The properties of space charge homocharge and dipoles heterocharge in various electrets have been in the focus of interest since the investigations of B. Gross on carnauba wax. In this article we present a review on the role and the interaction of charges and dipoles in the polymeric electret polyvinylidene uoride PVDF. It covers the importance of charge injection for the orientation of the dipoles at low elds as well as the role of charge trapping for the stabilization of the polarization. Investigations of the thermal stability of the polarization and the charge balance are leading to a detailed analysis of the depth and localization of the charge traps. Furthermore, it was possible to identify the space charge as ions created electrochemically during the poling process.
Abstract. The remanent polarization in PVDF and P(VDF/TrFE) is stabilized by trapped charges. We measured the remanent polarization P as function of temperature during heating from room temperature to 180 °C. The discharge current is then computed as the fint derivative -dP IdT. Assuming a Debye relaxation a.nd a continous distribution of binding energies g(E) of the trapped charges we can show that the discharge current I(T) is proportional to g(E=rnT), with m "" con.st. The constant rn can either be calculated numerically or by analytical approximation. With this method the distribution g(E) ca.n be determined. Thelie data a.re consistent with the extraordinary long life time of the remanent polarisation in PVDF and P(VDF/ThFE) at room temperature.
It is well known that poling of PVDF in high electric fields ( ...... lDOMV 1m) lellds to remanent polarization and a strong piezoelectric effect. The remanent polarization is etable for years at room temperature. We report on polarization reductions if poled films Are immersed in polar solvents like acetone. propanol , ethanol, and methanol for Ii few minutes . Also immersion in nonpolar solvents like hexane or carbon tetrachloride for some hours results in a small reduction of polarization. The amount of the reduction depends on tho dipole moment of the solvent molecules, the length of t.he influence of the solvent, and the poling parameters. We also observe an influence on tbe remanent poluization if the fllm!i have been immersed in a solvent for some minutes before poling. We find inhomogenious polarization distributions in these films different to distributions in untreated ftlms before poling. ]n the case of a polar solvent pretreatment, the developement of pollU'ization profiles with several m8xilll8 is observed. These pbenomena are clisctlased in terms of the interaction of solvent molecules with trapped charges stabilizing the polarization.
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