Preparation Of Graphene/Nanosilver Hybrid And The Basic Study Of The Application In The Epoxy Resin-based Conductive Adhesive

Polymer-based conductive composites (eg. electrical conductive adhesives, ECAs)which consist of polymer and conductive fillers have been considered as the new promisingmaterial for electronic packaging because of the advantages of low processing temperature,fine pitch interconnect and environmental friendliness. However, there are some issues suchas low electrical conductivity and poor mechanical strength which still need to be solved. Theformation of effective conductive paths in the ECAs is the key to improve the electricalconductivity. The nanomaterials such as graphene and silver nanostructure have been widelyused in the conductive composites as result of the excellent electrical and mechanicalproperties. In the study, pristine graphene with high quality was prepared by liquid-phaseexfoliation method. Then the silver nanoparticle (AgNP)/graphene hybrid were prepared byin situ method and Poly(amidoamine)(PAMAM) dendrimer functionalization. Also, the silvernanowire (AgNW) decorated graphene hybrid was generated. The graphene/Ag hyrid wereused to reinforce the electrical conductivity and shear strength of epoxy-based ECAs. Both ofthe structure and properties of ECAs were investigated.Graphene was prepared by direct exfoliation of natural graphite in low boiling-pointsolvent acetonitrile when1-cyanoethyl-2-ethyl-4-methyl imidazole (2E4MZ-CN) was used asstabilizer. The influence of the initial concentration of natural graphite, the concentration of2E4MZ-CN and the sonication time on the concentration of graphene in the dispersions wasinvestigated. The result of AFM and Raman indicated that single and few-layer graphenesheets were obtained. TEM, XRD and XPS demonstrated that the original crystallinestructure was preserved and there was few structure defects. The exfoliation of graphene wasinduced by the π-π interaction between graphene and the imidazole ring in2E4MZ-CN. TheSilver acetate (AgAc) was added to the exfoliated graphene dispersion. The complex wasgenerated between the AgAc and2E4MZ-CN. The AgNPs were in situ generated in theepoxy matrix and evenly distributed on the surface of graphene. The formation of AgNPs wasconfirmed by TEM, XRD and XPS. The ECAs have a volume resistivity of4.8E-5Ω·cmwhen the content of graphene and AgAc was0.6wt%and30wt%. The investigation ofinterfacial structure of the composites indicated that the generated AgNP/graphene hybrid sintered with microscale Ag flakes. The original structure in which the Ag flakes wereisolated by epoxy resin was changed, leading to the improvement of electrical conductivity.The liquid-phased exfoliated graphene was non-covalently functionalized by PAMAMdendrimer with terminal amino-group. FTIR, NMR and EA confirmed the synthesis ofPAMAM. The investigation of the structure morphology of PAMAM functionalized grapheneby TEM and XRD revealed that the intercalation of in situ synthesized PAMAM amonggraphene layers stabilized the graphene sheets and the functionalization did not disturb thestructure of graphene. The interaction sites between graphene and PAMAM dendrimer lied interminal amino-groups and interior tertiary amines. The AgNP/graphene hybrid wasgenerated by using PAMAM as both of the stabilizer and reducing agent. The mechanisminvestigation of the reduction of AgNPs by PAMAM revealed that the complex was formedbetween Ag ions and the amine groups in PAMAM. Then the AgNPs were generated byheating treatment of the complex under low and middle temperature. The investigation of thestructure of the hybrid demonstrated that the AgNPs with average size were uniformlydistributed on the surface of graphene sheets and there was nearly no isolated AgNPs outsidethe graphene sheets. The particle size generated in1.0G,2.0G and3.0G was13nm,12nm and7nm, respectively. The density of AgNPs generated in3.0G PAMAM was the largest. ThePAMAM/graphene/AgNP composite can cure the epoxy resin. There were two or three mainexothermic peaks in the curing system. The volume resistivity of the ECAs filled with0.6wt%graphene and24wt%AgAc reached at3E-5Ω·cm, the decrease of83%comparedwith the control samples. The investigation of the interfacial structure revealed that thegraphene sheets were uniformly dispersed in the matrix and the microscale Ag flakes werejointed together by the connection of graphene sheets, thus leading to the formation ofeffective conductive paths.The edge-functionalized graphene with4-aminobenzoyl group located at the edges wasprepared by edge functionalization method. The AgNWs with diameters of50±10nm andlength of8±5μm were prepared by using glycerol as reducing agent and PVP as stabilizer.The AgNWs display network structure. The graphene/AgNW hybrid was prepared throughthe simultaneous sedimentation process when the graphene were combined with AgNWs. Theinteractions between graphene and AgNWs resulted in the occurrence of co-assembling process and the simultaneous sediments. The investigation of the hybrid structure revealedthat the graphene sheets embedded in the AgNW network, leading to formation ofthree-dimensional (3D) network structure. The interactions between the graphene andAgNWs were investigated. The graphene sheets were separated by the AgNW network andthe graphene sheets can prevent the AgNWs from oxidation. Since the AgNWs can providedmany conductive paths at low percolation threshold and the graphene sheets enhanced theinterfacial contacts between the AgNWs, there was effective3D electrical conductivenetwork formed in the hybrid structure. The graphene sheets also enhanced the strength ofAgNW network. Thus there were synergistic effects on the reinforcement of the electricalconductivity and shear strength of the ECAs. The covalent interfacial bonding between theamino-group at the edge of edge-functionalized graphene and epoxy resin leaded to theeffective load transfer from the matrix to fillers, thus significantly improving the shearstrength of ECAs.