来源：金属熔化过程与表面吸附的计算化学研究 发布时间：2010年7月31日
新材料的设计与制备、结构、性质表征与应用是当今材料化学研究的几个主要发展方向。目前,材料化学的研究的重心已转移到分子、原子水平,即从微观相互作用出发定量描述材料的性质和在特定条件下的行为。本文工作以此为中心,应用计算化学方法围绕相变、CO在过渡金属表面吸附和NiTi合金表面氧化等几个问题展开,取得了一些有意义的结果。 首先应用分子动力学方法,采用QSC力场,系统研究金属Al、Pb、Cu、Ag、Au、Ni、Pd、Pt等在不同升温速率下和不同缺陷情况下的熔化过程。分子动力学模拟结果表明:(1)过热金属的熔化是一个动力学过程,具有一阶相变的特征,符合均匀成核动力学模型。(2)升温速率对过热金属的熔化过程有显著影响,升温速率增大有助于降低液相成核能垒,升温速率导致的金属过热存在极限温度。(3)在相同升温速率下,随缺陷率增加,金属熔点有下降趋势。同种金属相同缺陷率情况下,熔点温度随升温速率增加熔点也表现出升高趋势,但趋势随缺陷率增大而逐渐平缓。缺陷的存在可以显著提高原子的扩散能力,从而使体系的能量增加,降低液相成核的能垒。(4)随着升温速率的增加,过热晶体的稳定性有下降趋势:高升温速率下的非平衡升温过程对晶体内部周期性结构破坏非常明显;晶体内部周期性结构的破坏将显著降低过热晶体的相变势垒,促进过热金属晶体液相成核,导致过热金属稳定性下降,熔化相变能垒降低。由于升温速率对升温过程的主导作用,升温速率对过热金属晶体的过热和熔化行为的影响就表现为升温速率越高,熔点越高。 其次,在密度泛函理论构架下,系统研究了CO在Cu(100)、Ag(100)和Au(100)表面的吸附,同时对其中的规律和产生原因进行了归纳和阐述,弥补了人们对CO在Ag(100)表面和Au(100)表面吸附理论研究上的空白。研究结果表明:(1)采用包含相对论效应的超软赝势基组可以较为准确地确定CO在Cu、Ag和Au表面的最优吸附位、吸附结构和吸附能量。(2)确定CO在Cu、Ag和Au的(100)表面的吸附是活化化学吸附,最优吸附位是顶位。(3)CO与表面的相互作用强弱顺序为Cu>Au>Ag,与实验观测结果一致。(4)CO吸附伴随着电荷的转移,吸附后CO从金属表面获得电子。(5)CO与金属表面相互作用是CO分子轨道与表面杂化轨道之间相互作用的贡献。CO分子的3σ轨道、4σ轨道和1π轨道对吸附有贡献,但是不明显。而吸附作用主要是CO分子的5σ轨道和2π轨道与金属表面轨道的贡献,并根据Folisch等人的理论对计算结果进行了分析。【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