基于 FRFT-TFDR 的语音通信双绞线故障检测研究

语音通信双绞线故障点定位对断路、短路故障检测,以及防止机密信息窃听等具有重要意义。在基于时域反射法(TDR)和扩频时域反射法(SSTDR)的语音通信双绞线故障检测与定位中,由于双绞线的低频传输特性,反射信号衰减严重,无法实现远距离故障点定位。针对这一问题,提出了基于分数阶傅里叶变换的时频域反射法(FRFT-TFDR)。首先使用线性调频信号(LFM)作为检测信号,接着对接收信号加窗以提高信号的分辨力,然后进行分数阶傅里叶变换将接收信号变换到分数域,最后通过分析入射信号和反射信号谱峰值在u域内的位置差,并将其转换到时域内,从而定位故障位置。经实验验证,该方法可有效定位出不同断路故障点,实测时在1 557 m处的定位误差小于7%,满足语音通信双绞线远程故障检测需求。     

Chiral Seeded Growth of Gold Nanorods Into Fourfold Twisted Nanoparticles with Plasmonic Optical Activity

A robust and reproducible methodology to prepare stable inorganic nanoparticles with chiral morphology may hold the key to the practical utilization of these materials. An optimized chiral growth method to prepare fourfold twisted gold nanorods is described herein, where the amino acid cysteine is used as a dissymmetry inducer. Four tilted ridges are found to develop on the surface of single-crystal nanorods upon repeated reduction of HAuCl₄, in the presence of cysteine as the chiral inducer and ascorbic acid as a reducing agent. From detailed electron microscopy analysis of the crystallographic structures, it is proposed that the dissymmetry results from the development of chiral facets in the form of protrusions (tilted ridges) on the initial nanorods, eventually leading to a twisted shape. The role of cysteine is attributed to assisting enantioselective facet evolution, which is supported by density functional theory simulations of the surface energies, modified upon adsorption of the chiral molecule. The development of R-type and S-type chiral structures (small facets, terraces, or kinks) would thus be non-equal, removing the mirror symmetry of the Au NR and in turn resulting in a markedly chiral morphology with high plasmonic optical activity.     

Multi-Responsive Supercapacitors from Chiral Nematic Cellulose Nanocrystal-Based Activated Carbon Aerogels

The development of long-lived electrochemical energy storage systems based on renewable materials is integral for the transition toward a more sustainable society. Supercapacitors have garnered considerable interest given their impressive cycling performance, low cost, and safety. Here, the first example of a chiral nematic activated carbon aerogel is shown. Specifically, supercapacitor materials are developed based on cellulose, a non-toxic and biodegradable material. The chiral nematic structure of cellulose nanocrystals (CNCs) is harnessed to obtain free-standing hierarchically ordered activated carbon aerogels. To impart multifunctionality, iron- and cobalt-oxide nanoparticles are incorporated within the CNC matrix. The hierarchical structure remains intact even at nanoparticle concentrations of ≈70 wt%. The aerogels are highly porous, with specific surface areas up to 820 m² g⁻¹. A maximum magnetization of 17.8 ± 0.1 emu g⁻¹ with superparamagnetic behavior is obtained, providing a base for actuator applications. These materials are employed as symmetric supercapacitors; owing to the concomitant effect of the hierarchically arranged carbon skeleton and KOH activation, a maximum C_p of 294 F g⁻¹ with a capacitance retention of 93% after 2500 cycles at 50 mV s⁻¹ is achieved. The multifunctionality of the composite aerogels opens new possibilities for the use of biomass-derived materials in energy storage and sensing applications.     

Water-Assisted Growth of Twisted 3R-Stacked MoSe2 Spirals and Its Dramatically Enhanced Second Harmonic Generations

Enhanced second-harmonic generation (SHG) responses are reported in monolayer transition metal dichalcogenides (e.g., MX₂, M: Mo, W; X: S, Se) due to the broken symmetries. The 3R-like stacked MX₂ spiral structures possessing the similar broken inversion symmetry should present dramatically enhanced SHG responses, thus providing great flexibility in designing miniaturized on-chip nonlinear optical devices. To achieve this, the first direct synthesis of twisted 3R-stacked chiral molybdenum diselenide (MoSe₂) spiral structures with specific screw dislocations (SD) arms is reported, via designing a water-assisted chemical vapor transport (CVT) approach. The study also clarifies the formation mechanism of the MoSe₂ spiral structures, by precisely regulating the precursor supply accompanying with multiscale characterizations. Significantly, an up to three orders of magnitude enhancement of the SHG responses in twisted 3R stacked MoSe₂ spirals is demonstrated, which is proposed to arise from the synergistic effects of broken inversion symmetry, strong light–matter interaction, and band nesting effects. Briefly, the work provides an efficient synthetic route for achieving the 3R-stacked TMDCs spirals, which can serve as perfect platforms for promoting their applications in on-chip nonlinear optical devices.     

Magnetic Skyrmion Lattices in a Novel 2D-Twisted Bilayer Magnet

Magnetic skyrmions are topologically protected spin swirling vertices, which are promising in device applications due to their particle-like nature and excellent controlability. Magnetic skyrmions are extensively studied in a variety of materials and proposed to exist in the extreme 2D limit, i.e., in twisted bilayer CrI₃ (TBCI). Unfortunately, the magnetic states of TBCIs with small twist angles are disorderly distributed ferromagnetic and antiferromagnetic (AFM) domains in recent experiments, and thus the method to get rid of disorders in TBCIs is highly desirable. Here, intralayer exchange interactions up to the third nearest neighbors without empirical parameters and very accurate interlayer exchange interactions are used to study the magnetic states of TBCIs. The functions of interlayer exchange interactions obtained using first-principles calculations and stored in symmetry-adapted artificial neural networks are proposed. Based on these, the subsequent Landau–Lifshitz–Gillbert equation calculations explain the disorderly distributed FM-AFM domains in TBCIs with small twist angles and predict the orderly distributed skyrmions in TBCIs with large twist angles. This novel twisted 2D bilayer magnet can be used to design memory devices, monochromatic spin wave generators and many kinds of skyrmion lattices.     

Performance enhancement and optimization for rod-baffle heat exchangers with twisted oval tubes

Twisted oval tubes were proposed to be installed in rod-baffle heat exchangers (RBHXs) to enhance heat transfer performance. A comparison between RBHXs with twisted oval tubes and those with circular tubes is performed. The results show that the heat transfer coefficient (h) per unit pressure drop (Δp) of RBHXs with twisted oval tubes is larger than 36.01%~100.24% compared with that of circular tubes when Reynolds numbers are 4577~22 889, and heat transfer enhancement is more obvious under lower inlet velocity. Then the structural parameters of RBHXs with twisted oval tubes are optimized using the response surface model and genetic algorithms. The results show that the h decreases first and then remains constant with the increase of the twisted pitch length, and decreases first and then rises again with the increase of the ratio of long axis to short axis. The Δp almost keeps constant with increasing twisted pitch length and decreases by 36.67% with the increase in the ratio of long axis to short axis. The comprehensive performance index h/Δp of the RBHXs with twisted oval tubes is enhanced by an average of 26.42% compared with that of original structure. The research results have a good guideline for heat transfer enhancement and the structural design of RBHXs.     

Effect of aspect ratio on the aerodynamic performance and correlation of square section building exposed to twisted wind profile

This study investigates the aerodynamic performance of three square-section buildings with different aspect ratio (AR = 1:1, 1:4, and 1:6) exposed to twisted wind profile (TWP) by pressure measurement test. The effect of AR on the correlation of wind loads specifically for TWP is systematically revealed from both time–frequency domain and local–global perspective. Results show that compared with its counterparts in conventional wind profile (CWP), the effect of AR on aerodynamic load under TWP becomes significantly different and more prominent, which can be categorized into two types of patterns. For pattern low-rise building, TWP is more resembling the condition of CWP with certain attack angle. For pattern high-rise building, TWP results in stronger momentum exchange along building height but suppresses fluctuating feature associated with Karman vortex. As a result, under TWP, mean base moments of all buildings are enhanced except for longitudinal component of case AR = 1:4 and 1:6; while the fluctuating base moment for three AR cases is all reduced, which indicates that dynamic pattern of wake flow is suppressed. Moreover, the discrepancy of local wind load between case CWP and TWP concentrates on the lower-middle location for high-rise building but distributed evenly along all low-rise building height. Additionally, it is found that the effect of AR on aerodynamic correlation exhibits different mechanisms and patterns when building is under the impact of CWP or TWP.