Pyroelectrically Charged Piezoelectrets for Force Sensing

In this article, a pyroelectrically charged fluorinated ethylene propylene (FEP) piezoelectret force sensor is reported. The proposed pyroelectric charging technique provides reproducible and tunable charge generation. Pyroelectric charging is simple and compact when compared to other ferroelectret charging methods, such as corona discharge and dielectric barrier discharge and irradiation, which require high-voltage sources, sharp tips, and vacuum setups. In this pyroelectric charging study, temperature ramp rates of 1 °C/s (
dT

$=15^{\circ }\text{C}$
), 2 °C/s (
dT

$=30^{\circ }\text{C}$
), and 4 °C/s (
dT

$= 60\,\,^{\circ }\text{C}$
) are applied to a lithium niobate pyroelectric crystal using a programmable temperature control system. The effects of the temperature ramp applied to the pyroelectric crystal and the number of temperature ramps on the piezoelectret charging are assessed. The pyroelectrically charged FEP piezoelectrets show
${d}_{{33}}$
piezoelectric coefficients up to 68.6 pC/N. The output of the FEP sensors is linear with the force and independent of frequency in the range of 45–505 Hz. The charge retention was assessed, showing no degradation of the piezoelectric activity up to 66 h. Finally, the FEP ferroelectret sensors were used in a pressure sensor array, allowing the recording of a human step (gait analysis experiment).