Architected Cellular Ceramics via Direct Foam Writing

Abstract

Cellular architectures are ubiquitous in nature due to their low density, excellent mechanical properties, and multifunctionality. The ability to create synthetic materials with similar architectural complexity would benefit several applications. Many manufacturing methods are unable to scalably replicate the cellular motifs present in natural structures, because they are either limited in build volume, restricted to photopolymerizable materials, or lack the requisite precision. My Ph.D. thesis focuses on fabricating and characterizing architected cellular ceramics. Specifically, a new 3D printing technique is developed, referred to as direct foam writing, which enables the fabrication of hierarchically porous ceramics. Central to this technique is the design of a colloidal gel foam ink in which the continuous phase, an attractive alumina particle network, surrounds and interconnects dispersed bubbles. To formulate inks that satisfy the stability, microstructural, and rheological properties required for direct foam writing, we investigated the effects of several important parameters - surfactant concentration, surfactant hydrophobicity, pH, colloid volume fraction, and foaming intensity - on colloidal gel foam structure and elasticity. Notably, we identified a compositional range where colloidal gel foams display long term stability and high specific interfacial area. All stable foam compositions exhibited the desired rheology characteristics, i.e., solid-like behavior below a critical yield stress and shear thinning flow behavior, for direct writing. Moreover, we found that foam elasticity scales nearly linearly with specific interfacial area for these colloidal gel foams. We created ceramic architectures in the form of hexagonal and triangular honeycombs with closed-cell foam struts via direct foam writing. The sintered honeycombs have tailorable microstructure and geometry, programmable deformation modes, stiffness between 1.36 – 27.4 GPa, relative density as low as ~6%, and specific stiffness exceeding 10^7 Pa/(kg/m^3). As an additional demonstration, we also produced 3D woodpile structures possessing filamentary features composed of a hollow core and porous cellular shell which are inspired by natural stem-like structures. In summary, we developed and characterized colloidal gel foams which enable the direct writing of architected ceramics possessing multi-scale cellular features and exceptional specific stiffness. These materials may find application as lightweight structures, thermal insulation and tissue scaffolds.