The Impact of Synthesis Conditions and Crystal Quality of Two-dimensional Materials for Permeation Barriers

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Young, Katherine
Vogel, Eric M.
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Permeation barriers in applications for gas separation, hydrogen isotope permeation barriers, and corrosion coatings require similar properties, including high strength, chemical stability, limited permeation, and high aspect ratios. Two-dimensional materials possess many of these qualities and could serve as the thinnest possible permeation barriers. Two-dimensional materials are defined by an atomic thickness, with much larger lateral dimensions. For instance, graphene is an atomically thin, two-dimensional allotrope of graphite that has demonstrated high strength in comparison to its size, chemical stability, and impermeability to gases like He. For example, it has already displayed excellent corrosion resistance for short-term experiments, but there have been contradictory reports in the literature regarding the impact of defects and the associated permeation mechanisms. Likewise, its use as a gas separation barrier hinges on permeation through induced pores, but permeation through intrinsic defects is not well understood. The promising results of graphene suggest that other two-dimensional and ultrathin films may behave similarly, if not better. For instance, two-dimensional transition metal carbides (TMCs) were recently discovered and may also have relevant properties. Transition metal carbides exhibit beneficial properties normally attributed to metals or ceramics, such as high melting temperatures, high hardness and strength, and high corrosion resistance. Ultrathin TMCs have also demonstrated promising results for gas selectivity for gas separation, as well as high thermal stability in oxygen environments. However, the effects of intrinsic defects and crystal quality on permeation in ultrathin and two-dimensional TMCs are even less understood than graphene because they are newer. The crystal quality of two-dimensional materials is heavily dependent on the detailed conditions used for their synthesis. Chemical vapor deposition (CVD) is a promising synthesis technique for large-area, uniform coverage of 2D materials; however, it can lead to the formation of unwanted defects, such as grain boundaries. The development of a detailed and fundamental understanding of the impact of synthesis conditions on defects in the materials and the associated permeation mechanisms is needed in order for these materials to be used commercially in permeation applications. This work examines the effects of synthesis parameters and crystal quality on the permeation mechanisms of graphene and ultrathin TMCs. Graphene and ultrathin Mo2C are synthesized by chemical vapor deposition under varying process conditions, such as varying temperatures, process gas flow rates, substrate materials, and substrate pretreatments. Utilizing the existing extensive studies on graphene growth, this work describes synthesis techniques that are optimized to form graphene with varying grain sizes and point defect densities for studies as hydrogen isotope permeation and thermal oxidation barriers. Graphene is shown to significantly decrease the hydrogen isotope permeation through annealed Cu. Varying graphene grain sizes demonstrate similar permeation rates, suggesting that other defects, such as C vacancies, present a lower permeation resistance than line defects. Hydrogen isotope permeation is modeled using Henry’s law and displays Arrhenius behavior. Thermal oxidation experiments demonstrate a slightly different trend as the metal oxidation can also begin to damage the graphene film. Graphene with a high defect density, due to small grains or high levels of point defects, acts as a poor corrosion barrier, due to the relatively unhindered diffusion through these defects. However, increasing the graphene grain size displays diminishing returns because the graphene-metal interaction and its adhesion to different crystal orientations is more significant. Chemical vapor deposition of ultrathin and two-dimensional TMCs, such as Mo2C, was discovered well after graphene, and is therefore much less understood. Thus, a pioneering approach was taken in order to realize the role of the growth substrate and synthesis mechanism of the film. Utilizing alloys with differing compositions under varying synthesis conditions led to the definitive conclusion that the substrate acts as a diffusion barrier to the metal atoms before carburizing at the surface. However, the substrate is also required to dehydrogenate the hydrocarbon gas in order to successfully form ultrathin TMC flakes. Synthesis temperatures are limited in the same way, as they must remain high enough to efficiently pyrolyze the C precursor gas. Film coalescence is also limited when using alloys if the alloy components segregate during cooling. Successful CVD synthesis of Mo2C has led to preliminary studies using this material as a thermal oxidation barrier. In summary, this work studies CVD synthesis methods to achieve two-dimensional films with varying crystal quality to determine the effects of intrinsic defects on permeation through the films.
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