The Michael addition reactions of Z and E 4-methyl-benzaldoximes with propene were investigated theoretically by DFT method at B3LYP/6-31G* level.The calculation results show that both addition reactions are concerted...The Michael addition reactions of Z and E 4-methyl-benzaldoximes with propene were investigated theoretically by DFT method at B3LYP/6-31G* level.The calculation results show that both addition reactions are concerted processes accompanied by the migration of hydrogen from the atom oxygen to carbon.Both products Z and E nitrones have dipolar charge distributions and activities.Z isomer is more favorable in the reaction due to the barrier is lower.展开更多
In-situ Al2O3/TiAl composites were successfully synthesized from the starting powders of Ti, Al, TiO2 and Nb2O5. The oxidation behavior of the composites at 900°C in static air was investigated. The results indic...In-situ Al2O3/TiAl composites were successfully synthesized from the starting powders of Ti, Al, TiO2 and Nb2O5. The oxidation behavior of the composites at 900°C in static air was investigated. The results indicate that the composite samples present a much lower oxidation mass gain. Under long-time intensive oxidation exposure, the formed oxide scale is multi-layer. The formation of the outer TiO2 layer is fine and dense, the internal Al2O3 scale has good adhesiveness with the outer TiO2 scale, and the TiO2+Al2O3 mixed layer forming the protective oxide scale is favorable for the improvement of oxidation resistance. It is believed that the incorporation of Al2O3 particulates into the metal matrix decreases the coefficient of thermal expansion of the substrate, and forms a local three-dimensional network structure that can hold the oxide scale. The formation of the oxide scale with finer particle size, stronger adherence, less micro-defects and slower growth rate can contribute to the improvement of oxidation resistance. Nb element plays an important role in reducing the internal oxidation action of the materials, restraining the growth of TiO2 crystals and promoting the stable formation of the Al2O3-riched layer, which is beneficial to improve the oxidation properties.展开更多
Irradiation-induced atomic-scale defects and lattice disorder in Silicon Carbide(SiC) can significantly affect the material's mechanical properties.Currently there lacks a unified physical model capable of describ...Irradiation-induced atomic-scale defects and lattice disorder in Silicon Carbide(SiC) can significantly affect the material's mechanical properties.Currently there lacks a unified physical model capable of describing the law in which the properties of SiC scale with the accumulation of defects,especially in terms of the underlying physical mechanism.To develop fundamental models that are capable of describing the various physical properties of SiC as a function of microstructural change,molecular dynamics simulations of uniaxial tension were performed on a series of irradiation-amorphized SiC(a-SiC) samples with a range of imposed chemical disorder,which is defined as the ratio between the number of homonuclear bonds and heteronuclear bonds(χ≡ N C-C N Si-C).With increasing chemical disorder,significant alternation of mechanical response of a-SiC has been detected in terms of increasingly pronounced plastic flow.Meanwhile relevant mechanical properties,including Young's modulus,strength,yield stress and strain,as well as failure strain scale monotonically with chemical disorder while in distinct manners.Specifically slight chemical disorder(χ = 0.045) could induce substantial reduction of Young's modulus up to 2%,whereas strength basically linearly varies with chemical disorder until χ≈ 0.5 upon which the variations in mechanical properties tend to saturate.Further examination of the evolution of atomic structure of a-SiC reveals a crossover of deformation mechanisms from homogeneous elastic deformation to localized plastic flow,which accounts for the strong chemical disorder dependence of the mechanical properties as well as mechanical responses of amorphous SiC.This crossover is also manifested in switching of fracture mode from brittle failure dominated by lattice instability in the ligaments between topological disordered clusters to nanoductile failure preceded by percolation of nanocavities.Employing chemical disorder to measure the defect concentration of a-SiC could contribute to the quantification of the correlation between mechanical properties and the corresponding defective a-SiC structure.Moreover the distinct scale laws shown by Young's modulus and strength with chemical disorder and the proposed critical chemical disorder threshold could benefit the quantitative evaluations of the mechanical performances of SiC components in different irradiation environments.展开更多
US News 2017年世界工科大学400强中俄罗斯只有1所：#376M．V．Lomonosov Moscow State University。
这个结果似明显地不合理，似反映USNews评价工科大学存在偏重科学忽视工程经验的问题。钱学森一生强调技术科学。对钱学森对科技科...US News 2017年世界工科大学400强中俄罗斯只有1所：#376M．V．Lomonosov Moscow State University。
这个结果似明显地不合理，似反映USNews评价工科大学存在偏重科学忽视工程经验的问题。钱学森一生强调技术科学。对钱学森对科技科学一贯一致的论述，我反复思考后，展开更多
文摘The Michael addition reactions of Z and E 4-methyl-benzaldoximes with propene were investigated theoretically by DFT method at B3LYP/6-31G* level.The calculation results show that both addition reactions are concerted processes accompanied by the migration of hydrogen from the atom oxygen to carbon.Both products Z and E nitrones have dipolar charge distributions and activities.Z isomer is more favorable in the reaction due to the barrier is lower.
基金supported by the Special Program of the Education Bureau of Shaanxi Province of China (No.08JK240)the Breeding Program for Provincial Level Key Research Base of Shaanxi University of Technology (No.SLGJD0806)
文摘In-situ Al2O3/TiAl composites were successfully synthesized from the starting powders of Ti, Al, TiO2 and Nb2O5. The oxidation behavior of the composites at 900°C in static air was investigated. The results indicate that the composite samples present a much lower oxidation mass gain. Under long-time intensive oxidation exposure, the formed oxide scale is multi-layer. The formation of the outer TiO2 layer is fine and dense, the internal Al2O3 scale has good adhesiveness with the outer TiO2 scale, and the TiO2+Al2O3 mixed layer forming the protective oxide scale is favorable for the improvement of oxidation resistance. It is believed that the incorporation of Al2O3 particulates into the metal matrix decreases the coefficient of thermal expansion of the substrate, and forms a local three-dimensional network structure that can hold the oxide scale. The formation of the oxide scale with finer particle size, stronger adherence, less micro-defects and slower growth rate can contribute to the improvement of oxidation resistance. Nb element plays an important role in reducing the internal oxidation action of the materials, restraining the growth of TiO2 crystals and promoting the stable formation of the Al2O3-riched layer, which is beneficial to improve the oxidation properties.
基金supported by National Natural Science Foundation of China (Grant No. 10672086)National Basic Research Program of China (973 Program,Grant No. 2010CB631005)
文摘Irradiation-induced atomic-scale defects and lattice disorder in Silicon Carbide(SiC) can significantly affect the material's mechanical properties.Currently there lacks a unified physical model capable of describing the law in which the properties of SiC scale with the accumulation of defects,especially in terms of the underlying physical mechanism.To develop fundamental models that are capable of describing the various physical properties of SiC as a function of microstructural change,molecular dynamics simulations of uniaxial tension were performed on a series of irradiation-amorphized SiC(a-SiC) samples with a range of imposed chemical disorder,which is defined as the ratio between the number of homonuclear bonds and heteronuclear bonds(χ≡ N C-C N Si-C).With increasing chemical disorder,significant alternation of mechanical response of a-SiC has been detected in terms of increasingly pronounced plastic flow.Meanwhile relevant mechanical properties,including Young's modulus,strength,yield stress and strain,as well as failure strain scale monotonically with chemical disorder while in distinct manners.Specifically slight chemical disorder(χ = 0.045) could induce substantial reduction of Young's modulus up to 2%,whereas strength basically linearly varies with chemical disorder until χ≈ 0.5 upon which the variations in mechanical properties tend to saturate.Further examination of the evolution of atomic structure of a-SiC reveals a crossover of deformation mechanisms from homogeneous elastic deformation to localized plastic flow,which accounts for the strong chemical disorder dependence of the mechanical properties as well as mechanical responses of amorphous SiC.This crossover is also manifested in switching of fracture mode from brittle failure dominated by lattice instability in the ligaments between topological disordered clusters to nanoductile failure preceded by percolation of nanocavities.Employing chemical disorder to measure the defect concentration of a-SiC could contribute to the quantification of the correlation between mechanical properties and the corresponding defective a-SiC structure.Moreover the distinct scale laws shown by Young's modulus and strength with chemical disorder and the proposed critical chemical disorder threshold could benefit the quantitative evaluations of the mechanical performances of SiC components in different irradiation environments.
基金supported in part by the National Natural Science Foundation of China under Grant No.10675057Foundation of Liaoning Education Committee under Grant No.2007T086
文摘稀罕腐烂处理 K <SUP>+</SUP>→
π
<SUP>+</SUP>ν
和 K <SUB > L </SUB>→
在三地点的 Higgsless 模型的框架考虑的 π
<SUP>0</SUP>νare。到这二个腐烂过程的这个新物理模型的贡献与费米子为平常的计量器波色子的政变石楠来自新重计量器波色子和修正术语。我们的数字结果证明这二个腐烂过程的分叉的比率能被 40% 和 50% 相对标准模型预言的那些提高。
