Odile Majérus

Detection of First-Order Liquid/Liquid Phase Transitions in Yttrium Oxide-Aluminum Oxide Melts

We combine small-angle x-ray scattering (SAXS) and wide-angle x-ray scattering (WAXS) with aerodynamic levitation techniques to study in situ phase transitions in the liquid state under contactless conditions. At very high temperatures, yttria-alumina melts show a first-order transition, previously inferred from phase separation in quenched glasses. We show how the transition coincides with a narrow and reversible maximum in SAXS indicative of liquid unmixing on the nanoscale, combined with an abrupt realignment in WAXS features related to reversible shifts in polyhedral packing on the atomic scale. We also observed a rotary action in the suspended supercooled drop driven by repetitive transitions (a polyamorphic rotor) from which the reversible changes in molar volume (1.2 ± 0.2 cubic centimeters) and entropy (19 ± 4 joules mole–1 kelvin–1) can be estimated.

Effect of MoO3, Nd2O3, and RuO2 on the crystallization of soda–lime aluminoborosilicate glasses

The effect of adding Nd2O3, MoO3, and RuO2 separately or simultaneously on the crystallization of a soda–lime aluminoborosilicate glass during cooling from the melt or glass heating was studied by DTA, XRD (at room and high temperature), SEM, Raman, and optical absorption. Nd2O3 addition strongly reduces liquid–liquid phase separation and crystallization of calcium and sodium molybdates (CaMoO4 (powellite) and Na2MoO4) in Mo-rich compositions as long as Nd3+ ions remain solubilized in the glassy network. This suggests that (MoO4)2− entities and Nd3+ ions are close to each other in the glass structure (Nd3+ ions would prevent the clustering of molybdate entities). The effect of MoO3 addition in Nd-rich compositions is more complex since an increase of the solubility of Nd2O3 is observed, whereas the nucleation rate of an Nd-rich silicate apatite (Ca2Nd8(SiO4)6O2) in the bulk of the glass increases as soon as molybdates crystallized. The addition of RuO2 has a nucleating effect on apatite crystallization in the bulk but not on molybdates crystallization.

Structural and Crystallization Study of a Simplified Aluminoborosilicate Nuclear Glass Containing Rare-earths: Effect of ZrO 2 Concentration

Zirconium is an abundant element in nuclear wastes. In this paper, we present structural and crystallization results for a simplified glass composition belonging to the SiO 2-Al 2O3-B2O3Na 2O-CaO-ZrO 2-RE 2O3 system (RE = Nd or La) developed to immobilize highly concentrated waste solutions. The effect of varying ZrO 2 content on the structure and the crystallization tendency of this glass was studied using a multi-spectroscopic approach. Zr was shown to be located in six-fold coordinated sites with preferential charge compensation by Na + cations. Whereas a significant decrease of the proportion of BO 4 units was observed with ZrO 2 content, no effect was detected on the environment of AlO 4 units. However, a significant structural evolution of the silicate network occurred due to the formation of Si-O-Zr bonds. Whatever ZrO 2 concentration, the crystallization of only a rare earth silicate apatite phase was observed during either slow cooling from the melt or isothermal heat treatment. Whereas nucleation mainly occurred from the surface of the glass without ZrO 2, the introduction of zirconium induced apatite crystallization in the bulk. It is proposed that this nucleating effect of ZrO 2 is mainly due to changes induced in the neighborhood of Nd 3+ cations in glass structure.

Effect of Molybdenum and Ruthenium on the Crystallization Tendency of a New Nuclear Glass Containing High Rare-earth Concentration

The impact of Nd 2 O 3 , MoO 3 and RuO 2 addition on the competition between the crystallization of apatite Ca 2 Nd 8 (SiO 4 ) 6 O 2 and powellite CaMoO 4 phases which both may appear in High Level Waste nuclear glass (under certain specific conditions of cooling and glass composition) has been studied on a simplified composition belonging to the system SiO 2 -Na 2 O-CaO-Al 2 O 3 -B 2 O 3 . X-ray diffraction (at room temperature and high temperature) and scanning electron microscopy measurements have been performed on five glasses under two different thermal treatments. We show that RuO 2 acts as a nucleating agent for apatite. Moreover, neodymium and molybdenum cations seem to be very close in the glassy network as Nd 2 O 3 addition stops the phase separation of molybdates and inhibits the crystallization of CaMoO 4 . On the contrary, MoO 3 seems to favor the crystallization of apatite. For several samples, the evolution of the distribution of Nd 3+ cations after crystallization was followed by optical absorption spectroscopy.

Characterising density fluctuations in liquid yttria aluminates with small angle x-ray scattering

Small angle x‐ray scattering (SAXS) has been measured in the wavevector range 0.01<Q<1 A−1 for supercooled yttria‐alumina melts using a laser‐heated aerodynamic furnace. SAXS intensity rises gradually with temperature reflecting density fluctuations deriving from isothermal compressibility. With decreasing Q a minimum is located close to 0.1 A−1 at the foot of the inter‐atomic structure factor, below which SAXS rises, suggesting scatter from longer range fluctuating volumes.

Structure factor changes in supercooled yttria-alumina

Changes in the structure factor of yttria‐alumina liquids have been identified in the supercooled range. Different inter‐polyhedral configurations between AlO4 and YO6 groups distinguish low density and high density liquid phases. The coexistence of phases at high temperatures have been identified in simultaneous measurements of small angle x‐ray scattering.