Processing and switching mechanism of ferroelectric copolymer P(VDF/TrFE)
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The processing and properties of the ferroelectric copolymer P(VDF/TrFE) was investigated in this work, and is divided into two main parts. The first part focused on different processing routes, and the second part focused on the switching dynamics mechanisms P(VDF/TrFE). The processing focused on the characterization and ferroelectric properties of the extruded films from melt. The extruded films were moderately cooled and collected in air or fast cooled and collected in liquid nitrogen fume to study the effect of cooling rate. The extruded film collected in both ways were then hot-compressed at different temperatures ranging from 80℃ to 140℃ to control their thickness and study the effect of the quick heat treatment under pressure. All the extruded films collected in air and liquid nitrogen fume with or without hot-compression were characterized by XRD, 2-D XRD, SEM and DSC. X-ray Diffraction (XRD) showed that the extruded films collected in both ways had good crystallinity of the ferroelectric phase and a preferred orientation of (200)/(110). This preferred orientation and high crystallinity remained when the extruded films were hot-compressed between 80℃ to 120℃. 2-D XRD ring patterns showed that the preferred orientation (200)/(110) of extruded films hot-compressed at 120℃ increased compared to that of the extruded films without hot-compression. Differential Scanning Calorimetry (DSC) results showed the Curie point at 135℃ and a melting point at 148℃, which were consistent with the results of the high temperature XRD. The Scanning Electron Microscopy (SEM) images of extruded films showed close packed banded structures. The ferroelectric properties were investigated using the polarization hysteresis loop measurements. The best ferroelectric properties were achieved for the extruded film collected in air and hot-compressed at 140℃, and. a maximum remnant polarization of 0.09C/m2 was obtained. The study of the ferroelectric switching mechanisms focused on modelling switching under continuously changing and constant electric fields. A model for P(VDF/TrFE) was developed based on the application of rate theory for a continuously changing electric field at different loading rates. Ferroelectric switching of P(VDF/TrFE) under the continuously changing electric field was dominated by the 180˚ domain switching. The activation volume measured between 5 to 20Hz (4.3±0.8nm3) was estimated to be about four times greater than that measured between 20 to 100Hz (1.1±0.1nm3). The creep mechanism under constant electric fields showed that the decay of the rate of change of the ferroelectric polarization in P(VDF/TrFE) was dominated by hardening between 16 to 64kV/mm. The decay of the rate of change of the polarization was controlled by a mixture of hardening and exhaustion mechanisms between 80to 112kV/mm. The internal fields estimated from the partial unloading test showed a linear relationship with the polarization states.
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