Property Assessment and Application Exploration for Layered Polytetrafluoroethylene Piezoelectrets

Piezoelectrets are piezoelectric polymer foams with oriented macro-dipoles, having promising transducer applications. Here we report the properties of polytetrafluoroethylene (PTFE) piezoelectret films produced by a continuous production line and their application in film transducers. Such samples consist of a porous PTFE core and two compact PTFE outer layers. The results show that the samples are of significant longitudinal piezoelectric d(33) coefficients as large as 1300 pC/N in a quasi-static condition. The thermal stability of the PTFE piezoelectret is much superior to that of ordinary polypropylene (PP) piezoelectrets. After annealing treatments at RT, 90 and 120 degrees C for 25 h, 96, 82, and 49% of the initial d(33) values for PTFE samples were retained, respectively, while the d(33) value of PP samples dropped to 20% from a small initial value of 168 pC/N after 25 h annealing at 90 degrees C. The flexible piezoelectric film transducers based on PTFE piezoelectret films exhibited high sensitivity, excellent wetting resistance, and good mechanical stability. The typical sensitivity of the fabricated transducers ranges from 240 to 320 pC/g (g is the gravity of the earth.) for a load mass of 100 g. No significant difference in the sensitivity was observed when the transducer samples were stored in various environments. Such transducers succeed in detecting heart beating and identifying tactile and sliding signals as being employed in wearable devices and artificial skins. In addition, the normalized power of 0.06 mu W was output at the resonance frequency and match resistance for an energy harvester based on such transducers.