Yajun Gao

Shock-induced breaking of the nanowire with the dependence of crystallographic orientation and strain rate.

The failure of the metallic nanowire has raised concerns due to its applied reliability in nanoelectromechanical system. In this article, the breaking failure is studied for the [100], [110], and [111] single-crystal copper nanowires at different strain rates. The statistical breaking position distributions of the nanowires have been investigated to give the effects of strain rate and crystallographic orientation on micro-atomic fluctuation in the symmetric stretching of the nanowires. When the strain rate is less than 0.26% ps-1, macro-breaking position distributions exhibit the anisotropy of micro-atomic fluctuation. However, when the strain rate is larger than 3.54% ps-1, the anisotropy is not obvious because of strong symmetric shocks.

The interface and surface effects of the bicrystal nanowires on their mechanical behaviors under uniaxial stretching

Using molecular dynamics simulations, we have investigated systematically the mechanical deformation of bicrystalline metallic nanowires with [110]∥[100], [111]∥[100], and [111]∥[110] interfaces. When the size of the nanowire is larger than 20×20×60 (units: cell), the effect from the grain boundary is dominant in breaking as compared with the nanowire surface effect. For [110]∥[100] bicrystal, breaking occurred easily at the interface with no clear structural deformation of the grain interior. When the [111] direction was addressed, the sliding most likely took place in [100] region for [111]∥[100] but in both regions for [111]∥[110], causing obvious elongation of the nanowire. By exploring the stress-strain property and the stress concentration along the tensile direction, we elucidated how the interfacial microstructure affected the mechanical behavior. Reducing the wire size, the effect from the nanowire surface gradually becomes more pronounced, showing a new breaking position from the grain boundary ...

Uniaxial tension-induced breaking in the gold nanowire with the influence of defects and temperatures

Defects in metallic nanowires have raised concerns due to the influence on the properties of metallic nanowires in a nanoelectromechanical system. In this paper, the deformation and breaking of the [100] single-crystal gold nanowires containing vacancy defects are studied using molecular dynamics simulations at different temperatures. The statistical breaking position distributions show the deformation and breaking of the nanowires have a dependence on the applied temperature, and the sensitivity of the nanowire to vacancies is based on a competition between constructed vacancies and disordered crystalline structures induced by temperatures. At a low temperature of 100 K, a vacancy ratio of 25% has decided the breaking of the nanowire because microatomic fluctuation is in an equilibrium state. However, owing to acute atomic movements, the sensitivity of vacancies to breaking is not obvious before a vacancy ratio of 70% at a high temperature of 500 K.

Nanoscale interface of metals for withstanding momentary shocks of compression

The failure of the nanoscale metallic interface has raised concerns owing to the effect interfacial amalgamation has on its application in nanoelectronic devices. Single crystal copper [110] and [100], which are set as two components of [110]‖[100] nanocrystalline copper, are used to simulate the interfacial properties using molecular dynamics simulations. Repeated tension and compression cycles show that the two components of the interface can come into contact and separate without interfacial amalgamation. The [110]‖[100] interface could withstand momentary shocks of compression and heat produced by the momentary shocks. This property of the [110]‖[100] interface is dominated by crystalline orientations of interfacial structure, in comparison with [111]‖[100] and [111]‖[110] interfaces under the same conditions.

Temperature dependent exchange bias training effect in single-crystalline BiFeO3/Co bilayers

Single-crystalline BiFeO3 (BFO)/Co bilayers were prepared by combined pulsed laser deposition and magnetron sputtering on (001) SrTiO3 substrates. Exchange bias (EB) and accompanying training effect have been studied as a function of temperature (T) between 5 K and 300 K. A non-monotonic exchange field variation with sharp increase below 100 K has been observed. In the meanwhile, strong training effect was recorded when T < 100 K and it weakens monotonically with increasing T up to 300 K. These temperature dependent EB and training effect may be caused by the uncompensated spins in both the interfacial spin-glass (SG) phase at low temperature and the antiferromagnetic BFO layer at higher temperature. The low temperature EB training results can be well fitted by a modified Bineks model considering asymmetric changes of the pinning SG spins at the descending and the ascending branches.

Uniaxial Magnetic Anisotropy in Amorphous CoFeB Films on Different Orientational GaAs Substrates

In-plane uniaxial magnetic anisotropy (UMA) has been carefully studied for the same amorphous Co56Fe24B20 (CoFeB) film deposited on different orientational GaAs substrates. It was noted that a strong uniaxial anisotropy field (Hk) of ~270 Oe could be achieved with CoFeB film grown on GaAs(001) substrate, which is much larger than the largest value (150 Oe) reported before. In contrast, Hk was <;20 Oe when the same CoFeB film was deposited on (110) or (111) GaAs substrate. The angular dependence of Hk also behaved quite differently in these CoFeB films. Based on the surface morphology of the CoFeB/GaAs films, the possible mechanisms responsible for the different behavior of UMA were briefly discussed in terms of bond-orientational anisotropy model and random anisotropy model.

Investigation on the mechanical behaviour of faceted Ag nanowires

Using molecular-dynamic (MD) simulation, we investigated the mechanical response of faceted silver nanowires (NWs) to tensile deformations. The investigation considers different frustum heights, temperatures and 3D void effects to study the yield elasticity, incipient plasticity and ductility of the faceted NWs. The embedded-atom-method (EAM) potential is employed to describe the atomic interactions. To identify the defect evolution and deformation mechanism, the centrosymmetry parameter is implemented in our self-developed programme. A detailed discussion of surface-dominated deformation behaviour has been presented. It is found that the faceted NWs show brittle breaking behaviour in low temperature, which has never been reported in other NWs. This temperature-dependent breaking mode, as reported in this paper, might be useful in avoiding certain deformation mechanisms of metallic NWs in future.

Simulate the diffusion of hydrated ions by nanofiltration membrane process with random walk

We propose a new diffusion model describing the diffusion behaviours of hydrated ions in the process of nanofiltration (NF) based on the random walk (RW) theory when the NF membrane is uncharged or low charged. In this model, the hydration of ions and their deformation capacity are considered. The structure of the membrane is idealised into a lozenge shape and the diameter of membrane pore is defined as gapsize. A computer program named RW system in chemistry is developed to simulate based on this model. Six familiar ions Li+, Na+, Mg2+, Al3+, K+ and Ca2+ are investigated. Their characteristics are calculated by Gaussian 03, Pople, Inc., Wallingford, CT. The diffusivities of hydrated ions are calculated and discussed. The results show that the hydration of ions cannot be ignored in NF process when the membrane pore size is near the dimensions of the hydrated ions.

Enhanced ferroelectricity in orthorhombic manganites Gd1−xHoxMnO3

A series of orthorhombic manganites Gd(1-x)Ho(x)MnO(3) (0 <= x <= 0.5) are synthesized. The Ho doping leads to a transition of paraelectric phase into ferroelectric one, making Gd(1-x)Ho(x)MnO(3) multiferroic. It is argued that the enhanced electric polarization originates from the transition of the A-type Mn(3+) anti-ferromagnetic state into the spiral spin order driven by the reduction of Mn-O-Mn bond angle due to the doping. The noncollinear spin order of the A-site Ho(3+) spins at high doping level may also make contribution to the polarization. (c) 2011 American Institute of Physics. [doi: 10.1063/1.3535543]

Grain size effect on the plastic deformation of nanocrystalline silver

Abstract The plastic deformation of nanocrystalline Ag, with columnar grains, has been studied by molecular dynamics simulations. The nanocrystalline systems show two types of deformation mechanisms. One is the split of grain boundary that occurs before the activation of the dislocation in nanocrystalline Ag, and almost no dislocation debris and twins are left in the grains. Moreover, split of grain boundary is shown between the grains of nanocrystalline Ag. Another mechanism consists in partial dislocations dominating the process of plastic deformation. Plenty of stacking faults and twins remain in the grains of nanocrystalline Ag. It is revealed that different grain aspect ratios have induced the difference in deformation mechanisms of nanocrystalline Ag. When the grain aspect ratio is less than or equal to 1, the process of plastic deformation is dominated by partial dislocations. Otherwise, the process is dominated by split of grain boundary. The grain aspect ratio is the height in z direction to length in x direction ratio, which was found to noticeably impact yield strength, grain coarsening, indicating that the observed behaviour should have contributed to the plastic deformation significantly.