Home About us Contact
|
Home About us Contact | ||
|
|
||
Hydrogen Concentration (hydrogen + concentration)
Selected AbstractsTurbulent Dynamics of Beryllium Seeded Plasmas at the Edge of TokamaksCONTRIBUTIONS TO PLASMA PHYSICS, Issue 3-5 2010R.V. Shurygin Abstract Numerical simulation of turbulent MHD dynamics of beryllium seeded plasmas at the edge of tokamaks is performed. The model is based on the 4-fluid {,, n, pe, pi } reduced nonlinear Braginsky's MHD equations. Neutral hydrogen flow from the wall is described with a diffusion model. Beryllium line radiation is taken into consideration. The Be ion distribution over ionization states is calculated using the reduced model. Electron impact ionization, three body, photo- and dielectronic recombination and charge-exchange with neutral hydrogen are taken into account. Coronal equilibrium is not supposed. Simulations are performed for T-10 parameters. Radial distributions of averaged temperatures and their fluctuation levels, species flows, impurity radiation power, and impurity ions concentrations are obtained as functions of the Be concentration at the wall. The impurity radiation is shown to act on the turbulent oscillation level significantly if the total Be concentration at the wall exceeds 3 · 1011cm,3. The impurity turbulent transversal flow is directed inward and exceeds neoclassical flow significantly. The parallel conductivity and, as a consequence, turbulent transport are increased significantly by impurity radiation. The radiation loss dependence on the neutral Hydrogen concentration at the wall is also examined. The hydrogen concentration increasing the plasma density also rises. The relative beryllium concentration decreases. In total, these two effects are compensated, and the level of radiation losses is changed insignificantly (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Electrical and optical properties of thick highly doped p-type GaN layers grown by HVPEPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2008A. Usikov Abstract In this paper we report 3-7 ,m thick p-GaN growth by hydride vapor phase epitaxy (HVPE) on sapphire substrates. Mg impurity was used for doping. As-grown GaN layers had p-type conductivity with concentration NA -ND up to 3×1019 cm,3. Mg atom concentration was varied from 1017 to 1020 cm,3. Hydrogen concentration was about 10 times less than that for Mg, which may explain effective p-type doping for as-grown GaN layers. Micro-cathodoluminescence revealed a columnar-like structure of the GaN layers with a non-uniform distribution of material regions having dominant 362 nm or 430 nm luminescence. Use of these thick p-GaN layers to grow InGaN-based blue and green LEDs by the HVPE is demonstrated. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Preparation of ultra-high-molecular-weight polyethylene and its morphological study with a heterogeneous Ziegler,Natta catalystJOURNAL OF APPLIED POLYMER SCIENCE, Issue 6 2010G. H. Zohuri Abstract Ultra-high-molecular-weight polyethylene (PE) with viscosity-average molecular weight (Mv) of 3.1 × 106 to 5.2 × 106 was prepared with a heterogeneous Ziegler,Natta MgCl2 (ethoxide type)/TiCl4/triethylaluminum catalyst system under controlled conditions. The optimum activity of the catalyst was obtained at a [Al]/[Ti] molar ratio of 61 : 1 and a polymerization temperature of 60°C, whereas the activity of the catalyst increased with monomer pressure and decreased with hydrogen concentration. The titanium content of the catalyst was 2.4 wt %. The rate/time profile of the catalyst was a decay type with a short acceleration period. Mv of the PE obtained decreased with increasing hydrogen concentration and polymerization temperature. The effect of stirrer speeds from 100 to 400 rpm did not so much affect the catalyst activity; however, dramatic effects were observed on the morphology of the polymer particles obtained. A stirrer speed of 200 rpm produced PE with a uniform globulelike morphological growth on the polymer particles. The particle size distributions of the polymer samples were determined and were between 14 and 67 ,m. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source] Hydrogen response in liquid propylene polymerization: Towards a generalized modelAICHE JOURNAL, Issue 5 2006M. Al-haj Ali Abstract Liquid propylene batch experiments in the absence of a gas phase have been carried out using a highly-active MgCl2/TiCl4/phthalate/silane/AlR3 catalyst at varying temperatures (60-80°C) and molar hydrogen-monomer ratios of 0-10 mmol/mol. With increasing hydrogen concentration the polymerization rate increases rapidly, reaching a constant value at concentrations above 1.4 mmol/mol; pseudo-first-order catalyst deactivation constant increases; molecular weight decreases; polydispersity decreases slightly; but average molecular weight and polydispersity increase with increasing temperature. Polymerization rate, deactivation constant, and average molecular weight can be modeled based on a consistent dormant site mechanism assuming an (averaged) quasi-single-site model. © 2006 American Institute of Chemical Engineers AIChE J,2006 [source] Effects of external donors and hydrogen concentration on oligomer formation and chain end distribution in propylene polymerization with Ziegler-Natta catalystsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 2 2010Torvald Vestberg Abstract The effect of type and concentration of external donor and hydrogen concentration on oligomer formation and chain end distribution were studied. Bulk polymerization of propylene was carried out with two different Ziegler-Natta catalysts at 70 °C, one a novel self-supported catalyst (A) and the other a conventional MgCl2 -supported catalyst (B) with triethyl aluminum as cocatalyst. The external donors used were dicyclopentyl dimethoxy silane (DCP) and cyclohexylmethyl dimethoxy silane (CHM). The oligomer amount was shown to be strongly dependent on the molecular weight of the polymer. Catalyst A gave approximately 50 % lower oligomer content than catalyst B due to narrower molecular weight distribution in case of catalyst A. More n -Bu-terminated chain ends were found for catalyst A indicating more frequent 2,1 insertions. Catalyst A also gave more vinylidene-terminated oligomers, suggesting that chain transfer to monomer, responsible for the vinylidene chain ends, was a more important chain termination mechanism for this catalyst, especially at low hydrogen concentration. Low site selectivity, due to low external donor concentration or use of a weak external donor (CHM), was also found to increase formation of vinylidene-terminated oligomers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 351,358, 2010 [source] Hydrogen effects on crystallinity, photoluminescence, and magnetization of indium tin oxide thin films sputter-deposited on glass substrate without heat treatmentPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 2 2010Suning Luo Abstract Indium tin oxide (ITO) thin films were sputter deposited by using working gas containing hydrogen on glass substrate without any heat treatments. The films demonstrated X-ray diffraction due to polycrystalline ITO, blue-green photoluminescence (PL) due to oxygen defects in nano-structured ITO crystals, and paramagnetic behaviour in temperature dependence of magnetization overlapped with diamagnetic signal from the substrate. The carrier density n of the films was of the order of 1020,cm,3, and varied as an inverse of V-character with the hydrogen concentration [H] in the gas. The n value peaked at [H],=,1%. Spectral features at ,430 and ,470,nm of the PL emission were invariant with [H]. The order of the density of electrons N with spins obeying the Curie law was 1023,cm,3, and the variation in N with [H] was almost parallel to that in n with [H]. [source] Structural and defect changes of hydrogenated SiGe films due to annealing up to 600°CPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3-4 2010Petr Sládek Abstract In order to better understand the effects of the hydrogen incorporation on the defects and the disorder in the undoped nano/microcrystalline SiGe:H, we performed a comparative study on samples deposited under different plasma conditions. With variation of the pressure, we were able to change the structure of SiGe:H films. We have used the combination of the infrared spectroscopy, CPM, PDS and thermal desorption measurements to study the thermal dependence of defect density, disorder, as well as hydrogen concentration. The film mechanical properties were tested by depth sensing indentation technique. The results showing a different hydrogen bonding with the change of deposition conditions are interpreted as a whole by terms of the specific local hydrogen bonding environment, related to different growth mechanism. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Effect of hydrogen on the synthesis of carbon nanofibers by CO disproportionation on ultrafine Fe3O4ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 5 2009Wenxin Lu Abstract Carbon nanofibers (CNFs) are grown by catalytic CO disproportionation over ultrafine Fe3O4 catalyst at a hydrogen concentration of 0,29.17%, and the time-depending rates of CNFs growth are continuously monitored and the morphologies of the as-synthesized CNFs are analyzed. Increasing H2 concentration will lower CO dissociation energy and assist catalyst reconstruction so as to shorten the induction period and increase the growth rate of CNFs, but it will also increase the rate of catalyst deactivation because carbon hydrogasification is not possible and carbon diffusion in the catalyst particle is rate limiting. As a result of H2 -induced catalyst reconstruction and carbon deposition, the morphology of the CNFs changes from twisty to helical and to straight and becomes less entangled when the H2 concentration is increased. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd. [source] Bioenergetics of the formyl-methanofuran dehydrogenase and heterodisulfide reductase reactions in Methanothermobacter thermautotrophicusFEBS JOURNAL, Issue 1 2003Linda M. I. De Poorter The synthesis of formyl-methanofuran and the reduction of the heterodisulfide (CoM-S-S-CoB) of coenzyme M (HS-CoM) and coenzyme B (HS-CoB) are two crucial, H2 -dependent reactions in the energy metabolism of methanogenic archaea. The bioenergetics of the reactions in vivo were studied in chemostat cultures and in cell suspensions of Methanothermobacter thermautotrophicus metabolizing at defined dissolved hydrogen partial pressures (,pH2). Formyl-methanofuran synthesis is an endergonic reaction (,G°, = +16 kJ·mol,1). By analyzing the concentration ratios between formyl-methanofuran and methanofuran in the cells, free energy changes under experimental conditions (,G,) were found to range between +10 and +35 kJ·mol,1 depending on the pH2 applied. The comparison with the sodium motive force indicated that the reaction should be driven by the import of a variable number of two to four sodium ions. Heterodisulfide reduction (,G°, = ,40 kJ·mol,1) was associated with free energy changes as high as ,55 to ,80 kJ·mol,1. The values were determined by analyzing the concentrations of CoM-S-S-CoB, HS-CoM and HS-CoB in methane-forming cells operating under a variety of hydrogen partial pressures. Free energy changes were in equilibrium with the proton motive force to the extent that three to four protons could be translocated out of the cells per reaction. Remarkably, an apparent proton translocation stoichiometry of three held for cells that had been grown at pH2<0.12 bar, whilst the number was four for cells grown above that concentration. The shift occurred within a narrow pH2 span around 0.12 bar. The findings suggest that the methanogens regulate the bioenergetic machinery involved in CoM-S-S-CoB reduction and proton pumping in response to the environmental hydrogen concentrations. [source] A thermodynamic analysis of the anaerobic oxidation of methane in marine sedimentsGEOBIOLOGY, Issue 5 2008D. E. LAROWE ABSTRACT Anaerobic oxidation of methane (AOM) in anoxic marine sediments is a significant process in the global methane cycle, yet little is known about the role of bulk composition, temperature and pressure on the overall energetics of this process. To better understand the biogeochemistry of AOM, we have calculated and compared the energetics of a number of candidate reactions that microorganisms catalyse during the anaerobic oxidation of methane in (i) a coastal lagoon (Cape Lookout Bight, USA), (ii) the deep Black Sea, and (iii) a deep-sea hydrothermal system (Guaymas basin, Gulf of California). Depending on the metabolic pathway and the environment considered, the amount of energy available to the microorganisms varies from 0 to 184 kJ mol,1. At each site, the reactions in which methane is either oxidized to , acetate or formate are generally only favoured under a narrow range of pressure, temperature and solution composition , particularly under low (10,10 m) hydrogen concentrations. In contrast, the reactions involving sulfate reduction with H2, formate and acetate as electron donors are nearly always thermodynamically favoured. Furthermore, the energetics of ATP synthesis was quantified per mole of methane oxidized. Depending on depth, between 0.4 and 0.6 mol of ATP (mol CH4),1 was produced in the Black Sea sediments. The largest potential productivity of 0.7 mol of ATP (mol CH4),1 was calculated for Guaymas Basin, while the lowest values were predicted at Cape Lookout Bight. The approach used in this study leads to a better understanding of the environmental controls on the energetics of AOM. [source] The Atomic Layer Deposition of HfO2 and ZrO2 using Advanced Metallocene Precursors and H2O as the Oxygen Source,CHEMICAL VAPOR DEPOSITION, Issue 11-12 2008Charles L. Dezelah IV Abstract The atomic layer deposition (ALD) of HfO2 and ZrO2 thin films is investigated using (MeCp)2HfMe2, (MeCp)2Hf(OMe)(Me), (MeCp)2ZrMe2, and (MeCp)2Zr(OMe)(Me) as the precursors at deposition temperatures between 300 and 500,°C, with water vapor as the oxygen source. A self-limiting growth mechanism is confirmed at 350,°C for all the metal precursors examined. The processes provide nearly stoichiometric HfO2 and ZrO2 films with carbon and hydrogen concentrations below 0.5 and 1.0 at.-%, respectively, for representative samples. All films are polycrystalline as deposited, and possess a thin interfacial SiO2 layer. The capacitance-voltage (C - V) and current density-voltage (I - V) behavior is reported and discussed for capacitor structures containing films from this study. [source] | ||||||||||||||||