Showing papers on "Crystallization published in 1994"

Prediction of crystal–melt partition coefficients from elastic moduli

Abstract: MANY geochemical processes, such as crystallization of silicate magmas or planetary differentiation, require a knowledge of the way in which elements become partitioned between coexisting crystal and liquid phases1,2. But quantitative prediction of crystal/melt partition coefficients from thermodynamic principles has not previously been possible. By studying the partitioning of 15 elements between silicate minerals and their coexisting melts, we show here that the partitioning behaviour of any series of isovalent cations can be rationalized in terms of a simple model in which the size and elasticity of the crystal lattice sites play a critical role. We find that elasticity varies linearly with the formal charge of the cation. This model allows us to predict element partitioning behav-iour solely from the physical characteristics of the cation sites in the crystal.

Effect of crystallization temperature on the crystalline phase content and morphology of poly(vinylidene fluoride)

Abstract: The effect of temperature on the crystallization of α, β, and γ phases of PVDF from dimethylacetamide (DMA) solution was studied. Variation in the crystallinity content of these three phases was obtained as a function of temperature using infrared spectroscopy, differential scanning calorimetry (DSC) and x-ray diffraction techniques. Such variation is related to the dependence of the crystallization rate of each phase with temperature, and allowed a better understanding of some results found in the literature about the crystallization and interconversion of these phases. Micrographs of samples present morphologies that corroborate with the proposed explanations. © 1994 John Wiley & Sons, Inc.

Predicting protein crystallization from a dilute solution property

Abstract: A dilute solution parameter obtained from static light-scattering measurements is proposed as a predictor for protein crystallization experiments. The osmotic second virial coefficients, B22, have been measured for a variety of proteins in solvents that are known to promote crystallization and the values for B22 were found to lie within a fairly narrow range which we refer to as a crystallization slot. Solution conditions which were known not to favor crystallization of the proteins resulted in B22 values well outside the crystallization slot.

Observation of Coulomb-Crystal Formation from Carbon Particles Grown in a Methane Plasma

Abstract: A Coulomb crystal was successfully formed as a result of the growth of spherical and monodisperse carbon particles suspended in a methane plasma. The crystal structure was confirmed to be hexagonal from top- and side-view photographs. The particle growth was monitored by Mie-scattering ellipsometry and correlated with the formation process of the Coulomb crystal. The liquid-to-solid phase transition occurred when particle diameter grew to 1.3 µm, and when the Wigner-Seitz radius was about 90 µm.

Deformation-induced nanocrystal formation in shear bands of amorphous alloys

Abstract: AMORPHOUS alloys formed by rapid solidification of a metallic melt are of considerable technological interest as high-strength materials1–8. As they are not in thermodynamic equilibrium, these materials tend to crystallize on heating9,10. A high degree of crystallization leads to embrittlement, but if it can be arrested when the crystallites are of only nanometre dimensions, the resulting amorphous–nanocrystalline composite actually has greater strength than the original amorphous material11. There is consequently much interest in understanding the mechanisms of crystallization. Previous studies have suggested that mechanical deformation can induce crystallization12–16. Here we report the direct observation of crystallization within the shear bands of aluminium-based amorphous alloys induced by bending. The crystals are face-centred cubic aluminium, 7–10 nm in diameter, and seem to form as a consequence of local atomic rearrangements in regions of high plastic strain. We suggest that mechanical deformation might therefore be used to form high-strength amorphous–nanocrystalline composites.

Density-driven liquid–liquid phase separation in the system AI 2 O 3 –Y 2 O 3

Abstract: PHASE separation of liquid mixtures into two liquids with different compositions is a well-known phenomenon. It has been proposed1–9 that another type of liquid–liquid phase separation, driven by fluctuations in density rather than in composition, may occur in some elemental systems. Transitions between low- and high-density amorphous phases have been described for the one-component oxides H2O, SiO2and GeO2 (refs 10–17), and it has been suggested18–21 that a liquid–liquid phase transition might occur in supercooled water. If density-driven phase separation truly does occur in liquid mixtures, it should be possible to observe the coexistence of two liquids with the same composition but different density. Here we report the direct observation of such a situation. We observe two coexisting liquid phases in the supercooled melt of AI2O3–Y2O3 just above the glass transition at ambient pressure, both of which have the same composition. We propose that these two phases must differ solely in density, and that the transition is entropically driven. The occurrence of the phase transition in this system may explain why the crystallization of yttrium aluminium garnet, the host material for Nd3 +ions in YAG lasers, is sluggish22–25.

Orientation of rapid thermally annealed lead zirconate titanate thin films on (111) Pt substrates

Abstract: The nucleation, growth, and orientation of lead zirconate titanate thin films prepared from organometallic precursor solutions by spin coating on (111) oriented platinum substrates and crystallized by rapid thermal annealing was investigated. The effects of pyrolysis temperature, post-pyrolysis thermal treatments, and excess lead addition are reported. The use of post-pyrolysis oxygen anneals at temperatures in the regime of 350–450 °C was found to strongly affect the kinetics of subsequent amorphous-pyrochlore-perovskite crystallization by rapid thermal annealing. The use of such post-pyrolysis anneals allowed films of reproducible microstructure and textures [both (100) and (111)] to be prepared by rapid thermal annealing. It is proposed that such anneals and pyrolysis temperature affect the oxygen concentration/average Pb valence in the amorphous films prior to annealing. Such changes in the Pb valence state then affect the stability of the transient pyrochlore phase and thus the kinetics of perovskite crystallization.

Non-Isothermal and Isothermal Crystallization of Sucrose from the Amorphous State

Abstract: The crystallization of a model compound, sucrose, from the amorphous solid state has been studied non-isothermally using differential scanning calorimetry to determine crystallization temperature, Tc, and isothermally at 30°C by subjecting samples to 32.4% relative humidity and gravimetrically monitoring water vapor uptake and subsequent loss with time due to crystallization. From the measurement of glass transition temperature, Tg, and melting temperature, Tm, for sucrose alone and in the presence of absorbed water it was possible to predict Tc and thus to directly relate the plasticizing effects of water to its tendency to promote crystallization. Colyo-philization of sucrose with lactose, trehalose, and raffinose, all having Tg values greater than that of sucrose, increased Tc significantly, even at levels as low as 1 – 10% w/w. In the isothermal studies the time required for crystallization to commence, due to the plasticizing effects of water, i.e., the induction time, assumed to be mostly affected by rates of nucleation, was greatly increased by the presence of the additives at these low levels, with raffinose producing a greater effect than lactose and trehalose. Similarly, these additives reduced the rate of water loss, i.e., the rate of crystal growth, but now no significant differences were noted between the three additives. The possible relationships of nucleation and crystal growth and the effects of additives on molecular mobility are discussed.

Semiconductor, semiconductor device, and method for fabricating the same

Abstract: Method of fabricating semiconductor devices such as thin-film transistors by annealing a substantially amorphous silicon film at a temperature either lower than normal crystallization temperature of amorphous silicon or lower than the glass transition point of the substrate so as to crystallize the silicon film. Islands, stripes, lines, or dots of nickel, iron, cobalt, or platinum, silicide, acetate, or nitrate of nickel, iron, cobalt, or platinum, film containing various salts, particles, or clusters containing at least one of nickel, iron, cobalt, and platinum are used as starting materials for crystallization. These materials are formed on or under the amorphous silicon film.

On the super lateral growth phenomenon observed in excimer laser-induced crystallization of thin Si films

Abstract: This letter reports on the experimental findings, and provides a theoretical description of the super lateral growth (SLG) phenomenon observed in the pulsed laser‐induced solidification of amorphous thin Si films on SiO2. Experimentally, we report and elaborate on the isolated single‐crystal disk structure that is observed at the upper threshold of the SLG regime; the structure is revealed as an important microstructural feature for understanding the phenomenon. A theoretical discussion of the SLG phenomenon is provided in terms of the key factors that are suggested by our model—the interface response function of the solid, the nucleation kinetics of the solid, and a highly transient lateral‐thermal profile near the solid‐melt interface. Our model and analysis point out the important inadequacies associated with the vertical solidification rate/temperature gradient model, which is currently being utilized to explain the excimer laser crystallization of thin Si films on SiO2.

Crystallization behavior of iron-containing intermetallic compounds in 319 aluminum alloy

Abstract: The crystallization behavior of iron-containing intermetallic compounds in industrial grade 319 aluminum alloy has been investigated by means of thermal analysis and metallography. In the absence of manganese, the iron compound crystallizes in theβ phase, at all cooling rates ranging from 0.1 °C/s to 20 °C/s under normal casting temperatures (750 °C). However, when the melt is superheated to a high temperature (about 200 to 300 degrees above the liquidus temperature), the iron compound crystallizes in the α phase at high cooling rates. This is due to the fact that γ alumina, which forms at low melt temperatures (≤750 °C), acts as a nucleus for crystallization ofβ phase. When the melt is superheated to high temperature (≥85O °C), the γ alumina transforms to a alumina. This is a poor nucleus for the β-phase crystallization, and as a result, a phase forms. The importance of nucleation and growth undercooling for the crystallization of iron compounds is highlighted. In the presence of manganese, the iron compound crystallizes in a phase at low cooling rates and in both the α andβ phases at high cooling rates. This reverse crystallization behavior is explained in terms of phase diagram relationships.

Crystallization analysis fractionation: A new technique for the analysis of branching distribution in polyolefins

Abstract: A new technique to analyze the short-chain branching distribution (SCBD) in linear lowdensity polyethylene has been developed. The technique referred as crystallization analysis fractionation is based on a stepwise precipitation approach. By monitoring the polymer solution concentration during crystallization, the cumulative and differential SCBD can be obtained without the need of physical separation of fractions. The new technique has been shown to provide similar results to temperature rising elution fractionation but in a shorter time and with a simplified apparatus. It allows the simultaneous analysis of various samples and could also be used for analysis of polypropylene and other semicrystalline polymers that can be fractionated on the basis of crystallizability. © 1994 John Wiley & Sons, Inc.

Synthesis of SAPO-34: high silicon incorporation in the presence of morpholine as template

Abstract: Synthesis of small pore SAPO-34 molecular sieve has been achieved under optimum conditions, elucidated by a detailed study of the kinetics of crystallization, in the presence of morpholine as a template. Incorporation of high silicon content into the framework of SAPO-34 has been observed and corroborated by physico-chemical characterization studies. Magic-angle spinning (MAS) NMR investigation points to the existence of aluminosilicate domains in the SAPO framework.

Screening and optimization strategies for macromolecular crystal growth.

Abstract: Today the determination of successful crystallization conditions for a particular macromolecule remains a highly empirical process. Sparse-matrix and grid-screening procedures are rapid and economical means to determine preliminary crystallization conditions. During optimization the variable set (pH, precipitant type and precipitant concentration) utilized in these procedures is screened in an attempt to determine appropriate conditions for the nucleation and growth of single crystals suitable for X-ray diffraction analysis. Unfortunately, in many cases this strategy will not produce single crystals suitable for X-ray diffraction analysis. We have explored, in an empirical sense, other tools for use during optimization. First, a new screening protocol is evaluated which employs less classical precipitating agents. Second, a set of 24 electrostatic crosslinking agents are evaluated for their ability to promote crystallization. Third, a panel of more than 30 detergents are evaluated for their ability to prevent sample aggregation and influence crystal growth.

An approach to the formation and growth of new phases with application to polymer crystallization: effect of finite size, metastability, and Ostwald's rule of stages

Abstract: This article aims to link the mainstream subject of chain-folded polymer crystallization with the rather speciality field of extended-chain crystallization, the latter typified by the crystallization of polyethylene (PE) under pressure. Issues of wider generality are also raised for crystal growth, and beyond for phase transformations. The underlying new experimental material comprises the prominent role of metastable phases, specifically the mobile hexagonal phase in polyethylene which can arise in preference to the orthorhombic phase in the phase regime where the latter is the stable regime, and the recognition of “thickening growth” as a primary growth process, as opposed to the traditionally considered secondary process of thickening. The scheme relies on considerations of crystal size as a thermodynamic variable, namely on melting-point depression, which is, in general, different for different polymorphs. It is shown that under specifiable conditions phase stabilities can invert with size; that is a phase which is metastable for infinite size can become the stable phase when the crystal is sufficiently small. As applied to crystal growth, it follows that a crystal can appear and grow in a phase that is different from that in its state of ultimate stability, maintaining this in a metastable form when it may or may not transform into the ultimate stable state in the course of growth according to circumstances. For polymers this intermediate initial state is one with high-chain mobility capable of “thickening growth” which in turn ceases (or slows down) upon transformation, when and if such occurs, thus “locking in” a finite lamellar thickness. The complete situation can be represented by a P, T, 1/l (l ≡ crystal thickness) phase-stability diagram which, coupled with kinetic considerations, embodies all recognized modes of crystallization including chain-folded and extended-chain type ones. The task that remains is to assess which applies under given conditions of P and T. A numerical assessment of the most widely explored case of crystallization of PE under atmospheric pressure indicates that there is a strong likelihood (critically dependent on the choice of input parameters) that crystallization may proceed via a metastable, mobile, hexagonal phase, which is transiently stable at the smallest size where the crystal first appears, with potentially profound consequences for the current picture of such crystallization. Crystallization of PE from solution, however, would, by such computations, proceed directly into the final stage of stability, upholding the validity of the existing treatments of chain-folded crystallization. The above treatment, in its wider applicability, provides a previously unsuspected thermodynamic foundation of Ostwald's rule of stages by stating that phase transformation will always start with the phase (polymorph) which is stable down to the smallest size, irrespective of whether this is stable or metastable when fully grown. In the case where the phase transformation is nucleation controlled, a ready connection between the kinetic and thermodynamic considerations presents itself, including previously invoked kinetic explanations of the stage rule. To justify the statement that the crystal size can control the transformation between two polymorphs, a recent result on 1 -4-poly-trans-butadiene is invoked. Furthermore, phase-stability conditions for wedge-shaped geometries are considered, as raised by current experimental material on PE. It is found that inversion of phase stabilities (as compared to the conditions pertaining for parallel-sided systems) can arise, with consequences for our scheme of polymer crystallization and with wider implications for phase transformations in tapering spaces in general. In addition, in two of the Appendices two themes of overall generality (arising from present considerations for polymers) are developed analytically; namely, the competition of nucleation-controlled phase growth of polymorphs as a function of input parameters, and the effect of phase size on the triple point in phase diagrams. The latter case leads, inter alia to the recognition of previously unsuspected singularities, with consequences which are yet to be assessed.

Effect of Mannitol Crystallinity on the Stabilization of Enzymes during Freeze-Drying

Abstract: The stabilizing effect of mannitol during the freeze-drying of proteins was studied using L-lactate dehydrogenase (LDH, rabbit muscle), beta-galactosidase (Escherichia coli) and L-asparaginase (Erwinia chrysanthemi) as model proteins. Crystallization of mannitol was studied by powder X-ray diffraction and differential scanning calorimetry (DSC), in relation to the stabilizing effect. All the enzymes were protected concentration-dependently by amorphous mannitol, but the stabilizing effect was decreased with an increase in mannitol crystallinity. The heat-treatment of frozen solutions above crystallization temperature prior to drying enhanced mannitol crystallization and LDH inactivation. The importance of maintaining excipients in an amorphous state during freeze-drying, previously reported for Aspergillus oryzae beta-galactosidase (K. Izutsu et al., Pharm. Res., 10, 1233 (1993)), was confirmed using three different enzymes.

Characterization and Solid-State Properties of Processable N-Alkylated Polyanilines in the Neutral State

Abstract: Incorporation of flexible alkyl chains into polyaniline was accomplished through an N-alkylation method with the leucoemeraldine form in order to maximize the degree of alkylation. The number of carbons varied from butyl to octadecyl, and with dodecylated samples, the degree of substitution was also controlled. The solubility in common organic solvents improved remarkably with the alkylation. The polymers displayed interesting solvatochromism and thermochromism, which result from the conformational changes induced by the interactions between the polymer and the solvent molecules and from the cooperation of disordering of the side chains and twisting in the main chain upon heating. In the solid state, the polymers form a layered structure in which the distance between the backbones depends on the length of alkyl side chain as recorded by the d spacing in WAXD patterns. DSC studies revealed that glass and melting transitions decreased systematically by increasing the length of substituents. With increasing side-chain length, the degree of interdigitation increases and the side chains begin to crystallize in a hexagonal packing array as determined by DSC, WAXD, and FT-IR data. The critical length n for side-chain crystallization is a minimum of 14 carbons, which is much larger than in the case of polyacrylates and polythiophenes, indicating the more rodlike character of polyaniline backbones. In addition, with polymers of n≥16, a mesophase transition after melting of the side-chain crystallites can be detected by optical microscopy and DSC

Chemistry and crystal growth

Abstract: Single-crystal materials, along with other forms of condensed matter (ceramics, polymers, liquid crystals, etc.) are fundamental to modern technology. The basic research and production of new materials with “tailored” solid-state physical properties therefore necessitate not only chemical synthesis but also the production of single crystals of a particular morphology (either bulk or thin layer crystals) and well-defined crystal defects (doping). In this review, an attempt is made to broaden the traditional synthetic concept of chemistry to the process of single-crystal synthesis. The methods of the resulting approach, which takes into account the specific properties of solid materials, are discussed and illustrated by experimental set-ups for the solution of a range of problems in chemical crystallization. Also included is recent work on the growing of single crystals of high-temperature superconductors, organic non-linear optical compounds, and proteins.