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21.
A manganese(III)-substituted polyoxometalate of the "sandwich" structure, [MnIII2ZnW(ZnW9O34)2]10-, catalyzed the highly selective (>99.9%) epoxidation of alkenes, such as 1-octene, 2-octene, and cyclohexene with nitrous oxide. Reactions occurred in homogeneous media at 150 degrees C under 1 atm N2O. The epoxidation had a linear reaction profile; turnover frequencies of 0.5-1.4 h-1 were measured. The reactions were also stereoselective; for example, cis-stilbene gave cis-stilbene oxide. From ESR spectroscopy, it was shown that a Mn(II) octahedral species is reversibly formed by reaction between the original Mn(III) polyoxometalate and N2O. Therefore, it would appear that a Mn(V)-oxo active species is not formed; it is possible that the activation of nitrous oxide was by its oxidation by the Mn(III) polyoxometalate. 相似文献
22.
Ben-Daniel R de Visser SP Shaik S Neumann R 《Journal of the American Chemical Society》2003,125(40):12116-12117
We have demonstrated that a polyfluorinated alcohol, 2,2,2-trifluoroethanol, solvent enables haloperoxidase type activity and the oxychlorination of arenes (benzene and its alkylated derivatives) without a metal catalyst. The polyfluorinated alcohol has a dual function; it catalyzes electrophilic chlorination of less reactive arenes by several orders of magnitude and oxidation of chloride at lower H+ concentrations. DFT calculations show that a complementary charge template in the transition state explains the catalysis of the electrophilic chlorination. 相似文献
23.
Lipophilic quaternary ammonium cations are insoluble in water when paired with any anion except fluoride or hydroxide. This phenomenon is the basis for a novel method for total anion determination. Tetra-n-octylammonium fluoride (TOAF) is used for the direct titration of any anion or mixture of anions with the exception of hydroxide. Anions for which no direct analytical method exists, e.g., nitrate or hydrogenfluoride (HF2?), are simply assayed by conductimetric titration with TOAF reagent. 相似文献
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Revital Feldman Yehoshua Shimony Eyal Lebiush Yuval Golan 《Journal of Physics and Chemistry of Solids》2008,69(4):839-846
High-power pumped Nd:YAG elements may exceed their tensile strength under high thermally induced stress. Providing extra strength to such rods is essential for their employment in high-power lasers. The tensile strength of YAG elements was increased by chemical etching in concentrated phosphoric acid. The highest tensile strength was achieved by etching of fine-ground YAG components: an average for slabs, and for rods, which are 3.6 times and 5 times higher than those of non-etched elements, respectively. The measurements were carried out by four-point flexure strength test. We have established a dependency among the micro-roughness of YAG elements, the surface morphology obtained by etching, and the tensile strength: the tensile strength of the etched element improves for finer after-etch surface texture, which is obtained for finer initial micro-roughness.
To assure the withstanding of Nd:YAG rods under high thermal gradients, a new approach was employed, namely, increasing the pump-power applied to the Nd:YAG rod till fracture. Our results show an increase by more than 2.7 times in tensile strength of etched Nd:YAG rods as compared to standard commercial rods, which corresponds to a thermal loading of excess of 434 W/cm. 相似文献
26.
The particulate nature of matter is identified in science education standards as one of the fundamental concepts that students should understand at the middle school level. However, science education research in indicates that secondary school students have difficulties understanding the structure of matter. The purpose of the study is to describe how engaging in an extended project‐based unit developed urban middle school students' understanding of the particulate nature of matter. Multiple sources of data were collected, including pre‐ and posttests, interviews, students' drawings, and video recordings of classroom activities. One teacher and her five classes were chosen for an indepth study. Analyses of data show that after experiencing a series of learning activities the majority of students acquired substantial content knowledge. Additionally, the finding indicates that students' understanding of the particulate nature of matter improved over time and that they retained and even reinforced their understanding after applying the concept. Discussions of the design features of curriculum and the teacher's use of multiple representations might provide insights into the effectiveness of learning activities in the unit. 相似文献
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Steven Jackel Avi Meir Zvi Horvitz Inon Moshe Yehoshua Shimoni Yaakov Lumer Revital Feldman Izhak Hershko Yotam Pekin 《Optics & Laser Technology》2011,43(3):687-696
In this paper we investigate the source of failure in commercial, microchannel cooled CW diode bars placed in 12 bar horizontal arrays. The arrays were used to pump Nd:YAG rods in our 10 kW developmental laser. The laser was operated at low duty factor over a period of over 2 years. Experimental evidence indicated that the sudden, catastrophic failure was because of degraded cooling. We used optical microscopes, an X-ray microfocus imager, and a thermal neutron scattering camera to look inside the microcoolers. Our investigations revealed only one possible failure mechanism: cooling flow reduction because of delamination of the Au coating the walls of the microcoolers and the entrapment of Au flakes within the microchannel structures. We observed blisters in the microcoolers under working bars, and flake-like structures in the microcoolers under burnt-out bars (all taken from the laser). We observed no evidence of either massive blockages because of electrochemical deposits, or of corrosion/erosion in the microchannel walls. Integral operation times of the high flow-rate cooling system and of the diodes themselves were too short by one and two orders of magnitude, respectively, to explain the observed failures. Microchannel immersion times in the deionized water were, however, long enough to allow for corrosion of metals that may have been exposed through defects in the Au coatings. Three-dimensional heat flow simulations showed that blockage of multiple microchannels towards the edge of a bar can easily lead to catastrophic temperature increases. 相似文献
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