Coaxial ceramics based on core-shell materials with distinct properties are of great interest in a wide range of fields, such as catalysis, sensing devices or energy, due to their capacity to modulate their structural and functional properties. In this work, robocasting, a direct ink writing technique, has been employed to additive fabricate in a single-step coaxial patterned structures based on bi-component filaments with a core-shell arrangement, fully encapsulating the inner material. For that purpose, a novel uniaxial co-extrusion system has been designed employing two concentric syringes of different diameters, which allows the simultaneous printing of two pseudoplastic ceramic aqueous-based inks with a single piston. Matching the rheology of both inks and reducing the stresses linked to the different shrinkage during drying and sintering processes are key aspects that have been analysed in this work.
Inks based on boehmite and boehmite/graphene nanoplatelets (GNP) composites, the latter containing 18 vol.% of graphene fillers, were formulated to obtain printable coaxial struts with a good filamentary shape. Two distinct configurations have been robocast considering the different thermal and electrical properties of both materials: i) boehmite core/GNP composite shell and ii) GNP composite core/boehmite shell. The 3D-printed structures have been treated at 500 ºC for 2 h in N₂ atmosphere to transform boehmite to γ-Al₂O₃, keeping the integrity of the GNPs; to finally obtain two types of the 3D lattices, one with a γ-Al₂O₃ core of 0.4 mm diameter and 0.9 mm composite shell and the other with 0.6 mm composite core and 1.0 mm γ-Al₂O₃ shell. A complete study of the shrinkage, the porosity and the microstructure of these coaxial structures after the drying and the thermal treatment has been carried out. The compression strength and the thermal conductivity of both coaxial configurations have been analysed, the latter using the transient pulse source (TPS) method. The results have been compared to those measured in the corresponding 3D monolithic structures (γ-Al₂O₃ and γ-Al₂O₃/GNP) of alike geometries. Besides, finite element methods (FEM) have been used to simulate the heat flow in the coaxial structures and it has been validated with the experimental thermal conductivity data.