Hybrid composite mats composed of amorphous carbon, zinc oxide nanorods and nickel zinc ferrite for tunable electromagnetic interference shielding

Hybrid composite mats composed of amorphous carbon, zinc oxide nanorods and nickel zinc ferrite for tunable electromagnetic interference shielding

Accepted Manuscript Hybrid composite mats composed of amorphous carbon, zinc oxide nanorods and nickel zinc ferrite for tunable electromagnetic interf...

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Accepted Manuscript Hybrid composite mats composed of amorphous carbon, zinc oxide nanorods and nickel zinc ferrite for tunable electromagnetic interference shielding Shivam Gupta, Ching Chang, Chih-Huang Lai, Nyan-Hwa Tai PII:

S1359-8368(18)31196-X

DOI:

https://doi.org/10.1016/j.compositesb.2019.01.060

Reference:

JCOMB 6542

To appear in:

Composites Part B

Received Date: 18 April 2018 Revised Date:

12 December 2018

Accepted Date: 16 January 2019

Please cite this article as: Gupta S, Chang C, Lai C-H, Tai N-H, Hybrid composite mats composed of amorphous carbon, zinc oxide nanorods and nickel zinc ferrite for tunable electromagnetic interference shielding, Composites Part B (2019), doi: https://doi.org/10.1016/j.compositesb.2019.01.060. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Hybrid composite mats composed of amorphous carbon, zinc oxide nanorods and nickel zinc ferrite for tunable electromagnetic interference shielding Shivam Gupta, Ching Chang, Chih-Huang Lai and Nyan-Hwa Tai ⃰

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Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan 300, ROC

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*E-mail address: [email protected] (Nyan-Hwa Tai)

Abstract

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With the rapid growth of electronics and telecommunication industries, electromagnetic pollution is a serious concern to be addressed because it not only affects the sensitivity and performance of the devices but also affects human’s health. Here, we report lightweight hybrid composite mats, having porosity around 40%, composed of amorphous carbon, zinc oxide nanorods and nickel zinc ferrite for excellent electromagnetic interference (EMI) shielding in the X-band (8.2-12.4 GHz). The vibrating sample magnetometer measurement confirmed that the

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saturation magnetization value (Ms) of the composite materials enhances with the weight percentage of zinc oxide nanorods-nickel zinc ferrite (ZNF) powder, which leads to enhanced magnetic loss of the electromagnetic waves. With the thickness of 1.0 mm, the total EMI shielding effectiveness of the amorphous carbon composite was measured to be 25.70 dB which

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was further enhanced to 53 dB by the incorporation of the ZNF powder. Such high increment is attributed to the enhanced magnetic properties, interfacial polarization and dielectric properties

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of the composite. The synergistic combination of the materials results in the high reflection coefficient and absorption coefficient of the composites which were measured to be ~0.916 and ~0.083, respectively. Thus, the composites can shield up to 99.999% power of the electromagnetic waves which is shared by the 8.394% reflection and 91.605% absorption. Moreover, the magnetic, electrical and EMI shielding properties of the composites can be tuned by controlling the amount of ZNF powder in composites. Hence, the composite mats can be suitable for applications in defense and telecommunication. Keywords: A. Nano-structures; A. Hybrid; B. Magnetic properties; B. Electrical properties; EMI shielding

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1. Introduction: Owing to the rapid growth of the electronic and telecommunication industries, increasing usage of electromagnetic devices leads to serious electromagnetic pollution. Electromagnetic pollution

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is a serious concern to be addressed because it not only disturbs the functioning of electromagnetic devices, but also affects human’s health and may lead to serious disease such as leukemia [1] and brain tumor [2]. The metal sheets are considered to be the most readily available material for electromagnetic interference (EMI) shielding, however, their heavy

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weight, low flexibility, high cost, prone to oxidation and processing difficulty limit their use in practical applications [3]. The metal coatings such as electroplating, spraying, conductive paints and vacuum coatings were developed for EMI shielding, but their poor wear and environmental

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resistance make them non-preferential for practical applications [4, 5]. Intrinsically conducting polymers have also been widely developed for EMI shielding because of their advantage of absorption dominant shielding over metals which provides reflection dominant shielding due to their shallow skin depth [6-8]. However, owing to low aspect ratio of intrinsically conducting polymers, high concentration is required to achieve desired electrical and EMI shielding properties which often results in poor mechanical and thermal properties. Therefore, there is a

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need of the hour to develop and design a promising EMI shielding material. When the electromagnetic (EM) waves impinge on the shielding materials, reflection, absorption, multiple reflection and transmission of the waves simultaneously occurs [9]. For

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reflection of the EM waves, the shielding materials must have mobile charge carriers, however, high conductivity is not required, but connectivity of the path for mobile charge carriers is

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important, this is why percolation threshold plays an important role in the case of polymer composites containing conducting fillers. The absorption of the EM waves occurs attributing to the electric or magnetic dipoles present in the shielding materials which interact with the impinging EM waves. The multiple reflections of the EM waves is defined as the reflection from the various surfaces or interfaces and often neglected as the re-reflected waves are absorbed in the material. For a good EMI shielding material, the transmission of the EM waves should be negligible. The total EMI shielding effectiveness (SE) of the material is defined as the ratio of the incident and transmitted energies of the EM waves.

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In this regard, carbon materials and their composites have been considered to be the best candidate for EMI shielding owing to their light weight, flexibility, chemical inertness, low cost, environmental friendliness and easy processability [10-17]. Shen et. al [18] reported microcellular graphene foam prepared by simple hydrothermal thermal method and compared

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their results with graphene film prepared by thermal reducing of graphene oxide film, and they concluded that the graphene foam performed better EMI shielding effectiveness compared with graphene film due to its porous structure and the shielding was dominated by the absorption of the EM waves due to improved internal multiple reflection. Differently doped graphene were

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also prepared and it was found that the doping significantly enhances the electrical conductivity, carrier density, space charge polarization, dielectric polarization and trapping of EM waves, thus,

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facilitating the excellent EMI shielding dominated by absorption [19, 20]. Graphene coated polymer foams [21], flexible graphene-polymer composites [22-25], cellulose fiber-graphene aerogel [26] and ultrathin flexible graphene films [27] have been extensively developed for EMI shielding.

The combined use of conductive and magnetic fillers have also been extensively studied because the incorporation of the magnetic filler enhances the total EMI shielding of the composites by the

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means of magnetic losses such as magnetic domain movement, spin resonance and relaxation of magnetization [28]. Other than that, eddy current loss, interfacial polarization and natural loss significantly improves the absorption capabilities of the shielding materials [29]. Chen et al. used carbonyl iron as the magnetic filler and graphene as conductive filler in epoxy matrix and found

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that the magnetic carbonyl iron significantly enhanced permeability and magnetic loss, thus resulted in high EMI SE of 40 dB with a thickness of 4 mm. In their work, it was also observed

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that the magnetic and EMI shielding properties are dependent on the shape of the carbonyl iron [30]. Singh et al. uniformly decorated ϒ-Fe2O3 on the reduced graphene oxide (rGO) followed by the encapsulation of ϒ-Fe2O3 decorated rGO in polyaniline. The iron oxide nanoparticles decorated on rGO sheets in the core-shell structure acted as tiny dipoles and polarized microwave which resulted in improved absorption [31]. Hekmatara et al. reported Fe3O4 decorated multiwall carbon nanotubes (MWCNTs), which were used as fillers in epoxy matrix. It was also reported that the MWCNTs can be aligned in the epoxy matrix by using magnetic fields according to the requirement of the shield. It should be noted that it is extremely important to

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explore synergies arising from each constituent and maintain a good balance between them for high EMI shielding [32]. Carbon nanotubes-polymer composites [33-38] have also shown potential application on EMI

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shielding due to high aspect ratio of carbon nanotubes (CNTs) resulting in low percolation threshold. Arjmand et. al [39] studied the effect of synthesis catalyst (Ni, Fe and Co) on the microstructure of nitrogen doped CNTs and prepared their polymeric composites for EMI shielding application. It was found that different types of catalyst significantly affect the aspect

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ratio, structural defect and nitrogen content of the CNTs, hence, their composites showed significant difference in EMI shielding effectiveness. Jia et. al [40] described the effect of different CNTs-polymer mixing processes and their respective structures on EMI shielding

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effectiveness. Carbon black [41], carbon nano fiber [42, 43] have also been reported as fillers for polymer composites.

Chaudhary et. al [44] prepared freestanding and flexible mesocarbon microbead-multiwalled carbon nanotube composite paper and achieved EMI shielding effectiveness of 31 dB, which was further improved by the incorporation of iron oxide nanoparticles [45]. Several other reports [46-

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50] suggested that the incorporation of the magnetic nanoparticles in the carbon-polymer composite significantly enhances their EMI shielding performance due to magnetic losses, eddy current loss and natural resonance loss. Moreover, it also enhanced the dielectric loss and interfacial polarization which resulted in improved EMI shielding. Various ferrite materials such

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as barium ferrite [51-53], hexagonal-ferrite [54], Mn0.2 Ni0.4 Zn0.4 Fe2O4 ferrite [55] and nickel zinc ferrite [56, 57] have been adopted as the most preferred magnetic materials for EMI

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shielding application. Wang et. al [58] synthesized nickel zinc ferrite/polyaniline composite for the application of microwave absorption and concluded that the composite exhibited very high magnetic saturation of 43.7 emu/g resulting in high reflection loss of 17 dB at 11.1 GHz and wide bandwidth of 5 GHz for reflection loss below 10 dB. However, it has been challenging to attain high absorption dominant EMI shielding because the reflection is usually the primary mechanism for highly conductive EMI shields, thus, this work is mainly focused on high absorption loss of the EM waves with only 1.0 mm thick EMI shield. In this work, we report lightweight amorphous carbon composite mats filled with zinc oxide nanorods and nickel zinc ferrite with a thickness of 1.0 mm performing excellent EMI shielding 4

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in the X band (8.2-12.4 GHz). The amorphous carbon was extracted from the cucumber through the simple pyrolysis process while the ZNF powder was synthesized by the simple sol-gel method [59]. The electrical and EMI shielding properties of the composites were measured to understand the shielding mechanism and it was found that the amorphous carbon-ZNF

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composites can shield up to 99.999% power of the incident EM waves, moreover, the shielding efficiency can be tuned by adjusting the percentage of ZNF powder. Hence, the amorphous carbon-ZNF composites can be suitable for potential applications in defense and

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telecommunication. 2. Experimental

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2.1 Materials

Fresh cucumber was purchased from local markets. Iron nitrate (Fe-(NO3)3.9H2O), nickel nitrate (Ni-(NO3)2.6H2O) and Zinc nitrate (Zn-(NO3)2.6H2O) were purchased from Sigma-Aldrich®, Alfa Aesar® and J. T. Baker®, respectively. Citric acid, polyvinyl alcohol and ammonium hydroxide solution were purchased from Sigma-Aldrich®.

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2.2 Method

2.2.1 Syntheses of amorphous carbon and ZNF powder First, rind, seeds and soft pulp of the cucumber were removed. Then, it was cut into an

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appropriate size followed by washing with deionized (DI) water and placed into a tube furnace. The calcination was done in argon atmosphere at 800 °C for one hour with heating rate of 10

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°C/min. The as-prepared material was ground to obtain amorphous carbon flakes. To prepare ZNF powder, nickel nitrate (Ni-(NO3)2·6H2O, ferric nitrate (Fe-(NO3)3·9H2O) and zinc nitrate (Zn-(NO3)2·6H2O) with equal weight of 2.0 g each were dissolved in a deionized water. To chelate Ni2+, Zn2+ and Fe3+ ions in the solution, citric acid was added in the solution under continuous stirring. Ammonium hydroxide was added slowly to neutralize the solution (pH=7). Then, the neutralized solution was dried at 100 °C under continuous stirring and subsequently heated at 250 °C to form a loose powder. Eventually, the spinel phase was obtained by annealing the loose powder in the air at 1000 °C for one hour. The ratio of Ni to Zn was kept equal according to the report of excellent magnetic properties of Ni0.5Zn0.5Fe2O4 [60, 61]. 5

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2.2.2 Preparation of the composite mats The as-prepared carbon flakes were mixed with a specific amount of ZNF powder and DI water followed by sonication for 30 min. Polyvinyl alcohol (PVA) of 10 wt% was used as a binder in

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all the samples. PVA powder was dissolved in DI water and heated at 80 °C followed by mixing with the as-prepared homogenous solution containing carbon and ZNF, and the mixture was subsequently subjected to heat treatment at 100 °C to evaporate the water. Then, the as-prepared slurry was placed in a stainless steel mold (dimensions 2.5 cm × 1.5 cm × 1 cm) and pelletized

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by pressing at a pressure of 13 mega Pascal. The amorphous carbon composites loaded with 0 (0 vol%), 5 (0.23 vol%), 10 (0.5 vol%), 15 (3.95 vol%) and 20 wt% (5.64 vol%) of the ZNF

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powder were referred as M0, M5, M10, M15 and M20, respectively. 3. Characterization

The surface morphology and microstructure of amorphous carbon, ZNF powder and composites were examined by field emission scanning electron microscopy (FESEM, JEOL 6500F) and transmission electron microscopy (Philips TECNAI 20, 200 kV). X-ray diffraction (XRD) patterns of the samples were recorded in the scattering range of 20-80° at a scanning rate of

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2°/min (Shimadzu XRD600). Raman analysis was performed by HORIBA HR800 using a 633 nm excitation laser at room temperature. The thermal stability of the composite samples was studied in the range of room temperature to 1000 °C in argon atmosphere using thermogravimetric analysis (Seiko Instruments Inc., EXSTAR 6000). The magnetic properties of

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the composite samples were studied using vibrating sample magnetometer (PMC Micromag 3900). The electrical conductivities of the composite samples were measured by classic four

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probe method using a Keithley 2410 current source. The probes are made of beryllium copper coated with gold and the diameter of the probe is 100 µm. The spacing between the probes is fixed to 1.6 mm. For the electrical conductivity measurement, the samples were cut into 1 cm × 1cm × 0.1 cm and placed under the probes. A network analyzer (Agilent Technologies, E8364A, PNA Series Network Analyzer) was employed to measure the EMI shielding effectiveness in the range of X-band at room temperature using the rectangular waveguide method. To properly fit into the cavity groove of the sample holder with dimension 22.86 × 10.16 mm, the samples with a thickness of 1.0 mm were cut into rectangular shape with dimensions 25.50 × 12.50 mm and then fine modified to fit the mold. 6

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4. Results and Discussion The amorphous carbon was extracted from the fresh cucumber through a simple pyrolysis process in argon atmosphere at 800 °C and the ZNF powder was prepared by a simple co-

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precipitation method using citric acid. Cucumber, fast growing and long seasonal vegetable, is a widely cultivated all over the world. This vegetable contains more than 95% water and less than 5% polysaccharide which make it a very promising raw material for the fabrication of porous carbon. Porosity plays an important role by facilitating multiple reflections, thus resulting in

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enhanced absorption of the EM waves inside the material. The samples were prepared with different weight percentage of ZNF powder in amorphous carbon and PVA of 10 wt% was used as a binder in all the composite samples. The density of the composite was measured to be

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approximately 1.3 g/cm3. The porosities of all the samples were measured to be in the range of 38-42%. Fig. 1 depicts the schematic diagram of the composite preparation. 4.1 Surface morphology and structural analysis

The surface morphologies of the amorphous carbon, ZNF powder and M20 composites are shown in Fig. 2. The amorphous carbon has flat-sheet-like morphology with a length and width

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of several micrometers, which serves as highways for fast migration of electrons, as shown in Fig. 2(a). As can be seen in Fig. 2 (a), some flakes are quite big in size (50 µm) while some of them are quite small comparatively (5 µm). The high resolution image, as shown in the inset of Fig. 2(a), confirms the smooth surface morphology of the amorphous carbon sheets. Fig. 2(b)

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depicts the surface morphology of ZNF powder and it can be clearly observed that the zinc oxide nanorods were uniformly distributed in the nickel zinc ferrite. The high resolution image, as

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shown in the inset of Fig. 2(b), confirms the nanosheet-like structure of nickel zinc ferrite. The length and width of nickel zinc ferrite were estimated in the range of 200-300 nm, while the diameter and length of the zinc oxide nanorods were estimated to be in the range of 50-150 nm and 1-5 µm, respectively. The zinc oxide nanorods and nickel zinc ferrite nanosheets were uniformly distributed in the composite material, as shown in Fig. 2(c). The high resolution micrograph clearly confirmed the presence of zinc oxide nanorods and nickel zinc ferrite nanosheets, as shown in the inset of Fig. 2(c). As can be seen from the SEM micrograph of the M20 composite, the pore size of the composite can be estimated in the range of few micrometers.

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To further analyze the microstructural and morphological properties of amorphous carbon and ZNF powder, Transmission Electron Microscopy (TEM) was performed. Fig. 3(a) depicts the amplitude-contrast TEM micrograph of the carbon flakes. The length and width of the carbon flakes were measured to be several micrometers, as shown in Fig. 3(a). The phase-contrast high-

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resolution image shows the carbon flakes without the lattice fringes, as shown in Fig. 3(b), indicating amorphous structure of the carbon flakes. To further confirm the crystallinity of the carbon flakes, the selected area electron diffraction (SAED) pattern was recorded which also demonstrates the amorphous nature of the carbon flakes, as shown in the inset of Fig. 3(b). Fig.

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3(c) depicts the TEM micrograph of the ZNF powder which confirms the presence of zinc oxide nanorods and nickel zinc ferrite nanosheets simultaneously. The length and diameter of the zinc

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oxide nanorods were measured to be roughly 3 µm and 100 nm, respectively, which is in accordance with SEM analysis. Based on the structure factor of zinc oxide and the diffraction pattern shown in Fig. 3(d), it reveals that zinc oxide prefers to grow along the (011) plane because it has the symmetrical system, therefore the growth of zinc oxide consistently yields the {011} crystal plane as the dominant facet. By further focusing on nickel zinc ferrite nanosheets, as shown in Fig. 3(e), the length and width of the nanosheets were measured to be in the range of

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150-200 nm. Fig. 3(f) shows the SAED patterns of nickel zinc ferrite nanosheets. As can be seen easily, nickel zinc ferrite nanosheets show a good crystalline quality, which is consistent with the XRD results. Both Fig. 3(d) and 3(f) show dot patterns indicating the structures are composed of a single crystal.

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The typical XRD patterns of the amorphous carbon, ZNF powder and composite material are shown in Fig. 4(a). No peak can be detected for amorphous carbon, which confirms its

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amorphous nature. The diffraction pattern for ZNF powder was also recorded and it was concluded that the diffraction peaks at 2θ angle of 30.10, 35.41, 36.40, 43.03, 53.37, 56.90, 62.49, 74.67 and 78.92 correspond to the crystallographic planes of (220), (311), (222), (400), (422), (511), (440), (533) and (444), respectively, confirming the spinel structure of nickel zinc ferrite (JCPDS card number 00-008-0234) while the diffraction peaks at 2θ angle of 31.77, 34.40, 36.25, 47.54, 56.60, 62.86, 66.38, 67.96, 69.18, 72.56 and 76.95 correspond to the crystallographic planes of (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202), respectively (JCPDS card number 00-036-1451), confirming the hexagonal structure of zinc oxide nanorods. The XRD pattern of the composite material has shown a similar pattern as 8

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that of the ZNF powder, which confirms the successful incorporation of the ZNF in the composite material; moreover, no peak shift was detected which implies that the presence of amorphous carbon has no influence on the crystallographic structure of ZNF powder.

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Raman spectrum of amorphous carbon and ZNF powder were recorded at room temperature in the range of 1000-3000 cm-1 and 100-1000 cm-1, respectively, as shown in Fig. 4(b) and 4(c). The amorphous carbon has shown two major peaks denoted as D peak (1332 cm-1) and G peak (1575 cm-1). The G peak arises due to the E2g mode of carbon-carbon stretching which may be

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attributed to the clustering of sp2 phase in the amorphous carbon while the D peak arises due to the disordered structure of the amorphous carbon. The intensity ratio of D peak to G peak (ID/IG) is a direct indication of the degree of the crystallinity and the measured ratio of 1.17 indicating

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highly disordered structure of the carbon [62-64]. The Raman spectrum of nickel zinc ferrite confirmed the spinel structure belonging to Oh7 (Fd3m) space group [65, 66]. The spectrum has shown intensive Raman modes at 332 cm-1 (Eg), 478 and 560 cm-1 (F2g) and 700 cm-1 (A1g) with a shoulder feature at 660 cm-1. The shoulder may be attributed to the different local structures in the octahedral of nickel zinc ferrite i.e. one peak may arise due to all sites occupied by Fe ions in unit cell while another due to mixed occupancy of Ni, Zn and Fe ions. Moreover, the ionic radius

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of Zn ion is larger than that of Ni ion; therefore, it will cause more structural disorder of the oxygen sub-lattice. However, the Raman peaks of zinc oxide nanorods were not observed which may be buried under highly intense Raman peaks of nickel zinc ferrite.

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X-ray Photoelectron spectroscopy (XPS) is an important tool to analyze the chemical states of the materials. In this work, XPS was used to analyze amorphous carbon and ZNF powder, as

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shown in Fig. 5. The XPS spectra of amorphous carbon has shown two distinct peaks, which can be explained by the existence of carbon and oxygen atoms, as shown in Fig. 5(a). The C1s peak of the amorphous carbon was deconvoluted into 3 peaks located at 284.8 eV (sp2 carbon, C=C-C bonds), 286.3 eV (C-O bonds) and 288.1 eV (O-C=O bonds) [67-69]. The considerable peak area of the C-O and C=O bonds suggests a large amount of oxygen involved in the amorphous carbon, as shown in Fig. 5(b). The Fe2p photoelectron spectrum of the ZNF powder shows three peaks in the XPS pattern; one main peak of Fe3+2p3/2 at binding energy (BE) of 710.6 eV due to spin-orbit splitting of 2p3/2 and another main peak of Fe3+2p1/2 at BE of 723.7 eV owing to spinorbit splitting of 2p1/2 bands. The satellite peak of the main peak at BE of 717.5 eV is attributing 9

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to the ferric ions in the ferric structure, as shown in Fig. 5(c) [70]. The Ni2p photoelectron spectrum of the ZNF powder has shown main peak of Ni2+2p3/2 at BE of 854.5 eV and its satellite peak at 860.8 eV, as shown in Fig. 5(d). The photoemission of the Ni site of the Ni oxide constructed the main peak along with the satellite peak. The Zn2p photoelectron spectrum has

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shown a single sharp peak at BE of 1021.6 eV due to the tetrahedrally coordinated zinc ions, as shown in Fig. 5(e) [71]. Fig. 5(f) shows the O1s photoelectron spectrum of ZNF powder consisting well fitted peaks of O-Fe, O-Zn and O-Ni bonds. The binding energies of the O-Fe, O-

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Zn and O-Ni bonds appear at 531.9 eV, 529.6 eV and 530.9 eV, respectively [72].

Thermogravimetric analysis has been carried out in order to study the thermal degradation behavior of the ZNF powder and improvement in the thermal stability of the composite materials

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with weight percentage of ZNF powder, as shown in supporting information Fig. S1. The incorporation of ZNF powder has a significant thermal stabilizing effect on the composite materials.

4.2 Vibrating sample magnetometer (VSM) analysis

A highly sensitive VSM was used to examine the magnetic properties of the composite materials

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at room temperature and their respective hysteresis loops (magnetization (M) versus the applied magnetic field (H)) have been shown in Fig. 6. The saturation magnetization (Ms) value of the M0 composite was measured to be zero at an external field of 1 kOe because of the nonmagnetic behavior of the amorphous carbon. With the incorporation of 5 wt% ZNF powder, the composite

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material (M5) has shown typical ferromagnetic behavior with saturation magnetization (Ms) value of 1.4 emug-1 at an external field of 5 kOe due to highly magnetic nickel zinc ferrite.

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Furthermore, as the amount of ZNF powder increases from 5 to 20 wt% in the composite materials, the Ms value also increases from 1.4 to 6.9 emug-1 which confirms proper dispersion of the nickel zinc ferrite in amorphous carbon. The Ms values for M10 and M15 composite materials were measured to be 2.1 and 4.3 emug-1 at an external field of 5 kOe, respectively. In ferrite material, the complex permeability is directly related to the magnetization of the material which depends upon three magnetization mechanisms i.e. spin resonance, magnetic domain movement and relaxation of the magnetization. The increasing saturated magnetization with increasing wt% of ZNF powder enhances complex permeability of the composite materials, thus, the natural loss and impedance matching can be controlled by varying the percentage of ZNF 10

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powder in the composite materials. Moreover, the high permeability of the materials facilitates diversion of magnetic flux and eddy currents, thus, leading to high magnetic loss. 4.3 Electrical properties

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The electrical conductivities of composite materials with different ZNF powder loadings are shown in Table 1. The electrical conductivities were measured at room temperature using the standard four probe method. The electrical conductivity of the M0 composite was measured to be 75.21 S/m attributed to the highly conducting carbon flakes. The electrical conductivities of

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the composite materials decrease with the increasing weight percentage of ZNF powder and were measured to be 53.06, 38.72, 27.36 and 23.88 for M5, M10, M15 and M20 composite materials,

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respectively. The decrease in electrical conductivity is quite obvious owing to the incorporation of the non-conducting ZNF powder, as a result, enhanced interfacial resistance. The EMI shielding effectiveness strongly depends on the electrical and magnetic properties of the material. The incorporation of magnetic ZNF powder decreases the conductivity of the material but enhances permeability of the material, which resulted in magnetic loss. In addition, zinc oxide is a dielectric material and responsible for dipolar polarization and relaxation, which

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resulted in dielectric loss. Moreover, the incorporation of ZNF powder provides new interfaces for accumulation of virtual charges, thus leading to interfacial polarization. Therefore, it is concluded that the balance of electric and magnetic properties is extremely important to achieve the desired EMI shielding effectiveness.

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4.4 EMI shielding effectiveness

EMI shielding is the process in which the propagating EM waves are blocked by the conductive

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or magnetic materials. The total EMI shielding effectiveness (SE) of the material is defined as the ratio of the incident and transmitted energies of the EM waves, which can be written as = 10 log

= 20 log

= 20 log

(

)

(1)

When the EM waves impinge on the shielding materials, reflection, absorption, multiple reflection and transmission of the waves occurs. The total EMI shielding effectiveness is the sum of shedding due to reflection, absorption and multiple reflections, which can be expressed as

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(

)=

+

!

+

"

(2)

where Pinput and Poutput are the powers of the incident and transmitted waves, respectively, while SER, SEA and SEM represent the EMI shielding effectiveness due to the reflection, absorption

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and multiple reflection, respectively. It is usually accepted that the multiple reflections can be ignored if the SEA is higher than 10 dB as the re-reflected waves are absorbed in the material. The effectiveness of the shield is measured by the reduction of the incident power of the EM waves.

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For reflection of the EM waves, the shielding materials must have mobile charge carriers such as electrons or holes which interact with the electric and magnetic fields of the EM waves,

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therefore, the shielding material must be conductive although very high conductivity is not required. The impedance of the EM waves is defined as the ratio of E-field to H-field amplitudes. The impedance of the conductive materials is small compared to the impedance of the air, in which the waves are propagating; therefore, the larger mismatch leads to the reflection of the EM waves. The reflection loss is a function of the ratio of conductivity and permeability and the magnitude of the reflection coefficient reaches close to 1.0 for completely mismatched mediums

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at the interface. When the mediums are completely matched at the interface, the magnitude of the reflection coefficient becomes zero and the waves can penetrate into the material. In this case, effective shielding of the EM waves is achieved by the absorption of the EM waves. For absorption of the EM waves, the shielding materials must have electric and magnetic dipoles

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which interact with the electric and magnetic fields of the EM waves. The absorption loss is a function of the product of conductivity and permeability. Therefore, the balance of intrinsic

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physical properties including permittivity (ε), permeability (µ) and conductivity (σ) is extremely important to achieve desired shielding. When the EM waves impinge on the material, the sum of absorption (A), reflection (R), and transmission (T) must be 1, i.e., T + R + A = 1. The SEA and SER terms can be further explained by the following equations, respectively !

= 8.87&

(3)

= 10 log(1 − ()

(4)

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where d is the thickness of the shielding material, γ and R are the propagation constant of the waves in the conducting medium and reflection coefficient, respectively, which can be further explained as

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& = )*

+, -

(5)

(6)

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where ;′ and ;′′ are the real and imaginary parts of the permittivity, respectively, whereas µ and σ are the permeability and conductivity, respectively. The real (;′) and imaginary part (;′′) of the

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permittivity play an important role in the storage and loss of the electric energy. The displacement of charges caused by the external electric field induces material polarization which is directly associated with the real part of the permittivity (;′) while imaginary part (;′′) is associated with the electric energy dissipation. The characteristic impedance of free space is -

defined as => = * + 9 , which is approximate 377 Ω while impedance Zin is defined as 9

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9 EJKF (GH)

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|

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where =′ is the wave impedance and can be written such as

In this work, EMI shielding performance of the composite materials with different weight percentage of ZNF loading was measured in the X-band frequency range, as shown in Fig. 7. To calculate the EMI shielding of the composite materials, the scattering parameters (S11, S21) were obtained from the network analyzer. The scattering parameter Sij represents the power transmitted from port i to port j. The variation in total EMI shielding and reflection losses have been shown in Fig. 7(a & b) with respect to ZNF loading. At the mid-frequency of 10 GHz with 1.0 mm thickness, the M0 composite has shown total EMI shielding of 25.70 dB and shielding due to reflection was measured to be 3.17 dB, which further can be explained as the total power 13

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shielded of 99.73% (field shielded of 94.8%), while 48.19% is shared by reflection and 51.54% is shared by absorption. By the incorporation of 5 wt% ZNF, the total shielding increased to 30.8 dB and reflection loss increased to 7.37 dB at the frequency of 10 GHz. The incorporation of magnetic ZNF provides magnetic and electric dipoles in the composites, thus resulted in

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enhanced total EMI shielding. Moreover, the non-conducting ZNF gives rise to interfacial polarization, eddy current loss and magnetic loss due to spin resonance, magnetic domain movement and relaxation of the magnetization.

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By further increasing the weight percentage of ZNF from 5 wt% to 10 wt% and 15 wt%, the total EMI shielding as well as the reflection loss increases. At the frequency of 10 GHz, the total EMI shielding was measured to be 36.10 dB and 40.96 dB for M10 and M15 composites,

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respectively, whereas the reflection loss also increased to almost 9 dB for both M10 and M15 composites, as can be seen in Fig. 7(b). The highest reflection losses achieved by the M10 and M15 composites were 11.6 dB at 8.8 GHz and 13.94 at 8.6 GHz, respectively. The increase in reflection loss and total EMI shielding are an outcome of the increased percentage of ZNF nanoparticles. By further increasing the weight percentage of ZNF nanoparticles to 20 wt%, the total EMI shielding increased to almost 50 dB and the reflection loss also enhanced to 10.76 dB

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at the frequency of 10 GHz, which indicates that the power shielded by the reflection counts 8.394%, while the power shielded due to absorption counts 91.605% and the total power shielded is 99.999% (field shielded of 99.684%). Therefore, it can be concluded that the M20 composites provided absorption dominated shielding. According to the experimental results, it

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was observed that the shielding due to absorption increased with ZNF loading, as can be seen in Fig. 7(c), which indicates that the magnetic properties play an important role in electromagnetic

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absorption at higher ZNF loading. The absorption loss of the M0 composite was found to be 22.53 dB which was significantly increased by the incorporation of the ZNF loading and measured to be 38.36 dB for M20 composite. Therefore, the magnetic, electrical and EMI shielding properties of the composites can be tuned by controlling the amount of ZNF powder in composites. By further increasing the percentage of ZNF, the total shielding decreases drastically, therefore, 20 wt% ZNF filler is the optimum loading for the composite materials. It is found that the as-prepared amorphous carbon composites filled with zinc oxide nanorods and nickel zinc ferrite perform much better EMI shielding properties than those used other materials, as shown in Table 2. Therefore, it can be concluded that the ZNF has some synergetic effect on 14

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the EMI shielding properties of the composites; thus, the as-prepared composites hold great promises in EMI shielding applications. To further understand the mechanism of EMI shielding, the reflection coefficient (R), absorption

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coefficient (A), transmission coefficient (T) and absorption efficiency (Aeff) were calculated from the scattering parameters (S11, S21). The reflection and transmission coefficients were .

calculated such as ( = N ON = | P

66 |

.

=|

.. |

.

.

and Q = N R N = | P

.6 |

.

=|

6. |

.

, respectively.

The absorption coefficient and absorption efficiency were defined as S = (1 − ( − Q) and (6: : ) , (6: )

respectively. The calculated reflection, absorption and transmission coefficients

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STUU =

are shown in Fig. 8. At the frequency of 10 GHz, the transmission coefficient of M0 composite

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was calculated to be 0.0026 which decreased to 0.00001 for the M20 composite, as shown in Fig. 8(a). For all the ZNF incorporated composite materials, the transmission coefficient is always below 0.0026. The decrease in transmission coefficient confirms higher attenuation of the EM waves. The reflection coefficient increases with the weight percentage of ZNF, as shown in Fig. 8(b). The increasing reflection coefficient with decreasing transmission coefficient confirms absorption dominated shielding of the EM waves. For the M0 composite material at the

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frequency of 10 GHz, the absorption coefficient was calculated to be 0.479 which further decreases by the incorporation of the ZNF and reached to a minimum 0.08 for the M20 composite material, as shown in Fig. 8(c). Also, the absorption efficiency of the M0 composite material was calculated to be 99.44%, which further increased to 99.98% for the M20 composite

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material, as shown in Fig. 8(d).

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To further illustrate the EMI shielding mechanism, the schematic diagram of shielding mechanism has been shown in Fig. 9. According to the aforementioned discussion, the ZNF incorporated amorphous carbon can be successfully used for EMI shielding application. Moreover, the electric, magnetic and EMI shielding properties of the composite material can be tuned depending on the application. 5. Conclusions Amorphous carbon-ZNF composite mats have been successfully prepared with a thickness of 1.0 mm for excellent electromagnetic interference (EMI) shielding in the X band (8.2-12.4 GHz. The 15

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high electrical properties of amorphous carbon, high dielectric zinc oxide nanorods and rich magnetic properties of nickel zinc ferrite nanosheets played a vital role in achieving high EMI shielding. Although the electrical conductivities of the composite materials decrease by the incorporation of the non-conducting ZNF nanosheets, however, the magnetic properties enhance

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significantly. The synergistic combination of the materials results in the high reflection loss and absorption loss of the composites which were measured to be 10.76 dB and 38.36 dB, respectively. The highest EMI shielding of 53 dB, dominated by the absorption, was achieved by the incorporation of the 20 wt% ZNF nanosheets. Hence, the ZNF filled amorphous carbon

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composite mats can shield up to 99.999% power of the electromagnetic waves which is shared by the 8.394% reflection and 91.605% absorption, and can be suitable for potential applications

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in defense and telecommunication. Acknowledgement

The authors are thankful for the funding support from the Ministry of Science and Technology, Taiwan under the contract MOST 104-2221-E-007-029-MY3.

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Fig. 1. Schematic diagram of the experimental process.

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Figure Captions

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Fig. 2. SEM micrograph of (a) amorphous carbon flakes, (b) ZNF powder and (c) M20 composite.

Fig. 3. TEM micrograph of amorphous carbon flakes, (a) low resolution (b) high resolution of (a); inset showing the SAED pattern of the amorphous carbon, (c) ZNF powder and (d) nickel zinc ferrite and SAED pattern of (e) zinc oxide nanorods and (f) nickel zinc ferrite.

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Fig. 4 (a) XRD patterns of M0, ZNF and M20 composites. Raman spectrum of (b) amorphous carbon and (c) ZNF powder recorded at room temperature with 633 nm excitation. Fig. 5 (a) XPS spectra of amorphous carbon, (b) deconvoluted C1S spectrum of amorphous

and (f) O1s.

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carbon and deconvoluted photo electron spectrum of ZNF powder (c) Fe2p, (d) Ni2p, (e) Zn2p

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Fig. 6 Magnetization hysteresis loops of composites with different weight percentage of ZNF powder

Fig. 7. EMI shielding effectiveness of composites (a) transmission loss, (b) reflection loss and (c) reflection, absorption and transmission losses of the composites at the mid-frequency of 10 GHz. Fig. 8 (a) Transmission coefficient, (b) reflection Coefficient, (c) absorption coefficient and (d) Absorption efficiency of composites. Fig. 9. Schematic diagram of the EMI shielding mechanism. 22

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Table 1: Electrical conductivities of composites

M0

M5

M10

M15

M20

Electrical conductivity (S/m)

75.21

53.06

38.72

27.36

23.88

Standard deviation

1.47

2.03

1.64

1.02

0.84

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Materials

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Table 2. EMI shielding effectiveness of present work and other shielding materials Thickness Density (g/cm )

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(mm)

3

epoxy/graphene/carbonyl iron nanocomposites

EMI shielding

Ref.

(dB)

4

-

40

30

2

-

51

31

5.5

-

38

49

CNTs/Fe3O4/ phenolic foam

2

0.0441

62

50

Polylactide/graphite nanaocomposite

2

0.7

45

73

C/SiC composite

3

2

31

74

Polymethylmethacrylate-graphene

2.4

0.79

19

75

Carbon-cenosphere composite

2

0.30 

42.9

76

Carbon foam composite

2

0.11

30.5

77

CNT/polypropylene composite

2.2 

-

48.3

78

[email protected]–MWCNTs/epoxy

4

-

42

79

Nickel filaments- polyethersulfone-matrix

2.85

1.87

87

80

Graphene/ϒ-Fe2O3/polyaniline

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[email protected]/CNTs/ polyvinylidene fluoride

composites 23

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