Hierarchical Carbon Nanotube Coated Carbon Fiber: Ultra

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Applications of Polymer, Composite, and Coating Materials

Hierarchical Carbon Nanotube Coated Carbon Fiber: Ultra Lightweight, Thin and Highly Efficient Microwave Absorber Sandeep Kumar Singh, Mohammad Jaleel Akhtar, and Kamal K Kar ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 5, 2018

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ACS Applied Materials & Interfaces

Hierarchical Carbon Nanotube Coated Carbon Fiber: Ultra Lightweight, Thin and Highly Efficient Microwave Absorber Sandeep Kumar Singh†,*, Mohammad Jaleel Akhtar†,‡ and Kamal K. Kar †,§,* † Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur, India ‡Department of Electrical Engineering, Indian Institute of Technology Kanpur, Kanpur, India §Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering and Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur-208016, India *Corresponding Authors

KEYWORDS: carbon nanotube coated carbon fiber (CNTCF), trapping center, multiple reflection, microwave absorption, complex permittivity.

Abstract Strong EM wave absorption and light weight are the foremost important factors that drive the real world applications of the modern microwave absorbers. This work mainly deals with the design of highly efficient microwave absorbers, where the hierarchical carbon nanotube (CNT) forest is first grown on the carbon fiber through catalytic chemical vapor deposition method. The hierarchical carbon nanotube grown on carbon fiber (CNTCF) is then embedded in the epoxy matrix to synthesize light weight nanocomposites for their usage as efficient microwave absorbers. The morphological study shows that carbon nanotubes (CNTs) are self-assembled to form trapping center on the carbon fiber. The electromagnetic characteristics of resultant nanocomposites are investigated exclusively in the X-band (8.2-12.4 GHz) using the network

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analyzer. The synthesized nanocomposites, containing 0.35 and 0.50 wt% of CNTCFs, exhibit excellent microwave absorption properties, which could be attributed to the better impedance match condition and high dielectric losses. The reflection loss (RL) of -42.0 dB (99.99% absorption) with -10 dB (90% absorption) and -20 dB (99% absorption) bandwidth of 2.7 and 1.16 GHz, respectively is achieved for 0.35 wt% of CNTCF loading at 2.5 mm of thickness. The composite with 0.50 wt% of CNTCF loading illustrates substantial absorption efficiency with the RL reaching -24.5 dB (99.65% absorption) at 9.8 GHz and -10 dB bandwidth comprising of 84.5% of the entire X band. The excellent microwave properties are primarily obtained here due to the electric dipole polarization, interfacial polarization, and unique trapping center. These trapping centers basically induce multiple reflections and scatterings, which attenuate more microwave energy. This investigation opens a new approach for the development of extremely lightweight, small thickness and highly efficient microwave absorber for X-band applications.

Introduction With the speedy progress of electromagnetic technology in radar, wireless communication, local area network and other communication systems, the electromagnetic (EM) radiation and electromagnetic interferences (EMI) have become a serious problem for humankind and disturbing the electronic devices especially in the X band (8.2-12.4 GHz).1-5 The X-band frequency region of EM spectrum is widely used in wireless communication, satellites, terrestrial communication, motion detectors, traffic light crossing detection, weather radar and medical sciences.1 Therefore, it is extremely important to develop effective shielding techniques to curb or eradicate the resultant problem produced by EMI. Hence, the microwave absorbers have attracted considerable attention because of their EM wave attenuation properties. The modern microwave absorber needs to be lightweight, possessing small thickness in addition to showing


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strong EM wave absorption over a wide frequency band to fulfill the market demands.4, 6-7 At present, the carbon nanotubes (CNTs) with good electrical conductivity, large surface area, lightweight structure and high strength features are being steadily used as fillers in the nanocomposites, which are replacing the conventional microwave absorbing materials.8-13 However, the creation of agglomerations in the form of carbon nanotube (CNT) bundles deteriorates theirs EM absorption properties, which limit their uses in commercial applications.14 As a result, different strategies have recently been adopted on designing the CNT based hybrid morphologies to overcome this problem. The modification approaches contain decoration of CNTs with magnetic or dielectric particles.11, 13, 15-17 For example, Li et al. prepared Fe3O4 coated CNT and revealed that the composite with 30 wt% of Fe3O4 coated CNT exhibited a minimum reflection loss of -43.0 dB at 1.5 mm of thickness.11 Cao et al. dispersed 40 wt% ZnO coated MWCNT in the paraffin matrix, which showed -38 dB of minimum reflection loss at 5 mm of thickness with ~ 4 GHz of -10 dB (90% absorption) bandwidth.18 Shu et al. fabricated rGO/MWCNTs/ZnFe2O4

ternary

hybrid

composite

and

found

that

50

wt%

of

RGO/MWCNTs/ZnFe2O4 in paraffin matrix shown -23.8 dB of minimum reflection loss at 5 mm of thickness with very narrow -10 dB and -20 dB ( 99% absorption) bandwidth.19 Estevez et al. filled 0.083 vol % CNT-magnetic microwires hybrid (CNT; 3 wt%) into silicon resin, which exhibited -20.9 dB reflection loss at 5 mm of thickness with significant 3.1 GHz of -10 dB bandwidth.20 Peymanfar et al. in their work of fabricated CNTs along with Al doped-strontium hexaferrite nanoparticle that illustrated -24.93 dB of reflection loss at 6.5 mm of thickness using 20 wt.% of loadings.21 Yang et al. engineered FeNi decorated CNT hybrid structure, which exhibited the maximum reflection loss of -15.4 dB with 2.3 GHz of bandwidth using 2 wt% of loading exclusively in Ku band.22 Srivastava et al. explored good absorption properties from Ni



filled MWCNT/polystyrene composite, which exhibited -33 dB of reflection loss with 0.5 wt% filler loadings at 6 mm of thickness that solely devoted to S-band (2-4 GHz).23 Recently, Zheng et al. have reported Co60Fe15Si10B15 glass-coated microwires as X-band lightweight absorber with 0.017 wt.% of loading, which illustrated -25.7 dB of reflection loss but having a very low -10 dB bandwidth of 0.85 GHz and negligible -20 dB bandwidth.24 Lu et al. have reported that the SiO2 matrix containing 10 wt% grape like Fe2O3 coated MWCNT shows reflection loss of approximately -25 dB with 3.7 GHz of -10 dB bandwidth in the X band when the thickness of the composite was 3.2 mm.25 It may be noted from the above discussion that most of the recently reported works in the literature use magnetic/dielectric decorated CNT hybrid structure in order to make use of both dielectric and magnetic losses to further enhance the microwave absorption performances. However, the use of these engineered hybrid fillers increases thickness and weight of the resultant microwave absorber and low effective bandwidths are usually attained. Now, as it is well known, the lightweight is the most important parameter for practical application of modern EM wave absorber because it basically makes the absorber more energy efficient and cost effective.4,

26

Hence, it is highly desirable to develop microwave absorbers with strong

reflection loss at lower filler content so that the strong microwave absorption over a broad bandwidth is achieved without substantially increasing the overall weight and thickness of the resultant structure. Most Recently, Zhang et al. coined an innovative hybrid architecture by direct growing porous CNTs on carbon fiber (CF) surfaces using electrospinning and annealing methods to prevent the CNT bundle type agglomeration, which provided -44.5 dB of reflection loss using 20 wt% of loading in paraffin matrix.27 However, in their investigation, the electrospinning and annealing methods could only produce extremely low density of CNT formation on CF surfaces. As a result, it required 20 wt% of CNT-CF loading to achieve


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significant microwave absorption, which actually increased the overall weight of the resultant structure. In view of this, it is highly desirable to adopt quite efficient methods that can produce high density of CNTs on CF surfaces in order to make the resultant absorber a lightweight structure. It is found in recently reported literatures that among various available methods, the chemical vapor deposition (CVD) appeared to be the most effective method to synthesized densely populated CNTs on carbon fiber (CNTCF).28-29 These CNTCF structures have already been successfully employed for their effective use as electrodes in supercapacitors due to their high electronic conductivity and high surface area than those of individual CNTs and CFs.28 However, to the best of author’s knowledge, the high density CNTCF structures synthesized through the CVD have not been used in the literature to design microwave absorbers. The main aim of this work is to make use of the CVD procedure to synthesize high density CNTCF structures, which are ultimately used to design and develop light weight and highly efficient microwave absorbers in the X-band. The CVD is an advantageous process for the fabrication of highly populated CNTs on CF surfaces since the density and the tune length of CNTs can be easily controlled by changing CVD run time.14 It is particularly to be noted that because of the long tube length, the CNTs are not in a position to maintain their orientation perpendicular to the CF axis and hence few of them start bending resulting into formation of some hollow gap between the individual CNTs. These hollow spaces already have been well proven to enhance the EM absorption properties.30 When the incoming EM waves enter these hollow gaps, they get trapped and facilitate multiple reflections and scattering resulting in attenuation of more EM energy. The formation of trapping centers and long CNT tube lengths in the CNTCF hierarchal structure are achieved by raised growth time. The CNTCF with 0.35 wt.% of loading in epoxy composite demonstrates excellent microwave performance, which exhibits -



42.0 dB (99.99% absorption) reflection loss with -10 dB (90% absorption) and -20 dB (99% absorption) bandwidths of 2.7 and 1.16 GHz, respectively at 2.5 mm of thickness. Specially, this study not only offers a facile approach for the development of extremely lightweight and thin microwave absorber, which is a vital and valuable study in the practical application of CNTCF, but also provides insight into the absorption mechanism of a hierarchical CNTCF based microwave absorber.

Experimental Materials CFs having specific density of ∼1.8 g cm−3 and diameter of ∼8 µm were purchased via M/S Fortafil Industries Inc., United Kingdom (U. K.). NiSO4·6H2O with 99% purity, ammonium chloride with 99% of purity, Na3C6H5O7·2H2O with 98% of purity, 25% pure liquor ammonia were purchased from M/S Qualigens Fine Chemicals, India. In addition, 99% pure sodium hypophosphite was received from M/S Loba Chemie Pvt. Ltd., India. In this study, CNTCFs were fabricated by direct growing CNTs on the CFs substrate using CVD. Firstly, unidirectional CFs with the fiber diameter of ~ 8 µm were heated at 450 °C to remove polymer sizing. The heat treated CF strands were coated with nickel (Ni) by wellknown electroless coating method.28 The different chemicals ingredients utilized during the electroless coating bath and the function of these ingredients are same as reported in our previous work.28-29 In this work, the heat treated CFs were dipped in coating bath for 30 minutes. The temperature of coating bath was set at 85 °C to obtain the high density of Ni particles on CF surfaces. Moreover, pH was set to ~8.5 during the entire process. The synthesis of hierarchical CNTCFs was completed by the growth of CNTs on nickel coated CF through CVD using C2H2 (acetylene) as carbon precursor. Initially, the heat treatment of Ni coated CFs was carried out up


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to 500 °C using a horizontal quartz furnace with 200 ml/min of constant flow of nitrogen (N2) until the process get completed. Further, to avoid the oxide formation of Ni, the hydrogen gas (H2) was hosted (102 ml/min) for 0.25 h in the CVD chamber. Afterwards, the temperature was raised up to 700 °C and C2H2 was brought into the chamber with 200 ml/min of flow rate for 0.25 h; where N2 flow was still continued. Finally, the synthesized CNTCFs were collected after cooling it to room temperature. The density of CNTCF is found to be ~1.79 g/cm-3 (Pycnometer: ASTM D 854-00), which is lower than elsewhere reported MWCNT density (1.89 to 2.1 g/cm3).31-32 Therefore, due to high aspect ratio (confirmed in SEM and TEM analysis) and low density, CNTCF could be the most suited candidate for achieving the high absorption value even at a very low filler ratio. Figure 1 illustrates the schematic of the entire process to synthesize hierarchal CNTCF material. For electromagnetic characterizations, the rectangular shaped samples with dimensions of 22.8 X 10.16 X 2.5 mm3 were prepared by uniformly dispersing 1-2 mm long CNTCFs and CFs into the epoxy matrix. The contents of CNTCFs to epoxy resin were 0.10, 0.25, 0.35, 0.50, 0.75 and 1.00 % by weight. We prepared another epoxy composite with 0.10 wt% of CFs loading to examine the cause of governing phenomena that results in the difference in the absorption properties between composites filled with exact wt% of CNTCFs and CFs. Characterizations. Scanning electron microscopy study (SEM; Zeiss EVO MA-15) was performed on CF and CNTCF samples to examine the surface morphologies. Further, to testify the microstructural features, transmission electron microscopes (TEM, HR-TEM), FEI Titan G2 60-300) was carried out on CF and CNTCF materials. The Keithley programmable 220 was used for the electrical conductivity measurement in terms of four-probe method. The Raman spectra of CNTCF were taken from Raman microscope (Horiba Jobin Yvon LabRAM HR) armed with a



He-Ne laser source (λ = 632.7 nm). The N2 adsorption/desorption specific surface area of CNTCF was performed using Quantachrome Autosorb I-C, U.S.A. The complex permittivity was investigated through Agilent vector network analyzer (VNA) E8364B using the transmission/reflection waveguide method in the frequency range of 8.2-12.4 GHz.

Figure 1. Schematic to synthesize hierarchal CNTCF.

Results and discussions The SEM images of CFs and as synthesized CNTCF are shown in Figure 2a-f. Apparently, uniform and very dense CNT jungle kind of morphology are observed on the CF surface. The average diameter of CNTCFs are nearly three orders larger than un-coated CFs. This means that the grown CNTs are actually very thick, and fabrication efficiency is very high. The average length of grown CNTs was in the range between 5-10 µm. Specially, the high magnification SEM images of CNTCF that shown in Figs. 2(e)-(f) reveal that due to the long tube length many CNTs are self-assembled to form trapping centers. These trapping centers are quite often preferred to design microwave absorbing materials in order to attain the maximum absorption without substantially increasing the weight or thickness of the overall structure. Importantly, the


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entire CF surface covered by CNTs is the hierarchical CNTCF structure. This means that the CF weight share in the hierarchical CNTCF structure is quite small, and the CF has mainly been used to stop the CNTs bundle formation.

Figure 2. SEM images of (a) heat treated CF and (b)-(f) CNTCF with different magnifications.

To further testify the morphology of CNTs grown on the CF surface, the TEM and HRTEM imaging was carried out on the CNTCF samples as shown in Figure 3 (a)-(b). Figure 3a illustrates the low magnified view of CNTs. From Figure 3(b)-(c), it may be noted that the CNT exhibits the tubular and multiwalled structure with the outer diameter of ~35-45 nm and the inner diameter of ~12–25 nm. Further, the high resolution TEM image reveals 0.34 nm of inter-wall spacing in the CNT structure as shown in Figure 3(c), which is assigned to be (002) plane of multiwalled CNTs.



Figure 3. TEM (a) and HRTEM (b-c) images of MWCNTs.

Figure 4. Raman spectra of CNTCF.


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The fingerprint of Raman spectrum of CNTCF is presented in Figure 4. The presence of two characteristic peaks ~1330 and 1600 cm−1 corresponds to disordered D and graphitic G bands of carbon nanotubes, respectively.28 However, the ID/IG ratio of CNTCF is found to be 0.86, which shows high conducting nature of synthesized CNTs. In addition, the Brunauer-Emmett–Teller (BET) analysis of the CNTCF sample is carried out for the better assessment of the specific surface area.33 The N2 absorption/desorption isotherms received at 77.35 K has been shown in Figure 5 suggesting the absorption process to be of typeV in nature, which validates the capillary condensation of gas inside the porous structures of CNTCF. The calculated BET surface area of CNTCF is found to be 510.13 m2/g. This large BET value signifies that the CNTs are not accumulated/agglomerated in the form of bundles, which are confirmed by the SEM analysis (Figure 2a-f). Importantly, the CNTCF surface area is significantly higher than those reported for CNTs, which further validates the feasibility and importance of CNT synthesis on the CF substrate.34-36 The pores of CNTCF are distributed in the mesopore range with pore volume of 2.95 cm3/g for the smallest pore size of 1931 Å (radius) at a relative pressure of P/Po = 0.995. Moreover, the average pore radius determined using the Barrett–Joyner–Halenda (BJH) plots is 11.5 nm as shown in the inset of Figure 5, which confirms that majority of the BET surface area is contributed by the 3D mesopores.



Figure 5. N2 adsorption/desorption isotherms of CNTCF. Inset represents the BJH pore-size distribution plot of CNTCF.


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Figure 6. Schematic to show conductive network and electronics transports in (a) CNTCF, (b) CF, and (c) Resistor–capacitor circuit model containing resistors RMWCNT, RMWCNT–CF, RCF resistors and capacitor CMWCNT in CNTCF and RCF resistors in CF.



The frequency dependent permittivity profiles of CNTCF (0.1 wt.%) and CF (0.1 wt.%) in the epoxy media are shown in Figure 7a-c. From Figure 7a-b, it is obvious that both real part of complex permittivity (εr′) and imaginary part of complex permittivity (εr″) of CNTCF/epoxy composite are higher than that of CF/epoxy composite in the measured frequency range (8.2-12.4 GHz). Notably, εr′ and εr″ values increase from 3.6 to 4.5 and 0.17 to 0.28 at 8.2 GHz, respectively for CNTCF compared to CF system. The enhancement in real permittivity is assigned to the increased dipolar polarization and electrical conductivity in CNTCF/epoxy composite.37-38 In detail, the CNTCF contains numerous capacitor like structures (CMWCNT) that appears between MWCNT and CF interfaces (Figure 6a). With this formation, the charges have a tendency to align in the EM field, which is advantageous for polarization.38 In addition, the overall conductivity of CNTCF/epoxy composite (7x10-12 S/cm) is found to be higher than CF/epoxy composite (4.7x10-11 S/cm), which is due to the high aspect ratio of CNTCF hybrid structure (Figure 2:SEM and Figure 3: TEM). Therefore, at the same filler ratio, it is justifiable that CNTCF provides ample opportunities than CF to establish more conductive channel, which endow high dielectric losses. Due to this higher conductivity value, CNTCF with larger aspect ratio is favorable to create plentiful conductive networks at lower filler concentrations. The conductive network is mainly controlled by migrating electrons and hopping electrons in both systems as proposed in Figure 6a-b.8,39-41 When EM wave encounters the composite, the abundant available electrons in MWCNTs of CNTCF system migrate/hop along the CNT length or across MWCNTs. The high electron density in MWCNTs results high conductivity, which is an important parameter of CNTCF/epoxy composite that provide high electrical conductivity. The high dielectric loss is an extremely important parameter as it generates significant attenuation resulting into high absorption.8 According to the free electron theory, the imaginary


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permittivity may be typically expressed as εr″ ~ σ /2πfε0 where, f is the frequency, ε0 is the permittivity in free space and σ represents the effective electrical conductivity30. Thus, the parameter ε″ is strongly influenced by the effective electrical conductivity, which is decided with the help of electronic transport. In 2009, Cao’s group first established a perfect capacitor-resistor model to discuss the formation of the conductance network in the multiwalled carbon nanotubes/silica nanocomposite providing significant conductive loss.39 In 2010, the same group highlighted the capacitor-resistor model to describe the electronic transport and losses in short carbon fiber/silica composites.40 Later, in 2013, they fabricated CNT/SiO2 composite, and utilized this model to visualize the role of temperature on the electrical transport.41 Most recently, in 2018, Cao and his group fabricated graphene based nanohybrids, in which they applied the capacitor-resistor model to understand the formation of conductive network and associated electrical losses.42-43 In this work, a schematic presenting the direct current resistor circuit model highlighting the losses in CNTCF and CF systems based on the work by Cao’s research group is drawn as shown in Figure 6c. Herein, RMWCNT, RCF and RMWCNT+CNT are the leading resistors in the CNTCF system, whereas, the RCF is the only involved prime resistor in the CF system. Therefore, it is obvious that the CNTCF system contains more effective losses than due to CF alone. Later, the impact of interfaces on the dielectric losses in Fe3O4-MWCNTs, PANI-Fe3O4-MWCNTs and MWCNTs-ZnO hybrids was studied by Cao et. al. using the capacitor model.18,44 This model basically assumes the capacitor-like structure formation between Fe2O3, PANI, ZnO nanocrystals and CNTs resulting into the generation of interface polarization leading to high dielectric losses due to dissimilarity in their conductivities. Based on the above scheme, a capacitor-like model for the CNTCF system is adopted here as portrayed in Figure 6a. The difference between conductivities of CF and highly conductive CNTs in the



present situation induces strong interface polarization that further enhance the dielectric losses. Thus, the contributions from both the effective conductive losses and the interfacial polarizations result into an increase in the imaginary permittivity of the CNTCF/epoxy composite. In Figure 7c, CNTCF/epoxy system exhibits higher loss tangent value than CF/epoxy composite over the whole frequency range of 8.2-12.4 GHz. This further confirming that MWCNTs in CNTCF hybrid are the major contributor of dielectric losses. Owing to the high aspect ratio of CNTCFs as revealed by SEM and TEM, it is important to study the electromagnetic properties of CNTCF/epoxy composites. Figure 8a-b presents the real and imaginary permittivity of the CNTCF/epoxy composites with rising CNTCF contents in the X band. With elevated CNTCF loadings, the real and imaginary permittivity increases significantly as shown in Figure 8a-b, which can be explained by the effective medium theory.18 The εr′ of CNTCF/epoxy composite with 1.0 wt.% loading is ~ 6 times higher than composite filled with 0.1 wt.% of CNTCFs. Whereas, εr″ of 1.0 wt.% filled CNTCF/epoxy composite is uplifted enormously as compare to 0.1 wt.% of CNCTCFs loadings. On increasing filler concentrations, CNTCF provides ample opportunities in the composites to create more heterogeneous junction capacitance, and endow increased overall composite conductivity (Figure 7a), which results an increment in the real permittivity. The creation of direct current resistor network by CNTCF structure and the interface polarization (MWS relaxation) at interfaces between CF-MWCNT and among MWCNTs turned out to be the most important parameters for imaginary permittivity.8 Therefore, the rising CNTCF contents facilitate plentiful direct current resistors networks, which endows high conductive losses in the presence of EM wave that substantially enhances the imaginary permittivity of CNTCF/epoxy composites.36 On the other hand, the increasing filler loading causes an increment in the interface polarizations that leads to


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enhance the imaginary permittivity further. In Figure 8c, the tangent loss also follows the similar behavior i.e., a rising trend with increasing CNTCFs in the composite. The composites with CNTCF loadings of 0.10, 0.25, 0.35, 0.50, 0.75 and 1.0 show loss tangent values between 0.040.28, 0.18-0.25, 0.38-0.45, 0.61-0.62, 0.54-0.68 and 0.67-1.36, respectively in the frequency 8.212.4 GHz. To explore the cause of the complete loss mechanisms of the CNTCF hierarchal system, the DC electrical conductivity (σDC) is also measured. From Figure 8d, it should be noted that the DC conductivity increases with the increase of CNTCF concentrations in the epoxy matrix at room temperature. It is observed that the composite with 0 wt.% of CNTCF loading exhibits conductivity of the order of 10-13 S/cm. However, the conductivity value gets uplifted by an order of 7 magnitudes at 0.25 wt.% of loading, which shows the formation of percolating network or well-dispersed conductive channels required for the current to flow.45 The percolation threshold can be projected after performing data fitting on the following expression 41;

σDC (T) ∝ (W-WC)α

(1)

where, σDC represents the DC electrical conductivity of the composite, W is the filler concentration, Wc presents the exact percolate value and α is the critical electrical conductivity exponent32, which indicates the system dimensionality.32,41 The fitted conductivity plots of the composites are presented in the inset of Figure 8d. The observed value of percolation threshold is ~0.15 wt.%, which confirms the light weight feature of the hierarchal CNTCF. It should be especially noted that the fitted DC conductivity of the composites increases with the elevated concentration of CNTCF, which matches with the trend of increase in the imaginary permittivity with higher values of CNTCF in the matrix.39 The high aspect ratio of CNTCF easily creates



plentiful conductive networks in the epoxy matrix with significant filler ratio (Figure 2 and Figure 3). In detail, the contact conductivity between MWCNT interfaces is the leading contributor to the DC electrical conductivity that appears due to hopping electron in/on CNTCF system.37-39 Therefore, it is reasonable that with increasing filler contents, the MWCNTs contact networks improve that eventually enhances the overall contact conductivity of the samples. This results into an increase in the overall DC electrical conductivity of these samples.

Figure 7. Frequency-dependent (a) real, (b) imaginary permittivity and (c) loss tangent of CNTCF and CF mixed epoxy composite with 0.1 wt% of loading.


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Figure 8. Frequency dependent (a) real permittivity, (b) imaginary permittivity and (c) loss tangent of CNTCF/epoxy composites with increasing filler ratio, and (d) electrical conductivity of CNTCF/epoxy composites as a function of filler concentration (Inset presents fitted conductivity plot).

The microwave absorptions of single layer absorber are typically expressed as27, 30;

RL = 20 log10

( Zin − Z 0 )

(Z

1/ 2

µ  Z in = Z 0  r   εr 

in

(2)

+ Z0 )

 j 2π fd µ ε r r tanh  c 

  


(3)


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where, Zin and Z0 are the input impedance and the free space impedance, respectively, f is the frequency in GHz, d is the thickness and c is the velocity of light in free space. It is reported that impedance matching condition, Zin = Z0 is to be fulfilled in order to achieve maximum absorption of the interacting EM waves.46 To further compare the absorption performances, the reflection loss behaviour of CNTCF (0.1 wt.%)/epoxy and CF (0.1 wt.%)/epoxy composites at 2.5 mm of thickness are illustrated in Figure 9a. It can be seen that the optimum RL reaches -3.40 dB (54.3% absorption) and -0.97 (20% absorption) dB at 12.4 GHz for CNTCF and CF samples, respectively, which confirms the stronger microwave absorbing capability of CNTCF. This appearance can be understood from the following reasons. Firstly, due to higher permittivity, the CNTCF shows better impedance matching condition than that of CF. Secondly, the most crucial mechanism for microwave absorbing material is the dielectric relaxation. It is well reported that polarization and related relaxation processes enhance the microwave absorption performances.47 These relaxations are typically presented by the Cole-Cole semicircle. Based on Debye relaxation theory, the permittivity can be expressed as47-48; 2

(ε r ' − ε ∞ ) + (ε r '' ) 2 = (ε s − ε ∞ ) 2

(4)

where, εs represents the static dielectric constant and ε͚ is the dielectric constant at infinite (quite high) frequency. Hence, the plot of εr′′ vs εr′ would exhibit a single semicircle i.e., one Debye relaxation process.48 Figure 9b-c exhibit Cole-Cole plots of CF (0.1 wt%) and CNTCF (0.1 wt%) filled epoxy composites, respectively. It can be seen that six obvious overlapped semicircles are observed for CNTCF, which suggests diverse relaxation mechanisms under oscillating EM field such as interface polarization at heterogeneous junction between MWCNTs and CF, and interfacial polarizations among MWCNTs surfaces.25,44 In addition, the conductive loss produced by the highly conductive CNTCF networks also contributes greatly to enhance the electric dipole


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relaxation.48 However, the absence of heterogeneous junction interfaces in CF system make interface polarization weaker. Hence, less number of Cole-Cole semicircles in the CF/epoxy composite is obvious. The few distorted Cole-Cole semicircles in both systems are attributed to the Maxwell-Wigner relaxation effect.49 Thirdly, the formation of trapping centers by CNTs on CF is another important parameter for the improvement of microwave absorption. When the EM waves strike the CNTCF absorber surface, the waves get reflected and scattered multiple times before and after penetrating into these trapping centers.30 These multiple reflections induce relaxations resulting in additional EM wave attenuation giving rise to stronger microwave absorption. Thus, due to the presence of trapping center sponsored relaxations, electric dipole relaxation and interfacial relaxations, the CNTCF/epoxy composite exhibits much stronger EM wave absorption capability as compare to the CF/epoxy composite.



Figure 9. Frequency dependent reflection loss of (a) CNTCF (0.1 wt%)/epoxy and CF (0.1 wt%)/epoxy, Cole-Cole plots of (b) CNTCF (0.1 wt%)/epoxy and (c) CF (0.1 wt%)/epoxy composite. As discussed above, the microwave absorption capability of the synthesized nanocomposites would primarily depend upon its effective complex permittivity. The designed composite should usually possess high value of the dielectric loss in the specified frequency band in order to provide quite high absorption to the incident EM waves. The higher value of the dielectric loss leading to higher microwave absorption of the resultant composite in the present situation is usually provided by the CNTCF filler. However, for an effective microwave absorber another important parameter apart from the microwave absorption is the impedance matching of the resultant composite with the background medium. The impedance matching basically


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facilitates the transmission of the incident EM wave into the resultant nanocomposite so that the EM waves can be absorbed inside the material due to high dielectric loss. If the impedance matching of the resultant composite is not good with the background medium then it would lead to high reflection at the interface between the two media leading to poor microwave absorption of the EM wave inside the designed absorber. In other words, a synthesized composite possessing even high value of dielectric loss may not show good microwave absorption capability unless it also possesses good impedance matching properties.50 The impedance matching property of a composite primarily depends upon the real permittivity, which would be mainly decided by the epoxy present in the resultant composite. As the epoxy present in the composite system usually possesses very low dielectric loss, hence its main role is to tune the impedance of the resultant composite so that it is well matched with the background medium. The good matching helps in the maximum transmission of the EM wave inside the designed microwave absorber providing minimum reflection, which indirectly assists the microwave absorption process. In summary, it may be postulated that the epoxy resins possessing low dielectric loss may provide the function of EM wave transparency as most of the microwaves would be absorbed inside the composite without much reflection from its surface.51 Therefore, the microwave absorption performance of CNTCF/epoxy composite can be tuned by adjusting the weight (wt.) ratio of CNTCF to epoxy resin. The above fact is taken into consideration while designing the microwave absorbers using the CNTCF/epoxy composite in the present situation. Firstly, the microwave absorption performance of CNTCF/epoxy composites filled with rising CNTCF concentrations at 2.5 mm of thickness is investigated as shown in Figure 10. The RL intensity here first increases up to highest value, then decreases with further addition of CNTCF in the composites within the X band frequency domain. The RLs reach -3.40 (54.29%



absorption), -8.5 (85.87% absorption), -42.0 (99.99% absorption), -24.5 (99.65% absorption), 5.6 (72.46% absorption) and -3.8 dB (58.31% absorption) at 12.4, 12.4, 11.4, 9.8, 8.2, and 8.2 GHz of frequency with 0.10, 0.25, 0.35, 0.50, 0.75 and 1.0 wt% of CNTCF loadings, respectively. In present situation, it is obvious that 0.35 wt.% of CNTCF delivers the highest absorption (99.99% absorption) with 2.7 and 1.16 GHz of -10 and -20 dB broad effective bandwidths, respectively. The fabrication of CNTs directly on CF in the present situation quite significantly influences the dielectric losses of the resultant composite, which basically facilitates the microwave absorption with high efficiency and wide absorption bandwidths even at a very low filler ratio. In detail, as shown in Figure 6a, the capacitor-like structures formed at the abundant interfaces between CNTs and CF appear to be the major contributors to the microwave attenuation. This attenuation can be well tuned in the CNTCF due to the multiple interface polarization mechanisms that appeared between CNTs and CF, and among CNTs.44 Moreover, with the help of the electron hopping model and the conductive network, it has been demonstrated that the electrons in highly conductive CNTCF enhance the microwave absorption enormously due to the hoping between contact sites of CNTs (Figure 6a-c).38 Furthermore, the unique trapping center formation (SEM image: Figure 2) can generate more multiple reflection and scattering of EM wave, which greatly amplifies the overall absorption performance. Quite high permittivity values are sometimes not favorable for the absorber composites as it causes impedance mismatch with the surrounding resulting in poor absorption and the high EM wave reflection.46,48 Therefore, the absorption efficiency obviously decreases after 0.35 wt.% of loadings as presented in Figure 10. Hence, it may be observed here that using a very low CNTCF contents, a high RL value along with wide absorption bandwidth is achieved in the present situation, which shows the potential of CNTCF to synthesize commercial microwave absorbers.


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In this work, our main emphasis is to investigate the microwave absorption properties of CNTCF based nanocomposites exclusively in the X-band. However, in order to show the advantage of the proposed CNTCF based nanocomposite, its performance is compared with other recently proposed X-band microwave absorbers as shown in Table 1. It can easily be observed from this table that the proposed CNTCF (0.35%) based epoxy nanocomposite possesses excellent microwave absorption properties in the X-band over a broader effective bandwidth at a very small thickness of 2.5 mm. Hence, the proposed CNTCF based nanocomposite appears to be a promising candidate for next-generation high-performance lightweight and thin thickness microwave absorbers.

Figure 10. Frequency dependent reflection loss of CNTCF/epoxy composites at different filler concentrations at 2.5 mm of thickness.

Table 1. Comparison of microwave absorption properties of the CNTCF/epoxy absorber with other investigations.



Total filler (wt.%)

Organic filler share (wt.%)

20

--

70

--

40

70

60

--

1.3

--

0.5

--

1.0

1.00

LaFeO3/C/paraffin

40

13.17

Nano onion-like carbons/paraffin

40

doped Bahexaferrite/epoxy

Page 26 of 41

BW (GHz) (RL≤ -10 dB)

BW (GHz) (RL≤ -20 dB)

t (mm)

4.50

~1.10

3.0

53

2.50

~0.50

2.0

54

2.40

--

2.5

2

--

1.24

5.25

37

5.60

~ 2.50

2.0

55

~ 1.10

~ 0.20

6.0

23

~ 2.50

~ 1.9

2.5

56

-26.6 (X Band)

4.4

~ 0.8

2.94

57

40

- 42.04 (X Band)

0.54

~ 0.5

2.5

58

50

--

-29.0 (X Band)

2.6

~ 1.3

5.0

59

ZnOnws/RGO foam/PDMS

3.3

--

−27.8 (X Band)

4.2

~ 1.25

4.8

60

Cu0.25Ni0.25Zn0.5F e2O4/MWCNTs/epo xy

--

0.16

-37.7 (X Band)

~ 3.6

~ 1.0

2.5

9

Cobalt doped M-type strontium hexaferrite/LLDPE

60

--

-28.5 (X Band)

4.2

~ 0.9

3.0

61

Co60Fe15Si10B15 glasscoated microwires

0.017

--

- 25.7 (X Band)

0.81

~ 0.2

1.5

24

CNTCF/epoxy

0.35

0.35

-42.0 (X Band)

2.70

1.16

2.5

Samples Fe3O4/polypyrrole/ CNT/epoxy AACNT/BaFe12O19/ paraffin PA@MWNTs/paraffi n MWCNTs/Ni/epoxy FeCoNi-filled CNTs/epoxy Ni@MWCNT/ polystyrene RGO/silicon

RL (dB) -25.9 (X Band) -21.5 (X Band) -20.7 (X Band) -37.0 (S-Band) -28.2 (K-Band) -33.0 (S-Band) -37.8 (X Band)

Reference

Present work

BW= Bandwidth, t= thickness

To appraise the absorption efficiency of a microwave absorber, impedance matching ratio, Zr is considered as the most crucial parameter.54 Practically, the microwave absorber should exhibits high impedance matching ratio. To be specific, the absorber should enables the EM waves to


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completely penetrate into its interior and mostly be consumed without reflecting away from the absorber surface. The impedance matching ratio, Zr is typically expressed as62-63;

Zr =

zin = z0

µr εr

(5)

where, µr denotes complex permeability. Herein, µr taken as 1 because CNTCF is extremely nonmagnetic material. Therefore, if Zr = 1, the incident EM waves would be able to penetrate completely inside the absorber with zero reflection from its surface. It can be seen in Figure 11 that the impedance matching ratio of CNTCF/epoxy composites steadily changes from mismatch to nearly perfect matched condition, and Zr of CNTCF (0.35 wt.%)/epoxy composite is the nearest to 1. This further proves that CNTCF (0.35 wt.%)/epoxy composite facilitates the most favourable chances for incoming EM waves to be completely accommodate inside the sample, which subsequently results into maximum reflection losses to the EM waves.



Page 28 of 41

Figure 11. Impedance matching ratio of CNTCF/epoxy composites with different CNTCF loadings.

Further, the EM wave attenuation ability inside the absorber is an important parameter, which is characterized by the attenuation constant α and typically expressed as48, 62; α=

2πf c

( µ r " ε r " - µ r 'ε r ') +

( µ r " ε r " - µ r 'ε r ') 2 + ( µ r 'ε r " + µ r " ε r ') 2

(6 )

where, f represents the frequency and c denotes the velocity of light. It is clear that higher εr would lead higher attenuation. It is not hard to notice from Figure 12 that the α value in the present situation increases with the increasing CNTCF concentrations over the whole X band. The CNTCF-1.0 wt.% exhibits the maximum α among all samples. Specially, CNTCF-0.35 wt.% shows significantly high α value, which is higher than CNTCF-0.25 wt.% and CNTCF-0.10 wt.%. When we imitate on absorption performances and impedance matching ratio, the RL and Zr values of CNTCF-0.35 wt.% demonstration distinct advantage compared with other CNTCF filled composites. Thus we have forwarded a concept that, in a specific domain, the impedance matching ratio nearer to 1 is quite advantageous than attenuation constant.


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Figure 12. Attenuation constant of CNTCF/epoxy composites with different CNTCF loadings.

Along with complex permittivity, the thickness of the absorber composite also play a vital role to influence the absorption properties. The calculated relation between RL and thickness of optimized CNTCF (0.35 wt.%)/epoxy composite is shown in Figure 13a-b. It is obvious that with increasing composite thickness from 1.4 - 4.0 mm, the maximum RL is shifted towards the lower frequency region. This can be understood by the ¼ wavelength expression62, 64

; tm =

nc 4f m

ε µr

( n= 1, 3, 5..... )

(7)

r

where, tm represents matching thickness, fm denote matching frequency of the maximum RL intensity. εr and µr are complex permittivity and permeability at matching frequency, and c indicate the speed of light. It can clear that absorber composite easily has attained significant −10 dB bandwidths at thicknesses 2.2 – 4.0 mm in the measured frequency range (8.2-12.4 GHz).



Hence, the CNTCF (0.35 wt.%)/epoxy composite can be an attractive lightweight contender for overpowering the EMI and lessening the radar cross section.

Figure 13. (a) 3D plot of frequency and thickness dependent RL, and (b) RL vs thickness plot of CNTCF (0.35 wt.%)/epoxy composite. Conclusions The hierarchical CNTCF has been synthesized by CVD method, and the resulted structure was used as a filler material to design lightweight, low thickness and high-performance microwave absorber in X band applications. The formation of trapping centers by self-assembly of CNTs on CF turned out to be the crucial influential factor for the EM wave attenuation. The epoxy-based nanocomposites with different amounts of CNTCFs exhibit quite strong absorption capabilities. Reflection loss of -42.0 dB (99.99% absorption) with broad effective -10 dB and -20 dB bandwidth of 2.7 GHz and 1.16 GHz, respectively was achieved for 0.35 wt% of CNTCFs at a very small thickness of 2.5 mm. For 0.50 wt% CNTCF loading, the RL reached -24.5 dB (99.65% absorption) and the -10 dB bandwidth was 3.54 GHz covering 85 % of the entire X


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band frequency spectrum. Thus, the hierarchical CNTCF is a potential candidate for X band light-weight microwave absorption applications.

AUTHOR INFORMATION *Corresponding Author Email: [email protected], Phone: +91-512-2597687, Fax: +91-512-2597408 [email protected] Author Contributions All authors have given approval to the final version of the manuscript. Funding Sources The authors declare no competing financial interest. References 1. Gogoi, J. P.; Bhattacharyya, N. S., Expanded Graphite—Phenolic Resin Composites based Double Layer Microwave Absorber for X-Band Applications. Appl. Phys. Lett. 2014, 116 (20), 204101. 2. Pan, W.; He, M.; Bu, X.; Zhou, Y.; Ding, B.; Huang, T.; Huang, S.; Li, S., Microwave Absorption and Infrared Emissivity of Helical Polyacetylene@Multiwalled Carbon Nanotubes Composites. J. Mater. Sci. Mater. Electron. 2017, 28(12), 1-10. 3. Wen, F.; Zhang, F.; Liu, Z., Investigation on Microwave Absorption Properties for Multiwalled Carbon Nanotubes/Fe/Co/Ni Nanopowders as Lightweight Absorbers. J. Phys. Chem. C 2011, 115 (29), 14025-14030.



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54. Zhao, T.; Ji, X.; Jin, W.; Guo, S.; Zhao, H.; Yang, W.; Wang, X.; Xiong, C.; Dang, A.; Li, H.,

Electromagnetic

Wave

Absorbing

Properties

of

Aligned

Amorphous

Carbon

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BRIEFS Present work derived the strong microwave absorbing capability of hierarchical carbon nanotube coated on carbon fiber, which is prepared through catalytic chemical vapor deposition method.

‘‘TOC’’


Hierarchical Carbon Nanotube Coated Carbon Fiber: Ultra

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