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| Home > Publications database > Structural changes of Nd- and Ce-doped ammonium diuranate microspheres during the conversion to U$_{1-y}Ln_y$O$_{2\pm x}$ |
| Home > Publications database > Structural changes of Nd- and Ce-doped ammonium diuranate microspheres during the conversion to U$_{1-y}Ln_y$O$_{2\pm x}$ |
| Journal Article | FZJ-2020-03880 |
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2020
Elsevier Science
Amsterdam [u.a.]
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Please use a persistent id in citations: http://hdl.handle.net/2128/25872 doi:10.1016/j.jnucmat.2020.152454
Abstract: The structural changes of ammonium diuranate (ADU) microspheres, prepared by the sol-gel route via internal gelation, were investigated during thermal treatments in oxidative and reducing conditions. In particular, $in-situ$ simultaneous thermogravimetric analyses and $in-situ$ high-temperature scanning electron microscopy investigations were carried out on un-doped ADU microspheres, and ADU microspheres doped with 10 mol% neodymium or 10 mol% cerium. Calcination in air caused the surface of the particles to crack, but with increasing temperature up to 900 °C some healing, and additionally, shrinkage occurred. The extent of the fractures and the amount of shrinkage was, however, significantly more pronounced in a particle prepared with a tetravalent Ce precursor, as compared to particles prepared with a trivalent Nd or Ce precursor. The macroscopic behaviour could be related to the release of volatile decomposition products and the calcined compositions were identified as ($Ln$-doped) α-U$_3$O$_8$ by X-ray powder diffraction. During thermal treatment in reducing conditions a transition from a ($Ln$-doped) α-U$_3$O$_8$ phase to a ($Ln$-doped) UO$_{2±x}$ phase was observed. After exposure to a hydrogen containing gas mixture, this transition occurred rapidly in un-doped particles, and in particles prepared with trivalent Nd or Ce dopant precursors at 700 °C. Despite the fast reduction reaction, severe fractures appeared in the particles above temperatures of 850 °C, indicating that such behaviour is mainly attributed to sintering effects and less to the phase transition. In contrast, a more delayed reduction reaction was observed in particles prepared with a tetravalent dopant precursor and the described effects appeared less severe.
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