Degree
Doctor of Philosophy (PhD)
Department
Physics & Astronomy
Document Type
Dissertation
Abstract
While HEAs have been studied extensively for their material properties, their potential as catalysts remains widely unexplored. To explore the feasibility of CoCrFeNi as a catalyst for the hydrogen evolution reaction, the results of X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS) experiments are compared to electrochemical experiments and theoretical calculations. XPS shows that this surface forms passive oxides with a propensity of Cr > Fe > Co, with no evidence of Ni oxidation. From comparison to calculations, it is shown that the electrochemical activity is enhanced in a partially oxidized state. The lack of Ni oxidation suggests that Ni concentration plays a critical role in the enhanced electrochemical activity. The vibrational properties of CO, O2, and H2 adsorbed on a single-crystal, quaternary fcc HEA CoCrFeNi(110) surface have been probed using HREELS and modeled using density functional theory (DFT) calculations. We show strong evidence that CO adsorbs primarily on bridge and on-top sites, in compositionally varied local environments. Similar variation of adsorption sites is found with oxygen, which exhibits two broad groups of modes, initially forming Cr oxides. Unlike CO & O2, HREELS upon H2 adsorption shows only two much narrower modes and are consistent with adsorption on three-fold hollow sites. As new alloys are being developed for additive manufacturing (AM) applications, questions related to the temperature-dependent structural and compositional stability of these alloys remain. The accuracy of calculation of phase diagrams (CALPHAD) predictions are tested with energy-dispersive X-ray diffraction (ED-XRD) on electrostatically levitated samples at high temperatures. This technique leads to several advantages over other XRD techniques, primarily being in the viii simultaneous measurement of composition through X-ray Fluorescence (XRF) emissions. A set of binaries, ternaries, and CoCrFeNi alloy, all composed of 3d transition metals, are tested. It is found that while experimental results agree qualitatively with CALPHAD predictions for many binary and ternary alloys, they prove inaccurate for more complex alloys, such as the CoCrFeNi quaternary. Additionally, XRF measurements show that the alloys tested which contain Cu become Cu deficient at high temperatures.
Date
3-19-2024
Recommended Citation
McKay, Frank A. III, "SURFACE AND BULK PROPERTIES OF MULTI-COMPONENT METAL ALLOYS" (2024). LSU Doctoral Dissertations. 6366.
https://repository.lsu.edu/gradschool_dissertations/6366
Committee Chair
Phillip Sprunger