The initial stage of uranium oxidation: mechanism of UO(2) scale formation in the presence of a native lateral stress field.
The initial stage of uranium oxidation: mechanism of UO(2) scale formation in the presence of a native lateral stress field.
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DOI：
10.1021/jp062795n
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发表时间：
2006-11
期刊：
影响因子：
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通讯作者：
Z. Chernia;Y. Ben‐Eliyahu;G. Kimmel;G. Braun;J. Sariel
中科院分区：
文献类型：
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作者：
Z. Chernia;Y. Ben‐Eliyahu;G. Kimmel;G. Braun;J. Sariel
在本工作中，提出了一个α-铀的氧化模型。它描述了反应过程中在氧化膜中形成的内部横向应力场。弹性、保应力氧化物(UO(2+x))膜的厚度小于0.5微米。由UO(2)的X射线衍射法测得的线位移应变(β(2theta))计算出了6.5 Gpa的横向应力场。结果表明，在弹性生长区域内，(110)是UO(2)气固氧化的主要生长面。本文讨论的扩散限制氧化机理是基于已知的“2：2：2”团簇理论，该理论描述了萤石基超化学计量比氧化物的氧化机理。在本研究中，它适用于描述氧-阴离子跃迁。阴离子在[110]方向以高速率向氧化物-金属界面跳跃，因此使该管道路线成为UO(2)形成的主要生长方向。进一步认为，氧化膜纯弹性域的增长应完全归因于110处的阴离子跃迁。在相对较厚(>；0.35微米)的氧化膜中，通过晶界和裂纹各向同性扩散的阴离子被证明对整体氧化速率有显著影响，如果随后在后弹性区域发生雪崩破裂。应力通过直接控制跳跃速率来影响弹性域中的氧化。在后弹性区域，应力通过增强各向同性扩散间接地削弱了跳跃。表面粗糙度是阴离子跃迁的另一个阻碍因素。与各向异性跳跃相比，各向同性跳跃的扩散具有较低的激活能垒。因此，由于各向同性扩散，在较低的温度下表现出相对较强的影响。
In this work, an oxidation model for alpha-uranium is presented. It describes the internally lateral stress field built in the oxide scale during the reaction. The thickness of the elastic, stress-preserving oxide (UO(2+x)) scale is less than 0.5 microm. A lateral, 6.5 GPa stress field has been calculated from strains derived from line shifts (delta(2theta)) as measured by the X-ray diffraction of UO(2). It is shown that in the elastic growth domain, (110) is the main UO(2) growth plane for gas-solid oxidation. The diffusion-limited oxidation mechanism discussed here is based on the known "2:2:2" cluster theory which describes the mechanism of fluorite-based hyperstoichiometric oxides. In this study, it is adapted to describe oxygen-anion hopping. Anion hopping toward the oxide-metal interface proceeds at high rates in the [110] direction, hence making this pipeline route the principal growth direction in UO(2) formation. It is further argued that growth in the pure elastic domain of the oxide scale should be attributed entirely to anion hopping in 110. Anions, diffusing isotropically via grain boundaries and cracks, are shown to have a significant impact on the overall oxidation rate in relatively thick (>0.35 microm) oxide scales if followed by an avalanche break off in the postelastic regime. Stress affects oxidation in the elastic domain by controlling the hopping rate directly. In the postelastic regime, stress weakens hopping, indirectly, by enhancing isotropic diffusion. Surface roughness presents an additional hindering factor for the anion hopping. In comparison to anisotropic hopping, diffusion of isotropic hopping has a lower activation energy barrier. Therefore, a relatively stronger impact at lower temperatures due to isotropic diffusion is displayed.