Unmixing inducing polymerization of a sodium-molybdenum borosilicate network

Glass network polymerization critically influences rheological behavior and with it the kinetics and dynamics of nuclear waste immobilization. Molybdenum-bearing borosilicate melts may undergo unmixing and rheological changes, which is dominantly controlled by the associated chemical modifications of the melt network. Here, we obtain in-situ (500–940 °C) Raman spectra to probe structural changes of a sodium-molybdenum borosilicate melt undergoing unmixing. The extraction of alkali and molybdenum to form droplets induces polymerization of the residual borosilicate network. Conversely, the opposite phenomenon is observed during droplet re-dissolution. This work provides new insights into the polymerization of a molybdenum-bearing borosilicate composed of two composition sets due to a miscibility gap and has direct contributions for the immobilization of nuclear wastes

Étude des changements de rédox du cérium entre le verreet le liquide aluminosilicaté : Une approche multispectroscopique

International audienceRedox control of glasses is paramount both to their fusion process and to obtaining the desired properties of high technological glasses. However, the link between melting parameters, such as temperature, furnace atmosphere, or quenching rate, and the redox state of the final products is poorly understood. In this work, in situ x-ray absorption near-edge structure (XANES) data at Ce L3-edge data were acquired at high temperatures on cerium-containing sodium aluminosilicate glasses, allowing the determination of thermodynamic constants necessary to predict the cerium redox state over a wide temperature range (900–1500 °C). The results obtained were compared to the Raman spectra of samples quenched at different temperatures. Our findings demonstrate that the quench performed was fast enough to block the cerium oxidation state, meaning the redox measured at room temperature is representative of a high temperature state. This was further verified by room temperature Raman spectroscopy, where a relationship was found between the spectra and melting conditions. Wet chemical analysis, XANES at Ce L3-edge, Raman spectroscopy, and optical absorption spectroscopy were successfully used to determine the redox state of cerium in aluminosilicates

Charge Transfer Between Ce and Fe During Cooling of an Aluminosilicate Melt: An In Situ XANES Investigation

International audienceMultivalent elements are often incorporated to silicate glasses to enhance specific properties to the final product. However, these properties strongly depend on the redox state of the multivalent elements. While the redox behavior of glasses containing a single multivalent element is well studied, research on the in situ interaction between multiple multivalent elements is scarce. In this study, in situ XANES spectroscopy was used to investigate the high temperature redox state of both Ce and Fe in an aluminosilicate melt. The results were compared to room temperature measurements. Our findings demonstrate that, at high temperature equilibrium, Ce and Fe act independently. However, upon cooling below 900°C, a charge transfer process occurs between the two elements as described by the reaction Ce4+ + Fe2+ → Ce3+ + Fe4+. The existence of such a charge transfer, observed even in melts doped with very low Ce and Fe amounts, could suggests that both elements are not randomly distributed in the melt. The intensity of the charge transfer process depends on the CeO2/FeO ratio, with the element present in excess showing minimal change in redox state upon cooling. This explains the difference of room temperature redox state between samples

Influence de l’addition de cérium et de l’état redox sur la structure et la viscosité des silicates

International audienceThe viscosities of Ce-free and Ce-bearing (∼1.3 mol%,∼6.5 wt.% Ce2O3) soda lime silicate (window glass) melts were measured with respect to oxidation state. Experiments were performed isothermally using a concentric-cylinder viscometer on melts equilibrated with successively reducing CO–CO2 gas mixtures within a gas tight vertical tube furnace at 1 atm. Viscosity measurement and sampling were performed at the end of each melt reduction step. Further, viscosities in the glass transition temperature range were estimated using the shift factor method applied to glass transition temperature values determined using differential scanning calorimetry (DSC) measurements on quenched glasses. The Ce speciation at each stepwise melt reduction was probed using Ce L3-edge X-ray absorption near-edge structure (XANES) spectroscopy, while structural information upon Ce addition and reduction was provided by Raman spectroscopy. The viscosities of these materials remain constant at this level of Ce addition and do not vary significantly with redox state at high temperature. Conversely, viscosity values in the glass transition temperature range increase upon both Ce addition and reduction. Our analysis, based on the glass composition analyses obtained via electron probe microanalyzer (EPMA), viscosity calculations, and observations of silicate structural changes, leads to the conclusion that the observed viscosity increase around the glass transition temperature is explained by the high ionic field strength of Ce ions as well as the polymerization behavior of the silicate matrix occurring during reduction of Ce. Because of its low concentration, resulting from its low solubility, Ce redox changes exert only minimal effects on the viscosity of this melt

Growth and properties of defect-free ZnSe nanowires and nanoneedles

International audienceNot Availabl

A CdSe quantum dot in a ZnSe nanowire as an efficient high-temperature single-photon source

SPIE OPTO: Integrated Optoelectronic DevicesInternational audienc