Diffraction et Spectroscopie d'Absorption des Rayons-X

International audienceX-ray/neutron diffraction and X-ray absorption spectroscopy are complementary experimental techniques giving pair distribution functions. There are selective tools to obtain structural information, probing short and medium ranges (up to typically 1nm), such as coordination, bond distances but also valence state, even at a dilute concentration. Moreover, spatial or time resolution, specific environments (high pressure/temperature, controlled atmosphere) can be achieved. In this chapter, the theoretical basis of each techniques is given and case studies applied to glasses are used to illustrate the structural information that can be extracted

Intermediate range order in (Fe,Al) silicate network glasses: a neutron diffraction and EPSR modeling investigation

The local structural environment and the spatial distribution of iron and aluminum ions in sodosilicate glasses with composition NaFexAl1-xSi2O6 (x = 1, 0.8, 0.5 and 0) is studied by high-resolution neutron diffraction combined with structural modeling using the Empirical Potential Structure Refinement (EPSR) code. This work gives evidence of differences in the structural behavior of Al3+ and Fe3+, which are both often considered to act as network formers in charge-balanced compositions. The short-range environment and the structural role of the two cations are not composition dependent, and hence the structure of intermediate glasses can then be seen as a mixture of the structures of the two end-members. All Al3+ is 4-coordinated for a distance d[4]Al3+-O=1.76$\pm$0.01{\AA}. The high-resolution neutron data allows deciphering between two populations of Fe. The majority of Fe3+ is 4-coordinated (d[4]Fe3+-O=1.87$\pm$0.01{\AA}) while the remaining Fe3+ and all Fe2+ (~12% of total Fe) are 5-coordinated (d[5]Fe-O=2.01$\pm$0.01{\AA}). Both AlO4 and FeO4 are randomly distributed and connected with the silicate network in which they share corners with SiO4 tetrahedra, in agreement with a network-forming role of those species. On the contrary FeO5 tends to form clusters and to share edges with each other. 5-coordinated Fe is interpreted as network modifier and it turns out that, even if this coordination number is rare in crystals, it is more common in glasses in which they can have a key role on physical properties

Effets du froid, de l'humidité et des cycles de gel et de dégel sur les propriétés mécaniques des composites verre/époxy utilisés pour la fabrication de pales d'éoliennes

L’installation d’éolienne en milieu nordique comporte plusieurs défis d’envergure. Par exemple, la durabilité des composites utilisés pour la fabrication des pales n’y est pas garantie puisque les effets d’un tel climat sur ces matériaux sont mal connus. Ce projet vise donc à faire le lien entre les différentes théories actuellement proposées et de comparer leurs conclusions à des essais mécaniques sur des composites exposés à l’humidité, au froid et à des cycles de gel et dégel. Les composites utilisés sont moulés par infusion de résine et sont constitués de fibres de verre unidirectionnelles dans une matrice époxy. Les essais sont réalisés sur quatre familles d’éprouvettes, à température ambiante ainsi qu’à -40 °C. Ces familles sont divisées selon que les éprouvettes sont sèches ou saturées et qu’elles aient subi ou non 100 cycles de gel/dégel entre -40 °C et 40 °C. Des essais de traction, de compression et de flexion en poutre courte (ILSS) sont réalisés lors de ce projet. Les résultats ont montré que les équations présentes dans la littérature ne prédisent pas adéquatement les modules et la résistance des composites exposés à des températures froides. D’une manière générale, les cycles de gel et dégel n’ont pas influencé les propriétés mécaniques des composites. Le froid a produit une augmentation de la résistance en traction et en flexion en poutre courte d’environ 20 % et l’humidité a dégradé ces mêmes propriétés d’environ 30 %. Les modules ne sont pas significativement influencés par les conditions d’essais. Des micrographies des surfaces de rupture de l’essai ILSS n’ont pas montré de changement significatif du mode de rupture en fonction des conditions environnementales, mais suggèrent que la résistance de l’interface fibre/matrice du composite était faible. La comparaison des résultats obtenus avec ceux présentés dans la littérature fait ressortir le fait que le comportement à basse température est fortement lié à la nature de ses constituants ainsi qu’au procédé de moulage. Ainsi, pour des sollicitations en statique, certains composites pourraient être plus performants en milieu nordique, ce qui permettrait d’améliorer les produits s’ils sont spécifiquement conçus pour ce milieu

Temperature effects on the static, dynamic and fatigue behaviour of composite materials used in wind turbine blades

Many Canadian regions have strong winds that are interesting for wind energy production. However, these same regions are often quite remote and the Canadian climate is atypical for the wind energy industry. The high level of uncertainty about the turbines durability and the profitability of wind plants under such environments thus hinders the development of wind energy projects in Canada. Among the many uncertainties related to Canadian operating conditions, one specific concern is about the durability of wind turbine blades in Northern climates. The goal of this thesis is thus to clarify the effects of temperature and strain rate on the strength, stiffness and fatigue performance of composite materials as used in the wind energy sector. It thus focuses on glass fibre reinforced composites, which is the mainstream material for wind turbine blades. Wind turbine blades are basically beam exposed to a combination of axial, bending (in and out of the rotor plane) and torsional loads. In order to resist these loads, laminates used in the different parts of the blade are mostly made of a combination of longitudinal and ±45◦ plies. In order to improve the basic understanding of the mechanics of failure, two simple laminate configurations are studied, namely: • The unidirectional laminate loaded in the fibre direction, which is the main load bearing component of the blade structure. • The [±45]s bias-ply laminate, which provides shear stiffness to the blades structure. The temperatures considered are limited to those that could realistically be encountered in Canada’s climate, namely an extreme wintertime low of -40℃ to a summertime high of 60℃, which is deemed representative of a part exposed to direct sunlight in the summer. Similarly, fatigue frequencies are limited to a maximum of 24 Hz. It was found that the static strength and stiffness of both laminate configurations were strongly affected by both low and high temperatures. A significant increase of both properties was measured at low temperature, while high temperature strongly degraded them. However, while the high temperature fatigue durability followed the same trend as the static strength, the low temperature fatigue performance was only slightly affected, and even less so for unidirectional laminates. Both a vertical shift and a change in slope of the S–N curve with temperature was observed. At low temperature, this change of slope favours the fatigue strength under a high fatigue load, but reduces expected lives at lower load levels. This finding may be particularly significant in the context of wind turbine blade durability since they generally need to operate at low fatigue stresses, but over very long periods. Frequency effects were mostly not significant within the range explored. Nevertheless, experiments suggest that higher frequencies may have a slightly deleterious effect. An approach to predict the effect of temperature on the probabilistic S–N curve of fibre dominated composites with minimal experimental requirements is also proposed. This method is based on a cyclic strength degradation model, for which the parameters change with temperature is correlated with temperature effect on static strength. Since the latter is also an input for the cyclic strength degradation model, a function describing its temperature dependence is also suggested. The predictions obtained by the model are very good for both strength and fatigue life. Finally, models are developed for describing the static strength as well as the storage and loss modulus as a function of temperature across multiple transitions. The latter model also has provision for evaluating frequency effects on the storage modulus and glass transition temperature. These models provide a very good description of the dynamic response of polymers and composites on which they were validated (epoxies and epoxy based composites). Moreover, they provide a unambiguous definition of the glass transition temperature and allows for the evaluation of temperature and frequency effects on both the storage modulus without using the time-temperature superposition principle. Results show that if the time-temperature shift factors are calculated from the model, they are continuous across the glass transition. This suggests that the commonly expected discontinuity in this region may actually only be a side effect of neglecting the glass transition frequency dependence in conventional time-temperature superposition approaches

XANES determination of chromium oxidation states in glasses: comparison with optical absorption spectroscopy

The oxidation state of chromium in glasses melted in an air atmosphere with and without refining agents was investigated by Cr K-edge X-ray Absorption Near-Edge Structure (XANES) and optical absorption spectroscopy. A good agreement in the relative proportion of Cr(III) and Cr(VI) is obtained between both methods. We show that the chemical dependence of the absorption coefficient of Cr(III) is less important in XANES than in optical absorption spectroscopy. The comparison of glasses melted under different conditions provides an indirect assessment of the molar extinction coefficient of Cr(VI) in glasses

Rearrangement of the structure during nucleation of a cordierite glass doped with TiO2

Ordering of disordered materials occurs during the activated process of nucleation that requires the formation of critical clusters that have to surmount a thermodynamic barrier. The characterization of these clusters is experimentally challenging but mandatory to improve nucleation theory. In this paper, the nucleation of a magnesium aluminosilicate glass containing the nucleating oxide TiO2 is investigated using neutron scattering with Ti isotopic substitution and 27Al NMR. We identified the structural changes induced by the formation of crystals around Ti atoms and evidenced important structural reorganization of the glassy matrix

Démantèlement d’un grand sanctuaire civique de la cité des Aulerques Éburovices au III^e s. apr. J.-C.

International audienceThe excavation carried out since 2005 on the urbanized central temple of the cult complex at Le Vieil-Évreux (Eure) provide positive developments and a precise chronological sequence of the last decades of the life of the sanctuary, its official closing, its transformation into a castellum, and its well organized demolition. All these features attest to the permanency of the public authorities along these successive radical modifications.Les fouilles menées depuis 2005 sur le temple central du complexe cultuel urbanisé du Vieil-Évreux (Eure) apportent des éléments concrets et un phasage précis concernant la fin de la vie du sanctuaire, sa fermeture officielle, sa transformation en castellum et sa démolition très organisée. Tous les éléments témoignent de la permanence de la puissance publique dans toutes ces phases de transformations radicales