【作者】 张延志；

【导师】 汪小琳；

【作者基本信息】 中国工程物理研究院北京研究生部 ， 核燃料循环与材料， 2003， 硕士

【摘要】 本论文利用X射线衍射（XRD）并结合Rietveld方法较系统地对金属铀和八氧化三铀（U₃O₈）在不同温度、压力条件下的表层氧化情况和结构变化进行了分析研究。主要的研究结果如下： 对U₃O₈的相变和结构随温度的变化进行的分析研究结果显示：在300℃附近U₃O₈发生了相变，由底心正交结构（空间群为Amm2）的低温相转变为简单六方结构（空间群为P-62m）的高温相。在两种相结构各自稳定的温度范围内，U₃O₈的点阵参数以及衍射峰半高宽（FWHM）出现了有规律的变化，这种变化表明U₃O₈的晶体结构依赖于环境温度的变化。文中利用FWHM的变化对材料中的微晶尺寸和点阵畸变进行了一定的分析。 对金属铀表层在大气环境中的氧化情况进行分析研究，结果表明：所有温度条件下，金属铀的氧化都是在表层形成一定厚度的UO_2+x，只有在较高的温度条件下（如300℃）氧化物才会进一步转化为U₃O₈。室温～150℃下的氧化动力学曲线表明：金属铀表层氧化形成UO_2+x的过程中存在一个转折点，转折点前后形成两个反应阶段：非线性增长阶段和线性增长阶段。200℃条件下，金属铀表层所形成的氟化钙结构的UO_2+x的点阵参数a₀和FWHM随着氧化时间的增加逐渐减小，然后出现稳定。室温～150℃下金属铀表层的氧化反应符合阿累尼乌斯公式，活化能计算结果为52.4kJ／mol。 在论文中，对金属铀表层在纯氧环境中和低压（100～300Pa）环境中的氧化情况也进行了分析研究，结果表明：相同温度条件下，金属铀表层的氧化产物与大气环境中相同，点阵参数和FWHM的变化也很相似。但氧化速度有所差异，100℃条件下，第一阶段的氧化速率都比大气环境中的氧化速率慢。对比各种实验环境，发现300℃下U₃O₈的生成速度随着氧气分压的减小而变慢。更多还原

【Abstract】 The structure of triuranuim octaoxide (U3O8) and the oxidation of uranium metal were studied by X-ray diffraction (XRD) and Rietveld method in different partial pressure of oxygen and temperatures. The conclusions are reported as following:The structure change of triuranium octaoxide (U3O8) was studied by XRD at ambient atmosphere at temperature form 25 ℃ to 850℃. There are two kind of phases of U3O8 in the temperature range studied, and the transformation temperature is about 300℃. The low temperature phase is orthorhombic with space group Amm2, and it change to another phase with better symmetry around 300℃. Another phase, the high temperature phase, is hexagonal with space group P-62m, which is stable from 300℃ to 800℃. The lattice parameter and the full width of half maximum (FWHM) of XRD peaks of U3O8 were also investigated at different temperature, and it were found that they change with the temperature under rules. The mean crystallite size and microstrain of U3O8 in different temperature were also discussed in the paper.The specimens of uranium flat were heated in air, oxygen or low pressure, to investigate the formation of UO2+x and U3O8 on the surface layer of specimens. The processes of oxidation to UO2+x and U3O8 were analyzed by XRD to determine the extent of surface layer oxidation and the structure of oxidations. The XRD patterns were fitted by Rietveld method to refine the structure of different phases and to calculate quantificationally. The results indicate that the kinetic expression for the formation of UO2+x in 25-150℃ was initially nonlinear, but switched to linear at long reaction times, that is, there exist two stages in the UO2 growth kinetics: nonlinear portion and linear portion. The activation energy is about 52.4kJ/mol. The lattice parameter, a0, and FWHM of the UO2+x (fluorite type) decrease initially and stabilize later as the increasing of reaction times at 200℃. The UO2+X is progressively replaced by U3O8 as the oxidation proceeds at 300℃. The formation rate of U3O8 is insensitive to the partial pressure of oxygen, and decreased with the pressure of oxygen drop.更多还原