History and recent progress in ferroelectrets produced in Brazil

More than 30 years ago,a group of researchers in Tampere-Finland developed a thin foamed polymeric material for capacitive sensors.Such soft-voided films exhibited electrical charging characteristics,forming a powerful combination,which resulted in a smart-material with ferroelectric properties.The discovery of the electro-thermo mechanical film(ETMF)has sparked the curiosity of the electret community,leading to the development of several studies.At that time,ETMF became known as cellular electrets and,later,as ferroelectrets or piezoelectrets regarding their electromechanical properties.This paper provides a timeline review of the research on ferroelectrets produced in Brazil,between the years 1990 and 2020,towards demonstrating how the interest in the electret electrical charging mechanism has resulted in the use of ferroelectrets with well-controlled cavities for ultrasound applications.

Ferroelectrets:Heterogenous polymer electrets with high piezoelectric sensitivity for transducers

Nowadays,the demand for advanced functional materials in transducer technology is growing rapidly.Piezoelectric materials transform mechanical variables(displacement or force)into electrical signals(charge or voltage)and vice versa.They are interesting from both fundamental and application points of view.Ferrooelectrets(also called piezoelectrets)are a relatively young group of piezo-,pyro-and ferroelectric materials.They exhibit ferroic behavior phenomenologically undistinguishable from that of traditional ferroelectrics,although the materials per se are essentially non-polar space-charge electrets with artificial macroscopic dipoles(i.e.,internally charged cavities).A lot of work has been done on ferroelectrets and their applications up to now.In this paper,we review and discuss mostly the work done at University of Potsdam on the research and development of ferroelectrets.We will,however,also mention important results from other teams,and prospect the challenges and future progress trend of the field of ferroelectret research.

Resonance phenomena in dielectric media: A review and comparison of acoustic and electromagnetic modes

The theory and application of resonances and vibrational modes are part of the foundation of science.In this contribution,examples of acoustical resonators are highlighted,and compared to electromagnetic modes.As an example from architecture,we describe the phenomenon of whispering galleries;such modes are nowadays known in dielectric and optical resonators.A specimen of a semicircular whispering bench in Park Sanssouci in Potsdam is acoustically investigated and demonstrated to show low losses for sound propagation.A special acoustical bug is discussed which was used for the espionage of the U.S.ambassador in Moscow.The Sovyets could interrogate this passive device by radio waves.Its working principle was based on the electromagnetic resonance of the cavity that the sound-sensitive membrane was part of.The underlying relation between excitation and resonance is compared to the sound production in flue organ pipes.A stopped flue organ pipe was investigated using a piezoelectric film sensor inside the pipe body.The results show that even-numbered modes,which are usually suppressed in the radiated sound of a stopped pipe,are still present in the vibrations inside the resonator.

Nanogenerator-based bidirectional pressure sensor array and its demonstration in underwater invasive species detection

Lakes areas,which cause catastrophic damages in both commercial fishery and ecological systems.However,current assessment strategies may pose challenges for lake-wide abundance estimation and non-target anadromous species preservation.Here,we demonstrate an efficacious species-specific non-destructive sensing system based on porous ferroelectret nanogenerator for in-situ monitoring of lamprey spawning migration using their unique suction behavior.Simulations show that the porous structure enables a redistribution of surface charges under bidirectional deformations,which allows the detection of both positive and negative pressures.The quasi-piezoelectric effect is further validated by quantitative analysis in a wide pressure range of−50 to 60 kPa,providing detailed insights into transduction working principles.For reliable lamprey detection,a 4×4-pixel sensor array is developed and integrated with a complementary metal-oxide-semiconductor(CMOS)based signal processing array thus constituting a sensing panel capable of recording oral suction patterns in an underwater environment.