(732b) High Throughput Methodology for Study of Surface Segregation

Authors: 
Gellman, A. J., Carnegie Mellon University
Miller, J. B., Carnegie Mellon University
Priyadarshini, D., Carnegie Mellon University


Surface segregation is a ubiquitous phenomenon in all multicomponent materials. In all alloys the concentration of components in the surface layer is not the same at that in the bulk. The surface concentration is a thermodynamic property of composition and temperature. For an alloy AxB1-x the concentration of component A at the surface is a function of the bulk concentration, x, and temperature. Segregation is not just restricted to the topmost atomic layer of the alloy. The concentration of component A in the top few layers of the surface may differ from that of the bulk. Finally, segregation can be influenced by the presence of adsorbed species on the surface of the alloy. Because surface segregation is a continuous function of bulk composition, x, a complete understanding of segregation in a binary alloy requires the development of high throughput methods that allow concurrent measurements of surface segregation at all possible values of bulk composition.

PdxCu1-x alloys are used for hydrogen purification membranes. Segregation to the surface is likely to affect their properties. Surface segregation has been studied in a Pd70Cu30 alloy using both x-ray photoelectron spectroscopy (XPS) and low energy ion scattering (LEIS). The results show that the topmost atomic layer is rich in Cu. On the other hand, the near-surface region consisting of the immediate subsurface layers is rich in Pd. Furthermore, the adsorption of sulfur on the surface causes the complete elimination of Cu from the topmost layer.

A high throughput method for study of surface segregation has been developed that is based on the deposition of thin Composition Spread Alloy Films (CASFs) that contain all possible bulk compositions of the PdxCu1-x alloy. Spatially resolved surface analysis of the surface of the PdxCu1-x CASF has been achieved using XPS. The results show that the near-surface region of the alloy is Pd rich over a wide range of bulk Pd concentrations.