Constructing Machine-learned Interatomic Potentials for Covalent Bonding Materials and MD Analyses of Dislocation and Surface

As machine learning potentials for molecular dynamics (MD) simulations, Spectral Neighbor Analysis Potential (SNAP) and quadratic SNAP (qSNAP) were constructed for silicon (Si) and silicon carbide (SiC). The reproducibility of the basic material properties about perfect crystal, free surface and dislocation cores in Si and 3C-SiC was investigated. The coefficients of SNAP and qSNAP were optimized using liner regression to present energy and force obtained by DFT. In addition, hyperparameters (cutoff length and weights for optimization, here) were determined using genetic algorithm to reproduce elastic moduli obtained by DFT. Lattice constant and elastic moduli of Si crystal by MD using our SNAP or qSNAP agree well with the values of DFT, and they have higher accuracy than those by any empirical potential. Additionally, melting point and specific heat at constant pressure were calculated by MD correctly. Especially in qSNAP of Si, the surface energy of {100} and {111} planes and the reconstructed {100} surface structure were almost reproduced. For 3C-SiC, SNAP reproduces lattice constant and elastic moduli of DFT. Furthermore, edge dislocation cores were generated successfully. However, the potentials we constructed have insufficient reproducibility in the plastic region, so it is necessary to continue development

Fundamental study on functionality of synthetic sulfides: Evaluation of metal sulfides as solid lubricant

Replacement of exhaustible and harmful resources used as solid lubricants is required for sliding elements such as plain bearings. In particular, the substitution of lead containing in the lead bronze is considered an urgent task. Therefore, in this study, attention was focused on metal sulfides (Cu2S, Cu5FeS4, SnS, TiS2, etc) as a substitute material for lead. After synthesis of sulfide and preparation of sintered body, friction and abrasion test was carried out and applicability as solid lubricant was investigated. The tribological properties of the dry conditions were evaluated by a journal type high speed tester. As a result, the friction coefficient of the bronze specimen without sulfide was about 0.3, whereas the bronze specimen containing sulfide showed a friction coefficient of about 0.1, indicating that the sulfide reduced the frictional resistance. Among them, the specimens containing Cu2S and Cu5FeS4 exhibited a lower friction coefficient. It is considered that this is influenced not only by the effect of hardness but also by film formation by sulfide

Interfacial fracture strength property of micro-scale SiN/Cu components

AbstractThe strength against fracture nucleation from an interface free-edge of silicon-nitride (SiN)/copper (Cu) micro-components is evaluated. A special technique that combines a nano-indenter specimen holder and an environmental transmission electron microscope (E-TEM) is employed. The critical load at the onset of fracture nucleation from a wedge-shaped free-edge (opening angle: 90°) is measured both in a vacuum and in a hydrogen (H2) environment, and the critical stress distribution is evaluated by the finite element method (FEM). It is found that the fracture nucleation is dominated by the near-edge elastic singular stress field that extends about a few tens of nanometers from the edge. The fracture nucleation strength expressed in terms of the stress intensity factor (K) is found to be eminently reduced in a H2 environment

Sequence divergence and retrotransposon insertion underlie interspecific epigenetic differences in primates

内在性レトロウイルス配列によってヒトのエピゲノムが変化してきたことを発見！ --ヒトとチンパンジーのiPS細胞の比較解析から--. 京都大学プレスリリース. 2022-10-12.Changes in the epigenome can affect the phenotype without the presence of changes in the genomic sequence. Given the high identity of the human and chimpanzee genome sequences, a substantial portion of their phenotypic divergence likely arises from epigenomic differences between the two species. In this study, the transcriptome and epigenome were determined for induced pluripotent stem cells (iPSCs) generated from human and chimpanzee individuals. The transcriptome and epigenomes for trimethylated histone H3 at lysine-4 (H3K4me3) and lysine-27 (H3K27me3) showed high levels of similarity between the two species. However, there were some differences in histone modifications. Although such regions, in general, did not show significant enrichment of interspecies nucleotide variations, gains in binding motifs for pluripotency-related transcription factors, especially POU5F1 and SOX2, were frequently found in species-specific H3K4me3 regions. We also revealed that species-specific insertions of retrotransposons, including the LTR5_Hs subfamily in human and a newly identified LTR5_Pt subfamily in chimpanzee, created species-specific H3K4me3 regions associated with increased expression of nearby genes. Human iPSCs have more species-specific H3K27me3 regions, resulting in more abundant bivalent domains. Only a limited number of these species-specific H3K4me3 and H3K27me3 regions overlap with species-biased enhancers in cranial neural crest cells, suggesting that differences in the epigenetic state of developmental enhancers appear late in development. Therefore, iPSCs serve as a suitable starting material for studying evolutionary changes in epigenome dynamics during development

Fundamental study on functionality of synthetic sulfides : Evaluation of metal sulfides as solid lubricant

Replacement of exhaustible and harmful resources used as solid lubricants is required for sliding elements such asplain bearings. In particular, the substitution of lead containing in the lead bronze is considered an urgent task. Therefore, in this study, attention was focused on metal sulfides (Cu2S, Cu5FeS4, SnS, TiS2, etc) as a substitute material for lead. After synthesis of sulfide and preparation of sintered body, friction and abrasion test was carried out and applicability as solid lubricant was investigated. The tribological properties of the dry conditions were evaluated by a journal type high speed tester. As a result, the friction coefficient of the bronze specimen without sulfide was about 0.3, whereas the bronze specimen containing sulfide showed a friction coefficient of about 0.1, indicating that the sulfide reducedthe frictional resistance. Among them, the specimens containing Cu2S and Cu5FeS4 exhibited a lower friction coefficient. It is considered that this is influenced not onlyby the effect of hardness but also by film formation by sulfide

MOLECULAR DYNAMICS SIMULATION OF LI-ION COORDINATION STRUCTURE AND TRANSPORT PROPERTIES IN LIB ELECTROLYTES: INFLUENCE OF MOLECULAR SIZE

Lithium-ion batteries (LIBs) are currently attracting much attention because electric vehicles and large storage batteries are becoming widespread. For lightweight and efficient usability, LIB performance for safety, capacity, charge/discharge cycle characteristics, and electric current must be improved. Development of high performance LIB relies mostly on progress of materials used for electrolytes and electrodes. In particular, electrolytes are an important factor because they play a role in carrying charged substances, i.e. ions, between separated electrodes. The ion-carrying performance depends on the choice of electrolyte, and thus, in this study, we discuss the relationship between the geometric shape of molecules used as electrolytes and their physical properties, using molecular dynamics (MD) simulations. We showed that, as the smaller solvent is used, ionic conductivity of the system is further enhanced. MD simulations showed that there are two main reasons for this behavior. First, a smaller radius allows the solvent molecule to diffuse easily. Consequently, when solvent molecules frequently surround a lithium ion, and the diffusion coefficient of the lithium ion becomes larger. Second, because small solvent molecules naturally come close to ions, salt (composed of cations and anions) is forced to be dissociated because of the solvent molecules, and the degree of salt dissociation increases

Synthesis and Functional Evaluation of Sulfides : Application of Solid Lubricant and Thermoelectric Material

Sufides as MoS2 are known as solid lubricants. These materials are also known as thermoelectric materials. However, there is no material satisfying both functions at the same time. In this study, sulfides powders such as Cu2S (chalcocite), TiS2, SnS, Cu5FeS4 (bornite) were synthesized under vacuum condition. Some specimens of sintered sulfides were evaluated by friction tester and their lubricity. As a result it was found that SnS had indicated better friction properties. Other specimens were evaluated in terms of Seebeck coefficients, which is one of the thermoelectric properties. Experimental results of the Seebeck coefficients were compared with simulation results. For thermoelectric properties, the　Seebeck coefficient of Cu2S had matched with experiments and simulation. Totally, Cu2S (chalcocite) and SnS has superior properties both, as solid lubricant as well as thermoelectric material.This is a product of research which was financially supported in part by the Kansai University Subsidy for Supporting Young Scholars, 2016 – \u27Study on sulfide synthesis of superior solid lubricity and thermoelectric properties\u2

Effect of additional magnesium on mechanical and high-cycle fatigue properties of 6061–T6 alloy

The effect of additional solute magnesium (Mg) on mechanical and high-cycle-fatigue properties of 6061-T6 aluminum alloy is investigated in detail. By adding 0.5% and 0.8% Mg to the 6061-T6 alloy with a normal stoichiometric Mg2Si composition (base alloy), the alloy exhibits eminent strain-aging characteristics demonstrated by the emergence of serrated flow, the negative strain-rate-sensitivity and relatively weakened temperature dependency of flow stress. The Mg-added new alloy also shows higher work-hardening rate than the base alloy particularly at initial flow regime and at lower strain rate. The S-N curve of the new alloy shows a clear fatigue limit which is absent in the base alloy. The fatigue limit of the new alloy is shown to be controlled by the threshold against small crack growth. Moreover, the new alloy clearly exhibits a coaxing phenomenon (time-dependent strengthening) which is absent in the base alloy. The coaxing effect is attributed to the existence of a small quasi-non-propagating crack whose growth resistance gradually increases during stress amplitude step-ups.This study was financially supported in part by the Kansai University Grant-in-Aid for progress ofresearch in graduate course (2012) and also by the Kansai University Expenditures for Support ofTraining Young Scholars (2013)

Optical transmittance investigation of 1-keV ion-irradiated sapphire crystals as potential VUV to NIR window materials of fusion reactors

We investigate the optical transmittances of ion-irradiated sapphire crystals as potential vacuum ultraviolet (VUV) to near-infrared (NIR) window materials of fusion reactors. Under potential conditions in fusion reactors, sapphire crystals are irradiated with hydrogen (H), deuterium (D), and helium (He) ions with 1-keV energy and ∼ 1020-m-2 s-1 flux. Ion irradiation decreases the transmittances from 140 to 260 nm but hardly affects the transmittances from 300 to 1500 nm. H-ion and D-ion irradiation causes optical absorptions near 210 and 260 nm associated with an F-center and an F+-center, respectively. These F-type centers are classified as Schottky defects that can be removed through annealing above 1000 K. In contrast, He-ion irradiation does not cause optical absorptions above 200 nm because He-ions cannot be incorporated in the crystal lattice due to the large ionic radius of He-ions. Moreover, the significant decrease in transmittance of the ion-irradiated sapphire crystals from 140 to 180 nm is related to the light scattering on the crystal surface. Similar to diamond polishing, ion irradiation modifies the crystal surface thereby affecting the optical properties especially at shorter wavelengths. Although the transmittances in the VUV wavelengths decrease after ion irradiation, the transmittances can be improved through annealing above 1000 K. With an optical transmittance in the VUV region that can recover through simple annealing and with a high transparency from the ultraviolet (UV) to the NIR region, sapphire crystals can therefore be used as good optical windows inside modern fusion power reactors in terms of light particle loadings of hydrogen isotopes and helium.Iwano K., Yamanoi K., Iwasa Y., et al. Optical transmittance investigation of 1-keV ion-irradiated sapphire crystals as potential VUV to NIR window materials of fusion reactors. AIP Advances 6, 105108 (2016); https://doi.org/10.1063/1.4965927