Nanowire stiction is a crucial bottleneck for the development of M/NEMS devices.We present a model of a nano-beam stuck to the substrate in consideration of both surface elasticity and residual surface stress.The crit...Nanowire stiction is a crucial bottleneck for the development of M/NEMS devices.We present a model of a nano-beam stuck to the substrate in consideration of both surface elasticity and residual surface stress.The critical detachment length can be derived from the transversality condition using the variational method.The effects of the surface parameters on the adhesion of the nano-beam are discussed in detail.These analyses provide some suggestions for engineers in the design and fabrication of more accurate M/NEMS instruments.展开更多
Nanoporous metallic glasses(NPMGs)and crystalline/amorphous nanocomposites exhibit superior ductility over bulk macroscopic metallic glasses(MGs).Their fracture behaviors remain a mystery due to experimental technical...Nanoporous metallic glasses(NPMGs)and crystalline/amorphous nanocomposites exhibit superior ductility over bulk macroscopic metallic glasses(MGs).Their fracture behaviors remain a mystery due to experimental technical limitations.In this work,the fracture behaviors of pre-cracked NPMGs and crystal-impregnated nanoporous metallic glasses(CINPMGs)are investigated through large-scale molecular dynamics simulations,and the MG and crystal phases are amorphous Cu_(50)Zr_(50)and crystalline B2CuZr,respectively.Fracture toughness is determined by simultaneously considering the surface energy and plastic dissipated energy.Our results confirm the excellent plasticity of both the pre-cracked NPMGs and CINPMGs.The progressive necking and ductile rupture of ligaments are responsible for generating NPMGs with prominent ductility and fracture toughness.Meanwhile,secondary cracking is also triggered,which can consume energy without extending the major crack,thus further improving fracture resistance.It is also found that the fracture toughness of NPMGs can be improved by increasing the solid fraction of MG,and linear relation between fracture toughness and solid fraction can be expected.Crystal impregnation effectively inhibits global failure as the crystal phase shields the individual amorphous ligaments.Homogeneous plastic flow characterized by shear bands is observed in CINPMGs,and this homogeneous global deformation is facilitated by the crystalline/amorphous interface.Besides,the fracture toughness of CINPMGs is higher than that of the constituent single phases,regardless of the volume fraction of each phase.Ashby material charts manifest that these two types of materials demonstrate promising potential in the material selection library for advanced structural design.展开更多
In this study,we considered the wetting phenomenon on a general substrate from a new viewpoint of continuum mechanics.The analyses first show how the Wenzel and the Cassie models deviate the practical results in some ...In this study,we considered the wetting phenomenon on a general substrate from a new viewpoint of continuum mechanics.The analyses first show how the Wenzel and the Cassie models deviate the practical results in some special substrates,and then elucidate the mechanism of the triple contact line(TCL) moving.Based upon variational theory of the total free functional dealing with the movable boundary condition,we show that the macroscopic contact angle(MCA) expression is the corresponding transversality condition.It manifests that the MCA depends only on the chemical and geometric property at the TCL,and is not affected by the gravity of the droplet and the contact area beneath the liquid.Our continuum model also shows the exploration of the pinning effect on a sharp wedge or the interface between two different phases.This investigation will help designing super-hydrophobic materials for novel micro-fluidic devices.展开更多
The effect of the radial heat conduction on the effective thermal conductivity of carbon nanotube(CNT) bundles is studied by the nonequilibrium molecular dynamics(NEMD) method. The hexagonal CNT bundle consists of sev...The effect of the radial heat conduction on the effective thermal conductivity of carbon nanotube(CNT) bundles is studied by the nonequilibrium molecular dynamics(NEMD) method. The hexagonal CNT bundle consists of seven(10, 10) single-walled carbon nanotubes(SWCNTs). The radial heat conduction is induced by creating the vacancy defects in some segments of the constituent CNTs. Combined with the temperature differences and the inter-tube thermal resistances at the different segments,the radial heat flow in the CNT bundle is calculated. The maximum percentage of the radial heat flow is less than 7% with the presence of four defective CNTs, while the resultant decrement of the effective thermal conductivity of the bundle is about 18%.The present results indicate that the radial heat flow can significantly diminish the axial heat conduction in the CNT bundles,which probably explains the smaller effective thermal conductivity in the CNT assemblies compared to that of the individual CNTs.展开更多
The development of a high-performance cemented carbide drill bit is of great significance to the reduction of rock drilling-cost. The non-smooth features of a biological surface provide an insight into how they can ob...The development of a high-performance cemented carbide drill bit is of great significance to the reduction of rock drilling-cost. The non-smooth features of a biological surface provide an insight into how they can obtain low friction and good wear resistance with evolving surface morphology. By analyzing the mechanism of the surface of a dung beetle for reducing soil wear and adherence, we design a cemented carbide drill bit with a bionic surface, which is expected to have superior anti-wearing and anti-sticking properties for drilling the soft coal seam. Inspired from the characteristics of the head and pronotum surface of the dung beetle, optimized non-smooth surface of the drill bit was constructed. The working performance of this innovative drill was experimentally tested. With comparative experiments under the identical drilling conditions, the wear rates, drilling times of conventional drills and bionic drills were measured. Compared with the conventional counterpart, the drill designed exhibits better performance in reducing wear and sticking drilling-breaks, therefore achieving higher levels of efficiency. The diameter of the dome and pit on the bit surface is in the range of 0.8–1.2 mm, and the bionic drill bits could get better performance with preferable drilling speeds and wear rates.展开更多
A series of large-scale molecular dynamics(MD) simulations has been performed to study the effects of grain size and ligament diameter on the mechanical properties of nanocrystalline nanoporous gold. Such simulations ...A series of large-scale molecular dynamics(MD) simulations has been performed to study the effects of grain size and ligament diameter on the mechanical properties of nanocrystalline nanoporous gold. Such simulations indicate that the principal deformation mechanism is a combination of grain boundary sliding, grain rotation and dislocation movement. The results of uniaxial tensile tests reveal the presence of a reverse Hall-Petch relation between strength and nominal grain size, rather than the conventional Hall-Petch relationship in the present range of nominal grain size(7.9–52.7 nm). An increase of flow stress may possibly attribute to the lower total proportion of grain boundary sliding and grain rotation in the deformation of samples with larger grain size. The Young's modulus shows a linear relation with the reciprocal of nominal grain size, which depends largely on the volume fraction of grain boundaries and thus decreasing grain size leads to relatively lower Young's modulus. MD simulations on samples with ligament diameter ranging from 4.07 to 8.10 nm are also carried out and results show that the increasing ligament diameter resulted in decreased flow stress and increased Young's modulus.展开更多
基金by the National Natural Science Foundation of China under Grant Nos 10802099 and 11102140the Doctoral Fund of Ministry of Education of China(No 200804251520)+1 种基金the Natural Science Foundation of Shandong Province(No ZR2009AQ006)the Brain Korea Scholarship from Seoul National University。
文摘Nanowire stiction is a crucial bottleneck for the development of M/NEMS devices.We present a model of a nano-beam stuck to the substrate in consideration of both surface elasticity and residual surface stress.The critical detachment length can be derived from the transversality condition using the variational method.The effects of the surface parameters on the adhesion of the nano-beam are discussed in detail.These analyses provide some suggestions for engineers in the design and fabrication of more accurate M/NEMS instruments.
基金supported by the National Natural Science Foundation of China(Grant Nos.12072241 and 11972081)。
文摘Nanoporous metallic glasses(NPMGs)and crystalline/amorphous nanocomposites exhibit superior ductility over bulk macroscopic metallic glasses(MGs).Their fracture behaviors remain a mystery due to experimental technical limitations.In this work,the fracture behaviors of pre-cracked NPMGs and crystal-impregnated nanoporous metallic glasses(CINPMGs)are investigated through large-scale molecular dynamics simulations,and the MG and crystal phases are amorphous Cu_(50)Zr_(50)and crystalline B2CuZr,respectively.Fracture toughness is determined by simultaneously considering the surface energy and plastic dissipated energy.Our results confirm the excellent plasticity of both the pre-cracked NPMGs and CINPMGs.The progressive necking and ductile rupture of ligaments are responsible for generating NPMGs with prominent ductility and fracture toughness.Meanwhile,secondary cracking is also triggered,which can consume energy without extending the major crack,thus further improving fracture resistance.It is also found that the fracture toughness of NPMGs can be improved by increasing the solid fraction of MG,and linear relation between fracture toughness and solid fraction can be expected.Crystal impregnation effectively inhibits global failure as the crystal phase shields the individual amorphous ligaments.Homogeneous plastic flow characterized by shear bands is observed in CINPMGs,and this homogeneous global deformation is facilitated by the crystalline/amorphous interface.Besides,the fracture toughness of CINPMGs is higher than that of the constituent single phases,regardless of the volume fraction of each phase.Ashby material charts manifest that these two types of materials demonstrate promising potential in the material selection library for advanced structural design.
基金supported by the National Natural Science Foundation of China(Grant Nos.10802099,11272357 and 11102140)the Doctoral Fund of Ministry of Education of China(Grant No.20110141120024)+2 种基金the Natural Science Foundation of Shandong Province(Grant No.ZR2009AQ006)the Opening Project of State Key Laboratory of Explosion Science and Technology(Beijing Institute of Technology)(Grant No. KFJJ12-11M)the support from the Brain Korea 21 Program at Seoul National University
文摘In this study,we considered the wetting phenomenon on a general substrate from a new viewpoint of continuum mechanics.The analyses first show how the Wenzel and the Cassie models deviate the practical results in some special substrates,and then elucidate the mechanism of the triple contact line(TCL) moving.Based upon variational theory of the total free functional dealing with the movable boundary condition,we show that the macroscopic contact angle(MCA) expression is the corresponding transversality condition.It manifests that the MCA depends only on the chemical and geometric property at the TCL,and is not affected by the gravity of the droplet and the contact area beneath the liquid.Our continuum model also shows the exploration of the pinning effect on a sharp wedge or the interface between two different phases.This investigation will help designing super-hydrophobic materials for novel micro-fluidic devices.
基金supported by National Natural Science Foundation of China(Grant Nos.51476033,51376094)the Key Laboratory of Hydraulic Machinery Transients(Wuhan University)of Ministry of Educationthe Tianjin Key Laboratory of Advanced Functional Porous Materials
文摘The effect of the radial heat conduction on the effective thermal conductivity of carbon nanotube(CNT) bundles is studied by the nonequilibrium molecular dynamics(NEMD) method. The hexagonal CNT bundle consists of seven(10, 10) single-walled carbon nanotubes(SWCNTs). The radial heat conduction is induced by creating the vacancy defects in some segments of the constituent CNTs. Combined with the temperature differences and the inter-tube thermal resistances at the different segments,the radial heat flow in the CNT bundle is calculated. The maximum percentage of the radial heat flow is less than 7% with the presence of four defective CNTs, while the resultant decrement of the effective thermal conductivity of the bundle is about 18%.The present results indicate that the radial heat flow can significantly diminish the axial heat conduction in the CNT bundles,which probably explains the smaller effective thermal conductivity in the CNT assemblies compared to that of the individual CNTs.
基金supported by the National Natural Science Foundation of China(Grant Nos.51204122&51134018)the Natural Science Foundation of Jiangsu Province(Grant No.BK20140411)+3 种基金State Key Laboratory of Mechanics and Control of Mechanical Structures(Nanjing University of Aeronautics and AstronauticsMCMS-0414Y01)the National Basic Research Program of China(“973”Project)(Grant No.2014CB239203)the 111 Project(Grant No.B14006)
文摘The development of a high-performance cemented carbide drill bit is of great significance to the reduction of rock drilling-cost. The non-smooth features of a biological surface provide an insight into how they can obtain low friction and good wear resistance with evolving surface morphology. By analyzing the mechanism of the surface of a dung beetle for reducing soil wear and adherence, we design a cemented carbide drill bit with a bionic surface, which is expected to have superior anti-wearing and anti-sticking properties for drilling the soft coal seam. Inspired from the characteristics of the head and pronotum surface of the dung beetle, optimized non-smooth surface of the drill bit was constructed. The working performance of this innovative drill was experimentally tested. With comparative experiments under the identical drilling conditions, the wear rates, drilling times of conventional drills and bionic drills were measured. Compared with the conventional counterpart, the drill designed exhibits better performance in reducing wear and sticking drilling-breaks, therefore achieving higher levels of efficiency. The diameter of the dome and pit on the bit surface is in the range of 0.8–1.2 mm, and the bionic drill bits could get better performance with preferable drilling speeds and wear rates.
基金supported by the National Natural Science Foundation of China(Grant Nos.11102140&51575404)
文摘A series of large-scale molecular dynamics(MD) simulations has been performed to study the effects of grain size and ligament diameter on the mechanical properties of nanocrystalline nanoporous gold. Such simulations indicate that the principal deformation mechanism is a combination of grain boundary sliding, grain rotation and dislocation movement. The results of uniaxial tensile tests reveal the presence of a reverse Hall-Petch relation between strength and nominal grain size, rather than the conventional Hall-Petch relationship in the present range of nominal grain size(7.9–52.7 nm). An increase of flow stress may possibly attribute to the lower total proportion of grain boundary sliding and grain rotation in the deformation of samples with larger grain size. The Young's modulus shows a linear relation with the reciprocal of nominal grain size, which depends largely on the volume fraction of grain boundaries and thus decreasing grain size leads to relatively lower Young's modulus. MD simulations on samples with ligament diameter ranging from 4.07 to 8.10 nm are also carried out and results show that the increasing ligament diameter resulted in decreased flow stress and increased Young's modulus.
