Seminars

Prof. Nandika D'Souza
Department of Materials Science and Engineering, University of North Texas
September 4th , Friday 2-3pm, Room B155

Polymer Nanocomposites: What Lies Beneath

Polymer nanocomposites based on the dispersion of ceramic platelets and carbon nanotubes continue to be an interesting area of research. In this presentation we will present results that deviate from what an additive based reasoning would infer. In PET nanocomposites we will correlate unexpected increased permeability to interfacial phenomena at the PET-plate interface. The synthesis and consequent mechanical properties of a hybrid composite based on montmorillonite and carbon nanotube fillers in epoxy will be explored. Finally we will explore the potential for piezoresponsive sensors using carbon nanotubes and resolve an interesting conundrum based on a transition from resistive to conductive responses to stress application.

Dr. Peter V. Sushko
Department of Physics & Astronomy and London Centre for Nanotechnology
University College London, UK
September 15th , Tuesday 3-4pm, Room B155 *please note time and day*

Embedded cluster method for modeling defects in ionic and polar solids

Computer simulations of defects in solids are usually carried out using periodic and cluster models. In the first part of the talk I will briefly outline strong features and problematic issues of these models
and present a method for embedded cluster calculations of defects in ionic and ion-covalent materials. In this method, an accurate quantum-mechanical (QM) description of a part of a system, i.e. a QM
region, is combined with a lower level of theory description of the remaining part of the system (environment). I will discuss the strategies for deriving the three main components of the embedding
potential: i) long-range electrostatic potential, ii) short-range potential terminating the QM region, and iii) pair-wise and many-body inter-atomic potentials representing the environment.

In the second part of the talk I will illustrate how this method has been used to develop mechanisms of photo-stimulated processes in crystalline and disordered oxides with a few examples. In particular, I
will discuss modeling of charge trapping, atom emission, and impurity-induced modification of optical properties in alkali earth oxide nanoparticles and mechanisms of insulator-conductor transition and
polaron conductivity in a complex sub-nanoporous oxide 12CaO · 7Al₂O₃

Prof. Arun Pratap
Applied Physics Department, Maharaja Sayajirao University of Baroda, India
September 18th , Friday 2-3pm, Room B155 CANCELED

Iso-conversional versus Iso-kinetic methods of thermal analysis: Crystallization of amorphous alloys

Crystallization is a thermally activated process in non-crystalline and amorphous solids. The kinetics of the solid state phase transformations can be studied using thermal analysis techniques such as differential scanning calorimetry (DSC). For the kinetic analysis of the crystallization process under non-isothermal condition, the choice of a reliable method is very important. The methods for the analysis of non-isothermal data are, in general, derived by extending the formalism developed for isothermal conditions. Most methods for the kinetic analysis of crystallization processes rely on the iso-kinetic hypothesis to separate the kinetics of the transformation from its dependence on temperature. It is assumed that the transformation rate can be described by a differential equation separable in a (transformed fraction) and T (temperature) i.e. for continuous heating regime,  where b is the heating rate, k(T) is the rate constant, and f(a) is the kinetic function (reaction model). The Crystallization Kinetics of glasses have been studied with DSC and analysed using non-isothermal theoretical expressions.  The Avrami exponent (n), Frequency factor (A) and Activation Energy (E) of crystallization are evaluated using Matusita & Sakka (MS) and modified Kissinger equations.

Also, Isoconversional kinetic analysis has been applied to DSC data for the determination of these different kinetic parameters. The isoconversional methods calculate Ea values at progressive degrees of conversion, a without modelistic assumptions and hence this approach takes care of the variation of kinetic parameters with the fraction crystallized. The activation energy has been determined using both linear integral and differential isoconversional methods and also by the non linear isoconversional method suggested by Vyazovkin and Wight. These methods are found to give consistent results for E. A comparison has been made among various kinetic parameters obtained using different approaches to investigate the relative applicability and usefulness of the proposed methods.

Dr. Eric Chason
Division of Engineering, Brown University
September 25th , Friday 2-3pm, Room B155

Residual stress and morphology evolution in thin films: importance of non-equilibrium conditions

Thin films are often not in their equilibrium state, which is a driving force for their properties to change with time. In this talk, we will discuss results of two different studies in which the non-equilibrium
conditions modify the stress and microstructural evolution: 1) stress generation in polycrystalline films and 2) whisker formation in Sn coatings on Cu. In the case of polycrystalline films, we will review
experimental results of stress evolution under a variety of growth conditions. We will describe a simple kinetic model of competition between different mechanisms of stress generation and relaxation to
explain the results. In this picture, tensile stress is created as the grain boundary is formed and compressive stress is generated by atoms inserting themselves into the growing grain boundary. This simple model is shown to describe a large number of observations and predict the effect on stress of changing the growth conditions. In the case of whisker formation, we will show how an interfacial reaction between the Sn and Cu creates stress which acts as a driving force to extrude the whisker from the surface. These whiskers are a major reliability concern in Pb-free electronics manufacturing, and we will also describe how the presence of Pb prevents whiskers form forming. In both cases, the
studies are aided by the use of real-time in situ diagnostics that enable the kinetic evolution of the stress and morphology to be studied.

Dr. Francisco Armero
Department of Civil and Environmental Engineering, University of California at Berkeley
October 2nd, Friday 2-3pm, Room B155

Finite Elements with Embedded Discontinuities for the Modeling of Failure in Solids

The failure of solids and structures is often characterized by the appearance of discontinuous solutions of the mechanical boundary value problem, like the discontinuous displacements associated with cracks in brittle materials or with multi-scale treatments of shear bands and other localization bands in ductile failures. This situation has motivated the formulation of different techniques for the numerical resolution of these solutions. We present in this seminar several recent advances in the formulation of the so-called finite elements with embedded strong discontinuities. Exploiting the aforementioned multi-scale setting, these elements incorporate the kinematics of these highly non-smooth solutions through enhancements that are handled entirely at the element level, preserving the overall structure of the standard global mechanical/structural problem, and thus leading to efficient techniques for the numerical simulation of these failures in solids.

Specifically we discuss the formulation of finite elements incorporating high-order interpolations of the displacement jumps along the discontinuity, in both the infinitesimal and finite deformation ranges. The proposed strategy for the strain enhancement allows, in particular, the resolution of the enhanced kinematics without the characteristic overstiff response (or stress locking) that other alternatives may lead to. This strategy consists in the incorporation of the separation modes in the discrete strain field of the element, rather than the definition of a local discontinuous displacement field. A major advantage of this approach is the ability to extend it to highly complex situations, like the patterns involved in the branching of the discontinuities. In this way, we have recently formulated new finite elements that resolve locally these solutions, which we refer to simply as finite elements with embedded branching, and that are particularly appropriate in the modeling of dynamic fracture. After a discussion of these theoretical aspects, we present a series of representative numerical simulations illustrating the performance of the finite elements. We consider applications ranging from delamination in composites to ductile failures of elastoplastic solids in the quasi-static range, as well as dynamic fracture involving ductile/brittle mode transitions and crack branching in brittle materials.

Dr. Dennis M. Dimiduk
Air Force Research Laboratory
October 9th, Friday 2-3pm, Room B155

Mesoscopic Size-Effects in Microcrystal Deformation

New challenges emerge for property prediction as volume-element sizes approach those of the material structure and mechanistic processes. Thus, an important frontier in both metallurgy and mechanics is the development of a modeling framework that accurately represents length-scale effects and dislocation structure. Recent experimental studies show important intrinsic size effects exist separately from an evolving excess dislocation density at mesoscopic scales. Methods were introduced for preparing micron-scale samples via focused ion beam machining and, compression testing at room temperature using a commercially-available nanoindenter. The present studies experimentally examined a number of materials at sizes that approach those for the physical micromechanisms of deformation, but remain accessible to discrete dislocation modeling. Uniaxial compression tests of single crystals of LiF, pure Ni, Ni3Al alloys, Ni superalloys and fine grained polycrystalline Ni showed material-unique size dependent responses for sample sizes in the range from 0.5-40 micrometers. For several materials, including the LiF and Ni microcrystals, an intermittent deformation response was observed. By analysis of that intermittency, the deformation response of the Ni microcrystals was shown to exhibit the characteristic attributes of self-organized criticality (SOC). Some of the materials responses were represented within 3-D discrete dislocation kinetics
simulations which revealed important new micromechanisms for strengthening. Completely mechanistically self-consistent simulation of such results, by either discrete dislocation or continuum methods, stands as a challenge for emerging materials modeling approaches. The results suggest that a better understanding of dislocation nucleation, percolation, substructure evolution and SOC phenomena are need for understanding size-affected mesoscopic plasticity.

Prof. Amine Benzerga
Department of Aerospace Engineering, Texas A&M University
October 16th, Friday 2-3pm, Room B155

Size and Scale Effects in Crystal Plasticity

Plastic deformation takes place in solids due to fine-scale microstructural rearrangements. In crystalline materials, the latter are mediated by the generation and motion of lattice defects called
dislocations. There is an increasing amount of experimental evidence that the plastic behavior of crystals changes at micro- and nano-scales in a way that is not necessarily captured by state-of-the-art plasticity models. In this talk, I will analyze length scale effects in the plasticity of crystals by means of direct numerical simulations that resolve the scale of the carriers of plasticity, i.e., the dislocations
themselves. I will introduce a computationally efficient, atomistically informed dislocation dynamics framework which has the capability of reaching high dislocation densities and large strains at moderately low strain rates in finite volumes. I will then present our discovery of a new type of size effect in the hardening of crystals subject to nominally uniform compression. In an attempt to develop improved continuum models, I will present an analytic work targeted at characterization of structure, its evolution and resolution dependence. In light of such findings, I will discuss behavior transitions in the space of meaningful structural parameters, from forest-hardening dominated regime to an exhaustion hardening dominated regime. Various scalings of the flow stress with crystal size emerge in the simulations, which are compared with recent experimental data on micro- and nano-pillars.

Dr. S.A. Maloy
Advanced Fuel Cycle Initiative, Los Alamos National Laboratory
October 23rd, Friday 2-3pm, Room B155

Core Materials Development for Advanced Fuel Cycle Initiative

The Advanced Fuel Cycle Initiative is investigating methods of burning minor actinides in a transmutation fuel. To achieve this goal, the fast reactor core materials (cladding and duct) must be able to withstand very high doses (>300 dpa design goal) while in contact with the coolant
and the fuel. Thus, these materials must withstand radiation effects that promote low temperature embrittlement, high temperature helium embrittlement, swelling, accelerated creep, corrosion with the coolant, and chemical interaction with the fuel (FCCI). Research is underway that includes determining radiation effects in ferritic/martensitic steels at doses up to 200 dpa, testing and development of liners and coatings to prevent/reduce FCCI, and developing advanced alloys with improved irradiation resistance. A summary and status of these studies will be presented with plans for future research.

Arthur F. Voter
Los Alamos National Laboratory
November 4th, Wednesday 2-3pm, Room B155

Accelerated Molecular Dynamics Methods

A significant problem in the atomistic simulation of materials, as well as in other areas of chemistry and physics where atomistic simulations are used, is that molecular dynamics simulations are limited to nanoseconds, while important reactions and diffusive events often occur on time scales of microseconds and longer.  Although rate constants for these infrequent events can be computed directly using transition state theory (with dynamical corrections, if desired, to give exact rates),
this requires first knowing the transition state.  Often, however, we cannot even guess what events will occur.  For example, in vapor-deposited metallic surface growth, surprisingly complicated exchange events are pervasive.  In this talk, I will discuss the accelerated molecular dynamics approach, which we have been developing over the last decade, for treating these complex infrequent-event systems. The idea is to directly accelerate the dynamics to achieve longer times without prior knowledge of the
available reaction paths.  In some cases, we can achieve time scales with these methods that are many orders of magnitude beyond what is accessible to molecular dynamics.  I will give an
introduction to the three main methods in this class, hyperdynamics, parallel-replica dynamics and temperature accelerated dynamics, and discuss their relative merits.  I will present some illustrative and recent applications to materials problems, such as metallic surface growth, grain boundary slip under applied shear, radiation damage annealing in MgO, and tensile tests on metallic nanowires.  I will also discuss some of the ongoing challenges in making these methods as powerful and generally useful as possible.

Professor Justin Youngblood
University of North Texas
November 6th, Friday 2-3pm, Room B155

Photocatalytic Water Splitting in a Dye-Sensitized Solar Cell

Photochemical water splitting is now being intensively researched as a means of converting solar to chemical energy in the form of fuels. Hydrogen is a key solar fuel because it can be used directly in combustion engines or fuel cells, or combined catalytically with CO2 to make carbon-containing fuels. Different approaches to solar water splitting include semiconductor particles as photocatalysts and photoelectrodes, molecular donor-acceptor systems linked to catalysts for hydrogen and oxygen evolution, and photovoltaic cells coupled directly or indirectly to electrocatalysts.

Despite several decades of research, solar hydrogen generation is efficient only in systems that use expensive photovoltaic cells to power water electrolysis. Direct photocatalytic water splitting is a challenging problem because the reaction is thermodynamically uphill. Light absorption results in the formation of energetic charge-separated states in both molecular donor-acceptor systems and semiconductor particles. Unfortunately, energetically favorable charge recombination reactions tend to be much faster than the slow multi-electron processes of water oxidation and reduction. Consequently, visible light water splitting has only recently been achieved in semiconductor-based photocatalytic systems and it is still an inefficient process. This talk will describe recent results for overall water splitting using a composite material of metal oxides and a molecular photosensitizer.

Dr. Michel Dupuis
Pacific Northwest National Laboratory, Chemical and Materials Sciences Division
November 13th, Friday 2-3pm, Room B155

Charge Transport and Reactivity in Complex Environments

In this presentation we will highlight multi-scale investigations of charge transport and reactivity in complex environments, relevant to energy applications. In the first part of our talk we will describe
first-principles based studies of (1) e-/h+ transport, trapping, and energy redistribution in TiO2, (2) surface structure and material dependence of these properties, and (3) detailed reaction mechanisms of oxygenated species, all issues relevant to light-to-chemical energy conversions. In the second part of the talk we will describe multi-scale studies of proton transport in polymeric electrolyte membrane (PEM) relevant to chemical energy-to-electricity conversions. This work makes use of modern computational methodologies in DFT and ab initio and classical molecular dynamics MD from the molecular scale to the meso scale.