Naoki Miyazawa

Mechanical characterization of nanoporous Au modified with self-assembled monolayers

The surface of nanoporous Au was modified with self-assembled monolayers (SAMs) of 6-mercapto-1-hexanol and the hardness tests were performed on the SAM-modified and non-modified nanoporous Au to investigate the effects of SAM modification on the mechanical properties of nanoporous Au. In addition, the origin of the chemomechanical effects was investigated by first principles shear test simulations on an Au–S alloy. The SAM-modified nanoporous Au showed lower hardness than the non-modified nanoporous Au. The loading rate dependence tests showed that the activation volume was low for both, indicating that events of a short range play an important role in deformation of nanoporous Au, regardless of whether the nanoporous Au was modified with SAMs. It was suggested from the simulations that the lower hardness for the SAM-modified nanoporous Au is because movement of dislocation endpoints at the surface is facilitated by chemical effects of Au–S bonding.

An atomistic study of Y segregation at a {101¯1}–{101¯2} double twin in Mg

Segregation at a triple junction of grain boundaries has not been explained much because the structure of a triple junction is very complicated. The present paper describes Monte Carlo simulations by which Y segregation was investigated at a triple junction of a {101¯1}–{101¯2} double twin in Mg. Y atoms segregated at the extension sites in the {101¯1} and {101¯2} twin boundaries. However, they were not necessarily more segregated at the triple junction of the double twin, although the free volumes at the extension sites of the triple junction were larger on average than those of the other boundaries. Thus, the Y segregation behavior at the triple junction cannot be explained only by the free volume. The anisotropic factor of the atomic Voronoi polyhedron was developed to explain the Y segregation behavior at the triple junction. In addition, the shortest interatomic distance and coordination number affected Y segregation at the triple junction. Also, segregation at the triple junction strongly depended o...

Antimicrobial mechanisms due to hyperpolarisation induced by nanoporous Au

Nanomaterials such as nanoparticles exhibit remarkable antimicrobial activities. Nanoparticles directly disturb the cell membrane or cytoplasmic proteins because they pass through the cell wall. Nanoporous Au (NPG) is another antimicrobial nanomaterial, which cannot pass through the cell wall of bacteria but can still kill bacteria, utilising interactions between the surface of NPG and cell wall of bacteria. The origins of antimicrobial activities without direct interactions are unknown. It is necessary to elucidate these mechanisms to ensure safe usage. Here we show that the antimicrobial mechanism of NPG consists of two interactions: between the surface of NPG and cell wall, and between the cell wall and cell membrane. Fluorescent experiments showed that the cell wall was negatively hyperpolarised by NPG, and molecular dynamics simulations and first-principles calculations suggested that the hyperpolarisation of the cell wall leads to delicate structural changes in the membrane proteins, rendering them bactericidal. Thus, the hyperpolarisation induced by NPG plays a critical role in both interactions. The combination of molecular dynamics simulations and first-principles calculations allows a deeper understanding of the interactions between metallic surfaces and biomolecules, because charge transfer and exchange interactions are calculated exactly.

Atomistic study of inelastic deformation in aluminium grain boundary fractures

Abstract The plastic work of fracture in a deformable solid has been believed to be related to only the ideal brittle fracture energy. However, additional factors affecting the plastic work must also exist because the plastic work is a path function. In the present work, first-principles calculations and molecular dynamics simulations of tensile tests were performed on Σ3(1 1)[1 1 0] and Σ11(1 3)[1 1 0] symmetric tilt aluminium grain boundaries, where the grain boundary energy of the Σ11(1 3) grain boundary is higher than that of the Σ3(1 1)[1 1 0] gain boundary. The calculations showed that, although the ideal brittle fracture energy for the Σ11(1 3) grain boundary was almost the same as that for the Σ3(1 1) grain boundary, the plastic work for the former was larger than that for the latter, resulting in a larger fracture energy for the Σ11(1 3) grain boundary. Local inelastic deformation occurred around the atoms with high internal energy at the grain boundary for the Σ11(1 3) grain boundary. It is therefore suggested that the plastic work is a function of both the grain boundary energy and the ideal brittle fracture energy.

Atomic bond-breaking behaviour during grain boundary fracture in a C-segregated Fe grain boundary

Abstract First-principles fully relaxed tensile tests were performed on a C-segregated Fe Σ3 (1 1 1)/[] symmetrical tilt grain boundary (GB) to investigate the breaking behaviour of C–Fe bonds during tensile straining. Fe atoms around a C atom moved obliquely to the tensile direction, and C–Fe bonds stretched in the tensile direction without premature bond-breaking. Analyses of the electronic states during deformation showed that a variation in the charge density at the bond critical point was much larger for the C–Fe bond than for the P–Fe bond and that the C atom exhibited larger variations of s and p states involved in the covalent-like characteristics than the P atom. It is suggested that these lead to a high mobility of the C–Fe bonds. On the other hand, first-principles shear tests on the Fe GB imply that C-segregated Fe GB toughening is not associated with increased crack blunting by dislocation emission.

Atomic simulations of the effect of Y and Al segregation on the boundary characteristics of a double twin in Mg

There is usually a tradeoff between the strength and the ductility in solute strengthening of metallic materials. However, magnesium is an exception. A {10 1¯1}-{10 1¯2} double twin (DT) provides a fracture-initiation site in Mg. Hence, an investigation on effects of segregations on facture at the DT will give a clue for understanding an exception of Mg to the tradeoff relation. In the present work, we investigated segregation behavior of Y and Al at the DT and interactions of a screw dislocation with segregated DTs by Monte Carlo (MC) and molecular dynamics (MD) simulations. The MC simulations showed that the volumes of the atomic Voronoi polyhedron were changed by Y segregation, while the anisotropic factors of the atomic Voronoi polyhedron were changed by Al segregation. Also, the MD simulations indicated that Y segregation induced emission of twinning dislocations from the DT, while Al segregation suppressed the motion of the twinning dislocation. Thus, the boundary characteristics of the Y-segregated...