High-entropy ceramics (HECs) have quickly gained attention since 2015. To date, nearly all work has focused on five-component, equimolar compositions. This perspective article briefly reviews ...different families of HECs and selected properties. Following a couple of our most recent studies, we propose a step forward to expand HECs to compositionally complex ceramics (CCCs) to include medium-entropy and non-equimolar compositions. Using defective fluorite and ordered pyrochlore oxides as two primary examples, we further consider the complexities of aliovalent cations and anion vacancies as well as ordered structures with two cation sublattices. Better thermally insulating yet stiff CCCs have been found in non-equimolar compositions with optimal amounts of oxygen vacancies and in ordered pyrochlores with substantial size disorder. It is demonstrated that medium-entropy ceramics can prevail over their high-entropy counterparts. The diversifying classes of CCCs provide even more possibilities than HECs to tailor the composition, defects, disorder/order, and, consequently, various properties.
This title is part of UC Press's Voices Revived program, which commemorates University of California Press's mission to seek out and cultivate the brightest minds and give them voice, reach, and ...impact. Drawing on a backlist dating to 1893, Voices Revived makes high-quality, peer-reviewed scholarship accessible once again using print-on-demand technology. This title was originally published in 1965.
The resonant coupling of the fast magnetosonic wave to the Alfvén wave is considered in the ideal magnetohydrodynamic limit in a 3D equilibrium. It has previously been shown that the most efficient ...coupling occurs on particular paths that satisfy the “tangential alignment condition” (Wright et al., 2022, https://doi.org/10.1029/2022ja030294). In this article we show how this criterion is equivalent to a minimization principle which may lead to a deeper understanding of the physics of the wave coupling process.
Key Points
The Resonance Map formulation of resonant fast and Alfvén wave coupling is consolidated
It is shown that the locations where the strongest coupling occurs can be identified using a minimization principle
Four benzimidazolium hydroxide compounds, in which the C2‐position is attached to a phenyl group possessing hydrogen, bromine, methyl groups, or phenyl groups at the ortho positions, are prepared and ...investigated for stability in a quantitative alkaline stability test. The differences between the stability of the various protecting groups in caustic solutions are rationalized on the basis of their crystal structures and DFT calculations. The highest stability was observed for the m‐terphenyl‐protected benzimidazolium, showing a half‐life in 3 m NaOD/CD3OD/D2O at 80 °C of 3240 h. A high‐molecular‐weight polymer analogue of this model compound is prepared that exhibits excellent mechanical properties, high ionic conductivity and ion‐exchange capacity, as well as remarkable hydroxide stability in alkaline solutions: only 5 % degradation after 168 h in 2 m KOH at 80 °C. This is the most stable hydroxide‐conducting benzimidazolium polymer to date.
It takes backbone: Through a novel synthetic pathway, pendant benzimidazolium groups attached to a poly(phenylene) backbone, which acts as the sterically protecting group, produces the most alkaline‐stable polybenzimidazolium to date. Crystal structures and DFT calculations of model compounds are used to explain the observed alkaline stability.
A novel simulation grid is devised that is optimized for studying magnetohydrodynamic (MHD) wave coupling and phase mixing in a dipole‐like magnetic field. The model also includes flaring on the dawn ...and dusk flanks. The location of the magnetopause is quite general. In particular, it does not have to coincide with a coordinate surface. Simulations indicate the central role of global fast waveguide modes. These switch from being azimuthally standing in nature at noon, to propagating antisunward on the flanks. The field line resonances (FLRs) seen in the simulation results are three dimensional and not strictly azimuthally polarized. When a plume is present, the FLRs cross a range of 2 in
L shell, and have a polarization that is midway between toroidal and poloidal.
Key Points
A simulation grid optimized for MHD wave coupling is described
Global fast waveguide modes have a mixed standing/propagating nature
Field line resonances can have a polarization that is far from toroidal
The reconnection of two flux tubes with footpoints anchored to a plane, such as the photosphere, is considered. We focus on properties of the reconnected flux tubes, specifically their twist, which ...can be quantified using magnetic helicity. If the tubes are of equal flux (Φ) and are initially crossed we find the results are dependent upon the relative positioning of their footpoints: (i) nonequipartition of self-helicity is the typical situation; (ii) the total amount of self-helicity in the reconnected tubes lies between 0 and 2Φ2, corresponding to a total twist of between 0 and 2 turns. If the tubes are initially uncrossed the self-helicity of each reconnected tube depends upon footpoint arrangement. However, care needs to be taken when using these results as bringing the tubes together at the reconnection site can introduce twist or writhe, which will also need to be taken into account. In the case where the tubes are side by side and possess some overlap, reconnection may occur without distorting the tubes. For this situation the reconnected tubes will be crossed: (i) equipartition of self-helicity is never met, but can be approached in the limit of the footpoints being quasi-colinear; (ii) the overlying tube always has a self-helicity whose magnitude >Φ2/2 (it exceeds a half turn); the underling tube's self-helicity magnitude is always <Φ2/2 (less than a half turn). Our results have a broad application in developing models of reconnecting coronal magnetic fields, as well as in interpreting observations and simulations of these fields.
Sand corrosion, thermal expansion, and ablation properties of a new class of medium‐ and high‐entropy compositionally complex fluorite oxides (CCFOs) are examined as potential protective coating ...materials. Five binary oxides were mixed and sintered into dense, single‐phase CCFOs of the general formula: Hf(1‐2x)/3Zr(1‐2x)/3Ce(1‐2x)/3YxYbxO2‐δ (x = 0.2, 0.074, and 0.029). These CCFOs exhibit decreased molten sand infiltration and interaction at intermediate temperatures (1200‐1300°C) in comparison with a cubic yttria‐stabilized zirconia (YSZ) reference; however, at higher temperatures, the trend is reversed due to the increased chemical reactivity. The equimolar high‐entropy (Hf0.2Zr0.2Ce0.2Y0.2Yb0.2)O2‐δ exhibits no grain boundary penetration by molten sand at all examined temperatures (1200°C‐1500°C), although reaction and precipitation are significant. Moreover, these CCFOs exhibit higher intrinsic thermal expansion coefficients (CTE) than the YSZ reference, thereby being more compatible with Ni‐based superalloys. The 8YSZ‐like (Hf0.284Zr0.284Ce0.284Y0.074Yb0.074)O2‐δ exhibits the highest CTE in this series of CCFOs due to oxygen clustering effects. Finally, these CCFOs also exhibit lower emissivities and form unique faceted microstructures in ablative environments.
Optical images highlighting the process of the molten sand and ablation experiments.
Abstract
Antarctic subglacial lakes are studied for three main scientific reasons. First, they form an important component of the basal hydrological system which is known to affect the dynamics of ...the ice sheet. Second, they are amongst the most extreme viable habitats on Earth and third, if sediments exist on their floors, they may contain high-resolution records of ice sheet history. Here we present a new inventory of locations, dimensions and data sources for 379 subglacial lakes. Several major advances are responsible for the rise in the total number of lakes from the 145 known at the time of the last inventory in 2005. New radar datasets have been collected in previously unexplored regions of the ice sheet while digital data collection and processing techniques have allowed improvements to lake identification methods. Satellite measurements of ice surface elevation change caused by the movement of subglacial water have also been found to be widespread in Antarctica, often in places where radar data are absent. These advances have changed our appreciation of the Antarctic subglacial environment and have expanded our understanding of the behaviour of subglacial lakes.
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