单环苯并噁嗪树脂热解的ReaxFF反应动力学模拟

采用ReaxFF动力学方法模拟了非交联固化单环苯并噁嗪树脂在不同温度下的热解特性。模拟结果表明,N桥键的断裂是热解的主要引发反应,H_2,H_2O和大分子碳团簇是热解的主要产物。随着反应温度升高,H_2的数量急剧增加,而H_2O的数量反而降低,高温有利于促进相对分子质量较大的碳团簇的形成,还观察到了CO,NH_3,N_2和HCN等小分子产物。用ReaxFF动力学方法模拟所得的气体产物以及含类似石墨烯结构的碳团簇与实际实验结果一致,ReaxFF动力学模拟方法可以作为一种研究苯并噁嗪树脂高温热解反应的有效途径。     

Predicting cation ordering in MgAl2O4 using genetic algorithms and density functional theory

Genetic algorithms (GAs) together with classical pair potentials and density functional theory (DFT) are used to investigate cation order in MgAl₂O₄ (Spinel). To efficiently locate the global minimum/minima on the system potential energy surface, corresponding to the ordered and fully equilibrated low-temperature phase, local structural optimizations are essential. Such energy minimizations are expensive at the DFT level, but a comparison of the distribution of the energy minima from DFT and popular classical pair potentials allows one to rapidly tune the GA parameters. We show that GAs are able to find, not only the global minimum on the potential energy, but also other low-energy cation configurations representing possible frozen-in disordered or metastable phases after quenching. The nature of these low-energy configurations can help to interpret the extent of kinetic trapping which hampers the comparison between different experimental studies.     

Refractive indices of layers and optical simulations of Cu(In,Ga)Se2 solar cells

Cu(In,Ga)Se₂-based solar cells have reached efficiencies close to 23%. Further knowledge-driven improvements require accurate determination of the material properties. Here, we present refractive indices for all layers in Cu(In,Ga)Se₂ solar cells with high efficiency. The optical bandgap of Cu(In,Ga)Se₂ does not depend on the Cu content in the explored composition range, while the absorption coefficient value is primarily determined by the Cu content. An expression for the absorption spectrum is proposed, with Ga and Cu compositions as parameters. This set of parameters allows accurate device simulations to understand remaining absorption and carrier collection losses and develop strategies to improve performances.     

Low temperature annealing of metals with electrical wind force effects

Conventional annealing is a slow, high temperature process that involves heating atoms uniformly, i.e., in both defective and crystalline regions. This study explores an electrical alternative for energy efficiency,where moderate current density is used to generate electron wind force that produces the same outcome as the thermal annealing process. We demonstrate this on a zirconium alloy using in-situ electron back scattered diffraction(EBSD) inside a scanning electron microscope(SEM) and juxtaposing the results with that from thermal annealing. Contrary to common belief that resistive heating is the dominant factor, we show that 5 × 10~4 A/cm~2 current density can anneal the material in less than 15 min at only135?C. The resulting microstructure is essentially the same as that obtained with 600?C processing for360 min. We propose that unlike temperature, the electron wind force specifically targets the defective regions, which leads to unprecedented time and energy efficiency. This hypothesis was investigated with molecular dynamics simulation that implements mechanical equivalent of electron wind force to provide the atomistic insights on defect annihilation and grain growth.     

数字IO电路板的嵌入式测试性设计方案

近年来,随着DSP、FPGA等大规模集成电路的发展,电子系统的性能也在大大提高,但同时给电子系统带来了新的测试和故障诊断问题;为了解决电路板快速诊断维修问题,嵌入式测试正以全新的概念成为板级电路测试的研究方向;文中从嵌入式测试的基本概念出发,介绍了嵌入式边界扫描、非侵入式测试等先进的板级嵌入式测试技术,并阐述了模拟嵌入式测试性设计的难点和基础电路原则,同时给出了基于FPGA的嵌入式测试控制器设计方案;然后,面向数字IO电路板,针对其关键功能电路展开嵌入式测试性设计,简要说明了测试程序的开发与下载;根据测试验证结果,嵌入式测试性设计可以增强测试自动化、提高测试效率,从而能够更好地降低产品整个寿命周期的测试维修成本。     

Direct numerical simulations for assessment of gas-solid drag models in two-dimensional random arrays of particles

The momentum exchange between the phases plays a vital role in modelling of gas–solid flows and it is mathematically described by drag models. However, no consensus exists on which drag model gives the most accurate prediction of the drag force, and, despite the increase in available computing power, the same drag models are used in two-dimensional and three-dimensional simulations. In this study, direct numerical simulations of gas flow through multiple random configurations of static monodisperse particles are performed. The variations of solid volume fraction and particle Reynolds number are in the ranges of 0.05–0.4 and 13.7–136.9, respectively. The drag force exerted on particles is calculated and properly averaged. Based on the simulation results, thirteen drag models are compared and correction factors are introduced using the stochastic gradient descent algorithm. The correction factors provide a simple adjustment for the models to be used in 2D modelling.     

Prediction of thermodynamic properties for fluid nitrogen with molecular dynamics simulations

Molecular dynamics simulation results in the isochoric isothermal ensemble are reported for a two-center Lennard Jones model of fluid nitrogen characterized by the fixed molecular elongationL = 1 = 0.3292, New values of and were determined by fitting the vapor pressure and the saturated liquid density of the model to experimental data at 94,67 K. The required vapor liquid equilibrium data of the model were taken from a study using the NpT + test particle method. The resulting values are k = 36.32013 K (36.673 K) and = 0.32973 nm (0.33073 nm), with values in parentheses being those obtained previously from a Weeks Chandler Andersen-type perturbation theory. Then pressures and internal energies were calculated by molecular simulations for 110 state points in the temperature range from 72 to 330 K and for densities up to 35 mol · L¹. Comparison of the predictions based on the new parameters with the empirical equation of state of Jacobsen et al. shows good to excellent agreement except in the near-critical region. Moreover. for almost all state points the new parameters yield an improvement over old ones from perturbation theory.     

EPW: A program for calculating the electron–phonon coupling using maximally localized Wannier functions

EPW (Electron–Phonon coupling using Wannier functions) is a program written in Fortran90 for calculating the electron–phonon coupling in periodic systems using density-functional perturbation theory and maximally localized Wannier functions. EPW can calculate electron–phonon interaction self-energies, electron–phonon spectral functions, and total as well as mode-resolved electron–phonon coupling strengths. The calculation of the electron–phonon coupling requires a very accurate sampling of electron–phonon scattering processes throughout the Brillouin zone, hence reliable calculations can be prohibitively time-consuming. EPW combines the Kohn–Sham electronic eigenstates and the vibrational eigenmodes provided by the Quantum ESPRESSO package (see Giannozzi et al., 2009 [1]) with the maximally localized Wannier functions provided by the wannier90 package (see Mostofi et al., 2008 [2]) in order to generate electron–phonon matrix elements on arbitrarily dense Brillouin zone grids using a generalized Fourier interpolation. This feature of EPW leads to fast and accurate calculations of the electron–phonon coupling, and enables the study of the electron–phonon coupling in large and complex systems.

Program summary

Program title: EPWCatalogue identifier: AEHA_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEHA_v1_0.htmlProgram obtainable from: CPC Program Library, Queen's University, Belfast, N. IrelandLicensing provisions: GNU Public LicenseNo. of lines in distributed program, including test data, etc.: 304 443No. of bytes in distributed program, including test data, etc.: 1 487 466Distribution format: tar.gzProgramming language: Fortran 90Computer: Any architecture with a Fortran 90 compilerOperating system: Any environment with a Fortran 90 compilerHas the code been vectorized or parallelized?: Yes, optimized for 1 to 64 processorsRAM: Heavily system dependent, as small as a few MBSupplementary material: A copy of the “EPW/examples” directory containing the phonon binary files can be downloadedClassification: 7External routines: MPI, Quantum-ESPRESSO package [1], BLAS, LAPACK, FFTW. (The necessary Blas, Lapack and FFTW routines are included in the Quantum-ESPRESSO package [1].)Nature of problem: The calculation of the electron–phonon coupling from first-principles requires a very accurate sampling of electron–phonon scattering processes throughout the Brillouin zone; hence reliable calculations can be prohibitively timeconsuming.Solution method: EPW makes use of a real-space formulation and combines the Kohn–Sham electronic eigenstates and the vibrational eigenmodes provided by the Quantum-ESPRESSO package with the maximally localized Wannier functions provided by the wannier90 package in order to generate electron–phonon matrix elements on arbitrarily dense Brillouin zone grids using a generalized Fourier interpolation.Running time: Single processor examples typically take 5–10 minutes.References:

• [1] 

  P. Giannozzi, et al., J. Phys. Condens. Matter 21 (2009), 395502, http://www.quantum-espresso.org/.

     

Numerical simulations of particle migration in a viscoelastic fluid subjected to shear flow

Particle migration is a relevant transport mechanism whenever suspensions flow in channels with gap size comparable to particle dimensions (e.g. microfluidic devices). Several theoretical as well as experimental studies have been performed on this topic, showing that the occurring of this phenomenon and the migration direction are related to particle size, flow rate, and the nature of the suspending liquid.In this work we perform a systematic analysis on the migration of a single particle in a sheared viscoelastic fluid through 2D finite element simulations in a Couette planar geometry. To focus on the effects of viscoelasticity alone, inertia is neglected. The suspending medium is modeled as a Giesekus fluid.An ALE particle mover is combined with a DEVSS/SUPG formulation with a log-representation of the conformation tensor giving stable and convergent results up to high flow rates. To optimize the computational effort and reduce the remeshing and projection steps, needed as soon as the mesh becomes too distorted, a ‘backprojection’ of the flow fields is performed, through which the particle in fact moves along the cross-streamline direction only, and the mesh distortion is hence drastically reduced.Our results, in agreement with recent experimental data, show a migration towards the closest walls, regardless of the fluid and geometrical parameters. The phenomenon is enhanced by the fluid elasticity, the shear thinning and strong confinements. The migration velocity trends show the existence of a master curve governing the particle dynamics in the whole channel. Three different regimes experienced by the particle are recognized, related to the particle-wall distance. The existence of a unique migration behavior and its qualitative aspects do not change by varying the fluid parameters or the particle size.