| Summary: | 博士 === 國立交通大學 === 機械工程學系 === 98 === Development, characterization of a planar atmospheric-pressure nitrogen-based dielectric barrier discharge driven by a quasi-pulsed (distorted sinusoidal) power source and its applications using the post-discharge jet region are presented in this PhD thesis. The characterizations of the DBD system included the measurements of the direct image visualization, the electrical properties (current-voltage curve and power absorption), the optical properties (optical emission spectra and FTIR), the gas temperatures (thermocouple and optical emission spectra), and the ozone concentration. The measurements showed that abundant mestastable nitrogen was generated in the pure nitrogen DBD, while abundant ozone was created as long as the oxygen was added. Specially, observation of optical emissions in the post-discharge jet region was successfully explained by several kinetic mechanisms, which is important in understanding the fundamental mechanism in surface modification. This DBD system was then applied to several important applications, which included the modification of hydrophilic property of PP film, the surface cleaning of ITO glass, the inactivation of E. coli, B. subtilis and the sterilization of B. subtilis spore. Results of applications showed that the developed DBD system was highly effective in these applications under proper operating conditions, which they are briefly described in the following in turn.
For the stationary PP films, the contact angle (CA) decreases dramatically from 103?a (untreated) to less than 30?a (treated) with a wide range of O2/N2 ratios (< 1%) and treating distances (< 10 mm). In addition, the CA can still be maintained at ~40?a after 24 h of the aging test. For the stationary ITO glass, show that there exists two distinct regimes with lower CAs in the range of 20-30° (84° for untreated). The first one was the regime with an oxygen addition of less than 0.05% and a treating distance in the range of 6-16 mm. The second one was the regime with an oxygen addition larger than 0.06% and a treating distance in the range of 2-10 mm. The measurements showed that: 1) in the near jet downstream location (z<10 mm), both the metastable N2 and ozone photo-induced dissociation played dominant roles in surface modification, although their relative importance was unclear and requires further investigation; and 2) in the far jet downstream location (z>10mm), when the ratio of O2/N2 was small, only the long-lived metastable N2 played a major role in ITO cleaning and PP film surface modification. XPS measurements showed that improved hydrophilic property was obtained after DBD jet treatment with increasing O/C ratio.
For the inactivation of E. coli and B. subtilis, the results showed that the post-discharge jet region is very efficient in inactivating these two bacteria as previous studies using the discharge region, should the working gas contains appreciable oxygen addition, which in turn generates abundant ozone. In addition, the inactivation is more effective by using compressed-air APPJ as compared to that by oxygen APPJ, possibly through the assistance of nitrous oxide existing in the former. Results of survival rate show that both E. coli and B. subtilis bacteria (up to 10^7 CFU/mL) can be effectively inactivated using less than 18 passes (1.8 seconds of residence time in total) of exposure to the post-discharge jet region of compressed air and oxygen discharges at different treating distances in the range of 4-20 mm. For the sterilization of B. subtilis spore, the results showed that addition of only 2% CF4 into the air DBD is found to be very effective, which was otherwise very ineffective using the pure air DBD. It was found that the CF4/air (2%) APPJ treatment resulted in the efficient inactivation of the B. subtilis spores after 10 passes (residence time: 1.0 s) exposures for treatment distances (14 mm). Indirect evidence showed that highly reactive atoms, such as F atoms, were generated in the discharge and in the post-discharge jet region.
In conclusion, in the current thesis, a simple yet very effective planar nitrogen-based DBD system under atmospheric-pressure condition is presented and characterized experimentally. It was successfully applied using its post-discharge jet region, which was rarely seen in the literature, for surface modification and inactivation/sterilization. Recommendations for the future study are also outlined at the end of the thesis.
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