article posted 21 March 2016
Laurence Galoisy is an associate professor in Mineralogy at the Université Pierre et Marie Curie (Paris, France) where she conducts research at the Institute of Mineralogy, Material Physics and Cosmochemistry (IMPMC). She completed her PhD at the Université Paris - Diderot in 1991and was a post-doctoral fellow at CHIPR/SUNY in Stony Brook (USA) in 1992. From 1993 to 2012, she was associate professor at the Paris-Diderot University (Paris, France). Her research focuses on structure-property relationships in minerals and glasses, using (micro) spectroscopic techniques (optical absorption spectroscopy at high pressure and temperature, x-ray absorption spectroscopy), with interest on geochemical, industrial and archeological applications.
Zirconium as local probe for oxide glass and glass-ceramic structures
Galoisy L.*, Calas G., Dugué E. and Cormier L.
Institut de Minéralogie, de Physique des Matériaux et de
Cosmochimie (IMPMC) - Sorbonne Universités – UPMC
UMR CNRS 7590 - IRD UMR 206 – MNHN - 4 place Jussieu F-75005 Paris, France
Cations play a complex structural role in glasses, as they occur in different kinds of environments exerting a strong influence on the physical and chemical properties of this material. The amorphous nature of glass and often, the complex chemical composition, do not allow the construction of a unique structural model, as in crystalline compounds. If glasses lack periodicity and long-range order, they still retain a characteristic short-range order, which obeys the same crystal-chemical rules as in crystals. The combination of structural information given by a wide range of spectroscopic methods, by radiation scattering and numerical modelling, give insight on the structural organisation around cations.
Because of the variability of the local environment of Zr in crystalline oxides, this element was used as a structural probe in oxide glasses. Zr is generally found in a regular octahedra in glasses, an unusual coordination seldom encountered in minerals. A molecular scale approach helps understand the origin of the peculiar physicochemical properties provided by the presence of Zr in glasses, e.g. the resistance to alteration or the formation of glass-ceramics.
In France, the high level radioactive wastes produced by spent fuel reprocessing are incorporated into a borosilicate glass. A major challenge is the demonstration of a concept for safe disposal of the nuclear wastes. The waste form plays a central role to ensure radionuclide immobilization during the lifetime required by safety assessment scenarios. The leaching of glass by water is one of the most important mechanisms of long-term radionuclides release. ZrO2
contributes 2.75 % of the glass mass, but has a strong effect on the capability of glass to resist to alteration. During alteration in water, an amorphous gel is formed. The durability of the gel and its properties depend on the structural role played by Zr. In minor content in the glass, Zr is 6-coordinated with alkali or alkaline-earth elements as charge compensating cations. When the glass is altered under saturated conditions, Zr remains in octahedral sites in a protective gel. In open alteration conditions, Zr coordination changes to 7 in a non-protective gel and the dissolution rate of the glass increases. New generations of spent fuels require higher content of Zr in glasses. The structural role of Zr and the evolution of the gel will be discussed.
Glass–ceramics have a wide range of applications (architectural materials, cookware, bone implants, or hybrid optical materials). Glass–ceramics are made of randomly oriented fine-grained crystals embedded in a residual glass. The crystallization process starts with a nucleation event. Internal nucleation is induced by nucleating elements such as Zr, decreasing the crystallization temperature of the parent glass and increasing bulk crystallization rates. Zr initiate the first observable crystalline phases above Tg, in the form of t-ZrO2. Little information is available on the local structure around nucleating elements in the parent glass. By combining differential scanning calorimetry (DSC), Zr K-edge X-ray absorption near-edge spectroscopy (XANES) and electron-microscopy, an in situ investigation of the Zr environment during the thermal treatment of a ZrO2
glass, gives direct information on atomic-scale processes governing nucleation/crystallization of glasses at high temperature.