【作者】 李克艳；

【导师】 薛冬峰；

【作者基本信息】 大连理工大学 ， 功能材料化学与化工， 2009， 博士

【摘要】 电负性概念1932年由著名化学家Pauling提出,它表示分子中的原子将电子吸引向自身的能力。随着新材料的发展以及各学科之间的相互交叉与渗透,电负性如今已成为在化学、物理和材料科学等领域均具有广泛应用的基本原子参数。电负性与材料的超导性、磁性、光学等多种性能之间存在着密切的内在联系,它能够把复杂的分子现象以及材料的物理化学特性简单地参数化为由原子的一些性质来表示的定量关系,为新材料设计提供了理论依据。本论文考虑了原子在晶体中具体的成键环境将电负性概念进行了拓展,进而将其应用于材料的结构-性质关系研究以及超硬材料设计。具体工作如下:基于离子的有效静电势,提出了离子电负性模型,确定了周期表中82种元素不同价态、配位数和自旋态的电负性值,是迄今为止最为全面的电负性标度。由于精确地考虑了离子周围实际的化学环境,这些电负性值恰当地反映了离子吸引电子的能力。该电负性标度不但能够合理地估算离子的Lewis酸强度、水合自由能等一些有用的物理化学参量,而且能够预测三价镧离子在无机晶体中的电荷转移能,对于新材料的设计工作具有重要的指导意义。基于共价晶体中元素的静电势,提出了适用于共价晶体中元素的电负性模型,确定了周期表中58种元素在共价晶体中具有不同成键电子数和配位数的电负性值,在预测共价晶体的弹性模量、电子极化率等性质时得到了较为理想的应用。该电负性标度是对上述离子电负性标度的重要补充和完善,对于研究共价材料的结构-性质关系具有重要意义。根据Sanderson电负性均衡原理,提出了键电负性定义。基于单位体积的抓电子能,定义了原子硬度、离子硬度和键硬度,在这三个微观层次上研究了材料硬度的本质:材料的硬度本质上取决于其单位体积组成化学键的抓电子能,并由此建立了鉴别材料硬度的微观模型。该理论模型仅仅通过材料组成原子的电负性和晶体结构就能准确预测各种材料的硬度,并且将能形成超硬材料的原子组合挑选出来,对于人们设计新型超硬材料具有重要的理论指导意义和实际应用价值。根据该模型,又系统地研究了第ⅣA和ⅣB族氮化物各种相的硬度,发现对于由B、C、N等轻元素组成的化合物,类金刚石结构是最硬的结构,然而对于重元素化合物来说,必须尽可能地增大其平均配位数来达到高硬度。论文最后还基于键密度、键强和键的共价程度这三个决定材料硬度的主要因素将上述硬度模型进行了拓展,新拓展的模型也能很好地预测材料的硬度。更多还原

【Abstract】 The concept of electronegativity (EN) was proposed by Pauling in 1932, which describes "the power of an atom in a molecule to attract electrons to itself". With the development of new materials and the increase of interdisciplinary cooperation, EN has been a basic atomic parameter which is widely used in the fields of chemistry, physics and materials science. EN is closely related to many properties of materials such as superconductive, magnetic and optical properties, and it can be used to simplify the complicated molecular phenomena and properties of materials to some quantitative correlations, which provides a theoretical basis for the design of new materials, In this thesis, we develope the concept of EN by including the specific bonding environment of atoms in crystals, and then apply EN to the study of structure-property relationship of materials and the design of superhard materials. The detailed contents are listed as following:Based on the effective electrostatic potential of ions, an ionic EN model is proposed. According to this model, the EN values of 82 elements with different valence states, coordination numbers and spin states are quantitatively determined, which is the most comprehensive EN scale so far. Due to the detailed consideration of actual chemical environment of ions, these EN values well reflect the electron-attracting power of ions. This EN scale can not only be well used to estimate some useful physical and chemical parameters such as the Lewis acid strength and the hydration free energy of cations, it can also be used to predict the charge transfer energies of trivalent lanthanides in inorganic crystals, which provides a useful guide to the design of new materials.Based on the electrostatic potential of elements in covalent crystals, an EN model of elements in covalent crystals is proposed. According to this model, the EN values of 58 elements with different bonding electrons and coordination numbers are determined, which are satisfactorily used to predict the elastic moduli and electronic polarizabilities of covalent crystals. This scale is an important supplement of the ionic EN scale, which is very helpful to the study of the structure-property relationship of covalent materials.According to Sanderson’s idea of EN equalization, the definition of bond EN is proposed. Based on the electron-holding energy per unit volume, atomic stiffness, ionic stiffness and bond hardness are defined, and the nature of material hardness is investigated at these three microscopic levels. It is found that the hardness of materials is essentially determined by the electron-holding energy of its constituent chemical bonds per unit volume, by which a microscopic model for identifying the hardness of materials is established. By using this model, the hardness of various materials can be satisfactorily predicted solely in terms of EN of constituent atoms and crystal structure data. Moreover, the elemental combinations which may form superhard materials are selected, which is of great significance to the design of novel superhard materials. Furthermore, the hardness of various phases of group IVA and IVB nitrides is systematically studied on the basis of current model for hardness. It is found that for compounds made of light elements such as boron, carbon and nitrogen, the diamondlike structure is the hardest one among all possible structures, whereas high coordination number is the general requirement for heavy-element compounds to achieve high hardness. Finally, the hardness model is extended based on bond density, bond strength and degree of covalent bonding, which are three determinative factors for the hardness of materials. The newly developed model for hardness can also be well used to predict hardness.更多还原