Multi-scale porous materials: from adsorption and poro-mechanics properties to energy and environmental applications

Document available in extended abstract form only. 'Multi-scale Porous Materials under the Nano-scope'. Setting up the stage, one can list important engineering problems such as hydrogen storage for transportation applications, electric energy storage in batteries, CO2 sequestration in used coal mines, earthquake mechanisms, durability of nuclear fuels, stability of soils and sediment and cements and concrete cohesive properties in the context of sustainability. With the exception of health, these are basically the challenging engineering problems of the coming century that address energy, environment and natural hazards. Behind all those problems are complex multi-scale porous materials that have a confined fluid in their pore void: water in the case of clays and cement, an electrolyte in the case of batteries and super-capacitors, weakly interacting molecular fluids in the case of hydrogen storage devices, gas-shale and nuclear fuel bars. So what do we mean by 'under the nano-scope'? The nano-scope does not exist as a single experimental technique able of assessing the 3D texture of complex multi-scale material. Obviously techniques such as TEM are part of the answer but are not the 'nano-scope' in itself. In our idea, the 'nano-scope' is more than a technique producing images. It is rather a concept that links a suite of modeling techniques coupled with experiments (electron and X-rays microscopies, tomography, nano-indentation, nano-scratching...). Fig 1 gives an outline of this strategy for cement. It allows accessing material texture, their chemistry, their mechanical behavior, their adsorption/condensation behavior at all scales starting from the nano-scale upwards. The toolbox of the simulation aspect of the 'nano-scope' is akin to a statistical physics description of material texture and properties including the thermodynamics and dynamics of the fluids confined to their pore voids as a means to linking atomic scale properties to macroscopic properties and behaviors. The 'Art of simulation' includes the description of realistic multi-scale porous materials samples at atomic scales, the set up and the validity checking of transferable interatomic/intermolecular potentials, Grand canonical Monte Carlo and Molecular Dynamics simulation techniques with the goal of probing mechanical properties (elasticity, strength, fracture energy), adsorption condensation/evaporation processes (water: in clays, sediments and cements) and ion docking and exchange capacity (in clays and cement, including nuclear energy related species). By contrast (and as the necessary and complementary route), the engineering toolbox consists in continuum or discrete models, which are either based on or focused on field theories like continuum theories. They usually neglect thermal fluctuations and are assumed to a obey equilibrium thermodynamics at least in a macroscopic formulation. Both routes aim at predicting material properties. Ideally, the 'dream' would be to have a consistent engineering physical approach which is consistent from the scale of atoms to the scale of continuum theories, to tackle the challenging problems evoked here above. (author)