Showing papers on "Nucleation published in 1977"

Thermodynamic theory of size dependence of melting temperature in metals

Abstract: The way that small particles melt is a crucial clement in the construction of a thermodynamic treatment of the relation between particle size and melting temperature. There are indications that melting is initiated at the surface and that the solid–liquid interface sweeps rapidly through the solid at the melting temperature. The formal and physical elements of the indicated nucleation and growth criterion for melting are discussed and the existence of upper and lower limits on the melting temperature is outlined. Theoretical predictions show satisfactory agreement with experimental observations.

Studies on the α and β forms of isotactic polypropylene by crystallization in a temperature gradient

Abstract: Samples of isotactic polypropylene (PP) were zone-solidified in temperature gradients up to 300°C/cm at growth rates down to 3 μm/min. Oriented α-type spherulites were obtained only by nucleation. While β nucleation is extremely rare, the β phase is easily initiated by growth transformations along the oriented α front. Since the β phase was found to grow considerably faster than the α phase, the α-to-β transformation points diverge across the sample, interrupting growth of the oriented α fibrils. This causes subsequent nucleation to yield teardrop-shaped α spherulites. Differential scanning calorimetry (DSC) studies of zone-solidified PP show the β-phase to be favored by slow growth rates, high temperature gradients, and large degrees of superheat in the melt—all of which tend to suppress nucleation. Differential thermograms of largely β-PP obtained at a heating rate of 1°C/min show the actual melting and recrystallization of the β spherulites into the α form.

Aggregation and migration of ion‐implanted silver in lithia‐alumina‐silica glass

Abstract: The near‐surface nucleation and crystallization behavior of Ag+ ion‐implanted lithia‐alumina‐silica glasses has been studied. For room‐temperature Ag implants, crystallization of the glass ceramic phase was prevented by dissolution of Ag precipitates and migration of Ag atoms at temperatures below that necessary for formation of the glass ceramic phase. Crystallization was demonstrated after low‐temperature or low‐dose‐rate implantations. Optical spectroscopy was used to monitor the size of colloidal Ag particles and to detect the presence of the crystalline phase. Rutherford backscattering spectroscopy (RBS) was used to obtain the depth distribution of Ag atoms in the glass and thus monitor Ag migration. For samples implanted at room temperature and at relatively high dose rates (∼1 μA/cm2), aggregation of the Ag atoms into colloids occurred during implantation and also during subsequent annealing to temperatures ?350 °C. The RBS spectra indicate some migration of the Ag to the surface at these temperatu...

Relation of nucleation and crystal-growth rate to the development of granitic textures

Abstract: Analysis of crystal growth as a function of temperature for synthetic granite and granodiorite compositions in the system KAlSi/sub 3/O/sub 8/--NaAlSi/sub 3/O/sub 8/--CaAl/sub 2/Si/sub 2/O/sub 8/--SiO/sub 2/ under H/sub 2/O-saturated and undersaturated conditions yields quantitative data on the growth kinetics of quartz, alkali-feldspar, and plagioclase in these systems at 8 kbar and 400 to 900/sup 0/C. Measured growth rates vary from 3 x 10/sup -6/ cm/sec (about 3 mm/day) to 1 x 10/sup -10/ cm/sec (about 1 mm/yr), while nucleation density (nucleation sites/unit volume) varies from 0 to over 1 x 10/sup 8/ sites/cm/sup 3/. Crystal-growth rates commonly increase with increased undercooling. Maximum growth rates are lower in systems which contain a H/sub 2/O-rich vapor phase than in systems that are undersaturated with respect to H/sub 2/O. Number of nuclei (nucleation density), growth rate, and morphology for each of the minerals determines the texture of an ingeous rock. Results of this study show that long periods of time are not necessary to produce the textures commonly associated with igneous rocks. A relatively small number of large crystals may be produced by crystallization at a low undercooling in a few days. Volcanic (fine-grained) textures may be produced by crystallization at largemore » undercoolings, which produces a large number of small nuclei (high nucleation density, low growth rate). Coincidence of low nucleation density and high growth rate for alkali feldspar may explain the large alkali-feldspar crystals (phenocrysts or interstitial masses) sometimes found in granodiorites.« less

Non-equilibrium freezing behaviour of aqueous systems.

Abstract: The tendencies to non-equilibrium freezing behaviour commonly noted in representative aqueous systems derive from bulk and surface properties according to the circumstances. Supercooling and supersaturation are limited by heterogeneous nucleation in the presence of solid impurities. Homogeneous nucleation has been observed in aqueous systems freed from interfering solids. Once initiated, crystal growth is often slowed and, very frequently, terminated with increasing viscosity. Nor does ice first formed always succeed in assuming its most stable crystalline form. Many of the more significant measurements on a given system can be combined and displayed in the form of a 'supplemented phase diagram', the latter permitting the simultaneous representation of thermodynamic and non-equilibrium properties. The diagram incorporates equilibrium melting points, heterogeneous nucleation temperatures, homogeneous nucleation temperatures, glass transition and devitrification temperatures, recrystallization temperatures, and, where appropriate, solute solubilities and eutectic temperatures. Taken together, the findings on model systems aid the identification of the kinetic and thermodynamic factors responsible for the freezing - thawing survival of living cells.

On the Growth of Steam Droplets Formed in a Laval Nozzle Using both Static Pressure and Light Scattering Measurements

Abstract: : A series of experiments on steam condensation have been made in a Laval nozzle over a variety of starting conditions such that the onset of condensation occurs in the range -40 to 40 C. The homogeneous nucleation and growth of the new phase is documented with both static pressure and laser light scattering. Since even at onset the majority of the condensed phase is due to droplet growth the nucleation and growth are coupled and the availability of two measured quantities is helpful in comparing a particular combination of nucleation rate and growth law. For detailed calculations on one of the experiments there is excellent agreement with both measurements throughout the condensation zone and a theoretical calculation using the classical nucleation rate expression due to Volmer and a droplet growth law due to Gyarmathy.

Nucleation and growth of plagioclase, Makaopuhi and Alae lava lakes, Kilauea Volcano, Hawaii

Abstract: The Hawaiian lava lakes offer an unparalleled opportunity to study the processes that occur during the crystallization of basaltic magma. This paper presents estimates of the rates of nucleation and growth of plagioclase in the Kilauea lava lakes, Makaopuhi and Alae, and a discussion of the processes that control the nucleation and growth. The observed growth rates perpendicular to (010) vary from 1,7 to 11.0 × 10 −10 cm sec −1 . The nucleation rates vary from 6.8 × 10 −3 to 2.0 cm −3 sec −1 . In general the rates increase with increasing crystallization at any point, decrease with increasing distance from the surface, and are higher in the shallower lake, Alae. For the most part, nucleation appears to occur heterogeneously on previously existing crystals. The growth appears to be controlled by the interface attachment kinetics and not by diffusion in the melt. The observed results are in qualitative agreement with theoretical predictions.

Phase Separation Phenomena During the Formation of Asymmetric Membranes

Abstract: The formation of membranes from two systems has been studied. In the system polyurethane-dimethylformamide-water, the mechanism for the formation of the sponge-like structure proves to be a liquid-liquid phase separation with nucleation and growth of the diluted phase. This mechanism has been confirmed for the system modified polystyrene-polyisoprene-polystyrene/o-dichlorobenzene/(methanol-water). Crystallization and gelation is discussed. The membranes prepared showed hyperfiltration activity. The mechanism proposed here is believed to be valid for other systems, too.

Physics of thin films

Abstract: 1. Introduction.- 2. Methods of Preparation of Thin Films.- 2.1 Chemical and Electrochemical Methods.- 2.2 Cathode Sputtering.- 2.2.1 Principle of Diode Sputtering.- 2.2.2 Some Special Systems of Cathode Sputtering.- 2.2.3 Low-Pressure Methods of Cathode Sputtering.- 2.3 Vacuum Evaporation.- 2.3.1 Physical Foundations.- 2.3.2 Experimental Techniques.- 2.3.21 Evaporation Apparatus.- 2.3.22 Substrates and Their Preparation.- 2.3.23 The Most Important Materials for Evaporation.- 2.3.24 Evaporation Sources.- 2.3.25 Special Evaporation Techniques.- 2.3.26 Masking Techniques.- 3. Thin Film Thickness and Deposition Rate Measurement Methods.- 3.1 Balance Methods.- 3.1.1 Microbalance Method.- 3.1.2 Vibrating Quartz Method.- 3.2 Electrical Methods.- 3.2.1 Electric Resistivity Measurement.- 3.2.2 Measurement of Capacitance.- 3.2.3 Measurement of Q-factor Change.- 3.2.4 Ionization Methods.- 3.3 Optical Methods.- 3.3.1 Method Based on Measurements of Light Absorption Coefficient.- 3.3.2 Interference Methods.- 3.3.3 Polarimetric (Ellipsometric) Method.- 3.4 Deposition Rate Monitoring Using Transfer of Momentum.- 3.5 Special Thickness Monitoring Methods.- 3.5.1 Stylus Method.- 3.5.2 Radiation-absorption and Radiation-emission Methods.- 3.5.3 Work-function Change Method.- 4. Mechanism of Film Formation.- 4.1 Formation Stages of Thin Films.- 4.2 Nucleation.- 4.2.1 Capillarity Theory of Nucleation.- 4.2.2 Statistical (Atomistic) Theory of Nucleation.- 4.2.3 Influence of Individual Factors on Nucleation Process.- 4.2.4 Some Experiments for Verification of Nucleation Theories.- 4.3 Growth and Coalescence of Islands.- 4.4 Influence of Various Factors on Final Structure of Film.- 4.4.1 Special Properties of Films Deposited by Cathode Sputtering.- 4.5 Crystallographic Structure of Thin Films.- 4.6 Epitaxial Films.- 5. Composition, Morphology and Structure of Thin Films.- 5.1 Methods for Determination of Chemical Composition of Films.- 5.2 Electron Microscopy of Thin Films.- 5.2.1 Transmission Electron Microscopy.- 5.2.2 Electron-microscopic Examination of Surface by Replica Method.- 5.2.3 Special Types of Electron Microscopes for Direct Image-forming of Film Surface.- 5.2.31 Scanning Microscope.- 5.2.32 Reflection Microscope.- 5.2.33 Emission Microscopes.- 5.2.4 Tunnel Emission and Field Ionization.- 5.2.41 Field Electron Microscope.- 5.2.42 Field Ion Microscope.- 5.3 Diffraction of Electrons.- 5.3.1 Diffraction of High-Energy Electrons in Transmission and in Reflection.- 5.3.2 Low-Energy Electron Diffraction (LEED).- 5.4 X-ray Methods.- 5.4.1 X-ray Diffraction.- 5.4.2 X-ray Microscopy.- 5.5 Auger Spectroscopy.- 6. Properties of Thin Films.- 6.1 Mechanical Properties.- 6.1.1 Experimental Methods for Measurement of Mechanical Properties of Thin Films.- 6.1.2 Stress in Thin Films.- 6.1.3 Mechanical Constants of Thin Films.- 6.1.4 Adhesion of Thin Films.- 6.1.5 Rayleigh Surface Waves.- 6.2 Electrical and Magnetic Properties of Thin Films.- 6.2.1 Conductivity of Continuous Metal Films.- 6.2.2 Conductivity of Discontinuous Metal Films.- 6.2.3 Electrical Properties of Semiconducting Thin Films.- 6.2.4 Galvanomagnetic Effects in Thin Films.- 6.2.5 Superconductivity in Thin Films.- 6.2.6 Conductivity of Thin Dielectric Films.- 6.2.7 Dielectric Properties of Thin Films.- 6.2.8 Ferromagnetic Properties of Thin Films.- 6.3 Optical Properties of Thin Films.- 7. Application of This Films.- 7.1 Optical Applications.- 7.2 Applications in Electronics.- 7.2.1 Electric Contacts, Connections and Resistors.- 7.2.2 Capacitors and Inductances.- 7.2.3 Applications of Ferromagnetic and Superconducting Films.- 7.2.4 Active Electronic Elements.- 7.2.5 Microacoustic Elements Using Surface Waves.- 7.2.6 Integrated Circuits (IC).- 7.2.7 Thin Films in Optoelectronics and Integrated Optics.- 7.2.8 Further Applications.- References.

The mechanism of nucleation for in vitro collagen fibril formation.

Abstract: The formation of collagen fibrils from soluble monomers and aggregates by thermal gelation at neutral pH can be divided into two distinct stages: a nucleation phase and a growth phase. Turbidity studies of the kinetics of the precipitation reaction show that the lag-phase time or nucleation reaction time, t′l, is markedly temperature dependent while the growth reaction time is temperature independent. The activation energy of the nucleation reaction is essentially constant over the temperature range studied. In monitoring the nucleation-phase reaction by various physicochemical techniques, including viscosity, sedimentation equilibrium, and light scattering, no evidence for the formation of aggregates was observed. Enrichment of the initial collagen solution with aggregates accelerates nucleation, but de novo nuclei formation is still required even in highly aggregated collagen preparations. Removal of pepsin and pronase susceptible peptides lengthens the nucleation reaction time and increases the sensitivity of the rate of nuclei formation to changes in ionic strength. Electron microscope studies show the fibrils formed from the protease-treated collagen to be less well organized. With pepsin-treated collagen, subfibrils and obliquely striated fibrils are seen, showing that while microfibrils are formed interactions between them are modulated by the enzyme susceptible peptides in the same way that these regions modulate nuclei assembly. It appears that pepsin and pronase susceptible peptide regions of collagen play a more prominent role in the in vitro assembly of collagen molecules to form D-stagger nuclei and fibrils than do ionic interactions between helical molecular regions. A mechanism of nucleation of collagen fibrillogenesis is discussed.

Microphysical Processes Affecting Stratospheric Aerosol Particles

Abstract: Physical processes which affect stratospheric aerosol particles include nucleation, condensation, evaporation, coagulation and sedimentation. Quantitative studies of these mechanisms to determine if they can account for some of the observed properties of the aerosol are carried out. It is shown that the altitude range in which nucleation of sulfuric acid-water solution droplets can take place corresponds to that region of the stratosphere where the aerosol is generally found. Since heterogeneous nucleation is the dominant nucleation mechanism, the stratospheric solution droplets are mainly formed on particles which have been mixed up from the troposphere or injected into the stratosphere by volcanoes or meteorites. Particle growth by heteromolecular condensation can account for the observed increase in mixing ratio of large particles in the stratosphere. Coagulation is important in reducing the number of particles smaller than 0.05 micron radius. Growth by condensation, applied to the mixed nature of the particles, shows that available information is consistent with ammonium sulfate being formed by liquid phase chemical reactions in the aerosol particles. The upper altitude limit of the aerosol layer is probably due to the evaporation of sulfuric acid aerosol particles, while the lower limit is due to mixing across the tropopause.

Water at interfaces: Molecular structure and dynamics

Abstract: Recent advances in molecular mechanics and dynamics of water on inorganic surfaces are reviewed. The structures of hydroxylated surfaces and water-hydroxyl adducts, determined by LEED-Auger studies and infrared spectroscopy, are described to a detail only recently resolved. New mechanistic concepts of nucleation and freezing have ensued from the analysis of time-correlation functions, showing that low-frequency angular perturbations dominate the different behavior of water in bulk phases and at surfaces. While the rotational motion of water molecules in the liquid phase can be interpreted as rotation modulated by making and breaking hydrogen bonds with the neighbors, water adsorbed on nucleating catalysis undergoes an irreversible and complete reorientation in a fraction of the rotational period.

Grain Formation through Nucleation Process in Astrophysical Environment

Abstract: General picture of grain formation is preoented based on the nucleation theory. Grain formation process is described by a growth equation of grain radius and an equation of monomer consumption due to the growth of grains. These equations are characterized by two parameters. One depends on the physical conditions of the system and the other reflects the nature of grain materials. An overall feature of the grain formation process is illustrated by the use of an analytic expression of the solutions. After the vapor cools down to the saturated state, a waiting time is necessary until the grain formation begins effectively. Size distribution is relatively sharp in general. The representative size is closely related to the parameter which depends on the physical conditions. Growth by coalescence is not effective until the monomer sticking process is almost completed. The results are applied to the condensation in the primordial solar nebula. It is shown how the picture based on the chemical equilibrium calculations should be modified.

Precipitation of oxygen in silicon

Abstract: At high temperatures (1200 °C) the precipitation of oxygen in bulk dislocation‐free Czochralski silicon is inhibited by the absence of nucleation centers. A substantial supersaturation corresponding to a temperature drop of about 80 °C below the equilibrium saturation temperature is necessary for nucleation to occur. However, following a preliminary low‐temperature treatment (700 °C) precipitation occurs at a substantially smaller undercooling. This suggests that precipitation in crystals without a prior low‐temperature treatment occurs by homogeneous nucleation in the crystalline phase, in the absence of structural defects. Using classical nucleation theory the surface free energy of the precipitate is calculated to be about 100 erg/cm2 and the critical nucleus contains about 200 oxygen atoms.

Crystal nucleation in a three‐dimensional Lennard‐Jones system. II. Nucleation kinetics for 256 and 500 particles

Abstract: We have, by molecular dynamics computer simulation, observed homogeneous, spontaneous crystal nucleation in the 256‐particle Lennard‐Jones fluid at ρ*=0.91, 0.92, and 0.93, and in the 500‐particle fluid at ρ*=0.92, 0.93, and 0.94. For the 500‐particle system a nuclear region may be identified, and the nucleation and growth processes are found to be temporally separate. In all these events the nucleated phase was a body‐centered (nearly cubic) structure. We believe this to be a manifestation of the Ostwald step rule.