Effective electromechanical properties of cellular piezoelectret:A review

Due to the large quasi-piezoelectric d33 coefficient in the film thickness direction, cellular piezoelectret has emerged as a new kind of compliant electromechanical transducer materials. The macroscopic piezoelectric effect of cellular piezoelectret is closely related to the void microstructures as well as the material constants of host polymer. Complex void microstmctures are usually encountered in the optimum design of cellular piezoelectret polymer film with ad- vanced piezoelectric properties. Analysis of the effective electromechanical properties is generally needed. This article presents an overview of the recent progress on theoretical models and numerical simulation for the effective electromechanical properties of cellular piezoelectret. Emphasis is placed on our own works of cellular piezoelectret published in past several years.

Flexible piezoelectret film sensor for noncontact mechanical signal capture by multiple transmission media

Mechanical signal capture without physical contact has emerged as a highly promising research field and attracted tremendous attention due to its prosperous applications in household medical care,lifestyle monitoring and remote operation,offering users high level of safety,convenience and comfort.Moreover,noncontact sensing is ideal to maximize the immersive user experience in the human–machine interaction(HMI),eliminating interference to human activities and mechanical fatigue to the sensor,simultaneously.Herein,we report a self-powered flexible sensor integrated with irradiation cross-linked polypropylene(IXPP)piezoelectret film for noncontact sensing,featuring multi-functions to detect mechanical signals transmitted through solid,liquid and gaseous media and would facilitate their versatile practical applications.The folded-structure configuration of the sensor facilitates the improvement of the noncontact sensing sensitivity.For solid media,such as the rectangular wooden stick used in this study,the sensor can detect mechanical stimulus exerted at a distance of 100 cm.A system detection sensitivity up to 57 pC/kPa with a low detection limit of 0.6 kPa is achieved at a noncontact distance of 10 cm.Even when partly or completely immersed in water,the sensor effectively traces movement signals of human bodies underwater,demonstrating great advantages for non-inductive aquatic fitness training monitoring.Furthermore,due to the low acoustic impedance of piezoelectret film,speech recognition through gaseous medium is also achieved.We further introduce application demonstrations of the developed film sensors to monitor exercise postures and physiological signals without direct contact between human body and the sensor,displaying great potential to be incorporated into future smart electronics.This study commendably expands the application scope of piezoelectret materials,which will have profound implications for exploring novel intelligent human–machine interactions.

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.

A self-powered piezoelectret sensor based on foamed plastic garbage for monitoring human motions

Constructing piezoelectret based on foamed plastic garbage is an advisable strategy for obtaining self-powered flexible electromechanical sensors with good performances.Herein,a self-powered piezoelectret sensor with basic material of low density polyethylene(LDPE)foamed plastic garbage is proposed,with characteristics of easy fabrication,excellent flexibility,and high equivalent piezoelectric coefficient d33 value up to~1,100 pC/N.The output stability is verified by continuously stimulating a sensor for~180,000 cycles under low and high applied pressure,and the variations of peak outputs are less than 5.5%.Applications for measuring low-and high-pressure signals from human body are achieved.Assembled with a wristband,a sensor is demonstrated for detecting the human pulse waves.Moreover,real time human sitting information is wirelessly monitored with a smart chair based on 4 pixels sensors array.

THE EFFECTIVE ELECTROMECHANICAL PROPERTIES OF CELLULAR PIEZOELECTRET FILM:FINITE ELEMENT MODELING

Cellular space-charge polymer film, also called cellular piezoelectret, has very large piezoelectric effect due to their unique microvoid structure. In this article, the cellular piezoelectret film is considered to be a periodic composite material with closed-cell microvoids aligned periodically. Three dimensional finite element modeling is carried out to obtain the effective elastic modulus and piezoelectric coefficients. Sensitivity analysis was presented by modeling the effec- tive electromechanical properties with different individual variable, including material constants and void shape parameters. By assuming a relation between void shape and void volume fraction, the finite element model can simulate quite well the inflation experiments of the voided charged Polypropylene film published in literature. Finally, the finite element model is used to explore the voided charge polymer film with non-uniform distribution of the trapped charges on the internal surface of voids. It was found that the resultant overall piezoelectric coefficients will be more significant if charges are closely gathered in the central area, and sparse in the around area of the internal surface of voids.