Showing papers on "Nucleation published in 1984"

Nucleation and growth of thin films

Abstract: The purpose of this article is to describe the basic physical processes involved in the nucleation and growth of thin films of materials on solid surfaces. In this introduction the three modes of crystal growth which are thought to occur on surfaces in the absence of interdiffusion are described, and the relationships between the thermodynamics of adsorption and the kinetics of crystal growth are explored in general terms. This is followed by a brief review of atomistic nucleation theory, explaining the relations of such theories to experimental observables. In the next three sections, recent experimental examples of these three growth modes are given, which are interpreted where possible in terms of nucleation and growth theory. The last section discusses observations on the shapes of growing crystallites and the relation of such observations to nucleation and surface diffusion processes.

Kinetics of non-isothermal crystallization process and activation energy for crystal growth in amorphous materials

Abstract: An equation expressing the volume fraction,x, of crystals precipitating in a glass heated at a constant rate, α, was derived. When crystal particles grow m-dimensionally,x is expressed as In [- ln(1 -x)] = -n (nα - 1.052mE/RT + Constant whereE is the activation energy for crystal growth andn is a numerical factor depending on the nucleation process. When the nuclei formed during the heating at the constant rate,α, are dominant,n is equal tom + 1, and when the nuclei formed in the previous heat-treatment before thermal analysis run are dominant,n is equal tom. The validity and usefulness of this equation was ascertained by applying it to a Li2O·2SiO2 glass. A method for determining the values ofn andm from DSC curves was proposed and it was concluded that the modified Ozawa-type plot is very useful and convenient to obtain the activation energy for crystal growth.

Biliary proteins. Unique inhibitors of cholesterol crystal nucleation in human gallbladder bile.

Abstract: The onset time for cholesterol crystal nucleation of supersaturated normal human gallbladder biles is consistently prolonged when compared with biles from patients with cholesterol gallstone disease. Investigation of the factor(s) responsible for the suspended supersaturation (metastability) of normal human biles revealed that model bile solutions of cholesterol saturation index (CSI) and molar lipid composition identical to individual gallbladder bile specimens had much shorter crystal nucleation times, i.e., exhibited decreased metastability. Unsaturated normal biles, after supplementation with lecithin, cholesterol, and sodium taurocholate to a 'standard' supersaturated lipid composition, also demonstrated nucleation times three- to 15-fold longer than the comparable standard model bile. Total lipid extracts of normal biles, however, when similarly supplemented, did not differ in nucleation time from the control model solution. Gallbladder biles were fractionated by gel chromatography and the eluted fractions were pooled into two fractions. The fractions eluting in about the first 25% of the included volume when mixed with the supersaturated standard model bile induced a modest increase in nucleation time of approximately 1.5 times the control value. The fractions eluting in the second 25% of the included volume and which contained all of the bile lipids, were concentrated and supplemented with lipids to the standard composition. The nucleation times of these supplements were 3-10 times longer than the control nucleation times. Delipidated bile protein mixtures, purified by discontinuous sucrose gradient centrifugation, were recombined with purified lipids at the standard composition used previously. The nucleation times of these mixtures were significantly prolonged to the same extent as those associated with the second chromatographic fraction. These observations demonstrate that the delayed onset (inhibition) of cholesterol crystal nucleation observed in normal human gallbladder bile is produced by a factor(s) present in the biliary protein fraction.

Influence of composition, crystallization conditions and melt phase structure on solid morphology, kinetics of crystallization and thermal behavior of binary polymer/polymer blends

Abstract: Results of an investigation on the morphology, the crystallization and the thermal behavior of several binary crystallizable blends are reported. The composition, molecular mass and crystallization conditions strongly influence the crystallization and the thermal behavior as well as the overall morphology of crystallizable binary blends. Quantities such as nucleation density (N), radial growth rate (G) of spherulites, overall rate of crystallization (K), and equilibrium melting temperature (Tm) are strongly dependent upon composition, crystallization conditions, and molecular mass of components. The type of dependence is to be related to the physical state of the melt, which, at the crystallization temperature, is in equilibrium with or coexists with the developing solid phase. In the ease of compatible blends such as poly(ethylene oxide)/poly(methyl methacrylate) the depression observed for G and Tm is mainly to be attributed to the diluent effect of the non-crystallizable component. For such a blend it is found that, after crystallization, the non-crystallizable component is trapped in intralamellar regions increasing the distance between adjacent lamellae. Depression of G, in the case of incompatible blends such as isotactic polypropylene/rubbers is mainly accounted for by rejection and deformation of rubber drops. The coexistence during crystallization of different processes such as molecular fractionation and segregation, preferential inclusion or dissolution of molecules with lower molecular mass and/or high degree of steric disorder of the crystallizable component in the phase rich in non-crystallizable component and vice versa may explain some minima observed in the plots of T and Tm, vs. composition in the case of blends semicompatible in the melt. It was found that the addition of a second non-crystallizable component causes drastic variations on some morphological and structural quantities of the semicrystalline matrix (isotactic polypropylene or nylon 6) such as the shape, dimensions, and regularity of spherulites and interspherulite boundary regions and lamella and interlamella thickness. In some cases the formation of new boundary lines connecting occluded particles are also observed. Such phenomena may have great importance on crack propagation and on impact behavior as well as on the tensile mechanical properties of binary blends characterized by a semicrystalline polymer component with a relatively high Tg and a rubber-like component with a lower Tg.

Supercritical fluid molecular spray film deposition and powder formation

Abstract: Solid films are deposited, or fine powders formed, by dissolving a solid material into a supercritical fluid solution at an elevated pressure and then rapidly expanding the solution through a short orifice into a region of relatively low pressure. This produces a molecular spray which is directed against a substrate to deposit a solid thin film thereon, or discharged into a collection chamber to collect a fine powder. Upon expansion and supersonic interaction with background gases in the low pressure region, any clusters of solvent are broken up and the solvent is vaporized and pumped away. Solute concentration in the solution is varied primarily by varying solution pressure to determine, together with flow rate, the rate of deposition and to control in part whether a film or podwer is produced and the granularity of each. Solvent clustering and solute nucleation are controlled by manipulating the rate of expansion of the solution and the pressure of the lower pressure region. Solution and low pressure region temperatures are also controlled.

Coagulative nucleation and particle size distributions in emulsion polymerization

Abstract: A detailed theory is presented for nucleation kinetics in emulsion polymerization systems based on the coagulation of precursor particles (which may themselves be formed by either homogeneous nucleation or micellar entry). These precursor particles differ from true latex particles by a slower rate of polymerization and the lack of stability against coagulation. The coagulative nucleation theory combines extended Muller-Smoluchowski coagulation kinetics with DLVO theory. Expressions are provided for the time evolutions of the nucleation rate, particle number, and particle size distribution (PSD). With physically reasonable values for the parameters for the coagulation kinetics, agreement is obtained with data for styrene emulsion polymerization systems. In particular, excellent accord is obtained with the early-time evolution of the PSD, such data being especially sensitive to assumptions as to the nucleation mechanism. In addition, agreement is obtained with data on the dependence of particle number on surfactant and initiator concentrations.

Enhanced Densification of Boehrmte Sol-Gels by α-Alumina Seeding

Abstract: Boehmite sol-gels were seeded by adding <2.0 wt%α-alumina powder to a 20% boehmite hydrosol at pH =3. The seeded gel sintered to 98% of theoretical density after 100 min at 1200°C, whereas the unseeded gel had to be sintered at 1600°C to reach 94% of theoretical density. This difference results primarily from nucleation by the α-alumina seeds and enhanced transformation of the boehmite to α-alumina, such that a uniform, well-ordered, fine-grained α-alumina micro-structure forms prior to densification.

Ionotropic Nucleation of Calcium Carbonate by Molluscan Matrix

Abstract: The hydrophilic, sulfated fraction of the organic matrix found in molluscan shells appears to be involved in crystal nucleation. It is located primarily at the sites of initial nucleation. The hydrophilic fraction favors in vitro formation of calcified deposits, when it is fixed in place on the hydrophobic fraction. Calcium is bound by the hydrophillic fraction with high affinity and selectivity. Enzymatic desulfation reduces the calcium binding. However, the binding stoichiometry of one calcium for every two ester sulfates is not altered. The calcium binding induces local anion binding, which induces secondary calcium binding. This coordinated ion binding is known as ionotropy. The resultant local high concentration of ions is thought to bring about nucleation.

Measurements of homogeneous nucleation rates for n‐nonane vapor using a two‐piston expansion chamber

Abstract: The homogeneous nucleation rate J in supersaturated n‐nonane vapor was measured as a function of supersaturation S and temperature T using a newly developed expansion chamber. Homogeneous nucleation is observed only during a short supercritical nucleation pulse so that nucleation and droplet growth are decoupled under the considered experimental conditions. Reproducible nucleation pulses with a duration of about 1 ms are achieved by means of two spring‐operated pistons moving in a well‐defined time sequence. The number concentration of the droplets growing in the expansion chamber is determined immediately after the end of the nucleation pulse by means of constant angle Mie scattering. Taking into account the duration of the nucleation pulse we measured homogeneous nucleation rates ranging from about 5×105 to 5×109 cm−3 s−1. Homogeneous nucleation in n‐nonane vapor was observed in the temperature range from 199 to 241 K. Argon was used as the carrier gas. The measured nucleation rates were compared with t...

Nucleation, crystal growth and the thermal regime of cooling magmas

Abstract: Crystallization at the margin of a quiet cooling magma has been studied numerically, taking into account the kinetics of crystallization. The variables are the latent heat value, the growth and nucleation functions, the initial magma temperature, and the thermal contrast between magma and country rock. We have investigated a wide range of values for these parameters corresponding to natural conditions. We show that after a highly transient stage, crystallization tends toward an equilibrium between heat production (latent heat release) and heat loss. Given the small diffusivity of country rocks, latent heat release is the main factor controlling the temperature evolution. In order to minimize the latent heat release, crystallization occurs at a temperature where nucleation is small. This can be close to either the liquidus or the solidus, depending on the initial conditions. The main process controlling crystallization is nucleation and not crystal growth. Nucleation occurs as a series of sharp pulses followed by longer periods of crystal growth. The nucleation pulses give birth to thermal oscillations. These oscillations can be sustained if the interior magma temperature is above the liquidus independently of the heat loss mechanism. We show that the phenomenon occurs on the scale of a few centimeters which corresponds to the inch-scale layering of many ultrabasic complexes. The model allows us to calculate crystal sizes which are in good agreement with geological observations. The crucial parameters which determine crystal size variations near the margins of igneous bodies are the initial thermal conditions as well as the nucleation and growth functions. In the main cooling regime close to the liquidus, significant size variations can be created by small thermal disturbances.

Contribution of shear banding to origin of Goss texture in silicon iron

Abstract: A number of experiments involving cold rolling and annealing have been carried out on polycrystalline and single crystal silicon iron. It is shown that the microstructural state just beneath the surface of the sheet before cold rolling is an important factor in the successful development of Goss texture during simulated processing. A coarse grained surface zone encourages formation of shear bands during rolling and these become preferential sites for nucleation during primary recrystallization. Nucleation of secondary grains, which also occurs just beneath the sheet surface, appears to be dependent on the prior existence of the shear band structure. Single crystals with the orientation (111)[112] are prone to shear banding during cold rolling especially when strain aging conditions are established. During subsequent annealing the shear bands recrystallize first, producing new grains which have the Goss orientation. The texture after complete recrystallization is the same as that of the shear band ...

Modification in the aluminum silicon system

Abstract: Alloys in the range 0 to 24 wt pct Si have been examined by careful thermal analysis, macroand microscopical study with modification by a mixed alkali fluoride flux, and by separate additions of sodium, potassium, lithium, and strontium. Sodium and strontium exert similar effects and potassium and lithium differing and minor effects; with a mixed alkali flux the influence of sodium is dominant. The normal and modified eutectic arrests in the presence of primary aluminum both show comparable supercoolings and recalescence behavior relative to the horizontal growth temperatures, but this is not observed in hypereutectic alloys containing primary silicon. Both normal and modified eutectics grow radially inward from crucible walls, but the details of the growth fronts are very different: there is no nucleation from the bulk liquid in modified alloys and it is uncertain if this occurs in normal alloys. It is concluded that the structural modification in furnace cooled ingots, as in directionally grown samples, is primarily caused by modified growth of silicon. The mechanism(s) for such modification are briefly discussed.

A molecular theory of crystal nucleation from the melt

Abstract: We construct a molecular theory for the homogeneous nucleation of crystals from pure liquids by combining an order parameter theory of freezing with a square gradient approximation for the nonlocal dependence of free energy on density We show the results of our theory for the free energy, radius, and shape of crystal nuclei as a function of supercooling, and compare with the conventional nucleation theory based on the capillarity approximation We argue that the simple picture given by the conventional approach is qualitatively reasonable, although calculated values of the liquid–solid surface free energy may be inaccurate We point out some of the limitations of our approach and stress the need for additional structural data on supercooled liquids

Amorphization of Hf-Ni films by solid-state reaction

Abstract: We report on a detailed study of a new process to produce amorphous binary alloys by solid-state reaction of the elemental constituents from an initially thin-layer configuration. The Ni-Hf system was selected on the basis of the criteria that a fast diffuser (Ni) and a large binary heat of mixing drives the solid-state reaction at temperatures so low (∼ 300 °C) that nucleation and growth of the crystalline phases are suppressed and the amorphous phase becomes the lowest accessible state of free energy. Backscattering spectrometry, x-ray diffraction, and transmission electron microscopy are used to monitor the atomic composition profile and the microstructure of the samples. The kinetics of the amorphous phase formation is consistent with the diffusion-limited growth of a laterally uniform amorphous layer. The substantial and approximately linear composition gradient of the amorphous layer reflects the low atomic mobility of the atoms in the amorphous phase and the broad existence range of the amorphous phase in the binary Ni-Hf system. The latter is consistent with predictions based on the calculated equilibrium free-energy diagram of the system.

Nucleation behaviour of poly-3-hydroxy-butyrate

Abstract: Poly-3-hydroxy-butyrate (PHB) is a thermoplastic polyester produced by bacterial fermentation. Because of this bacterial origin PHB is a very pure polymer. This high purity in turn leads to very few (if any) heterogeneous nuclei, which gives a much wider scope for a systematic study of nucleation behaviour and the effect of nucleating agents than was possible before. It is shown that in pure PHB nucleation is sporadic. The nucleation rate may be measured over a temperature range of some 100° C. The nucleation rate data are recorded in the range where homogeneous nucleation is possible and are at least consistent with it. When foreign particles are added the nucleation rate is modified. Two distinct types of behaviour are observed. One may be interpreted quantitatively as the local raising of the crystal melting point due to the constraints of the actual presence of a surface; and the other as being due to epitaxial growth on the foreign surface. Detailed kinetic data are presented to support these conclusions.

Nucleation Theory Near the Classical Spinodal.

Abstract: We present a field-theoretic description of metastability and nucleation for arbitrary range interaction models near the limit of metastability, i.e., spinodal. We find that as the spinodal is approached, the size of the nucleating droplet diverges in all dimensions. The upper critical dimension is found to be six. For $dl6$, as the spinodal is approached the nucleating droplets become ramified and their free-energy cost goes to zero; thus the system nucleates before reaching the spinodal. The free-energy cost increases rapidly as the range of the interaction, $R$, is increased, so that even for $dl6$ the spinodal can be approached as close as desired by increasing $R$. The internal structure of the ramified droplets in all dimensions is mapped onto that of a percolation cluster. The lifetime, including both the free-energy cost of the nucleating droplet and a dynamic prefactor, diverges in all dimensions; this is due to "critical" slowing down as the spinodal is approached. For $dg6$, the free-energy cost of the ramified droplets diverges as the spinodal is approached; more compact droplets must also be considered. A crossover where the upper critical dimension changes continuously from 6, for spinodal behavior, to 4, for critical-point behavior, is found for the model; this crossover is hypothesized to be absent or unobservable for more realistic models. The initial growth of the droplet is found to take place through compactification rather than through radial accretion as occurs in nucleation near the coexistence curve.

Ductile failure by void nucleation, growth and coalescence

Abstract: A wide range of stress states has been generated using axisymmetric and plane strain notched tensile specimens which have been analysed by elastic-plastic finite element analysis. Observations of voids on metallographic sections have been combined with calculated deformation histories and the hole growth equations to determine the local conditions for hole nucleation and the strength of the particle-matrix interface. These calculations show that nucleation is a statistical event for which the radial stress at the particle/matrix interface has been determined. Subsequent calculations give the local conditions at void coalescence in the average material and in statistical inhomogeneities. The statistics of the inclusion distribution determine the size scale over which void coalescence must occur in order to create a crack-like defect.

The homogeneous nucleation of nonane

Abstract: The homogeneous nucleation rate of n‐nonane has been measured as a function of temperature and supersaturation ratio in a precision fast‐expansion chamber. The measured nucleation rate ranges from 102 to 105 drops/cm3 over the temperature range 215–270 K. The results have been compared to the classical theory and to the classical theory with the RKC replacement factor. The RKC theory functional form is the basis for an empirical rate equation to fit the data. A full listing of the thermodynamic constants used for the reduction of the data is given.