梯级溃坝洪水洪峰增强机制

我国在多条河流上修建了大量梯级水库, 梯级坝溃决诱发洪水大大超过单坝溃决洪水洪峰, 因此亟需加深对梯级坝溃决洪水洪峰增强机制的认识. 本文建立了梯级坝溃决洪水演进过程的一维浅水动力学模型, 发展了一套能捕捉激波、干湿边界和保平衡结构的数值求解方法, 通过大量算例, 系统研究了梯级坝溃决洪水演进过程的质量转化和能量转化机制. 研究结果表明, 梯级溃决中, 上游溃决诱发的洪水大大增大下游水库的质量和动量, 形成一个带动量的水塔, 同时在尾部残留一个动量较大的射流, 不断补充下游坝体溃决后水塔的质量和动量, 持续维持洪峰高度. 根据该射流-水塔机制, 建立了梯级坝溃决洪水演进过程对应的射流-水塔单坝溃决洪水过程等效模型, 该等效模型基本反映了梯级坝溃决诱发洪水的洪峰过程, 并成功预测了多个坝间距为百公里量级的梯级坝溃决洪水洪峰高程和流量, 可望为流域防洪和梯级坝设计提供理论依据.      

Multi-stable quantum droplets in optical lattices

Nonlinear Dynamics - We address the nonlinear dynamics of binary Bose-Einstein condensates with mutually symmetric spinor components trapped in an optical lattice. The interaction between the...     

A Comparison of Two Isoreticular Metal–Organic Frameworks with Cationic and Neutral Skeletons: Stability,Mechanism, and Catalytic Activity

Although many ionic metal–organic frameworks (MOFs) have been reported, little is known about how the charge of the skeleton affects the properties of the MOF materials. Herein we report how the chemical stability of MOFs can be substantially improved through embedding electrostatic interactions in structure. A MOF with a cationic skeleton is impervious to extremely acidic, oxidative, reductive, and high ionic strength conditions, such as 12 m HCl (301 days), aqua regia (86 days), H₂O₂ (30 days), and seawater (30 days), which is unprecedented for MOFs. DFT calculations suggested that steric hinderance and the repulsive interaction of the cationic framework toward positively charged species in microenvironments protects the vulnerable bonds in the structure. Diverse functionalities can be bestowed by substituting the counterions of the charged framework with identically charged functional species, which broadens the horizon in the design of MOFs adaptable to a demanding environment with specific functionalities.     

Palladium-Catalyzed Electrophilic Functionalization of Pyridine Derivatives through Phosphonium Salts

Herein, we report a highly efficient and practical method for pyridine-derived heterobiaryl synthesis through palladium-catalyzed electrophilic functionalization of easily available pyridine-derived quaternary phosphonium salts. The nice generality of this reaction was goes beyond arylation, enabling facile incorporation of diverse carbon-based fragments, including alkenyl, alkynyl, and also allyl fragments, onto the pyridine core. Notably, the silver salt additive is revealed to be of vital importance for the success of this transformation and its pivotal role as transmetallation mediator, which guarantees a smooth transfer of pyridyl group to palladium intermediate, is also described.     

Direct Intermediate Regulation Enabled by Sulfur Containers in Working Lithium–Sulfur Batteries

Polysulfide intermediates (PSs), the liquid-phase species of active materials in lithium–sulfur (Li-S) batteries, connect the electrochemical reactions between insulative solid sulfur and lithium sulfide and are key to full exertion of the high-energy-density Li-S system. Herein, the concept of sulfur container additives is proposed for the direct modification on the PSs species. By reversible storage and release of the sulfur species, the container molecule converts small PSs into large organosulfur species. The prototype di(tri)sulfide-polyethylene glycol sulfur container is highly efficient in the reversible PS transformation to multiply affect electrochemical behaviors of sulfur cathodes in terms of liquid-species clustering, reaction kinetics, and solid deposition. The stability and capacity of Li-S cells was thereby enhanced. The sulfur container is a strategy to directly modify PSs, enlightening the precise regulation on Li-S batteries and multi-phase electrochemical systems.     

A Robust Mixed-Lanthanide PolyMOF Membrane for Ratiometric Temperature Sensing

Temperature sensors play a significant role in biology, chemistry, and engineering, especially those that can work accurately in a noninvasive manner. We adopted a photoinduced post-synthetic copolymerization strategy to realize a membranous ratiometric luminescent thermometer based on the emissions of two lanthanide ions. This novel mixed-lanthanide polyMOF membrane exhibits not only the integrity and temperature sensing behaviour of the Ln-MOF powder but also excellent mechanical properties, such as flexibility, elasticity, and processability. Moreover, the polyMOF membrane shows remarkable stability under harsh conditions, including high humidity, strong acid and alkali (pH 0–14), which allowed the mapping of temperature distributions in extreme circumstances. This work highlights a simple strategy for polyMOF membrane formation and pushes forward the further practical application of Ln-MOF-based luminescent thermometers in various fields and conditions.     

微波瑞利散射法测定空气电火花激波等离子体射流的时变电子密度

大气压空气电火花激波等离子体射流的电子密度在亚微秒时间尺度上瞬变,其电子密度的测定很难.基于微波瑞利散射原理,本文测量了空气电火花冲击波流注放电等离子体射流的时变电子密度.实验结果表明:测量系统的标定参数A为1.04 × 105 V·Ω·m–2;空气流注放电等离子体射流的电子密度与等离子体射流的半径和长度有关,结合高速放电影像展示的等离子体射流的等效半径和等效长度,测定的电子密度在1020 m–3的量级,且随时间先快速增长至峰值再成指数衰减.此外,本文还探讨了等离子体射流的不同等效尺度对测定结果的影响;分析结果表明,采用时变等效半径和时变等效长度的计算结果最有效,且第1个快速波峰是由光电离的电离波导致的.     

Investigation of β-Ga_2O_3 films and β-Ga_2O_3/GaN heterostructures grown by metal organic chemical vapor deposition

In this work,(-201) β-Ga_2O_3 films are grown on GaN substrate by metal organic chemical vapor deposition(MOCVD). It is revealed that the β-Ga_2O_3 film grown on GaN possesses superior crystal quality, material homogeneity and surface morphology than the results of common heteroepitaxial β-Ga_2O_3 film based on sapphire substrate. Further, the relevance between the crystal quality of epitaxial β-Ga_2O_3 film and the β-Ga_2O_3/GaN interface behavior is investigated. Transmission electron microscopy result indicates that the interface atom refactoring phenomenon is beneficial to relieve the mismatch strain and improve the crystal quality of subsequent β-Ga_2O_3 film. Moreover, the energy band structure of β-Ga_2O_3/GaN heterostructure grown by MOCVD is investigated by X-ray photoelectron spectroscopy and a large conduction band offset of 0.89 eV is obtained. The results in this work not only convincingly demonstrate the advantages of β-Ga_2O_3 films grown on GaN substrate, but also show the great application potential of MOCVD β-Ga_2O_3/GaN heterostructures in microelectronic applications.     

衬底温度对Ga液滴在Al_(0.4)Ga_(0.6)As表面形成纳米结构的影响

本文研究了不同衬底温度对Ga液滴在Al_(0.4)Ga_(0.6)As表面形成纳米结构的影响,当300℃≤T≤380℃时,Ga液滴演化成纳米孔(Nanohole)和盘状结构(diffusion halo),纳米结构的尺寸随温度升高而增大.当T≥385℃时,盘状结构消失,形成一定平坦的Al_xGa_(1-x)As薄膜,Ga液滴在界面处继续向下刻蚀直至耗尽,形成平均直径为75 nm,平均孔深为5.52 nm的纳米孔.本文还通过盘状结构测出平均扩散长度△R,并拟合出Ga原子在Al_(0.4)Ga_(0.6)As表面的激活能E_A=0.78(±0.01) eV和扩散前因子D_0=0.15(×4.1~(±1))10~(-2 ) cm~2s~(-1).     

Chameleonlike metashells in microfluidics: A passive approach to adaptive responses

Heat energy can transfer via convection, conduction, and radiation. Based on convection and conduction in microfluidics, people have designed and fabricated many novel devices. However, almost none of them has adaptivity, thus restricting practical applications under different conditions. To solve this problem, here we propose a passive approach to adaptive responses. That is,we consider the thermal convection-conduction process in microfluidic structures where Darcy’s law and Fourier’s law are both valid. By carefully designing two key parameters(i.e., tensorial thermal conductivity and tensorial permeability) of a metashell,we theoretically reveal that its effective properties(i.e., effective thermal conductivity and effective permeability) can adaptively change according to the inside object, thus yielding the "chameleonlike metashell". Further, this metashell is passive since it requires no prior knowledge of the inside object. We also report that the chameleonlike behavior can occur for anisotropic inside objects, nonuniform external fields, or even complex shapes. All theoretical analyses agree well with finite-element simulations.The chameleonlike metashell can act as an intelligent metamaterial in microfluidics for its adaptive responses, and it can also benefit other physical fields where convection plays a role, such as mass diffusion.