CNRS Heterogeneous Materials Study Group

Limoges, France

CNRS Heterogeneous Materials Study Group

Limoges, France
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Joulia A.,CNRS Heterogeneous Materials Study Group | Joulia A.,University of Limoges | Joulia A.,French National Center for Space Studies | Vardelle M.,University of Limoges | Rossignol S.,CNRS Heterogeneous Materials Study Group
Journal of the European Ceramic Society | Year: 2013

Rare-earth zirconates and cerates have attracted particular interest for thermal barrier coating (TBC) applications due to their advantageous thermal properties, such as a low conductivity and efficient phase stability at elevated temperatures. This study focuses on synthesising La2Zr2O7, Gd2Zr2O7, La2Ce2O7-γ and La2(Zr0.7Ce0.3)O7-γ compounds via two soft chemistry processes, alkoxide and citrate synthesis. Thermal analysis, X-ray diffraction (XRD) and scanning electron microscope observations were used to analyse the powder after calcinations under air. Chemical reactivity tests under a reducing atmosphere were performed at 1400°C and investigated by XRD analysis. It was found that the lanthanum and gadolinium zirconates are the most stable and interesting materials under an Ar(g)/3%H2(g) atmosphere. © 2013 Elsevier Ltd.


Autef A.,CNRS Heterogeneous Materials Study Group | Joussein E.,CNRS Research Group on Water, Soil and Environment | Gasgnier G.,Imerys | Rossignol S.,CNRS Heterogeneous Materials Study Group
Journal of Non-Crystalline Solids | Year: 2013

Increasing 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.


Tognonvi M.T.,CNRS Heterogeneous Materials Study Group | Soro J.,CNRS Heterogeneous Materials Study Group | Rossignol S.,CNRS Heterogeneous Materials Study Group
Journal of Non-Crystalline Solids | Year: 2012

The purpose of this work is to identify the effect of the cation nature on mechanisms of the sand consolidation with alkaline silicate solution at low temperature (70 °C). Three diluted lithium, sodium and potassium silicate solutions with [Si] = 2.8 mol/l were used to agglomerate sand composed of grains which mean diameter is 340 μm. According to the cation, different behaviors were observed in terms of the drying time and the material cohesion. Essentially, the drying time increases with decreasing cation size. In contrast, the compressive strength raises when the cation size increases inducing intra-granular rupture highlighted by SEM observations. This could find an explanation in the cation hydration sphere of cations. The strength of the cation-water electrostatic interaction becomes less important as the size of the cations increase leading to more ionic bonds. Despite their strong consolidation, potassium-based materials have a high solubility in water. This result is consistent with the ionic nature of bonds. © 2011 Elsevier B.V. All rights reserved.


Autef A.,CNRS Heterogeneous Materials Study Group | Joussein E.,CNRS Research Group on Water, Soil and Environment | Gasgnier G.,Imerys | Rossignol S.,CNRS Heterogeneous Materials Study Group
Journal of Non-Crystalline Solids | Year: 2012

The 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 mixture. © 2012 Elsevier B.V. All rights reserved.


Prud'Homme E.,CNRS Heterogeneous Materials Study Group | Michaud P.,CNRS Heterogeneous Materials Study Group | Joussein E.,GRESE | Rossignol S.,CNRS Heterogeneous Materials Study Group
Journal of Non-Crystalline Solids | Year: 2012

The development of porous materials based on geopolymers allows the benefits of eco-consumption, good insulating properties and good mechanical properties to be combined. For geopolymers to be useful, the development of an understanding of their properties under various conditions is important. Attention was given to the structural evolution of porous materials with heating. The structural evolutions were investigated using thermal analysis and infrared spectroscopy. The formation of a crystalline phase was observed and identified as a zeolite. To elucidate the roles played by the raw materials, silicon concentration and potassium concentration on the formation of the crystalline phase, three parameters in the foam synthesis process were modified. The influence of silica fume and metakaolin on the appearance of the crystalline phase was examined through the reactivity of the raw materials in contact with a potassium solution. Different behaviors directly linked to the network composition were observed. Silica-fume-based samples showed phase crystallization and dehydroxylation due to the large concentration of silicon, which likely led to the formation of hydrated silica species. The formation of the zeolite-phase network upon the application of heat depended only on the silica dissolution and was linked to the composition of silica in terms of the silicon species and grain size. Studying the crystalline phase formation by variation of the amount of potassium allowed the determination of a maximum silicon concentration and a minimum potassium concentration. The formation of the zeolite phase occurred at a potassium concentration greater than 3.5 mol·L - 1 and a silicon concentration lower than 16.5 mol·L - 1, which confirmed the presence of F-zeolites. © 2012 Elsevier B.V. All rights reserved.


Meite M.,CNRS Heterogeneous Materials Study Group | Pop O.,CNRS Heterogeneous Materials Study Group | Dubois F.,CNRS Heterogeneous Materials Study Group | Absi J.,CNRS Heterogeneous Materials Study Group
International Journal of Fracture | Year: 2013

This paper focuses on the characterization of mixed-mode fracture parameters through use of two formalisms based on Crack Relative Displacement Factors and Stress Intensity Factors, respectively. The evaluation of Crack Relative Displacement Factors is based on a kinematic approach that integrates the experimental displacement field measured by a digital image correlation method. In parallel with this step, the stress intensity factor is calculated from a finite element analysis. The coupling between these two approaches allows for the identification of fracture parameters in terms of an energy release rate without any prior knowledge of material elastic properties. Depending on the mixed-mode configuration, the proportion of the energy release rate corresponding to opening and shear modes can be calculated. Moreover, the proposed formalism allows determining, in addition to fracture parameters, the local elastic properties in terms of reduced elastic compliance directly from the test sample. Experimental protocols are carried out using a Single-Edge notched specimen made from a rigid Polyvinyl Chloride polymer loaded at various mixed-mode ratio values. © 2012 Springer Science+Business Media Dordrecht.


Rachini A.,CNRS Heterogeneous Materials Study Group | Le Troedec M.,CNRS Heterogeneous Materials Study Group | Peyratout C.,CNRS Heterogeneous Materials Study Group | Smith A.,CNRS Heterogeneous Materials Study Group
Journal of Applied Polymer Science | Year: 2012

Natural hemp fibers were chemically modified using silane coupling agents to reduce their hydrophilic character. The existence of a chemical bond between coupling agents and hemp fibers was confirmed by ATR-FTIR spectroscopy, 29Si Nuclear Magnetic Resonance (NMR), thermogravimetric analysis (TGA), energy dispersive spectroscopy (EDS), and BET surface area measurements. It was shown that the initial concentration and the chemical structure of the organosilane coupling agent have an effect on the grafted quantity on the hemp fiber surfaces. The grafted quantity increased proportionally to the initial concentration of silane molecules. The presence of polar amino end group (NH2) in silane structure can cause an increase in the grafted quantity, compared with results obtained in the case of silane molecules containing methacryloxy groups. This effect is attributed to the formation of hydrogen bonds between NH2 and unreacted hydroxyl groups of hemp fibers. © 2011 Wiley Periodicals, Inc.


Lecomte-Nana G.,CNRS Heterogeneous Materials Study Group | Bonnet J.-P.,CNRS Heterogeneous Materials Study Group | Soro N.,CNRS Heterogeneous Materials Study Group
Journal of the European Ceramic Society | Year: 2013

Iron-enriched reference kaolins (KGa-1b, KGa-2 and KF) were used to study the effect of iron on the development of mullite phases during the sintering of kaolin-based materials. Up to 1050°C, primary mullite formation occurred at earlier temperature within iron-enriched kaolins than in the case of iron-free kaolins. At 1150°C, the presence of ferric ions tended to promote the transformation of the spinel (γ-Al2O3-like) phase into primary mullite. This action was correlated with an enhancement of the diffusion mechanism of the main constitutive species of the samples (Al, Si). In the range 1300-1400°C, iron-enriched kaolins exhibited an abnormal grain growth of secondary mullite crystals and a partial reduction of hematite (Fe2O3) into magnetite (Fe3O4). These two iron compounds reacted with mullite and cristobalite, leading to the occurrence of eutectic liquids as expected from phase equilibrium diagrams. © 2012 Elsevier Ltd.


Tognonvi M.T.,CNRS Heterogeneous Materials Study Group | Rossignol S.,CNRS Heterogeneous Materials Study Group | Bonnet J.-P.,CNRS Heterogeneous Materials Study Group
Journal of Non-Crystalline Solids | Year: 2011

This study is based on the understanding of the behavior of alkali silicates in a basic medium and aims to identify the mechanisms responsible for the formation of an irreversible gel and its consolidation. Commercial lithium, sodium and potassium silicate solutions were used to reveal the effect of the cation nature on the gelation process. Gels are obtained by acidifying alkaline silicate solutions with hydrochloric acid. A syneresis phenomenon during ripening that leads to the formation of a strongly consolidated solid has been observed. Whatever the cation, the gelation or syneresis mechanism would be similar. However, gelation time decreases and syneresis increases with the cation size. Gelation results from formation of small particles which grow in number and size and then gather to fill the available space. Ripening takes place through a dissolution/precipitation mechanism. © 2010 Elsevier B.V. All rights reserved.


Hounsi A.D.,University of Lomé | Lecomte-Nana G.L.,CNRS Heterogeneous Materials Study Group | Djeteli G.,University of Lomé | Blanchart P.,CNRS Heterogeneous Materials Study Group
Construction and Building Materials | Year: 2013

The present work aimed to investigate the influence of mechanical activation of raw kaolin on the final compressive strength of as-obtained geopolymers regarding the curing profile. A commercial raw kaolin containing 81.5 mass% of kaolin (labeled KBip) was used. Mechanical activation was performed by dry ball-milling of raw kaolin at 250 rpm for 1 h. The curing temperatures were 40 °C, 70 °C or 100 °C for 24 h, 48 h or 72 h. The compressive tests were conducted on geopolymers after the 2nd, 7th, 14th and 28th days of ageing. Mechanical activation was performed to improve mechanical properties. Results showed that without mechanical activation, the optimal curing condition was 24 h at 70 °C and the compressive strength was 15 MPa after 28 days of ageing. Under mechanical activation, improvement of the compressive strength was obtained with a curing time of 72 h at 70 °C (to reach 35% increase) or with a curing temperature of 100 °C (for 76% improvement). The formation of alkaline aluminosilicate gels and new crystalline hydrated phases controlled the strength development of geopolymers while the occurrence of carbonated species was responsible for the degradation of mechanical properties. © 2013 Elsevier Ltd. All rights reserved.

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