Duc Tam Ho

Negative Poisson’s ratios in metal nanoplates

The Poissons ratio is a fundamental measure of the elastic-deformation behaviour of materials. Although negative Poissons ratios are theoretically possible, they were believed to be rare in nature. In particular, while some studies have focused on finding or producing materials with a negative Poissons ratio in bulk form, there has been no such study for nanoscale materials. Here we provide numerical and theoretical evidence that negative Poissons ratios are found in several nanoscale metal plates under finite strains. Furthermore, under the same conditions of crystal orientation and loading direction, materials with a positive Poissons ratio in bulk form can display a negative Poissons ratio when the materials thickness approaches the nanometer scale. We show that this behaviour originates from a unique surface effect that induces a finite compressive stress inside the nanoplates, and from a phase transformation that causes the Poissons ratio to depend strongly on the amount of stretch.

Mechanical Failure Mode of Metal Nanowires: Global Deformation versus Local Deformation

It is believed that the failure mode of metal nanowires under tensile loading is the result of the nucleation and propagation of dislocations. Such failure modes can be slip, partial slip or twinning and therefore they are regarded as local deformation. Here we provide numerical and theoretical evidences to show that global deformation is another predominant failure mode of nanowires under tensile loading. At the global deformation mode, nanowires fail with a large contraction along a lateral direction and a large expansion along the other lateral direction. In addition, there is a competition between global and local deformations. Nanowires loaded at low temperature exhibit global failure mode first and then local deformation follows later. We show that the global deformation originates from the intrinsic instability of the nanowires and that temperature is a main parameter that decides the global or local deformation as the failure mode of nanowires.

Metal [100] Nanowires with Negative Poisson’s Ratio

When materials are under stretching, occurrence of lateral contraction of materials is commonly observed. This is because Poissons ratio, the quantity describes the relationship between a lateral strain and applied strain, is positive for nearly all materials. There are some reported structures and materials having negative Poissons ratio. However, most of them are at macroscale, and reentrant structures and rigid rotating units are the main mechanisms for their negative Poissons ratio behavior. Here, with numerical and theoretical evidence, we show that metal [100] nanowires with asymmetric cross-sections such as rectangle or ellipse can exhibit negative Poissons ratio behavior. Furthermore, the negative Poissons ratio behavior can be further improved by introducing a hole inside the asymmetric nanowires. We show that the surface effect inducing the asymmetric stresses inside the nanowires is a main origin of the superior property.

The effect of transverse loading on the ideal tensile strength of face-centered-cubic materials

We employed the Born-Hill-Milstein elastic-stability theory with the aid of molecular statics and density functional theory simulations to investigate the effect of transverse loading on the ideal tensile strength of six face-centered-cubic materials. The ideal strengths of the materials were found to be largely dependent on the transverse loadings. For the case in which the transverse loadings are symmetric to each other, the ideal strength is determined by a phase transformation from tetragonal to orthorhombic structures induced by elastic instability, and the ideal strength linearly increases or decreases with the applied tensile or compressive loadings, respectively. For asymmetric transverse loadings, the ideal strength decreases with increasing asymmetry of the applied loadings.

Strong correlation between surface stress and mechanical strain in Cu, Ag and Au

The surface stress, in addition to the surface energy and elastic modulus, is a fundamental measure of the surface effects in a nanostructure. Here, we investigate the strong correlation between the surface stress and mechanical strain in three noble-metal (001) surfaces. For this investigation, we have developed a simple and efficient method to calculate the surface stress directly from the strained configurations in molecular statics simulations. While the surface stress of the copper (001) surface along the strained direction is almost constant, that of the gold (001) surface decreases drastically as the strain increases. We explain these different responses of the surface stress to strain in terms of the different surface relaxation occurring in different noble-metal nanoplates.