| PVDF and its copolymer P(VDF-TrFE)are important components of the polymer family.Compared with other polymers,they possess relatively high dielectric constant,piezoelectric and ferroelectric properties,so they are widely applied in biomedicine,energy production,memory devices and pressure sensors.Since the fluorine atom is extremely electronegative with respect to hydrogen atoms and carbon atoms,the dipole moment in each cell of PVDF can reach(5-8)x10-30 C m which is the source of the excellent ferroelectric properties.In the four common crystalline phases of PVDF,the ?phase has the strongest ferroelectric properties.In the molecular chain of ?-PVDF,the C-C bond is serrated,and all the fluorine atoms are located on one side of the chain,the whole hydrogen atoms are located on the other side(all trans structure).The P-phase cell is composed of two monomer units of two all trans chain,this unique structure results in the strong ferroelectricity with a spontaneous polarization?10 ?c/cm2 in the ?-phase.The PVDF and its copolymer P(VDF-TrFE)are mostly semicrystalline and the non-ferroelectric a-phase is usually dominant,which lead to the weakening in the ferroelectric behaviour.Therefore,improving crystallinity and inducing the production of ? phase is an effective way to improve the ferroelectric properties of the PVDF and its copolymer.In this work,the effects of addition of BaTiO3 particles on the ferroelctric properties of the PVDF and P(VDF-TrFE)were investigated.The composite films were prepared by spin-coating method and the microstructure,phase composition and crystalline state of the films were characterized by SEM,AFM and XRD.The piezoelectric,dielectric and ferroelectric properties were studied in this work.The main conclusions are summerized as follows:(1)The pure P(VDF-TrFE)films were prepared on the Si substrate by spin coating method.The effects of different annealing temperatures on the crystalline morphology and crystallinity of pure P(VDF-TrFE)films were investigated.The results show that the films annealed at 120?,140?,160? and 180? possess large numbers of holes.The number of holes greatly reduced and the melting temperature and the enthalpy of fusion obviously decreased at an annealing temperature of 100 ?,which indicates that the crystallinity of polymer matrix decreases.The calculation results show that the crystallinity of the film reaches the maximum value of 59.2%at 140?,and the crystallinity of the film decreases to 34%when the annealing temperature is 100?.In order to obtain films with high crystallinity and less defects,the BaTiO3 ceramic powders were added and the effects on the crystallization were investigated.The results show that the addition of BaTiO3 effectively reduces the defects of the films.(2)A series of composite films with different BaTiO3 content were prepared,the effect of the BaTiO3 fillers on the ferroelectric and crystallized properties were investigated.It can be seen that the crystalline phase of composite films are mainly the a-phase and the y-phase.With the addition of BaTiO3 the large dendritic crystals change into irregularly oriented,fine and uniform needle-like crystals.According to the calculation,it is found that the addition of BaTiO3 increases the crystallinity of the polymer matrix.The ferroelectric measurement shows that the addition of BaTiO3 decreases the breakdown strength of composite films and improve the ferroelectric behaviour.Under the same electrical field(1200 kV/cm),the maximum polarization and the remnant polarization of the film with 2 wt%BaTiO3 are 4.2 ?C/cm2 and 2.6 ?C/cm2 respectively,while the values of the film without BaTiO3 are 1.5 ?C/cm2 and 0.3 ?C/cm2.(3)The BaTiO3 powders with an average particle size of 15 nm were added in the P(VDF-TrFE)solution to form uniform suspension,then a serious of films with different BaTiO3 content were prepared on the Pt/Si substrate by spin coating method.Ferroelectric test shows that the critical breakdown strengh of the films without BaTiO3 is 2200 kV/cm,the maximum polarization and the remnant polarization are 9.05 ?C/cm2and 6.89 ?C/cm2,respectively.With the increase of the content of BaTiO3,the dielectric breakdown strength decreased.The best ferroelectric properties were obtained from the composite film with 3 wt%BaTiO3,the maxmum polarization and the remnant polarization are 21.8 p.C/cm2 and 17.42 ?C/cm2,respectively.The results of XRD and DSC reveal that the addition of nano BaTiO3 powders can induce the transition of ? phase to ? phase,thus improving the ferroelectric properties. |
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