Molecular dynamics studies of the kinetics of phase changes in clusters III: structures, properties, and crystal nucleation of iron nanoparticle Fe331

Molecular dynamics (MD) computer simulations have been carried out to study the structures, properties, and crystal nucleation of iron nanoparticles with 331 Fe atoms or with diameter around 2 nm. Structure information for the nanoparticles was analyzed from the MD simulations. Three crystalline phases and one amorphous phase were obtained by cooling the nanoparticles from their molten droplets at different cooling rates or with different lengths of cooling time periods. Molten droplets froze into three different solid phases and a solid-solid transition from a disordered body-centered cubic (BCC) phase to an ordered BCC phase were observed during the slow cooling and the quenching processes. Properties of nanoparticle Fe₃₃₁, such as melting point, freezing temperature, heat capacity, heat of fusion, heat of crystallization, molar volume, thermal expansion coefficient, and diffusion coefficient, have been estimated. Nucleation rates of crystallization to two solid phases for Fe₃₃₁ at temperatures of 750, 800, and 850 K are presented. Both classical nucleation theory and diffuse interface theory are used to interpret our observed nucleation results. The interfacial free energy and the diffuse interface thickness between the liquid phase and two different solid phases are estimated from these nucleation theories.     

Phase separation and structure transition of undercooled Fe75Cu25 melts

The rapid quenching processes of Fe₇₅Cu₂₅ melt at different cooling rate are investigated by molecular dynamics simulation based on embedded atom method. Fe₇₅Cu₂₅ alloy ribbons are prepared by single roller rapid quenching. Liquid–liquid phase separation (LLPS) happens and the Cu-rich droplets embedded in the Fe-rich matrix can be observed both in simulation and experiments. Stronger interaction of homogeneous atom pairs than that of heterogeneous atom pairs leads to LLPS, controlled by nucleation growth mechanism in Fe₇₅Cu₂₅ melt, and quite different from that in Fe₅₀Cu₅₀ melt, which is controlled by spinodal decomposition mechanism. During the crystallisation process after LLPS, the new nuclei form only in Fe-rich regions; various multiply twinning boundaries are formed due to the minimisation of interfacial energy and only the homogeneous atomic stacking shows mirror symmetry along twinning boundary. The results provide atomic-scale understanding of phase separation mechanism and structure transition of Fe₇₅Cu₂₅ melt during rapid cooling processes.     

Atomistic mechanisms of phase separation and formation of solid solutions: model studies of NaCl, NaCl-NaF, and Na(Cl1-xBrx) crystallization from the melt

The crystallization of sodium chloride from its melt and mixtures with other sodium halides is investigated by means of transition path sampling molecular dynamics simulations. From this we explore the nucleation mechanisms of both the solidification and the melting process at the atomistic level of detail. By incorporation of impurities the nucleation picture of the eutectic mixtures changes considerably. Doping the NaCl crystal with fluoride ions, we observed the substitutional defects to act as favored nucleation centers for the melting transition. This phenomenon plays a critical role during the solidification process of NaCl-NaF melts of low NaF concentration and is demonstrated to account for the segregation of fluoride ions. While NaCl-NaF corresponds to a eutectic system, we also investigated NaCl-NaBr mixtures. The bromide ions were observed to behave very similarly to chloride ions. As a consequence, no phase separation occurs and Na(Cl1-xBrx) solid solutions are formed. At the example of these two prototypes we demonstrate the study of the atomistic mechanisms related to phase separation processes and solid solution formation during the nucleation and growth of crystals from multinary melts.     

On the behaviour of molecules of the quinoline group at the water—Mercury interface: Part III. The kinetics of the isoquinoline transition in the presence of an “inert” electrolyte

The reorientation transition, which involves two distinct monolayers of isoquinoline molecules, has been investigated by using the single-potential step and the double-potential step methods under various experimental conditions.The cathodic transients correspond to a liquid → solid transition. Their morphology and half-times have been measured at various initial and final potentials, and for several isoquinoline concentrations close to the saturation value. At small overvoltages, the reorientation process is determined by heterogeneous (“instantaneous”) nucleation, while progressive nucleation and growth prevail at higher potentials. Recourse to the double-potential step method affords an efficient way of assessing separately the influence of the overvoltage on the rate constants for nucleation and growth. Determination of the critical nucleus size and the activation energy for each of the two processes has been based on an adaptation of the Brandes theory developed for two-dimensional crystallization from a supersaturated vapour phase. The kinetics are markedly dependent on the initial state, which can be easily controlled by the potential and the bulk concentration. When the potential is stepped from the region where there is superadsorption, progressive nucleation is particularly fast, while starting from a partially depleted layer gives much slower transients with extensive tailing due to diffusional hindrance.The anodic transients show consonant characteristics which indicate that the initial film can be considered as a two-dimensional solid, and that boundary defects are acting as nucleation centres, during the solid → liquid transition triggered by the potential step.     

Nucleation and Growth of Amino Acid and Peptide Supramolecular Polymers through Liquid–Liquid Phase Separation

The transition of peptides and proteins from the solution phase into fibrillar structures is a general phenomenon encountered in functional and aberrant biology and is increasingly exploited in soft materials science. However, the fundamental molecular events underpinning the early stages of their assembly and subsequent growth have remained challenging to elucidate. Here, we show that liquid–liquid phase separation into solute‐rich and solute‐poor phases is a fundamental step leading to the nucleation of supramolecular nanofibrils from molecular building blocks, including peptides and even amphiphilic amino acids. The solute‐rich liquid droplets act as nucleation sites, allowing the formation of thermodynamically favorable nanofibrils following Ostwald's step rule. The transition from solution to liquid droplets is entropy driven while the transition from liquid droplets to nanofibrils is mediated by enthalpic interactions and characterized by structural reorganization. These findings shed light on how the nucleation barrier toward the formation of solid phases can be lowered through a kinetic mechanism which proceeds through a metastable liquid phase.     

液态合金NiAl凝固过程中微观结构转变的分子动力学模拟

采用分子动力学模拟方法对液态NiAl凝固过程进行了研究,考察了不同冷却速度下液态NiAl结构变化特点,原子间相互作用势采用F-S多体势,结构分析采用键取向序和对分析技术.计算结果表明,冷却速度对液态NiAl结构转变有重要影响,在不同的冷却速度下, NiAl凝固过程出现了明显不同,冷速为4×1013和4×1012 K/s时, NiAl快速凝固为无序的非晶体结构;而在较慢的8×1011 K/s冷速下, NiAl凝固为晶态结构.给出了不同冷却速度下液态NiAl结构转变的微观信息.     

金属间化合物Al~3Fe熔体结构的温度变化特性研究

利用分子动力学模拟技术,详细考察了在快速凝固条件下AL~3Fe熔体结构的温度变化特征。结果表明:Al~3Fe熔体中存在不同类型的原子基团.原子集团是以各种各样的键对和多面体的形式存在的.利用键对分析技术,计算出了不同温度下的键对类型数和二十面体的两类键取向序参数,分析了Al-Fe合金在快速凝固条件下非晶形成的演化特点。     

水性聚氨酯相变储能材料的相变机理

以聚乙二醇(PEG)为软段、六亚甲基二异氰酸酯-1,4-丁二醇-二羟甲基丙酸(HDI-BDO-DMPA)为硬段,制备了一系列水性聚氨酯相变储能材料(WPUPCM)。在相变过程中,由于软段PEG由聚合前的固-液相变转化为聚合后的固-固相变,因此,所制备的WPUPCM表现出固-固相变特性。为了研究其固态相变行为的本质和形成机理,揭示该类聚合物的能量贮存和转换机理,应用偏光显微镜、扫描电镜、原子力显微镜、X射线衍射仪分析研究了PEG、WPUPCM在相变过程中的晶体结构变化特征,讨论了WPUPCM固-固相转变的机理。     

Atomistic in situ investigation of the morphogenesis of grains during pressure-induced phase transitions: molecular dynamics simulations of the B1-B2 transformation of RbCl

The mechanistic details of the pressure-induced B1-B2 phase transition of rubidium chloride are investigated in a series of transition path sampling molecular dynamics simulations. The B2→B1 transformation proceeds by nucleation and growth involving several, initially separated, nucleation centers. We show how independent and partially correlated nucleation events can function within a global mechanism and explore the evolution of phase domains during the transition. From this, the mechanisms of grain boundary formation are elaborated. The atomic structure of the domain-domain interfaces fully support the concept of Bernal polyhedra. Indeed, the manifold of different grain morphologies obtained from our simulations may be rationalized on the basis of essentially only two different kinds of Bernal polyhedra. The latter also play a crucial role for the B1→B2 transformation and specific grain boundary motifs are identified as preferred nucleation centers for this transition.     

原子尺寸差异与非晶形成能力

采用分子动力学模拟技术，研究了纯Au及AuCu合金的熔化、非晶化和晶化过程.模拟结果表明,在冷却速率为5×1011 K•s－1至4×1012 K•s－1的范围内，液态Au总是形成晶体，且冷速越快，结晶温度越低；而AuCu合金则形成非晶，且冷速越快，非晶转变温度越高.验证了原子尺寸的不匹配有利于非晶形成这一规律.     

Molecular dynamics study of gas hydrate formation

Long-time-scale molecular dynamics simulations are presented of the spontaneous formation of methane hydrate at a methane/liquid water interface. The water film was prepared at 300 K, 30 bar and showed no significant hydrate order. On crash cooling to 250 K, 300 bar (about 20 K subcooling), the system showed a rapid growth of hydrate clusters. Contrary to popular models for hydrate nucleation, the clusters formed first as two-dimensional arrangements and only later into three-dimensional cage structures; the results are, however, consistent with the local order model proposed recently. The hydrate clusters showed clear signatures of the type II hydrate structure even though the type I structure is the thermodynamically stable form for methane hydrate; this is in accord with the results of recent diffraction experiments.     

Thermodynamics of crystalline nanonucleus formation from liquid phase

The energy of crystal nucleation from liquid phase was considered, with the following two stages taken into account: (1) the formation of metastable supercooled melt (solution), containing pre-nuclei with intermediate amorphous (quasicrystalline) structure, and (2) the transformation of amorphous clusters into solid crystalline nuclei having different structures. With growth of a nucleus the nucleation energy profile manifests 2–3 maxima corresponding to these stages, and the kinetics of the non-stationary nucleation has five characteristic variations.     

Powder X‐Ray Diffraction Investigation of Xylazine Hydrochloride Solid Phase Transformation Kinetics

The kinetics of the solid‐state phase transformation of xylazine hydrochloride form X to A has been investigated using powder X‐ray diffraction and differential thermal analysis. Three different kinetic models have been used to describe transition kinetics: the Avrami–Erofeev equation, the Cardew equation, and the methodology for simulation of solid‐state phase transition kinetics by the combination of nucleation and nuclei growth processes. The latter has been recently developed and has been tested in this paper for the case of a real solid‐state transition. The relative humidity, mechanical pressure, temperature, and sample‐preparation effect on phase‐transition kinetics have been investigated, and rate constant changes have been analyzed.     

金属Cu液固转变及晶体生长的分子动力学模拟

采用分子动力学模拟研究了液态Ｃｕ在不同冷却速度下的凝固特点,模拟采用ＥＡＭ作用势,计算了不同温度,不同冷却速度下Ｃｕ的偶相关函数,结果表明ＥＡＭ作用势能很好地描述液态Ｃｕ的结构特征,当冷却速度较快时,液Ｃｕ形成非晶;当冷却速度较慢时,液Ｃｕ形成晶体,分析了不同冷却速度下体系的相变热力学及相变动力学过程,最后采用液固两层构型法,描述了Ｃｕ晶体的生长过程。     

Molecular dynamics studies of the transient nucleation regime in the freezing of (RbCl)108 clusters

The freezing of supercooled liquids in the transient period before a steady state of nucleation is attained has been the subject of a number of theoretical treatments. To our knowledge, no published experimental studies or computer simulations have been carried out in sufficient detail to test definitively the behavior predicted by the various theories. The present molecular dynamics (MD) simulation of 375 nucleation events in small, liquid RbCl clusters, however, yields a reasonably accurate account of the transient region. Despite published criticisms of a 1969 treatment by Kashchiev, it turns out that the behavior observed in the present study agrees with that predicted by Kashchiev. The study also obtains a much more accurate nucleation rate and time lag than reported for MD studies of RbCl previously published in this journal. In addition, it provides estimates of the solid–liquid interfacial free energy and the Grànàsy thickness of the diffuse solid–liquid interface.     

When hydroquinone meets methoxy radical: Hydrogen abstraction reaction from the viewpoint of interacting quantum atoms

Interacting Quantum Atoms methodology is used for a detailed analysis of hydrogen abstraction reaction from hydroquinone by methoxy radical. Two pathways are analyzed, which differ in the orientation of the reactants at the corresponding transition states. Although the discrepancy between the two barriers amounts to only 2 kJ/mol, which implies that the two pathways are of comparable probability, the extent of intra‐atomic and inter‐atomic energy changes differs considerably. We thus demonstrated that Interacting Quantum Atoms procedure can be applied to unravel distinct energy transfer routes in seemingly similar mechanisms. Identification of energy components with the greatest contribution to the variation of the overall energy (intra‐atomic and inter‐atomic terms that involve hydroquinone's oxygen and the carbon atom covalently bound to it, the transferring hydrogen and methoxy radical's oxygen), is performed using the Relative energy gradient method. Additionally, the Interacting Quantum Fragments approach shed light on the nature of dominant interactions among selected fragments: both Coulomb and exchange‐correlation contributions are of comparable importance when considering interactions of the transferring hydrogen atom with all other atoms, whereas the exchange‐correlation term dominates interaction between methoxy radical's methyl group and hydroquinone's aromatic ring. This study represents one of the first applications of Interacting Quantum Fragments approach on first order saddle points. © 2018 Wiley Periodicals, Inc.     

Thermal and optical properties of newly synthesized dicholesteryl esters with a phenylene oxide link in the normal and solidified cholesteric phases

A series of liquid crystalline compounds with a central phenylene oxide group to which two cholesteryl groups are attached via two alkanoate spacers has been synthesized and investigated. All the compounds except the one with ethanoate spacers showed a cholesteric phase as a single mesophase. The phase transition temperatures, the corresponding enthalpy changes and the wavelengths of selective reflection associated with the cholesteric phases showed a strong odd-even effect as a function of the spacer length. By rapid cooling of the compounds from the cholesteric phase to 0°C, solid films maintaining a cholesteric molecular order were obtained. At room temperature, the solid film showed stable cholesteric colours controlled by changing the temperature at which the rapid cooling was begun. Heating the cholesteric solid film of the compound with hexanoate spacers gave two forms of crystals above 80°C, whose ratio changed depending on the colour of the starting solid films. This result suggests the existence of two conformational isomers in the liquid crystalline state. Since it is possible repeatedly to fix stable red, green and blue colours by thermal treatment of this compound, we may apply it to a rewritable full colour recording in the thermal mode.     

Thermal and optical properties of newly synthesized dicholesteryl esters with a phenylene oxide link in the normal and solidified cholesteric phases

A series of liquid crystalline compounds with a central phenylene oxide group to which two cholesteryl groups are attached via two alkanoate spacers has been synthesized and investigated. All the compounds except the one with ethanoate spacers showed a cholesteric phase as a single mesophase. The phase transition temperatures, the corresponding enthalpy changes and the wavelengths of selective reflection associated with the cholesteric phases showed a strong odd-even effect as a function of the spacer length. By rapid cooling of the compounds from the cholesteric phase to 0°C, solid films maintaining a cholesteric molecular order were obtained. At room temperature, the solid film showed stable cholesteric colours controlled by changing the temperature at which the rapid cooling was begun. Heating the cholesteric solid film of the compound with hexanoate spacers gave two forms of crystals above 80°C, whose ratio changed depending on the colour of the starting solid films. This result suggests the existence of two conformational isomers in the liquid crystalline state. Since it is possible repeatedly to fix stable red, green and blue colours by thermal treatment of this compound, we may apply it to a rewritable full colour recording in the thermal mode.     

Structural investigations of metal clusters by HREM

Silver clusters grown by the inert gas aggregation technique have been investigated by HREM. Undistorted cuboctahedra and icosahedra together with twinned particles have been observed. Three types of nucleation and growth mechanisms are proposed which can explain the observed particle structures. The untwinned large particles are created by the coalescence of liquid- like small clusters. The twins are produced by the coalescence of solid subunits.     

Dynamics of Monolayer–Island Transitions in 2,7‐Dioctyl‐benzothienobenzthiophene Thin Films

The order in molecular monolayers is a crucial aspect for their technological application. However, the preparation of defined monolayers by spin‐coating is a challenge, since the involved processes are far from thermodynamic equilibrium. In the work reported herein, the dynamic formation of dioctyl‐benzothienobenzothiophene monolayers is explored as a function of temperature by using X‐ray scattering techniques and atomic force microscopy. Starting with a disordered monolayer after the spin‐coating process, post‐deposition self‐reassembly at room temperature transforms the initially amorphous layer into a well‐ordered bilayer structure with a molecular herringbone packing, whereas at elevated temperature the formation of crystalline islands occurs. At the temperature of the liquid‐crystalline crystal–smectic transition, rewetting of the surface follows resulting in a complete homogeneous monolayer. By subsequent controlled cooling to room temperature, cooling‐rate‐dependent kinetics is observed; at rapid cooling, a stable monolayer is preserved at room temperature, whereas slow cooling causes bilayer structures. Increasing the understanding and control of monolayer formation is of high relevance for achieving ordered functional monolayers with defined two‐dimensional packing, for future applications in the field of organic electronics.