Mechanical response of lightweight hollow truss metal oxide lattices

Porous metal oxides are an important class of engineering materials with unique combinations of lightweight, mechanical, photovoltaic, catalytic and thermal properties. The structural stability and load-bearing capabilities of porous metal oxides can be improved if stretch/compression-dominated lattice designs are used instead of bending-dominated foam structures. Here, we introduce a simple, scalable technique that involves the dip-coating of 3D printed polymeric lattices, of simple cubic design, in a metal particle (Fe and Cu) suspension. Subsequent heat treatment in a furnace removed the polymeric core and binder, leaving behind a hollow-truss lattice structure composed of sintered and oxidized metal particles. Examination of its microstructure reveals that the hollow-truss lattices have three levels of hierarchy, namely, the length/ width of the lattice strut (similar to 1 mm), the thickness of the coating (similar to 0.1 mm) and the size of the pores/ particles (similar to 0.01 mm). This hierarchical arrangement of material enabled the hollow-truss metal oxide lattices to achieve similar to 1% relative density, which is lower than that achievable with ceramic foams. Under quasi-static compression, the hollow-truss lattices experienced multiple steps of fractures and exhibited highly serrated stress-strain curves. The relative modulus and relative strength of hollow-truss lattices were found to be related to the relative density by a power law relationship, with an exponent of 1.2 and similar to 1.3, respectively. A detailed analysis showed that the slight deviation of the mechanical properties from an ideal stretch-dominated design was primarily due to the presence of small amounts of porosity in the metal oxide coating. Nevertheless, the load-bearing efficiency exhibited by the hollow-truss metal oxide lattices was found to be comparable or superior to that of hollow-truss alumina micro- and nano-lattices, as well as ceramic foams.