Reactions of Metal Salts and Alkali Metal Nitrates. Role of the Metal Precursors and Alkali Metal Ions in the Resulting Phase of Zirconia

Zirconia powders are prepared by reaction of a zirconium precursor with an alkali metal nitrate. The major part of the reactions takes place before the melting points and then the reactions go slowly to completion at 450°C in the molten salts. The roles of the precursor and the alkali metal ion are discussed considering the reaction between two precursors, octahydrated zirconium oxychloride and zirconium tetrachloride, and two nitrates, LiNO₃ and NaNO₃, and some resulting physico-chemical differences. The obtained zirconia powders contain very small amounts of alkali metal ions which act as stabilizing agent. Their effect on the balance tetragonal-monoclinic ZrO₂ depends upon the homogeneity of their distribution which is related to their ability to diffuse inside the bulk of particles and their polarizing power when located mainly on the surface.     

Sur la structure des alkylcycloarsoxanes (RAsO)α

Organometallic compounds of general formula (RAsO)_α are associated as trimers (R = C₂H₅, C₃H₇, C₄H₉) or even as a tetramer (R = CH₃). Their physicochemical properties show them to be cyclic, the oxygen atoms being located in the bridging position. A “limited” electron delocalization may occur in these rings.     

Surface effects on Sm valence in amorphous Sm alloys

We present the results of XPS and X-ray absorption measurements performed on the amorphous Sm Au and La Sm Au compounds. The XPS Sm 3d$\text{5}\text{2}$ core level spectra in these compounds reveal that at the surface (5 ? 7 Å) the samarium ions present both the Sm²⁺ (4f⁶) and Sm³⁺ (4f⁵) configurations. When the concentration in samarium decreases it is shown that the Sm²⁺ configuration is strongly enhanced at the surface whereas the X-ray absorption measurements indicate on the contrary that in the bulk only the Sm³⁺ configuration is present. Comparison is made with similar findings in pure crystalline Sm and various crystalline rare earth compounds.     

13C NMR study of isoxazolines and isoxazolidines in the thiophene and benzo[b]thiophene S-oxide and S,S-dioxide series

The ¹³C NMR spectra were recorded for isoxazolines and isoxazolidines from 2,4,6-trimethylbenzonitrile oxide or α-phenyl-N-phenyl nitrone as dipolarophiles with the thiophene and benzo[b]thiophene S-oxide and S,S-dioxide series. The effects of the isoxazoline or isoxazolidine groups are not very different. Owing to the electronegativity of the oxygen atom, the main influence arises from the regioselectivity of the addition to the carbon atoms located at the α or β position from SO or SO₂.     

Optical retardation and structure of developable domains in tubular discotic liquid crystals

The simplest distorted texture of tubular discotic materials are “developable domains”. For small curvature, the tubes are expected to bend circularly around a straight line core. We have observed the birefringence fringes from such a texture in the hexapentoxytriphenylene, close to the isotropic transition. The optical retardation shows a cusp-like singularity above the core. It can be quantitatively described by the assumed circular texture for the tubes. The birefringence is Δn = 0.14 ± 0.01 for λ = 0.65 μm.     

Influence of the Brownian step size in off-lattice Monte Carlo simulations of irreversible particle aggregation

The influence of the Brownian step size in off-lattice Monte Carlo simulations of the aggregation and gelation of spheres is studied. It is found that the kinetics are strongly influenced if the step size is larger than the mean smallest distance between the sphere surfaces. The structure of the clusters and the gels is influenced, but only over length scales smaller than the step size. Using large step sizes leads to a narrower size distribution of the clusters. Implications of the present results are discussed for simulations reported in the literature in which the Brownian step size was chosen equal to the sphere diameter.     

Monte Carlo simulation of particle aggregation and gelation: I. Growth,structure and size distribution of the clusters

The application of a statistical method, the local polynomial regression method, (LPRM), based on a nonparametric estimation of the regression function to determine the critical micelle concentration (cmc) is presented. The method is extremely flexible because it does not impose any parametric model on the subjacent structure of the data but rather allows the data to speak for themselves. Good concordance of cmc values with those obtained by other methods was found for systems in which the variation of a measured physical property with concentration showed an abrupt change. When this variation was slow, discrepancies between the values obtained by LPRM and others methods were found.Received: 2 October 2003, Published online: 25 March 2004PACS: 02.50.-r Probability theory, stochastic processes, and statistics - 82.70.-y Disperse systems; complex fluids     

Surface modification of several carbon-based materials: comparison between CF4 rf plasma and direct F2-gas fluorination routes

Due to their extreme reactivity, fluorine and fluorinated gases may be used to modify the surface properties of numerous materials. In the following, the surface fluorination of some carbon-based compounds (graphite, graphitised carbon fibres, carbon blacks and elastomers) using CF₄ rf plasma technique and direct F₂-gas fluorination is proposed. From XPS studies, the different types of CF bonding obtained in the materials after treatment have been correlated either to the physico-chemical characteristics of the pristine material or to the experimental parameters of the fluorination. Reaction mechanisms are proposed.     

Unprecedented in situ oxidative ring cleavage of isoxazolidines: diastereoselective transformation of nitronic acids and derivatives into 3-hydroxymethyl 4-nitro tetrahydrofurans and pyrrolidines

[reaction: see text] Nitronic acids undergo an intramolecular 1,3-dipolar cycloaddition to unactivated double bonds, and the resulting isoxazolidines spontaneously evolve by an unprecedented in situ oxidative ring cleavage. The extension of this transformation to silyl nitronates results in a general diastereoselective construction of hydroxymethyl nitro functionalized tetrahydro-furans and -pyrrolidine having up to four consecutive stereogenic centers.