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Electronic Structure, Elastic and Thermodynamic Properties of Binary Intermetallics from Density Functional Theory

Author: LiYanFeng
Tutor: XuHui
School: Central South University
Course: Materials Physics and Chemistry
Keywords: Density Functional Binary intermetallic compounds Electronic structure Coefficient of elasticity Thermodynamics
CLC: TG111
Type: PhD thesis
Year: 2011
Downloads: 141
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Abstract

The goal of this paper is the use of the density functional theory and quasi nearly harmonic Germany thanks to theory, to study the electronic structure typical representative binary intermetallic compounds, thermodynamics and elastoplastic mechanical performance, as well as substitutional doping on the material properties impact. Calculated temperature structural intermetallic compound Ti-Al binary system Mn, Nb doping system of elastic-plastic, elastic modulus, electronic structure, and analysis of the substitutional doping on the geometric structure, electronic structure and key impact; calculated the electronic structure of the Department of Al-Sc four binary phase enthalpy of formation, Debye temperature, modulus of elasticity (with modulus) and free energy the analysis of AlSc and elastic properties under high pressure Al3Sc; calculated Ti -B system, the electronic structure and TiBx (x = 1,2, and pressure) and Ti1-xZrxB2 (, high-pressure conditions) the mechanical parameters and thermodynamic parameters, the analysis of the pressure change of Ti-Zr-B intermetallic compound system mechanical and thermodynamic properties, such as modulus of elasticity, the opposite sex, the Debye temperature and hot melt; design and calculation of the new superconducting materials of Mg1-xZnxB2 the electronic structure and elasticity coefficient, and derived based on Mc the formula Tc-Millan ab initio formula based on the calculation of the superconducting transition temperature Tc different Zn doping case and Zn doping on the mechanical properties of intermetallic compounds of Mg-Zn-B system. Paper system calculated the electronic density of states of the binary system, its doping system, bulk modulus B, elastic coefficient Cij, Debye temperature ΘD enthalpy of formation Ho the combined energy Ecoh, hot melt, free energy and Tc parameters calculated data comparison with known experimental data are consistent with many of the calculation results and conclusions of originality and the value of theoretical guidance. The main results show that: the tetragonal TiAl Nb substitutional Ti doping has little effect on the geometry of the plastic properties of the system, but the calculation of the electronic structure and layout, Nb-doped reduced system covalency and directional, and raised when the Nb-doped molar content of 8.33% -12.5% ??favor improved room temperature ductility. Mn alternate Al bit the the doping TiAl3 (Ti3Al8Mn) reduces the space of the key formed by the Al-Al covalent bond and the A12p-Ti3d hybridized bond anisotropy and high the fault energy barrier, and thus also to improve its ductility at room temperature . Al-Sc system the binary compounds Al2Sc Al3Sc structure is the most stable. OK when Al2Sc alloy formation strongest, AlSc2 worst; the strongest Al2Sc structural stability under high temperature, while the worst AlSc2 structural stability. The elastic coefficient of the the cubic phase AISc under high pressure and Al3Sc having a similar sensitivity to pressure. Weaker than AISc Al3Sc anisotropy with pressure increased, little with brittle plastic Al3Sc the Transition, brittle plastic AISc system depends on the pressure change as the pressure increases the plastic performance upgrade . The TiB2 electronic bond compared TiB more complex, the study found that the the TiB2 valence band shows the obvious parabolic and class sp with the characteristics, and the non-local extent of covalency than TiB. Under pressure, the modulus of elasticity of the Six-Party TiB2, and Ti1-xZrxB2, of sensitivity differences shear anisotropy factor of the Zr-doped system A2 \In the other hand, the doping of Zr also reduces the anisotropy of TiB2, and ductility of the system is gradually increased, but overall still shows the brittle nature. Thermodynamic aspects, system re-estimates the amount of Ti-B and Ti-Zr-B based thermodynamic parameters, calculations show that the, a TiB thermofusible value at all temperatures are the largest, TiB2 smallest, while Ti1-xZrxB2 thermofusible value As the pressure increases, Ti1-xZrxB2 each phase of the Debye temperature is approximately linear increase, and the Debye temperature value is gradually reduced with the increasing amount of Zr doping in between; System of superconducting binary compound MgB2 doped Zn computing research results show that with the increase of the amount of Zn, the lattice parameters a and c of the system significantly elongated, and showing the first increases and then decreases ; electronic structure, MgB2 at the Fermi surface of pure states of B atoms P electron covalent bonding, Mg2 ions reduce π with B atoms form a nearly full of holes 6 with thus enhancing the electro-acoustic coupling effect; Zn doped affect the total density of states at the Fermi level Ef, directly affects the of MgB2 phonon frequency and electron-phonon coupling strength, studies suggest that Zn substitutional doping conducive to the superconducting transition temperature Tc from increasing. The Tc formula derived by calculations show that the superconducting transition temperature Tc first increased gradually with the increase of the amount of Zn, that is, when the the Zn doping x value of 0.0833 and 0.125, the superconducting transition temperature of the system increased by 0.716 K and 0.108K, when the Zn content continues to increase, the destruction of the superconducting mechanism portion of the system thereby Tc decreased rapidly. With the increase of the amount of Zn-doped system the shear of the opposite sex A2 and compression anisotropic A3 as a whole showed a slight downward trend, when the the Zn doping amount x is about 0.14, the system appears brittle - ductile transition phenomenon.

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