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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Sublimating comets as the source of nucleation seeds for grain condensation in the gas outflow from AGB stars

A growing amount of observational and theoretical evidence suggests that most main sequence stars are surrounded by disks of cometary material. The dust production by comets in such disks is investigated when the central stars evolve up the red giant and asymptotic giant branch (AGB). Once released, the dust is ablated and accelerated by the gas outflow and the fragments become the seeds necessary for condensation of the gas. The origin of the requisite seeds has presented a well known problem for classical nucleation theory. This model is consistent with the dust production observed in M giants and supergiants (which have increasing luminosities) and the fact that earlier supergiants and most WR stars (whose luminosities are unchanging) do not have significant dust clouds even though they have significant stellar winds. Another consequence of the model is that the spatial distribution of the dust does not, in general, coincide with that of the gas outflow, in contrast to the conventional condensation model. A further prediction is that the condensation radius is greater that that predicted by conventional theory which is in agreement with IR interferometry measurements of alpha-Ori.

Whitmire, D. P.↗
NASA NTRS1991

Phase selection during crystallization of undercooled liquid eutectic lead-tin alloys

During rapid solidification substantial amounts of undercooling are in general required for formation of metastable phases. Crystallization at varying levels of undercooling and melting of metastable phases were studied during slow cooling and heating of emulsified PB-Sn alloys. Besides the experimental demonstration of the reversibility of metastable phase equilibra, two different principal solidification paths have been identified and compared with the established metastable phase diagram and predictions from classical nucleation theory. The results suggest that the most probable solidification path is described by the 'step rule' resulting in the formation of metastable phases at low undercooling, whereas the stable eutectic phase mixture crystallizes without metastable phase formation at high undercooling.

Fecht, H. J.↗
NASA NTRS1991

Estimation of homogeneous nucleation flux via a kinetic model

The proposed kinetic model for condensation under homogeneous conditions, and the onset of unidirectional cluster growth in supersaturated gases, does not suffer from the conceptual flaws that characterize classical nucleation theory. When a full set of simultaneous rate equation is solved, a characteristic time emerges, for each cluster size, at which the production rate, and its rate of conversion to the next size (n + 1) are equal. Procedures for estimating the essential parameters are proposed; condensation fluxes J(kin) exp ss are evaluated. Since there are practical limits to the cluster size that can be incorporated in the set of simultaneous first-order differential equations, a code was developed for computing an approximate J(th) exp ss based on estimates of a 'constrained equilibrium' distribution, and identification of its minimum.

Wilcox, C. F.↗
NASA NTRS1992

Microgravity nucleation and particle coagulation experiments support

This project is a part of a program at GSFC to study to formation and growth of cosmic dust grain analogs under terrestrial as well as microgravity conditions. Its primary scientific objective is to study the homogeneous nucleation of refractory metal vapors and a variety of their oxides among others, while the engineering, and perhaps a more immediate objective is to develop a system capable of producing mono-dispersed, homogeneous suspensions of well-characterized refractory particles for various particle interaction experiments aboard the Space Shuttle and Space Station Freedom. Both of these objectives are to be met by a judicious combination of laboratory experiments on the ground and aboard NASA's KC-135 experimental research aircraft. Major effort during the current reporting period was devoted to the evaluation of our very successful first series of microgravity test runs in Feb. 1990. Although the apparatus performed well, it was decided to 'repackage' the equipment for easier installation on the KC-135 and access to various components. It will now consist of three separate racks: one each for the nucleation chamber, the power subsystem, and the electronic packages. The racks were fabricated at the University of Virginia and the assembly of the repackaged units is proceeding well. Preliminary analysis of the video data from the first microgravity flight series was performed and the results appear to display some trends expected from Hale's Scaled Nucleation Theory of 1986. The data acquisition system is currently being refined.

Lilleleht, L. U.↗
NASA NTRS1991

Containerless processing and rapid solidification of Nb-Si alloys in the niobium-rich eutectic range

Containerless processing and rapid solidification techniques were used to process Nb-Si alloys in the Nb-rich eutectic range. Electromagnetically levitated drops were melted and subsequently splat-quenched from different temperatures. A variety of eutectic morphologies was obtained as a function of the degree of superheating or undercooling of the drops prior to splatting. Metallic glass was observed only in drops quenched from above the melting temperature. Microstructures of splats deeply undercooled prior to quenching were very fine and uniform. These results are discussed in terms of classic nucleation theory concepts and the expected heat evolution at different regions of the splat during the rapid quenching process. The locations of the coupled-zone boundaries for the alpha-Nb + Nb3Si eutectic are also suggested.

Hofmeister, W. H.↗
NASA NTRS1996

Effect of Pt Doping on Nucleation and Crystallization in Li2O.2SiO2 Glass: Experimental Measurements and Computer Modeling

Heterogeneous nucleation and its effects on the crystallization of lithium disilicate glass containing small amounts of Pt are investigated. Measurements of the nucleation frequencies and induction times with and without Pt are shown to be consistent with predictions based on the classical nucleation theory. A realistic computer model for the transformation is presented. Computed differential thermal analysis data (such as crystallization rates as a function of time and temperature) are shown to be in good agreement with experimental results. This modeling provides a new, more quantitative method for analyzing calorimetric data.

Narayan, K. Lakshmi↗
NASA NTRS1999

Undercooling Limits and Thermophysical Properties in Glass Forming Alloys

The primary objective of this program is to produce deeply undercooled metallic liquids and to identify factors that limit undercooling and glass formation. The main research objectives are: (1) Investigating undercooling limits in glass-forming alloys and identifying factors that affect undercooling; (2) Measuring thermophysical properties and investigating the validity of the classical nucleation theory and other existing theories in the extreme undercooled states; and (3) To investigate the limits of electrostatic levitation technology in the ground base and to identify thermophysical parameters that might require reduced-g environment.

Rhim, Won-Kyu↗
NASA NTRS2010

Modeling Nucleation and Grain Growth in the Solar Nebula: Initial Progress Report

The primitive solar nebula was a violent and chaotic environment where high energy collisions, lightning, shocks and magnetic re-connection events rapidly vaporized some fraction of nebular dust, melted larger particles while leaving the largest grains virtually undisturbed. At the same time, some tiny grains containing very easily disturbed noble gas signatures (e.g., small, pre-solar graphite or SiC particles) never experienced this violence, yet can be found directly adjacent to much larger meteoritic components (chondrules or CAIs) that did. Additional components in the matrix of the most primitive carbonaceous chondrites and in some chondritic porous interplanetary dust particles include tiny nebular condensates, aggregates of condensates and partially annealed aggregates. Grains formed in violent transient events in the solar nebula did not come to equilibrium with their surroundings. To understand the formation and textures of these materials as well as their nebular abundances we must rely on Nucleation Theory and kinetic models of grain growth, coagulation and annealing. Such models have been very uncertain in the past: we will discuss the steps we are taking to increase their reliability.

Nuth, Joseph A.↗
NASA NTRS2011

A Model of Silicate Grain Nucleation and Growth in Circumstellar Outflows

Based on its abundance, high bond energy, and recent measurements of its vapor pressure SiO is a natural candidate for dust nucleation in circumstellar outflows around asymptotic giant branch stars. In this paper, we describe a model of the nucleation and growth of silicate dust in such outflows. The sensitivity of the model to varying choices of poorly constrained chemical parameters is explored, and the merits of using scaled rather than classical nucleation theory are briefly considered, An elaboration of the model that includes magnesium and iron as growth species is then presented and discussed. The composition of the bulk of the grains derived from the model is consistent with olivines and pyroxenes, but somewhat metal-rich grains and very small, nearly pure SiO grains are also produced,

Paquette, John A.↗
NASA NTRS2020

Ultrafast Melting at Metal-Semiconductor Interfaces: A Molecular Dynamics Study

Quantifying the thermodynamics of interfacial melting is important for several applications such as laser machining and designing radiation hardened electronic devices. Due to the short time and length scales that are involved in the process, molecular simulations provide an ideal path for understating the interfacial melting phenomena. In this work we investigate tungsten (W) – silicon carbide (SiC) interfaces which are relevant in modern high-power electronic devices. Classical molecular dynamics simulations along with the two-temperature model (TTM) were used to study the radiation induced melting of the interface while a neural network-based machine learning framework was used to track the transient re-crystallization process. The damage endured by the semiconductor at the interface following the radiation event is discussed in detail while the re-crystallization of the metal is described in terms of the classical nucleation theory.

Ashwin Ravichandran↗
NASA NTRSDate not supplied

Inefficient Growth of Refractory Grains: Implications for Grains in the Solar Nebula

Introduction: We report the results of a sounding rocket experiment in which SiO nucleation and growth in an argon atmosphere was observed in microgravity starting from a vapor generated by evaporation of waterglass deposited on a heated Ta wire. The temperature and pressure at the point of nucleation was determined using a double wavelength Mach–Zehnder-type interferometer. The particle composition and grain size distribution were measured after payload recovery using a Transmission Electron Microscope (TEM; JEM-2100F. JEOL Ltd., Tokyo). The results were analyzed using Modified Classical Nucleation Theory (MCNT)[1]. We report a measurement of the SiO-silicate grain sticking coefficient of 0.016, indicating very inefficient grain growth from the vapor. In other words, for every 100 SiO molecules that collide with a growing grain only one to two are captured. In the only two previous studies of the growth of solid particles from the gas phase the calculated sticking coefficients were much lower. For zinc atoms colliding with zinc crystals only three out of 100,000 collisions resulted in grain growth [2]. Iron atoms colliding with growing iron particles similarly showed an efficiency of ~0.00002 [3]. For zinc, we can explain inefficient growth due to the difficulty of the colliding atom coming upon a vacancy in the growing crystal lattice before it evaporates from the grain surface. The much higher sticking coefficient of SiO molecules on silica grains is reasonable because the silica grains are highly amorphous or glassy and therefore provide a much higher defect density where SiO molecules can be captured into the grains. In the case of similar Fe experiment, the critical cluster was the dimer. In this case the forming dimer returns to the vapor phase by dissociation due to the excess energy from the formation of the Fe-Fe bond.

Joseph A Nuth↗
NASA NTRS1986

An analysis of refractory silicate nucleation via the theory of the physical spinodal

An analysis of experimental data has been conducted using the theory of the spinodal, as a test of its applicability to refractory vapors. A promising aspect of this approach is its prediction of a temperature and supersaturation at which there no longer exists a barrier to nucleation; such a phenomenon has been observed in both the SiO-H2 and MgSiO-H2 systems, where a small change in ambient temperature yielded 5-10 orders of magnitude increases in the nucleation rate within a few seconds.

Nuth, Joseph A., III↗
NASA NTRS1986

Theory of homogeneous nucleation - A chemical kinetic view

A simple function with two undetermined parameters has been used in place of the Thomson-Gibbs relation to relate the activation energy of the vaporization reaction to cluster size. The parameters are iterated to assume optimum values in numerical computation so experimental data may be correlated. Calculations show this approach closely predicts and correlates available data for water, benzene, and ethanol. The nucleation formulism is redeveloped with an emphasis on the chemical kinetic view. Surface tension of the liquid and free energy of droplet formation are not used in its derivation.

Yang, C. H.↗
NASA NTRS1988

The critical complex in nucleation is not the nucleus

A theory of nucleation is presented which combines features of classical nucleation. In this theory, the nucleus is a cluster of finite size with a sharp interface and spinodal decomposition, in which a fluctuation in density or composition generates the critical complex and nucleus. Thus, the nucleus is a product of nucleation. Schematic models for the classical and the proposed routes to nucleation are compared, focusing on the relation between pressure and volume in the models. In the proposed model, the critical complex, or critical state, is between the parent state and the nucleus.

Rasmussen, Don H.↗
DOE OSTI2020

Critical nuclei at hetero-phase interfaces

Two-step nucleation, in which a metastable intermediate phase acts as a precursor for nucleating a thermodynamically stable phase, has been widely observed in many materials systems and solid-state reactions. Among the advantages of two-step nucleation is that the stable phase may nucleate heterogeneously at the hetero-phase interface between the original and the precursory phases. Although heterogeneous nucleation (HN) theories for homo-phase grain boundaries and inert surfaces are well established, our understanding of HN at reactive hetero-phase interfaces remains incomplete. This deficiency stems from the discontinuity of the chemical potential driving force across the hetero-phase interface, which profoundly affects the fundamental properties of the nucleus in a way that is not properly accounted for in existing models. In this work, we incorporate these effects to extend the classical nucleation theory to HN at hetero-phase interfaces. Our extended model demonstrates that the nucleus shape along the minimum energy path is strongly size-dependent, and this additional degree of freedom can result in the reduction of the critical nucleus volume and associated activation energy barrier by orders of magnitude relative to conventional predictions. The simulation results are used to construct a sensitivity map in the parameter space of interfacial energy and bulk driving force ratios, which quantifies the difference in nucleation barriers predicted by different models.

36 MATERIALS SCIENCE↗
NASA NTRS1974

Nonstationary homogeneous nucleation

The theory of homogeneous condensation is reviewed and equations describing this process are presented. Numerical computer solutions to transient problems in nucleation (relaxation to steady state) are presented and compared to a prior computation.

Harstad, K. G.↗
DOE OSTI2022

Probing the Critical Nucleus Size in the Metal-Insulator Phase Transition of VO 2

In a first-order phase transition, critical nucleus size governs nucleation kinetics, but the direct experimental test of the theory and determination of the critical nucleation size have been achieved only recently in the case of ice formation in supercooled water. The widely known metal-insulator phase transition (MIT) in strongly correlated VO 2 is a first-order electronic phase transition coupled with a solid-solid structural transformation. It is unclear whether classical nucleation theory applies in such a complex case. Here we directly measure the critical nucleus size of the MIT by introducing size-controlled nanoscale nucleation seeds with focused ion irradiation at the surface of a deeply supercooled metal phase of VO 2 . The results compare favorably with classical nucleation theory and are further explained by phase-field modeling. This Letter validates the application of classical nucleation theory as a parametrizable model to describe phase transitions of strongly correlated electron materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

The Roles of Oil–Water Interfaces in Forming Ultrasmall CaSO 4 Nanoparticles

In natural and engineered environmental systems, calcium sulfate (CaSO 4 ) nucleation commonly occurs at dynamic liquid–liquid interfaces. Although CaSO 4 is one of the most common minerals in oil spills and oil–water separation, the mechanisms driving its nucleation at these liquid–liquid interfaces remain poorly understood. Here, in this study, using in situ small-angle X-ray scattering (SAXS), we examined CaSO 4 nucleation at oil–water interfaces and found that within 60 minutes of reaction, short rod-shaped nanoparticles (with a radius of gyration (R g ) of 17.2 ± 2.7 nm and a length of 38.2 ± 5.8 nm) had formed preferentially at the interfaces. Wide-angle X-ray scattering (WAXS) analysis identified these nanoparticles as gypsum (CaSO 4 ·2H 2 O). In addition, spherial nanoparticles measuring 4.1 nm in diameter were observed at oil–water interfaces, where surface-enhanced Raman spectroscopy (SERS) revealed an elevated pH compared to the bulk solution. The negatively charged oil–water interfaces preferentially adsorb calcium ions, collectively promoting CaSO 4 formation there. CaSO 4 particle formation at the oil–water interface follows a nonclassical nucleation (N-CNT) pathway by forming ultrasmall amorphous spherical particles which then aggregate to form intermediate nanoparticles, subsequently growing into nanorod-shaped gypsum. These findings of this study provide insights into mineral scaling during membrane separation and can inform more efficient oil transport in energy recovery systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗