Showing papers on "Nucleation published in 1995"

Nucleation of microtubule assembly by a gamma-tubulin-containing ring complex.

Abstract: The highly conserved protein gamma-tubulin is required for microtubule nucleation in vivo. When viewed in the electron microscope, a highly purified gamma-tubulin complex from Xenopus consisting of at least seven different proteins is seen to have an open ring structure. This complex acts as an active microtubule-nucleating unit which can cap the minus ends of microtubules in vitro.

Formation of Ti–Zr–Cu–Ni bulk metallic glasses

Abstract: Formation of bulk metallic glass in quaternary Ti–Zr–Cu–Ni alloys by relatively slow cooling from the melt is reported. Thick strips of metallic glass were obtained by the method of metal mold casting. The glass forming ability of the quaternary alloys exceeds that of binary or ternary alloys containing the same elements due to the complexity of the system. The best glass forming alloys such as Ti34Zr11Cu47Ni8 can be cast to at least 4-mm-thick amorphous strips. The critical cooling rate for glass formation is of the order of 250 K/s or less, at least two orders of magnitude lower than that of the best ternary alloys. The glass transition, crystallization, and melting behavior of the alloys were studied by differential scanning calorimetry. The amorphous alloys exhibit a significant undercooled liquid region between the glass transition and first crystallization event. The glass forming ability of these alloys, as determined by the critical cooling rate, exceeds what is expected based on the reduced glass transition temperature. It is also found that the glass forming ability for alloys of similar reduced glass transition temperature can differ by two orders of magnitude as defined by critical cooling rates. The origins of the difference in glass forming ability of the alloys are discussed. It is found that when large composition redistribution accompanies crystallization, glass formation is enhanced. The excellent glass forming ability of alloys such as Ti34Zr11Cu47Ni8 is a result of simultaneously minimizing the nucleation rate of the competing crystalline phases. The ternary/quaternary Laves phase (MgZn2 type) shows the greatest ease of nucleation and plays a key role in determining the optimum compositions for glass formation.

Dependence of apatite formation on silica gel on its structure : effect of heat treatment

Abstract: The prerequisite for glasses and glass-ceramics to bond to living bone is the formation of biologically active bonelike apatite on their surfaces in the body. Our previous study showed that a silica gel prepared by hydrolysis and polycon- densation of tetraethoxysilane in aqueous solution containing poly(ethy1ene glycol) induces apatite nucleation on its surface in a simulated body fluid. In the present study, the effects of heat treatment of silica gel on its catalytic effects in apatite nucleation was investigated in a simulated body fluid. I t was found that apatite forms on the surfaces of silica gels heat-treated below 8OO°C, but not on those heat-treated above 900°C. The volume of nanometer-range pores in the gel remarkably decreased by heat treatment above 900°C. The concentration of silanol groups in the silica gels gradually decreased with increasing heat treatment temperature. The rate of silica dissolution from the gel into the simulated body fluid decreased remarkably by heat treatment above 900°C. This suggested that a special type of silanol group which is formed by soaking the gel treated below 800°C into the simulated body fluid is respon sible for apatite nucleation.

Optimization of rates of protein folding: the nucleation-condensation mechanism and its implications.

Abstract: Small, single-module proteins that fold in a single cooperative step may be paradigms for understanding early events in protein-folding pathways generally. Recent experimental studies of the 64-residue chymotrypsin inhibitor 2 (CI2) support a nucleation mechanism for folding, as do some computer stimulations. CI2 has a nucleation site that develops only in the transition state for folding. The nucleus is composed of a set of adjacent residues (an alpha-helix), stabilized by long-range interactions that are formed as the rest of the protein collapses around it. A simple analysis of the optimization of the rate of protein folding predicts that rates are highest when the denatured state has little residual structure under physiological conditions and no intermediates accumulate. This implies that any potential nucleation site that is composed mainly of adjacent residues should be just weakly populated in the denatured state and become structured only in a high-energy intermediate or transition state when it is stabilized by interactions elsewhere in the protein. Hierarchical mechanisms of folding in which stable elements of structure accrete are unfavorable. The nucleation-condensation mechanism of CI2 fulfills the criteria for fast folding. On the other hand, stable intermediates do form in the folding of more complex proteins, and this may be an unavoidable consequence of increasing size and nucleation at more than one site.

Numerical evidence for bcc ordering at the surface of a critical fcc nucleus

Abstract: We report a computer-simulation study of the crystal nucleation barrier and the structure of crystal nuclei in a Lennard-Jones system at moderate supercooling. The stable structure of the Lennard-Jones solid is known to be face-centered cubic. We find that the precritical nuclei are predominantly body-centered cubic ordered. But, as the nucleus grows to its critical size, the core becomes fcc ordered. Surprisingly, however, the interface of the critical nucleus retains a high degree of bcc-like ordering.

Seismic Evidence for an Earthquake Nucleation Phase

Abstract: Near-source observations show that earthquakes initiate with a distinctive seismic nucleation phase that is characterized by a low rate of moment release relative to the rest of the event. This phase was observed for the 30 earthquakes having moment magnitudes 2.6 to 8.1, and the size and duration of this phase scale with the eventual size of the earthquake. During the nucleation phase, moment release was irregular and appears to have been confined to a limited region of the fault. It was characteristically followed by quadratic growth in the moment rate as rupture began to propagate away from the nucleation zone. These observations suggest that the nucleation process exerts a strong influence on the size of the eventual earthquake.

Effect of the pressure drop rate on cell nucleation in continuous processing of microcellular polymers

Abstract: Microllular plastics are cellular polymers characterized by cell densities greater than 109 cells/cm3 and cells smaller than 10 μm. One of the critical steps in the continuous production of microcellular plastics is the promotion of high cell nucleation rates in a flowing polymer matrix. These high nucleation rates can be achieved by first forming a polymer/gas solution followed by rapidly decreasing the solubility of gas in the polymer. Since, in the processing range of interest, the gas solubility in the polymer decreases as the pressure decreases, a rapid pressure drop element, consisting of a nozzle, has been employed as a continuous microcellular nucleation device. In this paper, the effects of the pressure drop rate on the nucleation of cells and the cell density are discussed. The experimental results indicate that both the magnitude and the cell density are discussed. The experimental results indicate that both the magnitude and the rate of pressure drop play a strong role in microcellular processing. The pressure phenomenon affects the thermodynamic instability induced in the polymer/gas solution and the competition between cell nucleation and growth.

NUCLEATION: Measurements, Theory, and Atmospheric Applications

Abstract: New experiments have succeeded in measuring actual rates of nucleation and are revealing the shortcomings of classical nucleation theory, which assumes that the molecular-scale regions of the new phase may be treated using bulk thermodynamics and planar surface free energies. In response to these developments, new theories have been developed that incorporate information about molecular interactions in a more realistic fashion. This article reviews recent experimental and theoretical advances in the study of nucleation of liquids from the vapor and of crystals from the melt, with particular emphasis on phenomena that relate to particle formation in the atmosphere.

Understanding and control of nucleation, growth, habit, dissolution and structure of two‐ and three‐dimensional crystals using `tailor‐made' auxiliaries

Abstract: Tailor-made auxiliaries for the control of nucleation and growth of molecular crystals may be classified into two broad categories: inhibitors and promoters. Tailor-made inhibitors of crystal growth can be used for a variety of purposes, which include morphological engineering and etching, reduction of crystal symmetry, assignment of absolute structure of chiral molecules and polar crystals, elucidation of the effect of solvent on crystal growth, and crystallization of a desired polymorph. As for crystal growth promoters, monolayers of amphiphilic molecules on water have been used to induce the growth of a variety of three-dimensional crystals at the monolayer-solution interface by means of structural match, molecular complementarity or electrostatic interaction. A particular focus is made on the induced nucleation of ice by monolayers of water-insoluble aliphatic alcohols. The two-dimensional crystalline structures of such monolayers have been studied by grazing incidence X-ray diffraction. It has become possible to monitor, by this method, the growth, dissolution and structure of self-aggregated crystalline monolayers, and indeed multilayers, affected by the interaction of solvent molecules in the aqueous subphase with the amphiphilic headgroups, and by the use of tailor-made amphiphilic additives

Structural evolution in epitaxial metalorganic chemical vapor deposition grown GaN films on sapphire

Abstract: The structural evolution of epitaxial GaN layers grown on basal plane sapphire has been studied by atomic force microscopy (AFM), x‐ray diffraction, and transmission electron microscopy (TEM). High‐temperature growth (1050–1080 °C) on optimized nucleation layers leads to clear, specular films. AFM on the as‐grown surface shows evenly spaced monatomic steps indicative of layer by layer growth. AFM measurements show a step termination density of 1.7×108 cm−2 for 5 μm films. This value is in close agreement with TEM measurements of screw and mixed screw‐edge threading dislocation density. The total measured threading dislocation density in the 5 μm films is 7×108 cm−2.

Effect of sample topology on the critical fields of mesoscopic superconductors

Abstract: THE superconducting state of a material can be suppressed by either increasing the temperature (T) or applying a magnetic field (H). For bulk samples, the form of the H–T phase boundary is mainly determined by the material itself; sample topology can be neglected because the surface-to-volume ratio is small1. But for mesoscopic samples, this ratio becomes very large and nucleation of the superconducting state should depend strongly on the boundary conditions imposed by the sample shape, analogous to the role of the confining potential on the energy levels in the quantum-mechanical 'particle-in-a-box' problem2. Here we describe measurements of the superconducting H-T phase boundary of a range of mesoscopic aluminium structures (lines, squares and square rings) which show clearly the effect of sample topology. The H-T phase boundaries determined experimentally are in excellent agreement with theoretical calculations.

Big bang simulation in superfluid 3He-B -- Vortex nucleation in neutron-irradiated superflow

Abstract: We report the observation of vortex formation upon the absorption of a thermal neutron in a rotating container of superfluid $^3$He-B. The nuclear reaction n + $^3$He = p + $^3$H + 0.76MeV heats a cigar shaped region of the superfluid into the normal phase. The subsequent cooling of this region back through the superfluid transition results in the nucleation of quantized vortices. Depending on the superflow velocity, sufficiently large vortex rings grow under the influence of the Magnus force and escape into the container volume where they are detected individually with nuclear magnetic resonance. The larger the superflow velocity the smaller the rings which can expand. Thus it is possible to obtain information about the morphology of the initial defect network. We suggest that the nucleation of vortices during the rapid cool-down into the superfluid phase is similar to the formation of defects during cosmological phase transitions in the early universe.

A New Look at Homogeneous Ice Nucleation in Supercooled Water Drops

Abstract: The classical theory for homogeneous ice nucleation in supercooled water is investigated in the light of recent data published in various physico-chemical journal on the physical properties of supercooled water and in the light of recent evidence that the cooperative nature of the hydrogen bonds between water molecules is responsible for a singularity behavior of pure supercooled water at −45°C. Recent rates for homogeneous ice nucleation in supercooled water drops field from field experiments at the cirrus cloud level and from cloud chamber studies were shown to be quantitatively in agreement with the laboratory-derived lowest temperatures to which ultrapure water drops of a given size have been supercooled. Using these verified nucleation rates together with the recent physical property data for supercooled water, the activation energy for the transfer of water molecules across the ice-water interface was computed using the classical nucleation rate equation. The thus computed values are signif...

Effect of Texture on the Rate of Hydroxyapatite Formation on Gel-Silica Surface

Abstract: An apatite layer can be formed on pure gel-silica soaked in simulated body fluid. The rate of formation depends on solution parameters and sintering temperature of the gelsilica. In this study, the effect of the texture of the gel-silica on the rate of hydroxyapatite formation was investigated. The apatite formation was monitored by means of Fourier-transform infrared reflection spectroscopy as well as by the measurement of changes in the ion concentration of the fluid. The induction time for apatite nucleation on the gel silica decreased as pore size and pore volume increased. The substrate parameters that affect nucleation are discussed and a mechanism that assumes pores as nucleation sites for hydroxyapatite is proposed.

Effect of Strain on Surface-Diffusion and Nucleation

Abstract: The influence of strain on diffusion and nucleation has been studied by means of scanning tunneling microscopy and effective-medium theory for Ag self-diffusion on strained and unstrained (111) surfaces. Experimentally, the diffusion barrier is observed to be substantially lower on a pseudomorphic Ag monolayer on Pt(111), 60 meV, compared to that on Ag(111), 97 meV. The calculations show that this strong effect is due to the 4.2% compressive strain of the Ag monolayer on Pt. It is shown that in general isotropic two-dimensional strain as well as its relief via dislocations have a drastic effect on surface diffusion and nucleation in heteroepitaxy and are thus of significance for the film morphology in the kinetic growth regime.

Low‐temperature growth of SiC thin films on Si and 6H–SiC by solid‐source molecular beam epitaxy

Abstract: Epitaxial growth of stoichiometric SiC on Si(111) and 2°–5° off‐oriented 6H–SiC(0001) substrates was carried out at low temperatures (800–1000 °C) by means of solid‐source molecular beam epitaxy controlled by a quadrupole mass spectrometry based flux meter. The films were obtained on Si‐stabilized surfaces showing (3×3) and (2×2) superstructures in the case of SiC(0001). The reflection high‐energy diffraction (RHEED) patterns and damped RHEED‐oscillations during the growth on 6H–SiC(0001) at T≳900 °C indicate that two‐dimensional nucleation on terraces is the dominant growth process.

Decomposition and primary crystallization in undercooled zr41.2ti13.8cu12.5ni10.0be22.5 melts

Abstract: Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 bulk metallic glasses were prepared by cooling the melt with a rate of about 10 K/s and investigated with respect to their chemical and structural homogeneity by atom probe field ion microscopy and transmission electron microscopy. The measurements on these slowly cooled samples reveal that the alloy exhibits phase separation in the undercooled liquid state. Significant composition fluctuations are found in the Be and Zr concentration but not in the Ti, Cu, and Ni concentration. The decomposed microstructure is compared with the microstructure obtained upon primary crystallization, suggesting that the nucleation during primary crystallization of this bulk glass former is triggered by the preceding diffusion controlled decomposition in the undercooled liquid state.

A preliminary study of the effect of ammonia on particle nucleation in the marine boundary layer

Abstract: A ternary nucleation model for the H2SO4-NH3-H2O system is presented in an effort to examine the effect of NH3 on heteromolecular homogeneous nucleation in the marine boundary layer (MBL). The results from this nucleation model suggest that ammonia could, in fact, enhance the nucleation rate over that of the binary system, H2SO4-H2O. The magnitude of this enhancement is introduced as an enhancement ratio, which, in principle, is applicable to any binary nucleation rate for H2SO4-H2O. Also presented are preliminary results from a simple aerosol model using this enhancement ratio. These results suggest that under conditions typical of the marine environment it may be possible to produce enough particles to balance the various particle sinks characteristic of the MBL.

Thermal stability of nanocrystalline Ni

Abstract: Thermal stability of electroplated nanocrystalline Ni of 10 and 20 nm grain size was investigated by differential scanning calorimetry (DSC). The temperature dependence and heat release ΔH during grain growth have been determined by linear anisothermal measurements (linear heating at 10 K min −1 ). The corresponding change in microstructure has been monitored in the temperature range between 373 K and 693 K using transmission electron microscopy (TEM). The TEM and DSC studies identified three exothermic reactions: “nucleation” and abnormal grain growth (353–562 K), normal grain growth (562–593 K) and growth towards equilibrium (643–773 K). The grain growth behaviour, and the similar heat releases ΔH = 18 J g −1 , and 16 J g −1 measured for the 10 nm and 20 nm Ni nanocrystals respectively in the DSC experiments may be related to the observed sulphur segregation at grain boundaries and triple junctions.

Numerical study of nucleation and growth of bubbles in viscous magmas

Abstract: The nucleation and growth processes of bubbles in viscous magmas with a constant decompression rate have been numerically investigated based on a formation which accounts for effects of viscosity, as well as diffusivity, interfacial tension, and decompression rate. The numerical solutions show two regimes in the nucleation and growth process, a diffusion-controlled regime and a viscosity-controlled regime, mainly depending on the decompression rate, initial saturation pressure and viscosity. The basic mechanism common to both regimes is that growth governs nucleation through depletion of degassing components. In basaltic eruptions the vesiculation is essentially controlled by diffusion, and the viscosity-controlled regime is limited to very high decompression rate and very small water content. When andesitic magma saturated by water at 10 MPa is decompressed through the propagation of rarefraction wave induced by a landslide, as took place in the Mount St. Helens 1980 eruption, the vesiculation is controlled by the viscosity up to 100 m depth. On the other hand, in a rhyolitic magma for the same situation, vesiculation is controlled by the viscosity over the whole depth of the magma column. In the viscosity-controlled regime, the vesicularity may be 90% or less as seen in silicic pumice, whereas in the diffusion-controlledmore » regime the vesicularity equals or exceeds 98% such as in reticulite in Hawaiian basalt. An observed variation of the number density of bubbles by several orders of magnitude in plinian eruptions and the correlation with the SiO2 content can be attributed approximately to the dependence of diffusivity of viscosity on SiO2 content and temperature, assuming the apparent correlation between SiO2 content and temperature of magma.« less

Relative Humidity and Temperature Influences on Cirrus Formation and Evolution: Observations from Wave Clouds and FIRE II

Abstract: Measurements in orographic wave clouds. and in cirrus sampled during FIRE II, are used to investigate ice nucleation in the upper troposphere. The dynamically and microphysically simpler quasi-steady-state wave clouds provide relatively ideal conditions for observing characteristics of ice nucleation. Conclusions from the wave cloud study are applied to help understand the formation and evolution of ice in the cirrus clouds observed during FIRE II. The wave cloud study extends analyses reported by Heymsfield and Miloshevich down to −56°C, in part by using an improved droplet size spectrometer with a detection threshold of 0.4 µm am a Video Ice Particle Sampler with a detection threshold of 5–10 µm. The measurements show a rapid transition from solution droplets to ice crystals characteristic of homogeneous ice nucleation throughout the temperature range from −35° to −56°C. The temperature dependence of the relative humidity and the droplet sizes when ice nucleation occurs is consistent with theor...

Kinetic study of the kaolinite-mullite reaction sequence. Part I: Kaolinite dehydroxylation

Abstract: The decomposition reaction of kaolinite has been investigated as a function of the defectivity of the starting material and the temperature of reaction. Time resolved energy-dispersive powder diffraction patterns have been measured using synchrotron radiation, both under a constant heating rate (heating rates from 10 to 100° C/min) and in isothermal conditions (in the temperature range 500 to 700° C). The apparent activation energy of the dehydroxylation process is different for kaolinites exhibiting a different degree of stacking fault density. The results of the analysis of the kinetic data indicate that the starting reaction mechanism is controlled by diffusion in the kaolinite particle. The diffusion process is dependent on the defective nature of both kaolinite and metakaolinite. At high temperatures, and at higher heating rates, the reaction mechanism changes and the resistance in the boundary layer outside the crystallites becomes the rate-limiting factor, and nucleation begins within the reacting particle. During the final stage of the dehydroxylation process the reaction is limited by heat or mass transfer, and this might be interpreted by the limited diffusion between the unreacted kaolinite domains and the metakaolinite matrix.

Laser‐induced crystallization phenomena in GeTe‐based alloys. I. Characterization of nucleation and growth

Abstract: The laser‐induced crystallization behavior of GeTe‐based amorphous alloy thin films has been quantitatively studied by local reflection measurements with a focused 780 nm laser. The use of multiple laser pulse sequences enables the nucleation rate and crystal‐growth speed to be separately deduced, allowing the compositional variation of both these processes to be followed. This not only gives detailed information on the crystallization mechanism, but also allows the fine tuning of phase change alloy compositions for use in erasable optical recording. The differences between the as‐deposited and melt‐quenched amorphous phases are also discussed. In particular, it is shown that the crystallization speed of the as‐deposited layer can differ by over an order of magnitude from that of the melt‐quenched amorphous layer. The as‐deposited state can, however, be transformed into a modified amorphous state equivalent to that obtained by melt quenching a previously crystalline layer. This allows the determination of...