Cellular electrets polymer is a new ferroelectret material exhibiting large piezoelectricity and has attracted considerable attentions in researches and industries. Property characterization is very important for this material and current investigations are mostly on macroscopic properties. In this work, we conduct nanoscale piezoelectric and ferroelectric characterizations of cellular polypropylene (PP) films using piezoresponse force microscopy (PFM). First, both the single-frequency PFM and dual-frequency resonance-tracking PFM testings were conducted on the cellular PP film. The localized piezoelectric constant d33 is estimated to be 7–11pC/N by correcting the resonance magnification with quality factor and it is about one order lower than the macroscopic value. Next, using the switching spectroscopy PFM (SS-PFM), we studied polarization switching behavior of the cellular PP films. Results show that it exhibits the typical ferroelectric-like phase hysteresis loops and butterfly-shaped amplitude loops, which is similar to that of a poly(vinylidene fluoride) (PVDF) ferroelectric polymer film. However, both the phase and amplitude loops of the PP film are intensively asymmetric, which is thought to be caused by the nonzero remnant polarization after poling. Then, the D-E hysteresis loops of both the cellular PP film and PVDF film were measured by using the same wave form as that used in the SS-PFM, and the results show significant differences. Finally, we suggest that the ferroelectric-like behavior of cellular electrets films should be distinguished from that of typical ferroelectrics, both macroscopically and microscopically.

This paper proposed a piezoelectric energy harvester based on a spring-mass-spring oscillator, of which the piezoelectrics operate in the d33 mode. Theoretical analysis reveals that the spring-mass-spring oscillator can not only generate a larger vibration than that of the ambient system but also buffer the force of possible accidental impact applied on the piezoelectric stacks. By using lead zirconate titanate (PZT-4) ceramics as model materials, we systematically characterized the performance of the energy harvester. Results show that at the resonance frequency, the harvester can output a satisfactory electric field. In addition, it has excellent fatigue resistance, e.g., under 9 g vibration acceleration for a long time about 12 h, the electric voltage output of the harvester nearly kept constant and only a slight fluctuation was observed.

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