Formation Of Crystal Nuclei Research Articles

Microreactor System with in-Situ pH Monitor for Synthesizing YVO4: Bi, Eu Nanophosphor

Recently, microreactor technology has received a great deal of attention. Most of the research carried out in this field is concerned with engineering aspects of microreactors. Microreactors consist of a series of small (10-1000μm) channels connected in various geometries that allow for the spatial and temporal manipulation of small amounts of fluids and reagents.[1] Extremely fast mixing by virtue of a short diffusion path is also an important advantageous feature of microreactors for highly homogeneous preparation.[2]We pay attention to the microreactor (MR), which is one of a solution synthesis method and has advantages that the proper pH value and temperature can be maintained during the reaction. Before, we proposed the combinatorial MR and reported that it enables the automatic and precise program control of reaction conditions such as temperature, pH and mixture ratio of sources [3]. In the reaction parameters, pH value is one of the key parameters, since it affects the reaction rate and the formation of crystal nucleus. Depending on the pH value, numbers of emission centers were changed, too. In other words, the solution pH control means number of emission centers control. In our former MR system, a pH value has been measured at the precursor product. A structure of mixer element, at where solution sources meets and the reaction takes place, is also important.In this study, to achieve more precise pH control, an in-situ pH monitoring of reacting solution has been attempted to use MR for YVO4: Bi, Eu nanophosphors preparation. A mixer glass cell has also been designed to realize the quick mixture of sources, which is MR system. In-situ pH monitor is not only the powerful tool for nanophosphors production process control but also every chemical production process.YVO4: Bi, Eu nanophosphor samples were synthesized by hybrid use of MR system and solvothermal method. The precursor was prepared by citric-acid-gel method using the MR system. The system consists of liquid sources, syringe pumps, a mixer cell, a mixing/reaction tube, and an auto-sampler. The mixer cell having a 1-mm-width Y-branch channel was fabricated by sandblast method with glass substrates. A pH sensor was placed at the outlet of the mixer cell.The acid solution (pH=5) sources and the basic solution (pH=12) sources were prepared Y(CH3COO)3°E4H2O, Eu(CH3COO)3°E4H2O, C6H5Na3O7°E2H2O, Bi(NO3)3°E5H2O, C2H6O2, H2O and Na3VO4, NaOH, H2O, respectively. These two sources were mixed at Y-branch and passed through a Teflon tube (1000μm diameter) in an ultrasonic bath maintained at 85 ºC. The in-situ pH monitor was performed during the synthesis. The monitored pH level was about 8.5. The complex ions were formed several types coordinate depend on pH levels in general. Due to the small reaction channels, the pH levels are precisely monitoring during the reaction, that is in-situ pH monitored in MR.The obtained precursor was carried out solvothermal process at 130 ºC for 6 h in the autoclave. After quenching to a room temperature, nanophosphor particles were collected by centrifugation.The spherical crystal grains of the YVO4: Bi, Eu nanophosphor sample were observed about 50 nm, which consists of smaller crystallites of about 10 nm in diameter. The observed XRD patterns of YVO4 are identified as those due to the tetragonal zircon structure of YVO4. According to the PL and PLE spectra, sharp emission peaks are found in the spectral range from 580 nm to 720 nm, and all peaks are attributed to the 4f6-4f6 electron transition of the Eu3+ centers. The broad PLE band is thought to be due to YVO4 host (250 - 300 nm) and charge transfer state of Bi3+ centers (300 - 400 nm) [4]. It is particularly worth noting of MR synthesis that the Bi3+center spectrum becomes higher than the previous reports. The internal quantum efficiency is 28 % at an excitation wavelength of 365 nm. It should be noted that the reproducibility of MR synthesis is entirely improved by employing the in-situ pH monitoring. REFERENCES [1] B. P. Mason, et al., Chem. Rev. 107, 2300, (2007)[2] J. Yoshida, et al., Chem. Eur. J. 14, 7450, (2008)[3] H. Okura, et al., Proc. IDW’10, 1097 (2010)[4] G. Blasse, A. Bril, J. Chem. Phys., 48, 217 (1968)

Crystallization and creep of the graphite nanosheets based poly(butylene adipate-co-terephthalate) biocomposites

Biodegradable poly(butylene adipate-co-terephthalate) (PBAT) composites containing graphite nanosheets were prepared by melt compounding. The results reveal that the nanosheets have full distribution throughout the PBAT matrix, although they are still dispersed as the multilayered stack. Those well distributed graphite nanosheets act as the heterogeneous nucleating agent to facilitate formation of crystal nucleus and also as the physical hindrance to retard crystal growth. But the nucleation dominates overall crystallization kinetics at experimental graphite loading levels because the composites show higher crystallization rates and lower activation energies than those of the neat PBAT. The results from creep study further confirm the role of physical hindrance played by graphite nanosheets because the creep strain of system is highly restrained by the presence of nanosheets. However, this inhibiting effect is attributed to suppressed relaxation of chain coins, rather than confinement to chain segments.

Ultrasonic-assisted synthesis of aqueous CdTe/CdS QDs in salt water bath heating

A novel simple method for fast and efficient synthesis of aqueous CdTe/CdS quantum dots (QDs) with core–shell structure was developed by using salt water bath heating with the ultrasonic-assisted technique in this paper. The formation of crystal nucleus was promoted by ultrasonic and CdTe/CdS QDs with blue fluorescence were synthesized only in 0.5h. The heat source was bath heating in salt water solution at 60% NaCl and the heating temperature could reach 105°C. The heating method solved the biggest drawback of low photoluminescence quantum yield (PLQY) of ordinal bath heating in water. The preparation was cheap, simple and had less pollution to the environment. The properties of the CdTe/CdS QDs were thoroughly investigated by ultraviolet–visible (UV–vis), photoluminescence (PL), transmission electron microscope (TEM), laser size analysis, fourier transform infrared spectra (FTIR), X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). Different CdTe/CdS QDs with core shell structure were efficiently synthesized and the maximum PLQY could reach 97.13% when refluxing at 105°C for 2h. These QDs exhibited uniform dispersity, high fluorescence intensity, good optical property and long life of fluorescent. The synthesis mechanism of the core shell structure of the QDs was inferred that the Cd2+ might coordinate with sulfur (S) as well as thiol propionate (–SCH2CH2COO−1) to constitute two relatively thick compound layers on the QDs surface as passive shells.

A new strategy for protein crystallization : Effect of ionic liquids on lysozyme crystallization and morphology

Protein crystallization is a complex physical and chemical process. The high-quality protein crystal is still a persistent bottleneck to the application of X-ray crystallography in structural biology. The additives may promote formation of crystal nucleus and subsequent growth in protein crystallization. As a distinct material, ionic liquids (ILs) have aroused great attention and interest for protein crystallization due to their unique properties. We reviewed the progress of protein crystallization and reported research about protein crystal morphology control by ILs, as crystal growth template, in aqueous solutions. ILs encourage changes in some cases in terms of growth morphology and crystal size. The effect of ILs on lysozyme growth morphology can be attributed to changing interaction among lysozyme molecules in aqueous solutions. This work can provide some initial insight into the preparation of high quality crystal and the development of new crystal form.

Unraveling the Coupling between Demixing and Crystallization in Mixtures

Using molecular simulation, we shed light on the coupling between the two nonequilibrium processes of demixing and crystallization in mixtures of fully miscible, size-matched, liquid metals. We show that the competition between these processes has a strong impact on the crystallization pathway, leading to the formation of crystal nuclei with large excesses (up to 95%) in the component of higher cohesive energy, and resulting in increases of more than 30% in the free energy barrier of nucleation. The competition between demixing and crystallization goes on to impact the growth process, as shown by the large variations in composition observed at the surface and inside the growing crystallite.

A simple one-step solvothermal synthesis of hierarchically structured ZnO hollow spheres for enhanced selective ethanol sensing properties

A simple one-step solvothermal method, using ethanolamine as solvent without any additives except zinc source, has been employed to synthesize hierarchically structured ZnO hollow spheres consisting of numerous orderly and radical nanorods with diameter of several tens nanometers and length of 2–3 μm. The ethanolamine and the solvothermal process play the critical role in the synthesis of the ZnO hollow spheres by the primary formation of ZnO crystal nucleus and subsequent transformation into nanorods, which self-assemble into hollow spheres. The morphology and structure of the spheres have been characterized by transmission electron microscopy, field emission scanning electron microscopy, X-ray powder diffraction, high-resolution transmission electron microscopy, and Brunauer–Emmett–Teller N2 adsorption–desorption analyses. The results also indicate that the sensor based on the prepared ZnO hollow spheres exhibit good ethanol sensing performance, which can be attributed to its structural defects and high surface-to-volume ratio that significantly facilitate the absorption of oxygen species and diffusion of target gas. Besides, the sensor shows high selectivity to ethanol because ZnO as a basic oxide is favored for dehydrogenation of ethanol.

Mechanism of silk fibroin scaffolds with oriented multichannels and its cytocompatibility

As a biomaterial, besides excellent biocompatibility and biodegradability, suitable macropores and pores structure should be provided to guide cell extension and migration. In present study, the silk fibroin (SF) scaffold with uniaxial channels was prepared by directional temperature field freezing technique. The average pore diameter, pore density and porosity of the scaffold with oriented channels are ∼128.7 µm, ∼158 mm−2 and ∼91.4 %, respectively. By controlling of the temperature gradient direction, the oriented multichannels of the scaffolds were formed in longitudinal easily. In process of the scaffolds fabrication, the directional growth of ice crystal could shear and draft to the silk fibroin molecule segments, which resulted in the new crystal nucleus formation in new zone and increase of β-sheet components in the scaffolds. In vitro, L929 cells were seeded on the scaffolds with oriented channels to evaluate the effect on cell behavior. Cell viability, adhesion and morphology were determined by methyl thiazolyl tetrazolium, confocal microscope and scanning electron microscope. The results showed that the cells anchored on the oriented channels, spread along the direction of the channels and hold a higher viability on the scaffolds with oriented channels. These new oriented multichannel scaffold could guide the adhesion and proliferation of L929 cells, which hold a potential in tissue engineering.

Kinetics and intimate mechanism of protein crystal nucleation

Experimental and theoretical investigations on protein crystal nucleation are reviewed. Various experimental applications of the classical principle, which requires separation of the nucleation and growth stages of the crystallization process, are described. Temperature control is used most frequently, hypergravity and concentration changes being auxiliary techniques. Nucleation time-lags are measured by imposing temperature evoked supersaturation gradients. Application perspectives are revealed. Nucleation rates are measured according to the classical principle mentioned above, and energy barriers for crystal nucleation and numbers of molecules constituting the critical nuclei are calculated. Surprisingly, although requiring unusually high supersaturation, protein crystal nucleation occurs much more slowly than that with small molecule substances. On this basis novel notions are suggested for the elementary mechanism of protein crystal bond formation. Due to the selection of the crystalline bonding patches a successful collision between protein molecules, resulting in the formation of a crystalline connection, requires not only sufficiently close approach of the species, but also their proper spatial orientation. Imposing a rigid steric constraint, the latter requirement postpones the crystal nucleus formation. Besides, it was shown that cluster coalescence is not a factor, hampering the protein crystal nucleation. The comparison of the model predictions with experimental results proved that nucleation kinetics is governed by kinetic (not by energetic) factors.

Fabrication and corrosion resistance of calcium phosphate glass-ceramic coated Mg alloy via a PEG assisted sol–gel method

To slow down the initial biodegradation rate of magnesium (Mg) alloy, crack-free CaO–P2O5–SrO–Na2O glass-ceramic coatings (CaPs) were synthesized using polyethylene glycol (PEG) assisted sol–gel method followed by a heat-treatment at 400°C. The thermal behaviors of the synthesized glass-ceramics were characterized by differential scanning calorimetry and thermogravimetric (DSC/TG). The glass transition temperature (Tg) of calcium phosphate system shifted to a lower temperature (~226°C) due to the addition of PEG, which promoted the crystal nuclei formation, thereafter induced a great amount of Ca2P2O7 at a relatively low temperature. The effects of PEG addition on the corrosion behaviors of the coated Mg alloys were investigated by the electrochemical measurements and immersion tests in simulated body fluid (SBF). It was found that the coatings comprised of massive crystalline phases could slow down the degradation rate and decrease the mass loss of the Mg alloy substrates from 78.04% to 3.81% in the 7th day test, showing a better anti-corrosion property than that of the calcium phosphate coatings without the addition of PEG. Moreover, protection effect of PEG assisted synthesis of calcium phosphate coating gradually declined after the initial soaking time, which was confirmed by electrochemical impedance spectroscopy (EIS) analysis. These results implied that crystallization induced by PEG addition would affect the corrosion resistance of the coated Mg alloy and subsequently influence the degradation rate.

Binary solvent mixture-induced crystallization enhancement for a white emissive polyfluorene copolymer toward improving its electroluminescence

The morphology of fluorescent polyfluorene (PF) derivatives has strong effects on their electroluminescent (EL) performance. Herein, the influence of processing solvents on the morphology of PF-based white emissive polymer and its correlation to the EL behavior is studied. It is found that the PF copolymer films prepared from chlorobenzene:toluene (CB:TOL) solvent mixtures show stronger tendency to form crystalline α phase PF upon thermal annealing than those prepared from pure TOL or CB solvents. The evaporation rate difference in solvent mixtures can assist in the formation of crystal nucleus and enhance the crystallinity of PF backbone after thermal annealing. The results also reveal that the solvent mixture-processed fluorescent PF-based white emissive polymer not only shows more efficient blue emission, but also more balanced electron and hole transport. Accordingly, both the white emission purity and the light-emitting efficiency of the light-emitting diodes based on PF-based white emissive polymer are greatly enhanced.

Crystal Nucleation in Glassy Poly(l-lactic acid)

Aging of glassy poly(l-lactic acid) (PLLA) allows formation of crystal nuclei which enhances/accelerates subsequent crystallization at temperatures above the glass transition. The effects of the time and temperature of aging on nuclei formation have quantitatively been probed by analysis of isothermal crystallization at 393 K, using fast scanning chip calorimetry and polarizing optical microscopy. Crystal nuclei begin to form on aging the glass of PLLA at 343 K after about 101 s. The time of nuclei formation increases exponentially with decreasing temperature, so that aging at 323 K requires a minimum time of 104 s, and the extrapolated time for generation of nuclei at 295 K is about 108 s. The aging-controlled increase of the nuclei density in glassy PLLA leads to a distinct decrease of the half-time of crystallization. The half-time of crystallization of nonaged PLLA at 393 K is about 600 s and decreases to less than half of this value due to aging at 343 K for a period of only 103 s. Nuclei formation o...

A novel silk fibroin scaffolds with oriented multichannels

Silk fibroin (SF) scaffold with uniaxial channels was prepared by directional temperature field freezing technique. The scaffold morphology was observed by scanning electron microscopy and structures of the scaffold were examined by Fourier Transform Infrared (FTIR) Spectroscopy and X-ray diffractometry (XRD) analysis. The results indicated that β-sheet content of the scaffolds with oriented channels was more than that of control group. The directional growth of ice crystal could shear and draft to the silk fibroin molecule segments, which resulted in the new crystal nucleus formation in new zone and increase of β-sheet component. Dependent on this approach, the new oriented multichannel scaffolds were fabricated.

Amorphous drugs and dosage forms

The transformation to an amorphous form is one of the most promising approaches to address the low solubility of drug compounds, the latter being an increasing challenge in the development of new drug candidates. However, amorphous forms are high energy solids and tend to recrystallize. New formulation principles are needed to ensure the stability of amorphous drug forms. The formation of solid dispersions is still the most investigated approach, but additional approaches are desirable to overcome the shortcomings of solid dispersions. Spatial separation by either coating or the use of micro-containers has shown potential to prevent or delay recrystallization. Another recent approach is the formation of co-amorphous mixtures between either two drugs or one drug and one low molecular weight excipient. Molecular interactions between the two molecules provide an energy barrier that has to be overcome before single molecules are available for the formation of crystal nuclei, thus stabilizing the amorphous form.

Crystal nucleation kinetics in confined systems

In confined systems formation of crystalline nuclei is influenced by depletion of a liquid phase due to the phase transition process. As the phase transition occurs, a portion of molecules is “transferred” from a liquid to the newly forming crystalline phase (depletion of a liquid). This work deals with homogeneous crystal nucleation from supercooled melt and supersaturated solution in confined systems. Kinetic equations describing crystal nucleation (including depletion of the mother phase) are solved numerically to determine the size distribution of nuclei, nucleation rate, and the total number of nuclei. These basic characteristics of nucleation process are compared with the solution of the standard model, when the condition within the liquid does not change in time. Depletion of the liquid in confined volume influences the formation of crystal nuclei at a lower energy barrier of nucleation. In contrast to the standard model the number of nuclei and the nucleation rate as a function of time reach some maxima and then decrease.

Preparation and characterization of ATO nanoparticles by various coprecipitation

A series of Sb doped SnO2 (ATO) nanoparticles, with Sb doping levels 0–20 at.% has been prepared by two different coprecipitation routes. Effect of preparation process, Sb doping concentration and calcination temperature on the crystallinity and morphology of ATO nanoparticles were investigated and analyzed. The prepared nanoparticles were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, selective area electron diffraction, X-ray diffraction and energy dispersive X-ray spectroscopy. Results indicated that the prepared ATO nanoparticles were tetragonal, and isostructural with rutile lattice structure as known from bulk SnO2. The ATO nanoparticles prepared via process I (homogeneous coprecipitation) presented obviously weaker crystallinity, smaller average crystallite size and harder agglomeration than that prepared via process II (heterogeneous coprecipitation). The crystallinity and average crystallite size of ATO nanoparticles prepared via process II increased with increasing calcination temperatures and reducing Sb doping concentrations, respectively. The increased crystallinity, dispersibility and average crystallite size for ATO nanoparticles prepared via process II may be due to the formation of ATO crystal nuclei, leading to an improved formation dynamics of ATO nanoparticles.

Predicted Optimum Composition for the Glass-Forming Ability of Bulk Amorphous Alloys: Application to Cu-Zr-Al.

Metallic glasses have been established to have unique properties such as ductility, toughness, and soft magnetism with promising engineering applications. However, the glass-forming ability (GFA) has not been sufficient to synthesize the bulk metallic glasses (BMGs) required for many engineering applications. Attempts to develop the understanding of the GFA required to predict the optimum alloys have not yet been proven successful. We develop here a computational model based on molecular dynamics simulations that explains the dramatic change of GFA with alloying small amounts of Al into Cu-Zr. We find that the high GFA to form BMGs depends on a combination of three factors, (a) a low thermodynamic driving force for crystallization, (b) a high melt viscosity, and (c) large ratios of icosahedral structures in the liquid phase. These computational methods to predict these factors that suppress formation of crystal nuclei and slow the dynamic motions in the liquids are practical for in silico prediction of new alloys with optimal GFA.

Nucleus in a droplet

Real-time transmission electron microscopy shows that the formation of crystal nuclei of organic molecules in solution occurs inside dense liquid nanoclusters.

An atomistic level description of guest molecule effect on the formation of hydrate crystal nuclei by ab initio calculations

In the present study, we report the results of a systematic investigation of cage-like water structures using the first-principles calculations. These results show that, in the case of methane hydrate, the following nucleation mechanism can be revealed. The formation of small water cavities filled with methane is the first step of the formation of methane hydrate. It is not necessary to occupy all dodecahedral cages by guest molecules. After that small cavities start to form the H-bonding network with surrounding water molecules and a small number of water molecules is enough for the formation of a stable hydrogen-bonding network. The structural information contained in such nuclei is conserved in the forming crystal. Moreover, the presence of a methane molecule between small cages is also important to prevent the adhesion of cavities. It found that the ozone molecule can also stabilize the small cage since the value of the interaction energy between the ozone guest and the water host framework is very close to that obtained for the methane case. However, ozone affects the structure of large cavities and hence, the second guest is necessary to stabilize the hydrate structure.

Structure of aqueous sodium metaborate solutions: X-ray diffraction study

A structural analysis of aqueous sodium metaborate solutions (NaBo2 · nH2O, n = 10, 15, 20) at 298 ± 0.5 K by a rapid liquid θ-θ X-ray diffractometry with a highly effective X’celerator detector is reported in present paper. The radial distribution functions (RDF) and theoretical partial radial distribution functions for B-O, O-O, Na-O, Na-B, and Na-Na atom pairs were obtained from precisely diffraction data processing. Structure of aqueous sodium metaborate solutions was given through model calculation and described in three different items: hydrated sodium ion, hydrated metaborate ion and ion association. Effects of concentration on the structure of the solutions were discussed in detail. The mechanisms of ion aggregation and the formation of crystal nuclei in the solution are proposed. The results show that a clear picture of the structure of aqueous sodium metaborate solution has been acquired.

Identification of nucleation rates in droplet-based microfluidic systems

Characterization of the rate of nucleation of crystals from solution continues to be of interest, both for investigations into fundamental molecular phenomena as well as for applications in the pharmaceuticals, biotechnology, and fine chemicals industries. Substantial experimental evidence indicates that nucleation in some solute-solvent systems does not agree with classical theory, especially at high supersaturations. An approach is proposed for computing bounds on the nucleation rate as a function of supersaturation that does not require an assumed analytical expression for the nucleation kinetics. The approach involves (1) a microfluidic platform that measures crystal nuclei formation in droplets, (2) the analytical solution of the Chemical Master equation for nucleation that takes finite-volume effects into account, and (3) a numerical algorithm that employs linear splines to construct upper and lower bounds on the nucleation rate from the experimental data produced by the microfluidic platform. The approach is demonstrated for mean induction times measured for the nucleation of paracetamol and glycine crystals in aqueous solution. The approach can be used to suggest dependencies for the development of new nucleation expressions and for providing kinetic information needed for the simulation of crystallizers that operate at high supersaturations, such as dual-impinging-jet and vortex-mixer crystallizers.