The assessment of the thermal stability across HfO2/Si and HfO2/SiO2 interfaces has been difficult due to lack of thermodynamic data. In this paper, we present the results of thermodynamic calculations intended to fill this gap. A thermodynamic model was developed by assuming that HfSiO4 is an ideal solution of HfO2 and SiO2 to a first order approximation. The theoretical results predict that the HfO2/Si interface is thermodynamically stable up to 1100°C, while the HfO2/SiO2 interface is thermodynamically unstable even at room temperature. Our experimental results from TEM and XPS analysis are consistent with these modeling predictions.

The phase transformations of anatase on Li-, H- and Na- intercalation are analyzed, for a variety of different insertion concentrations, using first principle calculations. Predicted structures are based on symmetry unconstrained optimization of all internal degrees of freedom and the unit cell shape and volume. The maximum insertion concentrations are determined, the phase stability of the predicted structures examined and the mechanism of the phase transformations discussed.

A balance of surface energies exists where grain boundaries meet the surface of a flat solid specimen. The energy balance leads to grain boundary grooving on the surface, and the establishment of the equilibrium dihedral angle. Triple junctions are defined at the intersections of three grain boundaries. Surface grooves are typically observed to be the deepest at the triple junctions. In this work, a simple model is constructed of a polycrystalline thin film using Surface Evolver numerical software. The equilibrium sur face groove depths at triple junctions are investigated as a function of triple junction line tension. Results show that line tension can affect grain boundary groove depths for grain sizes less than ∼1μm.

A Green's function method is described for multiscale modeling of point defects such as vacancies and interstitials at the atomistic level and extended defects such as free surfaces and interfaces at the macroscopic continuum level in a solid. The point defects are represented in terms of Kanzaki forces using the lattice-statics Green's function, which can model a large crystallite containing a million atoms without excessive CPU effort. The lattice-statics Green's function reduces to the continuum Green's function in the asymptotic limit which is used to model the extended defects by imposing continuum- model boundary conditions. Numerical results are presented for the displacement field on the free surface due to a vacancy in semi-infinite fcc copper.

Pb-Sn solders are known to coarsen and deform heterogeneously under shear thermomechanical cycling. Solder joint failure often occurs within these coarsened shear bands. The connection between microstructure and materials properties is well documented, which leads to the conclusion that an improved understanding of the interaction between stress and microstructural evolution will better enable the prediction of materials properties over time. In this paper, we simulate microstructural evolution of Pb-Sn solders using a novel two-phase version the Potts model, a kinetic Monte Carlo model that is coupled to a finite element mechanics model. Using this model, possible mechanisms for heterogeneous coarsening are explored.

Brilliant synchrotron microprobes offer new opportunities for the analysis of stress/strain and deformation distributions in crystalline materials. Polychromatic x-ray microdiffraction is emerging as a particularly important tool because it allows for local crystal-structure measurements in highly deformed or polycrystalline materials where sample rotations complicate alternative methods; a complete Laue pattern is generated in each volume element intercepted by the probe beam. Although a straightforward approach to the measurement of stress/strain fields through white-beam Laue microdiffraction has been demonstrated, a comparable method for determining the plastic-deformation tensor has not been established. Here we report on modeling efforts that can guide automated fitting of plastic-deformation-tensor distributions in three dimensions.

Both isotropic and anisotropic single-phase grain growth processes modeled using a modified Monte Carlo method exhibit parabolic growth kinetics, and the anisotropy degree affects only the rate of change of the mean grain area. In some cases, with significantly anisotropic grain boundary energies, the normalized grain size distributions are not time-invariant during the lattice evolution.

A discrete dislocation simulation of plastic deformation in metallic interconnects caused by thermal stress is carried out. The calculations are carried out using a two dimensional plane strain formulation with only edge dislocations. A boundary value problem is formulated and solved for the evolution of the thermal stress field and the evolution of the dislocation structure in the cross-section of the line as cooling proceeds. For lines with a small cross section (height or width less than 1 μm), the local concentration of stresses due to dislocation patterning strongly affects the overall stress build up and relaxation. The results show a clear dependence of the transverse stress development on the line aspect ratio.

Discrete element studies of powder compaction have become popular recently. A disadvantage of this technique is the need for simplification of the inter-particle contact behavior which limit the applicability of DEM to small relative densities. To overcome this problem, we analyze the compaction of powder by a 2-D finite element study of the compaction of 400 particles, each of which is discretized at a sufficient level to provide adequate detail of the interparticle interaction. The material is modeled as elastic-perfectly plastic. Simulations show that: (a) there is an effect of interparticle friction on the macroscopic response in the earlier stages of compaction, (b) there is significant rearrangement even in highly constrained compaction modes, (c) the absence of friction promotes inhomogeneous deformation in the compact, and (d) conditions for fragmentation develop in particles with loose lateral constrains.

The simulation of defect dynamics and evolution is a technologicaly relevant challenge for computational materials science. The diffusion of small defects in silicon unfolds as a sequence of structural transitions. The relative infrequency of transition events requires simulation over extremely long time scales. We simulate the diffusion of small interstitial clusters (I1, I2, I3) for a range of temperatures using large-scale molecular dynamics (MD) simulations with a realistic tight-binding potential. A total of 0.25 μ sec of simulation time is accumulated for the study. A novel real-time multiresolution analysis (RTMRA) technique extracts stable structures directly from the dynamics without structural relaxation. The discovered structures are relaxed to confirm their stability.

Predictions of component performance versus lifetime are often risky for complex materials in which there may be many underlying aging or degradation mechanisms. In order to develop more accurate predictive models for silica-filled siloxane foam components, we are studying damage mechanisms over a broad range of size domains, linked together through several modeling efforts. Atomistic and molecular dynamic modeling has elucidated the chemistry of the silica filler to polymer interaction, as this interaction plays a key role in this material's aging behavior. This modeling work has been supported by experimental data on the removal of water from the silica surface, the effect of the surrounding polymer on this desiccation, and on the subsequent change in the mechanical properties of the system. Solid State NMR efforts have characterized the evolution of the polymer and filler dynamics as the material is damaged through irradiation or desiccation. These damage signatures have been confirmed by direct measurements of changes in polymer crosslink density and filler interaction as measured by solvent swelling, and by mechanical property tests. Data from the changes at these molecular levels are simultaneously feeding the development of age-aware constitutive models for polymer behavior. In addition, the microstructure of the foam, including while under load, has been determined by Computed Tomography, and these data are being introduced into Finite Element Analysis codes to allow component level models. All of these techniques are directed towards the incorporation of molecular and microstructural aging signatures into predictive models for overall component performance.

Various theories have been developed to describe the diffusion-controlled growth of precipitates with shapes approximating needles or plates. The most comprehensive one is due to Ivantsov, Horvay and Cahn, and Trivedi (HIT theory), where all the factors that may influence the precipitate growth, i.e. diffusion, interface kinetics and capillarity, are accounted for within one equation. However, HIT theory was developed based on assumptions that transformation strain/stress and interfacial free energy are isotropic, which are not true in most of the real systems. An improved growth theory of precipitates of needle and plate shapes was developed in the present study. A new concept, the compression ratio, was introduced to account for influences from the anisotropy of transformation strain/stress and interfacial free energy on the precipitate morphology. Experimental evidence supports such compression effect. Precipitate growth kinetics were quantified using this concept. The improved HIT theory (IHIT theory) was then applied to study the growth of Widmanstatten austenite in ferrite in Fe-C-Mn steels. The calculated results agree well with the experimental observations.

The virial stress is the most commonly used definition of stress in discrete molecular systems. This stress includes two parts. The first part depends on the mass and velocity of atomic particles, reflecting an assertion that mass transfer causes mechanical stress to be applied on stationary spatial surfaces external to an atomic particle system. The second part depends on interatomic forces and atomic positions, providing a continuum measure for the internal mechanical interactions between particles. For the simple conditions of rigid translation and uniform tension, the virial stress yields apparently erroneous interpretations. It is shown that the virial stress violates balance of momentum and does not possess physical significance as a measure for mechanical interaction between material points. The lack of physical significance is both at the individual atom level in a time-resolved sense and at the system level in a statistical sense. As a stress-like quantity, the virial stress has the geometric interpretation of being a measure for momentum change in a fixed spatial region. It is demonstrated that the interatomic force term alone is a valid stress measure and can be identified with the Cauchy stress. The discussions here focus on an error in the theoretical derivation of the virial stress that led to the inclusion of the kinetic energy term and on the conceptual flaws in the argument commonly used for including it. The result here demonstrates that, for gas systems in macroscopic steady state, the isotropic statistical average of the internal stress tensor over time and space is not equal to the negative of the macroscopic pressure tensor for the system. The statistical average of the stress is equal to the negative of the pressure only at absolute zero when all molecular motions cease.

Descriptions, in which matter can be coarse grained to arbitrary levels, are necessary to study materials phenomenal simultaneously at various length scales. Attempts to do this in the static regime have already been developed. In this work, we present an approach that leads to dynamics for such coarse-grained models. Renormalization group theory is used to create new local potentials between nodes. Assuming that these potentials give an averaged description of node dynamics, we calculate thermal, mechanical and transport properties. If this method can be sufficiently generalized it may form the basis of a Molecular Dynamics method with time and spatial coarse-graining.

The electronic structures of six polycrystalline borocarbide superconductors YPd5-xRhxB3C0.4 (x=0, 0.05, 0.1, 0.15, 0.2, and 0.25) were studied by photoemission and photoabsorption spectroscopies and theoretical calculations. The valence-band (VB) photoemission spectrum is compared with the theoretical total and partial density-of-states (DOS) curves. The Pd L3-edge x-ray absorption near edge spectra (XANES) are compared with the calculated XANES spectra for these intermetallic compounds. The decrease of the superconducting transition temperatures (Tc) with addition of Rh dopant in these compounds is due to a decrease of the total DOS at the Fermi level (EF).

We present simulation results for Ni79Fe21 (5 nm)/Cu (3 nm)/Co (4nm) pseudo-spin valves. These simulations have been conducted on several different aspect ratios of rectangular dots. Distinct switches of the two magnetic layers were observed. At smaller aspect ratios, magnetization reversal proceeds through a leaf state in the soft layer and a flower state in the hard layer. For larger aspect ratios, reversal proceeds by nucleation and annihilation of domain walls. Our simulations show a reasonable agreement with the experimental results. Differences between the experimental and simulation results are discussed.

Multiparticle diffusion equations were modeled to simulate the dynamics of phase coarsening. Local environmental information and particle interactions within the microstructure are included in our simulations. These studies reveal that the growth rates of particles with the same radii can differ, and that particles with the average radius can grow, shrink, or remain conditionally stable. These results are in contrast to mean-field predictions, where particle growth rates are strictly deterministic. Multiparticle simulations prove that fluctuations occur in the particle growth rates, even at extremely low microstructural densities. Multiplicative noise provides a good basis to describe microstructural fluctuations.

Non-local elasticity kernels are calibrated based on the atomic scale structure of glasses obtained from atomistic simulations. A model Morse material, with interatomic interactions described by a pair potential, and Al, characterized by an embedded-atom potential are considered. The study is limited to linear isotropic non-local elasticity. The functional form of the derived kernels is significantly different than that usually assumed in non-local constitutive models (Gaussian). They have a range that extends up to the cut-off radius of the interatomic potential, are positive at the origin, and become negative approximately one atomic distance away. These kernels involve 2 internal length scales that are both derived from atomistics for the materials mentioned above. The kernel for Al is tensorial, a different function weighting each entry of the stiffness tensor. Model materials interacting by pair potentials may be described by a single function that weights the whole stiffness tensor. Two applications are briefly described in closure. The new non-local model improves upon the Gaussian one by predicting a more realistic wave dispersion relationship, with essentially zero group velocity at the boundary of the Brillouin zone. Next, the role of non-locality in defining the Peierls stress of a dislocation is studied within a Peierls-Nabarro model and it is shown that the predictions of the critical stress improve upon consideration of non-locality.

We apply Klemens' second-order perturbation theory to investigate thermal conductivity in wurtzite GaN films. Specifically, the effect of edge, screw, and mixed types of dislocations and their orientation with respect to the temperature gradient on the thermal conductivity values is analyzed. Using typical impurity profiles for GaN films, we study the relative contribution of different impurities into thermal resistance.

The successful integration of a porous low dielectric constant (k) material as an interlevel dielectric depends on the morphology of the embedded porosity. Simple site percolation models are utilized here to investigate porosity properties of low-k dielectrics with respect to the current technology trends. Significant differences between two generations of porous dielectrics, k < 2.4 and k < 2.1, are found. The porosity fraction in the latter is above the percolation threshold, which may have serious impact on the materials physical properties and its compatibility with production steps.