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Electronic Structure, Elastic And Thermodynamic Properties Of Binary Intermetallics From Density Functional Theory

Posted on:2012-09-28Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y F LiFull Text:PDF
GTID:1481303353988969Subject:Materials Physics and Chemistry
Abstract/Summary:PDF Full Text Request
The purpose of this dissertation is to demonstrate the electronic structure, thermodynamic and mechanical properties and Elemen doping performance of the typically representative binary intermetallic compounds, by using density functional theory and the quasi-harmonic Debye theory.The plastic, elastic modulus, electronic structure of the high-temperature Ti-Al intermetallic compounds and its Mn-doping, Nb-doping system are calculated, the affect of the doping on the geometric structure, electronic structure and the bond strength are analyzed. The electronic structure, formation enthalpy (binding energy), Debye temperature, elastic coefficient (including modulus) and the free energy of the Al-Sc binary compounds are calculated, the elastic properties of AlSc and Al3Sc in high-pressure are obtained. The electronic structure and the thermodynamic parameters of Ti-B binary compounds are calculated, the mechanical parameters and thermodynamic parameters of TiBx in atmospheric pressure and Ti1-xZrxB2 in high pressure are calculated. And the affect of the mechanical and thermodynamic properties of the Ti-Zr-B system are discussed, such as elastic coefficient, anisotropy, Debye temperature and heat capacity. The electronic structure and elastic coefficient of the Mg1-xZnxB2 s uperconducting intermetallic compounds are calculated, the ab initio formula of the Tc is derived according to the Mc-Millan's formula by BCS theory, which is used to calculate the Tc. The mechanical properties of Zn-doping on the Mg-B binary compounds are analyzed.This calculated parameters, such as DOS, bulk modulus, elastic coefficient, Debye temperature, enthalpy of formation, binding energy, heat capacity, the free energy and?*(?),et al, are in good agreement with the known experimental data, many calculated results of the paper have originality and the good reference value.It is found that: The plastic properties of the Ti-Al-Nb are not affected by the geometric structure of the tetragonal TiAl by Nb-doping in Ti site, but the electronic structure calculations and population analysis shows that Nb doping reduced the total covalence and direction of the ternary, and the room temperature plasticity is conducive to improved with the Nb content of 8.33%-12.5%. Mn doping in Al site in TiAl3 (Ti3Al8Mn) reduced the anisotropy of the key and the dislocation energy barrier which caused by the Al-Al covalent bond and A12p-Ti3d hybrid bond, and thus improve its room temperature brittleness.Al2Sc and Al3Sc are the most stable structure in Al-Sc binary compounds. The alloy's forming ability of Al2Sc is the strongest one at 0 K, but the AlSc2 is the worst one, as well as their structural stability at high temperatures. The elasticity coefficients of AlSc and Al3Sc with cubic phase have the similar sensitivity for the pressure. The anisotropy of Al3Sc is weaker than AlSc, thus the brittle-plastic transition of Al3Sc with the increasing pressure is not great, but the brittle-plastic transition of AlSc is larger depends on the pressure and the plastic could be upgraded as pressure increases.The electronic structure of TiB2 is more complex than that of TiB, the valence band of TiB2 shows a clear characteristic of parabolic and like-sp band, meanwhile, the degree of non-localization and covalence of TiB2 is larger than that of TiB. The elastic coefficients of the hexagonal TiB2 and Ti1-xZrxB2 have different sensitivity for the pressure. The shear anisotropy factor A2 of the Ti-Zr-B ternary has a special phenomenon which called "jump", and the pressure plays an important role in the transition from brittle to ductile. On the other hand, Zr doping reduces the anisotropy of TiB2, the plastic of Ti-Zr-B increased gradually, the thermodynamic parameters of Ti-B and Ti-Zr-B systems are re-estimated systematically, the results show that the heat capacities of TiB are the biggest at all temperatures, on the contrary of TiB2, while the data of Ti1-xZrxB2 are located between TiB and TiB2. The Debye temperature of Ti1-xZrxB2 increases linearly as the pressure increases, and the value of Debye temperature decreased with the increase of Zr doping content.The the lattice parameters (a and c) of MgB2 is significantly elongated by Zn doped. The DOS of Fermi energy of MgB2 is mainly provided by the p electrons of B and form covalent bonds, Mg+2 ions reduce the?band and the hole-type?band of B atoms is nearly full, thus enhancing the electron-phonon coupling; the Fermi level Ef is impacted by Zn doped, which directly affect the frequency of phonon of the MgB2 and the strength of the electron-phonon coupling. It is suggested that the Zn doping is benefit for the increasing of the superconducting transition temperature Tc. The calculation shows that with the increase of Zn content, the superconducting transition temperature Tc increases at first, and the theoretical value of Tc can increase 0.716K and 0.108K when x is equal to 0.0833 and 0.125, respectively; and then the Tc decreases rapidly with the increase of Zn content. Meanwhile, the anisotropy of shear and compression takes on a slight downward trend. When the x of Zn doping is about 0.14, the system exhibits brittle-plastic changes in behavior.
Keywords/Search Tags:Density functional theory, Electronic structure, Elasticity, Thermadynamics, Quasi-harmonic Debye
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