Polarization And Dynamic Shock Calibration Of Poly(vinylidene Fluoride) Film

Sensor is one of the key techniques in the information age.The piezoelectric sensor that can directly convert mechanical signals to electric signals plays an important role in the science and technology for the development of national defense and civil engineering.Polyvinylidene fluoride(PVDF),an organic semi-crystalline piezoelectric polymer,shows a strong piezoelectricity after mechanical stretch and poling under strong electric fields,and meanwhile has a low acoustic impedance,a strong chemical resistance,a good thermal stability,and especially an excellent flexibility and toughness,making it popular for integrated,miniaturized and multi-functional devices in the technical fields.Especially,in the field of dynamic pressure measurement like in underwater explosion pressure,detonation threshold,impact compression and safety parameters of inert materials,the PVDF thin-film piezoelectric sensor shows unique advantages,it does not require any external power supply,complex test circuit,and has a high respond speed,a fast frequency response,a broad pressure measurement ability,a micron or nanometer level thickness.In addition,the PVDF gauge can be made into different shapes and lead to less damage to the structural integrity of the measured object,which meet the development needs of modern non-destructive testing technology.Based on the above background,this thesis starting with the improvement of the PVDF polarization techniques systematically investigates the behaviors of PVDF under alternative poling fields and the response of PVDF under dynamic shocks.The thesis applies the solution nucleation approach to synthesize the original PVDF films and uses them for the uniaxial stretching and the static pressure experiments.The experimental results show that the mechanical stretch can evidently increase the fraction of the polarization phase in PVDF.The fraction after the stretch is 4 times of that of the original PVDF film.In the static pressure experiments,the feature peak of the polarization phase shows adequate fraction up to 22 GPa,which is the foundation of the PVDF film application for dynamic shock measurement.The thesis devises a PVDF poling system based on cyclic poling electric fields.The film with 20 ?m thinkness after cyclic polarization treatment has a remnant polarization intensity value of 7.25 ?C/cm2,higher than that of DC polarization and corona polarization.The repeatability of the remnant polarization intensity is within 4%,demonstrating that the cyclic alternating polarization method can effectively improve the remnant polarization of the PVDF piezoelectric film and maintain a good repeatability.We also study the effects of PVDF film polarization experiment parameters,including thickness,electrode area and field strength amplitude,on the polarization performance of PVDF film.The experimental results show that when the alternating polarization electric field strength exceeds 175 MV/m electric field,strength the remnant polarization strength of PVDF increases with the increasing electric field,until the field strength increases to a saturated electric field(approximately 3.5 times the coercive electric field),where the increase in polarization intensity slows down and gradually converges.It is found that the remnant polarization strength of PVDF does not change with the film area but depends on the thickness of the film.In experiments,a thicker film after polarization has a larger remnant polarization.It can be explained by that the monomers near the surface of the film are constrained by surface tension and are difficult to reverse the orientation under poling fields.In the thicker samples,the fraction of the effective dipoles that can be reversed is larger,which accordingly leads to the larger remnant polarization.We use the dipole flip polarization empirical model to simulate the hysteresis loop of the PVDF film under different experimental conditions.The simulation parameters reflect the polarization behavior of the PVDF film,such as,the field strength amplitude of the phase transition point and the polarization intensity.This empirical model has a good universality and reliability,and the model parameters can be used to describe the polarization behavior of PVDF film.This work designs a dynamic compression calibration system for the PVDF piezoelectric film within the stress range from 0.3 to 11 GPa,and investigates the response characteristics of the PVDF sensor under dynamic high pressure shock impact.The calibration system employs the charge method and the current method test circuit to collect the output charge of the sensor.The experimental results show that the PVDF pressure gauge can accurately record the input shock waveform,and the error between the measured waveform width and the theoretical pulse width is less than 2%.Because the remnant polarization of the PVDF piezoelectric film can fluctuate slightly in the synthesis,the raw calibration data points cannot be directly used for fitting the dynamic calibration curve.In order to obtain an accurate sensor dynamic calibration curve,we correct the charge released from the sensor by normalizing the polarization of PVDF.After correction,the released charge density is more regular and match better with the fitting calibration curve,especially at the high stress range.This study finds that the released charge density of PVDF piezoelectric film has a linear relationship with the bulk strain under dynamic shock wave.Based on this,we proposed the calibration equation on foundation of the Hugoniot equation of state(EOS).If the two parameters in the EOS are unknown and determined from fitting,the calibration equation will have a total of three fitting parameters.The three-parameter calibration equation has errors about 0.09 ?C/cm2,smaller than that of the two-parameter equation.We apply the three-parameter calibration equation to the experimental data from literature,which were measured within a broader stress range up to?35 GPa.The results show that the three-parameter calibration equation matches better with the data than the original fitting equations in the literature.Furthermore,we apply the machine learning models to evaluate the effect of the remnant polarization,the thickness,the saturated polarization field,the coercive electric field of the PVDF film on the eventual calibration relations.The results show that the Ridge and Lasso model both gives the statistical format of polarization and thickness effects.By comparison,the Lasso model matches better with the experimental data than the Ridge model.In addition,although the errors of the nonlinear SVM and ANN models are small,the predicted calibration curves are not practical in physics for over-relying on experimental data.This research provides experimental and theoretical views for the PVDF pressure gauge in the field of dynamic shock testing,improves its non-destructive testing capability,accuracy,and reliability in dynamic shock compression,and promotes its applications in the field of high-energy explosive explosion and ultra-high pressure testing.