Study On Fabrication And Properties Of Organic Ferroelectric Polymer Films

With excellent ferroelectricity,biocompatibility,and chemical stability,polyvinylidene fluoride(PVDF)and its copolymers stand out from organic polymer materials and are widely used in sensors,memories,etc.In order to test and improve the performance of devices made of PVDF and its copolymers films,it is necessary to investigate the preparation methods and corresponding properties.In this dissertation,the process of preparing P(VDF-TrFE)by spin-coating method was firstly explored.The relationship between the thickness of films prepared by spin-coating 2.5 wt%P(VDF-TrFE)solution and the angular velocity was obtained.Furthermore,the values of remanent polarization and the coercive electric field were measured at 7?C/cm~2 and 83.3 MV/m,respectively.Then,the ultrathin PVDF film was prepared by the Langmuir-Blodgett(LB)method and characterized using piezoelectric force microscope(PFM).The results showed that the average fluctuation was about 3nm for a film with an area of 900?m~2 and a thickness of 12 nm.Moreover,the film crystallizes readily into ferroelectric phase after annealing.The coercive electric field is 183.3 MV/m while the tolerable electric field is at least 666.7 MV/m.Subsequently,with the voltage pulse as stimulus,the transient response of a system that connect P(VDF-TrFE)ferroelectric capacitor in series with a resistor was experimentally investigated.The results showed that ferroelectric capacitor had no difference with ordinary capacitor when the amplitude of the voltage pulse is not sufficient to reverse the ferroelectric polarization.Only if the amplitude of voltage pulse exceeded the coercive voltage,the process of polarization inversion accelerated with increasing voltage amplitude.When the polarization inversion rate exceeded a certain threshold,a high current caused by the ultra-fast polarization inversion lead to high voltage drop on the series resistor,and the negative capacitance phenomenon became accessible.Finally,the Landau-Khalatnikov equation and Landau-Devonshire theory were applied to construct a mono-domain model of P(VDF-TrFE)ferroelectric capacitor for circuit simulation.The mono-domain model was connected in series with a resistor or a capacitor,and a voltage pulse is applied as excitation.Simulation results show that when the voltage pulse exceeded the coercive voltage of the model,as the amplitude of the voltage pulse increases,the polarization inversion rate would increase.In addition,the value of the resistance and capacitance could impact the duration of the negative capacitance phenomenon by changing the time constant of circuit.Moreover,the ferroelectric capacitor connected in series with an ordinary capacitor was able to amplify the voltage during the process of polarization inversion.