Hierarchically-porous metallic scaffolds via 3D extrusion and reduction of oxide particle inks with salt space-holders
3D ink-extrusion of powders followed by sintering is an emerging additive manufacturing method capable of creating metallic microlattices. Here, we study the creation of hierarchically porous Fe or Ni scaffolds by 3D extrusion of 0/90° lattices from inks consisting of fine oxide powders (Fe2O3 or NiO, < 3 µm), coarse space-holder particles (CuSO4, < 45 µm) and a polymer binder within a solvent. After space-holder leaching and debinding of the lattices, a sintering step densifies the metallic Fe or Ni powders created by oxide reduction with H2, while maintaining the larger pores templated by the space-holder particles within the printed struts. The resulting Fe or Ni lattices exhibit a hierarchical porosity: (i) open submillimeter channels within the lattice between the printed struts; (ii) open micropores (5–50 µm) within the struts, from leaching of space-holder particles; (iii) closed submicron pores within the micropore walls, from incomplete sintering of the metal microparticles created by oxide reduction. While the channel porosity is defined by the printed geometry, strut porosity is controlled by the sintering temperature, with the finest porosity removed first. Sintering at 873–1373 K for 1 h leads to strut porosities of 38–19% in Fe and 59–29% in Ni. In situ synchrotron X-ray tomography reveals the sintering kinetics and porosity evolution upon debinding, oxide reduction by H2 and sintering of the porous metal, which, combined, lead to ~50% shrinkage of the scaffolds as compared to the printed state. The space-holder templated pores are not eliminated by sintering and remain in the struts, but they shrink upon reduction and sintering of the surrounding metal matrix (e.g., by 40% in Fe and by 30% in Ni upon sintering at 1173 K for 1 h).