Observing crystal growth processes in computer simulations

A crystal is defined as a solid material that is inclusive of the molecules, atoms, and ions that are arranged in an orderly fashion and a repetitive pattern in a spatial arrangement. Crystal growth is a major step in the process of crystallization. The process of crystal growth and kinetics is a dynamic process that is impacted by several factors. Crystals of a compound are a form of solid state that is one the purest form of the solid. The crystals are produced through the process of crystallization that is inclusive of two primary processes. The first process is nucleation, and the second process is crystal growth. In low supersaturation or desupersaturation, crystals are grown and accelerated with nucleation. This results in larger crystal size and distribution. A supersaturated solution is present in thermodynamic equilibrium with the solid phase at a specified temperature. The state of supersaturation is critical for the operation of the crystallization experiment. The crystallization process in the pharmaceutical sciences also helps in improved downstream processes that include filtration, milling and drying. These applications demand a crystallization of even more complicated molecules that can be used as drugs and duplication at a large scale. This paper discusses the process of crystallization of paracetamol in the solvent, isopropanol. The developed crystals have been studied in this thesis by application of the process supersaturation. The developed crystals have also been analyzed by using the PAT tools. The bioavailability of the compounds is essentially dependent on three factors that include solubility, permeability, and dosage. Based on the solubility of the compounds, the physiological pH value and permeability parameters of drugs are classified into several categories. These properties are altered and are impacted by the process of crystallization and crystal growth in the chemical setting. Paracetamol has three polymorphs with the form I being most stable and form II being the least stable. The process of crystal growth and crystallization focuses on the inclusion of the development of the most stable crystal polymorph to be used as a drug and to be mass-produced and applied in the pharmaceutical industry. The crystal data of the molecule reveals that the crystal growth of the molecule is subject to change with alterations in temperature. The bond length and bond angles for the crystal polymorphs of the paracetamol remain unchanged. The optimized geometry for a free molecule crystal is planar, where the phenyl rings and the acetamide lie in different planes. Bulk crystallization of paracetamol has been done as per the controlled conditions in clinical conditions for saturation experiments. The induction and the nucleation time in the process are determined by the changes in the concentration of the solute in the process of crystal growth. The products of the process are microcrystals and are characterized for the strain, shape, and defect in the crystals using diffraction and microscopic studies. Through this thesis, the researchers aim to establish a method of crystal formation for paracetamol through the process of supersaturation and using isopropanol as a solvent with growth trajectory by second-order growth kinetics. This will help in the development of paracetamol crystals that are highly pure and can be used for replication in the medical industry for drug production and development.

The glass formation ability of an alloy depends on two competing processes: glass-transition, on one hand, and crystal nucleation and growth, on the other hand. While these phenomena have been widely studied before in nearly equiatomic Cu-Zr alloys, studies are lacking for solute/solvent-rich ones. In the present work, molecular dynamics simulations show that the addition of a small amount of Zr (1-10 at. %) to Cu drastically increases the incubation time and slows down crystal growth, thus, leading to an improved glass forming ability. The crystal nucleation and growth processes of a competing face-centered cubic (FCC) Cu crystalline phase are analyzed in detail. In particular, the values of the critical cooling rate, incubation period for crystallization, and growth rate of FCC Cu crystals in these Cu-rich alloys are obtained. The growth of a supersaturated FCC Cu solid solution is found to be polymorphic at the interface (except for alloys with 9 and 10 at. % Zr) though a Zr concentration gradient is observed within growing crystals at high enough Zr content. The crystal growth rate before crystal impingement is nearly constant in all alloys, though it decreases exponentially with the Zr content. Crystallization kinetics are also analyzed within the existing theories and compared with the experimental values available in the literature.

In the theoretical treatment of crystallization, it is commonly assumed that the relaxation processes of a liquid proceed quickly as compared to crystal nucleation and growth processes. Actually, it is supposed that a liquid is always located in the metastable state corresponding to the current values of pressure and temperature. However, near and below the glass transition temperature, Tg, this condition is commonly not fulfilled. In such cases, in the treatment of crystallization, deviations in the state of the liquid from the respective metastable equilibrium state have to be accounted for when determining the kinetic coefficients governing the crystallization kinetics, the thermodynamic driving force of crystallization, and the surface tension of the aggregates of the newly evolving crystal phase including the surface tension of critical clusters considerably affecting the crystal nucleation rate. These factors may greatly influence the course of the overall crystallization process. A theoretical analysis of the resulting effects is given in the present paper by numerical solutions of the J(ohnson)-M(ehl)-A(vrami)-K(olmogorov) equation employed as the tool to model the overall crystallization kinetics and by analytical estimates of the crystallization peak temperatures in terms of the dependence on cooling and heating rates. The results are shown to be in good agreement with the experimental data. Possible extensions of the theory are anticipated and will be explored in future analysis.

Protocol for growing insulin crystals of uniform size

Nonisothermal crystallization kinetics of potassium chloride produced by stirred crystallization

Fine-scale layering in igneous intrusions shows parallels with experimentally produced banding observed in crystallizing salt solutions in which recent advances have demonstrated the importance of crystal aging on the development of banding. These experiments, as well as numerical models of fine-scale layering development during crystal aging, reproduce features observed in fine scale layering that are not readily explained by periodic nucleation and crystal growth models. These include the phenomenon of “lost segments”, in which the development of a layer may be reversed such that the layer disappears over time but does so without affecting the overall spacing of later-developing layers, and the development of “doublet” layers, which results when crystals at the margins of developing layers preferentially grow at the expense of those in the center. While these results suggest that crystal aging is the dominant process in the development of fine-scale layering, crystal aging may enhance the textural and modal features of igneous layering initiated by a variety of other crystal settling, nucleation or growth processes.

The formation of wax crystals in mineral oils at low temperatures due to poor solubility of n-paraffinic components profoundly affects the oil rheology. For example, under quiescent cooling, wax crystals nucleate and grow to form macroscopic structures that cause development of a yield stress and produce rheology that is stress history dependent. The crystal formation and growth process is temperature history dependent. We have explored the effects of temperature history on the flow properties of lubricating mineral oils that do not contain performance chemical additives. The evolution of viscosity with decreasing temperature (T) is characterized by a strong increase in viscosity that occurs over a narrow 3–5 K range where the onset temperature (Tc) corresponds to that at which wax crystals become microscopically visible. Increasing the cooling rate depresses Tc and causes the activation energy in the transition region to increase. These changes correlate to a decrease in the average crystal size and a strong increase in the apparent steady state viscosity of the wax crystal dispersions at T≪Tc. Despite strong effects of stress history, we show that T≪Tc steady state flow properties are determined by the cooling rate and its apparent effects on the process occurring at the onset of crystal nucleation and growth. These results are discussed in the context of a homogeneous nucleation model for wax crystallization.The formation of wax crystals in mineral oils at low temperatures due to poor solubility of n-paraffinic components profoundly affects the oil rheology. For example, under quiescent cooling, wax crystals nucleate and grow to form macroscopic structures that cause development of a yield stress and produce rheology that is stress history dependent. The crystal formation and growth process is temperature history dependent. We have explored the effects of temperature history on the flow properties of lubricating mineral oils that do not contain performance chemical additives. The evolution of viscosity with decreasing temperature (T) is characterized by a strong increase in viscosity that occurs over a narrow 3–5 K range where the onset temperature (Tc) corresponds to that at which wax crystals become microscopically visible. Increasing the cooling rate depresses Tc and causes the activation energy in the transition region to increase. These changes correlate to a decrease in the average crystal size and a st...

During bone and dentin mineralization, the crystal nucleation and growth processes are considered to be matrix regulated. Osteoblasts and odontoblasts synthesize a polymeric collagenous matrix, which forms a template for apatite initiation and elongation. Coordinated and controlled reaction between type I collagen and bone/dentin-specific noncollagenous proteins are necessary for well defined biogenic crystal formation. However, the process by which collagen surfaces become mineralized is not understood. Dentin matrix protein 1 (DMP1) is an acidic noncollagenous protein expressed during the initial stages of mineralized matrix formation in bone and dentin. Here we show that DMP1 bound specifically to type I collagen, with the binding region located at the N-telopeptide region of type I collagen. Peptide mapping identified two acidic clusters in DMP1 responsible for interacting with type I collagen. The collagen binding property of these domains was further confirmed by site-directed mutagenesis. Transmission electron microscopy analyses have localized DMP1 in the gap region of the collagen fibrils. Fibrillogenesis assays further demonstrated that DMP1 accelerated the assembly of the collagen fibrils in vitro and also increased the diameter of the reconstituted collagen fibrils. In vitro mineralization studies in the presence of calcium and phosphate ions demonstrated apatite deposition only at the collagen-bound DMP1 sites. Thus specific binding of DMP1 and possibly other noncollagenous proteins on the collagen fibril might be a key step in collagen matrix organization and mineralization.

A new crystallization micromechanism for amorphous alloys

SiC has become the key player among wide bandgap semiconductors for power electronic applications. Since the first description of the physical vapour transport (PVT) growth process of SiC by Tairov and Tsvetkov (J. Crystal Growth, 43, 209(1978)), there has been steady progress in SiC-based crystal growth, epitaxy and device processing. The success of SiC compared to Si is related to its superior material properties such as extremely high electrical breakdown field and high thermal conductivity compared to the standard silicon counterpart. In addition, SiC device processing utilises much of the standard Si processing equipment. A major reason for the success of SiC in power electronic applications compared to other wide bandgap counterparts such as GaN, Ga2O3 and diamond is related to the availability of large diameter SiC wafer materials (150mm = standard, 200mm = developped). Bulk SiC growth is now a very well developed process with comparatively high yields. The extraordinary physical properties also include obstacles related to the strong chemical bonding and complex phase diagram of the material, which pose challenges to the growth process. Therefore, there are still a number of open questions related to the nucleation, progression and termination of the bulk growth process that require fundamental research in materials science and technology.The aim of this presentation is (i) to give an overview of the state-of-the-art PVT growth process and (ii) to discuss a current research topic dealing with the early stage of the growth process and the defect formation that can occur during the initial nucleation of SiC. We have applied 3D in-situ visualisation of the growth process using X-ray computed tomography to visualise island formation on the large seeding area. These data are related to growth process instabilities such as temperature variations during the seeding process and axial doping level changes from the seed to the newly grown crystal. Both process instabilities induce mechanical stress on the SiC lattice and act as sources for dislocation generation and multiplication. We will show a series of growth processes with varying growth parameters that shed light on the initial growth stage of SiC.As the crystal diameter of SiC increases from 150 mm to 200 mm, the results of this study become increasingly important.

This chapter reviews strategies and methods for achieving control over the crystallization process in order first to screen for polymorphism and subsequently to prepare various polymorphs in a controlled and robust manner. It describes the process of crystallization, i.e., nucleation and crystal growth, and the design of the strategies for crystallization with a monotropic or enantiotropic system. The topic of concomitant polymorphs, including thermodynamic and structural aspects is introduced, followed by a discussion of disappearing polymorphs. The chapter closes with comments on experimental design strategy for the control of crystallizations that may result in polymorphic structures.

Metallic glasses as non-equilibrium materials crystallize at a critical temperature, accompanying their embrittlement. The size of glassy alloys is dependent on their glass-forming ability. The development of an appropriate joining technique is important for the extension of industrial applications. In the present study, we examine crystallization behavior of Zr55Al10Ni5Cu30 glassy alloy upon annealing. In the beginning of the crystallization process, a clustering, a crystal nucleation and the crystal growth processes take place sequentially.The phase transition behavior was examined by differential scanning calorimetry (DSC). The kinetics of the crystallization process can be analyzed by the changes in the volume fraction of the crystalline phase which scales with the exothermic heat release and density measurement. The crystallization process in the Zr55Al10Ni5Cu30 alloy is found to be diffusion-controlled. The values of the Avrami exponent are between 2.3 and 2.8. Therefore, the velocity of nucleation and crystallization processes should scale with the diffusion velocity of the constituent elements in the metallic glass. The diffusion rate can be estimated using the diffusion coefficients of the constituent elements, as a function of time and temperature. The slope of the calculated iso-precipitation line of CuZr2 phase on the fixed TTT diagram is well fitted with the lines representing ductile and brittle behavior of the sample. The results indicate that the most harmful crystalline phase, which causes embrittlement is CuZr2. The influence of Si contamination on the crystallization behavior is also studied in the present work.

A micromechanism for crystallization of amorphous alloys I. An in situ TEM observation

Time-resolved reflection spectroscopic measurements are made for the kinetic analyses of the nucleation and growth processes of soft-type colloidal crystals of silica spheres (110 nm in diameter) in the presence of sodium chloride. Fast-scanning reflection spectra are taken using a continuous circulating-type stopped-flow cell system. The cell system is composed of a peristaltic pump and a quartz flow cell, which are connected with a PharMed tube in a closed circuit. The volume fraction of the spheres is 0.028. Induction periods range from 0.2 to 1.3 s and increase as salt concentration increases. Nucleation rates are 1 × 104 to 7 × 104 spheres/mm3s and decrease as salt concentration increases. The crystallization process has been observed from the sharpening and the increase in intensity of the reflection peaks. The crystal growth rate in the absence of salt is 23 μm/s, and decreases as salt concentration increases. The importance of electrostatic intersphere repulsion through the electrical double layers and the cooperative and synchronous fluctuation of colloidal spheres in the crystallization processes is supported.

In the present work variation of the two main parameters–viscosity and critical cooling rate, required for the recent upcoming optical data storage devices, have been analysed for different compositions of Se80-xSbxTe20 (0≤x≤9) system. Temperature dependence of viscosity has been determined using the activation energy for glass transition and for crystallization obtained from heating rate dependence of glass transition temperature and peak crystallization temperature from DSC measurements. Vicsocity has been estimated with the help of Vogel-Tamman-Fulcher equation. It has been found that for samples containing Sb≤4 at.%, the viscosity increases with the increase in Sb content. Further increase in Sb concentration causes a decrease in the viscosity. These results are consistent with the structural changes taking place in the Se–Te system with increase in Sb content. Using these values of viscosity and kinetics of crystal nucleation and growth process, the time-temperature-transformation curves were obtained. The critical cooling rate Rc is found to increase for Sb>4 at.%. For good reversible phase change optical recording, the critical cooling rate of the material should be high enough so as to get an erasure time of less than 1 µs. From the analysis of the data the suitability of the SeTeSb system for optical recording has been discussed.