1. Introduction
With the rapid development of communication technology and the widespread deployment of public mobile communication base stations, electronic signals now cover a vast majority of areas [
1,
2,
3,
4,
5]. However, the resulting EM pollution brings about a host of problems that seriously affect both human health and the normal operation of high-precision electronic equipment [
6,
7,
8]. To address these issues, lightweight and flexible EMI shielding films have gained extensive application in the field of intensive and lightweight electronic devices [
9,
10,
11,
12]. Nevertheless, current challenges faced by EMI shielding films also include low shielding efficiency, inadequate mechanical properties, and difficulties in large-scale production, which significantly restrict their application and development [
13,
14]. Therefore, there is an urgent need for the development of multifunctional EMI shielding films that are lightweight, flexible, highly effective and rapidly producible.
The thickness of EMI shielding materials is positively correlated with their shielding properties, making the improvement of EMI shielding film efficiency a key focus in research [
15,
16,
17,
18]. In order to address this issue, researchers have conducted a diverse range of investigations, primarily focusing on the utilization of multi-layer structures, “brick-mortar” structures, aerogel structures, asymmetric structures, sandwich structures and other related approaches [
19,
20,
21]. For example, Li et al. fabricated flexible and tough nanofibrillated cellulose/Fe
3O
4 and carbon nanotube/polyethylene films with multilayer alternating structures using an alternate vacuum-assisted filtration technique, which demonstrated an electromagnetic interference shielding effectiveness (EMI SE) value of 30.3 dB [
22]. Inspired by the structure of "brick and mortar," Gong et al. fabricated a multifunctional flexible composite membrane composed of PCC/MXene/polyvinyl alcohol (PMP) using a one-step vacuum-assisted filtration method, exhibiting an EMI SE value of 43.13 dB [
23]. Besides, Fu et al. proposed a laminated structural engineering strategy for the fabrication of an autonomous carbon nanotube-based aerogel film, which exhibits a compacted porous structure contributing to an enhanced EMI SE value of 35.1 dB through effective internal reflection loss [
16]. Among the aforementioned methods, sandwich-structured EMI shielding films can establish conductive networks by concentrating electrically/magnetically conductive fillers in specific layers. Therefore, the sandwich structure not only effectively enhances EMI shielding efficiency, but also provides structural support to the entire film and safeguards the intermediate shielding filler against friction, oxidation, and other forms of damage. As reported, Yao et al. prepared sandwich-structured Ti
3C
2T
x MXene/ANF films with an EMI SE greater than 49.7 dB by using a layer-by-layer construction method [
24]. However, at present, the preparation of sandwich-structured films through vacuum filtration is highly inefficient, necessitating the urgent development of a rapid and large-scale film fabrication method.
The incorporation of a high-performance polymer with exceptional mechanical properties and an effective shielding filler represents a viable approach to enhance the mechanical characteristics of EMI shielding composite films [
25,
26,
27]. Aramid fiber is a such kind of high performance polymer fiber which has been widely studied and developed in recent decades [
28,
29,
30,
31]. It is known for its excellent strength, high modulus and high heat resistance, [
32] and since Takayanagi et al. discovered the solubility of aramid, researchers have found a simple preparation method for aramid nanofibers (ANFs) [
33]. ANFs are extracted from macro aramid fibers by chemical etching and stripping, which is very simple and fast at present [
34,
35,
36]. ANFs inherit the excellent mechanical properties and thermal stability of macro aramid fibers. Wang et al. constructed a bidirectional conductive network to prepare a dual-function thermal management material. The film has a thermal stability of 31.3 W/mK and a mechanical strength of more than 100 MPa [
37]. At the same time, at the nano and micro level, it can be easily prepared into a film, which has become a basic building unit of high-performance composite materials and attracted our strong attention. For example Wang et al. added ND@PDDA to ANF/DMSO blending and scraped protonation film, which is of great significance for practical engineering applications [
38]. Zhou et al., prepared ANF@PPy thin films with an EMI SE of 41.69 dB when the amount of pyrrole (Py) monomer was 40 uL [
39]. In addition, among various EMI shielding fillers, silver nanowires with one-dimensional structure and high aspect ratio have ultra-high electrical conductivity up to 6.3 × 10
7 S/m. Therefore it is easy to construct excellent two-dimensional conductive network structures and obtain excellent flexibility, which can be widely used in EMI shielding films. For example, Zeng et al. prepared WPU/AgNWs nanocomposites with unidirectionally aligned micrometer-sized pores, where only 28.6 wt% of AgNWs could reach up to 64 dB in the X-band [
40].
In this work, the ANFs were prepared by dissolving aramid fibers in an alkaline solution of dimethyl sulfoxide using the aramid deprotonation method. Subsequently, a NM was utilized as a scaffold onto which the ANFs solution was applied and subsequently water-bathed, resulting in the formation of an aramid nanofiber film (ANF) supported by the scaffold. To create a conductive network structure with excellent EMI shielding properties, AgNWs solution was sprayed onto the surface of the film. Furthermore, to form a sandwich structure, another coating scraping step of ANFs solution was repeated to encapsulate the AgNWs conductive network structure inside. This sandwich-structured film allows for controllable EMI shielding efficiency by adjusting the loading amount of AgNWs. Additionally, due to its inner NM skeleton and outer ANF coatings, this film exhibits exceptional mechanical properties and thermal stability. Our work holds significant implications for facilitating rapid industrialized production of high-performance EMI shielding films.
2. Experimental Section
2.1. Chemicals and Materials
Poly-paraphenylene terephthalamide (PPTA) were bought from Yantai Tayho Advanced Materials Group Co., Ltd., China. Nylon mesh (NM, 200 mesh) were provided by Changzhou Hongli Hardware Co., LTD, China. Dimethyl sulfoxide (DMSO), potassium hydroxide (KOH), polyvinyl pyrrolidone (PVP, Mw≈5,8000) and glycerol were all purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., China. Silver nitrate (AgNO3) is supplied by Guangdong Guanghua Sci-Tech Co., Ltd., China. Sodium chloride (NaCl) is supplied by Tianjin baishi Chemical Industry Co., Ltd., China. Deionized water (DI water) is supplied by minling material. All chemicals were used without further purification.
2.2. Preparation of AgNWs
The AgNWs were synthesized using the well-established polyol reduction method [
41,
42]. 190 mL of glycerol and 5.86 g of PVP were added to a 250 mL three-necked flask,
followed by gradual heating from room temperature to 80 °C at a lower rotational speed. The temperature was then maintained between 80 and 90 °C until complete dissolution of PVP was achieved. Subsequently, the solution was allowed to cool down after discontinuing the heating process. Meanwhile, a mixture of 0.059 g of NaCl in 500 μL
of DI water and 10 mL of glycerol
was prepared by homogeneous mixing and preheated at 60 °C for at least 5 minutes.
Once the PVP-propanetriol solution reached a temperature of 55 °C, it was supplemented with 1.58 g of AgNO3 followed by addition of the preheated NaCl mixture. The whole reaction system was gradually heated up to 210 °C with a lower stirring speed and then the heating was stopped. The gray-green product was taken out, cooled to room temperature and then a large amount of DI water was added and stand for one week. After that, the mixture underwent three rounds of centrifugation at 4000 rpm using DI water as the washing agent. Ultimately, the AgNWs were dispersed in ethanol to yield an AgNWs solution.
2.3. Preparation of ANFs/DMSO Solution
The ANFs were prepared by the previously reported deprotonation process [
33,
43,
44]. Specifically, 1 g of KOH was dissolved in the mixed solvent containing 2 mL of DI water and 100 mL of DMSO. Then 1 g of PPTA was added. After stirring for 4 hours at room temperature, a dark red viscous ANFs/DMSO solution was obtained with a concentration of 10 mg/mL.
2.4. Preparation of NAAANF Films
The preparation process of NAAANF was illustrated in
Scheme 1a. The entire experiment was carried out at room temperature. Firstly, the ANFs/DMSO solution was coated on NM (
Scheme 1b), and then the complete film was immediately immersed in water for 5 minutes to ensure thorough protonation of ANFs. In this process, the reddish-brown film will gradually become transparent, which means that ANFs protonation was complete and formed a cross-linked ANF. Subsequently the wet film was dried at 60 °C for 30 minutes in a oven, resulting in the formation of NANF (
Scheme 1c). The inner EMI shielding layer was prepared by spraying the required amount of AgNWs on the NANF surface, and the film was denoted as NAANF (
Scheme 1d). Finally, the above steps were repeated to prepare another layer of ANF on the AgNWs coating to obtain NAAANF EMI shielding film (
Scheme 1e). These films were labeled as NAAANF0.2, NAAANF0.4, NAAANF0.6, NAAANF0.8 and NAAANF1.0 based on the amount of AgNWs added (ranging from 0.2 to 1.0 mg/cm
2).
2.5. Characterizations
The morphology and microstructure of AgNWs, ANFs, NANF, NAANF, and NAAANF were observed by field emission scanning electron microscopy (SEM, GeminiSEM500, Zeiss). The sample was sprayed with gold for 5 minutes prior to testing and the acceleration voltage during the test was 3 kV. Microscopic morphology of the raw materials ANFs and AgNWs was observed using transmission electron microscopy (TEM, JEM-1400Flash, JEOL) at an acceleration voltage of 120 kV. X-ray diffraction (XRD, D8 Advance, Bruker-AXS) of the sample used a copper target with diffraction angles ranging from 5 to 90°. Thermogravimetric analysis (TGA, TG209F1, NETZSCH) was used to analyze the thermal stability of the samples. The analysis was conducted in a nitrogen atmosphere, with a temperature range from 25 to 800 °C and a heating rate of 10 °C/min. A four-finger probe tester (FT-340, CHNT) was used to measure the resistance of different AgNWs concentrations. The EMI SE properties of the composite in the 8-12 GHz (X-band) microwave range were measured by waveguide method using vector network analyzer (VNA, AV3672, Ceyear). The measured scattering parameters (S11 and S21) were used to calculate the EMI SE of the materials, from which the total EMI SE (SET), absorbing shielding effectiveness (SEA), reflecting shielding effectiveness (SER), and power coefficients of absorptivity (A), reflectivity (R), and transmittance (T) were calculated as follow:
R = |S11|2
T = |S21|2
1=A + R + T
SER = − 10 log (1− R)
SEA = − 10 log (T/ (1− R)
SET =SEA + SER + SEM
The multiple reflection (SE
M) was generally ignored when SE
T > 15 dB [
45]. Tensile stress and strain test was performed on a universal testing machine (KJ-1065B, Kejian) at a speed of 10 mm/min to test the strip sample of 10 cm×1 cm [
46].
Scheme 1.
(a) The preparation process of NAAANF. Schematic structure and digital picture of (b) NM, (c) NANF, (d) NAANF, (e) NAAANF.
Scheme 1.
(a) The preparation process of NAAANF. Schematic structure and digital picture of (b) NM, (c) NANF, (d) NAANF, (e) NAAANF.
Figure 1.
(a) PPTA, (b) ANFs/DMSO solution and (c) ANFs solution. (d) SEM image and (e) TEM image of ANFs, (f) XRD of ANFs and PPTA, (g) TGA and (h) DTG curves of ANFs and PPTA.
Figure 1.
(a) PPTA, (b) ANFs/DMSO solution and (c) ANFs solution. (d) SEM image and (e) TEM image of ANFs, (f) XRD of ANFs and PPTA, (g) TGA and (h) DTG curves of ANFs and PPTA.
Figure 2.
(a) AgNWs solution. (b) SEM image, (c) TEM image and (d) XRD of AgNWs.
Figure 2.
(a) AgNWs solution. (b) SEM image, (c) TEM image and (d) XRD of AgNWs.
Figure 3.
SEM image of the NM (a1,a2,a3), NANF (b1,b2,b3), NAANF0.2 (c1,c2,c3), NAANF0.4 (d1,d2,d3), NAANF0.6 (e1,e2,e3), NAANF0.8 (f1,f2,f3), NAANF1.0 (g1,g2,g3) and NAAANF (h1,h2,h3).
Figure 3.
SEM image of the NM (a1,a2,a3), NANF (b1,b2,b3), NAANF0.2 (c1,c2,c3), NAANF0.4 (d1,d2,d3), NAANF0.6 (e1,e2,e3), NAANF0.8 (f1,f2,f3), NAANF1.0 (g1,g2,g3) and NAAANF (h1,h2,h3).
Figure 4.
(a) Surface resistance of NAAANF, (b) Electrical conductivity of NAAANF is demonstrated by small light bulbs, (c) EMI shielding performance at the X-band, (d) average EMI SET, SEA, and SER values, and (e) power coefficients of reflectivity (R) and absorptivity (A) values of NAAANF, (f) NAAANF0.4 EMI performance after 48h immersion in acidic, alkaline, and DI water, (g) This work compares with other recent studies on thickness and EMI performance, (h) Schematic of electromagnetic shielding mechanism.
Figure 4.
(a) Surface resistance of NAAANF, (b) Electrical conductivity of NAAANF is demonstrated by small light bulbs, (c) EMI shielding performance at the X-band, (d) average EMI SET, SEA, and SER values, and (e) power coefficients of reflectivity (R) and absorptivity (A) values of NAAANF, (f) NAAANF0.4 EMI performance after 48h immersion in acidic, alkaline, and DI water, (g) This work compares with other recent studies on thickness and EMI performance, (h) Schematic of electromagnetic shielding mechanism.
Figure 5.
(a) tensile strength–tensile strain curves, (b) tensile strength and tensile strain of NM, NANF and NAAANF, (c) TGA and (d) DTG curves of NM, NANF and NAAANF.
Figure 5.
(a) tensile strength–tensile strain curves, (b) tensile strength and tensile strain of NM, NANF and NAAANF, (c) TGA and (d) DTG curves of NM, NANF and NAAANF.
Table 2.
TGA related parameters of NM, NANF and NAAANF.
Table 2.
TGA related parameters of NM, NANF and NAAANF.
Sample |
T-5wt% (°C) |
Tmax1 (°C) |
Residue at 800 °C (wt %) |
NM |
387.4 |
398.0 |
7.64 |
NANF |
366.9 |
393.9 |
13.59 |
NAAANF |
373.7 |
393.8 |
20.57 |