Room Temperature Densification of Aerogel by Isostatic Compression

An alternative to the sintering process in densifying aerogels is the compaction by isostatic compression at room temperature. A porosimeter is used to compress the aerogel and to measure the amplitude of the shrinkage. We focus our study on the time dependent mechanisms such as densification kinetics and relaxation effects. These two mechanisms respectively increase or decrease the total densification.The densification kinetics is followed by the evolution of the volumetric shrinkage and stiffening with time. Hydroxylated and esterified aerogels show almost the same shrinkage evolution but the hydroxylated aerogels stiffen during compaction while the elastic bulk modulus of esterified aerogels decreases. Shrinkage is due to two opposing mechanisms: formation of new siloxane bonds that freeze the strained network, but also breakage of links between clusters which allows the restructuring of the solid phase.Relaxation is caused by the disentanglement of the clusters when the pressure is released (40% of the shrinkage can be recovered). However, if during the compression run a large quantity of siloxane bonds are formed, relaxation is not observed.     

Dualite pour les feuilletages transversalement holomorphes

Let F be a transversely holomorphic codimension n foliation on a compact orientable manifold M and E a foliated holomorphic vector bundle on M. We prove that the cohomology of M with coefficients in the sheaf of E-valued holomorphic base-like differential forms satisfies Serre duality. Some computations are given. In case n=1 we show that the base-like cohomology H^*(M/F,) is finite dimensional.     

Catalytic oxidation of α-pinene by transition metal using t-butyl hydroperoxide and hydrogen peroxide

The allylic oxidation of α-pinene 1 was investigated using various catalytic systems. By a proper choice of the latter, the reaction can be directed toward the selective synthesis of verbenol 2 or verbenone 3. High yield into verbenone was achieved under mild conditions with copper salts as catalysts and t-butyl hydroperoxide (TBHP) as oxidant. On the other hand, when Pd(acac)₂ was used with hydrogen peroxide, verbenol was obtained as the main product.     

Synthesis and characterization of ethylxanthato complexes of zinc(II) with P-donor ligands

Monomeric, five-coordinated bis(ethylxanthato)Zn^II(phosphine) complexes [phosphine = PPh₃, P(o-tolyl)₃, P(CH₂Ph)₃] have been synthesized by addition of the phosphine ligand (1:1 molar ratio) to CH₂Cl₂ solutions of [Zn(S₂COEt)₂]. Bidentate ligands Ph₂PCH₂CH₂PPh₂ (dppe) and Ph₂P(CH₂)₄PPh₂ (dppb) reacted in a 1:2 molar ratio to form dinuclear phosphine-bridged complexes. The Zn—P bonds are very labile and are probably broken in solution. The characterization of all the compounds has been carried out by elemental analyses and spectroscopic methods (i.r. and n.m.r.). The structure of binuclear [(S₂COEt)₂Zn(-dppb)Zn(S₂COEt)₂], determined by X-ray crystallography, shows a distorted trigonal bipyramidal environment for the Zn atoms, formed by two chelating xanthate and a bridging dppb ligand.     

Palladium(0)-catalyzed amination of allylic natural terpenic functionalized olefins

The palladium(0) catalyzed amination of allylic acetates and carbonates derivatives from terpenic olefins was carried out under mild conditions. The reaction offers a very good method for the preparation of allylic amines and thus to provide a useful entry to new functionalized terpenic olefin products. The mechanism involving a formation of p-allyl-palladium intermediate complex is in good agreement with the results obtained with the optically active substrates, as well as via an analysis of the observed regio-and stereoselectivity.      

Anticoccidial constituents from the stem bark of <Emphasis Type="Italic">Turraeanthus africanus</Emphasis>

In order to study some biological active products, phytochemical investigation of the stem bark of Turraeanthus africanus have led to the isolation of a novel compound 1, a new benzoic acid derivative, named turraeanthin C, and two known compounds sesamin (2) and stigmasterol. The structures of these compounds were established by spectral analysis, including two-dimensional nuclear magnetic resonance. The extract and the isolated compounds 1 and 2 showed noteworthy activity against Toxoplasma gondii intracellular parasite in mammals. Published in Khimiya Prirodnykh Soedinenii, No. 6, pp. 564–566, November–December, 2008.     

Large Scale Porous Structure in Gels and Relationship with Their Plastic Behavior

Silica gels (classical aerogels and composite aerogels) have been prepared by classical gelation and addition of silica soot in the gelifying solution before gelation. Due to the aggregation mechanisms, these structures are characterized by a fractal organization. The fractal network previously described in the literature (1–100 nm) which results from the aggregation mechanism of the organosiloxane is affected by the addition of the silica soots. Ultra Small Angle X-ray Scattering (USAXS) experiments (done at ESRF) shows that besides the fractal network built by the organosiloxane, the silica soots are forming another porous structure at a higher scale.The mechanical properties seem to be dependent on this large pore structure. Under isostatic pressure, aerogels display an irreversible shrinkage caused by plastic deformation. As a consequence of this plastic shrinkage it is possible to densify, by the pore collapse tending towards the silica glass. The densification mechanism is different from the one obtained by a sintering at high temperature. The pore collapse mechanism is favored by the large pores structure of the composite aerogels, in contrast to viscous sintering.     

On deformations of transversely homogeneous foliations

A transversely homogeneous foliation is a foliation whose transverse model is a homogeneous space G/H. In this paper we consider the class of transversely homogeneous foliations on a manifold M which can be defined by a family of 1-forms on M fulfilling the Maurer–Cartan equation of the Lie group G. This class includes as particular cases Lie foliations and certain homogeneous spaces foliated by points. We develop, for the foliations belonging to this class, a deformation theory for which both the foliation and the model homogeneous space G/H are allowed to change. As the main result we show that, under some cohomological assumptions, there exist a versal space of deformations of finite dimension for the foliations of the class and when the manifold M is compact. Some concrete examples are discussed.