| Electromagnetic radiation pollution not only affects the operation of electronic devices,but also endangers human health.Adopting high-performance electromagnetic interference(EMI)shielding materials to isolate electromagnetic radiation be perceived as an effective way to protect devices from EMI.The primary factor determining the EMI shielding performance of materials is electrical conductivity,so metal material with high electrical conductivity has been the first choice for preparing high-performance shielding material.With the rapid development of 5G communication technology and chip manufacturing technology,electronic components are evolving towards miniaturization,high integration density and multi-function,which also puts forward higher requirements for EMI shielding materials.Traditional metal materials are mostly metal frames,metal sheets,screen mesh made of copper and aluminum alloy.However,the further application of metallic material in EMI shielding is limited by their inherent defects,such as high density and rigidity.Therefore,to overcome the above problems of metal in EMI shielding,we started with the selection of metal filler and the construction of a three-dimensional network structure,finally prepared a lightweight,flexible metal/polymer composite material.This paper consists of five chapters.The first chapter is the introduction,which summarizes the background,mechanism and testing methods of EMI shielding,and reviews the recent research progress of metal/polymer materials in EMI shielding.The second chapter is the experimental part,which lists the basic information of reagents and instruments.In the third chapter,we described the preparation process of liquid metal/cellulose nanofiber(LM/CNF)composite film,and studied its properties such as mechanical property,EMI shielding performance,and reliability.At the same time,we verified the shielding mechanism of the LM/CNF film by electromagnetic simulation.The fourth chapter is the experimental section,where successfully prepared metallic ink/polymer composite material by the method of in-situ adsorption and sintering,which utilizes the metal-base ink compatibility with polymer matrix.Finally,we explored its various performance,including mechanical property,EMI shielding performance,and verified its EMI shielding mechanism by electromagnetic simulation.The fifth chapter is the conclusion and prospect.On the one hand,we summarized the experimental results of this thesis,and on the other hand further proposed the future research directions.The main experimental research contents of this thesis are as follows:(1)Flexible LM/CNF composites film with excellent thermal reliability for highly efficient and broadband EMI shielding.LM is a promising candidate for EMI shielding due to the superb electrical conductivity and easy processing.However,poor compatibility caused by enormous surface tension,insulated oxide shells formed during processing,and unmanageable fluidity at elevated temperature of LM severely hinder its application in the field of EMI shielding.Herein,we developed a novel processing strategy integrating ball-milling dispersion,freeze-drying and compression molding to achieve free-standing and flexible LM/CNF composites film,in which the oxide shells of LM droplets generated by ball-milling were broken by mechanical compression,and LM coalesced while confined by CNF to construct a continuously conductive path.As a result,the robust LM/CNF film shows the tensile strength of above 30 MPa,and it possesses electrical conductivity of 96000 S/m,leading to remarkable shielding effectiveness(SE)of above 65 d B with a thickness of only 300?m in a broad frequency range of 4-18GHz covering C-band,X-band and Ku-band.Moreover,LM/CNF film exhibits excellent structural stability and EMI shielding performance reliability after heating at120 ~oC.Besides,simulation in the frequency domain with ANSYS HFSS 2019 R2 was performed to intuitively understand the shielding mechanism of LM/CNF film.It is found that the SE of LM/CNF film against electromagnetic waves increases as of LM loading increase,and the loss of electromagnetic waves are mainly based on reflection.This study proposes a fresh scenario to achieve high-performance EMI shielding material and paves the way for potential applications of LM in electromagnetic radiation devices.(2)Structural design and EMI shielding performance of metallic ink/polymer foam compositesMeta/polymer foam with the advantage of lightweight,flexible,low volume content,compressible performance,and excellent shielding ability is applied in EMI shielding,especially in the aerospace field and in the board level package of the chip.However,the traditional preparation technology of metal/polymer foams,mainly included electroless plating and electroplating,existed serious problems of large pollution,complex production process and high cost.Therefore,it is urgent to develop a new method that meets the requirements of simple step,pollution-free,and low-temperature sintering for the preparation of metal/polymer foam.Herein,we demonstrated a novel synthesis strategy mainly via in-situ sintering of metal-organic decomposition(MOD)ink on modified melamine(ME)foam with polyethyleneimine(PEI)molecular.The Ag@ME foam provides a continuous 3D conductive network for electron transport,and exhibits a considered conductivity of 163.5 S/m and thus brings excellent EMI SE of 63 d B in the broad frequency of 8.2-40 GHz at an averages thickness of 5 mm,while the volume content is only 2.03%.Meanwhile,the Ag@ME foam shows outstanding near-field EMI shielding performance that the leaked electromagnetic radiation intensity decreases from-18 to-65 d B after placing the Ag@ME foam(2.03 vol%).Furthermore,we vividly first described the influence of changes in thickness or conductivity of the shielding materials for electromagnetic wave propagation in a rectangular waveguide at the X-band through CST Studio Suite2018 software,and demonstrated the shielding mechanism of Ag@ME foam.With the increase of low cost,non-invasive and non-pollution requirements,this novel strategy may become a potential fabrication method of metal/polymer composite foam. |
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