Fabrication and Evaluation of Electret Condenser Sensors with Layered Microgaps for Ultrasonic Measurement of Natural Rubbers

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    Microgaps formed by film lamination without bonding exhibit high electric breakdown strength. Poled PZT has extremely high polarization compared to a polymer electret. For this study, electret condenser sensors (FeECSs) were prepared with microgaps formed by film lamination and a poled PZT electret. Transmission properties of the prepared samples were examined when used as ultrasonic probes for soft media (natural rubber, silicone, and acrylic). Lamination of PFA and PPS film in the gap attenuated the resonant vibration of the PZT disk. That attenuation increased concomitantly with the number of film laminations. Addition of BN agglomerates at the interface between films in the gap increased the signal intensity of transmitted ultrasonic waves as well as the attenuation of the resonant vibration. Such improvement might result from the decrease in the contact area between the films in the gap. Some FeECS samples showed higher intensity and more of a rising edge slope of the signal than the piezoelectric sample showed, but time-degradation of the intensity was apparent: the rising edge slope of the signal decreased by around 40% after three days. Discharge in the electret (PZT) might cause time degradation, thereby necessitating improvement of the insulation of the ferroelectric ceramics.

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