Synthesis and microwave absorption performance of heat-treated RF/SiO_(2)aerogels

The well-designed composite with satisfactory electromagnetic microwave absorption at high temperatures remains a serious challenge.Herein,we fabricated a resorcinol-formaldehyde/silica dioxide composite aerogel(RF/SiO_(2))with a three-dimensional network structure using sol-gel,atmospheric pressure drying technique as well as heat-treated processes to achieve enhanced microwave absorption capabilities in the low frequency range.The pristine RF/SiO_(2)aerogel presented a typical micropores structure with a surface area,porous volume,and density of 146.82 m^(2)/g,62.40%,and 0.28 cm^(3)/g,respectively.Remarkably,the RF/SiO_(2)aerogel showed an effective absorption bandwidth of 3.56 GHz and a minimum reflection loss value of-46.10 d B at 2.25 mm after being heat-treated at 1500°C,while the maximum effective absorption bandwidth was 3.60 GHz at 2.30 mm.The intricate three-dimensional networks possessed remarkable impedance matching,multiple attenuation mechanisms,interfacial polarization,and dielectric loss,which were attributed to the exceptional ability to absorb electromagnetic microwaves.It offered a fresh approach to creating adaptable and effective microwave absorption materials in military defense.

Simultaneously realizing thermal andelectromagnetic cloaking by multi-physicalnull medium

Simultaneously manipulating multiple physical fields plays an important role in the increasingly complex integrated systems,aerospace equipment,biochemical productions,etc.For on-chip systems with high integration level,the precise and efficient control of the propagation of electromagnetic waves and heat fluxes simultaneously is particularly important.In this study,we propose a graphical designing method(i.e.,thermal-electromagnetic surface transformation)based on thermal-electromagnetic null medium to simultaneously control the propagation of electromagnetic waves and thermal fields according to the pre-designed paths.A thermal-electromagnetic cloak,which can create a cloaking effect on both electromagnetic waves and thermal fields simultaneously,is designed by thermal-electromagnetic surface transformation and verified by both numerical simulations and experimental measurements.The thermal-electromagnetic surface transformation proposed in this study provides a new methodology for simultaneous controlling on electromagnetic and temperature fields,and may have significant applications in improving thermal-electromagnetic compatibility problem,protecting of thermal-electromagnetic sensitive components,and improving efficiency of energy usage for complex onchip systems.

Dynamic Resistance and Energy Absorption of Sandwich Beam via a Micro-Topology Optimization

The current research of sandwich structures under dynamic loading mainly focus on the response characteristic of structure.The micro-topology of core layers would sufficiently influence the property of sandwich structure.However,the micro deformation and topology mechanism of structural deformation and energy absorption are unclear.In this paper,based on the bi-directional evolutionary structural optimization method and periodic base cell(PBC)technology,a topology optimization frame work is proposed to optimize the core layer of sandwich beams.The objective of the present optimization problem is to maximize shear stiffness of PBC with a volume constraint.The effects of the volume fraction,filter radius,and initial PBC aspect ratio on the micro-topology of the core were discussed.The dynamic response process,core compression,and energy absorption capacity of the sandwich beams under blast impact loading were analyzed by the finite element method.The results demonstrated that the overpressure action stage was coupled with the core compression stage.Under the same loading and mass per unit area,the sandwich beam with a 20%volume fraction core layer had the best blast resistance.The filter radius has a slight effect on the shear stiffness and blast resistances of the sandwich beams.But increasing the filter radius could slightly improve the bending stiffness.Upon changing the initial PBC aspect ratio,there are three ways for PBC evolution:The first is to change the angle between the adjacent bars,the second is to further form holes in the bars,and the third is to combine the first two ways.However,not all three ways can improve the energy absorption capacity of the structure.Changing the aspect ratio of the PBC arbitrarily may lead to worse results.More studies are necessary for further detailed optimization.This research proposes a new topology sandwich beam structure by micro-topology optimization,which has sufficient shear stiffness.The micro mechanism of structural energy absorption is clarified,it is significant for structural energy absorption design.

Microwave absorption properties of Ni/C@SiC composites prepared by precursor impregnation and pyrolysis processes

In the present study,the unique three-dimensional graphene coated nickel(Ni/C)foam reinforced silicon carbide(Ni/C@SiC)composites were first obtained via the precursor impregnation and pyrolysis(PIP)processes.The microstructure images indicated that the SiC fillers were successfully prepared in the skeleton pores of the Ni/C foam.The influence of the PIP cycles on the microwave absorption performances was researched,and the results indicated that after the primary PIP process,Ni/C@SiC-I possessed the optimal microwave absorbing performance with a minimum reflection loss(RL)of-25.87 d B at 5.28 GHz and 5.00 mm.Besides,the RL values could be below-10.00 dB from 5.88 GHz to 7.74 GHz when the corresponding matching thickness was 3.85 mm.However,the microwave absorption properties of Ni/C@SiC-II and Ni/C@SiC-Ⅲwere tremendously degraded as the PIP times increased.At last,the electromagnetic parameter,dielectric loss,attenuation constant as well as impedance matching coefficient were further investigated to analyze the absorbing mechanism,which opened a new path for the certain scientific evaluation of the absorbing materials and had extremely important to the defence technology.

连续亚麻纤维增强聚乳酸预浸带的制备及性能

采用自行设计和组装的连续纤维增强热塑性复合材料实验流水线制备了连续亚麻纤维增强聚乳酸(C-Flax/PLA)预浸带。通过力学性能、动态力学、傅里叶变换红外光谱、差示扫描量热及偏光显微镜分析等方法研究了C-Flax及相容剂对预浸带性能及结晶行为的影响。力学性能分析表明,C-Flax及相容剂对PLA材料的增强效果明显;当纤维的质量分数为55%时,预浸带拉伸强度达到235.75 MPa,较PLA提高了300%,已进入碳素结构钢的行列。动态力学性能分析表明,C-Flax/PLA预浸带的储能模量较PLA显著提高,且玻璃化转变温度向高温移动,预示着材料抵抗外力变形能力明显提升。结晶行为研究表明,加入C-Flax后,材料的Avrami指数(n)明显提高,通过与晶体生长速率常数(Z_c)及偏光显微镜图像比较发现,C-Flax起到了诱导结晶的作用,但阻碍了PLA晶体的生长。

Fabrication and catalytic behavior of hierarchically-structured nylon 6 nanofiber membrane decorated with silver nanoparticles

A hierarchically‐structured nylon 6 (PA6) nanofiber membrane decorated with silver nanoparticles (Ag NPs) was fabricated by electrospinning and impregnation methods. The as‐fabricated hierarchically‐structured Ag/PA6 nanofiber membrane (HS‐Ag/PA6 NM) exhibits a morphology in which Ag NPs are deposited on the surfaces of both thick fibers and thin fibers. The content and size of theAg NPs can be controlled by varying the concentration of the silver colloid solution. Compared with the non‐hierarchically‐structured Ag/PA6 nanofiber membrane, HS‐Ag/PA6 NM has a higher specificsurface area and exhibits a higher degradation rate for methylene blue of 81.8%–98.1% within2 h. HS‐Ag/PA6 NM can be easily recycled and exhibits good reusability. It retains a degradation rate for methylene blue of 83.5% after five consecutive cycles. The hierarchically‐structured nanofiber membrane is therefore a potential nanocatalyst.

Comparison of the osteogenic capability of rat bone mesenchymal stem cells on collagen,collagen/hydroxyapatite,hydroxyapatite and biphasic calcium phosphate

Collagen(COL),collagen/hydroxyapatite(COL/HA),HA and biphasic calcium phosphate were prepared as representative bone grafting materials with composition analogous to bone,and their structural characteristics were analyzed.The rat bone mesenchymal stem cells(BMSCs)were further seeded onto four groups of materials,and BMSCs grown in basic medium and standard osteogenic medium were set as controls of a reference model to show the basic and osteogenic behavior of cells without the intervention of materials.Cellular behaviors were characterized,including proliferation,spreading morphology and expression of osteogenesis factors.The rat BMSCs proliferated properly with time on four groups of materials as well on two groups of controls,and typical cuboidal,polygonal and extremely-elongated morphologies of cells were observed.According to the real-time polymerase chain reaction data,a higher osteogenic gene expression level was dependent upon the growing morphology but not the proliferation rate of cells,and the osteogenic differentiation capacity of cells onto four groups of materials varied in specific genes.In general,BMSCs exhibited the highest osteogenic capacity onto COL/HA,but the poorest onto HA.The growing behaviors of cells on materials were further discussed in comparison with the cases of OC and BC of the reference model.The present attempt to comparatively analyze cell experimental data with a reference model is expected to be useful for revealing the difference in the osteogenic capability of MSCs onto materials or even the bioactivity of materials.

激光雷达探测对流层气溶胶消光系数的统计分布

集后向散射激光雷达、侧向散射激光雷达及拉曼散射激光雷达为一体的激光雷达系统在探测对流层气溶胶消光系数时具有两个明显的优点,一是反演时不需要假设激光雷达比(LR),二是不存在近地面的过渡区和盲区。在2017年1月至2018年12月期间,利用一体化激光雷达系统在合肥进行了146天的数据探测,并对气溶胶消光系数进行了反演和统计分析,得出了气溶胶LR平均值为68.4 sr及LR平均值随月份的分布规律,还有气溶胶消光系数月平均、季平均及年平均情况。个例分析表明,高度在0.6 km以下的气溶胶消光系数随高度和时间的变化很复杂,传统后向散射激光雷达很难探测到。对比分析了利用LR经验值反演气溶胶消光系数引起的误差大小。这些探测结果为研究大气污染传输和大气污染防治提供了科学依据。

A review on China's constructed wetlands in recent three decades:Application and practice

Constructed wetlands(CWs)have been introduced to and developed in China for environmental engineering over the most prosperous three decades(1990–2020).To study the origin,development process,and future trend of CWs,this review summarized a wide range of literatures between 1990 and 2020 by Chinese authors.Firstly,the publication number over years,research highlights,and the author contributions with the most published papers in this field were conducted through bibliometric analysis.Secondly,the most principal components of CWs,substrates and macrophytes were summarized and analyzed.Thirdly,the typical application cases from traditional CWs,pond systems to combined pond-wetland systems were presented.In China,CWs were predominately distributed in the east of the so-called'Hu Huanyong Line'.Therefore CWs were limited by the socio-economic level and climatic conditions.It is unquestionable that the overall level of China's CWs has improved significantly,and one of the most prominent features has started towards the plural pattern development.There has been a trend of large-scale or low-cost CW application in the recent years.However,lifecycle research and management are required for better strategies in the future.

Concise and efficient direct-view generation of arbitrary cylindrical vector beams by a vortex half-wave plate

A concise, efficient, and practical direct-view scheme is presented to generate arbitrary cylindrical vector(CV)beams, including CV beams, vortex beams, and cylindrical vector vortex(CVV) beams, by a vortex half-wave plate(VHP). Six kinds of first-order and other high-order CV beams, such as azimuthally polarized(AP) beams, antivortex radial polarization mode beams, and three-order AP beams, are formed by simply rotating a half-wave plate. The Stokes parameters and double-slit interference of multitype CV beams are investigated in detail.The polarization parameters, including degree of polarization, polarization azimuth, and ellipticity, are obtained,which demonstrates the efficient generation of CV beams. In addition, the double-slit interference experiment is introduced in the setup, and fringe misplacement and tilt appear for CVV beams, in which the misplacement number M is 2P+1 for P ≤ 2 and 2P-1 for P ≥ 3, where P is the polarization order number, and the fringe tilt offset is positively related to the topological charge number l of CVV beams. In addition, new types of VHPs can be formed by cascading two or more VHPs when the types of available VHPs are limited, assisting in more flexible generation of multitype CV beams. It is experimentally demonstrated that arbitrary CV beams with high quality are effectively achieved by the proposed setup, and the double-slit interference method can be utilized to determine and analyze CV beams rapidly and concisely by practical performance, which shows the potential to be implemented as a commercial device.

Highly Oxygenated Dimeric Grayanane Diterpenoids as Analgesics:TRPV1 and TRPA1 Dual Antagonists from Rhododendron molle

Six highly oxygenated dimeric grayanane diterpenoids(1—6)including three new ones,bismollethers A—C(1—3),were isolated from the flowers of Rhododendron molle collected at Qichun,Hubei,China.The structures and relative configurations of 1—6 were determined by comprehensive spectroscopic data analysis and 13C NMR calculation with DP4+analysis.The absolute configurations of bismollethers A—C(1—3)and birhodomollein B(5)were indubitably assigned by single-crystal X-ray diffraction analysis,revealing the head-to-tail linking manner of diterpenoid monomers.Bismollether A(1)is a caged dimeric grayanane diterpenoid linked through two oxygen-bridges of C-2−O−C-14′and C-14−O−C-2′,featuring a unique 1,8-dioxacyclotetradecane motif.The plausible biosynthesis pathways of 1—6 are proposed.All the isolates displayed significant analgesic activities and their structure-activity relationships were discussed.Grayanane diterpenoid dimers were found to be TRPV1 and TRPA1 dual antagonists,and docking studies provide a structural basis to design potent analgesics.

Epoxymicranthols A-N,5,9-Epoxygrayanane Diterpenoids as Potent Analgesics from Rhododendron micranthum

Main observation and conclusion Fifteen 5,9-epoxygrayanane diterpenoids(1-15)including fourteen new ones,epoxymicranthols A-N(1-14),were isolated from the leaves extract of Rhododendron micranthum.Their structures were elucidated via extensive spectroscopic methods and ^(13)C NMR-DP4+analysis,and the absolute configurations of 1,3-10,14,and 15 were confirmed by single-crystal X-ray diffraction analysis.