Silicon carbide paste development for the extrusion based 3D printing of a heat exchanger employed in concentrated solar power applications

The next-generation concentrated solar power (CSP) applications consider the use of heat exchangers characterised by outstanding corrosion and oxidation resistances. These chemical and physical features are essential when using molten salts and supercritical CO2 as heat transfer fluids into a Brayton power cycle. To improve the heat exchange, the contact surface in between the working fluids have to be enhanced, implying the utilization of a more complex geometry. Due to common production technology limits, the Additive Manufacturing (AM) via Direct Ink Writing (DIW) has been considered as an alternative. In this context the control of rheological properties of the produced inks represents a crucial step for the printing. A suitable material able to build a heat exchanger for CSP applications is SiC. In this thesis the formulation of an innovative hydrogel based SiC ink has been developed, considering a greater solid amount than the usual receipts present in similar recent studies. The rheological properties of various ink formulations have been studied in terms of printability and possible improvements. The key rheological parameters were identified and linked to the printability aspects. Different production technologies have been compared to understand the best ink production. To perform the rheological studies, four algorithms were developed. Temperature Ramp algorithm allowed to understand the thermal reversible behaviour of the hydrogel. Shear Rate algorithm showed the correlation in between the shear rate of the screw used to print and the viscosity of the paste. Shear Stress and Frequency Sweep algorithm were implemented as further studies useful to understand the behaviour of the ink when subjected to a change in shear stress and frequency acted by the printing device. The pritability characteristics have been tested in a 3D printer, demonstrating the possibility of printing a ceramic structure in a complex geometry. To print with DIW an off-the-shelf printer for polymer filaments has been used. The printer was modified to be able to print pastes, implying some structural changes as well as programming manipulations. Scanning electron microscope has been used to analyse the microstructure and see the material distribution.