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

Crack path instabilities in DCDC experiments in the low speed regime

We studied the low speed fracture regime (0.1mm/s - 1nm/s) in different glassy materials (soda-lime glass, glass-ceramics) with variable but controlled length scale of heterogeneity. The chosen mechanical system enabled us to work in pure mode I (tensile) and at a fixed load on DCDC (double cleavage drilled compression) specimen. The internal residual stresses of studied samples were carefully relaxed by appropriate thermal treatment. By means of optical and atomic force (AFM) microscopy techniques fracture surfaces have been examined. We have shown for the first time that the crack front line underwent an out-of-plane oscillating behavior as a result of a reproducible sequence of instabilities. The wavelength of such a phenomenon is in the micrometer range and its amplitude in the nanometer range. These features were observed for different glassy materials providing that a typical length scale characterizing internal heterogeneities was lower than a threshold limit estimated to few nanometers. This effect is the first clear experimental evidence of crack path instabilities in the low speed regime in a uniaxial loading experiment. This phenomenon has been interpreted by referring to the stability criterion for a straight crack propagation as presented by Adda-Bedia et al. (Phys. Rev. Letters (1996) 76} p1497).Comment: 16 pages, 10 figures, submitted to Journal of Non-Crystalline Solid

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

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

We report on thermodynamic and optical measurements of the condensation process of $^4$He in three silica aerogels of different microstructures. For the two base-catalysed aerogels, the temperature dependence of the shape of adsorption isotherms and of the morphology of the condensation process show evidence of a disorder driven transition, in agreement with recent theoretical predictions. This transition is not observed for a neutral-catalysed aerogel, which we interpret as due to a larger disorder in this case.Comment: 11 page

Sub-critical crack growth in some phosphate glasses

International audienc

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

MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Guérison des fissures dans les verres d'oxydes par traitement thermique : influence de la composition.

Les fissures de surface diminuent fortement la durabilité des verres. Une meilleure compréhension des mécanismes de guérison des fissures par traitement thermique pourrait permettre d'améliorer la durabilité des verres. Nous avons récemment analysé l'autoguérison de fissures générées par indentation dans des verres de silicate type verre à vitre par des traitements thermiques au-dessus de la transition vitreuse[1]. Deux types principaux d'évolution des fissures ont été observés : (1) soit un changement significatif de morphologie des fissures conduisant à une restauration du matériaux, (2) soit une simple fermeture des fissures sans restauration. Le type d'évolution des fissures est principalement dépendant de la viscosité du verre : elle favorise ou empêche les changements induits par les forces capillaires. Nous avons également montré que la teneur en eau du verre et que le degré d'hygrométrie de l'atmosphère du four avait une influence prépondérante sur le type de mécanisme de modification des fissures, puisque ces paramètres influence beaucoup la viscosité. Pour compléter cette étude, nous nous intéressons maintenant à d'autres compositions verrières : un verre de borosilicate, un verre de phosphate, ainsi que la face " étain " du verre à vitre. Ces nouveaux résultats confirment les mêmes types de modification des fissures par traitement thermique, mais mettent en évidence que l'influence de l'atmosphère du traitement thermique est très dépendante de la composition du verre, comme on pouvait s'y attendre. Ces résultats montrent également que l'oxydation ou la réduction de certains ions va changer drastiquement les comportements des fissures lors de la guérison. L'étain, en particulier, favorise la réticulation du réseau en s'oxydant et modifie donc la viscosité localement. Par ailleurs, les résultats concernant le verre borosilicaté indiquent une dépendance très faible visà vis de l'atmosphère du traitement thermique. Pour compléter l'analyse, nous nous sommes intéressés à la mesure de la tension de surface avec un appareil développé dans notre laboratoire. Ces mesures accompagnées de mesures de viscosité basse température nous permettent de confirmer nos hypothèses sur le changement de morphologie des fissures conduit par les forces capillaires. [1] R. Girard, A. Faivre, F. Despetis, " Influence of water on crack self-healing in soda-lime silicate glass" J. Am. Ceram. Soc., 94 [8] 2402-2407 (2011)

Crack self-healing in glass: the effect of composition

Surface cracks lead to the decrease of the glass lifetime. A better understanding of cracks self-healing can give a method to increase the glass durability. In a previous study [1], the self-healing behavior of radial cracks generated indentation in float silicate glass has been analyzed when heat treated above glass transition temperature. Two main types of radial crack evolution are observed: (1) either very large crack morphological changes, or (2) a simple direct crack closure. We associate these two phenomena respectively to the effects of capillarity forces induced by surface energy minimization or to the effects of residual stress release. The type of crack evolution is proposed to be mainly depending on glass viscosity: it favors or impedes morphological changes driven by capillarity forces. Similar studies are also performed in others glass types, such as borosilicates or tin surface of float glass. We observe that the glass composition is also important as it probably determines potential hydration. But to go deeper in the analysis, we also focus on the measurement of surface tension in the typical temperature range of cracks thermal healing. These measurements are performed on a "home-made" apparatus, with a small mobile furnace moving along a glass fiber. These measurements of surface tension combined with low temperature viscosity measurements should allow us to confirm our hypothesis of cracks morphological changes driven by capillarity forces. [1] R. Girard, A. Faivre, F. Despetis, " Influence of water on crack self-healing in soda-lime silicate glass" (accepted by J. Am. Ceram. Soc.)

Influence of Water on Crack Self-Healing in Soda-Lime Silicate Glass

International audienceThe self-healing behavior of radial cracks generated by Vickers indentation in float glass is analyzed when heat treated at 620 degrees C under various atmospheres. Results evidence that two main driving forces influence radial crack evolution: release of residual stresses induced by initial indentation and capillary forces due to surface energy minimization. Depending on the viscosity level, viscous flow allows crack morphological changes driven by capillarity forces or not. Our results evidence that at 620 degrees C, the viscosity of the glass surrounding cracks can be significantly reduced by water diffusion and glass hydrolysis, or increased by glass des-hydration, as a function of the humidity level of the furnace atmosphere. Hydration and des-hydration of glass are shown to play a major role in the crack morphology changes during healing, respectively, favoring or impeding morphological changes driven by capillarity forces