Energy Harvesting based on Ferroelectrets with Transverse Piezoelectric Effect

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Harvesting environmentally available energy and converting it to electrical energy is gaining increasing importance for energizing sensor networks and data transmission. Possible applications are monitoring and transmitting human life functions or important measurement data of remote objects. In this project, we will be studying ferroelectret energy harvesters using two different types of ferroelectret film samples and new device configurations. In the last couple of years, we have demonstrated that harvesters, utilizing the longitudinal piezoelectric effect (d33) of ferroelectrets, deliver about 1 µW of electrical power with a seismic mass of 1 g and a sinusoidal acceleration corresponding to gravity. However, using the transverse piezoelectric effect (d31) in optimized energy harvesters with so called parallel tunnel ferroelectrets, a power output of up to 2 mW was achieved under the same conditions recently. These results, obtained with an energy harvester with a size of only 8 mm x 5 mm, have not been published yet. First, we will improve the existing parallel tunnel ferroelectrets and we will investigate a new tubular air cavity approach, in which several fluorethylenpropylen (FEP) tubes will be thermally fused to form a ferroelectret membrane. Then, by combining these new materials with a seismic mass in various membrane type energy harvesting configurations, we will investigate energy harvesting devices which are significantly improved and modified compared to those used in our aforementioned work.Further, a new class of energy harvesting devices, based on a cantilever structure, will be investigated. This structure will consist of a metal or polymer cantilever beam onto which the soft ferroelectret and a seismic mass is mounted. This approach features two advantages: 1) An independent choice of the mechanical properties, primarily determined by the cantilever, can be made. This allows us to optimize the cantilever e.g. for low resonant frequencies along with large deflections. 2) The piezoelectric activity of the soft ferroelectret can be maximized independently by designing for large change in tunnel thickness during cantilever bending. Separating these functions by combining the hard cantilever with a soft ferroelectret is one of the promising ideas of our proposed project and should result in large electrical power output.For our new class of aforementioned devices with its independent adjustment of resonance frequency and piezoelectric activity, no experimental results have been obtained yet. However, our estimates indicate a power output from such novel devices in the order of several mWs. Thus, our new harvesters will most likely outperform existing ceramic-based cantilever devices.