Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements

Designing heterointerface in BiOBr/g-C3N4 photocatalyst to enhance visible-light-driven photocatalytic performance in water purification

A rapid recombination of photo-generated electrons and holes is the intrinsic defect in graphitic carbon nitride (g-C3N4)-based photocatalysts. Inspired by natural photosynthesis, an artificially synthesized Z-scheme photocatalyst can efficaciously restrain the recombination of photogenerated electron-hole pairs and enhance the photoabsorption ability. Herein, a series of novel visible-light-driven (VLD) solid-state Z-scheme BiOBr/g-C3N4 heterostructured photocatalysts were successfully synthesized via a facile in-situ oxidation method which enabled the BiOBr nanosheets to grow on the surface of g-C3N4. The introduction of BiOBr on g-C3N4 to constitute a Z-scheme heterojunction system can effectively broaden the light absorption range and promote the separation of photo-generated electron-hole pairs. Besides, the Z-scheme mechanism proved by active species trapping experiments led to the persistence of photoinduced electrons in the CB of g-C3N4 while holes in the VB of BiOBr, thus ensuring the high redox ability of the charge carriers. Notably, the BC12% sample exhibited the optimized photocatalytic activity in which the RhB molecules were decomposed within 20 min under visible light irradiation, achieving 3.3- and 2.3-fold improvement than the bare g-C3N4 and pristine BiOBr, respectively. Meanwhile, the BC12% sample still revealed the superiority in both photodegrading MO and photoreducing Cr (VI). As an original strategy to obtain samples with highly dispersed heterointerface, this work could provide a facile route for the construction of g-C3N4-based solid-state Z-scheme heterojunction and very promising for practical application

Fluorine Effect for Improving Oxidation Resistance of Ti-45Al-8.5Nb Alloy at 1000 °C

In-depth analyses of the anti-oxidation behavior and structure of γ-TiAl alloys are of great significant for their maintenance and repair in engineering applications. In this work, fluorine-treated Ti-45Al-8.5Nb alloys and fluorine-treated oxidized specimens with artificial defects were prepared by isothermal oxidation treatment at 1000 °C. Several characterization methods, including SEM, EDS, XRD and TEM, were used to evaluate the surface microstructure of the fluorine-treated Ti-45Al-8.5Nb alloys and fluorine-treated oxidized specimens with artificial defects. The results indicate that the fluorine promoted the formation of an outer protective film of Al2O3, which significantly improved the oxidation resistance. The microcracks of oxidized specimens with the artificial defects provided a rapid diffusion passage for Ti and O elements during the 1000 °C/2 h isothermal oxidation treatment process, resulting in the quick growth of TiO2 toward the outside. The fine Al2O3 constituted a continuous film after the 1000 °C/100 h isothermal oxidation treatment. In particular, Al2O3 particles grew toward the substrate, which was ascribed to the good oxidation resistance and adhesion. These results may provide an approach for the repair of protective oxide film on the surface of blades and turbine disks based on γ-TiAl alloys

Constructing electrostatic self-assembled ultrathin porous red 2D g-C3N4/Fe2N Schottky catalyst for high-efficiency tetracycline removal in photo-Fenton-like processes

The traditional heterogeneous photo-Fenton reaction was mainly restricted by the fewer surface-active sites, low Fe3+/Fe2+ transformation and H2O2 activation efficiency of catalyst. This work designed and fabricated the efficient photo-Fenton Schottky catalysts via a facile electrostatic self-assembly of metallic Fe2N nanoparticles scattering on the surface of red g-C3N4 (ultrathin porous oxygen-doped 2D g-C3N4 nanosheets). The porous morphology and exceptional electrical structure of red g-C3N4 endowed more active sites and facilitated the photoexcited charge separation. Benefitting from the Schottky effect and unique dimensional coupling structure, the strong visible light absorption and fast spatial charge transfer were realized in the Schottky junction system. More strikingly, Fe2N as an efficient co-catalyst was in favor of the trap and export of e(-), leading to the Fe3+/Fe2+ transformation and H2O2 activation during the photo-Fenton process. Accordingly, the as-prepared catalysts revealed outstanding activity in photo-Fenton like degradation of tetracycline (TC) although under 5 W white LED light irradiation. Furthermore, the reasonable degradation pathway of TC and corresponding toxicity of the intermediates, as well as the photo-Fenton catalytic mechanism were interpreted and discussed in detail. This study would be a great aid in the development of various Schottky catalysts for heterogeneous photo-Fenton-based environmental remediation systems. (C) 2021 Elsevier Inc. All rights reserved