Configuration Design,Preparation,and Properties of Nanocarbon/Copper Composites

Author:Yang Ming

Supervisor:zhang fan fan tong xiang


Degree Year:2018





The rapid developments in modern industry fields including electronic information,transportion,and power energy are raising constant demands for high performance Cubased metal materials.The traditional alloying method can primarily impart Cu alloys with high strength,however,a simultaneous loss of ductility and electrical/thermal conductivities is always accompanied.Integrating metals with ceramic reinforcements is one of the most effective ways to obtain high-strength,high-conductivity and multi-functional metal-based materials.For example,Cu-matrix composites can take advantage of the properties of both components,i.e.,the high ductivity,deformability,and conductivity of the Cu-matrix and the high strength and modulus of the reinforcements.Therefore,there is a complementary and synergistic effect,which may leads to outstanding comprehensive properties beyond any of the single components.Carbon nanomaterials,typically 1D carbon nanotubes(CNTs)and 2D graphene(GR),are rendered as ideal reinforcements for metal-matrix composites(MMCs)due to their outstanding low-dimensional nanostructure and excellent physicochemical properties.Especially,the multiple geometries,dimensions,and architectures of nanocarbons provide us rich choices for intrinsic configuration design of nanocarbon-reinforced metal-matrix composites(NMCs).This thesis focuses on the development of high-sstrength,highconductivity Cu-matrix composites and concerns the following three key issues in the field of NMCs: i)the high efficient,uniform dispersion of nanocarbons in the NMCs;ii)the relationships between nanocarbon configuration and interface structure,microstructure,and macroscopic properties;iii)the influences of nanocarbon on the plasctic deformation and texture evolution of bulk NMCs.The major results are summarized as follows:1.Based on the concept of “intrinsic configuration desgin”,four types of nanocarbons including acid refluexed CNTs(R-CNTs),multi-layered carbon nanoribbons(CNRs),single-or few-layered graphene nanoribbons(GNRs),and leaf-like CNT-GNR hybrids(LCGHs)are fabricated by oxidizing,splitting or unzipping pristine multi-walled CNTs(MWCNTs).In detail,we have fabricated large amounts of GNRs via unzipping the MWCNT walls using the modified Hummers method,which facilitates a full exploitation of the intrinsic strength of all the walls.An easy,high-field,controllable gas-phase oxidation method,which can effectively evade the crystalline degradation in traditional liquid oxidation processes,is developed to synthesize CNRs via longitudinally splitting MWCNTs.The associated tube-opening mechanism and edge-selective oxidation process are studied with comparison to the non-selective oxidation in acid hot reflux.Inspired by the structurefunction relationship of plant leaves,we design a leaf-like carbon hybrid reinforcement.LCGHs are prepared by controllably exfoliating the outmost few layers of MWCNTs using the Hummers method.2.A three-step preparation method including solution-based electrostatic heteroaggregation,fast spark plasma sintering,and large-strain hot rolling is developed to fabricate fully densed and well dispersed NMCs.The influences of those configurational changes on the interfacial structure,load-transfer,strengthening mechanisms are further studied.Tensile and electrial tests reveal the relationships between the nanocarbon configuration and the macroscopic mechanical and electrical properties of NMCs: i)GNRs combine elegantly the structure and properties of 1D CNTs and 2D GR,enabling a simultaneous enhancement of strength,ductility and electrical conductivity of pure Cu.We investigate the unique balance of enhanced strength-ductility using strengthening mechanism analysis,shear-lag theory and fracture analysis,with highlight on the role of interface-dislocation interactions.ii)CNRs are more efficient than CNTs for both reinforcing the mechanical strength and electrical conductivity of pure Cu,underscoring the effect of such configurational change.The relationship between the nanofiller configuration,load transfer and macroscopic properties are obtained based on fractography analysis and finite element modelling.CNRs show planar geometry and irregular edges,high graphite crystalline,low oxidation degree,and high intrinsic electrical conductivity,which are conducive to high-strength and high-conductivity NMCs.iii)The tough “midrid” of LCGHs can avoid nanocarbon aggregation while the “lamina” parts provide large surface area and rich oxygen functionalities.Moreover,the leaf-like configuration enables more robust loadsharing and reinforcing role than its tubular counterpart,which verify that the intrinsic configuration design strategy is substantially effective for tuning the macroscopic properties of MMCs.Using FEM,shear-lag theory,and fracture analysis,we further prove that the robust interfacial bonding,RD-alignment of nanofillers,and especially the unique geometry of LCGHs are responsible for their enhanced load-bearing capacity.3.The influences of the rigid metal-nanocarbon hetero-interfaces on the dynamic recrystallization process and texture evolution during hot-rolling are further investigated using the GNR/Cu composite system.The GNR/Cu interfaces contribute to the atypical recrystallization-type and brass-type textures developed in composites within 0.5 vol.% and 3 vol.% GNRs,respectively,deviating from the normal Cu-type texture found in their pure Cu counterpart.Using EBSD,TEM,KAM analysis,and visco-plastic self-consistent simulations,it is proven that the hetero-interfaces may change the texture evolution of the Cu matrix in four ways including retarding dislocation cross slip,generating geometrically necessary dislocations,promoting the DRX process,and activating non-normal slip.This study suggests the possibility of manipulating the microstructure,texture and mechanical properties of traditional metallic materials through the design of heterophase interfaces.