Showing papers on "Crystallization published in 2005"

Mesocrystals: Inorganic superstructures made by highly parallel crystallization and controlled alignment

Abstract: Controlled self-organization of nanoparticles can lead to new materials. The colloidal crystallization of non-spherical nanocrystals is a reaction channel in many crystallization reactions. With additives, self-organization can be stopped at an intermediary step-a mesocrystal-in which the primary units can still be identified. Mesocrystals were observed for various systems as kinetically metastable species or as intermediates in a crystallization reaction leading to single crystals with typical defects and inclusions. The control forces and mechanism of mesocrystal formation are largely unknown, but several mesocrystal properties are known. Mesocrystals are exiting examples of nonclassical crystallization, which does not proceed through ion-by-ion attachment, but by a modular nanobuilding-block route. This path makes crystallization more independent of ion products or molecular solubility, it occurs without pH or osmotic pressure changes, and opens new strategies for crystal morphogenesis.

Crystal modifications and thermal behavior of poly(L-lactic acid) revealed by infrared spectroscopy

Abstract: Recently, we reported the isothermal crystallization behaviors of poly(l-lactic acid) (PLLA) from the melt and glassy states, respectively [J. Phys. Chem. B 2004, 108, 11514; Macromolecules 2004, 37, 6433]. Surprisingly, the quite different infrared (IR) spectral evolutions occur in the two crystallization processes at different temperatures in which the same crystal modification is expected to be formed. To clarify this unusual phenomenon, the crystal modifications and thermal behavior of PLLA samples prepared under different crystallization temperatures are investigated in detail by TEM, WAXD, and FTIR techniques. On the basis of the WAXD and IR data, a new crystal modification named the α form is proposed for the crystal structure of PLLA samples annealed at temperature below 120 °C. Such crystal modification with loose 103 helical chain packing is less thermally stable than the standard α form of PLLA. This assignment can explain all the experiment observations well. Other possible mechanisms for the ...

The Effect of Electrolyte Composition on the Fabrication of Self-Organized Titanium Oxide Nanotube Arrays by Anodic Oxidation

Abstract: We report on the fabrication of self-organized titanium oxide nanotube arrays of enhanced surface area prepared by anodic oxidation of a pure titanium sheet in electrolyte solutions containing potassium fluoride (KF) or sodium fluoride (NaF). The effects of electrolyte composition and concentration, solution pH, and the anodic potential on the formation of nanotubes and dimensions of the resulting nanotubes are detailed. Although nanotube arrays of length greater than 500 nm are not possible with hydrofluoric acid containing electrolytes [G.K. Mor, O.K. Varghese, M. Paulose, N. Mukherjee, C.A. Grimes, J. Mater. Res. 18, 2588 (2003)], by adjusting the pH of a KF containing electrolyte to 4.5 using additives such as sulfuric acid, sodium hydroxide, sodium hydrogen sulfate, and/or citric acid, we could increase the length of the nanotube-array to approximately 4.4 μm, an order of magnitude increase in length. The as-prepared nanotubes are composed of amorphous titanium oxide. Independent of the electrolyte composition, crystallization of the nanotubes to anatase phase occurred at temperatures ≥280 °C. Rutile formation occurred at the nanotube-Ti substrate interface at temperatures near 480 °C. It appears geometry constraints imposed by the nanotube walls inhibit anatase to rutile transformation. No disintegration of the nanotube array structure is observed at temperatures as high as 580 °C. The excellent structural and crystal phase stability of these nanotubes make them promising for both low- and high-temperature applications.

Nonclassical crystallization: mesocrystals and morphology change of CaCO3 crystals in the presence of a polyelectrolyte additive.

Abstract: Crystallization of calcite from differently concentrated calcium chloride solutions by the CO2 gas diffusion technique in the presence of polystyrene sulfonate yields crystal superstructures with unusual morphology. From the typical calcite rhombohedra as a starting situation, the morphology can be systematically varied via rounded edges and truncated triangles to finally concavely bended lens-like superstructures. Although these “crystals” are apparently well-faceted in light microscopy, electron microscopy analysis and BET reveal that the structures are highly porous and are composed of almost perfectly 3D-aligned calcite nanocrystals scaffolded to the final, partly nicely curved superstructures. At high supersaturations, superstructures with changed symmetry indicative of dipolar interaction potentials between the building blocks are found. The present model case also gives evidence for the importance of nonclassical, mesoscopic processes in crystallization in general.

Heterogeneity in polymer melts from melting of polymer crystals

Abstract: Semi-crystalline polymers containing amorphous and crystalline regions usually have intimately mixed chains. The resulting topological constraints (entanglements) in the amorphous regions limit the drawability in the solid state. By controlled synthesis the number of entanglements can be reduced. Ultimately, crystals composed of single chains are feasible, where the chains are fully separated from each other. If such separation can be maintained in the melt a new melt state can be formed. Here we show that through slow and carefully controlled melting such polymer crystals form a heterogeneous melt with more entangled regions, where the chains are mixed, and less entangled ones, composed of individually separated chains. Chain reptation, required for the homogenization of the entanglement distribution, is found to be considerably hindered. The long-lived heterogeneous melt shows decreased melt viscosity and provides enhanced drawability on crystallization. This novel route to create heterogeneous melt should be applicable to polymers in general.

Choosing the Crystallization Path Less Traveled

Abstract: Many organisms use a most unexpected strategy for forming large single crystals such as those that make up mineralized skeletal parts. Although nucleation and growth of a crystal occur from a solid disordered phase that has the characteristics of a melt phase, the process is accomplished at ambient temperatures and pressures. This strategy allows organisms to mold the crystals into unusual shapes and orient them at will.

Microstructure of Fragile Metallic Glasses Inferred from Ultrasound-Accelerated Crystallization in Pd-Based Metallic Glasses

Abstract: By utilizing ultrasonic annealing at a temperature below (or near) the glass transition temperature Tg, we revealed a microstructural pattern of a partially crystallized Pd-based metallic glass with a high-resolution electron microscopy. On the basis of the observed microstructure, we inferred a plausible microstructural model of fragile metallic glasses composed of strongly bonded regions surrounded by weakly bonded regions (WBRs). The crystallization in WBRs at such a low temperature under the ultrasonic vibrations is caused by accumulation of atomic jumps associated with the beta relaxation being resonant with the ultrasonic strains. This microstructural model successfully illustrates a marked increase of elasticity after crystallization with a small density change and a correlation between the fragility of the liquid and the Poisson ratio of the solid.

Poromechanics of freezing materials

Abstract: When subjected to a uniform cooling below the freezing point a water-infiltrated porous material undergoes a cryo-deformation resulting from various combined actions: (i) the difference of density between the liquid water and the ice crystal, which results in the initial build-up of an in-pore pressure at the onset of crystallization; (ii) the interfacial effects arising between the different constituents, which eventually govern the crystallization process in connection with the pore access radius distribution; (iii) the drainage of the liquid water expelled from the freezing sites towards the air voids; (iv) the cryo-suction process, which drives liquid water towards the already frozen pores as the temperature further decreases; (v) the thermomechanical coupling between the solid matrix, the liquid water and the ice crystal. We work out a comprehensive theory able to encompass this whole set of actions. A macroscopic approach first provides the constitutive equations of freezing poroelastic materials, including the interfacial energy effects. This approach reveals the existence of a thermodynamic state function—namely the liquid saturation degree as a function of the temperature only. The macroscopic ice-dependent poroelastic properties are then upscaled from the knowledge of the elastic properties of the solid matrix, of the pore access radius distribution, and of the capillary curve. The theory is finally illustrated by analysing quantitatively the effects of the cooling rate and of the pore radius distribution upon the cryo-deformation of water-infiltrated porous materials. The theory succeeds in accounting for the experimentally observed shrinkage of embedded air voids, while predicting the partial melting of the ice already formed when the cooling suddenly stops.

Nucleation and crystallization in diblock and triblock copolymers

Abstract: Crystallization of block copolymer microdomains can have a tremendous influence on the morphology, properties and applications of these materials. In this review, particular emphasis is placed on the nucleation, crystallization, thermal properties and morphology of diblock and triblock copolymers with one or two crystallizable components. The issues of the different types of nucleation processes (i.e., homogeneous nucleation and heterogeneous nucleation by different types of heterogeneities and surface nucleation) and their relation to the crystallization kinetics of the components is addressed in detail in a wide range of polymeric materials for droplet dispersions, blends and block copolymers. The case of AB double crystalline diblock copolymers is discussed in the light of recent works on biodegradable systems, while the nucleation, crystallization and morphology of more complex materials like ABC triblock copolymers with one or two crystallizable components are thoroughly reviewed.

Phase-change recording materials with a growth-dominated crystallization mechanism: A materials overview

Abstract: The influence of phase-change material composition on amorphous phase stability, crystallization rate, nucleation probability, optical constants and media noise is reported for materials with a growth dominated crystallization mechanism. Two material classes have been studied, doped Sb–Te and doped Sb-based compositions. The material properties of both are greatly influenced by their composition, and in a similar way. For both materials systems hold that the antimony content especially influences the crystallization rate, amorphous phase stability and media noise of the phase-change material. Compositions rich in antimony generally show high crystallization rates, low archival life stability and high media noise. The material properties are further influenced by the presence of dopants like tellurium, germanium, gallium, indium or tin. Germanium and tellurium reduce the crystallization rate, but are essential to increase the amorphous phase stability. Dopants like tin or indium are added to increase the c...

Can glass stability parameters infer glass forming ability

Abstract: Glass-forming ability (GFA) is the easiness to vitrify a liquid on cooling, while glass stability (GS) is the glass resistance against devitrification on heating; but it is questionable if there is any direct relationship between these two parameters. Therefore, to test this possibility, we assess and compare GFA and several GS parameters through quantitative criteria. GFA and GS were calculated for six stoichiometric glass forming oxides that only present surface (heterogeneous) crystallization in laboratory time scales: GeO2 ,N a 2O AE 2SiO2, PbO AE SiO2, CaO AE Al2O3 AE 2SiO2, CaO AE MgO AE 2SiO2 and 2MgO AE 2Al2O3 AE 5SiO2; plus Li2O AE 2SiO2 and Li2O AE 2B2O3 that, in addition to surface nucleation, also present homogeneous (internal) crystallization. We gauge GFA by the critical cooling rate, qcr, which was calculated from an estimated number of heterogeneous nucleation sites per unit surface, Ns, and from experimental crystal growth rates, u(T), assuming a detectable surface crystallized fraction Xc = 0.001. We define GS parameters by fourteen different combinations of the following characteristic differential thermal analysis (DTA) or differential scanning calorimetry (DSC) temperatures: the glass transition temperature (Tg), the onset crystallization temperature on heating ðT h Þ, the peak crystallization temperature on heating ðT h Þ, and the melting point (Tm). To obtain the experimental GS parameters for each glass we carried out DSC runs using coarse and fine powders, and completed the necessary data with literature values for Tm. The results for fine and coarse particles were quite similar. Most of the GS parameters that consist of three characteristic DSC temperatures show excellent correlation with GFA, however, rather poor correlations were observed for parameters that use only two characteristic temperatures. We thus demonstrated that certain, but not all GS parameters can be used to infer GFA. � 2005 Elsevier B.V. All rights reserved.

A procedure for setting up high-throughput nanolitre crystallization experiments. Crystallization workflow for initial screening, automated storage, imaging and optimization.

Abstract: Crystallization trials at the Division of Structural Biology in Oxford are now almost exclusively carried out using a high-throughput workflow implemented in the Oxford Protein Production Facility. Initial crystallization screening is based on nanolitre-scale sitting-drop vapour-diffusion experiments (typically 100 nl of protein plus 100 nl of reservoir solution per droplet) which use standard crystallization screening kits and 96-well crystallization plates. For 294 K crystallization trials the barcoded crystallization plates are entered into an automated storage system with a fully integrated imaging system. These plates are imaged in accordance with a pre-programmed schedule and the resulting digital data for each droplet are harvested into a laboratory information-management system (LIMS), scored by crystal recognition software and displayed for user analysis via a web-based interface. Currently, storage for trials at 277 K is not automated and for imaging the crystallization plates are fed by hand into an imaging system from which the data enter the LIMS. The workflow includes two procedures for nanolitre-scale optimization of crystallization conditions: (i) a protocol for variation of pH, reservoir dilution and protein:reservoir ratio and (ii) an additive screen. Experience based on 592 crystallization projects is reported.

Solvent Effect on Crystal Polymorphism: Why Addition of Methanol or Ethanol to Aqueous Solutions Induces the Precipitation of the Least Stable β Form of Glycine

Abstract: Crystal polymorphism, which embodies the ability of molecules to form diverse packing arrangements displaying different physical and chemical characteristics, is of paramount importance in fields such as pharmacology, solid-state chemistry, and material science. However, the conditions to induce the precipitation of various (metastable) polymorphs is invariably achieved by “mix and try” methods, which are kinetically driven. Various factors should be considered in trying to understand these complex processes, for example, the formation of structured clusters in solution prior to crystallization, the structure of growing surfaces that delineate emerging nuclei, the interaction between these surfaces and the solvent, as well as solvent–solute and solute– solute interactions. Herein, we attempt to unravel some of these factors to rationalize the preferred crystallization of the b form of glycine (gly) in water–alcohol solutions as opposed to the more stable a or g polymorphs. The thermodynamic stability of the three polymorphs of glycine at room temperature is in the order g>a> b. The a form (space group P21/n), grown from supersaturated aqueous solutions (33.3 g/100 mL water) at 25 8C, has a bipyramidal habit and is composed of centrosymmetric bilayers formed by strong NH···O hydrogen-bonding interactions between cyclic hydrogen-bonded zwitterionic molecular pairs. These bilayers are related along the b axis by glide symmetry through weak CH···O interactions (Figure 1a). Previous studies indicated that a-gly crystallizes primarily in aqueous solutions through hydrogen-bonded cyclic dimer growth units: Diffusion-coefficient measurements of supersaturated aqueous solutions of glycine point to the formation of clusters with an average of 1.8 molecular growth units. Furthermore, atomic force microscopy (AFM) and phase interferometry microscopy measurements established that steps approximately 1 nm in size were formed, which correspond to the thickness of a glycine bilayer. Grazingincidence X-ray diffraction studies on growing a-gly {010}

Crystallization kinetics and interfacial behaviors of polypropylene composites reinforced with multi-walled carbon nanotubes

Abstract: In this work, the carbon nanotubes (CNTs) were reinforced with polypropylene (PP) matrix resins to improve the electrical, thermal, and mechanical properties of CNTs/PP composites in different contents of 0, 1, 2, 3, and 5 wt% The volume resistivity was measured to discover the percolation threshold of the composites The crystallization kinetics, organizations, and microstructures of CNTs/PP composites were investigated with differential scanning calorimeter (DSC), X-ray diffraction (XRD), and scanning electron microscopy (SEM) analyses, respectively The Raman spectroscopy was also performed to obtain information on the CNTs/PP interactions As a result, the volume resistivity was decreased with increasing the CNT content that could be governed in a percolation-like power law with a relatively low percolation threshold And the crystallization exothermic peak shifted to a higher temperature, and the overall crystallization time was reduced by the increment of CNT content Also, the nucleant of CNTs affected the crystallization of PP, but was not linearly dependent on the CNT content that meant a saturation of the nucleant effect at low CNT content

Stereoselective Crystallization and Specific Interactions in Polylactides

Abstract: The stereoselective annealing conditions for polylactides to crystallize as α homocrystals (Tm = 180 °C) and/or higher melting temperature (Tm = 230 °C) η stereocomplex crystals are reported in this paper. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) studies provide evidence of the isothermal annealing conditions in which equimolar mixtures of optically pure poly(l-lactide) and poly(d-lactide) take exclusively the form α, exclusively the form η, or coexist as both α and η crystal polymorphs. Fourier transformed infrared (FTIR) spectroscopy studies reveal a shift to lower wavenumbers in the CO stretching band of polylactides in the stereocomplex with regard to that in the homocrystal. This band shift is accompanied by similar displacements in the C−H spectral bands. These results are interpreted in terms of H-bonding forces causing specific CH3···OC and CαH···OC interactions between both stereoisomers of polylactide. Molecular models support that the suggested hydrogen-bonding arrang...

A density-driven phase transition between semiconducting and metallic polyamorphs of silicon

Abstract: Amorphous and crystalline forms of silicon are well-known, tetrahedrally coordinated semiconductors. High-pressure studies have revealed extensive polymorphism among various metallic crystal structures containing atoms in six-, eight- and 12-fold coordination1,2. Melting silicon at ambient or high pressure results in a conducting liquid, in which the average coordination is greater than four (ref. 3). This liquid cannot normally be quenched to a glass, because of rapid crystallization to the diamond-structured semiconductor4. Solid amorphous silicon is obtained by synthesis routes such as chemical or physical vapour deposition that result in a tetrahedrally bonded semiconducting state. It has long been speculated that the amorphous solid and the liquid could represent two polymorphic forms of the amorphous state that are linked by density- or entropy-driven transformations5,6,7,8. Such polyamorphic transitions are recognized to occur among several different types of liquid and glassy systems9,10,11,12,13,14. Here we present experimental evidence for the occurrence of a density-driven polyamorphic transition between semiconducting and metallic forms of solid amorphous silicon. The experiments are combined with molecular dynamics simulations that map the behaviour of the amorphous solid on to that of the liquid state.

Heterogeneous surface crystallization observed in undercooled water.

Abstract: We report laboratory observations of higher freezing temperatures when an ice-forming nucleus is near the surface of an undercooled water drop than when the nucleus is immersed in the drop. The nucleation rate at the water surface is a factor of 10(10) greater than in bulk water, thereby complementing and providing evidence for homogeneous surface crystallization, which has been hypothesized recently. Interpretation of the data via classical nucleation theory shows that the free energy of formation of a critical ice germ is decreased by a factor of approximately 2 when the substrate is near the air-water interface. Furthermore, the analysis suggests that the jump frequency of molecules from the liquid to the solid may be greatly enhanced at the interface.

Synthesis of manganese oxide electrodes with interconnected nanowire structure as an anode material for rechargeable lithium ion batteries.

Abstract: Manganese oxide electrodes composed of interconnected nanowires are electrochemically synthesized in manganous acetate solution at room temperature without any template and catalyst. Annealing temperature affects the electrode morphology, crystallization, and electrochemical performance. Scanning electron microscope (SEM) results show that nanowires are uniformly distributed and sizes are about 12−18 nm in diameter; the diameter decreases to about 8−12 nm after annealing at 300 °C. X-ray diffraction (XRD) and transmission electron microscope (TEM) images indicate that nanowires have poor crystalline characteristics. The higher the annealing temperature, the higher the crystalline degree is in manganese oxide. The synthesized anode material shows a much larger capacity than the traditional graphite materials for lithium storage. After annealing at 300 °C, the electrode's reversible capacity reaches 800 mAhg-1, and the specific capacity retention remains nearly constant after 100 cycles.

Thermal Analysis of Amorphous Phases in Hydroxyapatite Coatings

Abstract: The amorphous phase in hydroxyapatite coatings has been examined by using X-ray diffractometry, Fourier transform infrared spectroscopy, optical microscopy, and thermal analysis methods. The amorphous phase mostly consists of a dehydroxylated calcium phosphate. When heated, crystallization of hydroxyl-rich areas produces hydroxyapatite, followed by diffusion of hydroxyl ions, thus increasing the amount of crystalline phase. Hydroxyl-deficient amorphous areas crystallize to oxyapatite at 700°C. Thus, crystallization occurs over a range of temperatures and is dependent on the hydroxyl content of the amorphous phase and the partial water-vapor pressure. The activation energies of crystallization to hydroxyapatite, diffusion of hydroxyl ions, and crystallization to oxyapatite are 274, 230, and 440 kJ/mol, respectively. Shrinkage from these processes leads to a crack network and decreases the mechanical strength of the coating.