Showing papers on "Crystallization published in 2001"

A Literature Review of Poly(Lactic Acid)

Abstract: A literature review is presented regarding the synthesis, and physicochemical, chemical, and mechanical properties of poly(lactic acid)(PLA). Poly(lactic acid) exists as a polymeric helix, with an orthorhombic unit cell. The tensile properties of PLA can vary widely, depending on whether or not it is annealed or oriented or what its degree of crystallinity is. Also discussed are the effects of processing on PLA. Crystallization and crystallization kinetics of PLA are also investigated. Solution and melt rheology of PLA is also discussed. Four different power-law equations and 14 different Mark–Houwink equations are presented for PLA. Nuclear magnetic resonance, UV–VIS, and FTIR spectroscopy of PLA are briefly discussed. Finally, research conducted on starch–PLA composites is introduced.

Real-space imaging of nucleation and growth in colloidal crystallization.

Abstract: Crystallization of concentrated colloidal suspensions was studied in real space with laser scanning confocal microscopy. Direct imaging in three dimensions allowed identification and observation of both nucleation and growth of crystalline regions, providing an experimental measure of properties of the nucleating crystallites. By following their evolution, we identified critical nuclei, determined nucleation rates, and measured the average surface tension of the crystal-liquid interface. The structure of the nuclei was the same as the bulk solid phase, random hexagonal close-packed, and their average shape was rather nonspherical, with rough rather than faceted surfaces.

Crystallization Technology Handbook

Abstract: Fundamentals of Crystallization Interaction Between Balances, Processes, and Product Quality Design of Crystallizers and Crystallization Processes Control of Crystallizers Reaction Crystallization Additives and Impurities Crystallization from the Melt Thermal Analysis and Economics of Processes Appendix 1: Physical Properties Appendix II: Examples of Large-Scale lndustrial Crystallizers.

Highly bioactive P2O5–Na2O–CaO–SiO2 glass-ceramics

Abstract: Glasses having a chemical composition between 1Na2O–2CaO–3SiO2 (1N2C3S) and 1.5Na2O–1.5CaO–3SiO2, containing 0, 2, 4 and 6 wt% P2O5, were crystallized to several volume percent through thermal treatments in the range 550–700 °C. These glasses and glass-ceramics were exposed to a simulated body fluid solution (SBF-K9 which is close to human plasma) for several time periods. Fourier transform infrared spectroscopy (FTIR) was used to determine the rate of hydroxy carbonate apatite (HCA) formation. Crystallization decreased the kinetics but did not inhibit the development of a HCA layer, even in fully crystallized ceramics. The onset time for crystallization of HCA varied from 8 h for a glass containing 6% P2O5 to 35 h for a fully crystallized 1.07Na2O–2CaO–3SiO2 ceramic. The HCA layer formation of these compositions in `in vitro' tests is much faster than in commercial bioactive materials such as synthetic hydroxyapatite ceramic, A/W glass-ceramic, Ceravital and Bioverit, for which the onset time usually takes at least seven days. FTIR and inductive coupled plasma studies confirmed the formation of an apatite layer which indicates bioactivity in the 1N2C3S crystal phase. X-ray diffraction experiments show that the phosphorus ions are kept in solid solution in the crystal phase. An apatite-like compound only appeared when the specimens were submitted to very long additional thermal treatments. The bioactivity of commercial materials is based on the apatite crystal phase, while the high level of bioactivity of this new generation of glass-ceramics is attained due to the combination of two mechanisms acting simultaneously; a non-phosphate bioactive crystal phase (1N2C3S) and the phosphorus ions in solid solution which are easily released from the structure, promoting a faster HCA layer formation similar to 45S5 Bioglass®.

A Systematic Examination of the Morphogenesis of Calcium Carbonate in the Presence of a Double-Hydrophilic Block Copolymer

Abstract: In this paper, a systematic study of the influence of various experimental parameters on the morphology and size of CaCO3 crystals after room-temperature crystallization from water in the presence of poly(ethylene glycol)-block-poly(methacrylic acid) (PEG-b-PMAA) is presented. The pH of the solution, the block copolymer concentration, and the ratio [polymer]/[CaCO3] turned out to be important parameters for the morphogenesis of CaCO3, whereas a moderate increase of the ionic strength (0.016 M) had no influence. Depending on the experimental conditions, the crystal morphologies can be tuned from calcite rhombohedra via rods, ellipsoids or dumbbells to spheres. A morphology map is presented which allows the prediction of the crystal morphology from a combination of pH, and CaCO3 and polymer concentration. Morphologies reported in literature for the same system but under different crystallization conditions agree well with the predictions from the morphology map. A closer examination of the growth of polycrystalline macroscopic CaCO3 spheres by TEM and time-resolved dynamic light scattering showed that CaCO3 macrocrystals are formed from strings of aggregated amorphous nanoparticles and then recrystallize as dumbbell-shaped or spherical calcite macrocrystal.

Structure Development during Shear Flow Induced Crystallization of i-PP: In Situ Wide-Angle X-ray Diffraction Study

Abstract: In situ synchrotron wide-angle X-ray diffraction (WAXD) was used to monitor crystallization of isotactic polypropylene (i-PP) in the subcooled melt at 140 °C after step shear. The melt was subjected to a shear strain of 1430% at three different shear rates (10, 57, and 102 s-1) using a parallel-plate shear apparatus. WAXD results were used to determine the type (α- and β-crystals), orientation, and corresponding mass fractions of i-PP crystals. It was found that formation of oriented α-crystals occurred immediately after application of the shear field. Subsequently, growth of primarily unoriented β-crystals was observed. WAXD patterns clearly showed that β-crystals grew only after the formation of oriented α-crystals in the sheared i-PP melt. The contribution of β-crystals to the total crystalline phase was as high as 65−70% at high shear rates (57 and 102 s-1) and low (20%) at low shear rates (10 s-1), which was attributed to the different amount of surface area of oriented α-crystal cylindrites generate...

Suppression of crystal nucleation in polydisperse colloids due to increase of the surface free energy

Abstract: The formation of small crystallites is governed by two competing factors: the free energy gained upon transferring constituent atoms, molecules or colloidal particles from the metastable liquid to the more stable solid, and the free energy needed to create the surface area of the crystallite. Because the ratio of surface area to bulk is large for small particles, small crystallites dissolve spontaneously under conditions where larger crystallites are stable and macroscopic crystal growth occurs only if spontaneously formed crystallites exceed a critical minimum size. On theoretical grounds, the probability of forming such critical crystal nuclei is expected to increase rapidly with supersaturation. However, experiments show that the rate of crystal nucleation in many systems goes through a maximum as the supersaturation is increased. It is commonly assumed that the nucleation rate peaks because, even though the probability of forming critical nuclei increases with increasing concentration, the rate of growth of such nuclei decreases. Here we report simulations of crystal nucleation in suspensions of colloidal spheres with varying size distributions that show that the probability that critical nuclei will form itself goes through a maximum as the supersaturation is increased. We find that this effect, which is strongest for systems with the broadest particle size distribution, results from an increase with supersaturation of the solid-liquid interfacial free energy. The magnitude of this effect suggests that vitrification at high supersaturations should yield colloidal glasses that are truly amorphous, rather than nano-crystalline.

Melting and crystallization in Ni nanoclusters: The mesoscale regime

Abstract: We studied melting and freezing of Ni nanoclusters with up to 8007 atoms ~5.7 nm! using molecular dynamics with the quantum-Sutten‐Chen many-body force field. We find a transition from cluster or molecular behavior below ;500 atoms to a mesoscale nanocrystal regime ~well-defined bulk and surface properties! above ;750 atoms ~2.7 nm!. We find that the mesoscale nanocrystals melt via surface processes, leading to Tm,N5Tm,bulk2aN 21/3 , dropping from Tm,bulk51760 K to Tm,336 5980 K. Cooling from the melt leads first to supercooled clusters with icosahedral local structure. For N.400 the supercooled clusters transform to FCC grains, but smaller values of N lead to a glassy structure with substantial icosahedral character. © 2001 American Institute of Physics. @DOI: 10.1063/1.1373664#

Laser induced crystallization of amorphous Ge2Sb2Te5 films

Abstract: The crystallization behavior of Ge2Sb2Te5 thin films has been analyzed by atomic force microscopy and optical reflection measurements on various time scales in order to determine the crystallization kinetics including the crystallization mechanism, the corresponding activation barrier, and the Avrami coefficient. On the minute time scale, thin amorphous films were isothermally crystallized in a furnace under a protective Ar atmosphere. From these measurements the activation energy for crystallization was determined to be (2.0±0.2) eV, in close agreement with previous studies using different techniques. The isothermal measurements also revealed a temperature dependent incubation time for the formation of critical nuclei, which is compared with recent theories. On the nanosecond time scale, Ge2Sb2Te5 was locally crystallized with a focused laser. Either crystalline spots of submicron size were generated in an as deposited amorphous film or amorphous bits in an otherwise crystalline film were recrystallized. For the formation of crystalline spots in an as deposited amorphous film a minimum time of (100±10) ns was found, which is identified as the minimum incubation time for the formation of critical nuclei. In contrast, the complete crystallization of melt-quenched amorphous bits in a crystalline matrix was possible in 10 ns. This is attributed to the presence of quenched-in nuclei inside the amorphous bits. The combination of optical measurements with atomic force microscopy reveals the formation and growth of crystalline bits and shows that the crystal growth in vertical direction strongly affects the reflectivity changes.

Polymer crystallization confined in one, two, or three dimensions

Abstract: We examine the crystallization behavior of polyethylene-b-poly(vinylcyclohexane) diblock copolymers, E/VCH, using a combination of transmission electron microscopy (TEM), dilatometry, and time-resolved small-angle X-ray scattering (SAXS). The glassy VCH matrix effectively restricts E crystallization to within the spheres, cylinders, gyroid channels, or lamellae formed by microphase separation in the melt. The VCH matrix can contract in response to crystallization of the E microdomains, so crystallization proceeds without cavitation. The crystallization kinetics strongly reflect the connectivity of the E microdomains: homogeneous nucleation and first-order crystallization kinetics for spheres or cylinders of E; conventional sigmoidal kinetics for the highly interconnected gyroid structure. Lamellar materials show an interesting two-step crystallization behavior: at higher temperature, heterogeneous nucleation permits the crystallization of lamellae interconnected through grain boundaries or defects, and ...

Crystallization in Foods

Abstract: Publisher Summary The crystalline structure of foods is important to product quality, texture, and stability. It is this crystalline structure and other structural elements that determine product appearance, mechanical properties during handling, mouthfeel during consumption, and shelf stability. To control crystallization, it is necessary to have an understanding of the phase behavior of the system, some knowledge of nucleation and growth kinetics, and the effects of both formulation and processing conditions on this kinetics. In foods, two circumstances for controlling the formation of crystals can be distinguished: those where the crystals provide an element of structure in the product and those where crystallization is a separation process. There are many factors that influence crystallization in food products. In many products, the goal of crystallization is to generate a certain texture or appearance that makes the product acceptable. Thus, nucleating many crystals that remain small within the product itself is often the goal. There must be many crystals with small mean size and narrow distribution. The crystals also must have the proper shape and/or polymorph to enhance stability of the product during storage and distribution. However, in other types of products, crystallization is undesired even though the system is supersaturated. In these cases, techniques are used to prevent crystallization from occurring during storage, because this leads to unacceptable product quality.

Wigner crystallization in mesoscopic 2d electron systems.

Abstract: Wigner crystallization of electrons in 2D quantum dots is reported. It proceeds in two stages: (i) via radial ordering of electrons on shells and (ii) freezing of the intershell rotation. The phase boundary of the crystal is computed in the whole temperature-density plane, and the influences of quantum effects and the particle number are analyzed.

Crystal size and temperature measurements in nanostructured silicon using Raman spectroscopy

Abstract: In this work we present a detailed structural characterization by Raman spectroscopy of hydrogenated amorphous silicon (a-Si:H) and of nanostructured silicon (ns-Si:H) thin films grown in radio-frequency plasma. The ns-Si:H thin films, also called polymorphous Si thin films, consist of a two-phase mixture of amorphous and ordered Si. The Raman spectra were measured at increasing laser intensities. Very low laser power densities (∼1 kW/cm2) were used to thoroughly analyze the structure of as-deposited thin films. Higher Raman laser powers were found to induce the crystallization of the films, which was characterized by the appearance of a sharp peak around 500 cm−1. This was attained faster in the ns-Si:H than in the conventional a-Si:H thin films because the silicon-ordered particles cause a heterogeneous nucleation process in which they act as seeds for crystallization. The laser power densities for film crystallization, crystal size, and surface temperature were determined from this Raman analysis. The ...

The Fabrication and Bandgap Engineering of Photonic Multilayers

Abstract: This communication describes a method to fabricate multilayer colloidal crystals formed by the layer-by-layer deposition of silica beads on a glass substrate. Each layer of the crystal consists of a three-dimensionally ordered array of close-packed colloids. These multilayer samples are amenable to templating methods for tuning the dielectric contrast of the material. The resulting photonic crystal structures exhibit optical properties which resemble the superposition of the properties of each individual crystal, with additional structure that suggests the onset of superlattice-type miniband formation. These multilayer structures thus afford new opportunities for engineered photonic behavior. Traditionally colloidal crystals are three dimensional periodic structures formed from monodisperse colloids. Because of their diffractive optical properties they are a type of photonic crystal and may have applications as optical filters and switches, high density magnetic data storage devices, chemical and biochemical sensors, or as removable scaffolds for the formation of highly ordered, macroporous materials. They are also useful as model systems for fundamental studies of crystal melting and phase transition behavior. The process of colloidal crystallization has been extensively studied, leading to the development of several methods to make high quality colloidal crystals with few crystalline defects. These techniques include electrostatically induced crystallization, gravity sedimentation, electro-hydrodynamic deposition, colloidal epitaxy, physical confinement and convective self-assembly. Bimodal AB2 and AB13 colloidal crystals with complex structures have also been observed in binary mixtures of hard-sphere colloids with specific radii ratios. Here we report a method to make a new form of colloidal crystal, a multilayer crystal, using successive deposition of crystals of colloids of arbitrary sizes. The multilayer colloidal crystal is schematically represented in Figure 1A. Spheres of different colors represent submicrometer silica or polystyrene colloids of different sizes. Each layer of the crystal is a close-packed array of colloids, and the overall structure consists of successively stacked crystals, formed of colloids of arbitrary diameters. The preparation of these structures is described in the experimental section. The high uniformity of the resulting crystals can be illustrated by the transmission (Fig. 1B) and reflection (Fig. 1C and D from different angles) photographs of a threelayer crystal. This sample is formed by consecutive deposition of 13 layers of 430 nm silica spheres, followed by 16 layers of 253 nm silica spheres, followed by 10 layers of 338 nm silica spheres. We describe the multilayer colloidal crystal pattern by listing the sphere size from bottom to top. For example, the sample in Figure 1 is referred to as 430 nm/253 nm/338 nm. The reflected colors are caused by Bragg diffraction of visible light by the three-dimensionally ordered arrays of submicrometer colloids. When two overlapping layers are made from colloids with extremely different sizes, most of the reflected light from the bottom layer will transmit through the upper layer, resulting in the transparent appearance of the second layer in Figure 1C (430 nm/253 nm). Crystalline quality is among the most important parameters in determining the performance of colloidal crystals in optical applications. Figure 2 shows the typical top-view and crosssectional scanning electron microscopy (SEM) images of each astepo of the sample shown in Figure 1. In Figure 2A, the hexagonal close-packed (hcp) arrangement of the top 430 nm layer is evident. The sharp peaks in the two-dimensional fast Fourier transform (FFT, inset) of a low-magnification image confirm the presence of long-range crystalline order extending over the largest length scales (40 ” 40 lm) accessible in a single image. The stacking of close-packed layers shown in Figure 2B demonstrates the high degree of order along the (111) crystallographic axis, perpendicular to the substrate.

Self-assembling three-dimensional colloidal photonic crystal structure with high crystalline quality

Abstract: High-quality colloidal crystal multilayers were fabricated from aqueous solutions by the vertical deposition method. The effect of the evaporation temperature on the crystalline quality of colloidal crystals was carried out. It is found that with the increase of the evaporation temperature, the colloidal crystal shows an increasing tendency towards equilibrium face-centered-cubic phase, and the resulted sample also shows few dislocations and vacancies when the balance in the processes of nucleus formation, particle transport, and crystallization can be kept. However, with the further increase of the evaporation temperature (above 55 °C), a vast amount of defects appear in the crystal because the fast water evaporation rate, which results in a fast crystal growth rate, will spoil the balance. Optical measurements correspond well to the microstructure results.

The stability of Cu/ZnO-based catalysts in methanol synthesis from a CO2-rich feed and from a CO-rich feed

Abstract: Water produced during methanol synthesis from a CO 2 -rich feed (CO 2 /CO/H 2 ) accelerated the crystallization of Cu and ZnO contained in a Cu/ZnO-based catalyst to lead to the deactivation of the catalyst. A small amount of silica added to the catalyst greatly improved the catalyst stability by suppressing the crystallization of Cu and ZnO. On the other hand, the catalysts both with and without silica were only slightly deactivated during methanol synthesis from a CO-rich feed containing a higher concentration of CO, because only a small amount of water was produced during the reaction, so no remarkable crystallization of Cu and ZnO contained in the catalyst occurred.

Large‐Pore Mesoporous Materials with Semi‐Crystalline Zeolitic Frameworks

Abstract: Crystallizing walls: Following the templated solid-state secondary crystallization of an amorphous mesoporous material the amorphous pore walls of the mesoporous precursor are partially reorganized into a crystalline material, the zeolite ZSM-5, as indicated by the dark-field transmission electron micrograph image shown. The bright spots correspond to the ZSM-5 nanocrystals.

Nanocrystallization of metallic glasses

Abstract: The paper summarizes briefly the current status of research in the field of nanocrystallization of metallic glasses especially highlighting the influence of glass composition and conditions of devitrification process on size, morphology and composition of crystallization products. Conventional crystallization creates a nanocrystalline structure only in glasses with particular compositions. Any metallic glass, decomposing in a primary crystallization process, can be converted into partially nanocrystalline material using non-conventional methods of heat treatment, e.g. high-temperature or low-temperature nanocrystallization. Temperature of devitrification process influences sizes and compositions of crystallization products for any volume fraction of crystalline phase. The change of crystallites sizes can change their morphologies. The change of a crystallite composition usually affects the lattice parameter but also can result in a change of crystallographic structure of the same phase or in formation of another phase. Composition of primary crystallites is dependent on temperature as well as on time of devitrification process. The lower the annealing temperature and the shorter the annealing time (smaller crystallites) the more the crystallites composition differs from the equilibrium state.

Protein crystallization in microfluidic structures

Abstract: A device for promoting protein crystal growth (PCG) using microfluidic channels. A protein sample and a solvent solution are combined within a microfluidic channel having laminar flow characteristics which forms diffusion zones, providing for a well defined crystallization. Protein crystals can then be harvested from the device. The device is particularly suited for microgravity conditions.