Andrew C. Buechele

Structural characterization of high-zirconia borosilicate glasses using Raman spectroscopy

Polarized Raman spectra were obtained for a collection of borosilicate glasses that have been developed as candidate compositions for the immobilization of wastes generated from the reprocessing of Zircaloy-clad spent nuclear fuel. Raman spectra were obtained for borosilicate glasses with zirconia compositions as high as 21 wt%, as well as for crystalline ZrO2 (baddeleyite) and crystalline ZrSiO4 (zircon). As zirconia content in the glass is increased, two trends in the spectra indicate that the partially polymerized silicate tetrahedral network becomes more depolymerized: one, the polarized mid frequency envelope near 450 cm ˇ1 , assigned to Si‐O‐Si symmetrical bend modes, decreases in area; and two, the higher frequency band assigned to Si‐O stretch in Q 2 units (silicate chains) increases in area, while band areas decrease for modes assigned to Si‐O stretch in more polymerized Q 3 and Q 4 units (silicate sheets and cages). These trends take place whether the glass composition is relatively simple or considerably more complex. As zirconia concentrations in the glass increase beyond 15 wt%, a series of sharp lines are observed in the spectra from baddeleyite crystals, and to a minor extent Zn‐Cr spinel phases, superimposed on broad features from the glass matrix. A low intensity, broad band near 1400 cm ˇ1 in the glass spectra is probably due to B‐O stretch modes within BO3 units. ” 2000 Elsevier Science B.V. All rights reserved.

X-ray absorption studies of the local environment of Zr in high-zirconia borosilicate glasses

Zirconium X-ray absorption spectroscopic (XAS) data were collected and analyzed for a series of zirconia borosilicate glasses developed as potential formulations for immobilization of nuclear fuel reprocessing wastes. For the first time, XAS are presented for silicate glasses with zirconia concentrations as high as 21 wt% and for zektzerite (LiNaZrSi6O15), a crystalline standard that is chemically similar to the glasses. Two other glass series were investigated where the CaO and ZnO contents were varied individually, while the Zr content was held constant. The analysis results indicate that Zr4+ is predominantly in octahedral sites in the glasses. Trends in the data also show that the Zr environments in the glasses are sensitive to changes in ZrO2 concentration, but are less sensitive to changes in Zn or Ca content. With addition of zirconia, the Zr site population shifts from mostly six-coordinated with minor amounts of seven-coordinated Zr, to having a larger fraction of seven-coordinated Zr sites, while maintaining a majority of octahedral Zr. The seven-coordinated Zr atoms probably act as nucleation sites in the glass; as the zirconia content increases to 15 wt% and higher, the numbers of these sites become large enough to promote ZrO2 baddeleyite crystallization. There is no evidence of Zr–Zr pair correlations, similar to those observed in baddeleyite, in any of the crystal-free glasses.

Absorption in the troughs of the far infrared spectra of NH3 and mixtures of NH3 and H2

Abstract The absorption coefficients in the troughs between the J-multiplets of the far infrared rotation-inversion spectrum of NH3 and NH3 broadened by H2 were measured at 296 K in the region from about 65–230 cm−1. Wave numbers were selected in the troughs that are removed from nearby weak absorption lines, and measurements of the absorption versus the density of NH3 and of H2 verified that the absorption was due to the wings of distant rather than nearby spectral lines. These measurements were compared with computations of the absorption arising from binary collisions of NH3–NH3 and NH3–H2 for uncoupled Lorentz lines. Except at the lowest and highest frequencies, the measured continuum absorption in the troughs was less than the computed absorption, a discrepancy that increased with increasing frequency. An improved representation of the measured absorption was obtained by fitting the Rosenkrantz line shape (Orton et al., unpublished) with the Ben-Reuven (Lightman and Ben-Reuven. J Chem Phys 1969;50:351–3) formulation of the line coupling coefficient of the inversion J-doublets. The application of these results to the spectrum of Jupiter shows clear differences between the spectrum of the above model and the Lorentz line shape, with the largest differences occurring in the clearest troughs.

Local environment of Zn in zirconium borosilicate glasses determined by X-ray absorption spectroscopy

The Zn environments in six zirconium borosilicate glasses were characterized using X-ray absorption spectroscopy (XAS). The Zn environment is of interest because the addition of ZnO to borosilicate glass produces beneficial eAects on certain bulk physical properties that are important in the production and long-term storage of radioactive waste glasses. Zn XAS data and analyses show that all glass samples investigated contain Zn aa surrounded by four oxygens in tetrahedral coordination, with most likely, Si second nearest neighbors. The Zn environments are essentially invariant in all of the glass samples investigated, despite significant diAerences in glass chemistry and sample homogeneity. The results indicate that tetrahedral Zn in these glasses takes on the role of a weakly bonded network former. As ZnO is added to a silicate glass, Zn tetrahedra most likely link to silicate tetrahedra, decrease the volume of voids between linked tetrahedra, and restrict diAusion of large ions. These inferences concur with observations that leach rates are reduced for zirconium borosilicate glass as ZnO is added. ” 2000 Elsevier Science B.V. All rights reserved.

Effect of surface layers on the dissolution of nuclear waste glasses

Explanation of the striking non-linear effect of glass composition on the aqueous dissolution represents an important challenge to existing dissolution mechanisms. Surface layers that are formed during glass dissolution may play an important role in this effect. One chemically reactive and one less-reactive nuclear waste glass (leachate concentrations differ by about a factor of 10) were reacted in deionized water. Two types of glass powders were used: Type A powders were pristine glass powders; Type B powders were the glass powders which had been reacted for 120 days to develop the surface layers. Both the solution concentrations and the surface layers were investigated. The experimental observations indicate that: (i) There is a range of glass compositions over which small differences in composition lead to large changes in both reaction rates and surface layer thickness; and (ii) The reaction rate is strongly affected by the formation of the surface layer (the layer appears to be protective) and cannot be explained in terms of saturation effects alone. The findings are contrary to the conclusion of a previous study and serve to highlight the inadequacy of existing dissolution models predicated on an overly simplistic mechanism, especially with regard to glass composition effects.

Raman studies of technetium in borosilicate waste glass

The characterization of technetium (Tc) environments in borosilicate glasses is important with regard to the long-term isolation of Tc in nuclear wastes from the environment by waste vitrification. In this study, Raman spectroscopy was used to detect Tc in various borosilicate waste glass formulations in which Tc valence and average coordination environment were previously characterized by X-ray absorption spectroscopy. Raman spectral comparisons between chemically equivalent Tc-containing and Tc-free borosilicate glass pairs indicate Tc-dependent features near 320 and 910 cm−1 resulting from internal vibrational modes of pertechnetate tetrahedral (Tc7+ in TcO4−), similar to those observed for pertechnetate aqueous solutions. Initial observations of the pertechnetate Tc–O symmetric stretch mode near 910cm−1 indicate a peak area dependence on pertechnetate concentration and a detection limit near 50 ppm Tc7+ in borosilicate glass. Pertechnetate Tc–O symmetric stretch mode frequency versus composition relationships for the borosilicate waste glasses suggest that pertechnetate tetrahedra are surrounded by network-modifying cations. However, Raman spectroscopy is insensitive to six-coordinated Tc4+ species in borosilicate glasses.

X-ray absorption characterization of Cr in forsterite within the MacAlpine Hills 88136 EL3 chondritic meteorite

Abstract Chromium K-edge X-ray absorption spectra were collected to characterize Cr in forsterite (Mg2SiO4) as well as sulfides within the MAC 88136 EL3 chondrite to determine Cr valence and to see whether forsterite within this meteorite can be used as a Cr2+-silicate standard. Spectra were measured on several areas within a nearly pure 100 × 200 μm forsterite grain containing 0.13 wt% Cr. XANES findings indicate highly reduced Cr2+ species, with no clear evidence of Cr3+ or Cr6+. EXAFS data indicate an average 2.02 Å Cr-O nearest-neighbor distance, consistent with Cr-O distances found in square-planar Cr2+O4 sites observed in synthetic crystalline silicates, and an average 2.69 Å Cr-Si second-nearest neighbor distance, consistent with Cr2+ substituting for Mg2+ in the forsterite M(1) site. Nearest-neighbor Debye-Waller factor and coordination number parameters indicate Cr2+ is likely entering forsterite in disordered sites that are possible intermediates between M(1) and squareplanar Cr2+O4 configurations. Preliminary Cr XAS measurements on sulfides within this meteorite also indicate Cr2+ in CrS6 octahedra.

Experimental and theoretical studies of absorption in microwindows of the ν4-band of methane and methane–hydrogen

Abstract The absorption coefficients of room temperature CH4 and CH4–H2 mixtures were measured in the 1300– 1360 cm −1 region of the ν4 band at about 295 K as a function of pressure. These results were analyzed at frequencies in the most transparent regions of various microwindows in order to study the far wings of self- and H2-broadened lines in the R-branch of the ν4 band. The experimental results were compared with synthetic spectra computed from the addition of Lorentzian line contributions. From such comparisons it appears that the high-frequency wings of the lines of the Q-branch decay more rapidly than do Lorentz wings. At higher wave numbers, the measured absorption tends to be equal and then less than the computed Lorentz absorption, although these deviations may not be significant in view of the uncertainties of the measurements and line parameters used in the computed spectra. Nonetheless, the computed absorption when including line coupling is in better overall agreement with the experiment and confirms the role of the Q-branch in producing the sub-Lorentzian absorption noted above.

Formulation, Testing, and Structural Characterization of High-Zirconium High-Level Waste Glasses

A range of compositions of high-zirconia borosilicate glasses were formulated and their structures investigated by a combination of techniques. These compositions have potential applications for high-level nuclear waste storage in combination with advanced reprocessing methods. Raman and Zr EXAFS data were collected for a series of glasses spanning a range of zirconia concentrations. The Raman spectra indicate that Zr acts as a silicate network modifier, where the silicate tetrahedral network depolymerizes as the zirconia content increases. Zr EXAFS analysis indicates that Zr is found in octahedral sites, and to a minor extent, sevencoordinated sites. As the zirconia content increases, the fraction of seven-coordinated Zr-sites increases; this may be the cause of ZrO 2 baddeleyite crystallization that was observed in some Zr-rich glasses investigated.

Alteration of Microstructure of West Valley Glass by Heat Treatment

Samples of West Valley reference glass WVCM-59 were subjected to isothermal heat treatment according to a systematic schedule of time-temperature combinations. Phases crystallizing during heat treatment were analyzed and quantified using an SEM equipped with an energy dispersive x-ray detector and image processing and analyzing capabilities. Only small amounts of noble metals (e.g. Rh, Pd) and RuO 2 were present in the as-melted glass, but these frequently serve as nucleation sites during heat treatment. Iron-group spinels containing Fe, Ni, Cr, and Mn in variable proportions were the most common phases observed, appearing in quantities up to 3.5 vol% in heat treated glass. The formation of a thoria-ceria phase occurred at temperatures of 900°C and below. Acmites formed at temperatures of 800°C and below. A lithium phosphate phase with a particularly interesting morphology developed after prolonged heat treatment at temperatures between 600–750°C. The same phase appeared in a range of glass compositions. However, canister cooling curves preclude such a phase in production glass. Furthermore, no such phase was found in actual canister-cooled glass.