21 Novembre – Thesis defense - Maria Isabel Rua Taborda
10 h Amphi Jean-Paul Dom - building IMS (Talence campus)
Printed ceramic Piezoelectric MEMS for Energy Harvesting: Towards Spark Plasma Sintering of multilayers.
An emerging application for piezoelectric MEMS (Micro-Electromechanical Systems) concerns the harvesting of mechanical vibratory energy. The fabrication of these piezoelectric MEMS in thick screen printed layers is an attractive low-cost approach. For MEMS energy harvesting applications, thick layers (1-100µm) are an attractive alternative to thin layers to maximize electromechanical coupling. Furthermore, the replacement of MEMS passive silicon substrates with more flexible metal substrates is also expected to improve performance. However, printed piezoelectric thick films maintain a residual porosity that is detrimental to the piezoelectric properties. The objective of this thesis is to develop a resonant mechanical energy harvester (frequency <100Hz) for the supply of autonomous systems. The printed device consists of a multilayer structure formed of Pb(ZrTiO3) as the active piezoelectric material sandwiched between two gold electrodes. The multilayer structure is screen printed on stainless steel (SS) substrate. The implementation of an innovative geometry led to an optimization of the structure resonance frequency and the power density. Finally, to improve the densification of PZT layers, the SPS (Spark Plasma Sintering) sintering technique combined with the screen printing technique was explored. During this thesis work, the energy harvesting device was fabricated by screen printing of all the layers. The manufacturing process was optimized, in particular, by solving problems of layer adhesion and structural deformation. Additionally, a modified zig-zag geometry was developed in collaboration with the University of Waterloo in Canada (UW). More specifically, this specific MEMS system is dedicated to smart grid technologies and operates based on a coupling of piezoelectric and electromagnetic effects. A frequency of 60Hz was obtained with an output power of 9 µW (load resistance 1 MΩ) for a current of 7A and 6.5mm wire-magnet distance. Compared to other piezoelectromagnetic devices in the MEMS-based literature, the normalized power density was significantly improved. Electronics associated with this device has also helped to highlight the potential of this microsystem. Although the performance of the EH was proven, the structure has limited the densification of the active layer of PZT (density ∽82%). Thus, to improve the performance of the device, the second part of this thesis focused on improving the densification of PZT using the SPS process. Optimization of the various SPS parameters (temperature, pressure, duration, and heating rate) was first carried out for the densification of PZT powders. Optimal SPS conditions were determined, and ceramics with densities close to 98% were obtained at temperatures as low as 850°C. The electromechanical properties close to those of PZT's commercial ceramics attest to the effectiveness of SPS. PZT powders could be densified without the addition of sintering aids, and the original use of a protective layer to protect the PZT from chemical reduction has allowed avoiding post-SPS heat treatment. These parameters were then transferred to simple Au/PZT/Au/SS multilayer structures. The main locks identified are the porosity of the active PZT layer, interface problems leading to interdiffusion between layers, delamination, or curvature problems. The various tests led to the design of a graphite SPS mold specially modified and optimized for the densification of this complex multilayer structure. Thanks to significant advantages in reducing sintering temperatures and sintering cycle, SPS is a promising process for the development of printed MEMS.