High-throughput Characterization of Surface Segregation in Copper Palladium Alloys.

摘要

High-throughput methodologies can be used to optimize alloy properties over complex composition space. To enable rapid determination of composition-structure-property relationships, we have developed a novel offset filament deposition tool to prepare composition spread alloy films (CSAFs). CSAFs are combinatorial materials libraries that contain all possible compositions of binary or higher-order alloys on a single, compact substrate. In this work, we describe the design and operation of the tool which can be used for quantitative deposition of CSAFs with up to four components.;A high throughput study of alloy structure and surface segregation was performed using the deposition tool. The tool was used to prepare Cu xPd1-x CSAFs, high throughput sample libraries with continuous lateral composition variation spanning the range x ∼ 0.05 to 0.95. Electron Backscatter Diffraction (EBSD) was used to identify the bulk phases in the CSAF. Phase changes as a function of alloy composition was observed and the results matched closely with the published Cu-Pd phase diagram. Spatially resolved Low Energy Ion Scattering Spectroscopy (LEISS) and X-ray Photoelectron Spectroscopy (XPS) were used to characterize the films' top-surface and near-surface compositions, respectively, as functions of alloy composition, x, temperature, and presence of sulfur. Films equilibrated by annealing at temperatures ≥700 K displayed preferential segregation of Cu to their top surfaces at all bulk compositions; segregation patterns did not, however, depend on local structure. The Langmuir-McLean thermodynamic model was applied to segregation measurements made in the temperature range 700--900 K in order to estimate the free energy of segregation as a function of bulk CuxPd1-x composition. Segregation measurements at x = 0.30 on thick (∼100 nm) CSAF compare well with results previously reported for a bulk, polycrystalline Cu 0.30Pd0.70 alloy, demonstrating the utility of the CSAF as a high throughput library for study of segregation. Segregation analysis performed in presence of adsorbed sulfur showed migration of Pd to the surface of the alloy. The driving force for Cu segregation, in sulfur modified films decreases across the entire composition space. This collection of encouraging results suggests the use of CSAFs for efficient study of alloy properties over their entire composition space.