Annealing for the improvement of the capabilities of parylene C as electret

Abstract

Parylene C is a polymer elaborated by chemical vapor deposition and is particularly useful for the coating of deformed structures. New‐generation soft energy harvesting electrostatic generators can be of complex shape. These generators need electret material in order for them to function. It could thus be interesting to incorporate parylene C in this device. However, this polymer suffers from bad retention of trapped charges after corona discharge and a poor thermal stability of these charges. In this work, parylene C polymers were subjected to specific annealing before corona charging. This induces strong changes in the crystallinity of the material. Atomic force microscopy and X‐ray diffraction quantified the crystalline nature of parylene C following the annealing temperature. Surface potential decay of corona‐charged parylene C demonstrated that annealing to about 150°C is beneficial for the trapping of charges for a long time (analysis over 12 days). At the same time, thermal stability of the electret parylene C is achieved up to 100°C. Spherulites with a specific area close to the surface of the polymer could explain such performances due to their influence on the distribution of traps. Dielectric analysis does not confirm that the decay of surface charges is correlated to the dynamics of polymer chains, although the moving of chains probably influences the flowing of these charges. In sum, annealed parylene C appears to be a new interesting candidate for electret‐based electrostatic generators. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 46908.

Outstanding performance of parylene polymers as electrets for energy harvesting and high‐temperature applications

“…This higher stability can derive from the symmetric characteristic of the polymeric chain of Parylene AF‐4 polymer (Figure ) and to its nonpolar nature. In fact, the polar nature of Parylene C can lead to partial implanted charge annihilation in the sample, thus compromising the surface potential stability of the electret in the first few days …”

“…One solution could possibly be to use annealing to modify trap density arrangement. Such an approach has recently been successfully proposed on Parylene C …”

“…In fact, the polar nature of Parylene C can lead to partial implanted charge annihilation in the sample, thus compromising the surface potential stability of the electret in the first few days. 29 Finally, as the difference between positive and negative homocharges stability is lower than 10%, Parylene AF-4 is suitable for energy harvesting applications with double polarity electret materials.…”

“…One solution could possibly be to use annealing to modify trap density arrangement. Such an approach has recently been successfully proposed on Parylene C. 29 The activation energies [extracted from eq. (2)] of 9-μm-thick Parylene AF-4 and VT-4 samples were found, respectively, equal to 1.23 and 1.16 eV, thus confirming the higher stability of the AF-4 variant.…”

“…However, Parylene‐C has a polar character, which does not contribute to the retention of the charges stored in the electret. In a recent study, annealing of Parylene‐C before corona charging was evaluated for its improvement as electret but performance remains inferior to that obtained with fluoropolymers. Indeed, some fluoropolymers are known for being very good electrets.…”