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Computational Chemistry Study on Melting Processes and Surface Adsorption of Metals

Author: LiuXin
Tutor: LiuChangHou;MengChangGong
School: Dalian University of Technology
Course: Chemical processes
Keywords: Molecular dynamics simulations QSC force field NiTi alloy surface electronic structure density of state phase transition
CLC: O647.31
Type: PhD thesis
Year: 2006
Downloads: 440
Quote: 2
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Abstract


Design and synthesis, characterization and application of new material are the main attracting fields for material chemistry research today. The development of material chemistry mainly depends on the depth of our understanding on the nature of the world around us, which is the chemical law for materials changing from one to another. And the most important part of it is the chemical nature lying behind the relationship between the various properties of a material and its microscopic structure. After observing experimental phenomena, analyzing results, and setting up empirical and semi-empirical correlations, researchers tend to study these basic laws at atomic or molecular level, that is to study the properties and behaviors of a material in a certain circumstance by studying the interaction inside it at microscopic level. This also lays the foundation of current work. In this work, methods of computational chemistry is applied to study several essential problems in material chemistry, including phase transition, surface oxidation of NiTi alloy and CO adsorption on transition metal surfaces.A brief review of the development of computational methods and their applications on several attractive fields in material chemistry is given first. And then, molecular dynamics simulations with QSC force fields are applied to study the melting of Al, Pb, Cu, Ag, Au, Ni, Pd, and Pt at different heating rates and different rates of defect. The results proves that melting of a superheated metal is a kinetic process and is a first order phase transition, and can be explained by the kinetic theory on homogeneous nucleation. The melting process is strongly affected by the heating rate, which will successfully eliminate the barrier of nucleation, and there is an upper limit for the heating rate induced superheating. For a metal heating at the same heating rate, its melting temperature will decrease with the increase of the rate of defect. The existence of defect will significantly increase the diffusibility of atoms, thus increasing the total energy and eliminating the nucleation barrier. A metal with higher defect rate exhibits a higher energy before the melting, but after melting there is no observable energy difference. Though defects exist, the effect of heating rate is still obvious. Stability study on superheated metals shows that the heating rate not only affects the melting and superheating behavior, but also affects the structure. The stability of superheated crystal is decreasing with the increasing heating rate. And local structure is obviously destroyed during non-equilibrium superheating forming disorder,which will also behave as defects. As the heating process is non-equilibrium and dominant, the overall effect of heating rates is the higher the heating rates the higher the melting temperature.After that, PBE, PW91 and revPBE gradient corrected functional within the framework of density functional theory are applied to study the CO molecule, bulk Cu, Ag, Au and their (100) surfaces. Based on these experimental comparable results, CO adsorption on Cu(100), Ag(100) and Au(100) surface is studied. It is found that relativistic ultrasoft pseudo potential can accurately predict the site preference, adsorption structure and adsorption energy for CO adsorption. The results show that CO is activated upon adsorption, and atop site is preferred on all 3 surfaces. Interaction between CO and Cu, Ag and Au surface is sorted as Cu > Au >Ag, which is accord with the experimental observations. CO adsorption is accompanied with charge transfer, and CO gains electrons from surface. The interaction between CO and the metal surface is the overall contribution of CO’s molecular orbital (mainly 5o and 2n orbital) and surface orbital. A schematic description is also given based on Folisch’s theory.And then, adsorption of 0 atom and O2 molecule on NiTi(lOO) surface is studied with relativistic pseudo potential and generalized gradient functional within the framework of density functional theory. The results show that, Ti terminated NiTi(lOO) surface exhibits the highest reactivity. Surface electrons move into the n anti-bonding orbital of O2 molecule, thus O2 molecule is activated and will dissociate upon adsorption. Bridge adsorption will cause the surface restruction, and the corresponding structure is most stable. Atop adsorption is very unstable and O2 will diffuse to form bridge adsorption or hollow adsorption. There are 4 possible adsorption sites for 0 atom to adsorb on NiTi(lOO) surface, and the interaction strength is sorted as 3-fold > hollow > bridge > atop. The atomic adsorption of O on NiTi(lOO) surface is chemical adsorption, which is accompanied with charge transfer from surface Ti atom to O atom. The O2 molecule or 0 atom adsorption on NiTi(lOO) surface will not significantly alter the surface electronic structure. The diffusion path between selected pairs of local energy minimum is also explored and the landscape of diffusion potential energy surface is calculated. Thermodynamics and kinetics studies on the diffusion of O atom, and also the structural analysis prove that O2 adsorption and dissociation and 0 atom diffusion are the important initial step for formation of surface oxide layer.

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CLC: > Mathematical sciences and chemical > Chemistry > Physical Chemistry ( theoretical chemistry ),chemical physics > The physical chemistry of surface phenomena > Adsorption > Adsorption theory
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