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Study On Controllable Preparation And Electromagnetic Shielding Performance Of Polymer Microcellular Composite Foams

Posted on:2021-10-30Degree:DoctorType:Dissertation
Country:ChinaCandidate:H M ZhangFull Text:PDF
GTID:1521307100474784Subject:Materials Processing Engineering
Abstract/Summary:PDF Full Text Request
Polymeric microcellular materials,which are prepared by supercritical fluid foaming process,exhibit the advantages of reduced weight and high strength,improved impact toughness and fatigue life,elevated thermal stability,as well as low dielectric constant and thermal conductivity.Polymer microcellular foams are widely used in various fields of the national economy.Recently,polymer microcellular composites,which are prepared by introducing the nanofillers into microcellular foams,have became the research hotspot in the field of electromagnetic shielding due to its advantages of light weight,corrosion resistance,easy processing,adjustable conductivity and high specific EMI shielding performance.However,the current composite foams show poor structural controllability,the types of nanofillers are too single and easy to agglomerate,and most are in poor electrical conductivity and high percolation threshold,in addition,the relationship between microcellular foaming behavior and EMI shielding performance is not clear enough.Therefore,in this paper,amorphous polymer polymethyl methacrylate(PMMA)and semi-crystalline polymer polyvinylidene fluoride(PVDF)were selected as typical matrix,MWCNTs@Fe3O4、Ni-chains、GNPs-MWCNTs、G-CNT hybrids were selected as the nanofillers,the nanofillers were introduced into polymer matrix by solution mixing and a series of microcellular nanocomposite foams with different structures were prepared by supercritical CO2 assisted batch foaming process.The influence of the physical aggregation state and the foaming process parameters on the structure and properties of composite foams were studied;To provide a certain theoretical and experimental basis for the development of microcellular nanocomposites with excellent conductivity and electromagnetic shielding meanwhile,the mechanical,thermal insulation,electrical conductivity and electromagnetic shielding performance of the obtained composite foams were systematically examined.The main results obtained in the thesis are as follows:(1)A light-weight multifunctional microcellular composite was prepared by the solution blending,compression-molding and supercritical CO2 foaming technology.When saturating the samples in 8.5 MPa and 40°C for 16 h and foaming it at 110°C for 60 s,the uniform microcellular foam with a density lower than 0.3 g/cm3,a cell density higher than 109 cells/cm3and a cell size lower than 10μm could successfully be prepared.Carbon nanotube-ferric tetroxide(MWCNTs@Fe3O4)hybrid with both magnetic and electrical conductivity was prepared by chemical co-precipitation method,and then added into the above microcellular foam system.MWCNTs@Fe3O4 was an excellent heterogeneous nucleating agent,the contents could significantly affect the density and microstructure of the microcellular foam.The MWCNTs@Fe3O4 hybrids which uniformly dispersed in the matrix can improve the thermal insulation and compression properties of the microcellular foam.Meanwhile,the microcellular composites presented excellent electrical conductivity,magnetic responsiveness and EMI shielding performance.Due to the presence of microcellular structure and magnetic nanofillers,PMMA/MWCNTs@Fe3O4 composite foams exhibited an absorption-dominated EMI-shielding feature.When the content of the hybrid filler is 7 wt%,the specific electromagnetic shielding effectiveness of the microcellular composites with an average cell size of 1.66μm and a density of 0.26 g/cm3 was about 50.1 d B/g·cm-3 in X-band.(2)Through the heat treatment-quenching process,the crystallinity of PVDF is significantly reduced,and the adsorption capacity of SC-CO2 is increased,and PVDF microcellular foam with uniform cell structure is prepared.The magnetic Ni-chains with an aspect ratio of 110 and a diameter of 480 nm were prepared by magnetic field-assisted chemical reduction.On this basis,a PVDF-based microcellular composite with the condensed magnetic-conductive Ni-chains network was prepared through the steps of solution blending,crystallinity adjusting,and supercritical CO2 foaming.The Ni-chains,which uniformly dispersed in the matrix,can improve the thermal insulation and tensile properties of the microcellular foam.Meanwhile,the composite foams were also endowed with excellent electrical conductivity,magnetic responsiveness and EMI shielding performance.As a result of the unique porous morphology,PVDF/10wt%Ni-chains foams present decreased mass density(~1.0 g/cm3),high tensile strength(~42.0 MPa),enhanced electrical conductivity(~0.01 S/m)and superior thermal insulation performance(~0.075 W/(m·K)).Furthermore,a 2 mm thinkness’s conductive-magnetic PVDF/10wt%Ni-chains foam exhibits a high EMI shielding effectiveness of 26.8 d B and an outstanding specific shielding effectiveness(SSEt)of 127.62 d B/g·cm-2,with an absorption-dominated shielding feature in X-band region.The enhanced absorption is attributed to the multiple reflections,dielectric loss,polarization loss and magnetic loss originated from the unique porous structure and condensed Ni-chains networks.(3)The solid and microcellular PMMA/GNPs-MWCNTs composites were prepared by solution blending,hot-pressing and microcellular foaming process,respectively.In solids,MWCNTs and GNPs display limited effect on the final electrical properties while they show obvious synergistic effect in influencing the mechanical performance.The well-dispersed MWCNTs-GNPs in polymer matrix provide plenty of 1D-2D interconnections for efficient stress transferring when exposed to stress tensile,but the distance between nanofiller is not enough for effective electron movements and the Schottky barrier between GNPs and MWCNTs is too high,therefore,the composites presented bad electrical properties.In microcellular composites,the bimodal PMMA/GNPs-MWCNTs microcellular composites exhibited better tensile strength and electrical conductivity than the single-filler foam system at the same loading.MWCNTs and GNPs reveal prominent synergistic effect in influencing the tensile and electrical properties of ternary composite foam systems.The obvious enhancement of electrical conductivity results from the in-stu exfoliation of GNPs,high-level orientation and redistribution of nanofillers,moderate physical foaming with bimodal microcellular structure as well as attenuated Schottky barriers at few-layer graphene/MWCNTs junctions.The MWCNTs could act as a bridge between the thin-layer graphene to promote the construction of a conductive network structure.(4)Herein,three kinds of fully carbon-based fillers(thermally reduced graphene oxide-carbon nanotubes(TG-CN),chemically reduced graphene oxide-carbon nanotubes(RG-CN),graphene nanoplates-carbon nanotubes(GN-CN)hybrids)were successfully synthesized and characterized and their corresponding PMMA/G-CNT composite foams with a relative density of 0.5 were prepared via batch-foaming process.PMMA/GN-CN foams exhibited a bimodal distribution cell structure,but both PMMA/RG-CN and PMMA/TG-CN microcellular composites exhibited a unimodal distribution cell structure.While the microcellular structure promotes the construction of conductive networks,all these PMMA/G-CNT composite foams with a relative density of 0.5 exhibit lower percolation threshold and higher conductivity and EMI SE.Among them,the PMMA/TG-CN system exhibits the highest conductivity,which is attributed to the higher intrinsic conductivity of the TG-CN hybrid filler and the better dispersion state.Specifically,a prominent electrical conductivity of 2.92 S/m and an absorption-dominated EMI SE of more than 30 d B in the X-band are achieved in the lightweight PMMA/TG-CN foams at 10 wt%loading.The increased EMI shielding performance is closely related to a large number of microcellular structures,dielectric losses caused by the TG-CN conductive network,increased dipole and interfacial polarization and a large number of multiple reflections.
Keywords/Search Tags:Thermoplastic polymer, Hybrid fillers, composite foams, Supercritical CO2 microcellular foaming, EMI shielding
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