| Polyvinylidene fluoride?PVDF?has strong piezoelectric and pyroelectric properties,stable chemical properties,and is easy to prepare large-area flexible film,which has been widely used in laser-induced impact pressure test,aviation and navigation,infrared camera,array sensor,biomedicine and other fields.Three common crystal types of PVDF are?phase,?phase and?phase,among which?phase is polar phase,with strong piezoelectric property.There are many spontaneous polarization in the cell,which can achieve high orientation under the effect of external electric field.Therefore,to improve the electrical performance of PVDF film,the content of?phase should be fundamentally improved.In recent years,many researchers at home and abroad have done a lot of research experiments to improve the content of?phase in PVDF films,and explored many experimental methods and conditions,such as high-pressure annealing,high-temperature and high-voltage DC electric field,low-temperature stretching,rapid cooling,etc.,but some of them are more cumbersome,and the experimental conditions are difficult to achieve,compared with the annealing and uniaxial annealing of PVDF films The tensile treatment is easy to operate and realize.At the same time,a large number of scholars have shown that annealing tensile treatment can improve the content of?phase in PVDF film to a certain extent.In this paper,polyvinylidene fluoride?PVDF?was used as the matrix,and two kinds of inorganic ceramic materials,Ba Ti O3and Ti C,were selected as fillers to prepare PVDF based composite films with different doping ratio.The films were annealed and stretched under certain conditions,and the required composite films were prepared by controlling different annealing temperature,annealing time,stretching rate and stretching ratio.The effects of annealing temperature,time,drawing rate and magnification on the morphology,crystal structure and dielectric properties of PVDF films were studied.The experimental results show that annealing and stretching treatment can obviously improve the crystallinity of PVDF pure film and composite film,reduce the defects such as holes in the film,promote the movement ability of the molecular chain in the crystal,and make the packing more evenly distributed.XRD analysis showed that the annealing and stretching treatment promoted the transition of PVDF from?phase to?phase,especially the high-speed stretching produced a large number of polar?phase.DMA analysis shows that annealing can improve the storage modulus of PVDF film,but tensile treatment can greatly reduce the storage modulus of PVDF film.The analysis of broadband dielectric spectrum shows that the dielectric constant of PVDF film increases first and then decreases with the increase of annealing temperature when the annealing temperature is controlled at 80oC-120oC.When the annealing temperature is 100oC,the maximum dielectric constant of PVDF film is 11.65.When the annealing temperature is120oC,the dielectric constant of 15wt%Ba Ti O3/PVDF composite film is12.08,the dielectric constant of 30wt%Ti C/PVDF composite film is 214.53,The experimental results show that the dielectric properties of the composite films can also be improved with the increase of annealing time,drawing rate and magnification.For example,when the annealing time increased from 2h to 4h,the dielectric constant of 15wt%Ba Ti O3/PVDF composite film increased from11.83 to 12.08,the tensile rate increased from 5mm/min to 15mm/min,and the tensile ratio increased from 87%to 112%,the dielectric constant of PVDF composite film is increasing.The dielectric loss and conductivity of PVDF pure film increased after annealing and stretching treatment,and the dielectric loss of15wt%Ba Ti O3/PVDF composite film decreased after annealing treatment.The dielectric loss of 30wt%Ti C/PVDF composite film increased sharply after annealing at 80oC and 120oc,which was in direct proportion to the increasing dielectric constant.After annealing at 100oC,the dielectric loss of 30wt%Ti C/PVDF composite film was stable between 0.1-0.25 at 103Hz.The conductivity of PVDF pure film and composite film increased continuously after annealing and stretching treatment.The conductivity of pure film increased from 5.8×10-6/S/cm to 2×10-5/S/cm.The conductivity of 15wt%Ba Ti O3/PVDF composite film increased from 7.5×10-6/S/cm to 3.4×10-5/s/cm.The conductivity of30wt%Ti C/PVDF composite film increased from 1.2×10-5/S/cm to 6.7×10-5/S/cm. |
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