Showing papers by "Zhiguo Wang published in 2008"

Atomistic simulations of the mechanical properties of silicon carbide nanowires

Abstract: Molecular-dynamics methods using the Tersoff bond-order potential are performed to study the nanomechanical behavior of [111]-oriented $\ensuremath{\beta}\text{-SiC}$ nanowires under tension, compression, torsion, combined tension-torsion, and combined compression-torsion. Under axial tensile strain, the bonds of the nanowires are just stretched before the failure of nanowires by bond breakage. The failure behavior is found to depend on size and temperatures. Under axial compressive strain, the collapse of the SiC nanowires by yielding or column buckling mode depends on the length and diameters of the nanowires, and the latter is consistent with the analysis of equivalent continuum structures using Euler buckling theory. The nanowires collapse through a phase transformation---from crystal to amorphous structure---in several atomic layers under torsion strain. Under combined loading the failure and buckling modes are not affected by the torsion with a small torsion rate, but the critical stress decreases by increasing the torsion rate. Torsion buckling occurs before the failure and buckling with a big torsion rate. Plastic deformation appears in the buckling zone by further increasing the combined loading.

Thermodynamic and Spectroscopic Studies of Lanthanides(III) Complexation with Polyamines in Dimethyl Sulfoxide

Abstract: The thermodynamic parameters of complexation of Ln(III) cations with tris(2-aminoethyl)amine (tren) and tetraethylenepentamine (tetren) were determined in dimethyl sulfoxide (DMSO) by potentiometry and calorimetry. The excitation and emission spectra and luminescence decay constants of Eu3+ and Tb3+ complexed by tren and tetren, as well as those of the same lanthanides(III) complexed with diethylenetriamine (dien) and triethylenetetramine (trien), were also obtained in the same solvent. The combination of thermodynamic and spectroscopic data showed that, in the 1:1 complexes, all nitrogens of the ligands are bound to the lanthanides except in the case of tren, in which the pendant N is bound. For the larger ligands (trien, tren, tetren) in the higher complexes (ML2), there was less complete binding by available donors, presumably due to steric crowding. FT-IR studies were carried out in an acetonitrile/DMSO mixture, suitably chosen to follow the changes in the primary solvation sphere of lanthanide(III) d...

Tuning the band structures of single walled silicon carbide nanotubes with uniaxial strain: A first principles study

Abstract: Electronic band structures of single-walled silicon carbide nanotubes are studied under uniaxial strain using first principles calculations. The band structure can be tuned by mechanical strain in a wide energy range. The band gap decreases with uniaxial tensile strain, but initially increases with uniaxial compressive strain and then decreases with further increases in compressive strain. These results may provide a way to tune the electronic structures of silicon carbide nanotubes, which may have promising applications in building nanodevices.

Effects of interatomic potential on He bubble creation by cascades in α-iron

Abstract: The effects of using different interatomic potentials in molecular dynamics (MD) simulations of the formation of He-vacancy clusters within displacement cascades in α-Fe are investigated using two sets of potentials. Simulations of cascades produced by primary knock-on atoms of energy Ep=1–20keV were performed in α-Fe containing a concentration of substitutional He atoms varying from 1to5at.% at an irradiation temperature of 100K. Although the effects of interatomic potentials on the nucleation of He-vacancy clusters induced by cascades are relatively small, the number and size of He-vacancy clusters produced are significantly different for the different potentials employed in this study. Thus, these differences may influence the microstructural evolution predicted in damage accumulation models that use the results from MD cascade simulations as input. The observed differences in postcascade configurations can be attributed mainly to the differences in the Fe–Fe and Fe–He potentials.

Molecular dynamics simulation on the buckling behavior of GaN nanowires under uniaxial compression

Abstract: Molecular dynamics simulation is one of the most promising methods for investigating the mechanical behavior of nanostructures, such as nanowires and nanotubes. Atomistic simulations are performed to investigate the buckling properties of [0 0 1]-, [1 1¯ 0]-, and [1 1 0]-oriented GaN nanowires under uniaxial compression; these three types of nanowires correspond to experimentally synthesized nanowires. The effects of simulation temperature and wire length on the buckling behavior are investigated. The simulation results show that critical stress decreases with the increase of wire length, which is in agreement with the Euler theory. Buckling occurs as a result of dynamic processes; buckling strain (and corresponding stress) decreases as temperature is increased.

Amorphous layer coating induced brittle to ductile transition in single crystalline SiC nanowires: an atomistic simulation

Abstract: Molecular dynamics simulations with Tersoff potentials were used to study the response of SiC nanowires with and without amorphous coating to a tensile strain along the axial direction. The uncoated nanowires show brittle properties and fail through bond breaking. Although the amorphous coating leads to a decrease in the Young's modulus of nanowires, yet it also leads to the appearance of plastic deformation under axial strain. These results provide an effective way to modify the brittle properties of some other semiconductor nanowires.

Mechanical behavior of gallium nitride nanotubes under combined tension-torsion: An atomistic simulation

Abstract: The tensile mechanical behavior of single crystalline gallium nitride (GaN) nanotubes under combined tension-torsion is investigated using molecular dynamics simulations with an empirical potential. The simulation results show that a small torsion rate (<0.010degps−1) does not affect the tensile behavior of GaN nanotube, i.e., the nanotubes show brittle properties at low temperatures; whereas at high temperatures, they behave as ductile materials. However, the failure stress decreases with increasing rate of torsion above 0.010degps−1, and the nanotube fails in a different manner. The torsion rate has no effect on the elastic properties of GaN nanotubes.

Nanomechanical behavior of single crystalline SiC nanotubes revealed by molecular dynamics simulations

Abstract: Molecular dynamics simulations with Tersoff potentials were used to study the response of single crystalline SiC nanotubes under tensile, compressive, torsional, combined tension-torsional, and combined compression-torsional strains. The simulation results reveal that the nanotubes deform through bond-stretching and breaking and exhibit brittle properties under uniaxial tensile strain, except for the thinnest nanotube at high temperatures, which fails in a ductile manner. Under uniaxial compressive strain, the SiC nanotubes buckle with two modes, i.e., shell buckling and column buckling, depending on the length of the nanotubes. Under torsional strain, the nanotubes buckle either collapse in the middle region into a dumbbell-like structure for thinner wall thicknesses or fail by bond breakage for the largest wall thickness. Both the tensile failure stress and buckling stress decrease under combined tension-torsional and combined compression-torsional strain, and they decrease with increasing torsional rat...

Atomistic simulation of the torsional buckling of single-crystalline GaN nanotubes

Abstract: The responses of single-crystalline gallium nitride (GaN) nanotubes under torsion have been investigated using an atomistic simulation based on a Stillinger–Weber potential. The simulation results show that the buckling behavior of GaN nanotubes is strongly dependent on the simulation temperature, strain rate, tube length and the wall thickness of the nanotubes. The critical torsional angle decreases with the increase of simulation temperature, strain rate and thickness of the nanotube, and increases with the increase in the tube length.

Atomistic level studies on the tensile behavior of GaN nanotubes under uniaxial tension

Abstract: Molecular dynamics method with the Stillinger-Weber (SW) potential has been employed to study the responses of GaN nanotubes (GaNNTs) to a uniaxial tensile load along the axial direction. It has been revealed that GaNNTs exhibits a completely different tensile behavior at different temperatures, i.e. ductility at higher deformation temperatures and brittleness at lower temperatures, leading to a brittle to ductile transition (BDT). Both the BDT temperature and the critical stress increases with increasing thickness of GaNNTs, and the critical stress at higher temperature are lower than those at lower temperature. These results on the tensile behaviors of GaNNTs in an atomic level will provide a good reference to its promising applications.

Orientation and temperature dependence of the tensile behavior of GaN nanowires: an atomistic study

Abstract: Gallium nitride (GaN) is a high-temperature semiconductor material of considerable interest. It emits brilliant light and has been considered as a key material for the next generation of high frequency and high power transistors that are capable of operating at high temperatures. Due to its anisotropic and polar nature, GaN exhibits direction-dependent properties. Growth directions along [001], [1−10] and [110] directions have all been synthesized experimentally. In this work, molecular dynamics simulations are carried out to characterize the mechanical properties of GaN nanowires with different orientations at different temperatures. The simulation results reveal that the nanowires with different growth orientations exhibit distinct deformation behavior under tensile loading. The nanowires exhibit ductility at high deformation temperatures and brittleness at lower temperature. The brittle to ductile transition (BDT) was observed in the nanowires grown along the [001] direction. The nanowires grown along the [110] direction slip in the {010} planes, whereas the nanowires grown along the [1−10] direction fracture in a cleavage manner under tensile loading.

Simulation on the effects of torsion strain on the mechanical properties of SiC nanowires under tensile and compressive loading

Abstract: Molecular dynamics simulations with Tersoff potentials were used to study the tensile and compressive mechanical behavior of SiC nanowires with torsion strain. The simulation results show that small torsion strain does not affect the mechanical behavior of SiC nanowires. However, large torsion strain induces the decrease of the critical stress. With large torsion strain, the collapse occurs in the nanowires before tensile failure and compressive buckling, and deformation zone occurs in the collapsed part.

Physical Properties of GaN Nanotubes as Revealed by Computer Simulation

Abstract: Single-crystalline wurtzite GaN nanotubes have been synthesized recently with proposed applications in nanoscale electronics, optoelectronics, and the biochemical sensing field In this work, molecular dynamics methods with a Stillinger-Weber potential have been used to investigate the melting behavior, thermal conductivity, and mechanical properties of wurtzite-type single-crystalline GaN nanotubes (1) The simulations show that the melting temperature of the GaN nanotubes increases with the thickness of the nanotubes to a saturation value, which is close to the melting temperature of a bulk GaN The results reveal that the nanotubes begin to melt at the surface, and then the melting rapidly extends to the interior of the nanotubes as the temperature increases (2) The thermal conductivity of nanotubes is smaller than that of the bulk GaN single crystal The thermal conductivity is also found to decrease with temperature and increase with increasing wall thickness of the nanotubes The change of phonon spectrum and surface inelastic scattering may account for the reduction of thermal conductivity in the nanotubes, while thermal softening and high-frequency phonon interactions at high temperatures may provide an explanation for its decrease with increasing temperature (3) The simulation results show that at low temperatures, the nanotubes show brittle properties, whereas at high temperatures, they behave as ductile materials The brittle to ductile transition temperature generally increases with increasing thickness of the nanotubes and strain rate (4) The simulation temperature, tube length, and strain rate all can affect the buckling behavior of GaN nanotubes The critical stress decreases with the increase of simulation temperature and tube length The tube length dependence of buckling is compared with those from the analysis of equivalent continuum structures using Euler buckling theory