Molecular dynamics simulation of the homogeneous crystallization of liquid rubidium

The homogeneous crystallization of liquid rubidium models containing 500, 998, and 1968 particles in the basic cube was studied by the molecular dynamics method. The liquid crystallized over the temperature range 70–182.5 K predominantly with the formation of a body centered cubic (BCC) structure. The mechanism of crystallization was different from that accepted in classic nucleation theory. Crystallization developed as an increase in the number of atoms with Voronoi polyhedra of the 0-6-0-8 and 0-4-4-6 types, the formation of bound groups (clusters) from these atoms, and growth of these groups as in the coagulation of an impurity from a supersaturated solution. At the initial stage, bound groups had a very loose structure and included a fairly large number of atoms with polyhedra of other types. The linear dimension of the largest group rapidly approached the basic cube size. The atoms with the 0-6-0-8 and 0-4-4-6 Voronoi polyhedra played a leading role in crystallization and activated the transition of bound group atoms with other coordination types into a BCC coordination. The probability of formation of a bound group of a given size was found to be independent of the volume of the liquid model. Cluster size fluctuations especially strong over the temperature range 180–185 K played an important role in the formation of 0608 clusters of a threshold (“critical”) size.     

Molecular simulation of the crystallization of aluminum from the supercooled liquid

We report hybrid Monte Carlo molecular simulation results on the crystallization of aluminum from the supercooled liquid. We simulate the entire crystallization process at P=1 atm and at temperatures 20% and 15% below the melting temperature. We demonstrate that crystallization takes place according to the same mechanism for the two degrees of supercooling considered in this work. We show that both nucleation and growth proceed into a random mixing of the hexagonal close packed structure and of the face centered cubic (fcc) phase, with a predominance of the stable fcc form. The concentration of icosahedral (Ih)-like atoms in the supercooled liquid is found to remain constant throughout nucleation and growth, showing that Ih-like atoms do not play an active role in the crystallization process. We also find that the crystallization mechanism of aluminum differs from that observed for simple fluids. While nucleation of simple fluids first proceeds into the metastable body centered cubic (bcc) phase, the fraction of bcc-like atoms in aluminum crystallites always remains very low.     

Supercooled states in the benzene-naphthalene system

Precrystallization supercooled states in the benzene-naphthalene system were studied by thermal analysis methods. A sharp change in the character of the crystallization of benzene and naphthalene, from quasi-equilibrium without supercooling to nonequilibrium-explosive with supercooling depending on liquid phase superheating with respect to the temperature of fusion was observed. Such a transition occurred monotonically for alloys. The phase diagram of the benzene-naphthalene system including metastable regions with supercooling under normal crystallization conditions was constructed. The rules governing Gibbs energy changes and mean supercoolings in crystallization depending on the concentrations of the components were determined. The supercooling values were also used to calculate such nucleation parameters as the size of critical nuclei, work of their formation, and number of unit cells and molecules in them. The molecular structure of benzene and naphthalene crystals and the structural changes of these substances during fusion that influenced the character of their subsequent crystallization were described.     

Comparative study of microstructural evolution during melting and crystallization

Molecular dynamics simulations, with the interaction between atoms described by a modified analytic embedded atom method, have been performed to obtain the atomic-scale details of isothermal melting in nanocrystalline Ag and crystallization from supercooled liquid. The radial distribution function and common neighbor analysis provide a visible scenario of structural evolution in the process of phase transition. The results indicate that melting at a fixed temperature in nanocrystalline materials is a continuous process, which originates from the grain boundary network. With the melting developing, the characteristic bond pairs (555), (433), and (544), existing in liquid or liquidlike phase, increase approximately linearly till completely melted. The crystallization from supercooled liquid is characterized by three characteristic stages: nucleation, rapid growth of nucleus, and slow structural relaxation. The homogeneous nucleation occurs at a larger supercooling temperature, which has an important effect on the process of crystallization and the subsequent crystalline texture. The kinetics of transition from liquid to solid is well described by the Johnson-Mehl-Avrami equation.     

Adsorption of Thiourea and Formation of Nickel-thiourea Complexes at Initial Stage of Nickel Deposition

The effect of thiourea (TU) on the nickel deposition process was analyzed by means of linear-sweep voltammetry. Raman spectroscopy and infrared reflectance spectroscopy were used to investigate the adsorption of TU and the formation of nickel-TU complexes on copper surface. The experimental results indicate that the nucleation and the preceding conversion step are involved in the deposition of nickel on copper electrodes. TU makes the onset nucleation potential negative due to the formation of nickel-TU complexes. which can accelerate the nickel deposition. Moreover, the S atom in the TU molecule adsorbed on copper surface facilitates the coordination of TU to Ni^2 . Meanwhile, TU might be adsorbed at a flatter orientation if no Ni^2 is on the surface, while at a perpendicular orientation when Ni^2 is coadsorbed.     

Nonequilibrium melting and crystallization of a model Lennard-Jones system

Nonequilibrium melting and crystallization of a model Lennard-Jones system were investigated with molecular dynamics simulations to quantify the maximum superheating/supercooling at fixed pressure, and over-pressurization/over-depressurization at fixed temperature. The temperature and pressure hystereses were found to be equivalent with regard to the Gibbs free energy barrier for nucleation of liquid or solid. These results place upper bounds on hysteretic effects of solidification and melting in high heating- and strain-rate experiments such as shock wave loading and release. The authors also demonstrate that the equilibrium melting temperature at a given pressure can be obtained directly from temperatures at the maximum superheating and supercooling on the temperature hysteresis; this approach, called the hysteresis method, is a conceptually simple and computationally inexpensive alternative to solid-liquid coexistence simulation and thermodynamic integration methods, and should be regarded as a general method. We also found that the extent of maximum superheating/supercooling is weakly pressure dependent, and the solid-liquid interfacial energy increases with pressure. The Lindemann fractional root-mean-squared displacement of solid and liquid at equilibrium and extreme metastable states is quantified, and is predicted to remain constant (0.14) at high pressures for solid at the equilibrium melting temperature.     

Nonisothermal bulk crystallization studies of high density polyethylene using light depolarizing microscopy

The quiescent nonisothermal bulk crystallization kinetics of two high-density polyethylene resins were investigated by a modified light-depolarizing microscopy (LDM) technique. The technique allows studies at average cooling rates up to 2500°C/min. The polymer was found to crystallize at a pseudo-isothermal temperature even at these very high cooling rates. The overall bulk crystallization rate increased rapidly as the cooling rate and supercooling increased. Crystallization kinetics was analyzed by Avrami analysis. Avrami exponents near 3 suggested spherical growth geometry and instantaneous nucleation at predetermined sites. Observation of spherulites by optical microscopy together with a number density of spherulites that changed little with increase in cooling rate or supercooling supported this model of crystallization behavior. Analysis of the half-time of crystallization based on the Lauritzen and Hoffman secondary nucleation theory indicated that the regime II-III transition was found to occur at a degree of supercooling of approximately 22°C. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 681–692, 1998     

Trichloro(1,1‐dimethylbiguanidium‐κN3)zinc(II)

The title compound, [ZnCl₃(C₄H₁₂N₅)], was prepared from aqueous solution and its structure determined. The coordination geometry around the Zn atom is a tetrahedron, with the central Zn atom bound to three Cl atoms and to one N atom of the biguanide ligand. The dihedral angle between the two guanidine groups is 67.86 (1)°.     

Extended-chain crystals. II. Crystallization of polyethylene under elevated pressure

A report on crystallization of polyethylene at elevated pressures to an extended-chain morphology is presented. The crystals have been characterized by electron microscopy and density determination. Pressure, supercooling (temperature), and crystallization time have been varied to find the best conditions for production of perfect crystals. At 10–30°C supercooling completely crystallized polyethylene was obtained between 4.5 and 7 kb crystallization pressure in 1–8 hr. Analysis of fracture surfaces of samples crystallized for different lengths of time shows an increase in size and number of crystal lamellae and an improvement of extended chain crystals in the early stages of crystallization. A further improvement of the less well crystallized material between the lamellae occurs after 15 min of crystallization time.     

Molecular simulation of the homogeneous crystal nucleation of carbon dioxide

We report on a molecular simulation study of the homogeneous nucleation of CO2 in the supercooled liquid at low pressure (P = 5 MPa) and for degrees of supercooling ranging from 32% to 60%. In all cases, regardless of the degree of supercooling, the structure of the crystal nuclei is that of the Pa3 phase, the thermodynamically stable phase. For the more moderate degree of supercooling of 32%, the nucleation is an activated process and requires a method to sample states of high free energy. In this work, we apply a series of bias potentials, which promote the ordering of the centers of mass of the molecules and allow us to gradually grow crystal nuclei. The reliability of the results so obtained is assessed by studying the evolution of the nuclei in the absence of any bias potential, and by determining their probability of growth. We estimate that the size of the critical nucleus, for which the probability of growth is 0.5, is approximately 240 molecules. Throughout the nucleation process, the crystal nuclei clearly exhibit a Pa3 structure, in apparent contradiction with Ostwald's rule of stages. The other polymorphs have a much larger free energy. This makes their formation highly unlikely and accounts for the fact that the nucleation of CO2 proceeds directly in the stable Pa3 structure.     

Effect of thermomechanical processing on the thermal stability of amorphous Fe-B alloys

Results from investigating the effect of thermomechanical processing on the thermal stability of amorphous Fe-B alloys are presented. It is shown that the combined thermomechanical processing of amorphous alloys raises the temperature of intense crystallization onset by 80 K for binary alloys; by 20–50 K, for multicomponent alloys. The greater expansion of the thermal stability interval of binary alloys relative to multicomponent alloys is explained by the presence of alloying dopants such as molybdenum, nickel, and silicon that inhibit the diffusion of boron and thus hinders nucleation and the growth of the crystalline phase. The enhanced thermal stability of amorphous alloys induced by thermomechanical processing is explained by the reduction in size of amorphous-phase frozen crystallization centers and by the formation of a nanostructured state.     

Formation of crystal nuclei near critical supersaturation in small volumes

This work deals with the nucleation of crystals in confined systems in response to the recent high interest in research on crystallization in emulsion and microemulsion droplets. In these confined systems, crystallization often occurs at high supercooling; thus, nucleation determines the overall crystallization process. A decrease in the volume of the confined mother phase leads to the higher supercooling needed for the phase transition. We have numerically solved kinetic equations in order to determine the conditions under which the first crystal nuclei are formed by homogeneous and heterogeneous nucleation from supercooled melt and supersaturated solution, depending on the volume of the mother phase. Supersaturation (or supercooling) increases with decreasing volume of the mother phase. The nucleation barrier depends linearly on the logarithm of volume of the mother phase in all cases under consideration, as follows from the numerical solution of kinetic equations.     

Morphology of cold-crystallized polyamide 6

The structure of cold-crystallized polyamide 6 (PA 6) has been analyzed by wide-angle X-ray scattering, atomic force microscopy, and polarizing optical microscopy. It has been found that ordering of initially fully amorphous and glassy PA 6 on slow heating to temperatures higher than the glass transition temperature results in formation of spatially non-organized short lamellae/nodules with a size depending on the maximum annealing temperature. In contrast, melt crystallization at low supercooling is connected with formation of spherulites and laterally extended lamellae. The observed experimental results demonstrate that crystals of qualitatively different morphology and higher-order organization can be generated by variation of the pathway of crystallization. It is assumed that the different structures obtained in melt- and cold-crystallized samples are related to heterogeneous and homogeneous nucleation, respectively.     

Effect of charge state and stoichiometry on the structure and reactivity of nickel oxide clusters with CO

The collision induced fragmentation and reactivity of cationic and anionic nickel oxide clusters with carbon monoxide were studied experimentally using guided-ion-beam mass spectrometry. Anionic clusters with a stoichiometry containing one more oxygen atom than nickel atom (NiO₂⁻, Ni₂O₃⁻, Ni₃O₄⁻ and Ni₄O₅⁻) were found to exhibit dominant products resulting from the transfer of a single oxygen atom to CO, suggesting the formation of CO₂. Of these four species, Ni₂O₃⁻ and Ni₄O₅⁻ were observed to be the most reactive having oxygen transfer products accounting for approximately 5% and 10% of the total ion intensity at a maximum pressure of 15 mTorr of CO. Our findings, therefore, indicate that anionic nickel oxide clusters containing an even number of nickel atoms and an odd number of oxygen atoms are more reactive than those with an odd number of nickel atoms and an even number of oxygen atoms. The majority of cationic nickel oxides, in contrast to anionic species, reacted preferentially through the adsorption of CO onto the cluster accompanied by the loss of either molecular O₂ or nickel oxide units. The adsorption of CO onto positively charged nickel oxides, therefore, is exothermic enough to break apart the gas-phase clusters. Collision induced dissociation experiments, employing inert xenon gas, were also conducted to gain insight into the structural properties of nickel oxide clusters. The fragmentation products were found to vary considerably with size and stoichiometry as well as ionic charge state. In general, cationic clusters favored the collisional loss of molecular O₂ while anionic clusters fragmented through the loss of both atomic oxygen and nickel oxide units. Our results provide insight into the effect of ionic charge state on the structure of nickel oxide clusters. Furthermore, we establish how the size and stoichiometry of nickel oxide clusters influences their ability to oxidize CO, an important reaction for environmental pollution abatement.     

Potassium di­cyano­argentate(I) 1,4,7,10,13,16‐hexaoxa­cyclo­octadecane

The crystal structure of the title compound, K[Ag(CN)₂]·C₁₂H₂₄O₆, conventionally denoted K(18‐crown‐6)Ag(CN)₂, where 18‐crown‐6 is 1,4,7,10,13,16‐hexa­oxa­cyclo­octa­decane, is characterized by closely packed linear chains formed by the coordination of the nitrile N atoms of the [Ag(CN)₂]⁻ anions to the K⁺ cations. The K atoms lie on centers of inversion and are additionally bound to the six O atoms of the crown ether.     

Observation of a transition in the water-nanoparticle formation process at 167 K

Rapid-scan Fourier transform infrared spectroscopy of the vapor/solid formation process of water nanoparticles in the 180-140 K temperature range at thermal-equilibrium conditions is reported. At 167 K a transition in the formation process was observed: the particle volume quintuples and the particle formation time triples within a temperature interval of +/-0.4 K caused by the temperature control. The authors interpret this behavior by an abrupt change in the nucleation rate of the H2O monomers in He buffer gas kept at 167 K and 200 mbar. A size and shape analysis of the particles during the formation process was carried out by application of the discrete dipole approximation method which delivers excellent accordance between experimental and calculated mid-IR spectra. Compared to other compact shapes (cube, prolate ellipsoid, and hexagonal prism) the ideal spherical shape fits the experimental spectra best. A distinct change in shape by particle conversion or agglomeration could be excluded to be involved in the formation process. As a possible explanation of the observed phenomenon, a transition from vapor/liquid/solid to vapor/solid nucleation with decreasing temperature is considered which was recently theoretically predicted by van Dongen and co-workers [J. Chem. Phys. 117, 5647 (2002); private communication; J. Chem. Phys. 120, 6314 (2004)].     

Two liquid phases of water in the deeply supercooled region and their roles in crystallization and formation of LiCl solution

The properties of supercooled liquid water and the mechanism of crystallization in it were investigated using time-of-flight secondary ion mass spectrometry and reflection absorption infrared spectroscopy. The self-diffusion of the water molecules commences at 136 K, and then the liquid-liquid phase transition occurs at 160-165 K. The latter is evidenced not only by the occurrence of fluidity but also by the formation of a LiCl solution. The infrared absorption band also changes drastically above 160 K due to crystallization of water (on the Au film) and the formation of LiCl solution (on the LiCl film). The immediate crystallization and dissolution of LiCl are thought to be characteristic of normal water that is created in a deeply supercooled region, indicating that viscous liquid water (T > 136 K) is transformed into supercooled liquid water at around 160 K. The crystallization kinetics is different between these two phases because the former (latter) involves nuclear growth (spontaneous nucleation). Without nuclei, crystallization is quenched below 160 K in the present experiment. It is suggested that the viscous liquid phase coexists at the surface or grain boundaries of metastable ice Ic.     

溶液间歇结晶过程成核与生长阶段的定量识别

为定量识别溶液间歇结晶过程中的成核和生长阶段,基于晶粒数目和粒度的变化对粒度分布(CSD)的二阶和三阶矩量影响程度的不同,定义并关联了无因次变量K和K^*.添加晶种KNO₃-H₂O溶液结晶过程模拟计算的结果表明,K和K^*值均呈先降后升的变化趋势,成核时单调下降,生长过程中单调上升;且K与K^*值较接近.测定了KNO₃-H₂O溶液自发成核结晶过程中溶液浓度和透光率的变化,用K^*判据定量识别出成核阶段和生长阶段,并与晶体线性生长速率模型检验的结果相吻合.K值的计算依赖于CSD和结晶动力学参数,而K^*作为成核和生长阶段的模型判据,由实验测定的溶液浓度和透光率计算得到.     

A real-time AFM study on crystal nucleation and growth in nanodroplets of isotactic polypropylene

Isotactic polypropylene (iPP) nanodroplets were prepared by using the classical droplet method in this study. The formation of nanodroplets allowed the controlled observation of polymer nucleation as well as access to crystal growth at exceptionally high supercooling in iPP. Three cases including the heterogeneous nucleation and fast crystallization in iPP droplets, the formation of multiple independent homogeneous nuclei within a single droplet and a single nucleus within a single droplet were detected by using atomic force microscopy (AFM) during gradually cooling after remelting the nanodroplets. Moreover, it is found that when the volume of droplet is larger than the value of ca. 130000 nm3, the first case was observed. Otherwise, the latter two cases appeared. The temperature at which the onset of nucleation was observed in individual droplets was found to be mainly dependent on height of the droplets when the size scale of the droplet is comparable to the size of the critical nucleus in at least one dimension, which indicates the nucleation behavior under confinement.     

Molecular dynamics simulations of cluster nucleation during inert gas condensation

Molecular dynamics simulations of vapor-phase nucleation of germanium in an argon atmosphere were performed and a unexpected channel of nucleation was observed. This channel, vapor-induced cluster splitting, is important for more refractory materials since the critical nucleus size can fall below the size of a dimer. As opposed to conventional direct vapor nucleation of the dimer, which occurs by three-body collisions, cluster-splitting nucleation is a second-order reaction. The most important cluster-splitting reaction is the collision of a vapor atom and a trimer that leads to the formation of two dimers. The importance of the cluster-splitting nucleation channel relative to the direct vapor nucleation channel is observed to increase with decreasing vapor density and increasing ratio of vapor to carrier gas atoms.