The phase transition of gallium phosphide （GAP） from zinc-blende （ZB） to a rocksalt （RS） structure is investigated by the plane-wave pseudopotential density functional theory （DFT）. Lattice constant a0, elasti...The phase transition of gallium phosphide （GAP） from zinc-blende （ZB） to a rocksalt （RS） structure is investigated by the plane-wave pseudopotential density functional theory （DFT）. Lattice constant a0, elastic constants cij, bulk modulus B0 and the pressure derivative of bulk modulus B0 are calculated. The results are in good agreement with numerous experimental and theoretical data. From the usual condition of equal enthalpies, the phase transition from the ZB to the RS structure occurs at 21.9 GPa, which is close to the experimental value of 22.0 GPa. The elastic properties of GaP with the ZB structure in a pressure range from 0 GPa to 21.9 GPa and those of the RS structure in a pressure range of pressures from 21.9 GPa to 40 GPa are obtained. According to the quasi-harmonic Debye model, in which the phononic effects are considered, the normalized volume V/Vo, the Debye temperature 8, the heat capacity Cv and the thermal expansion coefficient a are also discussed in a pressure range from 0 CPa to 40 GPa and a temperature range from 0 K to 1500 K.展开更多
Wide gap brazing（WGB） experiments using the butted specimen with a 1.5 mm gap were carried out for the repair of the failed K418B superalloy low-pressure turbine vanes.The high temperature creep rupture strengths of...Wide gap brazing（WGB） experiments using the butted specimen with a 1.5 mm gap were carried out for the repair of the failed K418B superalloy low-pressure turbine vanes.The high temperature creep rupture strengths of the brazed joints were tested,and the microstructures and fracture surfaces of the joints were observed.The results show that the microstructure of K418B alloy joint is composed of dense equiaxed grain,small and discrete compounds with a few micro-pores.During the creep rupture test,the cracks initiate preferentially at the micro-pore or the grain boundary,then propagate along the grain boundary till the fracture happens.The creep rupture strength at 700 ℃ of the brazed joints with 50% braze metal in the working part could exceed 90% that of the K418B superalloy,and the joints with 100% braze metal in the working part achieve 70% to 80% of the creep rupture strengths for the K418 B base metal.展开更多
The study of the heterogeneity of soil enzyme activities at different sampling locations in canopy gaps will help understand the influence mechanism of canopy gaps on soil ecological processes.In this paper,we analyze...The study of the heterogeneity of soil enzyme activities at different sampling locations in canopy gaps will help understand the influence mechanism of canopy gaps on soil ecological processes.In this paper,we analyzed the spatiotemporal variation of soil enzyme activities and soil physicochemical properties at different sampling locations(closed canopy,expanded edge,canopy edge,gap center)in different sampling time(December,February,April,June,August,and October)on the northern slope of the Tianshan Mountains,Northwest China.The results showed that soil catalase,cellulase,sucrase,and acid phosphatase activities were relatively high from June to October and low from December to April,and most of soil enzyme activities were higher at closed canopy than at gap center.Soil urease activity was high during December-February.The soil temperature reached the highest value during June-August and was relatively high at gap center in October,December,and February.Soil water content was significantly higher in December and April than in other months.Soil bulk density was higher at gap center than at closed canopy in December.Soil pH and soil electrical conductivity in most months were higher at closed canopy than at gap center.Soil organic carbon,soil total nitrogen,and soil total phosphorus were generally higher at gap center than at closed canopy.Furthermore,sampling time played a leading role in the dynamic change of soil enzyme activity.The key factors affecting soil enzyme activity were soil temperature and soil water content,which were governed by canopy gaps.These results provide important support for further understanding the influence mechanism of forest ecosystem management and conservation on the Tianshan Mountains.展开更多
Using the density functional theory, we have investigated the electronic and optical properties of two-dimensional Sc2C monolayer with OH, F, or O chemical groups. The electronic structures reveal that the functionali...Using the density functional theory, we have investigated the electronic and optical properties of two-dimensional Sc2C monolayer with OH, F, or O chemical groups. The electronic structures reveal that the functionalized Sc2C monolayers are semiconductors with a band gap of 0.44–1.55 eV. The band gap dependent optical parameters, like dielectric function, absorption coefficients, reflectivity, loss function, and refraction index were also calculated for photon energy up to 20 eV. At the low-energy region, each optical parameter shifts to red, and the peak increases obviously with the increase of the energy gap. Consequently, Sc2C monolayer with a tunable band gap by changing the type of surface chemical groups is a promising 2D material for optoelectronic devices.展开更多
We use a modified Becke-Johnson exchange plus a local density approximation correlation potential within the density functional theory to investigate the electronic structures of Hg1-xCdxTe and In1-xGaxAs with x being...We use a modified Becke-Johnson exchange plus a local density approximation correlation potential within the density functional theory to investigate the electronic structures of Hg1-xCdxTe and In1-xGaxAs with x being 0, 0.25, 0.5, 0.75, and 1. For both of the two series, our calculated energy gaps and dielectric functions （real part 61 and imaginary part 62） are in agreement with the corresponding experimental results with x being between 0 and 1. The calculated zero-frequency refractive index varies greatly with x for Hg1-xCdxTe, but changes little with for In1-xGaxAs, which is consistent with the real parts of their dielectric functions. Therefore, this new approach is satisfactory to describe the electronic structures and the optical properties of the semiconductors.展开更多
文摘The phase transition of gallium phosphide （GAP） from zinc-blende （ZB） to a rocksalt （RS） structure is investigated by the plane-wave pseudopotential density functional theory （DFT）. Lattice constant a0, elastic constants cij, bulk modulus B0 and the pressure derivative of bulk modulus B0 are calculated. The results are in good agreement with numerous experimental and theoretical data. From the usual condition of equal enthalpies, the phase transition from the ZB to the RS structure occurs at 21.9 GPa, which is close to the experimental value of 22.0 GPa. The elastic properties of GaP with the ZB structure in a pressure range from 0 GPa to 21.9 GPa and those of the RS structure in a pressure range of pressures from 21.9 GPa to 40 GPa are obtained. According to the quasi-harmonic Debye model, in which the phononic effects are considered, the normalized volume V/Vo, the Debye temperature 8, the heat capacity Cv and the thermal expansion coefficient a are also discussed in a pressure range from 0 CPa to 40 GPa and a temperature range from 0 K to 1500 K.
文摘Wide gap brazing（WGB） experiments using the butted specimen with a 1.5 mm gap were carried out for the repair of the failed K418B superalloy low-pressure turbine vanes.The high temperature creep rupture strengths of the brazed joints were tested,and the microstructures and fracture surfaces of the joints were observed.The results show that the microstructure of K418B alloy joint is composed of dense equiaxed grain,small and discrete compounds with a few micro-pores.During the creep rupture test,the cracks initiate preferentially at the micro-pore or the grain boundary,then propagate along the grain boundary till the fracture happens.The creep rupture strength at 700 ℃ of the brazed joints with 50% braze metal in the working part could exceed 90% that of the K418B superalloy,and the joints with 100% braze metal in the working part achieve 70% to 80% of the creep rupture strengths for the K418 B base metal.
基金supported by the National Natural Science Foundation of China (31760142)
文摘The study of the heterogeneity of soil enzyme activities at different sampling locations in canopy gaps will help understand the influence mechanism of canopy gaps on soil ecological processes.In this paper,we analyzed the spatiotemporal variation of soil enzyme activities and soil physicochemical properties at different sampling locations(closed canopy,expanded edge,canopy edge,gap center)in different sampling time(December,February,April,June,August,and October)on the northern slope of the Tianshan Mountains,Northwest China.The results showed that soil catalase,cellulase,sucrase,and acid phosphatase activities were relatively high from June to October and low from December to April,and most of soil enzyme activities were higher at closed canopy than at gap center.Soil urease activity was high during December-February.The soil temperature reached the highest value during June-August and was relatively high at gap center in October,December,and February.Soil water content was significantly higher in December and April than in other months.Soil bulk density was higher at gap center than at closed canopy in December.Soil pH and soil electrical conductivity in most months were higher at closed canopy than at gap center.Soil organic carbon,soil total nitrogen,and soil total phosphorus were generally higher at gap center than at closed canopy.Furthermore,sampling time played a leading role in the dynamic change of soil enzyme activity.The key factors affecting soil enzyme activity were soil temperature and soil water content,which were governed by canopy gaps.These results provide important support for further understanding the influence mechanism of forest ecosystem management and conservation on the Tianshan Mountains.
基金supported by the Fundamental Research Funds for the Central Universities,China(Grant No.30915014101)
文摘Using the density functional theory, we have investigated the electronic and optical properties of two-dimensional Sc2C monolayer with OH, F, or O chemical groups. The electronic structures reveal that the functionalized Sc2C monolayers are semiconductors with a band gap of 0.44–1.55 eV. The band gap dependent optical parameters, like dielectric function, absorption coefficients, reflectivity, loss function, and refraction index were also calculated for photon energy up to 20 eV. At the low-energy region, each optical parameter shifts to red, and the peak increases obviously with the increase of the energy gap. Consequently, Sc2C monolayer with a tunable band gap by changing the type of surface chemical groups is a promising 2D material for optoelectronic devices.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11174359,10874232,and 10774180)the National Basic Research Program of China(Grant No.2012CB932302)
文摘We use a modified Becke-Johnson exchange plus a local density approximation correlation potential within the density functional theory to investigate the electronic structures of Hg1-xCdxTe and In1-xGaxAs with x being 0, 0.25, 0.5, 0.75, and 1. For both of the two series, our calculated energy gaps and dielectric functions （real part 61 and imaginary part 62） are in agreement with the corresponding experimental results with x being between 0 and 1. The calculated zero-frequency refractive index varies greatly with x for Hg1-xCdxTe, but changes little with for In1-xGaxAs, which is consistent with the real parts of their dielectric functions. Therefore, this new approach is satisfactory to describe the electronic structures and the optical properties of the semiconductors.
