【作者】 郭晓菊；

【导师】 田永君； 徐波；

【作者基本信息】 燕山大学 ， 材料物理与化学， 2010， 博士

【摘要】 本论文包括两方面的主要内容:一是从化学键合的观点出发建立了硬质晶体的宏观力学性质——硬度和强度与晶体的微观结构参数之间的定量关系;二是采用第一性原理方法在轻元素与贵金属及轻元素化合物中预测了几种新型亚稳材料,尤其是超硬材料。将晶体的宏观力学性质与微观结构参数联系起来,不仅可以发展定量预测晶体宏观力学性质的理论方法,而且可以帮助我们从物理本质上理解相应的宏观性质。将硬度的半经验公式与化学键的布居数离子性标度相结合,使完全采用第一性原理可计算的参数来预测硬度成为可能。在我组前期工作的基础上,本文进一步探讨了富硼化合物和纤锌矿结构中布居数P_c的取值方法和影响化学键离子性的内在因素,预测了B₆O、B₁₃C₂、新型亚稳材料BC₂N、纤锌矿半导体、B₆N、B₆P和B₆As等复杂晶体的理论硬度,其中B₆O、B₁₃C₂、BC₂N和纤锌矿半导体的理论硬度与实验硬度相当吻合。对于过渡金属碳化物与氮化物而言,硬度的半经验公式并不适用。通过深入探究该类晶体与金刚石、c-BN等共价晶体化学键性质的差异,发现了影响晶体硬度的另外两个因素:化学键中很小的金属性成分使硬度急剧下降的负面影响,以及参与杂化的d价电子使硬度增大的正面影响。经修正后,得到了对高共价性晶体普适的硬度表达式。纳米超晶格的硬度异常增高现象引起了人们的广泛关注。如何将上述硬度模型拓展到处理超晶格的硬度问题,是不断完善该硬度模型的一个新目标。将影响超晶格中带隙宽度的量子限域效应引入到超晶格的硬度计算当中,导出了描述超晶格硬度的半经验模型。该模型认为:量子限域效应的存在使超晶格体系的硬度在小调制周期时得到大幅提升。强度是晶体力学性能的又一个重要指标。强度与硬度都由化学键的键强所决定。本论文首先将化学键的键强定义为化学键抵抗轴向拉伸变形所能承受的极限拉力,通过引入有效成键电子数这一新概念,建立起了化学键键强与键长及有效成键电子数的定量关系。该模型可以描述纯共价化学键、极性共价键与离子键的键强,并且可用于估算某些晶体材料沿特定方向的抗拉强度。与共价晶体内聚能及离子晶体晶格能比较后发现该模型给出的键强与内聚能、晶格能有相同的趋势。以5种结构的C₃N₄为初始结构,我们构建了5种可能的B₄C₃相。采用第一性原理计算方法得到了各B₄C₃相的平衡晶格常数、内聚能以及电子性质,采用硬度的半经验公式预测了4种半导体性B₄C₃相的理论硬度,其中,c-B₄C₃与cs-B₄C₃具有与c-BN可比的高硬度。为了确定前人实验合成的OsC的晶体结构,我们构建了7种可能的OsC结构。通过总能量计算、稳定性判断、X射线衍射模拟结果分析及硬度计算,我们认为实验中合成的OsC应具有NiAs结构而不是WC结构。更多还原

【Abstract】 This dissertation concentrates on two different topics: the first part is construction of the relations between macroscopic mechanical properties （including hardness and strength） and microscopic structural parameters of hard crystals in the view of chemical bonding, and the second part is prediction of several novel metastable materials, especially superhard materials, by first principles calculations.Connecting the macroscopic mechanical properties with the microscopic structural parameters not only can develop the theoretical methods of quantitatively predicting the mechanical properties of a crystal, but also help us to deeply understand their physical natures. To combine hardness semiempirical equations and population ionicity scale of a chemical bond makes the hardness predictions possible, in which all parameters can be calculated by first principles. Based on the previous works of our group, we further study the determination of population P_c in boron rich compounds and wurtzite-structures and the intrinsic factors influencing the chemical bond ionicity, predict the theoretical hardness of B₆O, B₁₃C₂, BC₂N, wurtzite semiconductors, B₆N, B₆P and B₆As complex crystals, where the theoretical hardness agrees well with the experimental Vicker’s hardness for B₆O, B₁₃C₂, BC₂N and wurtzite semiconductors.For transition metal carbides and nitrides, it was found that the semiempirical model is unsuitable for evaluating their hardness. By exploring the difference of chemical bond feature between these crystals and the covalent crystals of diamond and c-BN, we find another two factors determining the hardness of these crystals. One is a negative factor that a small metallic component of chemical bonds decreases obviously hardness, and second is a positive factor that d valence electrons increase hardness. After introducing the two factors, a universal hardness expression is established for covalence-dominant crystals.The abnormal enhancement of superlattice hardness attracts great attention of physical and material scientists. How to expand the hardness model of a crystal to treat the issue of superlattice hardness is a challenge work. By introducing the quantum confinement effect into the hardness calculation of superlattice, we derive an empirical model describing the superlattice hardness. In this model, the existence of the quantum confinement effect enhances significantly the hardness of superlattices with small stipulation period.Strength is another important mechanical property of crystals. Both strength and hardness is dependent on bond strength. In this dissertation, we define the bond strength as the maximum tensile force that a chemical bond can suffer when tensile deformation is along the axis of the bond. By introducing a new concept, effective bonding valence electrons, we construct the quantitative relationship between the bond strength and the bond length and the effective bonding valence electrons. This model can describe the bond strength of pure covalent, polar covalent and ionic bonds, and can predict the tensile strength of some crystals along specific directions. Compared to the cohesive energy of covalent crystals and the lattice energy of ionic crystals, the bond strength calculated with this model has the same tendency to them.Based on the five reported C₃N₄ structures, we construct five phases of B₄C₃. The equilibrium lattice constants, cohesive energy and electron properties are obtained by performing first-principles calculations. Theoretical hardness of four semiconducting B₄C₃ phases is estimated by using the semiempirical hardness model, where the cubic and cubic spinal phase has the hardness comparable to that of cubic boron nitride. In order to determine the crystal structure of previously-synthesized OsC, we construct seven possible OsC structures. Based on total energy calculation, mechanical stability judgment, X ray diffraction simulation, and hardness calculation, we conclude that the experimentally-synthesized OsC should be in NiAs structure not in WC structure.更多还原