Showing papers on "Nucleation published in 1986"

Kinetics of formation of silicides: A review

Abstract: The kinetics of silicide growth are classified into three different categories: (a) diffusion controlled, (b) nucleation controlled, (c) others (reaction rate controlled). These are analyzed with the aim of understanding both the phenomenology of growth and the specific atomic mechanisms of phase formation. Diffusion-controlled growth is discussed with respect to the Nernst-Einstein equation. Stress relaxation is considered as a possible cause of reaction-rate control. The relative merits of two different types of marker experiments are compared. A few silicides are discussed in terms of what can be inferred about diffusion mechanisms. The competition between reaction-rate and diffusion control phenomena is shown to have specific effects on the sequence of phase formation; it is also related to the formation of some amorphous compounds. Reactions between silicon and alloyed metal films are used to illustrate the respective influences of mobility and driving force factors on the kinetics of silicide growth; they can also be used to underline the dominance of nucleation over diffusion in some silicide formation processes.

Numerical Simulation of the Effects of Varying Ice Crystal Nucleation Rates and Aggregation Processes on Orographic Snowfall

Abstract: The Colorado State University cloud model is applied to the simulation of orogrophic cloud snowfall. A model of ice crystal aggregation processes and primary nucleation and secondary ice particle production of crystals is described. Sensitivity experiments demonstrated that aggregation plays an important role in controlling the fields of cloud liquid water content, ice crystal concentrations, and surface precipitation amounts. The sensitivity experiments also support observations that the air mass is often quite clean in upper levels of stable orographic clouds. Introducing a reduction of available nuclei that can be activated by deposition/sorption processes brought concentrations to within observed values. This study clearly emphasizes the need for a great deal more fundamental research in the physics of aggregation processes and primary and secondary nucleation of ice crystals.

Oxidation Kinetics of Silicon Carbide Crystals and Ceramics: I, In Dry Oxygen

Abstract: The oxidation kinetics of several single-crystal and polvcrystalline silicon carbide materials and single-crystal silicon in dry oxygen over the temperature range 1200° to 1500°C were fitted to the linear-parabolic model of Deal and Grove. The lower oxidation rates of silicon carbide compared to silicon can be rationalized by additional consumption of oxidant in oxidizing carbon to carbon dioxide. The (000J) Si face of the silicon carbide platelets exhibited lower parabolic oxidation rates than the (0001) C face, by a factor of 10 at 1200°C. Apparent activation energies increased from a value of ∼120 kJ/mol below 1400°C to a value of ∼300 kJ/mol above this temperature. The (0001) Si face exhibited this high activation energy over the entire temperature range. The controlled nucleation thermally deposited material exhibited the highest oxidation rates of the polycrystalline materials followed by the hot-pressed and sintered α-silicon carbides. In general, the oxidation rates of the polycrystalline materials were bracketed by the oxidation rates of the basal planes of the single-crystal materials. Higher impurity concentrations and higher density of nucleation sites led to a greater susceptibility to crystallization of the scale which significantly complicated the oxidation behaviors observed. When crystallization of the oxide scale occurred in the form of a layer of spherulitic cristobalite crystals, a retardation of the oxidation rates was observed. An accelerated oxidation behavior was found when this coherent layer was superseded by the formation of fine mullite crystals.

Organization of extracellularly mineralized tissues: a comparative study of biological crystal growth

Abstract: Biological mineralization processes are extremely diverse and, to date, it is an act of faith rather than an established principle that organisms utilize common mechanisms for forming crystals. A systematic analysis of the structural organization, as far as possible at the molecular level, of five different extracellularly mineralized tissues is presented to demonstrate that at least these mineralization processes are all part of the same continuum. The degrees of control exercised over crystal nucleation and crystal growth modulation are the basic variables. The five tissues, extracellularly mineralizing algae, radial and granular foraminifera, mammalian bone, mammalian enamel, and mollusk shell nacre, probably span the entire spectrum. Their crystal shapes, sizes, and the relations between the mineral phase and the organic phase, are primarily used to assess probable degrees of control exercised over crystal nucleation and modulation. Three different types of nucleation processes can be recognized: nonspecific, stereochemical, and epitaxial. Modulation of crystal growth after nucleation is either absent, achieved by adsorption of macromolecules onto specific crystal faces, or occurs by the prepositioning of matrix surfaces which interrupt crystal growth. The tissues in which active control is exercised over crystal growth all contain similar types of acidic matrix macromolecules. Significantly, the framework matrix macromolecules are all quite different and hence probably perform some tissue-specific functions. The study shows that there is a common basis for understanding these mineralization processes which is reflected in the nature of the protein-crystal interactions which occur in each tissue.

Ice Initiation in Natural Clouds

Abstract: Selected concentrations of ice crystal concentrations attributable to nucleation are compiled and summarized. The variability in the observations is discussed, and some conclusions related to natural precipitation formation and to seedability are discussed.

Dislocation reduction in epitaxial GaAs on Si(100)

Abstract: We have studied the nucleation and propagation of threading dislocations in GaAs on Si epitaxial layers, and have found several techniques which are effective in reducing their density. The use of substrates properly tilted off (100) reduces the dislocation density as the presence of steps helps create perfect edge dislocations with their Burgers vector parallel to the interface and thus do not propagate into the bulk epitaxial layer. Cross sections by transmission electron microscopy show that the incorporation of an InGaAs/GaAs strained‐layer superlattice reduces the density of threading dislocations above it by a factor of 10, clearly demonstrating the effectiveness of this technique. These methods lead to a dislocation density of 103 cm−2 near the surface of 2 μm layers which is five orders of magnitude lower than what has been obtained previously. We have also found that the density of oval defects is much lower for GaAs on Si than for GaAs on GaAs.

Transient nucleation effects in glass formation

Abstract: We extend to the non-isothermal case a numerical technique that was developed to treat transient homogeneous nucleation in a one-component system by modeling directly the reaction by which clusters are produced. Calculations are presented for the nucleation frequency during the quench and for the number of nuclei produced and the volume fraction transformed at the end of quench for different rates of cooling from the melt. Three model systems are considered: an alkali silicate which is a relatively good glass former, and two metallic glasses. These show a wide range of critical cooling rates for glass formation. In some systems transient effects are predicted to be critical for glass formation. A simple technique is presented for determining when transient effects are important based on a calculation using steady state nucleation frequencies and macroscopic growth velocities.

Nucleation and Growth of Colloidal Crystals

Abstract: bcc crystalline colloidal microsphere suspensions are shear melted into the metastable liquid phase. Recrystallization occurs via nucleation and growth of single crystallites at dilute sites. The nearly spherical growing crystals have rough interfaces with a time-independent interface velocity, $v$. The velocity is found to be consistent with an equation for normal growth with a limiting velocity determined by free-particle diffusion, ${v}_{0}\ensuremath{\sim}\frac{{D}_{0}}{\ensuremath{\xi}}$, where ${D}_{0}$ is the Stokes diffusion constant of independent noninteracting spheres, and $\ensuremath{\xi}$ is a length comparable to the particle spacing.

Nucleation and growth of monosized titania powders from alcohol solution

Abstract: Donnees sur la production de poudre monodispersee de TiO 2 par hydrolyse de Ti(OC 2 H 5 ) 4 dans une solution ethanolique

A bifurcation problem for a compressible nonlinearly elastic medium: growth of a micro-void

Abstract: In this paper, we carry out an explicit analysis of a bifurcation problem for a solid circular cylinder composed of a particularcompressible nonlinearly elastic material. This problem is concerned with the bifurcation of a solid body into a configuration involving an internal cavity. A discussion of its physical interpretation is then carried out. In particular, it is shown that this model may be used to describe the nucleation of a void from apre-existing micro-void.

Homogeneous nucleation rates for n‐alcohol vapors measured in a two‐piston expansion chamber.

Abstract: Homogeneous nucleation rates J in supersaturated n‐alcohol vapors (methanol through n‐hexanol) were measured in a two‐piston expansion chamber as functions of supersaturation S and temperature T. The measured nucleation rates were compared with the classical nucleation theory. We found that the slopes of the J–S curves are in good agreement with the theoretical prediction. However, the actual values of experimental and theoretical rates were generally found to differ significantly, Jexptl/Jtheor ranging from about 10−10 (methanol, 273 K) to 107 (n‐hexanol, 257 K). In particular, the experimental nucleation rates show a significantly weaker temperature dependence as compared to the classical nucleation theory, the difference being more pronounced for the higher alcohols. Thus, a temperature dependent correction to the classical nucleation theory is needed. For the lower alcohols, in particular methanol, we found that the change of the cluster distribution during the expansion strongly influences the nucleation process. The heat released during the formation of oligomers causes a significant temperature increase in the system and thus alters the nucleation conditions dramatically. For methanol vapor under the considered experimental conditions this effect leads to a reduction of the theoretically predicted nucleation rates by factors up to about 108. Since oligomer formation is prerequisite to nucleation, this effect has to be taken into account generally for nucleation experiments in adiabatically closed systems.

Significance of Internal Stresses for the Martensitic Transformation in Yttria-Stabilized Tetragonal Zirconia Polycrystals During Degradation

Abstract: Various aspects of the tetragonal (t) to monoclinic (m) transformation during degradation have been studied experimentally and theoretically in yttria-stabilized tetragonal zirconia polycrystals (Y-TZP), i.e., polycrystalline t-ZrO2 containing Y2O3 in solution. Transmission electron microscopy (TEM) revealed that protruding grains at the surface of Y-TZP specimens do not transform under corrosive conditions (250°C, humid atmosphere) even after an annealing time of 168 h. Eigenstresses due to anistropic thermal expansion In and around protruding and bulk grains have been calculated for Y-TZP containing 2 and 3 mol% Y2O3. The prominent role of these stresses on subsequent transformation nucleation during degradation is shown to agree qualitatively with an established free energy concept. The lack of complete transformation of m-ZrO2 is attributed to characteristics of the nucleation- and growth-controlled transformation process.

Nucleation rates and bubble stability in water-carbon dioxide solutions

Abstract: Using classical nucleation theory, homogeneous and heterogeneous nucleation rates are calculated for a supersaturated water/CO2 solution typical of a carbonated beverage. Heterogeneous rates are calculated for a smooth planar surface, and surfaces containing spherical and conical cavities or projections, and for a range of contact angles. These results are displayed. The stability of preexisting bubbles in a finite volume of water/CO2 solution is also examined.

Inclusion Phases and the Nucleation of Acicular Ferrite in Submerged Arc Welds in High Strength Low Alloy Steels

Abstract: Series of submerged arc welds of HSLA steel made with three different fluxes and metallic additions of Ti, Mo, and Cr have been examined to study the inclusions and their role in the nucleation of acicular ferrite. Inclusion phases and compositions have been analyzed by electron diffraction and X-ray microanalysis. These analyses have shown that the inclusions contained many different compounds, the proportions of each depending upon both the flux and metallic additions. Six inclusion phases have been identified: galaxite (Al2O3 ⋅ MnO), a titanium-rich compound (probably TiO), a copper sulfide, a manganese sulfide, a silica, and an aluminum-rich phase. No correlation was found between the amount of acicular ferrite in the weld metal and either average inclusion composition or individual inclusion phases. No epitaxial relationships between inclusions and adjacent ferrite grains could be identified. It has been concluded that inclusions nucleate acicular ferrite by acting as inert substrates according to the classical theory of heterogeneous nucleation. Because most inclusions are multi-phase and are touched by several ferrite grains, it has also been concluded that each inclusion can nucleate several ferrite grains, due to local regions of high surface energy on the inclusion.

Modeling of nonequilibrium melting and solidification in laser-irradiated materials.

Abstract: A computational model has been developed for treating various aspects of the complex melting and solidification behavior observed in pulsed-laser-irradiated materials. An important feature of the modeling is the capability of allowing nonequilibrium melting and solidification to occur at temperatures other than the thermodynamic phase-change temperatures. As a result, interfacial undercooling and overheating can be introduced and various types of nucleation events can be simulated. Calculations for pulsed-laser-irradiated silicon containing amorphous layers have shown a wide variety of behavior, including the formation and propagation of multiple phase fronts and buried molten layers. Although originally developed as a tool for studying problems arising in the field of laser annealing of semiconductors, the approach used in the modeling should be useful in treating many types of systems in which ultrarapid phase change and nucleation phenomena play important roles.

Roles of dislocations and grain boundaries in martensite nucleation

Abstract: In order to elucidate roles of dislocations and grain boundaries in martensite nucleation, the transformation temperature (Ms) of specimens austenitized at various temperatures and subjected to prestrain has been measured, using Fe-Ni, Fe-Ni-C, and Fe-Cr-C alloys. It is concluded that the plastic accommodation, in austenite, of the shape strain of the transforming martensite is a vital step in the nucleation event. Any factors impeding such plastic accommodation, such as the lack of dislocations, work hardening, and grain refinement, suppress the transformation. Contrary to the general belief, dislocations themselves do not act as favorable nucleation sites. Grain boundaries provide nucleation site, but only certain types of grain boundaries are qualified to be potential nuclei. A quantitative analysis shows that the increasing difficulty for the plastic accommodation with decreasing grain size is the main factor to depress Ms in fine-grained specimens.

Alpha alumina transformation in seeded boehmite gels

Abstract: The kinetics and surface area changes during the γ to α-Al 2 O 3 transformation sequence of an α-Al 2 O 3 seeded boehmite sol-gel are compared to an unseeded boehmite sol gel. The seeded gels exhibited increased kinetics, however the activation energy for the γ to α-Al 3 O 3 transformation was lowered only 17% indicating the α-Al 2 O 3 transformation is growth controlled. Transmission electron microscopy showed that each α-Al 2 O 3 seed acts as a multiple nucleation site for the θ to α-Al 2 O 3 transformation. The higher surface area of the seeded boehmite results from the prevention of vermicular microstructure development. Thus, by seeding boehmite sol gels with α-Al 2 O 3 the growth process during transformation to α-Al 2 O 3 is controlled resulting in a fine, uniform microstructure which sinters at significantly lower temperatures.

Rate-theory model of polymer crystallization.

Abstract: A new lattice model of polymer crystallization is described. Results of numerical solutions of rate equations for crystallization of a two-dimensional "crystal" are presented. It is shown that the crystal thickness varies inversely with supercooling and that kinetics similar to secondary nucleation occurs. This is true even though the growth face is intrinsically rough and hence there cannot be any nucleation. The presence of a low-entropy saddle point during growth is explained, and its influence on the crystal thickness and lamellar morphology is discussed.

Effect of stirring on crystallization kinetics of basalt: texture and element partitioning

Abstract: Isothermal crystallization experiments on basalt have been carried out using an infrared heating furnace to investigate the effect of stirring. When stirring was not applied (static experiment), the results agreed well with previous experiments. But when stirring was applied and a flow of Reynolds number=10−3∼−4 was present (dynamic experiment), considerably different results were obtained, especially in respect to the nucleation rate and the morphology of crystals. At ΔT=25° C essentially similar results were obtained on the nucleation rates and morphologies of crystals in both static and dynamic experiments. However, at supercoolings larger than 45° C, nucleation density increased drastically in dynamic experiments reaching up to ten times as large as that in static experiments. Crystals of plagioclase and clinopyroxene were small and adapted acicular morphology regardless of ΔT in dynamic experiments, and hyalopilitic textures were formed. A TTT-diagram shows that the nucleation incubation time is shorter in dynamic experiments than in static experiments. No compositional difference in major elements was found in plagioclase and clinopyroxene produced in both static and dynamic experiments. However, minor element concentrations, e.g., Mg in plagioclase and Ti, Al in clinopyroxene, were found to increase with both ΔT and flow velocity. All these results imply that although chemical diffusion in the melts did not play an important role in the dynamic experiments, interface kinetics were important. It is suggested that hyalopilitic texture commonly seen in natural basalt is mainly due to flow in magma.