Slow Crystal Growth Research Articles

Nucleation in small scale multicrystalline silicon ingots

Small scale solidification experiments were performed in order to study nucleation mechanisms of solar cell silicon. Ingots were grown in a Bridgman furnace; with a high rate (5cm/min), inducing dendrite-like grains; and at a slow rate (0.2mm/min), simulating the common slow crystal growth process. Two types of silicon were used, polysilicon and compensated material. The results showed that for the early stages of silicon solidification, the compensated material behaves similar to the polysilicon. A high undercooling of 11±3K was obtained for one of the fast cooled experiments. This suggests that Si3N4-coating is not the important factor for nucleation, but Si3N4-precipitates in the melt could contribute as inoculants. Grains with similar orientation were observed for both the solidification rates, which indicates that the most important issue for grain growth selection in PV silicon is control of the vertical growth, rather than nucleation substrates.

Effects of Humidity and Surfaces on the Melt Crystallization of Ibuprofen

Melt crystallization of ibuprofen was studied to understand the effects of humidity and surfaces. The molecular self-assembly during the amorphous-to-crystal transformation was examined in terms of the nucleation and growth of the crystals. The crystallization was on Al, Au, and self-assembled monolayers with –CH3, –OH, and –COOH functional groups. Effects of the humidity were studied at room temperature (18–20 °C) with relative humidity 33%, 75%, and 100%. Effects of the surfaces were observed at −20 °C (relative humidity 36%) to enable close monitoring with slower crystal growth. The nucleation time of ibuprofen was faster at high humidity conditions probably due to the local formation of the unfavorable ibuprofen melt/water interface. The crystal morphologies of ibuprofen were governed by the nature of the surfaces, and they could be associated with the growth kinetics by the Avrami equation. The current study demonstrated the effective control of the melt crystallization of ibuprofen through the melt/atmosphere and melt/surface interfaces.

Inkjet printable aqueous composite dye–polymer nanoparticles

Here we report the synthesis of polymer stabilised anthraquinone dye-based aqueous nanoparticles using evaporative precipitation from dichloromethane into water and their use as novel aqueous inkjet colorants. Polymer stabilised dye nanoparticles in the 100 nm size range are demonstrated and the mechanism of formation of stable nanoparticles and the role of polymer stabiliser design is discussed. Formation of stable small particle size nanoparticles requires a fast dye crystal nucleation rate followed by a subsequent slow growth rate so as to avoid Ostwald ripening. Rapid nucleation can be achieved by metering small amounts of water-miscible organic solution of the dye to the non-solvent water under rapid mixing, referred to as quasi-emulsion solvent diffusion (QESD). These conditions result in high-supersaturation which causes spontaneous nucleation, followed by a rapid decrease in local supersaturation and hence slow crystal growth rate.

Study on the Oiling-out and Crystallization for the Purification of Idebenone

Oiling-out, also termed as demixing or liquid–liquid phase separation (LLPS), frequently occurs in the crystallization process of pharmaceuticals and normally results in slow crystal growth, uncontrollable crystal morphology, and low purity of products. In this work, the oiling-out and cooling crystallization of idebenone were investigated. The cooling crystallization and the purity of its products were found to depend upon whether an oiling-out occurred in the course of cooling. At low initial concentrations (e.g., 12 mg/mL), oiling-out did not take place, and the crystallization products possessed higher purity. At moderate initial concentrations (e.g., 25 mg/mL) small quantities of oil droplets formed and then coalesced into larger droplets, which subsequently crystallized into products with lower purity. When high initial concentrations (e.g., 31 mg/mL) were employed, large quantities of oil droplets formed a stable oil phase, and thus no products could be harvested. In addition to initial concentration, other parameters such as solvent, cooling rate, and seeding also affected the occurrence of oiling-out. On the basis of the above results, the approach to purifying idebenone through cooling crystallization will be found.

Investigation of the crystal structure of a basic bismuth(iii) nitrate with the composition [Bi6O4(OH)4]0.54(1)[Bi6O5(OH)3]0.46(1)(NO3)5.54(1)

A basic bismuth(III) nitrate with the composition [Bi(6)O(4)(OH)(4)](0.5)[Bi(6)O(5)(OH)(3)](0.5)(NO(3))(5.5) formed in a slow crystal growth mode has an ordered crystal structure with the monoclinic space group P2(1) and lattice parameters a = 15.850(3), b = 14.986(3), c = 18.230(4) Å, β = 107.329(17)° and volume V = 4133.6 Å(3) (Henry et al. 2003). In a very fast crystal growth mode the complex ions disorder in another P2(1) cell with slightly different lattice parameters a = 15.8404(1), b = 15.1982(1), c = 18.3122(1) Å, β = 106.829(1)° and V = 4219.8 Å(3). This cell can be related to two smaller cells: a monoclinic C2/m cell with a = 13.7161(1), b = 15.1943(1), c = 10.2399(1) Å, β = 98.586(1)° and V = 2110.1 Å(3) and a trigonal R3 cell with a = 15.18650(6), c = 15.8416(1) Å (hexagonal setting) and V = 3164.1 Å(3). These smaller cells correspond to average structures and hence the X-ray data do not account for the difference in the structures of the two different complex ions. However, when analysing neutron powder diffraction data, it is possible to distinguish between the two complex ions using a trigonal R3 cell with a = 15.1865(1) and c = 15.8416(1) Å (hexagonal setting). In a Rietveld type structure model refinement with a total of 28 atom sites (4 Bi, 3 N, 15 O and 6 H), the composition of this sample is determined to be [Bi(6)O(4)(OH)(4)](0.54(1))[Bi(6)O(5)(OH)(3)](0.46(1))(NO(3))(5.54(1)).

Ultrasonic pretreatment for hydrothermal synthesis of SAPO-34 nanocrystals

Synthesis of SAPO-34 nanocrystals which has been recently considered as a challenging task was successfully performed by sonochemical method using TEAOH as structure directing agent (SDA). The products were characterized by XRD, SEM, EDX, BET and TGA. The average crystal size of the final product prepared sonochemically is 50 nm that is much smaller than that of synthesized under hydrothermal condition and the morphology of the crystals changes from uniform spherical nanoparticles to spherical aggregates of cube type SAPO-34 crystals respectively. In the case of sample synthesized sonochemically with aid of hydrothermal condition, the surface area is significantly upper than that of obtained by the conventional static hydrothermal technology with almost the same crystallinity. SAPO-34 framework synthesized by just ultrasonic treatment is unstable and a significant part of SAPO-34 nanocrystals is transformed to the dense phase of AlPO 4 structure, i.e., Cristobalite. Contrary to hydrothermal method that at least 24 h of the synthesis time is required to obtain fully crystalline SAPO-34, sonochemical-assisted hydrothermal synthesis of samples leads to form fully crystalline SAPO-34 crystals taking only 1.5 h. In a sonochemical process, a huge density of energy for crystallization is provided by the collapse of bubbles which formed by ultrasonic waves. The fact that small SONO-SAPO-34 crystals could be prepared by the sonochemical method suggests a high nucleation density in the early stages of synthesis and slow crystal growth after nucleation.

Controlling Zeolitic Imidazolate Framework Nano- and Microcrystal Formation: Insight into Crystal Growth by Time-Resolved In Situ Static Light Scattering

We report on a simple and straightforward method that enables the rapid room-temperature production of nanocrystals (finely tuned in size between ∼10 and 65 nm) and microcrystals (∼1 μm) of the prototypical microporous zeolitic imidazolate framework (ZIF) material ZIF-8. Control of crystal size is achieved in a novel approach by employing an excess of the bridging bidentate ligand and various simple auxiliary monodentate ligands with different chemical functionalities (carboxylate, N-heterocycle, alkylamine). The function of the monodentate ligands can be understood as a modulation of complex formation and deprotonation equilibria during crystal nucleation and growth. Using time-resolved static light scattering, the functioning of modulating ligands is monitored for the first time by in situ experiments, which offered significant insight into the crystal growth processes. Formation of nanocrystals is characterized by continuous, comparatively slow nucleation and fast crystal growth occurring on a time scale of seconds. Although nucleation and growth are not separated from each other, a significant narrowing of the particle size distribution during early stages results in rather monodisperse nanocrystals, before broadening of the particle size distribution occurs, as observed by complementary ex situ electron microscopy studies. Microcrystal growth is dominated by a particle−monomer addition mechanism, but indications for the operation of a coalescence process during early stages of growth have been also obtained. During later stages of microcrystal growth crystals change their shape from cubes to rhombic dodecahedra. The prepared phase-pure ZIF-8 nanoscale materials exhibit good thermal stability in air and large surface areas, which are comparable to those of large macrocrystals. Nanocrystal powders exhibit dual micro- and mesoporosity.

Nucleation of Polypropylene Homo- and Copolymers

Abstract The combination of moderately slow crystal growth at large undercoolings together with the practical absence of sporadic nucleation makes isotactic polypropylene (iPP) an ideal material for controlled nucleation. In this review the different types of nucleating agents – inorganic and organic, particulate and soluble – for the different crystal modifications of iPP (α, β and γ) are presented together with their working mechanism and criteria for activity. The interaction between polymer type, nucleating agent and processing conditions in determining mechanical and optical properties conclude the survey.

In Situ Study of Nanotemplate-Induced Growth of Lysozyme Microcrystals by Submicron Gisaxs

Nanoworld Institute, University of Genoa and Fondazione EL.B.A., Italy.AbstractA new in situ μGISAXS technique was used to study at ESRF the nucleation and growth mechanisms of lysozyme microcrystals with and without the thin Langmuir-Blodgett lysozyme film surface. Following recent studies on thaumatin crystal growth only in presence of LB monolayers (1.2) , in this work ultrasmall Lysozyme microcrystals are grown by classical hanging vapor diffusion and by its modification using homologous protein thin-film template displaying a long-range order. Intensity fluctuations in the μGISAXS pattern versus time for the LB-induced crystallization process appears associated to rapid seed formation and crystal growth, while the classical method continuous shift of intensity in the Yoneda region is compatible with slow crystal growth. The nucleation and growth mechanisms of lysozyme microcrystals are thereby studied at the thin lysozyme film surface by the new in situ μGISAXS technique developed at the microfocus beamline of the European Synchrotron Radiation in Grenoble. New insight on the nucleation and crystallization processes appear to emerge.Pechkova, E., Gebhardt, R., Riekel, C. & Nicolini, C. , In Situ GISAXS: I. Experimental Setup for Submicron Study of Protein Nucleation and Growth, Biophysical Journal(2010) doi:10.1016/j.bpj.2010.03.069 in pressGebhardt, R., Pechkova, E., Riekel, C. & Nicolini, C., In Situ mGISAXS: II. Thaumatin Crystal Growth Kinetic, Biophysical Journal(2010), doi:10.1016/j.bpj.2010.03, in press

Pressure-amorphized cubic structure II clathrate hydrate: crystallization in slow motion

A range of techniques has so far been employed for producing amorphous aqueous solutions. In case of aqueous tetrahydrofuran (THF) this comprises hyperquenching of liquid droplets, vapour co-deposition and pressure-induced amorphization of the crystalline cubic structure II clathrate. All of these samples are thermally labile and crystallize at temperatures above 110 K. We here outline a variant of the pressure-amorphization protocol developed by Suzuki [Phys. Rev. B, 2004, 70, 172108], which results in a highly crystallization resistant amorphous THF hydrate. The hydrate produced according to our protocol (annealing to 180 K at 1.8 GPa rather than to 150 K at 1.5 GPa) does not transform to the cubic structure II THF clathrate even at 150 K. We track the reason for this higher stability to the presence of crystalline remnants when following the Suzuki protocol, which are removed when using our protocol involving higher pressures and an annealing step. These crystalline remnants later serve as crystallization seeds lowering the thermal stability of the amorphous sample. Our protocol thus makes a purely amorphous THF hydrate available to the research community. We use powder X-ray diffraction to study the process of nucleation and slow crystal growth in the temperature range 160-200 K and find that the local cage structure and periodicity of the fully crystalline hydrate develops even at the earliest stages of crystallization, when the "clathrate crystal" has a size of about two unit cells.

Tuning the structure of non-equilibrium soft materials by varying the thermodynamic driving force for crystal ordering

The present work explores the ubiquitous morphological changes in crystallizing systems with increasing thermodynamic driving force based on a novel dynamic density functional theory. A colloidal ‘soft’ material is chosen as a model system for our investigation since there are careful colloidal crystallization observations at a particle scale resolution for comparison, which allows for a direct verification of our simulation predictions. We particularly focus on a theoretically unanticipated, and generic, morphological transition leading to progressively irregular-shaped single crystals in both colloidal and polymeric materials with an increasing thermodynamic driving force. Our simulation method significantly extends previous ‘phase field’ simulations by incorporating a minimal description of the ‘atomic’ structure of the material, while allowing simultaneously for a description of large scale crystal growth. We discover a ‘fast’ mode of crystal growth at high driving force, suggested before in experimental colloidal crystallization studies, and find that the coupling of this crystal mode to the well-understood ‘diffusive’ or ‘slow’ crystal growth mode (giving rise to symmetric crystal growth mode and dendritic crystallization as in snowflakes by the Mullins–Sekerka instability) can greatly affect the crystal morphology at high thermodynamic driving force. In particular, an understanding of this interplay between these fast and slow crystal growth modes allows us to describe basic crystallization morphologies seen in both colloidal suspensions with increasing particle concentration and crystallizing polymer films with decreasing temperature: compact symmetric crystals, dendritic crystals, fractal-like structures, and then a return to compact symmetric single crystal growth again.

Effect of Polymer Tacticity on the Performance of Poly(N,N-dialkylacrylamide)s as Kinetic Hydrate Inhibitors

Kinetic hydrate inhibitors (KHIs) are water-soluble polymers designed to delay gas hydrate formation in upstream gas and oilfield operations. Most classes of KHI polymers that have been discovered are vinylic polymers made by radical polymerization of vinyl monomers, giving atactic polymer structures. Atactic polymers may not be optimal for KHI performance. However, metallocene-mediated stereospecific coordination polymerization of polar vinyl monomers to stereoregular polymers can be carried out to give syndiotactic and isotactic polymers of certain monomers with molecular-weight control. This was carried out for several poly(N,N-dialkylacrylamide)s. We have carried out structure II gas hydrate KHI ramping tests in high-pressure autoclaves on the poly(N,N-dialkylacrylamide)s. Clearly, polymer tacticity does affect the performance of this class of polymers. Isotactic polyacryloylpyrrolidine (PAP) gave shorter induction times than atactic PAP. Assuming that the primary nucleation inhibition mechanism is perturbation of the water structure, these observations can be rationalized in that the side groups in isotactic PAP are more crowded together than in atactic PAP, which results in less water perturbation for the isotactic PAP and, therefore, less effect on disrupting hydrate nucleation. However, isotactic PAP gave longer periods of slow hydrate crystal growth than atactic PAP. This could be rationalized by considering that the isotactic PAP has a higher concentration of side groups along one side of the polyvinyl backbone than atactic PAP, which may give it better adsorption onto the growing hydrate crystal surfaces. All three different tacticity poly(N,N-dimethylacrylamide)s (PDMAA)s were poorer nucleation inhibitors than the PAPs. Syndiotactic PDMAA gave, on average, longer induction times than isotactic or atactic PDMAA. This can be rationalized by the fact that the syndiotactic polymer has alternating up and down dimethylamide side groups, giving the best surface/volume ratio for this polymer tacticity and, therefore, the best perturbation of the water structure. Again, this is assuming that is the dominant nucleation inhibition mechanism. The isotactic PDMAA clearly gave shorter slow crystal growth periods than the syndiotactic polymer, while the atactic polymer gave the longest times. For the isotactic polymer, this is the opposite of the results that we obtained with the PAPs, in which the isotactic polymer gave the longest hydrate slow growth periods.

Overcoming merohedral twinning in crystals of bacteriorhodopsin grown in lipidic mesophase

Twinning is one of the most common crystal-growth defects in protein crystallography. There are neither efficient rational approaches for the growth of nontwinned protein crystals nor are there examples of systematic studies of the dependence of the twinning-ratio distribution on crystallization conditions. The description of the twinning phenomenon has been covered even less for membrane-protein crystals and is non-existent for crystals grown using lipidic phases (in meso). In the present work, possibilities for overcoming merohedral twinning are investigated for crystals of the membrane protein bacteriorhodopsin (bR) grown in meso. It is shown that traditional crystallization additives are not effective in the case of the in meso crystallization of bR. The twinning ratio was determined for 310 crystals grown under different crystallization conditions. A correlation of the twinning ratio with the growth rate of the crystals was observed. Slow growth indicated that crystals had a noticeable chance of avoiding twinning. Model calculations were performed in order to rationalize this observation. The calculations confirmed the experimental observation that most crystals consist of two twin domains and showed that under this condition small changes in the probability of twin-domain formation lead to dramatic changes in the number of nontwinned crystals, which explains why slow crystal growth results in a considerable number of nontwinned crystals.

Real-Time In Situ Atomic Force Microscopy Imaging of Bismuth Crystal Growth

We have performed real-time atomic force microscopy (AFM) imaging of bismuth crystals that were grown under electrochemical control at low overpotentials to ensure a slow growth rate and allow in situ observation of the growth. A two step chronoamperometric potential was applied to a boron-doped diamond (BDD) working electrode with a short high overpotential, -0.4 V (2 s), to nucleate the bismuth, and then a long low overpotential for slow growth, -0.32 V (4.4 h). Growth rates of individual crystals and detailed growth mechanisms could be followed in real time because of the slow crystal growth. The close proximity of the AFM tip and tip holder to the working electrode appears to hinder the diffusion of bismuth to the BDD surface, as evidenced by the significantly lower density of crystals under the cantilever as compared to the rest of the electrode, therefore slowing down the growth process.

The distribution of impurities in lysozyme crystals

The effective distribution coefficient, K Ieff , of turkey egg white lysozyme into host hen egg white lysozyme crystals was measured for varying levels of relative supersaturation, impurity concentrations and temperatures. Turkey egg white lysozyme served as a test impurity of high homology to hen egg white lysozyme. K Ieff increased with increasing host protein concentration and temperature, consistent with a kinetically controlled incorporation process. The principles of slow crystal growth rate to limit impurity incorporation were applied to a commercial source of lysozyme with concomitant improvement in purity of re-crystallized protein.

Yielding in ethylene/methacrylic acid ionomers

Ethylene/methacrylic acid (E/MAA) ionomers exhibit a complex morphology – consisting of polyethylene crystals, amorphous polymer segments and ionic aggregates – as well as pronounced differences in mechanical properties compared with the E/MAA copolymers from which they are derived. Here, we illuminate the microstructural origins of the changes in one such property – the yield stress – imparted to E/MAA by partial neutralization with sodium. The yield stress reflects contributions from both polyethylene crystal plasticity and incomplete mechanical relaxation of the ion-containing amorphous phase; the amorphous phase, in turn, consists of ion-rich aggregates and ion-poor domains, with widely separated relaxation rates. The inability of the amorphous material immediately surrounding the ionic aggregates to relax, except at extremely low strain rates, greatly increases the yield stress of the ionomers. Only a minor fraction of the E/MAA groups must be neutralized to create ion-rich aggregates, and thus to achieve the limiting yield stress behavior. The slow growth of thin polyethylene crystals also has a marked influence; as they form after quenching from the melt, these secondary crystals bridge the gaps between the locally-vitrified amorphous regions, leading to a large increase in yield stress.

Perpendicular Branching in Crystal Growth of 3D Architecture‐Tuned Cadmium Hydroxide Arrays: From Oriented Tripods to Faceted Crystals

A chemical liquid-phase deposition method is used to synthesize perpendicularly branched nanotripods on substrates through continuous replenishment of nutrients into solution. A good control in crystal growth from oriented tripods to faceted crystals has been achieved, and more complex nanostructures are expected to be prepared by this slow crystal growth process.

Simple Route to Monodispersed Silica−Titania Core−Shell Photocatalysts

A monodispersed silica-titania core-shell photocatalyst was synthesized via a sol-gel route without the need of pH adjustment, cationic polyelectrolytes, or surfactants in a process where silica spheres were impregnated with hydrolyzed titanium tetrabutoxide, incubated at room temperature, and then condensed using an ethanol/water (1:1) solvent. Four coating cycles in a 10% v/v titania sol produced homogeneous titania shells. The quality of catalysts was assessed quantitatively using Rietveld analysis of powder X-ray diffraction patterns combined with X-ray fluorescence spectrometry. During calcination, the anatase-to-rutile transformation was delayed to 1000 degrees C, which is approximately 300 degrees C higher than usually observed. The thermal stability and surface area of titania were enhanced through the slow crystal growth of anatase. The photocatalytic activity of the core-shell photocatalysts calcined at 400-600 degrees C was found to be proportional to the thickness of titania but did not directly correlate with the surface area.

Role of chloride ions in the formation of Au@Ag core–shell nanocrystal structures by using a microwave–polyol method

Roles of chloride ion in the formation of [email protected] core–shell nanocrystal structures through a separated two-step process have been studied using a microwave (MW)-polyol method. The first step is to reduce HAuCl4 precursor to form Au cores and the second is to grow Ag shells epitaxially around the pre-existing Au nuclei by reducing AgNO3 at various Cl− concentrations. Without Cl− ions in the second step, dominant product is spherical Ag particles and few [email protected] particles can be produced. While, higher Cl− ion concentrations lead to the formation of a significant amount of AgCl due to the reaction between Cl− from HAuCl4 or NaCl and Ag+ from AgNO3, which will remarkably interfere with the formation of [email protected] nanocrystals. A small amount of Cl− anions (∼0.3 mM) has been found to be a key factor for the preferential formation of [email protected] nanocrystals with well-defined shapes. The role of Cl− anions has been discussed in terms of shape selective oxidative etching of spherical Ag nanoparticles and the formation of AgCl leading to slow face-selective crystal growth by decreasing the concentration of free Ag+ ions. Under our MW heating condition, spherical Ag particles or shells are preferentially etched, while single crystal cubic, twinned bi-pyramidal, and five-twinned nano-rod/-wire structures can survive and further grow into larger particles or [email protected] core–shell nanostructures with clear facets and shape edges. This finding provides new information in stability of twinned [email protected] core–shell and Ag nanoparticles against oxidative etching by Cl−/O2 (dissolved in solvent).

On the Slow Kinetics of Protein Crystallization

Although they require surprisingly high supersaturations, both nucleation and growth of protein crystals proceed substantially more slowly as compared to small molecule crystallization. The slow nucleation of the protein crystals is explained by the steric restriction for association of the protein molecules that is due to their highly inhomogeneous surfaces. Over their surfaces, the proteins exhibit a limited number of discrete patches that are the only attractive portions on the molecule. A simple model for protein crystal nucleation has been devised on this basis, which enables calculations of nucleus form, energy barrier, and probability for nucleus formation. It turns out that the unusually high supersaturations used experimentally are necessary for the formation of relatively small nuclei, because the probability for bigger fluctuations is negligibly small. On the other hand, the big size of the protein molecules contributes additionally for the slow growth of the protein crystals. It causes huge Burgers vectors of the dislocations. In turn, this leads to a locally increased chemical potential and an effective decrease of the local supersaturation at the step source.