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Journal of Materials Science and Engineering B 2 (6) (2012) 368-375

DAVID

PUBLISHING

CNT Based and Graphene Based Polymer Nanocomposites for Radar Absorbing Applications Chapal Kumar Das and Chakkirala Venkata Sudhakar Materials Science Centre, Indian Institute of Technology Kharagpur, West Bengal 721302, India Received: May 02, 2012 / Accepted: May 21, 2012 / Published: June 25, 2012. Abstract: In the ongoing search for newer materials for investigating the microwave absorption, carbon nanotubes and graphene merit a special consideration which gives their outstanding properties. While on one hand, the mechanically strong and electrically conductive properties of CNTs grab the attention for developing conductive polymer composites for microwave absorption, on the other hand, graphene is not left any far behind in this application owing to its outstanding mechanical, thermal and electrical properties. In this work, four different samples have been prepared with two of them using CNT as a filler while the other two utilizing dispersed graphene in Thermoplastic polyurethane (TPU) matrix. Barium hexaferrite (BaFe12O19) which exhibits both magnetic and dielectric properties has been added as an additional filler in two of those samples. The microwave absorbing characteristics have been investigated in X band region (8-12 GHz) on a Vector Network Analyzer, morphology and structure were studied through XRD, SEM and FESEM, and elemental composition was confirmed by EDX. It has been observed that a reflection loss of -16.3 dB has been obtained by CNT/Fe3O4/TPU system and -9.7 dB has been obtained by graphene/Fe3O4/TPU system, while CNT/Fe3O4/BaHF/TPU system achieved a reflection loss of -18.9 dB and graphene/ Fe3O4/BaHF/TPU system achieved -13.3 dB. Key words: Radar absorbing materials, CNTs, graphene, microwave absorption, reflection loss.

1. Introduction The quest for developing radar absorbing materials (RAMs) has been increasing ever since their development began way back in the World War-II era. One cannot imagine any doctrine of modern warfare in the 21st century in the all three dimensions namely the land, the air and the sea without the incorporation of stealth. This is the precise reason for the relentless efforts which is put in by the researchers all over the world in synthesizing materials that can absorb the electro-magnetic radiation in the microwave region [1-8]. Microwaves belong to a frequency band of 300 MHz to 300 GHz of the electromagnetic spectrum and there has been a special focus on engineering RAMs which absorb the microwaves in X-band (8-12 GHz). Carbon nanotubes (CNTs) and graphene have been attracting the materials scientists and engineers with 

Corresponding author: Chapal Kumar Das, research field:

radar absorbing materials. E-mail: [email protected].

their outstanding electrical and mechanical properties. While on one hand, the mechanically strong and electrically conductive properties of CNTs grab the attention for developing conductive polymer composites for microwave absorption, on the other hand, graphene is not left any far behind in this application owing to its outstanding mechanical, thermal and electrical properties [9-17]. An ideal radar absorbing material (RAM) caters for absorption of both the electrical and magnetic components of the microwave. To meet this requirement, CNTs and graphene have been dispersed to function as dielectric filler absorbing the electrical component of the EM wave and ferrites like Fe3O4 and Barium hexaferrite (BaFe12O19) have been included to absorb the magnetic component. A set of four RAMs have been synthesized by using solution casting technique for dispersion of fillers in TPU matrix. Compression molding has been carried out for fabrication of samples into the required shape and size.

CNT Based and Graphene Based Polymer Nanocomposites for Radar Absorbing Applications

As part of characterization of the RAMs, X-ray diffraction, Scanning electron microscopy (SEM) and Field emission scanning electron microscopy (FESEM) have been carried out to study the structure and surface morphology of the materials, elemental composition was examined by EDX and Vector Network Analysis has been carried out to evaluate the radar absorbing properties. While carrying out network analysis, the critical parameters of permittivity and permeability have been obtained for each sample. In electromagnetism, permittivity is a measure of how an electric field affects, and is affected by a dielectric medium. Permittivity is determined by the ability of a material to polarize in response to the field and thereby reduces the total electric field inside the material. Permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. In other words, it is the degree of magnetization that a material obtains in response to an applied magnetic field. Both permittivity and permeability are generally expressed as complex equations which consist of a real part and an imaginary part. The real part represents the energy stored while the imaginary part represents the energy lost. For each synthesized RAM, a total of four parameters which consist of the real and imaginary parts of both permittivity and permeability have been obtained from network analysis. Subsequently, these values have been substituted in appropriate mathematical equations with the help of a software to deduce the reflection loss and the concerned plots have been presented in this paper.

2. Experiment 2.1 Materials The MWCNTs (MWCNTs-1000) were procured from Iljin Nanotech Co. Ltd, Korea. The diameter, length and aspect ratio are 10-20 nm, 20 μm and (approximately) 1000, respectively. The density of MWCNT is 2.16 g/cm3. XGnP® brand graphene Nanoplatelets were procured from XG Sciences, Inc.

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East Lansing, Michigan, USA. Graphene Nanoplatelets which were used belong to Grade M which has an average particle thickness of about 6 nm, surface area of 120-150 m2/g and average particle diameter of 15 microns. The utilized TPU for developing RAMs belongs to commercial medical grade aliphatic, polyether (TecoflexVR EG 80A injection grade) procured from Lubrizol Advanced Materials (ThermedicsTM Inc. Polymer Products, USA). TecoflexVR EG 80A (around 35% of hard segments) has shore hardness of 72A, specific gravity = 1.04 and its constituent formulation contains methylene bis (cyclohexyl) diisocyanate (HMDI) as hard segment, polytetramethylene oxide (PTMO) as soft segment (molecular weight =1000 g/mol) and chain extender 1,4-butane diol. The utilized Fe3O4 in this work is of Stream Chemicals make with specifications of Iron (II, III) oxide, (Magnetite (95%)). 2.2 Purification of MWCNTs Acid treatment of CNTs has been carried out to purify them. Raw MWCNTs were dispersed in 70% pure nitric acid (HNO3) and heated at 130 °C for 3 hours. The mixture was then centrifuged and washed with deionized water several times until the pH of the black solution reached a value below 7. The resulting material was dried overnight at 110 °C in an oven [18] to obtain purified MWCNTs. 2.3 Synthesis of Barium Hexaferrite Barium hexaferrite has been prepared by ball milling of BaCO3 and α-Fe2O3 mixture followed by thermal heat treatments [19, 20]. BaFe12O19 was prepared by ‘mixed oxide’ approach. Here BaCO3 and α-Fe2O3 were mixed in (1:6) molar ratio and ball milled in dry condition for 6 hours. In order to carry out calcinations, the samples were kept in furnace and a temperature of 750°C was maintained for 1 hour. Further temperature was increased to 1300 °C and maintained at that temperature for three hours in air atmosphere to obtain BaHF powder.

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CNT Based and Graphene Based Polymer Nanocomposites for Radar Absorbing Applications

2.4. Synthesis of RAMs A total of four RAMs have been prepared whose compositions are mentioned below. Thermoplastic polyurethane has been utilized as a common polymer matrix for all the samples. For dispersion of dielectric and magnetic fillers in the polymer matrix, solution casting technique has been adopted. The compositions are as mentioned, RAM-1: CNT/Fe3O4/TPU; RAM-2: graphene/Fe3O4/TPU; RAM-3: CNT/Fe3O4/BaFe12O19/TPU and RAM-4: graphene/ Fe3O4/ BaFe12O19/TPU. Tetrahydrofuran (THF) has been utilized as solvent for dissolving the polymer and subsequently dispersing the nanofillers. TPU granules have been dissolved in THF by using a magnetic stirrer and subsequently CNTs/graphene have been dispersed in the matrix. For dispersing CNTs and graphene, magnetic stirring and sonication techniques have been used. After achieving an even dispersion of CNTs and Graphene, Fe3O4 has been dispersed in the solution by sonication. For synthesizing RAMs-3 and 4, after dispersing Fe3O4, BaFe12O19 powder was dispersed using sonication. Subsequent to obtaining an uniform distribution of fillers in the matrix incase of each RAM, the solution was dried in an oven at 70 oC for two hours. This step ensured that the solvent evaporates completely allowing the RAMs to be free from THF. In order to carry out the microwave absorption analysis on a Network Analyzer, there was a need to have the samples in a prescribed shape and size. To meet this requirement, the dried samples have been fabricated into rectangular slabs of 25 mm by 15 mm size with a thickness varying between 1.5 mm to 1.7 mm by compression molding. 2.5 Characterization The synthesized RAMs have been characterized by X-ray diffraction on a PW 1840 X-ray diffractometer using copper target (Cu Kα radiation) at a scanning rate of 3 oC/min. A Carl Zeiss-SUPRATM 40 FESEM with an accelerating voltage of 5 kV and a SEM of VEGA

LSU make with an accelerating voltage of 10 kV were employed to observe morphology of tensile fractured nanocomposites. A thin layer of gold was sputtered on the fractured surface of the specimens for electrical conductivity. Energy Dispersive X-ray analysis has been carried out to identify the elemental composition of prepared materials. The complex permittivity and permeability of the composite samples were measured by a Network Analyzer (Agilent E8364B PNA Series) using material measurement software 85071 in the frequency range of 8.2-12.2 GHz (X-band) region at room temperature and thus reflection loss has been calculated for various frequencies.

3. Results and Discussion 3.1 XRD Analysis The X-ray diffraction patterns of all the four RAMs are shown in Fig 1. Fig. 1a presents the XRD pattern of RAM-1. The well resolved peaks of RAM-1 conform to those of Fe3O4 as per JCPDS card No: 01-075-0449. Thus, presence of cubic Fe3O4 phase (spinel structure (MgAl2O4)) can be clearly established because of the presence of well resolved crystalline peaks of Fe3O4 (220), (311), (222), (400), (422), (511), (440), (620), (533) and (622) at 2θ = 30.28°, 35.67°, 37.32°, 43.36°, 53.8°, 57.36°, 62.99°, 71.48°, 74.55° and 75.56°, respectively [21]. XRD pattern of RAM-2 is presented in Fig. 1b. The presence of cubic Fe3O4 phase (spinel structure (MgAl2O4) can be confirmed because of the depiction of well resolved crystalline peaks of Fe3O4 (220), (311), (222), (400), (422), (511), (440), (620), (533) and (622) at 2θ = 30.28°, 35.67°, 37.32°, 43.36°, 53.8°, 57.36°, 62.99°, 71.48°, 74.55° and 75.56°, respectively, as JCPDS card No: 01-075-0449. In addition to the peaks of Fe3O4, the characteristic peak of graphene can be observed at 2θ =26.4o. Fig 1c shows the XRD pattern of RAM-3. In addition to the diffraction peaks of Fe3O4 as discussed in the earlier figures, the characteristic peaks of BaFe12O19 appear at 2θ = 39.8628° and 2θ = 47.23° which correspond to the crystal planes (114) and

CN NT Based and d Graphene Based B Polyme er Nanocomp posites for Ra adar Absorbing Applicatio ons

(a)

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(b)

(c) (d)) Fig. 1 (a) XRD pattern of RAM-1; (b b) XRD patttern of RAM-2; (c) XRD pattern p of RA AM-3; (d) XR RD pattern off RAM-4.

(205), respecctively, as peer JCPDS cardd No: 42-00002. If XRD patterrn of RAM--4 is observeed (Fig. 1d)), in addition to the diffracction peaks of Fe3O4, the characteristiic peaks of BaaFe12O19 appeear at 2θ = 266.74° and 35.36° which w are inndexed to cryystal planes (0006) and (110) (JJCPDS No: 422-0002) [20, 21]. Besides, the characteristiic peak of graaphene appearrs at 2θ = 26..42°. 3.2 Morphollogical Studyy Fig. 2 shoows the SEM pictures and Fig. 3 showss the FESEM picctures of the fractured surfaces of the specimens. In each of thhe nanocompposites, a proper impregnation of CNTs and a other maggnetic fillers into the TPU matrix m can bee seen. It is observed thhat a uniform disppersion of the fillers has been b achieveed in all the RAM Ms which in turn influenced in achievving uniform miccrowave absoorbing properrties across evvery sample. In Figs 2a, 2c,, 3a and 3c,, the thread like

uctures can be b identified as CNTs wh hile the sheett stru likee structures in Figs. 2b, 2d, 3b and d 3d can bee iden ntified as grapphene nanopllatelets. 3.3 EDX Analysiis The T preparedd specimens have been subjected too EDX X analysis in order to study and identify thee elem mental compposition of R RAMs. The EDX E picturess are shown in Figg. 4. It can bbe seen in Fig gs. 4a and 4bb thatt elements likke O, Fe and C are presen nt in RAMs-1 and d 2 which connfirms the presence of CNT Ts/ graphene,, Fe3O4 and TPU. In case of Fiigs. 4c and 4d d, in additionn to the t elements listed above,, Ba can also be observedd whiich confirms the presence of BaFe12O19 3 1 in RAMs-3 and d RAMs-4. Inn all the RAM Ms, presence of o Si elementt can n be attributeed to the preesence of traaces of usedd siliccone oil as a mould m releasee agent durin ng the processs of compression c m molding.

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CNT Based and Graphene Based Polymer Nanocomposites for Radar Absorbing Applications

(a)

Fig. 2

(b)

(c) (d) (a) SEM picture of RAM-1; (b) SEM picture of RAM-2; (c) SEM picture of RAM-3; (d) SEM picture of RAM-4.

(a)

(b)

(c) (d) Fig. 3 (a) FESEM picture of RAM-1; (b) FESEM picture of RAM-2; (c) FESEM picture of RAM-3; (d) FESEM picture of RAM-4.

CNT Based and Graphene Based Polymer Nanocomposites for Radar Absorbing Applications

(a)

Fig. 4

(b)

(c) (d) (a) EDX analysis of RAM-1; (b) EDX analysis of RAM-2; (c) EDX analysis of RAM-3; (d) EDX analysis of RAM-4.

(a)

Fig. 5

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(b)

(c) (d) (a) Reflection loss of RAM-1; (b) reflection loss of RAM-2; (c) reflection loss of RAM-3; (d) reflection loss of RAM-4.

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CNT Based and Graphene Based Polymer Nanocomposites for Radar Absorbing Applications

3.4 Microwave Absorption Analysis The microwave absorption characteristics of RAMs are plotted in terms of reflection loss (in dB) versus frequency (in GHz), as shown in Fig. 5. In case of RAM-1 (Fig. 5a), the maximum value of -16.3 dB is observed at a frequency of 12.4 GHz. RAM-2 shows a maximum reflection loss of -9.7 dB at 12.4 GHz (Fig. 5b). A reflection loss of -18.9 dB has been observed at 12.4 GHz (Fig. 5c) in case of RAM-3. RAM-3 shows an increasing trend of reflection loss with increase in frequency except in the zone of 8.6-8.9 GHz where a drop has been observed. In comparison with RAM-1, an increase of 2.6 dB has been observed due to addition of BaFe12O19. In case of RAM-4 (Fig. 5d), the best reflection loss of -13.3 dB has been observed at 12.4. In comparison with RAM-2, it can be summarized that there is an increase in reflection loss by about 2.6 dB by addition of BaFe12O19.

[6]

[7]

[8] [9]

[10]

[11]

[12]

4. Conclusions In this work, four RAMs have been synthesized in TPU matrix dispersing CNTs, graphene and ferrites as fillers. CNTs and graphene have been investigated as dielectric absorbers while the ferrites as magnetic absorbers. Their microwave absorption characteristics have been analyzed on a VNA in X-band region (8-12 GHz) and their reflection losses deduced.

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