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STUDY OF STRUCTURAL, THERMAL, HYDROGENATION AND COMPOSITIONAL PROPERTIES OF NI-BASED AMORPHOUS MEMBRANES USING EXPERIMENTAL AND COMPUTATIONAL METHODS
Materials Science and Engineering
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Hydrogen is considered as a promising source of energy because of its properties such as abundant availability, high energy conversion efficiency, low-density, solid-state storage potential, and most importantly zero emission from energy conversion devices, such as fuel cell. End product from hydrogen internal combustion engine is just water vapor. Hydrogen can be generated from various different sources and can be stored in solid, liquid and gaseous forms. One of the methods of producing hydrogen is in the form of syngas (mixture of methane, CO, CO2 and water vapor) obtained from coal gasification in a water shift reactor using hydrogen selective membrane. Palladium is a very expensive material and when used in its crystalline form, it is prone to hydrogen embrittlement during prolonged exposure to hydrogen at high temperature and pressure. It is important to find a cheaper alternative with excellent hydrogen permeation properties. Amorphous alloy membranes, such as NiNbZr membranes are good contenders due to their high glass forming ability and the hydrogen permeation properties comparable to Pd or Pd alloys. In spite of significant research performed on these glassy alloy materials, there is still a lack of understanding of the nature of the local atomic order involving formation of icosahedra, and the hydrogen interaction mechanisms with them. It is generally accepted that the multicomponent amorphous alloy membranes are most effective in hydrogen permeation. However, the effect of alloying element addition on cluster formation, permeation mechanisms, and thermal properties of certain membranes are also not fully understood.A combined experimental and computational approach is used to understand the undiscovered properties of Ni-Nb-Zr membranes. XRD studies on melt spun ribbons confirmed amorphicity of these membranes in all the conditions (at 25oC and at 400oC under hydrogen pressure) at the Molecular Foundry of Lawrence Berkeley National Laboratory. Depth profile analysis using the X-ray photoelectron spectroscopy (XPS) gave an insight in to the difference in the surface composition of both the sides of membranes and the effect alloy composition on the same. Addition of alloying elements such as V, Fe, Ti to the NiNbZr alloys are expected to alter thermal properties of these membranes because of difference in atomic size. We selected melt spun NiNbZr alloy with Fe additions for hydrogen solubility studies by volumetric methods. Density functional theory based Molecular dynamics (DFT-MD) studies revealed formation of icosahedral clusters. Upon hydrogenation, hydrogen atoms were seen to interact with these clusters in three different ways: (i) occupation of interstitial sites, (ii) substitution of an atom from a cluster, and (iii) replacement of the central atom. These results helped in understanding of hydrogen permeation process. Effect of addition of alloying elements on the structural and permeation properties showed how affinity towards hydrogen and size of certain alloying elements can affect icosahedral clusters in the membrane. Exploratory studies on developing numerous multicomponent alloys is a rather impossible task using melt spin spinning method. This is due to the fact that the bulk alloy needs to be fabricated first and annealed, then melt spinning needs to be performed, finally the hydrogen permeation studies that are very laborious, even to determine a single alloy property. An alternative expedient method is to produce numerous thin film alloys by PVD deposition on a Si single crystal wafer. Thus, thin film of NiNbZr with Ta additions on Si wafer substrate were fabricated at the CINT, Los Alamos National Laboratory that yielded numerous alloy compositions. In this study, we determined (i) stability, amorphicity and surface properties of amorphous melt spun ribbons of NiNbZr alloy membranes by using x-ray diffraction and x-ray photoelectron spectroscopy, thermal properties of NiNbZrFe, NiNbZrTi and NiNbZrV alloy membranes by differential scanning calorimetry and hydrogen solubility of NiNbZrFe alloy membrane using Sievert’s volumetric method, (ii) developed different compositions of thin films of NiNbZrTa alloys on Si substrate by using PVD methods, synchrotron x-ray diffraction to confirm their amorphicity and did chemical analyses by SEM-EDS system and (iii) developed theoretical models using Density Functional Theory based Molecular dynamics (DFT-MD) studies to understand icosahedral clustering and hydrogen interaction on a local atomic order.