A review of porous lightweight composite materials for electromagnetic interference shielding

Accepted Manuscript A review of porous lightweight composite materials for electromagnetic interference shielding Ashish Kumar Singh, Andrei Shishkin, Tarmo Koppel, Nikhil Gupta PII:

S1359-8368(18)30956-9

DOI:

10.1016/j.compositesb.2018.05.027

Reference:

JCOMB 5697

To appear in:

Composites Part B

Received Date: 26 March 2018 Accepted Date: 12 May 2018

Please cite this article as: Singh AK, Shishkin A, Koppel T, Gupta N, A review of porous lightweight composite materials for electromagnetic interference shielding, Composites Part B (2018), doi: 10.1016/ j.compositesb.2018.05.027. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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A review of porous lightweight composite materials for electromagnetic interference shielding Ashish Kumar Singh,1 Andrei Shishkin,2 Tarmo Koppel,3 and Nikhil Gupta1* 1

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Composite Materials and Mechanics Laboratory, Mechanical and Aerospace Engineering Department, New York University, Tandon School of Engineering, Brooklyn, NY 11201, USA. 2 Riga Technical University, Faculty of Materials Science and Applied Chemistry, Institute of General Chemical Engineering, Rudolfs Cimdins Riga Biomaterials Innovations and Development Centre, Pulka iela 3/3, Riga, Latvia. 3 Department of Work Environment and Safety, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia.

2.1 2.2

Shielding mechanism ....................................................................................................... 3 Characterization ............................................................................................................... 4

Porous EMI shielding materials .............................................................................................. 5 3.1

Syntactic foams ................................................................................................................ 5

3.1.1 3.2 3.3

Syntactic foams with conductive filler ............................................................................. 7 Foamed materials ............................................................................................................. 8

3.3.1 3.3.2 3.4

Conductive hollow particles ..................................................................................... 5

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Contents Abstract ........................................................................................................................................... 1 1 Introduction ............................................................................................................................. 2 2 EMI shielding mechanisms and characterization methods ..................................................... 3

Polymer based foam composites ............................................................................... 8 Metal reinforced carbon foams ................................................................................. 9

Effect of filler morphology and filler-matrix interaction ................................................. 9

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4 Conclusions ........................................................................................................................... 10 5 Acknowledgments................................................................................................................. 10 References ..................................................................................................................................... 10 Figure captions .............................................................................................................................. 18 Figures........................................................................................................................................... 19

Abstract Lightweight porous materials for electromagnetic interference (EMI) shielding applications are reviewed. EMI shielding refers to the capability of a material to protect from electromagnetic fields (EMFs) generated by electronic devices. Traditionally conducting metals are used in EMI shielding applications, which are slowly being replaced by conducting polymer based shields. This review is narrowly focused on understanding the approaches related to porous high EMI shielding composite materials that have very low density values. While conducting fillers can *

Corresponding author, Email: [email protected], Ph: +1-646-997 3080.

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increase the EMI shielding capabilities of polymers, they also increase the weight, which can be offset by inducing the porosity in the matrix. Porosity is found to be effective in providing higher shielding effectiveness at low filler volume fraction due to concentrating the filler in the solid polymers. However, use of gas porosity results in composites with low mechanical properties. Syntactic foams containing hollow particle fillers seem to be the best combination of EMI shielding capabilities and mechanical properties. These composites can be either filled with a second phase conducting filler, or hollow particles can be coated with a conducting layer, or hollow particles made of conducting materials can be used as fillers. The hollow particle wall thickness and volume fractions can be optimized to obtain the desired combination of properties in syntactic foams to enable their multifunctional applications.

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Keywords: Electromagnetic interference; electromagnetic force; porosity; hollow particle; syntactic foam.

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1 Introduction Electromagnetic interference (EMI) is the disturbance caused by radiation fields created by electronic devices such as communication antennas, household devices and cellular phones [1]. A mechanism for shielding from electromagnetic fields (EMFs) is important for reducing interference from other devices, noise, and malfunction. Interference from radiofrequency (RF) and microwave EMFs may affect sensitive electronic equipment [2, 3]. Long term exposure to electromagnetic fields poses serious risks to human health [4-8]. The environment has shown abrupt rise of EMFs, especially RF fields, due to the boom of digital communication. In places like hospitals [9], with a large number of equipment that generate EM radiations, it is important to achieve electromagnetic compatibility between equipment to ensure proper functioning of critical lifesaving systems and to ensure that the operators of machines are safe [10]. High power RF EMFs are ubiquitous in work environments, where processes such as RF welding, industrial microwave ovens or mobile communication with high power transceivers are used [11]. Operation of radio/TV signal transmitting stations and radars also contribute to a large amount of EMF radiations [12]. In Europe, public health concerns have led the government authorities to form special guidelines aimed at protecting children and other vulnerable groups from EMFs exposure [13, 14] and occupational laws aimed to reduce risk of EMFs exposure to pregnant women and people with medical implants in the workforce [15]. Medical implants such as cardiac pacemakers, insulin pumps and hearing aids may be at a risk if subjected to high intensity electromagnetic fields [16]. The authors’ earlier work investigated the interaction of electromagnetic waves with commercially available common building materials [17-19]. Industrial waste materials such as metallurgical slug, crumb rubber [20], and perforated steel [21, 22] are also studied for possible use in EMF shielding applications. Typical shielding, in dB, required for consumer electronic appliances are listed in Table 1 [23]. EMI shielding of 20 and 40 dB correspond to 99% and 99.99% attenuation of incident electromagnetic waves [24]. Academic and research interest in understanding the health hazards and effective methods of EM radiation shielding has substantially increased in the past decade [25, 26]. Data collected from only one database Sciencedirect shows that the number of publications related to EMI, its health hazards and- shielding has increased 180-330% during 2007-2017 period (Figure 1). The available literature shows that significant effort has been invested into the development of

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materials to shield humans and devices from EMI, which includes stray EMI emitted by consumer electronics or intentionally created EMI to disrupt military or space infrastructure.

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Electrically conductive materials such as metals exhibit excellent EMI shielding capabilities and have been used extensively in such applications [27-29]. Relatively higher density of metals compared to polymers and ceramics results in a tradeoff between the shield weight and shielding capability. Also, metals are susceptible to corrosion and environmental degradation, resulting in deterioration in integrity of the structure and loss in EMI shielding performance. Such considerations have resulted in the desire for using lighter materials for EMI shielding application and recent attention has been focused on developing polymeric EMI shielding materials. Since, most polymers have very low electrical conductivity, two approaches are employed to enhance their EMI shielding capability: (i) coating with conductive materials and (ii) blending with conductive materials to make composites [30]. A number of studies are available, where conductive fillers, especially nanoscale filers of carbon [31-33], such as carbon nanofibers (CNF) [34-37], carbon nanotubes (CNT) [38, 39], and graphene [40-42] as well as metallic nanowires and particles [24, 43-45] are incorporated in polymers to synthesize conductive composites with high EMI shielding capabilities. While the available body of literature on these materials is very large, the current review focuses only on porous composites, where porosity is innovatively used to develop lightweight materials with high EMI shielding capability. Effect of material properties and material microstructure on the shielding capabilities of porous materials is studied and relevant shielding mechanisms are identified. A combination of material properties and structural parameters can be particularly useful in developing new lightweight materials with high capabilities of EMI shielding.

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2 EMI shielding mechanisms and characterization methods 2.1 Shielding mechanism Performance of a shielding material is commonly expressed by the term shielding effectiveness (SE), in terms of decibel (dB) loss, which is the measure of its opacity to an incident EM wave at a given frequency band [46]. SE can be defined as the ratio of incoming (Pi) and outgoing (Po) power of an EM wave as it passes through a material and is given by (1) SE = 10 log10 ( Pi / Po ) An incident EM wave undergoes three primary interactions (Figure 2) with the material, namely absorbance (A), reflectance (R) and transmittance (T) [47-50]. The reflective component is comprised of reflection from the incident surface of the material and secondary reflection waves within the material, called internal/multiple reflections. Higher absorption results in the dissipation of EM energy in the form of heat energy, thereby increasing the temperature of the shield [48, 49]. In monolithic isotropic materials, the absorption can be estimated by measuring transmittance (T) and reflectance (R) by (2) A = 1− R − T In many cases, especially in composite or porous materials, the reflections from the internal surfaces may cause substantial reduction in transmitted signal. While this is truly not absorption by the material, the effect of such mechanism can be combined with the absorption to calculate the effective attenuation coefficient (α) of the material as =1− − (3)

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(

π f µσ

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, where f is the wave frequency, µ is the magnetic permeability and

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Highly conductive materials cause higher reflection loss leading in higher value of R. The capacitive and conductive components of the dielectric response of the material are set by the dielectric constant and the dielectric loss factor [48]. According to Schelkunoff’s theory [51], reflection, multiple reflection and absorption contribute to total shielding effectiveness ( SET ) of a material. The contribution of internal reflection to SET can be neglected if absorption loss ( SE A ) is less than 10 dB [52] and only reflection loss ( SE R ) and absorption loss ( SE A ) contribute to the total shielding effectiveness ( SET ), as given by (3) SET = SE A + SE R An electric field of plane wave attenuates exponentially as it passes through a conductive material and drops to 1/ e of the incoming intensity at a depth known as skin depth (δ), which is

σ is the electrical conductivity. If the thickness of the shield or any conductive particle inside it

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is larger than the skin depth, the effect of multiple reflections on SET can be ignored. 2.2 Characterization EMI SE tests are designed to measure the power of the transmitted wave as it passes through the material. The commonly used test configurations are [27]: 1. Open field method 2. Shielded box method 3. Shielded room method 4. Co-axial transmission line method

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The open field method is used to measure the EMI SE of any finished product as per the service requirements. This method does not focus on the performance of a given material, but the shield as a whole. The test is performed with the device at 30 m distance from a receiving antenna and the transmitted radiation intensity is recorded.

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Shielded box method is used for making comparative measurements of EMI SE of different shield materials. The specimen is mounted in a window in the wall of a metal box with a receiving antenna inside. A transmitting antenna is placed outside the box and transmitted intensity from the shield material is measured. Drawbacks of this method include difficulty to achieve good electrical contact between the specimen and the box and the limitation on operating frequency of the EM wave to be 500 MHz. To overcome these limitations, the shielded room method is employed, where all the components, namely the two antennas and the measuring systems are all isolated from each other in separate shielded rooms that increases the frequency range of the EM wave. The most widely used method for measuring EMI SE of shielding materials is the co-axial cable transmission line method. The test setup consists of transmitting and receiving co-axial cables, metal sample holder and a vector network analyzer (VNA). The VNA can transmit/receive and record EMI intensities dynamically over a much larger range of frequencies. Co-axial cables can produce EM waves over a larger frequency range with smaller losses than antennas. The main advantage of the co-axial transmission cable method is that the results can be resolved into

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reflected, transmitted and absorbed components. This gives a much better insight into the mechanisms of EMI shielding of a given material.

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3 Porous EMI shielding materials Several approaches are possible to create lightweight electrically conducting materials with high EMI shielding capabilities. A large body of literature is available on conducting composite materials; however, only the studies that specifically focused on measuring EMI shielding capabilities are included in this review. As a general guideline, most polymers have density values in the range 0.9-1.2 g.cm3, which is significantly lower than the density values of conducting metals such as copper (8.96 g/cm3). Adding conducting fillers will increase the density of the polymers. Therefore, innovative methods have been developed to incorporate porosity in the material to decrease the density of the composite below the density of the neat resins, while obtaining high EMI shielding capability. This section focuses only on such lightweight materials.

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3.1 Syntactic foams Syntactic foams are light-weight composite materials with hollow particles dispersed in a matrix [53]. These lightweight materials are widely used in weight sensitive applications such as marine vessels, aircraft structures, and thermal insulation of underwater pipelines [54, 55]. Syntactic foams offer the capability of tailoring mechanical, electrical, acoustic and thermal characteristics in a wide range of values by means of volume fraction and wall thickness of hollow particles [56-61].

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EMI SE of material is enhanced if the electrical conductivity of the material is high or contains filler with good electrical conductivity. These two approaches allow classifying the available studies into two categories where (i) either hollow particles of a conductive material are used, or particles are made conducting by providing a conductive coating or (b) a second conductive filler is used in syntactic foam matrix.

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3.1.1 Conductive hollow particles Several methods are available to produce metal coated hollow microspheres, including glass microballoons and cenospheres. These methods can be broadly classified as, i) chemical in-situ synthesis as metal coating followed by inner organic template removal [55, 62], ii) chemical synthesis including precipitation on the microspheres [63-66], and iii) physical vapor deposition on the microspheres [67, 68]. Several studies have suggested and demonstrated promising EMI SE by using metal coated hollow particles as fillers for making syntactic foam EMI shields [69]. Silver coated polypyrrole microballoons were produced by Panigrahi et al. [63] using template assisted emulsion polymerization. The process involved in situ chemical oxidative copolymerization of pyrrole (Py) on the surface of sulfonated polystyrene (PS) microspheres followed by the formation of hollow polypyrrole spheres (HPPy) by dissolving PS inner core in tetrahydrofuran (THF). HPPy and HPPy/Ag nanocomposites consisting of 2, 5 and 10 wt.% Ag, referred to as HPPy/Ag-2, HPPy/ Ag-5and HPPy/Ag-10, respectively, were developed and characterized for EMI shielding over a frequency range of 0.5-8 GHz. The Ag coated polypyrrole particles show increase in EMI SE with Ag content. EMI SE value for HPPy/Ag-10 was found to be ~59-23 dB over the 0.5-8 GHz (Figure 3a). A significant finding from this study 5

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was the sharply lower reflectance of HPPy compared to PPy microspheres, which is attributed to multiple internal reflections of the EM waves inside the HPPy microballoons. It is less likely for a wave to exit the outer surface of the hollow particle once it enters inside, as schematically shown in Figure 3b. The results also clearly demonstrate that the reflectance of HPPy-Ag-5 and HPPy-Ag-10 are significantly higher compared to PPy, HPPy or HPPy-Ag-2 due to the reflection of EM radiation from the outer surface of the HPPy shell coated with highly conducting Ag nanoparticles. Such hollow particles show promise as filler materials to be used in manufacturing syntactic foams.

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Zhang et al. [45] demonstrate the effect of functionalization of hollow carbon microballoons (HCMs) on the EMI SE of epoxy-HCM syntactic foam. HCMs were coated with poly-dopamine (PDA) by self-polymerization of dopamine, not only facilitating the dispersion of fillers but also enhancing bonding between matrix and HCMs resulting in better stress transfer between matrix and HCMs [45]. The PDA coating also serves as a reducing agent, allowing deposition of metallic particles that enhances EMI SE of the syntactic foam. Since the electrical resistivity of carbon is much higher than that of metals, a greater amount of carbon is needed to achieve the same SE. But metals are much heavier than carbon, so deposition of few nanoparticles on the surface of HCMs will allow for high EMI SE and keep the weight low. Silver nanoparticles were grown on HCMs by dispersing PDA-HCMs in silver nitrate solutions of varying concentration. Silver (Ag) nanoparticles get reduced on the PDA surface in different densities as shown in Figure 4, producing silver nanoparticle coated HCMs (Ag-PDA-HCM). All foams studied had a filler content of 30 vol. %.

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It was observed that presence of silver nanoparticles on HCM increases the EMI SE of the syntactic foam. As shown in Figure 5a, average EMI SE over 8-12 GHz frequency range of syntactic foams containing Ag-PDA-HCM with 28.5 and 30.5 wt.% Ag is 49.5 and 60.2 dB, respectively, which is higher than that required by many applications. While difference was not significant in EMI SE for PDA-HCMs and Ag-PDA-HCMs with lower Ag content, the difference is substantial for higher Ag contents. Since higher Ag content will increase the weight of syntactic foam, the study presented a comparison of the specific EMI SE of the syntactic foams with other lightweight foams from literature (Figure 5b). The specific EMI SE of AgPDA-HCMs was found to be at least 39 % higher than CNT-polystyrene [70] and CNF-epoxy foams [34] .

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Other metals such as Nickel (Ni) have also been used to coat hollow particles to improve the EMI SE of their syntactic foams. Xiaozheng et al. [67] coated cenospheres via ultrasonic assisted direct current magnetron sputtering method. Ni was deposited on cenospheres of diameter 20100 µm in different concentration by modulating the sputtering power (99, 142 and 187 W) achieving layer thickness of 26, 42 and 85 nm, respectively. 1 gm of the Ni coated cenospheres were then dispersed in melted paraffin matrix inside molds of 150 mm diameter and 60 mm height to ensure that the particles are isolated from each other. The material showed EMI SE of more than 25 dB in frequency rage 80-110 GHz with cenospheres coated at 187 W sputtering power as shown in Figure 6. This demonstrates the efficacy of Ni coated particle in EMI shielding even at very low volume fractions.

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Bora et al. [62] investigated the EMI SE of thin polyaniline films filled with nickel oxide coated cenosphere (NiOC). The coating of nickel oxide nanoparticles was achieved by chemical heterogeneous precipitation and thermal reduction method. In situ synthesis of polyaniline and NiOC composite (PNiOC) was carried out. Authors investigated EMI SE in the J-band (5.8–8.2 GHz), X-band (8.2–12.4 GHz) and Ku-band (12.4–18 GHz). An average EMI SE of ~24 dB, ~27–24 dB, ~21 dB was observed for 81±3 µm thicker flexible free standing PNiOC film in the J, X and Ku-band, respectively.

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Shishkin et al. [68] synthesised metallo-ceramic SF by spark plasma sintering method of metal coated cenospheres. Copper coated cenospheres (Cu@CS) and stainless steel coated cenospheres (steel@CS) were produced by means of vibration-assisted magnetron sputtering method. The particles were then consolidating using spark plasm sintering (SPS) achieving bulk densities from 0.904 - 1.496 g/cm3 and compressive strengths of 8.6 - 61.0 MPa, depending on sintering time and temperature. The Cu@CS syntactic foams showed electrical conductivities ranging from 2.05×104 and 3.6×104 S/m. Although EMI SE characterization was not performed on these materials, they are expected to show good performance in EMI shielding, due to high electrical conductivity of the material. As copper is one of the most widely used material in EMI shielding, Cu-coated hollow particles are promising in developing lightweight syntactic foams.

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3.2 Syntactic foams with conductive filler The effect of CNF to the EMI SE of phenolic resin based syntactic foam was studied by Zhang et al [34]. It was found that EMI SE increases with CNF content for EM waves of frequencies between 30 MHz to 1.2 GHz as shown in Figure 7a. Reflectance becomes dominant mode of EMI shielding at higher CNF content, which is a result of higher overall conductivity of the material. The study compared the EMI SE of CNF composites and CNF-reinforced syntactic foam (CNFSF). The syntactic foam had 28 vol. % hollow carbon microspheres (HCM) and CNF content was measured as a percentage of the resin. Figure 7b and c show the EMI SE for the two composites at 400 MHz as a function of CNF content and frequency, respectively. It can be observed that the EMI SE of CNFSF is higher than CNF composites for all volume fractions and frequencies studied. Reflectance was reported as the primary mode of shielding at 700 MHz.

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The introduction of hollow inclusions in the form of HCMs increases the amount of conductive material and conductive interface area inside the composite. Higher content of conductive material should increase EMI SE, but higher interface area should decrease it [34]. As the interfacial area increases inside a material, the multireflection effect grows in magnitude [35], but it is a negative contribution to overall SE if the absorption component is lower, as described in the Schelkunoff’s theory of shielding [51]. Accordingly, the EMI SE of CNFRSF should decrease as compared to CNF composite, but the experimental results by Zhang et al. show the opposite. This is because the introduction of cellular inclusions results in a higher interconnectivity of CNFs in the matrix, forming a longer conductive chain [34, 71], as shown in Figure 8. Metal matrix syntactic foams have also been studied for their EMI SE. Dou et al. [72] found that Aluminum matrix syntactic foam with 70 vol. % fly-ash cenosphere shows a maximum SE of 102 dB at 1 MHz frequency. However, it dropped to 32 dB at 1GHz, resulting in a specific SE of around 20 dB-cm3/g, which is much lower than polymer-based foams listed in Figure 5b. 7

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3.3 Foamed materials 3.3.1 Polymer based foam composites Foamed materials have also been of great interest in EMI shielding. Foaming agent reduces the viscosity of the polymer, facilitating the processing [37], preventing fiber aggregation and improving dispersion of filler material, which helps in lowering the electrical percolation threshold [39].

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SE of foamed polypropylene (PP)-CNF composites was studied by Ameli et al [71]. EMI shielding of PP-CNF composite with up to 10 vol. % CNF content was studied and compared with foamed composite. The foaming was achieved by purging the molding chamber with nitrogen prior to injection. 0.3 wt. % nitrogen was used to foam the composite that resulted in a void fraction of 25 vol.%. Comparison of EMI SE of foamed and solid composite is shown in Figure 9. Foamed composite exhibits higher SE than the solid PP-CNF composite for all CNF content between 8-12 GHz band frequency. The authors attribute this to a more random orientation of CNF leading to a higher chances of fiber interconnection. This leads to a larger electrically conductive network which shows better SE than the solid composite. In this case, absorption was found to be the primary mode of shielding, as opposed to what Zhang et al. reported in [34]. However, it should be noted that the wave absorption and reflection values reported by Ameli et al. are the average value in the X-band frequency range and those by Zhang et al. are at 700 Mhz [34].

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Yang et al. [70] investigated the SE of polystyrene-CNT foam composites over a frequency range of 8.2-12.4 GHz. A specific SE of 33.1 dB cm3/g was achieved by the foam with 7% CNT. This SE value was comparable to polystyrene-CNF foam composite with 20% CNF content, showing that CNTs are better at EMI shielding than CNFs. With increasing concentration of CNT, the SE was observed to increase, which was attributed to the increase in number of interconnected nanotubes that created a conductive network. Other polymers like polymethylmethacrylate (PMMA) have been used for creating foamed composites intended for EMI shielding applications. Zhang et al. [42] prepared a graphene reinforced PMMA foam composite that achieved up to 19 dB over a range of 8-12 GHz. The reflection loss was negligible in this composite and the total SE was due to primarily absorption loss as opposed to CNT or CNF filled foam composites [70, 73], where reflection loss dominated the total SE. Zhang et al. [74] achieved an average of 51.2 dB EMI SE over 8.2 – 12.4 GHz frequency range with a phthalonitrile based carbon foam (0.15 g/cm3). The study found that heat treatment temperature plays a key role in the effective SE of these carbon foams. Higher temperature (1000oC) leads to crystallization of carbon in the polymer that enhances the SE of the foam. Ling et al. developed lightweight microcellular polyetherimide (PEI)-graphene nanocomposite foam with a density of about 0.3 g/cm3 by a phase separation process. In this study, a water vapor induced phase separation process was applied to prepare the microcellular PEI-graphene composite foams with up to 10 wt.% graphene loading. Extensional flow generated during cell growth induced the enrichment and orientation of graphene on cell walls (Figure 10a). The foaming process significantly increased the specific EMI shielding effectiveness from 17 to 44 dB/(g/cm3). It was proposed that the spherical air bubbles in the foam structure enhanced the attenuation of incident electromagnetic microwaves by multiple reflection and decay between the 8

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cell wall and nanofillers (Figure 10b). As a result, the microwaves were absorbed and dissipated as heat before escaping from the material.

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Polymer foams reinforced with metals have been shown to exhibit high EMI SE at very low densities. Zeng et al. [43] developed an ultralight polyurethane-silver nanowire composite with specific SE of up to 2500 dB-cm3/g in the 8.2 - 12.1 GHz range, but the foam had very low mechanical strength ( σ y = 17 kPa ). Shen et al. [75] demonstrated the SE of polyetherimide (PEI)-graphene/Fe3O4 (PEI-G@Fe3O4) foam (0.40 g/cm3) at up to 18 dB over a frequency range of 8-12 GHz and 10% filler content. They observed that reflective loss of the material was almost negligible for all concentrations of filler and the absorption component increased with filler concentration. The higher loading of graphene and Fe3O4 absorbs more EM radiation and dissipates it as heat. Effect of Fe3O4 on the EMI SE of the composite was however not the explicit focus of the study. The specific EMI SE for this foam was 41.5 dB cm3/g and it is expected that the inclusion of Fe3O4 as filler will further reduce the specific EMI SE value, given the higher density of Fe3O4. This was confirmed by another study by Ling et al. [76], where PEI was used as matrix and only graphene as filler, and a specific EMI SE of 44 dB cm3/g was achieved over same frequency range and at same filler content of 10%. The study shows that foamed polymer composites with graphene filler have lower EMI SE than solid composites as shown in Figure 11(a, b) and specific EMI SE (Figure 11c). Figure 11d shows the comparison of specific EMI SE performance with PU-graphene foam [77], epoxy-graphene foam [78], PMMAgraphene [42], polystyrene-CNT foam [70], and polystyrene-graphene foam [69], obtained from literature.

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Similar results were obtained by Zhang et al. [79] with PPMA foam reinforced with iron oxide (III) decorated multi-walled carbon nanotubes (Fe3O4@MWCNT). Lightweight and multifunctional PMMA-Fe3O4@MWCNTs composite foams with density of 0.22–0.38 g/ cm3 were developed by supercritical carbon dioxide foaming process. The solid and foamed derivatives of the material were tested for EMI SE. It is observed that benefitting from the existence of microcellular structure and the Fe3O4@MWCNTs hybrids, the specific electromagnetic interference shielding effectiveness (EMI SE) of fabricated foams was significantly higher (Figure 12).

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3.3.2 Metal reinforced carbon foams Carbon foams reinforced with metals have also been developed for use in EMI shielding applications. Zhao et al. [44] demonstrated the synthesis of carbon foam with Ni nanoparticles (50-100 nm) that provided a reflection loss of 45 dB at 13.3 GHz.. Farhan et al. [80] achieved EMI SE of over 46 dB with carbon foams reinforced with 20 % SiC nanowires. This foam composite showed high absorption loss and a specific EMI SE of 79 dB cm3/g at 8.2 GHz. 3.4 Effect of filler morphology and filler-matrix interaction Any polymer-based material intended to be used as a shield for EMI needs to have conductive filler material to either absorb or reflect the incoming EM wave. These fillers exhibit distinct EMI shielding characteristics based on their shape, size and morphology. A filler with higher aspect ratio performs better at EMI shielding, as found by Zhang et al. [34]. CNF with aspect ratio of 500-1700 were 3 and 5 times more effective than long carbon fibers (LCF) with aspect ratio of 150-750 and chopped carbon fibers (CCF) with aspect ratio of 6-50, respectively. 9

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Similarly, CNTs have shown excellent EMI SE [38, 70] at very low volume fractions due to very high aspect ratios of up to 2000. Zhou et al. [81] studied the effect of shape and size of nickel coated particle on the electrical properties of silicon based composites. They found that larger, high aspect ratio nickel coated carbon fibers (CF) showed better electrical conductivity that smaller, lower aspect ratio graphite particles. This was due to the ease with which CF can make contact points among each other, increasing the overall volume conductivity, enhancing EMI SE. Smaller particles in high volume fractions make more contact points which increases contact resistance, lowering overall volume conductivity.

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Composites with high interface area like syntactic foams, or composites with weak filler to matrix bonding, will show high EMI SE only if the matrix is conductive. High interface area increases the multiple reflection of EM waves and if the matrix is conductive, the EM wave gets gradually absorbed and decays. If the matrix is nonconductive, multiple reflection is slightly detrimental to the overall SE, but can be neglected if absorption by the filler network is high enough [51, 71].

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4 Conclusions With the electronic industry seeking more miniaturization and weight saving, traditional EMI shielding materials like metals need to be replaced with lighter material. This paper analyzes various approaches available for developing very lightweight materials with high EMI shielding capabilities. More specifically, porous composite materials with density levels as low as 0.15 g/cm3 have been found studied for EMI shielding effectiveness. Innovative use of porosity, either in the matrix material or in terms of hollow particle fillers is found to provide benefit in EMI shielding. One of the limitations of polymers is their insulating nature, which can be changed by means of conducting reinforcement incorporated in high volume fraction. Large aspect ratio reinforcements such as carbon nanotubes are very effective in creating a conducting network in the polymer even at low volume fraction, whereas decrease in the aspect ratio of filler requires increased filler volume fraction to obtain the same EMI shielding performance. Coating of hollow particles with conductive materials is found to be an effective approach to synthesize materials with high EMI shielding at low density. Syntactic foams have high mechanical properties to enable their structural applications in marine and aerospace structures. Added property of EMI shielding can result in multifunctional materials.

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5 Acknowledgments This work is supported by the US Office of Naval Research grant N00014-10-1-0988 from the Solid Mechanics Program. The authors acknowledge European Regional Development Fund project Nr.1.1.1.1/16/A/007 “A New Concept for Sustainable and Nearly Zero-Energy Buildings” for their financial support. The views expressed in this article are those of authors, not of funding agencies. References 1. Markham, D., Shielding: quantifying the shielding requirements for portable electronic design and providing new solutions by using a combination of materials and design. Materials & Design, 1999. 21(1): p. 45-50

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64. Xu, P., Han, X., Wang, C., Zhou, D., Lv, Z., Wen, A., Wang, X., and Zhang, B., Synthesis of Electromagnetic Functionalized Nickel/Polypyrrole Core/Shell Composites. The Journal of Physical Chemistry B, 2008. 112(34): p. 10443-10448. 65. Meng, X.-f., Li, D.-h., Shen, X.-q., and Liu, W., Preparation and magnetic properties of nano-Ni coated cenosphere composites. Applied Surface Science, 2010. 256(12): p. 37533756. 66. Meng, X.-F., Shen, X.-Q., and Liu, W., Synthesis and characterization of Co/cenosphere core–shell structure composites. Applied Surface Science, 2012. 258(7): p. 2627-2631. 67. Yu, X. and Shen, Z., The electromagnetic shielding of Ni films deposited on cenosphere particles by magnetron sputtering method. Journal of Magnetism and Magnetic Materials, 2009. 321(18): p. 2890-2895. 68. Shishkin, A.D., M; Kozlov, V; Hussainova, I; Lehmhus, D, Vibration-Assisted Sputter Coating of Cenospheres: A New Approach for Realizing Cu-Based Metal Matrix Syntactic Foams. Metals, 2017. 7(1). 69. Xie, X., Wu, Y., Kong, Y., Zhang, Z., and Zhou, X., Synthesis and characterization of multilayer core–shell structure hollow spheres with low density, favorable magnetic and conductive properties. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2012. 408: p. 104-113. 70. Yang, Y., Gupta, M.C., Dudley, K.L., and Lawrence, R.W., Novel Carbon Nanotube−Polystyrene Foam Composites for Electromagnetic Interference Shielding. Nano Letters, 2005. 5(11): p. 2131-2134 71. Ameli, A., Jung, P.U., and Park, C.B., Electrical properties and electromagnetic interference shielding effectiveness of polypropylene/carbon fiber composite foams. Carbon, 2013. 60(Supplement C): p. 379-391. 72. Dou, Z., Wu, G., Huang, X., Sun, D., and Jiang, L., Electromagnetic shielding effectiveness of aluminum alloy–fly ash composites. Composites Part A: Applied Science and Manufacturing, 2007. 38(1): p. 186-191. 73. Yang, Y., Gupta, M.C., Dudley, K.L., and Lawrence, R.W., Conductive Carbon Nanofiber– Polymer Foam Structures. Advanced Materials, 2005. 17(16): p. 1999-2003. 74. Zhang, L., Liu, M., Roy, S., Chu, E.K., See, K.Y., and Hu, X., Phthalonitrile-Based Carbon Foam with High Specific Mechanical Strength and Superior Electromagnetic Interference Shielding Performance. ACS Applied Materials & Interfaces, 2016. 8(11): p. 7422-7430. 75. Shen, B., Zhai, W., Tao, M., Ling, J., and Zheng, W., Lightweight, Multifunctional Polyetherimide/Graphene@Fe3O4 Composite Foams for Shielding of Electromagnetic Pollution. ACS Applied Materials & Interfaces, 2013. 5(21): p. 11383-11391. 76. Ling, J., Zhai, W., Feng, W., Shen, B., Zhang, J., and Zheng, W.g., Facile Preparation of Lightweight Microcellular Polyetherimide/Graphene Composite Foams for Electromagnetic Interference Shielding. ACS Applied Materials & Interfaces, 2013. 5(7): p. 2677-2684. 77. Bernal, M.M., Molenberg, I., Estravis, S., Rodriguez-Perez, M.A., Huynen, I., LopezManchado, M.A., and Verdejo, R., Comparing the effect of carbon-based nanofillers on the physical properties of flexible polyurethane foams. Journal of Materials Science, 2012. 47(15): p. 5673-5679. 78. Liang, J., Wang, Y., Huang, Y., Ma, Y., Liu, Z., Cai, J., Zhang, C., Gao, H., and Chen, Y., Electromagnetic interference shielding of graphene/epoxy composites. Carbon, 2009. 47(3): p. 922-925.

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79. Zhang, H., Zhang, G., Li, J., Fan, X., Jing, Z., Li, J., and Shi, X., Lightweight, multifunctional microcellular PMMA/Fe3O4@MWCNTs nanocomposite foams with efficient electromagnetic interference shielding. Composites Part A: Applied Science and Manufacturing, 2017. 100: p. 128-138. 80. Farhan, S., Wang, R., and Li, K., Electromagnetic interference shielding effectiveness of carbon foam containing in situ grown silicon carbide nanowires. Ceramics International, 2016. 42(9): p. 11330-11340. 81. Zhou, H., Xia, Z.D., Wang, X.Y., Li, Z., Li, T.T., and Guo, F., Effect of shape and size of nickel-coated particles fillers on conductivity of silicone rubber-based composites. Polymer Composites, 2015. 36(8): p. 1371-1377.

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Table 1. Approximate shielding required for common consumer appliances [1].

Attenuation level 15-20 dB 15-20 dB 70-90 dB 70-90 dB 30-40 dB

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Device Notebook computer Desktop computer Cell phone Cable tap Workstation/server

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Figure captions Figure 1. Number of publications in Sciencedirect during the period 2000-2017 referenced by various keywords. Dashed (black) curves are plotted with y-axis on the left and solid (red) curves are plotted with the y-axis on the right and. Data extracted on March 12, 2018. Figure 2. Schematic representation of various mechanisms leading to EMI shielding [45]. Figure 3. (a) EMI SE PPy, HPPy and all HPPy/Ag particles over frequency range 0.5-8GHz and (b) trapping mechanism of EM wave through enhanced internal reflection in HPPy/Ag: an anticipated scheme [56]. Figure 4. SEM image of Ag-PDA-HCM prepared by different concentrations of AgNO3 (listed on the images). The images on the right are high magnification micrographs of the surface shown in the left image [38]. Figure 5. (a) EMI SE as a function of frequency (8 – 12 GHz) for syntactic foams with pristine and functionalized hollow carbon microballoons (HCMs) and (b) comparison of specific EMI SE of these syntactic foams with other foams in literature [38]. Figure 6. EMI SE against the measured frequency (80–110 GHz) for the uncoated cenospheres (a) and Ni-coated cenosphere particles at different sputtering power: (b) 99W; (c) 142W; (d) 187W [60]. Figure 7. (a) EMI SE of CNF composite as a function of CNF content, (b) comparison of EMI SE of CNFRSF and CNF composite at 400 MHz and (c) EMI SE of syntactic foam as a function of frequency for varying CNF contents [27]. Figure 8. Schematic illustration of effect of cellular air porosity on the inter-connectivity of fibers, (a) and (b) represent fiber alignment before and after cell formation [64]. Figure 9. Comparison of EMI SE of (a) solid and (b) foamed PP-CNF composite at varying CNF content. (c) contribution of reflection and absorption in total EMI SE of foamed and solid PPCNF composite at varying CNF content [64]. Figure 10. (a) TEM image of the dispersion of graphene in PEI-graphene nanocomposite foams: the orientation of graphene on cell wall and (b) schematic description of the microwave transfer across PEI-graphene nanocomposite foam [69]. Figure 11. EMI shielding efficiency of (A) solid PEI-graphene composite [69], (B) foamed PEIgraphene composite [69], (C) comparison of specific EMI SE of solid and foamed PEI-graphene composite at 9.6 GHz [69], and (D) comparison of specific EMI SE of PEI-graphene foam with other reported results. In part (D): a: obtained from ref. [70] b: obtained from ref. [71], c: obtained from ref. [35], d: obtained from ref. [63], f: obtained from ref. [62]. Figure 12. The specific EMI shielding efficiency of PMMA-Fe3O4@MWCNTs nanocomposites and foams [72].

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Figure 1. Number of publications in Sciencedirect during the period 2000-2017 referenced by various keywords. Dashed (black) curves are plotted with y-axis on the left and solid (red) curves are plotted with the y-axis on the right and. Data extracted on March 12, 2018.

Figure 2. Schematic representation of various mechanisms leading to EMI shielding [52].

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Figure 3. (a) EMI SE PPy, HPPy and all HPPy/Ag particles over frequency range 0.5-8GHz and (b) trapping mechanism of EM wave through enhanced internal reflection in HPPy/Ag: an anticipated scheme [63].

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Figure 4. SEM image of Ag-PDA-HCM prepared by different concentrations of AgNO3 (listed on the images). The images on the right are high magnification micrographs of the surface shown in the left image [45].

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Figure 5. (a) EMI SE as a function of frequency (8 – 12 GHz) for syntactic foams with pristine and functionalized hollow carbon microballoons (HCMs) and (b) comparison of specific EMI SE of these syntactic foams with other foams in literature [45].

Figure 6. EMI SE against the measured frequency (80–110 GHz) for the uncoated cenospheres (a) and Ni-coated cenosphere particles at different sputtering power: (b) 99W; (c) 142W; (d) 187W [67].

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Figure 7. (a) EMI SE of CNF composite as a function of CNF content, (b) comparison of EMI SE of CNFRSF and CNF composite at 400 MHz and (c) EMI SE of syntactic foam as a function of frequency for varying CNF contents [34].

Figure 8. Schematic illustration of effect of cellular air porosity on the inter-connectivity of fibers, (a) and (b) represent fiber alignment before and after cell formation [71].

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Figure 9. Comparison of EMI SE of (a) solid and (b) foamed PP-CNF composite at varying CNF content. (c) contribution of reflection and absorption in total EMI SE of foamed and solid PP-CNF composite at varying CNF content [71].

Figure 10. (a) TEM image of the dispersion of graphene in PEI-graphene nanocomposite foams: the orientation of graphene on cell wall and (b) schematic description of the microwave transfer across PEI-graphene nanocomposite foam [76].

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Figure 11. EMI shielding efficiency of (A) solid PEI-graphene composite [76], (B) foamed PEI-graphene composite [76], (C) comparison of specific EMI SE of solid and foamed PEI-graphene composite at 9.6 GHz [76], and (D) comparison of specific EMI SE of PEI-graphene foam with other reported results. In part (D): a: obtained from ref. [77] b: obtained from ref. [78], c: obtained from ref. [42], d: obtained from ref. [70], f: obtained from ref. [69].

Figure 12. The specific EMI shielding efficiency of PMMA-Fe3O4@MWCNTs nanocomposites and foams [79].

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