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.     

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(O ⁱ 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.     

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.     

Textural properties of low-density xerogels

The extent of shrinkage during drying is controlled by the balance between the capillary pressure developed in the pore liquid and the modulus of the solid network. One first method to obtain low-density xerogels consists in strengthening TEOS-based alcogels by providing new monomers to the alcogel after gelation. In the second method, low-density xerogels are produced by surface modification (silylation) of the wet gel with trimethylchlorosilane. The capillary pressure is reduced and the presence of non-reactive species on the surface makes the shrinkage reversible. A reduction of the capillary pressure can be achieved by introduction of a substituted alkoxide 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS) to a TEOS-based alcogel, synthesised in a single base-catalysed step. This additive acts as a nucleation agent leading to big silica particles (20 nm) with a low EDAS/TEOS ratio (0.03). The pores between those particles are also large and the drying stress is reduced. The textural properties of those three materials are compared: bulk densities of the samples modelled on the first and third method are varying in the same range (0.25–0.35 g/cm³) while xerogels obtained by the surface modification process are less dense (0.1–0.15 g/cm³). The biggest pores are observed in the third method.     

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.     

5-Amino-6-nitroquinoxalines from 6-Nitroquinoxalines by the Amination Reaction with Hydroxylamine

The yields of the direct amination of 6-nitroquinoxalines by hydroxylamine have been improved, opening thus a better access to 1,4,5,8-tetra-azaphenanthrenes.     

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.     

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.     

Answer to the comments made by N. Gibson,H. Rauscher and G. Roebben on the article by A.J. Lecloux (J Nanopart Res (2015) 17:447) regarding the use of the volume-specific surface area (VSSA) to classify nanomaterials

This article is an answer to the comments made by N. Gibson, H. Rauscher and G. Roebben on the article by Lecloux (J Nanopart Res 17:447, 2015) regarding the use of the volume-specific surface area (VSSA) to classify nanomaterials. The key point of discussion is the comparison of two models. The VSSA concept, developed for monodispersed spherical non-porous particle size distribution, was applied in two different ways to calculate the VSSA corresponding to a poly-dispersed distribution of particle size: on one side, a total VSSA value that is the number-weighted sum of the individual VSSAs corresponding to each particle size present with its frequency in the distribution. On the other side, a weight-averaged VSSA based on the measurement method, dividing the total surface area of all the particles by their total volume. The key questions are which model is better representing the experimental VSSA values obtained by analysis of the nitrogen adsorption isotherms and which model is the most appropriate to identify nanomaterials? This cannot be answered on a theoretical basis. Only a comparison with various experimental data could decide which model is the most appropriate.