High electromagnetic interference shielding effectiveness achieved by multiple internal reflection and absorption in polybenzoxazine/graphene foams

Electromagnetic interference (EMI) is an increasingly severe issue in modern life and high-performance EMI shielding materials are in desperate need. To achieve high EMI shielding effectiveness (EMI SE), a series of polybenzoxazine/graphene composites foams are developed using a simple sol–gel method. When the graphene loading increases from 1 to 20 wt%, the density of the composites foams drops from 0.4143 g/cm³ to 0.1654 g/cm³. Meanwhile, an electrically conductive path is formed at around 7 wt% of graphene. Below the percolation threshold, the dielectric constant increases with graphene content and composite foam with 5 wt% graphene shows dielectric constant of 10.8 (1 MHz). At the highest graphene content of 20 wt%, the electric conductivity reaches 0.02 S/cm, 10 orders of magnitude higher than pure polybenzoxazine foam. Benefiting from the high electrical conductivity and lightweight porous structure, the composite foam PF/20G delivers an EMI SE of 85 dB and a specific SE of 513.9 dB·cm³/g. Importantly, the EMI shielding is dominated by absorption attenuation, with PF/20G shows absorption ratio higher than 98% in the range of 8.4–11.0 GHz, which is believed to be caused by multiple internal reflection and absorption inside the conductive foam.     

Effect of microstructure induced by microcellular injection molding on electromagnetic interference shielding properties

Preparing lightweight and versatile products is the unremitting goal of industry to save resources and energy. Lightweight carbon fiber reinforced polypropylene (CF/PP) composite foams with high-performance electromagnetic interference (EMI) shielding materials were fabricated by microcellular injection molding (MIM) technology. The average length and distribution of CF in CF/PP composite foams were examined. Thanks to the introduction of foaming process, the average CF length of composite foams was 33.98% longer than that of solids, which effectively enhanced the electrical conductivity and EMI shielding properties. The effect of shot size, gas content, and injection rate on the electrical conductivity and EMI properties was investigated. With melt shot size of 2/3 of the cavity volume, gas content of 0.5 wt% N₂ and injection rate of 100 mm/s, optimal cellular structure of the composite material was obtained. The EMI shielding effectiveness (SE) reaches 36.94 dB, which is the highest value achieved by using MIM technology to the best of the authors' knowledge. In addition, the mechanical properties of cellular structure can still maintain good values, with the tensile strength and impact strength improved by 15.3% and 14.03%, respectively.     

Functionalized Graphene–PVDF Foam Composites for EMI Shielding

Novel foam composites comprising functionalized graphene (f‐G) and polyvinylidene fluoride (PVDF) were prepared and electrical conductivity and electromagnetic interference (EMI) shielding efficiency of the composites with different mass fractions of f‐G have been investigated. The electrical conductivity increases with the increase in concentration of f‐G in insulating PVDF matrix. A dramatic change in the conductivity is observed from 10^?16 S · m^?1 for insulating PVDF to 10^?4 S · m^?1 for 0.5 wt.% f‐G reinforced PVDF composite, which can be attributed to high‐aspect‐ratio and highly conducting nature of f‐G nanofiller, which forms a conductive network in the polymer. An EMI shielding effectiveness of ≈20 dB is obtained in X‐band (8–12 GHz) region and 18 dB in broadband (1–8 GHz) region for 5 wt.% of f‐G in foam composite. The application of conductive graphene foam composites as lightweight EMI shielding materials for X‐band and broadband shielding has been demonstrated and the mechanism of EMI shielding in f‐G/PVDF foam composites has been discussed.

     

Multifunctional Shape-Stabilized Phase Change Materials with Enhanced Thermal Conductivity and Electromagnetic Interference Shielding Effectiveness for Electronic Devices

A paraffin-based shape-stabilized composite phase change material (CPCM) is fabricated with dramatically enhanced thermal conductivity and excellent electromagnetic interference (EMI) shielding capacity. The as-prepared CPCMs are supported by graphene-based frameworks with many bubble-like micropores that are prepared by the addition of polystyrene microspheres into graphene oxide hydrogel as hard templates. These bubble-like micropores can encapsulate paraffin wax (PW) due to the strong capillary force between the graphene-based framework and PW and leading to enhanced shape stability of the as-prepared CPCMs. Moreover, the continuous thermally and electrically conductive network formed by graphene nanoplatelets endows the as-prepared CPCMs with a high thermal conductivity and an excellent EMI shielding effectiveness. When the ratio of graphene-based framework is 23.0 wt%, the thermal conductivity and latent heat of CPCM reaches 28.7 W m⁻¹ k⁻¹ and 175.8 J g⁻¹, respectively, and the EMI shielding effectiveness is higher than 45 dB in the frequency of 8.2–12.4 GHz. Their outstanding thermal and EMI shielding performance makes the as-prepared CPCMs promising candidates for use in thermal management and EMI shielding of electronic devices.     

Inorganic nanotubes reinforced polyvinylidene fluoride composites as low-cost electromagnetic interference shielding materials

Novel polymer nanocomposites comprising of MnO₂ nanotubes (MNTs), functionalized multiwalled carbon nanotubes (f-MWCNTs), and polyvinylidene fluoride (PVDF) were synthesized. Homogeneous distribution of f-MWCNTs and MNTs in PVDF matrix were confirmed by field emission scanning electron microscopy. Electrical conductivity measurements were performed on these polymer composites using four probe technique. The addition of 2 wt.% of MNTs (2 wt.%, f-MWCNTs) to PVDF matrix results in an increase in the electrical conductivity from 10^-16S/m to 4.5 × 10^-5S/m (3.2 × 10^-1S/m). Electromagnetic interference shielding effectiveness (EMI SE) was measured with vector network analyzer using waveguide sample holder in X-band frequency range. EMI SE of approximately 20 dB has been obtained with the addition of 5 wt.% MNTs-1 wt.% f-MWCNTs to PVDF in comparison with EMI SE of approximately 18 dB for 7 wt.% of f-MWCNTs indicating the potential use of the present MNT/f-MWCNT/PVDF composite as low-cost EMI shielding materials in X-band region.     

Electromagnetic interference shielding properties of polymer-grafted carbon nanotube composites with high electrical resistance

Poly(methyl methacrylate) (PMMA)-grafted multiwalled CNTs were prepared, and then dispersed into additional PMMA matrix, yielding highly insulated PMMA–CNT composites. The volume resistivity of PMMA–CNT was as high as 1.3 × 10¹⁵ Ω cm even at 7.3 wt% of the CNT. The individual CNTs electrically-isolated by the grafted PMMA chains in PMMA–CNT transmitted electromagnetic (EM) waves in the frequency range of 0.001–1 GHz, whereas the percolated CNTs in a conventional composite prepared by blending PMMA with the pristine CNTs strongly shielded the EM waves. This result suggests that the intrinsic conductivity of the CNT itself in PMMA–CNT does not contribute to the EM interference (EMI) shielding in the frequency range of 0.001–1 GHz. On the other hand, PMMA–CNT exhibited EMI shielding at the higher frequency range than 1 GHz because the dielectric loss of the CNT itself was rapidly increased over 1 GHz. At 110 GHz, PMMA–CNT with 7.3 wt% of the CNT had EMI SE of as high as 29 dB (0.57 mm thickness), though is slightly lower than that of the percolated conventional composite (35 dB). Thus, it is demonstrated that the highly insulated PMMA–CNT has the good EMI shielding at extremely high frequency range (30–300 GHz).     

Microcellular Conductive Carbon Black or Graphene/PVDF Composite Foam with 3D Conductive Channel: A Promising Lightweight,Heat-Insulating,and EMI-Shielding Material

In this study, a lightweight microcellular carbon-based filler/poly(vinylidene fluoride) (PVDF) composite foam is fabricated with a 3D conductive network that is thermally insulating, electrically conductive, and fabricated on a large scale. This composite can be used for high-efficiency thermal insulation and electromagnetic interference (EMI) shielding applications. The prepared composite demonstrates low density, high electrical conductivity, and excellent thermal insulation properties. The structure and density of the conductive network and the carbon-based filler content has a significant influence on the electrical conductivity of the prepared composite foam. Although the composite comprises microcellular PVDF beads of the same density, the conductivity of the composite-comprising strip beads is greater than that comprising spherical beads. In the same conductive network structure, as the size of the microcellular PVDF beads decrease, the conductive network becomes denser, which results in a higher conductivity. Furthermore, with an increase in the conductive filler content, the conductivity improves significantly. Excellent EMI shielding materials with optimal filler content and particle shapes, exhibiting EMI shielding effectiveness of up to 40–50 dB, are developed. The prepared composite foam possesses excellent application potential in the fields of ultra-light thermal insulation, conductivity, and EMI shielding.     

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.     

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     

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.     

Highly conductive and flexible bilayered MXene/cellulose paper sheet for efficient electromagnetic interference shielding applications

In this work, a robust and flexible bilayered MXene/cellulose paper sheet with superhigh electrical conductivity was prepared via vacuum-assisted filtration and a subsequent hot-pressing process for electromagnetic interference (EMI) shielding applications. By tightly assembling few-layered MXene (f-Ti₃C₂T_x) on the cellulose substrate via hydrogen bonds, an effective and interconnected conductive network was constructed in the paper sheet, resulting in a high electrical conductivity of 774.6–5935.4 S m^?1 at various f-Ti₃C₂T_x loadings. The highly conductive MXene layer can promptly reflect a great amount of incident EM waves, a process which preceded the transmission of EM waves in the cellulose matrix. Owing to the highly efficient reflection-dominated EMI shielding mechanism, the resultant bilayered MXene/cellulose paper sheets exhibit excellent EMI shielding effectiveness of 34.9–60.1 dB and specific EMI shielding efficiency of 290.6–600.7 dB mm^?1. Moreover, the MXene/cellulose paper sheets demonstrated improved mechanical strength (up to 25.7 MPa) and flexibility due to the mechanical frame effect acted by the cellulose substrate. Consequently, the robust and flexible bilayered MXene/cellulose paper sheet is a promising candidate for application in next-generation electric devices.     

In situ sintered silver decorated 3D structure of cellulose scaffold for highly thermoconductive electromagnetic interference shielding epoxy nanocomposites

This study presents a 3-dimensional (3D) network structure of cellulose scaffold (CS), which was in situ decorated with silver nanoparticles (AgNPs). The scaffold was then infiltrated with epoxy matrix and cured at elevated temperature to sinter the AgNPs; finally, highly thermoconductive epoxy composites (Ag@CS/epoxy) was obtained. The resultant Ag@CS20/epoxy composite reached a thermal conductivity of 2.52 W·m⁻¹·K⁻¹ at 2.2 vol% of filler loading, which shows an enhancement of over 11-folds in the thermal conductivity compared to the neat epoxy. The superb electrical conductivity value of over 53,691 S·m⁻¹ of the Ag@CS20/epoxy was achieved, which led to exceptional EMI SE values of 69.1 dB. Furthermore, surface temperatures during heating and cooling were also investigated to demonstrate the superior heat dissipating capacity of the Ag@CS/epoxy composite, which can be potentially put an application as thermal dissipating material in the next generation of electronics.     

Enhanced electrical conductivity of nylon 6 composite using polyaniline-coated multi-walled carbon nanotubes as additives

Aggregation in polymer composites is one of the major obstacles in the carbon nanotubes (CNTs) applications. Authentic CNTs are known to have very good electrical conductivity and mechanical strengths. Surface functionalization can avoid aggregation and help dispersion of CNTs, but reduces CNT’s electrical conductivities and mechanical strengths dramatically. It needs a good way to resolve the above dilemma situation; i.e., poor dispersion-good conductivity vs. good dispersion-poor conductivity. Herein, we demonstrate that in-situ polymerized polyaniline (PANI)-coated CNTs have good polymer matrix compatibility, and are superior electrically conductive fillers to nylon 6 composites. In this report, multi-walled CNTs (MWCNTs) were surface-modified with poly(acrylic acids) (PAA), followed by further coating with PANI. The electrical conductivity of (PANI-MWCNTs)-nylon 6 composite thin film was increased from 10⁻¹² to 7.3 × 10⁻⁵ S/cm in the presence of 1 wt% PANI-coated MWCNTs prepared by physical mixing of PANI and PAA-grafted MWCNTs. When in-situ polymerized PANI-coated MWCNTs were added, the electrical conductivity of MWCNTs-nylon 6 composite was further enhanced by 3 orders to be 3.4 × 10⁻² S/cm at the same 1 wt% loading of MWCNTs. Both Fourier-transformed infrared and uv-visible absorption spectra indicate that there exist very strong site-specific charge transfer interactions between the quinoid rings of PANI and MWCNTs, which results in the superior electrical conductivity of MWCNT-nylon 6 composite.     

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.     

Enhanced electrical conductivity in graphene-filled polycarbonate nanocomposites by microcellular foaming with sc-CO2

Electrically conductive polycarbonate (PC) foams containing a low concentration of graphene nanoplatelets (0.5 wt.%) were produced with variable range of expansion ratio by applying a high-pressure batch foaming process using sc-CO₂. The structure of the foams was assessed by means of SEM, AFM and WAXS, and the electrical conductivity was measured in the foam growing direction. Results showed that electrical conductivity of PC composite foams remarkably increased when compared to that of non-foamed PC composite, with both the electrical conductivity and the main cell size of the foams being directly affected by the resultant expansion ratio of the foam. This interesting result could be explained by the development of an interconnected graphene nanoparticle network composed by increasingly well-dispersed and reoriented graphene nanoplatelets, which was developed into the solid fraction of the foam upon foaming by sudden depressurising of the plasticised CO₂-saturated PC preform. Some evidences of morphological changes in the graphene nanoplatelets after foaming were obtained by analysing variations in graphene’s (0?0?2) diffraction plane, whose intensity decreased with foaming. A reduction of the average number of layers in the graphene nanoplatelets was also measured, both evidences indicating that improved dispersion of graphene nanoparticles existed in the PC composite foams. As a result, foams with a proper combination of low density and enhanced electrical conductivity could be produced, enabling them to be used in applications such as electromagnetic interference shielding.     

Waste cotton Fabric/MXene composite aerogel with heat generation and insulation for efficient electromagnetic interference shielding

Electromagnetic interference (EMI) shielding materials have become more and more indispensable due to serious electromagnetic-radiation pollution. Herein, waste cotton cellulose aerogels were prepared by dissolving waste cotton fabrics (WCF) in NaOH/urea aqueous solution, and MXene nanosheets were subsequently deposited on the cellulose aerogels by a facile dip coating method to obtain WCF/MXene composite aerogels. The WCF/MXene composite aerogels with highly porous network structure show remarkable electrical conductivity (8.2 Ω/sq of surface resistance), high EMI shielding effectiveness (EMI SE) in the range of 2–18 GHz (39.3–48.1 dB). The WCF/MXene aerogel possesses high SSE and SSE/t of 677.94–829.74 dB cm³ g^?1 and 3512.62–4299.17 dB cm² g^?1, respectively (2–18 GHz). In addition, the heating temperature of WCF/MXene composite aerogels reaches 199 °C when 3 V positive voltage is applied on them. The WCF/MXene composite aerogels possess excellent electromagnetic shielding effectiveness, heat generation property and insulation, which can be potentially used as multifunctional materials for EMI shielding, electrical-heating and high temperature protection.     

Epoxy composite foams with excellent electromagnetic interference shielding and heat‐resistance performance

Epoxy composite foams with improved heat‐resistant property and efficient electromagnetic interference shielding effectiveness (EMI SE) were fabricated through a two‐step foaming technique. A sort of novel and untraditional expandable microspheres was adopted to reduce the density of prepared materials. A multiscale conductive network system composed of multiwalled carbon nanotubes (MWCNTs) and nickel‐plated carbon fibers (NiCFs) was introduced in these foams. Benefitting from the synergistic effect between NiCFs and MWCNTs, the multiscale epoxy foam with best comprehensive performance achieved a greatly enhanced T_g at 178.3 °C and an exceptional specific EMI SE ranging from 52.8 to 72.6 dB cm³ g^?1 in X band (8.2–12.4 GHz) at low filler loading. These properties are greatly better than original epoxy foam with a T_g of 157.8 °C and specific EMI SE of 1.0–6.4 dB cm³ g^?1. Their shielding mechanisms were discussed and the results showed that reflection is dominating. The effects of microspheres content, foaming temperature, NiCFs content, and length were investigated. In general, we provided a feasible, convenient and cost‐effective method to fabricate light‐weight, heat‐resistant thermosetting epoxy foams with sufficient EMI shielding performance which has a potential to be applied in aerospace or electronic devices. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46013.     

Large-area reduced graphene oxide thin film with excellent thermal conductivity and electromagnetic interference shielding effectiveness

In this work, we fabricated reduced large-area graphene oxide (rLGO) with maximum surface area of 1592 μm² through a cost-effective chemical reduction process at low temperature. The product revealed large electrical conductivity of 243 ± 12 S cm⁻¹ and thermal conductivity of 1390 ± 65 W m⁻¹ K⁻¹, values much superior to those of a conventional reduced small-area graphene oxide (with electrical conductivity of 152 ± 7.5 S cm⁻¹ and thermal conductivity of 900 ± 45 W m⁻¹ K⁻¹). The rLGO thin film also exhibited not only excellent stiffness and flexibility with Young’s modulus of 6.3 GPa and tensile strength of 77.7 MPa, but also an efficient electromagnetic interference (EMI) shielding effectiveness of ∼20 dB at 1 GHz. The excellent performance of rLGO is attributed to the fact that the larger area LGO sheets include much fewer defects that are mostly caused by the damage of graphene sp² structure around edge boundaries, resulting in large electrical conductivity. The manufacturing process of rLGO is an economical and facile approach for the large scale production of highly thermally conducting graphene thin films with efficient EMI shielding properties, greatly desirable for future portable electronic devices.     

Part-A: Synthesis of polyaniline and carboxylic acid functionalized SWCNT composites for electromagnetic interference shielding coatings

Polyaniline (PANI) was synthesized by chemical oxidation process by using Ammonium persulphate (APS) as an oxidizer and HCl as a dopant. The effects of altering the stoichiometric ratio of monomer to oxidizer, addition time, reaction temperature and dopant concentration on the electrical conductivity of PANI were studied in detail. The synthesis procedure was optimized to yield the PANI with maximum electrical conductivity. The pure PANI thus synthesized exhibited the maximum electrical conductivity 5.6 Scm⁻¹. Different PANI–cSWCNT composites were prepared by ex-situ and in-situ methods and a comparative evaluation of electrical conductivity was carried out. From the electrical conductivity measurements, it was seen that maximum conductivity 27.12 Scm⁻¹ was achieved for 20% cSWCNT loaded PANI composite prepared by in-situ method and 12 Scm⁻¹ for the same composite prepared by ex-situ method. The efficacy of in-situ method, for conductivity enhancement was attributed to the formation of PANI coating over cSWCNT during the synthesis. This coating formation was further substantiated by FTIR, XRD, DSC, TGA, FESEM, and HRTEM analysis. The results of these studies confirmed that the in-situ prepared 20% cSWCNT loaded PANI composite is the most preferred filler for developing polyurethane based EMI shielding coatings.