The Effect of A-Site Cation on the Formation of Brannerite (ATi 2 O 6 , A = U, Th, Ce) Ceramic Phases in a Glass-Ceramic
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.470
The Effect of A-Site Cation on the Formation of Brannerite (ATi2O6, A = U, Th, Ce) Ceramic Phases in a Glass-Ceramic Composite System Malin C. Dixon Wilkins1, Martin C. Stennett1, and Neil C. Hyatt1 1 Immobilisation Science Laboratory, Department of Materials Science and Engineering, The University of Sheffield, Sheffield, UK
ABSTRACT
A range of stoichiometric and mixed A-site cation brannerite glass-ceramics have been synthesised and characterised. The formation of UTi2O6 in glass is reliant on ensuring all uranium remains tetravalent by processing in an inert atmosphere. ThTi2O6 forms in glass under both inert and oxidising atmospheres due to the lack of other easily available oxidation states. CeTi2O6 could not be made to form within this glass system. The formation of A0.5B0.5Ti2O6 phases depends strongly on the oxidation states of the A and B cations available in the process atmosphere, with the most successful compositions having an average final oxidation state of (A,B)4+. Mixed cation brannerite compositions that formed in argon include U0.75Th0.25Ti2O6 and U0.71Ce0.29Ti2O6. Those forming in air include U0.23Th0.77Ti2O6, Th0.37Ce0.63Ti2O6, and U0.41Ce0.59Ti2O6.
INTRODUCTION Brannerite (prototypically UTi 2O6) is a naturally occurring titanate mineral phase containing a particularly high fraction of uranium (> 55% uranium by weight), and has been suggested as a possible candidate host for high actinide content wastes.1 The analogous phase ThTi2O6 is relatively rare in nature,2 but naturally occurring Ubrannerites often have a high proportion of Th doped on the U site.3,4 Natural samples of CeTi2O6 with the brannerite structure have not been observed. It was first found that to successfully synthesise UTi 2O6 the oxygen partial pressure in the sintering atmosphere must be minimised to retain all U as U4+.5,6 Mixed cation brannerite ceramics have previously been investigated, with the substitution of lower valent cations (Ca2+, Y3+, Gd3+, and La3+) stabilising the presence of higher valent U in the brannerite structure.7–10 Brannerite ceramics have also been investigated as hosts
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for MOX (mixed oxide, i.e. mixed UO2 and PuO2) fuels, U0.9Ce0.1Ti2O6 and U0.81Ce0.09Gd0.1Ti2O6 (Ce as a surrogate for Pu) .11 A reasonable range of brannerite glass-ceramics have previously been reported in the literature, with the vast majority focussing on air-fired compositions following a batched ceramic stoichiometry of U0.5M0.5Ti2O6 (M is Tb3+, Dy3+, Y3+, and Eu3+).12–14 Pucontaining glass-ceramics have also been synthesised (Gd 0.2Pu0.5U0.5Ti2O6 and Gd0.1Hf0.1Pu0.2U0.6Ti2O6),13 which gives some indication that Pu is usefully soluble in UTi2O6, and does not interfere with the formation of these glass-ceramics. All o
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