| Owning to the unique physical properties and potential applications, ferroelectric materials have become an important research point in the field of modern information science and technology. Poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) ferroelectric copolymer owns outstanding dielectric, ferroelectric, piezoelectric, electric heating and pyroelectric properties. Recently, P(VDF-TrFE) has gotten great attention as a promising candidate for the uncooled pyroelectric infrared detector, ferroelectric random access memory (FeRAM) and applied piezoelectric sensor. Annealed under certain condition, P(VDF-TrFE)(70/30, mol%) can obtain high remanent polarization with high crystallinity. In this paper, Ag NPs doping P(VDF-TrFE) composite films were prepared by spin-coating, and the microstructure, crystallization, dielectric, ferroelectric and optical properties of the films were totally investigated. Ag NPs doping and bending deformation’s influence on P(VDF-TrFE) composite films was explained by the interface polarization theory and flexoelectricity effect. The main work and key results are as follows:1, The preparation of Ag NPs doping P(VDF-TrFE) composite films:The Ag NPs doping P(VDF-TrFE) composite films were spin-coated on the cleaned substrates and annealed in a heating oven at 135℃ for 4h in nitrogen atmosphere for crystallization. The diffraction peak of the β (110.200) phase appears at 2θ=19.90°.The presence of Ag NPs with the diameter of 8-20nm is proved by both the characteristic diffraction peak of Ag (111) phase at 20=38.1° in XRD patterns and surface morphologies characterized by AFM. It can also be concluded that the size of the Ag NPs increases with the concentration of AgNO3.2, Ag NPs’influence on composite films’electric and optical properties:Both remanent polarization (Pr) and saturated polarization (Pmax) of the composite films increase with the increasing density of Ag NPs. The value of Pr increases from 5.64μC/cm2 for the pristine P(VDF-TrFE) film to 7.80μC/cm2 for the composite films with the highest density of Ag NPs. The macroscopic electrical measurements indicated that the relative dielectric constant increases from 9.3 for the pristine P(VDF-TrFE) to 13.7 for the composite film with highest density of Ag NPs. The local polarization switching results from PFM suggested that the coercive voltage decreases from 5.2 V for the pristine P(VDF-TrFE) films to 2.9 V for the composite films with highest density of Ag NPs. The reasons for the improved properties are explained by the interfacial polarization occurs at the interface between Ag NPs and P(VDF-TrFE) matrix. Besides, UV-visible spectra reveals the surface plasmon resonance absorption of composite films at 415nm wavelength increases with the density of Ag NPs.3, Bent-deformation’s influence on composite films’electric properties:An Al-P(VDF-TrFE)-Al capacitor structure was prepared on the soft polyimide substrates. The ferroelectric, dielectric and pyroelectric properties of the bent P(VDF-TrFE) films were comprehensively characterized under controllable bent-deformation. The macroscopic electric measurements indicated that the remanent polarization and the pyroelectric voltage response increased from 6.14μC/cm2 and 2.95mV respectively for the films with maximum bending-downwards deformation to 6.87μC/cm and 3.35mV for the film with maximum bending-upwards deformation. Taking the regular change in relative dielectric constant and dielectric loss into consideration, we established a structure model related to flexoelectricity and explained the physical mechanism behind this phenomenon. After analyzing the stress and strain gradient in the bent P(VDF-TrFE) films, we suggested that the displacement of positive and negative atoms under the strain gradient, which disrupts the inversion symmetry, resulted in a net polarization inside the bent ferroelectric films. |
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