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http://repository.upenn.edu/mse_papers
Recent documents in Departmental Papers (MSE)en-usTue, 09 May 2017 08:23:56 PDT3600Monte Carlo Analysis of Stress-Directed Phase Segregation in Binary Thin Film Alloys Under Nonisothermal Annealing
http://repository.upenn.edu/mse_papers/241
http://repository.upenn.edu/mse_papers/241Wed, 25 May 2016 15:10:55 PDT
The use of patterned stress fields to direct phase separation in thin film alloys is investigated computationally with Monte Carlo simulations in which atomic interactions are represented by a Lennard-Jones potential. We show that careful design of annealing schedules based on consideration of the system phase diagram can lead to vastly enhanced patterning kinetics. In particular, by avoiding the low temperature formation of highly stable nuclei within the entire system, the kinetics of patterning are accelerated by rapid monomerdiffusion, rather than classical Ostwald ripening in which small precipitates must dissolve to feed larger ones.
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Alex M. Nieves et al.Comment on "Glide Systems and Peierls Stresses in f.c.c. and b.c.c. Metals From Phonon Energies"
http://repository.upenn.edu/mse_papers/240
http://repository.upenn.edu/mse_papers/240Wed, 25 May 2016 15:10:47 PDT
The theory of Boffi et al. of Peierls stresses in crystals is criticized on physical grounds on a number of points.
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G. Taylor et al.Local Fluctuations and Ordering in Liquid and Amorphous Metals
http://repository.upenn.edu/mse_papers/239
http://repository.upenn.edu/mse_papers/239Wed, 25 May 2016 15:10:31 PDT
A molecular-dynamics study of the structure and dynamics of monatomic liquids and glasses is presented. The local atomic structure and its development during the quenching process are analyzed in terms of fluctuations of atomic-level stresses and their correlations. This approach extends the basis for the description of the local structure from the usually employed scalar quantity, the local density fluctuation, to a tensorial quantity, the local stress fluctuation. It is shown here that the local stress fluctuations and their spatial and temporal correlations provide a detailed picture of the dynamics of the liquid and of the transition from an ideal fluid to a viscous liquid, and then to a glass. In particular, it is demonstrated that the shear stresses which are spatially uncorrelated at high temperatures become correlated below a temperature, T_{s}, which is about twice the glass transition temperature. At the same time the dynamic behavior of the liquid, characterized by the diffusivity, viscosity, and phonon states, changes sharply at this temperature. Implications of this apparent structural transition and its origin are then discussed.
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S. P. Chen et al.Radial Distribution Function and Structural Relaxation in Amorphous Solids
http://repository.upenn.edu/mse_papers/238
http://repository.upenn.edu/mse_papers/238Wed, 25 May 2016 15:10:19 PDT
A method of interpreting radial distribution functions (RDF) of amorphous metals is proposed in which the role of the local atomic structure is emphasized. It is found that the width and height of the peaks of the RDF are related to the second moment of the atomic-level hydrostatic stress distribution ⟨p^{2}⟩. The results of this analysis are then used to explain the details of the changes that occur in the RDF when structural relaxation takes place. The theoretical ▵RDF is found to be in excellent agreement with the results of a computer study and previous experimental results. It is further proposed that changes in ⟨p^{2}⟩ may be most easily accounted for in terms of changes in the density of the structural defects defined in terms of the local fluctuations in the hydrostatic stress. In this way the changes that occur in the structure of amorphous metal during structural relaxation, as represented by the RDF, may be explained in terms of the motion and annihilation of these structural defects. It is concluded that the number density of defects which could account for the observed changes in the experimental RDF is 10%. It is also found that while the hydrostatic stress distribution may be significantly changed during structural relaxation, the distribution of the atomic-level shear stresses remains unaltered.
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David J. Srolovitz et al.Many-Body Potentials and Atomic-Scale Relaxations in Noble-Metal Alloys
http://repository.upenn.edu/mse_papers/237
http://repository.upenn.edu/mse_papers/237Wed, 25 May 2016 15:10:07 PDT
We derive empirical many-body potentials for noble-metal alloy systems in the framework of the Finnis-Sinclair model [Philos. Mag. A 50, 45 (1984)] which is based on a second-moment approximation to the tight-binding density of states for transition metals [F. Cyrot, J. Phys. Chem. Solids 29, 1235 (1968)]. The most important extension of the model is a simple incorporation of interspecies interactions which involves fitting the alloying energies. The importance of properly accounting for the local atomic relaxations when constructing the potentials is emphasized. The observed principal features of the phase diagrams of the alloys are all well reproduced by this scheme. Furthermore, reasonable concentration dependences of the alloy lattice parameter and elastic constants are obtained. This leads us to suggest that fine details of the electronic structure may be less important in determining atomic structures than are more global parameters such as atomic sizes and binding energies.
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Graeme J. Ackland et al.Interatomic Forces and Atomic Structure of Grain Boundaries in Copper-Bismuth Alloys
http://repository.upenn.edu/mse_papers/236
http://repository.upenn.edu/mse_papers/236Wed, 25 May 2016 15:09:52 PDT
The many-body empirical potentials that describe atomic interactions in the copper-bismuth system were constructed using both experimental data and physical quantities obtained by ab initio full-potential linear muffin-tin orbital calculations for a metastable Cu_{3}Bi compound. These potentials were then used to calculate the structure of a grain boundary in copper containing bismuth, which was at the same time studied by high-resolution electron microscopy (HREM). Excellent agreement between the calculated and observed structures is shown by comparing a through-focal series of observed and calculated images. This agreement validates the constructed potentials, which can be used with a high confidence to investigate the structure and properties of other grain boundaries in this alloy system. Furthermore, this study shows, that HREM combined with computer modeling employing realistic empirical potentials can decipher with great accuracy the structure of boundaries containing multiple atomic species.
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Min Yan et al.Elastic Interfacial Waves in Discrete and Continuous Media
http://repository.upenn.edu/mse_papers/235
http://repository.upenn.edu/mse_papers/235Wed, 25 May 2016 15:09:37 PDT
Phonon spectra of bicrystals with relaxed grain-boundary structure display a variety of localized modes including long-wavelength acoustic modes. Continuum solutions for localized waves that incorporate atomic-level elastic properties of the interface via discontinuity relations agree well with the latter modes. In contrast, classical solutions that depend only on bulk elastic properties do not. This demonstrates that the distinct atomic structure of the interface is a controlling factor, and it is shown how local, atomic-level properties can be incorporated into continuum analyses of interfacial phenomena.
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E. S. Alber et al.<em>Ab Initio</em> Calculation of Phase Boundaries in Iron Along the bcc-fcc Transformation Path and Magnetism of Iron Overlayers
http://repository.upenn.edu/mse_papers/234
http://repository.upenn.edu/mse_papers/234Wed, 25 May 2016 15:09:28 PDT
A detailed theoretical study of magnetic behavior of iron along the bcc-fcc (Bain’s) transformation paths at various atomic volumes, using both the local spin-density approximation (LSDA) and the generalized gradient approximation (GGA), is presented. The total energies are calculated by the spin-polarized full-potential linearized augmented plane waves method and are displayed in contour plots as functions of tetragonal distortionc/aand volume; borderlines between various magnetic phases are shown. Stability of tetragonal magnetic phases of γ-Fe is discussed. The topology of phase boundaries between the ferromagnetic and antiferromagnetic phases is somewhat similar in LSDA and GGA; however, the LSDA fails to reproduce correctly the ferromagnetic bcc ground state and yields the ferromagnetic and antiferromagnetic tetragonal states at a too low volume. The calculated phase boundaries are used to predict the lattice parameters and magnetic states of iron overlayers on various (001) substrates.
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Martin Friák et al.<em>Ab Initio</em> Study of the Ideal Tensile Strength and Mechanical Stability of Transition-Metal Disilicides
http://repository.upenn.edu/mse_papers/233
http://repository.upenn.edu/mse_papers/233Wed, 25 May 2016 15:09:18 PDT
The ideal tensile test in transition metal disilicides MoSi_{2} and WSi_{2} with a C11_{b} structure is simulated by ab initio electronic structure calculations using the full-potential linearized augmented plane wave method. The theoretical tensile strength for [001] loading is determined for both disilicides and compared with that of other materials. A full relaxation of all external and one internal structural parameter is performed, and the influence of each relaxation process on energetics and strength of materials studied is investigated. Differences in the behavior of various interatomic bonds including tension-compression asymmetry are analyzed and their origin in connection with the changes of the internal structural parameter is traced. For comparison, the response of bonds in MoSi and CoSi with B2 structure to the [001] loading is also studied.
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Martin Friák et al.Bond-Order Potential for Molybdenum: Application to Dislocation Behavior
http://repository.upenn.edu/mse_papers/232
http://repository.upenn.edu/mse_papers/232Wed, 25 May 2016 15:09:07 PDT
The bond-order potential (BOP) for transition metals is a real-space semiempirical description of interactions between the atoms, which is based on the tight-binding approximation and the d-band model. This scheme provides a direct bridge between the electronic level modeling and the atomistic modeling, where the electronic degrees of freedom have been coarse grained into many-body interatomic potentials. In this paper we construct BOP in which both the attractive and the repulsive contributions to the binding energy are environmentally dependent due to both the nonorthogonality of the orbitals and the breathing of the screening charges. The construction of the BOP is described and tested in detail. First, the energies of alternative crystal structures (A15, fcc, hcp, simple cubic) are calculated and compared with those evaluated ab initio. The transferability of the BOP to atomic configurations that deviate significantly from the bcc lattice is studied by computing the energies along tetragonal, trigonal, and hexagonal transformation paths. Next, the phonon spectra are evaluated for several symmetrical crystallographic directions and compared with available experiments. All these calculations highlight the importance of directional bonding and the investigation of phonons demonstrates that the environmental dependence of the bond integrals is crucial for the phonons of the N branch not to be unphysically soft. Finally, the constructed BOP was applied in the modeling of the core structure and glide of the 1/2⟨111⟩ screw dislocation. The calculated structure of the core agrees excellently with that found in the recent ab initio calculations and the observed glide behavior not only agrees with available ab initio data but is in agreement with many experimental observations and explains the primary reason for the breakdown of the Schmid law in bcc metals.
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Matous Mrovec et al.Atomic Structure of a Grain Boundary in a Metallic Alloy: Combined Electron Microscope and Theoretical Study
http://repository.upenn.edu/mse_papers/231
http://repository.upenn.edu/mse_papers/231Wed, 25 May 2016 15:08:56 PDT
A synergistic high-resolution electron microscopy (HREM) and theoretical analysis of the structure of a grain boundary in copper containing bismuth is presented. The calculation of the structure of the boundary were carried out using N-body empirical potentials constructed using results of ab initio full-potential linear-muffin-tin-orbital calculations. Excellent agreement between the calculated and observed structures is shown by comparing a through-focal series of observed and calculated images. It is shown for the first time that HREM combined with computer modeling employing realistic empirical potentials can decipher with a great accuracy the structure of boundaries containing multiple atomic species.
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David E. Luzzi et al.Dislocation Screening and the Brittle-to-Ductile Transition: A Kosterlitz-Thouless Type Instability
http://repository.upenn.edu/mse_papers/230
http://repository.upenn.edu/mse_papers/230Wed, 25 May 2016 15:08:44 PDT
We propose a new model for the brittle-to-ductile transition based on the Kosterlitz-Thouless concept of dislocation screening. In this model, thermal fluctuations assisted by the applied stress drive the spontaneous generation of dislocations and the instability occurs well below the melting temperature. In the limit of zero stress, our model reduces to the Kosterlitz-Thouless theory of the melting transition, and, in the opposite limit of zero temperature, we obtain the Rice-Thomson result for the brittle-to-ductile transition.
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Mahadevan Khantha et al.Analytic Environment-Dependent Tight-Binding Bond Integrals: Application to <strong>MoSi<sub>2</sub></strong>
http://repository.upenn.edu/mse_papers/229
http://repository.upenn.edu/mse_papers/229Wed, 25 May 2016 15:08:35 PDT
We present the first derivation of explicit analytic expressions for the environmental dependence of the σ, π, and δ bond integrals within the orthogonal two-center tight-binding approximation by using the recently developed bond-order potential theory to invert the nonorthogonality matrix. We illustrate the power of this new formalism by showing that it not only captures the transferability of the bond integrals between elemental bcc Mo and Si and binary C11_{b} MoSi_{2} but also predicts the absence of any discontinuity between first and second nearest neighbors for the ddσ bond integral even though large discontinuities exist for ppσ, ppπ, and ddπ.
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Duc Nguyen-Manh et al.Prediction and Observation of the bcc Structure in Pure Copper at a <em><strong>Σ</strong></em>3 Grain Boundary
http://repository.upenn.edu/mse_papers/228
http://repository.upenn.edu/mse_papers/228Wed, 25 May 2016 15:08:20 PDT
We have used molecular dynamics and simulated annealing to study an asymmetrical Σ3 tilt grain boundary with ⟨211⟩ rotation axis in Cu. The boundary plane was inclined at 84° with respect to the {}(111) plane. A simple central force N-body interatomic potential was used. The most stable configuration shows a broad band of predominantly bcc structure in the boundary region. Samples of the bicrystal with the same misorientation and inclination of the boundary plane were observed in a 1250 kV transmission electron microscope, confirming the predicted structure with atomic resolution.
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C. Schmidt et al.Grain Boundary Structures in f.c.c, and b.c.c. Metals and Sites for Segregated Impurities
http://repository.upenn.edu/mse_papers/227
http://repository.upenn.edu/mse_papers/227Wed, 25 May 2016 15:08:11 PDT
The key features leading to low-energy grain boundaries in metals are discussed by reference to computer-simulated structures and geometrical analysis in terms of atom packing. Low energy is found to be associated with boundary structures consisting of relatively dense packing, and this can in turn be expressed in terms of the space-filling packing of coordination polyhedra. The geometrical method of analysis is shown to be well suited to the identification of interfacial sites for segregated impurities.
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R. C. Pond et al.Calculation of the Positions of the α- and β-bands in the Electronic Spectra of Benzenoid Hydrocarbons Using the Method of Limited Configuration Interaction
http://repository.upenn.edu/mse_papers/226
http://repository.upenn.edu/mse_papers/226Wed, 25 May 2016 15:07:58 PDT
The positions of the α- and β-bands in the electronic absorption spectra of twenty aromatic benzenoid hydrocarbons were calculated by the semiempirical method of limited configuration interaction in the π-electron approximation using the Huckel molecular orbitals. The agreement of the experimental and calculated values is good for the β-band whereas a systematic deviation is observed for the α-band. This deviation cannot be removed by extending the configuration interaction of the monoexcited states constructed from the molecular orbitals considered. However, the consideration of electronic repulsion enables us to explain the character of the dependences of the experimental excitation energies on the excitation energies obtained by the simple Huckel method of molecular orbitals. Using a suitable choice of semiempirical parameters different for various electronic transitions (showing no large mutual differences) yields semiempirical interpolation formulas for the; p-, α-, and β-bands which give very good agreement with the corresponding experimental excitation energies for the compounds studied.
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Josef Koutecký et al.Demonstration and Modeling of Multi-Bit Resistance Random Access Memory
http://repository.upenn.edu/mse_papers/225
http://repository.upenn.edu/mse_papers/225Wed, 27 Feb 2013 13:38:16 PST
Although intermediates resistance states are common in resistance random access memory (RRAM), two-way switching among them has not been demonstrated. Using a nanometallic bipolar RRAM, we have illustrated a general scheme for writing/rewriting multi-bit memory using voltage pulses. Stability conditions for accessing intermediate states have also been determined in terms of a state distribution function and the weight of serial load resistance. A multi-bit memory is shown to realize considerable space saving at a modest decrease of switching speed.
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Xiang Yang et al.Simulations and Generalized Model of the Effect of Filler Size Dispersity on Electrical Percolation in Rod Networks
http://repository.upenn.edu/mse_papers/224
http://repository.upenn.edu/mse_papers/224Wed, 27 Feb 2013 13:38:14 PST
We present a three-dimensional simulation of electrical conductivity in isotropic, polydisperse rod networks from which we determine the percolation threshold (ϕ_{c}). Existing analytical models that account for size dispersity are formulated in the slender-rod limit and are less accurate for predicting ϕ_{c} in composites with rods of modest L/D. Using empirical approximations from our simulation data, we generalized the excluded volume percolation model to account for both finite L/D and size dispersity, providing a solution for ϕ_{c} of polydisperse rod networks that is quantitatively accurate across the entire L/D range.
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Rose M. Mutiso et al.The Role of Confinement on Stress-Driven Grain Boundary Motion in Nanocrystalline Aluminum Thin Films
http://repository.upenn.edu/mse_papers/223
http://repository.upenn.edu/mse_papers/223Wed, 27 Feb 2013 13:38:13 PST
3D molecular dynamics simulations are performed to investigate the role of microstructural confinement on room temperature stress-driven grain boundary (GB) motion for a general population of GBs in nanocrystalline Al thin films. Detailed analysis and comparison with experimental results reveal how coupled GB migration and GB sliding are manifested in realistic nanoscale networks of GBs. The proximity of free surfaces to GBs plays a significant role in their mobility and results in unique surface topography evolution. We highlight the effects of microstructural features, such as triple junctions, as constraints to otherwise uninhibited GB motion. We also study the pinning effects of impurities segregated to GBs that hinder their motion. Finally, the implications of GB motion as a deformation mechanism governing the mechanical behavior of nanocrystalline materials are discussed.
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Daniel S. Gianola et al.Atomic and Electronic Structure of the BaTiO<sub>3</sub>(001) (√5×√5)<em>R</em>26.6° Surface Reconstruction
http://repository.upenn.edu/mse_papers/222
http://repository.upenn.edu/mse_papers/222Wed, 27 Feb 2013 13:38:11 PST
This contribution presents a study of the atomic and electronic structure of the (√5×√5)R26.6° surface reconstruction on BaTiO_{3} (001) formed by annealing in ultrahigh vacuum at 1300 K. Through density functional theory calculations in concert with thermodynamic analysis, we assess the stability of several BaTiO_{3} surface reconstructions and construct a phase diagram as a function of the chemical potential of the constituent elements. Using both experimental scanning tunneling microscopy (STM) and scanning tunneling spectroscopy measurements, we were able to further narrow down the candidate structures, and conclude that the surface is either TiO_{2}-Ti_{3/5}, TiO_{2}-Ti_{4/5}, or some combination, where Ti adatoms occupy hollow sites of the TiO_{2} surface. Density functional theory indicates that the defect states close to the valence band are from Ti adatom 3d orbitals (≈1.4 eV below the conduction band edge) in agreement with scanning tunneling spectroscopy measurements showing defect states 1.56±0.11 eV below the conduction band minimum (1.03±0.09 eV below the Fermi level). STM measurements show electronic contrast between empty and filled states’ images. The calculated local density of states at the surface shows that Ti 3d states below and above the Fermi level explain the difference in electronic contrast in the experimental STM images by the presence of electronically distinctive arrangements of Ti adatoms. This work provides an interesting contrast with the related oxide SrTiO_{3}, for which the (001) surface (√5×√5)R26.6° reconstruction is reported to be the TiO_{2} surface with Sr adatoms.
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John Mark P. Martirez et al.