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See detailMercury porosimetry: applicability of the buckling-intrusion mechanism to low-density xerogels
Alié, Christelle ULg; Pirard, René ULg; Pirard, Jean-Paul ULg

in Journal of Non-Crystalline Solids (2001), 292(1-3), 138-149

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 ... [more ▼]

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 Pirard's collapse model below the pressure of transition P-1 and Washburn's intrusion theory above P-t. 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 P-t and the intrusion law above P-t. The position of P-t 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. (C) 2001 Elsevier Science B.V. All rights reserved. [less ▲]

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See detailPreparation of low-density xerogels by incorporation of additives during synthesis
Alié, Christelle ULg; Ferauche, Fabrice; Pirard, René ULg et al

in Journal of Non-Crystalline Solids (2001), 289(1-3), 88-96

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 ... [more ▼]

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. (C) 2001 Elsevier Science B.V. All rights reserved. [less ▲]

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See detailThe use of additives to prepare low-density xerogels
Alié, Christelle ULg; Pirard, René ULg; Lecloux, A. J. et al

in Journal of Non-Crystalline Solids (2001), 285

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See detailTextural properties of low-density xerogels
Alié, Christelle ULg; Benhaddou, Arnaud; Pirard, René ULg et al

in Journal of Non-Crystalline Solids (2000), 270(1-3), 77-90

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 ... [more ▼]

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 (similar to 20 nm) with a low EDAS/TEOS ratio (similar to 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/cm(3)) while xerogels obtained by the surface modification process are less dense (0.1-0.15 g/cm(3)). The biggest pores are observed in the third method. (C) 2000 Elsevier Science B.V. All rights reserved. [less ▲]

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See detailPreparation of low-density xerogels through additives to TEOS-based alcogels
Alié, Christelle ULg; Pirard, René ULg; Lecloux, A. J. et al

in Journal of Non-Crystalline Solids (1999), 246(3), 216-228

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 ... [more ▼]

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 (similar to 20 nm) due to the nucleation process by EDAS, thus the pores between those particles are also large and the drying stress is reduced. (C) 1999 Published by Elsevier Science B.V. All rights reserved. [less ▲]

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See detailStructure of high temperature fluid selenium
Bichara, C.; Raty, Jean-Yves ULg; Gaspard, Jean-Pierre ULg

in Journal of Non-Crystalline Solids (1999), 250-252

Monte Carlo simulations based on a semi empirical tight binding model including dispersion forces were performed to study liquid selenium at temperatures between 600 and 2000 K. The atomic structures ... [more ▼]

Monte Carlo simulations based on a semi empirical tight binding model including dispersion forces were performed to study liquid selenium at temperatures between 600 and 2000 K. The atomic structures obtained are in agreement with the X-ray scattering and extended X-ray absorption fine structure (EXAFS) data in a range of temperatures and densities. A correlation between the conductivity of high temperature fluid selenium and the degree of branching and breaking of the selenium chains is observed [less ▲]

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See detailOn the structure of liquid IV-VI semiconductors
Raty, Jean-Yves ULg; Gaspard, Jean-Pierre ULg; Bionducci, M. et al

in Journal of Non-Crystalline Solids (1999), 250-252

The crystalline structure of most stoichiometric IV-VI semiconductors can be viewed as a distortion of the NaCl structure; the distortion amplitude depends on the components. As a result, corrugated ... [more ▼]

The crystalline structure of most stoichiometric IV-VI semiconductors can be viewed as a distortion of the NaCl structure; the distortion amplitude depends on the components. As a result, corrugated planes of covalently bonded atoms are linked by dispersion (Van der Waals) forces. Neutron scattering experiments were performed on the liquid phases of a series of IV-VI compounds (GeS, GeSe, GeTe, SnS, SnSe and SnTe). We show that the evolution of the local order is correlated with the conductivity measurements [less ▲]

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See detailEvolution of the Peierls distortion in liquid As xSb 1-x compounds
Raty, Jean-Yves ULg; Gaspard, Jean-Pierre ULg; Ceolin, R. et al

in Journal of Non-Crystalline Solids (1998), 232-234

The local order in the liquid binary alloy As xSb 1-x (0lesxles1) is studied by short-wavelength (lambda=0.7 Aring) neutron scattering. The coordination number (Z) and the interatomic distances (d) are ... [more ▼]

The local order in the liquid binary alloy As xSb 1-x (0lesxles1) is studied by short-wavelength (lambda=0.7 Aring) neutron scattering. The coordination number (Z) and the interatomic distances (d) are analyzed in terms of the As concentration: Z increases continuously, from a value of 3.5 for pure arsenic to a value of 6.3 for pure antimony, whereas d shows a departure from Vegard's rule. The interatomic distance increases in the range 0.15lesxles1.00 and is almost constant in the range 0.00lesxles0.15. A simple tight-binding model is developed, which indicates that the relevant parameter is the hardness of the core repulsion between the atoms [less ▲]

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See detailTexture control of freeze-dried resorcinol-formaldehyde gels
Kocklenberg, Régine; Mathieu, Bernard; Blacher, Silvia ULg et al

in Journal of Non-Crystalline Solids (1998), 225

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See detailHigh temperature catalysts through sol-gel synthesis
Lecloux, A. J.; Pirard, Jean-Paul ULg

in Journal of Non-Crystalline Solids (1998), 225

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See detailEvolution of the Peierls distortion in liquid AsxSb1-x compounds
Raty, Jean-Yves ULg; Gaspard, Jean-Pierre ULg; Céolin, R. et al

in Journal of Non-Crystalline Solids (1998), 232-234

The local order in the liquid binary alloy AsxSb1-x (0 ≤ x ≤ 1) is studied by short-wavelength (λ = 0.7 Å) neutron scattering. The coordination number (Z) and the interatomic distances (d) are analyzed in ... [more ▼]

The local order in the liquid binary alloy AsxSb1-x (0 ≤ x ≤ 1) is studied by short-wavelength (λ = 0.7 Å) neutron scattering. The coordination number (Z) and the interatomic distances (d) are analyzed in terms of the As concentration: Z increases continuously, from a value of 3.5 for pure arsenic to a value of 6.3 for pure antimony, whereas d shows a departure from Vegard's rule. The interatomic distance increases in the range 0.15 ≤ x ≤ 1.00 and is almost constant in the range 0.00 ≤ x ≤ 0.15. A simple tight-binding model is developed, which indicates that the relevant parameter is the hardness of the core repulsion between the atoms. © 1998 Elsevier Science B.V. All rights reserved. [less ▲]

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See detailFreeze-dried resorcinol-formaldehyde gels
Blacher, Silvia ULg; Pirard, René ULg; Pirard, Jean-Paul ULg et al

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 ▲]

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See detailAerogel compression theoretical analysis
Pirard, René ULg; Pirard, Jean-Paul ULg

in Journal of Non-Crystalline Solids (1997), 212(2-3), 262-267

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 ... [more ▼]

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. [less ▲]

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See detailOn the structure of liquid tellurium
Bichara, C.; Raty, Jean-Yves ULg; Gaspard, Jean-Pierre ULg

in Journal of Non-Crystalline Solids (1996), 205-207

The atomic structure and the bonding mechanism in liquid tellurium are investigated by a tight binding Monte Carlo technique. The chain structure is preserved but a third covalent bond emerges with an ... [more ▼]

The atomic structure and the bonding mechanism in liquid tellurium are investigated by a tight binding Monte Carlo technique. The chain structure is preserved but a third covalent bond emerges with an intermediate interatomic separation (3.15 Aring). In addition a bond length alternation inside the chain appears in agreement with recent EXAFS experiments and the valence angles are significantly reduced. The electronic structure is studied and particular attention is paid to the modifications of lone pair interactions in the liquid structure [less ▲]

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See detailLocal orders in II-VI liquid compounds
Gaspard, Jean-Pierre ULg; Raty, Jean-Yves ULg; Ceolin, R. et al

in Journal of Non-Crystalline Solids (1996), 205-207

The structure of liquid CdTe, HgTe and ZnTe is studied by neutron scattering. Upon melting, drastically different behaviors are found: CdTe and ZnTe keep their fourfold coordination in the melt and remain ... [more ▼]

The structure of liquid CdTe, HgTe and ZnTe is studied by neutron scattering. Upon melting, drastically different behaviors are found: CdTe and ZnTe keep their fourfold coordination in the melt and remain semiconductors, whereas HgTe becomes sixfold coordinated and metallic. This dissimilarity is discussed in terms of the electronegativity difference and of the repulsive potential hardness. Entropic aspects are also discussed [less ▲]

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See detailSynthesis of barium-titanate by the sol-gel process
LEMOINE, Christine; Gilbert, Bernard ULg; MICHAUX, Bernard et al

in Journal of Non-Crystalline Solids (1994), 175(1), 1-13

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 ... [more ▼]

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 drying 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 drying 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 degrees C from the cubic to the tetragonal form of BaTiO3 has been observed by polarization microscopy. [less ▲]

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