Dielectric barrier discharges during the generation of ferroelectrets: Optical spectroscopy for process monitoring

The transient light emission from the dielectric barrier discharges (DBDs) in cellular polypropylene ferroelectrets subjected to high electric fields was quantitatively measured and correlated with the piezoelectric d₃₃ coefficient. The spectrum shows strong emission from the second positive system (SPS) of molecular nitrogen N₂(C³Pi_urarrB³Pi_g) and the first negative system (FNS) of N₂ ⁺(B²Sigma_u ⁺ rarrX²Sigma_g ⁺), consistent with a DBD in air. When a dc ramp voltage is applied to the ferroelectret film, light emission starts above a threshold level, which coincides with the threshold voltage for obtaining piezoelectricity. Additionally, light emission from "back discharges" is observed when the voltage is decreased back to zero. Thus, spectroscopic analysis of the light emitted during charging of ferroelectrets can be employed to carefully monitor, better understand, and further optimize the underlying DBD process.