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Interfacial engineering of dense metallic membranes for stable and cost-effective hydrogen purification in fusion relavent environments
Job, Adam L.
Job, Adam L.
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2023
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2024-05-29
Abstract
Hydrogen is the most abundant element in the universe and plays a vital role in many natural and man-made processes. In particular, high purity hydrogen is of great importance for applications such as ammonia feedstock, semiconductor processing, and polymer electrolyte membrane fuel cells. Hydrogen isotopes play a critical role in fusion environments, where they serve as a fuel source for nuclear fusion reactions that release massive amounts of energy. This reaction is occurring at the center of our sun where hydrogen atoms are brought together under extreme pressure and temperature to form helium, releasing vast amounts of energy in the process. This process can be accomplished on earth by fusing isotopes of hydrogen: deuterium and tritium. A critical challenge facing fusion technology is the safe and effective management of tritium. Tritium is a radioactive beta-emitter with a short half-life (12.3 years), so it must be contained and regenerated on site.
Dense metallic membranes are the preferred method of hydrogen purification in fusion environments as they are perfectly selective for hydrogen. Vanadium is a material of critical importance as it has a high hydrogen permeability and has low induced radioactivity under neutron irradiation occurring from fusion environments. Palladium based membranes are currently the gold standard for metallic membrane separation, however vanadium offers the benefit of significantly cheaper than palladium. In this work, composite V-based membranes are engineered to understand their long-term stability as hydrogen permeable membranes.
First, hydrogen permeability in V was studied. It was found in order to achieve permeation, membranes needed extensive Ar sputter treatments to efficiently clean the surface of native oxides and must be operated at high temperature, however these membranes ultimately fail due to the inevitable formation of oxides on the surface originating from impurities in the V or gas streams. To achieve permeation at lower temperatures, thin Pd coatings (100 nm) are required to achieve maximum permeability. However, these membranes fail due to intermetallic diffusion between Pd and V and such alloys cannot permeate hydrogen effectively. The extent of Pd diffusion in V was quantified and correlated to membrane performance.
Next, ultrathin (20-100 nm) materials were developed to be used as intermetallic diffusion barriers. The goal for this work was to enable high-throughput membranes by using Pd as a catalyst for hydrogen, a ceramic barrier to prevent intermetallic diffusion, and bulk V for its high permeability. Al2O3 was the first candidate studied due to its ease of fabrication utilizing atomic layer deposition. Ultimately, Al2O3 was not found to be effective due to VOx forming at the interface that obstructs hydrogen permeance. For this reason, nitrides were considered. First, Mo2N was studied as a potential catalyst and an intermetallic diffusion barrier. As a catalyst, it was found that Mo2N decays to Mo leading to permeability decline due to Mo being fully miscible in V. As an intermetallic diffusion barrier, Mo2N would delaminate proceeding heat treatments causing poor membrane performance.
Finally, ZrN was developed as an intermetallic diffusion barrier. It was found that ZrN was effective at preventing intermetallic diffusion between Pd and V to a certain temperature. These composites membranes were found to be stable at temperatures up to 450°C, but increased temperatures lead to barrier breakdown and permeation decline. The effect of reactive sputtering conditions was studied and correlated with membrane performance with superior permeability observed from membranes fabricated with low electrical resistivity, suggesting that N vacancies and defects may play important roles. Additionally, these membranes improved with time. Stable performance (>200 h) at 425°C reached a permeability of 6 x 10-8 mol H2 m m-2 s-1 Pa-0.5, this result was 4X greater than that of Pd. Zr, having high affinity for oxygen, was shown to effectively getter oxygen from the bulk V that did not impede hydrogen permeability that could explain improved performance with time.
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