The Experts below are selected from a list of 270 Experts worldwide ranked by ideXlab platform
Takashi Nakayama - One of the best experts on this subject based on the ideXlab platform.
-
first principles study of stair rod dislocations in si and gaas Stacking Fault Tetrahedron defects
PHYSICS OF SEMICONDUCTORS: 28th International Conference on the Physics of Semiconductors - ICPS 2006, 2007Co-Authors: Ryo Kobayashi, Takashi NakayamaAbstract:Atomic and electronic structures of stair‐rod dislocations, which are produced at ridges of Stacking‐Fault Tetrahedron, are studied using the first‐principles calculations. It is shown that atoms at the dislocation core produce parallel or zigzag double dimer‐bond arrays along the dislocation. In case of GaAs, deep‐level bands appear in the band gap due to the misvalency of Ga‐Ga or As‐As dimer bonds. We found that the symmetry breaking of dimer‐bond length occurs between two neighboring arrays to stabilize such deep‐level bands.
-
First‐Principles Study of Stair‐rod Dislocations in Si and GaAs Stacking‐Fault Tetrahedron Defects
AIP Conference Proceedings, 2007Co-Authors: Ryo Kobayashi, Takashi NakayamaAbstract:Atomic and electronic structures of stair‐rod dislocations, which are produced at ridges of Stacking‐Fault Tetrahedron, are studied using the first‐principles calculations. It is shown that atoms at the dislocation core produce parallel or zigzag double dimer‐bond arrays along the dislocation. In case of GaAs, deep‐level bands appear in the band gap due to the misvalency of Ga‐Ga or As‐As dimer bonds. We found that the symmetry breaking of dimer‐bond length occurs between two neighboring arrays to stabilize such deep‐level bands.
-
thermal annihilation process of Stacking Fault Tetrahedron defect in si film epitaxy
Thin Solid Films, 2006Co-Authors: Ryo Kobayashi, Takashi NakayamaAbstract:Stacking-Fault Tetrahedron (SFT) is a nano- to micrometer size defect that is generated in epitaxialized films originating from impurity atoms on the substrate. The atomistic process of thermal annihilation of the SFT in Si(111) films was investigated using molecular dynamics simulations. We found that the SFT is annihilated by the movement of Shockley partial dislocations from the top surface to the SFT bottom apex.
-
electronic structures of natural quantum dot system si Stacking Fault Tetrahedron
Physica Status Solidi (c), 2005Co-Authors: Takashi Nakayama, Ryo KobayashiAbstract:Electronic structures of the Si Stacking-Fault Tetrahedron (SFT) are studied, by using the first-principles calculations. It is shown that the electron and hole states of bulk Si are localized around the SFT faces, the SFT ridges, and the SFT apexes. This feature occurs due to the unique atomic geometries in these structures, such as Stacking-Fault layers, dimer bonds, and the Si-dome structures. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
-
Electronic structures of natural quantum‐dot system; Si Stacking‐Fault Tetrahedron
Physica Status Solidi (c), 2005Co-Authors: Takashi Nakayama, Ryo KobayashiAbstract:Electronic structures of the Si Stacking-Fault Tetrahedron (SFT) are studied, by using the first-principles calculations. It is shown that the electron and hole states of bulk Si are localized around the SFT faces, the SFT ridges, and the SFT apexes. This feature occurs due to the unique atomic geometries in these structures, such as Stacking-Fault layers, dimer bonds, and the Si-dome structures. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Shengping Shen - One of the best experts on this subject based on the ideXlab platform.
-
size dependent stability of Stacking Fault Tetrahedron in coherent twin boundary bicrystal comparisons among al ni cu and ag
Computational Materials Science, 2018Co-Authors: Lianping Wu, Wenshan Yu, Shuling Hu, Shengping ShenAbstract:Abstract Both Stacking Fault Tetrahedron (SFT) and nanoscale twins have been found to significantly affect the plasticity of metals. In this study, molecular dynamics simulations are performed to investigate the size-dependent stability of SFT in four face-centered cubic (FCC) metal (i.e., Al, Ni, Cu, and Ag) coherent twin boundary (CTB) bicrystals under shear. It is found that different size-dependent SFT stability stems from the competition between CTB migration and SFT dissociation at the incipient plasticity of CTB bicrystal. For SFT of any sizes in Al and smaller size SFT in Ni, SFT always keeps stable so that CTB migrates at the incipient plasticity. For SFT of any sizes in Cu and Ag, and larger size SFT in Ni, initial CTB always keeps still and SFT dissociates instead. SFT size effect transforms the deformation mechanism at the incipient plasticity from CTB migration to SFT dissociation in Ni. In Cu and Ag, it reduces a lot of the critical stress corresponding to the incipient plasticity of CTB bicrystal. Whether CTB migration or SFT dissociation occurs first will cause completely different final configurations of SFT and CTB. This study may not only shed light on revealing the SFT-CTB interaction mechanism but also provide new insight into the plasticity of irradiated FCC nanotwinned metals.
-
shear response of grain boundary bicrystals with a Stacking Fault Tetrahedron
Computational Materials Science, 2018Co-Authors: Lianping Wu, Wenshan Yu, Shuling Hu, Shengping ShenAbstract:Abstract Stacking Fault Tetrahedrons (SFTs) are commonly seen in the polycrystalline and nanocrystalline materials due to the heavy plastic deformation, quenching and irradiation. In this study, we present the transformation of SFT near four Cu symmetric tilt grain boundaries (GBs) under shear. Atomistic structures involved in GB deformation and SFT configuration evolutions during the shear are analyzed. Our results show that the presence of SFT has small influence on critical stress corresponding to the incipient plasticity of GB bicrystal. Because GB deformation such as migration, sliding and structure evolvement occur at a smaller external shear stress not large enough to activate the destroy of SFT, and SFT near four GBs transforms only when interacting with partial dislocation or GB. Besides, the presence of an SFT does not substantially change the deformation modes of these four GBs. Of one particular case is GB deforming in the complex mechanism due to atom-shuffling, partial dislocation nucleation and local GB dissociations. Significant stress concentration arises around GB due to SFT, which may change the nucleation site of partial dislocations.
-
stability of Stacking Fault Tetrahedron in twin boundary bicrystal copper under shear
International Journal of Plasticity, 2017Co-Authors: Lianping Wu, Wenshan Yu, Shuling Hu, Shengping ShenAbstract:Abstract Stacking Fault Tetrahedrons (SFTs) appear nearly immobile in metals, its stability under shear loading is worth studying and discussion. In this paper, we present mechanism studies of SFT in twin boundary (CTB) bicrystal copper and single crystal copper under shear. Three different sizes of SFT and four different distances from SFT to CTB are considered. Detailed analysis of atomistic structures involved in CTB migration and SFT configuration evolutions during the shear are made. It is found that CTB bicrystal and single crystal embedded with a larger size of SFT have a smaller critical stress at their incipient plasticity. A complete transformation of SFT is assisted by dislocation motions and atomic diffusions in SFT area. Dislocation motion begins with the dissociation of two stair-rod dislocations on the SFT basal plane while the CTB migration direction with respect to SFT, resulting in the atomic diffusions, determines the final configuration of SFT. The continuous migrations of CTB towards and away SFT respectively lead to the collapse and flip over of SFT. This work, the first investigation of the stability of SFT in CTB bicrystal copper and single crystal copper under shear, may be helpful in revealing mechanical properties of irradiated metals.
Liang Zhang - One of the best experts on this subject based on the ideXlab platform.
-
dynamic interaction between grain boundary and Stacking Fault Tetrahedron
Scripta Materialia, 2018Co-Authors: Liang Zhang, Cheng Lu, Kiet Tieu, Yasushi ShibutaAbstract:Abstract We utilize molecular dynamics simulations to investigate the dynamic interaction between the grain boundary (GB) and the Stacking Fault Tetrahedron (SFT) in bicrystal copper. The grain boundary can migrate itself under the shear strain and can serve as a sink to remove SFT. The sink efficiency of grain boundaries is sensitive to their structural characteristics. The high-angle GBs can show a great ability to remove SFT even at an extreme low temperature, while the increase of temperature can facilitate the annihilation of SFT at the low-angle GBs. This study reveals a new possible GB-mediated damage healing mechanism of irradiated materials.
-
the formation and destruction of Stacking Fault Tetrahedron in fcc metals a molecular dynamics study
Scripta Materialia, 2017Co-Authors: Liang Zhang, Cheng Lu, Guillaume Michal, Guanyu Deng, Kiet TieuAbstract:Abstract Molecular dynamics simulations were conducted to study the formation and destruction of Stacking Fault Tetrahedron (SFT) in fcc metals. The Stacking Fault energy, the size of vacancy cluster and temperature were found to play a significant role in the formation of a perfect SFT. Also, it was found that the compressive stress can unzip the perfect SFT to a truncated one, and can facilitate the destruction of SFT by transforming the Faulted Frank loop to the unFaulted full dislocation loop. We provided the atomic details of how the unFaulting occurs using molecular dynamics method.
-
Stacking Fault Tetrahedron induced plasticity in copper single crystal
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2017Co-Authors: Liang Zhang, Cheng Lu, Kiet Tieu, Lihong Su, Xing ZhaoAbstract:Abstract Stacking Fault Tetrahedron (SFT) is the most common type of vacancy clustered defects in fcc metals and alloys, and can play an important role in the mechanical properties of metallic materials. In this study, molecular dynamics (MD) simulations were carried out to investigate the incipience of plasticity and the underlying atomic mechanisms in copper single crystals with SFT. Different deformation mechanisms of SFT were reported due to the crystal orientations and loading directions (compression and tension). The results showed that the incipient plasticity in crystals with SFT resulted from the heterogeneous dislocation nucleation from SFT, so the stress required for plastic deformation was less than that needed for perfect single crystals. Three crystal orientations ([1 0 0], [1 1 0] and [1 1 1]) were specified in this study because they can represent most of the typical deformation mechanisms of SFT. MD simulations revealed that the structural transformation of SFT was frequent under the applied loading; a metastable SFT structure and the collapse of SFT were usually observed. The structural transformation resulted in a different reduction of yield stress in compression and tension, and also caused a decreased or reversed compression/tension asymmetry. Compressive stress can result in the unFaulting of Frank loop in some crystal orientations. According to the elastic theory of dislocation, the process of unFaulting was closely related to the size of the dislocation loop and the Stacking Fault energy.
Ryo Kobayashi - One of the best experts on this subject based on the ideXlab platform.
-
first principles study of stair rod dislocations in si and gaas Stacking Fault Tetrahedron defects
PHYSICS OF SEMICONDUCTORS: 28th International Conference on the Physics of Semiconductors - ICPS 2006, 2007Co-Authors: Ryo Kobayashi, Takashi NakayamaAbstract:Atomic and electronic structures of stair‐rod dislocations, which are produced at ridges of Stacking‐Fault Tetrahedron, are studied using the first‐principles calculations. It is shown that atoms at the dislocation core produce parallel or zigzag double dimer‐bond arrays along the dislocation. In case of GaAs, deep‐level bands appear in the band gap due to the misvalency of Ga‐Ga or As‐As dimer bonds. We found that the symmetry breaking of dimer‐bond length occurs between two neighboring arrays to stabilize such deep‐level bands.
-
First‐Principles Study of Stair‐rod Dislocations in Si and GaAs Stacking‐Fault Tetrahedron Defects
AIP Conference Proceedings, 2007Co-Authors: Ryo Kobayashi, Takashi NakayamaAbstract:Atomic and electronic structures of stair‐rod dislocations, which are produced at ridges of Stacking‐Fault Tetrahedron, are studied using the first‐principles calculations. It is shown that atoms at the dislocation core produce parallel or zigzag double dimer‐bond arrays along the dislocation. In case of GaAs, deep‐level bands appear in the band gap due to the misvalency of Ga‐Ga or As‐As dimer bonds. We found that the symmetry breaking of dimer‐bond length occurs between two neighboring arrays to stabilize such deep‐level bands.
-
thermal annihilation process of Stacking Fault Tetrahedron defect in si film epitaxy
Thin Solid Films, 2006Co-Authors: Ryo Kobayashi, Takashi NakayamaAbstract:Stacking-Fault Tetrahedron (SFT) is a nano- to micrometer size defect that is generated in epitaxialized films originating from impurity atoms on the substrate. The atomistic process of thermal annihilation of the SFT in Si(111) films was investigated using molecular dynamics simulations. We found that the SFT is annihilated by the movement of Shockley partial dislocations from the top surface to the SFT bottom apex.
-
electronic structures of natural quantum dot system si Stacking Fault Tetrahedron
Physica Status Solidi (c), 2005Co-Authors: Takashi Nakayama, Ryo KobayashiAbstract:Electronic structures of the Si Stacking-Fault Tetrahedron (SFT) are studied, by using the first-principles calculations. It is shown that the electron and hole states of bulk Si are localized around the SFT faces, the SFT ridges, and the SFT apexes. This feature occurs due to the unique atomic geometries in these structures, such as Stacking-Fault layers, dimer bonds, and the Si-dome structures. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
-
Electronic structures of natural quantum‐dot system; Si Stacking‐Fault Tetrahedron
Physica Status Solidi (c), 2005Co-Authors: Takashi Nakayama, Ryo KobayashiAbstract:Electronic structures of the Si Stacking-Fault Tetrahedron (SFT) are studied, by using the first-principles calculations. It is shown that the electron and hole states of bulk Si are localized around the SFT faces, the SFT ridges, and the SFT apexes. This feature occurs due to the unique atomic geometries in these structures, such as Stacking-Fault layers, dimer bonds, and the Si-dome structures. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Yoshiharu Shimomura - One of the best experts on this subject based on the ideXlab platform.
-
Development of vacancy clusters in neutron-irradiated copper at high temperature
Journal of Nuclear Materials, 2000Co-Authors: Yoshiharu Shimomura, I. MukoudaAbstract:Pure copper was fission-neutron-irradiated at 473 and 573 K from 0.0003 to 0.14 dpa. In copper, which was irradiated at 573 K to 0.0003 dpa, the number of vacancies which were accumulated in the largest Stacking Fault Tetrahedron (SFT) was 276, while it was 470 in the smallest voids. This is explained by a model in which at 573 K a SFT converts to a void when the number of vacancies exceeds about 400. In 573 K irradiated copper, the number of vacancies in a SFT and a void of average size increases with the neutron fluence. The number of vacancies in a void increases more rapidly than that in a SFT. The reason appears to be that small vacancy clusters relax at 573 K to a string-like cluster, move as a cluster and coalesce. Experimental results are presented which show the movement of voids. In copper which was neutron-irradiated at 573 K, the number density of SFTs and voids peaked at 0.0003 dpa and decreased with fluence. The reason appears to be a low sink efficiency of dislocations for point defect absorption at 0.0003 dpa. Due to a low sink efficiency straight extended dislocations were decorated with many interstitial clusters. After jogs are formed on dislocations by joining with grown interstitial loops, the absorption efficiency of point defects increases significantly which lowers the density of SFTs and voids with increasing of dpa.
-
Computer simulation on the void formation in neutron-irradiated Cu and Ni at high temperature
Journal of Nuclear Materials, 1999Co-Authors: Yoshiharu Shimomura, I. Mukouda, Kenjiro SugioAbstract:Abstract Atomistic processes of evolution of damage structure in neutron-irradiated Cu and Ni at high temperature are investigated on the linkage of experimental results and computer-simulation. Interstitials and their clusters move to form a grouping of interstitial clusters which subsequently evolve to dislocation at high temperature. Vacancies aggregate to Stacking Fault Tetrahedron (SFT) and void. At high temperature gas atoms do not make a significant contribution to the nucleation of voids at low fluence of neutron irradiation such as 1018 n/cm2. SFT relax at a high temperature to a string of vacancy clusters, in which vacancies are connected one-dimensionally. The break-up of SFT occurs by jumping of an atom into a SFT. A loosely bound vacancy cluster of a string shape moves with low activation energy of 0.2 eV. Voids are formed by the coalescence of moving vacancy clusters of string shape at high temperature. This is due to low Helmholtz free energy of void-containing crystal than that of SFT-containing crystal. The role of gas atoms in the formation of voids is to degrade the mobility of movable vacancy clusters when captured by the clusters.
-
Damage evolution in neutron-irradiated Cu during neutron irradiation
Journal of Nuclear Materials, 1999Co-Authors: I. Mukouda, Yoshiharu ShimomuraAbstract:Abstract We fabricate Cu of residual-gas-free by melting them in vacuum of 10 −5 Pa. Both residual-gas-free specimen and as-received specimens which were estimated to contain 4 ppm hydrogen atoms were neutron-irradiated at 200°C and 300°C with temperature-controlled rig in Japan Material Testing Reactor (JMTR). Neutron fluence ranges from 5.3 × 10 18 to 1.0 × 10 20 n/cm 2 . Irradiated specimens were observed by electron microscopy. In copper, both Stacking Fault Tetrahedron (SFT) and voids were observed. The number density of voids decreased with increasing the fluence. The size of voids increased with the fluence. The voids formed uniformly in specimens at the low fluence, while some of voids were observed near dislocations at the high fluence. The number density of SFT increased with the fluence at 200°C. The number of vacancies which are accumulated in a void is 350 times larger than that in a SFT in a specimen irradiated to 5.3 × 10 18 n/cm 2 at 200°C. At low fluence the number density of voids is same for as-received specimens and residual-gas-free specimens. The difference of the number density of voids between these two specimens was observed at high fluence in which the density is low in the residual-gas-free copper. Results are modeled as follows. Small vacancy clusters move during an irradiation. Voids nucleate when the coalescence of small vacancy clusters occurs. The mobility of voids with gas atoms is lower than that without gas-atom. This causes the number density of voids in as-received copper to be larger than that in residual-gas-free copper.
-
Computer simulation of the clustering of small vacancies in nickel
Computational Materials Science, 1999Co-Authors: R Nishiguchi, Yoshiharu ShimomuraAbstract:Abstract Small clusters of vacancies were introduced in a crystal of 4000 atoms at randomly selected positions in Ni. Computer simulations were carried out with the isotropic potential of embedded atom method (EAM) (M.S. Daw and M.I. Baskes, Phys. Rev. B 29 (1984) 6443) at 1200, 1400, 1500, 1600 and 1700 K. After MD simulation, the results were output in an interval of 0.2 ps for 40–600 ps to observe the structural relaxation with AVS. In the present work, forty single vacancies were observed to cluster at 1700 K after MD run for 100 ps whereas four single vacancies were not observed for 600 ps. Further MD run is needed for vacancies to agglomerate. Triangular tri-vacancy (3v) also grew to a larger cluster from two tri-vacancies which were initially positioned closer than the other 3v. Four clusters of triangular 15-vacancy were introduced in a crystal. The clusters at first relaxed Stacking Fault Tetrahedron (15v-sft). To change the structure of clusters played an important role for the direction to agglomerate. At low temperature agglomeration did not occur during MD calculation.
-
vacancy clustering to Faulted loop Stacking Fault Tetrahedron and void in fcc metals
Radiation Effects and Defects in Solids, 1997Co-Authors: Yoshiharu Shimomura, R NishiguchiAbstract:Abstract An atomistic step of growth to a Faulted loop, a Stacking Fault Tetrahedron (sft) and a void by clustering of vacancies in fcc metals was studied by molecular dynamics computer simulation with an isotropic EAM potential due to Daw and Baskes [1]. In aluminum, a tri-vacancy relaxes to the Damask-Dienes-Weizer structure (3v-sft). A penta-vacancy relaxes to an octahedral 6v in which an atom is included. The relaxed 5v is stable so that a 6v grows to doubly linked relaxed 5v. This is a critical step of Faulted loop formation. By further absorption of vacancy, a cluster grows to an array of relaxed 5vs on a (111) plane and finally collapses to a Faulted loop. In gold, a stable structure of vacancy cluster below 15v is a void. A tri-vacancy in gold does not relax to the Damask-Dienes-Weizer structure. Above the size of 6v, partial relaxation of 3v-sft type was observed. The relaxation of a micro-void to a sft is the thermal activated process. In nickel, a void is the most stable cluster below 20v and a...