Interannual vegetation phenology estimates from global AVHRR measurements: Comparison with in situ data and applications

In a previous paper, we described a procedure to correct the directional effects in AVHRR reflectance time series. The corrected measurements show much less high frequency variability than their original counterparts, which makes them suitable to study vegetation dynamics. The time series are used here to estimate the start and ending dates of the growing season for 18 years from 1982 to 1999. We focus on the interannual variations of these phenological parameters.A database of in situ phenology observations is used to quantify the accuracy of the satellite-based estimates. Although based on a limited sampling of the Northern mid and high latitudes, the comparison indicates that i) the satellite phenological product contains meaningful information on interannual onset anomalies; ii) there is a higher degree of consistency over regions covered by Broadleaf Forests, Grasses and cereal Crops than over those covered by Needleleaf Forests or Savannas; and iii) the satellite phenological product is of lower quality in regions with mountainous terrain. In favorable conditions, interannual variations of the onset are captured with an accuracy of a few days.As this satellite-derived product captures the interannual onset variability at ground-truth sites, we confidently use it to larger scales studies. Mapped at a continental scale, the onset anomalies show coherent patterns at the regional (≈ 1000 km) scale for the mid and high latitudes of the Northern hemisphere, which is consistent with a meteorological forcing. In the tropics, there is larger spatial heterogeneity, which suggests more complex controls of the phenology. The relation between vegetation phenology and climate is further investigated over Europe by comparing the variability of the satellite-derived vegetation onset and that of the winter North Atlantic Oscillation index, at a fine spatial scale. The strong correlations observed confirm that this climate forcing parameter explains most of the onset variability over a large fraction of Northern Europe (earlier onsets for positive winter NAO), with lower impact towards the south and opposite effects around the Mediterranean basin. The NAO has a predictive character as the January-February NAO index is strongly correlated with the vegetation onset that occurs around April in Northern Europe.     

Stability analysis of a class of digital filters utilizing single saturation nonlinearity

A novel criterion for the global asymptotic stability of a class of digital filters utilizing single saturation nonlinearity is presented. An example showing the effectiveness of the present criterion is given.     

Molecular simulation of phase coexistence: Finite-size effects and determination of critical parameters for two- and three-dimensional Lennard-Jones fluids

Abstact The subject of this paper is the investigation of finite-size effects and the determination of critical parameters for a class of truncated Lennard-Jones potentials. Despite significant recent progress in our ability to model phase equilibria in multicomponent mixtures from direct molecular simulations, the accurate determination of critical parameters remains a difficult problem. Gibbs ensemble Monte Carlo simulations with systems of controlled linear system size are used to obtain the phase behavior in the near-critical region for two- and three dimensional Lennard-Jones fluids with reduced cutoff radii of 3, 3.5, and 5. For the two-dimensional systems, crossover of the effective exponent for the width of the coexistence curve from mean field ( = 1/2 in the immediate vicinity of the critical point to Ising-like (= 1/8) farther away is observed. Critical parameters determined by fitting the data that follow Ising-like behavior are in good agreement with literature values obtained with finite-size scaling methods. For the three-dimensional systems, no crossover to mean field-type behavior was apparent. Extrapolated results for the critical parameters are consistent with literature estimates for similar fluids. For both two- and three-dimensional fluids, system size effects on the coexistence curves away from the critical point are small, normally within simulation statistical uncertainties.Invited paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder. Colorado, U.S.A.     

Critical dynamics of a sheared micellar solution

We have investigated the dynamic behavior of a nonionic micellar solution of tetra-ethylene glycoln-decylether (C₁₀ E₄) in water near its critical point in the presence of shear. The non-Newtonian behavior of the viscosity can be represented by * = [ 1 +a(S₄)=]², where* is the viscosity in the absence of shear,S is the shear rate. ₄ is the lifetime of the critical Iluctuations,a is a system-dependent constant, and = 0.02 In addition, we have found that, before attaining a steady state, the sheared mixture undergoing phase separation shows significant shear-dependent rheological effects due to the presence of concentration domains.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24. 1994, Boulder, Colorado, U.S.A.     

基于偏振不敏感超材料的全光可调谐太赫兹慢光效应研究

太赫兹(terahertz,THz) 慢光效应可以有效地提升THz脉冲数据传输的安全性和存储性,而一般THz慢光器件对入射THz波偏振态变化敏感。本文设计了一种超材料结构,其微结构单元由一个十字型谐振器和4个U型谐振器构成。研究表明:基于超材料的THz慢光效应对线偏振光和圆偏振光均不敏感。通过对超材料结构的参数优化,获得到了最大群折射率为1 618的慢光效应。另外,本文在超材料微结构层和SiO₂衬底之间嵌入了一层二硫化钼(MoS₂)薄膜,当MoS₂的载流子浓度从1.7×10¹⁷ cm-3增大到5.1×10¹⁹ cm-3时,群折射率从1 566减小到26。实现了偏振不敏感全光可调谐的THz慢光效应。该研究有望为偏振不敏感和全光可调谐的THz慢光器件设计提供崭新的研究思路。     

The Edge Effects Boosting Hydrogen Evolution Performance of Platinum/Transition Bimetallic Phosphide Hybrid Electrocatalysts

Platinum (Pt) is regarded as a promising electrocatalyst for hydrogen evolution reaction (HER). However, its application in an alkaline medium is limited by the activation energy of water dissociation, diffusion of H⁺, and desorption of H*. Moreover, the formation of effective structures with a low Pt usage amount is still a challenge. Herein, guided by the simulation discovery that the edge effect can boost local electric field (LEF) of the electrocatalysts for faster proton diffusion, platinum nanocrystals on the edge of transition metal phosphide nanosheets are fabricated. The unique heterostructure with ultralow Pt amount delivered an outstanding HER performance in an alkaline medium with a small overpotential of 44.5 mV and excellent stability for 80 h at the current density of −10 mA cm⁻². The mass activity of as-prepared electrocatalyst is 2.77 A mg⁻¹_Pt, which is 15 times higher than that of commercial Pt/C electrocatalysts (0.18 A mg⁻¹_Pt). The density function theory calculation revealed the efficient water dissociation, fast adsorption, and desorption of protons with hybrid structure. The study provides an innovative strategy to design unique nanostructures for boosting HER performances via achieving both synergistic effects from hybrid components and enhanced LEF from the structural edge effect.     

Phase-Separated Polyzwitterionic Hydrogels with Tunable Sponge-Like Structures for Stable Solar Steam Generation

Solar steam generation (SSG) through hydrogel-based evaporators has shown great promise for freshwater production. However, developing hydrogel-based evaporators with stable SSG performance in high-salinity brines remains challenging. Herein, phase-separated polyzwitterionic hydrogel-based evaporators are presented with sponge-like structures comprising interconnected pores for stable SSG performance, which are fabricated by photopolymerization of sulfobetaine methacrylate (SBMA) in water-dimethyl sulfoxide (DMSO) mixed solvents. It is shown that driven by competitive adsorption, the structures of the resulting poly(sulfobetaine methacrylate) (PSBMA) hydrogels can be readily tuned by the volume ratio of DMSO to achieve phase separation. The optimized phase-separated PSBMA hydrogels, combining the unique anti-polyelectrolyte effects of polyzwitterionic hydrogels, demonstrate a rapid water transport capability in brines. After introducing photothermal polypyrrole particles on the surface of the phase-separated PSBMA hydrogel evaporators, a stable water evaporation rate of ≈2.024 kg m⁻² h⁻¹ and high solar-to-vapor efficiency of ≈97.5% in a 3.5 wt.% brine are obtained under simulated solar light irradiation (1.0 kW m⁻²). Surprisingly, the evaporation rates remain stable even under high-intensity solar irradiation (2.0 kW m⁻²). It is anticipated that the polyzwitterionic hydrogel evaporators with sponge-like porous structures will contribute to developing SSG technology for high-salinity seawater applications.     

CMOS图像传感器辐照损伤效应仿真模拟研究进展

CMOS图像传感器(CIS)工作在空间辐射或核辐射环境中遭受的辐照损伤问题备受关注。对CIS辐照损伤效应进行仿真模拟研究有助于深入揭示辐照损伤机理,进而开展抗辐射加固设计,有效提升CIS抗辐照能力。文章通过梳理国内外开展CIS辐照损伤效应仿真模拟研究方面的进展情况,结合课题组已开展的电子元器件辐照效应仿真模拟和实验研究基础,从CIS器件建模、时序驱动电路建模、辐照损伤效应建模、仿真模拟结果校验等方面探讨了CIS辐照损伤效应的仿真模拟方法,分析总结了当前CIS辐照效应仿真模拟研究中亟待解决的关键技术问题。     

Strain Rate Effects on Dynamic Fractures in Fine‐grained Granitic Rock

The effect of dynamic strain rates on failure responses of a fine‐grained granitic rock is studied experimentally and theoretically. Theoretical investigation employs a model incorporating dynamic fracture criterion with damage mechanics theory. Experimental investigation is conducted using split Hopkinson pressure bar device. In order to investigate the effects of microstructure on dynamic fracture failure under different loading rates, fragment debris of each tested specimen is collected and analyzed. It is found through the debris analysis that the granitic rock breaks down into the fragment debris in grain size scales and the effect of strain rates on the formation of fragment debris appears to be related to the microstructure of the rock. It is also found that dynamic inertia induced by the dynamic loading can reduce the effect of friction confinement generated by the contact between the cylindrical specimen and two split Hopkinson pressure bars on the dynamic responses of the specimen. Theoretical evaluations agree with the corresponding experimental observations.     

层次生物特征驱动的产品情感化设计方法

目的对产品情感化设计的实现途径进行研究。方法将工程设计领域的多生物效应方法引入到艺术设计领域,基于功能相似性确定多生物效应,抽取生物原型的不同特征指导产品情感化设计。结果形成基于多生物效应的产品情感化设计过程模型,从生物角度指导设计人员进行产品情感化设计。结论经实例验证,系统化、分层次地应用生物知识可以辅助产品情感化设计,有利于设计出实用性与艺术性统一的产品、提高设计方案的质量和产出效率。