Sandra Dirè

Sol–gel synthesis of siloxane–oxide hybrid coatings [Si(CH3)2O·MOx: M = Si, Ti, Zr, Al] with luminescent properties

Siloxane–oxide hybrid coatings have been prepared via the hydrolysis–condensation of diethoxydimethylsilane and various metallic alkoxides, M(OR)n with M = Si, Ti, Zr and Al. The rheological behaviour of the precursor sols and their hydrophobic character due to the presence of methyl groups, allows the formation of films several tens of micrometres thick, with no cracks. Preliminary structural characterizations are presented on the Ti-containing system using MAS NMR and X-ray absorption techniques. Rhodamine 6G and coumarin 4 have been embedded in these films. Some preliminary optical properties (absorption and emission) are presented. Most of the systems exhibit strong fluorescence, even for high dye concentration. This effect seems to be related to the particular structural nature of these matrices.

Electrical conductivity of tin oxide films prepared by the sol-gel method

Abstract A preliminary study concerning the electrical conductivity of tin oxide films obtained by a procedure of dipping sol-gel technique onto glass substrate is reported. The tin oxide-sol solution was prepared starting from stannous chloride. Crystallization of amorphous tin oxide was followed by means of XRD and DTA analyses. The morphology of the coatings was observed by SEM. By this method, it has been verified that tin oxide can be easily doped by means of zirconium and titanium oxides, giving films, in which electrical conductivity is strongly increased.

XPS studies of SiO2-TiO2 powders prepared by sol-gel process

Abstract SiO2, TiO2 and SiO2-TiO2 powders have been prepared via a sol-gel process using silicon tetraethoxysilane (TEOS) and titanium tetraisopropoxide Ti(OPri)4. X-ray photoelectron spectroscopy (XPS) is used for studying the chemical bondings of silicon, titanium and oxygen as a function of the air thermal treatment temperature (up to 1000°C). The lineshape of the Si 2p and Ti 2p peaks indicate that silicon and titanium are present as SiO2 and TiO2 oxide both for the starting and thermal-treated powders. These results are confirmed by consideration of O 1s and O KVV bands, which makes possible to distinguish between the single O-Ti and O-Si bonds and also to disclose the presence of cross-linking Si-O-Ti bonds that act as bridges between SiO2 and TiO2 moieties. Starting from 600°C, these bonds are broken and the formation of new Ti-O and Si-O bonds takes place. Furthermore, Si/Ti atomic ratios based both on curve-fitting measurements of the single O 1s components and on the Ti2p 3 2 and Si 2p peaks, show changes of the surface chemical composition of thermal-treated powders.

Silicon Oxycarbide Glasses from Sol-Gel Precursors

Silicon oxycarbide glasses have been prepared from sol-gel precursors containing not only Si-CH 3 , but also Si-H bonds. Three systems have been chosen containing various Si units but the same C/Si ratio. Their pyrolysis process has been mainly followed by 29 Si MAS-NMR and the composition of the final glass extracted. This study shows that a suitable choice of Si-CH 3 and Si-H functionnalized silicon alkoxides can lead to a strong decrease in the free carbon content and to an almost pure silicon oxycarbide phase.

Solid state NMR and IR characterization of wood polymer structure in relation to tree provenance.

(13)C nuclear magnetic resonance and mid-infrared spectroscopies were used for characterizing changes in the chemical structure of wood polymers (cellulose, hemicellulose and lignin) in relation to the tree growth location. Samples of three provenances in Europe (Finland, Poland and Italy) were selected for studies. The requirement was to use untreated solid wood samples to minimize any manipulation to the nanostructure of native wood. The results confirm that the chemical and physical properties of samples belonging to the same wood species (Picea abies Karst.) differ due to the origin. Both FT-IR and dynamic NMR spectroscopies were able to correctly discriminate samples originating from three different provenances in Europe. Such methods might be very useful for both, research and understanding of wood microstructure and its variability due to the growth conditions.

Unsupported SiO2-based organic–inorganicmembranes

Tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) have been used to prepare hybrid SiO 2 -based membranes. These self-supported materials were obtained from controlled polymerization reactions for various TEOS/MTES molar ratios ensuring the achievement of crack-free disks 8 cm in diameter and 10–40 µm in thickness. The rheological behaviour of precursor solutions was studied and gelling times were determined. The whole process, from starting solution to xerogel, was followed by FTIR spectroscopy, viscosity measurements and multinuclear solid-state NMR, and is discussed in terms of the hydrolysis–condensation kinetics of tetrafunctional and trifunctional silicon alkoxides. Density, shrinkage, elastic modulus (E), modulus of rupture (MOR) and elongation at break were all determined and related to preferential structural arrangements of networks according to the TEOS/MTES ratio.

Thermal Evolution and Crystallisation of Polydimethylsiloxane-Zirconia Nanocomposites Prepared by the Sol-Gel Method

Polydimethylsiloxane‐zirconia nanocomposites have been prepared by hydrolysis of diethoxydimethylsilane and zirconium n-propoxide in diAerent molar ratios. Transparent, homogeneous and non-porous xerogels have been obtained up to 70 mol% ZrO2 content. The starting xerogels have been pyrolyzed under argon atmosphere up to 1400C and the structural evolution of samples treated at diAerent temperatures has been followed by X-ray diAraction, transmission electron microscopy, infrared and 29 Si solid state nuclear magnetic resonance spectroscopies, thermal analyses and N2 sorption measurements. The polymer-to-ceramic conversion leads to the structural rearrangement of the siloxane component with the production at 600C of high surface area materials with pore sizes below 3nm. Samples are amorphous up to 800C. At 1000C, the structural evolution of the silicon moiety produces an amorphous oxycarbide phase whereas the primary crystallisation of tetragonal zirconia takes place, with crystallinity and crystallite sizes depending on the ZrO2 content. At 1400C, the silicon oxycarbide phase generates a mixture of amorphous silica and crystalline silicon carbide polymorphs. In this matrix, tetragonal and monoclinic ZrO2 phases are present with ZrO2 average crystallite dimensions never exceeding 20nm, for ZrO2 content50mol%. The tetragonal/monoclinic ratio as well as the crystallite sizes appear strictly related to the chemical composition. # 1999 Elsevier Science Ltd. All rights reserved

Synthesis and characterization of siloxane-titania materials

Abstract Diethoxydimethylsilane and titanium isopropoxide have been used for preparation, via the sol-gel process, of mixed siloxane-titania materials. Depending on the Ti/Si molar ratio, flexible to brittle gels can be obtained. The rheological properties of the sols can be adjusted in order to obtain films and transparent monoliths. Due to the high amount of hydrophobic methyl groups in these materials, thick films with an average thickness of 20 μm can be obtained. These films have been used for laser dye matrices. A characterization of the various steps of the hydrolysis-condensation process as well as of the structure of the final solid gel determined by liquid and solid state NMR and TiK edge X-ray absorption spectroscopy are presented. A structural model for the gel is proposed mainly based on siloxane chains and TiO2 based particles. Optical properties of Coumarin 4 and Rhodamin 6G incorporated in such films are discussed.

A comparative analysis of surface structure and surface tension of hybrid silica films

Unsupported SiO2-based films have been prepared by cohydrolysis of methyltriethoxysilane (MTES) and tetraethoxysilane (TEOS) in different molar ratios. Analysis of surface structure and surface tension have been performed; FTIR-ATR (attenuated total reflectance) results were compared with results obtained from atomic force microscopy (AFM) and dynamic contact angle (DCA) techniques. The chemical nature of the exposed surface is discussed on the basis of calculations of surface tension components using different theoretical models. Due to the different sampling depth of the techniques used, a complete and original description of the surface structure of SiO2-based films is proposed. The relative ratio between the exposed groups Si–OH, Si–O− and Si–CH3 depends on the TEOS/MTES ratio. Increasing %MTES produces a decrease of Si–OH and Si–O− surface content. The surface of pure MTES derived film appears to be a flat and dense arrangement of Si–Me groups pointing outward.

Micro- and nano-hydroxyapatite as active reinforcement for soft biocomposites.

Pectin-based biocomposite hydrogels were produced by internal gelation, using different hydroxyapatite (HA) powders from commercial source or synthesized by the wet chemical method. HA possesses the double functionality of cross-linking agent and inorganic reinforcement. The mineralogical composition, grain size, specific surface area and microstructure of the hydroxyapatite powders are shown to strongly influence the properties of the biocomposites. Specifically, the grain size and specific surface area of the HA powders are strictly correlated to the gelling time and rheological properties of the hydrogels at room temperature. Pectin pH is also significant for the formation of ionic cross-links and therefore for the hydrogels stability at higher temperatures. The obtained results point out that micrometric-size hydroxyapatite can be proposed for applications which require rapid gelling kinetics and improved mechanical properties; conversely the nanometric hydroxyapatite synthesized in the present work seems the best choice to obtain homogeneous hydrogels with more easily controlled gelling kinetics.