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41.
The cationic polymerization of cyclopentadiene (CPD) with 1-(4-methoxyphenyl)ethanol (1)/BF3OEt2 initiating system in CH2Cl2:CH3CN 4:1 (v/v) mixture at room temperature and in the presence of water ([H2O]/[BF3OEt2] up to 8) is reported. The number-average molecular weights of obtained polymers increased in direct proportion to monomer
conversion or initial monomer concentration (M
n ≤ 4,000 g mol−1) in agreement with calculated values, and were inversely proportional to initiator concentration. Polymer MWDs were relatively
narrow (M
w/M
n = 1.4–1.7) up to 60% of monomer conversion. It was also shown that regioselectivity of CPD polymerization with 1/BF3OEt2 initiating system did not depend significantly on water, monomer, or initiator concentration (1,4-structures content was
nearly 60% in all cases). 相似文献
42.
Sergei F. Burlatsky Vadim V. Atrazhev Mallika Gummalla Dave A. Condit Fuqiang Liu 《Journal of power sources》2009,190(2):485-492
Proper water management in a hydrogen-fueled polymer electrolyte membrane (PEM) fuel cell is critical for performance and durability. A mathematical model has been developed to elucidate the effect of thermal conductivity and water vapor diffusion coefficient in the gas diffusion layers (GDLs). The fraction of product water removed in the vapor phase through the GDL as a function of GDL properties/set of material and component parameters and operating conditions has been calculated. The current model enables identification of conditions wherein condensation occurs in each GDL component. The model predicts the temperature gradient across various components of a PEM fuel cell, providing insight into the overall mechanism of water transport in a given cell design. The water condensation conditions and transport mode in the GDL components depend on the combination of water vapor diffusion coefficients and thermal conductivities of the GDL components. Different types of GDLs and water transport scenarios are defined in this work, based on water condensation in the GDL and fraction of water that the GDL removes through the vapor phase, respectively. 相似文献
43.
A wavelength-independent method for optical gating, based on the optical Kerr effect, has been demonstrated. Using this method, we produced 100-ps, 10-kW, two-wavelength pulses (10.3 and 10.6 microm) with a signal-to-background ratio contrast of 10(5) by slicing a long CO2 pulse. The capability of gating consecutive pulses separated on a picosecond time scale with this method is also shown. 相似文献
44.
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46.
Sergei V. Kostjuk Alexei Yu. Dubovik Irina V. Vasilenkol Vyacheslav P. Mardykin Ludmila V. Gaponik Fyodor N. Kaputsky Lev M. Antipin 《Polymer Bulletin》2004,52(3-4):227-234
Summary
Quasiliving cationic polymerization of styrene was obtained in the system 2-phenyl-2-propano-/AlCl3·OBu2/Bu2O in a mixture of 1,2-dichloroethane and n-hexane (55:45 viv) at -15 °C. The molecular weights of the polymers (Mn) increased in direct proportion to the monomer conversion. However, the experimental Mns are essentially higher than theoretical ones, indicating that slow initiation relative to propagation takes place. The molecular weight distributions were broad (Mw/Mn2.5), probably due to the slow initiation and slow exchange between reversibly terminated and propagating species. 相似文献
47.
We give a direct proof by generic reduction that testing validity of formulas in a decidable rudimentary theory Ω of finite typed sets (Henkin, Fundamenta Mathematicæ 52 (1963) 323–344) requires space and time exceeding infinitely often(1) where n denotes the length of input. This gives the highest currently known lower bound for a decidable logical theory and affirmatively settles Problem 10.13 from (Compton and Henson, Ann. Pure Applied Logic 48 (1990) 1–79):
Is there a “natural” decidable theory with a lower bound of the form exp∞(f(n)), where f is not linearly bounded?The highest previously known lower (and upper) bounds for “natural” decidable theories, like WS1S, S2S, are of the form exp∞(dn), with just linearly growing stacks of twos.Originally, the lower bound (1) for Ω was settled in (12th Annual IEEE Symposium on Logic in Computer Science (LICS’97), 1997, 294–305) using the powerful uniform lower bounds method due to Compton and Henson, and probably would never be discovered otherwise. Although very concise, the original proof has certain gaps, because the method was pushed out of the limits it was originally designed and intended for, and some hidden assumptions were violated. This results in slightly weaker bounds—the stack of twos in (1) grows subexponentially, but superpolynomially, namely, as for formulas with fixed quantifier prefix, or as 2cn/log(n) for formulas with varying prefix. The independent direct proof presented in this paper closes the gaps and settles the originally claimed lower bound (1) for the minimally typed, succinct version of Ω. 相似文献
48.
Smith JT Wright SM Cross MA Monte L Kudelsky AV Saxén R Vakulovsky SM Timms DN 《Environmental science & technology》2004,38(3):850-857
Atmospheric nuclear weapons explosions and large-scale nuclear accidents may contaminate large areas of land with the long-lived radionuclides 137Cs and 90Sr. The mobility and bioavailability of these radionuclides in the environment is dependent primarily on soil characteristics and changes significantly over time after fallout (1-4). Radioisotope concentrations in different rivers and at different times after fallout vary over 2-3 orders of magnitude. Many previous studies have concentrated on the interactions of radiocesium and radiostrontium with various environmental components, but there are currently no operative models fortheirtransport over large spatial areas. We collected time-series measurements of 90Sr and 137Cs in 25 major European and Asian rivers and (using digital data sets with global coverage) determined characteristics of each of the rivers' catchments. This work has established, for the first time, a quantitative link between riverine transport of these radioisotopes and catchment and soil characteristics at a global scale. A generalized predictive model accounting for time changes in river concentrations and variation in catchment characteristics is developed. This can be used to predict the long-term riverine transport of these radiologically important radionuclides following any large-scale nuclear incident in North America, Europe, or (European and Asian) Russia. 相似文献
49.
Orlov SS Phillips W Bjornson E Takashima Y Sundaram P Hesselink L Okas R Kwan D Snyder R 《Applied optics》2004,43(25):4902-4914
We describe the design and implementation of a high-data-rate high-capacity digital holographic storage disk system. Various system design trade-offs that affect density and data-rate performance are described and analyzed. In the demonstration system that we describe, high-density holographic recording is achieved by use of high-resolution short-focal-length optics and correlation shift multiplexing in photopolymer disk media. Holographic channel decoding at a 1-Gbit/s data rate is performed by custom-built electronic hardware. A benchmark sustained optical data-transfer rate of 10 Gbits/s has been successfully demonstrated. 相似文献
50.
Lavrenko Vladimir A. Panasyuk Alla D. Firstov Sergei A. 《Powder Metallurgy and Metal Ceramics》2003,42(5-6):291-296
Potentiodynamic polarization, XRD, Auger electron spectroscopy, and scanning electron microscopy have been applied to determine the mechanisms of electrolytic corrosion of the intermetallides TiAl, TiAl3, and 2Ti3Al in 3% NaCl solution with the addition of MgSO4 in comparison with the corrosion of the pure metals (Al and Ti). There is comparatively high corrosion resistance in TiAl and Ti3Al because of the protective action from thin films of rutile TiO2. Evidence is obtained for the differences between these intermetallides from the appearance of pure titanium on deep anodic polarization (above +0.15 V for TiAl and +0.7 V for Ti3Al): the aluminum and titanium enter the solution in the forms of Al3+ and TiO2+ and intermetallides dissolve rapidly. The final solid-state products from anodic oxidation in that case are rutile, magnesium aluminate, and also magnesium and sodium titanates. In addition to pure titanium, TiAl and 2Ti3Al as constructional materials may be recommended for use in sea water. 相似文献