B. Michaux

Synthesis of barium titanate by the sol-gel process

Abstract 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 during 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 dyring 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°C from the cubic to the tetragonal form of BaTiO3 has been observed by polarization microscopy.

Synthesis of SiO2-TiO2 xerogels by sol-gel process

A new method to synthesize SiO2-TiO2 gels by sol-gel process has been developed. This technique uses tetraisopropylorthotitanate [Ti(OiPr)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.

Synthesis and Characterization of Monodisperse Spherical Zirconia Particles

Monodisperse zirconia spherical particles are prepared by hydrolysis of zirconium propoxide in 2-methoxyethanol in presence of decanoic acid as a shape stabilizer. The powder is analyzed by computer image analysis on TEM micrographs, TG-DSC, FTIR, X-ray diffraction and nitrogen adsorption isotherms. A competition phenomenon between aggregation and individual growth of the particles during precipitation is deduced from the observations.

Rheological characterization of the BaTiO3 sol–gel transition

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.

Rheological characterization of BaTiO3 sol-gel transition

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.

Silica-zirconia monoliths from gels

Zirconium propoxide and tetraethylorthosilicate were hydrolyzed in methoxyethanol which acts as stabilizer of zirconium and in presence of formamide or dimethylformamide as Drying Control Chemical Additives in order to obtain zirconia silica gels. The gels were dried at 70°C to get monolithic xerogels. The influence of zirconium content and DCCA on texture was determined. The evolution of these xerogels was investigated as a function of temperature between 70°C and 1000°C by thermogravimetry, differential scanning calorimetry, I.R. spectroscopy and R.X. diffractometry.

Synthesis and characterization of porous silica-alumina xerogels

Porous silica-alumina xerogels are synthesized through two methods, which differ by the aluminium precursor: aluminium tri-secbutoxide and aluminium nitrate nonahydrate. The silicium precursor is tetraethylorthosilicate. The porous texture is studied by nitrogen adsorption-desorption isotherms. It is found that the porous texture mainly depends on one parameter for each preparation method: hydrolysis catalyst in one case and aluminium content in the other case.

Preparation and characterization of porous BaTiO3 xerogels by sol-gel process

Abstract Homogeneous BaTiO3 gels are prepared by hydrolysis and polycondensation between titanium isopropoxide and barium hydroxide in the presence of methoxyethanol, methanol and water. Two drying methods are used: the first one under vacuum and the second one under a nitrogen flow. The influence of the drying method on the texture is studied through nitrogen adsorption-desorption isotherms analysis. A heat treatment is defined on the basis of the thermogravimetry (TG) and differential scanning calorimetry (DSC) results. The influence of the calcination temperature on the texture and the crystallization of the solids is discussed.