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MicroElectroMechanical Generators Based on High Performance Piezoelectric Materials

Final Report Summary - MICROGENS (MicroElectroMechanical Generators Based on High Performance Piezoelectric Materials)

Over the last few decades the use of wireless systems and consumer electronics has grown steadily. The overall performance of the computing systems increased rapidly, while becoming “greener”, in the attempt of minimizing the energy consumption by all possible means. The Internet of Things is expected to offer advanced connectivity services to a variety of embedded devices, from biomedical to security and from household objects to security systems.
The electronics all relied until recently on the use of rechargeable or disposable electrochemical batteries for providing electrical energy to devices. However, when using batteries, inherent problems occur because of their finite lifespan cycle. Therefore, the potential of various applications, especially in the field of compact wireless distributed systems was severely reduced as battery life limits the functionality of such devices.
Given the demand to extend the life span and reduce the volume of the electronics, researchers started investigating methods of scavenging electrical energy from the ambient energy sources surrounding the device. The process of acquiring small amounts of energy surrounding a system and converting it into usable electrical energy is called power energy harvesting. The amount of interest devoted to power harvesting increased significantly in the past decade. Converting energy from ambient vibrations, wind, heat or light using a variety of principles (inductive, piezoelectric, capacitive, thermoelectric, pyroelectric, electromagnetic, biomechanical etc.) could enable low power electronics to be operated indefinitely.

The overall objective of the Marie Curie European Re-Integration Grant (ERG), called “MicroElectroMechanical Generators Based on High Performance Piezoelectric Materials” – MICROGENS consisted into advancing the research frontier towards more integrated and more autonomous electromechanical energy harvesters, by using innovative piezoelectric materials for converting the commonly unused mechanical energy, which is dissipated into vibrations and shocks in various structures.
The project relied on strong collaboration between the former Intra European Fellowship host, the institute FEMTO-ST from Besançon, France, a facility with strong experience in piezoelectric micromechatronic systems and the fellow’s re-integration institution (Valahia University of Targoviste, Romania) which has a proven expertise in the sector of renewable energy sources.
The specific objective of the MICROGENS project consisted into the design, manufacturing and implementation of an innovative piezoelectric micro-electro-mechanical (PiezoMEMS) device made from advanced materials of the PMN-PT family on a regular silicon substrate, able to convert electrical energy from low levels of ambient vibrations and impulses (see attached image). The advantage of this approach would be increased efficiency, a further device integration and compatibility with the silicon technology. For this purpose, a clean-room compatible and suitable microtechnology process had to be developed.

The first project phase consisted into a comparative study of a series of candidate materials, regular or advanced (PZT, PZN-PT, PMN-PT...). Next research was performed on the gold eutectic bonding method which allowed wafer-level bonding between PMN-PT and silicon layers. The process compatibility had to be carefully investigated, as some phases such as resin curing or ion etching usually required temperatures beyond the PMN-PT material limits. Other microtechnology processes which were implemented into the flow chart included photolithography, wet etching, dry etching (DRIE), metal sputtering, lapping and polishing.
In parallel the design and re-design of harvesting patch structures was performed. After several iterations of technology improvements and device optimization, a final device was proposed. As observed in the image, the device has a size of 1.6 cm and consisted of five cantilevers of identical PMN-PT active size and different length seismic masses in order to harvest vibrating energy over an extended bandwidth. The width of each cantilever was 1mm and thickness ranged from 420µm at the base down to 40µm at the active area level. The device was successfully characterized on a custom-built testing platform which included a real-time processor. Tests have shown a power output gain of 2.5 times with respect to classical PZT materials, validating the suitability in millwatt power applications.

The works outputs may be summarized in 4 peer-reviewed papers, two invited lectures, a patent filing and a flow-chart report detailing the micro-fabrication phases, which may be used as the base for further development or industrialization.

In conclusion, the MICROGENS project allowed the fellow to investigate new applications and develop new skills in the area of energy harvesting, a sector complementary to his prior experience. The research shown the fesability of PMN-PT use in silicon-compatible technology and led to an innovative harvester device demonstrator. Moreover, this technology could be further employed in microrobotic (nano-positionning and handling) or in sensing (opto-electro-mechanic, atomic force microscopy etc.) applications.
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