锆合金电子结构与物性关联的第一性原理研究

发布时间：2018-04-24 01:34

本文选题：第一性原理 + 结构性质关联　；参考：《燕山大学》2015年博士论文

【摘要】：结构性质关联在材料科学中的地位举足轻重。只有充分掌握了材料结构和性质之间的关联信息,以目标性质为导向快速高效地设计开发新材料才有可能,否则便只能停留在高成本高周期的炒菜试错阶段。材料的宏观性能与其微观电子结构密切相关。对于锆合金,如何在电子尺度认识理解其一系列不同于其他合金体系的本征特性,建立其微观电子结构和宏观特性之间的关系,对于自下而上地设计与开发新型锆合金具有重要的理论与现实意义。结合文献调研和自己的研究工作,我们认识到在纯锆和许多锆合金中存在着一种特殊的赝原子现象,这种特殊的电子结构极有可能是锆基材料各种宏观特性的内在物理根源。本文基于密度泛函理论和Bader的电子密度拓扑分析方法,探讨了赝原子这一特殊电子结构在锆及其金属间化合物中的存在性及存在机制,并初步建立了其与锆合金一些宏观物性之间的关系。首先探讨对比了纯锆三个常见相的电子密度拓扑性质,发现随着锆四面体结构畸变程度的增大,Zr原子间的相互作用由α相中的赝原子变为ω相中的“香蕉键”再变为β相中常见的直线型键。在不同的密排六方金属中,随着元素电负性的降低,体系的赝原子特性越来越强。因此,赝原子的出现是由晶体结构和组成元素的电负性共同决定的。锆合金中特殊的赝原子结构可能会导致其具有特别的力学行为,如弹性各向异性、位错芯结构、位错滑移等。对ω-Zr和有序ω相Zr_2Al的对比研究表明,Al对Zr的有序取代未能产生赝原子,二者中均具有Zr-Zr香蕉键。但是,这两相的弹性性质差异很大,Zr_2Al具有更高的弹性模量和硬度以及更低的弹性各向异性。电子密度拓扑分析表明,Al对Zr的有序取代显著增大了对应化学键的方向性和键临界点处的电子密度,并且降低了键的各向异性。由于ω相在很多锆合金中都具有重要作用,这些结果对调控ω相的基本力学性质进而调控相关锆合金的性能具有指导意义。在金属间化合物Zr_2Cu的四方相中也发现了赝原子,并且在高压下其赝原子结构会发生塌陷,发生电子密度拓扑转变,进而导致Zr_2Cu晶体发生等结构相变。由于这个相变同时具有传统的等结构相变和结构相变的特征,因此我们提出一种新的相变类型叫做准结构相变。对Zr_2Cu的三个竞争相的研究发现,随着Zr原子堆积偏离四面体程度的增大,Zr原子间的成键形式由赝原子键(C11_b相)转变为C16相的二分叉键(香蕉键)再转变成E9_3相中锆八面体的直线型键。这对锆基非晶具有一定的隐含意义。在Zr基非晶中存在着许多畸变程度不同的Zr四面体结构,它们极有可能具有这三种不同的电子结构,这将会对Zr基非晶的物化性质具有决定性的作用。Zr_2Pd和ZrPd_2是晶体结构相同但性质却大不同的经典案例。尽管二者具有相同的Mo Si_2型结构和能形成氢化物的组成元素,但他们的储氢吸氢性能相差悬殊,即Zr_2Pd极易吸氢并形成一系列氢化物而ZrPd_2却几乎不固溶氢。这一反常现象一直以来未得到满意的解释。基于Bader的电子密度拓扑分析,我们发现其根源在于它们实际上具有不同的扩展结构(电子密度拓扑)。
[Abstract]:The relationship between structure and properties is very important in material science. Only by fully grasping the related information between material structure and nature, it is possible to design and develop new materials quickly and efficiently with the target nature as the guidance. Otherwise, it can only stay at the stage of high cost and high cycle stir fry. The structure is closely related. For zirconium alloys, it is of great theoretical and practical significance for the design and development of new zirconium alloys to understand the intrinsic properties of a series of different alloy systems in the electronic scale and to establish the relationship between the micro electronic structure and the macroscopic properties. We recognize that there is a special pseudo atomic phenomenon in pure zirconium and many zirconium alloys. This special electronic structure may be the intrinsic physical source of the macroscopic properties of zirconium based materials. This special electricity is discussed in this paper based on the density functional theory and the electronic density topological analysis of Bader. The existence and existence mechanism of substructure in zirconium and its intermetallic compounds and its relationship with some macroscopic properties of zirconium alloys are initially established. First, the topological properties of the electron density of the three common phases of pure zirconium are discussed and compared. It is found that the interaction of Zr atoms with the degree of structural distortion of zirconium tetrahedron is from the alpha phase. The pseudo atom becomes the "banana bond" in the Omega phase and then becomes a common linear bond in the beta phase. In the different dense six square metals, the pseudatomic properties of the system become stronger with the decrease of the electronegativity of the elements. Therefore, the appearance of the pseudo atoms is determined by the Electronegativity of the crystal structure and the constituent elements. The structure may result in special mechanical behavior, such as elastic anisotropy, dislocation core structure, dislocation slip and so on. A comparative study of Omega -Zr and ordered Omega phase Zr_2Al shows that the ordered substitution of Al for Zr has failed to produce pseudo atoms, and all of the two have Zr-Zr banana bonds. However, the elastic properties of this two phase are very different, and Zr_2Al has a higher level. The elastic modulus and hardness as well as the lower elastic anisotropy. The topological analysis of electron density shows that the ordered substitution of Al to Zr significantly increases the orientation of the corresponding chemical bonds and the electron density at the critical point of the bond, and reduces the anisotropy of the bond. The basic mechanical properties of the zirconium alloys are of guiding significance. The pseudo atoms are found in the Quartet phase of the intermetallic compound Zr_2Cu, and the pseudo atomic structure will collapse at high pressure and the topological transformation of the electron density will occur, which leads to the phase transition of the Zr_2Cu crystal. A new type of phase transition is called quasi structural phase transition. A new type of phase transition is called the quasi structural phase transition. A study of three competing phases of Zr_2Cu shows that the bonding form between the Zr atoms is transformed from the pseudo atomic bond (C11_b phase) to the two bifurcating bond (banana bond) of the C16 phase as the accumulation of Zr atoms deviates from the tetrahedron. It is converted into a linear bond of zirconium eight in the E9_3 phase. This has a certain implied meaning to the zirconium base amorphous. There are many Zr tetrahedron structures with different degrees of distortion in the Zr base amorphous. They are extremely likely to have these three different electronic structures, which will play a decisive role in the physical and chemical properties of the Zr base amorphous,.Zr_2Pd and ZrPd_. 2 are the classic cases of the same crystal structure but different properties. Although the two have the same Mo Si_2 structure and can form the constituent elements of the hydride, their hydrogen storage performance is very different, that is, Zr_2Pd is very easy to absorb hydrogen and form a series of hydrides, but ZrPd_2 is almost inSolid to dissolve hydrogen. This anomalous phenomenon has not been obtained all the time. Based on Bader's topological analysis of electron density, we find that the reason is that they actually have different extended structures (electron density topology).

【学位授予单位】：燕山大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG146.414

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