Thermal conductivity of three-dimensional multi-material core-shell filament structures obtained by material extrusion

Coaxial 3D structures based on ceramic materials with distinct properties are of great interest in a wide range of fields due to their enhanced ability to modulate structural and functional properties. In this work, 3D patterned structures based on bi-component filaments with a core-shell arrangement have been additively manufactured in a single step by material extrusion. A system has been designed consisting of two concentric syringes for simultaneous printing of pseudoplastic core and shell ceramic inks with a single pressure device. Aqueous boehmite and boehmite/graphene nanoplatelets (GNP) composite inks have been formulated. The rheology of both inks has been matched to ensure the printability and integrity of the boehmite (core)-composite (shell) layout and its reverse. The as-printed coaxial scaffolds have been treated at 500 degrees C for 2 h in nitrogen atmosphere to transform boehmite to gamma-alumina while the GNP remain undamaged. The thermal properties and the heat transfer of these robust coaxial structures have been experimentally analysed using the transient pulse source method and a high-resolution infrared camera, respectively. Besides, they have been theoretically simulated by finite element methods. These coaxial architectures promote higher thermal anisotropy as compared to mono-material scaffolds, allowing better control of the heat fluxes. The mechanical behaviour of the different lattice materials has been assessed through compression tests to calculate the strength and the apparent elastic modulus; and the fracture surface of the lattice struts after failure has also been examined.