Thermal stability of uranium nitride and oxynitride films in an ultra-high vacuum environment

Abstract Uranium nitride (UN 1.66 ) and oxynitride (UN 1·42 O 0.23 ) films have been prepared on the substrate of Si by radio frequency (RF) magnetron sputtering. The films have been studied by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Auger electron spectroscopy (AES). Then, vacuum heat treatment of uranium nitride and oxynitride films were in situ investigated in an ultra-high vacuum chamber of AES. The experimental results show that the UN 1.66 and UN 1·42 O 0.23 films fabricated by radio frequency magnetron sputtering method are dense and uniform. The surface of the UN 1.66 films changes into UN x O y at the beginning of the heat treatment. Then the surface changes into UO 2 with the increasing temperature. When the temperature exceeds 573 K, UO 2 phase gradually changes back into UN x O y phase due to the out diffusion of decomposed N atom in UN 1.66 subsurface with increased N content of the surface. UN 1.42 O 0.23 film exhibits good stability until 573 K. UN x O y and UO 2 mixed phases form on the surface of the UN 1·42 O 0.23 after initial oxidation. Owing to the decomposed nitrogen from the UN 1·42 O 0.23 subsurface, the UO 2 phase turns back into UN x O y again when the temperature exceeds 573 K.