Applications of electron microscopy in additive manufacturing of porous multi-ceramics structures

Ceramic additive manufacturing (AM), also known as ceramic 3D printing, allows the fabrication of 3D ceramic structures with complex geometries that are impossible to be built using traditional shaping methods [1]. Electron microscopy can assist in developing this “long-term game changer for manufacturers” by providing manufacturers with the characterization of the ceramic powders and the quality control of the printed structures. At Tampere University, the advanced electron microscopy techniques available at Tampere Microscopy Center are frequently employed to promote ceramic AM by controlling the quality of the prints and identifying microstructural defects that occur during the printing and heat treatment processes. Our recent study [2] demonstrated the potential of AM to fabricate a new generation of catalytic converters (CCs) by printing self-standing (substrate-less) honeycomb structures out of washcoat materials (gamma-alumina and ceria). Gamma-alumina is a common washcoat material due to its high surface area, however, it loses its surface area at high temperature, and therefore, it requires stabilizers such as ceria to avoid this phenomenon. The structures were printed using stereolithography technique and were sintered at two different temperatures (900 ℃ and 1100 ℃). The homogenous spatial distribution of alumina (orange) and ceria (cyan) powders within the sintered structure at 1100 ℃ was visualized using SEM-EDS mapping (Fig.1(a)). This confirms that ceria did not sediment during the printing process. Hierarchical porosity of the final structure was characterized by SEM, confirming different levels of porosity ranging from 1 mm (open channels intended for gas flow (Fig. 1(b)) to interconnected pores less than 10 μm (Fig. 1(c)). The stabilizing effect of ceria on gamma-alumina was studied by surface area measurements and analytical electron microscopy. The STEM and STEM-EDS images shown in Fig. 2(a)-(c) confirmed that no remarkable change in the particle size of alumina can be noticed upon the addition of ceria, indicating that ceria was not effective in stabilizing gamma-alumina at 900 ℃. On the other hand, Figs. 2(d)-(e) show that the presence of faceted ceria particles in the final sintered structure prevented the particle size growth of alumina at 1100 ℃, compared to the pure alumina sintered at 1100 ℃ (Fig. 2(f)).
As the ongoing steps of our research, we are currently using electron microscopy techniques to investigate the effect of organic and inorganic binders on the microstructure of the printed structures and correlate that to the mechanical strength of the structures.