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The profile of fatty acid methyl esters (FAME) of different vegetable oils produced under ultrasonic irradiation and conventional heating were compared. In the presence of potassium hydroxide as catalyst, the distribution of FAME was quite similar for both procedures, while in the case of sodium hydroxide ultrasonic irradiation gave better results. The FAME profile resulted from the reaction catalyzed by sulfuric acid was almost the same with the one resulted from the reaction catalyzed by KOH, while boron trifluoride can give rise to many artifacts, thus is not a reliable catalyst. 相似文献
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W. Langbein P. Borri U. Woggon M. Schwab M. Bayer S. Fafard Z. Wasilewski P. Hawrylak V. Stavarache D. Reuter A.D. Wieck 《Physica E: Low-dimensional Systems and Nanostructures》2005,26(1-4):400
We measure the dephasing time of the exciton ground state transition in InGaAs quantum dots (QD) and quantum dot molecules (QDM) using a sensitive four-wave mixing technique. In the QDs we find experimental evidence that the dephasing time is given only by the radiative lifetime at low temperatures. We demonstrate the tunability of the radiatively limited dephasing time from 400 ps up to 2 ns in a series of annealed QDs with increasing energy separation of 69–330 meV from the wetting layer continuum. Furthermore, the distribution of the fine-structure splitting δ1 and of the biexciton binding energy δB is measured. δ1 decreases from 96 to with increasing annealing temperature, indicating an improving circular symmetry of the in-plane confinement potential. The biexciton binding energy shows only a weak dependence on the confinement energy, which we attribute to a compensation between decreasing confinement and decreasing separation of electron and hole. In the QDM we measured the exciton dephasing as function of interdot barrier thickness in the temperature range from 5 to 60 K. At 5 K dephasing times of several hundred picoseconds are found. Moreover, a systematic dependence of the dephasing dynamics on the barrier thickness is observed, showing how the quantum mechanical coupling in the molecules affects the exciton lifetime and acoustic-phonon interaction. 相似文献
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The fracture of thick laminated graphite/epoxy composites has been the subject of an extensive research program. The program
was divided into three major areas of investigation which included laminate thickness, laminate stacking sequence, and part-through
surface flaws. The results from this program are reviewed with emphasis placed on their applicability to the design of thick
laminated composite structures.
Paper was presented at 1985 SEM Spring Conference on Experimental Mechanics held in Las Vegas on June 9–14, 1985. 相似文献
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二氧化硅悬浮体和水解聚丙烯酰胺体系的流变性 总被引:6,自引:0,他引:6
研究了二氧化硅悬浮体的流变性。在固体含量较少的情况下, 体系呈Newton型;固体含量达到一定程度以后, 产生三维结构, 为假塑性体系。三维结构的固体含量与pH值有关。实验证实, 在等电点(i,e, p)附近三维结构最为疏松。还研究了水解聚丙烯酰胺(HPAM)对悬浮体流变性能的影响。HPAM破坏了二氧化硅粒子聚集体结构, 呈絮凝体结构, 并出现触变性。当高聚物在固体表面覆盖一半时, 具有最大触变性和动剪切力。HPAM的水解度达到一定程度, 由于对固体粒子由“强”吸附转变为“弱”吸附, 固体粒子“屏蔽”了高分子间的相互作用, 出现了负触变性现象。在流动时粒子会脱附, 静止以后又产生吸附, 这种吸附与脱附是产生负触变性的主要原因。HPAM水解过度或用量过多, 均会减弱负触变性现象。 相似文献
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Greilich A Oulton R Zhukov EA Yugova IA Yakovlev DR Bayer M Shabaev A Efros AL Merkulov IA Stavarache V Reuter D Wieck A 《Physical review letters》2006,96(22):227401
Electron spin coherence has been generated optically in n-type modulation doped (In,Ga)As/GaAs quantum dots (QDs) which contain on average a single electron per dot. The coherence arises from resonant excitation of the QDs by circularly polarized laser pulses, creating a coherent superposition of an electron and a trion. Time dependent Faraday rotation is used to probe the spin precession of the optically oriented electrons about a transverse magnetic field. The coherence generation can be controlled by pulse intensity, being most efficient for (2n+1)pi pulses. 相似文献
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Oulton R Greilich A Verbin SY Cherbunin RV Auer T Yakovlev DR Bayer M Merkulov IA Stavarache V Reuter D Wieck AD 《Physical review letters》2007,98(10):107401
A key to ultralong electron spin memory in quantum dots (QDs) at zero magnetic field is the polarization of the nuclei, such that the electron spin is stabilized along the average nuclear magnetic field. We demonstrate that spin-polarized electrons in n-doped (In,Ga)As/GaAs QDs align the nuclear field via the hyperfine interaction. A feedback onto the electrons occurs, leading to stabilization of their polarization due to formation of a nuclear spin polaron [I. A. Merkulov, Phys. Solid State 40, 930 (1998)]. Spin depolarization of both systems is consequently greatly reduced, and spin memory of the coupled electron-nuclear spin system is retained over 0.3 sec at temperature of 2 K. 相似文献
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The transesterification of vegetable oil with short-chain alcohols, in the presence of base-catalyst, by means of low frequency ultrasound (28 and 40 kHz) in order to obtain biodiesel fuel was studied. By using ultrasounds the reaction time is much shorter (10-40 min) than for mechanical stirring. The quantity of required catalyst is 2 or 3 times lower. The molar ratio of alcohol/oil used is only 6:1. Normal chain alcohols react fast, while secondary and tertiary alcohols show some or no conversion after 60 min of reaction. Surprisingly, 40 kHz ultrasounds are much more effective in the reduction of the reaction time (10-20 min). Twenty eight kilohertz give slightly better yields (98-99%), but longer reaction time, while higher frequencies are not useful at all for the transesterification of fatty acids. 相似文献
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The relative rates of H-transfer between partners in ion-neutral complexes were compared with those in intramolecular rearrangements using results of first differential photoionization mass spectrometry measurements. Complex-mediated H-transfers are inferred to have rates of the same order as those for intramolecular hydrogen rearrangements, suggesting a similar range of motion of the reactive sites in both types of reactions. It is also concluded that at their fastest H-transfers take place between the partners in ion-neutral complexes within at most the time of several rotations of the partners in the complexes. Copyright 1999 John Wiley & Sons, Ltd. 相似文献