Paul C. Millett

Demonstrating the Temperature Gradient Impact on Grain Growth in UO2 Using the Phase Field Method

Grain boundaries (GBs) are driven to migrate up a temperature gradient. In this work, we use a phase field model to investigate the impact of temperature gradients on isotropic grain growth. GB motion in 2D UO2 polycrystals is predicted under increasing temperature gradients. We find that the temperature gradient does not significantly impact the average grain growth behavior because the curvature driving force is dominant. However, it does cause significant local migration of the individual grains. In addition, the temperature dependence of the GB mobility results in larger grains in the hot portion of the polycrystal.

Electric-field induced alignment of nanoparticle-coated channels in thin-film polymer membranes

Microscopic phase separation in immiscible polymer melts can be significantly altered by the presence of dispersed nanoparticles and externally applied electric fields. Inducing order or directionality to the resulting microstructure can lead to novel materials with efficient synthesis. Here, the coupled morphology of an immiscible binary polymer blend with dispersed nanoparticles in a thin-film geometry is investigated under the influence of an applied electric field using a unique mesoscale computational approach. For asymmetric binary blends (e.g., 70-30), the resulting microstructure consists of columnar channels of the B-phase perpendicular to the major plane of the film (aligned with the electric field), with the particles segregated along the channel interfaces. The simulations reveal the variability of the average channel diameter and the interfacial arrangement of the particles. The high density of exposed particles makes these structures viable candidates for catalytically active porous membranes or macromolecular manipulation devices.

Mesoscale modeling of intergranular bubble percolation in nuclear fuels

Phase-field simulations are used to examine the variability of intergranular fission gas bubble growth and percolation on uranium dioxide grain boundaries on a mesoscopic length scale. Three key parameters are systematically varied in this study: the contact angle (or dihedral angle) defining the bubble shape, the initial bubble density on the grain boundary plane, and the ratio of the gas diffusivity on the grain boundary versus the grain interiors. The simulation results agree well with previous experimental data obtained for bubble densities and average bubble areas during coalescence events. Interestingly, the rate of percolation is found to be highly variable, with a large dependency on the contact angle and the initial bubble density and little-to-no dependency on the grain boundary gas diffusivity.

Numerical simulations of bijel morphology in thin films with complete surface wetting.

Bijels are a relatively new class of soft materials that have many potential energy and environmental applications. In this work, simulation results of bijel evolution confined within thin films with preferential surface wetting are presented. The computational approach used is a hybrid Cahn-Hilliard/Brownian dynamics method. In the absence of suspended particles, we demonstrate that the model accurately captures the rich kinetics associated with diffusion-based surface-directed spinodal decomposition, as evidenced by comparison with previous theoretical and simulation-based studies. When chemically neutral particles are included in the films, the simulations capture surface-modified bijel formation, with stabilized domain structures comparable with the experimental observations of Composto and coworkers. Namely, two basic morphologies - bicontinuous or discrete - are seen to emerge, with direct dependence on the film thickness, particle volume fraction, and particle radius.

Dynamic Void Growth and Shrinkage in Mg under Electron Irradiation

We report in situ atomic-scale investigation of late-stage void evolution, including growth, coalescence and shrinkage, under electron irradiation. With increasing irradiation dose, the total volume of voids increased linearly, while the nucleation rate of new voids decreased slightly and the total number of voids decreased. Some voids continued to grow while others shrank to disappear, depending on the nature of their interactions with nearby self-interstitial loops. For the first time, surface diffusion of adatoms was observed to be largely responsible for the void coalescence and thickening. These findings provide fundamental understanding to help with the design and modeling of irradiation-resistant materials.

Numerical Simulations of Directed Self-Assembly in Diblock Copolymer Films using Zone Annealing and Pattern Templating

Bulk fabrication of surface patterns with sub-20u2009nm feature sizes is immensely desirable for many existing and emerging technologies. Directed self-assembly (DSA) of block copolymers (BCPs) has been a recently demonstrated approach to achieve such feature resolution over large-scale areas with minimal defect populations. However, much work remains to understand and optimize DSA methods in order to move this field forward. This paper presents large-scale numerical simulations of zone annealing and chemo-epitaxy processing of BCP films to achieve long-range orientational order. The simulations utilize a Time-Dependent Ginzburg-Landau model and parallel processing to elucidate relationships between the magnitude and velocity of a moving thermal gradient and the resulting BCP domain orientations and defect densities. Additional simulations have been conducted to study to what degree orientational order can be further improved by combining zone annealing and chemo-epitaxy techniques. It is found that these two DSA methods do synergistically enhance long-range order with a particular relationship between thermal gradient velocity and chemical template spacing.

The significance of the number of periods and period size in 2D photonic crystal waveguides

This work investigates the significance of the number of periods in two-dimensional photonic crystals. Models have been developed to study various photonic crystal properties (Reflection, Photonic crystal band gap). The numbers of photonic crystal periods, length of periods, and material properties have been investigated to determine their effect on the losses in the waveguide. The model focuses on a square period and has been designed to study transmission properties and the effects of period length. A finite difference frequency domain (FDFD) model has also been created to calculate the photonic band structure. Additionally, a simplified study focuses on the transmission of light through photonic crystal layers.

Simulations of Xe and U diffusion in UO2

Diffusion of xenon (Xe) and uranium (U) in UO{sub 2} is controlled by vacancy mechanisms and under irradiation the formation of mobile vacancy clusters is important. Based on the vacancy and cluster diffusion mechanisms established from density functional theory (DFT) calculations, we derive continuum thermodynamic and diffusion models for Xe and U in UO{sub 2}. In order to capture the effects of irradiation, vacancies (Va) are explicitly coupled to the Xe and U dynamics. Segregation of defects to grain boundaries in UO{sub 2} is described by combining the bulk diffusion model with models of the interaction between Xe atoms and vacancies with grain boundaries, which were derived from atomistic calculations. The diffusion and segregation models were implemented in the MOOSE-Bison-Marmot (MBM) finite element (FEM) framework and the Xe/U redistribution was simulated for a few simple microstructures.

Molecular Dynamics Study of the Effect of Dopant Atoms on Grain Boundary Sliding

Molecular dynamics simulations are used to study grain boundary sliding in pure and doped Cu bicrystals using both Lennard-Jones and Embedded-Atom Method potentials. Two tilt [100] grain boundaries are considered: the coincident site lattice Σ5 interface and a random high angle interface. Shear stress between 0.69 GPa and 1.61 GPa was applied to the bicrystals for a duration of 10 ps at ambient temperature (300K) and high temperature (800K). For the pure bicrystals, the sliding of the Σ5 interface with respect to the random interface was lower at 800K and higher at 300K. For the doped bicrystals, interstitial dopant atoms and substitutional dopant atoms with larger atomic radius were effective in retarding grain boundary sliding. These simulations will aid further work to determine how segregated dopant atoms alter the tensile properties of nanocrystalline metals.