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We designed catalytic nanorotors and investigated the rotational motion and energy conversion efficiency using reactive molecular dynamics in two dimensions. First, a two-arm nanorotor was constructed by decorating a slender beam with catalysts asymmetrically on its two long edges, while fixing the beam center as the rotational axis. Autonomous rotation was observed for the two-arm nanorotor immersing in a fuel environment. Here fuel molecules undergo exothermic combination reaction facilitated by the catalysts. It was found that the angular velocity increases with the catalyst coverage parabolically, while the rotary nanomotor efficiency stays roughly constant. These observations are consistent with a single-collision-momentum-transfer-based propulsion model. Furthermore, multi-arm nanorotors (up to eight arms) were constructed by carving radially distributed arms followed by decorating catalysts. For multi-arm nanorotors, both the angular velocity and the efficiency decrease as the number of arms increases. These behaviors contradict the aforementioned model, which are likely due to the deceleration from secondary collisions between products and the nanorotor arms. Our simulation results show that the optimal design for a nanorotor that maximizes its angular velocity and the motor efficiency is a two-arm nanorotor with nearly full coverage of catalysts.

Temperature‐dependent viscosity is critical to decipher two profound questions in condensed matter physics, namely the glass transition and the relaxation of amorphous solids. However, direct measurement of viscosity over a large temperature range is extremely difficult. Here, using classical molecular dynamics (MD) simulations, we report a novel method to calculate the equilibrium viscosity of supercooled liquid both above and below the glass transition temperature (Tg) and to estimate the nonequilibrium viscosity of glass down to room temperature. Based on the shoving model, we derived an analytical formula showing that the shear viscosity in logarithmic scale changes linearly with the shear‐induced variation in shear modulus or potential energy of the glass‐forming system. The shear viscosity as a function of steady‐state potential energy of liquid under different shear strain rates can be directly calculated in MD simulations; together with its equilibrium potential energy, one can extrapolate the zero‐strain‐rate equilibrium viscosity. We verified the proposed model by reliably calculating equilibrium viscosity near Tg of four glass‐forming systems (Kob–Andersen system, silica, Cu45.5Zr45.5Al9, and silicon) with different fragilities. Furthermore, our model can estimate the nonequilibrium viscosity of glass below Tg; the upper‐bound nonequilibrium viscosity of amorphous silica and silicon at room temperature are calculated to be ~1032 and 1025 Pa·s, respectively. [ABSTRACT FROM AUTHOR]

The strength of nanocrystalline (NC) metal has been found to be sensitive to strain rate. Here, by molecular dynamics simulation, we explore the strain rate effects on apparent Young's modulus, flow stress and grain growth of NC gold with small size. The simulation results indicate that the apparent Young's modulus of NC gold decreases with the decrease of strain rate, especially for strain rates above 1 ns−1. The rearrangement of atoms near grain boundaries is a response to the decrease of apparent Young's modulus. Indeed, the flow stress is also sensitive to the strain rate and decreases following the strain rate's decrease. This can be found from the change of strain rate sensitivity and activation volume with the strain rate. Temperature has little effect on the activation volume of NC gold with small grain size, but has an obvious effect on that of relatively large grain size (such as 18 nm) under low strain rate (0.01 ns−1). Finally, grain growth in the deformation process is found to be sensitive to strain rate and the critical size for grain growth increases following the decrease of strain rate. [ABSTRACT FROM AUTHOR]

Abstract: The yielding of metallic glasses (MGs) is generally considered to be dependent on pressure or normal stress, which is usually described by the Mohr–Coulomb yield criterion. Experimentally, the orientation of the shear band angle or fracture angles in MG is roughly 40° to 42° in uniaxial compression and 50° to 59° in uniaxial tension, both with respect to the loading direction. However, the amounts of deviation from the maximum shear stress direction (45°) under uniaxial tension and compression are quite different, which seems to be inconsistent with the Mohr–Coulomb yield criterion. Here we carried out molecular dynamics simulations to study the incipient plasticity of model Dzugutov glasses subjected to various uniaxial and biaxial stress states. The yield points are defined as the sharp rise of the population of atoms participating in non-affine deformations, in addition to the traditional offset method. The yield surfaces thus created agree well with the Mohr–Coulomb yield criterion. Furthermore, the orientations of the embryonic shear bands (instead of the fully-grown shear bands, or fracture planes) are measured, which also follow predictions based on the Mohr–Coulomb criterion. Our simulation results call for orientational measurement of incipient shear bands in experiments to critically examine the Mohr–Coulomb yield criterion in MG systems. [Copyright &y& Elsevier]

[Display omitted] Al 2 O 3 –SiO 2 binary glasses with structural and chemical heterogeneities were prepared by consolidating glassy nanoparticles with different compositions using classical molecular dynamics simulations. Consolidated glasses show both excellent ductility and enhanced flow strength. It was found that the structural heterogeneities such as over-coordinated network formers and neighboring oxygen induced during consolidation serve as plasticity carriers to increase the ductility. On the other hand, the enhanced yield strength in consolidated glasses is due to the chemical heterogeneity inherited from the starting glassy nanoparticles, which does not compromise the ductility. Furthermore, apparent work hardening behavior appears upon cold work in the consolidated glasses, with an increase in yield strength from ~3.3 to ~6.4 GPa after 40% cold work. Consolidation with glassy nanoparticles could be a viable way to synthesize strong, damage resistant and transparent oxide glasses that cannot be obtained through the traditional melt-quench process. [ABSTRACT FROM AUTHOR]

Metallic glasses, like many other glasses and ceramics, generally have zero tensile ductility. Distinct from common wisdom which seeks to toughen brittle glasses with ductile phases, we show that composites made from two brittle glasses can be ductile, strong and even harden under tension as a result of microstructure and stiffness contrast. Using molecular dynamics simulations, we designed and tested composites consisting of alternating wavy nanofilaments of two brittle glasses with different stiffness. The composites exhibit failure strain over 40% and strain hardening modulus over 2 GPa. The tensile ductility is determined by both the flexibility of nanofilaments and the propensity of crack deflection, with which a ductile composite design map can be constructed. The strain hardening is due to filament alignment analogous to linear polymer. The brittle-brittle composite design strategy proposed here is not material-specific and should be applicable to, for instance, ultra-hard materials as constituents in achieving new damage-resistance composites. [Display omitted] [ABSTRACT FROM AUTHOR]

Surface diffusion is an interesting and practically important phenomenon in many areas of chemistry and physics. One emerging problem is that of Li wetting on high-Z plasma-facing metals, especially tungsten, for application in tokamak reactors. We report construction of a Li-W binary force field to describe the interface between liquid Li and solid W. We show that this force field can simulate well the atomic processes of liquid Li diffusing across the W surface, as compared with results from first-principles calculations. We find that Li atoms have different wetting behaviors on different surfaces of W. Diffusion is the fastest on the (110) surface and the slowest on the (100) surface. Diffusion of second Li layer on the (110) surface is activated at relatively low temperatures above 262 K. In addition, the diffusion rate of single-layer Li changes with the coverage rate of Li on (110) surface. High diffusion coefficients at coverage rate of θ = 1/6, 1/3 and 1 are found to be related to the 2D phase transition of Li on the (110) surface. [ABSTRACT FROM AUTHOR]