World Journal of Nano Science & Technology 2(1): 42-46, 2013 ISSN XXXX-XXXX © IDOSI Publications, 2013 DOI: 10.5829/idosi.wjnst.2013.2.1.21138

CNT Based and Graphene Based Polymer Nanocomposites for Radar Absorbing Applications-II 1

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Chapal Kumar Das, 1C.H.V. Sudhakar, 1Mrinal Kanti Kundu, 2A.K. Saxena and 2Vinita Nigam

Materials Science Centre, Indian Institute of Technology Kharagpur, West Bengal- 721302, India 2 DMSRDE Kanpur, Kanpur-208013, India

Abstract: CNTs and Graphene have been dispersed in polymer matrices with a specific intent of investigating their capability to absorb the electrical component of the EM wave owing to their reputation as good dielectric materials. Four different samples have been prepared in which two samples contained CNT as the dielectric filler while the other two samples contained graphene for the same purpose. Fe3O4 has been used as common magnetic filler in all the four samples, to cater for the absorption of magnetic component of the microwave. Barium hexaferrite (BaHF) and Iron silicide (FeSi) which exhibit both magnetic and dielectric properties have been added each as additional filler in two of those samples. Polyphosphazene has been chosen as polymer matrix system in which the above mentioned fillers have been dispersed. Characterization of the prepared samples included the analysis of their microstructure and morphology by SEM, identification of the metal oxide fillers by XRD technique and elemental analysis by EDS. Microwave absorption properties were investigated in X band region (8 GHz to 12 GHz). It has been observed that a reflection loss of -14.8 dB has been obtained by RAM-1 (CNT/ Fe3O4/ Polyphosphazene), -23.4 dB by RAM-2 (Graphene/ Fe3O4/ Polyphosphazene), -20.1 dB by RAM-3 (CNT/ Fe3O4/ BaHF/ Polyphosphazene) and a reflection loss of -21.32 dB by RAM-4 (CNT/ Fe3O4/ FeSi/ Polyphosphazene). Key words: Barium hexaferrite

Polyphosphazene

Graphene

INTRODUCTION

X band

Iron silicide

GHz. The possible reason for this special drive could be that majority of combat radars operating in all the three dimensions namely the Army, the Navy and the Air Force belongs to X band. Ideally, a RAM should be devised such a way that it is capable of addressing both the electrical and the magnetic components of electromagnetic wave [1-3]. Amongst the newer materials being explored for preparing RAMs, the two important members of carbon family namely Carbon nanotubes (CNTs) and Graphene cannot go unnoticed. With their excellent mechanical and electrical properties, they have been proving to be one of the best options to be considered for causing the dielectric loss of the EM wave. On the other hand, spinel compounds like ferrites function as very good absorbers of the magnetic component. So, a RAM with a combination of the above mentioned constituents would prove to be ideal. In our work, CNTs and Graphene have been dispersed in a polymer matrix to function as dielectric absorber and ferrites like Fe 3O4 and Barium hexaferrite (BaFe12O19) have been included to function as magnetic absorbers [4-6].

There are two techniques employed to reduce the radar signature namely shaping and coating the objects with certain materials which can absorb the radio waves. The latter technique has grabbed the attention of materials scientists and engineers to devise materials which have the property to absorb a portion of the incoming electromagnetic wave so that the intensity of the reflected wave is reduced marginally. These newer materials, over a period of time, have been known as Radar Absorbing Materials (RAMs).With the advent of nanotechnology, RAMs are being synthesized based on nanocomposites. The constituents and formulations of these nanocomposites are being chosen such a way they effectively absorb microwaves. Microwaves belong to a frequency band of 0.3 to 300 GHz of the electromagnetic spectrum. Although there have been relentless efforts to devise RAMs for absorbing microwaves in general for decades, there has been a special interest to address the absorption of X band of frequencies ranging from 8-12

Corresponding Author: Chapal Kumar Das, Materials Science Centre, Indian Institute of Technology Kharagpur, West Bengal- 721302, India.

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MATERIALS AND METHODS

fractured site of nanocomposites, Scanning electron microscope (SEM) of VEGA LSU make with an accelerating voltage of 10 kV were employed. Energy dispersive X-ray spectroscope (EDS) analysis has been carried out to identify the elemental composition of the prepared materials. The complex permittivity and permeability of the composite samples were measured by Network Analyzer (Agilent E8364B PNA Series) using material measurement software 85071 in the frequency range of 8.4-12.4 GHz (X-band).

Materials: The MWCNTs (MWCNTs-1000) were procured from Iljin Nanotech Co. Ltd, Korea and MWCNTs have been purified by using conc. Nitric acid [7]. XgnP® brand graphene nanoplatelets were procured from XG Sciences, Inc. East Lansing, Michigan, USA. Polyphosphazene have been prepared and supplied by Defence Materials and Stores Research and Development Establishment (DMSRDE), Kanpur (India) [8, 9]. Fe3O4 were supplied by Strem Chemicals (Magnetite (95%). Iron silicide were provided by Alfa Aesar (99.9% (metal basis)).

RESULTS AND DISCUSSION

Synthesis of Barium Hexaferrite: Barium hexaferrite has been prepared by ball milling of BaCO3 and -Fe2O3 mixture followed by thermal heat treatments [10]. BaFe12O19 was prepared by ‘mixed oxide’ approach. Here BaCO3 and -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.

XRD Analysis: The X-ray diffractograms of all the RAMs have been presented in Figure 1(a-d) respectively. In case of RAM-1, the peaks shown in the Figure 1(a) have been identified as those belonging to Fe3O4 as per JCPDS card No: 01-075-0449. Because of the presence of well resolved crystalline peaks of Fe3O4 (220), (311), (222), (400), (422), (511), (440), (620), (533) and (622) at 2 = 30.28°, 35.67°, 37.32°, 43.36°, 53.8°, 57.36°, 62.99°, 71.48°, 74.55° and 75.56° respectively, presence of cubic Fe3O4 phase (spinel structure (MgAl 2O4)) can be clearly established. XRD pattern of RAM-2 is presented in Figure 1(b). The presence of cubic Fe3O4 phase (spinel structure 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 2 = 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 (002) can be observed at 2 = 26.4°. Figure 1(c) 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 2 = 39.8628° and 2 = 47.23° which correspond to crystal planes (114) and (205) respectively as per JCPDS card no: 42-0002. In case of RAM-4 (Figure 1d), characteristic peak of FeSi has been designated in conformity JCDPS card no- 04-011-5927 [11, 12].

Synthesis of Rams: A set of four RAMs have been prepared with the following formulations:RAM-1: CNT/ Fe3O4/ Polyphosphazene, RAM-2: Graphene/ Fe 3O 4/ Polyphosphazene, RAM-3: CNT /Fe3O4 /BaFe12O19 / Polyphosphazene and RAM-4: CNT/ Fe3O4/ FeSi / Polyphosphazene. Solvent utilized for dispersion of nanofillers into polymer matrix is Tetrahydrofuran. Initially, polyphosphazene has been dissolved in THF by stirring. This was followed by dispersion of CNTs / Graphene into the polymer matrix by stirring and sonication techniques. Once the dielectric fillers have been uniformly dispersed magnetic fillers likeFe3O4 / BaFe12 O19/FeSi have been dispersed by sonication. The next step was to dry the product and removal of solvent completely. This was achieved by drying in an oven at 70°C for 2-3 hours. The dried products have been fabricated by compression molding at 120°C at a pressure of 5 kgf/cm2 for duration of 15 minutes. They have been shaped into rectangular slabs of 25 mm by 15 mm size with a thickness varying between 1.5 mm to 1.7 mm.

Morphological Study: The study of the morphology of the RAMs was carried out by SEM technique. The pictures of the fractured surfaces of the specimens have been presented in Figures 2 (a-d). The common observations in all the pictures are that the thread like structures can be identified as CNTs and sheet like structures as Graphene. It can also be observed that CNTs and Graphene as well as other magnetic fillers have

Characterisation: X-ray diffraction has been carried out on a PW 1840 X-ray diffractometer using copper target (Cu K radiation). For studying the morphology of the 43

World J. Nano Sci. Technol., 2(1): 42-46, 2013

Fig. 1: XRD pattern of (a) RAM-1, (b) RAM-2, (c) RAM-3, (d) RAM-4

Fig. 2: SEM image of (a) RAM-1, (b) RAM-2, (c) RAM-3, (d) RAM-4 44

World J. Nano Sci. Technol., 2(1): 42-46, 2013

Fig. 3: EDS study of (a) RAM-1, (b) RAM-2, (c) RAM-3 and (d) RAM-4

Fig. 4: Reflection loss of (a) RAM-1, (b) RAM-2, (c) RAM-3, (d) RAM-4 impregnated themselves thoroughly into the polymer matrix and distributed themselves uniformly. In Figure 2 (a), (c), the thread like structures can be identified as CNTs while the sheet like structures in figure 2 (b), (d) can be identified as graphene nanoplatelets.

(a) and (b), presence Fe3O4 in Polyphosphazene matrix can be established. Presence of BaFe12O19 can be established in case of RAM-3 (Figure 3c). RAM-4 shows the presence of FeSi (Figure 3d). 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 Figure 4. Incase of RAM-1, reflection loss of

EDX Analysis: EDX analysis has been carried out in order to study and identify the elemental composition of RAMs. The EDX pictures are shown in Figure 3. From Figures 3 45

World J. Nano Sci. Technol., 2(1): 42-46, 2013

over -14 dB (~96% absorption) is observed spreading over a wider zone of 11.4-12.4 GHz while the best reflection loss of -14.83 dB has been achieved at 12.06 GHz (Figure 4a). In terms of percentage, it corresponds to microwave absorption of 96.7%. For RAM-2, the maximum reflection loss of -23.42 dB has been observed at 10.47 GHz (Figure 4b) which corresponds to absorption of 99.5%. A broad band of high reflection loss (over -16 dB) which corresponds to about 98% absorption has been observed over the complete X-band (8.2-12.2 GHz) which is highly desirable. Incase of RAM-3, best reflection loss of -20.1 dB has been observed at a frequency of 10.6 GHz and this corresponds to an absorption of approximately 99% (Figure 4c). We can also observe Reflection loss of more than -16 dB in the band 10.4-12.4 GHz which is a broad band of absorption. RAM-4 is found to exhibit low reflection loss at lower frequencies while absorption improves tremendously as we approach the higher side of X band, particularly between 9.8-10.7 GHz with the highest loss of -21.32 dB at 9.8 GHz which corresponds to 99.4% absorption (Figure 4d). Over the band of 9.4-11.4 GHz, a reflection loss of more than -16 dB is observed.

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CONCLUSION A set of RAMs have been synthesized with CNTs and Graphene as dielectric fillers and ferrites like Fe3O4, BaFe12O19 and FeSi as magnetic fillers. Polyphosphazene has been used as common polymer matrix. The microwave absorption characteristics of all the prepared RAMs have been analyzed on a Vector network analyzer in X band region (8.4-12.4 GHz) and the reflection losses of each RAM have been derived. It has been observed that the ranges of absorption achieved for each RAM extend over a reasonably broad band of frequencies within the X-band. REFERENCES 1.

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Andersson, N., R. Andersson, V. Bergman, E.K. Mattias and K. Malmberg, 12 May 2006. “Synthesis and Characterization of Radar Absorbing Nanocomposites,” Royal Institute of Technology, Stockholm, Sweden. Saville, P., January 2005. “Review of Radar Absorbing Materials,”. Defence Research and Development Canada-Atlantic, Technical Memorandu.

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