Electromagnetic Interference Shielding Polymer Composites: Structural Design And Performance Research

With recent emergence of portable and wearable electronics,it is imperative to design a novel electromagnetic interference（EMI）shielding material which possesses flexibility,ultra-thinness and excellent EMI shielding effectiveness（SE）to prevent the adverse effect of electromagnetic radiation.The conductive polymer composites（CPCs）has drawn tremendous attention because of their huge application potential,such as high mechanical deformation capability,controllable conductive properties and process-efficiency.Based on skin effect and shielding theory of schelkunoff,excellent conductivity and perfect conductive network are the prerequisites for CPCs electromagnetic shielding composites to achieve high electromagnetic shielding efficiency.At the same time,effective multi-interface reflection and absorption,as well as the dielectric loss and hysteresis loss,are important factors to realize the controllable electromagnetic shielding performance of CPCs electromagnetic shielding composites.Therefore,conducting network structure design for CPCs electromagnetic shielding composites has become an important way to realize its high performance.The electromagnetic shielding network of CPCs electromagnetic shielding composites is magnetically doped to provide controllable electromagnetic shielding performance.In this paper,we mainly focus on the structural design of electromagnetic shielding composites and the coordinated use of multi-functional filler,the main content can be divided into the following five parts:（1）Conductive ultrahigh-molecular-weight polyethylene（UHMWPE）composite with segregated nickel（Ni）structure which exhibited highly efficient electromagnetic interference（EMI）shielding performance was abricated in this work.By synthesizing nickel coated UHMWPE particles via electroless deposition and hen hot compressing,the prearranged thin Ni layer could only selectively locate at the boundary etween UHMWPE regions due to the specific high melting viscosity of UHMWPE,and thus constructed well connected segregate Ni conductive network in UHMWPE matrix.Owing to the advantage of egregated network structure,the UHMWPE/Ni composite shows a low percolation threshold and high onductivity,and exhibits an EMI shielding effectiveness（SE）of average 55 dB with Ni content of only 2.58vol%in X-band.This result indicates the segregated UHMWPE/Ni composite can serve as an ultraefficient material for EMI shielding applications.（2）A flexible and highly conductive nylon porous membrane（NPM）/nickel（Ni）composite film with outstanding electromagnetic interference（EMI）shielding performance is fabricated via a facile electroless deposition method.The NPM,with a large specific surface area,is amenable to the electroless deposition of Ni and the enhancement of interaction between Ni and NPM.Highly conductive Ni layers are uniformly deposited on the upper and lower surfaces of NPM to construct a sandwich structure that provides numerous interfaces to reflect,scatter,and absorb electromagnetic waves.The resultant NPM/Ni film exhibits an ultra-high EMI shielding effectiveness（EMI SE）of 77 dB with a film thickness of only 100μm.Moreover,the NPM/Ni film shows cyclic EMI shielding stability under repeated mechanical deformation,with 85%retention after 300 bending cycles.Our work provides an effective approach to fabricate reliable metal-polymer composite films that show potential for applications in EMI shielding.（3）With recent emergence of portable and wearable electronics,it is imperative to design a novel electromagnetic interference（EMI）shielding material which possesses flexibility,ultra-thinness and excellent EMI shielding effectiveness（SE）to prevent the adverse effect of electromagnetic radiation.Herein,we fabricate the ultrathin and flexible four corners needle zinc oxide whiskers/silver/waterborne polyurethane（T-ZnO/Ag/WPU）conductive films to realize different EMI shielding demand by tuning the nanofiller dispersion morphology.The T-ZnO/Ag/WPU composite film with uniform structure（prepared by blade coating method）shows excellent flexibility and ultrahigh EMI SE of over 42 dB at a thickness of only 0.02mm.Highly reliable shielding ability was achieved with 92%EMI SE retention after 1000 folding cycle tests.The T-ZnO/Ag/WPU composite film with deposited layer structure（prepared by casting method）exhibits an ultrahigh EMI SE of over 87 dB at a thickness of 0.25 mm.At the same time,the T-ZnO/Ag/WPU composite shows great potential in printable circuit.The ultrathin and flexible T-ZnO/Ag/WPU composite films with excellent EMI SE are highly promising for applications in next-generation flexible electronics,especially the portable and wearable electronic devices.（4）Highly efficient electromagnetic shielding materials entailing strong electromagnetic wave absorption and low reflection have become an increasing requirement for next-generation communication technologies and high-power electronic instruments.In this study,a new strategy is employed to provide flexible waterborne polyurethane（WPU）composite films with an ultra-efficient electromagnetic shielding effectiveness（EMI SE）and low reflection by constructing gradient shielding layers with a magnetic ferro/ferric oxide deposited on reduced graphene oxide（rGO@Fe₃O₄）and silver-coated tetra-needle-like ZnO whisker（T-ZnO/Ag）functional nanoparticles.Because of the differences in density between rGO@Fe₃O₄ and T-ZnO/Ag,a gradient structure is automatically formed during the film formation process.The gradient distribution of rGO@Fe₃O₄ over the whole thickness range forms an efficient electromagnetic wave absorption network that endows the film with a strong absorption ability on the top side,while a thin layer of high-density T-ZnO/Ag at the bottom constructs a highly conductive network that provides an excellent electromagnetic reflection ability for the film.This specific structure results in an“absorb-reflect-reabsorb”process when electromagnetic waves penetrate into the composite film,leading to an excellent EMI shielding performance with an extremely low reflection characteristic at a very low nanofiller content（0.8 vol%Fe₃O₄@rGO and 5.7 vol%T-ZnO/Ag）:the EMI SE reaches 87.2 dB against the X band with a thickness of only 0.5 mm,while the shielding effectiveness of reflection（SE_R）is only 2.4 dB,and the power coefficients of reflectivity（R）is as low as 0.39.This result means that only 39%of the microwaves are reflected in the propagation process when 99.9999998%are attenuated,which is the lowest value among the reported references.This composite film with remarkable performance is suitable for application in portable and wearable smart electronics,and this method offers an effective strategy for absorption-dominated EMI shielding.（5）In order to reduce the reflectivity R of the shielding composites to less than 0.1,the preparation of the electromagnetic shielding material with low reflection characteristics and high efficiency dominated by electromagnetic wave absorption mechanism can be truly realized.We propose a strategy of combining the design of the aligned structure and the layered shielding network to prepare a highly effective electromagnetic shielding composite foam with gradient structure.The rGO@Fe₃O₄ and CNT foam with aligned structure were prepared by vacuum freeze drying method,respectively.The T-ZnO@Ag/WPU composite film with deposition structure was prepared by casting method.The multi-interface electromagnetic wave reflection and absorption mechanism of the aligned structure,hysteresis loss and dielectric loss mechanism,is utilized to further limit the escape of the incident electromagnetic wave.Finally,the EMI SE of the composite foam reached more than 90 dB,and the average reflectivity as low as 0.02,meaning that only 2%of electromagnetic waves were reflected.