Gem Nitriding Mechanism Of Metal Uranium Surface And Fine Structure Of Nitrided Layer

The mechanism of glow plasma nitriding on uranium metal surface, the composition of nitride layer, and antioxidant mechanism of modified layer were studied in this paper. The hinder effect of oxygen impurity to the formation of nitride layer was discovered by experiments. In the early stage of the study, the oxygen concentration in nitride layer is high, and there is an oxide layer between the nitride layer and the metal substrate. Data of AES and XRD shows, with nitriding time prolonged, the thickness of modified layer increases, and the modified layer appears the trend of the nitrogen content increasing, the lattice constant decreasing.The electronic structure of elements in the modified layer was analyzed by XPS, and a variety of chemical states of uranium was found. With comparison of multiple sets of XPS data, it is realized that the less of oxygen impurity, the more of the component with metallic properties. Through long time nitriding, nitride samples with very low oxygen impurity were obtained. And the component with metallic properties is determined to be the U4N7, which is a reaction product of uranium with excess nitrogen. Another component with higher valence uranium is considered to be some kind of oxygen doped nitride.The fine structure of uranium nitride was analyzed by spherical aberration-corrected TEM and EELS. It was found that in a uranium nitride grain, nitrogen and oxygen atoms can share tetrahedral sits constituted by uranium atoms. There are some areas with high oxygen concentration in uranium nitride grains, which is due to the impurity oxygen atoms tend to gather together, forming UN2-xOx local structure. The density of states of U4N7 and UN2-xOx were calculated by first principles methods, and the calculated metallic properties are consistent with the experimental data of XPS. As the nitriding time prolonged, the concentration of oxygen impurity is reduced, which leads to the increase of the nitrogen concentration and the decrease of the crystal lattice constant. The oxygen atoms are doped into the uranium nitride crystal lattice, thus there is only one kind of lattice structure; there are two kinds of circumstances in the neighbor atoms of uranium, which are with oxygen and without oxygen, so there are two kinds of valences in uranium atoms.Monte Carlo method was used to simulate the growth mechanism of the modified layer. It is realized that the formation of the modified layer is the result of the co-diffusion of nitrogen and oxygen atoms. The initial oxide thickness and the nitriding temperature are the main influencing factors of the growth of nitride layer. Experiments show that ion bombardment plays an important role during uranium nitriding. Hollow cathode was utilized to generate high density plasma near the surface of uranium sample, without additional heating system, nitride layer was formed. Due to the reduction of oxygen impurity in the nitriding atmosphere, the accumulation of oxygen atoms between the nitride layer and the metal substrate disappeared, which is beneficial to the growth of the nitride.The mechanism oxidation resistance of nitride layer was analyzed, and it is found that antioxidant capacity of nitrogen-rich uranium nitride at room temperature comes from two aspects:first is the lower migration probability of atoms in the nitrogen and oxygen mixture layer, second is doped oxygen atoms reduce the number of vacancies in uranium nitride crystal lattice. When temperature is higher, the capacity of nitride lattice accommodating the oxygen atoms weakens, and the stability of the modified layer decreases. The initial oxidation rate of the clean uranium nitride surface is relatively fast, but the oxidation would stop after a certain thickness of oxide layer formation. The oxidation resistance of nitrogen-rich uranium nitride is stronger than UN.It is found in this study that the oxygen impurity has an important influence on the nitride layer. The oxygen atoms as impurities hinder the growth of the nitride layer but increase the stability of the layer. There is no sign of the presence of UN2 In the modified layer, but the CaF2-type local structure is formed by oxygen doping uranium nitride. Through this work, the formation process and the influencing factors of the nitride layer were deeply studied, and the glow plasma nitriding process was optimized, which create a foundation for the engineering application of this technology.