A kind of pollution known as electromagnetic interference (EMI), which results from ubiquitous usage of various electronic communication and military radar equipment, has been receiving increasing attention recently. However, large-area EMI shielding on transparent and/or curved surfaces, including building windows, curved glass wall, and special requirements spaces (SRSs), remains hard to achieve. In this paper, a silver nanofiber (AgNF) based flexible and transparent EMI shielding film was successfully assembled via a room-temperature roll-to-roll production method. For transparent application scenario, AgNF with 89% transmittance in visible range and 1 μm thickness shows ~20 dB shielding efficiency (EMI SE). On the other hand, total shielding (>50 dB) is obtained when the thickness of AgNF increases to 10 μm, while its transmittance in visible range remains higher than 75%. Considering the facile and scale-free production technology, this material can be readily applied in large-scale, transparent, and/or SRSs EMI shielding.
While various modern electronic equipment in many fields are improving people’s daily life, a kind of pollution known as electromagnetic radiation or electromagnetic interference (EMI) emerged. Among the concerned wave ranges, X-band electromagnetic wave with the frequency from 8 to 12 GHz plays an important role in military, weather radar, TV picture transmission, mobile device and telecommunication fields.1,2 This issue, however, if left unchecked can cause severe adverse effects to communication systems, sophisticated electronics and even human bodies.3,4,5,6,7 Foreseeably, EMI shielding will become more and more important for modern society due to the rapid growth of radio frequency radiation sources.8,9 The primary mechanism of EMI shielding is reflection. For reflecting the radiation by shielding materials, the shielding materials must have mobile charge carriers (electrons/holes) which interact with the electromagnetic fields in the radiation.10 The auxiliary mechanism of EMI shielding requires the absorption of electromagnetic radiation because the electrical and/or magnetic dipoles of the material interact with the radiation. High conductivity is the main factor that determines the reflectivity and absorption characteristics of shielding.11
The earliest method of preventing EMI was using a specific slurry, in which metal powders such as silver, copper and nickel are mixed into a diluent. The drawbacks of this method include poor binding force, low EMI shielding efficiency (EMI SE) and powder nonuniformity, seriously restricting its further development. Metal sheet made by electroless plating, vacuum deposition, and sputtering is another type of material used for EMI shielding.12,13,14,15 However, electromagnetism properties of these bulky materials are susceptible to cracks come from bending and stretching, which determine that metal sheets cannot be used in curve-surface and flexible situations. Recently, various nanomaterials have been developed to resist static hazard and electromagnetic wave pollution, including nano-metal,16,17,18,19 carbon,20,21,22 and polymer-supported composites.23,24,25,26,27,28,29,30 Albeit these materials with favorable EMI SE can be successfully applied to curve-surface coating and equipment casing, there has been little progress with EMI shielding of windows, since in this case sufficient shielding, optical transparence and dimensions have to be achieved.
It is generally agreed that the effectiveness of a reflection-type shielding material requires a continuous conductive structure with an electrical conductivity of at least 1 S m−1.31 As a good conductor, silver-based materials can easily meet and even exceed this requirement. However, for large-scale and transparent surface applications, an ineluctable contradiction is that in-plane continuous silver layer shows highly visible transmittance only if its thickness is far below skin depth (~650 nm for silver materials). Therefore, zero-dimensional silver nanoparticles coating material and two-dimensional silver film hardly to be considered as EMI shielding materials. On contrary, one-dimensional (1D) silver network is a promising structure to optimize the trade-off between transmittance and thickness due to its intrinsic low fill-factor and adjustable diameter. In recent years, more and more technologies have been proposed for continuous and large-scale fabrication of metal fibers and networks.32,33,34,35,36,37 Beyond solution reduction,38,39 successive multistep growth (SMG) method,40 and electrostatic spinning,41 1D silver include silver nanowires (AgNWs),42,43,44,45 silver nanotroughs,46 and silver fiber (AgNF)47 can be more cost-effectively fabricated in this day. Large-scale and room-temperature production of 1D silver is what people always pursuing. The challenge mainly consist in the annealing process for the reduction of silver ion, or the spattering technology for directly depositing of metallic silver, which are usually performed in a confined space such as a tube furnace or a sputtering apparatus. Such confined equipments will directly limit the scale of the final products.48 Anothoer key issues to be resolved is that room-temperature junction-fusing between crossing 1D silver network during fabrication process, which can greatly enhance material conductivity.
In this work, we fine investigate the technological parameter of blow-spinning, and propose a large-scale and transparent junction interconnected AgNF network for transparent surfaces SMI shielding. The AgNF network is fabricated via a roll-to-roll (R2R) production combining blow-spinning with in-situ UV irradiation.48 The forming of junction between crossing fibers is achieved by incorporation of lauryl sodium sulfate (SDS) to decrease precursor solution surface tension and adjusting of gas speed of blow-spinning. High transmittance in visible range and diameter larger than silver’s skin depth was obtained by finely adjusting other technological parameters such as spinning time, polymer concentration and needle type.
Figure 1a shows a simplified schematic of roll-to-roll blow-spinning process and UV treatment. Using this air-driven method (Fig. 1b), a fiber network can be easily fabricated with arbitrary size to cater to different environments, from portable device to building-scale applications, while maintaining the continuous structure of each fiber (Fig. 1c, d and S1, Supplemental Material). For the protection of precision instruments and portable devices, we designed an EMI shielding demo consisting of a grounded metal salver and different cover films (iron net and AgNF/PET). As shown in Fig. 1e, the signal strength dropped to about 40% when the mobile phone was covered by an iron net. When the cover was changed to AgNF/PET film, there was only about 20% signal left.
In a typical blow-spinning production, solution viscosity and needle type codetermine the diameter of final fibers, injection speed and air velocity codetermine the productivity. The transmittance of fiber network is codetermined by sample thickness, fibers density (so called fill-factor) and fibers diameter. The thickness of fiber network is in linear relation to the spinning time, and there is a negative correlation between the thickness of fiber samples and its optical transmittance in the visible region. Therefore, by controlling the spinning time and fibers diameter, the optical transmittance of AgNF can be varied (Fig. 2a, b). As an X-band EMI shielding material, AgNF with the diameter larger than 650 nm is demanded as mentioned before. By co-adjusting needle type and concentration of PVP, AgNF with the diameter from <500 to >1000 nm can be obtained. Wherein, G30 needle with an inner diameter of 0.16 mm and a PVP concentration of 5% are chosen to spin ~650 nm AgNF (Fig. 2d). Moreover, the combination of each single AgNF in network structure will severely affect its conductivity,49 and then further affect its electromagnetism property. SEM images of AgNF without and with the adding of sodium dodecyl sulfate (SDS) are shown in Fig. 2e, f, respectively. The junctions of AgNF with SDS naturally interconnected after 3 h UV irradiation. This junction interconnected structure mainly comes from the coefficient of surface tension reducing after SDS is added into the precursor solution, and the squeezing action of high-speed airflow on deposited precursor fibers, with is ductile because of the residual organic solvent.
Research has shown that EMI shielding materials usually need high conductivity of at least 1 S m−1.31 The fiber conductivity increases continuously with time under UV irradiation. Three hours of irradiation UV led to an equivalent result of annealing at 240 °C, after that precursor fibers completely reduce to silver, which then becomes an excellent conductor (Fig. 3a, b). Raman spectrum demonstrates the reaction of Ag+ ion (Fig. 3c, d). At the Raman shift about 1039 cm−1 was decreased after 3 h of UV irradiation, which attributing to NO3- ion. C–C bond at 1428 cm−1 from PVP was also decreased after 3 h of UV irradiation. Moreover, two characteristic peaks around 1350 and 1553 cm−1 were found, suggesting the existent of amorphous carbon coating (Fig. 3e). Under such a thin amorphous carbon layer, silver particles on fiber are uniform and consecutive (Fig. 3f, g and S2, Supplemental Material). XPS results of AgNF in Ag3d region showed well separated spin-orbit components with a gap of ~6.0 eV, corresponding to silver metal. Peak differentiation imitating results of C1s and O1s also indicate the same reaction process during UV irradiation (Figure S3, Supplemental Material).48,50 Figure 3h–j show the results of flexibility and durability test of AgNF and Ag films. As a metal-based fiber material, AgNF shows both high flexibility and bending stability (Fig. 3h). Compared with the Ag film obtained via magnetron sputtering, AgNF shows superior mechanical properties include the ultralow bending radius (Fig. 3i) and 1000 times bending durability (Fig. 3j), which as the result of two characteristics: (i) the specific structural advantage of one-dimensional materials (Figure S4, Supplemental Material); (ii) the inherent flexibility of metallic silver. The bendability of AgNF enables applying this material on curved surfaces, such as certain windows.
In addition to its remarkable transmittance and flexibility, AgNF also shows good microwave shielding performance. The EMI SE is a key index in EMI shielding related fields, and it can be expressed based on the S-parameters according to following equations:51
As shown in Fig. 4a, a comparison was made between AgNF with a density of 0.2 g cm−3 and other EMI shielding materials, including CNT,19,52,53 rGO,20,54 graphene,21,55,56,57,58 stainless steel fiber,7 silver NW,16 copper NW,17 nickel hybrid fiber,7,59 carbon fiber,7 and some ceramic materials.59,60,61 Moreover, the specific EMI SE of AgNF is orders of magnitude higher than that of graphene,22,62 copper,63 stainless steel,63 nickel fiber,63 carbon fiber,63 and CNT.20 Owing to the low density of fiber structure,37 ultralight AgNF film the specific EMI SE of AgNF reach ~5000 dB cm3 g−1, which is higher than most EMI shielding material have been used (Fig. 4b). Figure 4c shows the spectra between 8 and 12 GHz of AgNF for different thickness from 100 to 1 μm. It can be seen that the thickness of AgNF is a critical parameter for EMI shielding, with 100 μm AgNF film resulting in an EMI SE about 76 dB. Moreover, this value decreases with decreasing thickness. Results from the S-parameter, including EMI SER (|S21|) and EMI SEA (|S11|) (Fig. 4d, e) further confirmed the reflectivity of the AgNF film. The theoretical EMI SE values can be calculated using plan wave theory based on the measured electrical conductivity of AgNF (Figure S5, Supplemental Material).34 While σ/ωε0 ≫ 0, AgNF can be identified as a good-conductor and its EMI SE calculated according to following equations:64,65
where σ is the conductivity of material, ω is the angular frequency of electromagnetic wave, ε0 is the permittivity of vacuum, μ0 is the permeability of vacuum, d is the sample thickness, and δ is the skin depth of materials. As shown in Figure S6, Supplemental Material, the theoretical EMI SE values are slightly higher than the measured ones, owing to the structure differences between in-plane network and layered composites. The flexibility is an important character of EMI shielding materials. Highly flexible is one of the most important premise for mass production and transport, which also show significance in many fields such as curve-surface shielding. Figure S7, Supplemental Material, shows the EMI shielding performance of 100-μm-thick AgNF network on different bending stages from its plane state to the bending radius of 3.0 cm. The EMI SE of AgNF network shows slight increase with the reduction of bending radius, owing to more material was filled in to cover the cross section of waveguide when bending radius decrease.
In order to describe the working principle of the AgNF network structure and the behavior of plane electromagnetic wave during EMI shielding, time domain simulation was performed in computer simulation technology microwave studio (CST MWS) based on a metallic-silver-network approximate model. The EMI SER (|S21|) and EMI SEA (|S11|) results are shown in Fig. 4f, g, respectively, and show agreement with the VNA experimental results. Noted that the simulated result of S|21| is higher than that of test result, which mainly due to the uneven thickness of single-layer AgNF. As AgNF thickness decreases nearby skin depth, the diameter of singe fiber directly determined the local thickness. As shown in Fig. 2c, red line and error bar, some fibers diameter may drop below 650 nm, which will led to a local electromagnetic transmission. Therefore, it can be concluded that the AgNF network, when its thickness exceeds skin depth, behaves as a simple metallic-silver-network structure in 8–12 GHz microwave frequency band. The field distribution through structural unit is shown in Figure S8, Supplemental Material, with the incident wave propagating onto the structure unit from the right. Results show that total shielding for fill-factor of metallic-silver-network exceeds 20%, which is according well with the experimental results of AgNF. Field distribution of plane incident wave through different fill-factor AgNF of 20, 44, 55, and 80% (corresponding to the fiber thicknesses of 10, 20, 50, and 100 μm) are also simulated via CST MWS (Figure S9, Supplemental Material). A typical surface reflection can be found since AgNF with a fill-factor of 20%. To investigate its microwave blocking effect visually, point source field scanning was performed on an annular AgNF sheet (Fig. 4h, i), and subsequently performed a simulation with a 1 mm copper sheet under the same conditions (Fig. 4j, k). Results show a similar field distribution (total shielding), indicating an equivalent microwave shielding effect of the low fill-factor AgNF network and copper.
In conclusion, a flexible and transparent AgNF network structure was continuous fabricated via a R2R production combining blow-spinning with in situ UV irradiation. By controlling the spinning time, AgNF networks with different thicknesses and fill-factors were obtained. EMI SE was discussed in the range of 8–12 GHz. VNA experimental results demonstrate a remarkable EMI SE of AgNF, up to ~76 dB of EMI SER (|S21|) with a thickness of 100 μm. By comparison with the CST simulation results, we found that the shielding behavior of AgNF (when d ≫ δ) is very similar to that of simple metallic-silver-network and both of them exhibited total shielding for fill-factors higher than 20%. Moreover, it was found that the shielding effect of 100 µm AgNF for microwaves is equivalent to a 1 mm copper sheet. Therefore, we can conclude that remarkable EMI SE, unique flexibility and adjustable transparence at visible light of AgNF enable it to be applied for large-scale or complex transparent surface EMI shielding.
AgNO3 (99.8%) was purchased from Beijing Chemical Works. PVP (Mw = 1,300,000) and Sodium dodecyl sulfate (SDS, 99.0%) were purchased from Alfa Aesar. Acetonitrile (99.0%) was purchased from Aladdin. Fluorocarbon surfactant (Capstone FS-3100) was purchased from Shanghai Jianbang Industry Co., Ltd. PET with a thickness of 0.1 mm is commercial available. All reagents were used without further purification.
Preparation of AgNF network
In this work, we used a R2R production to prepare PET-based AgNF network. Typically, 1.975 g of PVP and 16.120 g of AgNO3 were dissolved in 21.400 g of acetonitrile. Then, 1 wt% of SDS was added into the solution to decrease surface tension to facilitate forming junctions between crossing fibers. A pale-yellow precursor solution was obtained after 30 min of stirring. For blow spinning, the precursor solution was jetted out from four air-propelled injectors to a rolling continuous track covered with PET. The speed of the continuous track was 0.5 m h−1 and the speed of injection was 1.0 mL h−1. Four UV lamps (250 W, Philips Lighting) located side by side were used to reduce Ag+. The distance between the UV light source and substrate was set to 10 cm. The AgNF based EMI shielding film was obtained after 3 h of UV irradiation.
Material microscopic characterization
The micro-morphological characterization of AgNF were measured via a field emission scanning electron microscope (FE-SEM, LEO-1530, Zeiss, Germany) and a transmission electron microscope (TEM, JEOL-2100, JEOL Ltd. Japan). The crystal lattice information of samples were obtained via an X-ray diffractometer (XRD, D/max 2500, Rigaku, Japan) with Cu Kα radiation (λ = 1.54178 Å) from 2θ = 10° to 90°. Raman spectra were obtained via a Raman spectrometer (LabRAM HR Evolution, HORIBA Jobin Yvon, French) with 325 nm excitation wavelength. An X-ray photoelectron spectrometer (Escalab 250Xi, Thermo Fisher, America) equipped with an Al Kα radiation source (1487.6 eV) and hemispherical analyzer with a pass energy of 30.00 eV and an energy step size of 0.05 eV was used. Thermogravimetric analysis and differential thermal gravity data for the measurement of PVP were obtained via a thermogravimetric analyzer (STA 449 F3, Jupiter, Germany). The transmittance of AgNF in visible range was collected via an UV–Vis-NIR spectroscope (Lambda 950, Perkin-Elmer, America) with quartz glass as reference. The sheet resistance of as-prepared AgNF was measured via a multifunctional four-point probe tester (ST-2258C, Suzhou Jingge Electronic Co., LTD, China).
EMI shielding characterization
EMI SE of as-prepared materials were measured via a vector network analyzer (PNA-L N5230C, Agilent Technologie, America) with 8–12 GHz calibration kit. Time domain simulation was performed in computer simulation technology microwave studio (CST MWS) based on a metallic-silver-network approximate model.
The authors declare no competing interests.
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This study was supported by the National Basic Research of China (Grants 2015CB932500), National Natural Science Foundations of China (Grant 51661135025 and 51522207), BUPT Excellent Ph.D. Students Foundation (Grant GX2017205), and State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications).
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