The insulation effect of the air cavity surrounding the riser in a 3D printed sand mold was studied. The influence of the air cavity on heat flux was theoretically analyzed. The results demonstrated that the heat flux...The insulation effect of the air cavity surrounding the riser in a 3D printed sand mold was studied. The influence of the air cavity on heat flux was theoretically analyzed. The results demonstrated that the heat flux of the air cavity in the 3D printed sand mold was significantly less than that of resin-bonded sand. The insulation effect of the air cavity in sand molds for a cylinder casting and a stress-frame casting were simulated using software COMSOL. The results illustrated that the air cavity could be used to insulate the riser and it was more suitable for a lower melting point metal casting. An air cavity with 10-15 mm width and 5-10 mm away from the riser can significantly prolong the solidification of the riser by over 10%. Meanwhile, the sand mold for the stressframe was made by 3D printing technology and poured with aluminum alloy A356 melt. The experiment results showed that the presence of the air cavity led to a 12.5% increase of the solidification time of its riser.展开更多
In-situ tests and numerical manipulations were conducted for comparison of superior/inferior quality of casing-tube-vacuumized insulation and polyurethane insulation of fi'eezing apparatuses in relation to non-unifor...In-situ tests and numerical manipulations were conducted for comparison of superior/inferior quality of casing-tube-vacuumized insulation and polyurethane insulation of fi'eezing apparatuses in relation to non-uniform cold distribution in the multi-coil fi'eez- ing layout and the contradiction between large-coil diameter layout of the freezing holes and stability of the headfi'ame base. Tests show that: (1) under current technology, vacuumized insulation is of short duration due to lack of appropriate applied vacuum; (2) after the freezing pipe is insulated with polyurethane, the temperature gradient is less than no insulation, and the temperature dif- ference between insulated and non-insulated apparatuses is stable as the freezing period increases. Finally, it is pointed out that polyurethane insulation is quite efficient in present technical freezing construction. Keywords: freezing pipe; insulation effect; polyurethane; energy conservation展开更多
The electron swarm parameters including the density-normalized effective ionization coefficients(α-η)/N and the electron drift velocities V e are calculated for a gas mixture of CF3I with N2 and CO2 by solving the...The electron swarm parameters including the density-normalized effective ionization coefficients(α-η)/N and the electron drift velocities V e are calculated for a gas mixture of CF3I with N2 and CO2 by solving the Boltzmann equation in the condition of a steady-state Townsend(SST) experiment.The overall density-reduced electric field strength is from 100 Td to 1000 Td(1 Td = 10-17V·cm2),while the CF3I content k in the gas mixture can be varied over the range from 0% to 100%.From the variation of(αη)/N with the CF3I mixture ratio k,the limiting field strength(E/N) lim for each CF3I concentration is derived.It is found that for the mixtures with 70% CF3I,the values of(E/N) lim are essentially the same as that for pure SF 6.Additionally,the global warming potential(GWP) and the liquefaction temperature of the gas mixtures are also taken into account to evaluate the possibility of application in the gas insulation of power equipment.展开更多
We report an experimental study of electron transport properties of MnSe/(Bi,Sb)_2Te_3 heterostructures,in which MnSe is an antiferromagnetic insulator,and(Bi,Sb)_2Te_3 is a three-dimensional topological insulator(TI)...We report an experimental study of electron transport properties of MnSe/(Bi,Sb)_2Te_3 heterostructures,in which MnSe is an antiferromagnetic insulator,and(Bi,Sb)_2Te_3 is a three-dimensional topological insulator(TI).Strong magnetic proximity effect is manifested in the measurements of the Hall effect and longitudinal resistances.Our analysis shows that the gate voltage can substantially modify the anomalous Hall conductance,which exceeds 0.1 e^(2)/h at temperature T=1.6 K and magnetic field μ_0H=5 T,even though only the top TI surface is in proximity to MnSe.This work suggests that heterostructures based on antiferromagnetic insulators provide a promising platform for investigating a wide range of topological spintronic phenomena.展开更多
Two-dimensional topological insulators(2DTIs)have attracted increasing attention during the past few years.New 2DTIs with increasing larger spin-orbit coupling(SOC)gaps have been predicted by theoretical calculations ...Two-dimensional topological insulators(2DTIs)have attracted increasing attention during the past few years.New 2DTIs with increasing larger spin-orbit coupling(SOC)gaps have been predicted by theoretical calculations and some of them have been synthesized experimentally.In this review,the 2DTIs,ranging from single element graphene-like materials to bi-elemental transition metal chalcogenides(TMDs)and to multi-elemental materials,with different thicknesses,structures,and phases,have been summarized and discussed.The topological properties(especially the quantum spin Hall effect and Dirac fermion feature)and potential applications have been summarized.This review also points out the challenge and opportunities for future 2DTI study,especially on the device applications based on the topological properties.展开更多
The electronic properties of TiO2-terminated BaTiO3(001) surface subjected to biaxial strain have been studied using first-principles calculations based on density functional theory. The Ti ions are always inward s...The electronic properties of TiO2-terminated BaTiO3(001) surface subjected to biaxial strain have been studied using first-principles calculations based on density functional theory. The Ti ions are always inward shifted either at compressive or tension strains, while the inward shift of the Ba ions occurs only for high compressive strain, implying an enhanced electric dipole moment in the case of high compressive strain. In particular, an insulator–metal transition is predicted at a compressive biaxial strain of 0.0475. These changes present a very interesting possibility for engineering the electronic properties of ferroelectric BaTiO3(001) surface.展开更多
Bi2Se3, as a three-dimensional topological insulator, has attracted worldwide attention for its unique surface states which are protected by time-reversal symmetry. Here we report the synthesis and characterization of...Bi2Se3, as a three-dimensional topological insulator, has attracted worldwide attention for its unique surface states which are protected by time-reversal symmetry. Here we report the synthesis and characterization of high-quality singlecrystalline Bi2Se3 nanowires. Bi2Se3 nanowires were synthesized by chemical vapor deposition(CVD) method via goldcatalyzed vapor-liquid-solid(VLS) mechanism. The structure and morphology were characterized by scanning electron microscopy(SEM), transmission electron microscopy(TEM), x-ray photoelectron spectroscopy(XPS), and Raman spectroscopy. In magnetotransport measurements, the Aharonov–Bohm(AB) effect was observed in a nanowire-based nanodevice, suggesting the existence of surface states in Bi2Se3 nanowires.展开更多
We report the photovoltaic effects of n-type topological insulator (TI) Bi2Te3 films grown on p-type Si substrates by chemical vapor deposition (CVD). The films containing large nanoplates with a smooth surface fo...We report the photovoltaic effects of n-type topological insulator (TI) Bi2Te3 films grown on p-type Si substrates by chemical vapor deposition (CVD). The films containing large nanoplates with a smooth surface formed on p-Si exhibit good p-n diode characteristics under dark and light illumination conditions and display a good photovoltaic effect under the broadband range from ultraviolet (UV) to near infrared (N1R) wavelengths. Under the light illumination with a wavelength of 1,000 nm, a short circuit current (Isc) of 19.2 μA and an open circuit voltage (Voc) of 235 mV are achieved. The maximum fill factor (FF) increases with a decrease in the wavelength or light density, achieving a value of 35.6% under 600 nm illumination. The photoresponse of the n-Bi2TeB/p-Si device can be effectively switched between the on and off modes in millisecond time scale. These findings are important for both the fundamental understanding and solar cell device avDlications of TI materials.展开更多
The importance of silicon in modem electronic devices has led to considerable interest in exploring the unconventional electronic properties of Si-based materials for future applications in spintronics and quantum com...The importance of silicon in modem electronic devices has led to considerable interest in exploring the unconventional electronic properties of Si-based materials for future applications in spintronics and quantum computing. Here, using density functional theory, we present the results of a systematic study of the effect of strain on Si(lll) thin films whose surfaces are functionalized with iodine. Films with an odd number of layers under biaxial strain are found to undergo a phase transition from a normal insulator to a topological insulator and ultimately to a metal. The spin-orbit coupling-induced topologically nontrivial band gap at the F point is found to be as large as 0.50 eV, which not only surpasses that of other Si-based topological materials, it is also large enough for practical realization of quantum spin Hall states at room temperature. No such nontrivial states are found in films for such a strain-induced transition with an even number of layers. Mechanisms are illustrated by a tight-binding model composed of s, px, and py orbitals. Equally important, we predict that iodized silicene, when stretched and hole-doped, would be a phonon-mediated super- conductor with a critical temperature of 9.2 K. This coexistence of a topological insulator and a superconducting phase in a single material is unusual; it has the potential for applications in electronic circuits and for the realization of Majorana fermions in quantum computations.展开更多
基金funded by the National Science and Technology Major Project of the Ministry of Science and Technology of China under Project No.2016YFB1100703
文摘The insulation effect of the air cavity surrounding the riser in a 3D printed sand mold was studied. The influence of the air cavity on heat flux was theoretically analyzed. The results demonstrated that the heat flux of the air cavity in the 3D printed sand mold was significantly less than that of resin-bonded sand. The insulation effect of the air cavity in sand molds for a cylinder casting and a stress-frame casting were simulated using software COMSOL. The results illustrated that the air cavity could be used to insulate the riser and it was more suitable for a lower melting point metal casting. An air cavity with 10-15 mm width and 5-10 mm away from the riser can significantly prolong the solidification of the riser by over 10%. Meanwhile, the sand mold for the stressframe was made by 3D printing technology and poured with aluminum alloy A356 melt. The experiment results showed that the presence of the air cavity led to a 12.5% increase of the solidification time of its riser.
基金supported by National S&T Support Program (2006BAB16B01)
文摘In-situ tests and numerical manipulations were conducted for comparison of superior/inferior quality of casing-tube-vacuumized insulation and polyurethane insulation of fi'eezing apparatuses in relation to non-uniform cold distribution in the multi-coil fi'eez- ing layout and the contradiction between large-coil diameter layout of the freezing holes and stability of the headfi'ame base. Tests show that: (1) under current technology, vacuumized insulation is of short duration due to lack of appropriate applied vacuum; (2) after the freezing pipe is insulated with polyurethane, the temperature gradient is less than no insulation, and the temperature dif- ference between insulated and non-insulated apparatuses is stable as the freezing period increases. Finally, it is pointed out that polyurethane insulation is quite efficient in present technical freezing construction. Keywords: freezing pipe; insulation effect; polyurethane; energy conservation
基金Project supported by the National Natural Science Foundation of China (Grant No. 51177101)
文摘The electron swarm parameters including the density-normalized effective ionization coefficients(α-η)/N and the electron drift velocities V e are calculated for a gas mixture of CF3I with N2 and CO2 by solving the Boltzmann equation in the condition of a steady-state Townsend(SST) experiment.The overall density-reduced electric field strength is from 100 Td to 1000 Td(1 Td = 10-17V·cm2),while the CF3I content k in the gas mixture can be varied over the range from 0% to 100%.From the variation of(αη)/N with the CF3I mixture ratio k,the limiting field strength(E/N) lim for each CF3I concentration is derived.It is found that for the mixtures with 70% CF3I,the values of(E/N) lim are essentially the same as that for pure SF 6.Additionally,the global warming potential(GWP) and the liquefaction temperature of the gas mixtures are also taken into account to evaluate the possibility of application in the gas insulation of power equipment.
基金Supported by the National Key Research and Development Program of China (Grant No.2016YFA0300600)the National Natural Science Foundation of China (Grant No.11961141011)the Strategic Priority Research Program of Chinese Academy of Sciences(Grant No.XDB28000000)。
文摘We report an experimental study of electron transport properties of MnSe/(Bi,Sb)_2Te_3 heterostructures,in which MnSe is an antiferromagnetic insulator,and(Bi,Sb)_2Te_3 is a three-dimensional topological insulator(TI).Strong magnetic proximity effect is manifested in the measurements of the Hall effect and longitudinal resistances.Our analysis shows that the gate voltage can substantially modify the anomalous Hall conductance,which exceeds 0.1 e^(2)/h at temperature T=1.6 K and magnetic field μ_0H=5 T,even though only the top TI surface is in proximity to MnSe.This work suggests that heterostructures based on antiferromagnetic insulators provide a promising platform for investigating a wide range of topological spintronic phenomena.
基金Project supported by the Beijing Natural Science Foundation,China(Grant Nos.Z190006 and 4192054)the National Natural Science Foundation of China(Grant Nos.61971035,61901038,and 61725107)+1 种基金Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XDB30000000)Beijing Institute of Technology Research Fund Program for Young Scholars(Grant No.3050011181814).
文摘Two-dimensional topological insulators(2DTIs)have attracted increasing attention during the past few years.New 2DTIs with increasing larger spin-orbit coupling(SOC)gaps have been predicted by theoretical calculations and some of them have been synthesized experimentally.In this review,the 2DTIs,ranging from single element graphene-like materials to bi-elemental transition metal chalcogenides(TMDs)and to multi-elemental materials,with different thicknesses,structures,and phases,have been summarized and discussed.The topological properties(especially the quantum spin Hall effect and Dirac fermion feature)and potential applications have been summarized.This review also points out the challenge and opportunities for future 2DTI study,especially on the device applications based on the topological properties.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.1574091,51272078,and 51431006)the Natural Science Foundation of Guangdong Province of China(Grant No.2015A030313375)+1 种基金the Science and Technology Planning Project of Guangdong Province of China(Grant No.2015B090927006)the Program for International Innovation Cooperation Platform of Guangzhou City,China(Grant No.2014J4500016)
文摘The electronic properties of TiO2-terminated BaTiO3(001) surface subjected to biaxial strain have been studied using first-principles calculations based on density functional theory. The Ti ions are always inward shifted either at compressive or tension strains, while the inward shift of the Ba ions occurs only for high compressive strain, implying an enhanced electric dipole moment in the case of high compressive strain. In particular, an insulator–metal transition is predicted at a compressive biaxial strain of 0.0475. These changes present a very interesting possibility for engineering the electronic properties of ferroelectric BaTiO3(001) surface.
基金Project supported by the National Key Basic Research Program of China(Grant Nos.2014CB921103 and 2013CB921103)the National Natural Science Foundation of China(Grant Nos.11274003 and 91421109)Collaborative Innovation Center of Solid-State Lighting and Energy-Saving Electronics,China
文摘Bi2Se3, as a three-dimensional topological insulator, has attracted worldwide attention for its unique surface states which are protected by time-reversal symmetry. Here we report the synthesis and characterization of high-quality singlecrystalline Bi2Se3 nanowires. Bi2Se3 nanowires were synthesized by chemical vapor deposition(CVD) method via goldcatalyzed vapor-liquid-solid(VLS) mechanism. The structure and morphology were characterized by scanning electron microscopy(SEM), transmission electron microscopy(TEM), x-ray photoelectron spectroscopy(XPS), and Raman spectroscopy. In magnetotransport measurements, the Aharonov–Bohm(AB) effect was observed in a nanowire-based nanodevice, suggesting the existence of surface states in Bi2Se3 nanowires.
文摘We report the photovoltaic effects of n-type topological insulator (TI) Bi2Te3 films grown on p-type Si substrates by chemical vapor deposition (CVD). The films containing large nanoplates with a smooth surface formed on p-Si exhibit good p-n diode characteristics under dark and light illumination conditions and display a good photovoltaic effect under the broadband range from ultraviolet (UV) to near infrared (N1R) wavelengths. Under the light illumination with a wavelength of 1,000 nm, a short circuit current (Isc) of 19.2 μA and an open circuit voltage (Voc) of 235 mV are achieved. The maximum fill factor (FF) increases with a decrease in the wavelength or light density, achieving a value of 35.6% under 600 nm illumination. The photoresponse of the n-Bi2TeB/p-Si device can be effectively switched between the on and off modes in millisecond time scale. These findings are important for both the fundamental understanding and solar cell device avDlications of TI materials.
文摘The importance of silicon in modem electronic devices has led to considerable interest in exploring the unconventional electronic properties of Si-based materials for future applications in spintronics and quantum computing. Here, using density functional theory, we present the results of a systematic study of the effect of strain on Si(lll) thin films whose surfaces are functionalized with iodine. Films with an odd number of layers under biaxial strain are found to undergo a phase transition from a normal insulator to a topological insulator and ultimately to a metal. The spin-orbit coupling-induced topologically nontrivial band gap at the F point is found to be as large as 0.50 eV, which not only surpasses that of other Si-based topological materials, it is also large enough for practical realization of quantum spin Hall states at room temperature. No such nontrivial states are found in films for such a strain-induced transition with an even number of layers. Mechanisms are illustrated by a tight-binding model composed of s, px, and py orbitals. Equally important, we predict that iodized silicene, when stretched and hole-doped, would be a phonon-mediated super- conductor with a critical temperature of 9.2 K. This coexistence of a topological insulator and a superconducting phase in a single material is unusual; it has the potential for applications in electronic circuits and for the realization of Majorana fermions in quantum computations.