Solid State Porous Metal Production: A Review of the Capabilities, Characteristics, and Challenges

Porous metals have been under development for nearly a century, but commercial adoption remains limited. This development has followed two primary routes: liquid state or solid state processing. Liquid state foaming introduces porosity to a liquid or semi-solid metal, and solid state foaming introduces porosity to a metal, which is fully solid. Either method may create pores by internal gas pressure or introducing metal around a template directly control porosity. Process optimization and commercial output has been primarily related to liquid state methods, as solid state processing is often more complex, diverse, and with lower throughput. Solid state methods, however, are often more versatile and offer greater control of pore characteristics. Ongoing advancements in solid state foaming have allowed for a wide array of metals and alloys to be made porous and the three-dimensional structure to be precisely tailored. In general, solid state processing remains limited to niche applications, often with modest dimensions (cm scale). Traditional solid state processes are being further refined and extended, and continuing developments to reduce cost, increase output, and control pore characteristics are likely to produce important advancements in coming years. The extensive variability of pore quantity and morphology makes solid state processes suitable, and often preferable, for an assortment of functional and structural applications, with electrodes and biomedical devices being among the most popular in current research. Various techniques for introducing porosity, the way these methods are applied, important considerations, typical outcomes, and current applications are reviewed.