The initial oxidation behavior of a UCxN1-x layer prepared by the pulsed laser ablation in the 1:1 N2+CH4 atmosphere

The uranium surface is C N-co-doped by the pulsed laser ablation technique in the 1:1 mixed atmosphere of CH ₄ and N ₂ . The results of X-ray diffraction, Auger electron spectroscopy and X-ray photoelectron spectroscopy show that the modified layer prepared is a UC _xN _1-x solid solution with trace metallic U. When contacted with O ₂, the NaCl-like structure of the outmost surface layer is soon transferred to a UO _2-x-like structured UO _2-x(C/N) _y ( y < x ), which can be subsequently oxidized to a CaF ₂-like structured UO _2-x(C/N) _y ( y ≈ x ), in which the C, N and O atoms share the tetrahedral interstitials of the U lattice. After the formation of the CaF ₂ -like structure, the surface C and N atoms are gradually replaced by O atoms during the following oxidation process. During the oxidation of the outmost surface layer, C is more preferentially oxidized than N to produce a product of UO _2-x(C/N) _y, in which the ratio of C/N is lower than that in the as-prepared layer. Besides, C is easily oxidized to CO and adsorbed on the layer surface while there is no obvious evidence for the escape and adsorption of N ₂ or NO, implying that the migration of N may be different from that of C during the oxidation of the UC _xN _1-x solid solution. After oxidation, a tri-layered structure, composing of UO ₂ located on the outmost surface, UO _2-x(C/N) _y and UC _xN _1-x distributed in the superficial layer, and a UC _xN _1-x solid solution with trace metal U in the inner layer, is formed. This structure of layer can effectively prevent the further migration of O atoms into the layer from the surface and maintain the existence of C and N in the inner layer, and thus provide an excellent passivation layer for U metal against O ₂.