Mechanical Response of Lightweight Hollow Truss Metal Oxide Lattices

Abstract

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. However, current techniques of fabricating orderly metal oxide lattices, such as selective laser sintering, are too costly or, in the case of alumina nanolattices synthesized through 2-photon polymerization and atomic layer deposition, not amenable to production scaling. 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 suspension. In the present study, iron particles and copper particles were separately used to demonstrate the feasibility of the technique. 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 (~ 1 mm), the thickness of the coating (~ 0.1 mm) and the size of the pores/ particles (~ 0.01 mm). This hierarchical arrangement of material enabled the hollow-truss metal oxide lattices to achieve ~ 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 curve. 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 ~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 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.