Abstract:

Objective With the development of wireless communication technology, the application of electronic devices in various scenarios has shown explosive growth, resulting in the increasingly electromagnetic radiation pollution. This not only can interfere the normal operation of electronic devices, but also seriously affect human health. Fabric-based electromagnetic shielding (EMI) material is an effective solution for protecting both the human body and sensitive electronic devices from electromagnetic radiation hazards, but traditional metalized or metal-coated EMI shielding textiles mainly rely on their high conductivity to reflect electromagnetic waves, inevitably causing secondary pollution. Therefore, it is necessary to develop high-performance EMI shielding textiles with low-reflection feature. However, according to Schelkunoff's theory, low reflectivity (R) and high shielding effectiveness (SE) exhibit the inherent incompatibility, especially on traditional textile substrates. Therefore, developing efficient EMI shielding textiles with low-reflection feature remains a huge challenge.

Method Impedance gradient structure can significantly reduce microwave reflection at the air-material interface, while enhancing energy dissipation through the "absorption-reflection-reabsorption" mechanism has been proven to be a promising solution. Herein, a NiCo₂O₄/Ni-W-P/MCSF composite fabric with Janus structure was successfully prepared by sequentially carboxyl-functionalized carbon nanotubes (MWCNTs) coating, localized electroless Ni-W-P plating, and hydrothermal-calcination of nickel cobalt oxide (NiCo₂O₄). The microstructure of composite fabric was characterized using scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. The surface resistivity and EMI shielding performance of composite fabric were tested using a multimeter and vector network analyzer. The influence of Ni-Co mass ratio on the EMI shielding performance of composite fabrics was investigated, and the modulation mechanism of Janus structure on the microwave reflection/absorption characteristics of composite fabrics was also explored.

Results Microstructure characterization confirmed that the MWCNTs coating and Ni-W-P plating on the fiber surface were dense and continuous, and the needle-like NiCo₂O₄ grows uniformly on fiber surface. Combined with cross-sectional analysis of composite fabric and individual fibers, a synergistic system with macroscopic Janus structure and microscopic fiber-radial heterostructure was successfully constructed. The results of surface resistance and EMI shielding properties showed that with the increase of Ni-Co mass ratio, a high impedance shell layer was covered on the fiber surface, resulting in an increase in surface resistance and a decrease in EMI SE value. However, the enhanced magnetic loss significantly improved the contribution for microwave absorption, achieving an absorption-dominated shielding mechanism. At an optimal Ni-Co mass ratio of 2∶3, the as-prepared NiCo₂O₄/Ni-W-P/MCSF composite fabric achieved an EMI SE of 63.14 dB within the K-band (18-26.5 GHz), coupled with an exceptionally low average R value of 0.095 (corresponding to >91% electromagnetic wave absorption), achieving the effective integration of the low reflection and high shielding properties onto a single textile substrate. Furthermore, the composite fabric also exhibited excellent superhydrophobicity (water contact angle of 152.1°) and good breathability.

Conclusion This study provides a significant reference for developing high-performance EMI shielding fabrics with low reflection and high EMI SE. The Janus structure architecture is achieved by constructing electrical-magnetic functional layers with distinct impedance characteristics on a 3-D spacer fabric substrate, and this asymmetric architecture establishes a special "absorption-reflection-reabsorption" dissipation pathway for electromagnetic waves, thereby overcoming the intrinsic incompatibility between low reflection and high shielding. The operational mechanism shows that when electromagnetic waves are incident from the NiCo₂O₄/MCSF side, the electrical/magnetic dual-functional conductive network with good impedance matching allows more electromagnetic waves penetration into the fabric interior, where the synergistic magnetic loss (from NiCo₂O₄), dielectric loss (from MWCNTs) and interface loss would attenuate the electromagnetic waves. Residual waves reaching the highly conductive NiCo₂O₄/Ni-W-P/MCSF side with severe impedance mismatch would be reflected back towards the NiCo₂O₄/MCSF side for secondary absorption. The as-prepared composite fabric exhibits great application potential for wearable electromagnetic protection.

Key words: Janus structure, electromagnetic shielding performance, 3D spacer fabric, nickel cobalt oxide, carbon nanotube, dip-coating method, hydrothermal method, impedance-asymmetric structure