High surface area thermally stabilized porous iron oxide/silica nanocomposites via a formamide modified sol-gel process

Iron oxide/silica (Fe:Si as 1:10 atomic ratio) composite materials have been prepared by calcination for 3 h at different temperatures (400-900 °C) of xerogel precursor obtained via a formamide modified sol-gel process. The process involved TEOS and iron(III) nitrate, nitric acid and formamide. Genesis of the composite materials from the xerogel precursor has been investigated by TGA, DSC, FTIR, XRD, SEM and EDX. Results indicated that all the calcined composites are mainly composed of amorphous iron oxide dispersed as finely divided particles in amorphous silica matrixes. Nitrogen adsorption/desorption isotherms revealed a reversible type I of isotherms indicative of microporosity. However, high S_BET surface area and microsporosity were observed for the calcined composite materials (e.g. S_BET = 625 m² g⁻¹, and S_αs = 556 m² g⁻¹ for the composite calcined at 400 °C). Formation of the porous texture was discussed in terms of the action of formamide, which enhanced strengthening of the silica gel network during evaporation of the more volatile components within the composite body during the drying process.     

LIMIT CYCLES OF A GENERALIZED LINARD DIFFERENTIAL EQUATION VIA AVERAGING THEORY

We apply the averaging theory of first and second orders to a generalized Liénard differential equation to study the maximum number of limit cycles of the system.     

Microlocalisation et estimations pour\bar \partial _b dans quelques hypersurfaces pseudoconvexes

Résume Nous étudions des estimations précises pour sur des hypersurfaces dont la matrice de Levi dans une base a une certaine forme. En particulier, nous donnons, par la méthode de microlocalisation, une démonstration très simple des estimations dites maximales lorsque les valeurs propres sont comparables. Nous obtenons en fait une précision supplémentaire (inégalité 3.17) qui permet de considérer des cas plus généraux. Finalement, de telles estimations, on déduit des estimations sous elliptiques optimales pour .

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Summary We study precise estimates for on hypersurfaces the Levi matrix of which has certain form. In particular, we give and simple proof of the so-called maximal estimates, using the microlocal analysis. In fact, we obtain a refined version (inequality 3.17), which permits the study of more general cases. Finally, from such estimates, we deduce optimal subelliptic estimates for .

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Oblatum 3-IX-1990     

LIMIT CYCLES FOR A CLASS OF SECOND ORDER DIFFERENTIAL EQUATIONS AND DUFFING EQUATIONS

By the averaging method of frst order, we study in this work the limit cycles for a class of second order diferential equations and Dufng diferential equations which can be seen as a particular perturbation of the harmonic oscillator.     

Effect of iron poly(acrylic acid-co-acrylamide) and melamine polyphosphate on the flammability properties of linear low-density polyethylene

Journal of Thermal Analysis and Calorimetry - This research is exhibited to visualize the squeezed flow of magneto-nanoliquid between two parallel disks. Transportations of heat and mass are...     

Production of Graphite During the Extinguishing Arc with New SF6 Alternative Gases

Most high voltage circuit breakers in operation use SF₆ as the arc interruption medium because of its high dielectric strength and good arc interruption properties. However SF₆ also displays a high global warming potential which motivates the investigation of possible alternatives such as CO₂–Novec?4710 and N₂–Novec?5110 mixtures proposed by different HVCB manufacturers. CB power test comparison between pure CO₂ and CO₂–Novec?4710 mixture shows that the latter created a large amount of graphite and reduced CB performances after several current breaking shots. Using Gibbs free energy method, a theoretical study has been conducted on SF₆ replacement candidates listed above. Aiming to understand graphite formation, several parameters impact were discussed: carrier gas such as CO₂ and N₂, pressure and O₂ initial mixture proportions. The study focuses on graphite condensation temperature and graphite production amount evolutions in respect to the study parameters as well as in respect to CB inter-contacts space condition in terms of pressure and temperature at current-zero. It has been shown that a high percentage of CO₂ in the mixture reduces graphite condensation temperature by 2600 K while adding N₂ do not make such an impact. Pressure increases graphite condensation temperature and O₂ reduces it. The amount of graphite produced is pressure independent but decreases with the increase of O₂ proportion in the initial mixture. It has been shown that adding a sufficient O₂ amount can prevent graphite production at all.

     

Reactivity of N-substituted alkenylidene hydrazinecarbothioamides toward tetracyanoethylene,an efficient synthesis stereoselective 1,3-thiazole compounds

Research on Chemical Intermediates - The reaction between N-substituted alkenylidene hydrazinecarbothioamides and two molar amounts of tetracyanoethylene (TCNE) in anhydrous THF at room temperature...     

Utilization of 2-ylidene-4-thiazolidinones in the synthesis of heterocyclic compounds. Part I: Synthesis of pyrazoles

2-Ylidene and 2,5-diylidene-4-thiazolidinones 2a–d were synthesized and converted into pyrazole derivatives 4a–d by reaction with hydrazine hydrate. A mechanism of this novel conversion is suggested.