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

Abstract

The uranium surface is CN-co-doped by the pulsed laser ablation technique in the 1:1 mixed atmosphere of CH4 and N2. The results of X-ray diffraction, Auger electron spectroscopy and X-ray photoelectron spectroscopy show that the modified layer prepared is a UCxN1-x solid solution with trace metallic U. When contacted with O2, the NaCl-like structure of the outmost surface layer is soon transferred to a UO2-x-like structured UO2-x(C/N)y (y < x), which can be subsequently oxidized to a CaF2-like structured UO2-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 CaF2-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 UO2-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 N2 or NO, implying that the migration of N may be different from that of C during the oxidation of the UCxN1-x solid solution. After oxidation, a tri-layered structure, composing of UO2 located on the outmost surface, UO2-x(C/N)y and UCxN1-x distributed in the superficial layer, and a UCxN1-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 O2.