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, 10 -15 m 2 .s -1 après formation de UO2,1 à environ 1500 °C. Ces études permettent donc d'observer que la stoechiométrie de l'oxyde d'uranium impacte son comportement lors du frittage. Dans le cas de cette étude, l'oxyde obtenu après oxydation de l'uranium (IV) étant U3O8, le coefficient de diffusion de l'uranium au sein de cet oxyde est probablement supérieur à celui mesuré dans UO2 stoechiométrique. Cette augmentation du coefficient de diffusion de l'uranium permet d'expliquer la détermination d, vol.86

, Diagramme d'Arrhenius déterminé après frittage des microsphères d'oxyde d'uranium entre 800 °C et 1000 °C sous 200 Pa d'air (soit 40 Pa environ d'oxygène) avec des grains polycristallins, Figure, vol.96

, Ces résultats préliminaires montrent donc que l'oxydation de l'uranium lors du frittage, influence fortement l'évolution globale du système, même à une échelle microscopique, ce qui permet d'obtenir des oxydes plus denses à basse température, vol.26, p.30

, Harada [31] a observé, lors de l'étude du frittage des pastilles de UO2, que l'oxydation de l'uranium (IV) au cours de la densification permettait d'augmenter la croissance granulaire ainsi que la densité finale des compacts frittés

, Les premières observations du stade initial du frittage des microsphères polycristallines de dioxyde d'uranium ont été réalisées in situ par MEBE-HT en travaillant sous vide dans la chambre du microscope avec un support en alumine. Ces conditions expérimentales ont permis de suivre le frittage des microsphères de UO2, en s'affranchissant d'une réaction entre les microsphères

, Le traitement d'images MEBE a ensuite permis de quantifier les modifications morphologiques observées en déterminant l'évolution des paramètres d'intérêt. L'exploitation des courbes obtenues, en utilisant une loi exponentielle (? = a0.exp(k.t)+ ?0), a ensuite permis de déterminer une énergie d'activation d'environ 335 ± 116 kJ.mol -1 qui est associée à des mécanismes de, Les résultats préliminaires obtenus ont permis de démontrer la faisabilité d'une telle étude expérimentale, à partir d'un oxyde d'actinide ayant un comportement chimique complexe, notamment en terme de redox

, Contrairement au cas de ThO2, l'évolution microstructurale de ces microsphères et l'allure des courbes obtenues sont assez similaires à celles décrites lors de la modélisation de ce stade du frittage

, Cette différence peut s'expliquer par la différence entre les précurseurs synthétisés dans les deux cas, ThO2.2H2O et UO2.3H2O, qui conduit à l'obtention de microsphères de porosités différentes après conversion thermique en oxyde

, Pa d'air, soit 40 Pa d'O2), l'influence de l'oxydation sur le frittage d'UO2 a pu être observée. Lors de cette étude, l'oxydation de l'uranium (IV) a conduit à une modification de la morphologie des microsphères, qui tend vers une forme plus ovale. Les résultats préliminaires obtenus ont montré d'une part que l'oxydation de l'uranium (IV) conduit à une diminution de la température de frittage, En travaillant ensuite sous atmosphère oxydante

, D'autre part que la vitesse de formation et de croissance du pont augmente de manière significative après oxydation. L'énergie d'activation déterminée dans ces conditions, d'environ 120 ± 10 kJ.mol -1 , est plus faible que celle d'UO2. La pression partielle en oxygène peut donc permettre de moduler la densification des échantillons d

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