A First Principles Study On The Effects Of Temperature, Pressure, And Special Chemical Bond On Strengths Of Materials

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.