排序方式: 共有100条查询结果,搜索用时 140 毫秒
61.
Fadi Dohnal Horst Ecker Helmut Springer 《Archive of Applied Mechanics (Ingenieur Archiv)》2008,78(12):935-947
A uniform cantilever beam under the effect of a time-periodic axial force is investigated. The beam structure is discretized
by a finite-element approach. The linearised equations of motion describing the planar bending vibrations of the beam structure
lead to a system with time-periodic stiffness coefficients. The stability of the system is investigated by a numerical method
based on Floquet’s theorem and an analytical approach resulting from a first-order perturbation. It is demonstrated that the
parametrically excited beam structure exhibits enhanced damping properties, when excited near a specific parametric combination
resonance frequency. A certain level of the forcing amplitude has to be exceeded to achieve the damping effect. Upon exceeding
this value, the additional artificial damping provided to the beam is significant and works best for suppression of vibrations
of the first vibrational mode of the cantilever beam. 相似文献
62.
The synthesis of end-capped thieno[3,2-f:4,5-f′]bis[1]benzothiophene was achieved from thiophene and 2,5-thiophenedicarboxaldehyde. Specifically, hexyl and dodecyl end-capping groups conferred reversible redox behavior as evidenced by cyclic voltammetry with oxidation potentials of 0.73 V versus Fc/Fc+ couple. An extensive spectrophotometric analysis is reported. 相似文献
63.
Frontispiece: Crown‐Shaped Tungstogermanates as Solvent‐Controlled Dual Systems in the Formation of Vesicle‐Like Assemblies 下载免费PDF全文
64.
Fortuna CG Bonaccorso C Qamar F Anu A Ledoux I Musumarra G 《Organic & biomolecular chemistry》2011,9(5):1608-1613
The synthesis and characterisation of new trans 2-(thiophen-2-yl)vinyl pyridinium, imidazolium and quinoilinium iodides is reported together with their solvatochromic shifts and EFISH characterization. 2-{(E)-2-[5'-(dibutylamino)-2,2'-bithien-5-yl]vinyl}-1-methyl pyridinium and quinolinium iodides display high μ.β(vec) values up to 1200 × 10(-48) esu. The promising non-linear optical (NLO) properties of this new family of chromophores, which can be further improved by the design of highly efficient systems exploiting the donor and acceptor properties of both heteroaromatic rings and substituents, make them suitable candidates for second harmonic generation imaging with interesting biological applications. 相似文献
65.
Most metals fail in a ductile fashion, i.e, fracture is preceded by significant plastic deformation. The modeling of failure in ductile metals must account for complex phenomena at micro-scale, such as nucleation, growth and coalescence of micro-voids. In this work, we start with von-Mises plasticity model without considering void generation. The modeling of macroscopic cracks can be achieved in a convenient way by the continuum phase field approaches to fracture, which are based on the regularization of sharp crack discontinuities [1]. This avoids the use of complex discretization methods for crack discontinuities and can account for complex crack patterns. The key aspect of this work is the extension of the energetic and the stress-based phase field driving force function in brittle fracture to account for a coupled elasto-plastic response in line with our recent work [3]. We develop a new theoretical and computational framework for the phase field modeling of ductile fracture in elastic-plastic solids. To account for large strains, the constitutive model is constructed in the logarithmic strain space, which simplify the model equations and results in a formulation similar to small strains. We demonstrate the modeling capabilities and algorithmic performance of the proposed formulation by representative simulations of ductile failure mechanisms in metals. (© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
66.
The synthesis and characterization of two novel series of octaazaphthalocyanines (AzaPc) bearing bulky phenoxy substituents are described. Target precursors to AzaPcs derivatives were prepared by using a nucleophilic aromatic substitution reaction between sterically hindered phenols (2,6-di-iso-propylphenol and 2,6-diphenylphenol) and 5,6-dichloropyrazine-2,3-dicarbonitrile. UV-vis and 1H NMR analyses confirm that steric isolation of the AzaPcs cores enforced both in the solution and in the solid state. This study explores the effectiveness of the steric factor imposed by the applied bulky phenoxy substituents on the packing behaviour of azaphthalocyanines and thereby improving their solubility and photo-physical properties. 相似文献
67.
Guo M Perez C Wei Y Rapoza E Su G Bou-Abdallah F Chasteen ND 《Dalton transactions (Cambridge, England : 2003)》2007,(43):4951-4961
The health benefits of cranberries have long been recognized. However, the mechanisms behind its function are poorly understood. We have investigated the iron-binding properties of quercetin, the major phenolic phytochemical present in cranberries, and other selected phenolic compounds (chrysin, 3-hydroxyflavone, 3',4'-dihydroxy flavone, rutin, and flavone) in aqueous media using UV/vis, NMR and EPR spectroscopies and ESI-Mass spectrometry. Strong iron-binding properties have been confirmed for the compounds containing the "iron-binding motifs" identified in their structures. The apparent binding constants are estimated to be in the range of 10(6) M(-1) to 10(12) M(-2) in phosphate buffer at pH 7.2. Surprisingly, quercetin binds Fe(2+) even stronger than the well known Fe(2+)-chelator ferrozine at pH 7.2. This may be the first example of an oxygen-based ligand displaying stronger Fe(2+)-binding affinity than a strong nitrogen-based Fe(2+)-chelator. The strong Fe-binding properties of these phenolics argue that they may be effective in modulating cellular iron homeostasis under physiological conditions. Quercetin can completely suppress Fenton chemistry both at micromolar levels and in the presence of major cellular iron chelators like ATP or citrate. However, the radical scavenging activity of quercetin provides only partial protection against Fenton chemistry-mediated damage while Fe chelation by quercetin can completely inhibit Fenton chemistry, indicating that the chelation may be key to its antioxidant activity. These results demonstrate that quercetin and other phenolic compounds can effectively modulate iron biochemistry under physiologically relevant conditions, providing insight into the mechanism of action of bio-active phenolics. 相似文献
68.
Rachel A. Scullion Andrew J. Surman Feng Xu Jennifer S. Mathieson De‐Liang Long Fadi Haso Tianbo Liu Leroy Cronin 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2014,126(38):10196-10201
The symmetry, structure and formation mechanism of the structurally self‐complementary { Pd84 } = [Pd84O42(PO4)42(CH3CO2)28]70− wheel is explored. Not only does the symmetry give rise to a non‐closest packed structure, the mechanism of the wheel formation is proposed to depend on the delicate balance between reaction conditions. We achieve the resolution of gigantic polyoxopalladate species through electrophoresis and size‐exclusion chromatography, the latter has been used in conjunction with electrospray mass spectrometry to probe the formation of the ring, which was found to proceed by the stepwise aggregation of {Pd6}− = [Pd6O4(CH3CO2)2(PO4)3Na6−nHn]− building blocks. Furthermore, the higher‐order assembly of these clusters into hollow blackberry structures of around 50 nm has been observed using dynamic and static light scattering. 相似文献
69.
This work outlines a variational-based framework for the phase field modeling of ductile fracture in elastic-plastic solids at large strains. The phase field approach regularizes sharp crack discontinuities within a pure continuum setting by a specific gradient damage model with geometric features rooted in fracture mechanics. Based on the recent works [1, 2], the phase field model of ductile fracture is linked to a formulation of gradient plasticity at finite strains in order to ensure the crack to evolve inside the plastic zones. The thermodynamic formulation is based on the definition of a constitutive work density function including the stored elastic energy and the dissipated work due to plasticity and fracture. The proposed canonical theory is shown to be governed by a rate-type minimization principle, which determines the coupled multi-field evolution problem. Another aspect is the regularization towards a micromorphic gradient plasticity-damage setting which enhances the robustness of the finite element formulation. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
70.
High-performance Tm–silica fibre lasers operating at 1.9 μm when pumped at 1319 or 1064 nm have been Q-switched using a rotating mirror mounted at an asymmetric angle. Pumping by 1319 nm gives better performance compared with 1064 nm pumping due to greater excited state absorption (ESA) at 1064 nm. A short Q-switched pulse duration of 25 ns and a peak pulse power of about 2.7 kW has been obtained from a 1.8 m fibre for 3.5 W launched pump power at 1.3 μm. The Q-switch performance has been studied for variation of fibre length and shown that the optimum length under these pumping conditions is around 2 m. 相似文献