Long-Qing Chen Pennsylvania State University
James Belak Lawrence Livermore National Laboratory
Heike Emmerich RWTH Aachen, Institute of Minerals Engineering (GHI)
ChrisM. Wolverton Northwestern University
XX1: Phase Stability
Tuesday AM, April 26, 2011
Room 2016 (Moscone West)
10:00 AM - **XX1.2
The Enigmatic Ag-Pt Phase Diagram and Yet Another Derivative Structure Algorithm.
Gus Hart 1 , Lance Nelson 1 , Rodney Forcade 2 Show Abstract
1 Physics and Astronomy, Brigham Young University, Provo, Utah, United States, 2 Mathematics, Brigham Young University, Provo, Utah, United States
The Ag-Pt phase diagram as published in the most recent phase diagramcompilations (Massalski, Pauling File) is entirely speculative below1000° C. Our recent first principles calculations and clusterexpansion-based modeling are largely consistent with the speculationsand harbor no big surprises. For example, the phase diagrams and ourcalculations both suggest a stable CuPt-like phase at 50 at.-\%Pt. However, an experimental study published after the compilationssupports a significantly different phase diagram. In this new phasediagram, the only stable phases at low temperatures are the elementalfcc Ag and Pt phases and one ordered phase at the unusualconcentration of 53±0.5 at.-% Pt. The experimental study showsthat the homogeneity range for the ordered phase is narrow (almost like aline compound), and its X-ray data suggests that the unit cell of thisphase contains 32 atoms with a stoichiometry of 15:17. We developed anew derivative structure enumeration algorithm specifically designedfor large unit cells with known concentrations. This is necessarybecause our original algorithm enumerated all concentrations and wastherefore limited to smaller unit cells. We have explored, viafirst-principles, the structural details of this enigmatic phase inthe Ag-Pt phase diagram. I will discuss our first-principles resultsfor Ag-Pt, and I will discuss the how the new algorithm is useful forlarge unit cells when partial structural information is known, a situation that occurs frequently in binary metallic phase diagrams.
10:30 AM - XX1.3
Phase-Stabilisers of Titanium Alloys.
Bengt Tegner 1 , Simon Macleod 2 , Hyunchae Cynn 3 , John Proctor 1 , Linggang Zhu 1 , William Evans 3 , Malcolm McMahon 1 , Graeme Ackland 1 Show Abstract
1 CSEC and SUPA, School of Physics and Astronomy, The University of Edinburgh, Edinburgh United Kingdom, 2 Institute of Shock Physics, Imperial College London, London United Kingdom, 3 PLS, Lawrence Livermore National Laboratory, Livermore, California, United States
Titanium alloys exhibit in three distinct crystal structure: alpha, beta and omega. In many situations, the high symmetry, more ductile beta-phase is desired, however it is only stable in Ti at high temperature. Thus additional alloying elements are used to stabilise the beta-phase at lower temperature. We present a joint experimental and ab initio study of the phase-stabilising behaviour of the commonly used substitutional elements aluminium, chromium and vanadium at high pressures and temperatures.Room temperature, angle-dispersive X-ray diffraction data of powdered Ti-6Al-4V were collected using gas-membrane diamond anvil cells at HPCAT at the 3rd generation synchrotron, the Advanced Photon Source, Chicago. Compression data were obtained up to 1 Mbar for varying states of hydrostaticity. We discuss the observed martensitic alpha to omega phase transformation and the results of equation of state fitting.In our ab initio study, we have used the plane-wave DFT code CASTEP to investigate various binary compositions of Ti-Al, Ti-Cr and Ti-V, as well as the ternary alloy Ti-6Al-4V, comparing equilibrium volumes, elastic properties and phase stability. We have used both the explicit positions of the alloying atoms in large supercells and the Virtual Crystal Approximation (VCA). Ti, V and Cr are adjacent in the periodic table, making this is a best-possible scenario for VCA, and indeed it works well showing that the phase stability is determined primarily by electron density rather than relative bond strength. We show that VCA cannot describe a mix of sp-bonded and transition metals, and fails for aluminium-containing alloys. Thus both chromium and vanadium stabilize the beta-phase by donating extra electrons, and one chromium atom acts equivalently to two vanadium atoms. Aluminium acts as an electron acceptor.Theoretical work is supported by the European project MaMiNa, which aims to develop new, more easily machinable, titanium alloys, looking at macro, micro and nano aspects. Experimental work is supported by US Department of Energy contract DE-AC52-07NA27344. HPCAT is supported by CIW, CDAC, UNLV and LLNL through funding from DOE-NNSA, DOE-BES and NSF. APS is supported by DOE-BES, under Contract No. DE-AC02-06CH11357.
11:15 AM - **XX1.5
A First Principles Investigation into Hydrogen Traps in bcc Fe.
William Counts 1 , Chris Wolverton 1 , Ron Gibala 2 Show Abstract
1 , Northwestern University, Evanston , Illinois, United States, 2 , University of Michigan, Ann Arbor, Michigan, United States
Hydrogen embrittlement of iron and steels is a classic but still unresolved problem in metallurgy. While hydrogen can freely move through the Fe lattice, its diffusion is hindered by lattice imperfections. Experimentally quantifying the binding energy of hydrogen to these defects has proven to be difficult. Fortunately, computational tools are ideally suited to study defect trapping because it is possible to isolate individual traps. Density functional theory (DFT) was used to quantify the binding energy of hydrogen to a number of different traps, including point defects, dislocations, and grain boundaries. We find defects that introduce some degree of free volume (vacancies and grain boundaries) are the stronger H traps than defects that do not (substitutional/interstitial solutes and screw dislocations). Furthermore, the effect of temperature (i.e. vibrational entropy) on the H binding energy was also considered for a limited number of defects. We observe that the H-vacancy binding free energy is temperature dependent while the H-solute binding free energy is temperature independent.
11:45 AM - XX1.6
Theoretical Study of Optical and Thermodynamic Properties of Ternary Transition Metal Nitrides.
Mouna Ben Yahia 1 , Abbas Hodroj 2 , Jean-François Pierson 2 , Marie-Liesse Doublet 1 Show Abstract
1 Equipe Chimie théorique Méthodologies et Modélisation, Institut Charles Gerhardt, Montpellier France, 2 Laboratoire de Science et Génie des Surfaces, Ecole des MInes , Nancy France
Transition metal nitrides are metallic systems. Their optical properties and high mechanical hardness make them interesting materials for coloured coatings in tooling industry or jewellery. Among them, TiN presents an attractive golden colour [1,2] which is the consequence of a selective absorption in the visible range.  In order to increase the colour range available, the addition of a third element M has been widely studied over the past ten years. Although numerous papers have been published on the development of decorative coatings, there are very few fundamental studies devoted to the effect of the substituent M on the colour of TiN. In particular, the effect of the substituent’s size, electronic nature (more or less electropositive) and rate on the crystal structure and the local symmetry of the TiN lattice has never been discussed as a systematic way to rationalize the evolution of the optical properties of Ti1-xM xN systems with x. To achieve that goal, one has first to understand the macroscopic behaviour of these coloured coatings at a microscopic level, starting from the principles of quantum mechanics. Among the computing methods available for solid state, the density functional theory (DFT) has already proven to be effective in predicting a wide variety of material properties. It was then used to correlate the intrinsic nature of the third element to the modifications induced on the optical properties of the substituted Ti1-xMxN compounds. In order to explore a representative collection of substituents, alkaline-earth (Mg, Ca), transitional (Y), post-transitional (Sn) and lanthanides (La) elements were chosen. Simple concepts of chemical bonds were then used to correlate the crystal and electronic structures of the Ti1-xMxN networks to the evolution of their optical properties. Temperature and pressure dependent phase diagrams were then computed to predict the achievable Ti/M substitution rates of each TiMN ternary phase. On the basis of these results, a qualitative model was proposed to rationalize the composition dependent optical spectra of Ti1-xMxN experimentally observed.  Ph. Roquiny, F. Bodart, G. Terwagne, Surf. Coat. Technol. 116/119 (1999) 278  C. Mitterer, P.H. Mayrhofer, W. Waldhauser, E. Kelesoglu, P. Losbichler, Surf. Coat. Technol. 108-109 (1998) 230 S. Niyomsoan, W. Grant, D. L. Olson, B. Mishra, Thin Solid Films 415 (2002) 187
12:00 PM - XX1.7
A Density Functional Theory Study of Atomic Order in Au-Pd Nanoparticles.
Tim Mueller 1 , Gerbrand Ceder 1 Show Abstract
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
In applications such as magnetic memory , lithium-ion batteries , and catalysis , it is important to understand how atoms are arranged in nanoparticles and how atomic order affects nanoparticle properties. Due to the prohibitive expense of using ab-initio methods such as density functional theory (DFT) to study atomic order in nanoparticles, theoretical studies have been limited to less accurate energy models, approximations based on bulk materials, or very small clusters. We demonstrate how the recently-developed Bayesian cluster expansion  addresses this problem, enabling researchers to study atomic order in nanoparticles with unprecedented computational efficiency and a level of accuracy close to that of DFT. As a demonstration of this method we present a study of atomic order in 2-nm cuboctahedral Au-Pd nanoparticles. We are able to evaluate the energies of millions of atomic arrangements per minute with an estimated prediction error of 1 meV / atom (0.1 kJ / mol) relative to DFT calculations. This combination of speed and accuracy enables us to identify ground state structures and model thermal disorder. Our results indicate that the atomic order in these particles is driven by a competition between core-shell behavior and a tendency to alloy, resulting in complex structures at a variety of compositions. In addition to providing insights into the structure and catalytic activity of Au-Pd nanoparticles, these results demonstrate the effectiveness of a method that could significantly improve our understanding of atomic order in nano-scale materials.1. Alloyeau, D., et al., Size and shape effects on the order-disorder phase transition in CoPt nanoparticles. Nature Materials, 2009. 8(12): p. 940-946.2. Meethong, N., et al., Size-dependent lithium miscibility gap in nanoscale Li1-xFePO4. Electrochemical and Solid State Letters, 2007. 10(5): p. A134-A138.3. Strasser, P., et al., Lattice-strain control of the activity in dealloyed core-shell fuel cell catalysts. Nature Chemistry. 2(6): p. 454-460.4. Mueller, T. and G. Ceder, Bayesian approach to cluster expansions. Physical Review B, 2009. 80(2): p. 024103.
12:15 PM - XX1.8
Accelerated Molecular Dynamics Study of Cu Crystal under Strains.
Shao-Ping Chen 1 , Arthur Voter 1 , Danny Perez 1 , Tim Germann 1 Show Abstract
1 T-1, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
We have used Accelerated Molecular Dynamics method to study the response of Cu crystal under strains at finite temperatures. The critical event of the generation of dislocation loop has been determined. The dynamics and energy barriers of this event and their relationship to the strength of the materials will be discussed.
12:30 PM - XX1.9
Modeling Aliovalent Transition Metal Dopants in Zirconia and Yttria-Stabilized Zirconia.
Bryce Meredig 1 , Chris Wolverton 1 Show Abstract
1 Materials Science & Engineering, Northwestern University, Evanston, Illinois, United States
We use Monte Carlo (MC) simulations based on empirical potentials, supported by density functional calculations, to investigate the thermodynamic properties of transition metal-doped zirconia (ZrO2) and yttria-stabilized zirconia (YSZ). In this talk, we focus primarily on Fe doping and study in particular the interesting phenomenon that Fe2+ is more soluble than Fe3+ in pure zirconia, but the situation evidently reverses in YSZ. We calculate full mixing enthalpy curves for Fe2+ and Fe3+ in ZrO2 and in YSZ at multiple yttria concentrations. We also quantify the energetic magnitude of short-range ordering in the Fe2+-ZrO2 and Fe3+-ZrO2 systems, by comparing random solution mixing enthalpies to MC-derived enthalpies that include short-range order. We correlate MC-derived atomistic defect configurations (between host atoms, dopants, and vacancies) with the shapes of the mixing curves and short-range ordering preferences. Finally, we present preliminary data on other transition metal dopants and comment on observed periodic trends in thermodynamic properties.
12:45 PM - XX1.10
Nucleation and Growth of Crystallization from the Amorphous State.
James Belak 1 , Daniel Orlikowski 1 Show Abstract
1 Condensed Matter and Materials Division, Lawrence Livermore National Laboratory, Livermore, California, United States
The classical theory of nucleation asserts that for homogeneous nucleation a critical cluster size or fluctuation must be achieved prior to nucleation of the new phase. Here we use large-scale molecular dynamics to examine this hypothesis in detail for crystallization from within an amorphous state. A nucleus of the crystalline state is introduced into the simulation and its stability is studied as a function of thermodynamic conditions (temperature) and size (radius). Clusters larger than the critical radius grow while clusters smaller that the critical radius shrink. By performing the simulations within the constant heat ensemble, we are able to compute the entire nucleation curve by direct numerical simulation. Results for crystallization in under-cooled liquid tantalum are presented within the Gibbs-Thomson-Tammann framework with emphasis on the lag time for crystallization and the structure of the critical nucleus and its neighboring liquid. Not surprising, significant under-cooling is required for a critical cluster size comparable to fluctuations in molecular dynamics simulations. Results will also be presented for a simple binary amorphous solid (NiAl) in an effort to quantify crystallization from different poly-a-morphic states, the role of inter-atomic diffusion, and recent in situ DTEM experiments.This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory, under contract No. DE-AC52-07NA27344.
Tuesday PM, April 26, 2011
Room 2016 (Moscone West)
2:30 PM - **XX2.1
Computational Thermodynamics of Phase Transformations.
Zi-Kui Liu 1 Show Abstract
1 , The Pennsyvania State University, University Park, Pennsylvania, United States
Classifications of phase transformations can be broadly presented in three schemes: thermodynamics, mechanism, and microstructure. These three schemes address three aspects of a phase transformation: thermodynamics, kinetics, and crystallography. In this presentation, the three schemes are briefly presented, and the focus will be on the thermodynamics scheme of phase transformations. In the thermodynamic scheme, a phase transformation is defined as first or second order if the first or second derivative of Gibbs energy with respect to temperature or pressure is discontinuous, and higher order phase transformations are usually not considered in the domain of materials science and engineering. In recent studies of ferromagnetic and ferroelectric phase transformations, we realized that from statistic point of view, all relevant phases, or more broadly defined as configurations, in a system contribute to the properties of the system. Experimental observations thus represent an average among configurations of statistically significant amount. With various computational techniques, we are able to access individual configurations, compute their energetics, and assemble them statistically. By analyzing the thermodynamic properties of such an assembly as a function of temperature or pressure, we obtain the statistic populations of each configuration as a function of temperature or pressure. In this presentation, we will discuss simple models to demonstrate the concept of multi-scale configurations and its versatile applications and insights to a wide range of first and second order phase transformations. Some specific examples of applications will also be presented.
3:00 PM - XX2.2
Thermodynamic Cartography and Structure/Property Mapping of Nanoscale Titania Photocatalysts.
Amanda Barnard 1 Show Abstract
1 Materials Science & Engineering, CSIRO, Clayton, Victoria, Australia
The design of optimal nanoparticle photocatalysts requires a detailed understanding of how fundamental materials parameters such as size, shape and structure relate to photoactivity; a field often referred to as structure/property mapping. Unfortunately, in the case of nanomaterials, the structure may vary in response to changes in their chemical and thermal surroundings, so it becomes necessary to generate sets of structure/property maps for all possible permutations, polymorphs and environmental conditions. Traditionally the approach has been experimental, based on a system of trial and error using high throughput synthesis and characterization, but an alternative approach is to use mathematical modeling and computer simulations to more rapidly sample the structure space. This study traces the development of a nanoscale phase diagram for anatase and rutile nanocrystals using thermodynamic cartography to predict the most probable structure as a function of size, shape, temperature and surface chemistry. These structural predictions are then used as the basis for a structure/property map of photocatalytic activity, by overlaying the population density of cationic sites on the active surface facets.
3:15 PM - XX2.3
Eutectics and Phase Diagrams of Molten Salts from Molecular Dynamics Simulations.
Saivenkataraman Jayaraman 1 , Anatole von Lilienfeld 2 , Aidan Thompson 1 Show Abstract
1 1435 - Multiscale Dynamic Materials Modeling, Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 1114 - Surface and Interface Sciences , Sandia National Laboratories, Albuquerque, New Mexico, United States
The use of alkali nitrate salt mixtures as heat transfer fluids in solar thermal power plants is limited by their relatively high melting point. Certain compositions of quaternary and higher dimensional mixtures of alkali and alkaline earth nitrates and nitrites have low melting points. However, the high dimensionality of the search space makes it difficult to find lowest melting compositions. Molecular simulations offer an efficient way to screen for promising mixtures.A molecular dynamics scheme general enough to identify eutectics of any HTF candidate mixture will be presented. The eutectic mixture and temperature are located as the tangent point between free energies of mixing for the liquid and a linear plane connecting the pure solid-liquid free energy differences. The free energy of mixing of the liquid phase is obtained using thermodynamic integration over ``alchemical'' transmutations sampled with molecular dynamics, in which particle identities are swapped gradually. Numerical results for binary and ternary mixtures of alkali nitrates agree well with experimental measurements.
3:30 PM - XX2.4
Grain Size Evolution in Thin Films: Comparison Between Experiments and Computational Results.
Chandra Pande 1 Show Abstract
1 , Naval Research Laboratory, Washington D.C., District of Columbia, United States
It is shown that experimental grain size distributions in thin films produced by a variety of methods show a definite departure from computer simulation results and also with some analytical results, whereas there is a good agreement with some analytical results and simulation results. The reason for this deviation is discussed in detail. In some experiments, it is proposed that either the self-similar distribution may not have been reached, or the width of the film acts an additional variable limiting the size of larger grains. Another possibility could be the presence of impurities or surface groves and other defects in the film interfering with the motion of the grain boundaries.
3:45 PM - XX2.5
Billion-atom Synchronous Parallel Kinetic Monte Carlo Simulations of Critical 3D Ising Systems.
Enrique Martinez 2 , Jaime Marian 1 Show Abstract
2 , LANL, Los Alamos, New Mexico, United States, 1 , LLNL, Livermore, California, United States
We present results of billion-atom critical Ising systems using a novel parallel kinetic Monte Carlo algorithm. The method solves the master equation synchronously by recourse to null events that keep all processors' time clocks current in a global sense. Boundary conflicts are resolved by adopting a chessboard decomposition into non-interacting sublattices. We analyze the statistical errors and the parallel efficiency of the method, and apply it to the calculation of scale-dependent critical exponents in billion-atom 3D Ising systems. Our results are found to be in very good agreement with state-of-the-art multispin simulations.
4:30 PM - **XX2.6
Alloying-driven Phase Stability in Group-VB Transition Metals under Compression.
Alexander Landa 1 , Per Soederlind 1 Show Abstract
1 Condensed Matter and Materials Division, Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California, United States
The change in phase stability of group-VB (V, Nb, and Ta) transition metals due to pressure and alloying is explored by means of first-principles electronic-structure calculations. It is shown that under compression stabilization or destabilization of the ground-state body-centered-cubic phase of the metal is mainly dictated by the band-structure energy that correlates well with the position of the Kohn anomaly in the transverse acoustic phonon mode. The predicted position of the Kohn anomaly in V, Nb, and Ta is found to be in a good agreement with data from the inelastic x-ray or neutron-scattering measurements. In the case of alloying the change in phase stability is defined by the interplay between the band-structure and Madelung energies. Alloying with a small amount of a neighboring metal can either stabilize or destabilize the body-centered-cubic phase relative to low-symmetry rhombohedral phases. We show that band-structure effects determine phase stability when a particular group-VB metal is alloyed with its nearest neighbors within the same d-transition series. In this case, the neighbor with less (to the left) and more (to the right) d electrons destabilize and stabilize the body-centered-cubic phase, respectively. When V is alloyed with neighbors of a higher (4d- or 5d-) transition series, both electrostatic Madelung and band-structure energies stabilize the body-centered-cubic phase. The opposite effect (destabilization) happens when Nb or Ta is alloyed with neighbors of the 3d-transition series. This surprising prediction invalidates current understanding of simple d-electron bonding that dictates high-symmetry cubic and hexagonal phases. Work performed under the auspices of the US DOE by LLNL under contract No. DE-AC52-07NA27344.
5:00 PM - XX2.7
Ruthenium Thermodynamics in Nuclear Waste Glasses.
Stephane Gosse 1 , Sophie Schuller 2 , Christine Gueneau 1 Show Abstract
1 Physico-Chemistry Department, CEA, Gif-sur-Yvette France, 2 Decontamination and Conditioning Department, CEA, Bagnols-sur-Cèze France
The platinoid elements (Pd, Ru, Rh) of very low solubility in high level radioactive borosilicate glasses precipitate both under (Pd-Te, Ru-Rh, Ru) metallic particles  and (RuO2, RhO2) oxide phases during the vitrification process . Composition and microstructures of these phases can affect significantly the physico-chemical properties and the electrical or thermal conductivities during melting in an induction melting cold crucible.Several studies are undertaken at CEA [1-2] in order to point out the reactions and the chemical interactions in the liquid and viscous states between the glass matrix and the platinoids issuing from the calcinated waste. Among these studies, a thermodynamic Fission-Products database (F-P Data) is being developed on the metallic (Pd-Rh-Ru-Te) and oxide (O-Pd-Rh-Ru-Te) systems. In this work based on the CALPHAD method, the Gibbs energies of each phase is modelled in order to provide an overall thermodynamic description of the platinoid phases in nuclear waste glasses.The objective of the database is to calculate phase diagrams and thermodynamic properties. This flexible tool also enables to calculate the relative stability between metallic and oxide phases in function of the oxygen potential (RedOx equilibrium) mainly fixed by the glass frit. At this point, the Pd-Rh-Ru-Te quaternary system has been modelled, oxygen has been introduced in some sub-systems.For example, some solidification routes are also calculated for typical Pd-Rh-Ru-Te compositions of LWR spent fuels. The calculated Pd-Rh-Ru-Te solidification paths are compared with the metallic phases analysed by EDS and XRD in simplified laboratory scale glass samples. These results enable to predict the composition of the Pd-Rh-Ru-Te phases at the thermodynamic equilibrium.Furthermore, the henceforth possible consideration of the RedOx equilibria for some Rh and Ru based phases makes it possible to explain the speciation between oxide and metallic (Rh,Ru) phases partly due to the Pd-Te interaction . Structure of Pd-Te precipitates in a simulated high-level nuclear waste glassL. Galoisy, G. Calas, G. Morin, S. Pugnet, C. Fillet, 1998J. Mater. Research, Vol. 13, N°. 5 Behaviour of ruthenium dioxide particles in borosilicate glasses and melts Rachel Pflieger, Leila Lefebvre, Mohammed Malki, Mathieu Allix, Agnès Grandjean, 2009J. Nuclear Mater, Vol. 389, N°3 Redox behavior of platinum-group metals in nuclear glassO. Pinet, S. Mure, 2009Journal of Non-Crystalline Solids, Vol. 355, pp. 221-227
5:15 PM - XX2.8
Thermodynamics of a Colloidal Phase Nanoparticles with Changes in Wetting Properties: 1. Phase Equilibria.
Seok Joon Kwon 1 , T. Alan Hatton 1 Show Abstract
1 Chemical Engineering, MIT, Cambridge, Massachusetts, United States
Theoretical study on the thermodynamic phenomena of a colloidal dispersions of nanoparticles (CNPs) in a solvent was presented. For the thermodynamic analysis, approximated lattice model as was used in the regular solution theory with different wetting conditions of NPs (partial and complete wetting conditions) in the solvent was employed. In the analysis, effects given by the long-range interaction among NPs were also considered and discussed from the point of view of interaction parameter that is responsible for the aspect of enthalpy of mixing. By calculating and comparing energy diagrams of single and binary phases with different wetting conditions, namely partial and complete wetting, we constructed phase diagrams considering size effects of the NPs and interaction parameters. In the phase diagrams, we found four and seven different phases for relatively small and large compositional fluctuation, respectively. Spinodal decomposition under small fluctuation leads to phase separation with different wetting condition. Seven phase found in the case of large fluctuation involves single and metastable phases with different wetting conditions, decomposed phases by instability, and hybridized metastable phases such as coexistence of single and binary phases having different wetting conditions each other. We also calculated the effective upper critical point for the phase separation obtained in the constructed phase diagrams, and found that it is determined by size and interaction parameter. Theoretical analysis provided here can provide a substantial benefit for the research on a variety of dispersions of NPs in polar or organic solvents.
5:30 PM - XX2.9
Elastic Strain Model for Formation of Nanostructures in YBCO Thin Film.
Jack Shi 1 , Judy Wu 1 Show Abstract
1 Physics & Astronomy, University of Kansas, Lawrence, Kansas, United States
A theoretical model on the formation of nanostructures in YBCO films has been developed based on the elastic strain theory. In this model, the surface interaction at an interface between two materials was modeled by a discontinuity of the tangential strain at the interface that equals the lattice mismatch between the two materials. Since the lattice mismatch is treated locally at an interface, in this model, we were able to consider both lattice mismatches between YBCO and dopant and between YBCO and substrate. With the experimentally observed configurations of BZO (BaZrO3) or BSO (BaSnO3) nanorods in YBCO film, we were able to solve the equilibrium strain and calculate the elastic energy analytically. The predictions of this model such as the energy-preferred configuration of the nenorods and the deformation of the YBCO lattice agree well with experimental results. In the case of non-vicinal STO substrate, the calculation showed that the nanorods are preferably aligned along the c-axis due to the anisotropy of the lattice mismatches between YBCO and BZO and the anisotropy in the YBCO elastic constants. In the case of the vicinal substrate, the calculation suggested that the effect of the vicinal substrate on the YBCO lattice is similar to that of a mismatched substrate, i.e. the ab-plane of the YBCO lattice is under tensile strain through a coherent interface with the inclined surface of the vicinal substrate. The tensile strain in the ab-plane results in a compressive strain or reduced lattice constant in the c axis of YBCO. Such characteristics of the interface between the YBCO film and vicinal substrate have been confirmed by a HRTEM study of the interface. For the configuration of the BZO nanorods in YBCO on the vicinal STO substrate, the model calculation confirmed the experimental observation that there is a threshold of the vicinal angle for the onset of the transition of the nanorod orientation. The nanorods are aligned along the c-axis when the vicinal angle is below the threshold and aligned in the ab-plane when the vicinal angle is above the threshold. The calculated threshold is in a good agreement with the experimental observations.
5:45 PM - XX2.10
Thermal Coarsening in Unique Nanotwinned Copper Materials.
Thomas LaGrange 1 , Mukul Kumar 1 , Bryan Reed 1 , Andrea Hodge 2 , Troy Barbee 1 , James Belak 1 , Joel Bernier 1 , Vasily Bulatov 1 , Ming Tang 1 Show Abstract
1 Physical and Life Sciences, Condensed Matter and Materials Division, Lawrence Livermore National Laboratory, Livermore, California, United States, 2 Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, California, United States
The motivation for this study is rooted in understanding the microstructural aspects required to withstand extended periods at elevated temperature and the extreme radiation conditions present in advanced nuclear energy systems. Conventional materials lack the required microstructural stability and exhibit excessive coarsening, hardening, and swelling. Grain boundaries in nanocrystalline materials can substantially reduce this degradation by acting as highly effective sinks for point defects, but these materials tend be unstable and are amenable to thermal coarsening. However, grain boundary networks consisting of a high fraction of annealing twins and twin variant boundaries can be stabilized against thermal coarsening and other grain-boundary-mediated degradation. Thus, nanocrystalline materials consisting of a network of twinned boundaries may be resilient to thermal coarsening. This presentation will discuss thermal annealing experiments an exotic Cu microstructure developed by pulsed, high-power sputter deposition. The growth mode under pulsed deposition develops large aspect ratio columnar grains that contain a high density of fine twins oriented parallel to the substrate in the plan of the film. Scanning electron imaging was used quantify the columnar grain coarsening, and the unique NanoMegas Astar system attached FEG transmission electron microscopy was used to map changes grain orientation and grain boundary area fraction. When these nanotwinned Cu samples were annealed at high temperatures (>773K) for extended periods, coarsening of the column grains occurs but little change in the twin density is observed. The thermal stability in the presence of the nanometeric sized twins will be discussed in the context of the local crystallographic environment along the columns and the relationship with the neighboring columns.Work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory, under contract No. DE-AC52-07NA27344. It was supported by the DOE Office of Basic Energy Sciences, Division of Materials Sciences and Engineering.
Long-Qing Chen Pennsylvania State University
James Belak Lawrence Livermore National Laboratory
Heike Emmerich RWTH Aachen, Institute of Minerals Engineering (GHI)
ChrisM. Wolverton Northwestern University
XX6: Poster Sesson II
Wednesday PM, April 27, 2011
Salons 7-9 (Marriott)
1:00 AM -
XX6.1 Transferred to XX2.11
XX4: Nucleation and Twinning
Wednesday AM, April 27, 2011
Room 2016 (Moscone West)
9:15 AM - **XX4.1
Thermodynamic Phase-field Model for Systems with Multiple Components and Phases: the Possibility of Metastable Phases.
W. Craig Carter 1 , Daniel Cogswell 1 2 Show Abstract
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States, 2 Chemical Engineering, MIT, Cambridge, Massachusetts, United States
A diffuse-interface model for microstructure with an arbitrary number of components and phases was developed from basic thermodynamic and kinetic principles. The model includes a composition gradient energy to capture solute trapping and is suited for studying phenomena where the width of the interface plays an important role. The model is formalized within a variational thermodynamic framework. An inhomogeneous free energy functional is obtained from a Taylor expansion of homogeneous free energy, revealing how the interfacial properties of each phase may be specified under a mass constraint. A diffusion potential is then defined as the driving force for nonlinear diffusion away from the dilute solution limit, and a multi-obstacle barrier function is introduced to constrain phase fractions. The model was used to simulate solidification via nucleation, premelting at phase boundaries and triple junctions, the intrinsic instability of small particles, and solutal melting resulting from differing diffusivities in solid and liquid. The shape of metastable free energy surfaces is found to play an important role in microstructure evolution and may explain why some systems premelt at phase boundaries while others do not.
9:45 AM - **XX4.2
Electromechanical Phenomena in Ferroelectrics and Energy Storage Materials: Synergy of Scanning Probe Microscopy and Phase Field Modeling.
Nina Balke 1 Show Abstract
1 CNMS, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Scanning Probe Microscopy has emerged as a powerful and versatile tool for probing nanoscale phenomena in functional oxide materials. Specifically, bias-induced phase transitions which are linked to a volume change in the investigated material are perfectly suited for this characterization technique. Examples are the piezoelectric effect in ferroelectric and electrochemical strain in battery materials. Recent technical developments in the field of Scanning Probe Microscopy like Band Excitation push the resolution limit of the technique and allow obtaining a new level of insight into nanoscale materials processes and to identify the role of single defects. This also opens the possibility to use this technique for new material classes like electrode materials for Li-ion batteries which was not possible before. In this talk we present recent advantages in Scanning Probe Microscopy for bias-induced phase transitions and demonstrate how the technique is used to not only characterize but also to manipulate material properties in ferroelectric and battery-related materials. In both cases, phase-field modeling provides valuable input in signal generation mechanism and provides models to explain the observed phenomena. In this talk we try to point out the synergy between experimental and theoretical methods and comment on future developments in this field.This material is based upon work supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number ERKCC61. Part of this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. Department of Energy.
10:15 AM - XX4.3
Experimental Determination of Nucleation Rates.
Joachim Bokeloh 1 , Gerhard Wilde 1 Show Abstract
1 Institute of Materials Physics , University Muenster, Münster Germany
Upon cooling a metallic melt, the nucleation rate changes from practically zero to virtually infinite in the small range of accessible crystallization temperatures, thus leaving only a narrow temperature window for experimental as well as for computational investigations. Both, the system size and the time scale of computational studies differ from those within reach of experimental studies by several orders of magnitude. Thus, for a meaningful comparison of computational and experimental works, the nucleation rates have to be extrapolated over several orders of magnitude. For this procedure, an accurate coverage of the nucleation rate over the complete accessible range is imperative. We present here data on the liquid undercooling behavior of Nickel obtained by repeated melting and crystallization in a DTA. This method allows acquiring a statistically meaningful data set under clean and reproducible conditions, from which nucleation rates can be determined. Supported by classical isothermal nucleation rate measurements, the nucleation rate of pure Nickel was determined over a range of six orders of magnitude. Nickel was chosen as a model system because it shows high levels of undercooling and a well refined embedded atom potential is available for concurrent simulations.
10:30 AM - XX4.4
Nucleation and Successive Microstructure Evolution Involving Dislocation Dynamics via Phasefield and Phasefield Crystal Method.
Heike Emmerich 1 Show Abstract
1 , Universität Bayreuth, Bayreuth Germany
It is well known, that the mechanical material properties of a material sample after solidification are strongly tied to its microstructure structure. Nevertheless, the precise laws governing the initial stage of this structuring process, i.e. nucleation and the successive transiental microstructure evolution scenario's, are still for from being fully understood.Here we show - after a thorough overview on the phasefield method and its relation to the phase-field crystal method -that the phase field method, which originally established itself to tackle the free boundary problem given by microstructure evolution, can also be employed to investigate the energetics of heterogenous nucleation in a solidifying sample.Moreover it is demonstrated, how thephasefield crystal method can shade more light in open questions regarding a quantitative formulation of nucleation statistics to thereby simulate the phase transition phenomena in solidification from nucleation to crystallization in larger domains thoroughly.This is demonstrated for the solid-liquid phase transformation case, as well as for solid-solid phasetransitions. In the case of the latter successive microstructure evolution can come along with plastic transformations.The contribution discussed one way totreat these efficiently in the phase-field framework.
10:45 AM - XX4.5
Statistical Analysis of Retention in Phase-change Memory Based on Mesoscale Simulation of Nucleation and Growth.
Yongwoo Kwon 1 , Young-Kwan Park 1 , Moon-Hyun Yoo 1 Show Abstract
1 , Samsung Electronics Co., Ltd., Hwasung-City, Kyunggi-Do, Korea (the Republic of)
We investigate the retention statistics in phase-change memory. We employ phase-field method, an effective tool for meso-scale simulation, to model the crystallization of chalcogenide glass via nucleation and growth in conjunction with electro-thermal simulation to estimate the size of the amorphous region resulted from reset operation and its variation in a memory cell array. Our analysis shows that if the crystallization process is nucleation-dominant the statistics of the retention time results in Weibull distribution while if the process is growth-dominant it does in lognormal distribution. We perform the meso-scale retention simulation for the small amorphous volume appearing in the length scale of memory cells whose critical dimension is less than 100nm, using the nucleation and growth parameters for GeSbTe presented in literature. The resulting statistics is lognormal suggesting that the retention failure mainly occurs by the growth-dominant crystallization.
11:30 AM - XX4.6
Quantifying Crystallization Processes Observed by Nanosecond In Situ Time-resolved Transmission Electron Microscopy.
Thomas LaGrange 1 , Geoffrey Campbell 1 , David Grummon 2 , Bryan Reed 1 Show Abstract
1 Physical and Life Sciences, Condensed Matter and Materials Division, Lawrence Livermore National Laboratory, Livermore, California, United States, 2 Chemical Engineering and Materials Science, Michigan State University, E. Lansing, Michigan, United States
Often a material’s macroscopic behavior under external stimuli is described through the observation of its microstructural features and dynamical behavior. Materials models and computer simulations that are used to predict material behavior in different environments, e.g., phase transformation kinetics under high pressures and temperatures, typically require experimental data for validation or interpretation of simulated quantities. However, most materials dynamics are extremely rapid, making it difficult to capture their transient, fine-scale features, especially on the length and time scale relevant for most mesoscale models. In effort to meet the need for studying fast material processes, we have constructed a nanosecond dynamic transmission electron microscope (DTEM) at Lawrence Livermore National Laboratory to improve the temporal resolution of in-situ TEM observations. The DTEM consists of a modified JEOL 2000FX transmission electron microscope that provides access for two pulsed laser beams. One laser drives the photocathode to produce the brief electron pulse and nanosecond exposure times. The other strikes the sample, rapidly heating, for example, an amorphous NiTi film to initiate crystallization. A series of pump-probe experiments with varying time delays enable, for example, the reconstruction of the average behavior and events occurring during rapid phase transformations. This presentation will discuss how the DTEM has been used to quantify the nucleation rates and crystallization kinetics of amorphous NiTi films at temperatures far above the glass transition. Using the standard Johnson-Mehl-Avrami-Kolomogrov analysis as a compator, the observed crystallization rates under pulsed laser heating were anomalously high as compared to slow-heating DSC crystallization experiments. Either new mechanisms are active higher temperatures, or the activation barrier for the same mechanism is strongly temperature dependent in a way that low-temperature measurements cannot detect. An answer may lie in understanding how mesoscale structure correlates to temperature dependent nucleation events. That is, polyamorphic transitions and changes in mesoscale structure at high temperatures may facilitate crystallization. Perhaps, exploratory MD modeling on this subject may shed some light.Work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory, under contract No. DE-AC52-07NA27344. It was supported by the DOE Office of Basic Energy Sciences, Division of Materials Sciences and Engineering.
11:45 AM - **XX4.7
First-principles Calculations of Free Energies Along Martensitic Transformation Paths.
Vidvuds Ozolins 1 Show Abstract
1 Materials Science & Engineering, UCLA, Los Angeles, California, United States
Accurate ab initio calculations of the free energies of high-temperature solid phases present a long-standing problem in phase diagram modeling, especially for harmonically unstable phases. We show that density-functional theory molecular dynamics simulations in conjunction with thermodynamic integration over lattice strains can be used to obtain the free energy along the Bain and Burgers transformation paths. For the prototypical case of W, we predict fcc/bcc energy and entropy differences that are in excellent agreement with the CALPHAD data. For Zr, we obtain hcp/bcc entropy differences in agreement with values derived from the measured phonon dispersion. The proposed methodology will find applications in first-principles calculations of thermodynamic properties and phase diagrams of metallic alloys, as well as in constructing accurate thermodynamic models of structural phase transformations.
12:15 PM - XX4.8
MD Study of the Nucleation and Growth of Deformation Twins in Polycrystalline Tantalum.
Luis Sandoval 1 , David Richards 1 Show Abstract
1 Condensed Matter and Materials, Lawrence Livermore National Laboratory, Livermore, California, United States
Recovered samples from high strain rate experiments clearly show that twin formation serves as an important plasticity mechanism in Tantalum. Despite years of study however, the nucleation and growth mechanisms of twining are still poorly understood, especially in bcc metals. Twins are typically thought to nucleate at grain boundaries via a cooperative emission of partials after a critical value of shear stress. We have used molecular dynamics (MD) simulation to observe the nucleation and growth of twin domains from grain boundaries and grain boundary junctions in polycrystalline cells, which have been prepared as arrangements of hexagon-columnar grains. Using a Finnis-Sinclair potential, we have examined the role of strain rate, temperature and hydrostatic pressure on the kinetic phenomena, in particular the twinning threshold and twin growth rates. We discuss how kinetic parameters extracted from MD simulations help inform a multiscale strength model for Tantalum that includes both twinning and slip as deformation mechanisms in the regime of high strain rates. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-461533
12:30 PM - XX4.9
Phase-field Model of Crystal Instability under Shear: Deformation Twinning.
Taewook Heo 1 , Saswata Bhattacharya 1 , Yi Wang 1 , Shenyang Hu 2 , Xin Sun 2 , Long-Qing Chen 1 Show Abstract
1 Materials Science and Engineering, Penn State University, University Park, Pennsylvania, United States, 2 , Pacific Northwest National Lab, Richland, Washington, United States
We report a phase-field description of microstructure evolution during deformation twinning. Deformation twinning results from the thermodynamic instability of a crystal under shear. The order parameters are proportional to the shear strains defined in terms of twin plane orientations and twinning directions. Using a face-centered cubic aluminum as an example, the deformation energy as a function of shear strain is obtained using first-principle calculations. The gradient energy coefficients are fitted to the twin boundary energies along the twinning planes and to the dislocation core energies along the directions that are perpendicular to the twinning planes. The elastic strain energy of a twinned structure is included using the Khachaturyan’s elastic theory. We simulated the twinning process and microstructure evolution under a number of fixed deformation magnitudes and predicted the twinning plane orientations and microstructures. It is shown that twinning may take place through either nucleation and growth or spinodal mechanism, and the relative volume fractions of twin variants has approximate linear dependence on the magnitude of deformation strain.
XX5: Microstructure Evolution
Wednesday PM, April 27, 2011
Room 2016 (Moscone West)
2:30 PM - **XX5.1
Suppression of Phase Separation in LiFePO4 Nanoparticles During Battery Discharge.
Daniel Cogswell 1 , Peng Bai 3 1 , Martin Bazant 1 2 Show Abstract
1 Chemical Engineering, MIT, Cambridge, Massachusetts, United States, 3 Automotive Engineering, Tsinghua University, Beijing China, 2 Mathematics, MIT, Cambridge, Massachusetts, United States
Nano-particulate lithium iron phosphate (LiFePO4) is revolutionizing high rate Li-ion batteries, but its dynamical properties remain poorly understood. A unique property of LiFePO4 is its strong tendency to separate into Li-rich and Li-poor phases in equilibrium, and it is widely believed that phase separation also occurs during battery discharge. Here we examine the feasibility of phase separation during battery discharge with the formulation of a phase-field model that includes rate-limiting Butler-Volmer kinetics and anisotropic elastic strain energy. The model developed here describes evolution of lithium in an open system in contact with an infinite reservoir at fixed chemical potential. A unique aspect of electrochemical systems investigated in this work is the ability to operate at constant current by varying an external voltage. An integral constraint is introduced to enforce global conservation, resulting in a differential-algebraic evolution equation. Through stability analysis and numerical simulation, we develop a general theory of electrochemically driven phase transformations. For small applied currents, spinodal decomposition or nucleation from wetted surfaces leads to phase separation and moving phase boundaries. Above a critical current (comparable to the exchange current), the spinodal disappears, and the particle fills homogeneously. At intermediate currents, the particle behaves as a “quasi-solid solution”, which is unstable but unable to fully phase-separate before filling. The corresponding overpotentials are small, below the thermal voltage (< 25 mV), so we predict that nano-LiFePO4 does not phase separate during fast battery discharge, which may explain its superior rate capability and cycle life. We conclude that prior observations of two-phase coexistence in nano-LiFePO4 may due to ex situ phase separation, which masks in situ solid-solution behavior. Anisotropic elasticity is found to play an important role in the onset and dynamics of phase separation and is responsible for the formation of striped morphologies that have been observed experimentally.
3:00 PM - XX5.2
Phase Field Modeling of Microstructure Evolution in Thermal Barrier Coating Systems.
Karim Ahmed 1 , Jie Deng 2 , Anter El-Azab 1 Show Abstract
1 Department of Scientific Computing , Florida State University, Tallahassee, Florida, United States, 2 , Sandia National Laboratories, Livermore, California, United States
Thermal barrier coatings (TBCs) are widely used to protect superalloys from high temperature, rapid temperature transient, oxidation and corrosion in high temperature environments. Both the efficiency and durability of TBC systems are strongly dependent on its microstructure. Understanding how the microstructure evolves in TBC systems is thus important for improving the design of such systems. A computational model for microstructure evolution in thermal barrier coatings has been developed. Based on the phase-field method, this model is used to understand the high temperature sintering of thermal barrier coatings. The model accounts for different sintering mechanisms including surface, grain boundary, and volume diffusion, coupled with the elastic effects arising from the thermal expansion mismatch in thermal barrier coating system. The numerical simulation results clearly illustrate the connection between the sintering behavior of thermal barrier coatings, the underlying diffusion processes and the morphological change during the sintering. Further, the dependence of the rate of the sintering process on temperature and strain is investigated. This work was supported by the Florida Center for Advanced Aero-Propulsion (FCCAP).
3:15 PM - XX5.3
Scale-bridging Simulation on Atomistic and Mesoscopic Length Scales.
Marco Berghoff 1 , Britta Nestler 2 Show Abstract
1 Institute of Materials and Processes (IMP), Karlsruhe University of Applied Sciences, Karlsruhe Germany, 2 Institute of Reliability of Components and Systems (IZBS), Karlsruhe Institute of Technology (KIT), Karlsruhe Germany
In the present analysis we study the process of early stage solidification using three modeling methods which are the molecular dynamics, phase-field crystal and the phase-field methods. While the molecular dynamics and the phase-field crystal methods are mostly in use at the atomistic scale, the phase-field method can make meaningful predictions at the mesoscale. To demonstrate the ability of the phase-field method in applications at the atomistic scale, we conduct a comparative study of growth of pure Ni in the early stage of solidification between molecular dynamics and phase-field simulations, starting from a nuclei (5nm) resulting from the molecular dynamics simulation. For this, the data from the molecular dynamics simulations, which are discrete atomic positions, is converted to a continuous field of the phase-field order parameter differentiating the phases, using the q6q6 operation. In addition, we tailor the parameters in the phase-field model to match those used in the Molecular dynamics model with a temperature dependent specific heat capacity and latent heat, and a free energy that is fitted to the molecular dynamics potential, derived using the Embedded Atom Method (EAM F85). We then compare the volume change of the nucleus as a function of time occurring in both simulation methods. As a substitute to molecular dynamics simulations, we also used the phase-field crystal method for generating data at the atomistic scale, and compared the results with phase-field simulations. The ultimate goal is to benchmark the atomistic simulations with the phase-field method at the smaller scale and thereafter conduct phase-field simulations at the mesoscale, which is outside the realm of atomistic simulation methods.
3:30 PM - XX5.4
Phase Field Simulation of Precipitates Morphology with Dislocations under Applied Stress.
Yongsheng Li 1 , Xiao-Ling Cheng 1 , Guang Chen 1 Show Abstract
1 Materials Science and Engineering, Nanjing University of Science & Technology, Nanjing China
A phase field dynamic model was developed to investigate the effects of dislocations and applied stress on the precipitation behavior and microstructure evolution in the model binary alloy systems. The local dislocation stress field was pre-existed in the system and was calculated by the Stroh's dislocation formula with homogeneous elastic modules approximation, the applied stress was acted on the system by using the inhomogeneous elastic modules. The simulation shows that the applied tensile stress during the ageing makes the precipitates elongate, and the elongate orientation of precipitates is perpendicular to the applied strain direction for the soft precipitates even if the dislocations or dislocation walls exist. Although the precipitates nucleate at the tensile stress regions of the dislocations or dislocation walls, the orientation of local morphology at the dislocation regions is predominated by the relative magnitude of applied stress and dislocation stress. The applied stress makes the phase decomposition faster as the alloy concentration decreasing.
3:45 PM - XX5.5
The Finite Phase Field Method - A Numerical Diffuse Iinterface Approach for Microstructure Simulation with Minimized Discretization Bias.
Janin Eiken 1 Show Abstract
1 , ACCESS, Aachen Germany
Phase-field approaches, being based on a diffuse representation of the phase boundaries, have become recognized as the method of choice for space-resolved simulation of microstructure evolution both during solidification and solid-state transitions. While early phase-field approaches were always discussed in the limit of infinitesimally small values of the interface width, modern phase-field approaches for applied computation require finite values, adjustable for numerical convenience. In ptrevious studies, much work was spend to compensate the bias of the finite interface width in computations of diffusion controlled processes. However, comparatively little attention was paid to the accurate reproduction of the sharp interface solution in computations of processes controlled by interface kinetics. In order to study the numerical bias of an existing phase-field model, simple benchmarks of interface motion, driven either by curvature or by an explicitly imposed driving force, have been performed. For this purpose, the phase-field equation has been solved numerically, using the finite difference method with varying values for the interface width, the grid spacing and the time step. Comparison of the simulation results with the analytical solutions revealed significant discrepancies. It has been found that the accurateness of the kinetic behavior is mainly determined by the number of grid points used to model the finite diffuse interface profile, i.e. by the ratio of interface width and grid spacing. The targeted high accuracy has only been obtained in simulations with at least 30 interface points, a number absolutely incompatible with efficient numerical performance (allowing for about five points). To overcome the problem, a new numerical phase-field approach is proposed which a priory takes into account the finite nature of the interface width, the grid spacing and the time step, and implicitly compensates the discretization bias. Benchmarks examples prove the high accuracy of the simulation results.
4:15 PM - XX5.6
Coupled Microstructure and Compositional Changes in Irradiated Alloys.
Santosh Dubey 1 , Anter El Azab 1 Show Abstract
1 Scientific computing, Florida State University, Tallahassee, Florida, United States
We present a reaction-diffusion model which captures species redistribution in a concentrated binary alloy under irradiation. In addition to vacancies, three types of interstitial configurations are considered: AA, BB and AB. The overall model tracks the space and time evolution of three species on the lattice (A atoms, B atoms and vacancies) and various interstitial types. Atomic displacement cascades are modeled as stochastic events in space and time, with each cascade event representing a source of point defects that is localized in space and time. With this model we have quantified the formation of various self-organized compositional patterns as a function of irradiation-specific control parameters. Unlike the previous works which were more or less focused on immiscible alloys with a natural tendency to phase separation (spinodal-type instability), we have dealt with a material system in which the components have a certain degree of miscibility and showed the formation of steady state compositional patterns. Using free boundary conditions, we have also quantified segregation of alloying elements and its effect on morphological changes at the free surface. This work has been extended to include defect microstructures (voids) using phase field formulation to see the effect of segregation on phase stability and evolution of defect microstructures. This work is supported by the NEAMS program via a subcontract from Idaho National laboratory.
4:30 PM - XX5.7
A Continuum Model of Dislocation Assisted Phase Separation in Irradiated Alloys.
Jeffrey Hoyt 1 , Mikko Haataja 2 Show Abstract
1 Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada, 2 Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey, United States
Experiments have demonstrated that phase separation in irradiated alloys can occur at temperatures above the limit of stability predicted by the Cahn theory of spinodal decomposition. Furthermore a model by Enrique and Bellon has shown that, under certain irradiation conditions, alloys can exhibit a stable composition pattern rather than complete phase separation. In this work we have investigated how these observations can be explained and/or modified by the presence of mobile dislocations. A continuum model developed by Haataja and Leonard has been applied to the case of irradiated materials and the linear stability with respect to both the composition and Burgers vector density fields will be discussed. In addition, numerical simulations, which demonstrate how misfit dislocations located at the interphase boundaries can assist the spinodal decomposition process, will be presented. Finally the continuum dislocation model is applied to the observation of one dimensional pattern formation of defects during the irradiation of pure metals.
4:45 PM - XX5.8
Characterization and Statistical Modeling of the Precipitation Kinetics in the Commercial Aluminum Alloy AA5182.
Zhenshan Liu 1 , Volker Mohles 1 , Olaf Engler 2 , Guenter Gottstein 1 Show Abstract
1 , Institut fuer Metallkunde und Metallphysik, Aachen Germany, 2 , Hydro Aluminium Deutschland GmbH, Bonn Germany
Precipitation kinetics in the wrought alloy AA5182 during homogenization was investigated by various experimental methods. The constituents generated during casting were identified by energy dispersive X-ray spectroscopy (EDS) analysis. Their volume fraction was measured by optical microscopy. The size evolution of dispersoids during the heat treatment was studied in TEM. The EDS analysis shows that the dispersoids are mainly Al3Fe, Al6Mn and alpha-Al(MnFe)Si. The dispersoids number was counted from a large number of election back scatter images to yield good statistics. Electrical resistivity and thermoelectric power (TEP) measurements were performed to evaluate the matrix composition. With the above information the thermodynamics based precipitation model ClaNG was calibrated for the alloy AA5182, i.e. unknown parameters like interface energies. ClaNG is capable of describing the simultaneous nucleation, growth and coarsening of all important precipitates in multi-component systems for arbitrary heat treatments. After calibration, the model was able to predict the volume and size distribution of dispersoids and the matrix composition for varied heat treatment, as shown by comparison with literature. The predictions were used to design and optimize the heating process with respect to the microstructure of the homogenized ingot.
5:00 PM - XX5.9
Phase-field Simulations of Recrystallization in a Sintering Process.
Nayely Pannier 1 , Mathis Plapp 1 , Marcel Filoche 1 , Bernard Sapoval 1 Show Abstract
1 , cnrs/ecole polyetchnique, Palaiseau France
The light-emitting material used in compact fluorescent lamps is a porous polycrystalline ceramic material with a typical grain size in the micron range. The grains are produced from nanocrystalline star-shaped aggregates (formed by precipitation from solution) by a heat treatment which induces recrystallization and sintering, leading eventually to the formation of monocrystalline grains with smooth surfaces. It is observed that the presence of a small amount of an immiscible liquid phase dramatically alters the kinetics of this process and the resulting grain structure.To elucidate the mechanisms underlying this morphological evolution, we construct a phase-field model based on the recent multi-phase-field formalism developed by Moelans et al. We take into account the various grain orientations as well as the liquid phase and the pores, each of which is represented by a phase field. The motion of the interfaces is controlled by the transport of two species (liquid and solid). The corresponding chemical potentials act as driving forces in the phase-field equations. We include mass transport through liquid and vapor as well as along grain boundaries and interfaces. We present numerical simulations of the sintering process for various parameters and initial conditions. In particular, the influence of the quantity of liquid on the final grain shape is examined.N.Moelans,B. Blanpain, P. Wollants, Phys Rev B 78,024113 (2008)
5:15 PM - XX5.10
Phase-field Simulations of Quantum Dot Growth for Photovoltaic Applications.
Larry Aagesen 1 , Luke Lee 2 , Michael Kuo 2 , Pei-Cheng Kuo 2 , Katsuyo Thornton 1 Show Abstract
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan, United States
Quantum dots have potential uses in several types of advanced photovoltaic cell designs, such as intermediate-band cells and hot carrier cells. To maximize cell efficiency, the quantum dots must be grown with a high degree of regularity in shape, size, and ordering. Selective area epitaxy has been used to grow quantum dots with greater regularity than possible using self-assembled growth. However, the morphology of the dots, which depends on the mask size and geometry as well as deposition parameters, remains difficult to control. The phase-field method has been used to simulate selective-area epitaxy in semiconductor systems used for advanced photovoltaic designs. The phase-field model is coupled with the smoothed boundary method, which is used to specify the contact angle between the quantum dots and mask. The simulation results are compared with experimental data, and the effects of mask geometry and sizes on the dot morphology are explored.
5:30 PM - XX5.11
A Phase-field-crystal and Classical Density Functional Theory Models Using an Accurate Fitting of Two-point Correlation Function.
Nirand Pisutha-Arnond 1 , Victor Chan 1 , Mrinal Iyer 2 , Vikram Gavini 2 , Katsuyo Thornton 1 Show Abstract
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States
We present a new fitting method to describe the two-body direct correlation function used in formulating the free energy functional of the Phase Field Crystal (PFC) model as well as classical density functional theory (CDFT). This fitting method takes a form of the ratio of polynomial functions, which is able to capture details beyond the first peak of the two-body direct correlation function while retaining the numerical stability of the original, second-order fit. The applicability of this fitting method is demonstrated by simulating the liquid-to-solid transition of iron. The bulk modulus, surface energy, and solid and liquid densities are calculated using this method. The results are compared with corresponding values obtained from experiments and other PFC models.
XX6: Poster Sesson II
Wednesday PM, April 27, 2011
Salons 7-9 (Marriott)
9:00 PM - XX6.10
Improving Functional Properties of Amorphous Silicone Polymers with Simulation.
Philip Shemella 1 , Teodoro Laino 1 , Oliver Fritz 2 , Alessandro Curioni 1 Show Abstract
1 , IBM Research - Zurich, Rüschlikon Switzerland, 2 , ABB Corporate Research - Switzerland, Dättwil Switzerland
The molecular properties of poly(dimethylsiloxane) (PDMS) and other silicone polymers are readily tuned for widespread applications. For those used as high-voltage insulation materials, reduced surface hydrophobicity results in accumulated water on the material surface which then cause leakage currents. Using all-atom molecular dynamics simulations on an amorphous system of more than one million atoms, we study the molecular motion of small PDMS-based molecules that may lead to the repair of the hydrophobic surface. The contributing factors to their diffusion, namely the structural and electrostatic environment, are discussed for various molecular sizes and environmental conditions. With an understanding of molecular motion based on atomic-level properties, materials may be designed with specific molecular components for enhanced functionality.
9:00 PM - XX6.11
Multi-million Fully Atomistic Molecular Dynamics Simulations of Yarn Formation from Carbon Nanotube Forests.
Leonardo Machado 1 , Sergio Legoas 2 , Douglas Galvao 1 Show Abstract
1 Applied Physics, State University of Campinas, Campinas, Sao Paulo, Brazil, 2 Physics Department, Federal University of Roraima, Boa Vista, Roraima, Brazil
An intense experimental effort is currently being made in order to improve thetechniques for the assembly of new carbon nanotube-based materials. Among these we can mention theyarns  and sheets  fabricated from carbon nanotube forests.Despite the large amount of experimental results involving the formation of these structures, little is knownabout the detailed atomistic mechanisms that govern these processes. In this work we present preliminaryresults from multi-million fully atomistic molecular dynamics (MD) simulations for the yarn formation fromstructural models of carbon nanotube forests. The simulations were carried out using the well-known NAMDcode .The considered models consisted of vertical arrays of bundles of single and multi-walled carbon nanotubes(CNTs) deposited on silicon oxide substrates, interconnected by other CNTs of shorter lengths and/orsmaller diameters. The models were built with structural information inferred from scanning electronmicroscopy data [1,2].In the simulations, a constant force was applied to the upper part of the CNT forest and then we analyzedthe system time evolution. We observed the formation of yarns composed by nanotube bundles andconnectors. Our results show that the connectors play a crucial role as a force mediator in the yarnformation. From the obtained results was possible to estimate the critical values of forces and forestmorphology that allow the yarn formation. These results provide valuable information that can be used tooptimize the experimental conditions leading to the realization of well-formed yarns and sheets from carbonnanotube forests. K. Jiang, Q. Li and S. Fan, Nature v.419, 801 (2002). M. Zhang, S. Fang, A. A. Zakhidov, S. B. Lee, A. E. Aliev, C. D. Williams, K. R. Atkinson, R. H.Baughman, Science v.309, 1215 (2005). NAMD, http://www.ks.uiuc.edu/Research/namd/.
9:00 PM - XX6.2
Hydrogenation Effects on the Structure and Morphology of Graphene and Single-walled Carbon Nanotubes.
Andre Muniz 1 , Dimitrios Maroudas 1 Show Abstract
1 Chemical Engineering, University of Massachusetts Amherst, Amherst, Massachusetts, United States
Chemical functionalization can be used for the modification and control of chemical, mechanical, and electronic properties of graphene layers and single-walled carbon nanotubes (SWCNTs). One example is hydrogenation, achieved by the exposure of these materials to a source of atomic hydrogen (e.g., a H2 plasma). This process has been considered for hydrogen storage purposes and for the control of the band gap of these materials for applications in carbon-based electronics. Hydrogen atoms are chemisorbed onto these carbon surfaces, introducing sp3-hybridized C-C bonds in a structure originally formed by delocalized sp2 C-C bonding. This locally induced sp2-to-sp3 bonding transition causes outward displacements of carbon atoms, resulting in structural and morphological changes on the graphene layers/walls. For practical applications of this hydrogenation process, a fundamental understanding of these structural transformations is of major importance.Toward this end, in this presentation, we report results of a computational analysis of the effects of atomic hydrogen chemisorption on the structure and morphology of graphene and SWCNTs. The analysis is based on classical molecular-dynamics (MD) and Monte Carlo (MC) simulations of compositional, structural, and strain relaxation. The results demonstrate that carbon nanotubes swell upon hydrogenation, as reported in experimental studies; this SWCNT swelling depends strongly on the hydrogen surface coverage. At low surface coverages, where sp2-hybridized C atoms are predominant, the strain levels associated with swelling are negligible; a critical H coverage (around 40-50%) is required, beyond which the sp3-hybridized C atoms prevail and the corresponding strain levels start increasing linearly with H coverage. Our compositional relaxation procedure generates configurations whose arrangements of H atoms are in excellent agreement with experimental observations. Detailed structural analysis of the relaxed hydrogenated surfaces demonstrates the tendency for clustering of hydrogenated and non-hydrogenated sites; this leads to surface morphologies characterized by ripples, containing mostly hydrogenated sites, surrounded by valleys, formed by long chains of non-hydrogenated sites. These features introduce surface roughness, which depends on the degree of hydrogenation and reaches its maximum levels at intermediate values of H coverage. Our findings are used to analyze the limitations on the maximum H storage capacity of these carbon-based materials upon their exposure to an atomic H flux and to provide explanations for experimental results reported in the literature.
9:00 PM - XX6.3
Development of an Experimentally Validated Predictive Model of Morphology Evolution in Organic Photovoltaics.
Olga Wodo 1 , Debora Marques 2 , Kang Wei Chou 2 4 , Alexander Hexemer 4 , Ruipeng Li 2 3 , Detlef Smilgies 3 , Kui Zhao 2 , Rachid Sougrat 2 , Aram Amassian 2 , Baskar Ganapathysubramanian 1 Show Abstract
1 Mechanical Engineering, Iowa State University, Ames, Iowa, United States, 2 Materials Science and Engineering, Division of Physical Sciences and Engineering, King Abdullah University for Science and Technology, Thuwal Saudi Arabia, 4 , Advanced Light Source, Berkeley, California, United States, 3 , Cornell High Energy Synchrotron Source, Ithaca, New York, United States
The past decade has witnessed considerable advances in organic photovoltaic technology both from the perspective of understanding the physical aspects of the underlying processes, as well as concurrent improvement in efficiencies. Despite these significant improvements in both fundamental understanding as well as device enhancements, there still remain several potential issues that thwart wide-spread use and profitable commercial production of organic photovoltaics. One major challenge is the weak control over the manufacturing process to get tailored morphologies. Current state-of-the-art approaches to understanding morphology evolution and tailoring manufacturing process for high efficiency organic solar cells are either limited to combinatorial experimental investigation or single scale analysis. Experimental techniques, however, provide limited data for analysis (limited to final morphology and mostly to lateral cross sections). The main reasons for that are related to the difficulty of attaining high spatial resolution and to the requirement of good contrast between components. In situ analysis is further complicated by incompatibility of many traditional characterization techniques with solution processes. These challenges hinder our ability to understand and subsequently control the interaction of multiple factors affecting morphology evolution. We describe an experimentally validated predictive computational framework that models the 3D evolution of morphology during solvent-based fabrication of organic solar cells. This computational framework is envisioned as a complementary tool to experimental characterization towards understanding property-structure-process relationships in tailored polymer solar cells. In our approach we use a phase field approach to describe morphology evolution. We model evaporation-induced phase-separation in ternary systems, which consist of conjugated polymer, fullerene derivative and solvent. The model takes into account both thermodynamic (e.g. interaction parameters between components) and kinetic parameters (e.g. diffusion coefficient). The computational framework is experimentally validated for a specific class of organic photovoltaics (P3HT:PCBM:solvent). We compare the 3D morphology predictions with experimental and characterization results, including post-deposition electron tomography and in situ time-resolved synchrotron X-ray scattering measurements performed at the Cornell High Energy Synchrotron Source and the Advanced Light Source (LBNL). This validated framework is subsequently used to investigate the effect of fabrication parameters (e.g. evaporation rate) and system parameters (e.g. type of solvent, blend ratio) on the morphology during two currently competitive solvent based fabrication techniques: spin coating and drop casting.
9:00 PM - XX6.4
Virtual Experiments of Extracellular Matrix-cell Interactions of Heart Valve Tissues.
Cory Burgett 1 , Hsiao-Ying Shadow Huang 2 Show Abstract
1 Computer Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Mechanical & Aerospace Engineering, North Carolina State University, Raleigh, North Carolina, United States
The objective of this project is to develop virtual experiments to study extracellular matrix-cell interactions in heart valves. The virtual experiments employ real microstructures of valvular tissues and finite element methods to simulate biaxial loading on tissues and depict changes in collagen fiber orientations and cellular deformations. The interactive content is made possible through the use of BioTester, photomicrographs, and an open source finite element software-OOF2, developed by National Institute of Standards and Technology. The extracellular matrix-cell interactions were studied in the fields of Cell Biology and Biochemistry. However, the mechanical interactions inside extracellular matrix were overlooked due to the limitations of physical experimental apparatus. The developed virtual experiments are designed for understanding mechanical interactions in biological tissues. The result of virtual experiment illustrates how organ-level biaxial loading translates into altered tissue stress states and cellular deformations.
9:00 PM - XX6.5
Packing Polyhedra on the Nanoscale - No Shaking Required.
Michael Gruenwald 1 , Asaph Widmer-Cooper 1 2 , Phillip Geissler 1 2 Show Abstract
1 Department of Chemistry, University of California at Berkeley, Berkeley, California, United States, 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Much progress has recently been made in enumerating and characterizing dense crystal packings of non-spherical shapes. Experimental realizations of these structures, however, have been few, especially on the microscale, where such ordered arrangements might generate novel material properties. While molecules and nanoparticles can in principle be induced to organize themselves into densely packed structures, dictating their spatial relationships requires precise control of particle shape, polydispersity, interactions and driving forces. Here we show with computer simulation and experiment that silver nanoparticles in a range of polyhedral shapes can self-assemble into their densest known packings under simple gravitational driving forces. Polymer molecules in our samples, whose adsorption prevents irreversible binding between nanoparticles, also induce depletion attractions that can stabilize less dense ordered structures. In the case of octahedra, controlling polymer concentration allows us to tune between the well-known Minkowski lattice and a novel packing with complex helical motifs.
9:00 PM - XX6.6
Modeling of Bismuth Embrittlement of a Copper Substrate.
Tristan Freiling 1 , Jacob Neth 1 , Matthew Pais 2 , Joshua Gonzales 1 , Patrick Pinhero 1 Show Abstract
1 Chemical Engineering, University of Missouri, Columbia, Missouri, United States, 2 , Idaho National Laboratory, Idaho Falls, Idaho, United States
Embrittlement of polycrystalline Cu by a liquid metal embitter species, such as Bi, is a well-documented phenomena that is still relatively poorly understood. It has been shown previously that Copper suffers from a reduction in ductility at intermediate temperatures with respect to its melting point and an increase at high temperatures below its melting point.(1) This ductility trough is commonly referred to as Intermediate Temperature Embrittlement (ITE). Further reduction in ductility is observed in the presence of a known embrittler by mechanisms such as Grain Boundary Embrittlement (GBE), Solid Metal Embrittlement (SME), and Liquid Metal Embrittlement (LME).(1) All of these effects act synergistically along with an applied load to cause intergranular fracture and ultimately material failure.Finite Element Method (FEM) was used to further understand these mechanisms and to accurately predict failure of Cu as a function of temperature, exposure time, Bi concentration, and applied load. Due to the strong dependence of intergranular fracture on the geometry of the grain structure, it is important to accurately model this structure. To accomplish this, grain sizes of Cu were determined at elevated temperatures using Scanning Electron Microscopy (SEM) and were modeled using Voronoi Tessellation and Delaunay Triangulation based on previous work(2). In addition, due to the debate(3,4) over the fundamental mechanisms of LME no quantification can be made about Cu’s loss of ductility with respect to the variables of interest. To remedy this issue, experimental load-displacement curves of Cu with varying concentrations of Bi deposited on the surface were used to calibrate a traction-separation model to predict void nucleation and coalescence. These components were then incorporated into the FEM model to refine it further. 1.Laporte, V.; Mortensen, A., Intermediate temperature embrittlement of copper alloys. International Materials Reviews 2009, 54 (2), 94-116.2.Luther, T.; Könke, C., Polycrystal models for the analysis of intergranular crack growth in metallic materials. Engineering Fracture Mechanics 2009, 76 (15), 2332-2343.3.Duscher, G.; Chisholm Matthew, F.; Alber, U.; Ruhle, M., Bismuth-induced embrittlement of copper grain boundaries. Nat Mater 2004, 3 (9), 621-6.4.Joseph, B.; Picat, M.; Barbier, F., Liquid metal embrittlement: a state-of-the-art appraisal. European Physical Journal: Applied Physics 1999, 5 (1), 19-31
9:00 PM - XX6.7
Investigation of the Thermodynamic Factor of Diffusion Coefficient for Lithium Ion Migration in Lithium Titanium Dioxide.
Zheng Liang 1 , Guangsha Shi 1 , Xu Lu 2 Show Abstract
1 Material Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Electrical Engineering , University of Michigan, Ann Arbor, Michigan, United States
The chemical diffusion coefficient for the Li-ion diffusion within the host crystal structure typically depends on the overall Li concentration. This chemical diffusion coefficient can be factored as a product of a self-diffusion coefficient D*, and a thermodynamic factor θ, according to D =θD*. The thermodynamic factor serves as a measure for the deviation of the Li chemical potential from thermodynamic ideality. It emerges when deriving Fick's first law from the more rigorous phenomenological flux expressions from irreversible thermodynamics that relate the Li flux, not to a gradient in concentration but to a gradient in chemical potential. The thermodynamic factor for interstitial diffusion can be conveniently expressed as a function of Li chemical potential μ, absolute temperature T, and Li concentration x (the fraction of interstitial Li sites within the host occupied by Li) derived from our model.Curve of θ is plotted as a function of lithium concentration at a given temperature manually from taking numerical derivatives. Also, this quantity can be obtained from grand canonical monte carlo simulation as the fluctuation of the number of lithium ions within a reference volume (i.e. in a monte carlo cell). Both the two methods agree with each other and we can gain further information from the thermodynamic factor – Li concentration curve.
9:00 PM - XX6.8
Novel Model for the Dendritic Growth of Monolayer in Dip Pen Nanolithography.
Hyojeong Kim 1 , Joonkyung Jang 1 Show Abstract
1 Nanomaterials Engineering, Pusan National University, Miryang Korea (the Republic of)
The dendritic growth of the self-assembly monolayer (SAM) has been studied for researchers in many areas, especially the epitaxial film growth . Using dip pen nanolithography (DPN) , P. Manandhar et al.  reported that a dendritic monolayer of organic molecules can be grown. In DPN, an atomic force microscopy (AFM) tip covered with ink molecules approaches the substrate, and the ink molecules transport from the tip to the substrate. A micrometer sized dendrite in the DPN of dodecylamine ink molecules  is in stark contrast to the isotropic and circular monolayer obtained in conventional DPN with alkanethiol ink molecules on the gold substrate. We suggest a simplistic random walk (RW) model for a dendritic SAM growth in DPN. In our RW model, we combine two different types of the previous suggested models – hopping down and serial pushing . The hopping down model assumes the interaction between the ink and the substrate is so strong that a molecule moves on top of the other molecules already adsorbed on the substrate only when reaches until the edge of the bottom layer. By contrast, the serial pushing model assumes a molecule moves when it is pushed by its neighbor on the substrate. We introduce the concept of directional coherence length in serial pushing which is the number of push-induced movements in the same direction. Due to a large directional coherence length, the SAM makes self-developing branches that are characteristic of a dendritic growth. Applying random Poisson process to the directional coherence length, our model reproduces various patterns observed in the previous MD simulation , such as a circle and hexagon with or without branches. Using a nonlinear fitting method, we characterized the molecular deposition in a molecular dynamics (MD) simulation. The molecular deposition in the MD simulation is found to be a random Poisson process. By applying the directional coherence length and the random Poisson deposition, we could reproduce a MD simulation  for the DPN of nonpolar molecules on a gold surface. It shows that the pushing mechanism of our RW model accurately explains the molecular motion of MD simulation.References P. Manandhar, J. Jang, G. C. Schatz, M. A. Ratner, S. Hong : Phys. Rev. Lett. 90,(2003) 115505. C. A. Mirkin : ACS Nano 1 (2007) 79. H. Kim, J. Jang : J. Phys. Chem. A 113 (2009) 4313. D. M. Heo, M. Yang, S. Hwang, J. Jang : J. Phys. Chem. C 112 (2008) 8791.
9:00 PM - XX6.9
Random Walk Model for the Growth of a Monolayer in Dip Pen Nanolithography.
Joonkyung Jang 1 Show Abstract
1 , Pusan National University, Miryang Korea (the Republic of)
We present a random walk (RW) model for monolayer growth in dip pen nanolithography (DPN). The monolayer in the RW model grows via a combination of hopping down and serial pushing of molecules deposited from the tip. The directional coherence in pushing induces branches of a monolayer that grow in preferential directions that are determined by the underlying lattice for the surface. The RW model accurately reproduces a molecular dynamics (MD) simulation for the DPN of nonpolar molecules on gold-like surfaces. The MD simulation shows the molecular deposition from the tip can be well described by a random Poisson process. The high directional coherence produces self-replicating branches in the monolayer that are characteristic of dendritic growth. With a change in directional coherence, our RW model produces diverse structures such as circles, hexagons, or dendrites.
Long-Qing Chen Pennsylvania State University
James Belak Lawrence Livermore National Laboratory
Heike Emmerich RWTH Aachen, Institute of Minerals Engineering (GHI)
ChrisM. Wolverton Northwestern University
XX7: Grain Boundary Migration
Thursday AM, April 28, 2011
Room 2016 (Moscone West)
9:30 AM - **XX7.1
Structural Disordering and Phase Transitions in Hot Grain Boundaries: Insights from Molecular Dynamics and Phase-field-crystal Simulations.
Mark Asta 1 3 , David Olmsted 1 , Saryu Fensin 2 , Dorel Buta 3 , Ari Adland 4 , Alain Karma 4 , Jeff Hoyt 5 Show Abstract
1 Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States, 3 Chemical Engineering and Materials Science, University of California at Davis, Davis, California, United States, 2 MST-8, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 4 Department of Physics and Center for Interdisciplinary Research, Northeastern University, Boston, Massachusetts, United States, 5 Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada
At high homologous temperatures the atomic structure of a grain boundary often displays pronounced structural disorder. In some cases this can involve "premelting" - the formation of nanometer-scale intergranular films with liquid like properties below the bulk melting point. The interfacial thermodynamic driving forces underlying grain boundary premelting are understood to be an important factor influencing grain coalescence behavior and microstructural evolution during the late stages of solidification, as they can give rise to a repulsive “disjoining pressure” which hinders the coalescence of two misoriented grains at nanometer-scale distances. We review results of recent molecular-dynamics and phase-field-crystal simulation studies investigating the structure and thermodynamic properties of high-temperature grain boundaries in model fcc and bcc metals. We describe a method for characterizing the structural contributions to the disjoining potential by equilibrium molecular-dynamics simulations, and present results describing a new type of "dislocation pariting" structural phase transition that competes with premelting at high homologous temperatures. The pairing transition involves coalescence of grain boundary dislocations and its formation is observed to prevent premelting at misorientation angles where it would otherwise be expected. This research is supported by the US Department of Energy, Office of Basic Energy Sciences.
10:00 AM - **XX7.2
Phase-Field Crystal Modeling of Equilibrium and Kinetic Properties of Grain Boundaries.
Alain Karma 1 , Ari Adland 1 Show Abstract
1 Physics, Northeastern University, Boston, Massachusetts, United States
This talk will survey recent progress made in using phase-field-crystal (PFC) simulations to explore a rich variety of grain boundary (GB) structures and GB motion under different driving forces as a function of both GB bicrystallography and temperature. The topics surveyed will include GB premelting, with special emphasis on its relationship to novel dislocation pairing transitions, grain boundary shearing of symmetrical and asymmetrical tilt boundaries, and grain rotation. Detailed quantitative or qualitative comparisons with atomistic simulations will be described for each topic. The need to reformulate the PFC model to correctly describe grain boundary motion associated with lattice translation will be emphasized.
10:30 AM - XX7.3
Comparing Calculated and Measured Grain Boundary Energies in FCC Metals.
Stephen Foiles 1 , Elizabeth Holm 1 , Gregory Rohrer 2 , Anthony Rollett 2 , Jia Li 2 , David Olmsted 3 Show Abstract
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 , Carnegie Mellon University, Pittsburgh, Pennsylvania, United States, 3 , University of California, Berkeley, Berkeley, California, United States
While the goal of computational simulation is to reproduce physical behavior, acquiring the experimental data to validate computational results can be difficult. The challenge is exacerbated in grain boundaries by the large crystallographic phase space that must be sampled. However, recent metallographic experiments and molecular dynamics simulations have independently produced large grain boundary energy data sets for Ni and for Al. By comparing measured and computed grain boundary energies, we find that Ni and Al behave rather differently. The Ni microstructures analyzed in experiments are highly twinned and mainly comprised of Σ3 and Σ9 boundary types. For these well-represented boundaries, we find excellent agreement between computed and measured grain boundary energies. However, for the less frequently observed boundary types, we find minimal correlation, highlighting the need for simulations to focus on experimentally relevant boundary structures. In contrast, grain boundary types are more uniformly distributed in Al, and computational and experimental grain boundary energies agree well over the range of boundary crystallography. In this system, simulations must survey a broad range of boundary types to achieve a realistic description of the boundary properties. Overall, experimental validation confirms that embedded atom method atomistic simulations of grain boundaries can reproduce grain boundary energies in FCC metals over a wide range of boundary crystallography.
10:45 AM - XX7.4
Grain Boundary Complexions and Transitions in Titania.
Shuailei Ma 1 , Naixie Zhou 2 , Jian Luo 2 , Martin Harmer 1 Show Abstract
1 Center for Advanced Materials and Nanotechnology, Lehigh University, Bethlehem, Pennsylvania, United States, 2 School of Materials Science and Engineering, Clemson University, Clemson, South Carolina, United States
With increasing temperature or doping activity, a grain boundary (GB) can undergo coupled structural and adsorption transitions, producing a series of discrete “interfacial phases.” These thermodynamically stable (or metastable) interfacial phases are named as “GB complexions”. The present work aims to validate the thermodynamically-predicted trends of complexion stability and transitions in well-controlled model experiments conducted on undoped and CuO-doped TiO2 bicrystals. Using HRTEM and aberration corrected HAADF-STEM imaging, we observed a striking example of a GB structural transition sequence upon dewetting a CuO nanolayer sandwiched between twisted TiO2 bicrystals. Specifically, the following succession of discrete GB “phases” were observed to co-exist at the same GB along the dewetting direction: an ordered CuO monolayer; a disordered CuO bilayer; a disordered CuO trilayer; and a CuO-enriched nanoscale (non-wetting) amorphous drop. The observed abrupt transitions from monolayer to bilayer, and from bilayer to trilayer, indicate that these GB transitions are first-order. This observation can be well explained with a premelting/prewetting type phenomenological thermodynamic model considering a structural oscillatory (solvation) interaction, strongly supporting the validity of the new GB complexion theory. A GB complexion (phase) diagram is constructed using a statistical interfacial thermodynamic model and computational thermodynamic methods.
11:30 AM - **XX7.5
Non-thermally Activated Grain Boundary Motion in FCC Metals.
Elizabeth Holm 1 , Eric Homer 1 , Stephen Foiles 1 , David Olmsted 2 Show Abstract
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 , University of California, Berkeley, Berkeley, California, United States
A recent, computational survey of grain boundary mobility in FCC metals revealed diverse boundary motion mechanisms over a range of temperatures. One example is a previously undescribed non-thermally activated motion, where grain boundary mobility does not follow Arrhenius behavior but instead increases as temperature decreases. A substantial minority of the 400 boundaries surveyed undergoes nonactivated motion in some temperature regime, and in extreme cases grain boundary mobility at 0.4 of the melting temperature can exceed mobility at 0.9 of the melting temperature. During nonactivated motion, boundary mobility often scales with the inverse of temperature, suggesting a phonon-damped mechanism. At a given temperature, high driving forces can nudge motion out of the activated regime and initiate nonactivated motion. Synthetic driving force molecular dynamics simulations of nonactivated boundaries elucidate how nonactivated motion depends on temperature, driving force, crystallography, and shear coupling. The implications for polycrystalline microstructural evolution will also be discussed.
12:00 PM - XX7.6
The Dynamics of Shrinking Grains: Molecular-dynamics Simulations and In-situ Electron Microscopy Studies.
David Olmsted 1 , Tamara Radetic 2 3 , Colin Ophus 2 , Ulrich Dahmen 2 , Mark Asta 4 1 Show Abstract
1 Materials Science and Engineering, University of California, Berkeley, California, United States, 2 National Center For Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Faculty of Technology and Metallurgy, University of Belgrade, Belgrade Serbia, 4 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Theoretical models and computer simulations of grain growth have shown that an island grain may rotate as it shrinks. This coupling between grain shrinkage and grain rotation can result from either coupled motion caused by motion of grain boundary dislocations, or attraction to a low-energy cusp. Here we undertake a study of the dynamics of shrinking island grains for <110> 90 degree tilt boundaries in Au, which feature higher disorientations than have been studied previously. The experimental study is based on in-situ electron-microscopy of mazed bicrystal samples, which provide a unique opportunity to investigate island grain dynamics in real time. Molecular-dynamics simulations of island grains with the same crystallography in Au have been undertaken in parallel with the experimental investigations. The combination of experiment and simulation leads to a detailed picture of the correlation between the rate of shrinking, grain shape and rotation.This research is supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Science and Engineering Division, of the US Department of Energy, under contract no. DE-AC02-05CH11231.
12:15 PM - XX7.7
Coarsening Kinetics of Grain-boundary-engineered Microstructures.
Bryan Reed 1 , Ming Tang 1 , James Belak 1 , Joel Bernier 1 , Vasily Bulatov 1 , Thomas LaGrange 1 , Mukul Kumar 1 Show Abstract
1 , Lawrence Livermore National Laboratory, Livermore, California, United States
Grain boundary engineering is a technique that increases the fraction of special boundaries in the grain boundary network to improve materials’ resistance to intergranular degradation such as cracking and corrosion. Coarsening of grain-boundary-engineered microstructures is also substantially slowed down by the presence of low-energy, low-mobility Σ boundaries. Because of the local crystallographic constraints imposed on triple junctions, the spatial distribution of similar or dissimilar boundaries is highly correlated, which has profound implications for the kinetic evolution of the network. Using simple microstructures with randomly distributed special and general boundaries in grain growth simulations thus will lead to erroneous results. To overcome this problem, we have developed a mathematical method to generate morphologically realistic microstructures that accurately capture the network correlation properties [B. W. Reed, M. Kumar, Scripta Mater. 54, 1029 (2006)]. With inputs from the algorithm, we perform phase-field simulations to study the dependence of grain boundary coarsening kinetics on the special boundary fractions and the statistical distribution of triple junctions. The effects of grain boundary energy and mobility anisotropy and links to atomistic calculations will also be discussed.This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and supported by the Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering of the U.S. Department of Energy.
12:30 PM - XX7.8
Stability and Microstructural Evolution of Grain Boundaries in Severely Deformed Metals.
Gerhard Wilde 1 , Gerrit Reglitz 1 , Harald Roesner 1 , Sergiy Divinski 1 Show Abstract
1 Institute of Materials Physics, University of Muenster, Muenster Germany
Severe plastic deformation of metals causes grain refinement due to the creation of large dislocation densities and the subsequent and / or concurrent socialization of the defects into cell- and grain boundaries. The resulting materials that have been termed “ultrafine grained materials” and that are characterized by average grain sizes in the range of a few hundred nanometers, have in some occasions shown unusual and highly desirable properties and property combinations such as increased strength and increased ductility at the same time. According to atomistic simulations and analytical modelling, so-called “non-equilibrium” grain boundaries with enhanced defect density and unique properties should be formed at the extreme strains that are involved in the respective synthesis methods. These “non-equilibrium” grain boundaries are described in the framework of extrinsic grain boundary dislocations that modify the local structure of the high angle grain boundaries and increase the excess free energy density of the interfaces, but do not contribute to the macroscopic misorientation of the adjacent grains.In our current work, detailed grain boundary diffusion studies using the radiotracer method after different thermomechanical treatment were performed to analyze the rates of atomic transport and the specific excess energy density of such grain boundaries in severely deformed pure metals like Cu, Ni, Ti and Ag. Additionally, the radiotracer analyses are combined with high resolution transmission electron microscopy and local strain field analyses with atomic resolution via geometric phase analysis to examine the stability and the time – and temperature – dependent evolution of grain boundaries in severely deformed metals. The comparison indicates that the theoretical and computational studies can capture important aspects of the properties and the evolution of such grain boundaries even quantitatively, but that they are not in agreement with the entire set of the experimentally observed behaviour. The authors gratefully acknowledge support by the Deutsche Forschungsgemeinschaft.
12:45 PM - XX7.9
Large Scale Simulations of Second-phase Evolution in Polycrystalline Structures.
Bala Radhakrishnan 1 , Sarma Gorti 1 Show Abstract
1 Computer Science and Mathematics, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Second phase coarsening by migration and coalescence plays a significant role when the second phase is in the nanoscopic length scale. Migration occurs predominantly by surface diffusion with essentially no mass transport through the bulk. Large-scale simulations of the coarsening process are carried out using a kinetic Monte Carlo technique. The simulations capture the known dependence of the migration velocity of the second phase on its size scale. Size distributions obtained using the simulations satisfy the usual lognormal function at low volume fractions. However, at large volume fractions, the size distribution changes from the lognormal at small simulation times to a distribution typical of Ostwald ripening at large simulation times, although the Ostwald ripening mechanism is completely turned off in the simulations. The reason for such a transition in the size distribution, and the implications of the findings of the simulation in practical applications such as in coarsening of supported catalysts in fuel cells and coarsening of nanoscale bubbles in irradiated nuclear fuels are discussed. Next, the coupled evolution of polycrystalline grain structure in the presence of migrating pores is simulated under isothermal conditions, and the effect of grain boundaries on second phase coarsening, and the effect of coarsening pores on grain structure evolution are quantified as a function of the grain boundary misorientation distribution. Finally, the evolution of the grain structure in the presence of nanoscale pores is simulated in the presence of a temperature gradient typically seen in nuclear fuels. For this purpose, the kinetic Monte Carlo approach is calibrated for pore migration in uranium oxide making use of published migration data from molecular dynamics simulations. The calibrated code is used to calculate the evolution of grain boundary percolation paths through the pores. The simulations are carried out in a massively parallel computing environment such that large computational domains that provide statistically significant results can be handled.
XX8: Functional Materials
Thursday PM, April 28, 2011
Room 2016 (Moscone West)
2:30 PM - **XX8.1
Elastic Heterophase Domains in Epitaxial Films and Multilayers.
Alec Roytburd 1 , Julia Slutsker 1 , Andrei Artemev 2 Show Abstract
1 Materials Science and Engineering, University of Maryland, College Park, Maryland, United States, 2 , Carleton University, Ottawa, Ontario, Canada
Theory and modeling of heterophase domain structures minimizing elastic interactions between epitaxial films and substrate are presented. The formation of polydomain films and multilayer as a result of solid-solid phase transformations of the first and second orders are discussed. It is shown that the evolution of heterophase polydomain structures under external fields is characterized by a negative compliance. Available experimental data on ferroelectrics, martensitic, and hydride-metal films are analyzed.
3:00 PM - **XX8.2
Microstructure Design of Lead-free Piezoelectric Ceramics.
Sukbin Lee 1 , Thomas Key 1 , Zhiwen Liang 1 , R. Edwin Garcia 1 , Shanling Wang 2 , Xavier Tricoche 3 , Gregory Rohrer 2 , Y. Saito 5 , C. Ito 5 , Toshihiko Tani 4 5 Show Abstract
1 Materials Engineering Department, Purdue University, West Lafayette, Indiana, United States, 2 Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States, 3 Department of Computer Science, Purdue University, West Lafayette, Indiana, United States, 5 , Toyota Central Research & Development Laboratories, Inc., Nagatuke, Aichi, Japan, 4 , Toyota Research Institute of North America, Ann Arbor, Michigan, United States
A novel methodology is presented to analyze the effect of microstructure on the macroscopic piezoelectric response for polycrystalline ceramics. The method combines position-dependent EBSD (electron backscatter diffraction) data collection and FIB (focused ion beam) micro-slicing to three-dimensionally reconstruct the morphology, crystallographic orientation, and grain boundary misorientation of every grain. Reconstructed 3D material representations of textured and untextured samples are used to generate a comprehensive numerical description that spatially resolves the polarization, elastic, and electromechanical fields. A Monte Carlo algorithm is combined with experimentally determined macroscopic MRD (multiples of a random distribution) data to assign a statistically representative polarization axis for each ferroelectric domain. The predicted macroscopic response is directly compared against experimental data and further extended to predict optimal and experimentally accessible crystallographic and ferroelectric textures.
3:30 PM - XX8.4
Surface Morphological Stabilization of Heteroepitaxial Thin-film Systems Driven by Electromigration.
Georgios Sfyris 1 , Rauf Gungor 1 , Dimitrios Maroudas 1 Show Abstract
1 Chemical Engineering, University of Massachusetts Amherst, Amherst, Massachusetts, United States
The competition between elastic strain energy and surface energy is responsible for surface morphological instabilities in stressed elastic solids; these include the well-known Asaro-Tiller or Grinfeld (ATG) instability under conditions that promote surface diffusion and the Stanski-Krastanow (SK) morphological instability in the heteroepitaxial growth of thin films on solid substrates. We have shown that applying on a stressed elastic conductor an external electric field of sufficient strength and proper direction can inhibit the ATG instability. Nevertheless, the role of surface electromigration in stabilizing the surface morphology of coherently strained epitaxial films remains largely unexplored. In this presentation, we report the results of a linear stability analysis for the electromigration-driven morphological response of an epitaxial film surface. The analysis is based on a three-dimensional model for the current-driven surface morphological evolution of a coherently strained epitaxial thin film on an elastic substrate. We consider an electrically conducting, coherently strained thin elastic film that has been grown epitaxially on an elastic substrate under the simultaneous action of an external electric field that is directed parallel to the film plane. In addition to practically infinitely thick substrates, we also consider thin substrates that are either clamped to a holder or compliant. Both the film and substrate materials are perfectly crystalline, they respond to stress according to isotropic linear elasticity in the small-displacement kinematic limit, and they have, in general, different elastic properties. The film/substrate interface is planar and fully coherent without any misfit dislocations. For such heteroepitaxial systems, the analysis addresses the morphological stability of the planar state of the epitaxial film surface. We have determined the critical electric-field strength for the stabilization of the planar epitaxial film surface morphology and the optimal applied field direction that maximizes its stabilizing effect. Furthermore, we demonstrate the effects of the substrate type and thickness on this critical field strength and elucidate the important kinetic effects of surface diffusional anisotropies on such morphological instabilities. Our analysis implies the possibility to control the onset of island formation in heteroepitaxial films by inhibiting the SK instability through the action of an external electric field in conjunction with substrate engineering. Our results generate experimentally testable hypotheses and motivate experimental measurements that can be compared directly with the theoretical predictions.
4:15 PM - **XX8.5
Crystalline to Amorphous Phase Transition in Tribocontacts.
Peter Gumbsch 1 2 , Lars Pastewka 1 , Gianpietro Moras 2 , Michael Moseler 1 Show Abstract
1 , Fraunhofer-Institut fuer Werkstoffmechanick IWM, Freiburg Germany, 2 , Karlsruhe Institute of Technology KIT, Karlsruhe Germany
Modelling friction and wear processes has to be chemically accurate to correctly describe bond breaking events and must yet be able to describe macroscopic materials degradation and materials losses in engineering applications. Our aim is to reduce the complexity of engineering systems as much as possible and therefore to study only some model systems, to demonstrate the usefulness of atomistic modelling approaches in this context. Our first approaches towards the simulation of wear processes of diamond are based on simulations employing specifically developed empirical potentials that can accurately describe bond breaking processes. It turns out that the phase transition from the crystalline solid to an amorphous diamond-like carbon (DLC) film is the key for the understanding of the polishing of diamond. Under the frictional loading, this phase transition occurs under conditions which are very far from equilibrium thermodynamics and is governed by atomic forces rather than energy landscape. The final step in the polishing of diamond corresponds to the wear or the burn-off of the DLC from the diamond surface.
4:45 PM - XX8.6
Formation of Carbon Nanostructures by Inter-layer Bonding in Bilayer Graphene and Multi-walled Carbon Nanotubes
Andre Muniz 1 , Dimitrios Maroudas 1 Show Abstract
1 Chemical Engineering, University of Massachusetts Amherst, Amherst, Massachusetts, United States
In multi-layered carbon structures, such as graphite and multi-walled carbon nanotubes (MWCNTs), inter-layer C-C bonds can be formed under high temperature and pressure, or due to exposure to ion irradiation or to a hydrogen plasma. In this presentation, we report a comprehensive computational analysis of different nanostructures that can be generated due to formation of inter-layer sp3 C-C bonds in bilayer graphene and MWCNTs upon exposure to atomic hydrogen fluxes. The analysis is based on a combination of classical molecular-dynamics (MD) simulations with first-principles density functional theory (DFT) calculations. We demonstrate that the resulting structures with inter-layer C-C bonds are stable and that their structural features are determined by the relative alignment of the adjacent graphene layers/walls in the original multi-layered/walled material. These structures may provide seeds for the nucleation of crystalline carbon phases embedded within the walls of MWCNTs; various crystalline phases can be generated, including the well-known cubic and hexagonal diamond phases, as well as new stable phases of carbon. The key parameter that determines the type and size of the generated nanocrystals is the chiral-angle difference between adjacent graphene layers/walls in the original structure. In the case of bilayer graphene with layers rotated with respect to each other (also called twisted bilayer graphene), we demonstrate the formation of a class of new nanostructures, consisting of an array of caged structures of various sizes and geometries, embedded within the graphene layers. The results of our analysis generate experimentally testable hypotheses regarding different routes for the synthesis of nanostructured carbon materials. The results also provide possible interpretations for the initial stages in the process of formation of diamond nanocrystals upon exposure of MWCNTs to hydrogen plasmas that has been reported in the literature.
5:00 PM - XX8.7
Numerical Simulation of Surface Roughness Effects on Partial-transient-liquid-phase Joining.
Christopher Bartlow 1 , Thomas Reynolds 1 , Sung Hong 1 , Andreas Glaeser 1 Show Abstract
1 Materials Science and Engineering, U.C. Berkeley, Berkeley, California, United States
Complex engineering designs incorporating ceramic components often require joining of multiple ceramic pieces or the bonding of ceramics to metallic parts. Partial-Transient-Liquid-Phase (PTLP) Joining has proven capable of rapidly creating high-temperature-capable, well-formed ceramic assemblies in which the bonded region is not strength limiting. PTLP joining involves the development of a thin liquid film between ceramic and metal surfaces that can flow to fill and eliminate potential strength-limiting interfacial flaws/defects. Successful implementation requires consideration of wetting properties involving dissimilar solids, control of surface features, and optimization of liquid-film thickness. This work focuses on the relationship between contact angles and length-scale-dependant roughness required to form strong ceramic joints. Computer simulations that allow the liquid-ceramic and liquid-metal contact angles, the inter-solid spacing, the surface roughness and the total liquid volume to be independently varied provide insight on interconnections between these material and process-dependent parameters, and the ultimate joint microstructure and properties. The ability to vary the surface roughness characteristics allows isolation of effects that hinge on the length scale. This topographical control allows quantifiable comparisons of surface roughness parameters, and the limitations of traditional roughness characterization are highlighted.
5:15 PM - XX8.8
The Mechanical Properties and Nanostructure of Silica Nanowires via Molecular Dynamics Simulations.
Lilian Davila 1 , Valerie Leppert 1 , Eduardo Bringa 2 Show Abstract
1 School of Engineering, University of California Merced, Merced, California, United States, 2 CONICET & Instituto de Ciencias Basicas, Universidad Nacional de Cuyo Mendoza, Mendoza Argentina
Inorganic nanostructures such as nanosprings, nanowires and nanorings are important morphologies of great scientific interest for future technological progress. We have focused our work on the nature and properties of silica nanowires. Nanowires have useful mechanical, electrical and optical properties that could make them useful in small-scale sensing and micro-system applications. We have performed large-scale molecular dynamics (MD) simulations to study the nature and mechanical properties of amorphous silica nanowires. The behavior of non-crystalline silica nanowires is studied using empirical interatomic potentials developed by Feuston and Garofalini. We have applied MD simulations to study the response of the silica nanowires to elevated compressive loads. We have centered our studies on the nanostructural changes occurring in the material and the correlation between the medium-range order (~10 nm), through the characteristic ring distribution of this material. Several glassy nanowires ranging in diameter from 1.4 nm to 20 nm are investigated. We also derived the elastic modulus of the nanowires from the stress-strain curves and found a distinctive dependence on nanowire diameter. For a nanowire length of ~14 nm and a diameter of ~4 nm, we do not observe any change in the amorphous structure through 25% uniaxial compression because the nanowires expand laterally to accommodate uniaxial stresses. A longer nanowire, with length of ~57 nm and diameter of ~4 nm, shows a buckling instability and reduced strength at similar strain conditions. In both cases, the ring size distribution reveals the glassy nanostructures remain essentially unaffected at elevated compressions. The ring structure and Young’s modulus for thicker nanowires, with diameters above ~6 nm and lengths of ~14 nm, increasingly resemble those typical of the bulk glass. Results are compared with recent experimental findings and theoretical predictions. Thin silica nanowire (diameter 4.3 nm) results are consistent with prior theoretical calculations using elasticity theory. This investigation contributes to an understanding of the nature of silica nanowires and their mechanical properties, influencing structure-dependent applications and design of nanoscale devices, with implications in nanotechnology, materials science, photonics and medicine.
5:30 PM - XX8.9
Interdiffusion-controlled Optical Properties in Nanocrystalline Heterostructures and Nanostructured Materials.
Luca Pavan 1 , Luca Cozzarini 1 , Vanni Lughi 1 Show Abstract
1 DI3 - Dept of Industrial and Information Engineering, University of Trieste, Trieste, TS, Italy
The interdiffusion kinetics of II-VI semiconductor nanocrystalline heterostructures have been investigated and exploited to control the optical properties of the nanocrystals themselves as well as of nanostructured materials fabricated starting from such nanocrystals. Near-spherical CdTe-CdS and CdSe-ZnSe core-shell nanocrystals (size range 3 to 8 nm) have been synthesized via colloidal approaches and then exposed to temperatures ranging between 240 and 325 °C for up to 200 minutes. The absorption spectrum has been monitored over time, as well as the nanocrystals' structure and morphology via X-Ray diffraction and Transmission Electron Microscopy. In order to correlate the wavelength at the absorption maximum with the stage of diffusion, a model has been developed assuming that the nanocrystal’s electron and hole confinement potentials could be modelled as a time-temperature-dependent combination of diffusion profiles: The model enabled calculation of the temperature-dependent interdiffusion coefficient, leading to the conclusion that surface curvature effects on the chemical potential should be taken in account when describing diffusion at the nanoscale. A novel nanostructured material consisting of CdTe nanocrystals embedded in a CdS matrix has then been synthesized. The fabrication of such material starts from colloidal nanoparticles assembled on a substrate; A thermal treatment at 250-500 °C follows, whereupon the particles are subject to diffusion. The results of this study have therefore been exploited for controlling the peculiar optoelectronic properties of such material - such as the presence of an intermediate electronic band within the matrix’s bandgap, which makes it a strong candidate for a number of applications including highly efficient photovoltaic absorber layers.