Publications

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Phase transformations under high strain rates (dynamic compression) are examined in situ on ZrW2O8, a negative thermal expansion ternary ceramic displaying polymorphism. Amorphization, consistent with prior quasi-static measurements, was observed at a peak pressure of 3.0 GPa under dynamic conditions, which approximate those expected during fabrication. Evidence of partial amorphization was observed at lower pressure (1.8 GPa) that may be kinetically restrained by the short (<∼150 ns) time scale of the applied high pressure. The impact of kinetics of pressure-induced amorphization from material fabrication methods is briefly discussed.

Negative and zero coefficient of thermal expansion (CTE) materials are of interest for developing polymer composites in electronic circuits that match the expansion of Si and in zero CTE supports for optical components, e.g., mirrors. In this work, the processing challenges and stability of ZrW₂O₈, HfW₂O₈, HfMgW₃O₁₂, Al(HfMg)_0.5W₃O₁₂, and Al_0.5Sc_1.5W₃O₁₂ negative and zero thermal expansion coefficient ceramics are discussed. Al_0.5Sc_1.5W₃O₁₂ is demonstrated to be a relatively simple oxide to fabricate in large quantity and is shown to exhibit single phase up to 1300 °C in air and inert N₂ environments. The negative and zero CTE behavior was confirmed with dilatometry. Thermal conductivity and heat capacity were reported for the first time for HfMgW₃O₁₂ and Al_0.5Sc_1.5W₃O₁₂ and thermal conductivity was found to be very low (~0.5 W/mK). Grüneisen parameter is also estimated. Methods for integration of Al_0.5Sc_1.5W₃O₁₂ with other materials was examined and embedding 50 vol% of the ceramic powder in flexible epoxy was demonstrated with a commercial vendor.

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C22-Gd films were deposited onto 316 stainless steel up to 2.4mm thick using three different cold spray conditions. No differences in visible coating adhesion was observed between grit-blasted and as-received 316 stainless steel substrates. All of the coatings produced had a density greater than 95% of bulk. EDS and backscattered imaging suggest that no secondary phases precipitated during coating fabrication, but potential micro-segregation of phases were likely present in the as-received material. The phase segregation was still observable in the as-deposited coatings. EDS could not resolve the differences in composition of the two phases.

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The supercritical carbon dioxide (s-CO2) Brayton cycle is currently being explored as a replacement for the steam Rankine cycle due to its potential for higher efficiency and lower cycle cost. 316 stainless steel is a candidate alloy for use in s-CO2 up to roughly 600 °C, but the mechanical effects of prolonged exposure of base and welded material in s-CO2 have not been analyzed. The potential for carburization makes this an important concern for the implementation of 316 and similar austenitic stainless steels in the s-CO2 environment. In this study, welded and base material of two types of 316–316L and 316H–were exposed in either s-CO2 or argon at 550 °C or 750 °C for 1000 h. 550 °C s-CO2 exposure yielded a thin (< 1 µm) Cr oxide with occasional nodules of duplex Fe oxide and Fe–Cr spinel that were up to 5 microns thick. However, tensile results from s-CO−2 exposure matched those of 550 °C thermal aging in Ar, indicating that no mechanically detrimental carburization occurred in either 316 variant after 1000 h exposure. Conversely, 750 °C s-CO2 exposure produced roughly 10 × the oxide thickness, with a more substantial Fe oxide (3–5 µm) on the majority of the surface and nodules of up to 40 µm thick. In comparison to aged samples, tensile testing of 750 °C CO2-exposed samples revealed ductility loss attributed to carburization. Projections of 316L performance in s-CO2 indicate that mechanically detrimental carburization—equal to that shown here for 750 °C, 1000 h—will likely be present after 7–14 years of service at 550 °C.

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Stockpile stewardship requires accurate and predictive models relying on the generation of extreme environments which is both incredibly difficult and profoundly necessary. Next generation pulsed power facilities (NGPPF), where these environments are created, may require a paradigm shift in equipment engineering/manufacture to fulfill this need. Therefore, this research aims to investigate the limitations, capabilities and efficacy of leveraging advancements in the field of additive manufacturing (AM) in order to produce novel power flow components for NGPPFs. This work focused on commercial 3D metal AM equipment producing several prototypes addressing prescient needs/shortcomings, and a technique wherein a lightweight polymer core is metalized. Ultimately, commercial 3D metal AM is considered a viable path forward but would require a sizeable investment and does not currently support the scale and complexity necessary for NGPPFs. Moreover, initial results from our composite technique are promising and is considered a realizable path forward given further investigation.

This study investigates the effectiveness of Cu as a corrosion barrier in supercritical carbon dioxide (s−CO2) by coating 316 stainless steel (316) with various thicknesses of Cu. 316 exposed to s−CO2 with 50 ppm CO showed a reduction in oxidation corresponding to the thickness of its Cu barrier coating. Additionally, a continuous Cu layer between the environment and the alloy was found to correlate to an elimination of carburization and corrosion-related mechanical degradation. This Cu coating technique could be applied over a variety of temperatures to improve the corrosion resistance of alloys that are susceptible to carburization.

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Grade 92 ferritic-martensitic steel is a candidate alloy for medium temperature (< 550 °C) components for the supercritical carbon dioxide (s-CO2) Brayton cycle. 1000 hours exposures were performed on base and welded material in s-CO2 at temperatures of 450 °C or 550 °C and compared to samples aged in Ar at 550 °C. Both s-CO2 exposures resulted in a duplex oxide growth and carburization, with 450 °C exhibiting carburization in a power law diffusion profile up to a depth of 200-250 µm, while 550 °C showed a linear profile up to a depth of 100 µm. The different profiles indicate much slower precipitation and coarsening of carbides at the lower temperature, allowing carbon to diffuse deeper into the material. However, 450 °C produced improved mechanical properties while 550 °C produced deteriorated properties. This was due to the higher density of carbon near the metal–oxide interface which leads to significant carbide coarsening and, subsequently, crack initiation and early failure. Additional exposure at 450 °C is predicted to increase deposited carbon, but further study would be needed to understand if and when carburization will produce a negative mechanical effect.

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