Satoshi Izumi

Dislocation nucleation from a sharp corner in silicon

By combining molecular dynamics simulation with reaction pathway sampling, we have observed the nucleation of a three-dimensional dislocation loop from a sharp corner in silicon and investigated the shear stress dependence of the activation energy and saddle-point configuration. The nucleated shuffle-set half-loop consisted of two 60° segments and one screw segment, each lying along a Peierls valley. The half-hexagonal shape is in good agreement with experiments at low temperature. Under high stress (90%–95% of athermal shear stress), the dislocation embryo is far from perfect, with half-size Burgers vector (about 0.2 nm) and a 0.4–0.7 nm radius forming a diffuse core region. A consequence is that the Rice–Thompson theory gives incorrect predictions regarding the activation energy and saddle-point configuration. With decreasing applied stress (less than 70%), the embryo approaches that of a perfect dislocation.

Reaction pathway analysis for dislocation nucleation from a sharp corner in silicon: Glide set versus shuffle set

Using reaction pathway sampling, we have investigated the shear stress dependences of the activation energies of shuffle-set and glide-set dislocation nucleation from a sharp corner in silicon. The gradient of the glide-set dislocation curve is lower than that of the shuffle-set dislocation, and the athermal stress of glide-set dislocation is largely higher than that of shuffle-set dislocation. As a result, the two curves have a cross point, which means that shuffle-set dislocation is likely nucleated at high stress and low temperature and glide-set dislocation is likely nucleated at low stress and high temperature. Our result clearly explains the mechanism of recent molecular dynamics on these two types of dislocation nucleation at different temperatures and stress regimes. With increased compressive stress on the slip plane, the activation energy of the shuffle-set dislocation nucleation is greatly decreased, while that of glide-set dislocation nucleation is slightly increased. That would explain why shuffle-set dislocations were found under compressive stress fields.

Reaction pathway analysis for dislocation nucleation from a Ni surface step

Threshold strain required for a thermally activated dislocation nucleation from a Ni surface step has been measured using an atomistic-based reaction pathway analysis. We show that the saddle-point configuration and the stress-dependent activation energy are strongly influenced by the presence of a surface step. Our results provide insight into the previous experimental findings concerning the mechanism on a coherency loss at the Ni∕Cu(001) interface. We conclude that the coherency strain caused by a lattice mismatch between Ni and Cu does not yield a sufficient driving force for the dislocation nucleation.

Development of empirical bond-order-type interatomic potential for amorphous carbon structures

A bond-order-type interatomic potential has been developed for reproducing amorphous carbon (a-C) structures. Several improvements have been incorporated into the conventional Brenner potential so that the material properties of carbon crystals remain unchanged. The main characteristics of the potential function developed in the present research are the use of a screening function instead of a cutoff function and the introduction of a dihedral angle potential around the bond between two threefold coordinated atoms. By using the developed interatomic potential, we can reproduce the material properties of a-C structures, such as the fraction of sp3-bonded atoms, radial distribution function, and ring statistics. It is found that the correction term enhances the formation of cluster structures in a-C, which is confirmed in the first-principles calculation.

Shuffle-set dislocation nucleation in semiconductor silicon device

We have found that a shuffle-set dislocation is nucleated in a semiconductor silicon device subjected to severe thermal processing. The dislocation transforms into a dissociated glide-set dislocation after annealing at 500°C. A possible mechanism for the nucleation of a perfect shuffle-set dislocation during thermal processing is that the dislocation nucleus was nucleated at a low temperature during prior ion-implantation processing.

Unsupervised statistical damage diagnosis for structural health monitoring of existing civil structures

Structural health monitoring is an important technology for ageing aerospace and civil structures. For this structural health monitoring, fiber optic sensors are increasing in popularity; however, several kinds of sensors are usually required, including sensors other than fiber optic sensors. Thus, a new technology for transforming conventional sensors into distributed sensors is required. The present study proposes Ethernet LAN technologies for the sensor integration required for structural health monitoring, and discusses the advantages of adopting this technology. Moreover, the paper describes an Ethernet-based health monitoring system and a statistical unsupervised damage detecting method for automatic damage diagnosis. Then, we create a system for monitoring the damage to an expressway tunnel jet-fan using system identification and statistical tools. Damage was detected from changes in a set of data measuring loads on the turnbuckles of the jet-fan. The resulting automatic diagnosis of damage to the jet-fan was successful.

Charge-transfer interatomic potential for investigation of the thermal-oxidation growth process of silicon

A charge-transfer interatomic potential, based on the hybrid-Tersoff potential that incorporates a covalent-ionic mixed-bond nature, was developed to reproduce the growth process of the thermal oxidation of silicon. A fitting process was employed with various reference structures sampled by MD. Actively exploring and learning the wide-range of phase space enabled us to develop a robust interatomic potential. Our interatomic potential reproduced the bulk properties of Si and SiO2 polymorphs well, in addition to the radial distribution function and bond angle distribution of amorphous SiO2. The covalent-ionic mixed-bond nature of the interatomic potential well reproduced the dissociation process of an oxygen molecule on the Si/SiO2 interface. The initial oxidation simulation was performed on the silicon surface. We grew the amorphous SiO2 layer by incorporating the oxygen molecules into the silicon network at the interface. The density of the SiO2 layer and the charge distribution at the interface showed go...

Nanostructural interpretation for elastic softening of amorphous carbon induced by the incorporation of silicon and hydrogen atoms

First-principles molecular dynamics simulation is used to investigate the elastic softening of amorphous carbon on the incorporation of silicon and hydrogen atoms, and the mechanisms responsible for this phenomenon are discussed from the viewpoint of atomic structure. With increasing silicon incorporation, it is found that the bulk moduli of silicon-incorporated amorphous carbon (a-C:Si) and silicon-incorporated hydrogenated amorphous carbon (a-C:Si:H) decrease, whereas the total number of sp3-bonded atoms increases. This is explained on the basis of interatomic bond structures such as: increasing silicon incorporation reduces the number of interatomic (both single and double) bonds between carbon atoms while increasing the number of interatomic bonds between silicon and carbon atoms. Furthermore, for a given density and silicon content, it is found that the bulk modulus of the a-C:Si structure is greater than that of the a-C:Si:H structure, though their interatomic bond structures are similar. The reduce...

Development of a bond-order type interatomic potential for Si–B systems

A bond-order type interatomic potential for Si–B systems is developed. We employed the bond-order type potential function proposed by Tersoff. Properties of Si–B crystals which involve a wide range of local atomic environments are used for fitting. The formation energies of various atomic structures that are thought to appear during B diffusion or Si–B clustering are also fitted. A genetic algorithm is used to find the optimized parameters. The resulting potential reproduced well the Si-interstitial-assisted diffusion of B as well as the physical properties used for fitting.

Fatigue Damage Evaluation for Thermal Striping Phenomena Using Analytical Spectrum Method

It is well known that fatigue damage accumulates around T-junctions of piping system where two kinds of fluid with different temperatures are mixed. This phenomenon is called thermal striping, and simple method for evaluating the fatigue damage derived from this phenomenon is greatly important for both design and maintenance stages. However, the evaluation of the thermal stress derived from the thermal striping is rather difficult since the evaluation requires complicated analyses such as fluid mixing, heat transfer, and heat conduction. In addition, the closed-form solution to describe the stress-range distribution under random loading has not been established yet since the rainflow cycle-counting method is not suitable to the analytical treatment. Though numerical calculation may be available for the evaluation, it requires time-consuming work and is not practical for design stage. For this reason, an analytical method for evaluating the fatigue damage directly from stress power spectrum density (PSD) is desired. In this paper, the feasibility study to apply analytical methods to the evaluation of thermal-striping damage is examined. The analytical methods are applied to the fatigue damage evaluation from the stress PSD that was obtained by the thermal striping experiment. Finally, the applicability and problem of each method will be discussed. In order to apply the envelope PSD to design, the property of safety margin associated with the PSD is investigated too.Copyright