Atomistic detailed hydration structures of poly(vinyl methyl ether)(PVME) have been investigated by molecular dynamics simulations under 300 K at various concentrations. Both radial distribution functions and the dist...Atomistic detailed hydration structures of poly(vinyl methyl ether)(PVME) have been investigated by molecular dynamics simulations under 300 K at various concentrations. Both radial distribution functions and the distance distributions between donors and acceptors in hydrogen bonds show that the hydrogen bonds between the polymer and water are shorter by 0.005 nm than those between water molecules. The Quasi-hydrogen bonds take only 7.2% of the van der Waals interaction pairs. It was found the hydrogen bonds are not evenly distributed along the polymer chain,and there still exists a significant amount(10%) of ether oxygen atoms that are not hydrogen bonded to water at a concentration as low as 3.3%. This shows that in polymer solutions close contacts occur not only between polymer chains but also between chain segments within the polymer,which leads to inefficient contacts between ether oxygen atoms and water molecules. Variation of the quasi-hydrogen bonds with the concentration is similar to that of hydrogen bonds,but the ratio of the repeat units forming quasi-hydrogen bonds to those forming hydrogen bonds approaches 0.2. A transition was found in the demixing enthalpy at around 30% measured by dynamic testing differential scanning calorimetry(DTDSC) for aqueous solutions of a mono-dispersed low molecular weight PVME,which can be related to the transition of the fractions of hydrogen bonds and quasi-hydrogen bonds at ～27%. The transition of the fractions of hydrogen bonds and quasi-hydrogen bonds at ～27% can be used to explain the demixing enthalpy transition at 30% at a molecular scale. In addition,at the concentration of 86%,each ether oxygen atom bonded with water is assigned 1.56 water molecules on average,and 'free' water molecules emerge at the concentration of around 54%.展开更多
共晶Si形貌与A356铝合金的动态、准静态压缩变形下的力学性能及抗氢脆性能的影响密切相关。因此文章通过Material Test System(MTS)及霍普金森压杆(SHPB)测试变质前后A356铝合金的动态/准静态压缩力学行为,并采用电化学充氢方法研究合...共晶Si形貌与A356铝合金的动态、准静态压缩变形下的力学性能及抗氢脆性能的影响密切相关。因此文章通过Material Test System(MTS)及霍普金森压杆(SHPB)测试变质前后A356铝合金的动态/准静态压缩力学行为,并采用电化学充氢方法研究合金的抗氢脆性能。结果表明,准静态压缩变形后,合金中板状共晶Si垂直于压缩方向破裂成颗粒状。细化后的共晶Si提高了合金的塑性,延缓了合金的失效。而动态压缩变形后,板状共晶Si变形不均匀,并且碎成块状的共晶Si的尖端在压缩过程中会切割基体,导致其附近出现裂纹等缺陷。随着应变速率增大,铸态A356合金的屈服强度及抗压强度逐渐增大,合金具有一定的应变速率敏感性。变质后,共晶Si得到细化,增大了Al/Si接触面积,共晶Si捕获原子氢后降低了其与基体的连结,导致合金在拉伸变形过程中裂纹更易沿其扩展,并且细化后的共晶Si会进一步降低合金的抗氢脆性能力。其中细化后残存的块状共晶Si在捕获原子氢后会出现脱粘现象,易成为裂纹萌发点。展开更多
Si dangling bonds at the interface of quasi-free-standing monolayer graphene （QFMLG） are known to act as scattering centers that can severely affect carrier mobility Herein, we investigate the atomic and electronic ...Si dangling bonds at the interface of quasi-free-standing monolayer graphene （QFMLG） are known to act as scattering centers that can severely affect carrier mobility Herein, we investigate the atomic and electronic structure of Si dangling bonds in QFMLG using low-temperature scanning tunneling microscopy/ spectroscopy （STM/STS）, atomic force microscopy （AFM）, and density functional theory （DFT） calculations. Two types of defects with different contrast were observed on a flat graphene terrace by STM and AFM; in particular, their STM contrast varied with the bias voltage. Moreover, these defects showed characteristic STS peaks at different energies, 1.1 and 1.4 eV. The comparison of the experimental data with the DFT calculations indicates that the defects with STS peak energies of 1.1 and 1.4 eV consist of clusters of three and four Si dangling bonds, respectively. The relevance of the present results for the optimization of graphene synthesis is discussed.展开更多
基金Supported by the National Natural Science Foundation of China (Grant Nos. 20474073, 20490220, 20674090 and 90612015)National Major Basic Research Project (Grant No. G1999064800)
文摘Atomistic detailed hydration structures of poly(vinyl methyl ether)(PVME) have been investigated by molecular dynamics simulations under 300 K at various concentrations. Both radial distribution functions and the distance distributions between donors and acceptors in hydrogen bonds show that the hydrogen bonds between the polymer and water are shorter by 0.005 nm than those between water molecules. The Quasi-hydrogen bonds take only 7.2% of the van der Waals interaction pairs. It was found the hydrogen bonds are not evenly distributed along the polymer chain,and there still exists a significant amount(10%) of ether oxygen atoms that are not hydrogen bonded to water at a concentration as low as 3.3%. This shows that in polymer solutions close contacts occur not only between polymer chains but also between chain segments within the polymer,which leads to inefficient contacts between ether oxygen atoms and water molecules. Variation of the quasi-hydrogen bonds with the concentration is similar to that of hydrogen bonds,but the ratio of the repeat units forming quasi-hydrogen bonds to those forming hydrogen bonds approaches 0.2. A transition was found in the demixing enthalpy at around 30% measured by dynamic testing differential scanning calorimetry(DTDSC) for aqueous solutions of a mono-dispersed low molecular weight PVME,which can be related to the transition of the fractions of hydrogen bonds and quasi-hydrogen bonds at ～27%. The transition of the fractions of hydrogen bonds and quasi-hydrogen bonds at ～27% can be used to explain the demixing enthalpy transition at 30% at a molecular scale. In addition,at the concentration of 86%,each ether oxygen atom bonded with water is assigned 1.56 water molecules on average,and 'free' water molecules emerge at the concentration of around 54%.
文摘共晶Si形貌与A356铝合金的动态、准静态压缩变形下的力学性能及抗氢脆性能的影响密切相关。因此文章通过Material Test System(MTS)及霍普金森压杆(SHPB)测试变质前后A356铝合金的动态/准静态压缩力学行为,并采用电化学充氢方法研究合金的抗氢脆性能。结果表明,准静态压缩变形后,合金中板状共晶Si垂直于压缩方向破裂成颗粒状。细化后的共晶Si提高了合金的塑性,延缓了合金的失效。而动态压缩变形后,板状共晶Si变形不均匀,并且碎成块状的共晶Si的尖端在压缩过程中会切割基体,导致其附近出现裂纹等缺陷。随着应变速率增大,铸态A356合金的屈服强度及抗压强度逐渐增大,合金具有一定的应变速率敏感性。变质后,共晶Si得到细化,增大了Al/Si接触面积,共晶Si捕获原子氢后降低了其与基体的连结,导致合金在拉伸变形过程中裂纹更易沿其扩展,并且细化后的共晶Si会进一步降低合金的抗氢脆性能力。其中细化后残存的块状共晶Si在捕获原子氢后会出现脱粘现象,易成为裂纹萌发点。
文摘Si dangling bonds at the interface of quasi-free-standing monolayer graphene （QFMLG） are known to act as scattering centers that can severely affect carrier mobility Herein, we investigate the atomic and electronic structure of Si dangling bonds in QFMLG using low-temperature scanning tunneling microscopy/ spectroscopy （STM/STS）, atomic force microscopy （AFM）, and density functional theory （DFT） calculations. Two types of defects with different contrast were observed on a flat graphene terrace by STM and AFM; in particular, their STM contrast varied with the bias voltage. Moreover, these defects showed characteristic STS peaks at different energies, 1.1 and 1.4 eV. The comparison of the experimental data with the DFT calculations indicates that the defects with STS peak energies of 1.1 and 1.4 eV consist of clusters of three and four Si dangling bonds, respectively. The relevance of the present results for the optimization of graphene synthesis is discussed.
