Robocasting process optimization of glass-based sealants for solid oxide technology

Solid oxide cells (SOCs) offer great potential for clean and efficient power generation from a wide variety of fuels, ranging from hydrocarbons to renewables, and for highly efficient conversion of electricity to hydrogen or synthesis gas via electrolysis. Performances of these devices are determined by the durability of different types of functional materials. The development of appropriate sealant materials represents one of the major challenges for the assembly of a durable SOC stack. The main role of the sealant is to prevent the mixing and leaking of fuel within the stack, to electrically isolate cells and to provide mechanical bonding with adjoining components. Thanks to the versatility of their compositions, glass-ceramics are likely to be the materials of choice for a proper sealant. These materials can be deposited in form of pastes or slurries (prior to the stack assembly) with different additive manufacturing techniques. Robocasting technique is an extrusion based additive manufacturing method, which offers the opportunity to create complex 3D geometries. This technique is advantageous for slurry-based depositions and can be used for the fabrication of dense ceramic and glass-ceramic structures. The aim of this study is to refine a novel robocasting process of glass-ceramic sealants for SOCs applications, by means of the ZMorph VX Mooltitool 3D Printer, within the project “NewSOC - Next Generation solid oxide fuel cell and electrolysis technology”. Two methods were analyzed, labeled as “Terpineol method” and “Pluronic method”, with two different glasses, labeled as NS9-1 and SoA. Ceramic pastes based on ethyl cellulose (EC) as binder and terpineol as organic solvent are currently used for screen printing. Recent studies explored inks based on a hydrogel, Pluronic F-127, which acts as a carrier for ceramic powders. Different designs were modelled by using SolidWorks software. A suitable glass thickness is fundamental to obtain a high-durable glass-ceramic sealant, therefore width and height of deposited paste were controlled by acting on different parameters in Voxelizer 2.0. Sealants were deposited onto Crofer22APU plates and a uniform pressure of 15 g/cm2 was applied on the joined samples. A 270 \mum Yttria-Stabilized-Zirconia YSZ spacer allowed to control the glass paste shrinkage and the final thickness of the sealant. The most appropriate thermal treatment was necessary to allow the burn out of the organic binder and the sinter-crystallization behavior of the glass. For these purposes the glasses were thermally analyzed and the microstructure of resulting sealants was evaluated through SEM analyses. The comparison of obtained results obtained allowed to assess the most appropriate choice in terms of binder, paste composition and sinter-crystallization treatment, with a specific focus on time of the process, heat treatment steps and the overall energy consumption. This MSc thesis has provided a deeper insight into innovative manufacturing processes for solid oxide cell components, focusing on a specific technique aimed at on minimization the use of solvents, towards a low amount of waste in the whole process, also considering the scalability of the process. Further research should focus on the preliminary characterization of joined samples with tests in dual-atmosphere exposure under an applied voltage, to simulate real-life operating high temperature conditions and monitor electrical resistivity.