Rick E. Williford

Mechanisms of vapor permeation through multilayer barrier films: Lag time versus equilibrium permeation

Multilayer, thin-film organic/inorganic composite barrier layers have recently been reported to achieve water vapor permeation rates of <10−5g∕m2∕day at 25°C∕40%RH on polyethylene terephthalate substrates. Using both transient and steady-state vapor permeation measurements combined with classical Fickian diffusion models, we determine the mechanism of vapor permeation through such barrier structures and show that results obtained to date are limited not by equilibrium diffusion but by lag-time effects caused by the extremely long effective path length for the diffusing gas. The implications for further improvement of flexible thin-film vapor barriers are discussed.

Three-dimensional thermo-fluid electrochemical modeling of planar SOFC stacks

A simulation tool for modeling planar solid oxide fuel cells is demonstrated. The tool combines the versatility of a commercial computational fluid dynamics simulation code with a validated electrochemistry calculation method. Its function is to predict the flow and distribution of anode and cathode gases, temperature and current distributions, and fuel utilization. A three-dimensional model geometry, including internal manifolds, was created to simulate a generic, cross-flow stack design. Similar three-dimensional geometries were created for simulation of co-flow, and counterflow stack designs. Cyclic boundary conditions were imposed at the top and bottom of the model domains, while the lateral walls were assumed adiabatic. The three cases show that, for a given average cell temperature, similar fuel utilizations can result irrespective of the flow configuration. Temperature distributions however, which largely determine thermal stresses during operation, are dependent on the chosen design geometry/flow configuration. The co-flow case had the most uniform temperature distribution and the smallest thermal gradients, thus offers thermo-structural advantages over the other flow cases.

Hybrid Nanogels for Sustainable Positive Thermosensitive Drug Release

A hybrid nanogel has been developed based on interpenetrating networks of thermosensitive poly(N-isopropylacrylamide) gels and tailored nanoporous silica. A sustainable positive thermo-responsive drug release profile is obtained. When the temperature rises, the polymer gel shrinks, squeezing the drug into the porous channels, and at the same time, opening the pores to the outside media. The drug slowly diffuses out of the porous channels. The overall release rate can be adjusted by changing the composition of the nanogel.

Effects of Cation Disorder on Oxygen Vacancy Migration in Gd2Ti2O7

Atomistic simulations were used to calculate defect formation and migration energies for oxygen vacancies in the pyrochlore Gd2Ti2O7, with particular attention to the role of cation antisite disorder. Oxygen occupies two crystallographically distinct sites (48f and 8a) in the ordered material, but the 8b sites become partially occupied with disorder. Because cation and anion disorder are coupled, oxygen vacancy formation and migration energetics are sensitive to the configuration of the cation disorder. The VO8a vacancy and VO8a + O8bi Frenkel defects are energetically favored in the ordered material, but VO8a is favored at higher disorder. The VO8a + O8bi Frenkel is favored for some disorder configurations. Eight possible oxygen vacancy migration paths converge toward a unique migration energy as cation disorder increases, reflecting a reversion towards the fluorite structure. Oxygen vacancy migration is determined by O48f → O48f transitions along the shortest edges of the TiO6 octahedra. The transition V48a → V48f is also possible for low disorder, and can activate the V48f → V48f migration network by depositing vacancies there. The reverse transition may occur at very high disorder to retard ionic conduction, and is consistent with Frenkel defect stabilities. Local regions of ordered and disordered material both appear necessary to explain the observed trends in ionic conductivity.

Computer simulation of displacement energies for several ceramic materials

Abstract Displacement energies ( E d ) are fundamental parameters controlling the production of radiation damage in materials, and as such, are useful for understanding and modeling the effects of radiation on materials. These energies are not easily determined experimentally for many ceramic materials. However, advances in computational methodologies and their application to ceramic materials provide a means to determine these energies in a number of materials of interest. Although computationally intensive molecular dynamics methods can be used to determine E d for the various cations and anions, energy minimization methods can also provide a more expedient means to obtain reasonable estimates of these energies. In this paper, the energy minimization code General Utility Lattice Program (GULP), which uses a Mott–Littleton approximation to simulate isolated defects in extended solids, is used to calculate displacement energies. The validity of using this code for these computations is established by calculating E d for several ceramics for which these energies are known. Computational results are in good agreement with the experimental values for alumina, MgO, and ZnO. Results are also presented for two ceramic materials, zircon and spinel, for which there are little or no experimental values yet available.

Native Vacancy Migrations in Zircon

Abstract Energy minimization methods were used to simulate the migration of Zr, Si, and O vacancies in zircon (ZrSiO 4 ). Two sets of interatomic potentials were employed for comparison: one with O–Si–O three-body terms for the SiO 4 , and one without. Results for Si were inconclusive, but consistent with maintaining the integrity of the SiO 4 molecular units. Both Zr and O vacancies can migrate on three-dimensional sublattice networks, thus supporting the experimentally observed diffusional isotropy. The predicted Zr vacancy migration energy (1.16–1.38 eV) was in good agreement with experiment if supplemented by Zr vacancy formation via Schottky or Frenkel defects (6.21–12.28 eV/defect). Oxygen vacancy migration energies were predicted to be 0.99–1.16 eV, somewhat lower than the experimental value of 4.64 eV measured in natural zircons, which thus may include significant contributions from vacancy formation mechanisms at 3.31–6.52 eV/defect.

Computer simulation of Pu3+ and Pu4+ substitutions in gadolinium zirconate

Abstract Atomistic computer simulations have been used to determine the energetics of a variety of defect reactions related to the incorporation of Pu 3+ and Pu 4+ into the pyrochlore and the fluorite-type structures of Gd 2 Zr 2 O 7 . The lowest energy states were found for Pu 3 + substitutions on Gd sites in the pyrochlore (−1.00 eV/Pu) and the fluorite-type (−1.55 eV/Pu) structures, so these defect reactions are the most likely configurations under reducing conditions that favor Pu 3+ ions. Slightly higher, but still exothermic, energies (−0.26 to −0.45 eV) were calculated for Pu 4+ substitutions on Zr sites for several fluorite-type cases, indicating that oxidizing conditions should favor Pu 4+ incorporation on Zr sites in Gd 2 Zr 2 O 7 hosts. Defect reactions involving cation vacancies or interstitials exhibited significantly higher energies, and are therefore not expected to occur. Mean field calculations indicated that the increases in crystal volume associated with Pu incorporation are minimized by the excess free volume associated with the Gd site in the pyrochlore structure. Volume changes upon thermal phase transformation from fluorite to pyrochlore are smaller for the material incorporating Pu by substitution than for the virgin material, with a slight advantage for the reducing conditions associated with Pu 3+ substitutions on Gd sites.

Displacement damage cross sections for neutron-irradiated silicon carbide

Displacements per atom (DPA) is a widely used damage unit for displacement damage in nuclear materials. Calculating the DPA for SiC irradiated in a particular facility requires a knowledge of the neutron spectrum as well as specific information about displacement damage in that material. In recent years significant improvements in displacement damage information for SiC have been generated, especially the energy required to displace an atom in an irradiation event and the models used to describe electronic and nuclear stopping. Using this information, numerical solutions for the displacement functions in SiC have been determined from coupled integro-differential equations for displacements in polyatomic materials and applied in calculations of spectral-averaged displacement cross sections for SiC. This procedure has been used to generate spectrally averaged displacement cross sections for SiC in a number of reactors used for radiation damage testing of fusion materials, as well as the ARIES-IV conceptual fusion device.

Computer simulation of Pu3+ and pu4+ substitutions in zircon

Energy minimization methods were used in atomistic computer simulations to determine the energetics of Pu3+ and Pu4+ substitutions and interstitials in zircon, including the effect of ion size. The lowest energy was found for Pu4+Zr substitutions. The lowest energy for Pu3+ substitutions was found for the defect cluster 2Pu3+′Zr+VO••. Mean field calculations of unit-cell volumes for 8% Pu substitutions were in agreement with X-ray diffraction (XRD) data.

Chemically assisted crack nucleation in zircaloy

Abstract Stress corrosion cracking models (proposed to explain fuel rod failures) generally address crack propagation and cladding rupture, but frequently neglect the necessary nucleation stage for microcracks small enough to violate fracture mechanics continuum requirements. Intergranular microcrack nucleation was modeled with diffusion-controlled grain-boundary cavitation concepts, including the effects of metal embrittlement by iodine species. Computed microcrack nucleation times and strains agree with experimental observation, but the predicted grain-boundary cavities are so small that detection may be difficult. Without a protective oxide film, intergranular microcracks can nucleate within 30 s at even low stresses when the embrittler concentration exceeds a threshold value. Indications were found that intergranular microcrack nucleation may be caused by combined corrosive and embrittlement phenomena.