Polarization switching at the nanoscale in ferroelectric copolymer thin films Appl. Phys. Lett. 99, 142904 (2011) Tunable temperature dependence of electrocaloric effect in ferroelectric relaxor poly(vinylidene fluoridetrifluoroethylene-chlorofluoroethylene terpolymer Appl. Phys. Lett. 99, 052907 (2011) Beneficial and detrimental fatigue effects of dielectric barrier discharges on the piezoelectricity of polypropylene ferroelectrets J. Appl. Phys. 110, 024108 (2011) Energy harvesting by nonlinear capacitance variation for a relaxor ferroelectric poly(vinylidene fluoridetrifluoroethylene-chlorofluoroethylene) terpolymer Appl. Phys. Lett. 98, 222901 (2011) Photocrosslinking of ferroelectric polymers and its application in three-dimensional memory arrays Appl. Phys. Lett. 98, 183302 (2011) Additional information on J. Appl. Phys. We describe the concept, the fabrication, and the most relevant properties of a piezoelectric-polymer system: Two fluoroethylenepropylene ͑FEP͒ films with good electret properties are laminated around a specifically designed and prepared polytetrafluoroethylene ͑PTFE͒ template at 300°C. After removing the PTFE template, a two-layer FEP film with open tubular channels is obtained. For electric charging, the two-layer FEP system is subjected to a high electric field. The resulting dielectric barrier discharges inside the tubular channels yield a ferroelectret with high piezoelectricity. d 33 coefficients of up to 160 pC/N have already been achieved on the ferroelectret films. After charging at suitable elevated temperatures, the piezoelectricity is stable at temperatures of at least 130°C. Advantages of the transducer films include ease of fabrication at laboratory or industrial scales, a wide range of possible geometrical and processing parameters, straightforward control of the uniformity of the polymer system, flexibility, and versatility of the soft ferroelectrets, and a large potential for device applications e.g., in the areas of biomedicine, communications, production engineering, sensor systems, environmental monitoring, etc.
ABSTRACT:The influence of the solvent-evaporation rate on the formation of a and b crystalline phases in solution-cast poly(vinylidene fluoride) (PVDF) films was systematically investigated. Films were crystallized from PVDF/N,N-dimethylformamide solutions with concentrations of 2.5, 5.0, 10, and 20 wt % at different temperatures. During crystallization, the solvent evaporation rate was monitored in situ by means of a semianalytic balance. With this system, it was possible to determine the evaporation rate for different concentrations and temperatures of the solution under specific ambient conditions (pressure, temperature, and humidity). Fourier-Transform InfraRed spectroscopy with Attenuated Total Reflectance revealed the b-phase content in the PVDF films and its dependence on previous evaporation rates. Based on the relation between the evaporation rate and the PVDF phase composition, a consistent explanation for the different amounts of b phase observed at the upper and lower sample surfaces is achieved. Furthermore, the role of the sample thickness has also been studied. The experimental results show that not only the temperature but also the evaporation rate have to be controlled to obtain the desired crystalline phases in solution-cast PVDF films.
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