Crystal Growth Theory Research Articles

Template, catalyst free growth and photoluminescence property of large scale ZnO nanorods

Uniform and large scale ZnO nanorods were successfully prepared by a simple chemical precipitation without any structure directing agents, the prominent merit in this mild process is that the final products do not need subsequent calcinations in an oven. Depending on the synthetic conditions used, the products with hexagonal shape consist mainly of high regularly nanorods with an average diameters of 120 nm or so, while the aspect ratio can be controlled about 8. The ZnO nanorods exhibit a strong ultraviolet emission and a very weak blue band at room temperature. The growth mechanism for the formation of ZnO nanorods was preliminarily explained by polar crystal growth theory and lowest energy arguments.

Development of a Suppression Method for Deposition of Radioactive Cobalt after Chemical Decontamination: (II) Consideration of Fe3O4 Plating Mechanism on Stainless Steel in Aqueous Solution at 363 K

Recently, chemical decontamination at the beginning of periodical inspection has been applied to many Japanese boiling water reactors in order to reduce radiation exposure. However, following the chemical decontamination, a dose rate increase can be seen in some plants after just a few operation cycles. The Hitachi ferrite coating (Hi-F Coat) process has been developed to reduce the recontamination by radioactive cobalt after the chemical decontamination. In this process, a fine Fe3O4 coating film is formed on the stainless steel base metal of the piping following the chemical decontamination in aqueous solution at 363 K. In this study, we investigated a Fe3O4 plating mechanism on the base metal in aqueous solution at 363K by measurements using a quartz crystal microbalance. We found that the Fe3O4 film grew in three steps. First, the Fe3O4 particles were produced on a stainless steel surface. Second, the Fe3O4 particles grew as dome shapes and the converged domes became filmlike. Third, the film grew and became a closely packed Fe3O4 film. Furthermore, we determined the equation of the time dependence of the Fe3O4 film amount using crystal growth theory. The equation predicted the film amount at 10,000 s within a margin of error of 5%.

CRYSTALLIZATION BEHAVIOR OF MAGNESIUM SALTS: A SUMMARY OF SOME EXPERIMENTAL OBSERVATIONS

Some efficient methods have been developed to chemically prepare some magnesium salt products with multiscale morphologies such as MgCO 3·3 H 2 O and 5 Mg ( OH )2· MgSO 4·2 H 2 O whiskers, Mg 5( CO 3)4( OH )2·4 H 2 O microplates and aggregates, and Mg ( OH )2 plates and spheres. The crystallization processes of the as-prepared products were studied by designing different reaction routes and corresponding growth morphologies. In these solution chemical systems, Mg 2+ cation was found to be easily coordinated by various ligands, and can be easily transformed into various compounds with variable compositions. The current crystallization observations of some magnesium salts can be readily simulated by the chemical bonding theory of single crystal growth. These results can guide the further studies of various magnesium salts with controllable morphologies.

Morphologic characteristics and growth interface stability of nano-micron FeS2 whiskers

Micromorphology is further studied on the basis of our previous researches concerned with the nano-micron FeS2 whisker. There are obvious differences in the intensive degree, diameter and micromorphology among the FeS2 whiskers growing in different stages. From the early to late stage, the intensive degree increases, the diameter decreases, and the surface micro-morphology changes following the regularity: protrusive nodulation → coarse → smooth → flat. According to the theory of crystal growth, the geological setting and processes of whisker formation, we discuss the stability and evolution of crystal growth interface of FeS2 whisker occurring in Gengzhuang gold deposit (Shanxi Province, China). The results suggest that the negative temperature gradient and the supercooling appear in the early stage of the whisker growth, whereas the positive temperature gradient of reposeful state appears in the late stage. In the whisker growth stage, the component concentration changes through the three stages: severely nonhomogeneous in the early stage, relatively homogeneous in the middle stage, more homogeneous in the late stage. The general changing process of the interfacial state is from unstable to stable. Micromorphology of FeS2 whisker in Gengzhuang is the result of synergism of temperature, component concentration and stability of crystal interface phase in hydrothermal system. The micromorphology not only reflects the physical and chemical characteristics of the hydrothermal system during the whisker growth, but also indicates the stability characteristics of the interface phase and records the changing process of the whisker growth.

Effect of electron beam irradiation on the crystallization of silk fibroin fiber‐reinforced poly(ε‐caprolactone) biocomposites

AbstractIn the medical industry, ionizing radiation has attracted increasing interest in recent years for the sterilization of biomedical materials due to its high effectiveness at relatively low cost and simple operation. In the study reported here, silk fibroin (SF) fiber was used to prepare novel reinforced poly(ε‐caprolactone) (PCL) biocomposites, and the effect of electron beam irradiation on both non‐isothermal and isothermal crystallization kinetics of the SF/PCL biocomposites was investigated. The models of Ozawa treatment, Avrami analysis and regime theory of crystal growth are applicable for describing the non‐isothermal and isothermal crystallization kinetics of the irradiated PCL and SF/PCL composites. Compared with non‐irradiated PCL and SF/PCL composites, the irradiated specimens exhibit the same crystallization regime (regime II) and similar lateral surface free energy (σ), except for smaller fold surface free energy (σe) and work of chain folding (q). The crosslinked PCL network formed in the irradiation process can act as a nucleation agent and accelerate the primary crystallization of PCL. However, the restriction effect of the crosslinked PCL network on the molecular chain mobility will reduce the overall crystallization rate of PCL, and this restriction effect appears stronger in the non‐isothermal crystallization process than in the isothermal crystallization process. Copyright © 2009 Society of Chemical Industry

Formation of PbS Nanowire Pine Trees Driven by Screw Dislocations

The basic characteristics of nanowire growth driven by screw dislocations were investigated by synthesizing hierarchical lead sulfide (PbS) nanowire "pine trees" using chemical vapor deposition of PbCl(2) and S precursors and systematically observing the effects of various growth parameters, such as hydrogen flow, temperature, pressure, and the growth substrates employed. Statistical surveys showed that the growth rate of the dislocation-driven trunk is about 6 mum/min and that of the vapor-liquid-solid (VLS) driven branch nanowire is about 1.2 mum/min under the typical reaction conditions at 600 degrees C, 900 Torr, and a hydrogen flow rate of 1.5 sccm. The onset of hydrogen flow plus the presence of fresh silicon have been identified as the critical ingredients for generating PbS nanowire trees reproducibly. To explain the experimental findings in the context of classical crystal growth theory, the former is suggested to create a spike in supersaturation of the actual sulfur precursor H(2)S and initiate dislocations with screw components that then propagate anisotropically to form the PbS nanowire trunks. Maintaining suitable hydrogen flow provides a favorable low supersaturation that promotes dislocation-driven trunk nanowire growth and enables the simultaneous VLS nanowire growth of branches. Furthermore, thermodynamic consideration and experiments showed that silicon fortuitously controls the supersaturation by reversibly reacting with H(2)S to form SiS(2) and that SiS(2) can also be a viable precursor for PbS nanowire growth. The key requirements of screw dislocation-driven nanowire growth are summarized. This study provides some general guidelines for further nanowire growth driven by screw dislocations.

Coarsening of polyhedral grains in a liquid matrix

The coarsening of polyhedral grains in a liquid matrix was calculated using crystal growth and dissolution equations used in crystal growth theories for faceted crystals. The coarsening behavior was principally governed by the relative value of the maximum driving force for growth (Δgmax), which is determined by the average size and size distribution, to the critical driving force for appreciable growth (Δgc). When Δgmax was much larger than Δgc, pseudonormal grain coarsening occurred. With a reduction of Δgmax relative to Δgc, abnormal grain coarsening (AGC, when Δgmax ≥ Δgc) and stagnant grain coarsening (SGC, when Δgmax < Δgc) were predicted. The observed cyclic AGC and incubation for AGC in real systems with faceted grains were explained in terms of the relative value between Δgmax and Δgc. The effects of various processing and physical parameters, such as the initial grain size and distribution, the liquid volume fraction, step free energy, and temperature, were also evaluated. The calculated results were in good agreement with previous experimental observations.

Instability of vicinal crystal surfaces with transparent steps: Transient kinetics and non-local electromigration

The steps at the crystal surfaces could be partially transparent in the sense that the migrating adatoms can cross the steps without visiting a kink position. In the case of significant transparency the velocity of a given step in a given moment is affected by atomic processes which took place at rather distant terraces in rather earlier moments. The reason is that an adatom needs time to cross many terraces before attaching to a kink position. That is why the theory of crystallization with non-local kinetics should account for the time dependence of the adatom concentrations on the terraces. Such a transient growth model (going beyond the quasi-static approximation of the BCF theory of crystal growth) is developed here for the limit of slow kinetics at the steps and fast surface diffusion. This model predicts instability of the vicinal surface (in agreement with the earlier studies) but rather different expression for the wavelength of the most unstable mode, which depends only on the ratio between the step transparency and the step kinetic coefficients.

Microstructural Evolution During Sintering with Control of the Interface Structure

This paper reports recent theoretical perspectives and experimental results on microstructural evolution during sintering in terms of the interface structure, which is either rough (atomically disordered) or faceted (atomically ordered). The paper presents theoretical predictions and calculations of grain growth during liquid‐phase sintering based on crystal growth theories. It is shown that various types of grain growth behavior, which may be normal, abnormal, or stagnant, can appear as a result of the coupling effects of the maximum driving force for growth and the critical driving force for appreciable growth. The predictions are also shown to be valid in the case of solid‐state sintering. A number of experimental observations showing the effect of some critical processing parameters have been found to be in excellent agreement with the predictions. Principles of microstructure development (grain growth control) during sintering are suggested. In addition, the effect of the interface structure on densification is briefly described and discussed.

DSC measurements and modelling of the kinetics of methane hydrate formation in water-in-oil emulsion

The kinetics of formation of clathrate hydrates of methane was investigated in a water-in-oil emulsion using high-pressure differential scanning calorimetry in the range 10–40 MPa, at various temperatures. At high driving force, the heat peak related to the formation of hydrates has a regular and symmetric shape, and its height and width depend on the gas pressure and sub cooling degree. At near equilibrium conditions, hydrate formation is delayed by more than 1 h, but is still clearly observable. A model based on crystal growth theory, coupled with a normal distribution of induction times to take into account the germination in a population of micro-sized droplets, is proposed to represent the hydrate formation rate versus time in the particular case of water-in-oil emulsions. It uses four parameters which appear strongly correlated to the experimental conditions: the growth rate constant, the over saturation of gas in the water phase, the average and standard deviation of the induction time distribution.

Water Mediates the Metastable Crystal-to-Stable Crystal Phase Transition Process in Phospholipid Aqueous Dispersion

The metastable crystal usually appears first during crystallization from solution, and it will then transform into the stable form. This metastable crystal-to-stable crystal phase transition process is important in the field of crystal growth and engineering. Crystallization behaviors of amphiphiles such as phospholipids are special as they have preassembled structures prior to crystallization, and the fundamental mechanisms regarding the transformation details of their polymorphic forms at the molecular level are not well understood. In this work, by using Fourier transform infrared spectroscopy combined with the hydrogen/deuterium (H/D) exchange method, we have demonstrated that water plays a critical role in mediating the metastable crystal-to-stable crystal phase transition process of a representative phospholipid, dilauroylphosphoethanolamine (DLPE). The H/D exchange method applied here is a powerful and simple approach to probe if water is involved in the crystal-to-crystal transformation process. The results indicate that water works like a catalyst for the physical process. Furthermore, the conformational changes of the lipid molecules during the crystal-to-crystal transition process have been examined. Understanding the details of the transition from a metastable crystal to a stable crystal will shed light on the crystal growth theory and engineering of the self-assembled amphiphiles in aqueous solutions.

電析デンドライトのフェーズフィールド解析

Effect of the phase-boundary potential at the electrode-electrolyte interface and the anisotropy of interfacial energy on the morphology of the electrodeposits have been investigated by phase-field simulation. In equilibrium system, the change in the phase-boundary potential due to the interfacial curvature satisfies the Gibbs-Thomson effect. The stability analysis of the electrode-electrolyte interface reveals that the marginal wavelength is proportional to the inverse of the square root of the growth velocity of the interface. It is found that the linear relationship between the marginal wavelength and tip radius during electrodeposition satisfies the crystal growth theory confirmed by the dendrite growth in solidification process.

Coarsening Behavior of Round‐Edged Cubic Grains in the Na1/2Bi1/2TiO3–BaTiO3 System

The coarsening behavior of 0.95Na1/2Bi1/2TiO3–0.05BaTiO3 (mole fraction, NBT–5BT) grains has been investigated with respect to the grain shape and compared with a calculation based on crystal growth theories. The powder compacts of NBT–5BT were sintered at 1200°C for various times up to 96 h. The shape of growing grains was a cube with flat {100} faces and rounded edges. The coarsening behavior during sintering up to a few hours was quite normal. However, with an increased sintering time, the relative grain size distribution broadened and apparently abnormal grains appeared. The abnormal grain coarsening behavior intensified and then decreased during extended sintering. This result shows that the coarsening behavior of partially faceted grains changes continuously with sintering time. Grain size distributions were also calculated under the assumption that the growth of grains was governed by the growth of facet planes with a low step free energy. The calculation showed the broadening of relative size distribution, the formation of abnormal grains, and then eventually narrowing of the distribution. This agreement between the measured and calculated distributions suggests the growth of round‐edged polyhedral grains to be governed by the growth of facet planes, which can occur via step formation and growth on the facets.

Growth rate changes of sodium chlorate crystals independent of growth conditions

Results of investigations of the growth rate changes inherent to the crystal are presented. It is shown that, in initial growth stage, there exist crystal growth rate changes independent of experimental conditions, with tendency to level during the time. Time evolution of sodium chlorate crystals growth rate dispersion is also presented. The results obtained show that these changes must be included in the interpretations of the growth rate changes affected by various parameters (supersaturation, temperature, fields, stress, impurities, etc.), which have not previously been taken into account. These results may improve the current crystal growth theories.

Fabrication of Two-Dimensional Arrays of Micron-Sized Gold Rings Based on Preferential Nucleation at Reentrant Sites

A templating method for fabricating two-dimensional (2D) arrays of micron-sized gold rings is reported. The microstructures are formed by electroless plating in a through-porous polymer membrane on a silicon substrate obtained from a closed-packed silica colloidal crystal. Our results show that the sizes of gold rings can be altered by varying electroless plating conditions for the porous polystyrene membranes. Moreover, we explain the growth mechanism of gold rings using the classical crystal growth theory that is preferential nucleation at reentrant sites.

Stranski, Krastanov, and Kaischew, and their influence on the founding of crystal growth theory

A reconsideration of the events and people associated with the birth of modern crystal growth theory is presented. The foundation of the new theory was enabled by validation of Gibbs and Volmer's thermodynamic theory with the new molecular-kinetic theory. For the first time it became possible to rigorously explain crystal growth at the atomic level. The new two-dimensional growth model opened the door to understanding the mechanisms of self-assembly, crystal defect formation, and nano-structures. Molecular-kinetic theory eventually embraced a second major tenet based on the model of spiral growth at dislocations. All of these initial discoveries occurred between 1927 and 1949, a time when the world was being torn apart by war. This article recounts some thoughts and actions of three of the most illustrious founders of crystal growth theory.

Abnormal Growth of Electrodeposited BiSb Alloy Nanotubes

BiSb alloy nanotube-nanowire array has been fabricated by the pulsed electrodeposition technique. It was found that the growth of the polycrystalline nanotubes is terminated by polycrystalline nanowires abruptly and then follows by single crystalline nanowires. The formation mechanism of the nanotubes and the origin of the transition from a polycrystalline nanotube to a polycrystalline nanowire were analyzed and discussed from the perspective of the crystal growth theory. The deep understanding of the transition is of high importance for the controlled synthesis and application in thermoelectric field of Bi-based nanotubes arrays.

Kinetic versus Thermodynamic Control over Growth Process of Electrodeposited Bi/BiSb Superlattice Nanowires

The growth mechanism of the electrodeposited single crystalline nanowires is generally considered to follow a three-dimensional to two-dimensional (2D) transition mode, and as for the 2D growth, it is ordinarily considered as a plane growth mode (layer-by-layer growth mechanism). We report in this Letter the growth of Bi/BiSb superlattice nanowires by adopting a charge-controlled pulse electrodeposition technique, and to our best knowledge, different growth modes of the nanowires, the 2D plane growth mode, the tilted plane growth mode, and the curved plane growth mode, were first observed. These growth modes were gathered and analyzed from the perspectives of crystal growth as well as kinetics and thermodynamics. It is shown that the superlattice nanowires are good structures for studying the growth mechanism of electrodeposited nanowires. This work will deeply benefit the understanding of the growth process of the electrodeposited nanowires and provide important experiment data to crystal growth theory.

Crystal nucleation and growth in glass-forming melts: Experiment and theory

A critical overview on basic assumptions of the classical theory of crystal nucleation and growth is given. It is shown how the classical theory can be generalized in order to overcome contradictions between theoretical predictions and experimental data. These generalizations refer both to the kinetic prefactor as to the method of determination of the work of formation of clusters of sub-, super- and critical sizes being essential ingredients of the theoretical description. In particular, generalizing the classical Gibbs’ theory of heterogeneous systems to thermodynamic non-equilibrium states it is shown that the properties of the clusters change significantly in dependence on their sizes. In particular, it is shown that the properties of the critical clusters deviate, as a rule, from the properties of the newly evolving macroscopic phases. As one result it turns out that the classical nucleation theory overestimates the work of critical cluster formation and underestimates the value of the steady-state nucleation rate. The application of the generalized Gibbs approach to phase formation allows one in addition a new interpretation of a variety of phenomena in crystallization processes of glass-forming melts, which could not be given a satisfactory explanation in terms of the classical theory so far, retaining on the other side the advantages of the classical approach.

Principles of Microstructural Design in Two-Phase Systems

When a polycrystal is in chemical equilibrium, the microstructure evolves as a result of grain growth under the capillary driving force arising from the interface curvature. As the growth rate of an individual grain is the product of the interface mobility and the driving force, the growth of the grain can be controlled by changing these two parameters. According to crystal growth theories, the growth of a crystal with a rough interface is governed by diffusion and its interface mobility is constant. In-contrast, the growth of a crystal with faceted interfaces is governed by the interface reaction and diffusion for driving forces below and above a critical value, respectively. As the growth rate is nonlinear for the regime of interface reaction control, the grain growth is nonstationary with annealing time. Calculations reveal that the types of nonstationary growth behavior including pseudo-normal, abnormal, and stationary are governed by the relative value of the maximum driving force, gmax, to the critical driving force for appreciable growth, gc. Recent experimental observations showing the effects of critical processing parameters on microstructural development also support the theoretical prediction. The principles of microstructural design are deduced in terms of the coupling effects of gmax and gc.