Ferroelectret Transducers Research Articles

Air-coupled ultrasound detection of natural defects in wood using ferroelectret and piezoelectric sensors

Air-coupled ultrasound was used for assessing natural defects in wood boards by through-transmission scanning measurements. Gas matrix piezoelectric (GMP) and ferroelectret (FE) transducers were studied. The study also included tests with additional bias voltage with the ferroelectret receivers. Signal analyses, analyses of the measurement dynamics and statistical analyses of the signal parameters were conducted. After the measurement series, the samples were cut from the measurement regions and the defects were analyzed visually from the cross sections. The ultrasound responses were compared with the results of the visual examination of the cross sections. With the additional bias voltage, the ferroelectret measurement showed increased signal-to-noise ratio, which is especially important for air-coupled measurement of high-attenuation materials like wood. When comparing the defect response of GMP and FE sensors, it was found that FE sensors had more sensitive dynamic range, resulting from better s/n ratio and short response pulse. Classification test was made to test the possibility of detecting defects in sound wood. Machine learning methods including decision trees, k-nearest neighbor and support vector machine were used. The classification accuracy varied between 72 and 77% in the tests. All the tested machine learning methods could be used efficiently for the classification.

Air-coupled ferroelectret ultrasonic transducers for nondestructive testing of wood-based materials

Air-coupled ultrasound (ACU) is used in through transmission to detect delamination, rot, and cracks in wood without altering the structure permanently. Novel ferroelectret transducers with a high signal-to-noise ratio enable high-precision structure recognition. Transducers made of cellular polypropylene are quite suitable for ACU testing due to their extremely low Young’s modulus and low density resulting in a favorable acoustic impedance for the transmission of ultrasonic waves between the transducer and air. Thus, structures with great dimensions, with a thickness of up to 300 mm and material densities below 500 kg/m3, can be inspected. Promising results were obtained under laboratory conditions with frequencies ranging from 90 to 200 kHz. The advantage of ACU transducers is that they do not require contact to the sample; they are accurate and cost-effective. Ultrasonic quality assurance for wood is an important avenue to increase the acceptance of wooden structures and toward sustainability in civil engineering in general.

Low-cost electro-acoustic system based on ferroelectret transducer for characterizing liquids

Several industrial applications require liquid characterization during manufacturing to ensure quality in their products. Examples of such products are those related to food and oil industries in which one of the concerns regards the different concentration of liquids in a mixture. In this context, it is presented a system based on ferroelectret transducers and low-cost technology to perform liquid analysis through acoustic measurements. The developed system is composed of three parts: a piezoelectric actuator, a medium chamber and a ferroelectret transducer (acoustic sensor). The system was assembled with a ferroelectret (with open tubular channels) housed on a cylindrical aluminum case (48 mm × 63 mm) in which a 24 dB preamplifier was mounted. The specifically designed chamber was manufactured in ABS plastic using a 3D printer and the piezoelectric actuator consisted in commercial piezoelectric ceramic which is connect to signal generator configured to generate a sinusoidal sweep between 0 Hz up to 50 kHz during 1 s. In order to characterize the device setup and validate the suitableness of the measuring system, three distinct mediums were analyzed (air, water and oil). Through the experimental tests it was identified, on time domain, distinct signatures for each medium, which were later used in a blind classification with a developed algorithm.

Enhanced electromechanical response of ferroelectret ultrasonic transducers under high voltage excitation

Polypropylene based ferroelectret films exhibit a strong electromechanical activity and provide a promising solution for the air coupled ultrasonic (ACUS) transducers. Ultrasonic transmission between two air coupled ferroelectret transducers in dependence on the amplitude and polarity of the high voltage exciting pulse revealed a strongly non-linear electromechanical response of the ferroelectret transmitter which provides an increase in the transmitter efficiency. The authors present a simple model describing both promotion and competition of the piezoelectric and electrostriction contributions, as well as increase in the transducer constant under high voltage excitation. Enlargement of the inverse transducer constant of the polypropylene ferroelectret film by a factor of 4 was demonstrated. The non-linear properties of the polypropylene ferroelectrets result in a strong increase in their ACUS figure of merit under the high voltage excitation, which exceeds the results of their technological optimisation. Consequently, enhancement of the ACUS system transmission by 12 dB and signal to noise ratio by 32 dB was achieved.

Piezoelectric and electrostrictive effects in ferroelectret ultrasonic transducers

Electromechanical response of polypropylene ferroelectret transducers under application of high-voltage pulses was measured by laser Doppler vibrometry and compared with results of ultrasonic through-air transmission between two ferroelectret transducers. The electromechanical response was completely explained by piezoelectric and electrostrictive effects. The electrostrictive effect dominates at high voltages and provides significant enlargement of the transducer constant, up to factor of 2.5. The induced strain of 1.7% was achieved at −2000 V. The nonlinear ultrasonic transmission was shown to be well described by the piezoelectric and electrostrictive response of transmitter, except in the range of high negative exciting voltages where some limitation of the transmitted signal was observed. This limitation seems not to be a fundamental one and does not abolish the advantages of high-voltage excitation of polypropylene ferroelectret transducers.

Cellular polypropylene polymer foam as air-coupled ultrasonic transducer materials

Cellular polypropylene polymer foams, also known as ferroelectrets, are compelling candidates for air-coupled ultrasonic transducer materials because of their excellent acoustic impedance match to air and because they have a piezoelectric d(33) coefficient superior to that of PVDF. This study investigates the performance of ferroelectret transducers in the generation and reception of ultrasonic waves in air. As previous studies have noted, the piezoelectric coupling coefficients of these foams depend on the number, size, and distribution of charged voids in the microstructure. The present work studies the influence of these parameters both theoretically and experimentally. First, a three-dimensional model is employed to explain the variation of piezoelectric coupling coefficients, elastic stiffness, and dielectric permittivity as a function of void fraction based on void-scale physics and void geometry. Laser Doppler vibrometer (LDV) measurements of the effective d(33) coefficient of a specially fabricated prototype transmitting transducer are then shown which clearly indicate that the charged voids in the ferroelectret material are randomly distributed in the plane of the foam. The frequency-dependent dynamic d(33) coefficient is then reported from 50 to 500 kHz for different excitation voltages and shown to be largely insensitive to drive voltage. Lastly, two ferroelectret transducers are operated in transmit-receive mode and the received signal is shown to accurately represent the corresponding signal generated by the transmitting transducer as measured using LDV.

Nonlinear electromechanical response of the ferroelectret ultrasonic transducers

The ultrasonic transmission between two air-coupled polypropylene (PP) ferroelectret (FE) transducers in dependence on the amplitude of the high-voltage exciting pulse revealed a strongly nonlinear electromechanical response of the FE transmitter. This phenomenon is described by a linear increase of the inverse electromechanical transducer constant \(t_{33}^{(1)}\) of the PP FE film with an increase of the exciting electrical pulse amplitude. Enlargement of \(t_{33}^{(1)}\) by a factor of 4 was achieved by application of 3500 V exciting pulses. The electrostriction contribution to \(t_{33}^{(1)}\) can be attributed to the electrostatic force between electrodes and the Maxwell stress effect. The nonlinear electromechanical properties of the PP FE result in a strong increase of its air-coupled ultrasonic (ACUS) figure of merit (FOM) under the high-voltage excitation, which exceeds results of the PP FE technological optimization. The FOM increase can be related to the increase of PP FE coupling factor and/or to the decrease of its acoustic impedance. A significant enhancement of the ACUS system transmission (12 dB) and signal-to-noise ratio (32 dB) was demonstrated by the increase of excitation voltage up to 3500 V. The nonlinear electromechanical properties of the PP FEs seem to be very important for their future applications.

Air-Coupled Ultrasonic Applications of Ferroelectrets

Improvement of ferroelectret transducers for air-coupled ultrasonic applications was demonstrated on the base of optimization of the ferroelectret electromechanical properties and adjustment of the ferroelectret transducers to the ultrasonic system. Controlled inflation of the polypropylene ferroelectret films results in an increase of the coupling factor in two and the figure of merit in ten times. Integration of a pre-amplifier with high impedance matching increases the ferroelectret transducer sensitivity in ∼ 40 dB. Ferroelectret transducers are characterized by the broad transmission band with smooth amplitude-frequency dependence and provide a well shaped and short pulse transmitted through the air.

Novel heat durable electromechanical film: processing for electromechanical and electret applications

New ferroelectrets were developed on the basis of foams from cyclo-olefin polymers and copolymers. The results obtained on the cyclo-olefin polymer foam demonstrate a significant improvement of the service temperature for ferroelectret transducer materials. Suitable compounding and preparation led to cyclo-olefin ferroelectrets with an electromechanical activity of around 15 pC/N, which is thermally stable at least up to 110degC. The properties in sensor and actuator applications are strongly dependent on the processing parameters related to film-making, sensor and actuator preparation, gas content and electric charging. The processing window for the film stretching was very narrow compared to the earlier developed polypropylene ferroelectrets. The film porosity, softness and thus the electromechanical activity are adjusted by gas-diffusion expansion. The activity of the electromechanically operating sensors and actuators was increased by stacking several layers of cellular cyclo-olefin film. For applications such as flat loudspeakers, the foamed films are tuned by tensioning them on a support frame. Correct tensioning was essential also for reducing the distortion levels