Optical emission characteristics of medium- to high-pressure N2 dielectric barrier discharge plasmas during surface modification of polymers

Abstract

The authors measured the band spectra (first and second positive systems) of the nitrogen molecule by optical emission spectroscopy with an aim to understand the mechanism of surface processing by medium- to high-pressure dielectric barrier discharge (DBD) plasmas. The experimentally measured and calculated spectra were compared to determine the vibrational and rotational temperatures of the N2 (C3Πu) state in the generated plasmas. The authors generated the N2 DBD plasmas at a driving frequency of 1–7 kHz and a discharge pressure of 20–105 Pa for the surface modification of a polyethylene terephthalate (PET) sample. It was found that the vibrational temperature was greatly affected by the N2 pressure while the rotational temperature remained constant in the N2 pressure range of 20–105 Pa. The emission intensity of N2 first positive system (B3Π → A3Σ) rapidly decreased at an increasing N2 pressure due to the collisional relaxation process of the B3Π state with N2 molecules. The N2+(B2Σu+→X2Σg+) radiative transition was observed in the low-pressure DBD plasmas, which was attributed to the direct electron impact ionization of N2 molecules. The surface characterizations of treated PET samples by contact angle measurement and atomic force microscopy indicate that the low-pressure N2 DBD plasma is an effective method for the surface modification of polymers. Analysis indicates the plasma characteristics such as electron temperature and ion energy are mainly dependent on the N2 pressure, which turn to determine the surface properties of treated PET samples.