Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session J6: Computational Modeling of Crystallization and Nucleation Phenomena |
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Sponsoring Units: DCOMP Chair: Jim Belak, Lawrence Livermore National Laboratory Room: 406 |
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J6.00001: Coupled Nucleation Processes in Metallic Liquids and Glasses Invited Speaker: Nucleation processes in condensed systems are often more complicated than expected from classical theory considerations. For example, our recent studies of glasses and deeply supercooled liquids demonstrate that the short- and medium-range order play an important role in the nucleation pathway. High-energy X-ray diffraction data from electrostatically levitated transition metal and alloy liquids demonstrate the frequent development of icosahedral short-range order (ISRO) with supercooling. This ordering has significant consequences for crystallization and vitrification of the liquids. It makes it difficult to nucleate ordered crystal phases, confirming a half-century old hypothesis by Frank. Measurements of the density and surface tension in several supercooled liquids suggest that it may be associated with a liquid/liquid phase transition. Quantitative measurements of the time-dependent nucleation rate in a Zr$_{59}$Ti$_{3}$Cu$_{20}$Ni$_{8}$Al$_{10}$ metallic glass and associated structural studies of the supercooled liquid demonstrate that it increases through the glass transition, providing support for a frustration model of the glass transition. In a Ti-Zr-Ni liquid the ISRO lowers the barrier for a metastable icosahedral quasicrystal, blurring the distinction between homogenous and heterogeneous nucleation. Our studies and those of others suggest that the nucleation of the ordered phase can be coupled with liquid phase transitions, including high order transitions. Coupling between other processes is also common for nucleation. For example, the coupling between the stochastic fluxes of interfacial attachment and long-range diffusion in the nucleation step can be critical when the initial and final phases have different chemical compositions. The implications of coupled nucleation processes on phase formation, stability and nanoscale crystallization are discussed. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:27PM |
J6.00002: Out-of-equilibrium processes in suspensions of oppositely charged colloids: liquid-to-crystal nucleation and gel formation Invited Speaker: We study the kinetics of the liquid-to-crystal transformation and of gel formation in colloidal suspensions of oppositely charged particles. We analyse, by means of both computer simulations and experiments, the evolution of a fluid quenched to a state point of the phase diagram where the most stable state is either a homogeneous crystalline solid or a solid phase in contact with a dilute gas. On the one hand, at high temperatures and high packing fractions, close to an ordered-solid/disordered-solid coexistence line, we find that the fluid-to-crystal pathway does not follow the minimum free energy route. On the other hand, a quench to a state point far from the ordered-crystal/disordered-crystal coexistence border is followed by a fluid-to-solid transition through the minimum free energy pathway. At low temperatures and packing fractions we observe that the system undergoes a gas-liquid spinodal decomposition that, at some point, arrests giving rise to a gel-like structure. Both our simulations and experiments suggest that increasing the interaction range favors crystallization over vitrification in gel-like structures. \\[4pt] In collaboration with Chantal Valeriani, Soft Condensed Matter, Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands and SUPA, School of Physics, University of Edinburgh, JCMB King's Buildings, Mayfield Road, Edinburgh EH9 3JZ, UK; Teun Vissers, Andrea Fortini, Mirjam E. Leunissen, and Alfons van Blaaderen, Soft Condensed Matter, Debye Institute for Nanomaterials Science, Utrecht University; Daan Frenke, FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands and Department of Chemistry, University of Cambridge, Lensfield Road, CB2 1EW, Cambridge, UK; and Marjolein Dijkstra, Soft Condensed Matter, Debye Institute for Nanomaterials Science, Utrecht University. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 1:03PM |
J6.00003: Nucleation of Ice Invited Speaker: The freezing of water into ice is a ubiquitous transformation in nature, yet the microscopic mechanism of homogeneous nucleation of ice has not yet been elucidated. One of the reasons is that nucleation happens in time scales that are too fast for an experimental characterization and two slow for a systematic study with atomistic simulations. In this work we use coarse-grained molecular dynamics simulations with the monatomic model of water mW[1] to shed light into the mechanism of homogeneous nucleation of ice and its relationship to the thermodynamics of supercooled water. Cooling of bulk water produces either crystalline ice or low- density amorphous ice (LDA) depending on the quenching rate. We find that ice crystallization occurs faster at temperatures close to the liquid-liquid transition, defined as the point of maximum inflection of the density with respect to the temperature. At the liquid-liquid transition, the time scale of nucleation becomes comparable to the time scale of relaxation within the liquid phase, determining --effectively- the end of the metastable liquid state. Our results imply that no ultraviscous liquid water can exist at temperatures just above the much disputed glass transition of water. We discuss how the scenario is changed when water is in confinement, and the relationship of the mechanism of ice nucleation to that of other liquids that present the same phase behavior, silicon [2] and germanium [3]. \\[4pt] [1] Molinero, V. {\&} Moore, E. B. Water modeled as an intermediate element between carbon and silicon. Journal of Physical Chemistry B (2008). Online at http://pubs.acs.org/cgi- bin/abstract.cgi/jpcbfk/asap/abs/jp805227c.html \\[0pt] [2] Molinero, V., Sastry, S. {\&} Angell, C. A. Tuning of tetrahedrality in a silicon potential yields a series of monatomic (metal-like) glass formers of very high fragility. Physical Review Letters 97, 075701 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:39PM |
J6.00004: Surface Induced Crystallization In Tetrahedral Liquids Invited Speaker: Freezing is a fundamental physical phenomenon that has been studied over many decades; yet the role played by surfaces in determining nucleation has remained elusive. While common wisdom regards surfaces as unfavorable nucleation sites, both atmospheric data and laboratory measurements on droplets of water support the hypothesis of surface-induced crystallization in some systems. In this talk I will discuss our recent work on employing accelerated molecular dynamics simulations to investigate nucleation in the presence of free surfaces in tetrahedral liquids with a negative slope of their melting line ($dP/dT<0$). Through conducting extensive study on nucleation rates and nucleation pathways in a few systems, {\em e.g.}, Si and Ge, we provide direct computational evidence of surface induced crystallization in supercooled systems with $dP/dT<0$. We show that the possibility of observing preferential nucleation in close proximity of free surfaces is related to the density decrease occurring upon freezing, and surface tension facilitating the initial nucleus formation. Furthermore, in contrast to the common assumption that regards surfaces as heterogeneous center, we identify the {\em homogeneous} nature of surface induced nucleation. This is related to both the local static and dynamical properties of liquid surface. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 2:15PM |
J6.00005: Growth and optical properties of embedded silicon nanocrystals Invited Speaker: The optoelectronic properties of nanostructured silicon (nc-Si) are governed by the interplay between the local chemical bonding features and the complex overall atomic structure. Interesting enough, a-Si has a larger optical absorption than the c-Si and, therefore, biphasic a-c silicon systems (i.e. nanocrystallites embedded into an amorphous matrix) are currently under investigation for next-generation photovoltaics. Biphasic systems undergo crystallization upon thermal annealing and, therefore, it is quite difficult to predict theoretically their finite-temperature optoelectronic properties. In this talk I will present our ongoing research on the growth and the optoelectronic properties of textured nanocrystalline silicon, here modeled as a distribution of cylindrical grains embedded into an amorphous matrix. As for the growth, I argue that by large-scale atomistic simulations it is possible to infer a continuum model for the crystallinity evolution upon thermal annealing.[1] In particular, at low crystallinity, it is proved that--consistently with the standard Kolmogorov-Johnson-Mehl-Avrami (KJMA) theory--the a-c phase transformation is dominated by the isolated grain evolution; conversely, at later stages deviations from the KJMA theory are observed, mainly due to atomic-scale features. I also prove that such effects can be included by using an improved phenomenological version of the KJMA theory.[2] As for the finite-temperature optoelectronic properties, I present a divide-and-conquer computational procedure, based on a combination of empirical tight-binding and model-potential molecular dynamics. This procedure is applied to investigate local and average optoelectronic properties of very large nanostructured silicon systems and to predict the variation of the optical absorption upon crystallinity.[3] I show that the optical absorption of a nc-Si sample corresponds to a simple linear combination between c-Si and a-Si phases and it is not affected by electron confinement within grains. Strain effects on combined absorption are discussed as well. \\[3pt] [1] A. Mattoni, L. Colombo, Phys. Rev. Lett. 99, 205501 (2007)\\[0pt] [2] A. Mattoni, L. Colombo, Phys. Rev. B 78, 075408 (2008)\\[0pt] [3] A. Mattoni, L. Colombo, submitted (2008) [Preview Abstract] |
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