【作者】 杨军；

【导师】 孙弘；

【作者基本信息】 上海交通大学 ， 凝聚态物理， 2011， 博士

【摘要】 本论文主要采用基于密度泛函理论的第一性原理计算方法,计算研究导电性超硬材料的理想强度等物理性质。导电性材料在国防、工业乃至日用生活中都有广泛的应用。在国防和工业应用中还会对导电性材料的硬度或强度有较高的要求。研究设计超硬、超强的导电性材料具有重要的理论和实用意义。论文的第一、二章简单介绍了相关的研究背景与理论基础知识。在第三章中,我们研究了由轻元素组成的导电性超硬材料类金刚石结构的BC₃（d-BC₃）,计算了d-BC₃在不同应变下拉伸与切变理想强度。对该材料的电子态密度计算结果显示d-BC₃不但在平衡结构位置甚至在大尺度拉伸和切变形变下依然保持金属性。通过计算分析d-BC₃在各方向上的理想强度结果,比较得到最弱方向上的理想切变强度,从而确定d-BC₃是一种迄今具有导电性的最硬超硬材料之一。为配合实验合成这种材料,我们又模拟了两种层状BC₃的亚稳态结构。并指出其中一种层状结构可以通过跨越较低的势垒合成类金刚石结构d-BC₃材料。在第四章中,我们对Fe₄N材料进行了计算研究。Fe₄N材料由于具有很高的饱和磁矩以及很高的硬度特别适用于需要耐磨磁性材料的应用领域,如设计高密度磁记录设备的磁头等等。由于在[001]方向上Fe和N原子的距离最近,可以形成较强的原子键,以往的理论和实验一般都认为Fe₄N结构的（001）晶面具有最大的强度。我们对Fe₄N结构在各低指数晶面,如（001）、（011）、（111）、（112）面,完成了基于第一性原理的拉伸强度和切变强度计算。结果发现（011）晶面具有比其它各晶面更好的机械性质,（001）面的切变强度比其它晶面的切向强度值高出35%。我们的计算结果表明如果实验上能生长高质量单相（011）Fe₄N薄膜,可以设计更理想的高密度记录磁头。在第五章中,我们计算研究了一种新型的特硬材料OsB₂。由单纯轻元素（B、C、N等）组成的（绝缘或导电）超硬材料的合成都需要极高的压力和温度,大规模生产需要很高的成本。近年来国外有研究小组提出利用过渡金属和共价键轻元素形成的化合物或许更加容易得到既导电又具有良好机械强度的新型超硬材料。一般来讲,共价轻元素（B、C、N等）的共价键是很强的化学键,并且具有良好的方向性,可抵抗材料的切向形变;而过渡金属元素的高密度价电子可使材料具有很高的体弹性模量,使材料结构不易被压缩。将共价元素掺入到过渡金属中就是结合了两种化学键的优点,从而达到增强结构的抗压缩和切向形变能力,从而提高结构的硬度。OsB₂是第一批采用这种机制合成的样品,实验发现在（001）面上它的Vickers硬度达到30GPa,高于一般的钢铁材料,有可能成为新型的钢铁器件的研磨切割工具,以及耐磨防腐涂层。但我们通过第一性原理计算研究发现OsB₂在一些晶面上的切向强度具有极大的各项异性,如（001）晶面上沿[010]方向的切向强度（约10GPa）只是[100]方向切向强度（约30GPa）的三分之一,OsB₂并不适合用来设计新型的钢铁器件的研磨切割工具。我们的计算结果表明简单的硬度实验并不能精确地探测OsB₂结构的强度特性,设计新型的超硬材料时应特别注意研究材料在原子尺度的结构畸变和失稳模式。在第六章中,我们研究了刻痕硬度实验中压头下正压力对材料硬度的影响。ReB₂是过渡金属和共价键轻元素化合物超硬材料的又一典型结构。ReB₂材料引起广泛的注意是因为实验发现这一材料可以在金刚石表面上留下划痕,显示出很高的（划痕）硬度。但刻痕硬度实验却显示ReB₂材料的维氏硬度只有20～30GPa,远低于金刚石的维氏硬度（约为100GPa）。为解释这一现象,我们采用第一性原理计算方法分别计算了ReB₂材料在没有外加正压力情况下结构的切向强度（对应于划痕硬度实验）和在外加正压力情况下结构的切向强度（对应于刻痕硬度实验）。计算结果表明外加正压力将减弱ReB₂材料的切向强度,造成材料刻痕硬度的下降。理论上计算得到在考虑了正压力情况下ReB₂切向理想强度为26GPa,与实验值很符合。论文中还进一步仔细分析了正压力造成ReB₂材料强度下降的微观物理机制。更多还原

【Abstract】 In this dissertation, the first-principles calculation method based on density function theory is used to study the physical properties of conductive super-hard materials, such as their ideal strengths. The conductive materials have wide applications in national defense, industry and daily life, where the hardness or strengths of conductive materials may become vital in their applications. It is of prime importance both theoretically and practically to study and design conductive materials with super- or ultra-hardness.In the first two chapters of this dissertation, we give a briefly introduction to the research background and basic theories of our studies.In chapter three, we present the studies on a new kind of light elements super-hard material, the diamond-like BC₃ structure （d-BC₃）. We calculate its tensile and shear strengths under different stain loads. The calculated electronic density of states reveals that d-BC₃ is metallic not only at equilibrium, but also under large tensile and shear deformation. With the analysis of tensile and shear strengths under strains along various high symmetrical directions, we found that d-BC3 structure is one of the hardest conductor studied to date. We also proposed and studied two types of meta-stable layered BC3 structures. One of these layered BC3 structures can become the precursor that will lead to the synthesis of d-BC3 at much reduced pressure and temperature.In chapter four, we present the results of extensive studies on Fe₄N for its potential applications as high density magnetic recording heads and recording materials because of its high hardness and large saturation magnetization. Due to the nearest distance between the strong N-Fe bonding chains in <001> direction in Fe₄N structure, previous theoretical and experimental studies of Fe₄N believe that the （001） plane is the crystal plane with the highest hardness or strength in Fe₄N structure. We carried out first principles calculations of tensile and shear strengths for Fe₄N on its （001）, （011）, （111） and （112） low index crystal planes. Our results show that the （011） plane of Fe₄N has superior mechanical properties with a shear stress 35% higher than those of other planes, possessing the highest scratching hardness suitable for designing high density magnetic recording heads. Our studies call for experimental efforts to grow high quality single phase Fe₄N films in （011） crystalline direction for designing better high density magnetic recording heads.In chapter five, we study a new type of ultra-hard materials, OsB₂. Synthesizing super-hard materials with only light covalent elements （such as B, C and N） in mass production is usually expensive, since their processes require extremely high pressures and temperatures. Recently, a new design principle is proposed to synthesize ultra-hard materials by combining light covalent elements （such as B, C and N） with electron-rich transition metals to obtain materials with good electric conductivity and superior mechanical strength. Generally speaking, light covalent elements can form strong, directional covalent bonds with high resistance against structural shear deformations, while the high density of valence electrons from transition metals results in high bulk modulus that prevents the material structures from being squeezed together, both of which enhance the resistance of the structures against large inelastic deformations, leading to increased hardness. A primary example and among the first synthesized following this principle is OsB₂, which shows a experimental Vickers hardness of 30 GPa, over that of steel, on its （001） plane, applicable in designing abrasives and cutting tools for ferrous metals as well as scratch-resistance coatings. However, our first- principles calculation results show highly anisotropic shear strength distributions in certain crystalline planes of OsB₂. For instance, the peak shear stress （about 10 GPa） in the [010] direction on the （001） plane of OsB₂ is only one third of that （about 30 GPa） in the [100] direction on the same plane. This prevents OsB₂ from being used as cutting tools for steels. Our calculations demonstrate that simple hardness experimental tests may not accurately detect the strength properties of OsB₂ and highlight the importance of exploring atomistic deformation and instability modes in designing new ultra-hard materials. In chapter six, we study the effects of normal pressures beneath indenters on the hardness of materials in indentation hardness tests. Rhenium diboride （ReB₂）, another typical crystal structure recently synthesized by combining covalent elements with transition metals, has attracted considerable interest for its high scratching hardness capable of scratching the diamond surface. However, the measured Vickers hardness （Hv） of ReB₂ is unexpectedly low ranging from 20GPa to 30GPa, which is well below that of diamond （about 100GPa）. To explain this seemingly contradictory hardness behavior of ReB₂, we report first-principles calculations of ideal shear strengths of ReB₂ by neglecting the normal pressures beneath indenters （similar to scratch hardness experiment） and including the normal pressures beneath indenters （similar to indentation hardness experiment） in the calculations. Our results show that the normal pressure beneath the indenter indeed reduces the shear strength of ReB₂. The calculated ideal indentation strength of ReB₂ under a Vickers indenter is about 26 GPa, which agrees well ReB₂ with that of experiments. Detailed atomistic physical mechanisms to explain the reduction of indentation strength of 2 by the normal pressures beneath indenters are discussed in the thesis.更多还原