Effect of Al 2 O 3 concentration on zirconolite (Ca (Zr, Hf) Ti 2 O 7) crystallization in (TiO 2, ZrO 2, HfO 2)-rich SiO 2–Al 2 O 3–CaO–Na 2 O glasses
ABSTRACT Glass-ceramic matrices containing zirconolite (nominally Ca(Zr,Hf)Ti 2 O 7) crystals in their bulk that would incorporate high proportions of minor actinides (Np, Am, Cm) or plutonium could be envisaged for their immobilization. Zirconolite-based glass-ceramics can be prepared by controlled crystallization of zirconolite in glasses belonging to SiO 2 –Al 2 O 3 –CaO–Na 2 O–TiO 2 –ZrO 2 – HfO 2 system. In this study, neodymium was used as tri-valent actinides surrogate. Increasing Al 2 O 3 concentration in glass composition had a strong effect on the nucleation rate I z of zirconolite crystals in the bulk, on the amount of neodymium incorporated in zirconolite phase and on the crystal growth rate of silicate phases (titanite + anorthite) from glass surface. These results could be explained by the existence of competition—in favor of aluminum—between Al 3+ and (Ti 4+ , Zr 4+ , Hf 4+) ions for their association with charge compensators cations to facilitate their incorpora-tion in the glassy network. Differential thermal analysis (DTA) was used to study exothermal effects associated with bulk and surface crystallization. 27 Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra showed that aluminum enters glasses network predomi-nantly in 4-fold coordination. Neodymium optical absorp-tion and fluorescence spectroscopies showed that the Al 2 O 3 concentration changes performed in this study had not significant effect on Nd 3+ ions environment in glasses.
Full-textDOI: · Available from: Daniel Caurant, Mar 14, 2014
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ABSTRACT: Glass–ceramic materials containing zirconolite (nominally CaZrTi2O7) crystals in their bulk can be envisaged as potential waste forms for minor actinides (Np, Am, Cm) and Pu immobilization. In this study such matrices are synthesized by crystallization of SiO2–Al2O3–CaO–ZrO2–TiO2 glasses containing lanthanides (Ce, Nd, Eu, Gd, Yb) and actinides (Th) as surrogates. A thin partially crystallized layer containing titanite and anorthite (nominally CaTiSiO5 and CaAl2Si2O8, respectively) growing from glass surface is also observed. The effect of the nature and concentration of surrogates on the structure, the microstructure and the composition of the crystals formed in the surface layer is presented in this paper. Titanite is the only crystalline phase able to significantly incorporate trivalent lanthanides whereas ThO2 precipitates in the layer. The crystal growth thermal treatment duration (2–300h) at high temperature (1050–1200°C) is shown to strongly affect glass–ceramics microstructure. For the system studied in this paper, it appears that zirconolite is not thermodynamically stable in comparison with titanite growing form glass surface. Nevertheless, for kinetic reasons, such transformation (i.e. zirconolite disappearance to the benefit of titanite) is not expected to occur during interim storage and disposal of the glass–ceramic waste forms because their temperature will never exceed a few hundred degrees.Journal of Nuclear Materials 07/2010; 402(1):38-54. DOI:10.1016/j.jnucmat.2010.04.021 · 2.02 Impact Factor
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ABSTRACT: Zirconolite (CaZrTi2O7) is a proposed ceramic for the disposal of plutonium. Density functional theory with the dispersion correction (DFT-D3) has been used to study the behaviour of the He defect in zirconolite. The lowest energy He interstitial site is located in the 〈0 1 0〉 channels and found to have a migration barrier of 1.46 eV. There was a significant charge state dependence on the binding energies of a He atom to the vacancies, with the neutral 5-fold coordinated Ti having the strongest binding followed by the Ca vacancies. Multiple He interstitials were studied to examine if He bubbles were likely to form in bulk zirconolite. It was found that it was unfavourable for He to cluster at the concentrations studied.Journal of Nuclear Materials 06/2013; 437(s 1–3):261–266. DOI:10.1016/j.jnucmat.2013.02.037 · 2.02 Impact Factor
- Advances in Applied Ceramics 10/2014; 113(7):394-403. DOI:10.1179/1743676114Y.0000000192 · 1.11 Impact Factor