Mianyang, China
Mianyang, China

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Wang X.-M.,CAS Beijing National Laboratory for Molecular | Wang C.-H.,Peking University | Wang C.-H.,Institute of Fluid Physics | Kuang X.-J.,Sun Yat Sen University | And 3 more authors.
Inorganic Chemistry | Year: 2012

A novel oxonitridosilicate phosphor host Sr 3Si 2O 4N 2 was synthesized in N 2/H 2 (6%) atmosphere by solid state reaction at high temperature using SrCO 3, SiO 2, and Si 3N 4 as starting materials. The crystal structure was determined by a Rietveld analysis on powder X-ray and neutron diffraction data. Sr 3Si 2O 4N 2 crystallizes in cubic symmetry with space group Pa3̄, Z = 24, and cell parameter a = 15.6593(1) Å. The structure of Sr 3Si 2O 4N 2 is constructed by isolated and highly corrugated 12 rings which are composed of 12 vertex-sharing [SiO 2N 2] tetrahedra with bridging N and terminal O to form three-dimensional tunnels to accommodate the Sr 2+ ions. The calculated band structure shows that Sr 3Si 2O 4N 2 is an indirect semiconductor with a band gap ≈ 2.84 eV, which is close to the experimental value ≈ 2.71 eV from linear extrapolation of the diffuse reflection spectrum. Sr 3-xSi 2O 4N 2:xEu 2+ shows a typical emission band peaking at ∼600 nm under 460 nm excitation, which perfectly matches the emission of blue InGaN light-emitting diodes. For Ce 3+/Li +-codoped Sr 3Si 2O 4N 2, one excitation band is in the UV range (280-350 nm) and the other in the UV blue range (380-420 nm), which matches emission of near-UV light-emitting diodes. Emission of Sr 3-2xSi 2O 4N 2:xCe 3+,xLi + shows a asymmetric broad band peaking at ∼520 nm. The long-wavelength excitation and emission of Eu 2+ and Ce 3+/Li +-doped Sr 3Si 2O 4N 2 make them attractive for applications in phosphor-converted white light-emitting diodes. © 2012 American Chemical Society.

Yang Y.,Rensselaer Polytechnic Institute | Huang H.,Rensselaer Polytechnic Institute | Huang H.,University of Connecticut | Xiang S.K.,Institute of Fluid Physics | Chason E.,Brown University
Applied Physics Letters | Year: 2010

The diffusion of adatoms into grain boundaries (GBs) of polycrystalline thin film during vapor deposition affects the stress that develops and the film's subsequent performance. This Letter reports a proposed mechanism of modifying the stress by controlling adatom diffusion into GBs through the use of surfactants. Based on polycrystalline kinetic Monte Carlo simulations of Cu 〈 111 〉 thin films with In surfactant, the authors demonstrate that the proposed mechanism is feasible. Further, the authors show that the reduction is due to the decrease in effective adatom diffusivity, which dominates over the increase in adatom concentration. © 2010 American Institute of Physics.

Liu Z.-L.,Luoyang Normal University | Yang J.-H.,Luoyang Normal University | Cai L.-C.,Institute of Fluid Physics | Jing F.-Q.,Institute of Fluid Physics | Alfe D.,University College London
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

First-principles and classical molecular dynamics simulations have been performed to study the structural and thermodynamic properties of Pd under pressure. By comparing the Gibbs free energy, in the quasiharmonic approximation (QHA), of the face-centered cubic (fcc) phase with those of the hexagonal-close-packed (hcp) and body-centered-cubic (bcc) phases we found that the fcc phase is stable up to 500GPa and 5000K. The predicted high-temperature elastic constants of fcc Pd agree well with experiments. The phonon dispersion curves are obtained at various pressures. In contrast with experiments we did not observe any phonon anomalies in Pd. We reproduced the thermodynamic properties of Pd accurately by taking into account the electron and phonon contributions to the free energy of Pd. The obtained thermal expansion coefficient, Hugoniot curves, and specific heat capacity compare well with experiments. In particular, the excellent agreement of the thermal expansion coefficients with experiment supports the validity of the QHA for Pd at high temperatures. Our QHA-based Hugoniot curves also show good agreement with experiments and our dynamic shock simulations. Shocks along [100] produced a melting temperature with a superheating of 18.3% at 226GPa, compared with our high-pressure melting curve of Pd from coexistence-phase simulations based on an embedded atom model. © 2011 American Physical society.

Qin R.,Institute of Fluid Physics | Wang C.-H.,Institute of Fluid Physics | Zhu W.,Institute of Fluid Physics | Zhang Y.,Institute of Computer Application
AIP Advances | Year: 2012

We perform first-principles calculations of mechanical and electronic properties of silicene under strains. The in-plane stiffness of silicene is much smaller than that of graphene. The yielding strain of silicene under uniform expansion in the ideal conditions is about 20%. The homogeneous strain can introduce a semimetal-metal transition. The semimetal state of silicene, in which the Dirac cone locates at the Fermi level, can only persist up to tensile strain of 7% with nearly invariant Fermi velocity. For larger strains, silicene changes into a conventional metal. The work function is found to change significantly under biaxial strain. Our calculations show that strain tuning is important for applications of silicene in nanoelectronics. © Author(s) 2012.

Wang K.,Hunan University | Xiao S.,Hunan University | Deng H.,Hunan University | Zhu W.,Institute of Fluid Physics | And 2 more authors.
International Journal of Plasticity | Year: 2014

The martensitic phase transition α → ε of iron is of particular interest to researchers and industrialists due to its technological and scientific significance in recent decades. Experimental and numerical studies have discovered and confirmed the phase transition mechanisms under shock loading. However, the relation between plasticity and the phase transition, which is of key importance in understanding the material behavior under dynamic loading, has not been made clear, and former NEMD simulations fail to reproduce the plasticity observed in experiments. In this work, a new embedded-atom-model potential for iron has been developed and validated. Large-scale NEMD simulations are performed with a variety of loading strengths along three low index crystallographic directions, i.e., [0 0 1], [1 1 0] and [1 1 1], and the phase transition mechanism is examined with the aid of the c axis analysis technique proposed in this work. The differences in shock response to the different loading directions are explained by rotation symmetry and compression mechanisms as the first step toward phase transformation of iron. Although no well-defined plastic process is observed for the shock along the [1 0 0] and [1 1 1] directions, nucleation, propagation and multiplication of dislocations are clearly observed, and the slip system associated with plastic slip is determined to be {1 1 2} 〈1 1 1〉 when loading along the [1 1 0] direction. © 2014 Elsevier Ltd. All rights reserved.

Wang Y.,Ningbo University | He H.,Institute of Fluid Physics | Wang L.,Ningbo University
Mechanics of Materials | Year: 2013

A Critical Damage Evolution (CDE) model considering void growth and coalescence is proposed for describing the dynamic tensile spall of ductile metals sustaining intense dynamic loading. Based on the percolation hypothesis, two physical parameters, named the critical void coalescence damage D c and the critical fracturing damage Df, are suggested to indicate the critical behavior of dynamic tensile fracture, namely the onset of void coalescence and the occurrence of catastrophic fracture, respectively. Finite element simulations of plate impact and sliding detonation tests have been carried out and compared to the experiments for steel. From the results obtained, we demonstrate that spall failure of ductile metals is dependent on these two critical damage parameters, which are intrinsic material parameters and thus independent of impact stress and strain rate. © 2012 Elsevier Ltd. All rights reserved.

Li T.,Institute of Fluid Physics | Hua C.,Institute of Chemical Materials | Li Q.,Institute of Fluid Physics
Propellants, Explosives, Pyrotechnics | Year: 2013

The shock sensitivities of plastic bonded explosives were studied with a thin flyer impact test by using two types of pressed RDX. The thin flyer, driven by an electrically exploding plasma, exerts a short-duration, high-pressure pulse to the samples to trigger a shock-to-detonation process. It was found that the duration and magnitude of the incident shock strongly influence the dominant mode of hot-spot formation, promoting a fast pore collapsing mechanism while suppressing other slower shear or friction mechanisms, as proposed by Chakravarty et al. [1]. The pressed PBX based on reduced sensitivity RDX had higher shock threshold pressure, compared to the pressed PBX based on commercial RDX. The difference was observed even with a certain portion of external extragranular defects. It is postulated that the internal crystal defects are more efficient than the external porosity in terms of the rapid reaction of hot spots. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Zheng J.,Institute of Fluid Physics | Gu Y.J.,Institute of Fluid Physics | Chen Z.Y.,Institute of Fluid Physics | Chen Q.F.,Institute of Fluid Physics
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2010

Experimental equations of state on generation of nonideal xenon plasma by intense shock wave compression was presented in the ranges of pressure of 2-16 GPa and temperature of 31-50 kK, and the xenon plasma with the nonideal coupling parameter Γ range from 0.6-2.1 was generated. The shock wave was produced using the flyer plate impact and accelerated up to ∼6km/s with a two-stage light gas gun. Gaseous specimens were shocked from two initial pressures of 0.80 and 4.72 MPa at room temperature. Time-resolved spectral radiation histories were recorded by using a multiwavelength channel pyrometer. The transient spectra with the wavelength range of 460-700 nm were recorded by using a spectrometer to evaluate the shock temperature. Shock velocity was measured and particle velocity was determined by the impedance matching methods. The equations of state of xenon plasma and ionization degree have been discussed in terms of the self-consistent fluid variational theory. © 2010 The American Physical Society.

Based on the single-mode potential model of Jacobs and Rikanati, Firstly, a viscous single-mode bubble evolution model of Rayleigh Taylor instability (RTI) is developed in this study. Viscous effects of RTI's early stage growth for low Atwood number have been explained. In addition, direct numerical simulations of single mode RTI are studied with Navier-Stokes equations and a transport-diffusive equation for miscible fluids, in which these equations are discritized with discontinuous Galerkin (DG) spectral element method. The turbulent mixing of RTI has kinetic energy dissipation, and the dissipation rate is determined by the inertial and viscous effects. Therefore, the numerical techniques must include a dissipation mechanism for kinetic energy. For this reason, the high accurate spectral element method is employed in this study. Agreement between the theoretical model and the numerical results shows that simulations of RTI is feasible using the mathematical miscible fluid model. The results also suggest that a high order numerical method may provide the capability of simulating small scale fluctuations in turbulent flows with RTI. © 2013 Elsevier Ltd.

Zhang X.,Institute of Fluid Physics | Zhang X.,University of Wyoming | Stanescu D.,University of Wyoming
Computers and Fluids | Year: 2010

This paper studies round jet with large eddy simulation (LES) method, in which spectral element technique is used as spacial discritization for the large eddy simulation Navier-Stokes equations. A local spectral discretization associated with Legendre polynomials is employed on each element of the structured mesh, which allows for high accurate simulations of turbulent flows. Discontinuities across the interfaces of the elements are resolved using a Riemann solver. An isoparametric representation of the geometry is implemented, with boundaries of the domain discretized to the same order of accuracy as the solution, and explicit low-storage Runge-Kutta methods are used for time integration. LES results of round jet are presented, in which the instantaneous and statistical turbulence structures of the round jet have been captured. The probability density function, and the spectral density function of the round jet that can reflect properties of turbulence have also been estimated. The work serves the purpose of allowing fast, convenient computations and comparisons with theoretical results and the ultimate goal is to develop it into an LES code featuring spectral accuracy with minimum dissipation and dispersion, a valuable tool for round jet computations. © 2009 Elsevier Ltd. All rights reserved.

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