【作者】 周玮；

【导师】 孙弘；

【作者基本信息】 上海交通大学 ， 凝聚态物理， 2013， 博士

【摘要】 对现代材料科学而言,精确地预测材料的机械性质一直是人们关注的重要课题之一。虽然对实际的体材料而言,机械性质通常是受其内部的缺陷、位错等因素影响。但从微观角度来研究、理解材料本身内在的性质,也是十分重要的。近年来,随着纳米技术的发展,在实验室内合成近乎完美的缺陷极少的的小颗粒样品已经成为可能。同时在机械性质测量手段方面,纳米刻痕硬度测量手段的出现,使测量可以仅仅发生在一块极小的区域,这也极大减小了测量时遇到缺陷或位错的可能性。在上述这类可以避开缺陷作用的实验中,往往会测得非常高的材料强度,甚至可以接近理论上预言的理想强度结果。所谓理想强度,是指使完美晶体材料发生结构失稳(如断裂,解理,发生塑性形变等)所需要的最小应力值。它决定了材料强度的上限。过去的大量研究表明,基于密度泛函理论(density functional theory,DFT)的第一性原理理想强度计算方法,可以准确有效地算出材料的理想强度值。理想强度的意义重大且适用性广,它不仅仅可以决定理想体材料的机械稳定性,同时也可以用于预测理想材料中的界面、表面的稳定性,或是用于研究纳米材料,如纳米管、纳米线、纳米带等结构的性质。但值得注意的是,目前几乎所有的理想强度计算工作,基本都只是研究在绝对零度,且不考虑外界压力的情况。然而,温度、压力这些环境因素,却是可以在很大程度上影响材料理想强度的大小。以金刚石为例,在常温下金刚石的静态硬度测量值在80GPa作用,是自然界存在最硬的材料。但当温度达到1200℃时,其硬度竟降至不足20GPa,较常温下降了70%以上。而对于部分金属材料来说,温度的效应有可能更为明显。因此,将温度、压力等等内外部因素的影响加入到理想强度的计算当中,是十分紧急而且必要的。在本论文中,我们尝试在计算方法上将各种内外部因素的作用引入第一性原理理想强度的计算过程,并讨论这些因素将会对理想强度带来怎样的影响。我们选择单质形式的硼、铝、碳材料作为研究对象,分别在不同的条件下研究了特殊化学键、温度、压强对单质材料理想强度的影响。研究的结果表明：1)在材料γ-B28中,三中心键的存在会导致新的成键断键模式的出现,从而显著降低材料的理想强度值。2)对于面心立方结构的金属铝来说,理想强度受到温度的影响十分敏感。即使仅仅是用室温下的理想强度计算结果同绝对零度的结果相比,也可能会有很大不同。同时,材料的形变失稳模式,也会发生根本性的变化。3)对于强共价键冷压石墨相的碳单质材料,外加高压将会抑制材料原有的形变模式,改变其理想强度。当压力达到一定值时,部分冷压石墨相材料的强度甚至可以超过金刚石。首先,我们研究了多中心键的存在,对γ-B28的理想强度的影响。γ-B28是一种近期才被预测、合成的新型硼单质材料。实验表明,这种硼材料的硬度在50GPa左右,是一种超硬材料。我们计算了当材料沿各种方向进行拉伸、剪切形变时,得到了“应力——应变”关系曲线。在大多数方向上,计算给出的结果同实验结果是一致的,显示γ-B28的强度在50-65GPa之间。但当材料沿[011]方向做拉伸形变,以及沿(001)[010]方向做剪切形变时,结果却有了明显的区别。同一般共价键材料的结果不同,γ-B28依照上述两种模式进行形变时,“应力——应变”曲线中并没有出现应力值的急剧下降。相反而是类似金属材料的结果,曲线始终呈现出缓慢连续变化的形态。为理解这种现象出现的原因,我们仔细计算并分析了在形变过程中,材料中的电荷密度分布(charge density distribution)情况以及电子局域函数(electron localization function, ELF)情况。这两种计算的结果均表明,在结构形变过程中,出现了一种“二中心键——三中心键——二中心键”的新型共价键演化模式。这种模式的存在,使得γ-B28在发生结构形变时,原胞内的电荷转移总是一个逐渐连续的过程,而没有出现强共价键突然断裂的情况。而这种新的共价键演化机制,也导致了γ-B28材料理想强度的显著降低。接下来,我们讨论了温度因素对单质金属铝材料的理想强度的影响。铝具有密度低,耐腐蚀,导电性、导热性好等优秀性质,因此在生活中得到广泛了的关注和应用。我们采用第一性原理分子动力学方法(Ab-Initio Molecular Dynamics Method,AIMD),计算了在不同有限温度下铝的理想强度变化情况。计算结果表明,随着温度的升高,铝的理想强度发生了明显的降低。而结构失稳模式同绝对零度下的结果相比,也有着很大区别。在高温情况下,绝对零度时所预言的声子失稳模式甚至全部消失。值得一提的是,同传统绝对零度下理想强度计算不同,通过第一性原理分子动力学办法来计算材料理想强度的方法,可以在一次独立的计算中将材料的弹性失稳效应,和动力学失稳效应同时考虑在内,这使得计算更为简洁、可靠。作为比较,我们采用SCAILD方法(Self Consistent Ab Initio Lattice Dynamical method,SCAILD)计算了有限温度下铝的声子谱线。不同温度下的声子谱结果同第一性原理分子动力学给出的结果完全一致,有力地支持了前文的计算工作。最后,我们还讨论了压力效应对冷压石墨相的影响。近年来,实验上发现对石墨进行冷压时可以得到新的透明超硬相,这种新相的强度甚至高于金刚石,可以在金刚石压头的表面留下明显的刻痕。这种冷压石墨相得到了广泛的关注,在理论上对此超硬相相继有一系列的结构推测工作发表。从拓扑学角度来看,新近提出的这些预测结构可以主要分为是由碳原子的6原子环,5+7原子环,和4+8原子环构成,或是由这几种结构混合构成。在所有的这些结构中,我们选取了W-carbon和Z-carbon这两种材料,分别作为5+7原子环,和4+8原子环结构的代表。为了得到可靠的结论,我们在不同的压力下对这两种材料做了大量的“应力——应变”关系的计算。这些工作包括在材料所有类似(001),(011),和(111)的低指数面上,沿所有低指数方向进行的剪切形变的计算。在此工作中,我们总计计算了528条完整的“应力——应变”曲线。同时,我们也将这些计算的结果同6原子环拓扑结构的金刚石做出了比较。结果显示,在高压情况下,W-carbon和Z-carbon的结构演化模式同低压或常压的情况相比有显著的区别。当处于常压环境或者压力较低的环境下时,材料在经历大尺度形变后往往会相变至密度较低的,类似石墨相的松散结构。而在高压情况下,低密度相的出现会受到抑制,高密度相的碳结构出现的可能性要增大许多。而这些致密的新相由于受到环境高压和形变模式的特殊约束,使其有可能会在强度上超过金刚石。与此同时我们指出,这种高压下形变模式改变,致密相更容易出现的情况,应不仅仅只会发生在W-carbon和Z-carbon两种材料上。理由是这些材料在高压下表现出的新颖形变模式,并非是由于某些特殊结构、特殊化学键的存在导致产生,而只是体系渴望被优化至焓值极小的直接结果。所以我们在对材料施加沿各个不同方向的形变时,这种新型形变模式都有出现。因此我们认为,本工作所阐述的高压环境下新的相变模式出现的情况,应是适用于所有冷压石墨相的一种整体趋势。更多还原

【Abstract】 Accurate prediction of the mechanical properties of materials is of primary importance inmodern material science. Though the strength of bulk materials usually depends on thedefects and dislocations inside materials, it is important to understand the microscopic originof how ideal crystalline materials become unstable. Recently, the development ofnano-technology makes it possible to synthesize nano-materials with nearly perfect structure.Also the nano-indentation experiments could examine strength in a very small region wheredefects and dislocations are nearly absent. High material strengths obtained in theseexperiments can be comparable with the theoretical ideal strength results.The ideal strength is defined as the minimal stress needed to deform a prefect crystal upto its structural failure. It sets an upper limit on the strength of materials. During the pastyears, it has been demonstrated that reliable ideal strength can be obtained by ab-initiocalculations based on density functional theory (DFT). Ideal strength can be used not only toexamine the mechanical stabilities of bulk materials, but also the stabilities of interface,surface and so on. The ideal strength of nano-structure, such as nanotube, nanowire,nanoribbon, can be precisely calculated, too. It should be noticed that nearly all the previousideal strength calculations are carried out under the conditions with absolute zero temperatureand ambient pressure. However, factors, such as temperature and pressure, may affect thematerial ideal strength to a large degree. For example, the static hardness of diamond will bedecreased by70%when raising the temperature to1200°C. In the cases of metals, thetemperature effects could be more remarkable. To develop an ideal strength calculationmethod that can take account of temperature and pressure is highly desirable and necessary. In this dissertation, we give a comprehensive study on how the intrinsic and extrinsicfactors will affect the ideal strength of materials. Systematical ab-initio calculations arecarried out to investigate the mechanical properties of elemental boron, aluminum, and carbonunder different conditions. The calculation results demonstrate that:1) the existence ofmulti-center bonds in!-B!"could lead to new bond deformation patterns, which may causesurprising reductions in ideal strength.2) Ideal strength could be sensitive to temperature. Themechanical properties of fcc aluminum at finite temperature are quite different from those atT=0K, even only raising the temperature to room temperature. The deformation modes willalso change.3) High pressure confinement could suppress the ambient shear deformationmodes in strong covalent carbon solids. In particular, some carbon allotropes exhibit giantshear strength enhancement, making them even stronger than diamond.We begin by exploring the role of multi-center bonds in mechanical properties of a newlydiscovered elemental boron phase, orthorhombic!-B!". This new boron form can besynthesized under high pressure and it is a superhard phase with the hardness of50GPa.Stress-strain relations are calculated under both tensile and shear loadings. Along mostdirections the strong peak stresses are obtained with the stress value between50and65GPa.But several lower peak stresses are also observed in both tensile and shear deformation modes.An exceptional and interesting result is, in the [011] tensile deformation path and (001)[010]shear deformation path, the sudden drops of stresses, typical for super-hard materials formedby light elements (e.g. B, C, N and O), are not seen in both situations. This is unexpectedsince the creep-like deformations usually only exist in metal stress-strain curves. Tounderstand this phenomenon, the charge density distribution and the electron localizationfunction (ELF) are calculated to examine the evolutions of the atom bonds. Both the chargedensity distribution and the ELF results show that a “two center bond–three center bond–two center bond” transformation procedure takes place with increasing deformations. Duringthe whole processes the charge distribution changes gradually with no rigid bond breaking. This new mechanism of covalent bonds transformation leads to great reductions in idealstrength.In the next part, we present a finite temperature study on the ideal strength of aluminum,an elemental metal that is remarkable for its low mass density and high corrosion resistance.Ab-Initio Molecular Dynamics Method (AIMD) was employed to take into account of thethermodynamic motions of atoms. The calculation results demonstrate that the ideal strengthof aluminum decreases rapidly with increasing temperature. The structure instability modesbecome very different from those predicted at T=0K. At high temperature, all the phononfailure modes disappear, which indicates the stabilization of the unstable phonons by the hightemperature effects. It is also worth mentioning that this method can treat both elasticinstability and dynamic instability in one unified calculation, which means the phononsoftening effects are already included in calculations. To further verify the calculation results,phonon spectra of aluminum are calculated at specific strains and different temperatures usingthe “Self Consistent Ab Initio Lattice Dynamical Method”(SCAILD). The obtained phononspectra results fully support those obtained by AIMD method.In the last part, pressure effects on ideal shear strength of cold compressed graphite phaseare examined. Recently great efforts are made to study the cold compressed graphite phasesince experiments show that graphite could transform to a super-hard phase harder thandiamond on cold compression. A large number of possible carbon structures are proposed toexplain this phenomenon. These phases can be classified into structures with6,5+7,4+8membered ring topologies or their mixtures. Among all these structures we choose W-carbonand Z-carbon as two representative examples, which have5+7and4+8membered ringtopologies, respectively. The aim of this work is to understand why these phases becomeharder than diamond under compression. Therefore, extensive stress-strain calculations areperformed in all inequivalent (001),(011), and (111) like planes under different pressures. Atotal of528stress-strain curves are fully calculated. The results are compared with diamond, which has a6membered ring topology. The results show that structural transformation modesof W-carbon and Z-carbon at high pressure are quite different from those at ambient or lowpressure. High-density phases are likely to be formed at high pressure, and its ideal shearstrength can be higher than that of diamond. This behavior is insensitive to structural details,and similar trends are thus expected for all proposed compressed graphite phases.更多还原