Effects of temperature on electromechanical properties and ultrasonic performance of piezoelectric cellular PP films

Abstract

Ferroelectrets are a recent addition to the family of piezoelectric polymers. Ferroelectrets are space-charge electrets with a heterogeneous and usually cellular foam structure. They are flexible, available in large areas and show strong piezoelectric response. Due to their low acoustic impedance they have strong application potential in air-borne ultrasonic transducers. Here, the influence of temperature on the electromechanical and ultrasonic properties of polypropylene (PP) ferroelectrets was addressed. PP ferroelectrets were subjected to repeated temperature variations. The characterization was performed by means of dielectric resonance spectroscopy (DRS) and Laser-Doppler Vibrometry (LDV). Profound variations in the piezoelectric properties during thermal cycling were observed and correlated to the changes in the elastic properties. Exposure to elevated temperatures resulted in a reduction in the piezoelectric response and an increase in the elastic stiffness constant which also shifted the thickness-extension resonance peaks towards higher values. Large differences in piezoelectric properties were discovered between the first and the subsequent cycles. To decrease the influence of temperature cycling on the piezoelectric properties of PP ferroelectrets, annealing was used to advantage. Annealing treatments with different durations and different annealing temperatures were investigated. A particular procedure yielding foam stiffening and a sufficiently high piezoelectric activity was determined and was employed for further investigations. The piezoelectric properties of the annealed ferroelectrets varied much less when compared to non-annealed ferroelectrets. Characterization of thermally cycled annealed and non-annealed films revealed relatively homogeneous phase and amplitude distributions of the surface-vibration below the resonance, which resembles a piston-like response. Piston-like response was not significantly deteriorated by thermal cycling below the resonance frequency. This was corroborated for all samples by measuring the acoustic directivity of them in the farfield, at frequencies below the resonance. At the resonance, much higher distortions of the response were observed as revealed from the phase images.