Transitions de phases et modes d'épitaxie complexes dans les couches minces d'oxydes de terres rares irradiées par un faisceau d'électrons

Rare earth oxide thin films are peculiar materials, especially in the case of the intermediate oxide series for cerium, praseodymium and terbium. Treatment by water-vapor and irradiation by means of an electron beam induce intricate transition phases and unexpected epitaxial modes. These phenomena are tentatively linked to shock-wave processus throughout the material.Les couches minces d'oxyde de terres rares sont des matériaux très particuliers. La série des oxydes intermédiaires de cérium, praséodyme et terbium en est une parfaite illustration. Les films traités à la vapeur d'eau, puis irradiés par un faisceau d'électrons sont le siège de transition de phases et de modes d'épitaxie complexes inattendus. Un processus d'ondes de chocs, induit par l'impact du faisceau au travers du matériau pourrait expliquer ces phénomènes

Dissolution of silica in solution required to form geopolymer binder

International audienc

Role of the silica source on the geopolymerization rate ALL or

International audienceThe synthesis of geopolymer-silica composites was achieved at room temperature to determine the role of the silica source (quartz or amorphous silica) on the polycondensation rate and the mechanical properties of synthesized materials. Then, samples with a composition range from 100% quartz to 100% amorphous silica were formed, compared and characterized by XRD, infrared spectroscopy, thermal analysis, SEM, and compression tests. The results give evidence that the increase of amorphous silica in the mixture favors the polycondensation reaction (i.e., "geopolymerization") to form consolidated materials whereas quartz led to heterogeneous materials without cohesion. These facts are explained by the modification of the Si/Al ratio in the geopolymer matrix due to the increase of quartz in the mixtur

Parameters that influence silica dissolution in alkaline media

International audienceGeopolymers have been the object of numerous studies because of their low environmental impact. The synthesis of these geomaterials is achieved by the alkaline activation of aluminosilicates. Alkaline activation is typically accomplished by the activation of potassium silicate or sodium silicate. These alkaline silicate solutions are relatively expensive. We thus attempted to create these solutions by the dissolution of potassium hydroxide and silica in water. This study focuses on the various parameters that can influence the dissolution of silica in basic media (pH>13). The samples used were amorphous silica, quartz sand and quartz ground to five different size distributions. The study of the dissolution of siliceous species was performed mainly by infrared spectroscopy by varying several factors. Stirring and solution volume played no significant role. However, the size distribution and crystallinity of silica were observed to significantly affect the kinetics of dissolution and the quantities of siliceous species in solution, which varied greatly according to the quantity of KOH introduced

Feasibility of aluminosilicate compounds from various raw materials: Chemical reactivity and mechanical properties

International audienceThe study of the feasibility of geopolymer formulations was realized from several aluminosilicate sources. Six of them led to the formation of geopolymer-type materials and were confronted with three other reference metakaolins. Several fundamental parameters were identified. A correlation was revealed between these parameters and the reactivity of the mixture, the microstructure and the behavior according to the time of consolidated materials from the various data issues from structural and mechanical characterizations. It has proved possible to predict the behavior of the reactive mixture until its evolution after consolidation for any raw material containing a metakaolinitic phase. All the results allowed to establish a model based on the existence of various geopolymer-type networks: (i) the first one in whom the aluminum atoms participate with the silicon as formers of the network (aluminum-rich) and (ii) the second where the aluminum atoms play the role of compensators in a network rich in silicon

Role of the silica source on the geopolymerization rate: A thermal analysis study

International audiencencreasing the amount of amorphous silica in a mixture containing silica and quartz favors a polycondensation reaction (i.e., geopolymerization) and improves the mechanical properties of the synthesized materials. The study aimed to investigate the polycondensation reaction during the consolidation step of geopolymer formation and examine the various equilibriums at different temperatures. In total, eleven compositions with various amounts of amorphous silica S (high reactivity) and quartz Q (low reactivity) (from 100%Q to 100%S) were synthesized in basic media with metakaolin. The synthesized samples were characterized by thermal analyses and mercury porosimetry. Correlations between the loss of water and the molar ratio of each composition were investigated. The existence of four reactions during the consolidation process was demonstrated: (i) the reorganization of the species; (ii) the dissolution of the metakaolin; (iii) the formation of oligomers; and (iv) the reaction of polycondensation. Moreover, two types of networks were shown, a silicate solution network for quartz-rich samples and a geopolymeric network for amorphous silica-rich samples. The nature of the primary network and the reactivity of the synthesized sample depend on the reactivity of the silica source used. © 2013 Elsevier B.V

Geopolymers shaping based on various metakaolins

International audienc

Impact of various aluminosilicate compounds in geopolymer foam formation to a Si/M=0.7 of silicate solutions

International audienc