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Metal oxide thermal reduction, enabled by microwave-induced plasma, was used to synthesize high entropy borides (HEBs). This approach capitalized on the ability of a microwave (MW) plasma source to efficiently transfer thermal energy to drive chemical reactions in an argon-rich plasma. A predominantly single-phase hexagonal AlB2-type structural characteristic of HEBs was obtained by boro/carbothermal reduction as well as by borothermal reduction. We compare the microstructural, mechanical, and oxidation resistance properties using the two different thermal reduction approaches (i.e., with and without carbon as a reducing agent). The plasma-annealed HEB (Hf0.2, Zr0.2, Ti0.2, Ta0.2, Mo0.2)B2 made via boro/carbothermal reduction resulted in a higher measured hardness (38 ± 4 GPa) compared to the same HEB made via borothermal reduction (28 ± 3 GPa). These hardness values were consistent with the theoretical value of ~33 GPa obtained by first-principles simulations using special quasi-random structures. Sample cross-sections were evaluated to examine the effects of the plasma on structural, compositional, and mechanical homogeneity throughout the HEB thickness. MW-plasma-produced HEBs synthesized with carbon exhibit a reduced porosity, higher density, and higher average hardness when compared to HEBs made without carbon. 

Microwave-induced plasma was used to anneal precursor powders containing five metal oxides with carbon and boron carbide as reducing agents, resulting in high entropy boride ceramics. Measurements of hardness, phase structure, and oxidation resistance were investigated. Plasma annealing for 45 min in the range of 1500–2000 °C led to the formation of predominantly single-phase (Hf, Zr, Ti, Ta, Mo)B2 or (Hf, Zr, Nb, Ta, Mo)B2 hexagonal structures characteristic of high entropy borides. Oxidation resistance for these borides was improved by as much as a factor of ten when compared to conventional commercial diborides. Vickers and nanoindentation hardness measurements show the indentation size effect and were found to be as much as 50% higher than that reported for the same high entropy boride configuration made by other methods, with average values reaching up to 38 GPa (for the highest Vickers load of 200 gf). Density functional theory calculations with a partial occupation method showed that (Hf, Zr, Ti, Ta, Mo)B2 has a higher hardness but a lower entropy forming ability compared to (Hf, Zr, Nb, Ta, Mo)B2, which agrees with the experiments. Overall, these results indicate the strong potential of using microwave-induced plasma as a novel approach for synthesizing high entropy borides. 

Abstract Decline and recovery timescales surrounding eclipse are indicative of the controlling physical processes in Io’s atmosphere. Recent studies have established that the majority of Io’s molecular atmosphere, SO₂and SO, condenses during its passage through Jupiter’s shadow. The eclipse response of Io’s atomic atmosphere is less certain, having been characterized solely by ultraviolet aurorae. Here we explore the response of optical aurorae for the first time. We find oxygen to be indifferent to the changing illumination, with [Oi] brightness merely tracking the plasma density at Io’s position in the torus. In shadow, line ratios confirm sparse SO₂coverage relative to O, since their collisions would otherwise quench the emission. Io’s sodium aurora mostly disappears in eclipse and e-folding timescales, for decline and recovery differ sharply: ∼10 minutes at ingress and nearly 2 hr at egress. Only ion chemistry can produce such a disparity; Io’s molecular ionosphere is weaker at egress due to rapid recombination. Interruption of a NaCl⁺photochemical pathway best explains Na behavior surrounding eclipse, implying that the role of electron impact ionization is minor relative to photons. Auroral emission is also evident from potassium, confirming K as the major source of far red emissions seen with spacecraft imaging at Jupiter. In all cases, direct electron impact on atomic gas is sufficient to explain the brightness without invoking significant dissociative excitation of molecules. Surprisingly, the nonresponse of O and rapid depletion of Na is opposite the temporal behavior of their SO₂and NaCl parent molecules during Io’s eclipse phase. 

Abstract The lunar surface is constantly bombarded by the solar wind, photons, and meteoroids, which can liberate Na atoms from the regolith. These atoms are subsequently accelerated by solar photon pressure to form a long comet‐like tail opposite the sun. Near new moon, these atoms encounter the Earth's gravity and are “focused” into a beam of enhanced density. This beam appears as the ∼3° diameter Sodium Moon Spot (SMS). Data from the all sky imager at the El Leoncito Observatory have been analyzed for changes in the SMS shape and brightness. New geometry‐based relationships have been found that affect the SMS brightness. The SMS is brighter when the Moon is north of the ecliptic at new moon; the SMS is brighter when new moon occurs near perigee; and the SMS peaks in brightness ∼5 h after new moon. After removing these effects, the data were analyzed for long term and seasonal patterns that could be attributed to changes in source mechanisms. No correlation was found between the SMS brightness and the 11‐year solar‐cycle, the proton or the He⁺⁺flow pressure, the density, the speed or the plasma temperature of the solar wind, but an annual pattern was found. A ∼0.83 correlation (Pearson's “r”) was found between the SMS brightness and a 4‐year average of sporadic meteor rates at Earth, suggesting a cause‐and‐effect. The new insights gained from this long‐term study put new constraints on the variability of the potential sources of the Na atoms escaping from the Moon.