Heitor Cury Basso

Template-based fluoroethylenepropylene piezoelectrets with tubular channels for transducer applications

We describe the concept, the fabrication, and the most relevant properties of a piezoelectric-polymer system: Two fluoroethylenepropylene (FEP) films with good electret properties are laminated around a specifically designed and prepared polytetrafluoroethylene (PTFE) template at 300 °C. After removing the PTFE template, a two-layer FEP film with open tubular channels is obtained. For electric charging, the two-layer FEP system is subjected to a high electric field. The resulting dielectric barrier discharges inside the tubular channels yield a ferroelectret with high piezoelectricity. d33 coefficients of up to 160 pC/N have already been achieved on the ferroelectret films. After charging at suitable elevated temperatures, the piezoelectricity is stable at temperatures of at least 130 °C. Advantages of the transducer films include ease of fabrication at laboratory or industrial scales, a wide range of possible geometrical and processing parameters, straightforward control of the uniformity of the polymer ...

Visible photoluminescence in amorphous ABO3 perovskites

The photoluminescence observed in ABO3 type perovskite in their highly structural disordered state can be explained by a model in which is assumed a distribution of electronic states localized within the energy band gap coupled to lattice local vibrational states. The model fits very well the experimental results and indicates that photoluminescence in the visible region can be considered as a general behavior of disordered solids.

Direct hysteresis measurements on ferroelectret films by means of a modified Sawyer–Tower circuit

Ferro- and piezo-electrets are non-polar polymer foams or film systems with internally charged cavities. Since their invention more than two decades ago, ferroelectrets have become a welcome addition to the range of piezo-, pyro-, and ferro-electric materials available for device applications. A polarization-versus-electric-field hysteresis is an essential feature of a ferroelectric material and may also be used for determining some of its main properties. Here, a modified Sawyer-Tower circuit and a combination of unipolar and bipolar voltage waveforms are employed to record hysteresis curves on cellular-foam polypropylene ferroelectret films and on tubular-channel fluoroethylenepropylene copolymer ferroelectret film systems. Internal dielectric barrier discharges (DBDs) are required for depositing the internal charges in ferroelectrets. The true amount of charge transferred during the internal DBDs is obtained from voltage measurements on a standard capacitor connected in series with the sample, but with a much larger capacitance than the sample. Another standard capacitor with a much smaller capacitance—which is, however, still considerably larger than the sample capacitance—is also connected in series as a high-voltage divider protecting the electrometer against destructive breakdown. It is shown how the DBDs inside the polymer cavities lead to phenomenological hysteresis curves that cannot be distinguished from the hysteresis loops found on other ferroic materials. The physical mechanisms behind the hysteresis behavior are described and discussed.Ferro- and piezo-electrets are non-polar polymer foams or film systems with internally charged cavities. Since their invention more than two decades ago, ferroelectrets have become a welcome addition to the range of piezo-, pyro-, and ferro-electric materials available for device applications. A polarization-versus-electric-field hysteresis is an essential feature of a ferroelectric material and may also be used for determining some of its main properties. Here, a modified Sawyer-Tower circuit and a combination of unipolar and bipolar voltage waveforms are employed to record hysteresis curves on cellular-foam polypropylene ferroelectret films and on tubular-channel fluoroethylenepropylene copolymer ferroelectret film systems. Internal dielectric barrier discharges (DBDs) are required for depositing the internal charges in ferroelectrets. The true amount of charge transferred during the internal DBDs is obtained from voltage measurements on a standard capacitor connected in series with the sample, but with...

Piezoelectricity in multi-air voids electrets

Multi-air voids space-charge electrets have been developed using two fluorethylenepropylene (Teflon-FEP) films. In this work, a new technique developed to assembler these films allows the control of the thickness and shapes of multi-air voids distributed on their surface. When an impulse voltage process charges these voids, it is possible to obtain samples with high piezoelectric coefficients. To explain this piezoelectric characteristic, a theoretical explanation based on the electret microphone mechanism is introduced. Measurements show the piezoelectric coefficients for different frequencies.

Laminated tubular-channel ferroelectret systems from low-density polyethylene films and from fluoroethylene- propylene copolymer films - A comparison

A template-based lamination technique for the manufacture of ferroelectrets from uniform electret films was recently reported. In the present work, this technique is used to prepare similar ferroelectret structures from low-density polyethylene (LDPE) films and from fluoro-ethylene-propylene (FEP) copolymer films. A comparative analysis of the pressure-, temperature-, and frequency-dependent piezoelectric properties has been performed on the two ferroelectret systems. It is observed that the FEP ferroelectrets exhibit better piezoelectric responses and are thermally more stable. The difference between the piezoelectric d33 coefficients of the two ferroelectret systems is partially explained here by their different elastic moduli. The anti-resonance peaks of both structures have been investigated by means of dielectric resonance spectroscopy and electroacoustic sound-pressure measurements. A difference of more than 10 kHz is observed between the anti-resonance frequencies of the two ferroelectret systems.

Three-layer ferroelectrets from perforated Teflon®-PTFE films fused between two homogeneous Teflon®-FEP films

In this contribution, we propose a new route towards polymer ferroelectrets with uniform voids: Two homogeneous Teflonreg-FEP films are separated by, but also thermally fused to a Teflonreg-PTFE film with several homogeneous holes. Closed voids with FEP top and bottom lids and PTFE walls are dius formed. After internally charging the top and bottom FEP layers with positive and negative polarity, respectively, the voids constitute large dipoles that can be deformed by mechanical or electrical stresses. Consequently, the three-layer sandwich exhibits direct and inverse piezoelectricity. The continuous layer-fusing process involves a press with hot cylindrical rolls that operate at temperatures up to 310degC, which may allow production of inexpensive transducer materials on a large scale. We report about the concept, the processing, the charging and the electromechanical performance of the three-layer ferroelectrets.

Temporal evolution of the re-breakdown voltage in small gaps from nanoseconds to milliseconds

A detailed understanding of electric breakdown in dielectrics is of scientific and technological interest. In gaseous dielectrics, a so-called re-breakdown is sometimes observed after extinction of the previous discharge. Although time-dependent re-breakdown voltage is essentially known, its behavior immediately after the previous discharge is not precisely understood. We present an electronic circuit for accurate measurements of the time-dependent re-breakdown voltage in small gaps from tens of nanoseconds to several milliseconds after the previous spark. Results from such experiments are compared with earlier findings, and relevant physical mechanisms such as heating of the gas, decay of the plasma, and ionization of excited atoms and molecules are discussed. It is confirmed that the thermal model is not valid at times below several microseconds.

Piezoelectret sensors detect geometry-related modifications of the acoustical signatures from partial discharges in an electrical equipment chamber

Piezoelectrets from thin electrically charged cellular polypropylene-foam films have been suggested for numerous transducer applications, such as force sensors for keyboards, flat microphones for conference environments, or devices for ultrasound generation and detection. In addition to polypropylene piezoelectrets, other materials systems - such as thermo-fused polymer-film stacks - were developed in order to generate piezoelectrets that are thermally more stable and contain geometrically well-controlled air cavities. Here, we demonstrate the viability of the recently developed piezoelectret systems that had been prepared by thermo-lamination with templates for the acoustical detection of electric discharges in air. Furthermore, we show that the resonance behavior of the air cavity in which the electric discharge occurs can also be assessed from the acoustic signal. For the experiments, a transducer prototype with a preamplifier directly connected to the laminated piezoelectret sensor inside an electrically shielded package was developed. Reproducible sound measurements according to the ASTM E976-10 standard were performed with the transducer prototype. The prototype was later employed in acoustical measurements of electric discharges in an air gap inside an acoustical chamber - in order to reduce external noise - and in a metallic chamber for reducing electrical interference. The recorded signals were compared by means of Fast-Fourier-Transform (FFT) analysis.

Hydrophobicity of polyurethane resins

The growth of polymeric insulator applications has shown a sharp upward trend thanks to the advantages of these insulators over the traditional porcelain and glass models. One of their most important features is their hydrophobicity, which allows water to be retained on their surface, preventing them from undergoing high current leakage. The present paper therefore aims to classify the degrees of hydrophobicity of castor-oil plant (Ricinus communis) derived polyurethane resins, which display special dielectric properties for the manufacture of polymeric insulators. The goniometric measurement technique was used through image processing. The main reason for using this technique was its guaranteed repeatability and the solution it offers for the problem of human subjectivity to which the visual inspection technique is prone.

Piezoelectric sensor based on electret thermoforming technology

Teflon FEP thermoformed piezoelectric transducers based on electret technology, which were introduced in 2005, basically consist of the fusion of two Teflon FEP films, creating air bubbles at the interface. The top and bottom of the air bubble surfaces are posterior charged to opposite polarities, forming large electric dipoles. Although this technology has enabled the production of transducers with piezoelectric coefficients above 300 pC/N, it is still in the laboratory stage. This paper proposes some improvements through a new methodology whereby the molding of one of the Teflon FEP films in a template is followed by fusion by hot lamination. This method enables control of the distribution and diameter of the bubbles and the production of piezoelectric transducers for specific applications such as presence sensors, thin keyboards, dynamic balances and pressure sensors.