Microengineered ferroelectret materials for energy harvesting

Abstract

Ferroelectrets are thin films of polymer foams which usually store charges in their internal voids and present a strong piezoelectric property after electrical charging. Ferroelectrets exhibit the similar piezoelectric properties, but the mechanism leading to those properties is completely different: in ferroelectrets, the properties is a result of deformation of charged voids, whereas piezoelectric materials rely on ion displacement in a lattice. For ferroelectrets, the interior voids contain opposite polarity charges on the top and bottom void surfaces and each void can be regarded as a dipole. When a ferroelectret material is compressed or expanded in its thickness direction, the dipole moments inside change in magnitude which leads to the changing of the compensation charge on the surface electrodes. Due to its outstanding piezoelectricity and material properties, ferroelectrets are widely utilized as functional materials in electromechanical sensors and actuators. Compared with piezoelectric materials, ferroelectret materials have several advantages. First, they have a very prominent piezoelectric effect and their piezoelectric coefficient is several times higher than that of traditional piezoelectric materials. Secondly, their production cost is relatively low. Again, most ferroelectrets are mainly composed of non-toxic materials, which do not cause environmental pollution. In addition, a ferroelectret film is light, thin and very soft; it can be made into different shapes and sizes, according to need. Finally, the unique acoustic impedance of ferroelectret materials makes it more useful in areas such as ultrasonic waves and underwater acoustic wave detection. Therefore, ferroelectret materials show great potential in vibration energy harvesting applications. Energy harvesting technology refers to collecting the energy from surroundings into electricity then supplying power to the system. The working principle of the piezoelectric vibration energy harvester is based on the piezoelectric effect of the piezoelectric materials. Under the action of the external vibration force, the piezoelectric layer in the device generates stress and strain, an electrical signal is formed. The piezoelectric energy harvester has the advantages of simple structure, long-life, high-energy density, and compatibility with the MEMS process. Polydimethylsiloxane (PDMS) has been demonstrated in the fabrication of ferroelectrets with controlled void layouts and geometries rather than the random voids associated with foams. Its primary advantages include low cost, fast simple fabrication and high levels of flexibility. However, PDMS is not a stable electret material and the surface charge stability on the void surfaces of the PDMS is poor, causing the effective piezoelectric coefficient of most samples to fall below 10 pC/N in one month, therefore some approaches are necessary to extend the charge stability of the PDMS ferroelectrets or find a novel ferroelectret material suitable for achieving a high level of activity over a large period of time. This report first focuses on some basic definitions and the background related to ferroelectret; This project also tries to use mathematical approach and simulation analysis to find an optimal cellular ferroelectret geometry structure with a high level of piezoelectric activity. This report presents approaches to enhance the surface charge density and stability of a PDMS based ferroelectret device by adding Polytetrafluoroethylene (PTFE) to the PDMS. This report also illustrates a new approach to obtain PDMS ferroelectret with random voids by using mechanical stirring to create cavities.

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Identifiers

ORCID for Mingming Zhang: orcid.org/0000-0002-5979-0014

ORCID for Stephen Beeby: orcid.org/0000-0002-0800-1759

ORCID for Russel Torah: orcid.org/0000-0002-5598-2860

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