Study On Preparation And Electromagnetic Protecting Performance Of 3D Printed Carbon/Polymer Composites Based On Fused Deposition Modeling

With the arrival of“Information Age”and even“Intelligent Age”,the large amount of electromagnetic wave（EMW）generated by all kinds of electronic devices inevitably brings negative problems such as electromagnetic radiation and electromagnetic interference（EMI）.Therefore,researchers have developed two types of electromagnetic protecting materials:EMI shielding materials and EMW absorbing materials.Carbon materials have excellent electrical and dielectric properties,becoming a popular choice for electromagnetic protecting materials.3D printing is a kind of manufacturing technology born and developed with the intelligent era.It has the advantages of product diversification,great structure designability and high material utilization rate.Some researchers have applied it to the manufacture of electromagnetic protecting materials.However,in such studies,3D printing technology was only regarded as a molding method,and its influence on the micro/macro structure and properties of electromagnetic protecting materials needs to be investigated.In this thesis,based on the fused deposition modeling（FDM）3D printing technology and carbon/polymer composites,the electromagnetic protecting materials which suit for the 3D printing technology with excellent performance was designed and prepared through the nano-modification of carbon materials and the adjustment of the composite ingredients.In full use of the advantages of 3D printing technology,structural design of composites was carried out to further optimize the electromagnetic protecting performance of 3D printed products,and further explore the impact of structure on performance.Integrated with the development of the material and the optimization of the structure design,high performance 3D printed electromagnetic shielding materials and microwave absorbing materials were prepared.（1）Considering the particularity of 3D printing technology,carbon nanofiber（CNF）/polycaprolactone（PCL）composites were taken as the research object to explore the effect of FDM process on the preparation and properties of EMI shielding materials.It was found that the 3D printing process would induce high-conductive CNFs to concentrate inside the 3D printing extruded threads and orientate along the printing direction,causing a high resistance“internal surface”between adjacent threads.As a result,it reduced the conductivity of 3D printing CNF/PCL,raised the percolation threshold,and caused a unique secondary percolation phenomenon.With the formation of the two-stage percolation network,with 40 wt%CNFs addition,the 3D printed40CNF/PCL showed excellent EMI shielding performance,and the total shielding efficiency（SE_T）reached 58.7 d B.By adjusting the packing density parameters,pore structures and heterogeneous interfaces could be introduced into the 3D printed materials,thus promoting multiple reflection and absorption of EMW inside the material,and achieving synchronous control of the weight and EMI shielding performance of materials.（2）Aiming at the adverse effects of 3D printing on the conductivity and EMI shielding performance of CNF/PCL,the conductive polymer polyaniline（PANI）was introduced into the composite material to partially replace CNFs.The CNF&PANI organic-inorganic hybrid conductive network was constructed in composites with pseudo doping from hexafluoroisopropanol.CNF&PANI/PCL showed higher electrical conductivity than CNF/PCL at the same addition amount.The addition of PANI specifically solved the problems of CNF orientation and“internal surface”.PANI acted as a bridge between CNFs and reduced the resistance at the internal surface.Therefore,the influence of 3D printing process on electrical conductivity was greatly reduced,and the applicability of 3D printing technology in electromagnetic shielding field was broadened.Compared with the 3D printed CNF/PCL with the same addition amount（10 wt%）,the conductivity of 3D printed 10-C3P1/PCL increased by about 10³ times,and the SE_T was increased by 16.7 d B to 39.8 d B.（3）For EMW absorbing materials research,a hierarchical nano-array structural EMW absorbing material（Co NC@CF）with minimum reflection loss(RL_min)of-61.4 d B was prepared by metal organic framework（MOF）in-situ growth on the surface of cut carbon fiber and reduction pyrolytic.Co NC@CF was introduced into 3D printing technology to prepare Co NC@CF/polylactic acid（PLA）composites suitable for FDM process.When Co NC@CF content was 10 wt%,3D printed 10Co NC@CF/PLA showed excellent EMW absorbing ability with RL_min of-45.5 d B due to the multiple loss mechanisms.In addition,Co NC@CF enhanced the mechanical properties of the 3D printed composites,and the interface bonding within the composites could be further improved by appropriate microwave processing.Tensile strength of 3D printed10Co NC@CF/PLA improved by about 68%compared with that of 3D printed PLA.The study actualized the simultaneous improvement of functional and mechanical properties of 3D printed material.（4）In order to prepare EMI shielding materials with low reflection characteristics,the EMI shielding composites with isolated asymmetric-multilayer structure was successfully designed and fabricated utilizing dual-nozzle 3D printing technology.Combining the above researches on EMI shielding materials and EMW absorbing materials,the Co NC@r GO/PCL with EMW absorbing ability and good impedance matching was prepared and combined with high conductivity CNF&PANI/PCL in layered distribution.According to the calculation simulation and experimental verification,it was proved that the multilayer structure lengthened the loss path of EMW in the material,and reduced the reflection of electromagnetic wave,so as to reduce the secondary pollution.The 12-layer sample showed a SE_T of 30.9 d B,and the ratio of reflected EMW energy was less than 50%at most frequencies.The 12-layer/Ag sample with asymmetric structure presented an ultra-high SE_T of 71.8d B.And SE_R of 12-layer/Ag was 5.4 d B,accounting for only 0.075 of SE_T.The loss path of EMW in the material was lengthened and guided through reasonable structural design,so as to realize the design and preparation of excellent performance of low reflection EMI shielding composite material.