Polarization profiles have been measured with a resolution of about in corona poled 95:5 P(VDF-TFE) copolymer by using the piezoelectrically induced pressure step (PPS) method. A polarization peak is observed near the positive side of samples poled in a negative corona by the constant current technique. The distribution of polarization is also nonuniform in the case of a constant voltage poling at , but profiles are rather uniform if a poling field is applied at . In multilayer samples, the upper film bombarded by corona ions remains absolutely nonpolarized. Thin transition nonpolarized zones are seen near surfaces of even well polarized P(VDF-TFE). It is shown that polarization uniformity in P(VDF-TFE) is strongly affected by injection of negative charge carriers from the virtual electrode formed either on the surface bombarded by corona ions, or between two contacting dielectric surfaces. Our data on P(VDF-TFE) are in accord with those reported on other ferroelectric polymers.
The polarization profiles dynamics in polyvinylidene fluoride (PVDF) films was studied in situ by the piezoelectrically induced pressure step method. It was shown that a substantially inhomogeneous polarization profile was formed during poling and reversal switching in medium fields with the maximum of polarization near the positive electrode. However, poling of virgin samples in strong fields resulted in formation of a homogeneous polarization profile that remained uniform and easily switchable at any subsequently applied field. It appeared that it was impossible to improve the polarization homogeneity in PVDF by application of strong fields, if original poling was performed in weak or medium fields. The observed features were explained by a model assuming injection of negative charges and formation of a permanent blocking layer at the boundary between polarized and non-polarized zones. The obtained results should be considered while selecting poling conditions in design and production of sensors and actuators based on PVDF. The results have a practical importance in design of piezo-and pyro-sensors and activators using the ferroelectric PVDF films, because they should be considered while selecting poling conditions.
A new method is proposed to study polarization-field hysteresis in ferroelectric polymers based on the appropriate treatment of experimentally recorded kinetics of the electret potential during the constant current corona poling The apparent conductivity and the dielectric constant measured in the same experiments are considered As an example, the P(E) dependence for PVDF is obtained and values of the coercive field and the remanent polarization are found 1
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