Graphene and other single-layer two-dimensional van der Waals (vdW) crystals have been of great research interest for the past two decades, demonstrating the possible interest for research on single-chain one-dimensional (1D) vdW materials. Furthermore, 1D vdW materials are predicted to have interesting physics and find applications in subnanometre electronic devices, as their sizes in both dimensions are close to physical limits. Here we prepared independent, single-chain 1D vdW crystals in which the single chains feature a definite chemical structure with a width of approximately 0.8 nm, a length reaching the micrometre scale and a single-chain ratio up to 90%. Atomically smooth vdW interfaces resulting from the inherent intrachain closed covalent bonding were observed, accompanied by good crystal quality and air stability. Low temperature electron transport measurements of the 1D vdW chains revealed typical single-electron tunnelling characteristics, demonstrating the possible applications of these materials in quantum technologies.

To meet the requirement for the low disturbance of wind tunnel flow in supersonic boundary layer transition simulation tests, a low-disturbance supersonic research wind tunnel with a nozzle size of 0.3 & times; 0.15 m(2) and test Mach number of 3.0 was built. It was then used to study key design technologies for increasing the size and Reynolds number of low-disturbance supersonic wind tunnel. The wind tunnel adopted a "blowdown + ejection" layout to achieve a wide pressure range of 50-450 kPa. A "sintered wire mesh + honeycomb + damping mesh" structure was adopted in the settling chamber with the flow straightening method to achieve velocity pulsations of less than 0.3% and pressure pulsations of less than 0.1% at the exit. A comprehensive design technique for the supersonic laminar nozzle was adopted, which included nozzle boundary layer suction and highly polished surface nozzle processing. At a total pressure of 450 kPa, a low-disturbance supersonic test area with an axial length of about 9 cm was realized. The airflow pressure pulsations in this area were less than 0.15%, and the Reynolds number based on the characteristic length reached 1.3 & times; 10(6).

Organic semiconductors have long spin lifetimes supporting non‐tunneling spin transport at room temperature, providing a window to control spin transport and enable charge‐spin co‐processing. Programmable diversity in backbones and side chains expands the design space for tuning structure‐transport relationships, yet links between structure and spin transport remain less explored than for charge transport. Focusing on repeat‐unit sequence effects on spin transport, a nonmonotonic dependence where intermediate sequence order optimizes π–π packing by balancing long‐range coherence and chain flexibility, maximizing spin‐transport efficiency is revealed. The three‐component regioregular copolymer tightens π–π spacing to 3.48 Å and extends coherence, yielding mobility 0.43 cm 2 V −1 s −1 and an on/off ratio near 6 × 10 6 . Stronger cohesion in a bicomponent alternating species flattens the backbone yet widens d π and shortens coherence; frontier levels and dihedral angles support this microstructural origin of lower mobility. Electron paramagnetic resonance gives a T1 of 101 ns for the alternating copolymer; spin valves show >8% room‐temperature non‐tunneling magnetoresistance ratio, ≈200% above bicomponent alternating species. This non‐monotonic design rule provides a synthesis strategy to extend spin lifetimes and spin diffusion length, thereby advancing conjugated polymers for applications in logic, memory, sensing, and wearable systems.

The transfer-free character of graphene growth on Silicon Carbide (SiC) makes it compatible with state-of-the-art Si semiconductor technologies for directly fabricating high-end electronics. Although significant progress has been achieved in epitaxial growth of graphene on SiC recently, the underlying nucleation mechanism remains elusive. Here, we present a theoretical study to elucidate graphene near-equilibrium nucleation on Si-terminated hexagonal-SiC(0001) surface. It is found that the ultra-large lattice mismatch between SiC(0001) surface and graphene and the highly localized electron distribution on SiC(0001) surface lead to a distinctive nucleation process: (i) Most of the magic carbon clusters on SiC(0001) show only C1 symmetry and are mainly composed of pentagonal rings; (ii) Two possible nucleation pathways are revealed, i.e., longitudinal and circular modes; (iii) Carbon clusters are more stable on flat terraces than near atomic step edges. Based on above findings, a graphene nucleation diagram on SiC(0001) is established and experimentally observed contradictories for graphene growth on SiC(0001) are answered. Our in-depth understanding on graphene nucleation on SiC(0001) extends nucleation mechanisms of 2D crystals and will benefit high-quality graphene growth on SiC(0001).

Reticular chemistry has been a cornerstone in the design of novel 2D materials. Despite numerous possibilities for topological arrangements, only a few with high symmetry can form stable networks. Here, starting from 2D carbons, four types of highly stable tessellations are discovered, which consist of chains of non-hexagonal rings separated by hexagonal ribbons. A modified Read-Shockley model is established to perfectly describe the stability of these highly stable frameworks, which is based on the interaction between non-hexagonal rings. Moreover, these four types of tessellations and the modified Read-Shockley model are found to be of general validity in designing highly stable 2D materials, which is verified by the calculations on polymorphs of boron nitride and molybdenum disulfide. Besides, among the studied 2D carbon allotropes, two semi-metallic structures with highly anisotropic Dirac cones and one semimetal with a Dirac nodal line at the Fermi level are discovered, as protected by their D2h symmetry. Spin-orbital coupling is further found to open small bandgaps for these three Dirac structures, making them nontrivial topological insulators. The in-depth understanding of the stability of 2D crystals in this study provides a new way for rational design of 2D crystals that may show peculiar electronic structures. Four classes of tessellations composed of chains of non-hexagonal rings separated by nanoribbons of hexagonal rings are established to construct highly stable 2D crystals. Based on the interaction between polygonal rings, a modified Read-Shockley model is further proposed to describe the stability of such 2D crystals, and peculiar topological electronic structures are found. image

The layer and stacking engineering of two-dimensional (2D) transition-metal dichalcogenides (TMDs) gives rise to novel phenomena and multiapplications; thus, TMDs have garnered considerable attention. However, the precisely customized fabrication of stacked 2D materials to date is largely limited to the lack of effective and controllable growth strategies, prone to the unpredictable stacking orders and randomly distributed nucleation sites. Here, we devise an optimized chemical vapor deposition approach for modulating the MoS2 single crystals from monolayer to multilayer with diverse stacking configurations. Significantly, the phototransistor based on monolayer MoS2 single crystal exhibits an ultrasensitive performance with a high photoresponsivity (R) of 3.3 × 104 A W-1 and a remarkable detectivity (D*) of above 1.7 × 1014 Jones at 405 nm light illumination. Ultralow-frequency and angle-resolved polarized Raman spectroscopy is used to systematically uncover the delicate interlayer interactions and crystallographic anisotropy. Moreover, the polarization-sensitive photodetectors using 1-3L MoS2 show a layer number-dependent anisotropic performance, with dichroism ratios of 1.36, 1.44, and 1.52. This work offers a promising method to not only enable the fabrication of new customized layer-, stacking-, and twist-2D materials but also provides the foundation for the development of advanced polarization-sensitive and optoelectronic devices based on stacking transitions.

2D nonlayered materials (NLMs) have garnered considerable attention due to unique surface structure and bright application prospect. However, owing to the strong interatomic forces caused by intrinsic isotropic chemical bonds in all directions, the direct synthesis of ultrathin and large area 2D NLMs remains a tremendous challenge. Here, the surface-assisted passivation growth strategy is designed to synthesize ultrathin and large size β-Bi2O3 crystals with the thickness down to 0.77 nm and the lateral size up to 163 µm. These results are primarily ascribed to the bonding between Se atoms and the unsaturated Bi atoms on the surface of β-Bi2O3, resulting in the surface passivation and promoting the obtaining of ultrathin β-Bi2O3. Strikingly, the photodetectors based on β-Bi2O3 flakes exhibit a high photoresponsivity of 71.91 A W-1, an excellent detectivity of 6.09 × 1013 Jones, a remarkable external quantum efficiency of 2.4 × 104%, an outstanding anisotropic photodetection and excellent UV imaging capability at 365 nm. This work sheds light on the synthesis of 2D ultrathin NLMs and promotes their applications in multifunctional optoelectronics.

This study examines boundary layer control and pressure recovery in low Reynolds number supersonic flow with chemical reactions in a chemical laser system. Our work prescribes a novel boundary layer control method for the optical cavity of a chemical laser system, and a design of a supersonic diffuser is compared and proposed to make a stable flow for the system. The flow characteristics of a low Reynolds number and internal reaction heat release were analyzed. Three types of experimental pieces were designed to passively control the boundary layer in the optical cavity. An active booster-type supersonic diffuser is proposed to study the pressure recovery problem of a low Reynolds number and chemical reaction supersonic flow generated by an optical cavity. A supersonic chemical reaction platform (SCRP) was established to conduct experimental research on boundary layer control and docking the active booster supersonic diffuser with the SCRP. The experimental results indicate that increasing the boundary layer pumping capacity within a certain range can reduce both the boundary layer thickness and the pressure on the optical cavity while simultaneously enhancing the SCRP energy power. The supersonic diffuser based on active gas pressurization can create the necessary conditions for the normal chemical reaction and improve the ability of the SCRP to resist high back pressure and airflow disturbance. Moreover, the chemical reaction energy release was full and stable with the docking of supersonic diffuser test pieces, resulting in energy power increases, which could be a significant improvement for the design of chemical laser systems.

为有效解决现有燃气引射气源存在的诸多缺点,适应一种引射系统大流量、小型化的需要,在综合分析各燃气引射气源方案的基础上,研制了一种以空气、酒精作为推进剂的多点喷射结构的燃气发生器,并开展了多种工况下的试验研究.结果表明:采用的多点喷射方案大大提高了空间利用率,有效满足了引射系统小型化的要求;喷雾性能好,喷雾锥角及粒径优于设计指标要求;点火可靠性高,解决了领域内现有燃气发生器点火可靠性低的缺点;点火迅速,燃烧平稳,可实现较为均匀的出口温度场;燃气发生器工作范围较宽,能在余气系数2.52～4.34 范围内稳定燃烧;高效的火焰筒内壁空气冷却方式有效保证了燃气发生器长时间工作运行.

Alkali-metal modification and transition-metal doping are effective strategies to dislodge the inertia of MoO3 and obtain greatly reduced OER overpotentials.

为有效解决高能化学激光器压力恢复系统无法实现小型化及机动性能等问题,提出了以高温气源替代常温空气作为引射气源的方案,并设计了一种基于航空发动机环形燃烧室结构的空气/酒精燃气发生器.采用CFD计算软件开展了燃气发生器的流场仿真计算,获取了燃气发生器冷态及燃烧工况下的速度、压力与温度场,并将部分仿真计算结果与理论计算及试验值进行了对比.研究表明:燃气发生器设计合理,扩压器及二股腔道转接处无气流分离,帽罩处无溢流现象;主燃区回流区明显,且尺度适中,有利于组织燃烧,并有效指导了高能电火装置的位置布局;空气流量配比合理,主燃孔及掺混孔的射流深度满足燃烧及掺混要求;燃烧室总压恢复系数达到9 6.7％,燃烧效率实现98.4％,实现了高效燃烧;此外,相同工况下的理论计算值及试验结果验证了数值计算方法的合理可行性.

A new mechanism of urea synthesis from N2O + CO is proposed, and potential DACs TM2/g-CN with high stability, activity and selectivity towards urea formation are screened out.

It is presently imperative to find efficient and practical catalysts for the hydrogen evolution reaction (HER) for hydrogen production with high efficiency. In this work, the catalytic HER activity of a class of two-dimensional (2D) metal-organic frameworks (MOFs), i.e., M-3(C6O3S3)(2) with M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Tc, Ru, Ta, W, and Re, is studied by using first-principles calculations. It is found that nonmetal atoms of the first coordination sphere to the central metal atom could be the active site for HER, and V-3(C6O3S3)(2) (delta G(H*) = 0.02 eV, on V), Cr-3(C6O3S3)(2) (delta G(H*) = -0.02 eV, on S, and delta G(H*) = -0.05 eV, on O), and Cu-3(C6O3S3)(2) (delta G(H*) = -0.03 eV, on S) are screened out as promising HER catalysts. To complete the picture of how the metal-ligand matching affects the activity, a different coordination microenvironment is considered by regulating the first coordination shell to, for example, the central V atom. It is of interest that the newly constructed moieties present high catalytic HER activity with delta G(H*) almost zero, proving the significance of metal-ligand interaction. In view of these results, we propose a descriptor delta epsilon(up arrow down arrow) that correlates the local electronic structure and the catalytic HER activity. In short, our results not only identify a series of efficient HER electrocatalysts but also unravel the underneath factors that affect the activity and thus provide new insights into the rational design of catalysts for other reactions.

针对化学氧碘激光器(Chemical Oxygen-Iodine Laser,COIL)光腔内边界层的被动控制方法,设计了三种实验件.光腔的上下壁板是可拆卸的,可以更换不同的实验件,以此比较边界层的控制效果.实验结果表明:开槽板、主流引射缝和开孔板在对光腔边界层的控制上都取得了一定的效果,改善了光腔特别是光腔后半部分的压力分布.在一定范围内,增加边界层的抽气量,可以进一步减小边界层厚度,降低光腔压力,同时提高COIL出光功率,但是当抽气量过大时,反而会降低出光功率.三种实验件中,主流引射的方式对抽气量最敏感,当抽气量增加至5％时,COIL出光功率已经明显下降;开孔板对抽气量不太敏感,抽气量从1％增加至7％,对COIL出光功率的影响并不明显.

NH 3 synthesis from NO occurs spontaneously on the Cu 2 Si monolayer and the importance of p-block elements in electrocatalysis is revealed.

航空发动机可以对空气增压,并且增加气流温度,理论上存在应用于超声速引射系统的可能.分析了气源对引射器性能的影响以及引气对航空发动机的影响,介绍了3种航空发动机在超声速引射系统中可能的布局方案.针对某领域内的排气参数要求,分别对3种布局方案进行了计算分析.计算结果表明,当上游气体压强为0.044×105 Pa和0.0293×105 Pa时,通过合理选择发动机的布局以及工作参数,发动机可以直接将上游气体排出或者作为驱动气源应用于超声速引射系统.

为满足先进涡扇发动机对变雷诺数平面叶栅试验的需求,设计了亚/跨/超声速来流高效变换、雷诺数和马赫数独立调节、压气机和涡轮平面叶栅试验为一体、换热与冷却试验能力兼具的变密度平面叶栅风洞,提出了风洞的总体设计方案.文章详细介绍了风洞引射器、半柔壁喷管及试验舱等部件设计问题,分析了流场调试及典型叶栅试验结果.调试结果表明:采用的部件设计技术实现了变密度平面叶栅风洞的主要功能,试验雷诺数可低至3.1×105 m–1,具备开展低雷诺数平面叶栅试验的能力.风洞流场调试结果满足《低速风洞和高速风洞流场品质要求》(GJB 1179A—2012),为研究亚/跨/超声速压气机和涡轮叶栅低雷诺数流动问题提供了重要试验平台.

According to the two-dimensional flow theory of the axial fan rotor blades and the aerodynamic characteristics of the low speed wind tunnel, combined with the fan aerodynamic efficiency and wind tunnel pressure loss coefficient, a new equation which points out the inherent relationship of the fan blade setting angle, fan rotating speed and flow rate in wind tunnel circuit is derived. So a new method for fan rotor blade setting angle adjustment to satisfy the fan performance at off-design point by getting the test results of fan operating parameters but without the fan total pressure rise in the low speed wind tunnel is developed. Following the new method, the fan rotor blade setting angle adjusting value was provided directly only with the fan rotating speed and flow velocity in the wind tunnel test section, the adjusting target was achieved successfully by the new blade setting angle, the cost of the wind tunnel commissioning test were saved. The test results show that, after increasing the fan rotor blade setting angle by 4.5 degrees, when the flow velocity in the wind tunnel test section reaches 60m/s, the fan rotating speed is 570rpm, the deviation from the predicted fan rotating speed value of 575rpm is 0.9%. For the same test section flow velocity, the predicted value and the real value of the fan rotating speed are in good agreement, it proves that this method is reliable and accurate in practical application.

Efficient cleaning of pollutants on the optical lens surface is a very important and urgent research topic in the field of laser processing. Therefore, based on the free jet theory of fluid dynamics, the flow field characteristics of the optical system lens are theoretically analyzed and numerically simulated, and the flow field distribution of the lens surface and far field is given. And then, combined with the existing solid particle pollutants adhesion type, influence law and calculation model, the adhesion force of solid particles with different diameters is calculated quantitatively. According to the gas dynamic pressure distribution on the surface of optical elements, the effectiveness of solid particle blowing on the surface of optical elements is analyzed and evaluated. Finally, according to the secondary dust phenomenon of mirror free jet purging, the jet outlet size, downstream collection port size and jet velocity that affect the flow field distribution characteristics of the lens surface are optimized. Finally, a self-priming mirror jet purging structure without backflow is given, which effectively eliminates the secondary dust phenomenon caused by mirror purging.

压缩空气弹射广泛应用于航空航天、舰船、陆基武器系统等领域,具有弹射速度快、无需热防护、可重复利用、安全环保及通用性能好等优势.通过对大量文献进行分类总结,概述了压缩空气弹射的应用研究领域,简要分析了压缩空气弹射在各行业内应用过程中存在的关键技术及发展前景.研究表明:研制高效、小型化的压缩空气弹射系统是各行业各领域发展的必然趋势,相关结果为压缩空气弹射的应用与研究提供有利参考.