Beihang University

About: Beihang University is a education organization based out in Beijing, China. It is known for research contribution in the topics: Control theory & Microstructure. The organization has 67002 authors who have published 73507 publications receiving 975691 citations. The organization is also known as: Beijing University of Aeronautics and Astronautics.

Microporous Co@CoO nanoparticles with superior microwave absorption properties

Abstract: Nanoporous metal materials with many potential applications have been synthesized by a chemical dealloying approach. The fabrication of nanoporous metal nanoparticles (NPs), however, is still challenging due to the difficulties in producing suitable nanoscale precursors. Herein, nanoporous Co NPs of 31 nm have been successfully prepared by dealloying Co–Al NPs, and surprisingly they possess micropores in a range from 0.7 to 1.7 nm and a large surface area of 50 m2 g−1. The crystalline size of the microporous NPs is 2–5 nm. Through the passivation process, the microporous Co NPs covered with CoO (Co@CoO) are generated as a result of the surface oxidation of Co. They exhibit better microwave absorption properties than their nonporous counterpart. An enhanced reflection loss (RL) value of −90.2 dB is obtained for the microporous Co@CoO NPs with a thickness of merely 1.3 mm. The absorption bandwidth corresponding to the RL below −10 dB reaches 7.2 GHz. The microwave absorption mechanism is discussed in terms of micropore morphology, core@shell structure and nanostructure. This novel microporous material may open new routes for designing high performance microwave absorbers.

Towards the discovery of new physics with lepton-universality ratios of $b\to s\ell\ell$ decays

Abstract: Tests of lepton-universality as rate ratios in $b\ensuremath{\rightarrow}s\ensuremath{\ell}\ensuremath{\ell}$ transitions can be predicted very accurately in the Standard Model The deficits with respect to expectations reported by the LHCb experiment in muon-to-electron ratios of the $B\ensuremath{\rightarrow}{K}^{(*)}\ensuremath{\ell}\ensuremath{\ell}$ decay rates thus point to genuine manifestations of lepton nonuniversal new physics In this paper, we analyze these measurements in the context of effective field theory First, we discuss the interplay of the different operators in ${R}_{K}$ and ${R}_{{K}^{*}}$ and provide predictions for ${R}_{{K}^{*}}$ in the Standard Model and in new-physics scenarios that can explain ${R}_{K}$ We also provide approximate numerical formulas for these observables in bins of interest as functions of the relevant Wilson coefficients Secondly, we perform frequentist fits to ${R}_{K}$ and ${R}_{{K}^{*}}$ The Standard Model disagrees with these measurements at $37\ensuremath{\sigma}$ significance We find excellent fits in scenarios with combinations of ${\mathcal{O}}_{9(10)}^{\ensuremath{\ell}}=\overline{s}{\ensuremath{\gamma}}^{\ensuremath{\mu}}{b}_{L}\ensuremath{\ell}{\ensuremath{\gamma}}_{\ensuremath{\mu}}({\ensuremath{\gamma}}_{5})\ensuremath{\ell}$ operators, with pulls relative to the Standard Model in the region of $4\ensuremath{\sigma}$ An important conclusion of our analysis is that a lepton-specific contribution to ${\mathcal{O}}_{10}$ is important to understand the data Under the hypothesis that new-physics couples selectively to the muons, we also present fits to other $b\ensuremath{\rightarrow}s\ensuremath{\mu}\ensuremath{\mu}$ data with a conservative error assessment and comment on more general scenarios Finally, we discuss new lepton universality ratios that, if new physics is the origin of the observed discrepancy, should contribute to the statistically significant discovery of new physics in the near future

Assembly of Hollow Carbon Nanospheres on Graphene Nanosheets and Creation of Iron–Nitrogen-Doped Porous Carbon for Oxygen Reduction

Abstract: Triblock copolymer micelles coated with melamine-formaldehyde resin were self-assembled into closely packed two-dimensional (2D) arrangements on the surface of graphene oxide sheets. Carbonizing these structures created a 2D architecture composed of reduced graphene oxide (rGO) sandwiched between two monolayers of sub-40 nm diameter hollow nitrogen-doped carbon nanospheres (N-HCNS). Electrochemical tests showed that these hybrid structures had better performance for oxygen reduction compared to physically mixed rGO and N-HCNS that were not chemically bonded together. Further impregnation of the sandwich structures with iron (Fe) species followed by carbonization yielded Fe1.6-N-HCNS/rGO-900 with a high specific surface area (968.3 m2 g–1), a high nitrogen doping (6.5 at%), and uniformly distributed Fe dopant (1.6 wt %). X-ray absorption fine structure analyses showed that most of the Fe in the nitrogen-doped carbon framework is composed of single Fe atoms each coordinated to four N atoms. The best Fe1.6-N...

Direction Controlled Driving of Tiny Water Drops on Bioinspired Artificial Spider Silks

Abstract: www.MaterialsViews.com C O M M Direction Controlled Driving of Tiny Water Drops on Bioinspired Artifi cial Spider Silks U N IC A By Hao Bai , Xuelin Tian , Yongmei Zheng ,* Jie Ju , Yong Zhao , and Lei Jiang * IO N Directional driving of liquid drops is of signifi cant interest in many applications, such as microfl uidic devices, [ 1–6 ] fog harvesting, [ 7 ] fi ltration, [ 8 ] and condensers. [ 9 ] For this purpose, great progress has been made in driving drops larger than hundreds of micrometers [ 9–23 ] by introducing chemical, [ 1 , 10 , 12–15 , 23 ] thermal, [ 16–19 ] or shape [ 20–22 ] gradients on surfaces. However, driving micrometer-sized drops is much more diffi cult because they encounter a larger contact angle (CA) hysteresis effect. [ 7 , 24 ] In nature, the wetted silk of cribellate spider offers new insights into solving this problem by combining different gradients together. [ 7 ] Here, inspired by the spider silk, we fabricated a series of artifi cial spider silks with spindle-knots in which the chemical compositions and surface nanostructures were subtly designed. Our investigations demonstrated that tiny water drops (tens of picoliters) could be driven with controllable direction (“toward” or “away from” the knot) by optimizing the cooperation of curvature, chemical, and roughness gradients on the fi ber surfaces. The study will pave the way for designing smart materials and devices to drive tiny water drops in a controllable manner. When a nylon fi ber was immersed into polymer solution and drawn out horizontally, a string of polymer drops, which became spindle-knots after being dried, formed on the fi ber due to the Rayleigh instability [ 25 ] of the polymer solution. The surface energy of the spindle-knots was tailored by choosing different polymers including poly(vinyl acetate) (PVAc), poly(methyl methacrylate) (PMMA), polystyrene (PS), and poly(vinylidene fl uoride) (PVDF), which have intrinsic water contact angles of 56.7 ° , 68.4 ° , 93.3 ° , and 92.7 ° , respectively (see Supporting Information, Figure S1). On the other hand, the surface roughness (porous nanostructures) of the spindle-knots was also designed through phase separation