Fabrication and encapsulation of micro-SOFCs. Improving the reliability of the system: integration of MEMS technology and novel thin film anode

In the last years, the huge proliferation of portable devices with increasing energy demands requires new technologies with higher power and storage density than batteries. Micro solid oxide fuel cells (micro-SOFCs) present the highest value of specific energy density and they allow a direct operation with hydrocarbons. The Ultra-SOFC project, managed by IREC (Catalunya Institute of Energy Research), is developing functional free-standing membranes integrated in silicon-based micro fabricated substrates. This thesis is carried out in the framework of the Ultra-SOFC project and it aims to develop innovative solutions in micro-SOFC state of art to increase reliability. Two main topics have been considered: (1) the optimization of the joining thermal treatment and characterization of a commercial glass-based sealant (carried out mainly at POLITO) and (2) a new ceramic anode deposited as a thin film (carried out at IREC). Chapter 1 introduces the fundamentals and challenges of a micro-SOFC power generator, the principles of functioning, the fabrication techniques, the different type of micro fuel cells and the different materials adopted until now. The second chapter gives a brief overview of the main Si-wafer joining techniques and the anodes tested for micro-SOFC devices. The third chapter focuses on the experimental procedures and characterization techniques used. A deep study on the glass-based sealant properties have been carried out by comparing two different thermal treatments applied for the joining of two silicon chips. A controllable fine line deposition technique has been developed (liquid deposition modelling, LDM). Simultaneously, the complete characterization route of a strontium-doped lanthanum titanite (LST) thin film ceramic anode is reported. The work has been carried out considering four different films, obtained with pulsed laser deposition (PLD) technique, by varying the chamber deposition pressure. Chapter 4 resumes the main results. Crystallizations at temperature above 675°C have been detected in the glass seal with XRD and SEM analyses. They should be the responsible of the CTE variation, allowing a good matching (4,716E-06 K-1) with the silicon substrate in the whole range of operating temperature. Moreover, by reaching 700°C a specific volume reduction of 40% is obtained, thus leading to a reduced porosity and an optimal wettability and adhesion of bonding surfaces. Deposition tests were performed and optimized; leak tests experiments are necessary to evaluate the glass-ceramic bonding gas tightness. Four types of films have been deposited with PLD showing different structures, thickness (100 nm) and roughness (2 nm) with SEM, ellipsometry and AFM analyses. In-plane electric conductivity tests have highlighted a strongly difference with the anodes of bulk SOFCs, in terms of maximum conductivity (0,2 S/cm). At list 20% of Titanium lack in each film is the main responsible of the low electric properties, since its reduction from Ti4+ to Ti3+ is the main phenomenon which allows the electrons migration. On the other side a very low activation energy has been measured (down to 0,07 ev) evincing the LST as a promising material for thin film anodes. Moreover, a grain size (hence pressure deposition) correlation with conductivity and activation energy has been demonstrated. Indeed, the larger the film grain size, the higher electrical conductivity and the lower activation energy have been obtained.