CO和超临界CO₂钝化金属铀的原子分子机理

【作者】 薛卫东；

【导师】 朱正和；

【作者基本信息】 四川大学 ， 原子与分子物理， 2002， 博士

【摘要】 本文应用密度泛函理论的B3LYP方法，在Gaussian 98程序中，对铀原子采用14个价电子（6s²7s²6p⁶6d¹5f³）的准相对论有效原子实势（RECP），及（6s5p2d4f）／[3s3p2d2f]收缩价基集合（P.J.Hay，1998），碳、氧原子采用6-311G^**全电子基集合，对UC、UO、CUO和UCO₂的分子结构、势能函数、光谱性质、热力学性质和动力学性质进行了系统研究，在本课题组提出了防止铀表面腐蚀的“钝化层”模型的基础上，应用热力学原理研究了铀在CO、CO₂气氛中的表面钝化反应，探讨了CO、CO₂对铀金属表面抗氧化机理。 防止铀的腐蚀，关键在于改变其表面的物理、化学性质。现有铀的防腐技术：如表面合金化，离子注入铌，阳极氧化，PVD，CVD及表面涂层等。美国曾用超临界二氧化碳清除核部件的表面有机物粘污，并探讨其与核材料的相容性问题；各国学者也在一直探讨铀与环境气体的相互作用机理，并寻找一种合适的抗腐蚀方法。国内汪小琳、刘柯钊以及本课题组发现，用CO、CO₂气体对铀样品表面有抗氧化作用。超临界CO₂流体除了有清洗功能外，还有对铀的抗氧化作用，因此研究超临界CO₂流体对金属铀的抗氧化机理显得尤为重要。 在原子分子反应静力学基础上，根据分子电子状态构造的群论原理，首次用B3LYP方法对CUO和UCO₂体系进行优化，发现角形结构（C_s）的CUO（³A″）分子比线形结构（³∑⁺）更稳定，UCO₂(C_2v构型)分子⁵A₁态的能量最低；然后，根据微观过程的可逆性原理、微观过程的传递性原理和能量最优原则，导出它们正确的电子状态和合理的离解极限，迄今未见有文献报道过这些分子的电子状态和离解极限。使用多体项展式理论方法，导出了CUO（³A″）分析势能函数，该势能表面准确地再现了平衡稳态结构构型及能量关系。使用其势能函数等值图讨论了U（³I_u）+CO（X¹∑⁺）反应的势能面静态特征，在这些反应通道上均无鞍点，即无能垒出现。此外，在（5s4p3d4f）／[3s3p2d2f]收缩价基集合（P.J.Hay，1983）下，通过对UOC、UCO和CUO三种分子结构、能量和电性质的讨论，发现UCO分子角形结构（³A″）的能量最低、完全离解能最大。研究了铀碳氧系统结构的多样性与特征，这会有利于进一步探明金属铀表面钝化机理。 四)！【大学博士学位论文 在导出的＊＊O分子（扩A”）多体项展式势能函数基础上，采用准经典的Monte（arlo轨迹法首次研究了 U＋CO＊，0）反应的碰撞轨迹，反应截面和产物散射角分布，根据反应截面与能量之间的关系，得出它是无阈能反应；并讨论了碰撞络合物形成的原因，U＋CO碰撞可直接生成稳定络合物CUO。在碰撞初始平动能低于刀5刀KJ．mOI＊时，该反应可以生成长寿命络合物CUO（X3A”）；其反应通道的反应截面随平动能增加而减少，从而表明该反应是无阈能反应。这一结论与前面用多体项展式理论计算的CUO势能曲线结果一致。这为U在CO气氛中的抗氧化研究提供了理论依据，具有重要意义。 根据热力学原理和固体能量理论，用从头计算和统计热力学方法计算得物质的热力学函数值。由算得的 CUO格抒 CUOk)分子在不同温度下的生成自由能变，分别讨论了CUO伦)分子和CUOk)的稳定性。 用B3LYP方法在6l ＊”基集合水平上优化CO。二聚体的几何构型和离解能时，获得与MPZ方法相当或更优的结果。文中首次给出了CO。二聚体C川构型的十二个正则振动频率分析图，其中代表 van der Waals分子特征的四个振动频率与文献值相符。从正则振动频率、离解能以及单体间距来看，CO力。确系弱结合的 van der W8lls分子。正因为有这个特性，在超临界下 CO。仍具有很强的动力学特征，该观点尚属首次提出。 B3LIP方法在 6刁 树基集合水平上对气态 CO。（X‘z。）分子全优化，直接得到了热力学性质嫡S和定压热容CP，其结果与实验值相当吻合，计算得到的 CO。分子的生成烩包含电子焰（为习93．31 kJ．mol”‘）和核部分烩（为1石53kJ．mol’‘）两部分，其相对误差为 0．37％。研究表明，用本理论方法计算气态CO。分子在latin下的热力学性质是可行的。这种理论计算CO。分子在latin下的热力学性质方法未曾见报道。 将 Benedict－WCbbl)方程应用于超临界 COz流体，采用非线性最小二乘法，拟合出适用于温度为310巧00K、压强为7．5上0刀Mpa范围的超临界CO。流体的有关参数，进而根据热力学公式可导出超临界状态下体系的嫡、热容和烩。分析发现，这些热力学函数具有明显的超临界特性，即在超临界点附近，体系条件的微小改变都将引起热力学性质的显著变化。其数据为进一步从理论上研究超临界CO。与金属铀表面反应的热力学行为提供了依据。 首次进行了超临界CO。流体与金属铀表面钝化形成覆盖层物质的反应 11 四川大学博士学伎论文一U（Q）＋COZ（SCF）＝UOZ(syC（GTSPhitC）和 ZU（Q）＋COZ（SCL）”UUZ（S）＋UC（S）w＃t力学研究。研究表明：这两个钝化反应在超更多还原

【Abstract】 The molecular structure, potential energy function, spectroscopic property, thermodynamics property and dynamics property of UC, UO, CUO and UCO2were systematically studied in this paper , by using Density functional B3LYP method and Gaussian98W program in which the 14 valence electrons (6s27s26p66d15f3) relativistic effective core potential (RECP) and the contract valence basis set(6s5p2d4f) /[3s3p2d2f] of uranium atom and full electronic basis set 6-311Gof carbon and oxygen were used. Based on "the bluntly thin skin" model which established by our research group for prevention of the surface corrosion of uranium, the blunt surface of uranium in CO, CO2 and the anti- oxidation mechanism of CO, CO2 for metallic U were studied under the thermodynamics principle.To prevent the corrosion of uranium, the key point is to change the surface physical and chemical property of uranium. The reported corrosion prevention technology of uranium includes the surface alloying of uranium, iron implantation of niobium, the oxidation of cathode, PVD, CVD and surface coating. In US the super critical CO2 was used to clean the surface organic stains of nuclear components and the compatibility between C02 and nuclear materials was discussed; In the meantime, the inter-reaction mechanism between uranium and environmental atmosphere and the suitable method to prevent corrosion were studied by many researchers. In China professor Wangxiaoling, Liukezhao and our research group found that besides the clean function, the super critical CO and COa gas had anti-oxidation function for the surface of uranium, so it was very important to study the anti-oxidation mechanism of the super critical CO and C02 gas for metallic U.Based on the Group Theory and the Atomic and Molecular Reaction Static, the CUO and UCO2 system were optimized by means of B3LYP method for the first time. It is shown that the CUO ( 3 A ) molecular with angular structure ( Cs ) was more stable than that of linear structure ( 3 + ) , and the energy of UCO2 ( C2v ) molecular at 5A1 state was the lowest. Then according to the convertible and transmit principle of macrocosmic process and the energy optimization principle, the correct electronic states and rational dissociation limit of those molecular were derived for the firsttime. By using the many-body expansion method, the analytical potential energy function of CUO (3A ) which reappeared the equilibrium stable state geometry and energy relationship accurately was also derived. According to the potential energy function figure potential surface static characteristics of U(3IU) +CO(X1+) reaction were discussed, it was shown that there was no saddle point and threshold energy in those reaction channels. Additionally by discussing the molecular structure, energy and electrical property of the UOC, UCO and CUO under the contract valence basis set (5s4p3d4f) /[3s3p2d2f], it was found that the UCO molecular with angular structure (3A ) had the lowest energy and the highest dissociation energy. In the meantime the structure variety and characteristic of uranium-carbon-oxygen system were discussed for the further study of the surface blunt mechanism of metallic U.Based on the many-body expansion potential energy function of CUO (X3A") molecular, the collision trajectory ,the reaction section as well as the distribution of scattering angle of products of U+CO (0,0) reaction were studied for the first time by means of the Monte-Carlo quasi-classical trajectory approach, and according to the relationship between reaction section and energy, the no threshold energy reaction could be concluded, it also indicated that the stable complex compound CUO could be formed directly after collision of U +CO, because of the long lived complex compound CUO (X3A " ) could be formed when collision initial transition energy was lower than 215.0KJ.mor1, and the reaction section of reaction decreased with the increase of transition energy, so it could be concluded that this reaction was no threshold energy reaction, this conclusion was co更多还原