Piezoelectric property of cross-linked polypropylene piezoelectret and its application in vibration energy harvester

Abstract

Piezoelectrets, also known as ferroelectrets, are space-charge electrets based polymer foams with strong piezoelectric effect. The piezoelectric effect in piezoelectrets originates from the regularly arranged dipolar space charges in the polymer matrix, achieved by properly charging the specific foam structure. The large figure of merit (FOM, d33·g33) in piezoelectrets implies that such kinds of materials are promising candidates in energy harvesters. In this article, the electron-irradiated cross-linked polypropylene (IXPP) foam sheets are rendered piezoelectric (i.e. to become piezoelectrets) by modification of the microstructure using hot-pressing process and polarization using corona charging at room temperature. The electromechanical properties of the fabricated IXPP piezoelectrets are investigated by measurements of quasi- and dynamic piezoelectric d33 coefficients, dielectric resonance spectrum, and isothermal decay at elevated temperatures. The energy harvesting from vibrations by using the IXPP piezoelectret films, at various vibration frequencies, load resistances, and seismic masses, are also studied. Results indicate that the quasi-static piezoelectric d33 coefficients of IXPP films up to 620 pC/N can be achieved. The variation of quasi-static piezoelectric d33 coefficient is dependent on the applied pressures, from 0.1 to 1.3 kPa, while it shows good linearity at larger pressures from 1.3 to 15 kPa. The typical values of Young's modulus in the thickness direction and the figure of merit (FOM) are 0.7 MPa and 11.2 GPa-1, respectively. The d33 coefficients will drop to 54%, 43%, 29% of the initial values after annealing the samples for 24 h at 50, 70, 90℃, respectively. At an exciting frequency of 820 Hz, the normalized output power of 65 μW/g2 is obtained from an IXPP film with an area of 3.14 cm2 and a seismic mass of 25.6 g around the optimum load resistance. Such thin, light and flexible IXPP piezoelectret films may be applied in vibration energy harvesters for powering low-power electronic devices.