Showing papers on "Crystallization published in 1999"

Control of crystal nucleation by patterned self-assembled monolayers

Abstract: An important requirement in the fabrication of advanced inorganic materials, such as ceramics and semiconductors, is control over crystallization1,2,3,4. In principle, the synthetic growth of crystals can be guided by molecular recognition at interfaces5,6,7,8,9,10,11,12,13,14,15,16. But it remains a practical challenge to control simultaneously the density and pattern of nucleation events, and the sizes and orientations of the growing crystals. Here we report a route to crystal formation, using micropatterned self-assembled monolayers17,18, which affords control over all these parameters. We begin with a metal substrate patterned with a self-assembled monolayer having areas of different nucleating activity—in this case, an array of acid-terminated regions separated by methyl-terminated regions. By immersing the patterned substrates in a calcium chloride solution and exposing them to carbon dioxide, we achieve ordered crystallization of calcite in the polar regions, where the rate of nucleation is fastest; crystallization can be completely suppressed elsewhere by a suitable choice of array spacing, which ensures that the solution is undersaturated in the methyl-terminated regions. The nucleation density (the number of crystals formed per active site) may be controlled by varying the area and distribution of the polar regions, and we can manipulate the crystallographic orientation by using different functional groups and substrates.

Crystallization of Biological Macromolecules

Abstract: The history and character of macromolecular crystals principles of macromolecular structure and the applications of x-ray crystallography the purification and characterization of biological macromolecules some physical and energetic principles practical procedures for macromolecular crystallization important considerations in macromolecular crystallization strategies and special approaches in growing crystals impurities, defects, and crystal quality the mechanisms and kinetics of macromolecular crystal growth crystal growth in unique environments.

Studies on nylon 6/clay nanocomposites by melt-intercalation process

Abstract: The preparation of nylon 6/clay nanocomposites by a melt-intercalation process is proposed. X-ray diffraction and DSC results show that the crystal structure and crystallization behaviors of the nanocomposites are different from those of nylon 6. Mechanical and thermal testing shows that the properties of the nanocomposites are superior to nylon 6 in terms of the heat-distortion temperature, strength, and modulus without sacrificing their impact strength. This is due to the nanoscale effects and the strong interaction between the nylon 6 matrix and the clay interface, as revealed by X-ray diffraction, transmission electron microscopy, and Molau testing. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1133–1138, 1999

Salt weathering: influence of evaporation rate, supersaturation and crystallization pattern

Abstract: Micro- and macroscale experiments which document the dynamics of salt damage to porous stone have yielded data which expose weaknesses in earlier interpretations. Previously unexplained differences are found in crystal morphology, crystallization patterns, kinetics and substrate damage when comparing the growth of mirabilite (Na2SO4. 10H2O) and thenardite (Na2SO4) versus halite (NaCl). The crystallization pattern of sodium sulphate was strongly affected by relative humidity (RH), while a lesser RH effect was observed for sodium chloride. Macroscale experiments confirmed that mirabilite (crystallizing at RH > 50 per cent) and thenardite (crystallizing at RH < 50 per cent) tend to form subflorescence in highly localized areas under conditions of constant RH and temperature. This crystallization pattern was more damaging than that of halite, since halite tended to grow as efflorescence or by filling the smallest pores of the stone in a homogeneous fashion, a result which contradicts Wellman and Wilson's theoretical model of salt damage. Low RH promoted rapid evaporation of saline solutions and higher supersaturation levels, resulting in the greatest damage to the stone in the case of both sodium sulphate and sodium chloride crystallization. At any particular crystallization condition, sodium chloride tended to reach lower supersaturation levels (resulting in the crystallization of isometric crystals) and created negligible damage, while sodium sulphate reached higher supersaturation ratios (resulting in non-equilibrium crystal shapes), resulting in significant damage. ESEM showed no damage from sodium sulphate due to hydration. Instead, after water condensation on thenardite crystals, rapid dissolution followed by precipitation of mirabilite took place, resulting in stone damage by means of crystallization pressure generation. It is concluded that salt damage due to crystallization pressure appears to be largely a function of solution supersaturation ratio and location of crystallization. These key factors are related to solution properties and evaporation rates, which are constrained by solution composition, environmental conditions, substrate properties, and salt crystallization growth patterns. When combined with a critical review of salt damage literature, these experiments allow the development of a model which explains variations in damage related to combinations of different salts, substrates and environmental conditions.

Mechanism of Zeolite A Nanocrystal Growth from Colloids at Room Temperature

Abstract: The formation and growth of crystal nuclei of zeolite A from clear solutions at room temperature were studied with low-dose, high-resolution transmission electron microscopy in field emission mode and with in situ dynamic light scattering Single zeolite A crystals nucleated in amorphous gel particles of 40 to 80 nanometers within 3 days at room temperature The resulting nanoscale single crystals (10 to 30 nanometers) were embedded in the amorphous gel particles The gel particles were consumed during further crystal growth at room temperature, forming a colloidal suspension of zeolite A nanocrystals of 40 to 80 nanometers On heating this suspension at 80°C, solution-mediated transport resulted in additional substantial crystal growth

Crystallization of Anatase from Amorphous Titania Using the Hydrothermal Technique: Effects of Starting Material and Temperature

Abstract: The crystallization of anatase from amorphous titania has been controlled using the hydrothermal technique Crystallite size and surface area can be controlled by careful alterations of the hydroth

Effect of crystalline phase, orientation and temperature on the dielectric properties of poly (vinylidene fluoride) (PVDF)

Abstract: The effect of crystalline phase, uniaxial drawing and temperature on the real (e′) and imaginary (e″) parts of the relative complex permittivity of poly (vinylidene fluoride) (PVDF) was studied in the frequency range between 102 and 106 Hz. Samples containing predominantly α and β phases, or a mixture of these, were obtained by crystallization from a DMF solution at different temperatures. α phase samples were also obtained from melt crystallization and from commercial films supplied by Bemberg Folien. Different molecular orientations were obtained by uniaxial drawing of α and β phase samples. The results showed that the crystalline phase exerts strong influence on the values of e′ and e″, indicating that the αa relaxation process, associated with the glass transition of PVDF, is not exclsively related to the amorphous region of the polymer. An interphase region, which maintains the conformational characteristics of the crystalline regions, should influence the process decisively. The molecular orientation increased the values of e′ for both PVDF phases and modified its dependence with temperature over the whole frequency range studied. The influence of the crystallization and molecular orientation conditions on the dc electric conductivity (σdc) were also verified. The value of σdc was slightly higher for samples crystallized from solution at the lowest temperature and decreased with draw ratio.

Crystallization, properties, and crystal and nanoscale morphology of PET–clay nanocomposites

Abstract: The crystallization process and crystal morphology of poly(ethylene terephathalate) (PET)–clay nanoscale composites prepared by intercalation, followed by in-situ polymerization, have been investigated by scanning electronic microscopy (SEM), transmission electronic microscopy (TEM), dynamic scanning calorimetry (DSC), and X-ray techniques, together with mechanical methods. Results of the nonisothermal crystallization dynamics show that the nanocomposites of PET (Nano-PET) have 3 times greater crystallization rate than that of pure PET. The thermal properties of Nano-PET showed heat distortion temperature (HDT) 20–50°C higher than the pure PET, while with a clay content of 5%, the modulus of Nano-PET is as much as 3 times that of pure PET. Statistical results of particle distribution show that the average nanoscale size ranges from 10 to 100 nm. The particles are homogenously distributed with their size percentages in normal distribution. The agglomerated particles are 4% or so with some particles size in the micrometer scale. The morphology of exfoliated clay particles are in a diordered state, in which the morphology of the PET spherulitics are not easy to detect in most of microdomains compared with the pure PET. The molecular chains intercalated in the interlamellae of clay are confined to some extent, which will explain the narrow distribution of the Nano-PET molecular weight. The stripe-belt morphology of the intercalated clay show that polymer PET molecular chains are intercalated into the enlarged interlamellar space. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1139–1146, 1999

Physical properties of solid molecular dispersions of indomethacin with poly(vinylpyrrolidone) and poly(vinylpyrrolidone-co-vinyl-acetate) in relation to indomethacin crystallization.

Abstract: Purpose. To measure solid-state features of amorphous molecular dispersions of indomethacin and various molecular weight grades of poly(vinylpyrrolidone), PVP, and poly(vinylpyrrolidone-co-vinylacetate), PVP/VA, in relation to isothermal crystallization of indomethacin at 30°C

Shear-Enhanced Crystallization in Isotactic Polypropylene. 1. Correspondence between in Situ Rheo-Optics and ex Situ Structure Determination

Abstract: The effects of “short term shearing” on the subsequent crystallization of a polydisperse Ziegler−Natta isotactic polypropylene are observed using in situ optical measurements and ex situ microscopy. Imposition of brief intervals of shear (0.25−20 s, less than a thousandth of the quiescent crystallization time) can reduce the crystallization time by 2 orders of magnitude (e.g., at 141 °C with a wall shear stress of 0.06 MPa). With increasing shearing time, the crystallization time saturates and highly anisotropic growth ensues. This transition to oriented growth correlates with changes in the transient behavior during flow and the semicrystalline morphology observed ex situ. During flow, we observe the generation of long-lived, highly oriented structures (evident in the transient birefringence) under all conditions that induce subsequent growth of highly oriented crystallites. In turn, the development of oriented crystallites observed in situ after cessation of flow correlates with development of a “skin-core” morphology (highly oriented skin on a spherulitic core) observed ex situ. Interestingly, the long-lived structures generated during flow appear at shorter times with increasing temperature (at fixed shear stress), the opposite of the trend one would expect on the basis of the temperature dependence of quiescent crystallization.

Helical Coordination Polymers from Achiral Components in Crystals. Homochiral Crystallization, Homochiral Helix Winding in the Solid State, and Chirality Control by Seeding

Abstract: An achiral anthracene−pyrimidine derivative (5-(9-anthracenyl)pyrimidine, 1) forms adduct 1·Cd(NO3)2·H2O·EtOH (2) in chiral space group P21. The metal ion is hexacoordinated with two pyrimidine ligands (equatorial cis), water and ethanol (equatorial cis), and two nitrate ions (axial trans). The chirality arises from a pyrimidine−Cd2+ helical array and is preserved not only in each crystal via homochiral interstrand water−nitrate hydrogen bonding but also in all the crystals in the same chirality as a result of single-colony homochiral crystal growth. Compound 1 also forms achiral (Pbca) trihydrate adduct 1·Cd(NO3)2·3H2O (3) having nonhelical pyrimidine−Cd2+ zigzag chains. Achiral zigzag polymer 3 and chiral helical polymer 2 are interconvertible with each other in the solid states upon exchange of volatile ligands (ethanol and water). The helix winding associated with the conversion of adduct 3 to 2 can be made homochiral by seeding.

Characterization of the time scales of molecular motion in pharmaceutically important glasses

Abstract: Increased interest in molecular time scales below the glass transition temperature, Tg, has arisen from the desire to identify the conditions (e.g., temperature) where the molecular processes which lead to unwanted changes in amorphous systems (e.g., chemical reactivity, crystallization, structural collapse) are improbable. The purpose of this study was to characterize the molecular mobility of selected amorphous systems (i.e., indomethacin, sorbitol, sucrose, and trehalose) below Tg using a combined experimental and theoretical approach. Of particular interest was the temperature where the time scales for molecular motion (i.e., relaxation time) exceed expected lifetimes or storage times. As a first approximation of this temperature, the temperature where the thermodynamic properties of the crystal and the equilibrium supercooled liquid converge (i.e., the Kauzmann temperature, TK) was determined. TK values derived from heat capacity and enthalpy of fusion data ranged from 40 to 190 K below the calorimet...

Synthesis of all-silica and high-silica molecular sieves in fluoride media

Abstract: Recent advances in the synthesis of all-silica and high-silica crystalline molecular sieves in fluoride media, with special regard to low framework density phases, are presented. The fundamental differences between the synthesis in hydroxide and fluoride media with respect to the properties of the materials obtained and the phase selectivity of the crystallization are discussed.

Cell-Mediated Crystallization of Calcium Oxalate in Plants

Abstract: Plants make crystals of calcium oxalate in an intriguing variety of defined shapes. [Figure 1][1] illustrates a commercial preparation of calcium oxalate, which consists of a mix of crystals with variable sizes and irregular shapes. In contrast, crystals synthesized by plants typically exhibit quite

Solder reaction-assisted crystallization of electroless Ni-P under bump metallization in low cost flip chip technology

Abstract: Solder reaction-assisted crystallization of electroless Ni–P under bump metallization in the Si/SiO2/Al/Ni–P/63Sn–37Pb multilayer structure was analyzed using transmission electron microscopy, scanning electron microscopy, energy dispersive x-ray, and electron probe microanalyzer. The electroless Ni–P had an amorphous structure and a composition of Ni85P15 in the as-plated condition. Upon reflow, the electroless Ni–P transformed to Ni3Sn4 and Ni3P. The crystallization of electroless Ni–P to Ni3P was induced by the depletion of Ni from electroless Ni–P to form Ni3Sn4. The interface between electroless Ni–P and Ni3P layer was planar. From the Ni3P thickness-time relationship, the kinetics of crystallization was found to be diffusion controlled. Conservation of P occurs between electroless Ni–P and Ni3P, meaning that little or no P diffuses into the molten solder. Combining the growth rates of Ni3Sn4 and Ni3P, the consumption rate of electroless Ni–P was determined. Based upon microstructural and diffusion r...

A study of low temperature crystallization of amorphous thin film indium–tin–oxide

Abstract: Deposition of tin-doped–indium-oxide (ITO) on unheated substrates via low energy processes such as electron-beam deposition can result in the formation of amorphous films The amorphous-to-crystalline transformation was studied in this system using in situ resistivity, time resolved reflectivity, glancing incidence angle x-ray diffraction, and transmission electron microscopy The resistivity of 180 nm thick In2O3(99 wt %SnO2) was monitored during isothermal anneals at 125, 135, 145, and 165 °C The dependence of the resistance on the volume fraction of crystalline phase was established using glancing incidence angle x-ray diffraction and a general two phase resistivity model for this system was developed These studies show that, upon annealing, as-deposited amorphous ITO undergoes both a structural relaxation and crystallization Structural relaxation of the amorphous material includes local ordering that increases the ionized vacancy concentration which, in turn, increases the carrier density in the

Time Scales for Viscous Flow, Atomic Transport, and Crystallization in the Liquid and Supercooled Liquid States of Zr 41.2 Ti 13.8 Cu 12.5 Ni 10.0 Be 22.5

Abstract: The shear viscosity of liquid Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 has been measured. At the liquidus temperature we find an extremely high viscosity of 2.5 Pa s, favoring glass formation. At deep supercooling the time scales for the diffusion of small and medium sized atoms as reported in the literature decouple from the internal relaxation time as probed by our viscosity measurements. Similarly, crystallization from the supercooled liquid state can be described with an effective diffusivity that scales with the viscosity at high temperatures and is Arrhenius-like at deep supercooling.

Influence of Structural and Topological Constraints on the Crystallization and Melting Behavior of Polymers. 1. Ethylene/1-Octene Copolymers†

Abstract: Studies of the crystallization, melting, and morphology of random ethylene/1-octene copolymers by a combination of differential scanning calorimetry and atomic force microscopy are presented. Two d...

Molecular-dynamics simulations of glass formation and crystallization in binary liquid metals: Cu-Ag and Cu-Ni

Abstract: We used molecular dynamics ~MD! to obtain an atomistic description of the melting, glass formation, and crystallization processes in metal alloys. These studies use the quantum Sutton-Chen many-body potentials for Cu, Ni, and Ag to examine the Cu4Ag6 and CuNi alloys. Using cooling rates in the range of 2 310 12 to 4 310 14 K/s, we find that CuNi and pure Cu always form a face-centered-cubic ~fcc! crystal while Cu4Ag6 always forms a glass ~with Tg decreasing as the quench rate increases!. The crystal formers have radius ratios of 1.025 ~CuNi! and 1.00 ~Cu! while the glass former ~CuAg! has a ratio of 1.13, confirming the role of size mismatch in biasing toward glass formation. @S0163-1829~99!05205-4#

Highly active thermally stable β-nucleating agents for isotactic polypropylene

Abstract: Calcium salts of suberic (Ca-Sub) and pimelic (Ca-Pim) acids were synthesized and implemented as in different grades of isotactic polypropylene (iPP). Propylene homopolymer, as well as random and block copolymers containing these additives, crystallized iPP into pure or nearly pure β modification in the isothermal and nonisothermal crystallization experiments. Recently, Ca-Sub proved to be the most effective β-nucleating agent of iPP. The Ca-Sub nucleating agent widens the upper crystallization temperature range of pure β-iPP formation up to 140°C. In this study the effect of the these additives on the crystallization, melting characteristics, and structure of the PP were studied. The degree of crystallinity of β-iPP was markedly higher than that of α-iPP. A widening in the melting peak of the samples crystallized in a high temperature range was first observed and discussed in regard to literature results of the same phenomenon for α-iPP. The morphology of the β-iPP samples was revealed by scanning electron microscopy. Independent of the type of polymer or nucleating agent, hedritic structures were found in the early stages of growth of the β-spherulites. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2357–2368, 1999

Crystallization in Block Copolymers

Abstract: Crystallization in block copolymers has a profound effect on their structure. This review article focusses on the morphology of semicrystalline block copolymers, and those containing two crystallizable blocks. The effect of crystallization on mechanical properties is briefly considered. The extent of chain folding upon crystallization is discussed, as is the orientation of crystal stems with respect to the microstructure. The effect of selective solvent on solution crystallization is also highlighted. Recent work on crystallization kinetics is summarized and finally the theories for crystallization in block copolymers are outlined.

Homogeneous Precipitation of TiO2 Ultrafine Powders from Aqueous TiOCl2 Solution

Abstract: Crystalline TiO2 powders were prepared by the homogeneous precipitation method simply by heating and stirring an aqueous TiOCl2 solution with a Ti4+ concentration of 0.5M at room temperature to 100°C under a pressure of 1 atm. TiO2 precipitates with pure rutile phase having spherical shapes 200-400 nm in diameter formed between room temperature and 65°C, whereas TiO2 precipitates with anatase phase started to form at temperatures >65°C. Precipitates with pure anatase phase having irregular shapes 2-5 µm in size formed at 100°C. Possibly because of the crystallization of an unstable intermediate product, TiO(OH)2, to TiO2xH2O during precipitation, crystalline and ultrafine TiO2 precipitates were formed in aqueous TiOCl2 solution without hydrolyzing directly to Ti(OH)4. Also, formation of a stable TiO2 rutile phase between room temperature and 65°C was likely to occur slowly under these conditions, although TiO2 with rutile phase formed thermodynamically at higher temperatures.

Structural and Kinetic Factors Governing the Formation of the γ Polymorph of Isotactic Polypropylene

Abstract: The molecular, thermodynamic, and kinetic factors that govern the formation and concentration of the α and γ polymorphs in metallocene-catalyzed isotactic poly(propylenes) have been studied with a set of polymers that have a wide range in molecular weight and defect contents. With these polymers it was possible to investigate the influence of molecular weight on γ formation at a fixed defect concentration, as well as the role of the defect concentration at constant molecular weight. The major experimental techniques used were wide-angle X-ray scattering and differential scanning calorimetry complemented by microscopy. From these studies the role of chain microstructure, the crystallization temperature, and the thermodynamic and kinetic requirement for the formation of the γ form could be established in more quantitative detail than heretofore. A particular important finding was the fact that at fixed defect concentration the fractional content of the γ polymorph goes through a maximum with crystallization...

In situ observation of nucleation and crystal growth in zeolite synthesis. A small-angle X-ray scattering investigation in Si-TPA-MFI

Abstract: The formation and consumption of nanometer scale precursor particles during the hydrothermal synthesis of Si-TPA-MFI from a clear solution has been studied in situ using a combination of X-ray scattering techniques and with electron microscpy. The combination of wide-, small-, and ultra-small-angle X-ray scattering allowed us to obtain information on a continuous range of length scales spanning over four orders of magnitude (0.17−6000 nm), covering all particle populations present during the complete course of the crystallization process. The use of high-brilliance synchrotron radiation allows us to perform time-resolved experiments. Two types of precursor particles were observed: 2.8 nm sized primary units and aggregates (≈10 nm). Variation of the alkalinity of the synthesis mixture revealed a strong correlation between the concentration of the aggregates and the rate of the crystal nucleation. The presence of the 2.8 nm sized primary units appears to be independent on the alkalinity. The addition of se...