Andre Lecloux

Study of the texture of monodisperse silica sphere samples in the nanometer size range

Abstract The texture of monodisperse silica spheres in the diameter range 8–200 nm, and in the form of powders and slabs is examined. The analysis of nitrogen adsorption—desorption isotherms shows the respective contributions of the voids between particles and the internal microporosity, as well as the influence of pelletization on texture. The importance of the adsorbent—adsorbate interactions on the selection of the standard isotherms to obtain a self-consistent picture of the solid texture is also emphasized.

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.

Textural properties and thermal stability of silica-zirconia aerogels

A solution of zirconium propoxide stabilized by methoxyethanol is mixed with silicon ethoxide in ethanol and hydrolyzed by small amounts of neutral, basified or acidified water. The obtained silica-zirconia gel is dried in supercritical conditions, yielding an aerogel. The modifications of the aerogel porous texture as a function of chemical composition and thermal treatment are investigated using adsorption-desorption isotherms analysis, mercury porosimetry, electron-microscopy and X-ray diffraction.

Texture and sintering of zirconium dioxide-yttrium oxide ceramics

Abstract Mercury porosimetry was used to investigate the behaviour of two commercial yttriastabilized zirconias during sintering. It was found to be advantageous to use as the initial powder a material characterized by a monodisperse distribution of small particle sizes in order to lower the temperature of total densification.

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.

Texture evolution of zirconium oxide xerogels prepared by sol-gel process

Abstract Zirconium propoxide was hydrolyzed in methoxyethanol in order to obtain zirconia gels. The gels were dried at 110°C to get xerogels. The textural and crystalline evolution of these xerogels was investigated as a function of temperature between 110 and 800°C. The influence of hydrolysis ratio of the zirconium propoxide, as well as the effect of stabilizers such as calcium, magnesium and yttrium cations, were studied in order to define the most suitable experimental conditions to obtain dense bulk zirconia ceramics.

The Generalized Broekhoff-De Boer Method

Abstract In this paper, the texture of four different porous solids is described using a consistent methodology of adsorption-desorption isotherm analysis, based on the BET theory, a generalized t-method and a generalization of the BdB method for pore size distribution calculation. It is demonstrated that the texture of porous solid obtained in this manner remains identical whatever the vapour used to carry out the adsorption-desorption isotherms, provided that the adsorbed molecules have nearly the same size and consequently the same access to the porous texture (i.e N2, O2, Ar, CO). This result clearly confirm the validity of this methodology.