Rheological characterization on the BaTiO3 sol-gel transition

in Journal of Sol-Gel Science and Technology (1997), 9

Synthesis and characterization of porous silica-alumina xerogels

in Journal of Sol-Gel Science and Technology (1997), 8

Rheological characterization of BaTiO3 sol-gel transition

in Journal of Sol-Gel Science and Technology (1997), 9(1), 5-15

BaTiO3 gels were prepared by hydrolysis and polycondensation reactions between titanium isopropoxide and barium hydroxide in presence of methoxyethanol, methanol and water. The rheology of the sol-gel ... [more ▼]

BaTiO3 gels were prepared by hydrolysis and polycondensation reactions between titanium isopropoxide and barium hydroxide in presence of methoxyethanol, methanol and water. The rheology of the sol-gel transition was studied with a rheometer allowing low amplitude sinusoidal oscillations. Experimental data show a continuous increase in the complex viscosity along with time, showing the progressive character of the transition. The influence of synthesis operating variables was studied. The gelation time, which definition is based on viscoelastic measurements, increases exponentially when the water content is increased, when the dilution due to the methoxyethanol is reduced or when the temperature is lowered. Different growth models were used for the characterization of the particles in the solution. These models suggest that the polymerisation first produces spherical particles (mass fractals) and that these spherical particles then agglomerate to form a linear network. [less ▲]

Aerogel compression theoretical analysis

in Journal of Non-Crystalline Solids (1997), 212(2-3), 262-267

Several authors have empirically shown that, in aerogels, a power law exists between the mechanical moduli and the bulk density. An exponent value can be determined from mercury porosimetry curves. The ... [more ▼]

Several authors have empirically shown that, in aerogels, a power law exists between the mechanical moduli and the bulk density. An exponent value can be determined from mercury porosimetry curves. The shrinkage of aerogels under mercury pressure follows a buckling mechanism which links the pore size to the exerted pressure. The present study relates the exponent to the pore volume distribution which can be described by a hierarchical model valid in a large range of pore size, so a physical meaning is given to the exponent. [less ▲]

Transition metal aerogel-supported catalyst

Patent (1996)

A process for preparing a supported catalyst comprising a transition metal selected from palladium, platinum, nickel, cobalt or copper on an aerogel support, which includes the steps of providing a ... [more ▼]

A process for preparing a supported catalyst comprising a transition metal selected from palladium, platinum, nickel, cobalt or copper on an aerogel support, which includes the steps of providing a mixture containing an alkoxide precursor of the aerogel, a chelate complex of the transition metal with a chelating agent having Si(OR)3 anchor groups, and an organic solvent in which the chelate complex is soluble; hydrolyzing the mixture by admixing it with water to form a gel; and converting the gel under supercritical conditions into the transition metal aerogel-supported catalyst. The supported catalyst has an especially homogeneous distribution of the metal component and is suitable, for example, for use as a hydrogenation catalyst. [less ▲]

SYNTHESIS OF SIO2-TIO2 XEROGELS BY SOL-GEL PROCESS

in Journal of Sol-Gel Science and Technology (1995), 4(3), 179-185

A new method to synthesize SiO2-TiO2 gels by sol-gel process has been developed. This technique uses tetraisopropylorthotitanate [Ti(O'Pr)(4)] and tetraethylorthosilicate [TEOS]: they are mixed in the ... [more ▼]

A new method to synthesize SiO2-TiO2 gels by sol-gel process has been developed. This technique uses tetraisopropylorthotitanate [Ti(O'Pr)(4)] and tetraethylorthosilicate [TEOS]: they are mixed in the same solvent and then directly hydrolysed. This one-step reaction is possible because of the use of 2-methoxyethanol, a protic polar solvent. This alcohol plays two different specific roles: it acts as a solvent as well as a stabilizer of titanium alkoxide towards the hydrolysis-precipitation reaction. So, by an accurate adjustment of the quantity of methoxyethanol in the mixture, we can control the reactivity of the titanium precursor. Monolithic and transparent xerogels were obtained whatever the composition. Three monolithic SiO2-TiO2 gels containing 20, 50 and 75 mol% of TiO2 were prepared and studied in details. By using the TG-DSC analysis, we can follow the evolution of the loss of water and organic residues. The structural evolution of gels during calcination is characterized by IR spectroscopy and X-Ray diffraction. [less ▲]

Synthesis of barium-titanate by the sol-gel process

in Journal of Non-Crystalline Solids (1994), 175(1), 1-13

Homogeneous BaTiO3 gels were prepared by hydrolysis and polycondensation reactions between titanium isopropoxide and barium hydroxide in the presence of methoxyethanol, methanol and water. Gel formation ... [more ▼]

Homogeneous BaTiO3 gels were prepared by hydrolysis and polycondensation reactions between titanium isopropoxide and barium hydroxide in the presence of methoxyethanol, methanol and water. Gel formation mechanisms were investigated by Raman scattering spectrometry. Two drying methods are carried out: either in air with a possible formation of barium carbonate or in a vacuum furnace, sheltered from CO2. The influence of the drying method on texture is studied through nitrogen adsorption-desorption isotherm analyses. A heat treatment is deduced from thermogravimetry and differential scanning calorimetry results. The influence of the calcining temperature on the texture and the crystallization of the solids has been measured. For the first time, the transformation at 900 degrees C from the cubic to the tetragonal form of BaTiO3 has been observed by polarization microscopy. [less ▲]

Modeling of the chemical processes in a longwall face underground gasifier at great depth

in In Situ (1993), 17(1), 83-104

In order to simulate the cavity growth of an underground gasifier in thin seams, at great depth, Wilks1,2 has developed an original model based on the distribution of fluid flows around an injection well ... [more ▼]

In order to simulate the cavity growth of an underground gasifier in thin seams, at great depth, Wilks1,2 has developed an original model based on the distribution of fluid flows around an injection well. In this paper, we aim to introduce within this model a description of chemical processes on the wall and within the fluid phase, in order to determine the temperature and concentration profiles along the coal wall. These profiles are dependent only on two factors: the transfer coefficients and the energy balance of the gas generator. A study of the sensitivity of the operating variables has been carried out. [less ▲]

Heavy water tracing test in the underground coal gasification process at Thulin (Belgium)

in Fuel (1990), 69

Réactions et réacteurs gaz-liquide-catalyseur solide

in Nouvelles de la Science et des Technologies (1988), 6