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We develop a ReaxFF reactive force field used for the molecular dynamics simulations of thermophysical properties of liquid Cu and Zr metals. The ReaxFF parameters are optimized by fitting to the first-principles density-functional calculations on the equations of state for bulk crystal structures and surface energies. To validate the force field, we compare the ReaxFF results with those from experiments and embedded-atom-method (EAM) potentials. We demonstrate that the present ReaxFF force field well represents structural characteristics and diffusion behaviors of elemental Cu and Zr up to high-temperature liquid regions. It reasonably reproduces the thermodynamic processes associated with crystal-liquid interface. In particular, the equilibrium melting temperatures show better agreement with experimental measurements than the results from EAM potentials. The ReaxFF reactive force field method exhibits a good transferability to the nonreactive processes of liquid systems. [ABSTRACT FROM AUTHOR]

Liquids attain a metastable state without crystallizing by cooling rapidly to a given temperature below the melting point. With increasing supercooling, the nucleation rate would show an increase based on the prediction of the classical nucleation theory. It is generally thought that the nucleation rate will reach the maximum upon approaching the glass transition temperature, Tg, for glass-forming liquids. We report that there exists a supercooled region above Tg in which the crystallization has actually been severely suppressed. Our molecular dynamics simulations show that the growth of embryos in the supercooled Cu60Zr40 melt is subjected to a strong anisotropic stress associated with the dynamic heterogeneity. Its long-range effect drives the embryo to grow into a ramified morphology so that the interface energy dominates over the embryo growth, leading to the suppression of nucleation. [ABSTRACT FROM AUTHOR]

Crystal growth of the glass-forming CuZr alloy is shown to be abnormally slow, which suggests a new method to identify the good glass-forming alloys. The crystal growth of elemental Cu, Pd and binary NiAl, CuZr alloys is systematically studied with the aid of molecular dynamics simulations. The temperature dependence of the growth velocity indicates the different growth mechanisms between the elemental and the alloy systems. The high-speed growth featuring the elemental metals is dominated by the non-activated collision between liquid-like atoms and interface, and the low-speed growth for NiAl and CuZr is determined by the diffusion across the interface. We find that, in contrast to Cu, Pd, and NiAl, a strong stress layering arisen from the density and the local order layering forms in front of the liquid-crystal interface of CuZr alloy, which causes a slow diffusion zone. The formation of the slow diffusion zone suppresses the interface moving, resulting in much small growth velocity of CuZr alloy. We provide a direct evidence of this explanation by applying the compressive stress normal to the interface. The compression is shown to boost the stress layering in CuZr significantly, correspondingly enhancing the slow diffusion zone, and eventually slowing down the crystal growth of CuZr alloy immediately. In contrast, the growth of Cu, Pd, and NiAl is increased by the compression because the low diffusion zones in them are never well developed. [ABSTRACT FROM AUTHOR]

It is previously predicted that the equilibrium shape of a rotating liquid drop evolves from the axisymmetric to the two-, three-, and four-lobed morphologies as the angular velocity increases. Although the two- and three-lobed shape bifurcations have been observed in experiments, the four-lobed equilibrium shape is scarcely reported. Here, we investigate the multiple-lobed shape bifurcations of rotating drops by using acoustic levitation, and in particular, follow the evolution of the four-lobed equilibrium shape. A new shape family of rotating drops characterized by five-lobed bifurcation is also observed, which is unexpected in the theoretical predictions. A numerical method is employed to simulate the shape evolution of acoustically levitated and rotating drops. And the results validate the existence of bifurcation point shifts among all the lobed-shape families due to the initial drop flattening induced by the acoustic radiation pressure, which plays a decisive role in the emergence of the five-lobed shape. [ABSTRACT FROM AUTHOR]

The interdiffusion processes in liquid Al–Cu and Ni–Cu alloys are studied by using molecular dynamics simulation method. The Maxwell–Stefan (MS) diffusivities are calculated over a wide composition range with both the Green–Kubo method and the Darken relation. Comparisons show that the Darken relation predicts well the MS diffusivity for Ni–Cu alloy, while overestimates the value for Al–Cu alloy, especially in the medium concentration region. Based on the calculated MS diffusivities and the activities of the alloys, the Fickian interdiffusivities are predicted. The results show strong dependences on the compositions of the alloys. In addition, the Fickian interdiffusivities of Al60Cu40 and Ni50Cu50 melts as a function of undercooling are estimated, which is proved to be beneficial in improving the quantitative predictions of dendrite growth velocity in solidification. [ABSTRACT FROM AUTHOR]

The supercooling capability of aqueous NaCl and KCl solutions is investigated at containerless state by using acoustic levitation method. The supercooling of water is obviously enhanced by the alkali metal ions and increases linearly with the augmentation of concentrations. Furthermore, the supercooling depends on the nature of ions and is 2–3 K larger for NaCl solution than that for KCl solution in the present concentration range: Molecular dynamics simulations are performed to reveal the intrinsic correlation between supercoolability and microstructure. The translational and orientational order parameters are applied to quantitatively demonstrate the effect of ionic concentration on the hydrogen-bond network and ice melting point. The disrupted hydrogen-bond structure determines essentially the concentration dependence of supercooling. On the other hand, the introduced acoustic pressure suppresses the increase of supercooling by promoting the growth and coalescence of microbubbles, the effective nucleation catalysts, in water. However, the dissolved ions can weaken this effect, and moreover the degree varies with the ion type. This results in the different supercoolability for NaCl and KCl solutions under the acoustic levitation conditions. [ABSTRACT FROM AUTHOR]

The China Spallation Neutron Source started delivering neutron beams to users in March 2018. To upgrade the beam power to 500 kW and improve the performance of the ion source, an RF-driven negative hydrogen (H−) ion source is under development. The source has a silicon nitride ceramic plasma chamber surrounded by a 4.5-turn antenna. The plasma is ignited by a pulsed DC spark gap and then driven by a 2 MHz solid-state amplifier with a repetition rate of 25 Hz. The commissioning of the source started in January 2019. When uncesiated, it produced about 20 mA beam at an RF power of 32 kW and pulse width of 450 μs. This paper describes the configuration of the ion source, several peculiar technologies used in it, and the first negative hydrogen (H−) ion beam extraction. [ABSTRACT FROM AUTHOR]

In contrast to other glass formers, silicon exhibits a thermodynamic discontinuity between its liquid and amorphous solid states. Some researchers have conjectured that a first-order phase transition occurs between two forms of liquid silicon: the high-density liquid (HDL) and the low-density liquid (LDL). Despite the fact that several computer simulations have supported a liquid–liquid phase transition (LLPT) in silicon, recent work based on surface free energy calculations contradicts its existence and the authors of this work have argued that the proposed LLPT has been mistakenly interpreted [J. Chem. Phys., 2013, 138, 214504]. A similar controversy has also arisen in the case of water because of discrepancies in the calculation of its free energy surface [Nature, 2014, 510, 385; J. Chem. Phys., 2013, 138, 214504]. Current evidence supporting or not supporting the LLPT is mostly derived from the thermodynamic stability of the LDL phase. Provided that the HDL–LDL transition is a first-order transition, the formation of LDL silicon should be an activated process. Following this idea, the nature of the LLPT should be clarified by tracing the kinetic path toward LDL silicon. In this work, we focus on the transformation process from HDL to LDL phases and use the mean first passage time (MFPT) method to examine thermodynamic and dynamic trajectories. The MFPT results show that the presumed HDL–LDL transition is not characterized by a thermodynamic activated process but by a continuous dynamic transformation. LDL silicon is actually a mixture of the high-density liquid and a low-density tetrahedral network. We show that the five-membered Si–Si rings in the LDL network play a critical role in stabilizing the low-density network and suppressing the crystallization. [ABSTRACT FROM AUTHOR]

Using molecular dynamics simulations, we examine the crystallisation process of the extended simple point charge model (SPC/E) water confined in a charged Pt nanochannel. The presence of the external electric field enhances the surface layering of water and promotes the super-cooled water to crystallise intoIcice within tens of nanoseconds.Icice is found to nucleate from the interior of the water lamina, and the Pt(111) surfaces do not show a remarkable promotion ofIcice nucleation. Structural analysis reveals that a two-dimensional hydrogen-bond network is built among the water molecules absorbed on the charged Pt surfaces, which influences the bonding of the molecules between the first and the second layers, and disturbs the formation of tetrahedral structures that matchIcice, finally resulting in the nucleation-free near the walls. Such arrangements of water molecules remain in the subsequent growth ofIcice and cause the slowdown of growth velocity while approaching surfaces. [ABSTRACT FROM AUTHOR]

Ultrasonic levitation of some small living animals such as ant, ladybug, and young fish has been achieved with a single-axis acoustic levitator. The vitality of ant and ladybug is not evidently influenced during the acoustic levitation, whereas that of the young fish is reduced because of the inadequacy of water supply. Numerical analysis shows that the sound pressures on the ladybug’s surface almost reach the incident pressure amplitude p0 due to sound scattering. It is estimated that 99.98% of the acoustic energy is reflected away from the ladybug. The acoustic radiation pressure pa on the ladybug’s surface is only 1%–3% of p0, which plays a compression role on the central region and a suction role on the peripheral region. [ABSTRACT FROM AUTHOR]

The classical molecular dynamics method is applied to simulate the surface tension of water over the wide temperature range from 193 to 398 K. The results show that the temperature dependence of surface tension obviously increases in supercooled region, and there exists a second inflection point in the surface tension-temperature curve at about 303 K similar to the first one at 525 K, which confirms the conjecture based on the experimental measurements. When the temperature approaches 180 K, the simulated surface tension displays a sharp rise like the anomalies of some bulk thermodynamic properties. [ABSTRACT FROM AUTHOR]

The supercooling and nucleation of acoustically levitated water drops were investigated at two different sound pressure levels (SPL). These water drops were supercooled by 13 to 16 K at the low SPL of 160.6 dB, whereas their supercoolings varied from 5 to 11 K at the high SPL of 164.4 dB. The maximum supercooling obtained in the experiments is 32 K. Statistical analyses based on the classical nucleation theory reveal that the occurrence of ice nucleation in water drops is mainly confined to the surface region under acoustic levitation conditions and the enlargement of drop surface area caused by the acoustic radiation pressure reduces water supercoolability remarkably. A comparison of the nucleation rates at the two SPLs indicates that the sound pressure can strengthen the surface-dominated nucleation of water drops. The acoustic stream around levitated water drops and the cavitation effect associated with ultrasonic field are the main factors that induce surface-dominated nucleation. [ABSTRACT FROM AUTHOR]

The density layering phenomenon originating from a free surface gives rise to the layerlike dynamics and stress heterogeneity in ultrathin Cu-Zr glassy films, which facilitates the occurrence of multistep relaxations in the timescale of computer simulations. Taking advantage of this condition, we trace the relaxation decoupling and evolution with temperature simply via the intermediate scattering function. We show that the β relaxation hierarchically follows fast and slow modes in films, and there is a β-relaxation transition as the film is cooled close to the glass transition. We provide the direct observation of particle motions responsible for the β relaxation and reveal the dominant mechanism varying from the thermal activated to the cooperative jumps across the transition. [ABSTRACT FROM AUTHOR]

Fivefold symmetry is supposed to have an important role in suppressing crystallization and promoting glass transition due to its structural incompatibility with crystal. In this paper, we study the correlation between the fivefold symmetry and the dynamical slowing down in glass-forming Cu-Zr liquids using the single-particle dynamics method based on molecular dynamics simulations. The dynamics of the glass-forming liquids is microscopically characterized by the jump cage motion for individual atoms; moreover, the cooperative jumps become more pronounced upon approaching the glass transition temperature. We find that the role of fivefold symmetry in the dynamical slowing down does not lie in caging atomic motion but, more importantly, in suppressing cooperative jumps. The atoms with a high degree of fivefold symmetry and involved in jump motions appear more sluggish compared to other jumps. This behavior significantly suppresses the cooperative jumps around them, leading to the slowing down of fast dynamics. The degree of suppression has a close relation to the glass-forming ability and contributes to the "strong" character of liquids. [ABSTRACT FROM AUTHOR]

The single-particle dynamics of the glass-forming Cu50Zr50 alloy, from the supercooled liquid well above the glass-transition temperature, Tg to the glassy state, is studied by using the molecular dynamics simulations. When the liquid is cooled below 1.2Tg, the dynamics heterogeneity characterized by the cage-jump motion becomes increasingly pronounced. The analyses based on the continuous time random walk method indicate that the liquid falls out of equilibrium in the present simulation time scale when it is cooled into the regime below 1.02Tg. However, we find that the jump length and the jump rate do not display the non-equilibrium behaviors even in the glassy state below Tg, which allows us to study the intrinsic dynamic characteristics through Tg. The mean waiting time between two successive jumps has a rapid growth following the Vogel-Fulcher-Tammann law as the non-equilibrium regime is approached, in analogy with the temperature behaviors of transport properties for fragile supercooled liquids. In contrast, the jump rate maintains the Arrhenius decay and the jump length has even a weaker temperature dependence when the liquid is cooled into glassy state. We find that a pronounced enhancement of the spatial correlation of jumps occurs accompanied by the glass transition: the string-like cooperative jumps dominate the fast motion instead of the uncorrelated and individual jumps. Our work offers an insight into the equilibrium effect of the single-particle dynamics in glass transition. [ABSTRACT FROM AUTHOR]

The dynamics of N(2) molecules blocked in open single-walled carbon nanotubes is investigated using molecular dynamics simulations. It is found that periodic axial and radial oscillations with extremely high frequency exist widely among these molecules. Between the two nanotube ends, N(2) molecules oscillate along or parallel to the nanotube axis, and their frequencies show an inverse length dependence in the range of 22 to 64 GHz. Accompanying the axial oscillation, the molecules oscillate radially with small amplitudes in the lateral potential well. The corresponding frequencies have a magnitude of several hundred gigahertz, and the maximum exceeds 1800 GHz. These periodic oscillations contribute to the molecular blockage in nanotubes.

Samples of Pb-Sn eutectic alloy with a high density of 8.5 x 10(3) kg/m(3) are levitated with a single-axis acoustic levitator, and containerlessly melted and then solidified in argon atmosphere. High undercoolings up to 38 K are obtained, which results in a microstructural transition of "lamellas-broken lamellas-dendrites." This transition is further investigated in the light of the coupled zone for eutectic growth and the effects of ultrasound. The breaking of regular eutectic lamellas and suppression of gravity-induced macrosegregation of (Pb) and (Sn) dendrites are explained by the complicated internal flow inside the levitated drop, which is jointly induced by the shape oscillation, bulk vibration and rotation of the levitated drop. The ultrasonic field is also found to drive forced surface vibration, which subsequently excites capillary ripples and catalyzes nucleation on the sample surface.

Single-axis acoustic levitation of the heaviest solid (iridium, rho=22.6 g cm(-3)) and liquid (mercury, rho=13.6 g cm(-3) on the Earth is achieved by greatly enhancing both the levitation force and stability through optimizing the geometric parameters of the levitator. The acoustically levitated Pb-Sn eutectic alloy melt (rho=8.5 g cm(-3)) is highly undercooled by up to 38 K, which results in a microstructural transition of "lamellae-broken lamellae-dendrites." The drastic enhancement of levitation capability indicates a broader application range of single-axis acoustic levitation.