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61.
Shultz DA Fico RM Bodnar SH Kumar RK Vostrikova KE Kampf JW Boyle PD 《Journal of the American Chemical Society》2003,125(38):11761-11771
A magnetostructural correlation (conformational electron spin exchange modulation) within an isostructural series of biradical complexes is presented. X-ray crystal structures, variable-temperature electron paramagnetic resonance spectroscopy, zero-field splitting parameters, and variable-temperature magnetic susceptibility measurements were used to evaluate molecular conformation and electron spin exchange coupling in this series of molecules. Our combined results indicate that the ferromagnetic portion of the exchange couplings occurs via the cross-conjugated pi-systems, while the antiferromagnetic portion occurs through space and is equivalent to incipient bond formation. Thus, molecular conformation controls the relative amounts of ferro- and antiferromagnetic contributions to exchange coupling. In fact, the exchange parameter correlates with average semiquinone ring torsion angles via a Karplus-Conroy-type relation. Because of the natural connection between electron spin exchange coupling and electronic coupling related to electron transfer, we also correlate the exchange parameters in the biradical complexes to mixed valency in the corresponding quinone-semiquinone radical anions. Our results suggest that delocalization in the cross-conjugated, mixed-valent radical anions is proportional to the ferromagnetic contribution to the exchange coupling in the biradical oxidation states. 相似文献
62.
Jeff A. Viaclovsky 《Transactions of the American Mathematical Society》2000,352(9):4371-4379
In this paper we will examine a class of fully nonlinear partial differential equations which are invariant under the conformal group . These equations are elliptic and variational. Using this structure and the conformal invariance, we will prove a global uniqueness theorem for solutions in with a quadratic growth condition at infinity.
63.
Jeff T. Gostick Marios A. Ioannidis Michael W. Fowler Mark D. Pritzker 《Electrochemistry communications》2009,11(3):576-579
The condition of liquid water breakthrough at the cathode of polymer electrolyte fuel cells (PEMFC) is studied experimentally and data on corresponding water saturation and capillary pressure are provided for gas diffusion layers (GDL) with and without a microporous layer (MPL). The data demonstrate that the GDL saturation at water breakthrough is drastically reduced from ca. 25% to ca. 5% in the presence of MPL. This observation is consistent with considerations of invasion percolation in finite-size lattices and suggests an explanation for the role of MPL in improving PEMFC performance at high current densities. 相似文献
64.
R. Grau S. W. Moissejew A. P. Georgijewski A. M. Michailowa A. Miermeister G. Buogo Maria Maitilasso C. Wilhelm G. Steiner F. Th. van Voorst J. Peltzer J. H. Glynn G. L. Taylor G. S. Adair Muriel E. Adair G. Kieselbach H. Diller und R. Stäsche 《Fresenius' Journal of Analytical Chemistry》1943,125(7-8):306-309
Ohne Zusammenfassung 相似文献
65.
Semen B. Kharchenko Rangaramanujam M. Kannan Jeff J. Cernohous Shivshankar Venkataramani Gaddam N. Babu 《Journal of Polymer Science.Polymer Physics》2001,39(21):2562-2571
With the increase in sophisticated synthesis methods, it appears that polymer architecture may be a tunable property. Therefore, the role of architecture in rheological and processing properties has received renewed attention, mainly because of dendrimer synthesis and metallocene‐catalyst technology. Linear polymers and hyperbranched polymers represent two ends of branching complexity. Some previous studies have suggested that hyperbranched polymers may behave like unentangled polymers, whereas others have proposed that they exhibit the properties of soft colloids. In an effort to compare the responses of linear and hyperbranched polymers, we synthesized starlike hyperbranched polystyrenes (HBPSs) of various branch lengths and numbers of branches. The HBPSs used in this study were unentangled or weakly entangled, allowing us to study the effect of branch density more readily. Two linear polystyrene (L‐PS) melts and two HBPSs were studied. Using a custom‐built rheooptical apparatus, we characterized the rheology and flow birefringence of these materials. To our knowledge, these are the first flow birefringence measurements on highly branched polymer melts. Our results suggest that the flow behavior of HBPS is significantly different from that of L‐PS: (1) HBPS shows nonterminal behavior in the low‐frequency rheological response; (2) when the stress‐optical rule (SOR) holds, the stress‐optical coefficient of HBPS is much lower than those of analogous linear polymers; and (3) when the branch density is high and the branch length is sufficiently low, the SOR fails for these homopolymer melts. A significant increase in the birefringence for a given amount of stress in the low‐frequency region suggests that there may be a soft core in these materials due to the strong preferential radial orientation of chain segments near the center of a molecule versus those near the periphery. The predominantly elastic response of the soft structures may be responsible for the enhanced form birefringence. Our preliminary results indicate that these materials may exhibit both polymeric and soft‐colloid natures. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2562–2571, 2001 相似文献
66.
67.
The idea of modulating an analog signal onto a light beam which is transmitted down an optical fiber is as old as fiber optic communication. It is only in the 1980s, however, that laboratories have focused serious attention on trying to move analog modulation rates into the microwave region, above 1 gigahertz. Progress in microwave fiber optics capability has been dramatic over the past five years, from laboratory curiosity status to 6 GHz production devices in 1984 and 10 GHz production devices in 1986. All the components required for assembling X-band transmission links are now available as standard products. 相似文献
68.
Richard F. Patterson Jeff Connor Jeannette Kline 《Central European Journal of Mathematics》2008,6(3):488-496
The notion of oscillation for ordinary sequences was presented by Hurwitz in 1930. Using this notion Agnew and Hurwitz presented
regular matrix characterization of the resulting sequence space. The primary goal of this article is to extend this definition
to double sequences, which grants us the following definition: the double oscillation of a double sequence of real or complex
number is given P-lim sup(m,n)→∞;(α,β)→∞|S
m,n
-S
α,β
|. Using this concept a matrix characterization of double oscillation sequence space is presented. Other implication and variation
shall also be presented.
相似文献
69.
Jeff Op de Beeck Wim De Malsche Joris Vangelooven Han Gardeniers Gert Desmet 《Journal of chromatography. A》2010,1217(39):6077-6084
We report on the possibility to perform HDC in micropillar array columns and the potential advantages of such a system. The HDC performance of a pillar array column with pillar diameter = 5 μm and an interpillar distance of 2.5 μm has been characterized using both a low MW tracer (FITC) and differently sized polystyrene bead samples (100, 200 and 500 nm). The reduced plate height curves that were obtained for the different investigated markers all overlapped very well, and attained a minimum value of about hmin = 0.3 (reduction based on the pillar diameter), corresponding to 1.6 μm in absolute value and giving good prospects for high efficiency separations. The obtained reduced retention time values were in fair agreement with that predicted by the Di Marzio and Guttman model for a flow between flat plates, using the minimal interpillar distance as characteristic interplate distance. 相似文献
70.
Thermal stability and reaction properties of Al-CuO system, a mixture of 50-200 nm aluminum nanoparticles passivated by nitrocellulose and 12 nm copper (II) oxide, were investigated with microstructure characterization, differential thermal analysis (DTA), and thermogravimetric analysis (TGA). Transmission electron microscopy observation confirmed that the passivation coating successfully hinders the oxidization. TGA revealed that the passivation shell does not influence the ignition temperature of the thermite reaction. Reaction chemistry of the nano-thermite was elucidated by heating the composite both in inert ambient and vacuum. It was found that the thermite reaction composes of three continuing steps: At 570 °C, Al is oxidized into Al2O3 by reacting with CuO, which forms Cu2O and produces a significant amount of heat. Subsequently two endothermic reactions occur. Starting at 800 °C, alumina reacts with Cu2O and forms CuAlO2. Above this temperature CuAlO2 will decompose and eventually produce alumina, Cu, and O2 at 1000 °C. Since the nano-thermite reaction pathway differs greatly from bulk thermite reactions, these results are important to develop a nano-thermite platform that can be used for a novel low cost, low temperature, and copper based microjoining and advance IC packaging. 相似文献