Lightweight, high electrical and thermal conducting carbon-rGO composites foam for superior electromagnetic interference shielding

Lightweight, high electrical and thermal conducting carbon-rGO composites foam for superior electromagnetic interference shielding

Composites Part B 160 (2019) 131–139 Contents lists available at ScienceDirect Composites Part B journal homepage: www.elsevier.com/locate/composite...

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Composites Part B 160 (2019) 131–139

Contents lists available at ScienceDirect

Composites Part B journal homepage: www.elsevier.com/locate/compositesb

Lightweight, high electrical and thermal conducting carbon-rGO composites foam for superior electromagnetic interference shielding

T

Pinki Rani Agrawala,c,1, Rajeev Kumarb,c,∗,1, Satish Teotiaa, Saroj Kumaria, D.P. Mondalb, Sanjay R. Dhakatea,c a

Advanced Carbon Products Group, Advanced Materials and Devices Division, CSIR-National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi, 110012, India CSIR-Advanced Materials and Processes Research Institute, Bhopal, 462026, India c Academy of Scientific and Innovative Research (AcSIR), India b

A R T I C LE I N FO

A B S T R A C T

Keywords: Carbon foam Decoration rGO Electrical and thermal conductivity Compressive strength and EMI shielding

Lightweight and high strength carbon-rGO composite foams were inventively fabricated by simple sacrificial template technique using reduced graphene oxide (rGO) and phenolic resin as a carbon source. The carbon-rGO composite foams were fabricated by two different routes. In one case, rGO was incorporated in phenolic resin and carbon foam developed by several heat treatments. In the other case, graphene oxide (GO) was decorated over carbon foam and converted into rGO decorated carbon foam by heat treatment. The EMI shielding of carbon-rGO composite foams was measured in the X-band frequency range (8.2–12.4 GHz) and mechanisms were systematically studied with respect to the rGO and porous structure. The EMI SE of carbon foams was increased from −23.2 to −50.7 dB by the decoration of 1.0 wt % rGO. The thermal conductivity achieves as high as 1.4 W/(m K) by incorporation of 4.0 wt % rGO in carbon foam. All the results indicated that this effort provided a novel, simple, low-cost concept for fabricating lightweight, high electrical and thermal conducting carbon-rGO composite foam for high-performance EMI shielding applications.

1. Introduction In modern society, the demand for newer wireless electronic and communication devices are (mobile, Laptop, radio etc.) significantly increases in the day to day life for convenience. These electronic devices create interference in other devices such as radio waves, emitted from the cellular phone at high frequency and tend to interfere with laptop or computers and also pollution such as noise pollution, air pollution and water pollution in the environment [1]. The electromagnetic waves also adversely affect the human health. For prevention of these waves, high-efficiency EMI shielding materials are looked-for civil, military, commercial and aerospace applications [2]. Metals, such as Cu, Ni, Al, Mg and Zn are the most extensively used as shielding materials [3] but these metals suffer from numerous problems such as high density, corrosion, uneconomical processing and lower specific shielding capability. Among other shielding alternatives, carbon materials are being used in EMI shielding because of their low density, high electrical and thermal conductivity, good corrosion resistance, thermal and environmental stability and processing advantages [4]. The mechanism of EMI shielding depends on absorption, reflection,

transmission and multi-reflection. The mechanism of shielding effectiveness is dominated by properties of the material. But to get a shielding material with both reflection and absorption dominating phenomena is still a challenge to the research community. Currently, the research community is focusing on the development of an absorption dominating materials rather than reflection dominating EMI shielding materials [5]. These includes, carbon materials such as mesocarbon microbeads [6], mesoporous carbon [7], carbon nanotubes [8,9], carbon black [10], carbon fibres [11], graphite [12], graphene [13], reduced graphene oxide [14,15] etc., which are lightweight as compared to metals, having good electrical/thermal conductivity and possess high-efficiency EMI shielding property because of thermally activated carrier hopping with defect states [16]. Alternatively, porous materials are very most important constituents especially for energy and environmental related applications. These materials have gone through rapid development and are used in many fields such as radar applications [17], EMI shielding [18], Li-ion batteries anode [19], supercapacitors [20], fuel cell [21] and support for many important catalytic processes [22]. Recently, carbon foam has gained a popularity due to its



Corresponding author.CSIR-Advanced Materials and Processes Research Institute, Bhopal, 462026, India. E-mail address: [email protected] (R. Kumar). 1 Both authors contributed equally. https://doi.org/10.1016/j.compositesb.2018.10.033 Received 20 June 2018; Received in revised form 24 September 2018; Accepted 13 October 2018 Available online 14 October 2018 1359-8368/ © 2018 Elsevier Ltd. All rights reserved.

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controlled to the decoration of different content of rGO on carbon foam. The schematic diagram for the fabrication of rGO decorated carbon foam is shown in Fig. 1, Schematic (b). The rGO decorated carbon foams were designated as; 0.5 wt % rGO decorated carbon foam (CFDrGO0.5) and 1 wt % rGO decorated carbon composite foam (CFDrGO1).

outstanding properties such as very lightweight, low density, high electrical and thermal conductivity and exceptional EMI shielding capability [23,24]. According to the literature, carbon foam has been developed by various researchers for multifunctional applications. Kumar et al. [25,26] developed various type pitch-based carbon foam specifically for EMI shielding. They investigate that EMI shielding properties of pitch-based carbon foam incorporated with ferrocene and MWCNTs is −81 dB and −85 dB, in which shielding effectiveness is mostly dominated by the reflection losses by −60 dB and −65 dB respectively at the frequency range of 8.2–12.4 GHz. To synthesize such materials which can be dominated by absorption phenomena during the EMI mechanism. It has been found that reduced graphene oxide (rGO) has desirable physical, chemical properties, tremendously high surface area, carrier mobility, abundant defects and nonmagnetic properties, therefore it has microwave absorption properties mostly owe to the dielectric loss [27]. Many researchers have reported different types of graphene and polymer-based composite foam as EM wave shielding materials. However, the use of these materials on a commercial scale is limited due to difficulty in their significant production. On the other hand, the outstanding properties of graphene can be suitable by using it as conducting filler in carbon foams prepared from other low-cost foam materials. The incorporation of graphene as reinforcement in phenolic resin based carbon foam has not been developed. Therefore, in the present investigation, a novel approach was adapted for the development of two type's carbon-rGO composite foams by polyurethane (PU) template method from a mixture of phenol formaldehyde resin and reduced graphene oxide (rGO). In the first experiment, rGO was incorporated in phenolic resin and converted into carbon foam by carbonization process. In the second experiment, phenolic resin based carbon foam was decorated by rGO. The resultant incorporated and decorated carbon-rGO composite foams were characterized by various techniques.

2.2. Characterization The microstructure of rGO was observed by HRTEM (Techno G30stwin, 300 kV instrument). The morphology of rGO and carbon foams was studied by scanning electron microscope (SEM, Leo model S-440). The crystal structure and phase of graphene and carbon foam samples were investigated by powder X-ray diffractometer (XRD, Bruker D8 Advance). Raman spectroscopy was used to find out the defects and phase identification of graphene and carbon foam using Renishaw Raman spectrometer, UK, with a laser as an excitation source at 514.5 nm. The surface area of carbon foams and carbon-rGO composite foams was measured by Autosorb 3B, Quantacrome Instruments by gas sorption technique. The compressive strength of carbon foam samples was measured using INSTRON machine model 4411. The electrical conductivity of the carbon foam samples was analyzed by a d.c. fourprobe contact method using a Keithley 224 programmable current source. The voltage drop was measured by a Keithley 197 an autoranging digital microvoltmeter. The carbon foam samples with a size of 26.8 × 13.5 × 2.0 mm3 were measured according to their EMI shielding in the frequency range of 8.2–12.4 GHz (X band) using waveguide method (VNA E8263B Agilent Technologies). The thermal conductivity of carbon foam samples was measured by flash diffusivity technique using thermal diffusivity system (model XFA-600 Linseis). Thermal stability of carbon foam was carried out by the thermal gravimetric analyzer (on Mettler Toledo TGA/SDTA 851E thermal analysis system in air) at a heating rate of 10 °C/min.

2. Experimental 3. Result and discussion 2.1. Preparation of carbon-rGO composite foams 3.1. Morphological and structural analysis Graphene oxide (GO) was prepared by modified Hummer's method [28] through the oxidation of natural graphite (NG) powder. The prepared GO powder was reduced by using heat treatment at 1000 °C in an inert atmosphere and converted to reduce graphene oxide (rGO). For the development of Carbon-rGO composite foams, polyurethane (PU) foam (density 0.030 g/cc, average pore size 0.45 mm) was used as a template [29]. In the first experiment, rGO in different weight fraction (0.5, 1, 2, 4 wt %) was dispersed in an organic solvent such as ethanol and mixed with the phenol-formaldehyde resin. After making a homogeneous solution of rGO/phenolic resin, PU foam slabs were dipped into rGO/phenolic resin solution so that solution can be filled inside the pore of PU foam. The rGO/phenolic resin impregnated PU foams were dried in an oven at 60–80 °C for 5 h for removing the solvent. The dried impregnated foams were cured at 300 °C in presence of air atmosphere for 2 h for increasing the cross-linking between the polymeric chains. Finally, the impregnated PU foams were converted into rGO incorporated carbon foam by the heat treatments at 1000 °C in an inert atmosphere (Fig. 1, Schematic (a)). The carbon foam was named as CFrGO0 for carbon foam whereas CFrGO0.5, CFrGO1, CFrGO2 and CFrGO4 named for 0.5, 1, 2 and 4 wt % of rGO incorporated carbon composites foam respectively. In the second experiment, rGO directly decorated over 1000 °C heattreated carbon foam by dip coating. Initially, synthesized GO were dispersed in ethanol solution for 2 h and then carbon foams were dipped in this solution and left for 2 h. After that, these foams were dried in an oven at 60–80 °C for the removal of ethanol. Subsequently, foams were kept in a tubular furnace at 950 °C in an inert atmosphere. During the heat treatment, GO decorated carbon foams were converted into rGO decorated carbon foam. The reaction parameters were

Fig. 2 (a) shows the scanning electron microscope (SEM) image of rGO. From the SEM image, it is observed that rGO has carpet or sheetlike layered structure and some wrinkles are present on the surface of rGO. This demonstrates that synthesized rGO having few-layer graphene sheets. Fig. 2 (b) represents the transmission electron microscope (TEM) image of rGO. Here TEM image reveals that the rGO has few layers which are intertwined with each other. Some parts of rGO are folded during processing. The powder X-ray diffraction (XRD) pattern of prepared GO and rGO was compared with natural graphite (NG) and depicted in Fig. 2 (c). In NG, the (002) plane is observed at 2θ = 26.48° with inter planar spacing (d002) of ∼0.3363 nm. When it is converted into graphene oxide by modified Hummer's method, the (002) peak shifted to lower diffraction angle 2θ = 11.26° [30] represents an increase in inter planar spacing (0.7854 nm) due to interaction and oxidation. Consequently, the formation of hydroxyl, epoxy and carboxyl groups on the edges graphene sheets. Furthermore, when GO was reduced by heat treatment at 1000 °C in an inert atmosphere, a broad peak of rGO showed a dramatic shift to higher diffraction angle 2θ = 24.44° corresponds to d-spacing value 0.3642 nm because of increasing disorder of the graphene layers [31]. Raman spectra of NG, GO and rGO is shown in Fig. 2 (d). The spectrum of NG shows that Raman active E2g mode at 1580 cm−1 (G-band) characterize sp2-hybridized carbon-carbon bonds and a very small D-band arises at 1355 cm−1 indicates some defects in NG. In Raman spectra of GO, one can clearly find a broader D-band and G-band, indicating a higher disorder of GO than that of graphite. Meanwhile, when GO was heat-treated at 1000 °C in an inert atmosphere for 3–4 min, the intensity of G-band is higher than D-band in rGO. It is important to note that intensity ratio of D and 132

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Fig. 1. Schematic diagram for the fabrication of carbon-rGO composite foam: (a) rGO incorporated and (b) rGO decorated carbon foam.

Fig. 2. (a) SEM image of rGO, (b) TEM image of rGO,(c) XRD Pattern and (d) Raman spectra of NG, GO and rGO. 133

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Fig. 3. (a) XRD pattern and (b) Raman spectra of carbon-rGO composite foams.

foams slightly increases from 0.30 to 0.36 gcm−3 when the rGO concentration increases from 0.5 to 4.0 wt % of phenolic resin. It is observed that the density of foam is the direct function of rGO concentration due to densification of carbon foam by rGO. The compressive strength of carbon composite foams depends on their bulk density, cell size and type of reinforcement. The compressive strength of rGO incorporated carbon foam is increased from 4.6 to 6.5 MPa when concentration of rGO is varied from 0 to 2.0 wt %. The maximum compressive strength is due to the uniform dispersion of rGO in phenolic resin up to rGO concentration of 2.0 wt %. The uniform dispersion of rGO creates maximum graphene-carbon matrix interface that facilitates effective matrix to reinforcement stress transfer. The reduction in compressive strength of samples having rGO concentrations above 2.0 wt % is due to increase in cell size and rGO agglomeration. In case of CFDrGO1, the uniform distribution of rGO can be seen (Fig. 4 (f)). Additionally, decorated graphene layers are clearly visible (Fig. 4 (f)) in CFDrGO1. The maximum improvement in the compressive strength of CFDrGO1 is achieved 7.2 MPa. This increment in compressive strength with the decoration of rGO is due to the reinforcement of graphene produced by in-situ reduction of GO.

G-bands [i.e. ID/IG ratio] is generally used to measure the imperfection in carbon material as it corresponds to relative population of sp3-hybridized of carbon atoms [32]. The ID/IG ratio of NG and rGO is found to be 0.1312 and 0.6921, respectively. The increasing ID/IG ratio indicate that some structural defects and disorder has remained in rGO. XRD pattern of carbon foam with various rGO concentrations is shown in Fig. 3 (a). In XRD pattern of the carbon foam (CFrGO0), there are two typical peaks at 2θ = 26.2° and 43.4° corresponds to diffraction plane (002) and (100), respectively. The incorporation of rGO influences the structure of carbon foam and as a consequence of diffraction peaks and its intensity. The interlayer spacing of CFrGO0 is 0.3734 nm while in case of rGO incorporated carbon foam, the interlayer spacing was observed at 0.3710 and 0.3695 nm for of 1.0 and 4.0 wt % rGO loading, respectively. In case of rGO decorated carbon foam, peaks are observed at relatively higher diffraction angle as compared to rGO incorporated foam because of a uniform coating of rGO over the surface of carbon foam. The interlayer spacing of 1.0 wt % rGO decorated carbon is lower i.e. 0.3433 nm as compared 1.0 wt % rGO incorporated carbon foam. Fig. 3 (b) represents the characteristic Raman spectra of carbon foam with different loading of rGO and it consists of majorly three bands: a disorder induced D-band (∼1350 cm−1), G-band (∼1580 cm−1) and 2D-band (∼2700 cm−1). The 2D-band is the second most prominent band, which is always observed in graphite material. From Fig. 3(b), it can be seen that increasing the rGO concentration in rGO incorporated carbon foam leads to decrease in ID/IG ratio. The ID/IG ratio of CFrGO0 is 0.9869 and it decreases to 0.8463 by increasing the rGO loading to 4.0 wt % (CFrGO4). In case of rGO decorated carbon foam (CFDrGO1), the ID/IG ratio is 0.9073. The distribution of rGO in carbon foam was investigated by SEM micrograph, and results are shown in Fig. 4(a–f). The SEM image of CFrGO0 which have a solid structure based on cell connected through open faces is shown in Fig. 4 (a). At rGO incorporation of 2.0 wt %, the cell wall of carbon foam seemed to be smooth and a few graphene sheets are observed on the ligaments and surface of carbon foam as shown in Fig. 4 (b). A uniform distribution of rGO (2.0 wt %) in carbon matrix can also see in high magnification image of CFrGO2 (Fig. 4 (c)) Fig. 4 (d) shows the SEM image of 0.5 wt % rGO decorated carbon foam (CFDrGO0.5) which indicates that rGO is uniformly decorated on carbon foam surface. Fig. 4 (e) shows the SEM images of 1.0 wt % rGO decorated carbon foam (CFDrGO1) which indicates that increasing rGO concentration on carbon foam surface shows better dispersion (Fig. 4 (f)) compared to rGO incorporated carbon foam.

3.3. Electrical and EMI shielding properties Electrical conductivity is a critical parameter for EMI shielding because it is an intrinsic ability of a material for absorbing electromagnetic radiation. The influence of rGO on electrical conductivity of carbon-rGO composite foams is investigated and results are shown in Fig. 5 (a). Initially, carbon foam with 0 wt % rGO (CFrGO0) shows electrical conductivity of 24.4 Scm−1. After incorporation of rGO, the electrical conductivity is dramatically increased and it is reached up to 52.4 Scm−1 with 4.0 wt % of rGO, indicates the formation of conductive percolating network among rGO nanosheets. On the other hand, rGO decorated carbon foam has higher electrical conductivity (58.5 Scm−1) with rGO loading of only 1.0 wt % which suggested that presence of thin layers of rGO on carbon foam might decrease the percolation threshold of conductive network. Furthermore, in rGO decorated carbon foam, the extent of increase in electrical conductivity at lower rGO loading (1.0 wt %) is higher as compared to 1.0 wt % rGO incorporated carbon foam, and this is due to the degradation of properties of rGO embedded in resin derived carbon foam. Electromagnetic (EM) interference shielding of the materials is referred to three main constituents i.e. reflection (SER), absorption (SEA) and multiple reflections (SEM) [33]. Reflection of EM radiations is the outcome of interactions between EM waves and free charges on the surface of materials. Therefore, electrically conductive networks with a large number of charge carriers are helpful to EMI shielding effectiveness by reflection. Meanwhile, the absorption is a measure of capacity of materials to attenuate EM energies into thermal energies. Any types of energy dissipation and energy consuming responses induced by EM radiations, such as localized currents generated in conductive networks

3.2. Physical and mechanical properties Table 1 shows the bulk density, compressive strength and BET surface area of the rGO incorporated/decorated carbon foams. The bulk density of carbon foam prepared without rGO is 0.30 gcm−3 (sintering temperature is 1000 °C). While, bulk density of carbon-rGO composite 134

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Fig. 4. SEM images of (a) CFrGO0, (b) CFrGO2, (c) uniform distribution of rGO in carbon foam (CFrGO2), (d) CFDrGO0.5, (e) CFDrGO1 and (f) uniform decoration of rGO on carbon foam.

If Z02 ≫ Z1, where Z0 (Z02 = μ0 / ε0) and Z1 (for conducting materials, = 2πfμ/ σ ) are the impedance of air and carbon-graphene foam respectively, then reflection components SER is specified as [35].

Table 1 Density, compressive strength and BET surface area of carbon-rGO composite foams. Sample

CFrGO0 CFrGO0.5 CFrGO1 CFrGO2 CFrGO4 CFDrGO0.5 CFDrGO1

Density (gcm−3)

Compressive Strength (MPa)

BET Surface Area (m2g−1)

0.30 0.32 0.35 0.36 0.36 0.30 0.31

4.5 5.0 6.3 6.5 6.0 6.5 7.2

8.6 – 22.3 – 35.0 93.6 97.5

Z12

SER = 20log

SEA =

SET(dB) = −10log (PI/PO)

(1)

|S11 |= √PR/ PI

(2)

|S21 |= √PT / PI

(3)

T = |PT/PI| = |S21|

2

A=1−R−T

(4) (5) (6)

The SE values contributed by reflection (SER) absorption (SEA) and total SET of materials is determined by SER(dB) = −10log(1 − R)

(7)

SEA(dB) = −10log[T/(1 − R)]

(8)

SET(dB) = SER + SEA

(9)

8.686d = 8.686d πfσμ dB δ

(11)

Where d is shielding thickness of the materials. Equations (10) and (11) designate that reflection and absorption components of materials are proportional to log square root and square root of σ at specific frequencies. Additionally, electrical conductivity of shielding materials has an intimate connection with their EMI shielding performance [1]. In the present investigation, rGO incorporated and decorated carbon foam has been prepared and their EMI shielding measurements are carried out in the X-band frequency range (8.2–12.4 GHz). Fig. 5(b–d) shows variation in absorption, reflection and the total EMI SE of the samples, respectively. From Fig. 5 (b), it is clearly shown that SET consistently increases with increasing content of rGO, which can be explained by enhanced microwave absorption SEA by rGO. In the meantime, reflection component SER, fluctuated significantly against frequency although its average value remained unchanged irrespective of rGO contents because the overall interconnected foam structure did not change. It is shown in Fig. 5 (b) that SET value for as such carbon foam (CFrGO0) is −23.2 dB at a thickness of 2.0 mm. SET is equally shared by SE due to the reflection (−11.2 dB) as well as absorption (−12.0 dB) at a frequency of 8.2 GHz. From the experimental data, it is well evident that on loading of 0.5 wt % rGO in carbon foam (CFrGO0.5), SET is increases initially to −30.2 dB and SE due to SEA and SER are found to be −23.7 dB and −6.5 dB, respectively at frequency 8.2 GHz (Fig. 5 (c) and (d)). Whereas, on varying rGO loading up to 4.0 wt % in carbon foam, SET reaches to −44.6 dB because SEA and SER are found to be −37.4 and −7.2 dB, respectively. However, in case of 1.0 wt % rGO decorated carbon foam (CFDrGO1), absorption (SEA) is improved to −44.6 dB with reflection (SER) of −6.1 dB, therefore, the maximum value of total shielding effectiveness is −50.7 dB obtained at the same frequency. The enhancement in total EMI shielding of both type of carbon foam, incorporated and decorated

Where R represents reflectance and T transmittance respectively i.e. and A, can be calculated.

2

(10)

The absorption component of EMI shielding (SEA) of carbon-rGO composite foam, is known as

and polarization/relaxation processes of dipoles and free charges, contributes to absorption effectiveness of materials. Furthermore, the contribution of SEM is important for low absorbing materials and it's negligible if SE is > 10 dB therefore, reflection and absorption are the dominant mechanisms of carbon-rGO composites foam [34]. The total EMI shielding effectiveness is the logarithmic power ratio of incoming (PI) to outgoing (PO) of EM waves, measured by scattering (S) parameters, S11 and S21 as:

R = |PR/PI|2 = |S11|2

μ0 σ ⎞ Z0 = 20 log ⎜⎛ ⎟ dB 4Z1 4 2 π fμε0 ⎠ ⎝

135

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Fig. 5. (a) Electrical conductivity, (b) Total (SET), (c) Absorption (SEA) and (d) Reflection (SER) of carbon-rGO composite foams in the frequency range of 8.2–12.4 GHz (A = CFrGO0; B=CFrGO0.5; C= CFrGO1; D = CFrGO2; E = CFrGO4; F=CFDrGO0.5 and G = CFDrGO1). Fig. 6. Comparison of the recently reported graphene/ polymer foams and carbon-based porous composites and their corresponding shielding performance in the X-band (8.2–12.4 GHz) (PEI: polyetherimide [34]; Epoxy [35]; PI: polyimide [36]; PVDF: polyvinylidene fluoride [37]; CNF: carbon nanofiber [38]; PS: polystyrene [39]; PU: polyurethane [40]; PDMS: Poly dimethyl siloxane [41]; and Carbon [42]).

rGO heterojunction which helps to create widely continuous networks that facilitate electron transport in the carbon foam faster at low loading of rGO. The incident radiation directly interacts with rGO on the surface which leads to higher shielding effectiveness because of incident radiation scattering. Fig. 6 shows a comparison of SE of various polymer based graphene composite foams in the X-band [36–44], from which we can see clearly that carbon-based materials showed large number of advantages compared to polymer-based materials. In the present study, an effective SET of −50.7 dB is obtained by carbon-rGO composite foam with sample thickness only of 2.0 mm in the X-band region. Apart from this, high surface area and porosity will contribute to total EMI shielding of carbon foam samples. The BET surface area of carbon foam samples measured by sorption of nitrogen. It is observed

by rGO, indicates significant synergy between carbon and rGO. The porous structure and seamless interconnected conductive network of carbon-rGO composite foam are responsible for synergy with improved microwave dissipation. The incident EM waves pass into the carbon foam could be reflected at these cell-matrix interfaces because of large impedance mismatch between air, carbon and graphene sheets, which may improve the transfer of EM energy to be dissipated as heat [23] leading to the improvement in SE absorption. Furthermore, incorporated rGO framework supplied the basic pathway for long-range electromagnetic field-induced currents, leading to rapid decay of incident microwaves, whereas decorated rGO expanded the network and provided abundant interfaces for surface current attenuation. Additionally, decoration of rGO on carbon foam surface makes carbon136

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Fig. 7. Schematic representation of mechanism of EMI shielding in carbon-rGO composite foam.

exhibits SE as high as −50.7 dB and also shows absorption-dominant shielding mechanism. Additionally, in lightweight shielding applications, the concept of specific EMI SE (ratio of SE to density) is often used to evaluate the shielding capability of conductive foams [26]. Here, specific EMI SE of obtained samples at 8.2 GHz is summarized in Fig. 8 (b). As shown in Fig. 8 (b), rGO decorated carbon composite foam exhibits much higher specific EMI SE than as such carbon foam. The specific EMI SE of CFDrGO1 was improved by nearly 2 times larger than that of CFrGO1 and reached the maximum specific SE of 159.3 dBcm3g−1.

that with increasing rGO content, surface area increases continuously from 8.6 to 35 m2g-1 with 4 wt % rGO content in carbon foam. However, in case of 0.5 wt % rGO decorated carbon foam (CFDrGO0.5), the surface area is 93.6 m2g-1 and further increase to 97.5 m2g-1 on decoration of 1.0 wt % rGO content. The carbon–rGO composite foam fabricated with high surface area has greater chances of interaction with EMI radiation. This will cause high power absorption within shielding material results in improved EMI shielding properties. The interaction mechanism of electromagnetic waves with carbon and rGO is shown in Fig. 7 through a schematic diagram. Furthermore, it is interesting that the above synergy in EMI shielding is quite consistent with synergy in electrical conductivity increase arising from rGO. The theoretical relationship between electrical conductivity and EMI SE can be explained in terms of skin depth of materials. For conductive materials, ‘‘skin depth” is defined as the thickness of material below the outer surface where electric current flows. Skin effect causes effective resistance of conductor to increase at higher frequencies where skin depth is smaller. If electrical conductivity, σ (S / cm) ≫ 2πfε0 , skin depth, δ, is given by Ref. [1]. 1

2 ⎞ δ = ⎜⎛ ⎟ ⎝ fμσ ⎠

2

t ⎞ = −8.68 ⎛ ⎝ SEA ⎠ ⎜

3.4. Thermal properties The thermal conductivity of carbon-rGO composite foams was dominated by three factors including density, specific heat capacity and thermal diffusivity. It is calculated by the following equation: κ = α · ρ ·Cp

(13)

Where κ is thermal conductivity (W/(m.K)), α is thermal diffusivity (m2/s), Cp is specific heat (J/kg.K), and ρ is density (kg/m3). The thermal conductivity of carbon-rGO composites is depicted in Fig. 9 (a). Initially, thermal conductivity of carbon foam (CFrGO0) is 0.21 W/ (m.K) at room temperature. It is attributed to the random structure which conducts heat much less effectively than the aligned graphitic planes. This result is remarkable for a material with such a low density (0.30 g/cm3). The low thermal conductivity of carbon foam is due to the phenolic resin because most of the resins are vitreous, thus they are isotropic in thermal conductivity. There is an extraordinary increase in thermal conductivity for the carbon-rGO composites foam with the increase of graphene content as expected. After incorporation of graphene in carbon foam 0 to 4 wt %, thermal conductivity increased from 0.21 to 1.4 W/(m.K). However, in case of rGO decorated foam, thermal conductivity increases up to 1.2 W/(m K) by the decoration of only 1.0% rGO on carbon foam. This indicates that decoration of 1% rGO is effectively increased thermal conductivity as compared to 1% rGO incorporation. Thermal conductivity of carbon foam depends upon the structure of foam and most of the heat is transferred by ligaments and



(12)

where f is frequency, σ is electrical conductivity, ε0 is vaccum permittivity and μ is magnetic permeability of materials with a relationship of μ = μ0 μr (π0 = 4π × 10−7 H/m and μr = 1) for carbon-rGO composite foam due to non-magnetic nature. The variation in skin depth of carbon-rGO composite foams with different loading of rGO at frequency range 8.2–12.4 GHz is shown in Fig. 8 (a). From equation (12) it is clearly seen that the conductivity is inversely proportional to the square of skin depth, a slight increase in conductivity may result in a large decrease in skin depth of a shield, and finally improvement in its absorption component (SEA). For example, skin depth of as such carbon foam is approximately 2.0 mm at 8.2 GHz, while for 1.0 wt % rGO decoration on carbon foam, this value decreases obviously to about 0.55 mm. The decrement in skin depth has significantly enhanced their SEA that is directly proportional to skin depth [45]. Consequently, foam 137

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Fig. 8. (a) Variation in skin depth and (b) Specific EMI shielding carbon-rGO composite foams (A = CFrGO0; B=CFrGO0.5; C=CFrGO1; D = CFrGO2; E = CFrGO4; F=CFDrGO0.5 and G = CFDrGO1).

4. Conclusions

cell wall. This increase in thermal conductivity of carbon-rGO composite foam might be attributed to interconnected structure and reduction in the number of microcracks or defects [46]. Thus, fewer defects in the foam contribute high heat transfer. In rGO decorated foam (CFDrGO1), the extent of increase in thermal conductivity is higher as compared to rGO incorporated carbon foam (CFrGO1) and this is due to rGO decoration in the cracks which is provided a better cohesion between matrix (carbon foam) and filler (graphene). Thermal stability of carbon-rGO composite foams is investigated by TGA analysis. Fig. 9 (b) represents the TGA curves of CFrGO0, CFrGO4 and CFDrGO1 which have been studied in oxidative environment at the rate of 10 °C/min up to 1000 °C. The CFrGO0 shows weight loss in two steps. At the first, it shows a very fine wt. loss, which is started from a temperature below 400 °C, however, in later step major weight loss takes place between temperatures 400 °C to 700 °C. The final major loss corresponds to the complete pyrolysis of carbon foam fragments into smaller fraction and gaseous by-products. Whereas, incorporation (4 wt %) and decoration (1 wt %) of rGO in carbon foam change the thermal degradation steps absolutely and major weight loss process takes place above 500 °C. This indicates that thermal degradation temperature increases with incorporation and decoration of rGO in carbon foam. This is due to the fact that rGO being highly crystalline in nature. Therefore, graphene particles result in a stabilizing effect on thermal pyrolysis of carbon matrix and increase thermal stability of carbon foam. Additionally, as evident from TG traces, with the incorporation/decoration of rGO, corresponding ash residue is almost same in all the samples i.e. 1.6% which indicates that no other impurities presence in carbonrGO composite foams.

In the present work, carbon-rGO composite foams are developed via simple and cost-effective template method. The results revealed that rGO incorporation or decoration has improved mechanical strength, electrical conductivity, EMI shielding as well surface area of carbon foam significantly. The rGO also facilitate graphitization process via internal stress between carbon matrix and rGO filler during processing. The shielding effectiveness (SET) increases from −23.2 to −50.7 dB (in the X-band frequency region) with 1.0 wt % rGO decoration on carbon foam and sample thickness is only 2.0 mm. The SET, is dominated by SEA and it increases from −12 to −44.6 dB (275% increment) by the decoration of 1.0 wt % rGO on carbon foam. The reason is mainly due to enhanced microwave absorption derived from porous structure via multiple scattering and reflections and as a consequence of high surface area (97 m2g-1) of carbon-rGO composite foam. Furthermore, rGO contents improved not only in EMI SE of composite foam but also their electrical and thermal properties. This novel rGO decorated carbon foam is believed to be providing a new strategy for designing highperformance lightweight EMI shielding materials and indicating advantages of rGO for EMI shielding materials in areas such as nextgeneration electronics and aerospace. Conflicts of interest There are no conflicts of interest. Acknowledgments Authors are highly grateful to the Director, CSIR-NPL to publish the results. The authors are also thankful to Dr. S.K. Dhawan and Dr.

Fig. 9. (a) Thermal conductivity and (b) thermal stability of carbon-rGO composite foams. 138

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