Ruy Alberto Pisani Altafim

Treatment with orthophosphoric acid enhances the thermal stability of the piezoelectricity in low-density polyethylene ferroelectrets

Ferroelectrets have been fabricated from low-density polyethylene (LDPE) films by means of a template-based lamination. The temperature dependence of the piezoelectric d33 coefficient has been investigated. It was found that low-density polyethylene ferroelectrets have rather low thermal stability with the piezoelectric coefficient decaying almost to zero already at 100 °C. This behavior is attributed to the poor electret properties of the polyethylene films used for the fabrication of the ferroelectrets. In order to improve the charge trapping and the thermal stability of electret charge and piezoelectricity, LDPE ferroelectrets were treated with orthophosphoric acid. The treatment resulted in considerable improvements of the charge stability in LDPE films and in ferroelectret systems made from them. For example, the charge and piezoelectric-coefficient decay curves shifted to higher temperatures by 60 K and 40 K, respectively. It is shown that the decay of the piezoelectric coefficient in LDPE ferroelec...

Unipolar ferroelectrets - Following the example of the electret microphone more closely

We report first investigations on a ferroelectret transducer material that is based on the same layer sequence as a simple electret microphone. Only one electrically charged polymer film is employed instead of two as in previous tubular-channel ferroelectrets. The new materials design has major advantages such as a reduced number of polymer films to be processed and a reduced amount of expensive electret material, as well as the possibility to choose the polarity of the charged layer for optimal stability. A drawback seems to be a lower piezoelectric coefficient, since the values obtained in our preliminary experiments amount to roughly half of those found on bipolar ferroelectrets made from two polymer-electret films. Nevertheless, the d33 values observed on our samples are still twice as high as those of polyvinylidene fluoride (PVDF) and can be further improved, e.g. through modifications of the ferroelectret geometry, the layer sequence and the electret properties or via multi-layer stacking as already demonstrated for polypropylene piezoelectrets.

Piezoelectret-based hydrophone: an alternative device for vibro-acoustography

Piezoelectric polymers are highly desirable for ultrasound applications since their low acoustic impedance is much closer to the impedances of air and water than those of traditional piezoceramics. However, the piezoelectric effect observed in these poled polymers is limited to a few dozen picocoulombs per newton and requires very low-noise amplification. A different class of polymeric material known as piezoelectrets, presents piezoelectric effect in the same order of magnitude as those found on piezoceramics. This new class of materials has been explored in a wide range of applications such as flexible keyboards and airborne ultrasound transducers. Based on these polymers, we present here a new transducer for vibro-acoustography (VA), which is an ultrasonic image technique employed in medical diagnosis or material analysis based on ultrasound scattered by a target object. A calibration was carried out using a standard hydrophone, which is normally employed in VA. The average sensitivity of the transducer in the continuous wave mode was 1.712 mV Pa−1(−182.7 dB re 1 V Pa−1) with a maximum sensitivity of 18.25 mV Pa−1 (−154.8 dB re 1 V Pa−1) at its resonance frequency around 40 kHz. Subsequently, measurements in the burst mode and of directional sensitivity were taken and are presented here together with VA images obtained from a chicken bone and a metal sphere.

Space charge deposition in tubular channel ferroelectrets: A combined fluorescence imaging/LIMM study with finite element analysis

Ferroelectrets, i.e., soft materials with electric charges deposited on the surfaces of internal voids, are well known for their potential in transducer applications and energy harvesting. Due to their regular geometry and optical transparency, tubular channel ferroelectrets (manufactured by laminating polymer films around a polytetrafluoroethylene template which is later removed) are well-suited for studying the process of charge deposition. Understanding how space charges are formed on the internal surfaces will lead to improvements in the charge density and in the piezoelectric performance of these films. In this work, the inception voltage for dielectric barrier discharges (and hence the onset of charge deposition) was measured using two independent techniques, fluorescence imaging and the laser intensity modulation method (LIMM). The results (around 1.4–1.7 kV, depending on the void height) are in agreement within ±50 V. The internal electric field distribution was calculated using finite element ana...

Laminated composite based on polyester geotextile fibers and polyurethane resin for coating wood structures

New environmental laws have restricted the use of hardwood trees in overhead power lines structures, such as, poles and cross-arms, leading companies to seek alternative materials. Reforested wood coated with polymeric resin has been proposed as an environmental friendly solution, with improved electrical properties and protection against external agents,

Well-defined piezoelectrets fabricated with 3D printing technology

Three-dimensional (3D) printing is a feasible solution for fast prototyping with precision in micrometer range. On the other hand, piezoelectrets are polymeric films with internal cavities, which has been electrically charged, resulting in a material with piezoelectric behavior. In the present paper, both, technology and concept were combined to produce structures with well-defined cavities and electromechanical properties. 3D printing can be performed in large range of materials, however for piezoelectret production the polymer must be electrically stable as in electrets. For this study, acrylonitrile butadiene styrene (ABS) was chosen as the base structural material and different voided structures were prepared. Results from pressure dependence and temporal stability are here presented.

Low-cost lightning current peak detector for installation on communication towers

Communication towers are spread all over the countries and are normally placed in open fields what makes them susceptible to lightning strikes. Therefore, using those structures to study lightning effect is a natural solution. However, installing measuring systems on every tower is a task that requires huge investment due to the costs of such systems. In this paper, a non-expensive system, with an estimated cost of less than one thousand dollars, for measuring lightning impulse current peaks that can be installed on high telecommunication towers is presented. The system is composed of a self-integrating Rogowski coil, coaxial cables and a mini digital oscilloscope allocated at the base of the tower. The coil is fixed on a platform at the towers end, next to the lightning-rod discharge protection system, and it is connected to the mini oscilloscope through a coaxial cable. Once lightning strikes the tower, a voltage proportional to the lightning current is produced on the self-integrated coil and transferred to the oscilloscope. Thus, using calibration patterns, the current peak can be determined.

Space charge build-up in tubular channel ferroelectrets

The charge deposition process in tubular channel ferroelectrets, (i. e. voided FEP polymer films with internal electric charges) has been studied using a combination of near-UV fluorescence light emission imaging and non-destructive space charge measurements with the laser intensity modulation method (LIMM). By galvanically isolating the diode laser from the detection circuit, the LIMM sensitivity was improved by nearly one order of magnitude. It was shown that the onset of light emission as a function of charging voltage precisely correlates with the deposition of space charge. This confirms previous studies where the presence of space charge was indirectly detected from the samples piezoelectric properties.

Thin Teflon-FEP capacitors with open-tubular channels employed on low-pressure measurements for smart-grid monitoring systems

In order to improve quality and efficiency in electrical energy distribution, power supply companies have sought for better ways to monitor their equipment, stimulating research in smart-grid projects. Equipment subjected to constant monitoring are power and distribution transformers, in which severe faults can be avoided through preventive maintenance, such as, detecting partial discharges in early stages, conducting oil chemical analysis or monitoring transformer internal pressure. In the present contribution we discuss the use of thin voided capacitors made of fluoroethylenepropylene (FEP) films to measure pressure changes in sealed chambers, as in distribution transformers. Voided film structures are very sensitive to pressure variations due to their low mechanical resistance in the thickness direction. Here voided capacitors were fabricated using the same thermal lamination process reported to produce piezoelectrets with open tubular channels. In order to measure pressure disturbance an electrical circuit was specifically designed for detecting variations in capacitance and results correlating these changes with pressure increase are presented.

Comparative analysis between impulsive detection methods applied on partial discharge acoustic signals

Partial discharges (PD) plays an important role on power transmission lines and equipment failures therefore previously detect this high-voltage phenomena can prevent severe damages and avoid further outlay. Spark gaps discharges are among the most common PD in power transformers and other efficiency energy equipments. And in order to detect this effect optical and acoustic sensors are normally employed into monitoring systems. However optical sensors are expensive for transmission power lines and detecting acoustical signals from PD can be difficult since they are normally followed by other background sounds. In order to improve detection sound systems in this work we compare two methods that are regularly used on impulsive signal processing, the Variance Method (VM) and the Conditional Median Filter (CMF), to detect PD acoustic signals. From the presented results it is observed that the CMF is more appropriated for detecting impulsive PD although the VM detected less false positive discharge and can still be further optimized.