Showing papers on "Crystallization published in 2003"

Principles of crystal nucleation and growth

Abstract: In the most general sense, biomineralization is a process by which organisms produce materials solutions for their own functional requirements. Because so many biomineral products are derived from an initial solution phase and are either completely crystalline or include crystalline components, an understanding of the physical principles of crystallization from solutions is an important tool for students of biomineralization. However, crystal growth is a science of great breadth and depth, about which many extensive texts have been written. In addition, there are already other thorough reviews that specifically address the crystal growth field of study as it relates to biomineral formation. Consequently, the goals of this chapter are both modest and specific. It is intended to provide: 1) a simple narrative explaining the physical principles behind crystallization for those who are completely new to the topic, 2) a few basic equations governing nucleation and growth for those who wish to apply those principles—at least in a semi-quantitative fashion—to experimental observations of mineralization, and 3) an overview of some recent molecular-scale studies that have revealed new insights into the control of crystal growth by small molecules, both organic and inorganic. This last topic gets to the heart of what makes crystallization in biological systems unique. Every day, many tons of crystals are produced synthetically in non-biological processes, but by-and-large, the degree of control over nucleation and growth achieved by deterministic additions of growth modifiers or the presence of a controlling matrix is very minor. More commonly, crystal growers view modifying agents as unwanted impurities and work extremely hard to eliminate them from the starting materials. Indeed, the degree to which living organisms are able to control the crystallization process is most striking when contrasted to the products of such synthetic crystallization processes. This contrast applies to both the compositional differences that …

Crystallization and Reduction of Sol-Gel-Derived Zinc Oxide Films by Irradiation with Ultraviolet Lamp

Abstract: Structural changes in sol-gel films with photo-irradiation were investigated using zinc oxide (ZnO) derived from zinc acetate. The exposure of the films to an ultraviolet lamp induced hexagonal ZnO crystals in a relatively dense amorphous structure. On the other hand, the formation of zinc metal was found in a porous gel film. The photo-induced crystallization and reduction are ascribed to the electronic excitation in the metastable non-crystalline states.

Controlled formation of highly organized mesoporous titania thin films: from mesostructured hybrids to mesoporous nanoanatase TiO2.

Abstract: In this paper, we report the complete synthesis and characterization procedures to generate highly organized and oriented mesoporous titania thin films, using poly(ethylene oxide) (PEO)-based templates. Controlled conditions in the deposition, postsynthesis, and thermal treatment steps allow one to tailor the final mesostructure (2D hexagonal, p6m, or 3D cubic, Im3m). Various techniques were used to determine the time evolution of the mesostructure. Spectroscopic techniques (UV/vis, (17)O NMR) and EXAFS/XANES have been used to follow the chemical changes in the Ti(IV) environment. Crossing these techniques spanning all ranges permits a complete description of the chemistry all the way from solution to the mesostructured metal oxide. A critical discussion on all important chemical and processing parameters is provided; the understanding of these features is essential for a rational design and the reproducible construction of mesoporous materials.

Screening of Protein Crystallization Conditions on a Microfluidic Chip Using Nanoliter-Size Droplets

Abstract: Protein crystallization is a major bottleneck in determining tertiary protein structures from genomic sequence data. This paper describes a microfluidic system for screening hundreds of protein crystallization conditions using less than 4 nL of protein solution for each crystallization droplet. The droplets are formed by mixing protein, precipitant, and additive stock solutions in variable ratios in a flow of water-immiscible fluids inside microchannels. Each droplet represents a discrete trial testing different conditions. The system has been validated by crystallization of several water-soluble proteins.

General Synthesis of Single-Crystal Tungstate Nanorods/Nanowires: A Facile, Low-Temperature Solution Approach

Abstract: The general large-scale synthesis of a family of single-crystalline transition metal tungstate nanorods/nanowires is easily realized by a hydrothermal crystallization technique under mild conditions using cheap and simple inorganic salts as precursors. Uniform tungstate nanorods/nanowires such as MWO4 (M = Zn, Mn, Fe), Bi2WO6, Ag2WO4, and Ag2W2O7 with diameters of 20–40 nm, lengths of up to micrometers, and controlled aspect ratios can be readily obtained by hydrothermal transformation and recrystallization of amorphous particulates. This novel and efficient pathway toward various kinds of related low-dimensional tungstate nanocrystals under mild conditions could open new opportunities for further investigating the novel properties of tungstate materials.

Crystallization Behavior and Morphology of Biodegradable Polylactide/ Layered Silicate Nanocomposite

Abstract: An intercalated polylactide (PLA)/layered silicate nanocomposite was prepared by simple melt extrusion of PLA and organically modified montmorillonite. The detailed crystallization kinetics and morphology of neat PLA before and after nanocomposite preparation were studied by using polarized optical microscopy, light scattering, differential scanning calorimetric, and wide-angle X-ray diffraction analyses. The overall crystallization rate and spherulitic texture of pure PLA were strongly influenced in the presence of montmorillonite particles.

Crystallization and phase stability of CaSO4 and CaSO4- based salts

Abstract: Calcium sulfate occurs in nature in form of three different minerals distinguished by the degree of hydration: gypsum (CaSO4·2H2O), hemihydrate (CaSO4·0.5H2O) and anhydrite (CaSO4). On the one hand the conversion of these phases into each other takes place in nature and on the other hand it represents the basis of gypsum-based building materials. The present paper reviews available phase diagram and crystallization kinetics information on the formation of calcium sulfate phases, including CaSO4-based double salts and solid solutions. Uncertainties in the solubility diagram CaSO4–H2O due to slow crystallization kinetics particularly of anhydrite cause uncertainties in the stable branch of crystallization. Despite several attempts to fix the transition temperatures of gypsum–anhydrite and gypsum–hemihydrate by especially designed experiments or thermodynamic data analysis, they still vary within a range from 42–60°C and 80–110°C. Electrolyte solutions decrease the transition temperatures in dependence on water activity. Dry or wet dehydration of gypsum yields hemihydrates (α-, β-) with different thermal and re-hydration behaviour, the reason of which is still unclear. However, crystal morphology has a strong influence. Gypsum forms solid solutions by incorporating the ions HPO4 2−, HAsO4 2−, SeO4 2−, CrO4 2−, as well as ion combinations Na+(H2PO4)− and Ln3+(PO4)3−. The channel structure of calcium sulfate hemihydrate allows for more flexible ion substitutions. Its ion substituted phases and certain double salts of calcium sulfate seem to play an important role as intermediates in the conversion kinetics of gypsum into anhydrite or other anhydrous double salts in aqueous solutions. The same is true for the opposite process of anhydrite hydration to gypsum. Knowledge about stability ranges (temperature, composition) of double salts with alkaline and alkaline earth sulfates (esp. Na2SO4, K2SO4, MgSO4, SrSO4) under anhydrous and aqueous conditions is still very incomplete, despite some progress made for the systems Na2SO4–CaSO4 and K2SO4–CaSO4–H2O.

Structural Characterization of CeO2−TiO2 and V2O5/CeO2−TiO2 Catalysts by Raman and XPS Techniques

Abstract: Structural characteristics of ceria−titania and vanadia/ceria−titania mixed oxides have been investigated using X-ray powder diffraction (XRD), Raman spectroscopy (RS), and X-ray photoelectron spectroscopy (XPS) techniques. The (1:1 mole ratio) mixed oxide was obtained by a coprecipitation method, and a nominal 5 wt % V2O5 was deposited over its surface by a wet impregnation technique. Both of the materials were then subjected to thermal treatments from 773 to 1073 K and were characterized by the above-mentioned techniques. The XRD results suggest that the CeO2−TiO2 mixed oxide calcined at 773 K primarily consists of poorly crystalline CeO2 and TiO2-anatase phases and that a better crystallization of these oxides occurs with increasing calcination temperature. The “a” cell-parameter values suggest some incorporation of titanium into the ceria lattice. Impregnation of vanadia on ceria−titania enhances the crystallization of CeO2 and TiO2 oxides. However, no crystalline V2O5 could be observed from XRD and R...

Highly Porous TiO2 Anatase Optical Thin Films with Cubic Mesostructure Stabilized at 700 °C

Abstract: TiO2 optical thin films stable to 700 °C, exhibiting 35% volume porosity, more than 100 m2·g-1 in surface area, fully nanocrystalline anatase framework, and organized mesostructure (cubic Im3m derived), have been stabilized by careful delayed rapid crystallization (DRC) thermal treatments. In-situ time-resolved SAXS and WAXS investigations were simultaneously performed during such treatments. They revealed that a slow and progressive heating to a temperature just below that of the formation of anatase (Tc ≈ 400 °C), followed by a long pretreatment at this temperature, stabilizes the amorphous network. A following rapid increase of temperature up to temperatures as high as typically 700 °C, followed by a short residence time at this high temperature, provokes the homogeneous formation of crystalline small nanoparticles and the total elimination of organic residues. The crystallization is accompanied by matter migration through diffusing sintering and pore merging along the [111] directions of the cubic str...

Determination of the crystalline phases of poly(vinylidene fluoride) under different preparation conditions using differential scanning calorimetry and infrared spectroscopy

Abstract: A method with good precision has been developed to quantitatively measure the degree of α-, β-, and γ crystallinity in poly(vinylidene fluoride) (PVDF) by means of infrared spectroscopy. The phase composition of solution-deposited PVDF films was found to be strongly influenced by the presence of hydrophilic residues on the silicon substrate, the relative humidity present at film deposition, the spatial position on the substrate, and the thermal treatment of the deposited film. Films produced on pristine surfaces gave predominantly α-phase PVDF, but when a layer of polar solvent (acetone or methanol) remained on the surface, the films produced were predominantly γ phase. Higher humidity promoted a higher fraction of γ crystallinity in the solution-deposited PVDF films. Solution-cast films had highly variable composition across the substrate, whereas spin-cast films were uniform. High-temperature annealing of PVDF films normally converts the polymer to the γ phase, but annealing the film while still attached to the silicon substrate inhibited this phase transformation. Low-temperature annealing of freestanding films led to a previously unreported thermal event in the DSC, a premelting process that is a kinetic event, assigned to a crystalline relaxation. Higher-temperature annealing gave a double endotherm, assigned to melting of different-sized crystalline domains. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1093–1100, 2003

Nucleation ability of multiwall carbon nanotubes in polypropylene composites

Abstract: The nonisothermal crystallization of multiwall carbon nanotube (MWNT)/isotactic polypropylene (iPP) nanocomposites was investigated The results derived from the differential scanning calorimetry curves (onset temperature, melting point, supercooling, peak temperature, half-time of crystallization, and enthalpy of crystallization) were compared with those of neat iPP The data were also processed according to Ozawa's theory and Dobreva's approach These results and X-ray diffraction data showed that the MWNTs acted as α-nucleating agents in iPP Accordingly, MWNT/iPP was significantly different from neat iPP: A fibrillar morphology was observed instead of the usual spherulites © 2003 Wiley Periodicals, Inc J Polym Sci Part B: Polym Phys 41: 520–527, 2003

Effect of surface nanocrystallization on friction and wear properties in low carbon steel

Abstract: A nanocrystalline (nc) surface layer of about 10 μm thick was fabricated on a low carbon steel plate by means of surface mechanical attrition treatment. The grain size is about 15 nm in the top surface layer, and it increases with an increase of depth from the treated surface. Nanoindentation measurements indicated that hardness is enhanced in the nc surface layer relative to the matrix. Experiments show that the friction coefficient decreases and the wear resistance increases with the nc surface layer. The improvement in friction and wear properties may be attributed to the harder nc surface layer which reduces the degree of plowing and micro-cutting under the lower load and the degree of plastic removal and surface fatigue fracture under the higher load, respectively.

The Binary System Isotactic Polypropylene/Bis(3,4-dimethylbenzylidene)sorbitol: Phase Behavior, Nucleation, and Optical Properties

Abstract: The phase behavior of the binary system consisting of the commercial nucleating and clarifying agent 1,3:2,4-bis(3,4-dimethyldibenzylidene)sorbitol (DMDBS, Millad 3988) and isotactic polypropylene (i-PP) was investigated over the entire concentration range by means of differential scanning calorimetry (DSC), rheology, and optical microscopy. Experimental phase diagrams were constructed from data obtained in melting and crystallization studies, and a simple binary monotectic is advanced. Distinct regimes in the phase diagram, which apparently dictate nucleation and clarification of i-PP by DMDBS, are discussed. A maximum increase in the crystallization temperature of i-PP due to the nucleating action of DMDBS was observed in compositions containing between 0.2 and 1 wt % of the latter. Liquid−liquid phase separation was observed at elevated temperatures for i-PP/DMDBS mixtures comprising more than 2 wt % of DMDBS. A study of the optical properties of the i-PP/DMDBS system revealed that values for haze and ...

Steps toward interstellar silicate mineralogy - VII. Spectral properties and crystallization behaviour of magnesium silicates produced by the sol-gel method

Abstract: Amorphous silicate particles are generally assumed to be the main dust component in the envelopes of oxygen-rich evolved stars and may be considered the precursors of the pure crystalline enstatite and forsterite particles detected by ISO We present optical constants in the broad wavelength range 02-500 µm for a unique series of pure amorphous Mg-silicates (Mg/Si in the range 07-24) They have been prepared by the sol-gel process, a chemical technique based on the condensation of Mg- and Si-hydroxides in a liquid phase The salient feature of these Mg-silicates is the very small content of Si-OH bonds in the silicate network, which considerably reduces the activation energy of crystallization and, thus, decreases the temperature threshold for crystallization as well as crystallization time The astrophysical relevance of our sol-gel silicates is shown by a comparison of optically thin model spectra based on dust emissivities with ISO-SWS spectra of AGB stars and with 10 µm emission profiles of such stars obtained by ground-based spectroscopy As paradigmatic cases of AGB spectra with respect to the appearance of the silicate bands, TY Dra (slender bands and deep trough between them) and R Cas (broad bands and widely filled-up trough) were used, for which ISO-SWS spectra are available The dust emissivity derived from TY Dra can be excel- lently reproduced by the models, suggesting that the dust grains consist indeed of pure amorphous Mg-silicates Satisfactory agreement was also found with the mean 10 µm profiles of some groups of AGB stars and supergiants Spectra with strong dust emission in the silicate trough like R Cas require additional contributions by other dust components, probably oxides A rough orientation on the spectral properties of such potential trough opacity contributors has been obtained by subtracting a pure silicate spectrum (TY Dra) from a spectrum with a nearly filled trough and a less pronounced 20 µm band (R Cas) In agreement with other amorphous silicates, the spectral index of the new silicate analogues amounts to −2