Jean-Paul Pirard

The importance of standard isotherms in the analysis of adsorption isotherms for determining the porous texture of solids

Abstract The basic principle of the t - and α s -methods is to compare the adsorption isotherm under test to a standard isotherm in a reduced form, corresponding to a nonporous reference solid. It is demonstrated that the standard isotherm has to be chosen according to the intensity of the adsorbentadsorbate interactions, expressed by the C BET constant. A set of experimental reference isotherms, corresponding to definite ranges of the C BET constant value between 20 and about 3000, is proposed. These isotherms are shown to be very well fitted by the n -layers-BET equation, with n nearly equal to 4. It is also shown that the t - and α s -methods are strictly relevant to the BET theory and that these two methods are two equivalent ways of analyzing adsorption isotherms. In particular, the microporosity of solids can be assessed, within the limits of the BET model, provided that the reference isotherm is correctly chosen. All these results are proved to be valid whatever the nature of the adsorbent-adsorbate pair used. Some isotherms obtained with N 2 , Ar, CO, O 2 , CO 2 are reported.

INTERPRETATION OF MERCURY POROSIMETRY APPLIED TO AEROGELS

The observation of aerogels submitted to a pressure of mercury indicates that this porous material is compacted and not intruded by the mercury. Consequently, the classical Washburn equation cannot be applied. A relation is established between the pressure P of compaction and the size L of the largest pores. The size of pores is estimated by using the nitrogen adsorption-desorption isotherms analysis and SEM measurements. A relation is found in which P is proportional to L −4 The new relation is applied to mercury porosimetry. Finally, a mechanical model is proposed that reproduces successfully the behavior of aerogels under high pressure of mercury.

Preparation of low-density xerogels through additives to TEOS-based alcogels

A new process for preparing silica xerogels with similar textural properties to silica aerogels by drying under vacuum has been studied. The xerogels are produced by adding, before gelation, 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS) to tetraethylorthosilicate (TEOS)-based alcogels, synthesised in a single base-catalysed (NH3) step. It is hypothesized that EDAS acts as a nucleation agent leading to silica particles with a hydrolysed EDAS core and a shell principally made of hydrolysed TEOS. The EDAS concentration and the basicity of the aqueous NH3 solution are important parameters influencing the resistance of the gel to drying stress. A decreasing EDAS/TEOS ratio or an increasing concentration of NH3 at constant EDAS content leads to less shrinkage during drying and so the final pore volume is larger. Gels prepared with a low EDAS/TEOS ratio (about 0.03) contain large particles (∼20 nm) due to the nucleation process by EDAS, thus the pores between those particles are also large and the drying stress is reduced.

Convective Drying of Wastewater Sludges: Influence of Air Temperature, Superficial Velocity, and Humidity on the Kinetics

Abstract The influence of air temperature, velocity, and humidity during convective drying of two different sludges (A and B) is investigated through a 33 factorial design. For sludge A, a constant drying flux period is observed, while sludge B is characterized by a long decreasing drying flux phase. A sensitivity analysis shows that temperature is the main operating parameter affecting the drying kinetics. Mass and heat transfer coefficients as well as water evaporation capacities are calculated from drying curves. Transfer coefficients are related to operating conditions through dimensionless relations. For both sludges, a linear relation is found between water evaporation capacity and the maximal measured drying flux.

Measurement of Shrinkage and Cracks Associated to Convective Drying of Soft Materials by X-ray Microtomography

Abstract Traditionally, the measurement of shrinkage occurring during drying is performed by destructive or poorly accurate techniques such as volume displacement methods. Cracks detection and quantification are realised either by destructive techniques or sophisticated but expensive nondestructive ones (NMR imaging). X-ray microtomography in combination with image analysis provides an accurate, nondestructive and easy to use technique to determine simultaneously shrinkage and crack extent. Results reported in this article concern drying of wastewater sludges whose management will become a real challenge in the years to come. These results show a clear relation between drying kinetics and crack development. This could be related to the development of internal diffusional limitations inducing moisture gradients and mechanical stresses leading to cracks formation.

Alignment of glial cells stimulates directional neurite growth of CNS neurons in vitro

Olfactory ensheathing cells (OECs) together with olfactory nerve fibroblasts (ONFs) and neonatal astrocytes are potent stimulators of neurite growth in adulthood and during development, respectively. Since it is known that alignment of glial cells is important for the correct outgrowth of axon tracts, it was hypothesized that the alignment of glial cells stimulates directional and enhanced neurite outgrowth. Adult OEC/ONF and neonatal astrocytes were cultured either on biodegradable poly(d,l)-lactide matrices or in Petri dishes for 4 days. Thereafter neonatal cerebral cortical neurons were added. After a 2-days coculture period the cultures were fixed and processed for a combined MAP-2 and phosphorylated neurofilament (RT97) staining. The neurite growth (neurite elongation and neurite formation) and the neurite direction were assessed. We show that (1). OEC/ONF cultures are more potent in stimulating the length of the longest neurite of cocultured neurons, (2). alignment of glial is achieved in vitro on our biomatrices, (3). aligned glial/biomatrix complexes do not enhance neurite growth, and (4). aligned glial/biomatrix complexes direct neurite outgrowth. These data have significant implications for in vivo experiments focusing on glial transplantation. Transplanting glial/biomatrix complexes may stimulate the directional regrowth of severed axons across a lesion site.

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.

Characterization of hyperporous polyurethane-based gels by non-intrusive mercury porosimetry

Evaporative drying of polyurethane-based gels produces xerogels. Supercritical drying after replacement of interstitial liquid by supercritical CO2 produces aerogels. SEM micrographs show that both materials are made up of small size particles gathered up in filament-shaped, strongly cross-linked aggregates. Density measurements show that they both have a large pore volume. When submitted to mercury porosimetry, the behavior of these materials is similar to that of inorganic aerogels, as previously observed. Mercury does not penetrate the pore network, but the whole material is densified. The usual Washburn equation cannot be used to analyze the mercury porosimetry. A well-suited equation based on a buckling model of filament-shaped aggregates has been developed in order to determine the pore volume distribution of mineral dried gels. This equation is also valid for analyzing the texture of organic hyperporous materials like polyurethane dried nanoporous gel.

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.

Aerogel compression theoretical analysis

Abstract 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.