The past,present,and future of piezoelectric fluoropolymers:Towards efficient and robust wearable nanogenerators

Polyvinylidene difluoride(PVDF)derivatives in metal/PVDF/metal(MPM)sandwich structures have been studied extensively since 1969.Cousin copolymers of the same family have been discovered with fascinating piezoelectric,pyroelectric,electrocaloric,and ferroelectric properties.Solution processing,flexibility,lightweight,and thermal stability make this class of materials complementary to inorganics.Thus,PVDF based polymers potentially compete with inorganic materials for a broad range of technologies such as energy generators,loudspeakers,coolers,and memories.However,the stable non-electroactiveα-phase and hydrophobic nature of PVDF are the main barriers for developoing high performing and robust MPM devices in electronic applications.In this review,we present an up-to-date overview on different methods to induce the electroactiveβ-phase and improve the adhesion strength with metals to ensure robust and durable MPM devices.We go through advantages and disadvantages of several methods and pinpoint future opportunities in this research area.A special attention is paid to wearable piezoelectric nanogenerators for energy harvesting from human body motion,where flexible PVDF derivatives are compared with rigid piezoelectric ceramics.While the piezoelectric coefficient of PVDF(d_(33)~24–34 pm/V)is one order lower than ceramic materials,novel co-polymers of PVDF display d_(33)>1000 pm/V upon bias.This shows promise to bring piezoelectrics to flexible and large-area applications such as smart textiles.We also discussed challenges to improve wearability,such as light weight,breathability,and flexibility.

Review on piezoelectric actuators:materials,classifications,applications,and recent trends

Piezoelectric actuators are a class of actuators that precisely transfer input electric energy into displacement,force,or movement outputs efficiently via inverse piezoelectric effect-based electromechanical coupling.Various types of piezoelectric actuators have sprung up and gained widespread use in various applications in terms of compelling attributes,such as high precision,flexibility of stoke,immunity to electromagnetic interference,and structural scalability.This paper systematically reviews the piezoelectric materials,operating principles,representative schemes,characteristics,and potential applications of each mainstream type of piezoelectric actuator.Herein,we intend to provide a more scientific and nuanced perspective to classify piezoelectric actuators into direct and indirect categories with several subcategories.In addition,this review outlines the pros and cons and the future development trends for all kinds of piezoelectric actuators by exploring the relations and mechanisms behind them.The rich content and detailed comparison can help build an in-depth and holistic understanding of piezoelectric actuators and pave the way for future research and the selection of practical applications.

New pattern recognition system in the e-nose for Chinese spirit identification

This paper presents a new pattern recognition system for Chinese spirit identification by using the polymer quartz piezoelectric crystal sensor based e-nose. The sensors are designed based on quartz crystal microbalance（QCM） principle,and they could capture different vibration frequency signal values for Chinese spirit identification. For each sensor in an8-channel sensor array, seven characteristic values of the original vibration frequency signal values, i.e., average value（A）,root-mean-square value（RMS）, shape factor value（S_f）, crest factor value（C_f）, impulse factor value（I_f）, clearance factor value（CL_f）, kurtosis factor value（K_v） are first extracted. Then the dimension of the characteristic values is reduced by the principle components analysis（PCA） method. Finally the back propagation（BP） neutral network algorithm is used to recognize Chinese spirits. The experimental results show that the recognition rate of six kinds of Chinese spirits is 93.33% and our proposed new pattern recognition system can identify Chinese spirits effectively.

Wave‑Shaped Piezoelectric Nanofber Membrane Nanogenerator for Acoustic Detection and Recognition

With the rapid development of internet of things and wearable electronics,how to conveniently power uncountable sensors remains a huge challenge.Energy harvesting strategy is suggested to collect and convert environmental energies into electrical energy.Thereinto,piezoelectric polymers are utilized as fexible harvesters to convert mechanical energy.The latter widely distributes in both our daily life and industrial environment.Intrinsic piezoelectric property further drives piezoelectric polymers to construct fexible self-powered strain sensors.However,relatively low piezoelectric performance restricts their application in detection and conversion of weak mechanical excitations.Herein,wave-shaped 3D piezoelectric device was fabricated by embossing electrospun polyvinylidene fuoride nanofbers.This 3D structured device presents better longitudinal and transverse piezoelectric performance than usual fat-type one.This wave-shaped piezoelectric device was developed for acoustic detection and recognition with a frequency resolution better than 0.1 Hz.This wave-shaped device was capable of frequency spectrum analyses of various sound sources from human and animals and well presents its potential for future wearable acoustic sensors and transducers.