Beneficial and detrimental fatigue effects of dielectric barrier discharges on the piezoelectricity of polypropylene ferroelectrets

Abstract

Cellular polypropylene (PP) ferroelectrets combine a large piezoelectricity with mechanical flexibility and elastic compliance. Their charging process represents a series of dielectric barrier discharges (DBDs) that generate a cold plasma with numerous active species and thus modify the inner polymer surfaces of the foam cells. Both the threshold for the onset of DBDs and the piezoelectricity of ferroelectrets are sensitive to repeated DBDs in the voids. It is found that the threshold voltage is approximately halved and the charging efficiency is clearly improved after only 103 DBD cycles. However, plasma modification of the inner surfaces from repeated DBDs deteriorates the chargeability of the voids, leading to a significant reduction of the piezoelectricity in ferroelectrets. After a significant waiting period, the chargeability of previously fatigued voids shows a partial recovery. The plasma modification is, however, detrimental to the stability of the deposited charges and thus also of the macroscopic dipoles and of the piezoelectricity. Fatigue from only 103 DBD cycles already results in significantly less stable piezoelectricity in cellular PP ferroelectrets. The fatigue rate as a function of the number of voltage cycles follows a stretched exponential. Fatigue from repeated DBDs can be avoided if most of the gas molecules inside the voids are removed via a suitable evacuation process.

Ferroelectrets: Recent developments

“…Higher or thinner thickness leads to higher breakdown voltage. On the other hand, the DBD charging of ferroelectrets generates a cold plasma that seriously deteriorates the chargeability of the cavities and hence the piezoelectricity of ferroelectrets [59]. Nevertheless, the different polarization states in ferroelectrets can be used in sensor applications [60].…”

Fatigue and recovery of cellular-polypropylene piezoelectrets during and after dielectric-barrier discharges

“…Therefore, we investigated the influence of the fatigue process on the stability of the piezoelectricity in cellular PP ferroelectrets [11]. Considering the partial recovery of the d 33 coefficient in fatigued samples, we also studied the influence of the post-fatigue storage period on the stability of the piezoelectricity.…”

“…When fatigued samples are stored in the lab for sufficiently long periods, the chargeability of the cavities partially recovers. However, repeated DBDs are detrimental to the stability of the internally trapped charges and hence of the piezoelectricity [10,11].…”

Synthesis of polar polynorbornenes with high dielectric relaxation strength as candidate materials for dielectric applications

“…7 Such foams show a high piezoelectric response, but humidity negatively influences their performance. 8 Quasi-permanent polarization can be introduced in amorphous polar polymers by poling in a strong electric field when heated above T g and cooled below T g. 9 Research in this direction attracted the attention of polymer chemists in the 1990s, as for example Hall Jr., 10 Feast, 11 and Wegner. 12 Polyacrylonitrile and poly(vinylidenecyanide vinyl acetate) (PVVA) show a large dielectric relaxation strength (De), but they are either environmentally unfriendly or have a high T m .…”

“… 7 Such foams show a high piezoelectric response, but humidity negatively influences their performance. 8 …”