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Abstract: Uranium is a risk-driving radionuclide in both radioactive waste disposal and contaminated land scenarios. In these environments, a range of biogeochemical processes can occur, including sulfate reduction, which can induce sulfidation of iron (oxyhydr)oxide mineral phases. During sulfidation, labile U(VI) is known to reduce to relatively immobile U(IV); however, the detailed mechanisms of the changes in U speciation during these biogeochemical reactions are poorly constrained. Here, we performed highly controlled sulfidation experiments at pH 7 and pH 9.5 on U(VI) adsorbed to ferrihydrite and investigated the system using geochemical analyses, X-ray absorption spectroscopy (XAS), and computational modeling. Analysis of the XAS data indicated the formation of a novel, transient U(VI)–persulfide complex as an intermediate species during the sulfidation reaction, concomitant with the transient release of uranium to the solution. Extended X-ray absorption fine structure (EXAFS) modeling showed that a persulfide ligand was coordinated in the equatorial plane of the uranyl moiety, and formation of this species was supported by computational modeling. The final speciation of U was nanoparticulate U(IV) uraninite, and this phase was evident at 2 days at pH 7 and 1 year at pH 9.5. Our identification of a new, labile U(VI)-persulfide species under environmentally relevant conditions may have implications for U mobility in sulfidic environments pertinent to radioactive waste disposal and contaminated land scenarios.

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Abstract: Here we present the first real-time three dimension (3D) observations of damage evolution in a composite tube under torsion. An in-situ torsion test of 1/1 45° (diamond) braided carbon fibre-epoxy circular composite tube was performed on a loading rig and the damage process was characterised by synchrotron X-ray computed tomography (CT). A number of damage modes and their damage sequence has been identified and monitored globally and in more detail within a representative region of interest. In particular, intra-tow cracks and inter-tow debonding have been found to occur almost simultaneously at low shear strains (1.5%). It is noteworthy that inter-tow debonding was initially trapped/limited within repeated braid units before propagating and connecting with other damage modes in 3D. The area fraction of inter-tow debonds was quantified at different stages and it was found to dramatically increase with increasing shear strain beyond 1.5%. The total volume fraction of the observed intra-tow cracks of various forms was seen to grow rapidly beyond shear strain of 2.0%. Beyond the peak shear stress (at shear strain of 2.5%), fibre micro-buckling and kink bands occur in the tows subjected to torsion induced axial compression at crimped regions close to tow crossovers. Tow crossovers control many aspects of damage propagation under torsion, positively by localising inter-tow debonds and negatively by initiating fibre micro-buckling.

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Abstract: Mantle oxygen fugacity (fO2) governs the physico-chemical evolution of the Earth, however current estimates from commonly used basalt redox proxies are often in disagreement. In this study we compare three different potential basalt fO2 proxies: Fe3+/Fetot, V/Sc and V isotopes, determined on the same submarine lavas from a 700 km section of the Reykjanes Ridge, near Iceland. These samples provide a valuable test of the sensitivities of fO2 proxies to basalt petrogenesis, as they formed at different melting conditions and from a mantle that towards Iceland exhibits increasing long-term enrichment of incompatible elements. New trace element data were determined for 63 basalts with known Fe3+/Fetot. A subset of 19 lavas, covering the geographical spread of the ridge transect, was selected for vanadium isotope analyses. Vanadium is a multi-valence element whose isotopic fractionation is theoretically susceptible to redox conditions. Yet, the δ51 VAA composition of basaltic glasses along the Reykjanes Ridge covers only a narrow range ( δ51 VAA = −1.09 to −0.86‰; 1SD = 0.02–0.09) and does not co-vary with fractionation-corrected Fe3+/Fetot (0.134–0.151; 1SD = 0.005) or V/Sc (6.6–8.5; 1SD = 0.1-1.3) ratios. However, on a global scale, basaltic δ51 VAA may be controlled by the extent of melting. The V/Sc compositions of primitive (MgO > 7.5 wt%) basalts show no systematic change along the entire length of the Reykjanes Ridge. Typical peridotite melting models in which source Fe3+/Fetot is constant at 5% and that account for the increased mantle potential temperature nearer the plume center and the fO2 dependent partitioning of V, can reproduce the V/Sc data. However, while these melting models predict that basalt Fe3+/Fetot ratios should decrease with increasing mantle potential temperature towards Iceland, fractionation-corrected Fe3+/Fetot of Reykjanes Ridge lavas remain nearly constant over the ridge length. This discrepancy is explained by source heterogeneity, where an oxidized mantle pyroxenite component contributes to melting with increasing proximity to Iceland. Comparison of observed and modeled Fe3+/Fetot indicate that source variation in fO2 is present under the Reykjanes Ridge, with higher Fe3+/Fetot closer to Iceland. This source variability in fO2 cannot be resolved by V isotopes and redox-sensitive trace element ratios, which instead appear to record magmatic processes.

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Abstract: Mechanically-exfoliated two-dimensional ferromagnetic materials (2D FMs) were discovered to possess long-range ferromagnetic order and topologically nontrivial skyrmions in few-layers. However, owing to the dimensionality effect, such few-layer systems usually exhibit much lower Curie temperature (TC) compared to their bulk counterparts. It is therefore of great interest to explore effective approaches to enhance their TC, particularly in wafer-scale for practical applications. Here, we report an interfacial proximity-induced high-TC 2D FM Fe3GeTe2 (FGT) via A-type antiferromagnetic material CrSb (CS) which strongly couples to FGT. A superlattice structure of (FGT/CS)n, where n stands for the period of FGT/CS heterostructure, has been successfully produced with sharp interfaces by molecular-beam epitaxy on 2-inch wafers. By performing the elemental specific X-ray magnetic circular dichroism (XMCD) measurements, we have unequivocally discovered that TC of 4-layer Fe3GeTe2 can be significantly enhanced from 140 K to 230 K because of the interfacial ferromagnetic coupling. In the meanwhile, an inverse proximity effect occurs in the FGT/CS interface, driving the interfacial antiferromagnetic CrSb into a ferrimagnetic state as evidenced by a double-switching behavior in hysteresis loops and the XMCD spectra. Density functional theory calculations show that the Fe-Te/Cr-Sb interface is strongly FM coupled and doping of the spin-polarized electrons by the interfacial Cr layer gives rise to the TC enhancement of the Fe3GeTe2 films, in accordance with our XMCD measurements. Strikingly, by introducing rich Fe in 4-layer FGT/CS superlattice, TC can be further enhanced to near room temperature. Our results provide a feasible approach in enhancing the magnetic order of few-layer 2D FMs in wafer-scale and render opportunities for realizing realistic ultra-thin spintronic devices.

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Abstract: Rh-Containing artificial metalloenzymes based on two mutants of sterol carrier protein_2L (SCP_2L) have been shown to act as hydroformylases, exhibiting significant activity and unexpectedly high selectivity in the hydroformylation of a range of alkenes. Here we report modifications of the catalyst performance by site directed mutagenesis, and studies of the biophysical properties of these catalysts. Catalysts were prepared based on single methionine mutants of SCP_2L for hydroformylation studies. Multinuclear (1H, & 13C), multi-dimensional (2D) solution NMR spectroscopy, EXAFS and XANES were employed to probe the structure. Biophysical studies using 2D [1H 13C] HSQC NMR spectroscopy of 13C-methyl methionine labeled catalysts were used to investigate changes in protein conformation. Hydroformylation studies employing the methionine mutants as catalysts revealed the significant effects of mutation on hydroformylation activity. Among the methionine mutant catalysts M1A of V83C and A100C, and M112A of V83C exhibited significantly higher activity than their parent proteins with improved selectivity. A metal binding role for methionine was suggested by EXAFS and XANES data on selenomethionine variants.