EMI shielding effectiveness of carbon based nanostructured polymeric materials: A comparative study

The microstructure, electromagnetic interference (EMI) shielding effectiveness (SE), DC electrical conductivity, AC electrical conductivity and complex permittivity of nanostructured polymeric materials filled with three different carbon nanofillers of different structures and intrinsic electrical properties were investigated. The nanofillers were multiwall carbon nanotubes (MWCNT), carbon nanofibers (CNF) and high structure carbon black (HS-CB) nanoparticles and the polymer was acrylonitrile-butadiene-styrene (ABS). In addition, the EMI SE mechanisms and the relation between the AC electrical conductivity in the X-band frequency range and the DC electrical conductivity were studied. The nanocomposites were fabricated by solution mixing and characterized by uniform dispersion of the nanofillers within the polymer matrix. It was found that, at the same nanofiller loading, the EMI SE, permittivity and electrical conductivity of the nanocomposites decreased in the following order: MWCNT > CNF > CB. MWCNT based nanocomposites exhibited the lowest electrical percolation threshold and the highest EMI SE owning to the higher aspect ratio and electrical conductivity of MWCNT compared to CNF and HS-CB. The AC conductivity in the X-band frequency range was found to be independent of frequency.     

Multiwalled carbon nanotubes–BaTiO3/silica composites with high complex permittivity and improved electromagnetic interference shielding at elevated temperature

Composites with silica matrix and mixed filler of multiwalled carbon nanotubes (MWCNTs) and BaTiO₃ powder were fabricated. Excellent uniform dispersion of MWCNTs can be obtained using a two-step mixing method. Both of the real and imaginary parts of complex permittivity increased with increasing MWCNT content and measured temperature. The electromagnetic interference (EMI) shielding results showed that the absorption mechanism is the main contribution to the total EMI shielding effectiveness (SE). Compared with the EMI SE resulting from reflection, the absorption showed more dependence on the MWCNT content, measured temperature and frequency. The total EMI SE is greater than 20 dB at 25 °C and 50 dB at 600 °C in the whole frequency range of 12.4–18 GHz with a 1.5 mm composite thickness, which suggests that the MWCNT–BaTiO₃/silica composites could be good candidates for the EMI shielding materials in the measured frequency and temperature region.     

The electrical conductivity and electromagnetic interference shielding of injection molded multi-walled carbon nanotube/polystyrene composites

Multi-walled carbon nanotube (MWCNT)/polystyrene (PS) composites were injection molded into a mold equipped with three different cavities. A high alignment of MWCNTs in PS was achieved by applying high shear force to the melt. The effects of gate and runner designs and processing conditions, i.e., mold temperature, melt temperature, injection/holding pressure and injection velocity, on the volume resistivity of the composites were investigated in both the thickness and in-flow directions. The experiments showed that volume resistivity could be varied up to 7 orders of magnitude by changing the processing conditions in the injection molded samples. The electromagnetic interference shielding effectiveness (EMI SE) of the molded composites was studied by considering the alignment of the MWCNTs. The EMI SE decreased with an increase in the alignment of the injection-molded MWCNTs in the PS matrix. This study shows that mold designs and processing conditions significantly influence the electrical conductivity and shielding behavior of injection molded CNT-filled composites.     

Effect of oxyfluorination on electromagnetic interference shielding of polypyrrole-coated multi-walled carbon nanotubes

The polypyrrole-coated multi-walled carbon nanotubes (MWCNTs) were prepared by in situ chemical oxidative polymerization of pyrrole on the surface of MWCNTs for the novel electromagnetic interference (EMI) shielding materials. The oxyfluorination treatment on MWCNTs introduced the hydrophilic functional groups resulting in well distribution and higher interfacial affinity between polypyrrole (PPy) and MWCNTs. The PPy phases formed on MWCNTs were observed by SEM. The thickness of PPy on the surface of MWCNTs decreased as increasing the hydrophilic groups on MWCNTs by the oxyfluorination treatment. The PPy-coated MWCNT composites showed the remarkable increases in permittivity, permeability, and EMI shielding efficiency (SE). The EMI SE of PPy-coated MWCNTs increased up about 28.6 dB mainly based on the absorption mechanism.     

Electrical conductivity and EMI shielding effectiveness of polyurethane foam–conductive filler composites

The morphological, electrical, and thermal properties of polyurethane foam (PUF)/single conductive filler composites and PUF/hybrid conductive filler composites were investigated. For the PUF/single conductive filler composites, the PUF/nickel‐coated carbon fiber (NCCF) composite showed higher electrical conductivity and electromagnetic interference shielding effectiveness (EMI SE) than did the PUF/multiwall carbon nanotube (MWCNT) and PUF/graphite composites; therefore, NCCF is the most effective filler among those tested in this study. For the PUF/hybrid conductive fillers PUF/NCCF (3.0 php)/MWCNT (3.0 php) composites, the values of electrical conductivity and EMI SE were determined to be 0.171 S/cm and 24.7 dB (decibel), respectively, which were the highest among the fillers investigated in this study. NCCF and MWCNT were the most effective primary and secondary fillers, and they had a synergistic effect on the electrical conductivity and EMI SE of the PUF/NCCF/MWCNT composites. From the results of thermal conductivity and cell size of the PUF/conductive filler composites, it is suggested that a reduction in cell size lowers the thermal conductivity of the PUF/conductive filler composites. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44373.     

Electromagnetic interference shielding effectiveness of MWCNT filled poly(ether sulfone) and poly(ether imide) nanocomposites

Multiwalled carbon nanotube (MWCNT) filled poly(ether sulfone) (PES) and poly(ether imide) (PEI) composites were prepared with different MWCNT weight fractions (0.5–5wt%) by a solution mixing technique. Their electrical conductivities, electromagnetic interference (EMI), shielding effectiveness (SE), return loss (RL), and absorption loss (AL) were investigated. Morphologies of the fracture surfaces of nanocomposites studied by scanning electron and transmission electron microscopy showed relatively good MWCNT dispersion and distribution. The electrical conductivity of compression molded samples measured at room temperature indicated that the electrical percolation network was achieved already at 0.5% loading. The measurements of shielding effectiveness (SE) carried out in the frequency range of 8 to 12 GHz (X‐band range) showed that SE increases with measurement frequency and with filler loading, whereby no significant differences could be observed between PES and PEI as matrices. The nanocomposites based on both matrices with 5 wt% loading of MWCNT exhibited shielding levels at 8 GHz between 42 and 45 dB in comparison with the pure polymers which showed value in the range of 1 to 2 dB. RL and AL showed significantly lower values for the composites as compared to unfilled polymers, but no systematic trends were observed on frequency. POLYM. ENG. SCI., 54:2560–2570, 2014. © 2013 Society of Plastics Engineers     

Effects of hybrid fillers on the electrical conductivity,EMI shielding effectiveness,and flame retardancy of PBT and PolyASA composites with carbon fiber and MWCNT

The effects of hybrid fillers of carbon fiber (CF) and multiwall carbon nanotube (MWCNT) on the electrical conductivity, electromagnetic interference shielding effectiveness (EMI SE), flame retardancy, and mechanical properties of poly(butylene terephthalate) (PBT)/poly(acrylonitrile-co-styrene-co-acrylate) (PolyASA) (70/30, wt %) with conductive filler composites were investigated. The CF was used as the main filler, and MWCNT was used as the secondary filler to investigate the hybrid filler effect. For the PBT/PolyASA/CF (8 vol %)/MWCNT (2 vol %) composite, a higher electrical conductivity (1.4 × 10⁰ S cm⁻¹) and EMI SE (33.7 dB) were observed than that of the composite prepared with the single filler of CF (10 vol %), which were 9.0 × 10⁻² S cm⁻¹ and 23.7 dB, respectively. This increase in the electrical properties might be due to the longer CF length and hybrid filler effect in the composites. From the results of aging test at 85 °C, 120 h, the electrical conductivity and EMI SE of the composites decreased slightly compared to that of the composite without aging. The results of electrical conductivity, EMI SE, and flame retardancy suggested that the composite with the hybrid fillers of CF and MWCNT showed a synergetic effect in the PBT/PolyASA/CF (8 vol %)/MWCNT (2 vol %) composite. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48162.     

Fabrication and characterization of NBR/MWCNT composites by latex technology

To develop a rubber composite with excellent electrical properties, a sort of synthetic rubber, acrylonitrile butadiene rubber (NBR) with CN dipoles as matrix, multi‐walled carbon nanotubes (MWCNTs) as filler, was synthesized. NBR composites reinforced with 0.5, 1.5, 3, 10, and 20 phr MWCNT contents were fabricated by latex technology. The electrical conductivity, dielectric characteristics, and electromagnetic interference (EMI) shielding effectiveness at room temperature of NBR/MWCNT composites were investigated. MWCNTs were found well dispersed into NBR matrix even for 20 phr content by FESEM observation. The electrical conductivity increased with an increment of MWCNT content. The dielectric constant was over 10⁴ at 10³ Hz frequency for 10 and 20 phr MWCNTs‐reinforced NBR composites. It was attributed to the increased electrons and interface polarization. The improved conductivity and dielectric permittivity resulted in an enhanced EMI shielding effectiveness. The EMI shielding effectiveness reached 26 dB at 16.7 GHz frequency for NBR/20 phr MWCNT composite with 1.0 mm thickness. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Theoretical and experimental studies on EMI shielding mechanisms of multi-walled carbon nanotubes reinforced high performance composite nanofibers

Electrically conductive composite nanofibers of polyvinylpyrrolidone (PVP) filled with multi-walled carbon nanotubes (MWCNTs) were prepared by electrospinning process. The complex permittivity and electromagnetic interference shielding effectiveness (EMI SE) of all composite nanofibers were measured in the X band frequency range 8.2–12.4 GHz. The electrical conductivity, real and imaginary part of permittivity, and EMI shielding behaviors of the composite nanofibers were reported as function of MWCNTs concentration. Electrical conductivity of MWCNTs/PVP composite nanofiber followed power law model of percolation theory having a percolation threshold ?_c = 0.72 vol% (~1 wt.%) and exponent t = 1.71. The total EMI SE of MWCNTs/PVP composite nanofibers increased up to 42 dB mainly base on the absorption mechanism. The EMI SE measured from experiments was also compared with the approximate value calculated from theoretical model. The obtained theory results confirmed that the selected model presented acceptable performance for evaluating the involved parameters and prediction of the EMI SE of composite nanofibers. The ability of the theoretical model to predict the EMI shielding by reflection and absorption was found to be a function of the frequency, thickness, permittivity, and conductivity.     

Multiwalled carbon nanotube/cement composites with exceptional electromagnetic interference shielding properties

Multi-walled carbon nanotube (MWCNT)/portland cement(PC) composites have been fabricated to evaluate their electromagnetic interference (EMI) shielding effectiveness (SE). The results show that they can be used for the shielding of EMI in the microwave range. The incorporation of 15 wt.% MWCNTs in the PC matrix produces a SE more than 27 dB in X-band (8.2–12.4 GHz), and this SE is found to be dominated by absorption. Furthermore, the structural analysis, surface morphology and surface interaction of MWCNTs with PC matrix have been explored using XRD, SEM and X-ray photoelectron spectroscopy technique.     

Electrical conductivity,dielectric and microwave absorption properties of graphene nanosheets/magnesia composites

Herein, a novel microwave absorbing material with Graphene nanosheets (GNSs) as microwave absorbing filler and magnesia (MgO) as matrix were prepared by hot-pressing sintering. The composites were highly dense with a homogeneous distribution of GNSs. Electrical conductivity, dielectric and microwave absorption properties in X-band were investigated. The results revealed that the electrical conductivity of the GNSs/MgO composites showed a typical percolation-type behavior with a percolation threshold of 3.34 vol%. With GNSs content increased to 3 vol%, the real permittivity, imaginary permittivity and dielectric loss tangent of the composites increased from ～9, ～0 and ～0 to 26–43, 23–28 and 0.55–0.96, respectively. By adjusting the GNSs content, thickness and frequency, the 2.5 vol% GNSs/MgO composite shows the minimum reflection loss of ?36.5 dB at 10.7 GHz and the reflection loss below ?10 dB (90% absorption) ranges from 9.4 to 11.4 GHz with 1.5 mm thickness, exhibiting excellent microwave absorption properties.     

Polypropylene/carbon nanotube nano/microcellular structures with high dielectric permittivity,low dielectric loss,and low percolation threshold

Nano/microcellular polypropylene/multiwalled carbon nanotube (MWCNT) composites exhibiting higher electrical conductivity, lower electrical percolation, higher dielectric permittivity, and lower dielectric loss are reported. Nanocomposite foams with relative densities (ρ_R) of 1.0–0.1, cell sizes of 70 nm–70 μm, and cell densities of 3 × 10⁷–2 × 10¹⁴ cells cm⁻³ are achieved, providing a platform to assess the evolution of electrical properties with foaming degree. The electrical percolation threshold decreases more than fivefold, from 0.50 down to 0.09 vol.%, as the volume expansion increases through foaming. The electrical conductivity increases up to two orders of magnitude in the nanocellular nanocomposites (1.0 > ρ_R > ∼0.6). In the proper microcellular range (ρ_R ≈ 0.45), the introduction of cellular structure decreases the dielectric loss up to five orders of magnitude, while the decrease in dielectric permittivity is only 2–4 times. Thus, microcellular composites containing only ∼0.34 vol.% MWCNT present a frequency-independent high dielectric permittivity (∼30) and very low dielectric loss (∼0.06). The improvements in such properties are correlated to the microstructural evolution caused by foaming action (biaxial stretching) and volume exclusion. High conductivity foams have applications in electromagnetic shielding and high dielectric foams can be developed for charge storage applications.     

Improved dielectric and electromagnetic interference shielding properties of ferrocene-modified polycarbosilane derived SiC/C composite ceramics

In order to enhance the dielectric and electromagnetic interference shielding (EMI) properties, the SiC/C composite ceramics were fabricated by pyrolysis of ferrocene-modified polycarbosilane. The microstructure evolutions, dielectric properties, EMI and microwave absorption properties of SiC/C composite ceramics were investigated. The increases of both ferrocene contents and annealing temperatures led to the increases of crystallizations of SiC and carbons. Crystallized carbons including carbon nanowires, turbostratic carbons, onion-like carbons and graphene-like carbons were obtained in the materials. The carbon nanowires were longest when the 5 wt.% ferrocene-modified polycarbosilane was annealed at 1250 °C. These carbons played a more important role than SiC in the increases of dielectric and EMI properties. The average real and imaginary permittivities of materials increased from 4.4 and 0.7 to 38.9 and 39.6, respectively. The materials exhibited high total shielding effectiveness, high absorption shielding effectiveness and low reflection shielding effectiveness, which were 36.6, 30.1 and 6.5 dB, respectively.     

Effects of carbon fiber modification with multiwall CNT on the electrical conductivity and EMI shielding effectiveness of polycarbonate/carbon fiber/CNT composites

The effect of carbon fiber (CF) modification with multiwall carbon nanotube (CNT) on the electrical, mechanical, and rheological properties of the polycarbonate (PC)/CF/CNT composite was investigated. The CF and multiwall CNT (MWCNT) were treated with sulfuric acid and nitric acid (3:1 wt %) mixture, to modify the CF with the CNT. For the PC with acid-treated CNT (a-CNT) modified acid-treated CF (a-CF) (PC/a-CF/a-CNT) composite, the electrical conductivity, and the electromagnetic interference shielding effectiveness (EMI SE) showed the highest values, compared with those of the PC/a-CF and PC/a-CF/CNT composites. The EMI SE of the PC/a-CF (10 wt %)/a-CNT (0.5 wt %) composite was found to be 26 (dB at the frequency of 10.0 GHz, and the EMI SE was increased by 91.2%, compared to that of the PC/a-CF composite at the same amount of total filler content. Among the composites studied in this work, the PC/a-CF/a-CNT composite also showed the highest values of relative permittivity (ε_r) and dielectric loss factor. The above results suggest that the CF modification with the a-CNT significantly affected the electrical conductivity and EMI SE of the composite, and the hybrid fillers of the a-CNT and a-CF resulted in good electrical pathways in the PC/a-CF/a-CNT composite. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47302.     

The effect of surface chemistry of graphene on rheological and electrical properties of polymethylmethacrylate composites

Graphene sheets with different oxygen contents were prepared to functionalize the electrically insulating polymethylmethacrylate (PMMA). The influences of surface chemistry of graphene on rheological, electrical and electromagnetic interference (EMI) shielding properties of its PMMA composites were investigated. The appearance of frequency-independent storage modulus at low frequency suggests a solid-like viscoelastic behavior and the formation of an interconnected network of graphene in the matrix. Due to the favorable interfacial interactions arising from polarity matching, the graphene with a C/O ratio of 13.2 (graphene-13.2) shows a better dispersion in PMMA than those with lower C/O ratios, and thus its PMMA composites exhibit lower rheological and electrical percolation thresholds. The EMI shielding properties of the graphene/PMMA composites exhibit similar dependence on the oxygen content of graphene. A high EMI shielding effectiveness of ～30 dB was obtained for the PMMA composite with 4.2 vol.% of graphene-13.2 with microwave absorption as the dominant EMI shielding mechanism.     

Dielectric,electromagnetic absorption and interference shielding properties of porous yttria-stabilized zirconia/silicon carbide composites

SiC was infiltrated into porous yttria-stabilized zirconia (YSZ) felt by chemical vapor infiltration (CVI), and continuous SiC matrix layer was formed around YSZ fibre. When 86.9 wt.% SiC is introduced into the porous YSZ felt, the mean values of the real part of the permittivity and dielectric loss tangent of porous YSZ felt increase from 1.16 and 0.007 to 8.2 and 1.31, respectively. The electromagnetic interference (EMI) shielding efficiency (SE) increases from 0.069 dB to 16.2 dB over the frequencies ranging from 8.2 GHz to 12.4 GHz. The reflection loss of the composites with a thickness of 5 mm at 8–18 GHz is smaller than ?6.5 dB, and the bandwidth below ?10 dB is 5 GHz at room temperature, which increases to 5.9 GHz at 800 °C. The considerable increases in EMI SE and microwave absorption properties are attributed to the formation of continuous SiC matrix layer composed of SiC nanocrystals in the porous YSZ felt, which is beneficial for the production of induced electric current and the enhancement of dielectric loss.     

Dielectric and microwave absorption properties of polymer derived SiCN ceramics annealed in N2 atmosphere

Porous SiCN ceramics were successfully fabricated by pyrolysis of a kind of polysilazane. The effects of annealing temperature on the microstructure evolution, direct-current electrical conductivity, dielectric properties, and microwave absorption properties of SiCN in the frequency range 8.2–12.4 GHz (X-band) were investigated. With the increase of annealing temperature, SiC, Si₃N₄ and free carbon nanodomains are gradually formed in the SiCN. Both the SiC and free carbon nanodomains lead to the increases of the complex relative permittivity and loss tangent of SiCN. With the increase of the annealing temperature, the average real permittivity, imaginary permittivity and loss tangent increase from 4.4, 0.2 and 0.05 to 13.8, 6.3 and 0.46, respectively. The minimum reflection coefficient and the frequency bandwidth below −10 dB for SiCN annealed at 1500 °C are −53 dB and 3.02 GHz, indicating good microwave absorption properties.     

Probing the engineered sandwich network of vertically aligned carbon nanotube–reduced graphene oxide composites for high performance electromagnetic interference shielding applications

Herein, we developed a strategy for fabrication of iron oxide infiltrated vertically aligned multiwalled carbon nanotubes (MWCNT forest) sandwiched with reduced graphene oxide (rGO) sheets network for high performance electromagnetic interference (EMI) shielding application which offers a new avenue in this area. Such engineered sandwiched network exhibits enhanced shielding effectiveness compared to conventional EMI shielding materials. This network of exotic carbons demonstrates the shielding effectiveness value more than 37 dB (>99.98% attenuation) in Ku-band (12.4–18 GHz), which is greater than the recommended limit (∼30 dB) for techno-commercial applications.     

Excellent electromagnetic interference shielding and mechanical properties of high loading carbon-nanotubes/polymer composites designed using melt recirculation equipped twin-screw extruder

This paper describes for the first time a facile, scalable and commercially viable melt blending approach involving use of twin-screw extruder with melt recirculation provision, for uniform dispersion of up to 4.6 vol% multiwall carbon nanotubes (MWCNTs) within polypropylene random copolymer (PPCP). Morphological characterization of PPCP/MWCNT nanoscale composites (NCs) was done using scanning electron microscopy and transmission electron microscopy, which show good dispersion of MWCNTs in the PPCP matrix even at high loadings and confirm the formation of true NCs. The improved dispersion leads to the formation of electrically conducting three dimensional networks of MWCNTs within PPCP matrix at very low percolation threshold (∼0.19 vol%). The attainment of dc conductivity value of ∼10⁻³ S/cm, tensile strength of ∼42 MPa and good thermal stability for 4.6 vol% MWCNTs loading NC along with electromagnetic interference (EMI) shielding effectiveness (SE) value of −47 dB (>99.99% attenuation), demonstrate its potential for making light weight, mechanically strong and thermally stable EMI shields. These NCs also display specific SE value of ∼−51 dB cm³/g which is highest among unfoamed polymer NCs.     

Using a non-covalent modification to prepare a high electromagnetic interference shielding performance graphene nanosheet/water-borne polyurethane composite

We prepared flexible, lightweight, and high electromagnetic interference (EMI) shielding performance graphene nanosheet (GNS)/water-borne polyurethane (WPU) composites. WPU, with sulfonate functional groups, was used as the polymer matrix. By adsorbing the cationic surfactant (stearyl trimethyl ammonium chloride) on the surface of the GNSs (S-GNSs), restacking and aggregation of the GNSs have been efficiently suppressed, which also attracted sulfonate groups from the WPU matrix. Because of the favorable interfacial interactions arising from electrostatic attraction, the S-GNS exhibited good compatibility with the WPU matrix. Such a homogeneous dispersion contributed to the construction of an electrical conductive network. The S-GNS/WPU composite exhibited a low electrical conductivity percolation threshold and an outstanding enhanced electrical conductivity of approximately 5.1 S/m. A high EMI shielding effectiveness of approximately 32 dB was obtained by the WPU composites with contents of 5 vol.% (approximately 7.7 wt.%) S-GNSs.