Showing papers on "Nucleation published in 1993"

Synthesis and characterization of nearly monodisperse CdE (E = sulfur, selenium, tellurium) semiconductor nanocrystallites

Abstract: A simple route to the production of high-quality CdE (E=S, Se, Te) semiconductor nanocrystallites is presented. Crystallites from ∼12 A to ∼115 A in diameter with consistent crystal structure, surface derivatization, and a high degree of monodispersity are prepared in a single reaction. The synthesis is based on the pyrolysis of organometallic reagents by injection into a hot coordinating solvent. This provides temporally discrete nucleation and permits controlled growth of macroscopic quantities of nanocrystallites. Size selective precipitation of crystallites from Portions of the growth solution isolates samples with narrow size distributions (<5% rms in diameter). High sample quality results in sharp absorption features and strong «band-edge» emission which is tunable with particle size and choice of material

Giant energy product in nanostructured two-phase magnets.

Abstract: Exchange hardening of nanostructured two-phase systems composed of an aligned hard phase and a soft phase with high magnetization is investigated using an approach which yields analytic nucleation fields from the micromagnetic vector equation, and accounts for interactions between the soft regions. In suitable structures the nucleation field is proportional to the volume-averaged anisotropy constant. For example, a multilayer composed of alternating 2.4 nm hard-magnetic ${\mathrm{Sm}}_{2}$${\mathrm{Fe}}_{17}$${\mathrm{N}}_{3}$ layers and 9 nm ${\mathrm{Fe}}_{65}$${\mathrm{Co}}_{35}$ layers can have an energy product as high as 1 MJ/${\mathrm{m}}^{3}$ (120 MG Oe), with a rare-earth content of only 5 wt %. Giant energy products may also be achieved in suitable cellular and disordered structures.

Probabilistic modelling of microstructure formation in solidification processes

Abstract: A new approach to the modelling of grain structure formation in solidification processes is proposed. Based upon a two-dimensional cellular automata technique, the model includes the mechanisms of heterogeneous nucleation and of grain growth. Nucleation occurring at the mould wall as well as in the liquid metal are treated by using two distributions of nucleation sites. The location and the crystallographic orientation of the grains are chosen randomly among a large number of cells and a certain number of orientation classes, respectively. The growth kinetics of the dendrite tip and the preferential 〈100〉 growth directions of cubic metals are taken into account. The model is then applied to small specimens of uniform temperature. The columnar-to-equiaxed transition, the selection and extension of columnar grains which occur in the columnar zone and the impingement of equiaxed grains are clearly shown by this technique. The calculated effect of the alloy concentration and cooling rate upon the resultant microstructure agree with experimental observations.

Self-nucleation and recrystallization of isotactic polypropylene (α phase) investigated by differential scanning calorimetry

Abstract: The crystallization behavior after partial or complete melting of the α phase of iPP is examined by combined differential scanning calorimetry (DSC) and optical microscopy: calorimetric results are directly correlated with corresponding morphologies of microtome sections of DSC samples. On partial melting at various temperatures (hereafter referred to as Ts) located in a narrow range (4°C) below and near Tm, the number of nuclei increases (as in classical self-nucleation experiments), by several orders of magnitude; on subsequent cooling, the crystallization peak is shifted by up to 25°C. After partial melting in the lower part of the Ts range and recrystallization, the polymers display a prominent morphology “memory effect” whereby a phantom pattern of the initial spherulite morphology is maintained. After partial melting in the upper part of the Ts range the initial morphology is erased and self-nucleation affects only the total number of nuclei. The present experimental procedures make it possible to define, under “standard” conditions, the crystallization range of the polymer and in particular, the maximum crystallization temperature achievable when “ideally” nucleated. © John Wiley & Sons, Inc.

Effect of Surface Wettability on Active Nucleation Site Density During Pool Boiling of Water on a Vertical Surface

Abstract: Pool boiling of saturated water at 1 atm pressure has been investigated. In the experiments, copper surfaces prepared by following a well-defined procedure were used. The cumulative number density of the cavities and their shapes were determined with an optical microscope. The surface had a mirror finish and had a surface Ra (centerline average) value of less than 0.02 μm. The wettability of the surface was changed by controlling the degree of oxidation of the surface. In the experiments with the primary surface, the wall heat flux and superheat were determined with the help of thermocouples embedded in the test block. The density, spatial distribution, local distribution, and nearest-neighbor distance distribution of active nucleation sites in partial and fully developed nucleate boiling were determined from still pictures.

A luminescent silicon nanocrystal colloid via a high-temperature aerosol reaction

Abstract: We describe a high-temperature aerosol apparatus for the synthesis of 3-8-nm, surface-oxidized Si crystallites. The particles are made by homogeneous gas-phase nucleation following pyrolysis of dilute disilane in He. The particles are collected as a robust ethylene glycol colloid, and characterized by transmission electron microscopy, X-ray powder Brag scattering, IR, high-pressure liquid chromatography, and optical spectroscopy. The particles exhibit a shell structure, with a crystalline Si core, of bulk lattice constant, capped with an approximately 1.2-nm shell of silicon dioxide

Silicide formation and silicide‐mediated crystallization of nickel‐implanted amorphous silicon thin films

Abstract: The nucleation and growth of isolated nickel disilicide precipitates in Ni‐implanted amorphous Si thin films and the subsequent low‐temperature silicide‐mediated crystallization of Si was studied using in situ transmission electron microscopy. Analysis of the spatial distribution of the NiSi2 precipitates strongly suggested the occurrence of site saturation during nucleation. NiSi2 precipitates were observed in situ to migrate through the amorphous Si thin films leaving a trail of crystalline Si at temperatures as low as ∼484 °C. Initially, a thin region of epitaxial Si formed on {111} faces of the octahedral NiSi2 precipitates with a coherent interface which was shown by high‐resolution electron microscopy to be Type A. Migration of the NiSi2 precipitates led to the growth of needles of Si which were parallel to 〈111〉 directions. The growth rate of the crystalline Si was limited by diffusion through the NiSi2 precipitates, and an effective diffusivity was determined at 507 and 660 °C. A mechanism for the enhanced growth rate of crystalline Si is proposed.

Scaling of the critical slip distance for seismic faulting with shear strain in fault zones

Abstract: THEORETICAL and experimentally based laws for seismic faulting contain a critical slip distance1–5, Dc, which is the slip over which strength breaks down during earthquake nucleation. On an earthquake-generating fault, this distance plays a key role in determining the rupture nucleation dimension6, the amount of premonitory and post-seismic slip7–10, and the maximum seismic ground acceleration1,11. In laboratory friction experiments, D c has been related to the size of surface contact junctions2,5,12; thus, the discrepancy between laboratory measurements of Dc (∼10−5m) and values obtained from modelling earthquakes (∼10−2m) has been attributed to differences in roughness between laboratory surfaces and natural faults5. This interpretation predicts a dependence of Dc on the particle size of fault gouge2 (breccia and wear material) but not on shear strain. Here we present experimental results showing that Dc scales with shear strain in simulated fault gouge. Our data suggest a new physical interpretation for the critical slip distance, in which Dc is controlled by the thickness of the zone of localized shear strain. As gouge zones of mature faults are commonly 102–103 m thick12–17, whereas laboratory gouge layers are 1–10 mm thick, our data offer an alternative interpretation of the discrepancy between laboratory and field-based estimates of Dc.

Self‐nucleation and enhanced nucleation of polymers. Definition of a convenient calorimetric “efficiency scale” and evaluation of nucleating additives in isotactic polypropylene (α phase)

Abstract: A simple, convenient and reliable calorimetric efficiency scale is proposed for the evaluation of nucleating additives for polymers. The scale is based on conventional differential scanning calorimetry cooling runs and makes use of a crystallization range determined in self-nucleation experiments. It can be correlated with spherulite sizes, and indicates the potential range of improvement of nucleating additives. Typical nucleating agents for isotactic polypropylene are evaluated; at best they rate at 60 to ca. 70% on this efficiency scale. © 1993 John Wiley & Sons, Inc.

Template mineralization of self-assembled anisotropic lipid microstructures

Abstract: THE high-fidelity replication and functional specificity of biomaterials and biopolymers is inspiring a renewal in the development of biomimetic strategies for materials synthesis1,2. Approaches involving the templating or nucleation of inorganic materials by compressed monolayers of amphiphiles3–5, the use of supramolecular lipid or protein cages in the preparation of nanoscale inorganic structures6–8 and the mineralization of bacterial fibres9 indicate the potential of controlled crystallization at inorganic–organic interfaces. Here we report the controlled formation of tubular inorganic–organic composites by using self-assembled lipid tubules as templates for the crystallization of inorganic oxides. We use microstructures formed by a sugar-based lipid galactocerebroside, doped with small amounts of an anionic sulphated derivative, to induce nucleation of magnetic and non-magnetic iron oxides. By varying the reaction conditions, we can create either tube-like or lamellar disk-like composites. Our results suggest that the variety of microstructures formed by chemically modified sugar-based lipids may provide a route to the production of mineral-containing fibres and other ceramic–organic composites.

Entropy-driven formation of a superlattice in a hard-sphere binary mixture

Abstract: A MIXTURE of two dissimilar species (A and B) may freeze to form a substitutionally ordered crystal, the structure of which can vary from a lattice with only a few atoms per unit cell to a complex 'superlattice'. For example, a mixture of sodium and zinc can form a solid with the AB13 structure with 112 atoms per unit cell1 (Fig. la). One might suspect that very specific energetic interactions are needed to stabilize a structure as complex as this. But recent experiments2,3 show that the AB13 structure is also formed in mixtures of spherical colloidal particles with different diameters, which interact only via simple repulsive potentials. This raises the possibility that the formation of an AB13 superlattice might be sup-ported by entropic effects alone. To investigate this possibility, we present here computer simulations of a binary mixture of hard spheres. Our calculations show that entropy alone is indeed sufficient to stabilize the AB13 phase, and that the full phase diagram of this system is surprisingly complex. Our results also suggest that vitrification or slow crystal nucleation in experimental studies of colloidal hard spheres can prevent the formation of equilibrium phases.

Textured growth of diamond on silicon via in situ carburization and bias‐enhanced nucleation

Abstract: Ordered diamond films have been deposited on single‐crystal silicon substrates via an in situ carburization followed by bias‐enhanced nucleation. Textured diamond films with greater than 50% of the grains oriented D(100)//Si(100) and D〈110〉//Si〈110〉 were grown in both a horizontal and vertical microwave plasma chemical vapor deposition reactor. Separate diamond films from each of the two reactors were analyzed both by scanning electron microscopy and Raman spectroscopy. The in situ carburization is speculated to form an epitaxial SiC conversion layer, thus providing an economical alternative to obtaining epitaxial diamond films on single‐crystal SiC.

Interaction of oxygen with Al(111) studied by scanning tunneling microscopy

Abstract: The interaction of oxygen with Al(111) was studied by scanning tunneling microscopy (STM). Chemisorbed oxygen and surface oxides can be distinguished in STM images, where for moderate tunnel currents and independent of the bias voltage the former are imaged as depressions, while the latter appear as protrusions. An absolute coverage scale was established by counting O adatoms. The initial sticking coefficient is determined to so=0.005. Upon chemisorption at 300 K the O adlayer is characterized by randomly distributed, immobile, individual O adatoms and, for higher coverages, by small (1×1) O islands which consist of few adatoms only. From the random distribution of the thermalized O adatoms at low coverages a mobile atomic precursor species is concluded to exist, which results from an internal energy transfer during dissociative adsorption. These ‘‘hot adatoms’’ ‘‘fly apart’’ by at least 80 A, before their excess energy is dissipated. A model is derived which explains the unusual island nucleation scheme by trapping of the hot adatoms at already thermalized oxygen atoms. Oxidation starts long before saturation of the (1×1) O adlayer, at coverages around ΘO≂0.2. For a wide coverage range bare and Oad covered surfaces coexist with the surface oxide phase. Upon further oxygen uptake both chemisorbed and oxide phase grow in coverage. Oxide nucleation takes place at the interface of Oad islands and bare surface, with a slight preference for nucleation at upper terrace step edges.Further oxide formation progresses by nucleation of additional oxide grains rather than by growth of existing ones, until the surface is filled up with a layer of small oxide particles of about 20 A in diameter. At very large exposures up to 5×105 L they cover the entire surface as a relatively smooth, amorphous layer of aluminum oxide. The difference in Al atom density between Al metal and surface oxide is accommodated by short range processes, with no indication for any long range Al mass transport. Based on our data we discuss a simpler two step model for the interaction of oxygen with Al(111), without making use of an additional subsurface oxygen species. The complex spectroscopic data for the O/Al(111) system are rationalized by the wide coexistence range of bare and Oad covered surface with surface oxide and by differences in the electronic and vibronic properties of the surface atoms depending on the number of neighboring O adatoms in the small Oad islands.

Homogeneous nucleation rates for water

Abstract: We have investigated homogeneous nucleation of water droplets in the vapor phase Nucleation in water vapor has been previously studied a number of times using various experimental techniques The nucleation pulse technique used here allows accurate measurements of homogeneous nucleation rates J vs supersaturation S in the range from 105 to 109 cm−3 s−1 The advance reported here consists of the fact that the J–S curves are measured at a selectable constant temperature We determined the nucleation rates in supersaturated water vapor mixed with various carrier gases as functions of supersaturation and temperature (217

Low temperature perovskite formation of lead zirconate titanate thin films by a seeding process

Abstract: A two-step seeding process has been developed to lower the transformation temperature and modify the grain structure of ferroelectric lead zirconate titanate (PZT) thin films with high Zr/Ti ratio. Previous study has shown that nucleation is the rate-limiting step for the perovskite formation. Therefore, any process that enhances the kinetics of nucleation is likely to decrease the transformation temperature. In this process, a very thin (45 nm) seeding layer of PbTiO3, which has a low effective activation energy for perovskite formation, was used to provide nucleation sites needed for the low temperature perovskite formation. In this study, we have shown that the pyrochlore-to-perovskite phase transformation temperature of PbZrxTi1−xO3 films of high Zr/Ti ratio (e.g., x = 53/47) can be lowered by as much as 100 °C. The grain size of these films can also be substantially modified by this two-step approach.

Vapor pressures of solid hydrates of nitric acid - Implications for polar stratospheric clouds

Abstract: Thermodynamic data are presented for hydrates of nitric acid: HNO3.H2O, HNO3.2H2O, HNO3.3H2O, and a higher hydrate. Laboratory data indicate that nucleation and persistence of metastable HNO3.2H2O may be favored in polar stratospheric clouds over the slightly more stable HNO3.3H2O. Atmospheric observations indicate that some polar stratospheric clouds may be composed of HNO3.2H2O and HNO3.3H2O. Vapor transfer from HNO3.2H2O to HNO3.3H2O could be a key step in the sedimentation of HNO3, which plays an important role in the depletion of polar ozone.

Diamond nucleation by hydrogenation of the edges of graphitic precursors

Abstract: HOW diamond films grow by chemical vapour deposition is now fairly well understood1,2, but the mechanism by which the diamond phase first nucleates is still unclear. Evidence is accumulating that atomic hydrogen, known to be important in diamond growth1,2, also plays an important role in nucleation3,4. The nature of the carbon precursor to diamond has been much debated2–9; although there is some evidence that graphite is formed before diamond nucleation2,5,6 and that diamond grows epitaxially on the graphite edges7, others have suggested10,11 that graphite formation is detrimental to diamond nucleation. Here we present calculations that suggest that diamond films can nucleate by the initial condensation of graphite and subsequent hydrogenation of the {1¯100} prism planes along the edges of the graphite particles. If nucleation really does occur in this manner, the understanding that our model provides should assist in the development of methods for growing large diamond single crystals (now limited in part by secondary nucleation of independent crystals) and highly oriented epitaxial diamond films.

Reaction diffusion patterns in the catalytic CO‐oxidation on Pt(110): Front propagation and spiral waves

Abstract: The dynamic behavior of elliptical front propagation and spiral‐shaped excitation concentration waves associated with the catalytic oxidation of CO on a Pt(110)‐surface was investigated by means of photoemission electron microscopy (PEEM) The properties of these patterns can be tuned through the control parameters, viz, the partial pressures of CO and O2 and the sample temperature Over a wide range of control parameters the transition between two metastable states (COad and Oad covered surface) proceeds via nucleation and growth of elliptical reaction‐diffusion (RD)‐fronts Front velocities and critical radii for nucleation are determined by the diffusion of adsorbed CO under reaction conditions If at constant pO2, T the CO partial pressure is increased beyond a critical value a transition to qualitatively different dynamic behavior takes place The elliptical fronts give way to oxygen spiral waves of excitation spreading across the CO‐covered areas For fixed experimental conditions a broad distribution of spatial wavelengths and temporal rotation periods was found This effect has to be attributed to the existence of surface defects of μm‐size to which the spiral tip is pinned These data lead to a dispersion relation between the front propagation velocity and the wavelength, respectively, period In addition, the dynamics of free spiral‐shaped excitation waves was investigated under the influence of externally modulated temperature Now the spiral starts to drift, resulting in distortion of the Archimedian shape and a pronounced Doppler effect

Direct imaging of surface cusp evolution during strained-layer epitaxy and implications for strain relaxation.

Abstract: We have directly imaged the evolution of surface cusps during strained-layer epitaxy The cusps arise naturally as a result of gradients in the surface chemical potential High stress concentrations at the cusp tip have important implications for strain relaxation in the film via dislocation nucleation

Does C60 have a liquid phase

Abstract: Above a substance's liquid–vapour critical point (i>Tc), the distinction between the liquid and vapour phases disappears. Below the triple point (T t), meanwhile (at which solid, liquid and vapour coexist), only the solid and vapour are stable. The liquid range, T c/T t, depends on the nature of the intermolecular forces: for argon, T c/Tt = 1.8, whereas for sodium the ratio is 7.5. But might there be molecular substances that have no liquid phase at all? Here we present results which suggest that C60 is such a substance. We map out the phase diagram using computer simulations in which the C60 molecules are represented by spheres interacting via Lennard-Jones potentials summed over all 60 carbon atoms. We find that the sublimation line passes above the metastable liquid-vapour coexistence curve. By drawing an analogy with the aggregation of colloidal particles, we expect that solid C60 formed by nucleation from the vapour phase will be amorphous rather than crystalline.

Activity of substrates in the catalyzed nucleation of glass-forming melts. II. Experimental evidence

Abstract: Methods for determining the nucleating activity of substrates are considered. Experimental data on the activity of nucleating catalysts in various organic and inorganic glass-forming melts are summarized and discussed in terms of the theoretical approach developed in Part I of the present investigation. The nucleation of the same melt induced by different substrates is analysed. In this respect, the data for sodium metaphosphate, lithium metaphosphate, poly(ethylene terephthalate), poly(vinyl chloride), sodium tetraborate nucleated with noble metal cores, for poly(decamethylene terephthalate), lithium disilicate, poly(ethylene) and water initiated by oxide and halide catalysts give a direct proof for the validity of the concepts, developed in Part I.

Thermal stability and grain growth behavior of mechanically alloyed nanocrystalline Fe-Cu alloys

Abstract: X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry were used to study the thermal stability of highly supersaturated nanocrystalline FexCu100−x alloys (10 ~80. For 60<=x<=80 fcc and bcc phases coexist. Heating to elevated temperatures leads to structural relaxation, phase separation, and grain growth of the metastable nanocrystalline solid solutions. Single-phase fcc and bcc alloys undergo significant strain release but no appreciable grain growth prior to phase separation. After phase separation pronounced grain growth sets in. In contrast, samples in the two-phase region show some grain growth and significant chemical redistribution even at low temperatures. The phase separation of single-phase fcc and bcc alloys proceeds via different mechanisms: fcc solid solutions decompose by forming small Fe precipitates, while demixing in bcc alloys starts by segregation of Cu atoms to bcc grain boundaries before nucleation of Cu precipitates. These results show that the stability and grain growth behavior of nanocrystalline alloys is strongly affected by the microstructure of the material.

Effects of a Magnetic Field on the Formation of CaCO3 Particles

Abstract: Characteristics of CaCO3 crystals formed by mixing CaCl2 and Na2CO3 solutions, which were exposed to magnetic fields before mixing, were investigated by measuring the variation of absorbance of suspensions of CaCO3 particles, observing sedimentary CaCO3 particles at the bottom of a cell by a video system, and analyzing the crystal structure of CaCO3 particles by X-ray diffractometer. It is found that (l) the nucleation frequency of CaCO3 particles is suppressed but the growth of particles is accelerated, if the exposed magnetic flux density is greater than about 0.3 T and the exposure time is greater than 10 min; (2) the magnetic effect is attributable mainly to the magnetic exposure on Na2CO3 solutions but not to that on CaCl2 solutions; (3) the magnetic effect was observed even if CaCl2 and Na2CO3 solutions were mixed at 120 h after the magnetic exposure; and (4) the formation of aragonite structure of CaCO3 crystals is acceleratedd by the magnetic exposure.

Etching of Silicon in NaOH Solutions I . In Situ Scanning Tunneling Microscopic Investigation of n‐Si(111)

Abstract: The etching of n-type silicon (111) has been investigated by means of in situ scanning tunneling microscopy (STM) observations performed over a wide range of bias of the sample A special procedure has been used to observe topography changes at potentials close and positive of the rest potential Irrespective of the bias, images show that the surface consists in atomically smooth terraces separated by 31 A high steps At cathodic bias, the etching occurs principally at terrace edges and (111) terraces are most probably H terminated, which prevents their reconstruction, as could be seen in atomically resolved pictures taken in situ Triangular etch pits nucleate when the potential approaches the rest potential