Lightweight and Easily Foldable MCMB-MWCNTs Composite Paper

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Light Weight and Easily Foldable MCMB-MWCNTs Composite Paper with Exceptional Electromagnetic Interference Shielding Anisha Chaudhary, Saroj Kumari, Rajeev Kumar, Satish Teotia, Bhanu Pratap Singh, Avanish Pratap Singh, S. K. Dhawan, and Sanjay R Dhakate ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12334 • Publication Date (Web): 01 Apr 2016 Downloaded from http://pubs.acs.org on April 13, 2016

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ACS Applied Materials & Interfaces

Light Weight and Easily Foldable MCMB-MWCNTs Composite Paper with Exceptional Electromagnetic Interference Shielding Anisha chaudharya,c, Saroj Kumaria* Rajeev Kumara, Satish Teotiaa, Bhanu Pratap Singha, Avanish Pratap Singhd, S.K. Dhawanb, Sanjay R. Dhakatea a b

Physics & Engineering of Carbon, Division of Material Physics and Engineering,

Polymeric and Soft Materials Group, Division of Material Physics and Engineering,

CSIR-National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi-110012, India c d

Academy of Scientific and Innovative Research(AcSIR)

Department of Physics, Atma Ram Sanatan Dharam College, New Delhi-110021

Abstract Light weight and easily foldable with high conductivity multi-walled carbon nanotubes (MWCNTs) based mesocarbon microbeads (MCMB) composites paper is prepared using a simple, efficient and cost effective strategy. The developed light weight and conductive composite paper have been first time reported as an efficient electromagnetic interference (EMI) shielding material in X-band frequency region having low density of 0.26 g/cm3. The investigation revealed that composite paper shows an excellent absorption dominated EMI shielding effectiveness (SE) of -31 dB to -56 dB at 0.15 to 0.6 mm thickness, respectively. Specific EMI-SE of as high as -215 dBcm3/g exceeds the best values of metal and other low density carbon-based composites. Additionally, light weight and easily foldable ability of this composite paper will helps in providing stable EMI shielding values even after constant bending. Such intriguing performances opens the framework to design a light weight and easily foldable composite paper as promising EMI shielding material, especially in nextgeneration devices and for defence industries. Keywords: MCMB, MWCNTs, Composites, Flexible Paper, Low density, EMI shielding * Corresponding author. Tel.: +91-11-45608285 E-mail address: [email protected] (Saroj Kumari)

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1. Introduction The advancement in the communication technology along with many electrical and electronic devices have becomes an integral part of day-to-day life due to their ability to make our life easier. However, these electronic devices generate electromagnetic radiations which adversely degrade the device performance and are harmful for our society. At the same time, the growth of scientific devices, military instruments and commercial/industrial equipment are also in a process which has also developed a threat to electromagnetic radiation pollution.1-3 Therefore, there is an urgent need to shield these devices/instruments from electromagnetic radiation which raises the considerable research interest to prepare appropriate electromagnetic interference (EMI) shielding materials in either by absorption or reflection mechanism. Many metals based shielding materials have been explored due to their high conductivity, high permeability and very shallow skin depth.4-7 However, their heavy weight, low flexibility, poor dispersion and processing difficulty results in their inferior shielding effectiveness (SE) and make them unsuitable as shielding materials. Metal coated plastics or metal fibers based plastics are also trying as shielding materials, but not found suitable for specific applications where flexibility is necessary. In the present era of nanotechnology and nanomaterials, much attention has been paid towards the development of a thin, lightweight, flexible corrosion resistant and cost effective new generation smart materials with strong absorption over a wide frequency range. For this purpose a wide variety of carbon nanomaterials such as carbon nanotubes, graphene, graphene oxides and their composites, conducting polymers with or without transition metal oxides have been explored widely8-11 because of their light weight, high aspect ratio and excellent electrical and magnetic properties as compared to traditional metal based shielding materials.12-16 MCMB are one of the commercially available carbon materials which are the micro carbon spheres prepared from mesophase pitches. Formerly, MCMB has been used for making high density and high strength composites materials which were used in special applications such as high strength carbon artifact, column packing material for chromatography, feedstock for super activated carbon, and as a precursor for high-density.17-18 MCMB is also a well-studied anode material in lithium ion batteries.19-20 Even though MCMB are conducting carbon materials with good thermal and chemical stability still their use for EMI shielding applications are not studied thoroughly. Till date, there is only one such study in which 2 ACS Paragon Plus Environment

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MCMB based carbon matrix is used to form a composite with iron oxide for EMI shielding applications.21 Very high EMI SE of this composite is reported however, thickness and flexibility remains a problem. Also the density of MCMB is one of the issues which limit their use in lightweight portable devices. As MCMB cannot be transformed into a flexible and lightweight film alone, hence some strategies have been taken into consideration in which combination of CNTs and MCMB is used to form a thin and light weight composite paper. Two main issues can be solved using CNTs in preparation of composite paper. Firstly, it can form an interconnected conductive network in which high density MCMB can easily entangle without using any binder and form a porous, foldable thin sheet having low density as compared to MCMB. Secondly, high electrical conductivity of the composite paper can be achieved. MCMB is cheap, highly crystalline materials due to their spherical shape with good conductivity which can be utilized for cost effective EMI shielding application. By replacing some portion of MCMB with CNTs can improve their conductivity but at the same time overall cost remains low because the major carbon material is MCMB in which CNTs acts as filler. Thus, a combination system helps in forming a flexible lightweight composite paper which otherwise not possible in case of MCMB alone and also improves the properties of MCMB, as CNTs are superior conducting materials. Therefore, taking advantages of CNTs (high aspect ratio) with highly conducting mesocarbon microbeads (MCMB) matrix, we demonstrate first time a novel approach to fabricate very thin, lightweight, foldable and highly conductive mesocarbon microbeadsMWCNTs (MCMB-MWCNTs) composite paper using homogenization and vacuum assisted filtration process and EMI shielding properties were measured in X- band frequency range (8.2-12.4 GHz). The composite paper showed very high EMI shielding at very low thickness as low as 0.15 mm. Although, fabrication process is very simple but novel in the view of the fact that it is first time utilized to fabricate lightweight, foldable MCMB based composite paper for EMI shielding application, which is yet not possible with other processes. In addition, the market value of coal tar pitch (precursor for MCMB synthesis) is very low compared to other carbon materials. Therefore, the need for cost effective materials and techniques with the advantages of lightweight, flexibility and low density, motivated us to attempt a combination of MCMB and MWCNTs composite paper for EMI shielding applications. 3 ACS Paragon Plus Environment


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2. Experimental 2.1. Material Coal tar pitch (CTP) was supplied by Konark Industry Pvt. Ltd., India. The solvents required for extraction of MCMB such as toluene (99.0%), quinoline (99.0%) and acetone (99.0%) were purchased from Qualigenes Fine Chemicals, Navi Mumbai, India. Aluminium chloride was supplied by Loba Chemie Pvt. Ltd. Mumbai. MWCNTs used in this study were synthesized by CVD technique. 2.2. Synthesis of MCMB and MWCNTs For synthesis of MCMB, coal tar pitch (contain 2.5 wt.% quinoline insolubles (QI)) was blended with 2 wt.% of AlCl3 by simple mixing process in which AlCl3 works as catalyst. This homogeneous mixture was heated isothermally in a reaction furnace at constant temperature 410°C in an inert atmosphere for 3-4 hr. After cooling at room temperature, a black product was then obtained which contain MCMB. These MCMB were mixed with tar oil and quinoline in a soxhlet assembly in which MCMB, secondary QI was separated by filtration and washed with toluene and acetone. The properties of coal tar pitch and MCMB is given in Table-1. These MCMB were carbonized at 1400oC in inert atmosphere of nitrogen. MWCNTs have been synthesized using catalytic chemical vapour deposition (CVD) 22

. In this process, solution of toluene (hydrocarbon source) and ferrocene (iron catalyst) was

injected in the quartz reactor in two zone furnace having the temperature 200 °C of first zone and 750 °C of second zone. Other experimental details is given elsewhere.22 2.3. Preparation of MCMB-MWCNT composite paper Carbonized MCMB was ball milled with MWCNTs for 3 hr to obtain a fine powder. This powder was mixed in acetone using ultrasonicator for 2 h which was then homogenized for 10 min to make a uniform dispersion. The solution was filtered by filter paper using vacuum assisted filtration technique. Finally, composite paper was peeled off from filter paper and dried overnight at room temperature. Thus, obtained MCMB-MWCNTs composite paper was very easily foldable and light in weight. This procedure was repeated by a varying mass fraction of MWCNTs from 10 to 25 wt.% to obtain different MCMB-MWCNTs composite


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papers. The schematic representation for fabrication of MCMB-MWCNTs composite paper is given in Figure-1.

Table 1: Properties of coal tar pitch and MCMB Properties

Coal tar pitch

MCMB

Quinoline Insoluble (%)

2.5

93.06

Toluene Insoluble (%)

16.0

99.32

Specific gravity

1.28

1.32

Coking Value (%)

43.6

90

Optical Texture

Isotropic

Anisotropic

Ash Content (%)

0.129

2.8



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Figure- 1:.Schematic representation of fabrication process of MCMB-MWCNTs composite paper. (inset of schematic shows (a, b and c) multiple folds and (d) unfolds MCMBMWCNTs composite papers

2.4. Characterization Surface morphology of the MCMB and MCMB-MWCNTs composite paper was examined by a scanning electron microscope (SEM, VP-EVO, MA-10, Carl-Zeiss, UK) operated at 10 kV. X-ray diffraction (XRD) of the samples were carried out in the scattering range of 10–80o on D-8 Advanced Bruker diffractometer using CuKa radiation (λ=1.5418Å) at scanning rate of 5° per min. Raman spectroscopy was done by using Renishaw in-Via Raman spectrometer, UK. The wave length for excitation laser was 514.5 nm provided by ionized argon. BET surface area was calculated by Autosorb iQ automated gas sorption analyzer from Quantacrome Instruments, USA (model no. ASIQM0000-4 and N2 adsorption isotherm from Germany). Thermal behaviour of samples is investigated by the TGA analysis which is done by thermo gravimetric analyzer, Mettler Toledo TGA/SDTA 851E thermal analysis system in air at the heating rate of 10°C/min. TGA is an important technique to find out the weight (wt.) loss of materials, presence of impurities along with the percentage of material. Classical four probe contact method was used to determine electrical conductivity using Keithley 2602A as a constant current source. EMI shielding measurements have been carried out by using Agilent E8362B vector network analyzer in the 8.2-12.4 GHz (X-band) microwave range. The samples of thickness 0.15 mm were cut in a rectangular shape and placed in the copper sample holder (with dimensions 26.8 mm x 13.5 mm) connected between the waveguide flanges of network analyzer.

3. Results and Discussion 3.1. Morphological and structural properties of MCMB-MWCNTs composites paper Figure 2(a) shows an SEM image of carbonized MCMBs, with diameter range between 1 to 5 µm. Spherical structure of MCMB is clearly seen in inset of Figure 2(a). Tubules of MWCNTs with diameter 20-70 nm and lengths varies to several microns which can be seen in Figure 2(b). In the MCMB-MWCNTs composite paper, MCMB are uniformly distributed in entangled mat of MWCNTs as shown in Figures 2(c) and 2(d). MWCNTs make an interconnected conducting network in which mesoporous MCMB are entrapped. Also, during 6 ACS Paragon Plus Environment

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ball-milling collision and friction of the steel balls have large impacts on the MWCNTs. This creates defects and crack formation on the surface of the MWCNTs.23 These defects created on the surface of MWCNTs sponsor to the physical bonding between MCMB-MWCNTs. In this interconnected mat MCMB are entrapped. Also the interconnected MWCNTs network contributes to the improvement of conductivity of composite paper.

Figure 2: SEM micrographs of (a) MCMB (b) MWCNTs (c) MCMB-MWCNTs composite paper (d) MCMB-MWCNTs composite paper at higher magnification XRD measurement was performed in order to investigate the crystalline nature of MCMB, MWCNTs and MCMB-MWCNTs composite paper. Figure 3(a) shows typical X-ray diffraction pattern of MCMB, MWCNTs and MCMB-MWCNTs composite paper. The small diffraction peak for MCMB observed at 2θ angle of 25.8o corresponds to the (002) crystallographic plane of turbostatic carbon. With the incorporation of MWCNTs (25 wt.%) in composite paper, the intensity and sharpness of (002) and (100) peaks increased. Additionally, the peak also slightly shifted to a higher degree (26.12o) confirming



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improvement in crystallinity of composite paper, this is due to the incorporation of crystalline MWCNTs (26.2o). Interlayer spacing (d-spacing) of MCMB and composite paper is calculated from (002) peak and it is observed that d-spacing decreased from 0.343 to 0.339 nm for MCMB and composite paper, respectively. This confirms that crystallinity improved on incorporation of MWCNTs in MCMB. Moreover, as seen from a diffraction pattern of composite paper, peak width is higher as compared to MCMB which can be ascribed to the small size of the graphitic domains.

Figure 3: XRD patterns (a) and Raman spectra (b) of MCMB, MWCNTs and MCMBMWCNTs composite paper Figure 3(b) shows the Raman spectra of MCMB, MWCNTs and composite paper having 25 wt % of MWCNTs. Three major peaks can be seen in Raman spectra at 1346 cm-1, 1596 cm-1 and 2707 cm-1 for all samples. The intensities of these peaks are much lesser in MCMB due to turbostatic (semi-graphitic) nature of MCMB heat treated at 1400oC but sharp and intense peaks are observed in composite paper which is due to the incorporation of crystalline MWCNTs in MCMB. Peak at 1346 cm-1 is a longitudinal optical phonon induced by disordered carbon and typically coupled with the vibration of carbon atoms by means of dangling bonds and related to sp3 bonded carbon (like in diamond), labelled as the D-band. The 1596 cm-1 peak (G-band) is a tangential shear mode of carbon atoms and very much related to the vibration of sp2-bonded carbon atoms (matching to the E2g mode) in a 2D hexagonal lattice, as seen in layer of graphene12. Third peak appeared at 2707 cm-1 corresponds to the overtone of the D-band known as Ǵ-band and comes in the second-order Raman spectra of crystalline graphite. A small distinguishable Raman peak can be seen at 8 ACS Paragon Plus Environment

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2941 cm-1 in Figure 3(b) for composite paper and this feature is attributed to D + G combination mode and also is induced by disorder. The intensity ratio of the D and G bands (ID/IG) further quantifies the relative levels of disordered and graphitic carbons in the MCMB MWCNTs and composite paper. It is observed that ID/IG ratio for MCMB and MWCNTs is 0.75 and 0.45 respectively while 0.49 in case of MCMB-MWCNTs composite paper. Clearly, shows that on MWCNTs incorporation with MCMB ID/IG ratio decreases from 0.75 to 0.49 as MWCNTs have highly graphitic structure which contributes to ordered graphitic structure of composite paper. 3.2. Thermal gravimetric analysis (TGA) TGA curves of MCMB and MCMB-MWCNTs composite paper containing 10 wt.% MWCNTs have been studied in oxidative atmosphere upto temperature 1000oC at the rate 10oC/min and presented in Figure 4. It is observed that MCMB go through a two-step decomposition process, in first step upto temperature 450oC, very less amount of wt. loss takes place and in second step major wt. loss occurs between temperatures 450oC to 800oC. While in case of MCMB-MWCNTs composite paper major weight loss takes place between temperatures 600oC to 900oC. This clearly indicates that MWCNTs has stabilizing effect on thermal pyrolysis of MCMB. This is due to the result of incorporation of more thermally stable materials as reinforcements as MWCNTs are thermally more stable and thus increases initial step of decomposition from temperature 450oC to 600oC. Increase in thermal stability of MWCNTs incorporated MCMB composite paper in oxidizing environment is also due to increase in degree of graphitization which can reduce number of edge carbon atoms which is responsible for oxidation reaction12. Additionally, from the TGA analysis ash content is found to be 2.8% and 5.6% for MCMB and MCMB-MWCNTs composite paper, respectively after complete pyrolysis. Ash content in composite is high as compared to MCMB is due to the presence of catalyst added during the synthesis process of MWCNTs.



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Figure 4: TGA curves of MCMB and MCMB-MWCNTs composite paper containing 10 wt.% MWCNTs. 3.3. Physical properties of MCMB-MWCNTs composites paper Effect of MWCNTs on density and porosity of composite paper is studied. We observe that the density of MCMB-MWCNT composite paper is decreases as we increases the MWCNT content in the composite. Density of composite paper decreases from 0.59 to 0.26 g/cm3 with the variation in MWCNTs content from 10 wt.% to 25 wt.%. It is because of density of MWCNTs is very less as compare to MCMB. Composite paper prepared in this study has very low density, light weight and high flexibility as compared to other low density carbon based composites reported in the literature (e.g. low density ferrocene carbon foam

24

and

25

elastomers foam nanocomposites. ) In contrast to the density, porosity of composite paper increases by increasing MWCNTs concentrations in MCMB-MWCNT composite paper. Surface area of composite paper increases from 89 to 205 m2/g with increasing MWCNTs loading in composite paper. 3.4. Electrical conductivity MCMB-MWCNTs composites paper Electrical conductivity is an important parameter for studying the EMI shielding properties. Figure 5(a) shows the variation of the electrical conductivity at room temperature for 10 ACS Paragon Plus Environment

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MCMB-MWCNTs composites paper having different MWCNTs wt.% loading. Initially, the electrical conductivity of 10 wt.% MCMB-MWCNTs composites paper is found to be 7.9 Scm-1 and it increased to 11.2 Scm-1 with 25 wt.% MWCNTs loading. The increment in conductivity is because of MWCNT which gives the increment in conduction way of electron, which is specifically identified with structure of fortifying material. In addition, mesopores framed in caught mat of MWCNTs are interconnected, thus leading system was shaped which permits a constant charge appropriation all through the surface. 3.5. Electromagnetic interference (EMI) shielding It is well known that performance of EMI shielding materials is measured in terms of the shielding effectiveness (SE). The EMI Shielding effectiveness is the logarithmic ratio of incoming power (Pi) to outgoing power (Pt) of radiation and expressed in decibel (dB). EMI shielding effectiveness is a negative quantity as Pt is always less than Pi. When the microwave radiation impinges on surface of shielding materials (with thickness higher than skin depth) three type of phenomena namely reflection (SER), absorption (SEA) and multiple reflections (SEM) can be observed 9, 26 and sum of these entire phenomenon’s (SER, SEA & SEM) is the total shielding effectiveness i.e., total EMI SE (SEtotal). Therefore, the total EMI shielding effectiveness can be written as 12, 27-29

= + + = 10log ⁄

(i)

Where, Pi and Pt are the incident and outgoing power respectively. Thus, in order to provide the shield against the EM waves, it must be either reflected (multiple reflections) or absorbed by the shielding materials. But, if the reflecting surfaces or interfaces are separated by a distance larger than the skin depth, losses due to multiple reflections can be ignored. Thus, the total shielding effectiveness (SET) can be expressed as 30-32

= +

(ii)

In the present study, we measured EMI-SE of light weight, free standing MCMBMWCNTs composite paper at MWCNTs loadings of 10, 15, 20 and 25 wt.%. EMI-SE is studied by two methods. In first method, using single layer of different carbon papers and corresponding results are presented in Figure 5(b-d). In another one, stacking method is used in order to increase the thickness of composite paper by applying multiple layers of composite paper of same composition. Figure 5(b-d) shows the variation of the absorption, reflection and total EMI SE as a function of MWCNTs loading in MCMB-MWCNTs 11 ACS Paragon Plus Environment


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composite papers (single layer) in the frequency range of 8.2 to 12.4 GHz (X-band). The total shielding effectiveness, SET at 10 wt.% MWCNTs loading in composites paper with single layer is -16.4 dB, in which absorption loss is –8.5 dB and reflection losses is -7.9 dB. While, in case of 25 wt.% loading of MWCNTs composites paper, the absorption loss improved to 19.7 dB but reflection loss remains only -11.0 dB which consequences maximum SET value of (-30.7 dB) is obtained. It clearly demonstrate that SET is dominated by absorption (SEA) rather than reflection (SER) in 25 wt. % MCMB-MWCNTs composites paper. The increases in SEA with increasing MWCNTs content can be correlated with increase in electrical conductivity and surface area of MCMB-MWCNTs composites paper.

Figure 5: Electrical conductivity (a), EMI-SE of MCMB-MWCNTs composite paper: SET (b), SEA(c) and SER (d) in frequency range of 8.2 to 12.4 GHz (X-band) with MWCNTs loading The EM radiation at high frequencies penetretes only near surface region of an electrically conducting material. This phenominon is known as skin effect. The depth, at which field drops to 1/e of the incident value is called as skin depth and expressed as27, 33: =

=

8.68

$%&


(iii)

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where f is frequency, σ is electrical conductivity and µ is permeability and t is the thickness of composite paper. Hence, from the above formula skin depth decreases with increasing frequency, permeability or conductivity and vice versa. Therefore, it is expected that the use of carbon nanotube with high aspect ratio and high conductivity could be possibly result in achieving promising EMI shielding value. A strong skin effect was observed due to the small unit size and high aspect ratio of the MWCNTs. Figure 6 (a) shows the variation in skin depth with frequency for different MWCNTs loading (wt.%) and figure 6 (b) shows the skin depth variation with MWCNTs at frequency of 8.2 GHz. It is observed that with the increase in frequency and MWCNTs loading the skin depth of the MCMB-MWCNTs composite paper decreases. Due to absorption, EMI SE increases at higher frequency, result reduce skin depth because it is inversely proportional to the absorption loss.

Figure 6: Variation of skin depth of MCMB-MWCNTs-composite paper with (a) frequency and (b) MWCNTs loading Furthermore, it is also observed that SET increases monotonically at fixed frequency and thickness with increasing wt. % of MWCNTs in the composite paper. The enhancement of the EMI SE in the composites paper is attributed to formation of the interconnected conductive network by MWCNTs, during the processing, MWCNTs can make interaction with MCMB, which will interact with the incident EM radiation which help in contributing to the absorption capacity. Consequently, the electric field strength drops with each skin depth to 1/e of the incident value and this loss or drop is said to be as the absorption loss which will also contribute to shielding.33-35 Other factors which will contribute in total EMI shielding are the high surface area and porosity of the MCMB-MWCNTs composite paper. Higher the available surface area for 13 ACS Paragon Plus Environment


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interaction with the electromagnetic radiation higher will be the EMI shielding due to absorption. The thin film composite paper fabricated with high surface area thus has greater chances of interaction with EMI radiation. This will cause greater power absorption within the shielding material and results in improved EMI shielding properties. The interaction mechanism of electromagnetic waves with MCMB and MWCNTs is shown in figure 7 through a schematic diagram.

Figure 7: Schematic representation of mechanism of EMI shielding Furthermore, it is important to mention here that density of composite paper is very less compared to other composites and graphene based materials, which reaches to 0.26 g/cm3 on highest MWCNTs loading (25 wt.%). As a result, the mesoporous MCMB and interconnected mesopores of MWCNTs allowed composite paper to have a very much smaller density and high flexibility than to most to the polymeric foam composites (e.g. ~0.79 g/cm3 for RGO/PMMA foams.28) Moreover, SE is directly proportional to the thickness of the materials upto a certain level therefore; at very small thickness it is a major challenge to attain effective shielding performance. The total shielding performance achieve is -31dB, which meets the optimal SE requirements for commercial applications. In this work, from the data presented in Table 2, it is noted that the MCMB-MWCNTs composite papers with thickness around 0.15 mm shows highly competitive shielding material to other polymeric graphene-based composites or other composites with thickness greater than 1 mm. For example, polymeric RGO/Fe3O4 composite foams with thickness of 2.5 mm shows a shielding performance of 15~19 dB only.36 Hence,


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the MCMB-MWCNTs composite papers of low density, high porosity, and high electrical conductivity here provide a promising material for potential lightweight electromagnetic shielding applications and for other areas where there is the requirement of light weight and thin film materials. To elaborate the influence of thickness on SE, a combination of layered system with MWCNTs variation was formulated. It was found that, total EMI SE increases as the thickness of the composite films increases by increasing the number of composite layers. This is primarily due to the fact that number of free electrons available to interact with EM radiation is more with more amounts (not concentration) of MWCNTs present. As the most part of the power of the incident radiation is absorbed by the depth of the structure at the interface of each layer by multiple absorptions and reflection phenomena. This suggest that reflection is less at the interface of the layer due to which entering power maximized and thus most part of the power is absorbed by the MCMB together with interconnected network structure of MWCNTs and dissipated. Additionally, higher values of SEA compare to SER put forward the idea of using MCMB-MWCNTs composite paper primarily as EMI absorbing materials.



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Figure 8: Shielding effectiveness of MCMB-MWCNTs composite paper: SET (a), SEA (b), SER (c) and specific EMI shielding effectiveness (d) in frequency range of 8.2 to 12.4 GHz with varying composite layers Figure 8 (a-c) shows the total shielding effectiveness of stacked layers of MCMBMWCNTs composite paper with varying MWCNTs wt%. On increasing the thickness using layers from one to four a sharp rise in total SE was observed. The maximum total shielding effectiveness on 25 wt.% MWCNTs loading achieved was -56 dB with 0.6 mm thickness obtained by stacking 4 layers of composite paper. Subsequently SEA increased to -48 dB while major change was not observed in SER. The significance of these values at this small thickness could be more appropriately described in terms of specific EMI SE (EMI SE divided by density). Figure 8(d) shows the specific EMI SE of composite paper at varying MWCNTs loading. It was observed that specific EMI SE increases from -119 to -215 dBcm3/g on increasing MWCNTs loading from 10 to 25 wt.%. It is worth noting that the our low density composite paper shows very high 16 ACS Paragon Plus Environment

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specific shielding performance compare to metals and lightweight polymer composite, foams and other graphene based composites (Table 2). After analysing the specific EMI shielding data of reported literature work, only few studies are found to have the high specific EMI shielding value as compare to our results. In these studies,37-38 SWNTs/ DWCNTs based macro film are used as EMI shielding materials. But we are concentrated on developing the MCMB-MWCNTs composite paper with low cost and high EMI shielding properties. Thus, by using materials with varying thickness by layered method the possible use of applications increases in view of the fact that it offer the flexibility to use as many as layers of composite paper for the requisite EMI shielding application.

Table 2: Comparison of Electromagnetic interference shielding effectiveness of different materials reported in literature Density

Thickness

Frequency

EMI SE

Specific

(mm)

(GHz)

(dB)

EMI SE

-3

Materials

(g/cm )

(dB cm3/g) Graphene/PEVA39

-

0.36

8.2-12.4

23-27

-

Fe3O4/Graphene paper40

0.78

0.3

8.2-12.4

21-24

26-31

Fe3O4/RGO PEI Composite foam36

0.40

2.5

8.2-12.4

14.3 − 18.2

35-45

Fe3O4/RGO/PANI31

-

2.5

12.4-18

26

-

Carbon foam/MWCNTs12

0.54

2.75

8.2-12.4

85

163

PMMA/CNT composite41

-

0.57

110

29

-

PS /Graphene composite42

0.45

2.5

8.2-12.4

29

64.4

CNT/PS Composite Foam9

0.56

-

8.2-12.4

19

33.1

Graphene/PMMA composites28

0.79

2.4

8.2-12.4

13-19

17-25

Polyimide/Graphene composite

0.28

0.8

8.2-12.4

17–21

60-75

-

3

12.4-18

57

-

43

foam

MnO2 decorated Graphene 44

nanoribbons



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MWCNTs/Phenolic resin

Page 18 of 22

0.51

0.14

12.4-18

32.4

63.5

MWCNTs-Epoxy composite46

-

0.35

8.2-12.4

19

-

MCMB/Fe3O4 composite21

1.6

2.5

8.2-12.4

MCMB-MWCNTs composite

0.26

0.15-0.6

8.2-12.4

composite paper

45

75 31-56

47 119-215

paper (Our work)

4. Conclusion In summary, a novel approach is used to fabricate free standing, light weight, flexible and thin MCMB-MWCNTs composite paper which shows promising EMI shielding properties in X band (8.2-12.4 GHz). MWCNTs having high surface area and high aspect ratio is used to fabricate MCMB based composite paper which is highly conducting and possess very low density 0.26 g/cm-3 at 25 wt.% MWCNTs loading. EMI shielding effectiveness of -31 dB is achieved at 0.15 mm thickness only and maximum EMI Shielding value of -56 dB was obtained at 0.6 mm thickness by stacking four layers of composite paper. The high value of shielding effectiveness at very less thickness shows that MCMB-MWCNTs composite paper could be one of the best candidates for EMI shielding applications as compared to other conventional materials. Also, the specific EMI shielding of composite paper is -215 dB cm3/g, which is very high as compared to most of the low density polymeric composites with high thickness. This signifies that the simple strategy based free standing, light weight and thin MCMB-MWCNTs composite paper with advantages of flexibility and high surface area can be used for stealth technology, civil, military and next-generation flexible electronic devices. Acknowledgements Authors are grateful to the Director, CSIR-NPL to publish the results. Authors are thankful to Dr. R. P. Pant and Mr. Jai Tawale for doing XRD measurements and SEM characterization, respectively. Supporting Information TEM micrograph of MWCNTs showing embedded iron nanoparticles inside the tubes (Figure S1) and SEM micrograph of MCMB-MWCNTs composite paper composite paper at 18 ACS Paragon Plus Environment

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lower magnification (Figure S2). Density, open porosity and BET surface area of composite paper with MWCNTs loading are given in Figure S3 (a-b) and table (table S1) shows the error limits in the EMI shielding of MCMB-MWCNTs composite paper with varying composite layers. References 1. Li, N.; Huang, Y.; Du, F.; He, X.; Lin, X.; Gao, H.; Ma, Y.; Li, F.; Chen, Y.; Eklund, P. C., Electromagnetic Interference (EMI) Shielding of Single-Walled Carbon Nanotube Epoxy Composites. Nano Lett. 2006, 6 (6), 1141-1145. 2. Yang, H. J.; Cao, M. S.; Li, Y.; Shi, H. L.; Hou, Z. L.; Fang, X. Y.; Jin, H. B.; Wang, W. Z.; Yuan, J., Enhanced Dielectric Properties and Excellent Microwave Absorption of SiC Powders Driven with NiO Nanorings. Adv. Opt. Mater. 2014, 2 (3), 214-219. 3. Huang, Y.; Li, N.; Ma, Y.; Du, F.; Li, F.; He, X.; Lin, X.; Gao, H.; Chen, Y., The Influence of SingleWalled Carbon Nanotube Structure on the Electromagnetic Interference Shielding Efficiency of its Epoxy Composites. Carbon 2007, 45 (8), 1614-1621. 4. Panigrahi, R.; Srivastava, S. K., Trapping of Microwave Radiation in Hollow Polypyrrole Microsphere through Enhanced Internal Reflection: A Novel Approach. Sci. Rep. 2015, 5. 5. Che, R.; Peng, L. M.; Duan, X. F.; Chen, Q.; Liang, X., Microwave Absorption Enhancement and Complex Permittivity and permeability of Fe Encapsulated within Carbon Nanotubes. Adv. Mater. 2004, 16 (5), 401-405. 6. Thomassin, J.-M.; Lou, X.; Pagnoulle, C.; Saib, A.; Bednarz, L.; Huynen, I.; Jérôme, R.; Detrembleur, C., Multiwalled Carbon Nanotube/poly (ε-caprolactone) Nanocomposites with Exceptional Electromagnetic Interference Shielding Properties. J. Phys. Chem. C 2007, 111 (30), 11186-11192. 7. Cho, H.-S.; Kim, S.-S., M-Hexaferrites with Planar Magnetic Anisotropy and their Application to High-Frequency Microwave Absorbers. IEEE Trans. Magn. 1999, 35 (5), 3151-3153. 8. Thomassin, J.-M.; Pagnoulle, C.; Bednarz, L.; Huynen, I.; Jerome, R.; Detrembleur, C., Foams of Polycaprolactone/MWNT Nanocomposites for Efficient EMI Reduction. J. Mater. Chem. 2008, 18 (7), 792-796. 9. Yang, Y.; Gupta, M. C.; Dudley, K. L.; Lawrence, R. W., Novel Carbon Nanotube-Polystyrene Foam Composites for Electromagnetic Interference Shielding. Nano Lett. 2005, 5 (11), 2131-2134. 10. Cao, M.-S.; Yang, J.; Song, W.-L.; Zhang, D.-Q.; Wen, B.; Jin, H.-B.; Hou, Z.-L.; Yuan, J., Ferroferric Oxide/Multiwalled Carbon Nanotube vs Polyaniline/Ferroferric Oxide/Multiwalled Carbon Nanotube Multiheterostructures for Highly Effective Microwave Absorption. ACS Appl. Mater. Interfaces 2012, 4 (12), 6949-6956. 11. Ling, J.; Zhai, W.; Feng, W.; Shen, B.; Zhang, J.; Zheng, W. g., Facile Preparation of Lightweight Microcellular Polyetherimide/Graphene Composite Foams for Electromagnetic Interference Shielding. ACS Appl. Mater. Interfaces 2013, 5 (7), 2677-2684. 12. Kumar, R.; Dhakate, S. R.; Gupta, T.; Saini, P.; Singh, B. P.; Mathur, R. B., Effective Improvement of the Properties of Light Weight Carbon Foam by Decoration with Multi-Wall Carbon Nanotubes. J. Mater. Chem. A 2013, 1 (18), 5727-5735. 13. Kumar, R.; Kumari, S.; Dhakate, S. R., Nickel Nanoparticles Embedded in Carbon Foam for Improving Electromagnetic Shielding Effectiveness. Appl. Nanosci. 2014, 1-9. 14. Shen, B.; Zhai, W.; Zheng, W., Ultrathin Flexible Graphene Film: An Excellent Thermal Conducting Material with Efficient EMI Shielding. Adv. Funct. Mater. 2014, 24 (28), 4542-4548. 15. Yousefi, N.; Sun, X.; Lin, X.; Shen, X.; Jia, J.; Zhang, B.; Tang, B.; Chan, M.; Kim, J. K., Highly Aligned Graphene/Polymer Nanocomposites with Excellent Dielectric Properties for High-Performance Electromagnetic Interference Shielding. Adv. Mater. 2014, 26 (31), 5480-5487. 19 ACS Paragon Plus Environment


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Lightweight and Easily Foldable MCMB-MWCNTs Composite Paper

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