Cellular-foam polypropylene ferroelectrets with increased film thickness and reduced resonance frequency

Ferroelectrets are piezoelectric materials suitable for acoustic applications such as airborne ultrasonic transducers. Typical ferroelectrets exhibit resonance frequencies in the high kHz to low MHz range. In order to decrease the transducer resonance frequencies to the low kHz range, processes such as gas-diffusion expansion and electric charging were adjusted to cellular films which are initially twice as thick as in earlier studies. The demonstrated film expansion and electric charging lead to mechanically soft cellular structures which show high piezoelectric activities with coefficients up to 130 pC/N. Due to the simultaneously increased film thicknesses, the resonance frequencies are lowered down to about 233 kHz

Piezo- and ferroelectric P(VDF-TrFE) films with inkjet-printed PEDOT:PSS electrodes: Preparation parameters and property evaluation

Thin layers of, poly-(3, 4-ethylenedioxythiophene) with polystyrene sulfonic acid (PEDOT:PSS) were inkjet printed on polymer films of poly(vinylidene fluoride - trifluoroethylene) (P(VDF-TrFE)). Different ink formulations and the printing parameters were studied in order to optimize PEDOT:PSS layer deposition and the resulting film properties such as the layer homogeneity and conductivity. Printed PEDOT:PSS films were deposited at a temperature of only 30°C, they were partially transparent and they can be deposited with a unique shape or form. Electrical poling and piezoelectric properties of P(VDF-TrFE) layer with inkjet printed PEDOT:PSS electrodes were investigated. It was determined that layers printed from PEDOT:PSS inks with special surfactants and additives are suitable electrodes for ferroelectric polymers and piezoelectric transducers

Piezoelectric cellular PP films with enhanced performance for low frequency ultrasound

Piezoelectric cellular PP films are a relatively new type of electromechanical transducer material. They have strong application potential in air-borne ultrasonic transducers because of their low acoustic impedance and high piezoelectric activity. Cellular PP transducers with varied structure in the density range between 251 and 606 kg/m(3) are processed. They show low acoustic impedances between 0.024 and 0.027 MRayls and strong piezoelectric activity with coefficients up to 800 pm/V. Acoustic measurements revealed a relative bandwidth of 35% at -6 dB (resonance frequency around 150 kHz) and an acoustic transmit sensitivity of 6 mPa/V at 30 cm far on principal axis for circular samples of 16 mm diameter. The vibratory pattern of the transducer films were determined below, at and above the resonance frequencies. At frequencies below the resonance, the transducer acts like an ideal piston. In addition, we demonstrate that the transducer samples can be operated at frequencies close to either the first or the second resonance. To do this, fabric samples were contactless inspected in through transmission mode at 140 kHz and 400 kHz

Fiber-reinforced Composite Structures with Embedded Piezoelectric Sensors

Embedding of piezoelectric materials into fiber-reinforced composites is constituted as promising technology to develop ultra-sensitive structural health monitoring functionalities, e. g. strain measurement or acoustic emission testing. Here, we present latest results of embedding piezoelectric polymer foils into fiber-reinforced composite structures. The applied processing technique allows the implementation of several piezoelectric transducers at different positions within the fiber-reinforced composite. For studying the functionality non-destructive impact testing and three-point bending experiments were carried out to characterize the composite. Exemplarily, the functionality of the embedded sensors is demonstrated by means of triggering an embedded light emitting diode which is switched on after the piezoelectric transducer sensed mechanical strain