Absorption-Dominated Electromagnetic Wave Suppressor Derived

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Absorption-Dominated Electromagnetic Wave Suppressor Derived from Ferrite-Doped Cross-Linked Graphene Framework and Conducting Carbon Sourav Biswas,† Injamamul Arief,‡ Sujit Sankar Panja,*,† and Suryasarathi Bose*,‡ †

Department of Chemistry, National Institute of Technology, Durgapur, WB India 713209 Department of Materials Engineering, Indian Institute of Science, Bangalore, India 560012

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S Supporting Information *

ABSTRACT: To minimize electromagnetic (EM) pollution, two key parameters, namely, intrinsic wave impedance matching and intense absorption of incoming EM radiation, must satisfy the utmost requirements. To target these requirements, soft conducting composites consisting of binary blends of polycarbonate (PC) and poly(vinylidene fluoride) (PVDF) were designed with doped multiwalled carbon nanotubes (MWCNTs) and a three-dimensional cross-linked graphene oxide (GO) framework doped with ferrite nanoparticles. The doping of α-MnO2 onto the MWCNTs ensured intrinsic wave impedance matching in addition to providing conducting pathways, and the ferrite-doped cross-linked GO facilitated the enhanced attenuation of the incoming EM radiation. This unique combination of magnetodielectric coupling led to a very high electromagnetic shielding efficiency (SE) of −37 dB at 18 GHz, dominated by absorption-driven shielding. The promising results from the composites further motivated us to rationally stack individual composites into a multilayer architecture following an absorption−multiple reflection−absorption pathway. This resulted in an impressive SE of −57 dB for a thin shield of 0.9-mm thickness. Such a high SE indicates >99.999% attenuation of the incoming EM radiation, which, together with the improvement in structural properties, validates the potential of these materials in terms of applications in cost-effective and tunable solutions. KEYWORDS: Polymer blends, Nanomaterials, Mechanical properties, Conductivity, EMI shielding

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INTRODUCTION Rampant use of electronic and communication devices emitting electromagnetic (EM) radiation as a byproduct invites possible repercussions in terms of EM radiation pollution. This trend often leads to inevitable issues in compromising device performances and human health.1−3 Thus, curbing the EM radiation has become a paramount factor in product design. Metals, despite being the most common EM shielding materials, suffer from drawbacks such as high density, poor corrosion resistance, and cumbersome processing. As a prerequisite to obtaining higher shielding effectiveness of material systems without metallic shields, it is necessary to investigate carbonaceous nanomaterials that, when blended with polymers, can be used ti fabricate lightweight and tough polymer-based shielding materials.4−10 Carbon nanotubes (CNTs) and graphene-based fillers are particularly favored because of their high aspect ratios and skin effects, which refer to a high-frequency bandwidth.10−21 It must be noted that the shielding effectiveness (SE) of such conductive materials largely comes from the reflection mechanism, although the reflected EM waves are still undesirable because of potential secondary pollution.22 EM absorbers, on the other hand, can provide a © 2016 American Chemical Society

more viable alternative to conventional reflection-based interfaces by converting the EM energy into thermal energy and dissipating it through the surface. The absorption phenomenon can be correlated with the magnetic and dielectric properties of the nanofillers.22−34 To reproduce desirable shielding efficiencies, the coupling of both conductivity and magnetodielectric loss has to be taken into consideration.35−39 Moreover, the methodology of incorporating particles into the polymer matrix, the matrix−filler interaction, the amount of particles inside the matrix, and the overall morphological structure are also crucial in the design of new high-performance lightweight EM shield materials. In light of the above-mentioned facts, we concentrate on the recent trend toward improving both the conductive and consolidated loss parts of shielding materials utilizing selectively filled binary immiscible polymer blends as model systems. Conventionally, conducting multiwalled carbon nanotubes (MWCNTs) with a very large impedance mismatch are directly Received: November 19, 2016 Accepted: December 30, 2016 Published: December 30, 2016 3030

DOI: 10.1021/acsami.6b14853 ACS Appl. Mater. Interfaces 2017, 9, 3030−3039

Research Article

ACS Applied Materials & Interfaces Scheme 1. Synthesis of MWCNT−MnO2 Nanoparticles

Scheme 2. Synthesis of MDA-Modified Cross-Linked r-GO/Fe Nanoparticles

properties and exfoliating the component of the model polymer blend system, thus lowering the impedance mismatch for new improved absorption-driven attenuation of incident radiation. The designed screening material with hybrid nanostructured systems was found to demonstrate ca. 92% absorption of the total incident EM waves, whereas MWCNTs alone exhibited ca. 75% reflection. The remarkable wave attenuation of individual soft composites further led us to design a multilayer assembly (of 0.9 mm) following a rational arrangement that resulted in an exceptional SE when compared with those of the individual layers.

incorporated into the matrix system to construct a conducting framework. We observed an enhancement of the SE by absorption merely by lowering the mismatch and broadening the interaction of the incident EM waves with the shield surface. Chemical modification of the MWCNT surface and doping with dielectric nanomaterials such as α-MnO2 readily enhance the dielectric losses. α-MnO2 was chosen as a dielectric dopant because of its high dielectric loss in the gighertz frequency range.40,41 Although α-MnO2 nanoparticles are antiferromagnetic, a recent study revealed that they contribute positively toward inflicting significant permeability loss in the higher frequency range.42 The magnetic permeability loss is further augmented following the incorporation of cross-linked reduced graphene oxide (r-GO)/Fe nanostructures. Numerous studies have revealed that the decoration of magnetic nanoparticles onto graphene sheets has a boosting effect on the total SE of the polymer matrix.35−37 Nevertheless, we dealt simultaneously with both dielectric polarization losses and eddy current losses by incorporating cross-linked magneto-doped reduced graphene oxide sheets. We were able to blend different types of nanostructured materials in a more coherent way by chemically conjugating them with characteristic material

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EXPERIMENTAL SECTION

Materials. Poly(vinylidene fluoride) (PVDF; Kynar-761, MW = 440 000 g/mol) was provided by Arkema. Polycarbonate (PC; Lexan143R) was obtained from Sabic (MFI 11 g/10 min). Pristine MWCNTs (length = 1.5 μm, diameter = 9.5 nm) were procured from Nanocyl SA (Sambreville, Belgium). FeCl3, iron sulfate heptahydrate (FeSO4·7H2O), manganese sulfate (MnSO4), and Larginine were obtained from Sigma-Aldrich. Analytical grades of chloroform, 28% ammonia solution, ethanol, N,N-dimethylformamide (DMF), graphite flakes, H2SO4, H3PO4, and tetrahydrofuran were obtained from commercial sources. 3031

DOI: 10.1021/acsami.6b14853 ACS Appl. Mater. Interfaces 2017, 9, 3030−3039

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Figure 1. TEM images of (A) MWCNTs, (B) MWCNT−MnO2 nanoparticles (inset: magnified TEM image), (C) GO, and (D) cross-linked r-GO/ Fe. (E) Elemental mapping of cross-linked r-GO/Fe. Characterization. Transmission electron microscopy (TEM) and high-angle annular dark-field (HAADF) images were obtained on an FEI Technai F30 instrument operated at an accelerating voltage of 300 kV. Energy-dispersive analysis by X-rays (EDAX) mapping was performed using the same instrument. A Sirion XL30 field-emission gun (FEG) scanning electron microscope at an accelerating voltage of 10 kV was utilized to determine the morphology of the PC/PVDF blends. A Perkin-Elmer GX FT-IR instrument was used to obtain FTIR spectra. The magnetic properties of the synthesized nanoparticles were assessed using a Lakeshore vibratory sample magnetometer (VSM) with an applied force ranging from −8000 to 8000 Oe at room temperature. Raman spectra were recorded using a LabRam HR (UV) system. X-ray diffraction (XRD) was recorded using an X'Pert Pro system from PANalytical. A Cu Kα radiation source (l = 1.5406 Å, 40 kV, and 30 mA) was used to determine the XRD profiles of different nanoparticles. Room-temperature electrical conductivities of the blends were measured using an Alpha-N analyzer from Novocontrol (Montabaur, Germany) in the frequency range from 0 0.1 Hz to 10 MHz. A universal testing machine (UTM) was used to assess the mechanical properties of different blends, at a cross-head speed of 5 mm min−1. Before the estimation, dumbbell-shaped specimens were formed with a compression molding machine. EM interference shielding was measured on an Anritsu MS4642A vector network analyzer (VNA). A Damaskos MT-07 coaxial connector and a KEYCOM waveguide were used to measure the shielding efficiency of a multilayer architecture. The multilayer assembly was prepared by the laboratory-scale hot pressing of thin layers at 155 °C and a pressure of 2 bar. Synthesis of MWCNT−MnO2 Nanoparticles. The synthesis of MWCNT−MnO2 nanoparticles involved a two-step procedure. Initially, carboxyl acid-functionalized MWCNTs were prepared by harsh acid treatment with HNO3.43 The dried acid-functionalized MWCNTs (100 mg) were redispersed in water (100 mL) by bath sonication. Then, manganese sulfate (1 mmol) was added, and the solution was kept stirring for nearly 10 min in an ice bath. Meanwhile, L-arginine (20 mmol) was taken in a separate beaker and dissolved in 40 mL of deionized (DI) water. The L-arginine solution was then added dropwise to the precursor containing manganese sulfate and MWCNTs. The resulting solution was held at a constant temperature of 10 °C and stirred for 3 h. Afterward, the solution was centrifuged

and washed with ethanol. The obtained hybrid particles were dried in an oven overnight at 80 °C. This synthetic procedure is illustrated in Scheme 1. Synthesis of MDA-Modified r-GO/Fe. The synthesis of MDAmodified r-GO/Fe consisted of two steps. Initially, MDA molecules were attached to the GO surface, as reported in our previous work,44 by refluxing GO with MDA in DMF solution at 105 °C for 24 h. The resulting dry powder of MDA−GO (20 mg) was redispersed in 20 mL of DI water. At the same time, a 2:1 FeCl3/FeSO4 mixture was was taken in a separate beaker and dissolved in 10 mL of water. These two solutions were then mixed together and transferred into a Teflon-lined steel autoclave, to which 5 mL of 28% ammonia was then added. The autoclave was kept at 90 °C for 6 h. Finally, the resulting mixture was centrifuged, washed several times with water, and then vacuum-dried at 80 °C. The detailed procedure is schematically represented in Scheme 2. Preparation of Blends. All of the blends with different nanoparticles were prepared using a HAAKE MiniLab II melt compounder under a nitrogen atmosphere at a temperature of 260 °C and a rotational speed of 60 rpm. Generally, when all of the nanoparticles were mixed together during the melt-mixing, because of the lowering of the polarity mismatch, the nanoparticles were preferentially localized in the PVDF phase. All of the blends contained 3 wt % MWCNTs and 5 wt % r-GO/Fe nanoparticles.

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RESULTS AND DISCUSSION Characterization of Different Synthesized Nanoparticles. The morphologies and structures of α-MnO2 doped MWCNTs and MDA-cross-linked GO/Fe were extensively characterized by transmission electron microscopy (TEM), as shown in Figure 1. The anchoring of the α-MnO 2 nanostructures onto the defect sites of the MWCNTs is clearly demonstrated in the TEM images, whereas the pristine MWCNTs exhibited smooth outer wall surfaces (Figure 1A,B). We estimated the weight fraction of MnO2 nanoparticles attached to the MWCNTs to be nearly 20 wt %, as inferred from thermogravimetric analysis (TGA; Figure S1A). Moreover, FT-IR spectra of both the pristine MWCNTs and the α3032

DOI: 10.1021/acsami.6b14853 ACS Appl. Mater. Interfaces 2017, 9, 3030−3039

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also confirmed the formation of a three-dimensional crosslinked structure (Figure S3). The formation mechanism of MDA−GO and subsequent adhesion of Fe onto the crosslinked GO networks were investigated by X-ray photoelectron spectroscopy (XPS), as shown in Figure S4. The N 1s peaks appearing in MDA−GO and MDA-cross-linked r-GO/Fe indicate N-doped cross-linked structures, whereas the C 1s spectrum of r-GO/Fe further supports our claim that epoxy groups exclusively hosted the incoming Fe on the MDA-crosslinked GO basal planes. Characterization of Blend Structures and Preferential Localization of Nanoparticles. A lack of specific interactions between two immiscible polymer components generally results in a heterogeneous morphology. Our previous study revealed that a cocontinuous type of morphology was obtained when these two polymers were mixed in a 50:50 weight ratio.47 It is indeed an interesting finding that all of the nanoparticles were selectively localized in PVDF because of polarity mismatch, despite the fact that the PC component is thermodynamically more favorable.48 With inputs from SEM, a cocontinuous type of morphology was observed to be retained even after the addition of different nanoparticles (Figure 2A,B). For SEM, the other component (PC) was selectively etched out for better spatial resolution. In Figure 2C, the image of a cryofractured sample without etching is shown. The smooth PC component indicates the absence of nanoparticles, whereas the PVDF exhibits a coarse surface morphology as a result of the selective localization of nanoparticles. The interfacial adhesion between the constituent polymers is a prime factor governing the mechanical properties of blend systems.49−51 Therefore, the coarse morphology inevitably leads to catastrophic failure because of the poor stress transfer at the interface. However, the tensile properties and Young’s modulus were significantly improved following the addition of mechanically strong filler nanoparticles, whereas the elongation properties were compromised. The overall mechanical properties of the individual blends are plotted in Figure S5. A high-resolution SEM image confirmed the preferential localization of nanoparticles in PVDF (Figure 2D). Additionally, EDAX analyses of cryofractured PC-etched samples confirmed the presence of all nanoparticles in the PVDF phase (Figure 2E). The restricted migration of the nanoparticles was further studied by the dissolution of a particular phase in a selective solvent. The clear solution obtained after the extraction of PC by chloroform clearly ruled out any migration of nanoparticles into the PC component, whereas the PVDF extraction resulted in a blackcolored solution in DMF (inset of Figure 2E). Furthermore, crystallization behavior is known to be one of the key factors dictating the structural performance of blend structures, more specifically when one of the components of the blend (PVDF, in this case) is a semicrystalline polymer. The presence of any external rigid nanoparticles and their distribution inside the blend can essentially affect the crystallization behavior.52,53 The crystallization temperature was studied in this work by differential scanning calorimetry (DSC), suggesting that the preferential localization of the nanoparticles in the PVDF influenced the crystallization temperature (Figure S6). Electromagnetic Shielding Properties of Various Blend Structures. The extent of the attenuation of incident EM radiation is manifested in terms of the total shielding effectiveness (SET) in the X and Ku frequency bands, which is expressed in dB. The total shielding effectiveness (SE) consists of the contributions of three different scattering mechanisms,

MnO2-bound MWCNTs further demonstrated the possible adhesion of α-MnO2 onto the MWCNT walls (Figure S1B). The band at 1707 cm−1 corresponds to the CO stretching frequency of the carboxyl acid groups formed after the acid treatment of the MWCNTs. The band at 527 cm−1 in the MnO2-bound MWCNTs indicates MnO stretching. From Xray diffraction studies, we concluded that the attached nanoparticles were mainly α-MnO2 (JCPDS card 44-0141) ( Figure S1C). Prior to the attachment of α-MnO2, the MWCNTs were subjected to a strong acid treatment to create defects on the side walls of the MWCNTs, which, in turn, increased the intensity ratio of the D-band peak to the G-band peak (ID/IG) in the Raman spectrum from 1.24 for the pristine MWCNTs to 1.30 for MWCNT−MnO2 (Figure S1D). GO, on the other hand, exhibited a planar sheetlike morphology prior to undergoing any modification, whereas postmodification, significant coarsening of the otherwise planar GO surface was observed, as evident from the TEM images (Figure 1C−E). This can be attributed to the attachment of Fe3O4 nanoparticles onto the surface and gallery spaces of the MDA-cross-linked GO network. The corresponding elemental mapping analysis of the MDA-cross-linked GO/Fe indicated a nearly uniform elemental distribution of Fe across the GO sheets (Figure 1E). For the MDA-cross-linked r-GO/Fe, the concentration of attached Fe3O4 was nearly 65 wt %, as confirmed by TGA (Figure S2A). FT-IR spectra further confirmed the presence of Fe on the cross-linked MDA−GO network (Figure S2B). Typically, the peaks of GO correspond to the stretching frequencies of hydroxyl (OH), carboxyl (C O), aromatic (CC), carboxy (CO), alkoxy (CO), and epoxy (COC) groups at 3336, 1727, 1620, 1385, 1224, and 1080 cm−1, respectively, as shown. The absence of the major peaks corresponding to O-containing functional groups in MDA-cross-linked r-GO/Fe confirmed the reduction of GO, whereas the presence of N−H bending peak at 1552 cm−1 was assigned to cross-linked MDA−GO network. The X-ray diffraction patterns of the unmodified GO and MDA-r-GO/ Fe are shown in Figure S2C. The characteristic peaks correspond to (001) and (100) are associated with GO.45 The broad (002) peak for MDA-cross-linked r-GO/Fe sheets signifies the removal of functional groups from GO and the formation of reduced GO. The appearance of peaks associated with Fe 3O4 were indexed and matched with standard data (JCPDS card 00-001-1111), further hinting at possible anchoring of Fe3O4 onto the MDA−GO networks. We also obtained Raman spectra for MDA−GO and r-GO/Fe (Figure S2D). The shifting of the D and G bands in the GO and MDAcross-linked r-GO/Fe provides information in terms of intercalation and subsequent anchoring of metal nanoparticles.46 The I D/IG ratio increased with the extent of GO reduction [GO (0.92) < MDA−GO/Fe (1.02)]. This is as expected given that the magnitude of the ID/IG ratio is proportional to the extent of GO reduction and is decreased with defects. For efficient EM absorption, the initial permeability is expected to be as high as possible. Therefore, magnetic inclusions on a nonmagnetic matrix serve as a means to introduce permeability into the system. Magnetization studies of MDA-cross-linked GO/Fe nanostructures were performed in a vibrating sample magnetometer (VSM) at room temperature, as shown in Figure S2E. For MDA-crosslinked GO/Fe, the saturation magnetization and coercivity were observed to be ca. 21 emu g−1 and 116 Oe, respectively. The scanning electron microscopy (SEM) image of r-GO/Fe 3033

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namely, reflection relative to the surface of the absorber, absorption of the incident energy, and multiple internal reflection. However, multiple internal reflection can be ignored if the total SE is greater than 10 dB.3 Theoretically, SET can be estimated from the ratio of the incident and transmitted electric field intensities. We estimated SET by using scattering parameters obtained directly from the vector network analyzer (VNA). Figure 3A shows the frequency-dependent total shielding efficiencies of various blends in the range of 8−18 GHz, whereas Figure S7 shows the total shielding effectiveness values of various blends between 2 and 8 GHz. A considerable shielding efficiency was detected upon the addition of MWCNTs, whereas the neat blends were transparent to EM radiation. Preferential localization of the MWCNTs in the energetically favored component of the immiscible polymer blend led to a significant enhancement in the ac electrical conductivity (Figure S8A). The perceived changes in ac conductivity can be attributed to the formation of interconnected conducting networks within the polymer matrix. It is noteworthy that the double percolation effect also helps enhance the ac electrical conductivities of the blend systems.54 The presence of an insulating polymer between MWCNT networks enhances the energy barrier and facilitates the electron-hopping mechanism for charge transport. We based this conclusion on the power-law fitting of the ac conductivity data.54 The exponent n is a measure of the charge-transport mechanism in a three-dimensional network (Figure S8B). Such hopping electrons could enhance the microcurrent in the MWCNT network when the concentration of fillers in the blend is sufficient. Wen et al.55 suggested that this microcurrent network also has a very positive effect on EM shielding effectiveness. Therefore, materials should have adequate electrical conductivity and good network connectivity to interact with the EM radiation. However, after the incorporation of α-MnO2 and cross-linked r-GO/Fe nanoparticles along with the MWCNTs, we observed a change in the shielding mechanism (Figure 3B,C). Generally, any chemical modification of the MWCNT surface can affect the πconjugation and, in turn, compromise the intrinsic conductivity of the MWCNTs themselves. In this work, we deliberately applied a harsh acid treatment to create defects on the MWCNT surface where α-MnO2 could easily be anchored. In fact, the conductivity of the acid-treated MWCNTs was lower than that of the pristine MWCNTs. By reducing the

Figure 2. SEM images of (A) PC/PVDF with MWCNT (PC component etched out by chloroform), (B) PC/PVDF with MWCNT−MnO2 and r-GO/Fe (PC component etched out by chloroform), and (C) PC/PVDF with MWCNT−MnO2 and r-GO/Fe (without etching). (D) High-resolution SEM image of PC/PVDF with MWCNT−MnO2 and r-GO/Fe, where the PC component was etched out by chloroform (arrows indicate the preferential localization of the MWCNTs at the PVDF component). (E) EDAX spectrum of cryofractured PC-etched PC/PVDF with MWCNT−MnO2 and crosslinked r-GO/Fe sample, confirming the localization of the nanoparticles at the PVDF. Inset: Solution dissolution test of PC/PVDF with MWCNT−MnO2 and cross-linked r-GO/Fe, including (i) a clear solution after PC extraction by chloroform confirming the absence of migration of any nanoparticles at that component and (ii) a blackcolored solution of PVDF extraction by DMF confirming the preferential localization of the nanoparticles at that component.

Figure 3. (A) Total shielding effectiveness, (B) total shielding by absorption, and (C) percentages of the absorption and reflection components of total shielding. 3034

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Figure 4. (A) Consolidated tangent loss parameters and (B) attenuation constants of various samples. (C) Thickness-oriented total shielding effectiveness values of blends containing MWCNTs and MWCNT−MnO2 + cross-linked r-GO/Fe.

permeability. The magnetic losses in doped substrates mainly originate from magnetic hysteresis, magnetic resonance, domain-wall displacement, and eddy current effects.57 The contribution of hysteresis loss is insignificant, given that the magnetic field of the incident EM waves is very weak, and the domain-wall resonance usually occurs at much lower frequency (megahertz).57 Therefore, natural ferromagnetic resonance and eddy current effects are generally taken as the principal loss mechanisms in the gigahertz range for ferromagnetic absorbers. The eddy current loss can be expressed as μ″/f(μ′)2 = 2πμ0d2σ.58 From our experimental results, we observed that the eddy current losses were almost independent of the operating frequency (Figure S10), which clearly indicates that the observed magnetic permeability losses were mainly caused by eddy current effects.59 As dissipated energy is associated with the loss parameters, we evaluated the consolidated loss parameters by evaluating the dielectric tangent loss (tan δε = ε″/ε′) and the magnetic tangent loss (tan δμ = μ″/μ′). We found that, after the addition of MWCNT−MnO2 and cross-linked r-GO/Fe nanoparticles to the blends, the total losses increased (Figure 4A). As discussed previously, the main contributions to the the total dielectric losses were from interfacial polarization and conductivity losses. Initially, the charge-transport mechanism of electron hopping generates conductivity losses. However, further oxidation of the MWCNT surface leads to enhanced conductivity losses, whereas the addition of a high-dielectricconstant material such as α-MnO2 at the defect sites of the MWCNTs ameliorates the dielectric losses due to the presence of more lossy materials.42 The interfacial polarization of the MWCNT−MnO2 and MWCNTs + cross-linked r-GO/Fe network further ensured positive contributions to the dielectric losses. It is important to note that the cross-linked r-GO/Fe nanoparticles acted as a magnetodielectric material. Thus, the interfacial polarizations between the cross-linked networks also appear to have enhanced the dielectric losses. Moreover, the eddy current losses helped to generate significant magnetic losses, contributing to the total loss as both the MWCNT− MnO2 and cross-linked r-GO/Fe nanoparticles were present in the matrix. Consequently, when the external electromagnetic field encountered the designed material, the EM waves come across a variety of microscopic boundaries owing to the inclusions that constitute the heterostructure. Furthermore, subsequent variations in the local field can have considerable effects on the absorption of energy at such heterogeneous

conductivity, we were able to maximize the interaction of the incident EM waves with the shield surface by reducing the reflection properties. Interestingly, after the addition of the αMnO2-doped MWCNTs, the blend conductivity was unaltered, whereas the SE increased greatly following the alteration in the overall shield mechanism. Further addition of cross-linked rGO/Fe nanoparticles also enhanced the total shielding effectiveness (SET). As stated earlier, reflection is regarded as a primary mechanism of shielding when high conductivity and the dielectric constant are of primary concern.1 Moreover, the reflection of incident EM waves originates from the surface of the material as a result of impedance mismatching. On the other hand, absorption mainly arises as a result of energy losses and subsequent dissipation as heat. Theoretical findings further reinforce our understanding that, when the total power of a harmonic electromagnetic field with a sufficient angular frequency encounters the surface of a material by entering a volume through the surface, it contributes partly to increasing the field energy stored inside the volume, whereas the rest is dissipated as heat.56 The losses during attenuation originate mainly from the dielectric and magnetic losses, which can be evaluated from the scattering parameters as the relative complex permittivity (εr) and permeability (μr) parameters, where εr = ε′ − iε″ and μr = μ′ − iμ″. These complex permittivity and permeability parameters are frequency-dependent, and the real parts mainly represent the storage ability, whereas the imaginary parts correspond to the losses (Figure S9).56 The dielectric loss is primarily attributable to conductivity and polarization losses. The polarization loss can be further resolved into ionic polarization, electronic polarization, dipole orientation polarization and, interfacial polarization.57 The interconnected network of MWCNTs inside the polymer matrix beneficially transports electrons through hopping and creates a dip in conductivity. Ionic and electronic polarizations can be excluded because they are generated in the infrared range. The energy consumption is also negligible in terms of dielectric orientation polarization. Therefore, we conclude that the dielectric losses arise mainly as a result of interfacial polarization and conductivity losses. Magnetic permeability, on the other hand, is attributable to the incorporation of cross-linked r-GO/Fe nanoparticles into the blend systems. A high initial permeability also predicts high magnetic losses in the materials.57 An increased saturation magnetization (Ms) is generally favorable for higher initial 3035

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Figure 5. Mechanisms of shielding with blends containing (A) MWCNTs and (B) MWCNT−MnO2 + cross-linked r-GO/Fe nanoparticles. (C) Total shielding efficiency of a multilayer assembly where MWCNT−MnO2 + cross-linked r-GO/Fe nanoparticles are contained in the blends in the top and bottom layers and only MWCNTs are contained in the blend in the middle layer. (D) Mechanism of shielding of the multilayer assembly.

reflection. Impedance mismatching is the key factor for the reflection of incident EM waves from the surface of the MWCNT-based blend. To achieve zero reflection at the front surface of the shield material, the c-haracteristic impedance of the shield should be equal or close to that of free space.58,60 The characteristic impedance is always determined by the relationship between the real parts of the complex permittivity and permeability parameters. If the real part of the complex permittivity is much higher than the real part of the complex permeability, then most of the incident electromagnetic waves will be reflected off the surface because of a low surface resistance rather than penetrating into the shield. Therefore, we calculated the ratio of real parts of the permittivity and permeability parameters, which should be close to 1 for better impedance matching. From Figure S13, it is evident that MWCNT-containing blends showed higher ratios and, thus, their effective microwave absorption was hindered by the poorly matched intrinsic characteristic impedance. Conversely, the gap between the real parts of the complex parameters became narrower, which led to impedance matching, thereby enhancing the absorption efficiency when both of the nanomaterials, namely, the MWCNT−MnO2 and cross-linked r-GO/Fe, were present in PVDF. However, the thickness of a shield material is also an important parameter.3 EM attenuation can be correlated with the skin depth of the individual material and increases with increasing thickness. Skin depth is the intensity of penetration, which is inversely related to the absorption efficiency of the screening material. It was found that the skin depth decreased readily from 4.9 mm when only MWCNTs were present to 1.3 mm after the incorporation of cross-linked r-GO/Fe nanoparticles along with the MWCNTs,

boundaries because absorption depends quadratically on the electric field intensity.56 Because of Maxwell−Wagner−Siller polarization, virtual charges accumulate at the interface of the two different materials, and the whole energy is dissipated through Joule heating.56 It is worth noting that the thermal conductivity also increased after the addition of heterogeneous nanoparticles to the blend structure. Thus, the generated heat was easily distributed through the surface by the highly conducting network structure (Figure S11). It is interesting to note that the thermal conductivities of the blends gradually increased from 0.16 to 0.56 W m−1 K−1 upon the incorporation of the MWCNTs and cross-linked r-GO/Fe nanoparticles, as this is an important parameter that is essential for the design of efficient shielding devices. Therefore, we observed that, through the synergetic contributions of both dielectric and magnetic components (Figure S12), the shielding mechanism can be altered from a reflection-driven process toward an absorptiondominated process through the dissipation of heat energy throughout the surface of the substrates. In addition to the aforementioned tangent loss parameters, the attenuation constant and impedance matching capabilities are also important factors that enhance the absorption abilities of materials.57 It was found that the attenuation constant increased significantly following the addition of α-MnO2 and cross-linked r-GO/Fe nanoparticles as a result of the increased permittivity and permeability parameters (Figure 4B). Terefore, the addition of heterogeneous nanoparticles to the blends simultaneously enhanced the loss parameters and attenuation constant, which are the main reasons for the attenuation of EM waves predominantly through absorption, whereas the MWCNT-exclusive blends could provide screening only by 3036

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ACS Applied Materials & Interfaces Table 1. Total Shielding Effectiveness Values of Various Blends with Different Thicknesses composition

thickness (mm)

SET (dB)

0.9 0.9 0.9 0.3 0.9 0.3 0.9 0.3 0.9 0.3 0.9 0.9

−8 −7 −16 −10 −11 −7 −21 −15 −36 −29 −57 −35

PVDF with 3 wt % MWCNTs PC with 3 wt % MWCNTs PC/PVDF with 3 wt % MWCNTs PC/PVDF with 3 wt % MWCNT−COOH PC/PVDF with 3 wt % MWCNT−MnO2 PC/PVDF with 3 wt % MWCNT−MnO2 + r-GO/Fe multilayer assembly 1a multilayer assembly 2b a

PVDF with 3 wt % MWCNT−MnO2 + r-GO/Fe in the top and bottom layers and PC/PVDF with 3 wt % MWCNT in the middle layer. bPVDF with 3 wt % MWCNT−MnO2 + r-GO/Fe in the middle layer and PC/PVDF with 3 wt % MWCNT in the top and bottom layers.

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CONCLUSIONS In summary, we have demonstrated that the preferential localization of different nanomaterials in a selective component of a model immiscible polymer system readily enhances the absorption-driven shielding efficiency as a result of better impedance matching. The higher consolidated loss parameters and attenuation constant also corroborated higher EM absorption following the doping of magnetic and dielectric nanoparticles with carbonaceous nanomaterials. Consequently, a sequential stack of individual layers with different shielding mechanisms was found to provide an astonishing SET of −57 dB following an absorber−multiple reflector−absorber pathway.

indicating a boost in the EM attenuation properties. When we enhanced the shield thickness, SET no longer changed after the latter skin depth was reached. Figure 4C clearly chows the enhancement in shielding effectiveness with thickness at 18 GHz, and the two-dimensional surface contour maps in Figure S14 show the measured changes in SET with thickness in the 8−18 GHz frequency range. To tune EM attenuating devices for a wide range of applications in electronics and telecommunications, individual shielding materials with different shielding mechanisms are used to construct multilayer assemblies of various polymer blends (Figure 5A,B). In this work, the blends with heterogeneous nanoparticles were chosen as the outer layers of a multilayer assembly to maximize the interaction of the EM waves with the surface of the material, which finally resulted in an astonishing 92% absorption of incident EM waves (Figure 5B). As discussed earlier, this resultant shielding efficiency is mainly due to the synergetic effects of both permittivity and permeability parameters, further supported by better impedance matching and a higher attenuation constant. However, the EM waves penetrating from both outer layers are now encountered in the middle layer of the assembly, which is globally conductive in nature. As a result, the majority of penetrating waves are again reflected back and interact with the outer layers of the multilayer assembly because of the impedance mismatching with the middle layer. Consequently, most of the penetrating waves are again absorbed by the outer layers of the assembly. It should be noted that, when different dielectric materials are used, a small amount of virtual charge can also accumulate at the interfacial position of the two layers that exerts a positive effect on the shielding effectiveness. Finally, the waves penetrating from the middle layer again interact with the absorber and maximize the attenuation up to >99.999% of the incident EM waves (Figure 5C). Simply by changing the layer position while maintaining the thickness, we were also able to evaluate the shielding performances of other possibilities, which are listed in Table 1; however, the absorber−reflector−absorber assembly was found to exhibit better shielding effectiveness than other constructions. A schematic representation of the multilayer assembly is presented in Figure 5D, and Figure S15 includes optical images of the construction process and an SEM image of the multilayer assembly.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b14853. TGA, FT-IR, XRD, and Raman analyses of MWCNTs and MWCNT−MnO2 nanoparticles; TG, FT-IR, XRD, Raman, and VSM analyses of GO and r-GO/Fe; SEM micrograph of 3D cross-linked r-GO/Fe; XPS spectra of DO, GO-MDA, and r-GO/Fe; mechanical properties of various blend structures; DSC crystallization temperatures of various blends; total shielding effectiveness values of various blends at frequencies of 2−8 GHz; conductivities of various blends; complex permittivity and permeability parameters of various blends; eddy current loss as a function of frequency; thermal conductivities of various blends; synergetic effects of permittivity and permeability parameters; ratio of real permittivity and permeability as a function of frequency; 2D contour plot of various blends; and optical and SEM images of a multilayer assembly (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Suryasarathi Bose: 0000-0001-8043-9192 Notes

The authors declare no competing financial interest. 3037

DOI: 10.1021/acsami.6b14853 ACS Appl. Mater. Interfaces 2017, 9, 3030−3039

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ACS Applied Materials & Interfaces

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from DST and INSA (India).

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