. The RL value of −20 dB is considered to be 99% microwave absorption according to whereis the refractive index andis the velocity of light. The RL value of −20 dB is considered to be 99% microwave absorption according to eqn (8) and (11) , which is believed as an adequate level of absorption. In order to minimize the impedance mismatch, the best way is to increase the effective permeability or decrease the effective permittivity. Hence, high-performance microwave absorbing materials have been considered extensively to prevent incident EM wave radiation. These materials convert EM energy into thermal energy through dielectric loss and/or magnetic loss by the balance outcome of integralities between the relative permittivity and/or permeability. Moreover, technological fields desire not only efficient shielders, but also fulfil some necessitous criteria such as being lightweight, having a minimum thickness, corrosion- and chemical resistance, good flexibility, tunable morphology, ease of processing, and cheapness. 7

2.2 Factors affecting the EMI performance

2.2.1 Permittivity and permeability. (14) Therefore, permittivity and permeability are crucial parameters to design an effective EMI shielding material, as explained in the previous section. For electrical shielding, conductivity and polarization loss are two key factors that are responsible for the dielectric loss ( ε′′ ). Polarization loss could be based on electronic, ionic, dipole orientation (raised by bound charges) and interfacial polarization (due to trapping of space charge). Based on free electron theory, the dielectric loss is given by ε ′′ = σ /2π ε 0 f or ε ′′ ∝ σ , where σ is the conductivity, which indicates that a high electric conductivity enhances ε ′′. Ionic polarization and electronic polarization works only at the very high frequency region (above 1000 GHz) hence their effects can be neglected in the low microwave frequency region. Dipole polarization comes into the picture due to the presence of defects and residual groups in the material etc. The interfacial polarization and respective relaxation appear to be due to trapped space charges at the interfaces. In this case the relaxation process can be investigated by a Cole–Cole semicircle obtained from the Debye dipolar relaxation process. The relationship between ε ′ and ε ′′ is ( ε ′ − ε ∞ ) 2 + ( ε ′′) 2 = ( ε s − ε ∞ ) 2 (15) where ε s and ε ∞ are the static and relative dielectric permittivity at higher frequencies. If polarization relaxation takes place then an ε ′′ versus ε ′ plot will be a single semicircle. This plot is popular as the Cole–Cole semicircle plot. This type of polarization mostly appears in hierarchical and multi-interface composites. Ideal EMI shielders require impedance matching characteristics of composites which are influenced by permittivity and permeability according to following equation: 8 Therefore, permittivity and permeability are crucial parameters to design an effective EMI shielding material, as explained in the previous section. For electrical shielding, conductivity and polarization loss are two key factors that are responsible for the dielectric loss (). Polarization loss could be based on electronic, ionic, dipole orientation (raised by bound charges) and interfacial polarization (due to trapping of space charge). Based on free electron theory, the dielectric loss is given by′′ =/2πor′′ ∝, whereis the conductivity, which indicates that a high electric conductivity enhances′′. Ionic polarization and electronic polarization works only at the very high frequency region (above 1000 GHz) hence their effects can be neglected in the low microwave frequency region. Dipole polarization comes into the picture due to the presence of defects and residual groups in the material 9,10 and mainly depends on the fabrication processes, chosen materials, annealing temperatureThe interfacial polarization and respective relaxation appear to be due to trapped space charges at the interfaces. In this case the relaxation process can be investigated by a Cole–Cole semicircle obtained from the Debye dipolar relaxation process. The relationship between′ and′′ iswhereandare the static and relative dielectric permittivity at higher frequencies. If polarization relaxation takes place then an′′′ plot will be a single semicircle. This plot is popular as the Cole–Cole semicircle plot. This type of polarization mostly appears in hierarchical and multi-interface composites. On the other hand, magnetic loss comes from natural ferromagnetic resonance, exchange resonance and eddy current loss in the microwave frequency band. The natural resonance frequency f r correlates to an anisotropy field H a which can be expressed by the natural-resonance equation: f r = γH a /2π, where γ/2π is the gyromagnetic ratio. The anisotropy field H a is given by H a = 2K/μ 0 M s , where K is the anisotropy constant and M s is the saturation magnetization. A high saturation magnetization (or a smaller anisotropy field) is ascribed to a red shift of the resonance frequency. In other words, a smaller anisotropy field improves the absorption bandwidth. For an excellent microwave-absorbing material, magnetic shielding requires conservation of its magnetic permeability over the GHz range, but it can be seen that at the cut-off frequency f r , permeability sharply decreases according to the Snoek’s limit, f r (μ − 1) ∝ M s . Hence, a high M s is required at high frequency f r . Magnetic metals and their alloys (Fe, FeNi, FeCo) possess high M s and good permeability, although their high conductive behavior produces eddy current losses resulting in reduced permeability at lower frequencies (in the MHz range). Fortunately, ferrites are semiconducting in nature, but these ferrites possess a significantly lower M s value and hence the f r occurs at the low GHz range. Therefore, the above-mentioned situations limit their use in the GHz range to the maximum bulk ferromagnetic materials. To overcome the above problem, researchers have focused on nano- or micro-sized materials because these low-dimension materials lower the eddy current loss.

2.2.2 Snoek’s limit. μ ( ϖ ) = 1 + χ sr ( ϖ ) + χ dw ( ϖ ) (16) where (17) and (18) where ϖ , ϖ sr and ϖ dw are the rf magnetic field, the spin resonance and domain wall motion resonance frequencies, respectively. The terms K sr and K dw define the static spin and domain wall motion susceptibilities while β is a damping factor of the domain wall motion. It was observed that only the spin rotational component remains in the higher frequency region; nevertheless the domain wall motion contribution diminishes. Thus at high frequencies (above 100 MHz), complex permeability is governed only by the spin rotational component. In terms of magnetization K sr and ϖ sr can be written as (19) and (20) where M s is the saturation magnetization, K 1 is the crystalline anisotropy, and γ is the gyro magnetic ratio. ϖ sr K sr = 2 C ′π γM s (21) Snoeks limit confers a boundary on the microwave permeability spectrum in magnetic materials. The complex permeability belongs to two type of magnetizing mechanisms: the domain wall and the spin rotation motion, where domain wall and spin rotational term contribution is of the resonance type and relaxation type. 11–13 Thus permeability is given bywhereandwhereandare the rf magnetic field, the spin resonance and domain wall motion resonance frequencies, respectively. The termsanddefine the static spin and domain wall motion susceptibilities whileis a damping factor of the domain wall motion. It was observed that only the spin rotational component remains in the higher frequency region; nevertheless the domain wall motion contribution diminishes. Thus at high frequencies (above 100 MHz), complex permeability is governed only by the spin rotational component. In terms of magnetizationandcan be written asandwhereis the saturation magnetization,is the crystalline anisotropy, andis the gyro magnetic ratio. At resonance frequency ϖ sr = ϖ r = 2πf r ϖ r ( μ − 1) ∝ M s (22) This is called Snoeks limit, which gives a limitation on the permeability in the case of ferrite.

2.2.3 Size, shape and morphology. E eddy ∝ area). It is believed that anisotropy energy dominates at the small size of the nanostructures due to the breaking of some exchange bonds. The change in anisotropy energy modifies the spin relaxation time or frequency. Apart from the bulk magnet situation, permeability in nanomaterials is governed by relaxation mechanisms, in contrast to the intrinsic resonance which predicts a constant permeability until relaxation. In the superparamagnetic state, spin fluctuation remains very fast due to its small size, hence relaxation occurs at higher frequencies. The dimensions of magnetic particles have a great impact on permeability. It is observed that below a critical small size, eddy current losses decreases due to the decrease in induced eddy voltage (∝ area). It is believed that anisotropy energy dominates at the small size of the nanostructures due to the breaking of some exchange bonds. The change in anisotropy energy modifies the spin relaxation time or frequency. Apart from the bulk magnet situation, permeability in nanomaterials is governed by relaxation mechanisms, in contrast to the intrinsic resonance which predicts a constant permeability until relaxation. In the superparamagnetic state, spin fluctuation remains very fast due to its small size, hence relaxation occurs at higher frequencies. 7,14–18 Furthermore, some complicated structures consisting of high porosity and large surface area introduced multi-interfaces that accumulate to bound charges at the interfaces, causing the Maxwell–Wagner effect. In addition, several surfaces within complicated geometries possess unsaturated bonds that are responsible for dipole polarization. Therefore, multi-interfaces are beneficial for electromagnetic attenuation due to conductivity loss and interfacial/dipole orientation polarization. Many Fe and Fe-alloy based systems have been reported that confirm the effect of magnetic anisotropy and relaxation processes. Bayat et al. have observed the effect of particle size and the thickness of material on the EMI performance of Fe 3 O 4 /CFs composites. When the particle size varies from 10–20 nm to 20–30 nm, then SE Total also varies from 47 dB to 68 dB. The above observation shows that larger size particles improve the electrical conductivity as they boost the graphitization of the carbon matrix. Thus, larger Fe 3 O 4 NPs increase the magnetic permeability of the composite and hence improve the shielding efficiency of the composite. Similarly, thickness variation revealed that a 0.1 mm to 0.7 mm sample thickness enhances SE T from 24 dB to 68 dB. This happened due to an increase in the conductive network, which enhances the SE A and total SE Total .

2.2.4 Temperature and time. It is a well known fact that heat treatment increases disorder and creates defects in the form of vacancies, dangling bonds or substitutions in materials, as observed in the ferrite system in which reflection loss is reversed by the annealing temperature. 19 These defects create an extra energy level around the Fermi level and hence enhance attenuation rather than reflection. Furthermore, reaction time and temperature also influence reflection loss, as reported for FeCo/ZnO composites 20 because of structural changes that occur as the time and temperature increase.

2.2.5 Mass ratio. σ = σ 0 ( V − V c ) c (23) where σ is the electrical conductivity of the materials, σ 0 is natural conductivity, V is the volume fraction of filler, V c is the volume fraction at the percolation threshold and c is the critical exponent. At the percolation threshold, conductive networks form within matrices. The percolation threshold depends on certain factors like the shape, morphology, aspect ratio and conductivity of the filler. Moreover, it also depends on the distribution, concentration and compatibility of the filler with the host matrix. 3 O 4 coated CNTs, reflection loss does not only depend on the Fe 3 O 4 coating structure, but is also related to the CNT-to-Fe 3+ mass ratio. This is because the mass ratio ultimately generates dielectric relaxation processes and also enhances the magnetic loss in the form of the eddy current effect. Generally, the electrical properties of any of material depend on the percolation threshold value of conductivity:whereis the electrical conductivity of the materials,is natural conductivity,is the volume fraction of filler,is the volume fraction at the percolation threshold andis the critical exponent. At the percolation threshold, conductive networks form within matrices. The percolation threshold depends on certain factors like the shape, morphology, aspect ratio and conductivity of the filler. Moreover, it also depends on the distribution, concentration and compatibility of the filler with the host matrix. 21 Above the percolation threshold, the properties of the composites start decreasing. For example, in elastomer composites a high volume fraction of filler (mostly metals) in the host matrices decreases the resilience of composites. For this region, a low volume fraction is most desirable. For example, Li and coworkers observed that, in nano Fecoated CNTs, reflection loss does not only depend on the Fecoating structure, but is also related to the CNT-to-Femass ratio. This is because the mass ratio ultimately generates dielectric relaxation processes and also enhances the magnetic loss in the form of the eddy current effect. 22

2.2.6 Thickness. min , of the microwave power occurs when the sample thickness, t , of the absorber approximates a quarter of the propagating wavelength multiplied by an odd number, that is (24) where n = (1, 3, 5, 7, 9…), so that n = 1 corresponds to the first dip at low frequency. The propagating wavelength in the material ( λ m ) is given by (25) Minimal reflection, RL, of the microwave power occurs when the sample thickness,, of the absorber approximates a quarter of the propagating wavelength multiplied by an odd number, that iswhere= (1, 3, 5, 7, 9…), so that= 1 corresponds to the first dip at low frequency. The propagating wavelength in the material () is given by The matching condition results in the cancellation of the incident and reflected waves at the surface of the absorber material, e.g. the dips for t = 7 mm occurred at the sample thicknesses 1.0(λ m /4), 3.0(λ m /4) and so on. Hence, with increasing sample thickness, reflection peaks shift toward the lower frequencies. Apart from sample thickness, coating on the surface of the Fe component also changes the microwave absorption properties. This can be attributed to EM wave dimensional resonance, which increases with the increase of coating thickness. Du et al. have shown the influence of shell thickness on the absorption properties of Fe 3 O 4 @C composites. The thickness of the carbon shell in Fe 3 O 4 @C was controlled in the range of 20–70 nm.23 A critical thickness of carbon shells shows superior dielectric behavior.

3 Measurement techniques

1 , S 2 ) indicate the incident and transmitted waves in terms of complex scattering S parameters ( i.e. S 11 or S 22 and S 21 or S 12 , respectively. These are known as the forward reflection coefficient ( S 11 ), the reverse reflection coefficient ( S 22 ), the forward transmission coefficient ( S 12 ) and the backward transmission coefficient ( S 21 ). Different conversion approaches such as the short circuit line (SCL), NIST iterative, delta-function method, new non-iterative, transmission line theory and Nicolson–Ross–Weir (NRW) technique have been adopted to obtain the characteristic parameters ( i.e. ε , μ , RL and Z ). The above conversion techniques also have some benefits and limitations. For instance, the short circuit line (SCL) method can estimate ε only, while the NIST iterative approach provides ε and μ but with the limitation μ = 1. Among them all, the NRW technique (presented by Nicolson and Ross in 1970 and by Weir in 1974) gives a direct calculation of complex permittivity and permeability from the input S -parameters. Therefore, the transmission line theory and the Nicolson and Ross and Weir algorithm are the more popular methods due to their ease of use. Z (Ω), RL (dB), SE A (dB), SE T (dB) and SE R (dB) can be obtained by using the following equations (26) log | S 11 | RL = 20log (27) (28) where T is the transmittance (29) where R is the reflectance (30) Summation of the reflectance ( R ), transmittance ( T ) and absorbance ( A ) is always equal to 1; R + T + A = 1 (31) Experimentally, network analyzer instruments are used to measure EMI shielding efficiency. There are two types of network analyzer: scalar network analyzers (SNA) and vector network analyzers (VNA). As its name indicates, the SNA measures signal amplitudes only, that is why it is not useful for measuring complex signals. On the other hand, the vector network analyzer (VNA) measures signal magnitude along with various phases. Therefore the VNA is a highly demanded and widely used instrument. In a VNA, its two ports (S, S) indicate the incident and transmitted waves in terms of complex scatteringparameters ( Fig. 3 ),orandor, respectively. These are known as the forward reflection coefficient (), the reverse reflection coefficient (), the forward transmission coefficient () and the backward transmission coefficient (). Different conversion approaches such as the short circuit line (SCL), NIST iterative, delta-function method, new non-iterative, transmission line theory and Nicolson–Ross–Weir (NRW) technique have been adopted to obtain the characteristic parameters (, RL and). The above conversion techniques also have some benefits and limitations. For instance, the short circuit line (SCL) method can estimateonly, while the NIST iterative approach providesandbut with the limitation= 1. Among them all, the NRW technique (presented by Nicolson and Ross in 1970 and by Weir in 1974) gives a direct calculation of complex permittivity and permeability from the input-parameters. Therefore, the transmission line theory and the Nicolson and Ross and Weir algorithm are the more popular methods due to their ease of use. 24 Parameters(Ω), RL (dB), SE(dB), SE(dB) and SE(dB) can be obtained by using the following equationswhereis the transmittancewhereis the reflectanceSummation of the reflectance (), transmittance () and absorbance () is always equal to 1;

Fig. 3 Reflected and transmitted EM wave in a filled transmission line.

Some researchers have also studied impedance matching by means of the delta-function method, in which the delta-function shows the impedance matching degree. The delta-function is given by following equation:9,25

| Δ | = |sin h 2 ( Kfd ) − M | (32)

K

M

δ

ε

δ

m

c

4 Materials used for EMI shielding

4.1 Iron (Fe) ingredient

2 kg −1 at 293 K, and a curie temperature, T C = 1043 K, above room temperature. Furthermore, iron is a very soft magnetic material compared to cobalt and possesses low magnetocrystalline anisotropy. For a few decades, design of Fe based nanostructures has increased greatly because nanostructured materials have many advantages such as a high aspect ratio, good porosity and the high magnetic moment (superparamagnetic behavior) of the nanomaterials compared to bulk materials. σ ∼ 10 7 S cm −1 ) and the strong skin effect at GHz high frequency. This is the main reason that Fe structures have been part of rather few studies. Some other studied Fe-based microwave materials include nanoparticles (NPs) and dendrite-like micro-structures that crystallize in bcc structures prepared by ball milling and hydrothermal process, respectively. The reflection loss of Fe NPs was observed by pelleting in a paraffin matrix, so for Fe/paraffin = 4/1, RL min = 11 dB at 13.6 GHz. A complete energy dissipation of the EM wave occurs means no reflection and satisfies the impedance matching condition Z = Z 0 , indicating the absence of an actual absorbing resonance. In case of dendrite-like micro-structures, M s is found to be higher than Fe nanoparticles but less than the bulk and hence attains an RL min = −25.0 dB (matching frequency 2.5 GHz, matching thickness 3 mm). The excellent microwave absorption properties of Fe dendritic microstructures could be result of their hierarchical morphology, providing surface defects and a large surface area. For the development of high performance microwave absorption materials, magnetic nanostructures have been of great interest in the last few years. Their low cost facile synthesis along with the high biodegradability and biocompatibility advantages of iron and other components have made them desirable materials relative to other transition-metals in terms of potential applications. In the earth’s crust, the transition metal iron is the fourth most ubiquitous material that forms the inner as well as the outer surface of the earth. Iron is one of the most promising candidates for several applications including catalysis, microwave absorption, water pollution treatment and magnetic materials and many others. Ion can exhibit from the +2 to the +7 oxidation state, nevertheless the +2 and +3 states are more common due to the ease of hopping of the charge carriers. Fe is well known as the highest room temperature ferromagnetic material with a high saturation magnetization of 218 A mkgat 293 K, and a curie temperature,= 1043 K, above room temperature. Furthermore, iron is a very soft magnetic material compared to cobalt and possesses low magnetocrystalline anisotropy. For a few decades, design of Fe based nanostructures has increased greatly because nanostructured materials have many advantages such as a high aspect ratio, good porosity and the high magnetic moment (superparamagnetic behavior) of the nanomaterials compared to bulk materials. 26 Pure Fe is found either in the body-centered cubic (bcc) structure or face-centered cubic (fcc) structures, but exhibits extreme sensitivity of the structure of iron to changes in air conditions (orthorhombic, spinel) and hence the properties (such as electrical, magnetic, optical) of the Fe material. The most common iron species are iron oxides, ferric oxide, magnetite, ferrous oxides (FeO) and iron hydroxide (FeOOH), as depicted in Fig. 4 . Although the fabrication of a magnetic iron nanostructure is quite difficult, much effort has been made to prepare Fe nanostructures using ball milling, DC arc plasma and sputtering methods. 27 Among these, Fe nanostructures such as nanoflakes, nanoparticles and core–shell (Fe as core coated with oxide shell) structures are evidently the more common structures, because oxide shells not only prevent Fe from oxidation in the presence of air, but also prevent the forefront reflections as previously observed in pure Fe sheets that show negligible microwave absorption, due to the good conductivity of Fe elements (∼ 10S cm) and the strong skin effect at GHz high frequency. This is the main reason that Fe structures have been part of rather few studies. Some other studied Fe-based microwave materials include nanoparticles (NPs) and dendrite-like micro-structures that crystallize in bcc structures prepared by ball milling and hydrothermal process, respectively. The reflection loss of Fe NPs was observed by pelleting in a paraffin matrix, so for Fe/paraffin = 4/1, RL= 11 dB at 13.6 GHz. A complete energy dissipation of the EM wave occurs means no reflection and satisfies the impedance matching condition, indicating the absence of an actual absorbing resonance. In case of dendrite-like micro-structures,is found to be higher than Fe nanoparticles but less than the bulk and hence attains an RL= −25.0 dB (matching frequency 2.5 GHz, matching thickness 3 mm). The excellent microwave absorption properties of Fe dendritic microstructures could be result of their hierarchical morphology, providing surface defects and a large surface area. 22,28–30 More importantly iron occurs in various shapes, size and dimensions, such as nanowires, nanoparticles, nanorods, nanotubes, hollow fibers, microspheres and dendrite-like microstructures 31 which enhance the reflection loss, but can moderate the conductivity. To overcome the above problem, iron oxides such as ferric oxide, magnetite and ferrous oxides (FeO) have been preferred for the design of effective microwave absorption materials because they are semiconductors (highly resistive). 32

Fig. 4 Different type of iron components.

4.1.1 Ferrites. Ferrites have iron oxide as their main constituent, along with other metal oxides. These materials have been used from more than half a century due to their interesting magnetic properties. Compared to iron, ferrites possess high resistivity (0.1–10 −5 Ω-m), high saturation magnetization and a tunable anisotropy field which make them a preferable choice in a wide range of applications such as bubble devices, the memory cores of computers and microwave devices, recording media, magnetic motors etc. Depending upon the crystal structure, ferrites can be classified into the following types:

4.1.1.1 Spinel ferrite. Spinel ferrites are given by formula PFe 2 O 4 , where tetrahedral and octahedral interstitial sites are designated with P (divalent metal ions like Cu, Co, Mn, Ni, Zn) and Fe, respectively. Considering its applicability in the microwave region, the spinel ferrites can be utilized as microwave-absorbing materials, because these ferrites have large magnetic losses and moderate conductivity (semiconductor property). However, spinel ferrites in MW-absorbing applications are restricted because of their low natural magnetic resonance frequency.

4.1.1.2 Garnet. 3 Fe 5 O 12 where Pe stand for a trivalent ion e.g. a rare earth element. These ferrites have a similar structure to spinel ferrites but with some extra sites (a dodecahedral c axis). Doping of cations in these sites may be helpful because lattice interaction with these sites may tune the physical properties of ferrites. This is described by PeFewhere Pe stand for a trivalent iona rare earth element. These ferrites have a similar structure to spinel ferrites but with some extra sites (a dodecahedralaxis). Doping of cations in these sites may be helpful because lattice interaction with these sites may tune the physical properties of ferrites. 33 Like spinel ferrites, garnet ferrites are soft ferromagnetic materials with high remanence, a large saturation magnetization and low coercivity. Moreover, the good chemical stability and EM compatibility of these ferrites show their potential for EMI suppression.

4.1.1.3 Ortho-ferrites. 3 , where Pe is a large trivalent metal or rare earth ion such as Bi or Y. These ferrites exhibit a weak/canted antiferromagnetism with affluent magnetic properties. For instance, ortho-ferrite shows a phase transition from paramagnetic to antiferromagnetic at 620–750 K. Moreover, these kind of ferrites possess excellent multi-ferroelectricity and tunable magnetic properties in which the interaction between Fe 3+ and Pe 3+ ions decides the magnetic properties of the ferrites. The general formula for these ferrites is PeFeO, where Pe is a large trivalent metal or rare earth ion such as Bi or Y. These ferrites exhibit a weak/canted antiferromagnetism with affluent magnetic properties. For instance, ortho-ferrite shows a phase transition from paramagnetic to antiferromagnetic at 620–750 K. Moreover, these kind of ferrites possess excellent multi-ferroelectricity and tunable magnetic properties in which the interaction between Feand Peions decides the magnetic properties of the ferrites. 34

4.1.1.4 Hexagonal ferrites. Hexagonal ferrites have a high magnetocrystalline anisotropy field and a planar anisotropy that improves their natural resonance in the upper gigahertz range. This property of hexagonal ferrites increases their versatility in a variety of applications. These ferrites crystallize in a hexagonal structure. Apart from spinel ferrites, the magneto-plumbite structure of these ferrites enables theme to working in the entire GHz range due to their high intrinsic magnetocrystalline anisotropy. 35 Hence, some of them have gained considerable technological importance in recent years. There are six type of hexagonal ferrites:

4.1.1.4.1 M-Type. M-Type ferrites are given by the formula PFe 12 O 19 where P = Ba, Sr, Mg, Pb etc. These ferrites are composed of the form SRS*R*, in which R and S indicate the three and two oxygen-ion layer blocks. The large magneto-crystalline anisotropy, inexpensive price, high Curie temperature and competent saturation magnetization properties of these kind of ferrites stand them as effective microwave materials.

4.1.1.4.2 Y-Type. 2 Q 2 Fe 12 O 22 where P = Ba, Sr, Mg, Pb, and Q = Cu, Co, Zn etc. The magnetic properties of these type of ferrites are greatly susceptible to their crystalline structure, especially in presence of a magnetic environment. Thus, the addition of divalent, trivalent and tetravalent species in these hexaferrites controls their magnetic characteristics in order to obtain improved microwave absorption. The Y-type ferrites are ferrimagnetic materials, generally given by the formula PFewhere P = Ba, Sr, Mg, Pb, and Q = Cu, Co, ZnThe magnetic properties of these type of ferrites are greatly susceptible to their crystalline structure, especially in presence of a magnetic environment. Thus, the addition of divalent, trivalent and tetravalent species in these hexaferrites controls their magnetic characteristics in order to obtain improved microwave absorption. 36

4.1.1.4.3 W-Type. 2 Q 2 Fe 16 O 27 . The crystal structures of these ferrites are closely related to the M-type. The characteristics of these ferrites depend on their particle size or morphology, synthesis method and the distribution of the cations in the crystal structure. These hexagonal ferrites are made up of the structure SSRS*S*R* in which R is a three oxygen-ion layer block with a composition of PFe 6 O 11 , S is a two oxygen-ion layer block with the composition of Fe 6 O 8 , called the spinel block. In the above equation, an asterisk indicates the rotation of the block by 180° along the hexagonal axis. The W-type structure composed of spinel blocks is twice as thick with respect to the M-type hexagonal structure. W-Type ferrites are given by the formula PFe. The crystal structures of these ferrites are closely related to the M-type. The characteristics of these ferrites depend on their particle size or morphology, synthesis method and the distribution of the cations in the crystal structure. These hexagonal ferrites are made up of the structure SSRS*S*R* in which R is a three oxygen-ion layer block with a composition of PFe, S is a two oxygen-ion layer block with the composition of Fe, called the spinel block. In the above equation, an asterisk indicates the rotation of the block by 180° along the hexagonal axis. The W-type structure composed of spinel blocks is twice as thick with respect to the M-type hexagonal structure. 37,38

4.1.1.4.4 X-Type. X-Type ferrites are represented by the formula P 2 Q 2 Fe 28 O 46 . These are composed by the structre 3(SRS*S*R*). X-Type hexagonal ferrites can be considered as a mixture of M and W-type hexagonal ferrites. In comparison with M and W-type hexagonal ferrites, these ferrites possess a larger Curie temperature and saturation magnetization, and hence work as excellent microwave absorbing materials.

4.1.1.4.5 Z-Type. 3 Q 2 Fe 14 O 41 . These hexagonal ferrites have much good permeability and a higher resonance frequency ( f r ) in comparison with spinel ferrites. That is why these ferrites are only used in microwave devices like antennas, inductors and absorbers etc. 39 The Z-type ferrites are given by the formula PFe. These hexagonal ferrites have much good permeability and a higher resonance frequency () in comparison with spinel ferrites. That is why these ferrites are only used in microwave devices like antennas, inductors and absorbers

4.1.1.4.6 U-Type. 4 Q 2 Fe 36 O 60 . Among the hexagonal ferrites, the U-type ferrites possess better thermal stability, a large magnetic anisotropy ( H a ) and a large saturation magnetization ( M s ). The U-type hexagonal ferrites are represented by the formula PFe. Among the hexagonal ferrites, the U-type ferrites possess better thermal stability, a large magnetic anisotropy () and a large saturation magnetization (). 35 Therefore these ferrites have been used in many studies on EMI applications.

4.1.2 Ferric oxide (Fe 2 O 3 ). 2 O 3 is the most common oxide of iron. Therefore, it is one of the most extensively used biomaterials in different applications like cell separation and drug delivery etc. Fe 2 O 3 occurs in an amorphous form and consists of four polymorphs (alpha, beta, gamma and epsilon). 2 O 3 has a rhombohedral–hexagonal type structure, whereas γ-Fe 2 O 3 shows a cubic spinel structure, as shown in 2 O 3 and ε-Fe 2 O 3 polymorphs have cubic bixbyite and orthorhombic structures. The α- and β-Fe 2 O 3 are termed antiferromagnetic and paramagnetic materials, respectively. Hence these are extremely useful in photocatalysis, conversion of pigments, solar energy and water treatment. In contrast, γ and ε-Fe 2 O 3 possess ferromagnetism 2 O 3 and γ-Fe 2 O 3 have been widely investigated in EMI shielding applications. Different composites comprising α-Fe 2 O 3 in attractive morphologies such as popcorn-like α-Fe 2 O 3 , coin-like α-Fe 2 O 3 , watermelon-like α-Fe 2 O 3 microspheres, α-Fe 2 O 3 nanorods and hollow γ-Fe 2 O 3 have been studied and have shown excellent microwave performance. In general, thermo-chemical, two step hydrothermal, solvothermal, chemical reduction and sol–gel approaches are some of the reported methods which have been employed to prepare Fe 2 O 3 based composites. Among the iron oxides, biocompatible Feis the most common oxide of iron. Therefore, it is one of the most extensively used biomaterials in different applications like cell separation and drug deliveryFeoccurs in an amorphous form and consists of four polymorphs (alpha, beta, gamma and epsilon). 31 The multitudinous polymorph structures α and γ named as hematite and maghemite, respectively. The α-Fehas a rhombohedral–hexagonal type structure, whereas γ-Feshows a cubic spinel structure, as shown in Fig. 5a . On the other hand, the β-Feand ε-Fepolymorphs have cubic bixbyite and orthorhombic structures. The α- and β-Feare termed antiferromagnetic and paramagnetic materials, respectively. Hence these are extremely useful in photocatalysis, conversion of pigments, solar energy and water treatment. In contrast, γ and ε-Fepossess ferromagnetism 40 so that these are particularly useful in bio-medicine. α-Feand γ-Fehave been widely investigated in EMI shielding applications. Different composites comprising α-Fein attractive morphologies such as popcorn-like α-Fe, coin-like α-Fe, watermelon-like α-Femicrospheres, α-Fenanorods and hollow γ-Fehave been studied and have shown excellent microwave performance. In general, thermo-chemical, two step hydrothermal, solvothermal, chemical reduction and sol–gel approaches are some of the reported methods which have been employed to prepare Febased composites. 41–46

Fig. 5 Crystal structure of (a) Fe 2 O 3 , (b) Fe 3 O 4 , and (c) FeO materials.

4.1.3 Magnetite (Fe 3 O 4 ). 3 O 4 is the most comprehensively investigated magnetic nanostructure because of its ease of synthesis, high biocompatibility, superparamagnetic nature, high chemical stability, low toxicity etc. Several low cost preparation methods of Fe 3 O 4 nanostructures can be found in the literature such as sol–gel, solvothermal, co-precipitation and magnetic separation methods etc. As a result, magnetite has versatile applications in fields of magnetic storage devices, food analysis, magnetic resonance imaging (MRI), segregation of biomolecules, hyperthermia, and EMI applications, 3 O 4 nanostructures possess a cubic inverse spinel structure with two Fe 3+ and one Fe 2+ valence state in which oxygen frames a fcc closed-pack structure, as depicted in 2+ and Fe 3+ . 3 O 4 nanostructure make it a favorable candidate for magnetic/electric attenuation sources in the EMI shielding mechanism. Fe 3 O 4 has an abundant number of morphologies e.g. it occurs in sandwich-like Fe 3 O 4 , dendritic forms, and as nanorods, nanoparticles, microspheres and nanospindles. Thus, Fe 3 O 4 can be considered as a good choice for energy applications, including EMI. Among all the Fe oxides, Feis the most comprehensively investigated magnetic nanostructure because of its ease of synthesis, high biocompatibility, superparamagnetic nature, high chemical stability, low toxicitySeveral low cost preparation methods of Fenanostructures can be found in the literature such as sol–gel, solvothermal, co-precipitation and magnetic separation methodsAs a result, magnetite has versatile applications in fields of magnetic storage devices, food analysis, magnetic resonance imaging (MRI), segregation of biomolecules, hyperthermia, and EMI applications, 47 particularly in the field of magnetism owing to its high magnetic moment. Moreover, Fenanostructures possess a cubic inverse spinel structure with two Feand one Fevalence state in which oxygen frames a fcc closed-pack structure, as depicted in Fig. 5b . It is an indispensable kind of half-metallic material in which electron hopping takes place between the Feand Fe 48,49 Consequently, the outstanding magnetic/dielectric properties of the Fenanostructure make it a favorable candidate for magnetic/electric attenuation sources in the EMI shielding mechanism. Fehas an abundant number of morphologiesit occurs in sandwich-like Fe, dendritic forms, and as nanorods, nanoparticles, microspheres and nanospindles. Thus, Fecan be considered as a good choice for energy applications, including EMI.

4.1.4 Wüstite (FeO). II ) oxide (FeO) has a cubic (rock salt) structure in which iron and oxygen atoms are octahedrally coordinated to each other, as depicted in et al. prepared for the first time Fe@FeO dispersions in a polyurethane (PU) matrix. 2 exhibits better performance than Fe@FeO and Fe@FeO/PU composites because the silica shell significantly reduces the eddy current loss and causes an upsurge in the anisotropy energy. Iron() oxide (FeO) has a cubic (rock salt) structure in which iron and oxygen atoms are octahedrally coordinated to each other, as depicted in Fig. 5c . FeO is not stable at normal temperature and hence shows high temperature and pressure stability only above 560 °C ( ref. 50 ) which results in high costs of preparation and limits its potential application. Therefore, FeO has rarely been studied. Zhuprepared for the first time Fe@FeO dispersions in a polyurethane (PU) matrix. 51 It was seen that Fe@FeO NPs became magnetically harder after being dispersed in the PU matrix. Fe@FeO/PU possess a significant eddy current effect hence RL is >20 dB even at larger absorber thicknesses. Nevertheless, a coating of SiOexhibits better performance than Fe@FeO and Fe@FeO/PU composites because the silica shell significantly reduces the eddy current loss and causes an upsurge in the anisotropy energy.

4.1.5 Iron oxy-hydroxide (FeOOH). Iron( III ) oxy-hydroxide occurs in following forms: goethite (α-FeOOH), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH) and feroxyhyte (δ-FeOOH). These are widely used in electrode materials and lithium batteries. Iron( III ) oxy-hydroxide has poor magnetic as well as electrical properties, which are a primary requirement for EMI applications. Therefore iron( III ) oxy-hydroxide materials are not very popular among material scientists.

4.1.6 Carbonyl iron (CI). Finally, carbonyl iron (CI) is another captivating magnetic absorbing material that has attracted much attention due to its virtuous properties including superior saturation magnetization, a high Curie temperature, and a high magnetic loss with low permittivity. Interestingly, the magnetic properties of CI are tunable in accordance with its size, morphology and shape. In fact, planar anisotropy as observed in CI nanoflakes, effectively improves the Snoek’s limit which increases the permeability and resonance frequency at the same time. Besides, in the high frequency range, such flakes-type structures can ignore the skin effect. 52

Although these Fe materials offer several advantages, their high density, heavy weight, processing difficulties, flexibility and narrow absorption bandwidth impede their further application. Fe materials suffer from the skin depth problem; on the other hand ferrites are restricted by Snoek’s limit. As we explained earlier, to design an excellent microwave absorbing material, one needs to optimize its permeability and permittivity, due to the magnetic/dielectric loss capabilities of EM energy. Hence, poor permittivity in comparison to permeability is the main drawback of these Fe oxides. Accordingly, scientists have mainly concentrated on materials which show the complementary relation between permittivity and permeability. In this direction, conducting polymers and carbon based materials have attracted the attention of researchers. Many strategies have been employed to develop effective shielding materials.

5 Anchoring of metal oxides

Anchoring of transition metal oxides such as ZnO, ZrO 2 , MnO 2 , SnO 2 , BaTiO 3 , TiO 2 , SiO 2 with Fe ingredient enhance the permittivity of EMI preventing materials. Thus the combination of these oxides with Fe ingredients significantly improve the dielectric losses and magnetic losses in materials by mean of double attenuation mechanism which is accountable for superior microwave absorption performance. Their cheap, natural richness and environmentally friendly properties make them more accessible for EMI shielding. To date, numerous Fe and transition metal oxides with great EM properties have been explored. However, these semiconductor oxides are restricted at the high GHz range due to their lack of permittivity. Moreover, processing-related difficulties, agglomeration during synthesis and poor dispersion are major drawbacks in the use of Fe/metal oxides composites.

6 Conducting polymers (CPs)

−3 ) than iron (7–8 g cm −3 ), gentle processing and preparation conditions, structural flexibility, and most importantly tunable conductivity (0.1–10 −10 S cm −1 ). Conductive polymers have various applications in sensing, metal corrosion protection, and specifically in energy storage like electromagnetic shielding and microwave absorption. The peculiarities of conducting polymers are believed to depend on their doping level, dopant ion size, water content and protonation level. Two well-known methods have been reported to prepare CPs: CPs are either prepared by electrochemical oxidative polymerization, or by the chemical oxidative polymerization method. Chemical oxidative ( in situ ) polymerization is the most frequently used method to prepare such polymer composites, and is also known as the chemical encapsulation technique. In this method, a filler such as Fe 3 O 4 nanoparticles are first dispersed in a liquid monomer. The polymerization reaction is initiated by heat/radiation, the diffusion of the appropriate initiator takes place, then the organic initiator/catalyst is set on the surface of the nanoparticles under the required temperature, pressure and stimulation (stirring) conditions, as shown in para -phenylene) (PPP) and poly(phenylenevinylene) (PPV) are of particular interest due to their easy availability, environmental sustainability, cost-effectiveness and versatile doping chemistry. In comparison to conventional metals and semiconductors, conducting polymers (CPs) possess exclusive properties such as a lower density (1–1.3 g cm) than iron (7–8 g cm), gentle processing and preparation conditions, structural flexibility, and most importantly tunable conductivity (0.1–10S cm). Conductive polymers have various applications in sensing, metal corrosion protection, and specifically in energy storage like electromagnetic shielding and microwave absorption. The peculiarities of conducting polymers are believed to depend on their doping level, dopant ion size, water content and protonation level. Two well-known methods have been reported to prepare CPs: CPs are either prepared by electrochemical oxidative polymerization, or by the chemical oxidative polymerization method. Chemical oxidative () polymerization is the most frequently used method to prepare such polymer composites, and is also known as the chemical encapsulation technique. In this method, a filler such as Fenanoparticles are first dispersed in a liquid monomer. The polymerization reaction is initiated by heat/radiation, the diffusion of the appropriate initiator takes place, then the organic initiator/catalyst is set on the surface of the nanoparticles under the required temperature, pressure and stimulation (stirring) conditions, as shown in Fig. 6 . In fact, fabrication of polymer nanocomposites is a hybridization process between the organic/inorganic polymer matrix and the inorganic/organic nanofiller to achieve a single material which comprises integrated properties with respect to the matrix and filler only. 53 This method also helps the modulation of shell thickness in the case of a core–shell structure just by controlling the weight ratio of the monomer and the Fe-based nanostructure, which influences the EM wave absorption properties effectively. According to dissipation mechanisms, microwave absorbing materials show dielectric loss and magnetic loss. In microwave absorbing materials, conducting polymers (CP) serve as dielectric loss materials which makes them the most attractive candidate. 54 Among the various conducting polymers polyaniline (PANI), polypyrrole (PPy), poly(3,4-ethylenedioxythiophene) (PEDOT), polythiophene (PT), polyfuran (PF), poly(-phenylene) (PPP) and poly(phenylenevinylene) (PPV) are of particular interest due to their easy availability, environmental sustainability, cost-effectiveness and versatile doping chemistry.

Fig. 6 The in situ polymerization method of preparation of conductive and insulating polymers.

6.1 Polyaniline (PANI) polymer

−10 S cm −1 . Moreover, PANI is a biocompatible and anti-corrosive polymer which has a controllable dielectric loss ability and is feasible for composition with micro/nano-sized magnetic metals. 3 O 4 and Fe 2 O 3 and dielectric materials such as TiO 2 , SiO 2 , and ZnO are widely used in polymer composites. At broad GHz range, however, these dielectrics suffer from a lack of permittivity. For this purpose, carbonaceous materials such as graphene, MWCNT and RGO have also been used with these polymers. Among the different conducting polymers, polyaniline (PANI) is one of the most commonly used polymers as a host material for micro/nano-sized nanofillers owing to its unique physico-chemical properties. These polymers show improved mechanical properties (tensile strength and elongation at break), thermal stability and particularly enhanced electrical conductivity and magnetic properties; these are the prerequisites for the design of effective EMI shielding materials. In comparison with other CPs, PANI is one of the oldest CPs and was first highlighted in 1862 due to the oxidation of an aniline monomer in sulphuric acid. The conductivity of PANI lies between 0.1 and 10S cm. Moreover, PANI is a biocompatible and anti-corrosive polymer which has a controllable dielectric loss ability and is feasible for composition with micro/nano-sized magnetic metals. 55 Over the last two decades, many efforts have been made to prepare composites comprising polymers and nanofillers. However, improvement of the electric and magnetic properties of the filler/polymer composites are insufficient to design effective EMI shielding materials; one important factor that is still required is how to combine influentially the permeability and permittivity of the these composites. To fulfil these conditions ferromagnetic materials possessing high permeability such as Fe, Feand Feand dielectric materials such as TiO, SiO, and ZnO are widely used in polymer composites. At broad GHz range, however, these dielectrics suffer from a lack of permittivity. For this purpose, carbonaceous materials such as graphene, MWCNT and RGO have also been used with these polymers.

6.2 Polypyrrole (PPy) polymer

After the PANI polymer, polypyrrole (PPy) is another most promising conductive polymer because of its tunable stability and ease of preparation, but suffers from poor mechanical strength and processability problems along with insolubility and infusiblity. 56 These drawbacks hinder its commercial application. To conquer the above problems, magnetic/metal nano-fillers as inorganic filler can be used with PPy to integrate the electro-magnetic properties of polymer composites. These nanoscale fillers have received increased interest due to their intriguing properties arising from their large surface area and nanosize in the host matrix. When the proper combination of magnetic nanofillers along with dielectric materials are encapsulated within the PPy polymer matrix then these polymer composites provide a new perspective to tune the dielectric and permeability properties of magnetic and dielectric materials in a different way by the control of the polymer structure and functionalisation.

6.3 Poly(3,4-ethylenedioxythiophene) (PEDOT) polymer

2 O 3 , Fe or Fe 3 O 4 components. Among the conductive polymers, a polythiophene derivative poly(3,4-ethylenedioxythiophene) (PEDOT) possesses a moderate band gap, controllable electrical conductivity, attractive electrochemical activity. Interestingly, the light weight, easy synthesis processes, good environmental stability, and dielectric loss ability properties of PEDOT make it a promising microwave absorbing material. It is a well known fact that poor EM impedance matching is attributable to magnetic or dielectric loss only. For this purpose, to achieve excellent microwave absorption performance PEDOT has been used with magnetic γ-Fe, Fe or Fecomponents. 47

6.4 Polythiophene (PT) polymer

Similar to other conducting polymers, polythiophene (PT) is used in anti-corrosion devices, rechargeable batteries and chemical sensors. Similar to other CPs, the conductivity of the PT materials could be controlled from a conducting to an insulating nature by only changing the polymerization route. Nevertheless, its poor solubility restricted its commercial use in many applications. Besides, the inert sulphur atom in thiophene enhances the oxidation potential, which makes the fabrication of polythiophene more complicated. 57 Therefore, these polymers have been the subject of few studies.

7 Nonconducting polymers nanocomposites

in situ polymerization methods are used for the preparation of these polymer-containing composites. In the solution mixing method the polymer and filler are dissolved or dispersed in a common solvent and undergo a stirring and sonication process until the complete mixing/blending of matrix and filler occurs, followed by casting and drying of the as-prepared composites. In melt blending/mixing, the polymer is melted at high temperature. To avoid the use of a solvent, the mixing of filler and matrix (polymer) takes place at high temperature followed by cooling and drying, as shown In situ polymerization processes have been generally used to synthesize nanocomposites having insoluble and thermally unsteady matrices (insulating polymers) that can not be developed by solution/melting methods. For more details, some of these extrinsic polymers are explained in later section. Though the conductive polymers have many advantages, they suffer from a lack of flexibility and processability during the large scale production of materials. Herein, insulating polymers like rubber and resin have been utilized as alternative substrates for conductive polymers. This is because non-conducting/extrinsic polymer synthesis processes are very cheap, easy, time sparing and environmentally stable. In addition, they can be prepared on large-scale quantities. To overcome the poor electrical, thermal and mechanical properties of these insulating polymers, metal, alloy and carbon nano fillers are often mixed into the polymer matrices to enhance the mechanical strength, conductivity and permeability, which improves reflection well as absorption, depending on the filler characteristics. Mostly facile solution mixing, melt mixing andpolymerization methods are used for the preparation of these polymer-containing composites. In the solution mixing method the polymer and filler are dissolved or dispersed in a common solvent and undergo a stirring and sonication process until the complete mixing/blending of matrix and filler occurs, followed by casting and drying of the as-prepared composites. In melt blending/mixing, the polymer is melted at high temperature. To avoid the use of a solvent, the mixing of filler and matrix (polymer) takes place at high temperature followed by cooling and drying, as shown Fig. 6 and 7 polymerization processes have been generally used to synthesize nanocomposites having insoluble and thermally unsteady matrices (insulating polymers) that can not be developed by solution/melting methods. For more details, some of these extrinsic polymers are explained in later section.

Fig. 7 Method of preparation of extrinsic polymers: (1) solution mixing and (2) melt mixing methods.

7.1 Polyvinylidene fluoride (PVDF) polymer

2 O 4 /RGO. 2 O 4 , RGO and PVDF, and PVDF and MnFe 2 O 4 . This analysis shows that PVDF takes a part in impedance mismatching and improved the performance. The fantastic piezoelectric behavior, light weight, compact size, good flexibility, and excellent dielectric properties of the PVDF thermoplastic open up the door to wide applications in various fields. Polyvinylidene fluoride (PVDF) (transparent to light) is a semi-crystalline polymer having significant thermal stability and good chemical resistance among polymers. It occurs in five crystalline phases α, β, γ, δ and ε, each with different chain conformations. Hence, PVDF as a matrix in nanocomposites is one of the key parameters for a wide range of applications. Pure PVDF has poor EMI shielding properties, 6 but the addition of Fe based nanoparticles within the PVDF matrix improves its conductivity and enhances its response by capitalizing on the nature and properties of the nanoscale filler. In this direction, Zheng and coworkers investigated the microwave properties of the PVDF polymer with the nanofiller MnFe/RGO. 58 They found that the composites had a minimum reflection loss of 29.0 dB at 9.2 GHz at a 5 wt% loading of filler. Moreover, a high dielectric loss and magnetic loss occur due to synergistic effect between RGO and MnFe, RGO and PVDF, and PVDF and MnFe. This analysis shows that PVDF takes a part in impedance mismatching and improved the performance.

7.2 Thermosetting polymers

Versatile thermosetting-resin-based composites offer good adhesion, resistance to corrosion, high strength and stability. These polymers commonly establish a good dispersion and interfacial adhesion between the filler and the polymers. Epoxy resins are one of the most important thermosetting resins, especially for industrial applications. In general, polyurethanes (PU) and polyethylene terephthalate (PET) polymers (after epoxy) are used to suppress EMI pollution. However, these polymers do not respond in presence of EM waves due to their insulating behavior. Therefore, they are widely used with conducting polymers and carbon materials along with Fe materials. Moreover, thermosetting polymers like epoxy compounds are also used as binders with Fe materials that prevent the aggregation of Fe nanostructures and serve as ideal dispersing materials.

7.3 Elastomeric polymers

e.g. low Young’s modulus). Ethylene-vinyl acetate (EVA), ethylene-propylene-diene monomer (EPDM) and nitrile rubber (NBR) are some examples of these synthetic rubbers. Apart from some weaknesses, rubber has excellent weathering resistance, resistance to aging, and chemical resistance along with good compatibility with many kinds of fillers. Elastomers are polymers which exhibit visco-elasticity and are bounded with weak intermolecular forces. These polymers are insulating in nature and have poor physico-mechanical properties (low Young’s modulus). Ethylene-vinyl acetate (EVA), ethylene-propylene-diene monomer (EPDM) and nitrile rubber (NBR) are some examples of these synthetic rubbers. Apart from some weaknesses, rubber has excellent weathering resistance, resistance to aging, and chemical resistance along with good compatibility with many kinds of fillers. 59 Therefore, these elastomers have been used with magnetic Fe ingredients and conducting polymers or carbon materials, which improve its conductivity and enhance the EMI performance.

7.4 Other polymers

p -phenylenevinylene) (PPV), polypropylene (PP), polyvinyl butyral (PVB), polyvinyl alcohol (PVA), polyethylenimine (PEI) and polycarbonate, along with blends (PC (polycarbonate)/SAN [poly(styrene- co -acrylonitrile)]) and polymer composites have also been studied. Akinay et al. synthesized polyvinyl butyral (PVB)/Fe 2 O 4 and (PVB)/NiFe 2 O 4 composites and observed that the composites exhibit good RL min performances in the 1–14 GHz range. In NiFe 2 O 4 /PVB composites, percolation of NiFe 2 O 4 particles within the PVB matrix resulted in good RL min values. In contrast, the overall microwave absorption performance was better in Fe 3 O 4 /PVB in comparison with (PVB)/NiFe 2 O 4 . In similar way, Yao et al. reported better EMI performance of PVC/graphene/Fe 3 O 4 composites. PVC composites have negligible EMI SE T due to their insulating behavior. In comparison to pure PVC, the addition of 5 wt% graphene and 5 wt% Fe 3 O 4 nanoparticles form sufficient conducting interconnected graphene–Fe 3 O 4 networks in the insulating PVC matrix. Hence graphene/Fe 3 O 4 /PVC obtained an improved S T value compared to PVC/Fe 3 O 4 and PVC/graphene composite. Apart from the polymer matrices discussed earlier, other polymers such as polyvinylpyrrolidone (PVP), polyvinyl chloride (PVC), poly(-phenylenevinylene) (PPV), polypropylene (PP), polyvinyl butyral (PVB), polyvinyl alcohol (PVA), polyethylenimine (PEI) and polycarbonate, along with blends (PC (polycarbonate)/SAN [poly(styrene--acrylonitrile)]) and polymer composites have also been studied. Akinay 60 synthesized polyvinyl butyral (PVB)/Feand (PVB)/NiFecomposites and observed that the composites exhibit good RLperformances in the 1–14 GHz range. In NiFe/PVB composites, percolation of NiFeparticles within the PVB matrix resulted in good RLvalues. In contrast, the overall microwave absorption performance was better in Fe/PVB in comparison with (PVB)/NiFe. In similar way, Yao 61 reported better EMI performance of PVC/graphene/Fecomposites. PVC composites have negligible EMI SEdue to their insulating behavior. In comparison to pure PVC, the addition of 5 wt% graphene and 5 wt% Fenanoparticles form sufficient conducting interconnected graphene–Fenetworks in the insulating PVC matrix. Hence graphene/Fe/PVC obtained an improvedvalue compared to PVC/Feand PVC/graphene composite.

8 Carbonaceous materials

Carbonaceous materials with unique characteristics such as low density, high permittivity, excellent conductivity, high chemical, thermal and mechanical stability are a current fields of growing interest scientifically as well as technically. These materials offer a great opportunity to fabricate a lot of varieties of new generic materials, with tunable optical, electrical, mechanical and magnetic properties. Most importantly, the high permittivity of carbonaceous materials establishes complementary behavior between the Fe ingredients and the carbon based materials that make it suitable for EMI applications. It is a well known fact that in the universe, after the evolution of hydrogen, helium, and oxygen, carbon (C) is the fourth most common chemical element. Pure carbon occurs in two main ordered lattice structures: diamond and graphite, shown in Fig. 8(a and b) . Diamond has many industrial uses like cutting, and polishing of equipment, along with some scientific applications. Moreover, diamond is the hardest natural material, highly thermally conductive and electrically insulating (band gap ∼ 5.5 eV), as well as valuable and venerable; these properties cause it to be disfavoured in potential energy applications. On the other hand, graphite is soft, lubricating and electrically conductive. Furthermore, carbon possesses various allotropes, as depicted in Fig. 9(a–c) , comprising 2D graphene, 0D buckminsterfullerene and 1D carbon nanotubes (single wall and multi wall). These lightweight carbonaceous materials and their derivatives with Fe ingredients serve as excellent candidates for the design of effective EM reflection/or absorption materials. A brief introduction to some carbon materials which are usually used in EMI shielding applications, along with their pro and cons, is given in a later subsection.

Fig. 8 (a) Crystal structure of diamond and (b) graphite.

Fig. 9 (a) 2D graphene, (b) carbon nanotube, (c) fullerene, (d) expanded graphite, (e) graphene oxide and (f) reduced graphene oxide.

8.1 Graphite/expanded graphite

2 SO 4 acid treatment, is mostly used to prepare the composites. e.g. , EG is 2-dimensional, consisting of a small stack of graphite layers, low cost and has poor resistivity and high mechanical stability ( Graphite is a traditional carbon material which has a layered lattice consisting of hexagonal rings of carbon atoms attached by weak van der Waals forces in different planes. Within a plane, the carbon atoms are joined together by covalent bonds. As a low cost, lightweight lubricant graphite possess good electrical conductivity, a high aspect ratio, and good mechanical and thermal stability that establish it as attractive filler in several potential applications in the fields of electronic, optical and energy devices. However, the major drawback of graphite is its poor dispersion in solvents. Therefore, functionalized graphite, produced by HCl and HSOacid treatment, is mostly used to prepare the composites. 62 Apart from conventional graphite, more and more interest is being extended to expanded graphite. Expanded graphite (EG), obtained by thermal treatment, has many advantages,, EG is 2-dimensional, consisting of a small stack of graphite layers, low cost and has poor resistivity and high mechanical stability ( Fig. 9d ). The major problem of using these materials is their poor magnetic properties that restrict their practical application. Therefore, anchoring of Fe ingredients with graphite or expanded graphite integrates their magnetic properties due to the synergistic effect between iron and the graphite and thus enhancing the EMI performance in the microwave region.

8.2 Graphene

2 bonding, as depicted in 2 g −1 theoretical value) and extraordinary electrical and thermal stability. Moreover, the theoretical dielectric loss of graphene is found to be superior than conventional oxide materials like ZnO, TiO 2 or SnO 2 . Graphene (G or GNS) is defined as a 2-dimensional (2D) allotrope of carbon atom formed by a single atomic layer of a honeycomb hexagonal lattice that hybridizes by spbonding, as depicted in Fig. 9a . Graphene has an amazing mechanical strength with good elasticity, excellent electrical conductivity, superior thermal conductivity, extremely high surface area (∼2630 mtheoretical value) and extraordinary electrical and thermal stability. Moreover, the theoretical dielectric loss of graphene is found to be superior than conventional oxide materials like ZnO, TiOor SnO 5 Several preparation methods of graphene, including top-down or bottom-up approaches and chemical vapor deposition (CVD) have been reported till now. However, these physical methods do not offer the large scale production of graphene. Additionally, the lack of surface functionalities and the excessively high carrier mobility of graphene is also harmful for EM absorption, creating impedance mismatching between air and the material. Hence graphene’s derivatives such as graphene oxide (GO) and reduced graphene oxide (RGO) are more broadly used as alternative to graphene in practical applications.

8.2.1 Graphene oxide (GO). i.e. COOH, O–H) expand the layer separation within graphite along the c axis and make it hydrophilic. This hydrophilicity enables us to extract graphene oxide after water sonication ( e.g. hydrogen bond) among the organic groups on the GO surface and the polymers. As reported by Samadi et al. for Fe 3 O 4 –GO/PVDF composites show better electromagnetic microwave absorption than pure PVDF. GO in Fe 3 O 4 –GO/PVDF composites does not only affect the reflection loss and absorption bandwidth but also has a great impact on the α-to-β phase transformation of the PVDF crystals. To evaluate quantitatively the EMI performance by GO we shall discuss its electro-magnetic properties. The disruption of sp 2 bonding in GO diminishes its electrical properties. Hence GO acts as an electrical insulator, directly this is not very useful. However, the Fe components improve its conductivity to a certain extent. Furthermore, to recover the honeycomb structure of GO, different methods like reflux, hydrothermal and sol–gel approaches have been employed. It has been demonstrated that when graphite is oxidized with strong oxidizing agents, the resulting attached oxygen functionalities, carboxyl, carbonyl, hydroxyl and epoxy groups (COOH, O–H) expand the layer separation within graphite along theaxis and make it hydrophilic. This hydrophilicity enables us to extract graphene oxide after water sonication ( Fig. 9e ). The most appealing property of GO is easy dispersion in either kind of solvent (organic or inorganic), because organic groups pave the way for GO to be modified easily by other materials. Moreover, GO can be well dispersed in a polymer matrix because of the strong and specific interactions (hydrogen bond) among the organic groups on the GO surface and the polymers. As reported by Samadi 63 for Fe–GO/PVDF composites show better electromagnetic microwave absorption than pure PVDF. GO in Fe–GO/PVDF composites does not only affect the reflection loss and absorption bandwidth but also has a great impact on the α-to-β phase transformation of the PVDF crystals. To evaluate quantitatively the EMI performance by GO we shall discuss its electro-magnetic properties. The disruption of spbonding in GO diminishes its electrical properties. Hence GO acts as an electrical insulator, directly this is not very useful. However, the Fe components improve its conductivity to a certain extent. Furthermore, to recover the honeycomb structure of GO, different methods like reflux, hydrothermal and sol–gel approaches have been employed.

8.2.2 Reduced graphene oxide (RGO). 4 or NaOH etc. Reduced graphene oxide (RGO) is the most studied carbon derivative due to its cost effective preparation, good flexibility, superior electric/thermal conductivity and attractive barrier properties. et al. investigated the microwave absorption properties of chemically reduced graphene oxide. They observed that residual defects and organic groups within RGO not only improved the individual impedance matching but also produced energy transitions from the continuous states to the Fermi level. Furthermore, these peculiarities introduce relaxation polarization, defect polarization relaxation and electronic dipole relaxation which favor EM wave penetration and absorption. Compared with graphite and carbon nanotubes, reduced graphene oxide has a higher dielectric/magnetic loss by means of microwave absorption. Thus, due to the unique properties of RGO and Fe-based materials, as well as the synergistic effect between them, many reduced graphene oxide/Fe based composites for EMI shielding have been investigated. He and coworkers 3 O 4 - and Fe 2 O 3 -based composites due to their ease of preparation. In most of these cases the chemical reduction method is employed to fabricate the Fe- and RGO-based composites. In this method, Fe 3 O 4 nanoparticles and GO are ultrasonicated/stirred followed by the addition of a reducing agent/surfactant and heat treatment by re-fluxing or hydro-thermal means, etc. which reduced the GO into RGO, as shown in Among all the carbon-derived materials, reduced graphene oxide (RGO) is a most promising material with diverse applications in several branches of science. In RGO, the oxygen functional group is removed using a reducing agent such as hydrazine hydrate, NaBHor NaOHReduced graphene oxide (RGO) is the most studied carbon derivative due to its cost effective preparation, good flexibility, superior electric/thermal conductivity and attractive barrier properties. 64 Moreover, RGO comprises remanent functional groups and defects within the sheet which improve impedance mismatch, defect polarization relaxation and electronic dipole relaxation ( Fig. 9f ). All these groups and defects increase absorption rather than reflection, as can be seen in graphite and carbon nanotubes. For example, Wang 65 investigated the microwave absorption properties of chemically reduced graphene oxide. They observed that residual defects and organic groups within RGO not only improved the individual impedance matching but also produced energy transitions from the continuous states to the Fermi level. Furthermore, these peculiarities introduce relaxation polarization, defect polarization relaxation and electronic dipole relaxation which favor EM wave penetration and absorption. Compared with graphite and carbon nanotubes, reduced graphene oxide has a higher dielectric/magnetic loss by means of microwave absorption. Thus, due to the unique properties of RGO and Fe-based materials, as well as the synergistic effect between them, many reduced graphene oxide/Fe based composites for EMI shielding have been investigated. He and coworkers 66 mixed reduced graphene oxide (RGO) nanosheets with the flaky carbonyl iron (FCI) as depicted in Fig. 10(a and b) . They observed that FCI/RGO composites (−65.4 dB at 5.2 GHz at thickness 3.87 mm) lead to better microwave absorption properties compared with pure FCI (−13.8 dB at 13.7 GHz at thickness of 2.28 mm), as shown in Fig. 11(a and b) . More interestingly, they used the delta-function method to see the contribution of typical dielectric dispersion behavior in FCI/RGO. It is anticipated that a smaller delta value gives better impedance matching. Since FCI/RGO possesses a larger area close to zero, which can directly explain the better matching of the characteristic impedance in FCI/RGO composites. Therefore, recent investigations have mainly concentrated on RGO and Fe-, Fe- and Fe-based composites due to their ease of preparation. In most of these cases the chemical reduction method is employed to fabricate the Fe- and RGO-based composites. In this method, Fenanoparticles and GO are ultrasonicated/stirred followed by the addition of a reducing agent/surfactant and heat treatment by re-fluxing or hydro-thermal means,which reduced the GO into RGO, as shown in Fig. 12

Fig. 10 (a) SEM images of FCI and (b) RGO-coated FCI 66 – reproduced by permission of the Royal Society of Chemistry.

Fig. 11 (a) Reflection loss mapping of FCI and (b) RGO-coated FCI with absorbers thickness from 1 mm to 5 mm in the frequency range of 2.0–18.0 GHz 66 – reproduced by permission of The Royal Society of Chemistry.

Fig. 12 Chemical reduction method of preparation for Fe and RGO based composites.

8.3 Carbon nano tubes (CNTs)

−3 ), and wall integrity of CNTs enable them to serve them as superb nanofillers for improving the properties of composites. Carbon nanotubes are unique one-dimensional (1D) nanostructures that can be understood hypothetically as a 1D quantum wire. These nanotubes belong to the fullerene family. Structurally, CNTs are a long, hollow structure with cylindrical walls framed by a honeycomb lattice (similar to graphene). Carbon nanotubes have received much recognition due to their intriguing electronic, mechanical (tensile strength is >60 GPa) and thermal properties. 67,68 CNTs may be semiconductors or metallic depending on their structure and diameter. Furthermore, the high aspect ratio, low mass density (∼1.6 g cm), and wall integrity of CNTs enable them to serve them as superb nanofillers for improving the properties of composites. 22,62 There are two main types of carbon nanotubes: single walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs).

8.3.1 Single walled carbon nanotubes (SWCNTs). 2 hybridized carbon atom, similar to fullerenes. A single sheet of carbon comprises the wall thickness all around the circumference (diameter ∼ 1.4 nm). The structure of SWCNTs is a cylindrical tube including six-membered carbon rings similar to graphite. Single walled nanotubes are a crucial type of carbon nanotube owing to their good electric properties compared to MWCNTs. The electrical properties of SWCNTs are distinctly different from their larger diameter MWCNTs counterparts due to their smaller diameters and larger aspect ratios. Because of this, the EM-absorbing properties of MWCNTs and SWCNTs are expected to be altogether different. et al. in Fe 3 O 4 /SWCNT composites, Single walled carbon nanotubes (SWCNTs) are an allotrope of sphybridized carbon atom, similar to fullerenes. A single sheet of carbon comprises the wall thickness all around the circumference (diameter ∼ 1.4 nm). The structure of SWCNTs is a cylindrical tube including six-membered carbon rings similar to graphite. Single walled nanotubes are a crucial type of carbon nanotube owing to their good electric properties compared to MWCNTs. The electrical properties of SWCNTs are distinctly different from their larger diameter MWCNTs counterparts due to their smaller diameters and larger aspect ratios. Because of this, the EM-absorbing properties of MWCNTs and SWCNTs are expected to be altogether different. 69,70 The main flaws of SWCNTs are the complicated synthesis procedure, extremely high conductivity and poor magnetic properties. These characteristics of SWCNTs inhibit their use as excellent microwave absorbing materials. Although the incorporation of Fe materials improves their magnetic and electrical properties, as studied by Kuchiin Fe/SWCNT composites, 71 SWCNTs still have been the subject of rather few studies.

8.3.2 Multi walled carbon nanotubes (MWCNTs). etc. properties of MWCNTs. These structural disorders might be Stone–Wales defects, atomic defects or in the form of vacancies and incomplete bonding defects etc. As the result of their high aspect ratio, large surface area and low percolation threshold, MWCNTs are favored as effective fillers rather than SWCNTs in terms of EMI shielding potential; despite this, their comparatively high cost limits their application to some extent. Multiwalled carbon nanotubes (MWCNTs) are one of the most preferable CNTs. Structurally, MWCNTs possess multiple layers of graphite superimposed and rolled in on themselves to make a tube shape. Moreover, these can be considered as a collection of concentric SWCNTs consisting of different diameters, lengths and natures. The distance between each layer is well known to be approximately 0.34 nm. 72 MWCNTs are most promising 1D materials due to their attractive properties. Note that structural disorders, appearing in pristine MWCNTs during their development, are responsible for the unusual electrical and opticalproperties of MWCNTs. These structural disorders might be Stone–Wales defects, atomic defects or in the form of vacancies and incomplete bonding defectsAs the result of their high aspect ratio, large surface area and low percolation threshold, MWCNTs are favored as effective fillers rather than SWCNTs in terms of EMI shielding potential; despite this, their comparatively high cost limits their application to some extent. The available literature on CNTs demonstrates that pure CNTs manifest low absorption but a significantly larger skin depth. However, the addition of Fe species to CNTs greatly improves their microwave absorption, as predicted by Che et al. and Qi et al. in the case of CNTs/CoFe 2 O 4 and Fe/CNTs composites, respectively. The combination of Fe compounds with CNTs (e.g. Fe/CNTs or CNTs/CoFe 2 O 4 ) presents better matching between the dielectric and magnetic losses. Moreover, observations have revealed that a fine dispersion of CoFe 2 O 4 nanoparticles within the CNT matrix weakened the congregation of the CoFe 2 O 4 particles, resulting in dipolar interaction and the resonance absorption effect, owing to the shape anisotropy.3,73

8.3.3 Carbon fibers (CFs). Similar to other carbon materials, carbon fibers (CFs) also possess a high mechanical strength modulus and a low density but a poor thermal expansion coefficient. CFs composed of fibers between (50 to 10 μm in diameter) mainly consisted of carbon atoms. Nevertheless, their lower magnetism and high conductivity increases impedance mismatching in EMI due to increasing their skin depth, similar to CNTs. Hence, modification of CFs with Fe, Fe 3 O 4 , Fe 2 O 3 or alloys could be a useful approach to handle the above problem. Still, the high cost of CFs limits their potential for extensive use as effective fillers. Apart from these fillers, graphitic carbon, carbon black and carbon coils have also been investigated for EMI applications. Although the large surface area of these fillers improves many properties, the major impedance to using these materials as fillers is the requirement for a high weight % ratio, which deteriorates the mechanical properties in case of these polymers.

9 Strategies for the preparation of effective EMI shielding materials

μ which lead to a larger impedance matching value. Subsequently, these magnetic composites shows strong interface polarization (in case of multi-interface materials) which offer advantages with respect to conversion of the incidence EM thermal energy into thermal energy. Keeping this in mind, different strategies for EMI shielding materials and magnetic absorbers have been proposed by scientists, as shown in As we discussed in earlier sections, Fe-related materials offer significant improvements of the complex permeabilitywhich lead to a larger impedance matching value. Subsequently, these magnetic composites shows strong interface polarization (in case of multi-interface materials) which offer advantages with respect to conversion of the incidence EM thermal energy into thermal energy. Keeping this in mind, different strategies for EMI shielding materials and magnetic absorbers have been proposed by scientists, as shown in Fig. 13 , and explained in later subsections.

Fig. 13 Iron can be used with carbonaceous materials, conducting polymers, dielectric materials or insulating polymers.

9.1 Hierarchical/porous structure or divalent/trivalent ion substitution in ferrites

2 O 3 and Fe, 3 O 4 , 3 O 4 , α-Fe 2 O 3 , 3 B/Y 2 O 3 , 3 O 4 and α-Fe 2 O 3 , 3 O 4 ( et al. have shown that, in the case of octahedral Fe 3 O 4 nanoparticles, their anisotropic structure has excellent magnetic characteristics on account of its shape anisotropy. Moreover, the octahedral structure is also advantageous for reflection scattering from multi edges. Zhang et al. made a comparative study of sphere-shaped particles and flake-shaped carbonyl iron particles. They found an optimal reflection loss (12.2 dB at 4.4 GHz at 1 mm thickness) in flake carbonyl iron particles, which was better than that in sphere-shaped particles. Similarly, porous Fe materials hav