Atomic scale simulation of deformation in magnesium single crystals

The deformation behaviour of magnesium single crystals under plane strain conditions has been examined using molecular dynamics modelling. The simulations were based on an existing atomic potential for magnesium taken from the literature. A strain of 10% was applied at rates of 3x10⁹s^-1 and 3x10⁷s^-1. The simulations predicted the formation of mechanical twins that accommodated extension in the c-axis direction of the hexagonal unit cell. However, the predicted twin is not of the same kind found in magnesium, but is that commonly observed in titanium. It is believed that further analysis of the physical properties predicted by this interatomic potential will shed more light on the atomic processes controlling twinning in Magnesium alloys. It also highlights the need for improvements to the interatomic potential such that more accurate deformation behaviour can be attained.

Measurement of anisotropies of the kinetic energy and anharmonicity in pyrolytic graphite by neutron Compton scattering

Neutron Compton scattering (NCS) measurements of the anisotropy of the momentum distribution and the mean Laplacian of the interatomic potential ∇2V have been performed using electron volt neutrons, with wave vector transfers between 24 Å−1 and 98 Å−1. The measured momentum distribution of the atoms displays significantly more anisotropy than a calculation using a model density of states. We have observed anisotropies in ∇2V for the first time. The results suggest that the atomic potential is harmonic within the graphite planes, but anharmonic for vibrations perpendicular to the planes.

Single and multi-step phase transformation in CuZr nanowire under compressive/tensile loading

A novel stress induced martenistic phase transformation is reported in an initial B2-CuZr nanowire of cross-sectional dimensions in the range of 19.44 x 19.44-38.88 x 38.88 angstrom(2) and temperature in the range of 10-400 K under both tensile and compressive loading. Extensive Molecular Dynamic simulations are performed using an inter-atomic potential of type Finnis and Sinclair. The nanowire shows a phase transformation from an initial B2 phase to BCT (body-centered-tetragonal) phase with failure strain of similar to 40% in tension, whereas in compression, comparatively a small B2 -> BCT phase transformation is observed with failure strain of similar to 25%. Size and temperature dependent deformation mechanisms which control ultimately the B2 -> BCT phase transformation are found to be completely different for tensile and compressive loadings. Under tensile loading, small cross-sectional nanowire shows a single step phase transformation, i.e. B2 -> BCT via twinning along {100} plane, whereas nanowires with larger cross-sectional area show a two step phase transformation, i.e. B2 -> R phase -> BCT along with intermediate hardening. In the first step, nanowire shows phase transformation from B2 -> R phase via twinning along {100} plane, afterwards the nanowire deforms via twinning along {110} plane which cause further transformation from R phase -> BCT phase. Under compressive loading, the nanowire shows crushing along {100} plane after a single step phase transformation from B2 -> BCT. Proper tailoring of such size and temperature dependent phase transformation can be useful in designing nanowire for high strength applications with corrosion and fatigue resistance. (C) 2009 Elsevier Ltd. All rights reserved.

Anharmonic Effects in Germanium

A precise X-ray investigation is carried out to probe the lowest-order anharmonic contribution of the atomic potential of the germanium atom. A total number of 1052 reflections (h + k + l = 4n and 4n +/- 1) are precisely measured at room temperature using a spherical single crystal of germanium and using a Nonius CAD-4 X-ray diffractometer with crystal monochromatized MoKalpha radiation. A least-square refinement program is used to refine the harmonic and anharmonic thermal parameters of the crystal. The refinement gives beta(Ge) = (-0.749 +/- 1.79) x 10-(16) J nm-3 with B(Ge) = (0.528 +/- 0.004) x 10(-2) nm2. The reliability index (R) amounts to 1.71% for germanium.

Dynamics of Plasmon in Graphene Oxide

Dynamic effects of plasmon such as scattering with defect boundaries and oxygen impurities in the graphene oxide are investigated. Study of plasmon dynamics helps in understanding electronic, opto-electronic and biological applications of graphene based nanostructures. Tuning or control over such applications is made possible by graphene nanostructure engineering. We have modeled defects with increased smoothing of defect edge in graphene keeping area of the defect constant. Scattering of plasmons in graphene with defects is modeled using an electromagnetic field coupled inter-atomic potential approach with finite element discretization of the atomic vibrational and electromagnetic field degrees of freedom. Our calculations show pi + sigma plasmon red shifting under sharp defect edges whereas pi plasmon show high extinction efficiency. Strong localization of electric fields near the sharp defect edges is observed. Observations on plasmons and its dynamics draws attention in designing novel optoelectronic devices and binders for bio-molecules.

Molecular/Cluster Statistical Thermodynamics Methods To Simulate Quasi-Static Deformations At Finite Temperature

The rapid evolution of nanotechnology appeals for the understanding of global response of nanoscale systems based on atomic interactions, hence necessitates novel, sophisticated, and physically based approaches to bridge the gaps between various length and time scales. In this paper, we propose a group of statistical thermodynamics methods for the simulations of nanoscale systems under quasi-static loading at finite temperature, that is, molecular statistical thermodynamics (MST) method, cluster statistical thermodynamics (CST) method, and the hybrid molecular/cluster statistical thermodynamics (HMCST) method. These methods, by treating atoms as oscillators and particles simultaneously, as well as clusters, comprise different spatial and temporal scales in a unified framework. One appealing feature of these methods is their "seamlessness" or consistency in the same underlying atomistic model in all regions consisting of atoms and clusters, and hence can avoid the ghost force in the simulation. On the other hand, compared with conventional MD simulations, their high computational efficiency appears very attractive, as manifested by the simulations of uniaxial compression and nanoindenation. (C) 2008 Elsevier Ltd. All rights reserved.

基于跨尺度模型的界面研究

探索和建立不同尺度理论之间的关联模式是科学研究的重要课题，本文基于跨尺度模型着重探讨了金属陶瓷界面的凝聚能和原子结构问题。本文遵循原始Peierls-Nabarro模型的基本思想，提出了一种处理一维界面失配位错组的新方法。在这个推广的Peierls-Nabarro模型中，本文得到了一个简单而且准确的解析解，此解反映了失配位错的核结构、能量与失配度、剪切模量之间的依赖关系。当界面剪切模量较强而失配度较小时，界面的结构可以用一组奇导师Volterra位错来描述，这与一些原子模拟结果一致。采用这一简单的模型，引入第一原理计算得到的数据，此模型可以估算金属陶瓷界面的凝聚能。一维界面失配位错组的Peierls-Nabarro模型还被解析推广描述一大类较宽的位错。在模型中我们引进了一个参数a，通过控制参数a，我们可以系统地改变失配位错芯的宽度、剪切应力的分布和弹性恢复力。随着a增加，位错宽度增加，同时弹性恢复力和失配位错应力的幅度减少。当界面剪切模量强和失配度小时，失配位错的宽度近似线性反比于弹性恢复力的幅度大小。同时当界面剪切模量和失配度固定时，失配能、弹性能和总的界面能随a的增加而减少。界面能和恢复力律形式密切相关，当界面剪切模量弱和失配度大时，这种依赖关系更强。考虑到界面常常是在晶格两个方向都有失配，本文还引进了描述界面周期失配位错的二维广义Peierls-Nabarro模型，使得我们能够定量地研究界面的结构和能量。文中定量分析了广义堆垛能γ面对界面失配位错的结构和能量的影响，分析了位错网中两种位错组的相互作用。当界面剪切模量τ_0变大和失配度f变小时，随着位错核区占整个界面的比重下降，γ面的形状对界面能量和结构影响减弱，结果两种位错组之间的相互作用也减弱。此外γ面的变化还有可能导致位错网结构的转变，也就是导致界面结构的转变。应用此模型，本文还研究了金属－陶瓷Ag/MgO(100)界面，给出了界面的能量和原子结构。文中得出结论：在Ag/MgO(100)界面将会形成{1/2<110>; <110>}类型的位错网。此外由于界面失配位错的形成，Ag/MgO(100)界面凝聚能的理论值900mJ/m~2将减少214mJ/m~2，最终成为686mJ/m~2。基于第一原理赝势平面波的总能计算，文中给出了金属陶瓷Al/MgO(100)界面弛豫和未弛豫时的广义堆垛能面。然后结合第三章发展的广义二维Peierls-Nabarro模型，详细研究了金属陶瓷Al/MgO(100)界面的原子结构和界面能。文中得出的“在Al/MgO(100)界面将会形成{1/2<110>; <110>}类型位错网”的推论，证实了Vellinga等的猜测；文中还预测了凝聚能的理论是在600mJ/m~2（未弛豫情形）和670mJ/m~2（弛豫情形）之间。这个应用表明此方法能够容易地建立连续介质理论和第一原理计算之间的联系，实现理论上的跨尺度。本文最后提出了一种得到界面原子有效对势的反演方法。通过反演金属－MgO陶瓷界面的第一原理计算的凝聚能曲线，我们得到了一些金属原子和陶瓷离子之间的对势，此对势反映了金属陶瓷键合的特性。本文的反演方法提供了通过第一原理计算数据来拟合界面原子对势的一种可行性途径。这种方法可归结为第一类尺度关联理论，即单向的跨尺度关联模式。

Molecular dynamics simulation of nanoindentation of Fe3C and tetrahedral Fe4C

Study of nanomechanical response of iron carbides is important because presence of iron carbides greatly influences the performance and longevity of steel components. This work contributes to the literature by exploring nanoindentation of nanocrystalline Fe3C and tetrahedral-Fe4C using molecular dynamics simulation. The chemical interactions of iron and carbon were described through an analytical bond order inter-atomic potential (ABOP) energy function. The indentations were performed at an indentation speed of 50 m/sec and a repeat trial was performed at 5 m/sec. Load-displacement (P-h) curve for both these carbides showed residual indentation depth and maximum indentation depth (hf/hmax) ratio to be higher than 0.7 i.e. a circumstance where Oliver and Pharr method was not appropriate to be applied to evaluate the material properties. Alternate evaluation revealed Fe3C to be much harder than Fe4C. Gibbs free energy of formation and radial distribution function, coupled with state of the average local temperature and von Mises stresses indicate the formation of a new phase of iron-carbide. Formation of this newer phase was found to be due to deviatoric strain rather than the high temperature induced in the substrate during nanoindentation

Solid-liquid interfacial free energy out of equilibrium

The properties of the interface between solid and melt are key to solidification and melting, as the interfacial free energy introduces a kinetic barrier to phase transitions. This makes solidification happen below the melting temperature, in out-of-equilibrium conditions at which the interfacial free energy is ill defined. Here we draw a connection between the atomistic description of a diffuse solid-liquid interface and its thermodynamic characterization. This framework resolves the ambiguities in defining the solid-liquid interfacial free energy above and below the melting temperature. In addition, we introduce a simulation protocol that allows solid-liquid interfaces to be reversibly created and destroyed at conditions relevant for experiments. We directly evaluate the value of the interfacial free energy away from the melting point for a simple but realistic atomic potential, and find a more complex temperature dependence than the constant positive slope that has been generally assumed based on phenomenological considerations and that has been used to interpret experiments. This methodology could be easily extended to the study of other phase transitions, from condensation to precipitation. Our analysis can help reconcile the textbook picture of classical nucleation theory with the growing body of atomistic studies and mesoscale models of solidification.

Liquid alumina: detailed atomic coordination determined from neutron diffraction data using empirical potential structure refinement

Greaves, George; Jenkins, T.E.; Landron, C.; Hennet, L., (2001) 'Liquid alumina: detailed atomic coordination determined from neutron diffraction data using empirical potential structure refinement', Physical Review Letters 86 pp.4839-4842 RAE2008

Coherent atomic oscillations and resonances between coupled Bose-Einstein condensates with time-dependent trapping potential

The atomic tunneling between two tunnel-coupled Bose-Einstein condensates (BECs) in a double-well time-dependent trap was studied. For the slowly varying trap, synchronization of oscillations of the trap with oscillations of the relative population was predicted. Using the Melnikov approach, the appearance of the chaotic oscillations in the tunneling phenomena between the condensates was confirmed.

Analytical functions for the calculation of hyperspherical potential curves of atomic systems

We present angular basis functions for the Schrödinger equation of two-electron systems in hyperspherical coordinates. By using the hyperspherical adiabatic approach, the wave functions of two-electron systems are expanded in analytical functions, which generalizes the Jacobi polynomials. We show that these functions, obtained by selecting the diagonal terms of the angular equation, allow efficient diagonalization of the Hamiltonian for all values of the hyperspherical radius. The method is applied to the determination of the 1S e energy levels of the Li + and we show that the precision can be improved in a systematic and controllable way. ©2000 The American Physical Society.

Investigating routes toward atomic layer deposition of silicon carbide: Ab initio screening of potential silicon and carbon precursors

Silicon carbide (SiC) is a promising material for electronics due to its hardness, and ability to carry high currents and high operating temperature. SiC films are currently deposited using chemical vapor deposition (CVD) at high temperatures 1500–1600 °C. However, there is a need to deposit SiC-based films on the surface of high aspect ratio features at low temperatures. One of the most precise thin film deposition techniques on high-aspect-ratio surfaces that operates at low temperatures is atomic layer deposition (ALD). However, there are currently no known methods for ALD of SiC. Herein, the authors present a first-principles thermodynamic analysis so as to screen different precursor combinations for SiC thin films. The authors do this by calculating the Gibbs energy ΔGΔG of the reaction using density functional theory and including the effects of pressure and temperature. This theoretical model was validated for existing chemical reactions in CVD of SiC at 1000 °C. The precursors disilane (Si2H6), silane (SiH4), or monochlorosilane (SiH3Cl) with ethyne (C2H2), carbontetrachloride (CCl4), or trichloromethane (CHCl3) were predicted to be the most promising for ALD of SiC at 400 °C.