Vanadium Oxidation States and Structural Role in Aluminoborosilicate Glasses: An Integrated Experimental and Molecular Dynamics Simulation Study.

Vanadium-containing glasses have aroused interest in several fields such as electrodes for energy storage, semiconducting glasses, and nuclear waste disposal. The addition of V2O5, even in small amounts, can greatly alter the physical properties and chemical durability of glasses; however, the structural role of vanadium in these multicomponent glasses and the structural origins of these property changes are still poorly understood. We present a comprehensive study that integrates advanced characterizations and atomistic simulations to understand the composition-structure-property relationships of a series of vanadium-containing aluminoborosilicate glasses. UV-vis spectroscopy, X-ray photoelectron spectroscopy, and X-ray absorption near-edge structure (XANES) have been used to investigate the complex distribution of vanadium oxidation states as a function of composition in a series of six-component aluminoborosilicate glasses. High-energy X-ray diffraction and molecular dynamics simulations were performed to extract the detailed short- and medium-range atomistic structural information such as bond distance, coordination number, bond angle, and network connectivity, based on recently developed vanadium potential parameters. It was found that vanadium mainly exists in two oxidation states: V5+ and V4+, with the former being dominant (∼80% from XANES) in most compositions. V5+ ions were found to exist in 4-, 5-, and 6-fold coordination, while V4+ ions were mainly in 4-fold coordination. The percentage of 4-fold-coordinated boron and network connectivity initially increased with increasing V2O5 up to around 5 mol % but then decreased with higher V2O5 contents. The structural role of vanadium and the effect on glass structure and properties are discussed, providing insights into future studies of sophisticated structural descriptors to predict glass properties from composition and/or structure and aiding the formulation of borosilicate glasses for nuclear waste disposal and other applications.

Sol-Gel Synthesis and Characterization of Gels with Compositions Relevant to Hydrated Glass Alteration Layers.

During the processes associated with glass corrosion, porous hydrated glass alteration layers typically form upon exposure to aqueous conditions for extended time periods. The impacts of the alteration layer on glass durability have not been agreed upon in the glass science community. In particular, the formation mechanisms of hydrated glass alteration layers are still largely unknown and require further investigation, but these layers often require months to years to develop and are often too thin to adequately characterize. Meanwhile, sol-gel-derived silicate gels are relatively easy to synthesize in bulk with custom compositions relevant to hydrated glass alteration layers. If alteration layers and synthetic silicate gels demonstrate physical and chemical properties that are sufficiently similar, synthetic silicate gels could be used as analogues for hydrated glass alteration layers in future studies. However, synthetic gels must first be prepared and evaluated before comparisons between glass alteration layers and synthetic silicate gels can be made. This work focuses entirely on the synthesis and observed physical properties of synthetic silicate gels. A future work will compare the characteristics of synthetic gels described in this work with altered waste glass formed in similar pH environments. In this study, synthetic gels were made with custom compositions at various pH values to evaluate the effect of pH on gel structure and morphology. Several other variables were examined also, such as composition, drying, and aging. Gels were produced by sequential additions of organometallic precursors in a single container. Gels were analyzed with several techniques including small-angle X-ray scattering, gas adsorption, and He pycnometry to determine the effects of the variables on physical properties. Results show that gels prepared at pH 3 consistently contained fewer primary particles with diameters larger than 7.2 nm and fewer pores with diameters larger than 30 nm compared to gels synthesized at pH 7 and 9. Composition was shown to have no discernable effect on primary particle and pore sizes at any pH.

Investigating the Durability of Iodine Waste Forms in Dilute Conditions.

To prevent the release of radioiodine during the reprocessing of used nuclear fuel or in the management of other wastes, many technologies have been developed for iodine capture. The capture is only part of the challenge as a durable waste form is required to ensure safe disposal of the radioiodine. This work presents the first durability studies in dilute conditions of two AgI-containing waste forms: hot-isostatically pressed silver mordenite (AgZ) and spark plasma sintered silver-functionalized silica aerogel (SFA) iodine waste forms (IWF). Using the single-pass flow-through (SPFT) test method, the dissolution rates respective to Si, Al, Ag and I were measured for variants of the IWFs. By combining solution and solid analysis information on the corrosion mechanism neutral-to-alkaline conditions was elucidated. The AgZ samples were observed to have corrosion preferentially occur at secondary phases with higher Al and alkali content. These phases contained a lower proportion of I compared with the matrix. The SFA samples experienced a higher extent of corrosion at Si-rich particles, but an increased addition of Si to the waste led to an improvement in corrosion resistance. The dissolution rates for the IWF types are of similar magnitude to other Si-based waste form materials measured using SPFT.