Effects of temperature and grain size on deformation of polycrystalline copper-graphene nanolayered composites

Nanoscale friction behavior and deformation during copper chemical mechanical polishing process

CONTEXT

The mechanical characteristics and deformation behavior of Cu material under the nanoscratching through a diamond tooltip on the workpiece are studied using molecular dynamics (MD) simulation. Effects of scratching velocity, scratching depth, workpiece temperature, and grain size on the total force, shear strain, pile-up, shear stress, workpiece temperature, and phase transformation are investigated. The results reveal that increasing the scratching velocity leads to higher oscillation in total force, greater shear strain and shear stress, higher pile-up on the workpiece surface, and higher workpiece temperatures. The effect of the scratching velocity on phase transformation shows that most of the dislocation is a transformation structure from the FCC structure to the HCP, BCC, and other structures in all workpieces during the nanoscratching process. In addition, with increasing the scratching depth, material pile-up becomes more prominent, consequently elevating the contact area between the diamond tooltip and the workpiece, which simultaneously leads to an increase in total force, shear strain, pile-up, shear stress, and workpiece temperature. The MD simulation results revealed that the subsurface region of nanoscratched Cu single-crystal experiences the formation of stacking faults, vacancy defects, and cluster vacancies. In studying the effect of workpiece temperature, the results show that higher temperatures lead to the decline of scratching force, high plastic deformation, increased shear strain and stress, lower pile-up height, and high transition from the FCC structure to both other and BCC structures. For polycrystalline structures, the force curves occur in the oscillation state in all cases of different grain sizes because of the dislocation deformation during the cutting process. The maximum force decreases with diminishing grain size, attributed to the inverse Hall-Petch relation. As the grain size increases, leading to a decrease in the shear strain, stress, and an uneven pile up; also, the HCP structure rises with decreasing grain boundary and the partial dislocation and stacking fault mobilize inside grains.

METHODS

By using molecular dynamics (MD) simulation based on the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) software, all molecular interactions were described by the Lennard-Jones (LJ) and embedded atom method (EAM) potentials. In order to mitigate the effects of temperature fluctuations, the system employs an isothermal and isobaric (NPT) ensemble for precise temperature control. The temperature was set as 300 K and the time step was 1 fs (femtosecond).

Effects on the Microstructure Evolution and Properties of Graphene/Copper Composite during Rolling Process

Rolling treatments have been identified as a promising fabrication and deformation processing technique for graphene/metal composites with high performance. However, it is still a challenge to choose appropriate rolling parameters to achieve high strength, ductility and electrical conductivity of the composite simultaneously. In this study, graphene/Cu composites were prepared with an in situ growth method and rolling treatment. The effects of rolling deformation and temperature on the microstructural evolution of graphene and Cu grains, interface bonding between graphene and the matrix, mechanical and electrical properties were systemically investigated. The cold-rolled composite with 85% deformation displayed a maximum ultimate strength of 548 MPa, a high elongation of 8.8% and a good electrical conductivity of 86.2% IACS. This is attributed to oriented graphene arrangement and matrix grain refinement. Our research provides a comprehensive understanding for the rolling behavior of graphene/Cu composites, and can promote the development of graphene-based composites with high performance.

Effects of temperature and repeat layer spacing on mechanical properties of graphene/polycrystalline copper nanolaminated composites under shear loading

In the present study, the characteristics of graphene/polycrystalline copper nanolaminated (GPCuNL) composites under shear loading are investigated by molecular dynamics simulations. The effects of different temperatures, graphene chirality, repeat layer spacing, and grain size on the mechanical properties, such as failure mechanism, dislocation, and shear modulus, are observed. The results indicate that as the temperature increases, the content of Shockley dislocations will increase and the maximum shear stress of the zigzag and armchair directions also decreases. The mechanical strength of the zigzag direction is more dependent on the temperature than that of the armchair direction. Moreover, self-healing occurs in the armchair direction, which causes the shear stress to increase after failure. Furthermore, the maximum shear stress and the shear strength of the composites decrease with an increase of the repeat layer spacing. Also, the shear modulus increases by increasing the grain size of copper.

Pure thermal spin current and perfect spin-filtering with negative differential thermoelectric resistance induced by proximity effect in graphene/silicene junctions

The spin-dependent Seebeck effect (SDSE) and thermal spin-filtering effect (SFE) are now considered as the essential aspects of the spin caloritronics, which can efficiently explore the relationships between the spin and heat transport in the materials. However, there is still a challenge to get a thermally-induced spin current with no thermal electron current. This paper aims to numerically investigate the spin-dependent transport properties in hybrid graphene/silicene nanoribbons (GSNRs), using the nonequilibrium Green’s function method. The effects of temperature gradient between the left and right leads, the ferromagnetic exchange field, and the local external electric fields are also included. The results showed that the spin-up and spin-down currents are produced and flow in opposite directions with almost equal magnitudes. This evidently shows that the carrier transport is dominated by the thermal spin current, whereas the thermal electron current is almost disappeared. A pure thermal spin current with the finite threshold temperatures can be obtained by modulating the temperature, and a negative differential thermoelectric resistance is obtained for the thermal electron current. A nearly zero charge thermopower is also obtained, which further demonstrates the emergence of the SDSE. The response of the hybrid system is then varied by changing the magnitudes of the ferromagnetic exchange field and local external electric fields. Thus, a nearly perfect SFE can be observed at room temperature, whereas the spin polarization efficiency is reached up to 99%. It is believed that the results obtained from this study can be useful to well understand the inspiring thermospin phenomena, and to enhance the spin caloritronics material with lower energy consumption.