. .. Et-oxyde-sur-stoechiométrique, 160 7.1.1 Formation des différents agrégats d'oxygène à température nulle, vol.158

?. and .. .. , 2.3 Comparaison des structures simulées à la phase expérimentale, Modélisation de la phase U 4 O 9

?. and .. .. , 194 7.3.1 Stabilité de la phase U 4 O 9?y dans UO 2 à température nulle, Étude du diagramme de phase entre UO 2 et U 4 O 9, 0200.

, Conclusion du chapitre : description du diagramme de phase à l'échelle atomique

, Justement, sur la figure 7.21, le nombre de cuboctaèdres calculés sur les sites d'U 4 O 9?y chute drastiquement à partir de 1200K puis diminue plus doucement à partir de 1400K. Il est nul à 1600K. Lors de la transition, la pseudo-PDF des COTs est modifiée. Nous voyons apparaître deux pics correspondant aux vecteurs 1/2<310> et 1/2<222>. La pseudo-PDF des COTs change également après le pic des premiers premiers voisins COTs 1/2<321>. A partir de 1400K, même si le nombre de COTs sur sites U 4 O 9?y n'est pas tout à fait nul, la pseudo-PDF des COTs ne varie plus. La variation du paramètre de maille est plus importante à haute température dans la phase U 4 O 9?y désordonnée, Une faible proportion de cuboctaèdres seulement occupent les sites U 4 O 9?y. A 1400K, les COTs semblent complètement désordonnés

, Le nombre moyen de COTs identifiés est de l'ordre de 50% du nombre de COTs initialement présent à 1350K (la fin du régime transitoire), puis décroit lentement. A 2000K, nous identifions toujours une faible fraction de COTs. Du à l'agitation thermique, ils sont plus mobiles et leur structure ne peut pas être déterminée précisément. Les agrégats se forment, migrent et se dissolvent et se reforment etc... rapidement. Nos calculs indiquent en tout cas une forte hétérogénéité de concentration locale d'oxygène et des cuboctaèdres identifiables à une position donnée, nous observons toujours des cuboctaèdres, même s'ils sont désordonnés

, Nous notons également sur la figure 7.18 représentant les pseudo-fonctions de distribution de paires de cuboctaèdres que les intensités des pics 1/2<211>, 1/2<220>, 1/2<310> et 1/2<222> correspondant à des courtes distances entre COTs restent très faible à haute température (>1400K)

, Sur nos simulations, le pic principal a une intensité plus élevée que cela. Ainsi, même dans la phase U 4 O 9?y désordonnée, les COTs ont tendance à vouloir rester éloignés les uns des autres. On peut alors penser que la sphère de rayon égal à 1,871 a 0 (la distance premiers voisins COTs dans U 4 O 9?y , soit la norme du vecteur 1/2<321> a 0 ) définisse un volume d'exclusion partiel pour les cuboctaèdres, Cela nous indique que les COTs ne sont pas répartis complètement aléatoirement dans la cellule

, Si l'on diminue encore la température sur les simulations réalisées à partir d'UO 2+x , nous n'observons pas de transition vers la phase U 4 O 9?y , même avec des temps de simulation de l'ordre de 500 ps. La structure désordonnée des cuboctaèdres d'UO 2+x est conservée. Les pseudo-PDFs à température décroissante présentées sur la figure 7.19 sont similaires. De même, nous ne voyons pas d'évolution notable du rayon des COTs ou du paramètre de maille. Nous pouvons enfin comparer ces derniers résultats aux expériences. Comme nous l'avons déjà énoncé plusieurs fois, la phase U 4 O 9?y est stable jusqu'à 1400K puis disparaît au profit de l'UO 2+x. Dans nos calculs, la transition s'opère à des températures plus faibles, entre 1150K et 1350K. Néanmoins, d'après le diagramme de phase présenté en figure 7.1 au début de ce chapitre, U 4 O 9?y est stable monophasé jusqu'à environ 1280K à la stoechiométrie exacte O/U = 2,234375. Il Une fois ces modifications ajoutées au code "fix_gcmc" (GCMC : Grand Canonical Monte Carlo) de LAMMPS, nous calculons les énergies de suppression (multipliées par-1) et d'insertion pour différents rayons d'insertion à une température donnée de 1000K dans UO 2. Les distributions de ces énergies sont représentées sur la figure 7.27 et ajustées par des fonctions Lorentziennes. Nous pouvons voir que pour un rayon de 2,0 Å, l'énergie d'insertion moyenne est de 2,2 eV. La variation moyenne d'énergie lorsque l'on supprime un atome d'oxygène est très élevée et est égale à 8,4 eV. Le potentiel chimique préservant la stoechiométrie d'UO 2 sera dans ce cas égal à la moyenne de l'énergie d'insertion et de l'opposée de l'énergie de suppression, Les simulations réalisées en température décroissante à partir d'une phase UO 2+x constituée initialement d'interstitiels placés aléatoirement nous permettent de confirmer que la phase désordonnée dont nous venons de discuter est bel et bien UO 2+x. En effet, nous observons qu'entre 1600K et 2000K, les structures obtenues par les deux types de calcul sont tout à fait similaire, aussi bien au niveau du nombre de COTs identifié, de leur pseudo-PDFs (voir figure 7.19), de leur rayon ou des grandeurs thermodynamiques. A 1400K et 1500K, les deux structures obtenues par les deux approches sont légèrement différentes

. La-probabilité-de-supprimer-un-atome-d&apos;oxygène-est-le-rapport-de-l, intégrale de la distribution des énergies de suppression entre-3.1 eV et +? à son intégrale totale. Autrement dit, la probabilité d'insérer ou de supprimer des atomes, égale à l'intégrale de recouvrement des distribution d'insertion et de suppression est très faible

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