MECHANICAL CHARACTERIZATION OF GLASS/STEEL JOINED SAMPLES FOR SOFC

The European Union has the goal to reduce greenhouse gas emissions by 80% to 95% compared to 1990 levels by 2050, mainly by introducing more shares of renewable energy sources. Solid oxide technologies are key enabling technologies for allowing for such integration. Solid oxide cells (SOCs) devices are efficient and clean power generator and highly efficient converter of electricity to hydrogen (SOFC), fuels and chemicals using high temperature electrolysis (SOEC). These devices can balance the fluctuating nature of electricity from renewable sources by changing dynamically operation mode between SOFC and SOEC (rSOC) in a reversible way. Although their development still faces various problems with high-temperature materials compatibility, durability and integration processes, SOCs are expected to enter the commercial markets in the near future.The enhanced efficiency and durable electrochemical energy conversion in SOC systems can be achieved by suitable material choice and processing and a reliable integration and joining technology. In this scenario, glass ceramic materials play a key role in the sealing and joining of metallic interconnects, having in mind that SOCs work in the temperature range between 650°C and 850°C, depending on the stack configuration. One of the major challenges in the design of SOC devices is the selection of materials to join and seal the metallic interconnects and thus build a SOC stack. The seals must provide hermetic conditions and exhibit thermo-chemical and thermo-mechanical compatibility with the metallic interconnect.The glass-sealing materials have been identified as a critical element in several studies and mechanical issues of the sealing materials have an important impact on SOC performance and on their degradation rate. Furthermore, the mechanical reliability of glass-ceramic/metal joints over several thermal cycles might become even more critical at high temperatures.The glass seal/metallic interconnect interface play a crucial role in determining the measured interfacial strength and it can be affected by the interconnect surface, rugosity etc. The main goal of this MSc thesis is to evaluate the compatibility between the Crofer22APU (a ferritic steel) and a glass-ceramic seal and subsequently to optimise and improve the joining process.The torsion test on hourglass-shaped specimens of Crofer22APU is a reliable method to measure the “pure” shear strength of glass-to-metal seals with only negligible contribution from other stress components.A laser treatment was exploited to increases the interconnector surface roughness; subsequently, an alumina layer was deposited on the interconnect modified surface, to increase the adhesion.The experimental activity is divided in two parts. A preliminary section is dedicated to the best surface treatment by analysing modified and joined samples with SEM, EDS, HSM and profilometry techniques. In the second section, torsion tests are performed on the Crofer22APU glass ceramic joined samples to evaluate different failure modes of the joined area, depending on the surface modification. This study has shown that a proper combination of different surface treatments is crucial for the processing of the joining between the glass ceramic and metallic interconnect and how the glass/metal interface can be affected by the interconnect surface and rugosity, providing an important insight regarding the integration of metallic and glass-ceramic seal materials in SOC devices.