With the continuous improvement of the integration of large-scale integrated circuits, electronic devices have become smaller and more powerful, but the accompanying electromagnetic radiation pollution and interference have become more and more serious. This is in aerospace, military, artificial It is particularly prominent in the fields of intelligence, 5G communications, and wearable electronic devices. At this time, electromagnetic shielding (EMI) materials are critical to the reliable operation of precision electronic equipment, information security and human health. Although metal EMI materials have excellent electrical conductivity, EMI shielding and thermal management properties, their high density, poor flexibility, inability to resist chemical corrosion, and difficulty in processing and forming, have severely restricted their application. The emergence of conductive polymers overcomes the shortcomings of the heavy weight of metal EMI materials, but this material has a high penetration threshold, and usually requires a large amount of fillers to obtain the ideal electrical conductivity and EMI performance, resulting in flexibility, mechanical properties (especially strength and Toughness) and workability deteriorate, and its EMI shielding effect is much lower than that of metal materials.
Therefore, the development of high-efficiency EMI shielding materials with super flexibility and excellent mechanical properties is still a huge challenge. MXenes is a unique two-dimensional (2D) transition metal carbide or nitride. It has excellent electrical conductivity, hydrophilicity and mechanical properties. It can be used as an electromagnetic shielding material. It combines polymer and one-dimensional (1D) organic Nanofibers mixed with MXene can enhance the mechanical properties of MXene, but the interface adhesion between these composites and MXene is weak. The addition of polymers and organic nanofibers leads to insulation of the contact parts of MXenes, which inevitably reduces material conductivity and EMI Shielding performance. Simply mixing various materials together is equivalent to creating a "brick house". Although the mechanical and EMI performance of composite materials has been improved, it is difficult to manufacture both super-flexibility, excellent mechanical properties and high-efficiency EMI. Performance materials.
Introduction
Ma Jianzhong from Shaanxi University of Science and Technology and Professor Gu Junwei from Northwestern Polytechnical University have innovated in the microstructure of composite materials. The traditional "brick house" was built into a two-story "small foreign building", which produced super flexible and excellent mechanical properties. Double-layer ANF-MXene/AgNW nanocomposite with high-efficiency EMI performance. When the MXene/AgNW content is 20 wt%, the double-layer nanocomposite exhibits excellent mechanical properties: tensile strength 235.9 MPa, fracture strain 24.8%, electrical conductivity reaches 922.0 S·cm-1, and EMI shielding efficiency is 48.1 dB. When the content of MXene/AgNW is 80 wt%, the maximum conductivity of the material is as high as 3725.6 S·cm-1, and the EMI shielding efficiency is 80 dB, which can shield 99.999999% of incident electromagnetic waves. In addition, this double-layer nanocomposite material is also an excellent Joule heater, the material saturation temperature can reach 41.2 ℃ under 1 V voltage, which is very suitable for wearable treatment applications.
In order to prepare Kevlar nanofibers ANF, the researchers used aromatic polyamide (PPTA) fibers as raw materials, and weakened PPTA by deprotonation in a potassium hydroxide/dimethylsulfoxide (KOH/DMSO) mixed solution. The hydrogen bonding interaction between the polymer chains produced a highly stable and uniform ANF dispersion with a diameter of about 10 nm and a length of several microns.
The researchers used Ti3AlC2 (MAX) as the raw material to selectively etch away the Al layer with HCl/LiF to prepare multilayer Ti3C2Tx (m-Ti3C2Tx), and then further ultrasonic stripped m-Ti3C2Tx into a single layer of Ti3C2TxMXene. The researchers synthesized AgNW by the liquid-phase polyol method, with an average diameter of about 50 nm and an aspect ratio of about 800.
After synthesizing the required raw materials, the researchers used two-step vacuum assisted filtration (TVAF) and hot pressing technology to successfully prepare a double-layer structured nanocomposite material: the first step, the concentration of 0.5 mg/mL ANF dispersion (80 mL) is vacuum filtered onto the nylon porous membrane (vacuum filtration I); then, a certain amount of MXene and AgNW is ultrasonically processed to obtain a homogeneous mixture solution; the second step is vacuum filtration to deposit it on the ANF/nylon membrane The upper part (Vacuum Filtration II); Finally, dry at 60°C and 1MPa hot pressure, and peel off the double-layer nanocomposite from the nylon membrane. As a comparison, the researchers also used a one-step vacuum method to prepare this material, which is called a "brick house."
The double-layer structure prepared by TVAF is completely different from the one-step method. The upper surface of the double-layer structure is dark gray (MXene/AgNW), and the lower surface is yellow (ANF color). The surfaces are all the same black. Not only that, the double-layer structure shows a leaf-like MXene/AgNW nanostructure on the upper surface. This structure is composed of 1D AgNWs as a conductive skeleton (vein) and 2D Ti3C2TxMXene as a connection (sheet). After the two complement each other, A continuous and efficient 3D conductive network is constructed on ANF to realize the rapid transmission of electrons. In contrast, the one-step homogeneously mixed composite material shows a uniformly distributed "brick house" structure of ANF, Ti3C2TxMXene and AgNWs.
Mechanical and electrical properties of double-layer nanocomposites
Researchers found that as the content of MXene/AgNW in the material increases, a more effective conductive network is formed, and the conductivity of the material increases: when the content of MXene/AgNW increases from 5wt% to 10wt%, double-layer and uniformly mixed nanocomposites The conductivity increased from 0.10 and 4.2 to 0.65 and 157.2 S·cm-1, respectively. Compared with uniformly mixed nanocomposites, the double-layer structure exhibits stronger electrical conductivity, which is inseparable from the more efficient MXene/AgNW conductive network of the double-layer structure. When the content of MXene/AgNW is 80 wt%, the conductivity of the double-layer structure is as high as 3725.6 S·cm-1, while the conductivity of the uniformly mixed structure is only 98.9 S·cm-1, which is 37 times higher.
The super flexibility and excellent mechanical properties of the double-layer structure are more important for the design of high-performance EMI shielding materials. The double-layer nanocomposite material can be folded in half and can withstand a weight of 500 g without any cracks or breaks, showing excellent mechanical properties. The double-layer nanocomposite exhibits very stable real-time relative resistance during repeated bending and stretching. R/R0 is between 0.996 and 1.008, and the radius of curvature is 4 mm when the bending angle is 200°. The "SUST" LED lamp has stable brightness when repeatedly bent and stretched, indicating that the material has excellent flexibility and conductivity stability under applied stress/strain fields.
Electromagnetic shielding performance of double-layer nanocomposites
The study found that in the frequency range of 8.2-12.4 GHz (X band), both double-layer and uniformly mixed nanocomposite materials exhibit good electromagnetic shielding performance, but the double-layer structure performance is even better: when the content of MXene/AgNW is 10% The EMI SE of the double-layer nanomaterial is 35.5 dB, which is much higher than the industry standard of 20dB, while the homogeneous mixed material is only 9.8 dB; when the content of MXene/AgNW is 80 wt%, the EMI of the double-layer nanomaterial increases to 80.0 dB. The EMI of the homogeneously mixed material is increased to 50.9 dB. It can be seen from the above results that only need to add 10wt% of MXene/AgNW, the double-layer nanomaterial electromagnetic shielding efficiency can reach 99.97%, which can meet the needs of most applications, and after adding 80 wt% of MXene/AgNW, double The layered nanocomposite material can shield 99.999999% of incident electromagnetic waves.
Thermal management performance of double-layer nanocomposites
In addition to excellent mechanical and EMI shielding performance, the double-layer nanocomposite material also has excellent thermal management performance and can be used as a high-performance Joule heater. When the content of MXene/AgNW is 20 wt%, the saturation temperature of the double-layer nanocomposite is 41.2℃ at a low voltage of 1 V, which is very suitable for wearable treatment applications; at 2.5 V, the surface temperature of the double-layer nanocomposite is 15 It can quickly exceed 115°C within seconds. The surface temperature of the double-layer nanocomposite can be easily adjusted by changing the voltage, and the surface temperature is evenly distributed. Even if the material is bent, the thermal management performance will not be compromised. The temperature can be maintained at about 75°C within 1 hour.
summary
The research group of Professor Ma Jianzhong of Shaanxi University of Science and Technology and Professor Gu Junwei of Northwestern Polytechnical University used two-step vacuum assisted filtration (TVAF) and hot pressing technology to prepare the nanocomposite EMI material ANF-MXene/AgNW with a double-layer structure. This material has super flexibility, excellent mechanical properties and high-efficiency EMI performance, and is also an efficient Joule heater. When the material is folded in half, it can still withstand a weight of 500 g without any cracks or fractures, showing excellent toughness and mechanical properties. When the content of MXene/AgNW is 80 wt%, the conductivity of the double-layer structure is as high as 3725.6 S·cm-1, which can shield 99.999999% of incident electromagnetic waves, while the conductivity of the uniform mixed structure is only 98.9 S·cm-1, which is completely higher 37 times, such excellent performance is inseparable from the double-layer structure, the rich hydrogen bond interaction between ANF and each component. The double-layer nanocomposite material can reach 41.2°C in a few seconds at a low voltage of 1 V, and is very stable; at 2.5 V, the surface temperature can quickly exceed 115°C in 15 seconds, showing excellent thermal management performance.