Nuclei Growth Processes Research Articles

Molecular dynamics simulation of solidification epitaxial growth in a nanoscale molten pool.

In the hot working process, the liquid metal part formed by the heat source on the workpiece is known as molten pool. Since the solidification process of the molten pool determines the mechanical properties of the structure after hot working, the molten pool solidification under the condition of rapid solidification has attracted the attention of researchers. In thisstudy, to understand the influence of the microstructure and morphology of the base metal on the solidification of the molten pool, a simulation system of epitaxial growth during the solidification of the molten pool is established based on molecular dynamics (MD), and the details of the epitaxial growth of the molten pool solidification are dynamically monitored. The results show that the nano molten pool produces two atomic layers of pre-melting on the base metal before solidification, and then, the molten pool continues to grow with the exposed and ordered atoms of the base metal as the nuclei. The transformation process of the final obtained solidification morphology is consistent with the results observed by in situ TEM experiments. These phenomena reveal the mutual guidance between the molten pool and the base metal during the solidification of the molten pool as well as the genetic effect of the parent metal on the crystallization of the molten pool. In addition, the crystal growth of molten pool solidification follows the growth pattern of directional solidification, from equiaxed to columnar, but the average grain size of each zone is smaller than that of directional solidification. Even the nucleation rate and dislocation density are an order of magnitude higher than in directional solidification. Therefore, the simulation results lay a foundation for the in-depth study of the molten pool solidification process at the atomic scale.

Ice Nucleation of Confined Monolayer Water Conforms to Classical Nucleation Theory

We confirmed that monolayer water confined by parallel graphene sheets spontaneously crystallizes from a structurally and dynamically heterogeneous liquid phase under moderate supercooling via direct molecular dynamics simulation. Square-lattice-like geometric order is observed at the early stage of nucleation and is preserved during the entire nucleus growth process. The diffusion coefficient and free energy profile in the cluster space extracted from a Bayesian trajectory analysis agree well with the classical nucleation theory (CNT) prediction and yield thermodynamic quantities exhibiting linear temperature dependence. The effectiveness of maximum cluster size as the descriptor of ice nucleation dynamics in the CNT framework can be attributed to the dynamical time scale decoupling and strong structural pattern dependence of density fluctuation in the liquid phase.

Hydrothermal synthesis of MoS2 with different morphology and its performance in thermal battery

Three different kinds of hierarchical MoS2 microspheres are successfully synthesized by a simple hydrothermal route, and their morphology, thermal stability and electrochemical performance in thermal battery are characterized. The flower-like porous MoS2 microsphere with good crystallinity consists of MoS2 nano-sheets with thickness of about 10 nm. Its evolution process is proposed, and includes three stages: a nucleus growth process following the principle of Ostwald ripening, MoS2 nano-sheets growth and self-assembly. The dependence of the macroscopic electrochemical properties on the microscopic structure and morphology is discussed. Since the MoS2 with flower-like porous microsphere structure presents less weight loss, higher decomposition temperature, outstanding thermal stability and excellent compatibility with electrolyte, it performs longer discharge time and larger capacity when used as the cathode in the thermal battery, compared to MoS2 with other structures, FeS2 or CoS2. The possible discharge mechanism of MoS2 is proposed, and it can be divided into three steps, which are the generation of intercalation compounds containing lithium ions (LiXMoS2), desulfurization generating low-valent intercalation compound and reduction from low-valent intercalation compound to Mo, respectively.

A new method for preparing mono-dispersed nanoparticles using magnetized water

We studied the use of magnetized water on the size of the nanoparticles. Magnetized water found to reduce the diameter of the nanoparticles during a homogeneous precipitation process, which is a combination of nucleation and nuclei growth processes. We found that the modified water, which demonstrated different physical properties especially on the surface tension and viscosity, significantly influenced the both processes. Therefore, the nucleation process was initially prolonged in the homogeneous precipitation process due to the lower critical size of nucleus and higher rate of nucleation, and consequently formed smaller particles and a larger number of particles. Furthermore, the growth rate of nanoparticles was hindered owing to the higher viscosity of the water and restriction in the mass transport process. As a result, the precipitated particles with the magnetized water were eventually structured smaller particle diameter compared to the bulk. The presented method in here indicated a low cost, straightforward, and feasible technique for industrial application. In addition, this method could open a new promising perspective on nanomaterial synthesis in order to facilitate the production of monodispersed nanoparticles. Molecular dynamic confirmed that surface tension decreased as the external magnetic field was applied. Moreover, the density profile illustrated that the average number of hydrogen atoms is greater than oxygen atoms.

Kinetic study of the dehydration reaction of lithium sulfate monohydrate crystals using microscopy and modeling

Simulation of gas–solid reactions occurring in industrial processes requires a robust kinetic model to be applicable in a wide range of complicated reaction conditions. However, in literature it is often seen that even the same reaction under specific controlled conditions is interpreted with different kinetic models. In the present work, a phenomenological model based on nucleation and nuclei growth processes is presented to study the kinetics of the dehydration reaction of lithium sulfate monohydrate single crystals. The two elementary processes of the reaction, nucleation and nuclei growth, are characterized and quantified as a function of temperature by using optical microscopy experiments. The in-situ measured characteristics of the dehydration reaction provided confirmatory evidence that the rate of nucleation obeys an exponential law and the rate of nuclei growth is approximately constant. With knowledge acquired from the optical observations as inputs of the kinetic model, the fractional conversion of the dehydration reaction was calculated and compared with experimental results from thermogravimetric analysis (TGA). A satisfactory comparison was found both in isothermal and non-isothermal conditions. It is demonstrated that this knowledge-based model has a great potential to represent the gas–solid reaction kinetics in a wide range of process conditions regarding temperature, pressure and particle geometry.

Comparison of preparation methods for iron–alumina oxygen carrier and its reduction kinetics with hydrogen in chemical looping combustion

ABSTRACTSeven preparation methods, namely sol‐gel, co‐precipitation, hydrothermal synthesis, low heating solid‐state reaction, freeze granulation, combustion synthesis, and mechanical mixing, were used to synthesize Fe2O3/Al2O3 oxygen carrier for chemical looping combustion. A comprehensive physicochemical characterization (i.e. productivity, crushing strength, crystalline characteristics, microstructure, and chemical reactivity with hydrogen) was carried out, and the effects of preparation methods and processes on the oxygen carrier performance were explored. Taking into consideration various physicochemical indices, the sol‐gel method and the freeze granulation method were preferred for oxygen carrier preparation. Following that, a critical chemical reaction in in situ gasification chemical looping combustion, the reduction reaction between oxygen carrier and hydrogen, was clarified in terms of reaction kinetics through the non‐isothermal kinetics analysis and the double extrapolation method. Temperature programmed reduction experiments of the sol‐gel‐derived Fe2O3/Al2O3 particle and hydrogen were performed using a chemisorption analyzer. The reduction mechanisms and kinetics parameters for the two‐stage reaction (reduced from Fe2O3 to Fe3O4 and then from Fe3O4 to FeAl2O4) were determined. In the first stage, the reduction reaction is described by the surface reaction model with an order of 2; on the other hand, the conversion from Fe3O4 to FeAl2O4 is dominated by the nucleation and nuclei growth process. © 2014 Curtin University of Technology and John Wiley & Sons, Ltd.

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

ABSTRACTThe 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.

A new methodology for the simulation of solid state phase transition kinetics by combination of nucleation and nuclei growth processes

A new methodology for the simulation of solid state phase transition kinetics has been developed by combining the influence of nucleation rate, nuclei growth rate and the power p characterizing the contact area between the growing particles. The equations used in this methodology were well known, and have been used previously for creating some of the most popular solid-state kinetic equations. The developed methodology made possible calculations of separate rate constants for two processes affecting the rate of phase transition—nucleation (described with K1) and nuclei growth (described with K2). Similar phase transitions were also approximated with the well-known single constant Avrami–Erofeev equation, but we successfully calculated both constants according to the new methodology, which allowed a separate evaluation of these two processes and explained the different induction periods. The effects of empirically adjusted constants on theoretically calculated kinetic curves were thus determined.

Formation mechanism of MgB 2 in 2LiBH 4 + MgH 2 system for reversible hydrogen storage

The formation conditions of MgB 2 in 2LiBH 4 + MgH 2 system during dehydrogenation were investigated and its mechanism was discussed. The results show that direct decomposition of LiBH 4 is suppressed under relative higher initial dehydrogenation pressure of 4.0×10 5 Pa, wherein LiBH 4 reacts with Mg to yield MgB 2, and 9.16% (mass fraction) hydrogen is released within 9.6 h at 450 °C. However, under relatively lower initial dehydrogenation pressure of 1.0×10 2 Pa, LiBH 4 decomposes independently instead of reacting with Mg, resulting in no formation of MgB 2, and 7.91% hydrogen is desorbed within 5.2 h at 450 °C. It is found that the dehydrogenation of 2LiBH 4 + MgH 2 system proceeds more completely and more hydrogen desorption amount can be obtained within a definite time by forming MgB 2. Furthermore, it is proposed that the formation process of MgB 2 includes incubation period and nucleus growth process. Experimental results show that the formation process of MgB 2, especially the incubation period, is promoted by increasing initial dehydrogenation pressure at constant temperature, and the incubation period is also influenced greatly by dehydrogenation temperature.

Characterization of electrodeposited magnetic Co/Pt multilayered nanostructures

Electrodeposition will prove to be promising for the preparation of magnetic metal nanostructures. Multilayered fcc Co/Pt nanostructures grown on a Pt(111) single-crystal substrate by electrodeposition under potential control exhibit a remanent perpendicular magnetization and a large magnetic coercivity, which depend on the multilayer growth mechanism. A possible interfacial alloying below the Pt overlayers induces a magnetic moment on the Pt atoms, leading to asymmetry between Pt/Co and Co/Pt interfaces of the multilayers. The role of the codeposition of Ni as an inhibitor for the nucleus growth process as well as for the dissolution of Co during the Pt deposition is of great importance for the occurrence of the multilayered nanostructure.

Study of the electrosynthesis of poly(3-butylthiophene) and its semiconductor properties

The mechanism of the electrosynthesis of poly(3-butylthiophene) (PBuT) was studied by cyclic voltammetry and potential step methods in comparison with polybithienyl. The anodic oxidation polymerization of the 3-butylthiophene underwent two steps: oligomer formation and further polymerization to form the polymer. The doping level of the PBuT increases with the cycle number of the potential sweeps during polymerization. The current responses to the potential steps indicate a nucleation and nuclei growth process which is repeated layer to layer. The differential capacity (Cd) and photocurrent were measured at the PBuT films in the aqueous electrolyte solution. The Cd−2 vs. E plot shows two regions of linearity, one with a negative slope and the other with a positive slope in different potential regions, which give the same flat-band potential. This indicates that the PBuT film exhibits both p-type and n-type features of a semiconductor at differrent potential regions. The cathodic photocurrent spectrum was analysed by the (jphhν)2/n vs. hν plots, giving band gap energies of 2.41 eV for n=1 and and 2.01 eV for n=4.

Domain Growth Scaling at the Isotropic-to-Cholesteric Liquid Crystal Transition

The phase-ordering kinetics for the growth of the ordered cholesteric liquid crystalline phase from the disordered isotropic melt is investigated at isothermal conditions after a temperature quench. The growth of a characteristic length, here the nucleus diameter, follows a universal growth law L(t) ≈ tn with the growth exponent changing from n = 1/2 in the vicinity of the transition to n = 1 for large quench depths. The short-term nucleus growth process is found to be dependent on the sample confinement with increasing growth exponents as the cell gap is increased. Nevertheless, no crossover from three-dimensional to two-dimensional growth could be observed as the nucleus diameter reached the dimension of the cell gap. A long-term coarsening process was found to be independent of the quench depth and confinement dimension, obeying a square-root growth law L(t) ≈ t1/2.

The incipient kinetics of hydride growth on cerium surfaces

The incipient hydriding stage of cerium is characterized by nucleation and growth of the hydride phase on the surface of the metal. The kinetics of this initial growth process were studied utilizing a hot-stage microscope (HSM) equipped with a video camera. Growth velocities of nuclei were determined as a function of reaction temperature and hydrogen pressure over a grid of temperatures and pressures which enabled the maintenance of near isothermal conditions (25–100°C, 10–1000 mbar H2). A linear pressure dependence of the growth velocities was obtained, with temperature-dependent slopes. An Arrehenius plot of these slopes yielded a straight line with apparent activation energy of 6.3 kcal (g atomic H)−1 (0.27 eV). The results were compared with those of the previously studied massive hydriding stage, where the reaction is characterized by a contracting envelope morphology with a hydride-metal boundary moving at a constant (temperature-pressure dependent) velocity. Under similar experimental conditions, the velocities of the initial nuclei growth process are significantly higher than the corresponding boundary velocities at the massive stage. Different types of controlling mechanism are proposed to account for these observed differences between the incipient and the massive hydriding stages.

Kinetics of the isothermal dehydration of Li 2SO 4·H 2O in vacuo

Although the dehydration of lithium sulfate monohydrate in vacuum (≈10 −4 Torr) has a simple appearance, the kinetics is not so simple as the reaction mechanism, which can be expressed by a single function model F(α). The kinetics is represented by the combination of A 2 and R 2 functions. The initial acceleratory stage described by the A 2 equation seems to be related to the nucleation and nuclei growth process controlled by the diffusion of dissociated water molecules. When the two-dimensional phase boundary is formed around the specimen by the growing nuclei, the dehydration seems to be controlled by the advance of the interface and to have the characteristics of the R 2 model function. The values of activation energies and pre-exponential factors were 22.1 kJ mol −1 and 1.45 s −1 for the A 2 process, and 29.3 kJ mol −1 and 12.1 s −1 for the R 2 process, respectively.

Some reactivity parameters in the reduction of cobalt chloride by hydrogen

The order of the reduction of cobalt chloride with respect to hydrogen, the overall activation energy and the activation energy of the nucleus growth process have been determined. Palladium, nickel, supported and unsupported platinum accelerate the reaction. This latter phenomenon is explained by an activation of hydrogen on the metals and a migration of activated hydrogen to CoCl2 . Special experiments show that the migration of spill-over hydrogen takes place through jump-over processes and migration on alumina and silica-alumina surfaces. The number of hydroxyl groups and their acidity strength strongly influences the rate of reduction.