| Nowadays,with the rising and flourishing of electronic information technology,human-computer interaction,5G communication and various smart devices have been widely and abundantly used,which make electromagnetic radiation ubiquitous.Electromagnetic radiation usually affects the human nervous system,interferes with the normal operation of sophisticated equipment and leaks important personal privacy and state secrets.As a result,the development of electromagnetic interference（EMI）shielding materials can assist in meeting all aspects’urgent protection needs.The traditional metallic EMI materials with excellent conductivity and electromagnetic shielding effectiveness（SE）greatly limit the application of flexible lightweight,small refinement and highly integrated terminal devices,because of its poor flexibility,high density,difficult to process molding and chemical resistance and other defects.In contrast,flexible conductive polymer composites（FCPCs）are emerging as potential materials in the field of electromagnetic shielding,often offering distinctive research advantages as well as facilitating multifunctional designs of fillers and structures.However,in order to avoid the mechanical property deterioration of FCPCs caused by the excessive use of fillers,the key to achieve a good shielding effect lies in the construction of conductive fillers and the design of hierarchical structure.Notably,how to utilize the fillers reasonably and effectively and ensure the stable formation of multi-layered structure interface is one of the key concerns to achieve efficient electromagnetic shielding materials.In response to these problems,many studies have been conducted to enhance the interference dissipation of electromagnetic waves（EMW）by introducing multi-component conductive particles with different functions into the polymer as multiple scattering/reflecting interfaces,which largely reduces the amount of fillers and increases the utilization of fillers.However,the enhancement of SE is still limited and needs further improvement.It is worth noting that the effective structural design of composites not only reduces the percolation threshold value,but also contributes to the high performance of polymers.In this study,we focus on the structural point of view.On one hand,the layer structure is employed to regulate the distribution of conductive fillers inside FCPCs to achieve the synergistic effect of different interfaces of multi-component particles;On the other hand,layer structure is utilized to induce the formation of FCPCs surface microstructure achieving a composite structure with multi-structured interface distribution,which enhances the interference scattering ability of EMW improving the shielding performance and reduces the reflection of composites.（1）Polydimethylsiloxane（PDMS）/multi-walled carbon nanotubes（MWCNTs）-nickel（Ni）-antimony oxide（Sb2O3）composites are prepared by a solution blending and layer-by-layer casting method.First,the synergistic combination of different ratios with functional fillers in conventional composites（CC）is discussed and the optimal mixing ratio is determined.Second,symmetrical structure（SS）and asymmetrical structure（AS）composites are further efficiently designed to obtain high shielding effectiveness/absorption coefficients（SE/A）based on controlled distribution of fillers.Due to the electro-magnetic-dielectric synergistic distribution of the filler and the Salisbury screen effect with a specific geometry,the SE/A of the SS and AS composites achieve～57.4 d B/～0.75 and～55.7 d B/～0.80,respectively.Finally,comparing with CC,the layer structure helps to realize the multi-interface synergistic loss effect of electric-magnetic-dielectric particles,interface polarization,multiple reflections and Salisbury screen effect,which largely improve the overall EMW absorption performance of composites.（2）The above work suggests that surface layer conductivity has a significant effect on EMI SE.High surface conductivity tends to facilitate the achievement of high microwave shielding performance.In order to further explore the effect of the surface structure on microwave interference,the surface microstructures are designed accordingly.In this study,a metallic Cu wrinkle structure with different periodic units is effectively achieved by regulating the substrate thickness and pre-stretching under sputtering deposition.The wrinkles effectively increase the specific surface area of the composites and affect the surface conductivity.The increase of substrate thickness and pre-stretching makes the tendency of denser wrinkles and greater amplitude as well as smaller wavelengths,in which the SE value of the composites increases and the reflection is reduced.In order to deeply evaluate the mechanism of surface microstructure for EMW shielding,EM simulations are performed to analyze the changes of electric,magnetic fields and surface currents as well as the corresponding attenuation distribution after microwave interference by surface micro-wrinkles of composites.It is found that metal wrinkles can seriously affect the variation of surface field and the difference of energy distribution is more obvious as the increase of thickness and pre-stretching.In addition,the formation of wrinkles makes the surface charges have an oriented movement,which greatly enhances the conductive loss.Finally,the appearance of a certain low-energy region between the wrinkles as a result of interfacial scattering interference,along with the enhancement of the Salisbury screen effect,which largely favors the reflectivity reduction,is also basically consistent with the experimental results.（3）In order to deeply explore the interfering effect of different heterogeneous structures with high conductive surfaces on microwaves,the microcrack structure is introduced on the basis of the previous work to construct a multi-interface structure of wrinkles-cracks-interlayer.The relative percentage and distribution of microcracks and micro-wrinkles are balanced by tuning the substrate modulus,thickness,and pre-stretching.The formation of microcracks makes the surface layer with conductive anisotropy,and the phenomenon is more obvious when the scale of microcracks is large.In addition,the electric field,surface current distribution of microcracks,and the attenuation effect of the surface layer on microwaves are simulated,and it is found that the size and distribution of different degrees of microcracks affect the difference in the density of surface charges.Meanwhile,the microcracks and micro-wrinkles sandwich composite structure with high percentage achieve high shielding performance of～47.6d B and low reflection coefficient of～0.58.Therefore,the enhanced EMI SE of the composite is mainly attributed to a combination of micro-capacitance effect,scattering interference from multiple heterogeneous interfaces,and conduction loss. |
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