Project nature: Experimental, Data analysis, Simulation
X-Ray Photoelectron spectroscopy is a powerful, non-destructive tool to reliably evaluate near surface compositions of thin films up to about 5 nm depth. Angle resolved XPS adds additional information which encodes the depth dependent concentrations of various materials of interest. The inverse problem of reconstructing the in-depth concentrations from experimental photoelectron (PE) intensities can be solved either for simple cases of well-defined layers or by ignoring effects of interfaces between thin layers.
This (bachelors/masters) assignment aims to study and develop a method to unravel the PE intensity contributions from layers with defined interfaces and stacking order. The work done here will grant you hands on experience of thin film deposition and characterization using XPS. It will also challenge your skills in data processing, programming and maths.
High Entropy Alloys (HEA) have shown to be difficult to manufacture due to the complexity of mixing 5 or more elements together. Experimentally testing all permutations would make co-deposition using multiple targets beneficial. However, physical limitations prevent the co-deposition of more than 3 elements due to the space required for each target. This assignment looks into the preparation and use of mosaic targets: a targets made out of smaller facets of different elemental targets. The goal is to be able to create targets for the ADC setup at the XUV optics lab that can deposit an alloy at predetermined stoichiometries.
High Entropy Alloys (HEA) are a recent material category which shows excellent mechanical and chemical properties. One such property is delayed oxide transitions, with studies showing that for some thick film HEAs self-limiting oxide growth can be achieved. However, this effect is poorly understood for surfaces and ultrathin films (<100nm). This assignment focusses on figuring out the physical and chemical effects taking place in low (2 elements), medium (3 or 4 elements) or high (5 or more elements) entropy alloys in oxidizing environments, based on transition metals.
The low molecular weight and high reactivity of hydrogen make it a suitable candidate for various applications as fuel, etchant, and reducing agent. Typically hydrogen molecules (H2) dissociate on the material surface, producing hydrogen atoms/radicals (H*). H* penetrate the material and cause embrittlement, blistering, interface defects, chemical erosion, and/or reduction. To protect materials in hydrogen environments, coatings of materials with low hydrogen diffusivity – hydrogen permeation barriers – are used. Metals, oxides, nitrides, carbides, and graphene/graphite are reported as hydrogen permeation barriers. Permeation of hydrogen through these materials at elevated temperatures in H2 environments is well reported in the literature. However, their efficiency to inhibit hydrogen diffusion in H* environments is yet unknown. In such environments, metal compounds are susceptible to reduction, which is anticipated to enhance hydrogen permeability through them.
The assignment entails evaluating hydrogen permeability through metal compound(s) in H* environment and investigating way(s) to reduce it. For instance, TiN film (100 nm thick) is reported to be stable in atomic deuterium environment at 127 0C [1]. However, Kura et al. demonstrated high hydrogen permeability through nanocrystalline TiN membranes (600 nm thickness; 23 – 14 nm grain size) [2]. Hopping transport of hydride ions via a bond exchange mechanism between Ti-H terminal groups covering internal grain surfaces is stated as a hydrogen diffusion mechanism in nanocrystalline TiN films. In order to minimize hydrogen diffusion through nanocrystalline TiN one way is thought to be the scaling (coating) of grain boundaries with transition metal oxide [3, 4].
Fig. 1: Schematic of methodology for evaluating hydrogen permeability through materials.
The proposed methodology for the assignment is depicted in Fig 1. The candidate materials will be deposited via (reactive) DC-magnetron (co-)sputtering. Microstructure, stoichiometry, and optical properties of the as-deposited samples will be analyzed via XRD, XRR, XPS, and ellipsometry. following that, samples will be exposed to H* with ellipsometry performed during the exposures. Metrology will be performed on the samples again after H* exposure. The hydrogen permeability through the candidate materials will be evaluated by comparing the samples before and after H* exposures and using in-situ ellipsometry data.
[1] J. Prasad, G.M. Nuesca, J.A. Kelber, Atomic hydrogen cleaning of a TiN surface, Applied surface science 74(1) (1994) 115-120.
[2] C. Kura, Y. Kunisada, E. Tsuji, C. Zhu, H. Habazaki, S. Nagata, M.P. Müller, R.A. De Souza, Y. Aoki, Hydrogen separation by nanocrystalline titanium nitride membranes with high hydride ion conductivity, Nature Energy 2(10) (2017) 786-794.
[3] Z. Qi, F. Zhu, Z. Wu, B. Liu, Z. Wang, D. Peng, C. Wu, Influence of yttrium addition on microstructure and mechanical properties of ZrN coatings, Surface and Coatings Technology 231 (2013) 102-106.
[4] H. Ju, P. Jia, Microstructure, Oxidation Resistance and Mechanical Properties of Nb–Y–N Films by Reactive Magnetron Sputtering, Protection of Metals and Physical Chemistry of Surfaces 56(2) (2020) 328-332.
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