Ruy Altafim Professor Ruy Altafim from the University of São Paulo, talks to Electronics Letters about the paper ‘Hydrophone based on 3D printed polypropylene (PP) piezoelectret’, page 203. Our field of research has always been focused on electrets, poled dielectric materials, applied to high voltage sensors. In the mid-2000s, we encountered a technology named electro-mechanical films, which became known as ferroelectrets or piezoelectrets. These films contain air voids, which when electrically charged may exhibit intense ferroelectric properties. Since then, we have developed new methods for producing ferroelectrets for a variety of applications. With the development of 3D printing, we realised that voided films, required for a material to become a piezoelectret, could be produced by this procedure. After demonstrating that 3D printing was suitable for this technology we started looking for applications in the high voltage field and this work is the first result of this research. These sensors, due to their lightness, flexibility and high performance, are also attractive for many other areas of research, for example in medical ultrasonography, sonar, tactile sensors, etc. Piezoelectrets are polymeric films with internal cavities with some polymers possessing unique properties such as trapping electrical charges for longer periods of time than others. Thus, when exposed to high electrical fields, these foamed films become poled as an electret, with strong piezoelectric properties. The main concerns in this field are thermal stability and control of cavities geometries, which directly affect their electrical charging and resonance frequency responses. In this work, we have demonstrated that by using 3D printing methods, we can produce piezoelectrets with controlled voids and they are already being employed in realistic applications, such as ultra-sonic sensors for detecting partial discharges. This was the first time a piezoelectret was produced using 3D printing in a single extrusion step, and where this one-step-processed material could be used in a realistic application. The advantage of this production method lies in the simplicity of the 3D printing process, which allowed us to create any imaginable voided geometry and reproduce it many times. With this technology, we also expand the concept of three-dimensional material as, by adding functionality to it, it allows us to create a four-dimensional material. The major issue we faced was finding a polymer that could be used as a base material for ferroelectrets as it needed to retain electrical charge for long periods and had to be extruded in a commercial 3D printer. In a previous work, we demonstrated that 3D printing could be applied to this technology using ABS (acrylonitrile butadiene styrene), but charge stability was very much reduced for realistic applications. In this report, we managed to work with polypropylene (PP), which was the base polymer in which the ferroelectrets were fabricated. In the short term we believe that by demonstrating the feasibility of 3D printing in the ferroelectret manufacturing process, others will produce piezoelectrets with materials that are thermally more stable than PP, and with different void geometries, which could lead to improvements in piezoelectret sensitivity. In the long term, we hope to see 3D printed piezoelectrets being used to produce customised applications with a very reduced cost, for instance, specially designed sensitive pads or keyboards, pressure sensors with well-defined resonance frequencies, etc. Now we are working on developing customised keyboards using 3D printed piezoelectrets, so the focus has been the development of the electronics to communicate with the sensors and there is research on going to apply such ultra-sonic sensors to detect partial discharge in power transformers and compare their efficiency with previous ferroelectrets. The most significant change we made in the field of ferroelectrets was the demonstration that electromechanical films could not only be produced by the foaming process, but also by bonding layers of electret films with regular spacers. Later, we showed that by fusing electret films and thermoforming air bubbles, a much more controlled structure could be fabricated. From bubble films, we developed piezoelectrets with well-defined open tubular channels, which allowed us to control all geometrical parameters of the cavities and to better understand the physics behind this technology. Others have followed us and different methods for fabricating piezoelectrets with well-defined cavities are being developed. In the future we foresee more controlled piezoelectrets being developed with nanoscale technology or with more thermally stable materials, stretched and flexible piezoelectrets with pre-printed electrodes, and 3D printed applications with embedded piezoelectrets.
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Jan 26, 2024
Kun Zhu + 13
Open Access