In the field of high-performance electromagnetic interference (EMI) shielding, shielding materials with excellent shielding effect, low consumption, and good mechanical flexibility have always been urgently needed by researchers. At present, the honeycomb structure composed of nano-structured frame materials and micron-sized holes shows its potential to replace conventional metal shielding layers due to its low density, good flexibility, ease of processing, chemical stability, and adjustable EMI shielding performance Great potential. Therefore, the design of the synergy of frame materials and their microstructures has always been the focus of research.
In recent years, a new type of two-dimensional metal carbides and nitrides, so-called MXenes, have attracted widespread attention due to their metal conductivity, excellent mechanical properties, and large specific surface area. However, the weak gelling ability and the characteristics of easy agglomeration limit its application for assembly into low-density aerogels. Therefore, designing composite materials with good conductivity, excellent EMI shielding performance, and low density based on MXenes is facing great challenges.
The Swiss Federal Institute of Materials Science and Technology Gustav Nyström team used ultra-thin cellulose nanofibers (CNF) to collaboratively construct ultra-low-density MXenes aerogels with oriented bionic cell walls. This study revealed that the angle between the directional cell wall and the direction of the electric field of the incident EM wave has a significant impact on the EMI shielding performance, thus proposing a new microstructure design strategy. When the density of this aerogel is only 8.0 and 1.5 mg/cm3, its EMI shielding effect can reach 74.6 or 35.5 dB, respectively. The shielding effectiveness of the normalized surface is as high as 189400 dB cm2/g, which greatly exceeds other EMI shielding materials reported so far. The research was published in "Advanced Science" as a paper entitled "Nanocellulose-MXene Biomimetic Aerogels with Orientation-Tunable Electromagnetic Interference Shielding Performance".
The author prepared a stable CNF dispersion through the oxidation of biomass cellulose fibers, and then grinded it to weaken the hydrogen bonds between adjacent cellulose chains. These CNFs have a large number of hydrophilic groups at the end, which can make MXene and CNF aqueous dispersions evenly mixed without adding other surfactants. The similarity of hydrophilicity and surface groups makes CNF uniformly deposited on MXene nanosheets, so that the former can be used as an effective structure directing agent in the aerogel manufacturing process. Figure 1e shows the unidirectional ice-like freeze-drying method for preparing anisotropic honeycomb MXene/CNF composite aerogel. In this synthesis process, the formed pyramidal crystals do not contain the MXene layer with CNF on the surface, thereby forming a directional interconnected MXene mixed cell wall. The assembly of this intact cell wall can be explained by the effective crosslinking of MXene with hydroxyl-containing groups in the CNF, which ultimately helps to form a low-density and strong composite aerogel. The SEM image clearly observed the oriented cell wall and the unidirectional pores with a gap of about 20 μm in the longitudinal plane.
The experimental results show that regardless of the CNF content, the pores of adjacent aerogels are almost the same, and the thickness of the cell wall is also similar, because the thickness of the pore wall depends on the solution concentration, and the diameter of the pore channel is induced by the temperature Gradient effect. The adjustment of CNF content can effectively regulate the compressive strength, modulus, electrical properties and EMI shielding effect of an anisotropic composite aerogel. By adding 33 wt% CNF, the compressive modulus of the composite aerogel can reach 9.61 kPa, which is about ≈350% higher than the modulus of the pure MXene aerogel. This result highlights the critical role of CNF, which can produce super Light and strong aerogel. However, too much CNF will inevitably reduce conductivity and reduce the number of charge carrier soft aerogels, which greatly damages its EMI shielding effect. Due to the interaction of CNF and MXene, the aerogel can provide high EMI shielding performance even at a low CNF content. The study finally selected 17 wt% CNF as the optimal content to prepare a composite aerogel.
Aerogel EMI shielding performance
The ultra-light composite aerogel is mainly attributed to the structural orientation of CNF. Figure 3a shows that the pure MXene aerogel with a thickness of 2 mm (density of 2.0 mg/cm3) has an EMI shielding effectiveness of 25.5 dB, which is much lower than the MXene/CNF composite aerogel with a thickness of 2 mm (35.5 dB). 1.5 mg/cm3. In addition, MXene/CNF aerogel with a density of 8.0 mg/cm3 can exhibit a more excellent EMI shielding effect of 74.6 dB. In addition, the greater the thickness of the aerogel, the better its EMI shielding performance. The MXene/CNF composite aerogel with a thickness of 1 mm (density ≈ 1.5 mg/cm3) has a shielding performance of 28.4 dB, which can be increased to> 45 dB when the thickness is increased to 3 mm, corresponding to 99.99% of the incident EM attenuation. As shown in Figure 3d, the aerogel in this study has achieved excellent shielding performance up to 189400 dB cm2/g, which is much higher than other shielding materials, including carbon-based, metal-based and other MXene-based systems. The excellent EMI shielding performance of the MXene/CNF composite aerogel is attributed to the synergy between the one-dimensional CNF and two-dimensional MXene nanosheets, making it possible to achieve directional bionic cell walls.
Summary: Using the inherent characteristics and structure of MXene and the interaction between CNF and MXene nanosheets, the authors prepared an ultra-low density and high flexibility MXene/CNF composite aerogel. The aerogel is composed of oriented and complete cell walls through a bionic strategy. Its excellent EMI shielding effect depends on the angle between the oriented cell wall and the surface. This ultra-light aerogel exhibits a shielding performance that far exceeds the EMI shielding materials reported in other literatures, making the further application of MXene/CNF composite materials with extremely high EMI shielding efficiency in electronic devices possible.
Original link: https://onlinelibrary.wiley.com/doi/10.1002/advs.202000979