| Uranium nitride is an important nuclear fuel,which has been widely concerned due to its unique physical and chemical properties.Under various conditions,the corrosion behavior of materials has always been the focus of attention.However,the results scatter in the literature in different projects under various conditions.As a strongly correlated electron system,the uranium nitride is complex due to the intrinsic electronic properties of U5f,and less was revealed,especially on the high nitrogen-contained uranium nitride UN2(-x).In this paper,serial uranium nitride samples in different stoichiometry are successfully deposited on Si substrates by magnetron sputtering deposition,where the high N content sample UN1.85 is included.Furthermore,UN layers are fabricated on uranium metal surface by pulse laser nitriding.The electronic structure including core levels and valence band are characterized by X-ray photoelectron spectroscopy(XPS)and Auger electron spectroscopy(AES).The in-situ oxidations of each sample are carried out in the analysis chamber under UHV condition in different atmospheres,and the influence of stoichiometry on the surface behaviors are studied,and the mechanism is included.Under UHV condition,oxidation occurs on UN surface with different O2 exposure in partial pressure of?5e-8mbar.And the initial oxidation product is identified to be a UO2-like ternary compound UO2-xNy,with the corresponding O1s and N1s features locating at 530.3eV and 396.3eV,respectively.Additionally,the final oxidation product with O2 under room temperature is UO2 with similar U4f7/2 peak position at 380.3eV,but no Nls signal detected Studies also show that the non-stoichiometric uranium nitrides with N/U ration less than 1(UN0.1,UN0.5 and UN0.68)are the mixture of U and UN in different proportions,and the oxidation behaviors are not the combination of two independent oxidation of both phases,but correlative to each other,where both components are involved in the reactions,resulting in the formation of UO2-xNy with different oxygen vacancy concentrations.And the more the UN content,the less the oxygen vacancy concentration with relative lower U4f peak positions.The U4f features is broadening towards higher BE side with the increasing N content in urnaium nitrides,and slightly chemical shift of U4f peaks is observed in higher N-content uranium nitride comparing the relative lower one.For example,the U4f7/2 peak of UN and UN1.85 lies at 377.5eV and 377.7eV,while 377.3eV for the other uranium nitride species with N/U ratio less than 1.Remarkable influence of N content on the U5d electronic structure is observed with doublet characteristics of U5d spectra for uranium nitrides included in this thesis.Meanwhile,the curve fitting implies the weak doublet structure in U5d spectra of U metal,which is not mentioned before in literature.The positions of double peaks in U5d5/2 spectra of UN are 93.1eV and 94.3eV,and those for UN1.85 are 94.5eV and 93.1eV with great difference in the doublet intensity.During oxidation,N is gradually displaced by O atom in a gradual displacement mechanism resulting in the formation of ternary uranium oxy-nitride.And the N,O contents of the ternary uranium oxy-nitrides vary in different oxidation stage.Moreover,unique electronic structure of each ternary uranium oxy-nitride is revealed by the corresponding specific U4f,O1s and N1s features.The depth profile shows the thickness of oxidation layer on UN1.85 which stored in lab environment for more than 2 years is similar to that oxidized with 2000L O2 under UHV condition,and the good corrosion resistance of high N-content uranium nitride is proved,which is attributed to the lower chemical activity and better stability of the UNxOy surface.High N content uranium nitrides layer on uranium metal surface are obtained by plasma ion immersion implanation(PIII),and the surface structure and the oxidation behaviors are systematically studied.The influence of composition to its corrsion behavior and the mechanism of the failure of nitride layer are discussed.It is revealed that the nitride layer on uranium surface is mainly composed of UN2-x,and a certain amount of oxygen in the form of UNxOy,and the corresponding U4f7/2 peak locates at 378.9 eV.During oxidation,several species with successively U4f7/2 peak at 378.9 eV,379.9 eV and 380.3 eV are found to be attributed to different UNxOy.Generally,the lower O content UNxOy is further oxidized to another UNxOy with the descent of N content,and the U4f feature shifts to higher BE side,while the O1s and N1s peak to the opposite direction.The final oxidation product in O2 atmosphere is supposed to be U? contained species.The surface structure of the nitrided uranium surface is summarized as:UO2+x+UNx1Oy1(379.9eV)/UNx2Oy2(378.9eV)+UN2-x/UO2-xNy+UNx1(x2)Oy1(y2)+UN2-x+U/UBefore nitriding,oxides on uranium surface are inevitable even after surface sputtering,and there is an O-contained layer in form of UO2-xNy at interface with plenty of oxygen vacacies.During oxidation,the oxygen vacancies are occupied by the oxygen atom,and the gradual displacement of N by O occurs with the generation of UO2,which is further oxidized to uranium oxides in higher valence state,such as U4O9 and U3O8.Lattice mismatch and internal stress due to the generation of U4O9 and U3O8 at the interface may result in the failure of nitriding layer.And the removement of surface oxide layer before nitriding is suggested.In this paper,the corrosion behavior and the influence of stoichiometry on the electronic structure of uranium nitrides are obtained,and the oxidized species of uranium nitrides are clarified,especially of uranium mononitride.The surface morphology of different uranium nitride samples stored under different ambience is compared,and the good corrosion resistence of high N content uranium nitrides is identified.The coexistence of U and UN in different proportions is corresponding to the non-stoichiometric uranium nitride UNx(x<1),and a coorelative oxidation behavior of both components and the influence of UN content to the products are also revealed.The compositions and surface structure are achieved by depth profile,and the oxidation behavior of the nitride uranium surface and interface are included.The gradual displacement mechanism yields the formation of a seriel of intermediate U-N-O ternary compounds with different N and O content,the failure behavior and mechanism of the interface of plasma nitriding modified layer are also included.The results shed light on the surface chemistry and surface modification of uranium nitrides.However,studies are never enough for the comprehensive understanding of the complex uranium-nitrogen system. |
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