The effect of microstructural representation on simulations of microplastic ratcheting

This paper assesses the sensitivity of cyclic plasticity to microstructure morphology by examining and comparing the microplastic ratcheting behavior of different idealized microstructures (square, hexagonal, tessellated, and digitized from experimental data). This analysis demonstrates the sensitivity of computational accuracy to the various approximations in microstructural representation. The methodology used to perform this study relies on a coupling between microstructural characterization, mechanical testing and numerical simulations to investigate the influence of the microstructure on the purely tensile uniaxial microplastic ratcheting behavior of pure nickel polycrystals. The morphology and deformation behavior of polycrystals were characterized using electron back-scatter diffraction (EBSD), while a finite element model (FEM) of crystal plasticity was used in a computational framework. The predicted cyclic behavior is compared to experimental results both at the macroscopic and microstructural scales. The stress–strain response is less sensitive to the details of the microstructural representation than might be expected with all representations displaying similar macroscopic constitutive response. However, the details of the plastic strain distribution at the microstructural scale and the related estimations of damage mechanics vary substantially from one microstructural representation to another.     

On a simple optimal stopping problem

L.A. Shepp has posed and analyzed the problem of optimal random drawing without replacement from an urn containing predetermined numbers of plus and minus balls. Here Shepp's results are extended by improving the bounds on values of perturbed urns, deriving an exact algorithm for the urn values and computing the stopping boundary for urns of up to 200 balls.     

Chemical treatment of US Department of Energy high level and low level waste to obtain a pure radiochemical fraction for determination of californium alpha-decay content

We have developed a chemical separation technique that allows the radiochemical determination of the californium -decay content in Department of Energy (DOE) high level wastes from the Hanford and Savannah River sites. The chemical separation technique uses a series of column extraction chromatography steps that use Eichrom Industries' lanthanide and actinide +3 oxidation state selective Ln-resin^® and the transuranic selective +4 oxidation state TRU-resin^® to obtain intermediate product phases in dilute nitric acid. The technique has been demonstrated on three types of authentic DOE high and low level waste samples. We obtain discrimination from Pu -activity by a factor of over 200 and from ²⁴⁴Cm -activity by a factor approaching 1700. Californium recoveries are measured by addition of a ²⁴⁹Cf spike and are in the range of 50% to 90% in the synthetic samples and are in the range of 1.4% to 48% for the authentic DOE waste samples.     

Nonlinear structural mechanics based modeling of carbon nanotube deformation

A nonlinear structural mechanics based approach for modeling the structure and the deformation of single-wall and multiwall carbon nanotubes (CNTs) is presented. Individual tubes are modeled using shell finite elements, where a specific pairing of elastic properties and mechanical thickness of the tube wall is identified to enable successful modeling with shell theory. The effects of van der Waals forces are simulated with special interaction elements. This new CNT modeling approach is verified by comparison with molecular dynamics simulations and high-resolution micrographs available in the literature. The mechanics of wrinkling of multiwall CNTs are studied, demonstrating the role of the multiwalled shell structure and interwall van der Waals interactions in governing buckling and postbuckling behavior.     

Nuclear magnetic resonance investigation of H, H2 and dopants in hydrogenated amorphous silicon and related materials

Nuclear magnetic resonance has been successfully applied to the study of the microstructure of hydrogenated amorphous silicon and related materials. It has been used to determine the local bonding and structural environment of the host atoms, the hydrogen, and the dopants. First, we review some of these NMR experimental results on the hydrogen microstructure in hydrogentaed amorphous semiconductors and compare the results on plasma deposited hydrogenated amorphous silicon (a-Si:H), remote hydrogen plasma deposited a-Si:H, thermally annealed a-Si:H, doped a-Si:H, microcrystalline Si and amorphous (Si, Ge):H alloys. A common feature is that these materials exhibit a heterogeneous distribution of hydrogen bonded to the semiconductor lattice in dilute and clustered phases. In addition, the lattice contains voids of varying number and size that contain non-bonded molecular hydrogen whose quantity is altered by deposition conditions and thermal treatment. Second, we review some aspects of the local bonding structure of dopants in a-Si:H. A significant fraction of the dopants are found to be in dopant-hydrogen clusters similar to those proposed to explain hydrogen passivation in crystalline silicon. Implications of the determined local structure on the doping efficiency are discussed.     

RDF studies of the superionic conductor RbAg4I5 using EXAFS

Measurements of the extended X-ray absorption fine structure (EXAFS) on the Ag K-shell absorption in RbAg₄I₅ show that in all three crystalline phases the equilibrium position of the Ag ions is near the center of tetrahedra formed by the iodine atoms. In addition, Ag-Ag correlations, which persist into the superionic α-phase, are observed.     

Light-emitting diode-compatible probes for indirect detection of anions in CE

A range of compounds were evaluated as probes for the indirect detection of inorganic ions using CE and light-emitting diodes (LEDs) as the light source. Emphasis was placed on examining probes likely to absorb strongly in the UV-Vis region near 350-430 nm as compounds, which absorb at longer wavelengths tend to be bulkier and adsorb onto the capillary wall. These probes should act as a replacement for the very effective but carcinogenic probe chromate. Two probes were identified and evaluated: p-nitrophenol and 4-hydroxy-3,5-dinitrobenzoic acid. The former showed the most potential with low-mobility anions, while the later had a moderate electrophoretic mobility and was more suitable for a wider mobility range of analytes. However, neither could match the efficiencies and LOD of chromate for the separation of the fast inorganic ions such as chloride, nitrate and sulphate. Nevertheless, application of the 4-hydroxy-3,5-dinitrobenzoic acid system to the determination of oxalate in Bayer liquors showed excellent sensitivity and selectivity.     

Preparation and properties of a monomeric high-spin Mn(V)-oxo complex

Oxomanganese(V) species have been implicated in a variety of biological and synthetic processes, including their role as a key reactive center within the oxygen-evolving complex in photosynthesis. Nearly all mononuclear Mn(V)-oxo complexes have tetragonal symmetry, producing low-spin species. A new high-spin Mn(V)-oxo complex that was prepared from a well-characterized oxomanganese(III) complex having trigonal symmetry is now reported. Oxidation experiments with [FeCp(2)](+) were monitored with optical and electron paramagnetic resonance (EPR) spectroscopies and support a high-spin oxomanganese(V) complex formulation. The parallel-mode EPR spectrum has a distinctive S = 1 signal at g = 4.01 with a six-line hyperfine pattern having A(z) = 113 MHz. The presence of an oxo ligand was supported by resonance Raman spectroscopy, which revealed O-isotope-sensitive peaks at 737 and 754 cm(-1) assigned as a Fermi doublet centered at 746 cm(-1)(Δ(18)O = 31 cm(-1)). Mn Kβ X-ray emission spectra showed Kβ' and Kβ(1,3) bands at 6475.92 and 6490.50 eV, respectively, which are characteristic of a high-spin Mn(V) center.     

Collective mechanical behavior of multilayer colloidal arrays of hollow nanoparticles

The collective mechanical behavior of multilayer colloidal arrays of hollow silica nanoparticles (HSNP) is explored under spherical nanoindentation through a combination of experimental, numerical, and theoretical approaches. The effective indentation modulus E(ind) is found to decrease with an increasing number of layers in a nonlinear manner. The indentation force versus penetration depth behavior for multilayer hollow particle arrays is predicted by an approximate analytical model based on the spring stiffness of the individual particles and the multipoint, multiparticle interactions as well as force transmission between the layers. The model is in good agreement with experiments and with detailed finite element simulations. The ability to tune the effective indentation modulus, E(ind), of the multilayer arrays by manipulating particle geometry and layering is revealed through the model, where E(ind) = (0.725m(-3/2) + 0.275)E(mon) and E(mon) is the monolayer modulus and m is number of layers. E(ind) is seen to plateau with increasing m to E(ind_plateau) = 0.275E(mon) and E(mon) scales with (t/R)(2), t being the particle shell thickness and R being the particle radius. The scaling law governing the nonlinear decrease in indentation modulus with an increase in layer number (E(ind) scaling with m(-3/2)) is found to be similar to that governing the indentation modulus of thin solid films E(ind_solid) on a stiff substrate (where E(ind_solid) scales with h(-1.4) and also decreases until reaching a plateau value) which also decreases with an increase in film thickness h. However, the mechanisms underlying this trend for the colloidal array are clearly different, where discrete particle-to-particle interactions govern the colloidal array behavior in contrast to the substrate constraint on deformation, which governs the thickness dependence of the continuous thin film indentation modulus.