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Fast Decrease (fast + decrease)

Distribution by Scientific Domains

Polymers and Materials Science	50%

Selected Abstracts

Effect of the Media on the Quantum Yield of Singlet Oxygen (O2(1,g)) Production by 9H -Fluoren-9-one: Solvents and Solvent Mixtures

HELVETICA CHIMICA ACTA, Issue 2 2003
Claudia
We have investigated the effect of a series of 18 solvents and mixtures of solvents on the production of singlet molecular oxygen (O2(1,g), denoted as 1O2) by 9H -fluoren-9-one (FLU). The normalized empirical parameter E derived from ET(30) has been chosen as a measure of solvent polarity using Reichardt's betaine dyes. Quantum yields of 1O2 production (,,) decrease with increasing solvent polarity and protic character as a consequence of the decrease of the quantum yield of intersystem crossing (,ISC). Values of ,, of unity have been found in alkanes. In nonprotic solvents of increasing polarity, ,ISC and, therefore, ,, decrease due to solvent-induced changes in the energy levels of singlet and triplet excited states of FLU. This compound is a poor 1O2 sensitizer in protic solvents, because hydrogen bonding considerably increases the rate of internal conversion from the singlet excited state, thus diminishing ,, to values much lower than those in nonprotic solvents of similar polarity. In mixtures of cyclohexane and alcohols, preferential solvation of FLU by the protic solvent leads to a fast decrease of ,, upon addition of increasing amounts of the latter. [source]

Fast and Almost Complete Nitridation of Mesoporous Silica MCM-41 with Ammonia in a Plug-Flow Reactor

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2010
Fumitaka Hayashi
The title reaction proceeded well to yield silicon (oxy)nitride at 973,1323 K using a plug-flow reactor. The degree of nitridation was studied as a function of temperature and time of nitridation, the sample weight, and the flow rate of ammonia. It was dependent on the reaction temperature and the amount of ammonia supplied per sample weight. The nitridation at 1273 K for 10,25 h yielded the oxynitride with 36,39 wt% nitrogen, which was very close to 40 wt% of Si3N4. Characterization with X-ray diffraction, field-emission scanning electron microscopy and transmission electron microscopy measurements, and nitrogen adsorption revealed the conversion of MCM-41 to the corresponding oxynitride without essential loss of the mesoporous structure, the decrements of the lattice constant and the pore diameter by 20,35%, and the increments of the wall thickness by ca. 45%. Solid-state 29Si nuclear magnetic resonance spectra during the nitridation clearly showed fast decrease in SiO4 species and slow in SiO3(OH). Various intermediate species, SiOxNy(NH2 or NH)z, were observed to be formed and finally, ca. 70% SiN4 species, ca. 20% SiN3(NH2 or NH), and ca. 10% SiON2(NH2 or NH) were produced, being consistent with the results of the above mentioned elemental analysis. [source]

Galvanisches Verzinken von Magnesiumlegierungen

MATERIALWISSENSCHAFT UND WERKSTOFFTECHNIK, Issue 2 2007
H. Pokhmurska
magnesium alloy; zinc coating; electro plating of zinc; corrosion behaviour Abstract Magnesiumlegierungen sind sehr korrosionsanfällig, was ihre Anwendung unter korrosiven Bedingungen einschränkt. Eine Möglichkeit, um Magnesium vor aggressiven Umweltbedingungen zu schützen, ist das Beschichten. Es werden Ergebnisse über das Korrosionsverhalten der aus verschiedenen Elektrolyten galvanisch verzinkten Magnesiumknetlegierung AZ31 gezeigt. Die Bewertung der Korrosionsprozesse in chloridhaltigen Lösungen erfolgte durch elektrochemische Messungen. Durch einstündige Auslagerung der beschichteten Proben wurde festgestellt, dass dicke und dichte galvanische Zinkschichten auf AZ31 die Korrosionseigenschaften verbessern. Eine Erhöhung der Immersionszeit führt jedoch zu einer Verschlechterung der Korrosionseigenschaften. Elektrolytische Zinkschichten, die durch einen konsekutiven alkalisch / sauren Prozess erzeugt werden, stabilisieren die Schicht und verbessern somit die Korrosionsbeständigkeit des beschichteten AZ31. Zinc-Plating of Magnesium Alloys Magnesium alloys are highly susceptible to corrosion that limits their application when exposure to corrosive service conditions is needed. One of the ways to prevent corrosion is to coat the magnesium-based substrate to avoid a contact with an aggressive environment. Results concerning corrosion behaviour of wrought AZ31 magnesium alloy with electrolytic zinc coatings deposited from different electrolyte solutions are described. Evaluation of corrosion processes in chlorides containing solutions was performed by electrochemical measurements. It was found that thick and dense electrolytic zinc coatings formed on AZ31 significantly improve the corrosion behaviour of magnesium alloy after one hour immersion of zinc coated magnesium alloys in corrosive media. Further increase of immersion time leads to relatively fast decrease of corrosion properties. Electrolytic zinc coatings obtained in consecutive alkaline / acidic process demonstrate an improvement of corrosion resistance of coated AZ31. The time to coating degradation strongly increases. [source]

Silicon incorporation in CVD diamond layers

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 11 2005
J. Barjon
Abstract The silicon incorporation in diamond is an important issue as silicon is widely used as a substrate for the growth of polycrystalline thin films. Incorporated silicon impurities are suspected to come from the hydrogen etching of the silicon substrate. To clearly establish this point we introduced a solid source of silicon during the growth of a homoepitaxial diamond layer on a HPHT diamond substrate. A quantitative SIMS analysis revealed concentrations of silicon up to 3 × 1019 cm,3 in the diamond layer. Then we propose a scenario for the contamination of polycrystalline diamond grown on silicon substrates: after nucleation, the progressive paving of the silicon surface by 3D grains causes a fast decrease of its incorporation. At coalescence, the silicon substrate is completely covered by a 2D diamond film and the silicon concentration in diamond reaches a residual level. The investigated MPCVD and HFCVD diamond layers grown on silicon substrates present comparable features. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]