Piezoelectricity in cellular electret films

Abstract

Permanently charged films with a cellular or porous structure represent a new family of polymer electrets. These materials show piezoelectric properties with high piezoelectric constants. The electromechanical response equations of such films are derived for their operation as sensors and as actuators. Experimental results are also presented for cellular polypropylene (PP). In particular, measurements of the direct and inverse piezoelectric constants in the frequency range 0 to 10 kHz and of the variation of these constants across the surface of the films are discussed. These measurements, performed by direct application of stress or by the use of a profilometer, an accelerometer and an interferometer yield a frequency-independent piezoelectric $33 constant of 5 220 pC1N. Assuming reasonable charge distributions and charge densities, the calculated piezoelectric constants are in good agreement with the measured values. The theoretical model shows the reciprocity of the piezoelectric constants.

Energy harvesting study on single and multilayer ferroelectret foams under compressive force

“…It indicates that the 1 st layer (top layer), deforms more than the 2 nd layer (middle layer), and the 2 nd layer deforms more than the 3 rd layer (bottom layer). As the output voltage is directly proportional to deformation [6,45], which is the change of thickness in this case, this simulation implies that the 1 st layer produces higher voltage than the 2 nd layer, and the 2 nd layer produces higher voltage than the 3 rd layer. To support this simulation, a three-layer PP sample is fabricated by stacking the layers with all edges sealed.…”

“…Thus the force F is directly proportional to ∆v, and the impulse with higher momentum has higher value of ∆v. As the peak voltage V of an output pulse is directly proportional to the applied force in a simplified mathematical model [6,15,45], the output voltage in a mode with high momentum is higher than those one with low momentum. This explains the difference in the positive output pulses in different walking modes.…”

“…FERROELECTRET is a thin and flexible cellular polymer foam that can store charges in its internal voids [1][2][3][4], thus being able to generate electric pulse under mechanical deformation such as compression and bending [5][6]. Due to its high piezoelectric voltage constant g 33 [7], ferroelectret has already been used extensively in sensor applications, such as blood pressure pulse transducer [8], touch sensor [9], ultrasonic transducer [10], prosthetic skin [11], and accelerometer [12].…”

“…Due to its high piezoelectric voltage constant g 33 [7], ferroelectret has already been used extensively in sensor applications, such as blood pressure pulse transducer [8], touch sensor [9], ultrasonic transducer [10], prosthetic skin [11], and accelerometer [12]. Among the ferroelectret materials that have been developed, cellular polypropylene (PP) is one of the most researched [2][3][4][5][6], and being one of the first commercialized ferroelectrets [13][14][15]. Charged cellular PP exhibits a high piezoelectric charge constant d 33 , which is comparable to commercial PZT [7].…”

Intermittently-powered energy harvesting step counter for fitness tracking

“…Ferroelectret materials are flexible cellular polymer foams that, similarly to a piezoelectric source, convert mechanical energy into electricity when they are bent or compressed [9]. In an energy harvesting insole made with ferroelectret materials, the mechanical stress is generated by the heel striking against the floor when walking or running.…”