We investigated the electronic state of an (Am,U) mixed oxide with the fluorite structure using the all-electron full potential linear augmented plane wave method and compared it with those of Am2O3, ...AmO2, UO2, and La0.5U0.5O2. The valence of Am in the mixed oxide was close to that of Am2O3 and the valence of U in the mixed oxide was pentavalent. The electronic structure of AmO2 was different from that of Am2O3, particularly just above the Fermi level. In addition, the electronic states of Am and U in the mixed oxide were similar to those of trivalent Am and pentavalent U oxides. These electronic states reflected the high oxygen potential of AmO2 and the heightened oxygen potential resulting from the addition of Am to UO2 and also suggested the occurrence of charge transfer from Am to U in the solid solution process.
•The electronic states of Am oxides, U oxides, and CaF2-type (Am,U) mixed oxide are investigated.•The valence of Am in the (Am,U) mixed oxide was close to that of Am2O3.•The valence of U in the mixed oxide was pentavalent.•The electronic states of these oxides reflected their oxygen potential.•Charge transfer from Am in AmO2 to U in UO2 in the solid solution process of the mixed oxide.
The effect of p-block elements (Ga, Ge and In) addition on grain boundary structures and room temperature mechanical responses was investigated on extruded Mg binary alloys with fine-grained ...structures. Grain boundary segregation was confirmed in the Mg–Ga and Mg–In alloys, whereas the Mg–Ge alloy did not show such microstructures associated with solubility. Grain boundary segregation affected the plastic deformation of the Mg–Ga and Mg–In alloys. In particular, the Mg–In alloy had a large strain rate dependency and exhibited good deformability at low strain rate regimens. First-principles calculations indicated that p-block elements produce a bond-weakening effect at grain boundaries, and atomistic distances at grain boundaries varies according to the element. Solute atom which brings about both bond-weakening and bond-expansion effects to Mg atoms is effective in enhancing the contribution of grain boundary sliding in deformation.
We found that porous UO2 grain was formed by the precipitation of helium injected by HIP. Scanning electron microscopy analysis showed that polyhedral negative crystals were formed in the sample. The ...shape of the negative crystal changes dramatically with the conditions of helium precipitation. A truncated octahedron-type, an octa-triacontahedron-type, and a pentacontahedron-type negative crystal were observed. Our study implies that the shape of the negative crystal should change depending on the helium inner pressure enclosed in the negative crystal. It is difficult to arbitrarily control the shapes of these polyhedral negative crystals embedded in a solid medium. However, the shape of the negative crystal can easily be controlled by the helium injection method. In this article, we call the shape controlled negative crystal an image crystal. Here, we report a relationship between the surface energy and the shape and discuss the transformation mechanism of the image crystal. Our detailed observation indicates that the growth process of the image crystal can be explained by a step free energy model rather than an attachment energy. We could not find a cuboctahedron-type negative crystal of which the surface area is larger than that of a pentacontahedron-type negative crystal with the same volume.
Static annealing process of 423 K for 2.5 h is adequate to segregate alloying elements at {101¯2} twin boundaries in various Mg binary alloys. These segregated twin boundaries play a role in ...obstruction of dislocation slips; thus, they contribute to increase in hardness. Internal friction tests reveal that, irrespective of the solute elements, induced twin boundaries are effective in enhancing damping capacity, owing to their reversible motion, i.e., growth and shrinkage. In contrast, by comparison of the loss factor of specimens with/without twin boundary segregation, segregation leads to a decrease in damping capacity. The energy barrier required for twin boundary sliding to occur is closely related to the loss factor. When solute element having a characteristic of high (or low) energy barrier exists at twin boundaries, such an alloying element prevents (or enhances) the occurrence of twin boundary motion; as a result, shows a low (or high) loss factor.
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