Textural properties and thermal stability of silica-zirconia aerogels

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

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 ▲]

Freeze-dried resorcinol-formaldehyde gels

in Journal of Non-Crystalline Solids (1997), 212(2-3), 250-261

The texture of resorcinol-formaldehyde gels, previously synthesized by Pekala et al. [1-6], but dried by freeze-drying was studied. The freeze-dried gels present a complex porous structure from nanometer ... [more ▼]

The texture of resorcinol-formaldehyde gels, previously synthesized by Pekala et al. [1-6], but dried by freeze-drying was studied. The freeze-dried gels present a complex porous structure from nanometer to micrometer length-scale. Taking a standard freezing rate, we study in detail the influence of the molar ratio resorcinol/catalyst on the texture, This molar ratio is one of the parameters under consideration which can lead to a tailoring of the dried gel texture in order to develop adsorbents or catalyst supports of controlled porosity. Characterization methods include nitrogen adsorption-desorption (including the classical and the fractal interpretations), mercury porosimetry, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). [less ▲]

AC Properties of Carbon Black Filled Polymer Blends

in Materials Research Society Symposium Proceedings (1996), 411

The macroscopic electrical properties of carbon black filled polyethylene (PE)/poly-styrene (PS) blends depend strongly on the selective localization of carbon black particles in one of the phases of the ... [more ▼]

The macroscopic electrical properties of carbon black filled polyethylene (PE)/poly-styrene (PS) blends depend strongly on the selective localization of carbon black particles in one of the phases of the polyblend. The electrical and dielectric properties of these materials can be understood in the framework of the double percolation theory. It appears that in the vicinity of the carbon black percolation threshold, the electrical conduction is dominated by tunneling. The recent theory of Brouers and Sarychev which includes quantum tunneling in the percolation scaling laws provides the basis for a discussion of the observed frequency dependence of the conductivity and the dielectric constant in the range 10-3 –10° Hz and its relation to the blend morphology. [less ▲]