Allen, Samuel
Professor of Materials and Engineering, MIT, <smallen@mit.edu>

Baca, Adra
Member of Tech. Staff/Materials Research, AMP, Inc., <asbaca@amp.com>

Binder, Kurt
Professor, University of Mainz, Germany, <binder@chaplin.physik.uni-mainz.de>

Boyce, M.
Professor of Mechanical Engineering, MIT, <mcboyce@mit.edu>

Breedis, John
Materials Engineer, AMP, Inc., <jjbreedi@amp.com>

Cassenti, Brice
Senior Principal Engineer, United Technologies Res. Ctr., <cassentibn@utrb.utc.com>

Chang, Jim C.I.
Director, Aerospace & Mat Sci Directorate, AFOSR, Bolling AFB, DC, <jim.chang@afosr.af.mil>

Chizmeshya, Andrew
Research Scientist, Dept. of Physics, Arizona State University, <chizmesh@mrg1.la.asu.edu>

Cohen, Marvin
Professor of Physics, UC Berkeley, <cohen@jungle.berkeley.edu>

Condat, Marc
Vice Manager, Chemical Sciences Dept., Centre National de la Recherche Scientifique, <marc.condat@chrs-dir.fr>

Coronell, Dan
Section Manager, Equipment Stimulating Group, <dan_coronell@emialsps.mot.com>

Demkov, Alex
Dr. Sr Staff Scientist, Predictive Eng. Lab., Motorola Inc., <alex_demkov@email.sps.mot.com>

DePristo, Andrew E.
Dr., Interim Program Manager, Materials Science Div., DOE

Derby, James
R&D, Materials, EG&G, <jderby@egginc.com>

Fitzsimmons, Timothy
Div. of Materials Sciences, US DOE, <tim.fitzsimmons@oer.doe.gov>

Fossey, Stephen
Materials Res. Eng., US Army Natick RD&E Center, <sfossey@natick-emh2.army.mil>

Gandin, Charles André
Postdoctoral Fellow, Ecole Polytechnque Federale de Lausanne, <gandin@epfl.ch5>

Gilmer, George
Member of Technical Staff, Bell Laboratories, <ghg@lucent.com>

Grujicic, Mica
Prof., Mech. Eng., Clemson Univ., <mica@ces.clemson.edu>

Harris, John F.
Research Director, Molecular Simulations Inc., <jfh@msi.com>

Heaney, Michael B.
Sr. Staff Scientist, Raychem Corp., <mheaney@raychem.com>

Jensen, Klavs
Professor of Chemical Engineering, MIT, <kfjensen@mit.edu>

Joannopoulos, John
Professor of Physics, MIT, <joannop@mit.edu>

Kaburaki, Hideo
Principal Researcher, Group Leader, Ctr for Promotio n of Computational Sci & Eng., Japan Atomic Energy Resear ch Institute, kaburaki@sugar.tokai.jaeri.go.jp

Kai, Zhang
Development Eng., Parker Hannifin.

Kniazzeh, Alfredo
Polaroid, Waltham, <Kniazzeh@polaroid.com>

Kohn, Robert V.
Professor, Courant Institute, New York University, <kohn@cims.nyu.edu>

Kremer, Kurt
Director, Max-Planck Institute, Mainz, <kremer@th01.mpip-mainz.mpg.de>

Kriven, Waltraud (Trudy)
Professor, Mat Sci & Eng, U Illinois Urbana-Chanpaign, <w-kriven@uiuc.edu>

Kubin, Ladislas
Research Director, CNRS-ONERA, France, <kubin@onera.fr>

Lipton, Robert
Professor, Dept. of Material Sciences, Worcester Polytechnic Inst., <lipton@wpi.edu>

Mailhiot, Christian
Division Leader, Physics Dept., Lawrence Livermore Nat. Lab., <mailhiot@llnl.gov>

Martin, Georges
Research Director, CEN-Saclay, France, <martin@srmp12.saclay.cea.fr>

Marx, Klaus
FV/FLT, Robert Bosch GmbH, klaus.marx@pcm.bosch.de

Monette, Liza
Program Leader, Advanced Composite, Exxon R&E, Annan dale, NJ, <lmamone@erenj.com>

Moulin, Antoine
Laboratoire de Metallurgie Structurale, Universite P aris Sud, <amoulin@isma.isma.u-psud.fr>

Olson, Greg
Professor of Materials Science, Northwestern Univ., <olson@elmo.tech.nwu.edu>

Rafii-Tabar, Hashem
Head of Res., Nano-Sci Simul Grp, Schl Math Sci, Uni v Greenwich, UK, <H.Rafii-Tabar@gre.ac.uk>

Rice, James R.
Professor of Engineering Sciences, Harvard Univ., rice@husm.harvard.edu

Richmond, Owen

Research Director, ALCOA Technical Center, <owen.richmond@aloca.com>

Rodgers, Brendan
Chief Engineer, Goodyear Tire & Rubber Co., <usgtrvs9@ibmmail.com>

Rodin, Gregory
Assoc. Prof., Aeorspace Eng. & Eng. Mechanics, Univ. Texas at Austin, <gjr@ticam.utexas.edu>

Schmauder, Siegfried
Professor Dr. rer. nat., MPA Stuttgart, University of Stuttgart, <schmauder@mpa.uni-stuttgart-de>

Suresh, Subra Professor of Materials Science and Engineering, MIT, <ssuresh@mit.edu>

Suter, Ulrich W.
Professor of Polymer Science, ETH Zurich, suter@ifp.mat.ethz.ch

Sutton, Adrian
Department of Materials, Oxford Univ., <adrian.sutton@materials.oxford.ac.uk>

Tadmor, Ellad
Postdoc, Div. of Eng & Applied Sci, Harvard, Gordon McKay Laboratory, <tadmore@cmt.harvard.edu>

Tang, Meijie
Physicist, H-Div./P&ST, Livermore NL, <meijie@llnl.gov>

Tasaki, Ken
Ph.D., Chief Scientist, Mitsubishi Chemical America, <tasaki@mcaca.com>

Tewary, Vinod
Physicist, Materials Reliability Div., NIST, <tewary@boulder.nist.gov>

Thomas, Edwin
Professor, DMSE, MIT, <elt@mit.edu>

Vanderbilt, David
Professor pf Physics, Rutgers Univ., <dhv@physics.rutgers.edu>

Visintainer, Jim
Dr., R&D Assoc., Goodyear Tire & Rubber Co.

Wang, Cai-Zhuang
Physicist, Iowa State U & DOE, <wangcz@ameslab.gov>

Wang Jian-Sheng
Research Scientist, Div. Eng. & Applied Science, Har vard, <wang@husm.harvard.edu>

Williams, James C.
<jim.c.williams@ae.ge.com>

Wolf, Dieter
Senior Scientist, Argonne National Laboratory, <dieter_wolf@qmgate.anl.gov>

Wong, Channy
SMTS, Sandia National Lab, <ccwong@sandia.gov>

Wu, Shi-Yu
Professor, Dept. of Physics, University of Louisville, <sywu0001@capella.physics.louisville.edu>

Zhang, Shuguang
Principal Res. Sci., Biology, MIT

Zhu, Jing
Physicist, H-Div., Livermore NL, <zhu1@llnl.gov>

Abstracts

Plenary Talks:

Many of the constitutive equations which have been developed and used over the past 10-15 years consist primarily of sets of algebraic and quasilinear ordinary differential equations for particular alloys with time as independent variable and various dependent variables representing averaged aspects of microstructure, both physical and chemical. These constitutive equations are combined with the classical conservation laws to simulate spaciotemporal behavior of material products and processes.

In the past few years we have begun to try to link coupon-scale behavior to atomic-scale behavior through multiscale models, and to represent temporal and spaciotemporal complexity of behavior (particularly plastic flow instabilities) by refinements of the constitutive models and attention to instabilities resulting from nonlinearities. Examples will be described of the effects of plastic nonnormality and dynamic strain aging on plastic flow instabilities.

In order to predict and optimize the grain structures formed in investment cast parts, a new three-dimensional (3D) Cellular Automaton -Finite Element (CAFE) model has been developed. Such a model not only takes into account the crystallographic orientation of the grains, but also the growth kinetics of the dendrites and the heterogeneous nucleation of grains at the inner surface of the mold or within the bulk of the liquid. All these microscopic phenomena calculated at the scale of the fine grid of the CA are coupled with heat flow computations based upon the coarser mesh of the model.

As will be shown, this 3D CAFE model is able to predict, for given geometries and casting conditions, the formation of stray crystals, the transitions from columnar to equiaxed grain morphologies, the grain competition and the evolution of the grain texture in the columnar zone.

Starting with the basic concepts, this presentation will also show some of the latest developments: two dimensional (2D) modeling of grain movement and its effect on the final macrostructures.

Abstract: We present simulations of 3D dynamic fracture which suggest that a persistent elastic wave is generated in response to a localized perturbation of a propagating crack front, e.g., by a local heterogeneity of critical fracture energy (Morrissey and Rice, EOS, Trans. AGU, 1996; also, submitted to J. Mech. Phys. Solids, 1997). The wave propagates along the moving crack front and spreads, relative to its origin point on the fractured surface, at a speed slightly below the Rayleigh speed. The simulations were done using the spectral elastodynamic methodology of Geubelle and Rice (J. Mech. Phys. Solids, 1995). They model failure by a displacement-weakening cohesive model, which corresponds in the singular crack limit to crack growth at a critical fracture energy. Confirmation that crack front waves with properties like in our simulation do exist has been provided by Ramanathan and Fisher (submitted to Phys. Rev. Let., 1997). Through a derivation based on the linearized perturbation analysis of dynamic singular tensile crack growth by Willis and Movchan (J. Mech. Phys. Solids, 1995), those authors found by numerical evaluation that a transfer function thereby introduced has a simple pole at a certain w/k ratio, corresponding to a non-dispersive wave.

Further, we show that as a consequence of these persistent waves, when a crack grows through a region of small random fluctuations in fracture energy, the variances of both the local propagation velocity and the deformed slope of the crack front increase, according to linearized perturbation theory, in direct proportion to distance of growth into the randomly heterogeneous region. That rate of disordering is more rapid than the growth of the variances with the logarithm of distance established by Perrin and Rice (ibid, 1994) for a model elastodynamic fracture theory based on a scalar wave equation. That scalar case, which shows slowly decaying (as t**-1/2) rather than persistent crack front waves, is analyzed here too. Simulations of cracking through heterogeneous toughness show that the wave effects can cause the effects of heterogeneities to cascade along wave paths so that, e.g., strong fluctuations of crack velocity, and possibly instantaneous arrest of propagation, are induced at heterogeneities which would normally be too weak to strongly affect the crack motion.

It has been found experimentally that smooth tensile fracture surfaces in glass (Wallner, Z. Physik, 1939) and tungsten (Hull and Beardmore, Int. J. Fracture Mech., 1965) can exhibit long-lived pulse markings, now called Wallner lines, produced by disturbances at the intersection of the main crack front and the specimen surface, or at internal heterogeneities. The crack front waves discussed, at least if some generalization of them exists that includes small out-of-plane perturbations of the crack front, may provide an explanation of such lines.

With the advent of scaleable parallel computers, computational approaches are being extended for providing immediate insights into the nature of fracture dynamics. We have studied the rapid brittle fracture of solids using molecular dynamics for 10-millions of atoms and finite-element continuum mechanics. We have been able to follow the crack propagation over sufficient time and distance intervals so that a comparison with experiments is feasible. Most important, we can "see" what is happening on the atomic scale.

A detailed comparison between laboratory and computer experiments demonstrates that many of the recent laboratory findings occur in our simulation experiments, one of the most intriguing being a dynamic instability of the crack tip and its associated properties. Microscopic processes have been identified, and an explanation for the limiting is discovered. The origin of the instability dynamics at the atomic level is best seen in a video of the fracture simulations. In a 100-million atom simulation, we have discovered a dynamic brittle-to-brittle transition in the rapid cleavage of fcc solids, immediately leading to the initiation of plastic failure, crack arrest and the spontaneous proliferation of dislocations. We will discuss how this problem scales to the future teraflop regime in scientific computing.

Multimedia versions of our 2D and 3D atomistic simulation studies of fracture are available via the World Wide Web:
2D fracture: http://www/almaden.ibm.com/vis/fracture/prl.html
3D fracture: http://www.tc.cornell.edu/~farid/fracture/100million

This talk reviews recent work on interfaces in polymer blends, including adsorbed block copolymers as surfactants. It is emphasized that the interfacial profile and width depends sensitively on both the lateral (L) and perpendicular (D) linear dimension of the simulation box. It is shown that simulations help to understand analogous fluctuation broadening of interfacial widths in experiment. Evidence is presented that analogous phenomena occur also for antiphase domain walls in solid ordered binary alloys.

I will describe the work Tchavdar Todorov and I have done at Oxford to model the mechanical evolution of these contacts during fracture, and our simultaneous calculations of the conductance. We explain the origin of the vast majority of the jumps in the conductance in terms of mechanical instabilities within the contact. I will also describe the very close interactions between experiment and simulation that has led to the present consensus that mechanical instabilities are indeed the principal cause of the jumps.

Finally, if time permits, I will describe our recent thinking about electromigration. This is where one asks what effect the current flow has on the mechanical evolution of the contact, rather than the effect the mechanical evolution of the contact has on the current flowing through the contact.

In this presentation, a new interdisciplinary research program at MIT on microstructure and mechanical performance of polymeric materials is discussed. A current focus of the program is the problem of toughening semi-crystalline polymers. The commercial market for semi-crystalline polymers has been expanding; but these materials exhibit both notch and temperature embrittlement. Recently, we have demonstrated the ability to tailor polymer microstructure in order to produce a super-tough semi-crystalline polymer which also exhibits an enhanced stiffness. Furthermore, this toughness is maintained at low temperatures. The microstructure tailoring was accomplished by exploiting our fundamental understanding of the underlying deformation mechanisms and the operative material length scales which govern mechanical performance. Both experimental and modelling efforts of this program will be presented. The experimental effort focuses on probing the mechanical properties (stiffness, strength, toughness) and the microstructure. The modelling effort focuses on simulating the deformation of the heterogeneous material system by incorporation of continuum material models which account for the material behavior at the crystallographic level. The experiments and the simulations show the importance of the role of the local material microstructure in producing macroscopic toughening in these heterogenous material system.

Posters:

1. The design of cathode oxides for rechargeable Li batteries using the first-principles pseudo potential method

   G. Ceder, M.K. Aydinol, A. Van der Ven
   Department of Materials Science and Engineering, Massachusetts Institute of Technology
   Abstract: In principle, the properties of a material of a given composition and structure can be determined solely from the basic laws of physics. Such "first-principles" calculations are the dream of any materials developer, as they allow one to obtain information on materials without synthesizing them. In our research on rechargeable lithium batteries, we have used computational modeling to direct the search for cathode-active materials, leading to the first realization of a "quantum engineered" high-performance material.

   By combining the first-principles pseudopotential method with basic thermodynamics we establish a clear relation between the structure, chemistry and intercalation voltage for Li in metal oxides. Contrary to conventional wisdom, which dictates that cathode oxides should contain a transition metal to charge-compensate for variation in Li+ ion concentration, first-principles computations suggest that even better properties can be achieved with p-block metals. These results have led to a set of design criteria for new, higher energy density materials. Experiments on these novel materials will also be shown.

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2. Predicting effects of alloying on ductility of MoSi₂ from first principles

   U. V. Waghmare and E. Kaxiras*, V. Bulatov**, M. S. Duesbery***
   *Department of Physics, Harvard University, **Department of Mechanical Engineering, MIT, ***Fairfax Materials Research, Inc.
   Abstract: We investigate the possibility of enhancing the ductility of MoSi₂ by sustitutional alloying, through the changes that this introduces to the relevant surface and unstable stacking fault energies. We obtain these energies using the ab-initio pseudopotential total energy method based on a conjugate gradient algorithm. Effects of V, Nb, Tc substitution for Mo, and Mg, Al, Ge, P substitution for Si are investigated. Incorporating these results in a fracture mechanics model for dislocation nucleation and the Griffith criterion for brittle failure, we predict the effects of substitutional alloying on ductility of MoSi₂ for all the substitutional elements considered.

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3. Thermoelastic modeling of complex ceramics: A non-empirical DFT approach

   A. V. G. Chizmeshya, G. H. Wolf, W. T. Petuskey
   Materials Research Science and Engineering Center, Arizona State University, Tempe, AZ 85287
   Abstract: Among the most difficult properties to extract reliably from simulations of complex ceramics is the elastic/stiffness tensor, and in particular, its pressure and temperature dependence. Large unit cells containing rare-earth and transition metal atoms often vitiate the application of ab initio methods to this problem, while simplified schemes based on empirical interaction potententials usually reproduce one physical property at the expense of betraying another. We describe an efficient but quantitative simulation approach to the calculation of thermoelastic properties that has been used extensively within our group as a counterpart to experimental investigations. The variational principle is used to determine the minimum Gibbs free energy of a crystal composed of compressible Kohn-Sham atoms/ions. Many-body effects emerge naturally from a coupling between the ionic and electronic degrees of freedom, and all salient properties, including a thermodynamically consistent finite-T phonon spectrum can be readily obtained. Elastic properties are computed from a long wavelength analysis of the finite temperature and pressure quasi-harmonic phonon spectrum. We demonstrate the methods' performance by calculating a wide range of thermal and mechanical properties of some technologically important ionic compounds including oxide perovskites, corundum, aluminum nitride and the layered perovskites KCa(2)Nb(3)O(10), K(2)Ca(3)Nb(4)O(13).

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4. First-principles study of a nanoscale friction using local orbitals in adaptive real-space coordinates

   Greg S. Smith, Normand A. Modine, Umesh Waghmare, Efthimios Kaxiras
   Department of Physics, Harvard University
   Abstract: We introduce a computational method for the efficient treatment of systems with complex structure and highly inhomogeneous electronic density distributions, in the context density functional theory. The method uses a local orbital basis represented on an adaptive real-space grid. We apply this approach to study the properties of an atomically flat interface between a nanoscale MoO₃ crystal and a MoS₂ substrate, a system found experimentally to exhibit a large friction anisotropy [1]. This is an ideal example for the study the effects of microscopic structure on macroscopic phenomena such as friction. Based on our calculations, we obtain estimates of the critical force which must be applied through an atomic force microscope in order to move the MoO_3 crystal, for different shapes of the crystal and different relative orientations with respect to the substrate.
   [1] P.E. Sheehan and C.M. Lieber, Science 272, 1158 (1996).

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5. Test of Herring's scaling laws at the nanoscale

   P. Zeng, P. C. Clapp, J. A. Rifkin
   Center for Materials Simulation, Institute of Materials Science, University of Connecticut, Storrs, CT 06269-3136
   Abstract: Molecular Dynamics techniques with Embedded Atom Method potentials has been used to study sintering in arrays of pure Cu nanofibers. The sintering studies on multi-particle arrays several hundred degrees below the melting point of pure Cu show unexpectedly large contributions from plastic deformation processes, mechanical rotations, amorphization and highly driven surface and grain boundary diffusion effects. These results strongly indicate that the standard sintering theories developed for micron scale powders do not apply at the nanoscale. A detailed test of Herring's scaling law has also been performed and it is found to fail over the entire range of sintering sizes at the nanoscale. Reasons for this failure will be offered. Computer movies will be displayed to illustrate the dynamics of the competing sintering processes.

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6. Can micron-scale sintering and grain growth theories be applied at the nanoscale?

   P. C. Clapp, P. Zeng, S. Zajac J. A. Rifkin
   Center for Materials Simulation, Institute of Materials Science, University of Connecticut, Storrs, CT 06269-3136
   Abstract: We are using Molecular Dynamics techniques with Embedded Atom Method potentials to study sintering, surface diffusion and grain boundary mobility in nanoparticle arrays. Preliminary results of the sintering studies on multi-particle arrays several hundred degrees below the melting point of pure Cu and Au show unexpectedly large contributions from plastic deformation processes, mechanical rotations and highly driven surface and grain boundary diffusion effects. These results strongly indicate that the standard sintering theory developed for micron scale powders (e. g. Ashby sintering diagrams) will have to be heavily revised, if not abandoned, before accurate predictions of nanoscale sintering kinetics will be possible. Computer movies will be displayed to illustrate the dynamics of the competing sintering processes.

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7. Nanoporous semiconductors

   A. Demkov*, and O. Sankey**
   

   *Predictive Engineering Laboratory, Motorola, Inc., Mesa, AZ
   **Department of Physics, Arizona State University

   Abstract: We investigate theoretically a new class of nanoporous semiconductor phases. First we describe novel phases of Si which we call silisils. Silisils may be thought of as zeolites without oxygen; their structures are derived by reducing the (4;2)-connected nets of zeolites to simple 4-connected nets. Two of these structures have bandgaps almost twice that of the diamond phase. These materials have unusual electronic properties, and we will discuss the latest experimental results. By reducing the (4;2)-connected nets of AlPO molecular sieves, a novel class of binary semiconductors such as GaAS is introduced. We use an ab-initio local-orbital quantum molecular-dynamics method to investigate these GaAs materials and their properties. The most important result is that the total energies of the nanoporous Si and GaAs structures are in the range of 0.1-0.2 eV/atom above the corresponding ground state structure (diamond or zinc-blende). These energy differences are significantly less than any of those for the high pressure phases.

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8. Initial stages of oxidation of silicon (001) surfaces: A case study in managing multiple scales within the DFT

   N. A. Modine, G. Zumbach, G. Smith, Efthimios Kaxiras
   Department of Physics, Harvard University
   Abstract: The oxidation of Si surfaces is a problem of central importance to electronic device applications. First-principles calculations for the oxidation of Si surfaces are challenging due to the presence of 3 length scales: short wavelengths are required to represent accurately the 2p orbitals of O, moderate length scales are associated with the Si atoms, and a comparatively low resolution is needed for efficient simulation of the interlayer vacuum region. Our recently developed Adaptive Coordinate Real-space Electronic Structure (ACRES) method [1] treats this range of length scales efficiently by using a regular mesh in curvilinear space, which is mapped by a change of coordinates to an adaptive mesh in real space. Use of parallel computing makes this approach particularly efficient from a computational point of view.

   Using the ACRES method, we study several mechanisms of incorporating a sub-monolayer coverage of oxygen into the characteristic (2X1) dimer reconstruction of the Si(001) surface. Based on our results, we propose a physically motivated two step pathway for the initial incorporation of an oxygen atom into the dimerized surface, and we explain what formerly appeared to be puzzling Ultraviolet Photoelectron Spectroscopy measurements which indicated that each initial oxygen atom saturates two dangling surface bonds.

   We also discuss how the Linear Combination of Atomic Orbitals (LCAO) approximation can be integrated easily into the ACRES framework, and we note that the freedom in boundary conditions allowed by a real-space method make ACRES an ideal starting point for linking the DFT to effective methods for the treatment of longer length scales.
   [1] N.A. Modine, G. Zumbach and E. Kaxiras, Phys. Rev. B 55, 10289 (1997).

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9. Modeling quantum effects in the Raman spectra of carbon nanotubes

   R. Saito*, T. Takeya*, T. Kimura*, G. Dresselhaus** M. S. Dresselhaus**
   *University of Electro-Communications, Tokyo, Japan, **MIT
   Abstract: Using non-resonant bond polarization theory, the Raman intensity of a single-wall carbon nanotube is calculated as a function of the polarization of light and the chirality of the carbon nanotube. The force constant tensor for calculating phonon dispersion relations in the nanotubes is scaled from those for two-dimensional graphite. The calculated Raman spectra do not depend much on the chirality while their frequencies clearly depend on the diameter. The polarization and sample orientation dependence of the Raman intensity shows that the symmetry of Raman modes can be obtained by varying the direction of the nanotube axis with fixed polarization vectors of light.

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10. Models for low dimensional thermoelectricity

    M. S. Dresselhaus (MIT), T. Koga (Harvard University), X. Sun (MIT), S. B. Cronin, (MIT), K.L. Wang, UCLA, G. Chen, UCLA

    Abstract: Enhanced ZT has been predicted theoretically for low dimensional electronic systems under appropriate experimental conditions. Enhanced ZT has been observed experimentally within 2D quantum wells of PbTe, and good agreement between theory and experiment has been obtained. The advantages of low dimensional systems for thermoelectric applications are described, and prospects for further enhancement of ZT are discussed.

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11. Intrinsic Crossover Mechanism for Thermal Conduction in Rare-Gas Crystals

    Hideo Kaburaki and Sidney Yip
    Abstract The thermal conductivity of solid Argon crystals has been calculated by the equilibrium Green-Kubo method. We derived the temperature dependence of thermal conductivity in the high temperature region by the molecular dynamics method and compared the results with experiment. We have found that the heat-flux correlation function consists of two stages and the long-time stafe disappears as the temperature approaches the melting point.

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12. Tight-binding simulation of the amorphous-crystal interface in silicon

    Noam Bernstein, Efthimios Kaxiras Michael Aziz
    Division of Engineering and Applied Sciences, Harvard University
    Abstract: We study the structural features of the interface between crystalline and amorphous Si in the (001) plane using a non-orthogonal tight-binding model. This tight-binding Hamiltonian was optimized for the types of structures and local bonding distortions expected in defective crystalline and amorphous structures as well as the transition states between metastable configurations [1]. An analysis of the energetics of the resulting interface models indicates the presence of a number of atoms near the interface that can be moved with little energetic cost. Structural features include defects such as dimerized atom pairs in the <110> chains in the predominantly crystalline regions, as well as <110> chains in the predominantly amorphous regions that lose their coherence over the distance of a few atoms. Pathways for processes leading to the repair of the defects and extension of the crystal are calculated, and found to have energy barriers in the range 1.2 -- 2.2 eV.

    [1] N. Bernstein and E. Kaxiras, MRS Symposium Proceedings, Vol. 408, p. 55, edited by E. Kaxiras, J. Joannopoulos, P. Vashishta and R.K. Kalia (Materials Research Society, Pittsburgh, 1996).

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13. Tight-binding simulation of the amorphous-crystal interface in silicon

    N. Bernstein, E. Kaxiras, M. Aziz
    Division of Engineering and Applied Science, Harvard Univ.

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14. Environment-dependent interatomic potential for bulk silicon

    Martin Z. Bazant and Efthimios Kaxiras*, Joao Justo, Vasily Bulatov, Sidney Yip**
    *Harvard University, **MIT
    Abstract: Empirical interatomic potentials are essential for the prediction of material properties because they extrapolate the results of ab initio electronic structure calculations to the larger systems needed for atomistic simulations of crystalline defects and disordered phases. Many advances in computation have been made, but the theoretical validation of this linking of scales is unsatisfactory for covalently bonded solids (such as Si, Ge and C). Although more than thirty fitted potentials have been proposed in the prototypical case of Si, realistic simulations of plastic deformation, diffusion, crystallization, melting and other important bulk phenomena are still problematic. In order to guide the the fitting process, we use analytic techniques to extract features of potentials directly from ab initio energy calculations. Elastic constant relations test models of sp2 and sp3 hybrid covalent bonding, and inversion of cohesive energy curves sheds light on the covalent to metallic transition and the nature of angular forces. These results are captured by a functional form with only a few fitting parameters which we call the Environment-Dependent Interatomic Potential (EDIP). The parameters were fitted to a large data base of ab initio results for bulk structures and point and planar defects. The fitting of EDIP for Si has already led to unprecedented transferability for bulk defects and condensed phases, and extensions to other elements and alloys may be possible.

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16. Transformation toughening of ceramic composites by transformation weakening of interphases

    W. M. Kriven, S. J. Lee, C. M. Huang, D. Zhu, Y. Xu, S. M. Mirek
    Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign Urbana, IL 61801
    Abstract: A new concept for debonding of interphases between fibers and matrix or between laminates in oxide ceramic composites is introduced. It is based on a thermal or shear stress induced phase transformation which is accompanied by a volume contraction or significant shape change, leading to microcracks. Thermally induced transformation above the critical particle particle size was demonstrated in enstatite (MgO*SiO2) due to an orthorhombic protoenstatite (PE) to monoclinic clinoenstatite (CE) transformation which is accompanied by a 5.5% volume contraction during cooling at 865 =B0C, forming intragranular microcracks. Thermal or shear stress induced transformation was also observed in (Ca, Al)-doped cristobalite (SiO2) where the cubic (b) to tetragonal (a) transformation occurs on cooling at 265 =B0C, with a ~3.2 % volume contraction. Laminates of mullite/cordierite separated by cristobalite interphases were fabricated to optimize thermal expansion mismatch. Crack propagation was observed within the transformed interphases and graceful failure was observed in 4-point flexure testing.

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17. On the role of length-scale in the prediction of failure of composite structures: Assessment and Needs

    S. Mark Spearing, Paul A. Lagace Hugh L. N. McManus
    Technology Laboratory for Advanced Composites, Department of Aeronautics and Astronautics, MIT
    Abstract: The role of modeling in the design of structural composite components against failure is discussed. Composite materials fail due to damage processes operating at several length-scales. The interactions between these processes, and between the scales at which they act, offer the principal challenges to applying mechanism-based models at structural scales beyond the ply level. A methodology is proposed to increase theefficiency of the design process, analogous to the "building block" approach, which provides a framework for integrating mechanism-based models with the current experimentally-based design process. The available models are reviewed, and their key elements identified. General concepts are illustrated via a discussion of the particular issues pertaining to notched components. Key steps needed to allow the evolution of the design process to the envisioned process are identified.

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18. Multiscale Atomistic Continuum Modelling of Crack Propagation in 2-D Metallic Plates

    H. Rafii-Tabar. Lu, Hua & M. Cross
    Abstract: A novel multiscale modelling of brittle fracture in an Ag plate with macroscopic dimensions is proposed in which the crack propagation is identified with the stochastic movement of the crack tip atom through the material. The model couples the atomistic dynamics of the crack growth at the nanoscopic scale with the continuum-based theories of fracture mechanics. The linkage is established via Ito stochastic calculus. The atomistic aspect of the modelling is based on molecular dynamics simulation method using a many-body interatomic potential. The continuum-based computations employ the finite-element. Well-known crack characteristics at the nano-scale, such as the mirror-to-mist-to-hackle transitions, are obtained as well as the stochastic trajectory of the crack propagation on the macroscopic scale.

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19. Study of the Plasticity of Silicon at a mesoscopic scale by numberical 3D simulation

    Antoine Moulin
    Abstract: Featuring an exhaustive study of the Frank-Read source emission and a study of the yield pint phenomenon.

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20. Application of the fast multipole method to Microdynamics

    Gregory Rodin
    Abstract:I will explain how the fast multipole method can be applied to large-scale three-dimensional micromechanics problems that involve a large number of interacting defects like second-phase particles or dislocations.

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23. A Mesoscopic Approach to Dislocation Mobili Mechanical Response in bcc Single Crystals

    
    Tang, Meijie, L P. Kubin and C R.
    Abstract: This work will present a method that links single dislocation properties (activation energy and mobility) to the macroscopic mechanical response in b cc single crystals. It will show that realistic dislocation behavior as well as stress vs. s train curves can be obtained, and address the credibility of extracting single dislocation pro perties from low temperature mechanical testings.

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24. Why L12 intermetallics are brittle and how to make it ductile

    Wang, Jian-Sheng
    Abstract: The origin of the intrinsic and extrinsic brittleness of L12 intermetallics are discussed and the alloying principles to improve the ductility are suggested.

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27. The size and shape of self-assembled micelles

    P.H. Nelson, T.A. Hatton, G.C. Rutledge
    Department of Chemical Engineering, MIT
    Abstract: Equilibrium size and shape distributions of self-assembled micelles are investigated using a course grained Monte Carlo simulation technique. The micellar size distributions are shown to include a Gaussian peak of spherical micelles, in combination with an exponential tail of cylindrical micelles.

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31. Time and strain dependence of the mechanical behavior of elastomers

    J. S. Bergstrom, M. C. Boyce
    Department of Mechanical Engineering, MIT
    Abstract: The mechanical behavior of elastomeric materials is known to be rate dependent and to exhibit hysteresis upon cyclic loading. Although these features of the rubbery constitutive response are well-recognized and important to its function, few models attempt to quantify these aspects of response perhaps due to the complex nature of the behavior and its apparent inconsistency with regard to current reasonably successful static models of rubber elasticity.

    By performing a careful experimental investigation, it has been possible to probe the material response to different strain histories and to find the influence of different microstructural parameters such as concentration of filler particles and number density of crosslinking sites. From the experimental data a constitutive model based on reptational relaxational motion of chain molecules has been developed. In the model, the macroscopic mechanical behavior is determined by the dynamic interaction between two networks acting in parallel: a perfect network giving the equilibrium response and an elastically `inactive' network that deforms with the perfect network during fast macroscopic deformations but, when given sufficient time, relaxes towards a lower energy state.

    By comparing the predictions from proposed model both with the experimental data and with molecular dynamics simulations we conclude that the constitutive model predicts the rate-dependence and relaxation behavior well.

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32. Molecular dynamics simulation of boundary lubricated interfaces

    S. Yim, N. Saka, N. Sonwalkar
    Department of Mechanical Engineering, MIT
    Abstract: A molecular dynamics simulation study of friction in boundary lubrication was conducted in order to investigate the atomic-scale behavior of the lubricant molecules during the sliding motion. The simulated system consisted of two silicon (001) semi-infinite substrates lubricated by a thin, three layer film of dodecane. Silicon was modeled using the Stillinger-Weber potential, and the dodecane with the Consistent Force Field potential function; a novel scheme was used to generate the silicon-dodecane interaction potentials. The simulations show that the dodecane molecules strongly prefer to adsorb into ledges on the silicon surface. The orientation of the adsorbed molecules depends heavily on the concentration of the lubricant at the Si surface, showing a tendency to stand up at high lubricant concentrations. In sliding, the dodecane layers adsorbed on the silicon surfaces behave as a solid, whereas the middle layer exhibits more liquid-like characteristics. The friction coefficient of this well-lubricated case was calculated to be 0.07, well within the range for boundary lubricated systems.

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33. Materials Processing Center (MPC) and Industry Collegium: The industry and government link to materials research at MIT

    L. C. Kimerling, G. B. Kenney, C. Reif
    Materials Processing Center, MIT
    Abstract: The Materials Processing Center (MPC) is a focal point for the 150+ member, multi-disciplinary materials community around MIT, and a bridge to domestic and international industry. MPC's purpose is to provide an environment where students and professionals from industry, government and academia can collaborate to identify and address pivotal issues in materials processing and manufacturing. Sectors of research include biomaterials, transportation, structural materials, energy, primary materials, and electronics. A proactive forum for the exchange of knowledge exists through the MPC Industry Collegium. The Collegium serves as a direct link between on-going materials research and industry needs, providing a one-on-one conduit between industry personnel and MIT faculty, staff and students. Industry partners receive special publications andaccess to focused workshops and symposia, and have the opportunity to create and promote cooperative or sponsored research programs with knowledgeable experts and students who are committed to working with industry. Collegium members may also send Visiting Scientists to participate in cooperative research at MIT.

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34. Lattice Monte Carlo simulations as link between ab-initio calculations and macroscopic behavior of dopants and defects in silicon

    Marius M. Bunea* and Scott T. Dunham**
    *Physics Department and Department of Electrical and Computer Engineering**, Boston University
    Abstract: We use recent ab-initio dopant/vacancy binding energies (Pankratov et al., Nelson et al., Ramamoorthy and Pantelides) to calculate hopping rates of vacancies for use in lattice Monte Carlo (LMC) simulations of diffusion and aggregation in silicon. The lattice Monte Carlo simulations consider the biased nature of hop frequencies in the neighborhood of dopants, with interactions up to ninth nearest neighbor distances included. We use these LMC simulations to investigate the expected macroscopic diffusion behavior, as well as the process by which dopant/defect aggregation occurs. Specific phenomena investigated include dopant fluxes in the presence of a vacancy gradient, collective phenomena leading to greatly enhanced diffusivity at high doping levels, and the time dependence of effective diffusivity due to the formation of dopant/vacancy clusters.

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