High-Throughput Characterization of Surface Segregation in Cu_xPd_1–x Alloys

Abstract

A high throughput methodology for the study of surface segregation in alloys has been developed and applied to the Cu x Pd 1Àx system. A novel offsetfilament deposition tool was used to prepare Cu x Pd 1Àx composition spread alloy films (CSAFs), high throughput sample libraries with continuous lateral composition variation spanning the range x = 0.05À0.95. 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, and temperature. Electron backscatter diffraction (EBSD) was used to identify the bulk phases in the CSAF as a function of alloy composition, x. Films equilibrated by annealing at temperatures g 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 LangmuirÀMcLean thermodynamic model was applied to segregation measurements made in the temperature range 700À900 K in order to estimate the enthalpy (ΔH seg ) and entropy (ΔS seg ) of segregation as a function of bulk Cu x Pd 1Àx composition. Segregation measurements at x = 0.30 on the CSAF compare well with results previously reported for a bulk, polycrystalline Cu 0.30 Pd 0.70 alloy, demonstrating the utility of the CSAF as a high throughput library for study of segregation.

Segregation at the surfaces of CuxPd1−x alloys in the presence of adsorbed S

“…[38] The values of ΔH seg and ΔS seg at x ~ 0.3 on the CSAF matched those we measured for the single composition Cu 0.3 Pd 0.7 alloy sample, providing validation of the high throughput approach for characterization of segregation phenomena.…”

“…In our prior high-throughput study of surface segregation on a clean Cu x Pd 1-x CSAF, we observed that Cu preferentially segregates to the top surface of the alloy at all bulk compositions in the range x = 0.05 to 0.95 and temperatures in the range T = 500 -900 K. [38] We used a Langmuir-McLean analysis to extract ΔH seg and ΔS seg , the enthalpy and entropy of Cu segregation to the alloy top-surface, as functions of bulk composition, x, and showed that Cu segregation is exothermic (ΔH seg < 0) over all x. [38] The values of ΔH seg and ΔS seg at x ~ 0.3 on the CSAF matched those we measured for the single composition Cu 0.3 Pd 0.7 alloy sample, providing validation of the high throughput approach for characterization of segregation phenomena.…”

“…[38,39] Briefly, experiments were carried out in an ultrahigh vacuum chamber with a base pressure of 2 x 10 -10 Torr. The chamber is equipped with a monochromated x-ray source (for spatially resolved x-ray photoemission spectroscopy, XPS), a He + ion gun (for spatially resolved low energy ion scattering spectroscopy, LEISS), and a sputter gun for substrate cleaning.…”

“…[3,36,37] We have extended our study of surface segregation to all of Cu x Pd 1-x composition space using a high-throughput approach based on the use of Composition Spread Alloy Film (CSAF) sample libraries. [38] As illustrated in Figure 1, CSAFs are thin films with continuously variable, lateral composition gradients that can be deposited such that every composition of a binary or ternary alloy appears on a compact (~ 1 cm 2 ) substrate. When coupled with spatially resolved methods for measuring their composition, structure and functional properties, CSAFs allow rapid characterization of the composition-structure-property relationships that form the basis for rational design of alloys for applications such as H 2 separation.…”

Thermal interface conductance across metal alloy–dielectric interfaces

“…[24] and Au x Pd 1-x [25] phase diagrams) by conductive heat transfer from the resistively heated sapphire substrate. The annealing ensured that the CSAFs contained face centered cubic crystalline solid solutions, without intermetallics, across all compositions [26]. After cooling, the chamber was vented to remove the sample for composition and thickness vs. position analysis.…”