René Pirard

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.

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.

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.

Mercury porosimetry: applicability of the buckling-intrusion mechanism to low-density xerogels

Mineral materials can be either crushed or invaded by mercury during mercury porosimetry experiments. It has been shown here that many low-density xerogels exhibit the two volume variation mechanisms successively, compaction followed by intrusion, when submitted to mercury porosimetry and that a unimodal pore size distribution can be obtained by applying Pirards collapse model below the pressure of transition Pt and Washburns intrusion theory above Pt. To confirm the validity of the use of the buckling law, one low-density xerogel was wrapped in a tight membrane (intrusion is prevented and the sample is crushed during the whole porosimetry experiment). The analysis of the mercury porosimetry data of the wrapped sample by the buckling law leads to a continuous unimodal distribution similar to the distribution of the unwrapped sample obtained by applying the buckling law below Pt and the intrusion law above Pt. The position of Pt is characteristic of the tested material: when submitted to mercury pressure, aerogels and low-density xerogels only collapse in case of very small aggregates whereas they are crushed and then intruded in case of larger silica aggregates. The fact that compacted slabs of monodisperse non-aggregated silica spheres (of the same size range as the xerogels and aerogels) show only intrusion during mercury porosimetry experiments implies that the particles need to be aggregated so that the compaction mechanism takes place. The position of the change of mechanism from crushing to intrusion is not directly related to the size of the elementary particles but is linked to the size of the aggregates of silica particles.

Multiscale Texture Characterization of Wastewater Sludges Dried in a Convective Rig

Abstract Secondary sludges from two different wastewater treatment plants are conditioned and dewatered in the same way before drying in a lab-scale convective rig at the same operating conditions. Several techniques are used to characterize the texture of the dried materials over a wide range of scales from nm up to mm. Texture and porosity of the dried products are studied by SEM imaging, nitrogen absorption isotherms (0.5<d p <50 nm), mercury porosimetry (7.5 nm< d p <150 µm) and X-ray microtomography (spatial resolution = 41 µm). The image analysis of cross-sections reconstructed by microtomography also allows following shrinkage and textural evolution during drying.

Image analysis, impedance spectroscopy and mercury porosimetry characterisation of freeze-drying porous materials

Abstract Image analysis and impedance spectroscopy have been used as potential tools for the characterization of the texture of ultramacroporous PLA foams prepared by freeze-drying. It has been shown that these two techniques actually provide valuable information on the structure of this material. They are complementary to mercury porosimetry, which does not allow morphological details to be distinguished. Image analysis of SEM micrographs of transverse cross-sections at two different magnifications gave information on both the macroporosity (1 μm 10 μm). Impedance spectroscopy was used to investigate the transport properties of the three-dimensional porous matrices by measurement of ionic conduction.

Preparation of low-density xerogels by incorporation of additives during synthesis

Abstract Low-density xerogels were prepared by incorporation of an additive to alcogels prior to gelation. The additives studied are 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS), 3-aminopropyltrimethoxysilane (AMS), propyltrimethoxysilane (PMS), tetramethylorthosilicate (TMOS) and 3-aminopropyltriethoxysilane (AES) using tetraethylorthosilicate (TEOS) as main silica precursor. Samples were also prepared with EDAS as additive and TMOS as main silica reagent. When the additive contains methoxy groups, it reacts first, forms nuclei on which the main reagent TEOS reacts to form the silica particles. The nucleation mechanism by the additive occurs only in case of a difference of reactivity between additive and main silica precursor. The other group of the additive (amine, alkyl group, …) influences only the gelation time. In case of ethoxy groups (series AES/TEOS) or methoxy groups (series EDAS/TMOS) for both additive and main reagent, there is no nucleation by the additive.

Characterization of porous texture of hyperporous materials by mercury porosimetry using densification equation

Abstract The purpose of this paper is to propose a method of analyzing the mercury porosimetry data in the case of materials called hyperporous. This class of material does not undergo intrusion by mercury; instead, it shrinks under the mercury isostatic pressure and its density increases. The phenomenon is partially or completely irreversible. The proposed method enables computing the pore volume distribution as a function of the pore size in the same way as Washburns method does in the case of mercury intrusion.

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.