Metallic nano-foams –structure and synthesis

Metal foams are so-called cellular materials composed of a metal ligament structure and space. A common feature of metal foams is their very high porosity, less than 50% by volume. Their structures are highly varied: periodic like "honeycombs", stochastic with random distribution of material, partially organized like the assembly of hollow spheres. There are also open and closed porosity foams. Metal foams have been manufactured for over a century, around the foundry trades. However, interest in these materials has grown over the last twenty years, in the wake of major societal challenges (environment, energy, health, etc.). Metal foams are now part of the family of architectural materials, where the organization of the material is modeled and controlled by assembly techniques such as additive manufacturing, or by dispersion in a liquid state (foaming). Cellular architecture is designed to give rise to targeted properties, notably multifunctionality, while making structures lighter. Industrial development is beginning in the fields of transport, housing, energy and medicine: shock and vibration absorbers, shielding, heat exchangers, sound insulation, catalyst supports, electrodes, prostheses.

In the vast family of metallic foams, nano-foams stand out. While they share the main characteristics outlined above, the nanometric structural scale (or scales) will require adapted modes of elaboration and characterization, and will produce properties specific to the nanometric state. Compared with their larger siblings, nanofoams are still fairly confidential, but academic research has been very dynamic since 2010. In 1927, Raney patented the manufacture of nickel catalysts with a very high specific surface area, laying the foundations for one of the major techniques in the manufacture of nanofoams, based on the selective dissolution (dealloying) of a Ni-Al-Si alloy.

The dominant feature of nanofoams is their structural dimensions. IUPAC (International Union of Pure and Applied Chemistry) nomenclature defines pore size in terms of pore diameter: micropores are smaller than 2 nm, mesopores are between 2 and 50 nm, and macropores are larger than 50 nm. However, it is customary to classify nano-porous materials as those with pore diameters between 1 and 100 nm. With a porosity rate of over 50%, the dimensions of the ligaments of material will also be in these orders of magnitude. On these dimensional bases, nanofoams develop extremely high specific surface areas, of great interest for interactions and exchanges taking place via the surface (catalysis, insertions, heat exchange, flow). Far beyond this, nanometric dimensions bring the specific properties of metallic matter in this state: mechanical strength, optical properties with electron plasmon confinement, magnetic properties...