Ref HAL: hal-01921452_v1
DOI: 10.3390/gels1020256
WoS: 000461751200007
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27 Citations
Résumé:

Sets of silica gels: aerogels, xerogels and sintered aerogels, have been studied in the objective to understand the mechanical behavior of these highly porous solids.The mechanical behaviour of gels is described in terms of elastic and brittle materials, like glasses or ceramics. The magnitude of the elastic and rupture modulus is several orders of magnitude lower compared to dense glass. The mechanical behaviours (elastic and brittle) are related to the same kinds of gel characteristics: pore volume, silanol content and pore size. Elastic modulus depends strongly on the volume fraction of pores and on the condensation reaction between silanols. Concerning the brittleness features: rupture modulus and toughness, it is shown that pores size plays an important role. Pores can econsidered as flaws in the terms of fracture mechanics and the flaw size is related to the pore size. Weibull’s theory is used to show the statistical nature of flaw. Moreover, stresscorrosion behaviour is studied as a function of environmental conditions (water and alcoholicatmosphere) and temperature.

Ref HAL: hal-01236173_v1
DOI: 10.1007/s10853-015-9253-0
WoS: WOS:000359811300015
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2 Citations
Résumé:

Subcritical crack growth and crack closure are analysed using the double-cleavage-drilled-compression method, in two calcium phosphate based glasses. The measurements of the crack velocity as a function of temperature, from ambient to 250°C, are reported for two different relative humidity levels. Studied glasses present unusual trends characterized by a non gradual shift of V-KI curves versus temperature. Moreover, when the load is released and crack closes, expulsion of a “liquid-like” phase is observed forming droplets along the crack path. These droplets are only formed at certain temperatures, but both under high and low water humidity. They evolve toward residues when temperature is decreased.

Ref HAL: hal-01100853_v1
DOI: 10.1111/jace.13140
WoS: WOS:000344779900021
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7 Citations
Résumé:

The evolution of radial cracks generated by Vickers indentation on “tin side” of float glass is analysed when heat treated at 620°C under various atmospheres and results are compared with a previous similar study performed on the “air side” of the same glass. Results evidence that the presence and the oxidation state of tin ions has a strong influence on crack morphological changes. If the oxidation state of tin remains quasi unchanged during the heat treatment, the mechanism responsible of the crack evolution is the spheroidization and there is no strong difference between the crack morphological changes observed on the “air” and “tin side” in this case. For oxidative atmospheres, contrary to what is observed on the “air side”, no crack spheroidization occurs. The cracks stay quasi unchanged for dry air or close for humid air. Increase of the viscosity at the top surface due to Sn oxidation is proposed to explain this spheroidization impediment. It is also shown that in these two cases, there is an efficient blunting and healing of the cracks, as four point bending tests indicate a significant sample strengthening with thermal treatment.

Ref HAL: hal-01061376_v1
DOI: 10.5539/apr.v6n4p16
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Résumé:

Silica ionogels were synthesized from tetramethoxysilane (T MOS ),methyltrimethoxysilane (MT MS ) and 1-ethyl- 2-methylimidazolium Acetate (EMI MAc: Ionic Liquid) in different proportions .The textural characterizations showed an effect of these concentrations on the corresponding aerogels: pore size distributions and effective surfaces. The structure of the aerogels was measured with a SAXS (Small- Angle XRay Scattering) apparatus and was typical of acid catalyzed aerogels. Conductivity voltage measurements, operated on the ionogels, were carried out using an electrical 4 wire-electrodes set up. The electrical voltage temporal response of the EMIMAc silica ionogel was modelled by a RLC series circuit which characteristics depended on the synthesis.

Ref HAL: hal-00858215_v3
Ref Arxiv: 1309.1243
DOI: 10.1103/PhysRevLett.113.085301
WoS: 000341115200007
Ref. & Cit.: NASA ADS
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8 Citations
Résumé:

High resolution measurements reveal that condensation isotherms of $^4$He in a silica aerogel become discontinuous below a critical temperature. We show that this behaviour does not correspond to an equilibrium phase transition modified by the disorder induced by the aerogel structure, but to the disorder-driven critical point predicted for the athermal out-of-equilibrium dynamics of the Random Field Ising Model. Our results evidence the key role of non-equilibrium effects in the phase transitions of disordered systems.

Commentaires: 5 p + suppl. material

Ref HAL: hal-00842669_v1
Ref Arxiv: 1506.07750
DOI: 10.1007/s10909-012-0740-2
WoS: 000318175500027
Ref. & Cit.: NASA ADS
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3 Citations
Résumé:

We have studied 4 He confined in a 95% porosity silica aerogel in the vicinity of the bulk liquid gas critical point. Both thermodynamic measurements and light scattering experiments were performed to probe the effect of a quenched disorder on the liquid gas transition, in relation with the Random Field Ising Model (RFIM). We find that the hysteresis between condensation and evaporation present at lower temperatures disappears at a temperature T ch between 25 and 30 mK below the critical point. Slow relaxations are observed for temperatures slightly below T ch , indicating that some energy barriers, but not all, can be overcome. Above T ch , no density step is observed along the (reversible) isotherms, showing that the critical behavior of the equilibrium phase transition in presence of disorder, if it exists, is shifted to smaller temperatures, where it cannot be observed due to the impossibility to reach equilibrium. Above T ch , light scattering exhibits a weak maximum close to the pressure where the isotherm slope is maximal. This behavior can be accounted for by a simple model incorporating the compression of 4 He close to the silica strands.

Commentaires: 6 pages

Ref HAL: hal-00780352_v1
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Résumé:

Silica gels were synthesized from TMOS and MTMS in different proportions changing the ionic liquid concentration. EMIMac have significant influence in the gelation processe. The textural characterizations showed an effect of these concentrations on the corresponding aerogels: pore size distributions and effectives surfaces. The structure of the aerogels was measured with a SAXS apparatus and was typical of acid catalyzed aerogels. Conductivity measurements, operated on the ionogels, were carried out using an original electrical 4 electrodes set up which allowed us to study the ionic conductivity at the pore scale. The results obtained showed that one can model the electrical temporal response of the EMIMAc silica ionogel with a RLC series circuit.