The change of the temperature coefficient of resistivity (a) with the particle size, dp, and the grain size, dc, in the nanostructured Ag bulk samples was investigated. dp and dc were controlled by heating the nano-Ag...The change of the temperature coefficient of resistivity (a) with the particle size, dp, and the grain size, dc, in the nanostructured Ag bulk samples was investigated. dp and dc were controlled by heating the nano-Ag powders over the temperature range from 393 to 453 K. The electrical resistance measurements of the nanostructured Ag bulk samples obtained by compacting the Ag powders after heat treatments showed a change in the sign of a with dP and dc. When dp and dc are smaller or equal to 18 and 11 nm below room temperature or 20 and 12 nm above room temperature, respectively, the sign of the temperature coefficient of resistivity changes from positive to negative. The negative a arises mainly from the high resistivity induced by the particle interfaces with very lowly ordered or even disordered structure, a large volume fraction of interfaces and impurities existing in the interfaces, and the quantum size effect appearing in the nano-Ag grains.展开更多
The highly charged ion Ar^12＋ with an energy of 3 Me V is used for irradiating metallic glass （Cu47Zr45Al8）98.5Y1.5 and polycrystalline metallic W at the irradiation fluences of 1× 10^14 ions/cm2, 1 × 10^...The highly charged ion Ar^12＋ with an energy of 3 Me V is used for irradiating metallic glass （Cu47Zr45Al8）98.5Y1.5 and polycrystalline metallic W at the irradiation fluences of 1× 10^14 ions/cm2, 1 × 10^15 ions/cm^2 and 1 × 10^16 ions/cm^2. The main structure of metallic glass remains an amorphous phase under different irradiation fluences according to x-ray diffraction analysis. The scanning electron microscope observation on the morphologies indicates that no significant irradiation damage occurs on the surface and cross section of the metallic glass sample after different fluences of irradiation, while a large area of irregular cracks and holes were observed on the surface of metallic W at a fluence of 1 ×10^16 ions/cm^2, with cracks and channel impairments at a certain depth from the surface. The root-mean-square （rms） roughness of metallic glass increases with increasing fluence of Ar^12＋, while the reflectance decreases with increasing irradiation fluence. A nano-hardness test shows that the hardness of metallic glass decreases after irradiation. Under certain a higher capability of resistance to Ar^12＋ irradiation in conditions, metallic glass （Cu47 Zr45Al8 ）98.5 Y1.5 exhibits comparison with polycrystalline W.展开更多
Density functional theory calculations are conducted to investigate the stability and interactions among small helium （He） clusters in bulk tungsten （W）. The lowest energy structure of each cluster for sizes n= 1 ...Density functional theory calculations are conducted to investigate the stability and interactions among small helium （He） clusters in bulk tungsten （W）. The lowest energy structure of each cluster for sizes n= 1 to 6 is determined. With the formation of He dusters, He defects form in bulk W. Tile thermodynamics of the dusters are investigated in the temperature range of 1000-2300K using molecular dynamics. This study provides the information essential to understand smaB He duster behavior in bulk W.展开更多
This paper describes a comprehensive model for predicting the evolution of the velocity and temperature fields in electromagnetically-driven flows during solidification.The electromagnetic field was formulated using t...This paper describes a comprehensive model for predicting the evolution of the velocity and temperature fields in electromagnetically-driven flows during solidification.The electromagnetic field was formulated using the mutual inductance method,which accounts for the metal,chill blocks,and magnetic shields.The model solves the heat transfer equation throughout the system,as well as the fluid flow equations in the liquid and mushy regions.A two-zone model for the mushy region that accounts for dampening of momentum by the turbulent field has been developed.The turbulence in the molten region was determined using the k-s model.Calculations were performed for unidirectional solidification in a bottom chill mold placed in a stationary magnetic field.Computed results show that the flow field at the beginning of solidification shows typical four recirculating loops,and later evolves to two recirculating loops as solidification progresses.The magnitude of the velocity in the bulk liquid was found to decrease as solidification progresses.展开更多
基金the National Natural Science FOundation of China under grant! No.19974041the National Major Fundamental ResearCh Program-Nal
文摘The change of the temperature coefficient of resistivity (a) with the particle size, dp, and the grain size, dc, in the nanostructured Ag bulk samples was investigated. dp and dc were controlled by heating the nano-Ag powders over the temperature range from 393 to 453 K. The electrical resistance measurements of the nanostructured Ag bulk samples obtained by compacting the Ag powders after heat treatments showed a change in the sign of a with dP and dc. When dp and dc are smaller or equal to 18 and 11 nm below room temperature or 20 and 12 nm above room temperature, respectively, the sign of the temperature coefficient of resistivity changes from positive to negative. The negative a arises mainly from the high resistivity induced by the particle interfaces with very lowly ordered or even disordered structure, a large volume fraction of interfaces and impurities existing in the interfaces, and the quantum size effect appearing in the nano-Ag grains.
基金Supported by the National Natural Science Foundation of China under Grant Nos 11079012 and 11375037the National Basic Research Program of China under Grant No 2010CB832901
文摘The highly charged ion Ar^12＋ with an energy of 3 Me V is used for irradiating metallic glass （Cu47Zr45Al8）98.5Y1.5 and polycrystalline metallic W at the irradiation fluences of 1× 10^14 ions/cm2, 1 × 10^15 ions/cm^2 and 1 × 10^16 ions/cm^2. The main structure of metallic glass remains an amorphous phase under different irradiation fluences according to x-ray diffraction analysis. The scanning electron microscope observation on the morphologies indicates that no significant irradiation damage occurs on the surface and cross section of the metallic glass sample after different fluences of irradiation, while a large area of irregular cracks and holes were observed on the surface of metallic W at a fluence of 1 ×10^16 ions/cm^2, with cracks and channel impairments at a certain depth from the surface. The root-mean-square （rms） roughness of metallic glass increases with increasing fluence of Ar^12＋, while the reflectance decreases with increasing irradiation fluence. A nano-hardness test shows that the hardness of metallic glass decreases after irradiation. Under certain a higher capability of resistance to Ar^12＋ irradiation in conditions, metallic glass （Cu47 Zr45Al8 ）98.5 Y1.5 exhibits comparison with polycrystalline W.
基金Supported by the National Natural Science Foundation of China under Grant No 11605007the Funding from the China Scholarship Council under Grant No 201506465019
文摘Density functional theory calculations are conducted to investigate the stability and interactions among small helium （He） clusters in bulk tungsten （W）. The lowest energy structure of each cluster for sizes n= 1 to 6 is determined. With the formation of He dusters, He defects form in bulk W. Tile thermodynamics of the dusters are investigated in the temperature range of 1000-2300K using molecular dynamics. This study provides the information essential to understand smaB He duster behavior in bulk W.
基金Item Sponsored by National Science Foundation under grant CMMI-0856320
文摘This paper describes a comprehensive model for predicting the evolution of the velocity and temperature fields in electromagnetically-driven flows during solidification.The electromagnetic field was formulated using the mutual inductance method,which accounts for the metal,chill blocks,and magnetic shields.The model solves the heat transfer equation throughout the system,as well as the fluid flow equations in the liquid and mushy regions.A two-zone model for the mushy region that accounts for dampening of momentum by the turbulent field has been developed.The turbulence in the molten region was determined using the k-s model.Calculations were performed for unidirectional solidification in a bottom chill mold placed in a stationary magnetic field.Computed results show that the flow field at the beginning of solidification shows typical four recirculating loops,and later evolves to two recirculating loops as solidification progresses.The magnitude of the velocity in the bulk liquid was found to decrease as solidification progresses.
