Nicolas Tessier-Doyen

Dispersion of phyllosilicates in aqueous suspensions: Role of the nature and amount of surfactant

HYPOTHESES The present work aims at investigating the effect of pH values and additives on the dispersion of two 1:1 dioctahedral phyllosilicates in the presence of water. Two model clays are used for this purpose, BIP kaolin and NZCC halloysite, presenting the same surface chemistry but different morphologies. The effect of sodium hexametaphosphate, sodium silicate and sodium carbonate is discussed. EXPERIMENTS Kaolin and halloysite powders were first characterized using X-ray diffraction, thermogravimetric analysis and scanning electron microscopy. Subsequently, suspensions containing 8 mass% of each clay were prepared with or without additives. Experimental measurements regarding the pH values, the zeta potential and the rheological behavior were performed to determine the most suitable additive. FINDINGS Results show that the conformation of halloysite particles changes regarding pH values of suspensions and is strongly related to the surface charges of these particles. At their natural pH values, halloysite and kaolin suspensions exhibit zeta potentials equal to -50 and -20 mV respectively. This trend indicates that halloysite-based suspensions are well dispersed compared to kaolin-based suspensions. Sodium hexametaphosphate is the most suitable dispersant for both clays. The rheological characterization regarding further applications in casting process indicates a shear-thinning behavior for all studied compositions.

Thermal conductivity calculation of bio-aggregates based materials using finite and discrete element methods

This work is about the calculation of thermal conductivity of insulating building materials made from plant particles. To determine the type of raw materials, the particle sizes or the volume fractions of plant and binder, a tool dedicated to calculate the thermal conductivity of heterogeneous materials has been developped, using the discrete element method to generate the volume element and the finite element method to calculate the homogenized properties. A 3D optical scanner has been used to capture plant particle shapes and convert them into a cluster of discret elements. These aggregates are initially randomly distributed but without any overlap, and then fall down in a container due to the gravity force and collide with neighbour particles according to a velocity Verlet algorithm. Once the RVE is built, the geometry is exported in the open-source Salome-Meca platform to be meshed. The calculation of the effective thermal conductivity of the heterogeneous volume is then performed using a homogenization technique, based on an energy method. To validate the numerical tool, thermal conductivity measurements have been performed on sunflower pith aggregates and on packed beds of the same particles. The experimental values have been compared satisfactorily with a batch of numerical simulations.

Microstructural effects associated to CTE mismatch for enhancing the thermal shock resistance of refractories

This work is devoted to the study of thermomechanical properties of several industrial and model refractory materials in relation with the evolution of their microstructure during thermal treatments. The aim is, in particular, to highlight the role of thermal expansion mismatches existing between phases which can induce damage at local scale. The resulting network of microcracks is well known to improve thermal shock resistance of materials, since it usually involves a significant decrease in elastic properties. Moreover, this network of microcracks can strongly affect the thermal expansion at low temperature and the stress/strain behaviour in tension. Even if these two last aspects are not so much documented in the literature, they certainly also constitute key points for the improvement of the thermal shock resistance of refractory materials. Evolution of damage during thermal cycling has been monitored by a specific ultrasonic device at high temperature. Beyond its influence on Youngs modulus, this damage also allows to decrease the thermal expansion and to improve the non-linear character of the stress/strain curves determined in tension. The large increase in strain to rupture, which results from this non-linearity, is of great interest for thermal shock application.

Clay Structural Transformations during Firing

Silicate ceramics with clays are some of the most complicated ceramic systems because of the very complex relationship between the behavior of mineral materials during the ceramic processing and the transformations during heating. A major challenge is to predict the phase transformations in silicate ceramics, since complex relationship occur between the microstructural and structural characteristics of fired product and the physical properties. Clay minerals undergo strong structural transformations during heating, simultaneously to a complex path of thermal transformations. Individual reactions cannot simply identify since they are closely related and overlapped. At temperature above 800°C, new phases are recrystallized and many of the reactions are strongly topotactic. Mullite is the most important phase, which recrystallizes with a range of morphology and stoichiometry. Variables affecting the mullite formation include the clay mineral type and behavior during heating, the possible occurrence of liquid and impurities as Fe. It results in large variations of the stoichiometry and shape of mullite crystals, which are embedded in a low ordered phase to form a micro-composite microstructure. This presentation will review recent research, looking at structural transformations in some typically used phyllosilicate systems : (i) structural transformation of kaolinite and mica phases were identified at temperature up to 1100°C. They evidence a residual structural order of high temperature phases which is favorable to the topotactic recrystallization of mullite; (ii) from the high temperature form of phyllosilicates, an organized network of mullite can be obtained; (iii) the composition of a local and transient liquid and the presence of minor elements as Fe has a significant influence on the mullite morphology; (iv) mechanical properties are closely related to size and organization degree of the mullite network; (v) the process itself influence the kinetic of structural transformation and particularly the powder compact density and the thermal cycle. These research in silicate ceramics evidence multiple and complex challenges, providing opportunities for future development.

Properties Related Phase Evolution in Multilayer Silicate Ceramics

The use of ceramic processes inducing a microstructural organization at the grain scale favors the improvement of strength and toughness. With layered structures, it is possible to design the microstructural characteristics of materials, leading to increased threshold strength. Layered structures can be arranged to control the local residual stresses causing elastic mismatches between dissimilar materials and crack deflection at interfaces. In this way, multilayer composites from kaolinite and alumina or mullite fibers were shaped by tape casting and staked by thermo-compression, or by centrifugation. During sintering, they show at strong anisotropic behavior, which is in correlation with different activation energy for sintering. Mullite growth is also anisotropic, inducing the formation of an organized micro composite microstructure. The mechanical and elastic properties are correlated with the organization degree of mullite crystals, due to the formation of an interconnected mullite network in the microstructure. It is also shown that variations of mechanical and elastic properties are correlated with the texture index obtained by Quantitative Texture Analysis from X-ray data. The anisotropy of the elastic properties is evidenced by different values of Young’s modulus in directions parallel and perpendicular to the casting direction. Beside, the crack growth resistance is governed by discontinuities along layer boundaries and fiber interfaces.