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21.
The magnetic flux density in the cavity of a sintered tube of Yttrium Barium Copper Oxide (YBCO) is measured as a function of H, the axial magnetic field impressed and removed after zero field cooling (ZFC). We note that, in a short thick-walled hollow cylinder of a weak-linked polycrystalline high-Tc superconductor, the magnetic field in the cavity of the tube, Hz hole, rises above the applied field H when it is initially increasing. Consequently the standard approach to determine the critical current density from an ascending sweep of H alone, or a descending sweep alone cannot be exploited. However the field dependence of the intergranular critical current density Jcm can be reliably obtained from the horizontal traversals of Hz hole vs H. Comparison of the experimental data with model calculations for the horizontal traversals of the hysteresis curves show that the intergranular critical current density in the YBCO tube has a Kim-like field dependence. The magnetization of the grains has a significant influence on the flux density in the cavity of the tube. 相似文献
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Strachan JD Bitter M Ramsey AT Zarnstorff MC Arunasalam V Bell MG Bretz NL Budny R Bush CE Davis SL Dylla HF Efthimion PC Fonck RJ Fredrickson E Furth HP Goldston RJ Grisham LR Grek B Hawryluk RJ Heidbrink WW Hendel HW Hill KW Hsuan H Jaehnig KP Jassby DL Jobes F Johnson DW Johnson LC Kaita R Kampershroer J Knize RJ Kozub T LeBlanc B Levinton F La Marche PH Manos DM Mansfield DK McGuire K McNeill DH Meade DM Medley SS Morris W Mueller D Nieschmidt EB Owens DK Park H Schivell J Schilling G 《Physical review letters》1987,58(10):1004-1007
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Rene J. LeBlanc Christopher T. Williams 《Journal of molecular catalysis. A, Chemical》2004,220(2):207-214
The adsorption of the chiral modifier cinchonidine on platinum in ethanol as a function of temperature has been studied with surface-enhanced Raman spectroscopy (SERS). The temperature range chosen was from 30 to 70 °C, within which both the activity and selectivity of cinchonidine-modified Pt catalysts have been shown to change dramatically. Platinum surfaces were modified with 260 μM cinchonidine in ethanol, and examined both in pure ethanol and in the modifying solution itself. Adsorbed cinchonidine under pure ethanol was found to partially desorb as the temperature was raised, accompanied by an increase in the average tilt of the quinoline group with respect to the surface. In contrast, the presence of solution-phase cinchonidine resulted in an increase in the cinchonidine surface coverage and average tilt as temperature was raised. In a previous study [J. Mol. Catal. A 212 (2004) 277] we showed that hydrogen causes a dramatic enhancement in the SERS response of adsorbed cinchonidine. This was attributed to a conversion of cinchonidine to 10,11-dihydrocinchonidine on the Pt surface and a more flat orientation of the quinoline group. In both pure ethanol and in 260 μM cinchonidine, the presence of hydrogen causes a significant decrease in the alkaloid SERS bands at temperatures above 40 °C. In addition, the average tilt of the quinoline group increases significantly at these elevated temperatures. The temperature-dependence of 10,11-dihydrocinchonidine adsorption was also investigated, and is almost identical to that observed for cinchonidine in the presence of hydrogen. This lends further support to the conclusion that cinchonidine is being hydrogenated on the Pt surface in the presence of hydrogen. The significant changes observed on the cinchonidine-modified Pt surface above 40 °C correlate well with reported decreases in enantioselectivity and turn-over frequency at similar temperatures during ethyl pyruvate hydrogenation. 相似文献
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THE U.V. PHOTOCHEMISTRY OF CYTIDYLIC ACID 总被引:1,自引:0,他引:1
Abstract The ultraviolet (u.v.) irradiation of the 3' isomer of cytidylic acid (Cp) produces the hydrate (Cp*) with water added across the 5–6 double bond. The yield of this photo-product has been measured by, (a) separating the photoproduct by electrophoresis and (b) by observing the loss in absorbance. When corrections are made for reversal of the hydrate during the experiment, both methods gave the same result. Cross sections and quantum yields for the production of the hydrate were measured over the wavelength range 220 to 290 nm and over a pH range from 1 to 10. The quantum yield is markedly dependent on pH being higher by a factor of 6 to 10 for the neutral form. We have also demonstrated the existence of a very short lived photoproduct (half life 8–9 min) in both Cp3' and Cp5': The nature of this short lived product is not known. 相似文献
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Ryan M. Hensarling Emily A. Hoff Arthur P. LeBlanc Wei Guo Santosh B. Rahane Derek L. Patton 《Journal of polymer science. Part A, Polymer chemistry》2013,51(5):1079-1090
In this work, a postpolymerization surface modification approach is reported that provides pendent thiol functionality along the polymer brush backbone using the photolabile protection chemistry of both o‐nitrobenzyl and p‐methoxyphenacyl thioethers. Poly(2‐hydroxyethyl methacrylate) (pHEMA) brushes were synthesized via surface‐initiated atom transfer radical polymerization, after which the pHEMA hydroxyl groups were esterified with 3‐(2‐nitrobenzylthio)propanoic acid or 3‐(2‐(4‐methoxyphenyl)‐2‐oxoethylthio)propanoic acid to provide the photolabile protected pendent thiols. Addressing the protecting groups with light not only affords spatial control of reactive thiol functionality but enables a plethora of thiol‐mediated transformations with isocyanates and maleimides providing a modular route to create functional polymer surfaces. This concept was extended to block copolymer brush architectures enabling the modification of the chemical functionality of both the inner and outer blocks of the block copolymer surface. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013 相似文献
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M V Ovchinnikov E LeBlanc I A Guzei R J Angelici 《Journal of the American Chemical Society》2001,123(46):11494-11495
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