Monte Carlo modeling of low-energy electron-induced secondary electron emission yields in micro-architected boron nitride surfaces

Surface erosion and secondary electron emission (SEE) have been identified as the most critical life-limiting factors in channel walls of Hall-effect thrusters for space propulsion. Recent wall concepts based on micro-architected surfaces have been proposed to mitigate surface erosion and SEE. The idea behind these designs is to take advantage of very-high surface-to-volume ratios to reduce SEE and ion erosion by internal trapping and redeposition. This has resulted in renewed interest to study electron-electron processes in relevant thruster wall materials. In this work, we present calculations of SEE yields in micro-porous hexagonal BN surfaces using stochastic simulations of electron-material interactions in discretized surface geometries. Our model consists of two complementary parts. First we study SEE as a function of primary electron energy and incidence angle in flat surfaces using Monte Carlo simulations of electron multi-scattering processes. The results are then used to represent the response function of discrete surface elements to individual electron rays generated using a ray-tracing Monte Carlo model. We find that micro-porous surfaces result in SEE yield reductions of over 50% in the energy range experienced in Hall thrusters. This points to the suitability of these micro-architected surface concepts to mitigate SEE-related issues in compact electric propulsion devices

Geological characteristics and origin of the Watershed W deposit, North Queensland

Tungsten is considered a strategic metal by various countries, including Australia. Between 1998 and 2016 Australia has been steadily increasing its tungsten production, but it is still far smaller than those of the main producers (e.g., China, Russia). Watershed with its current resources of 49.2 Mt averaging 0.14% WO₃ is considered one of the biggest undeveloped tungsten deposits outside of China, and if developed would boost Australia's tungsten production. The main goal of this PhD thesis is to improve our understanding of the Watershed tungsten deposit and how to explore for similar deposits in northeast Queensland. This goal was approached by documenting the geological, geochemical and structural characteristics of the Watershed deposit, as well as the timing, mineral paragenesis and fluid characteristics of the mineralizing system. The Watershed tungsten deposit lies within the Mossman Orogen, which comprises multiple deformed Silurian-Ordovician metasedimentary rocks of the Hodgkinson Formation intruded by Carboniferous-Permian granites of the Kennedy Igneous Association. The Hodgkinson Formation in the Watershed area comprises skarn-altered conglomerate, psammite and slate units, which record at least four deformation events ranging from early ductile folding and shearing events (D₁ to D₃) to the development of later brittle-ductile shear zones (D₄) associated with veining and four separate stages of retrograde metamorphism/alteration (Retrograde Stages 1 to 4). Peak metamorphic assemblages (garnet, actinolite, quartz, clinopyroxene, titanate) in the host rocks to mineralisation formed during D₁-₂. Multiple felsic dykes intrude the metasedimentary rocks at Watershed and include: (a) Carboniferous, monzonite dykes (zircon U/Pb age of 350 ± 7 Ma) emplaced during D₁₋₂; and (b) Permian granite plutons and dykes (zircon U/Pb ages of 291 ± 6 Ma, 277 ± 6 Ma and 274 ± 6 Ma), and diorite (zircon U/Pb age of 281 ± 5 Ma) emplaced during D₄. An early (syn-D₁₋₂) mineralization event involved the syn-tectonic growth of disseminated scheelite in monzonite dykes and adjacent skarn-altered conglomerate, and was associated with the emplacement of the monzonite, which appears to have enriched the Hodgkinson Formation in W-Be- B-Sc-Cu-Mo-Re. The bulk of the economic scheelite mineralization formed in syn-D₄ shear-related, quartz-oligoclase veins and associated vein haloes (with a muscovite Ar-Ar age of 276 ± 6 Ma). The veins developed preferentially in skarn-altered conglomerate and terminate abruptly when they encounter slate. Vein opening occurred synchronous with four retrograde alteration stages in skarn-altered conglomerate. The margins of the D₄ veins contain feldspar, scheelite and quartz, which represents Retrograde Stages 1 and 2. During Retrograde Stage 1 early sanidine (overgrown by plagioclase, An₁₅₋₅₅) formed with minor quartz. Retrograde Stage 2 is characterised by intergrown scheelite and plagioclase (An₃₋₄₃) overgrowing early plagioclase, phlogopite and trace apatite. Further vein opening during Retrograde Stage 3 infilled the central part of the vein with quartz, which is cross cut by muscovite, calcite and minor chlorite, tourmaline and fluorite. Fractures that formed during Retrograde Stage 4 cut textures belonging to the previous stages and contain pyrrhotite, arsenopyrite with lesser pyrite, chalcopyrite, and sphalerite. Scheelite can incorporate small amounts of REE, and the origin of the scheelite grains (i.e. intrusion-related vs metamorphic) has been investigated using the relative abundance of contained LREE, MREE and HREE. Using ternary REE plots, early D₁₋₂ scheelite in monzonite coincides with the compositional field for scheelite that forms during magmatic-hydrothermal processes, whereas late D₄ vein-hosted scheelite is compositionally similar to pure hydrothermal scheelite. The Eu and Mo contents of scheelite, coupled with graphite inclusions in scheelite and the presence of pyrrhotite and arsenopyrite in scheelite-bearing veins, show that D₁₋₂ scheelite precipitated from a relatively oxidized fluid, while vein-hosted D₄ scheelite record a shift to more reduced conditions as a result of fluid interaction with carbonaceous shale. Whole rock geochemistry of the various rock types within the deposit indicates that the Watershed deposit is characterized by an enrichment of W-Be-B-Sc-Cu-Mo-Re. These elements were probably introduced by hydrothermal fluids during D₄ veining. The fluid interacted with the skarn-altered conglomerate to leach REE, Y and Nb plus skarn-related elements (i.e., Ca-F-P-Fe-Sr), and add Rb, Cs and Li in vein haloes. Whole rock geochemistry of psammite units along a 2 km transect north of the deposit shows a regional footprint that is characterised by enrichment in W-Cu-Mo-Ca-Fe-Mn-Li. Fluid inclusions in D₄ vein scheelite and quartz from Retrograde Stage 2 preserve a low salinity H₂O-NaCl-CH₄ fluid (XCH₄ < 0.01). The fluid inclusions show evidence for fluid-fluid mixing between low- (close to 0 wt.% NaCl) and medium-salinity (< 8 wt.% NaCl) fluids. The P-T conditions during mineralisation were determined at ca. 300°C and 1-1.5 kbar (i.e. depths of 3.5-6 km) indicating a high geothermal gradient, linked to the emplacement of Permian granites. Those P-T conditions are similar to those recorded in lode-gold deposits in the Hodgkinson Gold Field and elsewhere. The oxygen fugacity was calculated at 0.6 to 0.8 log₁₀ values below the FMQ buffer, consistent with the reduced mineralogy and geochemical signatures. δ¹⁸OVSMOW values obtained for scheelite (+3.4 to +7.3‰), plagioclase (+7.0 to +11.8‰) and quartz (+12.6 to +15.5‰), which formed during Retrograde Stage 2, and δDVSMOW (−73.4 to −62.7‰) and δ¹⁸OVSMOW (+11.5 to +13.2‰) values for muscovite that formed during Retrograde Stage 3 are indicative of a metamorphic origin for the mineralising fluids, with a possible magmatic component. Sulphur isotope (δ³⁴SCDT) values for sulphides formed during Retrograde Stage 4 in veins are consistent with the presence of seawater sulphate (i.e. basinal brine) in the system. Metamorphic fluids probably originated from prograde devolatilization reactions during metamorphism of the Hodgkinson Formation. Our findings indicate that tungsten was sourced from Carboniferous monzonite, which enriched the metasedimentary rock units of the Hodgkinson Formation during the early stages of deformation/ metamorphism. Continued ductile deformation and associated metamorphism during D₁₋₃ caused devolatilization reactions in the host rocks and remobilisation of tungsten. Permian scheelite mineralisation during D₄ involved a metamorphic-hydrothermal fluid with minor magmatic input that deposited tungsten at 300°C and 1-1.5 kbar (<6 km depth). This tungsten was transported as NaWO₄⁻, HWO₄⁻ and WO₄²⁻ complexes along extensional shear zones. Calcium was supplied by the skarn-altered conglomerate that hosts the scheelite-bearing veins. It is proposed that the precipitation of scheelite was promoted by the interaction between the relatively acidic hydrothermal fluids and the alkaline, carbonate-rich, skarn-altered conglomerate host rock, lowering the solubility of the tungsten complexes and co-precipitating scheelite and Na-rich plagioclase during Retrograde Stage 2. Considering a continuum model for this deposit type (i.e. mineralization could form between 2-20 km depth) it is feasible to consider the potential for mineralization at depth

Tidally-induced migration of TESS gas giants orbiting M dwarfs

According to core-accretion formation models, the conditions under which gas giants will form around M dwarfs are very restrictive. Also, the correlation of the occurrence of these planets with the metallicity of host stars is still unknown due to the intrinsic faintness of M dwarfs in the optical and some intricacies in their spectra. Interestingly, NASA's TESS mission has started to create a growing sample of these systems, with eleven observed planets located in close-in orbits: contrary to what is expected for low stellar masses. Tidal interactions with the host star will play a key role in determining the fate of these planets, so by using the measured physical and orbital characteristics of these M-dwarf systems we numerically analyse the exchange of rotational and orbital angular momentum, while constraining the energy dissipation in each system to calculate whether host stars are spun up or spun down, depending on the relationship between the gain and loss of angular momentum by the stellar rotation. We also study the coupled orbital and physical evolution of their gas giant companion and calculate orbital circularization time-scales, as well as the time needed to undergo orbital decay from their current orbital position to the Roche limit. The thorough study of tidal processes occurring over short and long time-scales in star-planet systems like those studied here, can help constrain tidal dissipation rates inside the star and planet, complement tidal theories, and improve estimations of unconstrained properties of exoplanetary systems.Comment: Accepted for publication in MNRAS. 11 pages, 6 figures. V2: a newly discovered M-dwarf system was adde

Implementación del hidrógeno en la legislación mexicana para el desarrollo energético nacional

La energía es uno de los motores más importantes que mueven a las sociedades. El desarrollo de un sistema energético a base de hidrógeno (H2) está respaldado por dos grandes fuerzas, la seguridad energética y la problemática medioambiental. Considerando la celda de combustible de óxido sólido (SOFC) como el principal receptor del H2, se obtendría el potencial de crear un sistema energético eficiente, limpio y sostenible en las próximas décadas. El factor más importante a mejorar a corto plazo es el desarrollo de mejores sistemas de almacenamiento del H2. En México, el marco institucional para la promoción y aplicación de energías alternas es muy limitado, puesto que se prefiere desarrollar tecnologías para el aprovechamiento de fuentes tradicionales de energía; hay desconocimiento por parte de las autoridades de la magnitud y el posible aprovechamiento del H2 como fuente alterna de energía. La normativa mexicana no hace referencia al H2 como alternativa energética. Por ello, considerando todos los beneficios ambientales, económicos y sociales que se promueven con su uso y aplicación, es pertinente que se incorpore en el marco jurídico mexicano a fin de fomentar la investigación, promoción y aplicación de este elemento como fuente alterna de energía