A first-principles local stress analysis has been applied in order to reveal stress distribution in the vicinity of semiconductor hetero-interfaces of GaAs/XAs (X = Al, As). The weighted Bader integration scheme based on the Yu-Trinkle algorithm is found to be so effective that stress density is accurately integrated to the local stress over the Bader atomic volume. Applying the developed local stress calculation to semiconductor interfaces, we found that interface stress is localized, in the range of a single atom, in the vicinity of the interface. We also found that the nanoscopic stress state reflects the electronic bonding of atoms in the vicinity of the interfaces.
Intermittent crystalline plasticity with power-law behaviors, which is reminiscent of self-organized criticality, has been reported in recent experimental and numerical studies. In this study,we showthat the compressiveloading condition can provide different statistical features of intermittent plasticity in metals from those under tensile loading. Employing an embedded atom method potential for aluminum we performed molecular dynamics simulations for uniaxial tensile and compressive deformation. It is shown that the power spectra of tensile stress have power-law decay region. Power-law distribution of stress drop and waiting time of plastic deformation events are also observed. However, under the compressive loading large-scale deformation events which result in a plateau and larger cutoffon the stress drop distri.bution are observed. This difference is originated from the geometrical feature of slip systems that dominate interaction between dislocations in crystals.
The intrinsic stress at hetero interface is one of key factors to induce the structural instability of micropatterning in semiconductor devices, and it is essential to quantitatively predict the intrinsic stress in terms of atomic structure. In our previous study, we established the method to predict the lateral undulation buckling of micropattern using continuum buckling theory and .nite element method. However, there is a possibility that several-nanometer surface oxidized layer, which is formed during oxygen plasma etching process, involves the intrinsic stress of the order of 1 GPa and affects the buckling criteria. Since the experimental measurement of the stress of such a nano-scale layer is beyond the present technology, an atomistic simulation could be a tool to discuss the possibility. In this study, we make a molecular dynamics model for the plasma etching and predict the intrinsic stress of the oxidized layer on the surface of amorphous silicon (a-Si). Our approach reveals that the layer thickness depends on the incident energy of oxygen and that the intrinsic stress exceeds 1 GPa. Therefore, we conclude that it would be possible for the oxidized a-Si film to trigger the buckling of micropatterning even if the initial a-Si structure involves no stress.
Dislocation nucleation from free surfaces is one of the important process of plastic deformation of a size reduced pillar, in which pre-existing dislocations is not sufficient to take plastic deformation. In this work, nucleation of a basal dislocation from a free surface of an elliptic cylindrical Mg single crystal is considered. Molecular dynamics simulations of tensile deformation of crystals show that the nucleation site of a dislocation changes when the ratio between axes of the ellipse changes, though the critical resolved shear stress is almost the same. Dependence of the activation free energy of nucleation of a dislocation on the nucleation site is also studied by applying metadynamics method to the atomistic simulation. Then, we found that the activation free energy of nucleation of a dislocation lowers at the site where a dislocation is actually nucleated in molecular dynamics simulations.
Crack propagation in Magnesium was investigated using molecular dynamics (MD) simulations. By using a disk-shaped atomic cell containing a crack under the anisotropic linear elastic displacement, the deformation field around the crack tip was first atomistically resolved. In order to consider effects of surface energy on crack propagation, two kinds of interatomic potentials were adopted. For an embedded atom method (EAM) potential, basal and first-pyramidal surfaces are favorable cleavage planes due to relatively low surface energies. Thus, MD simulations using the EAM showed unstable crack propagation on those planes. For the other generalized embedded atom method (GEAM) potential which provides the higher surface energies instead, the basal plane has the larger critical stress intensity factor by the Griffith’s formula because it is proportional to square root of surface energy. Consequently, the dislocation nucleation and deformation twin nucleation were occurred in the vicinity of the crack tip. The other MD analyses of defect interaction between a crack and twin boundaries (TBs) were also carried out. Twinning dislocations of (1012) twin were generated by reaction with the basal dislocations emitted from the crack tip, and thus (1012) TB easily moved. On the other hand, a void or deformation twin was generated along the boundary region where the dislocations from the crack tip piled up, and then the crack propagation near (1011) TB arose.
Understanding the diffusion properties of hydrogen in metals, especially interactions between hydrogen and lattice defects such as dislocations, is essential in order to improve the performance and reliability of equipment associated with hydrogen. We investigate the effect of edge dislocations on hydrogen diffusion in face-centered-cubic (fcc) metals using molecular dynamics simulations of palladium-hydrogen as a model system. We find that the hydrogen diffusion in a perfect crystal without the dislocations is isotropic, whereas that in a system with the dislocations has anisotropy. In addition, our results predict high hydrogen diffusivity along the dislocation lines in high hydrostatic stress region with high hydrogen density after hydrogen accumulation caused by the interactions between hydrogen atoms and the dislocations. The activation energy of the hydrogen diffusivity in the system with the dislocations is estimated to be 0.10 eV, which is 38 % smaller than that in the perfect crystal.
CCS (Carbon dioxide Capture and Storage) projects have been focused on as one of state-of-art technologies to prevent the CO2 emission in the atmosphere. In a project, the CO2 collected from factories is injected into the geological reservoir at deep underground through well and stored in the reservoir. It is expected that the well will have gradual degradations due to high temperature, high pressure and high concentration CO2 at deep underground. The process of the degradations should be investigated to prevent from leaking the CO2 through the well. In this study, the variation of pore structure and air permeability by the carbonation of the hardened cement paste exposed to the supercritical CO2 under water, salt water and sealed condition was examined focusing on the carbonation progress. The pore structure of small sample (diameter 4mm and height 4 mm) became denser at 1 day of exposure all cases because of the precipitation of CaCO3 in capillary pores while the carbonation of C-S-H occurred at 7 days to make pores coarse in the case of supercritical CO2 in water. It was experimentally found that the slight carbonation makes pores denser and leads to the reduction of the air permeability. The hardened cement paste using oil well cement with fly ash had denser pore structure to make permeability smaller than that using only oil well cement. It is attributed to Pozzolanic reaction of fly ash at high temperature. Thus, it was suggested that fly ash has a potential to apply to the CCS well from a view point of the prevention of CO2 gas leakage.
Steel is commonly used for marine structures. However steel is easily corroded under marine environment, therefore various protection methods against steel corrosion have been developed. Among them, paint coating and cathodic protection are often used together. When cathodic protection is applied to defective paint-coated steel, cathodic protection may lead to paint degradation around defects. Namely, paint coating is blistered or disbonded by the generation and accumulation of hydroxide ions induced by cathodic reaction. Cathodic protection can control corrosion reaction under submerged zone. However in tidal zone, there is a dry period in which paint-coated steel is exposed in air. At that time cathodic protection cannot work. So paint degradation in tidal zone may lead to severe corrosion. Based on the above backgrounds, the following items were investigated in the present study; (1) investigation of influence of defect existence on paint degradation on paint-coated steel with cathodic protection, (2) investigation of influence of paint type, paint thickness and current density on paint degradation on defective paint-coated steel with cathodic protection, (3) clarification of paint degradation and corrosion on defective paint-coated steel with cathodic protection under tidal zone. Experimental results obtained in this study indicated that the paint degradation and severe corrosion around defects might be occurred due to cathodic protection but could be controlled by using proper paint conditions.
Tensile tests were carried out for pure Al and its alloy thin films sputtered on Polyimide film. The effect of annealing temperature (at 200, 400 and 500℃) on deformation behavior was investigated. All thin films were fabricated by DC-Magnetron sputtering. Deformation behavior was measured using X-ray method. In Al-Ta-Si(A), (B) of non-annealed and 200℃ specimens, a relatively wide elastic region was observed. On the other hand the elasticity region was narrow for other materials. 0.2% proof stress of pure Al was almost constant irrespective of annealing temperature. For other materials, the value decreased with increasing annealing temperature. In that case, the grain size increased with annealing temperature for each material. In the case of pure Al, Al-Si and Al-Si-Ti, in all annealing temperatures, FWHM was almost constant in elastic region and increased when the plastic deformation occurred. On the other hand, in the case of Al-Ta-Si(A) and (B) of non-annealed specimen, the initial value of FWHM was high enough, and change of FWHM was not remarkable even in plastic region. For the relation between the proof stress and grain size, scatter was relatively large, however the high-strength tendency with decreasing grain size was confirmed. After tensile loading, intergranular cracks could be observed on a specimen surface for each material. Cracks initiated from splashes and large grains.
In order to investigate the delamination behavior along the interface between a submicron thick polymer (epoxy) film and substrate (silicon) due to the time-dependence deformation (creep), we develop in-situ observation techniques of the interface crack using a cantilever specimen (Stainless steal/Si/epoxy/glass). Under the unidirectional loading, an interface crack is initiated from the free edge of Si/epoxy and propagates stably along the interface, which leads to the delamination fracture of the specimen. The result of unidirectional loading test with the different loading speed shows the different Si/epoxy delamination behavior. This indicates that the Si/epoxy interface crack propagation depends on the loading speed due to the creep of epoxy thin film. To evaluate the effect of creep on the delamination, the constant loading test is conducted. It is revealed that Si/epoxy interface crack propagates with the passage of time although the load is constant. These results conclude that the interface crack between Si and epoxy propagates due to the creep. We also conduct the unidirectional loading test for the specimen with different thick epoxy thin film. The average interface crack propagation rate is increased with decreasing the thickness of epoxy thin film. Considering that the deformation of epoxy thin film is constrained by hard material of glass and Si, this result indicates that the creep zone, where the creep strain dominates, is localized near the Si/epoxy interface crack tip (Small Scale Creep (SSC) condition). It is concluded the delamination behavior between Si and submicron thick epoxy film is dominated by SSC condition.
A technology for efficiently designing a multilayer metal film with strong adhesion to a soft resin was developed by combining a molecular simulation with an orthogonal array. In this technique, the adhesion strength of the interface between the metal film and resin is evaluated by calculating the adhesive fracture energy by use of a molecular-dynamics simulation. The most significant factor in the adhesion strength can be found by using an orthogonal array. This paper describes an application of this technique to the design of a metal film with strong adhesion to a soft resin made of hydrocarbon chains. By carrying out a sensitivity analysis with the orthogonal array, the mismatches among four metal-film factors (the short-side and long-side lattice mismatches with the resin ⊿a and ⊿b values, the surface energy density, G , and the cohesive energy, H of an atomic-layer laminated metal film) were found to be the most dominant factors in the adhesion strength. The author also found that reducing the mismatches is effective for increasing the adhesion strength. The results of molecular-dynamics simulations showed that a copper/cobalt/nickel-laminated film is the most appropriate structure with strong adhesion to the hydrocarbon-chain-based resin.