| Acoustic-to-electric energy conversion device can be devided into two categories according to application types,one is acoustic sensor to detect and record sound,another is energy generator to conversion acoustic energy to electric energy.Acoustic-to-electric energy conversion devices based on piezoelectric polymer are widely studied and used in varies applications,due to flexibility,easy processing,easy forming and durability.However,the performance of them is not good.Both output voltage and energy are low.Besides,most of the piezoelectric polymers need to be processed by a tedious multistep method,which are of energy consumption and high cost.Many researchers’ studies have indicated that nanofibers electrospun from piezoelectric polymers,such as poly(vinylidene fluoride)(PVDF)nanofibers and poly(vinylidene fluoride-co-trifluoroethylene)(P(VDF-Tr FE))nanofibers,have high β crystal phases content and piezoelectricity without stretching and poling treatments.These piezoelectric nanofibres have shown great potential for making mechanical sensors and energy generators However,little has been reported on acoustic-toelectric energy conversion of polymer nanofibres in research literature to date.This work shows a high-sensitivity acoustic sensor and a high-output acoustic energy generator from electrospun PVDF and P(VDF-Tr FE)nanofiber web.Research contents are shown as follows:(1)Test setup and evaluation system for acoustic-to-electric energy conversion deviceThe performance of the acoustic-to-electric energy conversion device is tested using a commercial PC speaker as the sound source.The sound frequency is controlled by a computer and an Audio Tester software.A sound level meter is used to record the acoustic wave pressure.An electrochemistry working station is used to record electric signals.To eliminate the noise signals from the background sound in the testing environment,electronic instrument and connecting cables,fast Fourier transform(FFT)technique is used to process the output signals.FFT technique is also used to analyze the frequency spectrum of output signals.The performance of the acoustic sensor is evaluated by output voltage and sensitivity.The performance of the acoustic energy generator is evaluated by peak output voltage,peak output current,internal resistance,peak instantaneous power,instantaneousa areal power density,instantaneousa volume power density,power,energy and energy conversion efficiency.(2)Prototype of acoustic sensor from electrospun piezoelectric nanofiber webThe prototype of acoustic sensor is designed.The acoustic sensor prototype is prepared by sandwiching a piece of PVDF nanofibre web as sensory material,with two aluminum electrode films and two transparent polyethylene terephthalate(PET)vibration substrate films.The structure of sensor is optimized by studying the effects of sensor parameters,such as vibration substrate materials,device size,nanofiber web thickness,electrode materials and size,on sensory properties.Evaluation parameters are output voltage and signal-to-noise ratio.The optimized structure parameters are: vibration substrate materials,two plastic films;sensor size,16 cm2,length-width ratio,1:1;nanofiber web thickness,0.128 mm;areal density,5~6 mg/cm2;electrode size,16cm2.The proposed reasons for decreasing the output signals are the closure of device structure,aluminum electrode and single-side stand test setup.The research above is the foundation of preparing highoutput acoustic sensor.(3)High-sensitivity acoustic sensor from electrospun piezoelectric nanofiber webThe output of the acoustic sensor prototype is low.Improved methods are put forward based on the drawback of sensor prototype.The improved high-sensitivity acoustic sensor from electrospun piezoelectric nanofiber web is prepared.Sensory test setup and evaluation system for improved acoustic sensor are designed and set up.The sensor structure and nanofibers are optimized by exploring the effects of parameters,such as through hole diameter,nanofiber web size and thickness,nanofiber diameter and arrangement,on the sensor output and sensitivity.Optimized parameters are shown as follow: PET plastic vibration substrate with 12.8 mm diameter though hole;nanofiber web thickness is 40 μm and size is 3×4 cm2;the average diameter of nanofiber is 310 nm.The highest sensitivity and output voltage of improved sensor is 266 m V/Pa and 3.10 V,more than five times higher than that of commercial piezoelectric acoustic sensor with same structure.The high-sensitivity acoustic sensor can be used in detecting and distinguishing sound waves in the low to middle frequency region.The acoustic sensor is able to record human sound and music.The signals are similar with the ones recorded by a commercial microphone.The devices show higher sensitivity to the sound of the pressure level above 100 d B,which is very suitable for detecting noise.(4)High-output acoustic energy generator from electrospun piezoelectric nanofiber webThe high-output acoustic energy generator from electrospun piezoelectric nanofiber web is prepared by improving the structure of acoustic sensor.Acoustic-to-electric conversion property test setup and evaluation system for acoustic energy generator are designed and set up.The as-prepared acoustic energy generator can convert acoustic energy to electric energy.The energy generator structure and nanofibers are optimized by exploring the effects of parameters,such as hole number,nanofiber web thickness,nanofiber diameter and arrangement,on internal resistance,output voltage and current.Optimized parameters are shown as follow,PET plastic vibration substrate with 8 holes,hole diameter is 4.9cm,nanofiber web thickness is 20μm,the average diameter of nanofiber is 240 nm.The devices generate higher electric outputs under a sound with pressure level above 100 d B.The optimized device can generate voltage and current outputs of 14.5 V and 28.5 μA with a volume power density output of 306.5 μW/cm3 and energy conversion rate of 60.3%,which are more than five times higher than these of commercial piezoelectric P(VDF-Tr FE)films.The electricity generated from the nanofiber device can be used to directly drive microelectronic devices,such as LEDs,and conduct electrochemical reactions,such as electrochemical polymerization of EDOT and corrosion protection of steel,without using any a storage unit.These incredible features make electrospun nano-nonwovens particularly useful for conversion of noise,a white pollution,into usable electricity.(5)Finite element modelling of vibration of acoustic-to-electric device in soundTo study the effect of device structure on acoustic-to-electric property,finite element modelling software COMSOL is used to simulate and calculate the vibration of device in sound waves.The vibration of 1-hole and 8-hole devices is simulated and analyzed,including the center of nanofiber,average velocity of devices,surface vibration energy,overall vibration energy.The overall vibration energy of 2-hole device with different hole distance is also calculated.The calculation data is compared with the data from vibration meter test to analyze the mechanism of the effect of device structure on device output.The vibration energy of both nanofiber web and PET plastic of 8-hole device is higher than that of 1-hole device under sound of 100-115 d B.However,there is little difference between two devices under sound of 60-100 d B.As a result,the output energy of 8-hole device is evidently higher than that of 1-hole device under sound of high sound pressure level.When sound pressure level is low,the output difference between two devices is little.Multi-hole structure enhances the vibration of the overall device,thus more absorbed sound energy can be transferred into vibration of device.Therefore,more mechanical energy can be converted to electric energy output,increasing energy conversion rate. |
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