Theoretical Analysis and Design of Ultrathin Broadband Optically Transparent Microwave Metamaterial Absorbers

Optically Transparent Microwave Metamaterial Absorber (OTMMA) is of significant use in both civil and military field. In this paper, equivalent circuit model is adopted as springboard to navigate the design of OTMMA. The physical model and absorption mechanisms of ideal lightweight ultrathin OTMMA are comprehensively researched. Both the theoretical value of equivalent resistance and the quantitative relation between the equivalent inductance and equivalent capacitance are derived for design. Frequency-dependent characteristics of theoretical equivalent resistance are also investigated. Based on these theoretical works, an effective and controllable design approach is proposed. To validate the approach, a wideband OTMMA is designed, fabricated, analyzed and tested. The results reveal that high absorption more than 90% can be achieved in the whole 6~18 GHz band. The fabricated OTMMA also has an optical transparency up to 78% at 600 nm and is much thinner and lighter than its counterparts

The Dynamic Change of Vegetation Cover and Associated Driving Forces in Nanxiong Basin, China

Natural climate change and human activities are the main driving forces associated with vegetation coverage change. Nanxiong Basin is a key ecosystem-service area at the national level with a dense population and highly representative of red-bed basins, which are considered as fragile ecological units in humid regions. In this study, the authors aimed to determine the trends in vegetation cover change over past two decades and the associated driving forces in this study area. The Normalized Difference Vegetation Index (NDVI) of 2000–2015, derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) remote sensing dataset along with the application of statistical methods and GIS (geographic information system) techniques were used to quantify vegetation cover change. The results show that human-induced factors can explain most variations at sites with significant cover change. That is to say that human activities are the main drivers of vegetation dynamics in this study area, which shows a significant reduction trend in vegetation cover during the industrialization and urbanization processes of the study period and noticeable recovery trend in 2000–2015 under the plantation and enclosed forest policy

Kinetic Stability of the Streptavidin–Biotin Interaction Enhanced in the Gas Phase

Results of the first detailed study of the structure and kinetic stability of the model high-affinity protein–ligand interaction between biotin (B) and the homotetrameric protein complex streptavidin (S₄) in the gas phase are described. Collision cross sections (Ω) measured for protonated gaseous ions of free and ligand-bound truncated (residues 13–139) wild-type (WT) streptavidin, i.e., S₄^<i>n</i>+ and (S₄+4B)^<i>n</i>+ at charge states <i>n</i> = 12–16, were found to be independent of charge state and in agreement (within 10%) with values estimated for crystal structures reported for S₄ and (S₄+4B). These results suggest that significant structural changes do not occur upon transfer of the complexes from solution to the gas phase by electrospray ionization. Temperature-dependent rate constants were measured for the loss of B from the protonated (S₄+4B)^<i>n</i>+ ions. Over the temperature range investigated, the kinetic stability increases with decreasing charge state, from <i>n</i> = 16 to 13, but is indistinguishable for <i>n</i> = 12 and 13. A comparison of the activation energies (<i>E</i>_a) measured for the loss of B from the (S₄+4B)¹³⁺ ions composed of WT streptavidin and five binding site mutants (Trp79Phe, Trp108Phe, Trp120Phe, Ser27Ala, and Tyr43Ala) suggests that at least some of the specific intermolecular interactions are preserved in the gas phase. The results of molecular dynamics simulations performed on WT (S₄+4B)¹²⁺ ions with different charge configurations support this conclusion. The most significant finding of this study is that the gaseous WT (S₄+4B)^<i>n</i>+ ions at <i>n</i> = 12–14, owing to a much larger <i>E</i>_a (by as much as 13 kcal mol^–1) for the loss of B, are dramatically more stable kinetically at 25 °C than the (S₄+4B) complex in aqueous neutral solution. The differences in <i>E</i>_a values measured for the gaseous (S₄+4B)^<i>n</i>+ ions and solvated (S₄+4B) complex can be largely accounted for by a late dissociative transition state and the rehydration of B and the protein binding cavity in solution

Local zone-wise elastic-plastic constitutive parameters of Laser-welded aluminium alloy 6061 using digital image correlation

The mechanical properties of aluminium alloys can be affected by the local high temperature in laser welding. In this paper, an inversion identification method of local zone-wise elastic-plastic constitutive parameters for laser welding of aluminium alloy 6061 was proposed based on full-field optical measurement data using digital image correlation (DIC). Three regions, i.e., the fusion zone, heat-affected zone, and base zone, of the laser-welded joint were distinguished by means of microstructure optical observation and micrometer hardness measurement. The stress data were obtained using a laser-welded specimen via a uniaxial tensile test. Meanwhile, the local strain data of the laser-welded specimen were obtained by the DIC technique. Thus, the stress–strain relationship for different local regions was established. Finally, the constitutive parameters of the Ramberg–Osgood model were identified by least-square fitting to the experimental stress–strain data. Experimental results revealed that the mechanical properties of the local zones of the welded joints clearly weakened, and these results are consistent with the results of the hardness measurement