EMI shielding and conductivity of carbon nanotube-polymer composites at terahertz frequency

High-permittivity Solvents Increase MXene Stability and Stacking Order Enabling Ultraefficient Terahertz Shielding

2D transition metal carbides and nitrides (MXenes) suggest an uncommonly broad combination of important functionalities amongst 2D materials. Nevertheless, MXene suffers from facile oxidation and colloidal instability upon conventional water-based processing, thus limiting applicability. By experiments and theory, It is suggested that for stability and dispersibility, it is critical to select uncommonly high permittivity solvents such as N-methylformamide (NMF) and formamide (FA) (εr  = 171, 109), unlike the classical solvents characterized by high dipole moment and polarity index. They also allow high MXene stacking order within thin films on carbon nanotube (CNT) substrates, showing very high Terahertz (THz) shielding effectiveness (SE) of 40-60 dB at 0.3-1.6 THz in spite of the film thinness < 2 µm. The stacking order and mesoscopic porosity turn relevant for THz-shielding as characterized by small-angle X-ray scattering (SAXS). The mechanistic understanding of stability and structural order allows guidance for generic MXene applications, in particular in telecommunication, and more generally processing of 2D materials.

Multifunctional Elastic Nanocomposites with Extremely Low Concentrations of Single-Walled Carbon Nanotubes

Stretchable and flexible electronics has attracted broad attention over the last years. Nanocomposites based on elastomers and carbon nanotubes are a promising material for soft electronic applications. Despite the fact that single-walled carbon nanotube (SWCNT) based nanocomposites often demonstrate superior properties, the vast majority of the studies were devoted to those based on multiwalled carbon nanotubes (MWCNTs) mainly because of their higher availability and easier processing procedures. Moreover, high weight concentrations of MWCNTs are often required for high performance of the nanocomposites in electronic applications. Inspired by the recent drop in the SWCNT price, we have focused on fabrication of elastic nanocomposites with very low concentrations of SWCNTs to reduce the cost of nanocomposites further. In this work, we use a fast method of coagulation (antisolvent) precipitation to fabricate elastic composites based on thermoplastic polyurethane (TPU) and SWCNTs with a homogeneous distribution of SWCNTs in bulk TPU. Applicability of the approach is confirmed by extra low percolation threshold of 0.006 wt % and, as a consequence, by the state-of-the-art performance of fabricated elastic nanocomposites at very low SWCNT concentrations for strain sensing (gauge factor of 82 at 0.05 wt %) and EMI shielding (efficiency of 30 dB mm^-1 at 0.01 wt %).

Progress in polymers and polymer composites used as efficient materials for EMI shielding

The explosive progress of electronic devices and communication systems results in the production of undesirable electromagnetic pollution, known as electromagnetic interference. The accumulation of electromagnetic radiation in space results in the malfunction of commercial and military electronic appliances, and it may have a negative impact on human health. Thus, the shielding of undesirable electromagnetic interference has become a serious concern of the modern society, and has been a very perspective field of research and development. This paper provides detailed insight into current trends in the advancement of various polymer-based materials with the effects of electromagnetic interference shielding. First, the theoretical aspects of shielding are outlined. Then, the comprehensive description of the structure, morphology and functionalization of the intrinsic conductive polymers, polymers filled with the different types of inorganic and organic fillers, as well as multifunctional polymer architectures are provided with respect to their conductive, dielectric, magnetic and shielding characteristics.

MXene-based ultra-thin film for terahertz radiation shielding

We have successfully fabricated Ti-based MXenes flakes, Ti₃C₂T_x, by chemical etching, then prepared it as an organic dispersion and finally spin-coated it on polyimide plastic substrate for terahertz wave shielding. The shielding effectivity of the 12 μm ultra-thin film can reach up to 17 dB measured by the terahertz time-domain spectra. We can attribute the excellent phenomenon to the intrinsic absorption of triple-layered Ti₃C₂, due to the similar double-peak type refraction curves, which have been respectively observed from the experimental samples and the simulation ones. High conductivity and strong THz absorption indicate the Ti₃C₂T_x MXene is the absorptive electromagnetic shielding material. Comparing with other kinds of THz shielding materials, the Ti-based MXenes might be a potential candidate for the next generation of ultra-thin and lightweight THz shielding.

Multilayered salt water with high optical transparency for EMI shielding applications

Electromagnetic interference (EMI) shielding for visual observation applications, such as windows utilized in military or aerospace, is important but difficult to realize due to conventional materials having difficulty in achieving sufficient transparency and EMI shielding simultaneously. In this paper, we present multilayered structures based on salt water for simultaneous highly optical transparency (OT) and EM shielding effectiveness (SE) performance. In the proposed structures, planar acrylic and glass were used as two types of clear substrates to hold salt water. The measured OT of both acrylic/salt water/acrylic and glass/salt water/glass structures was higher than 90% with a nearly uniform light transmission, which introduced a negligible impact on optical observation. Furthermore, both simulations and experimental results demonstrated that the SE of the multilayer structure was higher than 20 dB in the X-band from 7.5 to 8.5 GHz. Moreover, the SE was significantly enhanced by increasing the thickness of the salt water layer. Especially, both OT and SE of the multilayered structures were improved simultaneously by increasing the salinity of the salt water. These proposed structures demonstrate great potential in EMI shielding observation applications.

Pattern randomization: an efficient way to design high-performance metallic meshes with uniform stray light for EMI shielding

Here, we proposed an ingenious grid pattern design method called pattern randomization to obtain metallic meshes with uniform stray light. The periodicity of a grid is weakened by the pattern randomization. By comparing the diffraction patterns of one-dimensional periodic grid, one-dimensional aperiodic grid and concentric rings structure, we found that the "radial homogenization" and "angular homogenization" can uniform the high-order diffracted energy. The pattern randomization is proposed to achieve the "radial homogenization" and "angular homogenization" two-dimensional grid while ensuring connectivity. For collimated incident beam, the metal grid with a randomness (90%, 90%) obtained by pattern randomization method generates uniform stray light, while it maintains high visible light transmittance and high electromagnetic shielding efficiency (SE). The simulated results are experimentally verified that the high-order diffraction spots can be effectively suppressed. The coefficient Cv is reduced from 1078.14% to 164.65%. Meanwhile, the randomness of the designed grid structure hardly affects the visible light transmittance and shielding efficiency. The metallic mesh with a shielding efficiency about 17.3 dB in the Ku-band, a relative transmittance higher than 94% in the visible light band and an ultra-uniform diffraction pattern is obtained.

Graphene-based plastic absorber for total sub-terahertz radiation shielding

Increasing the requirements on telecommunications systems such as the need for higher data rates and connectivity via the Internet of things results in continuously increasing amounts of electromagnetic radiation in ever-higher telecommunications bands (up to terahertz). This can generate unwanted electromagnetic radiation that can affect the operation of electronic devices and human health. Here, we demonstrate that nonconductive and lightweight, graphene-based composites can shield more than 99.99% of the electromagnetic energy in the sub-THz range mainly via absorption. This contrasts with state-of-the-art electromagnetic radiation shielding materials that simply redirect the energy of the radiation from a protected area via conduction-based reflection mechanisms. This shifts the problem of electromagnetic pollution from one place to another. We have demonstrated that the proposed composites can be fabricated by industrial compatible methods and are characterized by specific shielding efficiency values that exceed 30 dB cm3 g-1, which is more than those for typical metals used today. Therefore these materials might help to solve the problem of electromagnetic environmental pollution.

Efficient terahertz anti-reflection properties of metallic anti-dot structures

We report the use of micrometer-sized copper (Cu) anti-dot structures as a novel terahertz (THz) anti-reflection coating (ARC) material and their superior performance over conventionally used metallic (Cu) thin films. Cu anti-dot structures of two different thicknesses (7 and 10 nm) with varying anti-dot diameters (100, 200, and 300 μm, inter-anti-dot separation fixed at 100 μm) are deposited on silicon substrates by RF magnetron sputtering and e-beam evaporation. The anti-reflection performance of these samples is studied in the frequency range of 0.3-2.2 THz. While continuous metallic (Cu) thin film minimizes the Fabry-Perot (FP) peak, it also suppresses the primary transmission peak, reducing the advantage due to the former effect. On the contrary, the anti-dot arrays reduce both the absolute amplitude of the FP peak and the amplitude ratio (AR) of the FP peak to the primary peak, making them a superior material for ARC applications. The AR can be further manipulated by varying the anti-dot size. A universal conductivity phase-matching condition, which is a prerequisite for the disappearance of the FP peak, is observed in these samples. The enhanced anti-reflection performance promotes these anti-dot structures as an efficient terahertz ARC material.

Double-layer interlaced nested multi-ring array metallic mesh for high-performance transparent electromagnetic interference shielding

We report a nested multi-ring array metallic mesh (NMA-MM) that shows a highly uniform diffraction pattern theoretically and experimentally. Then a high-performance transparent electromagnetic interference (EMI) shielding structure is constituted by the double-layer interlaced NMA-MMs separated by transparent quartz-glass substrate. Experimental results show that double-layer interlaced NMA-MM structure exhibits a shielding effectiveness (SE) of over 27 dB in the Ku-band, with a maximal SE of 37 dB at 12 GHz, normalized optical transmittance of 90%, and minimal image quality degradation due to the interlaced arrangement. It thus shows great potential for practical applications in transparent EMI shielding devices.

Terahertz conductivity engineering in surface decorated carbon nanotube films by gold nanoparticles

We report the controllable conductivity of single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes with their surface walls decorated by gold nanoparticles (Au NPs) with varying concentration in terahertz (THz) frequency range. Colloidal Au NPs of nominal diameter ∼15  nm are synthesized by the reduction of gold chloride solution using tri-sodium citrate. A simple chemical route is followed to attach Au NPs on the surfaces of both types of carbon nanotubes (CNTs). The attachment of Au NPs on the sidewalls of CNTs is confirmed by UV-visible spectroscopy and scanning electron microscope images. THz spectroscopic measurements are carried out at room temperature in transmission geometry in the frequency range of 0.3-2.0 THz. It is found that the THz conductivity of the surface decorated SWNT composites can either be increased or decreased by ±15% than that of the as-prepared SWNT composites by carefully choosing the Au NP concentration. The conductivity variation is qualitatively explained in terms of carrier trapping potential for low Au NP density, and alternative carrier conduction pathways at higher Au NP density and analyzed with the help of a modified universal dielectric relaxation model.

Generation of uniform diffraction pattern and high EMI shielding performance by metallic mesh composed of ring and rotated sub-ring arrays

We present an optical transmission model and a fast shielding effectiveness (SE) evaluation method for the inductive mesh comprising metallic rings with rotated sub-ring arrays, which can be extended for designing and optimizing other ring-based mesh structures. The theoretical analysis and experimental verification show that the established model and method are valid. A Ku-band SE >17 dB (98% energy attenuation) is observed for a triangular ring mesh with rotated sub-rings, and a normalized visible transmittance >95% is obtained with an ultra-uniform diffraction pattern, thus indicating the possibilities of our approach for high-optical-transmittance, strong-SE, reduced-image-degradation shielding applications.

Verification and improvement of equivalent refractive index models for evaluating the shielding effectiveness of high-transmittance double-layer metallic meshes

The validity of real and complex equivalent refractive index models (ERIMs) is verified for a shielding effectiveness evaluation of high-transmittance double-layer metallic meshes. Theoretical and experimental studies show that the real ERIM is invalid for thin substrates and inaccurate for thick substrates for double-layer meshes, although it has long been used successfully for single-layer meshes. However, the complex ERIM shows more reasonable results not only for double-layer but also for single-layer meshes, and the evaluation accuracy is further improved by modifying the equivalent reactance coefficient using least-squares fitting. Therefore, the modified complex ERIM is applicable in most conditions.

Achieving an ultra-uniform diffraction pattern of stray light with metallic meshes by using ring and sub-ring arrays

We provide theoretical and experimental evidence that introducing metallic rings and sub-rings in mesh unit cells significantly decreases the high-order diffraction energy. Moreover, rotating the sub-rings results in increased uniformity in the diffraction distribution without affecting the transmittance. Experiments show that the triangular ring mesh with rotated sub-rings exhibits a normalized visible transmittance greater than 95% as well as an ultra-uniform diffraction pattern of stray light, whose maximal normalized high-order diffraction energy is lower than 0.0167%. This kind of metallic mesh will be favorable in transparent electromagnetic interference shielding devices and touch screens.

Electromagnetic interference shielding in 1–18 GHz frequency and electrical property correlations in poly(vinylidene fluoride)–multi-walled carbon nanotube composites

A dilute acid-treatment of MWNT showed a low electrical percolation in PVDF and significant improvements in EMI shielding properties.