Condensation of helium in aerogels and athermal dynamics of the Random Field Ising Model

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.Comment: 5 p + suppl. materia

Critical behavior of the liquid gas transition of 4 He confined in a silica aerogel

6 pagesInternational audienceWe 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

Etude de la corrosion sous contrainte de certains verres d'aluminophosphates de calcium (effet de la teneur en alumine)

MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Slow crack growth in silica aerogels: A review

International audienc

Crack Healing by Thermal Treatment in Float Glass: The Effect of Tin

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

Effect of CaO and Al2O3 addition on the properties of K2O-Na2O-P2O5 glass system

This work reports on the effect of substituting Al2O3 for P2O5 and substituting CaO for (Na2O+K2O) in the ternary 0,46 (K2O + Na2O) 0,54 P2O5 glass system, certain of these compositions being eventually of interest for bio-medical applications. Different properties are analysed here. When CaO replaces (Na2O+K2O) (constant O/P ratio) or when Al2O3 replaces P2O5 (increasing O/P ratio), it is shown that the density of the glasses increased, the coefficient of thermal expansion decreased and the glass transition temperature increased. A partial and limited study of dissolution behaviour in water indicates that the dissolution rates decreased by orders of magnitude with the introduction of CaO or Al2O3, in agreement with the trends reported in the literature. The pH values of the dissolution solutions were also measured. The pH quickly decreased and saturated to around 2.5 for glasses with no alumina, i.e with the highest P2O5 content. On another hand, when Al2O3 was substituted for P2O5 (i.e. when P2O5 content decreased and O/P increased), the pH initially increased to more basic values (around 7) and always remained above 5. Some interpretation of the structural role of CaO and Al2O3 are proposed to explain these properties change with composition

Evidence for a disorder induced phase transition in the condensation of 4He in aerogels

4 pages latexWe report on thermodynamic and optical measurements of the condensation process of $^4$He in two silica aerogels of same porosity 95%, but different microstructures resulting from different pH during synthesis. For a base-catalyzed aerogel, the temperature dependence of the shape of adsorption isotherms and of the morphology of the condensation process show evidence of a disorder induced transition,in agreement with recent theoretical predictions. This transition is not observed for a neutral-catalyzed aerogel, which we interpret as due to a larger disorder in this case

Calcium alumino-phosphate used as bio-active glasses for hydrogel/glass bio-composites

International audienceDans le domaine de l'ingénierie des tissus osseux (branche de la médecine qui s'occupe de la régénération, de la réparation ainsi que de la thérapie de l'os et des articulations qui lui sont associées), les bioverres ont connu un développement très important depuis le 1969. Malgré leur fragilité inhérente, les verres bioactifs ont de nombreuses propriétés extrêmement bien adaptées au domaine de l'ingénierie des tissus osseux. Ces matériaux, utilisés comme substitutif osseux, se dégradent progressivement dans l'organisme pour libérer des éléments qui favorisent la repousse osseuse. Des verres à base de phospho-silicate ont été développés et sont actuellement commercialisés. Les verres de phosphate ont été moins étudiés, et présentent cependant d'autres avantages, comme un fibrage plus aisé. Nous présentons ici des résultats concernant l'étude de verres de phosphate et d'alumino-phosphate de calcium, sodium et potassium. Notre objectif, à terme, est d'introduire ces bio-verres dans des hydrogels afin de développer de nouveaux produits bioactifs, alliant les propriétés des bio-verres et de ces hydrogels

Evidence for a disorder induced phase transition in the condensation of 4He in aerogels

4 pages latexWe report on thermodynamic and optical measurements of the condensation process of $^4$He in two silica aerogels of same porosity 95%, but different microstructures resulting from different pH during synthesis. For a base-catalyzed aerogel, the temperature dependence of the shape of adsorption isotherms and of the morphology of the condensation process show evidence of a disorder induced transition,in agreement with recent theoretical predictions. This transition is not observed for a neutral-catalyzed aerogel, which we interpret as due to a larger disorder in this case