Uranium-niobium Alloy Surface Oxidation Behavior Of Electron Energy Loss Spectroscopy Study

As a method of surface analysis, Electron energy loss spectroscopy (EELS) can provide structural information from a specimen. The corrosion resistance of uranium is greatly enhanced by alloying with niobium. In this study the adsorption process and the initial stage of oxidation reactions of oxygen on uranium-niobium alloys surface at room temperature were studied with electron energy loss spectroscopy and Auger energy spectroscopy (AES) in this paper. By comparison, the initial oxidations of clean, high-purity polycrystalline niobium and uranium metal surfaces for low exposures to dry oxygen were also studied.Firstly, EELS of uranium and niobium with different energy electrons or energy resolution has been obtained in Auger electron energy spectroscopy instrument, in order to study their influence on EELS. Else, oxidations of niobium at different exposure pressure of oxygen have also been studied. Then, EELS of Nb, U, U-2.3Nb and U-6Nb were obtained in the experiments. At the same time, AES was used to clarify and confirm the behavior observed in the EELS studies. It is showed that experimental results of surface and bulk plasmon losses of clear surface uranium agreed with theoretical calculations. There are distinct changes about shifts or intensity in bulk plasmon, surface plasmon, and interband electron transitions losses as a result of oxidation of specimens, and adsorption and oxidation processes can be studied through these changes. At room temperature, the formation of oxide layers on uranium and uranium-niobium alloys were found to occur rapidly upon exposure to oxygen, and the resultant oxide in each case was near-stoichiometric UO2. Due to formation of niobium oxide in uranium-niobium alloys, the diffusion of O^- (O^2-) and U⁴⁺ in the interface region was prevented, and the corrosion resistance of uranium-niobium alloys to oxygen is greatly enhanced by alloying with niobium. EELS is more surface signal and sensitive than AES. But EELS ranges from 0 to l00eV, it is difficult to distinguish.Finally, EELS and AES were combined to study initial oxidation processes of uranium, niobium and uranium-niobium alloys in an ultra-high vacuum chamber at the temperature of 373K, 473K, 573K and 673K. Conclusions can be doped out as below. At temperatures from 298K to 473K, increasing reaction temperature is of great benefit to oxidation reactions of oxygen on specimens surface. When surface temperatures of uranium and uranium-niobium alloys increase higher than 573K, heating was observed to deplete surface oxygen by accelerating its transport into the bulk and separate carbon out to their surface. So there were uranium carbide formed on the surface and thicker layer of UO₂ formed in the bulk. With surface temperature increasing, the O/U ratio dropped sharply as the surface oxygen diffused into the bulk. The uranium carbide content and the deepness of the oxides were increased with treatment temperature.