研究目的
The design of a corrosion-resistant modi?ed layer for uranium surface is both challenging and attractive. Several methods have been applied to improve the corrosion resistance of uranium, including radio-frequency magnetron sputtering, the plasma N-doped, ion implantation, pulsed laser modi?cation. The pulsed laser modi?cation technique has several unique advantages such as high concentration of doping elements, low substrate temperature, fast treatment, simple vacuum chamber and precise position control, which is favorable for radioactive material passivation.
研究成果
The N-doped modi?ed layer on uranium surface prepared by pulsed laser irradiating can improve the corrosion resistance of uranium material e?ectively. The N-doped modi?ed layer is a composite layer with a gradual transition structure from UO2-xNy and UNxOy and to UN and eventually from UN to metallic U from surface to interior. The UNxOy with U 4f7/2 BE at 379.6 eV in the oxygen-rich region is inert under the condition of exposure to O2, while the metallic U, UN and UN1-xOy (partial N atom replaced by O) are relatively vulnerable to oxidize to uranium oxides. The N-doped modi?ed layer has good stability and excellent ability to prevent the di?usion of O atoms into the interior. After thermal treatment for 24 h at a temperature of 423 K and a vacuum of 10?4 Pa, the microstructure of the N-doped modi?ed layer has only a slight change which almost can be ignored. The N-doped modi?ed layer on uranium surface provides excellent corrosion resistance for uranium. The results of electrochemical polarization tests showed that its self-corrosive potential and self-corrosive current were ?5.75 × 10?2 V vs. SCE and 1.37 × 10?7 A/cm2, respectively, while the values of the uranium were ?5.67 × 10?1 V vs. SCE and 8.08 × 10?7 A/cm2, respectively. The corrosion resistance of the N-doped modi?ed layer was further improved because of the residual metallic U in the N-doped modi?ed layer was oxidized or the micro-defects in the N-doped modi?ed layer are reduced after vacuum thermal oxidation.
研究不足
The physical process of pulsed laser modi?cation on uranium surface is very complex, whether the formation of oxygen-containing nitrides in the process of laser modi?cation remains unclear due to the absence of direct evidence.
1：Experimental Design and Method Selection:
The N-doped modi?ed layer on uranium surface was prepared by pulsed laser irradiating in nitrogen atmosphere. The microstructure was characterized by XRD, AES and XPS. The initial oxidation behavior and the thermal stability of N-doped modi?ed layer were studied by XPS. The corrosion protective properties of the N-doped modi?ed layers before and after thermal treatment were evaluated through potentiodynamic polarization tests.
2：Sample Selection and Data Sources:
The substrate used for N-doping modi?cation was α-phase depleted uranium circular sheet (Φ10 mm × 3 mm). The uranium sheets were polished then cleaned with acetone followed by ethanol.
3：List of Experimental Equipment and Materials:
X-ray di?raction (XRD) analysis in θ-2θ geometry using an X’Pert PRO system with a Cu Kα radiation (λ =
4：5406 ?). Auger electron spectroscopy (AES) depth pro?les. X-ray photo-electron spectroscopy (XPS) analysis was performed using a Thermo ESCALAB 250 model with a resolution of 6 eV at pass energy of 20 eV and a base pressure of ~10?9 mbar. The electrochemical tests were carried out using a Gamery 600 electrochemical workstation equipped with three electrodes. Experimental Procedures and Operational Workflow:
The irradiation chamber was evacuated to 10?2 Pa and baked for 2 h at a temperature of 323 K to reduce the amount of H2O adsorbed on the inner wall of the irradiation chamber before it was ?lled with nitrogen atmosphere. The uranium samples were irradiated with an excimer laser (wave length of 248 nm, pulsed width of 25 ns). The laser scanned along X direction with a speed of
5：2 mm/s. After scanning a line in X direction, the laser moved 5 mm along Y direction. The two steps repeated until full area of the uranium sample was irradiated. Data Analysis Methods:
The phase structure of the N-doped modi?ed layer on uranium surface prepared by pulsed laser irradiating was analyzed by X-ray di?raction (XRD) analysis in θ-2θ geometry using an X’Pert PRO system with a Cu Kα radiation (λ = 1.5406 ?). Scans were collected over a 2θ range of 20–70° with a step of 0.01°. The elements distributions of the modi?ed layer were investigated by Auger electron spectroscopy (AES) depth pro?les. The cylindrical mirror analyzer was adopted for measurements with the electron beam energy of 3 keV and current of 100 nA. During the depth pro?le process, the sputtering of modi?ed layers was carried out by a Ar+ ion gun with the energy of 4 KeV and the spot size of 1 mm2. X-ray photo-electron spectroscopy (XPS) analysis was performed using a Thermo ESCALAB 250 model with a resolution of 0.6 eV at pass energy of 20 eV and a base pressure of ~10?9 mbar. The XPS data were accumulated using monochromatic Al Kα radiation. The spectra were calibrated using the binding energy (BE) of Ag 3d5/2 to be 368.24 eV acquired on the clean surface of a separate Ag foil, and the contaminant C1s peak of the as received samples lies in 284.8 eV.
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