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In quasi-steady operation, convection currents in a Bridgmandevice, used for producing a semi-conductor crystal, createinhomogeneities that may make the crystal unusable. It has oftenbeen suggested that additional forces due to rotation or magnetismmight be efficacious in reducing the segregation of the elementsof the alloy. It has been found that, over a wide range of rotationrates, there is no improvement in performance due to rotationabout the vertical axis. However, numerical results that havebeen obtained previously (Lee & Pearlstein, J. Crys. Growth240, 2002) indicate that, when effects of centrifugal buoyancyare introduced, a substantial reduction in segregation is achieved.In the work reported here, by contrast, in which we extend previouslarge-Rayleigh-number asymptotic analysis to include centrifugalbuoyancy, we find no improvement in radial segregation, butrather increasing segregation with increasing rotation rate. 相似文献
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Field SJ Thornton NP Anderson LJ Gates AJ Reilly A Jepson BJ Richardson DJ George SJ Cheesman MR Butt JN 《Dalton transactions (Cambridge, England : 2003)》2005,(21):3580-3586
Protein film voltammetry of Paracoccus pantotrophus respiratory nitrate reductase (NarGH) and Synechococcus elongatus assimilatory nitrate reductase (NarB) shows that reductive activation of these enzymes may be required before steady state catalysis is observed. For NarGH complementary spectroscopic studies suggest a structural context for the activation. Catalytic protein film voltammetry at a range of temperatures has allowed quantitation of the activation energies for nitrate reduction. For NarGH with an operating potential of ca. 0.05 V the activation energy of ca. 35 kJ mol-1 is over twice that measured for NarB whose operating potential is ca. -0.35 V. 相似文献
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Cheesman MR Oganesyan VS Watmough NJ Butler CS Thomson AJ 《Journal of the American Chemical Society》2004,126(13):4157-4166
Fully oxidized cytochrome bo3 from Escherichia coli has been studied in its oxidized and several ligand-bound forms using electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) spectroscopies. In each form, the spin-coupled high-spin Fe(III) heme o3 and CuB(II) ion at the active site give rise to similar fast-relaxing broad features in the dual-mode X-band EPR spectra. Simulations of dual-mode spectra are presented which show that this EPR can arise only from a dinuclear site in which the metal ions are weakly coupled by an anisotropic exchange interaction of J 1 cm-1. A variable-temperature and magnetic field (VTVF) MCD study is also presented for the cytochrome bo3 fluoride and azide derivatives. New methods are used to extract the contribution to the MCD of the spin-coupled active site in the presence of strong transitions from low-spin Fe(III) heme b. Analysis of the MCD data, independent of the EPR study, also shows that the spin-coupling within the active site is weak with J approximately 1 cm-1. These conclusions overturn a long-held view that such EPR signals in bovine cytochrome c oxidase arise from an S' = 2 ground state resulting from strong exchange coupling (J > 10(2) cm-1) within the active site. 相似文献
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Ahmed Waqar Chowdhury Z. Z. Kazi S. N. Johan MR. Akram Naveed Oon C. S. Abdelrazek Ali H. 《Journal of Thermal Analysis and Calorimetry》2021,144(4):1517-1534
Journal of Thermal Analysis and Calorimetry - In recent decades, the growth of heat transfer using nanomaterials in the conventional base fluid has caught the attention of researchers... 相似文献
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Vincent KA Tilley GJ Quammie NC Streeter I Burgess BK Cheesman MR Armstrong FA 《Chemical communications (Cambridge, England)》2003,(20):2590-2591
Addition of an equivalent of a polyaminocarboxylate ligand (L) to a solution of a redox protein and the aqua Eu2+ ion results in the instantaneous in situ generation of a very powerful reductant Eu(II)-L that can rapidly drive an electron stoichiometrically onto a redox centre having an extremely negative reduction potential (lower than -1 V): this is exemplified by straightforward generation of the super-reduced state of the Fe-protein of nitrogenase. 相似文献
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S.D. Gopal Ram G. Ravi MR. Manikandan T. Mahalingam M. Anbu Kulandainathan 《Superlattices and Microstructures》2011,50(4):296-302
In the present work, a controlled growth of ZnO nanostructures by manipulating Zn metal ion concentration by the chelating action of ethylene diaminetetra acetic acid in hydrothermal method is studied. EDTA produces metal–chelate complex by the formation of bidentate ligand with Zn2+ in the solution and diminishes the reactivity of Zn metal cations. Concentration of EDTA in the mother solution was varied in different ranges like 3, 5 and 10 mM while retaining the zinc metal salt and the NaOH concentration the same. Three different morphologies of wurtzite structured ZnO nanostructures such as nanorods-bunch, separate/discrete uniformly sized hexagonal nanorods and tapered flower petals like shapes are achieved by 3, 5 and 10 mM strengths of EDTA, respectively. The medium concentration 5 mM of EDTA is found to have moderate control over producing ZnO nanostructures of uniform diameter and a high aspect (length to diameter) ratio. An array of vertically aligned free standing ZnO nanorods with uniform spacing is successfully achieved by the addition of 5 mM of EDTA in the mother solution and the same is studied for its fluorescence property at an excitation of 325 nm and it has exhibited a characteristic UV emission of ZnO around 383 nm. 相似文献
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Bruce Victor Cheesman Raymond Frederick Martin White 《Magnetic resonance in chemistry : MRC》1984,22(12):764-770
Vicinal thallium–hydrogen coupling constants are used to discuss conformations in dicyclohexylthallium chloride, bis(4-methylcyclohexyl)thallium chloride and bis(4-tert-butylcyclohexyl)thallium chloride. Thallium does not have a very strong preference for equatorial positions in dicyclohexylthallium chloride, whereas bis(4-alkylcyclohexyl)thallium chlorides exist largely in one conformation. Bis(4-methylcyclohexyl)thallium chloride exists in three isomeric forms; the major product appears to be the cis-isomer (equatorial methyl, axial thallium), with the other two isomers probably containing thallium trans to the methyl group (axial thallium being preferred). The preference for the cis-isomer (equatorial tert-butyl, axial thallium) of bis(4-tert-butylcyclohexyl)thallium chloride is such that other isomers are not obtained. 相似文献
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Comerford DW Cheesman A Carpenter TP Davies DM Fox NA Sage RS Smith JA Ashfold MN Mankelevich YA 《The journal of physical chemistry. A》2006,110(9):2868-2875
Experimental and modeling studies of the gas-phase chemistry occurring in dilute, hot filament (HF) activated B2H6/H2 and B2H6/CH4/H2 gas mixtures are reported. Spatially resolved relative number densities of B (and H) atoms have been measured by resonance enhanced multiphoton ionization methods, as a function of process conditions (e.g. the HF material and its temperature, the B2H6/H2 mixing ratio, and the presence (or not) of added CH4). Three-dimensional modeling of the H/B chemistry prevailing in such HF activated gas mixtures using a simplified representation of the gas phase chemistry succeeds in reproducing all of the experimentally observed trends, and in illustrating the key role of the "H-shifting" reactions BHx + H <= => BHx-1 + H2 (x = 1-3) in enabling rapid interconversion between the various BHx (x = 0-3) species. CH4 addition, at partial pressures appropriate for growth of boron-doped diamond by chemical vapor deposition methods, leads to approximately 30% reduction in the measured B atom signal near the HF. The modeling suggests that this is mainly due to concomitant H atom depletion near the HF, but it also allows us a first assessment of the possible contributions from B/C coupling reactions upon CH4 addition to HF activated B2H6/H2 gas mixtures. 相似文献