|Ph.D Student||Kravchuk Tatyana|
|Subject||Investigation of Adsorption Phenomena and Various|
Properties of alpha-Cu-Al Alloys, Cu and Al
Oriented Single Crystal Surfaces Using
Electron and Ion Spectroscopy
|Department||Department of Chemistry||Supervisor||Professor Alon Hoffman|
|Full Thesis text|
Bimetallic alloys have great importance for different fields of science and technology . It has been known for quite a long time that the properties of bimetallic surfaces can differ from those of the individual metals composing them. There has been a substantial effort in the surface science community to investigate the surface properties of bimetallic systems. A major goal is to identify the phenomena that accompany the formation of new metal-metal bonds, and to determine the dependence of alloy surface properties on the interacting metals.
The motivation for studying Cu-Al alloys has scientific as well as technological components. From a fundamental point of view, the Cu-Al alloy represents a prototype for bimetallic alloys, making it an attractive research subject. In this study we investigate the physical and chemical properties of a-Cu-Al(5, 12, 17 at. %)(100) alloy surfaces. The aim of this research is to understand the effect of alloying Cu and Al on various surface properties. To that end, the surface properties of a-Cu-Al alloys were compared to those of Cu(100) and Al(100). The interactions of ions and photons with the studied surfaces were examined as a function of experimental parameters.
Our results point clearly to a significant enhancement, in comparison with pure copper, of a-Cu-Al(100) oxidation kinetics, due to small quantities of aluminum in the copper matrix. The electron density modifications on the alloy surface may lead to local reactivity changes, resulting in an alteration of the probability for electron transfer to the adsorbed molecules. Our experiments demonstrate the depletion of electron density on Al and the transfer to a neighboring Cu site. Therefore, the electron transfer and the subsequent creation of O2- oxidation precursor is expected to be more efficient on the Cu sites than on the Al sites on the alloy surfaces. However, the thermodynamic stability of Al-O is higher than that of Cu-O. These conclusions explain the enhanced reactivity of a-Cu-Al alloys and shed some light on the mechanism of initial oxidation: the oxidation starts from saturation of very reactive aluminum sites, and then both, the aluminum and copper sites, react simultaneously. The subsequent oxidation process depends on the atom's mobility within the solid. The competition between oxygen diffusion in the sub-surface region and the segregation of aluminum to the surface, determines the oxidation processes. In general, the initial oxidation is faster than the final one. Moreover, all alloy samples display strong oxygen-induced segregation of aluminum and ion-induced preferential sputtering of aluminum.