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1. Influence of water activity on belite (β‐C 2 S) hydration

   https://doi.org/10.1111/jace.17608  

   Cook, Rachel ; Ma, Hongyan ; Okoronkwo, Monday ; Sant, Gaurav ; Kumar, Aditya ( December 2020 , Journal of the American Ceramic Society)

   The hydration of the two most reactive phases of ordinary Portland cement (OPC), tricalcium silicate (C₃S), and tricalcium aluminate (C₃A) is successfully halted when the activity of water () falls below critical thresholds of 0.70 and 0.45, respectively. It has been established that the reduction in relative humidity (RH) and suppresses the hydration of all anhydrous phases in OPC, including less explored phases like dicalcium silicate, that is, belite (β‐C₂S). However, the degree of suppression, that is, the critical threshold, for β‐C₂S, standalone has yet to be established. This study utilizes isothermal microcalorimetry and X‐ray diffraction techniques to elucidate the influence ofon the hydration of‐C₂S suspensions via incremental replacements of water with isopropanol (IPA). Experimentally, this study shows that with increasing IPA replacements, hydration is increasingly suppressed until eventually brought to a halt at a critical threshold of approximately 27.7% IPA on a weight basis (wt.%_IPA). From thermodynamic estimations, the exact criticalthreshold and solubility product constant of‐C₂S () are established as 0.913 and 10^−12.68, respectively. This study enables enhanced understanding of β‐C₂S reactivity and provides thermodynamic parameters during the hydration of β‐C₂S‐containing cementitious systems such as OPC‐based and calcium aluminate‐based systems.

    

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2. Flash sintering produces eutectic microstructures in Al 2 O 3 –LaPO 4 versus conventional microstructures in 8YSZ–LaPO 4

   https://doi.org/10.1111/jace.17786  

   Yang, Yingjie ; Mumm, Daniel R. ; Mecartney, Martha L. ( April 2021 , Journal of the American Ceramic Society)

   While monazite (LaPO₄) does not flash sinter even at high fields of 1130 V/cm and temperatures of 1450°C, composite systems of 8YSZ–LaPO₄and Al₂O₃–LaPO₄have been found to more readily flash sinter. 8YSZ added to LaPO₄greatly lowered the furnace temperature for flash to 1100°C using a field of only 250 V/cm. In these experiments,‐Al₂O₃alone also did not flash sinter at 1450°C even with high fields of 1130 V/cm, but composites of Al₂O₃–LaPO₄powders flash sintered at 900‐1080 V/cm at 1450°C. Alumina–monazite (Al₂O₃–LaPO₄) composites with compositions ranging from 25 vol% to 75 vol% Al₂O₃were flash sintered with current limits from 2 to 25 mA/mm². Microstructures were evaluated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A eutectic microstructure was observed to form in all flash sintered Al₂O₃–LaPO₄composites. With higher power (higher current limits), eutectic structures with regular lamellar regions were found to coexist in the channeled region (where both the current and the temperature were the highest) with large hexagonal‐shaped‐Al₂O₃grains (up to 75 m) and large irregular LaPO₄grains. With lower power (lower current limits), an irregular eutectic microstructure was dominant, and there was minimal abnormal grain growth. These results indicate that Al₂O₃–LaPO₄is a eutectic‐forming system and the eutectic temperature was reached locally during flash sintering in regions. These eutectic microstructures with lamellar dimensions on the scale of 100 nm offer potential for improved mechanical properties.

    

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3. High‐resolution microstructural and compositional analyses of shock deformed apatite from the peak ring of the Chicxulub impact crater

   https://doi.org/10.1111/maps.13541  

   Cox, Morgan A. ; Erickson, Timmons M. ; Schmieder, Martin ; Christoffersen, Roy ; Ross, Daniel K. ; Cavosie, Aaron J. ; Bland, Phil A. ; Kring, David A. ; IODP–ICDP Expedition 364 Scientists ( August 2020 , Meteoritics & Planetary Science)

   The mineral apatite, Ca₅(PO₄)₃(F,Cl,OH), is a ubiquitous accessory mineral, with its volatile content and isotopic compositions used to interpret the evolution of H₂O on planetary bodies. During hypervelocity impact, extreme pressures shock target rocks resulting in deformation of minerals; however, relatively few microstructural studies of apatite have been undertaken. Given its widespread distribution in the solar system, it is important to understand how apatite responds to progressive shock metamorphism. Here, we present detailed microstructural analyses of shock deformation in ~560 apatite grains throughout ~550 m of shocked granitoid rock from the peak ring of the Chicxulub impact structure, Mexico. A combination of high‐resolution backscattered electron (BSE) imaging, electron backscatter diffraction mapping, transmission Kikuchi diffraction mapping, and transmission electron microscopy is used to characterize deformation within apatite grains. Systematic, crystallographically controlled deformation bands are present within apatite, consistent with tilt boundaries that contain the <c> (axis) and result from slip in <> (direction) on(plane) during shock deformation. Deformation bands contain complex subgrain domains, isolated dislocations, and low‐angle boundaries of ~1° to 2°. Planar fractures within apatite form conjugate sets that are oriented within either {, {, {, or. Complementary electron microprobe analyses (EPMA) of a subset of recrystallized and partially recrystallized apatite grains show that there is an apparent change in MgO content in shock‐recrystallized apatite compositions. This study shows that the response of apatite to shock deformation can be highly variable, and that application of a combined microstructural and chemical analysis workflow can reveal complex deformation histories in apatite grains, some of which result in changes to crystal structure and composition, which are important for understanding the genesis of apatite in both terrestrial and extraterrestrial environments.

    

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4. The effect of O 2 and pressure on thiosulfate oxidation by Thiomicrospira thermophila

   https://doi.org/10.1111/gbi.12352  

   Houghton, Jennifer L. ; Foustoukos, Dionysis I. ; Fike, David A. ( June 2019 , Geobiology)

   Microbial sulfur cycling in marine sediments often occurs in environments characterized by transient chemical gradients that affect both the availability of nutrients and the activity of microbes. High turnover rates of intermediate valence sulfur compounds and the intermittent availability of oxygen in these systems greatly impact the activity of sulfur‐oxidizing micro‐organisms in particular. In this study, the thiosulfate‐oxidizing hydrothermal vent bacteriumThiomicrospira thermophilastrain EPR85 was grown in continuous culture at a range of dissolved oxygen concentrations (0.04–1.9 mM) and high pressure (5–10 MPa) in medium buffered at pH 8. Thiosulfate oxidation under these conditions produced tetrathionate, sulfate, and elemental sulfur, in contrast to previous closed‐system experiments at ambient pressure during which thiosulfate was quantitatively oxidized to sulfate. The maximum observed specific growth rate at 5 MPa pressure under unlimited O₂was 0.25 hr⁻¹. This is comparable to theμ_max(0.28 hr⁻¹) observed at low pH (<6) at ambient pressure whenT. thermophilaproduces the same mix of sulfur species. The half‐saturation constant for O₂() estimated from this study was 0.2 mM (at a cell density of 10⁵cells/ml) and was robust at all pressures tested (0.4–10 MPa), consistent with piezotolerant behavior of this strain. The cell‐specificwas determined to be 1.5 pmol O₂/cell. The concentrations of products formed were correlated with oxygen availability, with tetrathionate production in excess of sulfate production at all pressure conditions tested. This study provides evidence for transient sulfur storage during times when substrate concentration exceeds cell‐specificand subsequent consumption when oxygen dropped below that threshold. These results may be common among sulfur oxidizers in a variety of environments (e.g., deep marine sediments to photosynthetic microbial mats).

    

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5. Diffusion interactions between crossing fibers of the brain

   https://doi.org/10.1002/mrm.28702  

   Buldyrev, Sergey V. ; Meng, Xiangyi ; Reese, Timothy G. ; Mortazavi, Farzad ; Rosene, Douglas L. ; Stanley, H. Eugene ; Wedeen, Van J. ( February 2021 , Magnetic Resonance in Medicine)

   Recent observations of several preferred orientations of diffusion in deep white matter may indicate either (a) that axons in different directions are independently bundled in thick sheets and function noninteractively, or more interestingly, (b) that the axons are closely interwoven and would exhibit branching and sharp turns. This study aims to investigate whether the dependence of dMRI Q‐ball signal on the interpulse timecan decode the smaller‐than‐voxel‐size brain structure, in particular, to distinguish scenarios (a) and (b).

   High‐resolution Q‐ball images of a healthy brain taken with s/mm²for 3 different values ofwere analyzed. The exchange of water molecules between crossing fibers was characterized by the fourth Fourier coefficientof the signal profile in the plane of crossing. To interpret the empirical results, a model consisting of differently oriented parallel sheets of cylinders was developed. Diffusion of water molecules inside and outside cylinders was simulated by the Monte Carlo method.

   Simulations predict that, agreeing with the empirical results, must increase withfor largeb‐values, but may peak at a typicalthat depends on the thickness of the cylinder sheets for intermediateb‐values. Thus, the thickness of axon layers in voxels with 2 predominant orientations can be detected from empiricaltaken at smallerb‐values.

   Based on the simulation results, recommendations are made on how to design a dMRI experiment with optimalb‐value and range ofin order to measure the thickness of axon sheets in the white matter, hence to distinguish (a) and (b).

    

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