Strain Rate Sensitivities of Face-Centred-Cubic Metals Using Molecular Dynamics Simulation

We use dislocation theory and molecular dynamics （MD） simulations to investigate the effect of atom properties on the macroscopic strain rate sensitivity of f cc metals. A method to analyse such effect is proposed. The stress dependence of dislocation velocity is identified as the key of such study and is obtained via 2-D MD simulations on the motion of an individual dislocation in an fcc metal. Combining the simulation results with Orowan＇s relationship, it is concluded that strain rate sensitivities of fcc metals are mainly dependent on their atomic mass rather than the interatomic potential. The order of strain rate sensitivities of five fcc metals obtained by analysing is consistent with the experimental results available.     

A Simple Theoretical Method to Predict the Hardness of Pure Metal Crystals

Design of superhard bulk materials requires predicting their hardness, challenging current theories for material design. By introducing a concept of condensing force （CF）, it is shown via ab initio calculations for fcc （Ni, Cu, Al, Ir, Rh, Au, Ag, Pd） and hcp Re crystals that materials with larger CF can have greater hardness. Since the calculation of CF is easy, this method might prove a convenient way to evaluate the hardness of newly designed materials.     

Temporal Characteristic of M--M Transition Lasers in Strontium Atom Vapour

The kinetic process of Sr atom metastable-metastable transition lasers in He-St longitudinal pulsed discharge is analysed and a concise self-consistent physical model is developed. The temporal evolutions of discharge parameters, main paxticle densities, the electron temperature, and the lasing pulses are numerically calculated. The results provided by the model agree well with the experiment, and the temporal behaviour of each laser pulse is explained successfully by the simulation results.     

Mechanism of Strain Rate Effect Based on Dislocation Theory

Based on dislocation theory, we investigate the mechanism of strain rate effect. Strain rate effect and dislocation motion are bridged by Orowan＇s relationship, and the stress dependence of dislocation velocity is considered as the dynamics relationship of dislocation motion. The mechanism of strain rate effect is then investigated qualitatively by using these two relationships although the kinematics relationship of dislocation motion is absent due to complicated styles of dislocation motion. The process of strain rate effect is interpreted and some details of strain rate effect are adequately discussed. The present analyses agree with the existing experimental results. Based on the analyses, we propose that strain rate criteria rather than stress criteria should be satisfied when a metal is fully yielded at a given strain rate.     

Review on the symmetry-related properties of carbon nanotubes

In this work we review the basic properties of carbon nanotubes from the standpoint of group theory. The zone folding scheme is reviewed in the light of the helical symmetry of the nanotube. The group theory for chiral and achiral nanotubes is reviewed, and the representations of the factor group of the wavevector k are obtained. The similarities and differences between the formalism of the group of the wavevector and that of line groups are addressed with respect to the irreducible representations and quantum numbers associated with linear and angular momenta. Finally, we extend the results of group theory to illuminate the electronic and vibrational properties of carbon nanotubes. Selection rules for the optical absorption and double resonance Raman scattering are discussed for the case where the electron–electron interaction is negligible (metallic nanotubes) and for the case where exciton binding energies are strong and cannot be neglected.     

HPHT Synthesis of Micron Grade Boron-Doped Diamond Single Crystal in Fe-Ni-C-B Systems

Micron grade boron-doped diamond crystals with octahedral morphology are successfully synthesized in a Fe-Ni- C-B system under high pressure and high temperature （HPHT）. The effects of the additive boron on synthesis conditions, nucleation and growth, crystal morphology of diamond are studied. The synthesized micron grade diamond crystals were characterized by optical microscope （OM）, scanning electron microscope （SEM）, x-ray diffraction （XRD） and Raman spectroscopy. The research results show that the V-shaped section of synthetic diamond moves downwards to the utmost extent due to 0.3a wt% （a is a constant.） boron added in the synthesis system. The crystal colour is black, and the average crystal size is about 25μm. The crystal faces of synthetic diamond are mainly （111） face. The synthesis of this kind of diamond is few reported, and it will have important and widely applications.     

Mechanical Properties and Anisotropy in PbWO4 Single Crystal

The mechanical properties of PbWO4 （PWO） crystals grown by the vertical Bridgman method are systematically investigated using the microindentation technique. In the present work, the Vickers microhardness Hν, fracture toughness Kc, yield strength σy and friability index Bi of PbWO4 crystals are measured. The Vickers microhardness Hν on the （100） wafer is about 140 MPa, which means that PWO is a little soft＇＇ scintillator. The anisotropy of mechanical properties is also investigated under a steady load of 0.5 kg. The （100） wafer of the crystal exhibits combined mechanical properties more excellent than those of （111） and （001） wafers, and the values of Kc, σy, and Bi are 0.538 MPa＆#12539;m1/2, 51.11 kg/mm2 and 284.96 νm-1/2, respectively.     

Ab initio study of Nb2SC and Nb2S2C: Differences in coupling between the S and Nb-C layers

Using ab initio calculations, we have studied the structurally related compounds Nb₂SC and Nb₂S₂C. In Nb₂S₂C (space group , prototype Bi₂Te₃), S atoms are nearest neighbours, while in Nb₂SC (space group P6₃/mmc, prototype Cr₂AlC) this is not the case. The calculated equilibrium volume for these two phases deviates by 1.6-3.7% to previously-published experimental data and the bulk modulus-to-c₄₄ ratios obtained are 1.5 and 5.9, respectively. These results indicate a resemblance of Nb₂S₂C to hexagonal BN and graphite. Furthermore, we have demonstrated that the uniform compression method is adequate for estimating the elastic properties of Nb₂SC, a so-called MAX phase. It is our ambition that these calculations will stimulate further experimental research on these compounds.     

Theoretical Study on Structural and Elastic Properties of ZnO Nanotubes

A theoretical investigation on the structural and elastic properties of ZnO nanotubes is carried out by using atomistic calculations based on an inter-atomic pair potential within the shell-model approach. The calculation results are presented for the bond length, bond angle, radius dilation, strain energy, Young modulus and Poisson ratio as a function of tube radius. For small tube radius these properties depend on the helicity of the tube, while for the tube radius larger than 6.0A, they are independent of the tube radius and helicity except for the strain energy which decreases with increasing tube radius.     

Structural Investigation of Solid Methane at High Pressure

High pressure studies of solid methane are performed using both classical simulated annealing and first-principles methods. A series of simulated annealing and geometry optimization reveal a monoclinic P21/b structure with the unit cell containing four methane molecules. The phonon dispersion curves and vibrational density of states indicate that this structure is stable in the pressure range 10-90 GPa. The electronic band structure and density of states show that this structure has not metalized until 90 GPa.     

Quantum-Confined Stark Effects in a Single GaN Quantum Dot

Using analytical expressions for the polarization field in GaN quantum dot, and an approximation by separating the potential into a radial and an axial, we investigate theoretically the quantum-confined Stark effects. The electron and hole energy levels and optical transition energies are calculated in the presence of an electric field in different directions. The results show that the electron and hole energy levels and the optical transition energies can cause redshifts for the lateral electric field and blueshifts for the vertical field. The rotational direction of electric field can also change the energy shift.     

Theoretical Studies on Defects of Kaolinite in Clays

Using the first-principles methods, we study the formation energeties and charge doping properties of the extrinsic substitutional defects in kaolinite. Especially, we choose Be, Mg, Ca, Fe, Cr, Mn, Cu, Zn as extrinsic defects to substitute for Al atoms. By systematically calculating the impurity formation energies and transition energy levels, we find that all group-Ⅱ defects introduce the relative shallow transition energy levels in kaolinite. Among them, MgAl has the shallowest transition energy level at 0.08 eV above the valence band maximum. The transition- elemental defects FeAl, CrAl, and MnAl are found to have relative low formation energies, suggesting their easy formation in kaolinite under natural surrounding conditions. Our calculations show that the defects CuAl and ZnAl have the high formation energies and deep transition energy levels, which exclude the possibility of their formation in natural kaolinite.     

Fabrication of a Mono-Domain Alignment Ferroelectric Liquid Crystal Device Using a Polar Self-Assembled Monolayer

A mono-domain ferroelectric liquid crystal device (FLCD) is fabricated using a novel method. The cell used in this method is an asymmetric cell, typically the combination of a polar self-assembled monolayer (SAM) for one substrate and a rubbed polyimide for the other substrate. A defect-free alignment of ferroelectric liquid crystal is fabricated without applying a dc voltage to remove degeneracy in the layer structure. The contact angles of self-assembled monolayer and PI-2942 are measured and the polarity of SAM is higher than the PI alignment. It is found that the polarity of self-assembled monolayer is a key factor in the formation of mono-domain alignment of FLC.     

Influence of High Atomic Hydrogenation on the Electronic Structure of Zigzag Carbon Nanotubes： A First-Principles Study

Using density functional theory, we study high hydrogenated zigzag single-walled carbon nanotubes from （7,0） to （11,0）. Two structure transitions are classified： type A is a metallic transition and type B is a ＂semiconductive transition＂ according to the energy band structure. The charge density transforms only at the C-C bonds without hydrogenated sites. The sp^3 hybridization is mainly enhanced for all the C-C bonds in the vertical axial direction for type-A configurations, and the sp^3 hybridization mainly increases for all C-C bonds along the axial direction for the type-B case.     

Observational Constraints on the Unified Dark-Energy--Dark-Matter Model

We investigate the constraints on a generalized Chaplygin gas （GCG） model using the gold sample type-Ia supernovae （She Ia） data, the new Supernova Legacy Survey （SNLS） Sne Ia data and the size of baryonic acoustic oscillation peak found in Sloan Digital Sky Survey （SDSS）. In a spatially flat universe case we obtain, at a 95.4% confidence level, A8 = 0.76^＋0.07 -0.07 and α= 0.028^＋0.322 -0.2382 Our results are consistent with the ACDM model （α= 0）, but rule out the standard Chaplygin gas model （α= 1）.     

Crystal quality boosts responsiveness of magnetic shape memory single crystals

Magnetic shape memory alloys are promising materials to replace giant magnetostrictive materials and piezoelectrical ceramics in actuating devices due to the large magnetically induced strains. Ni-Mn-Ga is the most intense studied system due to its relatively high operational temperatures and the huge magnetically induced strains reported. Up to now the application of these materials is still limited by the operational temperature range. Additionally twin boundary mobility suffers from structural defects increasing the magnetic fields needed for significant and reproducible strains. The sample quality is affected by crystal inhomogeneity, porosity and impurities. Here new results are reported for the Ni-Mn-Ga class based on a set of single crystals grown by the SLARE method, recently developed by Mecklenburg et al. Single crystalline samples of Ni_49.7Mn_29.3Ga₂₁ of tetragonal martensitic structure exhibit a magnetic field induced strain of more than 4% below 170 mT and 6.5% at only 340 mT. Furthermore the operational temperature regimen could be expanded up to 65 °C.     

Investigation on Guided-Mode Characteristics of Hollow-Core Photonic Crystal Fibre at Near-Infrared Wavelengths

Guided-mode characteristics of hollow-core photonic crystal fibre （HC-PCF） are experimentally and theoretically investigated. The transmission spectrum in the range from 755 to 845nm is observed and the loss is measured to be 0.12dB/m at 800nm by cut-back method. Based on the full-vector beam propagation method and the full-vector plane-wave method, the characteristics of mode field over propagation distance 1 m are simulated, and the results show that the propagation efficiency can be above 80%. Compared with the fundamental guided mode well confined in air core within shorter propagation distance, the second-order guided mode leaks into the cladding region and gradually attenuates due to larger refractive index difference. The primary loss factors in HC-PCF and the corresponding solutions are elementarily discussed.     

Mechanism of Current Oscillations in Gallium ArsenidePhotoconductive Semiconductor Switches

Semi-insulating photoconductive semiconductor switch with an electrode gap of 4 mm, triggered by a laser pulse with energy of 0.5md, and applied bias of 2.5kV, the periodicity current oscillation with a cycle of 12ns is obtained. It is indicated that the current oscillation is one mode of transferred electron effect, namely quenched domain mode. This mode of trans-electron oscillator is obtained when the instantaneous bias electric field drops below the sustaining field （the minimum electric field required to support the domain） before the domain reaches the anode, which leads to the domain disappears somewhere in the bulk of the switch and away from the ohmic contacts. We mainly analyse the time-dependent characteristic of the mode, the theoretical analysis results are in excellent agreement with the experiment.     

Emergence of Strange Spatial Pattern in a Spatial Epidemic Model

Pattern formation of a spatial epidemic model with nonlinear incidence rate hI^2 S/（1 ＋ αI^2） is investigated. Our results show that strange spatial dynamics, i.e., filament-like pattern, can be obtained by both mathematical analysis and numerical simulation, which are different from the previous results in the spatial epidemic model such as stripe-like or spotted or coexistence of both pattern and so on. The obtained results well extend the finding of pattern formation in the epidemic model and may well explain the distribution of the infected of some epidemic.     

Theoretical investigation on structural stability of InN thin films on 3C-SiC(0 0 1)

The structural stability of InN thin films on 3C-SiC(0 0 1) substrate is systematically investigated based on an empirical interatomic potential, which incorporates electrostatic energy due to covalent bond charges and ionic charges. The calculated energy differences among coherently grown 3C-InN(0 0 1), 3C-InN(0 0 1) with misfit dislocations (MDs), and 2H-InN(0 0 0 1) imply that the coherently grown 3C-InN(0 0 1) is stable when the film thickness is less than 7 monolayers (MLs) while 2H-InN(0 0 0 1) is stabilized for the thickness beyond 8 MLs. This is because InN layers in 2H-InN(0 0 0 1) are fully relaxed by one MD. The analysis of atomic configuration at the 3C-InN(0 0 1)/3C-SiC(0 0 1) interfaces reveals that the coordination number of interfacial atoms is quite different from that in the bulk region. Thus, 3C-InN(0 0 1) with MDs on 3C-SiC(0 0 1) is always metastable over entire range of film thickness, consistent with the successful fabrication of 2H-InN(0 0 0 1) on 3C-SiC(0 0 1) by the molecular beam epitaxy. These results suggest that the mismatch in atomic arrangements at the interface crucially affects the structural stability of InN thin films on 3C-SiC(0 0 1) substrate.