The Structures And Mechanical Properties Of Nickel Nanowires

The researches on the structures of the one-dimensional nanowires are of great significance in the development of their other properties.Due to the wide applications in areas of nanocomposite strengtheners and components in nanoelectromechanical system(NEMS) devices,knowledge on mechanical properties of nanowires is extremely important.However,the one-dimensional nano-structures, such as metal nanowires,nanotubes and carbon nanotubes,are not available easily in experiment,molecular dynamics(MD) simulation shows its importance in the research of the structures and properties of nanowires.In the paper,MD simulation is performed to explore the structures and mechanical properties of nickel nanowires.The Ni nanowires can keep face-centered-cubic(FCC) structure when their cross-sizes are large enough.However,the energy and properties are diverse for nanowires with different orientations and free surfaces.FCC <001>,<110>, <110>{111} and <111> oriented Ni nanowires have been constructed and relaxed to explore their structural stabilities by calculating their size dependence of the total energy per atom and contraction ratio.The finding indicates that the energy and contraction ratio decrease as size increases,and the <110>{111} nanowires show the lowest level among the simulated nanowires.These are attributed to the ratio of surface atoms and different surface orientations.When the size is small enough,as well as non-closed packing on the free surfaces,large surface stress will induce the structural transforming and closed packing on the surface.Three measures have been taken to explore the structures of ultrathin Ni nanowires named relaxing-annealing circle,stretching and cylindrical folding respectively.According to the closed packing on the surface of ultrathin nanowires,atomic structures of(m,n) Ni nanowires are constructed by cylindrical folding of the 2D triangular FCC(111) lattice sheet.However,not all constructed(m,n) nanowires are stable,thus,the nanowires are relaxed by energy minimization way.The non-helical(5,5) and(6,6) structures show much lower structural energy after relaxation in comparison with the energy and radius of stable nanowires.In addition,some nanowires the same as the cylindrical folding(m,n) nanowires can be obtained by relax-annealing(structure and energy optimization) thin nanowires with FCC structure.The(5,5) structure which turns from(6,5) structure can be formed at the necking part of tensioned FCC nanowires along the <001> crystallographic direction.The Ni nanowires with multi-shell structure(MS) have also been explored.The finding indicates that(12,6)(6,3) structure shows lower energy and the(11,6)(4,2),(13,7)(6,3) and(14,8)(7,4) structures show a little change in atomic configurations during relaxation and can be also regarded as being relative stable.The mechanical properties of the relaxed stable nanowires are the other aspect of our researches.We explore the size effects of the different orientations FCC Ni nanowires and make a summary on the Young's modulus,yielding stress and yielding strain as a function of the nanowires' size.The strain rate effects are exemplified by loading FCC <001> nanowires.The nanowires undergo plastic slipping deformation at low strain rates and the yielding stress and strain are on the same level.At high strain rates,the nanowires change from crystalline structure to amorphous phase continuously and the yielding stress increases as the strain rate increases.The mechanical properties of ultrathin nanowires such as Young's modulus,yielding stress and strain are calculated by tensioned simulation.The deformation mechanisms of ultrathin Ni nanowires are explored by describing the snapshots of atomic configurations at several strains.The non-helical(5,5) and(6,6) nanowires show high yielding stress but low plasticity.The helical structure,such as(7,4) structure,has a good performance in plastic deformation.The helical nanowires are prone to plastic deformation by decreasing the number of helical strands m.