Scientific Publications 2008
2008. "Experimentally determined dissolution kinetics of Na-rich borosilicate glass at far from equilibrium conditions: Implications for Transition State Theory." Geochimica et Cosmochimica Acta 72(12):2767-2788. doi:10.1016/j.gca.2008.02.026 Abstract Abstract—The dissolution kinetics of five chemically complex and two chemically simple borosilicate glass compositions (Na-B-Si±Al) were determined over a range of solution saturation values by varying the flow-through rates (1 to 100 mL d-1) in a dynamic single-pass flow-through (SPFT) apparatus. The chemically complex borosilicate glasses are representative of prospective hosts for radioactive waste disposal and are characterized by relatively high molar Si/(Si+Al) and Na/(Al+B) ratios (>0.7 and >1.0, respectively). Analysis by x-ray absorption spectroscopy (XAS) indicates that the fraction of ivB to iiiB (N4) varies from 0.66 to 0.70. Despite large differences in bulk chemistry, values of 29Si peak shift determined by MAS-NMR varies only by about 7 ppm (29Si = -94 to -87 ppm), indicating small differences in polymerization state for the glasses. Forward rates of reaction measured in dynamic experiments converge (average log10 rate [40°C, pH 9] = -1.87±0.79 [g/(m2•d)]) at high values of flow-rate (q) to sample surface area (S). Dissolution rates are independent of total Free Energy of Hydration (FEH) and this model appears to overestimate the impact of excess Na on chemical durability. For borosilicate glass compositions in which molar Na > Al + B, further addition of Na appears to stabilize the glass structure with respect to hydrolysis and dissolution. Compared to other borosilicate and aluminosilicate glasses, the glass specimens from this study dissolve at nearly the same rate (0 to ~55×) as the more polymerized glasses, such as vitreous reedmergnerite (NaBSi3O8), albite, and silica. Dissolution of glass follows the order: boroaluminosilicate glass > vitreous reedmergnerite > vitreous albite > silica glass, which is the same order of increasingly negative 29Si chemical shifts. The chemical shift of 29Si is a measure of the extent of bond overlap between Si and O and correlates with the forward rate of reaction. Thus, dissolution appears to be rate-limited by rupture of the Si—O bond, which is consistent with the tenants of Transition State Theory (TST). Therefore, dissolution at far from equilibrium conditions is dependent upon the speed of the rate-controlling elementary reaction and not on the sum of the free energies of hydration of the constituents of boroaluminosilicate glass.
2008. "Ligand field effects on the multiplet structure of the U4f XPS of UO2." Surface Science 602(5):1114-1121. doi:10.1016/j.susc.2008.01.010 Abstract Ab initio, fully relativistic four component theory was used to determine atomic and interatomic many-body effects for the 4f X-ray photoelectron spectrum of an embedded UO8-12 cluster representing UO2. Many-body effects were included through the use of configuration interaction wavefunctions that allow the mixing of XPS allowed and XPS forbidden configurations. Charge transfer configurations were not included. This work extends our earlier studies on simulations of the U 4f XPS for the free U4+ cation. While the main XPS features are similar in both cases, ligand field effects changed the multiplet structure in important ways that better simulated experimental data for UO2. Neither initial nor final state covalency significantly reduced the 4f-5f exchange integrals, and the differences between the atom and cluster model was due to ligand field splitting of the 5f band and increased distributions of intensity from XPS allowed to XPS forbidden peaks. The prominent 7 eV satellites associated with UO2 were absent in the simulations, and provides further evidence that these satellites are due to charge transfer and not other interatomic effects.
2008. "Direct observations of thermally induced structural changes in amorphous silicon carbide." Journal of Applied Physics 104(3):033503, 1-5. doi:10.1063/1.2960342 Abstract Thermally induced structural relaxation in amorphous silicon carbide (SiC) has been examined by means of in situ transmission electron microscopy (TEM). The amorphous SiC was prepared by high-energy ion-beam-irradiation into a single crystalline 4H-SiC substrate. Cross-sectional TEM observations and electron energy-loss spectroscopy measurements revealed that thermal annealing induces a remarkable volume reduction, so-called densification, of amorphous SiC. From radial distribution function analyses using electron diffraction, notable changes associated with structural relaxation were observed in chemical short-range order. On the basis of the alteration of chemical short-range order, we discuss the origin of thermally induced densification in amorphous SiC.