The Ferroelectric And Dielectric Properties Of Poly(Vinylidenefluoride-Trifluoroethylene) Copolymer Composite Films

Polyvinylidene fluoride(PVDF) and its copolymers as the most interesting organic ferroelectric and piezoelectric material, have a widely application such as piezoelectric sensor, energy harvesting generator, pyroelectric sensor and nonvolatile memories. PVDF homopolymer mainly possesses four crystalline phases:non-polar a phase(TGTG’)、polar (3phase (TTTT)、less polar y phase (TTTGTTTG’) and weak polar δ phase with the similar molecular conformation as a phase. The non-polar a phase usually occupied the dominant phase instead of the polar β phase with the most superior ferroelectric and piezoelectric properties. Ferroelectric polarization of pure PVDF is weak because of the lower degree of crystalline which could be greatly improved by introduce copolymers such as trifluoro ethylene (TrFE) and tetrafluoro ethylene (TeFE).P(VDF-TrFE)(70/30, mol%) exhibits higher polarization because of higher abundance of the polar all-trans conformation and the crystallize convert into β-phase at a particular annealing condition. Compared with the ferroelectrics with a perovskite structure, the dielectric constant of PVDF is small (-10), which is a weak point in the field of applications where a dielectric should be with a high permittivity. Consequently, how to enhance the dielectric constant of the PVDF is an ongoing topic. In this work, silver nitrate, AgNO3, has been incorporated into P(VDF-TrFE) copolymer dispersion mediums, which are annealed at two different atmospheres, i.e., air and hydrogen atmosphere.We finally obtain the P(VDF-TrFE)/Ag composite films through the reduction of AgNO3under hydrogen annealing.The main work and results are as follows:l.The P(VDF-TrFE)/Ag single layer thin films are prepared by spin coating. The as-grown films were annealed at135°C for12hrs under airand pure hydrogen with the flow rate at30ml/min-1under1.5atmrespectively. Two intensive peaks at2θ=19.9and2θ=38.7areobserved in all the samples, which correspond to ferroelectric β phase(110,200) and γ phase (211) respectively as shown in XRD patterns. Thediffraction peak at2θ=38.1is attributed to Ag (111), suggesting thatthe formation of silver nano-particle which decomposed from the silvernitrate under hydrogen atmosphere at135°C. A slight blue shift is foundin the IR spectrum of the P(VDF-TrFE)/Ag composite, e.g., the band at1078cm-1β-phase shift to1073cm-1. It can be related to the change ofthe molecule conformation of the P(VDF-TrFE) copolymer as theincorporation of Ag particles.These tiny particles shown in SEM imagesare considered to be Ag particles as suggested the formation of Ag(111)phase in XRD pattern.2.The ferroelectric properties of P(VDF-TrFE)/Ag composite films areanalyzed.The ferroelectric polarization Pr shows a continuous increasewith the increasing Ag content.The Pr of P(VDF-TrFE)/Ag composite filmswith dopant of initial8%AgNO3reach up to5.67μc/cm^2and increaseby30%compared with the pure P(VDF-TrFE) thin film. The polarizationvalue and coervice voltage of the composite thin films under hydrogenannealing are larger than the films under air annealing. The resultssuggest that the polar β phase and γ phase are improved byincorporated Ag particles into P(VDF-TrFE) polymer films. The changes ofcoercive voltage may be due to suppression of defect density in H2annealing condition.3. The dielectric properties of P(VDF-TrFE)/Ag composite films areanalyzed. The dielectric constant shows a continuous increase with theincreasing Ag content. The Pr of P(VDF-TrFE)/Ag composite films withdopant of initial10%AgNO3reach up to16.79and increase by17%compared with the value14.37of the pure P(VDF-TrFE) thin film. The loss tangent for the samples with Ag doping films are lower than pureP(VDF-TrFE) thin film and independent with the doping ratio. Theconductive silver nanoparticles distributed in the insulted polymer matrixcan form into a large number of minicapacitors and rapidly increasethe dielectric constants of P(VDF-TrFE) thin film. The dielectric lossindependent on doping ratio for the effective conductive path not builtas shown in SEM images.4. The dielectric properties under heating and cooling process ofcomposite thin films are analyzed. The dielectric constant decreaseswith increasing frequency and the real permittivity peak independentof frequency. The heat stagnation reach to36K indicate a typical firstorder transition.5. The temperature dependence of dielectric properties at differentkinds of bias voltage are analyzed. There are three imagery permittivitypeak at330K、360K and388K corresponding to dielectric relaxationcaused by defection and impurity interface polarization,ferroelectric-paraelectric transition and polymer melting transition. Thetemperature corresponding to imagery permittivity peak around330Kincreasing with the increasing frequency shows a typical dielectricdisperse both for pure P(VDF-TrFE) thin film and P(VDF-TrFE)/Agcomposite film with10%doping ratio under2V dc bias. The relaxationtime fitted by cole-cole formation decrease with increasingtemperature. The relaxation time distribution of pure P(VDF-TrFE) thin filmdecrease because of the weaker interaction of dipoles with theincreasing temperature. On the contrary, the relaxation time distributionof P(VDF-TrFE)/Ag composite thin films increase because of the strongerinteraction of Ag nano-particles and dipoles with the increasingtemperature.