Composites Part A 123 (2019) 232–241
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Composites Part A journal homepage: www.elsevier.com/locate/compositesa
Ultra-light 3D reduced graphene oxide aerogels decorated with cobalt ferrite and zinc oxide perform excellent electromagnetic interference shielding eﬀectiveness Shivam Guptaa, Sanjeev Kumar Sharmab, Debabrata Pradhanb, Nyan-Hwa Taia, a b
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, ROC Materials Science Centre, Indian Institute of Technology, Kharagpur, India
A R T I C LE I N FO
A B S T R A C T
Keywords: A. Graphene B. Electrical properties B. Magnetic properties E. Autoclave EMI shielding eﬀectiveness
In this study, three dimensional interconnected graphene aerogels decorated with cobalt ferrite nanoparticles and ZnO nanorods were prepared for the application of absorption dominant electromagnetic interference (EMI) shielding. Because of low density, high porosity and large surface area, the graphene aerogel showed total EMI shielding eﬀectiveness of 25.07 dB at a thickness of 5 mm, which was further improved to 42.10 dB with the inclusion of cobalt ferrite nanoparticles. The incorporation of magnetic cobalt ferrite not only enhanced the total EMI shielding of the aerogels but also improved the power absorption from ∼37.738% to ∼87.788%. By further incorporating ZnO nanorods along with cobalt ferrite nanoparticles into the aerogels, the total EMI shielding eﬀectiveness and power absorption enhanced to 48.56 dB and ∼93.655%, respectively. In this work, the EMI shielding performances of diﬀerent samples are compared and the underlying mechanism of such excellent absorption dominant EMI shielding is discussed in detail.
1. Introduction Due to the fast growth of electronics, broadcasting and telecommunications, it is impossible to totally avoid human being from the exposure to all sorts of electromagnetic (EM) ﬁelds; therefore, EM pollution has become a serious concern of the modern world . Apart from the public health impact of EM pollution, such as leukemia  and brain tumor , it also leads to the ineﬃcient functioning of the miniaturized devices kept close together because of space constraints [4,5]. The practice of blocking external EM waves penetration into sensitive areas as well as blocking EM waves to be transmitted outside the controlled area is known as electromagnetic interference (EMI) shielding. The long range applications of EMI shielding materials include commercial and scientiﬁc electronics, antenna systems, medical devices, space exploration and military electronic devices. EMI shielding materials are widely used in military applications such as stealth which is used to reduce the detectability of the target by absorbing radar signal known as radar absorbing materials [6,7]. Different kinds of advanced radar absorbing materials and designs (often classiﬁed) have been extensively developed by various countries for stealth military planes in which reﬂection of waves has to be minimized .
Traditionally, metals have been widely used as eﬃcient EMI shielding materials owing to their ability to conduct both electricity and heat. However, the metal shields are heavy, expensive, prone to oxidation, less ﬂexible and bulky, thus, ineﬃcient for modern technologies and diﬃcult to design. Moreover, metal shields mainly function by reﬂection, thus, not suitable for stealth technology . Furthermore, the shielding eﬃciency of the metal shields greatly degrades in the joints . Owing to high magnetic permeability, light weight and suﬃciently soft, mumetals have also been considered as an EMI shielding material, but galvanic corrosion and high cost of mumetals hamper their practical application as an EMI shield . Intrinsically conducting polymers (ICPs) [12–14] have also been studied for the application of EMI shielding because they do not require conductive ﬁllers for shielding. Moreover, ICPs provide absorption dominant EMI shielding as compared with metals which mainly function by reﬂection owing to their shallow skin depth, but their poor mechanical and thermal properties along with poor processability limit their practical application as shielding material. Owing to their excellent electrical conductivities, high ﬂexibility, low cost, chemical inertness and easy processability, carbon materials and their composites have been regarded to be the ideal candidate for the application of eﬀective EMI shielding [15–18]. Graphene [19–21],
Corresponding author. E-mail address: [email protected]
https://doi.org/10.1016/j.compositesa.2019.05.025 Received 28 January 2019; Received in revised form 8 May 2019; Accepted 19 May 2019 Available online 20 May 2019 1359-835X/ © 2019 Elsevier Ltd. All rights reserved.
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followed by sealing in an autoclave at 140 °C for 16 h. After a solvothermal reaction of 16 h, the autoclave was cooled down to room temperature naturally. The resultant white product was washed thoroughly with DI water followed by vacuum ﬁltration using a 1.0 µm pore sized membrane. After ﬁltration, the white product was peeled oﬀ from the ﬁlter paper. Finally, the ﬁltered white product was dried in an oven at 70 °C overnight (12 h).
carbon nanotubes [22–24], graphite , carbon nanoﬁber [26,27], carbon microspheres , carbon black , carbon-cenosphere  and activated carbon  have been extensively reported for EMI shielding. Among all these, three dimensional (3D) graphene aerogels or foams have demonstrated highly eﬃcient performance in the ﬁeld of EMI shielding due to their light weight and absorption dominant shielding [32,33]. Song et al  reported graphene aerogel–carbon texture hybrid, which showed transmission and reﬂection losses of 15 dB and 6 dB, respectively. Shen et al  reported compressible graphene-coated polymer foams which were prepared by a simple dip coating method. With 5 wt% loading of graphene on polyurethane sponge, the sponge exhibited total EMI shielding eﬀectiveness of 34.7 dB with thicknesses of 6 cm. The highly porous structure of the graphene-coated sponge decreased the impedance mismatch at the foam/air interface, thereby facilitating deep penetration of incident EM waves into the foam and subsequently resulting in absorption dominant shielding. Moreover, the incorporation of the magnetic materials inside the shield has also been beneﬁcial for absorption of the EM waves as reported extensively [36–38]. Sahoo et al reported 3D graphene/noble metal nanocomposites, which was prepared by wet shaping technique or freeze casting. The silver and platinum decorated graphene aerogels showed total EMI shielding eﬀectiveness of 28 dB and 24 dB, respectively. However, the reﬂection of the EM waves was found to be the dominant contributor in the EMI shielding in most of the studies. The reﬂection-dominant EMI shielding materials can protect the enclosed devices from damage by the outer interference but the reﬂected EM waves may disturb the other nearby devices. On the other hand, the absorption-dominant EMI shielding materials can signiﬁcantly absorb and alleviate the EMI pollution completely. In addition, the absorptiondominant EMI shielding materials can also absorb the EM wave generated by the enclosed devices to prevent the self-disturbance. Moreover, the stealth technology speciﬁcally require high absorptiondominant EMI shield because the reﬂected waves increase the detectability of the military planes. This paper is thus focused on the absorption-dominant EMI shielding and its dependence on the incorporation of the dielectric and magnetic materials. In this work, the 3D interconnected graphene aerogels decorated with cobalt ferrite (CFO) nanoparticles and ZnO nanorods were prepared for the application of absorption dominant EMI shielding. Various characterization techniques were employed to examine the morphology, structure and physical properties of the as-prepared aerogels. The aerogels have shown a 3D interconnected graphene network uniformly decorated with cobalt ferrite nanoparticles and ZnO nanorods. Without incorporation of cobalt ferrite nanoparticles and ZnO nanorods, the aerogel has shown total EMI shielding eﬀectiveness of 25.07 dB at a thickness of 5 mm, which was further improved to 42.10 dB with the incorporation of cobalt ferrite nanoparticles. By further incorporating ZnO nanorods into the aerogels, the total EMI shielding enhances to 48.56 dB. Moreover, it was found that the shielding is mainly attributed to the absorption of the EM waves and the aerogels can absorb up to ∼93.655% power of the incident EM waves. Hence, the as-prepared aerogels can be considered suitable for stealth technology as radar absorbing material. Moreover, the aerogels can also be used as shielding material in commercial and scientiﬁc electronics, antenna systems, medical devices, space exploration and military electronic devices.
2.2. Preparation of CFO nanoparticles The CFO nanoparticles were synthesized using the ﬂash pyrolysis route . Iron nitrate Fe(NO3)3·9H2O and cobalt nitrate Co (NO3)2·6H2O were used as precursors of Fe and Co ions, respectively. Firstly, cobalt nitrate was dissolved in DI water and NH4OH solution was used to precipitate hydrated hydroxides. Then, the iron nitrate was mixed in the as-prepared solution. Further, the citric acid was added into the solution to chelate the metal ions. The ratio of citric acid to metal ions (Co ions and Fe ions) was maintained as 1: 1. NH4OH solution was used to maintain the pH level of the as-prepared solution. Furthermore, the requisite amounts of ethylene glycol and glycine were also added to the solution as a chelating agent and fuel, respectively. Finally, the as-prepared solution was heated in a furnace at 350 °C for 2 h. The low density black powder was obtained and subsequently subjected to annealing at 800 °C for 2 h to remove the residual impurities of iron precursor completely. 2.3. Preparation of the aerogel Graphene oxide (GO) was prepared by the modiﬁed Hummers method as described in our previous work . Then, GO was mixed with DI water and sonicated for 1 h. CFO nanoparticles and ZnO nanonanorods were added into the GO solution and the solution was stirred for 30 min for proper dispersion. After that, the as-prepared solution was placed into a Teﬂon vessel, which was then sealed in an autoclave at 150 °C for 5 h. The as-prepared aerogel was washed with DI water several times and dried by freeze-drying. A schematic representation of the aerogel preparation has been depicted in Fig. 1. The aerogels were deﬁned as G-XY which represents the ratio between GO:CFO:ZnO as starting materials. The weight ratio of GO was always ﬁxed to 10 while the weight ratios of CFO nanoparticles and ZnO nanorods were changed and represented as X and Y, respectively, as shown in Table 1. 3. Characterizations The surface morphologies and microstructures of ZnO nanoroads, cobalt ferrite and aerogels were investigated by ﬁeld emission scanning electron microscopy (MERLIN ZIESS and FESEM, JEOL 6500F). X-ray diﬀraction (XRD) patterns were collected in the scattering range of 15–80°on a Bruker D2 Phaser at a scanning rate of 4.27°/min. A Keithley 2410 current source was employed to measure the electrical conductivities of the aerogels through classic four-probe method. The magnetic properties of the CFO nanoparticles and aerogels were examined by vibrating sample magnetometer (PMC Micromag 3900). To measure the S-parameters (S11, S21, S12, S22) using the rectangular waveguide method for EMI shielding eﬀectiveness, a vector network analyzer (Agilent Technologies, PNA Series, E8364A) was used in the frequency range of 8.2–12.4 GHz (X-band) at room temperature. The rectangular waveguide possesses the cavity groove of 22.86 × 10.16 mm; therefore, the aerogels were cut into rectangular dimensions of 23 × 11 mm and then slightly modiﬁed to ﬁll the cavity. The thicknesses of all the samples were 5 mm.
2. Experimental 2.1. Preparation of ZnO nanorods ZnO nanorods were prepared through the solvothermal method using ZnCl2 and Na2CO3 as the starting materials . First, ZnCl2 of 0.267 g and Na2CO3 of 26.67 g were mixed with 80 ml deionized water (DI) and the solution was magnetically stirred at room temperature for 30 min. Then, the solution was placed into a 125 ml Teﬂon vessel
4. Results and discussion The graphene aerogels decorated with CFO nanoparticles and ZnO 233
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Fig. 1. Schematic representation of the aerogel preparation. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)
be seen in Fig. 2(d). SEM micrograph of the G-51 aerogel conﬁrmed that the aerogel possess 3D interconnected conductive network, as shown in Fig. 2(e), indicating continuous channels for the transportation of the charge carriers, which could improve the electrical conductivity and EMI shielding performance. Moreover, the porous cellular structure (700–1000 μm) of the aerogel could be beneﬁcial for the multiple reﬂections, thus may improve the absorption of the EM waves. As it can be seen in Fig. 2(f), the high resolution SEM micrograph on the pore wall conﬁrms the uniform distribution of the CFO nanoparticles and ZnO nanorods in the aerogel. No signiﬁcant change was observed in the morphologies of the CFO nanoparticles and ZnO nanorods, which conﬁrms the successful decoration of the ZnO nanorods and CFO nanoparticles on the graphene sheets. The typical XRD patterns of the G-00, ZnO nanorods, CFO nanoparticles and aerogel G-51 are shown in Fig. 3(a). Comparing with the characteristic diﬀraction peak of graphite at 26.58°, the XRD pattern of G-00 has shown a broad peak around 24° which was due to the shortrange order in stacked sheets. The interlayer spacing of G-00 was calculated to be 0.37 nm which is slightly larger than that of graphite suggesting that the small amount of residual oxygen-containing functional groups are still present. In the XRD pattern for the ZnO nanorods, typical peaks at 31.77, 34.42, 36.25, 47.54, 56.60, 62.86, 66.38, 67.96, 69.18, 72.56 and 76.95° correspond to the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), (2 0 1), (0 0 4) and (2 0 2) planes (JCPDS card 036-1451), respectively, can be detected. The diﬀraction pattern shows hexagonal structure of ZnO nanorods. The XRD pattern of CFO nanoparticles has shown two diﬀerent phases; one from the cobalt ferrite and another from the residual phase of iron oxide (burnt ochre). The diﬀraction peaks at 18.28, 30.08, 35.43, 37.05, 43.05, 56.97, 62.58 and 74.01° are corresponding to the (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (5 1 1), (4 4 0) and (5 3 3) planes of cobalt ferrite (JCPDS card 022-1086) whereas the peaks at 24.13, 33.15, 35.61, 40.85, 49.52, 54.09, 62.45 and 63.99° are corresponding to the (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 6), (2 1 4) and (3 0 0) planes of residual iron oxide (JCPDS card 033–0664), respectively. Further, the CFO nanoparticles were annealed at 800 °C for 2 h to remove the impurities of iron precursor completely, as shown in Fig. S1. The XRD pattern of the aerogel G51 has shown all the main peaks of the rGO,
Table 1 Weight ratio of diﬀerent samples as starting materials. Samples
Weight ratio of GO:CFO:ZnO
G-00 G-10 G-30 G-40 G-50 G-01 G-11 G-31 G-41 G-51
10:0:0 10:1:0 10:3:0 10:4:0 10:5:0 10:0:1 10:1:1 10:3:1 10:4:1 10:5:1
nanorods were prepared by a simple solvothermal process followed by freeze-drying. A schematic representation of the aerogel preparation has been depicted in Fig. 1. The as-prepared aerogel has ultra-low density of 8 mg cm−3 which is categorized in ultra-light material. The porosity of the aerogel was measured to be ∼95%. The aerogel is so light that it can stand on grass hairs, as shown in Fig. 1. Owing to high surface area, such porous 3D interconnected networks can facilitate multiple reﬂections through various surfaces inside the network, thus enhance the absorption of the EM waves by trapping it inside the network. 4.1. Morphology and crystalline phase Fig. 2 depicts the surface morphologies of ZnO nanorods, CFO nanoparticles and aerogels. ZnO has shown a rod like morphology with diameter ranging from 100 to 200 nm and length in several micrometers, as shown in Fig. 2(a). As shown in the Fig. 2(b), the high aspect ratio of the ZnO nanorods is attributed to the high concentration of the Na2CO3 which introduced alkaline conditions and suppressed lateral growth . The surface morphology of the CFO nanoparticles has been shown in Fig. 2(c) & (d). It can be clearly observed that the particles are spherical and uniform along with a notable tendency of agglomeration. The high resolution SEM micrograph conﬁrms the particle sizes of the CFO nanoparticles to be in the range of 50–100 nm, as can 234
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Fig. 2. SEM micrographs of (a, b) ZnO nanorods (c, d) CFO nanoparticles and (e, f) aerogel G51; (b), (d) and (f) are the high resolution images of (a), (c) and (e), respectively. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)
4.2. Electrical properties As shown in Fig. 3(b), the electrical conductivities of the aerogels were measured using a classical four-probe system at room temperature. The conductivity of the G-00 aerogel was measured to be 405 S/m, which is due to the greatly conductive graphene sheets interconnected together. With the inclusion of the non-conducting CFO nanoparticles, the conductivity of the aerogels decreased. The conductivities of the G10, G-30, G-40 and G50 aerogels were measured to be 148, 86, 48 and 42 S/m, respectively. The impedance of the plane wave in free space is around 377 Ω. High conductive materials have low impedance, thus, show a high reﬂection of the EM waves at the interface due to impedance mismatch. The decrease in conductivity can improve impedance matching at the interface; as a result, EM waves can deep penetrate into the material and be absorbed inside the material. The inclusion of the non-conducting material also generates interfacial polarization as the charges accumulate around the non-conducting material . These accumulation centers act as scattering centers for EM waves, thus enhance overall absorption of the EM waves. With the incorporation of the ZnO nanorods into the aerogel, the conductivity of the G-01 was measured to be 291 S/m which is higher than that of G-10 due to the slightly conductive nature of ZnO nanorods. The electrical conductivities of the G-11, G-31, G-41 and G-51 are measured to be 119, 68, 41 and 39 S/m, respectively. ZnO is a dielectric material and can enhance dielectric loss of the material through dipole polarization and relaxation, thus, can improve overall absorption of the EM waves.
4.3. Magnetic properties Magnetic properties of the CFO nanoparticles and aerogels were examined at room temperature using a highly sensitive vibrating sample magnetometer (VSM) and their respective hysteresis loops have been shown in Fig. S2 & Fig. 4. At an external ﬁeld of 15 kOe, the saturated magnetization of the as-prepared CFO nanoparticles was found to be 100 emu g−1 while the annealed CFO nanoparticles have shown saturated magnetization of 83 emu g−1, as shown in Fig. S2. The higher saturated magnetization of the as-prepared CFO nanoparticles was attributed to the mixed phases of cobalt ferrite and iron oxide. Therefore, the as-prepared cobalt ferrite nanoparticles were used for further study. The saturated magnetizations of the G-00 and G-01 aerogels were found to be approximately zero due to the non-magnetic behavior of graphene sheets and ZnO nanorods, as shown in Fig. 4. With the incorporation of cobalt ferrite nanoparticles, the saturated
Fig. 3. (a) XRD patterns of RGO, ZnO, CFO and aerogel G51 and (b) Electrical conductivities of the aerogels. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)
ZnO nanorods and CFO nanoparticles, which demonstrates the successful decoration of the CFO nanoparticles and ZnO nanorods on the graphene sheets.
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measured in the frequency range of 8.2–12.4 GHz (X-band). EMI waves propagate at the right angle to the plane consisting electric ﬁeld (E) and magnetic ﬁeld (H) which are also perpendicular to each other . The ratio of electric ﬁeld to magnetic ﬁeld amplitudes is deﬁned as the impedance of the EM wave and shown in Eq. (2). As it is known, the μ0 characteristic impedance of the free space is described as Z0 = , which is almost 377 Ω  whereas impedance Zin is deﬁned as
Zin = Z′
Z0cosh(γd ) + z ′sinh(γd ) z ′cosh(γd ) + Z0 sinh(γd )
where μ0 and ε0 are permeability and permittivity of free space, respectively, and Z′ is the wave impedance and can be expressed such as
|E| = |H|
jωμ σ + jωε
where σ, μ, γ and d are the conductivity, permeability, wave propagation constant and thickness of the shielding material, respectively. The propagation constant can be further explained as 1/2
ε′μ ⎡ ⎛ (σ + ωε″) ⎞2 ⎤ 1+ + 1⎥ ⎢ 2 ωε′ ⎠ ⎝ ⎣ ⎦
ω = 2π f; f = frequency (3)
where ε′ and ε″ are the real and imaginary parts of permittivity, respectively. The real (ε′) and imaginary (ε″) parts of the permittivity signify electric energy storage and electric energy dissipation capabilities of the shield, respectively. An external electric ﬁled causes displacement of charges in a dielectric material, thus induces polarization which is directly related to the real permittivity (ε′) whereas subsequent relaxation resulting in electric energy dissipation is associated with the imaginary permittivity (ε″) [45,46]. The conductive shields have mobile charge carriers (electron and holes) which interact with the impinging EM ﬁelds and produce a counter ﬁeld known as scattered ﬁeld or induced ﬁeld. The impedance of the conductive shields is lower than that of free space, therefore, most of the impinging EM waves reﬂects back due to impedance mismatch at the air/shield interface . The reﬂection loss (SER) can be deﬁned as
Fig. 4. Magnetization hysteresis loops of (a) CFO nanoparticles decorated aerogels and (b) CFO nanoparticles and ZnO nanorods decorated aerogels. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)
SER = 10log(1 − R); magnetization of the G-10 aerogel was measured to be 2.07 emu g-1 at an external ﬁeld of 15 kOe. With respect to the percentages of as-prepared CFO in the aerogel, the saturated magnetization of the aerogels have been found low, which may be attributed to the sparse distribution and wrapping of the nanoparticles with graphene sheets resulting in the less interaction of the nanoparticles with each other, as shown in Fig. 4(a). The value of saturated magnetization increased proportionally to the weight ratio of the cobalt ferrite nanoparticles in the aerogels. The saturated magnetizations of the G-30, G-40 and G-50 aerogels are 5.46, 7.02 and 9.65 emu g−1, respectively. The high saturated magnetization is favorable to the high permeability of the aerogels. When the EM waves impinge on a surface, there are typically three kinds of mechanisms that occur, i.e. relaxation of the magnetization, magnetic domain movement and spin resonance. Moreover, the natural resonance and eddy current loss play an important role in the absorption of the EM waves, thus enhancing the overall magnetic loss. By further incorporating non-magnetic ZnO nanorods in the aerogels, the saturation magnetization of the aerogels slightly decreased. The saturated magnetizations of the G-11, G-31, G-41and G-51 aerogels are 1.60, 4.82, 6.15 and 8.00 emu g−1, respectively, as can be seen in Fig. 4(b).
Z − Zin ⎞ Reflection constant(R) = ⎛ 0 ⎝ Z0 + Zin ⎠ ⎜
The absorption dominant EMI shields possess magnetic and electric dipoles which respond to the magnetic and electric ﬁelds of the EM waves, respectively. The absorption loss (SEA) is a function of the product of conductivity and permeability. The SEA can be deﬁned as
SEA = 8.87γd
Other than the reﬂection and absorption of the impinging EM waves, multiple reﬂections (SEM) at various surfaces or interfaces in the shield are also an important phenomenon. A large surface area or interface is the key requirement for multiple reﬂections. However, it is usually accepted that the re-reﬂected waves are absorbed inside the shield if the absorption loss is higher than 10 dB . To calculate the power coeﬃcients and EMI shielding, the scattering parameters (S21, S11, S22, S12) were obtained from the vector network analyzer, where Sij illustrates the power transmitted from port i to port j. The reﬂection coeﬃcient (R) and transmission coeﬃcient (T) can be calculated according to following formulas R = and T =
ET 2 EI
ER 2 EI
= |S11 |2 = |S22 |2
= |S21 |2 = |S12 |2 , while the absorption coeﬃcient was
calculated as A = (1 − R − T ) . All the calculations were performed at the mid-frequency of 10 GHz. As shown in Fig. 5, the reﬂection, absorption and transmission coeﬃcients of the aerogel G-00 were calculated to be 0.4456, 0.5513 and 0.0031, respectively, which is owing to the highly conducting graphene sheets acting as reﬂector. However, the porous
4.4. EMI shielding eﬀectiveness In this work, the shielding eﬀectiveness of the aerogels has been 236
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provided more absorption of the EM waves. The low transmission coeﬃcient conﬁrmed high EMI shielding, but mainly contributed by the reﬂection of the EM waves. By the incorporation of the CFO nanoparticles, the reﬂection coeﬃcient enhanced and measured to be 0.6510 for G-10 aerogel indicating better impedance matching at the free-space/specimen interface. The absorption coeﬃcient also decreased to 0.3160 which conﬁrms enhanced absorption of the EM waves. However, the transmission coeﬃcient enhanced to 0.0330 which indicates higher transmission of the EM energy. Although the total transmission increased while incorporating the CFO nanoparticles but the absorption coeﬃcient enhanced signiﬁcantly, which conﬁrms improved absorption of the EM energy. By further increasing the weight ratio of the CFO nanoparticles, the reﬂection coeﬃcients of the G-30, G40 and G50 aerogels were calculated to be 0.7716, 0.8147 and 0.8700, respectively, as shown in Fig. 5(a). The signiﬁcant increase in reﬂection coeﬃcients indicates enhanced impedance matching at the interface and enhanced wave penetration inside the aerogel. The inclusion of the non-conducting CFO nanoparticles signiﬁcantly decreased the conductivity of the aerogel, which resulted in enhanced impedance matching at the interface. The absorption coeﬃcients of the G-30, G40 and G50 aerogels were calculated to be 0.2209, 0.1845 and 0.1299, respectively, as can be seen in Fig. 5(b). The signiﬁcant decrease in the absorption coeﬃcients conﬁrms improved absorption of the EM waves. The inclusion of the nonconducting CFO nanoparticles produced new interfaces in the shielding material and the charges accumulated around these non-conducting nanoparticles. As mentioned, these accumulated charges act as scattering centers for EM waves and known as interfacial polarization or space charge polarization. With an increasing ratio of the CFO nanoparticles, the interfacial polarization enhanced which led to more absorption of the EM waves. Moreover, the high saturated magnetization of the CFO nanoparticles led to high magnetic losses such as magnetic domain movement, spin resonance and relaxation of the magnetization. Apart from these losses, the natural resonance and eddy current losses which are introduced due to the magnetic nanoparticles are also accountable for improved absorption of the EM waves. Therefore, the transmission coeﬃcients of the G-30, G40 and G50 aerogels were decreased to approximately 74.76 × 10−4, 7 × 10−4 and 0.61 × 10−4, respectively, as shown in Fig. 5(c). The low transmission coeﬃcient conﬁrmed very high EMI shielding of the CFO incorporated aerogels. By further incorporating ZnO nanorods in the aerogels, the reﬂection coeﬃcients of the G-31, G41 and G51 aerogels were calculated to be 0.83, 0.91 and 0.92, respectively, as shown in Fig. 6(a). The enhanced reﬂection coeﬃcient conﬁrmed improved impedance matching at the interface due to the decreased conductivities of the aerogels. The absorption coeﬃcient of the G-31, G41 and G51 aerogels were calculated to be 0.1652, 0.0878 and 0.0707, respectively, as can be seen in Fig. 6(b). The decrease in absorption coeﬃcient conﬁrmed improved absorption of the EM waves with the incorporation of the ZnO nanorods. ZnO is a dielectric material and accountable for dipole polarization and relaxation, which enhanced dielectric loss. When an external electric ﬁeld is applied, the dielectric material responds by a shift in charge distribution by aligning the positive charges with the electric ﬁeld and the negative charges against it, thus generating energy dissipation owing to change of dipole moment with time. Moreover, the incorporation of the ZnO introduced newer interfaces inside the shielding material i.e. interfaces between RGO/CFO, RGO/ZnO and CFO/ZnO which resulted in enhanced interfacial polarization. The transmission coeﬃcient also further decreased to approximately 6.12 × 10−4, 1.48 × 10−4 and 0.13 × 10−4 for G-31, G41 and G51 aerogels, which conﬁrmed the enhanced shielding of the EM waves, as shown in Fig. 6(c). By further increasing the weight ratio of the CFO or ZnO nanorods, the aerogel became fragile, thus G-51 is the optimum loading of CFO nanoparticles and ZnO nanorods. Blockage of EM waves by the means of conductive or magnetic materials is known as EMI shielding. The ratio of the incident and
Fig. 5. Power coeﬃcients of CFO nanoparticles decorated aerogels (a) reﬂection Coeﬃcient, (b) absorption coeﬃcient and (c) transmission coeﬃcient. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)
interconnected graphene network still allowed a large fraction of EM waves to penetrate inside and absorbed. The large surface area and porosity of the aerogels facilitated multiple reﬂections of the EM waves inside the material, thus provided high absorption of the EM waves. Apart from multiple reﬂections, the residual polar groups on the RGO surface and defects functioned as scattering centers for EM waves and 237
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Fig. 7. EMI shielding eﬀectiveness of CFO nanoparticles decorated aerogels (a) reﬂection loss and (b) transmission loss. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)
respectively, whereas subscripts “input” and “output” represent incident and transmitted waves, respectively. When EM waves impinge on the EMI shields, the phenomenon of reﬂection, absorption, transmission and multiple reﬂections occur simultaneously. The total EMI shielding eﬀectiveness is determined by three factors; primary reﬂection when EM waves ﬁrst impinge on the material i.e. reﬂection loss, internal absorption of the waves i.e. absorption loss and secondary (multiple) reﬂections inside the material i.e. multiple reﬂection losses , and can be written as
SETotal (dB ) = SER + SEA + SEM The reﬂection and transmission losses of the aerogels have been shown in Fig. 7(a) & (b), respectively. The transmission and reﬂection and loss of the G-00 aerogel was found to be 25.07 dB and 2.56 dB, respectively, which is owing to the interconnect graphene network. It was found that the reﬂection loss increased with weight ratio of the CFO nanoparticles in the aerogel, which is owing to the decreased conductivity of the aerogels resulted in improved impedance matching at the interface. As can be seen, the transmission loss of the G-00 aerogel was found higher than that of G-10 and G-30, however, the reﬂection losses of the G10 and G-30 were found higher than that of G00, which suggests that the absorption in the G-10 and G30 aerogels are far higher than that of G-00 aerogel. The reﬂection of the EM waves is the dominant mechanism of the shielding in G-00 aerogel. The
Fig. 6. Power coeﬃcients of CFO nanoparticles and ZnO nanorods decorated aerogels (a) reﬂection Coeﬃcient, (b) absorption coeﬃcient and (c) transmission coeﬃcient. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)
transmitted energies or ﬁelds of the EM waves in decibels is deﬁned as the total EMI shielding eﬀectiveness (SE) of the shield , and can be expressed as
Pinput ⎞ ⎛ Einput ⎞ = 20log ⎛ Hinput ⎞ (dB ) SEtotal = 10log ⎜⎛ ⎟ = 20log ⎜ ⎟ ⎜ ⎟ P ⎝ output ⎠ ⎝ Eoutput ⎠ ⎝ Houtput ⎠ where P, E and H represent power, electric ﬁeld and magnetic ﬁeld, 238
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improved and measured to be 31.49 and 42.10 dB, respectively. In other words, aerogel G-50 can shield up to ∼99.994% power of the incident EM wave which was shared by ∼12.997% reﬂection and ∼86.997% absorption. Clearly, aerogel G-50 provided absorption dominant EMI shielding owing to its 3D interconnected graphene network uniformly decorated with CFO nanoparticles. Comparing with chemically derived graphene foam  and graphene aerogels/epoxy composite  with EMI shielding eﬀectiveness of 20 and 35 dB, respectively, the CFO decorated graphene aerogel performed much higher EMI shielding eﬀectiveness, thus conﬁrming the synergistic eﬀects of CFO decoration. By only incorporating ZnO nanorods and comparing with G-00 aerogel, the reﬂection loss of G-01 aerogel increased slightly from 2.56 to 4.22 dB while the transmission loss also increased from 25.07 to 28.10 dB, which indicates the crucial role of the ZnO nanorods, as shown in Fig. 8(a) & (b), respectively. The enhanced EMI shielding eﬀectiveness may attribute to the enhanced dielectric loss and interfacial polarization. Beneﬁtting from both dielectric ZnO nanorods and magnetic CFO nanoparticles, the reﬂection and transmission loss of the aerogels have improved signiﬁcantly. The reﬂection losses of the G-11, G-31, G-41 and G-51 were measured to be 4.81, 7.80, 10.55 and 11.50 dB, respectively, as shown in Fig. 8(a). The enhancement in reﬂection losses is attributed to impedance matching due to the incorporation of the non-conducting CFO nanoparticles and ZnO nanorods. The transmission losses of the G-11, G-31, G-41 and G-51 were measured to be 22.48, 32.13, 38.29 and 48.56 dB, respectively, as shown in Fig. 8(b). Wan et al.  reported cellulose ﬁber-graphene aerogel with transmission and reﬂection losses of 47.8 and 5 dB, respectively. Comparing with cellulose ﬁber-graphene aerogel, the G-51 aerogel performed slightly better transmission loss but surpassed reﬂection loss. The increased reﬂection and transmission losses conﬁrmed improved absorption of the microwaves. In other words, the aerogel G51 can shield up to ∼99.998% power of the incident EM wave which was shared by ∼7.074% reﬂection and ∼92.924% by absorption. The improved absorption of the EM waves is contributed by the dielectric losses due to the ZnO nanorods and magnetic losses due to the CFO nanoparticles. Other than that, the multiple reﬂections due to the excellent 3D interconnected porous structure and interfacial polarization also played key roles in absorption. Furthermore, the average absorption percentages of the aerogels have been calculated in the entire Xband range using the S-parameters such as Aaverage % = (1 − S11 − S21) × 100 , as shown in Fig. 9. The G-00 aerogel has shown average absorption of 37.738%, which increased to 87.788% for G-50 with the incorporation of the CFO nanoparticles. By further incorporating ZnO nanorods, the average absorption of the G-51 was calculated to be 93.655%. Such high absorption dominant shielding materials are suitable for the application of radar absorbing. Comparing with previously reported graphene-based aerogels, as shown in Table 2, the cobalt ferrite nanoparticles and ZnO nanorods decorated graphene aerogels performed better in terms of total EMI shielding eﬀectiveness and reﬂection loss. To better understand the shielding mechanism, the illustrative representation of the detailed shielding mechanism is depicted in Fig. 10. Based on the prior discussion, the CFO nanoparticles and ZnO nanorods incorporated aerogels can be the best candidate for the absorption of the microwaves. Furthermore, the magnetic, electrical and EMI shielding properties of the aerogels can be adjusted by varying the loading of the CFO nanoparticles and ZnO nanorods in the aerogels.
Fig. 8. EMI shielding eﬀectiveness of CFO nanoparticles and ZnO nanorods decorated aerogels (a) reﬂection loss and (b) transmission loss. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)
Fig. 9. Average power absorption of aerogels. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)
reﬂection losses were further improved by increasing the weight ratio of the CFO nanoparticles and measured to be 7.3 and 8.8 dB for G-40 and G-50 aerogels, respectively. The transmission losses were also
In conclusion, the CFO nanoparticles and ZnO nanorods decorated 3D interconnected graphene aerogels have been successfully prepared for the application of the EMI shielding. The aerogels have shown ultralow density of 8 mg cm−3 which is categorized in ultra-light material. SEM micrographs have conﬁrmed uniform decoration of CFO 239
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Table 2 Shielding eﬀectiveness of this work and diﬀerent graphene based aerogels. Samples
Reﬂection loss (SER) (dB)
Total EMI shieling (SET) (dB)
Cellulose ﬁber/graphene aerogel 3D graphene aerogel-Carbon texture hybrids Chemically derived graphene foam Phenolic resin-enhanced 3D graphene aerogels/epoxy composites Thermally reduced graphene oxide aerogels 3D graphene foam/polydopamine Thermally annealed anisotropic graphene aerogels/epoxy composites Reduced graphene oxide aerogels decorated with cobalt ferrite and zinc oxide
5 3 1 4 2.5 0.032 4 5
2.83 70 0.06 10.8 4.5 0.05 9.2 8
5 7 2 – 4.6 9.2 – 11.50
47.8 37 20 35 40.2 26.5 32 48.56
       This work
Fig.10. Schematic representation of the shielding mechanism. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)
nanoparticles and ZnO nanorods on the graphene sheets. With the incorporation of CFO nanoparticles, aerogel G-50 can shield up to ∼99.994% power of the incident EM waves shared by ∼12.997% reﬂection and ∼86.997% absorption while with the incorporation of ZnO nanorods, aerogel G-01 can shield up to 99.845% power of the incident EM waves shared by ∼37.803% reﬂection and ∼62.042% absorption. Beneﬁtting from dielectric ZnO nanorods and magnetic CFO nanoparticles, aerogel G-51 can shield up to ∼99.998% power of the incident EM waves shared by ∼7.074% reﬂection and ∼92.924% absorption. Aerogel G-51 has shown average absorption of 93.655% in the entire X-band. Clearly, aerogels have shown the absorption dominant shielding mechanism. Hence, aerogels can be considered suitable for stealth technology as radar absorbing material. Moreover, aerogels can also be used as shielding material in commercial and scientiﬁc electronics, antenna systems, medical devices, space exploration and military electronic devices.
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