George W. Arnold

Phosphate glass dissolution in aqueous solutions

Rates of aqueous dissolution are reported for a series of phosphate glasses having the composition (50 − X)M2O·(X)CaO·50P2O5. Dissolution reactions involving the consumption of H+ and OH− were monitored using pH stat titration techniques, solution analyses using inductively coupled plasma emission, and depth profiling of corroded glass surfaces for H and other elements using elastic recoil detection analysis and Rutherford backscattering. These analytical results indicate that the phosphate glasses dissolve uniformly due to acid- or base-catalyzed hydration of the polymeric phosphate network. The rate and nature of this hydration appears to be controlled by the surface pH and/or charge which develops at the glass/ solution interface. The implication of these results on dissolution models for both the phosphate and silicate glasses is discussed.

The surface chemistry of dissolving labradorite feldspar

Abstract Elastic recoil detection (ERD) analysis was used in conjunction with Rutherford backscattering (RBS) analysis to determine depth profiles of hydrogen, silicon, aluminum and calcium in labradorite crystals reacted under various pH conditions. The inventory of hydrogen in the mineral is strongly affected by solution pH. Hydrogen extensively infiltrates the mineral during reaction for 264 hours with solutions in the pH range 1–3. Infiltration is accompanied by extensive removal of sodium, calcium and aluminum from the mineral. This incongruent reaction proceeds to several hundreds of angstroms of depth and produces a silicon-rich surface which is amorphous to electron diffraction. The amount of hydrogen in the reacted layer is much less than is predicted from knowledge of the quantity of cations leached from the feldspar. These low inventories of hydrogen suggest that hydrogen-bearing groups in the reacted layer repolymermize subsequent to ion exchange and depolymerization reactions. This repolymerization eliminates hydrogen from the layer. At higher pH conditions (pH > 5), hydrogen inventories in the crystals decrease with time relative to an unreacted reference crystal. Hydrogen does not infiltrate beyond the first few unit cells of feldspar. Thus, dissolution in slightly acid, near-neutral, and basic solutions proceeds at the immediate surface of the feldspar. Within the limit of the RBS technique, there is no evidence for incongruent dissolution at these conditions.

Recoil refinements: Implications for the 40Ar/39Ar dating technique

Integration of the neutron energy distribution for a water-moderated reactor with the most recent cross-section data yields mean recoil energies of 177 keV for 39K (n, p) 39Ar, 969 keV for 40Ca (n, α) 37Ar, and 140 eV for 37Cl (n, γ) 38Cl (β) 38Ar. These estimates are insensitive to the anisotropy of reaction products. Utilizing Monte Carlo simulations of collision cascades, we calculate a mean recoil range of 1620 A for 39K (n, p) 39Ar, 3780 A for 40Ca (n, α) 37Ar, and 11 A for 37Cl (n, γ) 38Cl (β) 38Ar. Rutherford backscatter (RBS) measurements of argon implantation experiments into albite confirm the veracity of these estimates. Integration of the recoil range distributions yields a mean depletion depth in a semi-infinite medium of 820 A for 39Ar, 1950 A for 37Ar, and 6 A for 38Ar. The concentration gradients generated by recoil-redistribution between thin slabs were then incorporated into standard diffusion equations. If the exsolution lamellae are the effective diffusion dimension for argon, then the calculations indicate that the argon release rates and 40Ar/39Ar age spectrum derived from incremental heating of minerals exsolved at the micron to submicron scale are significantly affected by recoil-redistribution. The age spectra will be discordant even if the feldspar has not experienced a complex or slow cooling history. Incremental step apparent ages will increase with the fraction of 39Ar released as the potassium poor lamellae degas. The age spectra will exhibit decreasing apparent ages with increasing fraction 39Ar released as the potassium feldstar lamellae degas. The overall profile of the age spectrum will depend upon the composition of the feldspar and the size distribution of the lamellae, if the lamellae are the effective argon diffusion dimension. In principal, these calculations can be used to discriminate between different models for argon diffusion in minerals. Finally, the 11 A mean recoil distance calculated for 38Ar indicates that it is not a proxy for anion-sited excess argon. Instead, published correlations of 38Ar with excess 40Ar probably reflect the degassing of fine-grained, Cl-rich inclusions.

Atomic displacement and ionization effects on the optical absorption and structural properties of ion‐implanted Al2O3

Ion‐induced lattice atom displacements in single‐crystal Al2O3 give rise to an optical absorption band at 204 nm as well as to expansion of the implanted volume. The 204‐nm absorption per unit energy into atomic processes is found to increase rapidly with decreasing incident ion mass. In contrast, light ions (H+, D+, He+) show less volume expansion per unit energy into atomic processes than do heavier ions. Furthermore, the volume expansion induced by heavy ion implantation can be completely relieved by H+‐ion implantation or by ionizing electron irradiation (8.16 keV). A simple model characterizes the results. The 204‐nm absorption is found to be proportional to the ion energy into electronic processes, and the expansion is proportional to the ion energy into atomic processes linearly reduced by the ion energy into electronic processes. The implications for defect production in CTR insulators are discussed.

Mechanisms for alkali leaching in mized-NaK silicate glasses

Abstract The kinetics of alkali removal from (1-X) Na2O·XK2O·3SiO2 glasses were studied using pH stat titration techniques, solution analyses, and elemental depth profiling by Rutherford backscattering spectrometry and elastic recoil detection. In the first stage of leaching ( t 1 2 kinetics), the interdiffusion coefficients measured for the exchange of alkali cations by H3O+ are orders of magnitude greater than the alkali diffusion coefficients in bulk glass and show no evidence of the mixed-alkali effect. At longer times, the rate of alkali removal becomes constant with time, but selective alkali leaching is still observed rather than uniform dissolution. These results support a model where the rate determining step for alkali leaching is the rate at which molecular water diffuses into the glass.

the surface of labradorite feldspar after acid hydrolysis

Abstract After reaction with a pH Both the surface area of the reacted feldspar and the porosity increase with acid hydrolysis. Modeling of nitrogen sorption onto the surface suggests that the pores have a nominal radius of ∼ 20–80 A or less. This distribution of pore sizes resembles other acid-reacted silicate materials, such as glass, chrysotile and kaolinite. Although the mineral surface clearly becomes more porous during acid hydrolysis, the increase in powder area also does not coincide with an increase in the flux of dissolved Si from the powder. We thus attribute most of this increase in area to spallation of the silica-rich surface from the feldspar upon drying.

The effect of molecular structure on borosilicate glass leaching

The leaching behavior of several sodium borosilicate glasses has been characterized using a combination of pH stat titrations. elemental solution analyses, elemental depth profiling via elastic recoil detection analysis and Rutherford backscattering. sodium diffusion measurements, and 11B NMR measurements on both leached and unleached glass. Leaching results indicate that the molecular structure of the glass controls glass dissolution by establishing the distribution of ion exchange sites, hydrolysis sites, and the access of water to those sitas. There is no correlation between sodium leaching and sodium diffusion in the unaltered glass. For most borosilicates in most environments, network hydrolysis controls the kinetics of glass dissolution.

LATTICE EXPANSION AND STRAIN IN ION‐BOMBARDED GaAs AND SI

Measurements of the surface topography of ion‐bombarded GaAs and Si show a marked elevation of the irradiated portion of the surface relative to the surrounding unirradiated material. The average lattice atom displacement brought about by the expansion can cause anomalously high yields of displaced atoms as measured by backscattering of particles incident along channeling directions.

Characterization of the near-surface region of glass implanted with light elements

Abstract Ion-implantation of glasses modifies physical properties such as density, refractive index, surface stress, hardness, and chemical durability. Compositional changes can also occur due, e.g., to radiation-enhanced diffusional losses of alkali ions, crystallization, phase separation, and H incursion. Quantitative depth-profiling of the implanted ion distributions and changes in elemental constituent concentrations can be accomplished by means of ion-beam analysis. Elastic recoil detection (ERD) is used for light elements (H to N) while Rutherford backscattering (RBS) is utilized for larger masses. Chemical information from X-ray photoelectron spectroscopy (XPS) supplements the compositional information obtained from ERD and RBS to supply detailed data about the surface region or in-depth after sputter etching. These techniques, have been utilized, in combination with optical spectroscopy, to study the effects of ion-implanted H, Li, B, N, O, and Si on fused silica. Evidence has been obtained for the chemical incorporation of these elements in the silica network. The results allow deeper understanding of the relationship of structure to implant incorporations, as is important for the application of ion implantation wave guide formation in optoelectronic applications.

Ion implantation in silicate glasses

Abstract This review examines the effects of ion implantation on the physical properties of silicate glasses, in particular, induced changes in optical properties, volume changes and consequent refractive index changes, and hardness alterations. In addition, the effects of chemical modification, either through direct interactions with the implanted species or through alkali depletion and crystallization, are discussed. The use of metal implants to form colloidal nanosize particles for increasing the non-linear refractive index is emphasized due to the importance for optoelectronic applications.