Waterproof and Low-Cost Piezoelectrets with High Piezoelectric Responses

Piezoelectrets, which are made of porous polymers, reveal a pronounced piezoelectric response, and they are widely used in many fields. This work presents preliminary results for the fabrication and piezoelectric activity of a novel piezoelectret that was fabricated with polypropylene solid polymer films as a shell and polyethylene solid polymer films as a support. After hot pressing treatment and corona poling, this novel piezoelectret revealed a high piezoelectric response. For a sample with a 300 mu m thick air gap, the average quasi-static piezoelectric coefficient was about 582 pC N-1 and the dynamic piezoelectric coefficient was about 379 pC N-1 at 50 Hz. Moreover, these piezoelectric coefficients remained nearly constant after more than 2 x 10(5) excitations. The distinctive feature of this novel piezoelectret was water resistance; the piezoelectret could be stored in a high humidity environment or even in water for 15 h without any loss of piezoelectric activity. Finally, one theoretical model for calculating the maximum charge density of the novel piezoelectret was given, which revealed that the d(33) value was linearly dependent on the air gap thickness. Due to the water resistance and large amount of piezoelectric activity, the novel piezoelectret could be potentially used in various applications with high humidity.