【作者】 张桂凯；

【导师】 汪小琳；

【作者基本信息】 中国科学技术大学 ， 核燃料循环与材料， 2014， 博士

【摘要】 阻氚涂层是聚变堆氚安全防护领域的关键科学与技术问题之一。在众多候选材料中,Al2O3由于其较低的氢渗透率成为当前阻氚涂层材料研究的重点。然而,氢同位素在Al203中行为及其作用机理尚不明确,因此,Al203阻氚涂层的方案设计、制备工艺调制和性能调控之间难以协调,往往顾此失彼,阻氚效果尚未得到理想的发挥。针对Al203阻氚涂层中氢行为涉及的基本科学问题,采用基于密度泛函理论的第一性原理计算、热力学概念、过渡态搜索和速率理论相结合的方案,分别从热力学和动力学角度系统研究了典型氧化物阻氚涂层材料a-Al203表面(中)氢的吸附、解离、侵入、存在形式和扩散等行为的详细微观机制以及α-Al2O3/FeAl界面对这些行为的影响效应,从基础上提出了在典型阻氚涂层工作环境下a-Al203阻滞氢渗透的作用机理和增强其阻氚性能的指导性原则。论文的研究成果为Al203阻氚涂层的性能调控和制备工艺研究提供了科学依据,促进了氧化物阻氚涂层材料中氢同位素行为研究的进展。论文主要研究成果有,1.研究了α-Al2O3(0001)和(1-102)表面的氢吸附、解离、侵入和扩散行为。首次获得了a-Al203表面(次表面)氢行为的详细机制,同时预测出氢原子优先通过α-Al2O3(1-102)面扩散侵入α-Al2O3中,并得到红外光谱实验的验证。H2分子以接近平行方式物理吸附在α-Al2O3(0001)和a-Al203(1-102)的表面后,在室温附近自发解离成共吸附在Al、O原子上的H原子。此后,在一定的温度范围内,这些氢原子将优先在α-Al2O3表面扩散；随温度的升高,吸附在氧原子上的氢原子先围绕该氧原子旋转,再跳跃到下一层氧原子上,从而进入次表面,此后氢继续以“旋转-跳跃”方式依次进入α-Al2O3内部。其中,H原子由a-Al203表面到次表面的“旋转”需克服较高能垒,同时次表面对氢原子的束缚作用相对表面减弱,这使得H原子难以侵入α-Al2O3次表面。一旦侵入,随后的扩散相对容易发生。氢由(1-102)表面到次表面的扩散能垒比由(0001)的扩散能垒小,而由(1-102)面侵入的平衡常数大,因此氢原子将优先通过α-Al2O3(1-102)面扩散侵入α-Al2O3中。这归因于α-Al2O3各晶面的氢原子起始侵入点的密度差异,从而引起了侵入过程中氢原子相互干扰程度不同。2.研究了α-Al2O3中氢的存在形式及扩散行为,对a-Al203中阻滞氢渗透的主要作用区域提出新见解。研究发现,氢在α-Al2O3主要以Hip和[VAl3-H+]q两种形式存在,但稳定性不如自由氢分子。在富氢平衡状态下,氢在cα-Al2O3中主要以Hi+形式存在,并伴有[VAl3--H+]2-和Ho+形式存在的氢。H在α-Al2O3中的输运主要是通过Hi+扩散完成,[VAl3--H+]2-和Ho+在一定的高温下才能解离成Hi+参与扩散。Hi+在a-Al203中以“旋转-跳跃”方式沿c轴方向成螺旋形式在八面体空隙间扩散,相应的扩散能垒低于H原子由α-Al2O3表面到次表面的扩散能垒,这表明α-Al203阻滞氢渗透主要是表面到次表面扩散能垒相对较高所致,与以往人们认为阻滞氢渗透主要是α-Al2O3内部氢扩散能垒相对较高的看法迥异,提出我们今后研究α-Al203阻滞氢渗透行为时应对表面与体相间区域加以考虑。部分α-Al2O33的本征点缺陷VAl3-,Oi2--和Vo0等能捕获Hi+形成[VAl3--H+]2-和Ho+,为α-Al2O3阻氚性能的调控提供了一种重要思路。此外,研究也获得了Hi+,[VAl3--H+]2-和Ho+的局域原子结构和H-O键的伸缩振动频率,为辨另α-Al2O3中H相关红外光谱峰的局域原子结构提供了重要参考。3.研究了α-Al2O3/FeAl界面的电子结构及其对α-Al2O3中氢行为的影响效应。发现界面对组元α-Al2O3的电子结构和氢行为的影响是局域性的,界面氢原子对界面的结合强度和原子结构没有损伤效应。根据实验结果,考虑了铁、铝混合氧化物界面和氧化铝界面两种界面,发现α-Al2O3与FeAl合金间的界面连接主要涉及离子键和金属键；由于界面对组元α-Al2O3FeAI电子结构的影响是局域性的,因此组元cα-Al2O3中氢原子的稳定性相对其表面上氢原子显著降低,这与纯a-Al203中的情况相同；但在界面上,组元FeAI中氢原子的稳定性显著升高。组元α-Al2O3中氢原子的扩散是α-Al2O3/FeAl中氢渗透过程的速率决定步骤,其中的氢原子扩散机制及动力学限制途径与纯α-Al2O3中的类似。α-Al2O3/FeAl的铁、铝混合氧化物界面相对更有利于阻滞氢的渗透和提高界面的结合强度。随界面处氢原子的聚集,α-Al2O3/FeAl界面的理论界面连接功有所增加,其上的Fe-O、Al-O或Fe-Al等键也无明显变化,所以界面氢原子对界面的结合强度和原子结构没有损伤效应。4.发展了研究阻氚涂层阻滞氢渗透作用机理的方法,通过热力学概念、含氢α-Al2O3系统原子化学势的变化规律、第一性原理计算得出的吸附能和过渡态速率理论为主的动力学理论的有效结合,给出了典型阻氚涂层工作环境下α-Al2O3阻滞氢渗透的作用机理。这一结果将通常的基于Fick定律的表象方法向原子级方法前推了一步,已被日本九州大学的Hashizume K研究组用于解释α-Al2O3的氚自显影实验结果。平衡状态下α-Al2O3中Hi+的浓度极低(稳定性低)是α-Al2O3阻滞氢渗透在热力学上的主要原因。在动力学上的作用机理为,在500～700℃温度范围,当氢渗透压力大于17kPa时,氢原子围绕α-Al2O31-102)面第3层氧原子旋转阻滞α-Al2O3中的氢渗透；当压力小于1kPa时,氢分子在α-Al2O3(1-102)面第2和第4原子层的铝原子之间位置上方的极性解离阻滞α-Al2O3中的氢渗透。这两种动力学控制机制很好地与氢渗透和氚自显影实验吻合。5.基于α-Al2O3阻滞氢渗透的热力学和动力学作用机理,提出了增强α-Al203阻氚涂层阻氚性能的指导性原则。在热力学上增强α-Al2O3阻氚性能的关键原则是降低a-Al203中H的稳定性,以增加非自发反应的难度,这有望通过在α-Al2O3中形成[VAl3-H+]2-和Ho+等氢相关缺陷达到。在动力学上的关键原则是控制Hi+的扩散,这有望通过在a-Al203表面形成负l价的氧离子中心使表面的氢分子极性分解转化为非极性分解,或在α-Al2O3中适当增加本征点缺陷数量以捕获Hi+等方法达到。更多还原

【Abstract】 Tritium permeation barrier (TPB) is one of key scientific and technological issues for preventing the tritium loss from fusion plants. α-Al2O3is of special interest because of low hydrogen permeability. However, from the microcosmic point view, less insight is available regarding interaction of hydrogen with α-Al2O3material and corresponding mechanism of thermodynamics and kinetics, thus the design, processing and optimization of α-Al2O3TPBs rely on trial-and-error, making such TPBs often exhibiting lower efficiency than anticipated based on the bulk properties of this material.Focusing on basic scientific issues of the interaction of hydrogen with α-Al2O3TPB, adsorption, dissociation and diffusion of hydrogen on and into α-Al2O3surfaces, dominant H-related defects and its diffusion in bulk α-Al2O3, and effects of α-Al2O3/FeAl interface on these H-behaviors have been investigated thermodynamically and kinetically in a systematic way by the first-principle calculations and transition state theory. Based on above basic and essential steps of H-permeation in α-Al2O3, mechanisms of α-Al2O3resisting H-permeation under typical TPB working condition, and principles for the optimum performance have proposed from the basic point of view, to guide design, processing and performance optimization of α-Al2O3TPBs for engineering practical applications. The major results are as follows.1. Mechanisms for adsorption, dissociation and diffusion of hydrogen on α-Al2O3surface2. A H2molecule, with parallel configuration, absorbs on α-Al2O3(0001) and α-Al2O3(1-102) surface, and then dissociates heterolytically near room temperature, with one H atom adsorbing on a top Al atom site, and another H atom adsorbing on a top O atom site,which is thermodynamically spontaneous. It is possible that at finite temperatures a number of events involving surface diffusion of such H atoms before their migration into the bulk takes place. Bulk diffusion process of the H atom involves two steps on every O atomic layer of α-Al2O3:(1) the reorientation step in which hydrogen atom remains bonded to the same O atom and (2) the hopping step in which breaking and reforming of O-H bond take place. The rate-limiting barrier of H diffusion into the bulk from the (1-102) and (0001) surface is1.41～1.58eV, while H atoms in bulk α-Al2O3are significantly less stable than on surfaces, thus H diffusion preferentially occurs via surface path rather than bulk one. The rate-limiting barrier of H diffusion into the bulk from the (1-102) surface is0.17eV less than that of the (0001) surface, and the equilibrium constant for adsorbed H on the (1-102) surface in equilibrium with absorbed H in bulk α-Al2O3is larger than that of the (0001) surface, thus significant diffusion of H into bulk can occur more readily from the α-Al2O3(1-102) surface compared to the α-Al2O3(0001) surface, in well agreement with results of infrared spectroscopy, which is related to the different crystal structure of surfaces resulting in the different interference from other hydrogen atoms.3. State, local configuration and diffusion of H-related defects in bulk α-Al2O3We predict that the stable forms of H related defects in α-Al2O3are charged H interstitials (Hiq) and hydrogenation of the bulk VAl3-([VAl3--H+]q) under hydrogen-rich condition, which are less energetically stable than the gas phase H2. As the system of α-Al2O3and H reaches equilibrium, H in α-Al2O3is mainly in present of Hi+state, which is the most stable among H-related defects, and also likely to exist in the form of [VAl/3--H+] and Ho+. Hi+is the predominant diffusion species in α-Al2O3, and [VAl3--H+]2-and Ho+can release trapped hydrogen during high temperature annealing, and then contribute to the H-transport in α-Al2O3. The Hi+diffusion process, involving H-reorientation around oxygen atom from an octahedral interstitial site to an adjacent one followed by H-hopping within the adjacent one, has a barrier of1.26eV, lower than suface-to-subsurface diffusion barriers of H in α-Al2O3, thus it is rather reasonable that the overall diffusivity of H in α-Al2O3is governed by the surface-to-subsurface diffusion. Hi+will be trapped by the VAl3-, Vo0, Oi2-, increasing the activation energy of H migration and decreasing the H mobility, which is favored for low H-transport in α-Al2O3TPB. Local vibration mode and local structures for Hi+,[VAl3--H+]2-and Ho+have been obtained and is very helpful for us to identifying actual local structures responsible for the IR observed peaks.3. The effect of α-Al2O3/FeAl interface on stability and diffusion of hydrogen in α-Al2O3partThe interfacial binding involves cation-anion and metal-metal interactions. Due to the localized distributions of the interface on density of sate of α-Al2O3part of α-Al2O3/FeAl, H-surface interaction on the α-Al2O3/FeAl resembles that on pure α-Al2O3(0001) case, H interstitials in the α-Al2O3part is significantly less stable than on surface, and in interface region consisting of the only Al-oxide (Al/O interface) or the Al, Fe mix-oxide (Al/Fe/O interface), and in FeAl part are significantly more stable. The α-Al2O3component takes a critical role in preventing hydrogen diffusion during the α-Al2O3/FeAl TPB operation. H diffusion into the α-Al2O3part of both slabs must overcome a rate-limiting barrier of about1.66-2.02eV at surface-to-subsurface step, like in α-Al2O3case. For the bulk path, the migration of H atom can occur more readily in the α-Al2O3part of the slab with the Al/O interface compared to that with the Al/Fe/O interface, while H atoms in the former are less stable, thus the α-Al2O3/FeAl barrier with Al/Fe/O interface is predicted to be more effective in resisting H-permeation against the underlying steel. The adhesion work decreasing and hydrogen-related structural damage do not occur in the interface region, suggesting that the appearing or trapped H atoms in such interface region will not be weak linked and accelerate the failure of α-Al2O3/FeAl TBP.4. Mechanisms, in atomic scale, of α-Al2O3resisting hydrogen isotopic permeation under typical TPB working conditionsWe have combined thermodynamic concepts, chemical potentials of atoms in the system of H and α-Al2O3, density functional theory calculations with rate theory of transition state theory to identify the kinetic and thermodynamic mechanism of α-Al2O3resisting H-permeation. The high formation energy of Hi+is the dominate term in the activation energy for H-transport in bulk α-Al2O3, suggesting that the low H concentration (low stability) in α-Al2O3is the thermodynamical bottleneck for H-permeation through α-Al2O3. As for the kinetic mechanism, H-permeation through α-Al2O3is governed by the H reorientation around oxygen in the third atomic layer of α-Al2O3(1-102) surface at the hydrogen pressure of above17kPa, whereas that is suppressed by H2dissociation above the site between B-site Al and D-site Al on the α-Al2O3(1-102) surface at the hydrogen pressure lower than1kPa. Our DFT calculations successfully reproduce hydrogen permeation and tritium imaging plate experiment observations. These results are very important to comprehend the results from the common method based on Fick’s law.5. Principles for optimum suppressing-performance of α-Al2O3TPBThermodynamical principle for optimum suppressing H-permeation for α-Al2O3TPB would be to decrease energetical stability of Hi+, and kinetics principle would be to enhance migration barrier for H,+.更多还原