Influence of thermal history on the structure and properties of silicate glasses

We studied a set of float glass samples prepared with different fictive temperature by previous annealing around the glass transition temperature. We compared the results to previous measurements on a series of amorphous silica samples, also prepared with different fictive temperature. We showed that the modifications on the structure at a local scale are very small, the changes of physical properties are moderate but the changes on density fluctuations at a nanometer scale are rather large: 12 and 20% in float glass and silica, for relative changes of fictive temperature equal to 13 and 25% respectively. Local order and mechanical properties of silica vary in the opposite way compared to float glass (anomalous behavior) but the density fluctuations in both glasses increase with temperature and fictive temperature

In situ measurements of density fluctuations and compressibility in silica glass as a function of temperature and thermal history

In this paper, small-angle X-ray scattering measurements are used to determine the different compressibility contributions, as well as the isothermal compressibility, in thermal equilibrium in silica glasses having different thermal histories. Using two different methods of analysis, in the supercooled liquid and in the glassy state, we obtain respectively the temperature and fictive temperature dependences of the isotheraml compressibility. The values obtained in the glass and supercooled liquid states are very close to each other. They agree with previous determinations of the literature. The compressibility in the glass state slightly decreases with increasing fictive temperature. The relaxational part of the compressibility is also calculated and compared to previous determinations. We discussed the small differences between the different determinations

The future for Mediterranean wetlands: 50 key issues and 50 important conservation research questions

Wetlands are critically important for biodiversity and human wellbeing, but face a range of challenges. This is especially true in the Mediterranean region, where wetlands support endemic and threatened species and remain integral to human societies, but have been severely degraded in recent decades. Here, in order to raise awareness of future challenges and opportunities for Mediterranean wetlands, and to inform proactive research and management, we identified (a) 50 key issues that might affect Mediterranean wetlands between 2020 and 2050, and (b) 50 important research questions that, if answered, would have the greatest impact on the conservation of Mediterranean wetlands between 2020 and 2050. We gathered ideas through an online survey and review of recent literature. A diverse assessment panel prioritised ideas through an iterative, anonymised, Delphi-like process of scoring, voting and discussion. The prioritised issues included some that are already well known but likely to have a large impact on Mediterranean wetlands in the next 30 years (e.g. the accumulation of dams and reservoirs, plastic pollution and weak governance), and some that are currently overlooked in the context of Mediterranean wetlands (e.g. increasing desalination capacity and development of antimicrobial resistance). Questions largely focused on how best to carry out conservation interventions, or understanding the impacts of threats to inform conservation decision-making. This analysis will support research, policy and practice related to environmental conservation and sustainable development in the Mediterranean, and provides a model for similar analyses elsewhere in the world

Are Farm-Reared Quails for Game Restocking Really Common Quails (Coturnix coturnix)?: A Genetic Approach

The common quail (Coturnix coturnix) is a popular game species for which restocking with farm-reared individuals is a common practice. In some areas, the number of released quails greatly surpasses the number of wild breeding common quail. However, common quail are difficult to raise in captivity and this casts suspicion about a possible hybrid origin of the farmed individuals from crosses with domestic Japanese quail (C. japonica). In this study we used a panel of autosomal microsatellite markers to characterize the genetic origin of quails reared for hunting purposes in game farms in Spain and of quails from an experimental game farm which was founded with hybrids that have been systematically backcrossed with wild common quails. The genotypes of these quail were compared to those of wild common quail and domestic strains of Japanese quail. Our results show that more than 85% of the game farm birds were not common quail but had domestic Japanese quail ancestry. In the experimental farm a larger proportion of individuals could not be clearly separated from pure common quails. We conclude that the majority of quail sold for restocking purposes were not common quail. Genetic monitoring of individuals raised for restocking is indispensable as the massive release of farm-reared hybrids could represent a severe threat for the long term survival of the native species

Elastic Moduli of Permanently Densified Silica Glasses

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Magnetism of spinel microcrystals in a Cr3+ -doped cordierite glass : an E. S. R. study

We have used Electron Spin Resonance to characterize the Cr3+ clusters which grow, during various heat treatments, in a 0.8 % Cr2 O3-doped cordierite glass. We found useful to compare the magnetic properties of these clusters to those of powdered MgCr2O 4. In MgCr2O4 the room-temperature E.S.R. spectrum consists of a single exchange-narrowed dipolar line. An approximate value of the exchange field between the two sublattices at T = 0 K is He ∼ 2 × 106 G, explaining the disappearance of the E.S.R. signal below TN = 15 K. Room-temperature E.S.R. results confirm that the Cr3+ clusters built up during a 100 H-825 °C or a 6 H-850 °C heating have a MgCr2xAl2(1-x) O4 composition, and precise the x value (x ∼ 0.9 ) . At low temperature these clusters are antiferromagnetic fine grains, with a high anisotropy field (Ha ≳ 10 kG). After a 4 h-875 °C + 2 h-900 °C heating, the Cr3+ concentration in the clusters is smaller, whereas after 2 h-875 °C + 10 min-1 050 °C heating, the Cr3+ ions are randomly distributed in MgAl2O4.Nous avons utilisé la Résonance Electronique pour caractériser les amas d'ions Cr3+ qui se forment au cours de divers traitements thermiques dans un verre de cordiérite dopé à 0,8 % en Cr2O3. Nous avons jugé bon de comparer les propriétés magnétiques de ces amas à celles de MgCr2O4 en poudre. A température ambiante le spectre R.P.E. de MgCr2O4 est constitué d'une raie dipolaire rétrécie par échange. Nous avons trouvé qu'à T = 0 K le champ d'échange He entre les deux sous-réseaux est d'environ 2 x 106 G, ce qui explique l'impossibilité d'observer en bande X le signal de résonance en dessous de TN = 15 K. Les résultats obtenus par R.P.E. à température ambiante confirment que les amas de Cr3+ formés au cours d'un recuit 100 H à 825 °C ou 6 H à 850 °C ont une composition du type MgCr2xAl2(1-x) O4, et permettent de préciser la valeur de x (x∼0,9). A basse température, ces amas forment des grains fins antiferromagnétiques, avec un champ d'anisotropie élevé (Ha ≳ 10 kG) . Pour les échantillons ayant subi un recuit de 4 h-875 °C et 2 h-900 °C, la concentration en Cr3+ dans les amas est plus faible. Après un recuit de 2 h-875 °C et 10 min-1 050 °C les ions Cr3+ sont distribués aléatoirement dans une matrice de Al2O3

Memory effect in the plasticity of a silicate glass densified at room temperature

International audienceMemory effects are a classic feature of disordered materials. With Raman-microspectroscopy we evidence a memory effect in the deformation of densified soda-lime silicate glasses, and with molecular dynamics simulations we identify the network reconfigurations it originates from. These results pave the way toward a better understanding of plastic instability, damage, and rupture properties in iono-covalent glassy materials

Low-frequency Raman scattering under high pressure in diamond anvil cell: Experimental protocol and application to GeO2 and SiO2 boson peaks

International audienceLow frequency vibrations, a universal feature of amorphous solids which is responsible for thermodynamical anomalies at low temperature, are complicated to record in high pressure device, notably because of the closeness of the elastic line. We first present an experimental protocol allowing to record the in situ low-frequency Raman scattering of samples under high pressure in the diamond anvil cell apparatus with a high quality. This protocol is particularly adapted to study the evolution of the boson peak of glassy materials. The second part is dedicated to the study of the boson peak of a-GeO2, a typical strong glass. The results, which clearly show the non-Debye behaviour of this material at the beginning of the compression where an anomalous compression takes place, are compared to previous measurements on a-SiO2. Interpretation in terms of local structural transitions and discussions about the elastic nanoheterogeneities of the amorphous state are addressed

Permanent Ge coordination change induced by pressure in La 2O3-B2O3-GeO2 glass

In this study, we report the results of an in situ X-ray absorption spectroscopy investigation of the La2O3-B 2O3-GeO2 (LBG) glass compressed in a diamond anvil cell until 17 GPa at ambient temperature. A pressure-induced coordination change of germanium from fourfold to sixfold is shown here. The analysis of Ge K-edge X-ray absorption near-edge structure spectra supports a model of mixing of fourfold and sixfold coordinated Ge sites during the compression/ decompression cycle. This transformation proves to be irreversible: about 25% of sixfold coordinated Ge still exists at ambient pressure. This phenomenon underlines a polymorphism property of the LBG glass, whose origin lies in the complexity of composition

Silica under hydrostatic pressure: A non continuous medium behavior

International audienceThe homogeneous/inhomogeneous structure of glasses is still a debated question. Hydrostatic high pressure experiments allow us to determine if a glass behaves as an elastic continuous random network or if a nanometer scale heterogeneity has to be taken into account. In order to get information on the homogeneous/inhomogeneous structure of glasses, in situ high pressure Raman experiments are performed on silica in the elastic domain up to 4.7 GPa. A strong decrease of the Boson peak intensity is observed between 1 bar and 3 GPa. We show that this decrease does not correspond quantitatively to the effect of pressure on a homogeneous elastic medium. From the interpretation of the narrowing of the main Raman band width under pressure as a narrowing of the θ inter-tetrahedral Si–O–Si angle distribution it is shown that the decrease of the Boson peak intensity is correlated to the decrease of the intrinsic inhomogeneity of the silica glass. These results confirm the occurrence of an intrinsic inhomogeneity at a nanometer scale even in a single component glass like SiO2 which is very important for the interpretation of the optical or mechanical properties of the glasses