Atomistic calculations of structural and elastic properties of serpentine minerals: The case of lizardite

The physical properties of the hydrous phyllosilicate lizardite have been investigated by atomistic simulation using the GULP code based on transferable semi-empirical interatomic potentials. Lizardite behavior was first investigated during structure relaxation at room temperature. The Helmholtz free energy is minimum for an equilibrium structure that is in agreement with experiment. The bulk, shear, and Young modulii for lizardite were calculated along with the Poisson ratio. From the shear and bulk modulii, we also calculated translational and longitudinal acoustic wave velocities that are important quantities for tectonophysics models. As expected, lizardite is stiffer in the a direction parallel to the layers than in the c perpendicular direction; the variation of the unit cell parameters with pressure is in good agreement with experiment. The cohesive energy between two successive layers along c direction was calculated at 0.33 eV (i.e., 0.11 eV per OH bond) in good agreement with recent ab initio calculations. Upon pressure and temperature variations, we evidenced that structural changes are mainly pressure induced; pressure being accommodated by a decrease of the c parameter up to 10 GPa. We also found that the change of slope in the derivative of the c cell parameter with respect to pressure occurring around 2 GPa originates from the bending of the interlayer hydroxyl groups with respect to the layer normal direction

Agglomerates of wet particles: effect of size distribution

International audienceWe analyze the strength of agglomerates of wet frictional particles subjected to axial compression by means of particle dynamics simulations. The numerical model accounts for the cohesive and viscous effects of the binding liquid up to a debonding distance [1]. We show that wet agglomerates undergo plastic deformation due to the rearrangements of primary particles during compression [2]. The compressive strength is characterized by the plastic threshold before the onset of failure by the irreversible loss of wet contacts between primary particles [3]. The agglomerate plastic threshold is proportional to the characteristic cohesive stress defined from the liquid-vapor surface tension and the mean diameter of primary particles, with a pre-factor that is a nearly linear function of the debonding distance and increases with size span. We analyze the effect of particle size distribution and show that the plastic strength is an increasing function of the size ratio when the size of the particles in the largest size class is increased.References1. F. Radjai, F. Dubois, Discrete-element modeling of granular materials (Wiley-Iste, 2011)2. T-Trung. Vo, P. Mutabaruka, J-Y. Delenne, S. Nezamabadi, F. Radjai, EPJ Web Conf. 140, 08021 (2017)3. T-Trung. Vo, P. Mutabaruka, S. Nezamabadi, J.Y. Delenne, E. Izard, R. Pellenq, F. Radjai, Mechanics Research Communications 92, (2018

Binary CoOx–SiO 2 Porous Nanostructures for Catalytic CO Oxidation

International audienceThrough the integration of sol-gel chemistry, lyotrope mesophases and emulsions, the first CoOx-SiO2(HIPE) series of cobalt nano-oxides were embedded within silica macromesocellular self-standing hosts. These binary CoOx-SiO2 porous nanostructures (MUB-100(x)) series present an average of 95% porosity. We found out that high cobalt concentration maintains the hexagonal-2D organization of the mesoscopic voids when applying the thermal treatment at 700°C. Their specific surface areas fall between 400-500 m 2 g-1 when assessed from Ar physi-sorption measurements. At the microscopic length scale, as revealed through magnetic investigations, the low cobalt content foams MUB-100(1) and MUB-100(2) are made of the amorphous -Co(OH)2 phase coexisting with the silica network, while increasing the cobalt concentration during the one pot syntheses (MUB-100(3) and MUB-(4) materials) favors the formation of the spinel Co3O4 and olivine Co2SiO4 crystalline nanoparticles embedded within silica. When it turns towards the CO oxidation catalytic performance, the MUB-100(4) is able to totally convert the CO flow before 200°C (starting at 125°C) while achieving 50% of conversion for a light-off temperature T50 of 145°C, revealing the good efficiency of the MUB-100(4) in CO oxidation with which up to 4 catalytic cycles have been performed without disrupting drastically the catalytic performances while reaching thermodynamic stability from the cycle 2 to the cycle 4