Barium Titanate-bismuth Ferrite/Polyvinylidene Fluoride Nanocomposites Sensors:Preparation And Application In Construction Quality Monitoring

The issues of construction process cannot be found in time which will affect the construction quality,and even endanger structural safety.Therefore,it is desiderate to monitor the construction quality.Generally,the common non-destructive monitoring methods for construction quality include radiographic testing,ultrasonic method,stress wave method,infrared thermography technology.The stress wave method possesses the advantages of high sensitivity,good real-time,and not limited by the shape of building structure.The piezoelectric sensors can realize the interconversion of mechanical and electrical energy,which is a key device for stress wave method to realize the construction quality monitoring.The common piezoelectric sensors are lead zirconate titanate(PZT)sensors,but its brittleness makes it easy to damage during the monitoring process.Polyvinylidene fluoride(PVDF)flexible sensor can effectively replace PZT sensor,but its received signal is susceptible to temperature,and its piezoelectric performance is low.Therefore,it is necessary to improve the piezoelectric performance and temperature stability of flexible PVDF sensors.Based on this background,this work studies from material development to sensor practical application.The results are as follows :(1)The overall properties of BT-BFO/PVDF nanocomposite piezoelectric films was determined by the material level,and the optimal dosage of barium titanate–bismuth ferrite(BaTiO3-BiFeO3,BT-BFO)nanoparticles.The microstructure,crystal phase structure,dielectric,electrical conductivity,and piezoelectric properties were tested to determine the electrospinning conditions of BT-BFO/PVDF nanocomposite piezoelectric film,and investigate the influence of the BT-BFO nanoparticles on PVDF nanocomposite piezoelectric film.The β phase fraction in the20wt% BT-BFO/PVDF nanocomposite piezoelectric film reached a maximum value of 75%,which was 41% higher than the pure PVDF sample.The dielectric constant(ε),piezoelectric strain constant(d33)and piezoelectric voltage constant(g33)of the films were significantly enhanced to 55.71,16.19 p C/N and 30.48 m Vm/V,respectively.When the dielectric loss remained at a low level(0.035)compared with the traditional PVDF piezoelectric film.In summary the piezoelectric properties of BT-BFO/PVDF nanocomposite piezoelectric film were improved.(2)The temperature stability of BT-BFO/PVDF nanocomposite piezoelectric sensor was studied from the sensor level.The optimal monitoring scheme and appropriate temperature range were determined.The piezoelectric coefficient d33 of the sensor after packaging was not much different from that of the piezoelectric film,and the signal amplitude of the 20BT-BFO/PVDF nanocomposite piezoelectric sensor reached 2.13 V.The frequency and amplitude of the excitation signal were determined by the excitation-reception device based on stress wave method,which will reach 100 k HZ and 10 V,respectively.As compared with the traditional PVDF sensor,the dielectric constant change of the 20BT-BFO/PVDF nanocomposite piezoelectric sensor was less than 15%,and the amplitude fluctuation of the received signal was less than 7% achieved at the temperature range of 20～120°C.Additionally,the peak dielectric loss at 65°C was reduced.These results indicated that the monitoring signal of the BT-BFO/PVDF nanocomposite piezoelectric sensor is not easily affected by temperature in the range of 20 ～ 120 °C.(3)Based on BT-BFO/PVDF nanocomposite piezoelectric sensor,the method for monitoring the density of grouting sleeve and the strength of massive concrete was proposed from practical application.The nanocomposite piezoelectric sensors were used to monitor the semi-grouting sleeves under five grouting compactness(87%,90%,93%,96%,100%)conditions.The signal amplitude ratio SR obtained by the normalized processing was proposed to characterize the defect degree of grouting sleeve.The above-correlation between grouted density and SR was established.The nanocomposite piezoelectric sensors were used to monitor the strength of concrete blocks and massive concrete at different ages.Meanwhile,the correlation between strength ratio and signal amplitude ratio was established,and the compressive strength of concrete at different ages was predicted.Moreover,the hydration heat of massve concrete would not affect the signal amplitude.In a word,based on BT-BFO/PVDF nanocomposite piezoelectric sensor,which showed the strength prediction formula of concrete block still applied to massive concrete.