Riccardo Ceccato

Pyrolysis study of methyl-substituted Si-H containing gels as precursors for oxycarbide glasses, by combined thermogravimetry, gas chromatographic and mass spectrometric analysis

Monolithic and transparent gels were prepared by mixing various ethoxide silicon precursors containing Si—CH3 and Si—H groups, the composition ensuring the same number of C—H and Si—H bonds. Pyrolysis of these samples was followed under helium flow by connecting thermogravimetry, gas chromatographic and mass spectrometric analysis, to study the conversion of the gels into oxycarbide materials. In addition to the usual direct thermal and mass spectra analysis (TG–MS), a TG–GC–MS arrangement, allowing gas chromatographic separation of the species simultaneously evolving during thermodecomposition followed by mass spectral analysis, was successfully achieved. Experimental results indicate that mass loss occurs in three steps, each characterized by specific reactions. At low temperatures, densification of the siloxane network derives from further condensation reactions. At intermediate temperatures, a remarkable rearrangement of the siloxane chains occurs, with the release of volatile silanes and several siloxane fragments due to Si—H and Si—O bond exchanges. At higher temperatures, the development of methane was detected and attributed to Si—C bond cleavage. Pyrolysis of gels containing only Si—CH3 or Si—H groups was also studied for comparison.

Production of biodiesel from Jatropha curcas L. oil catalyzed by SO42-/ZrO2 catalyst: Effect of interaction between process variables

This study reports the conversion of Jatrophacurcas L. oil to biodiesel catalyzed by sulfated zirconia loaded on alumina catalyst using response surface methodology (RSM), specifically to study the effect of interaction between process variables on the yield of biodiesel. The transesterification process variables studied were reaction temperature, reaction duration, molar ratio of methanol to oil and catalyst loading. Results from this study revealed that individual as well as interaction between variables significantly affect the yield of biodiesel. With this information, it was found that 4h of reaction at 150°C, methanol to oil molar ratio of 9.88 mol/mol and 7.61 wt.% for catalyst loading gave an optimum biodiesel yield of 90.32 wt.%. The fuel properties of Jatropha biodiesel were characterized and it indeed met the specification for biodiesel according to ASTM D6751.

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.

Development of mullite-SiC nanocomposites by pyrolysis of filled polymethylsiloxane gels

Abstract A modified processing route, which is based on the active filler controlled pyrolysis technique, was followed here to form a homogeneous mullite-SiC nanocomposite. An Al 2 O 3 -filled polymethylsiloxane was prepared by the in-situ formation of the SiCO network via a sol-gel process. The filled gels obtained in this way show an excellent machinability with conventional surface grinding techniques. Heat treatment up to 1500°C initiated crystallization of mullite, which showed intragranular SiC precipitation. Based on microstructure characterization in conjunction with thermal analysis, a formation mechanism is proposed which includes the glass transition temperature of SiCO as well as its phase sparation into SiO 2 plus nanosized SiC crystallites, giving rise to this new mullite-SiC nanocomposite.

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

Unsupported SiO2-based organic–inorganicmembranes. Part 2: Surface features and gas permeation

Gas permeation of unsupported hybrid membranes, obtained by co-hydrolysis of various Si(OEt) 4 /MeSi(OEt) 3 (TEOS/MTES) mixtures, is studied with Ar, He and N 2 . The separation performance of these hybrid membranes is higher than for pure SiO 2 sol–gel derived membranes and depends on chemical composition: a considerable increase in separation factor α(He/N 2 ) is found as the amount of MTES increases; moreover, gas permeability decreases with increasing organic modification of the network. Membrane characterization is performed by N 2 adsorption–desorption measurements and low-temperature differential scanning calorimetry (DSC) and dynamic contact angle (DCA) analyses. Data are related to chemical composition, affecting both the chemical nature of the surface and gas permeation. Results indicate that gas permeation through hybrid membranes may favour the Knudsen flow model or surface diffusion mechanism, depending on the TEOS/MTES ratio.

Optical Study of The Matrix Effect on the ESIPT Mechanism of 3-HF Doped Sol-Gel Glass

In this work we report our preliminary results on the synthesis and characterization of the optical properties of 3-hydroxyflavone (3-HF) doped sol-gel derived glass, to be applied as a wavelength shifting medium in GaAs based near UV-VIS detectors in order to extend the device response down to 350 nm. As already reported by several authors, the intramolecular excited-state proton transfer process (ESIPT) in the 3-HF molecule is strongly affected by the chemical environment. For this reason the evolution of the emission features of the synthesized glasses was investigated by changing the chemical nature of precursor alkoxides used in the sol-gel process. The influence of chemical surroundings on the ESIPT process was studied by analysing the samples with fluorescence spectroscopy. FT-IR spectroscopy was performed in order to study the chemical interactions between the host matrix and the encapsulated dye molecule. The main result of this work is that the Stokes-shifted emission from the 3-HF tautomeric form is enhanced at increasing trifunctional alkoxide amount and at decreasing polarity of the non-hydrolizable groups.

Hybrid Gels as Host Matrices of Perfumed Essences

The variety of SiO2-gel structures allows the use of sol-gel derived materials as host matrices for prolonged release of chemicals. In this work, the release of perfumed essences is studied in the case of hybrid SiO2 materials obtained from alkyl-modified silicon alkoxides. Thermogravimetric analysis in isothermal conditions at various temperatures provide data on the kinetic release process. The effects of physical parameters (porosity reduction, vapour pressure) and chemical interactions (organic molecules with matrix) are discussed for the interpretation of the results.

Synthesis and characterization of strontium-substituted hydroxyapatite nanoparticles for bone regeneration.

The production of stable suspensions of strontium-substituted hydroxyapatite (Sr-HA) nanopowders, as Sr ions vector for bone tissue regeneration, was carried out in the present work. Sr-HA nanopowders were synthesized via aqueous precipitation methods using Sr2+ amount from 0 to 100mol% and were characterized by several complementary techniques such as solid-state Nuclear Magnetic Resonance spectroscopy, X-ray diffraction, Infrared spectroscopy, N2 physisorption and Transmission Electron Microscopy. The substitution of Ca2+ with Sr2+ in HA is always isomorphic with gradual evolution between the two limit compositions (containing 100% Ca and 100% Sr), this pointing out the homogeneity of the synthesized nanopowders and the complete solubility of strontium in HA lattice. Strontium addition is responsible for an increasing c/a ratio in the triclinic unit cell. A significant variation of the nanopowders shape and dimension is also observed, a preferential growth along the c-axis direction being evident at higher strontium loads. Modifications in the local chemical environment of phosphate and hydroxyl groups in the apatite lattice are also observed. Stable suspensions were produced by dispersing the synthesized nanopowders in bovine serum albumin. Characterization by Dynamic Light Scattering and ζ-potential determination allowed to show that Ca2+→Sr2+ substitution influences the hydrodynamic diameter, which is always twice the particles size determined by TEM, the nanoparticles being always negatively charged as a result from the albumin rearrangement upon the interaction with nanoparticles surface. The biocompatibility of the suspensions was studied in terms of cell viability, apoptosis, proliferation and morphology, using osteosarcoma cell line SAOS-2. The data pointed out an increased cell proliferation for HA nanoparticles containing larger Sr2+ load, the cells morphology remaining essentially unaffected.

Probing the chemical environment of 3-hydroxyflavone doped ormosils by a spectroscopic study of excited state intramolecular proton transfer

Abstract The spectroscopic properties of 3-hydroxyflavone (3-HF) molecules entrapped in films and in monoliths of sol–gel derived organically modified silicates (Ormosils) xerogels are studied by excitation and fluorescence spectroscopy as a function of the sol–gel precursors used for the synthesis. Different molar ratios of tetraethoxysilane (TEOS), methyltriethoxysilane (MTES) and phenyltriethoxysilane (PTES) as precursors are used for the sol preparation. Emission and excitation spectra in the ultraviolet–visible range and photo-degradation curves as a function of time are collected with a spectrofluorimeter. The 3-hydroxyflavone optical properties change in the different networks, owing to the effects of the chemical environment on the excited state intramolecular proton transfer and to the solubility of the dye molecules in the different sol–gel systems. It turns out that the spectroscopic features can be used to probe the chemical state of the dye molecules microenvironment.