Work Function Calculation And Its Applications In The Study On Electrochemical Phenomena Of Materials

Surface electrochemistry is a complex subject including a series of phenomena such as catalysis, oxidation, adsorption, corrosion and field electron emission etc., which is always a focus of material science research. Electrochemical phenomena are generally accompanied by electron transfer and flow, and it is a redox process that the cathode gains and the anode lost electrons. We start with the work function which has closed relationship with electrons, studied the electrochemical corrosion and field emission of materials by using the first-principles based on density functional method. The electrochemical activity of the surface was elucidated by the ability of the material bound electron. From this point, the reason and trend of the electrochemical reactions are clarified.The main results obtained in the thesis are divided into three parts as following:(1) Al alloys contain some secondary phases to improve their properties. These secondary phases have different potentials to the Al matrix, which greatly affect the local corrosion behaviors of Al alloys. In order to reveal the physical nature of Al alloy corrosion, we use the first-principles method based upon density functional theory to calculate the work function of the main secondary phases (Al2Cu, Al3Ti, and Al7Cu2Fe).The difficulty of electrons escaping from various crystal planes has analyzed, and the potential difference between the secondary phases and Al matrix is obtained. We find that different crystal planes exposed to the outmost layer significantly impacted on the potential difference. Different atomic types at the outmost layer play different roles in Al alloy corrosion, even for the same crystal surface. The causes of galvanic corrosion are thus revealed. Furthermore, our method using slab model to study Al alloy corrosion provide a new theoretical approach for research of galvanic interaction on other alloy surface.(2) The surface is direct contact with the external environment, so the surface properties sometimes play a decisive role in the material. Surface energy and work function are two fundamental physical parameters of metallic surface, they are important to understand a wide range of surface phenomena such as surface stability, crystal growth, the formation of grainboundaries and surface segregation of trace elements. The material surface is anisotropic and various forging means made the surface component more complex. Therefore, it is far from enough to describe the surface electrochemical phenomena by the work function and surface energy of only few close packed surfaces. In view of this, we calculated the surface energies and work functions of six close packed surfaces for 19 common bcc and fcc metals in periodic table. Through calculation, we provide an accurate and comprehensive database for experiment and a roughly inverse proportional relationship is found between the surface energy and work function.(3)The electron work function is one of the key factors that affect the field emission. The transition-metal carbides, especially the Hafnium carbide (HfC), as a promising field emission material attracted more attentions due to their extraordinary properties including high melting temperature, good anti-corrosion and low work function. In order to reveal the surface properties of HfC materials, we calculated the work functions and surface energies of six close packed planes. The ability of the gain and loss of electrons and the surface stability are discussed. For the polar surface of (1 11) and (311) plane of HfC, a new method is taken to calculate the surface energy of the different surface terminations. Additionally, when the field emission materials are working, the surfaces often adsorb some small molecules. These adsorbates can be expectedto have an apparent effect on work functions. We focused on work function changes induced by oxygen atom adsorption on HfC(111) plane as a function of coverage. An unexpected decrease of the work function is found at low coverage, and a reasonable explanation for this anomaly is given based on the method of Roman et al.