| Summary: | 博士 === 國立臺灣大學 === 化學工程學研究所 === 101 === Discharges in solution provide the possibility to generate reactive species in liquid phase in demand. The discharge behavior has been proved highly relevant with the behavior of bubble where the plasma is ignited, however the methods to actively manipulate the bubble dynamics and composition are still limited. In this study, strategies to tailor the plasma behavior via different power types are proposed.
Unlike previous study in which plasma was driven by a direct-current power, a time-varying voltage was used to ignite the plasma. Two distinct bubble modes can be observed by adjusting the applied voltage and frequency: at 150 V, when a relatively low frequency (e.g. 50 Hz ) is used, the plasma is sustained inside a gas bubble with diameter of few mm; when a relatively high frequency (e.g. 500 Hz) is used, a jetting flow is observed. Depending on the bubble dynamics, we therefore called them as bubble and jetting mode, respectively. According to our study, it is known that frequency serves as an effective factor to control the heating in the vicinity of electrode surface, thus influencing the bubble and plasma behavior. The current and electron density in plasma are changed in response to the frequency. However, when plasmas are driven by an AC power source, plasmas are ignited by positive and negative polarity alternatively, and thus three issues are raised, namely damage of electrode, plasma instability, and poor power efficiency. A rectified AC power source was used in order to further improve the system. For both bubble mode and jetting mode, the discharge current for positively rectified AC- (PRA) driven plasma is only one tenth of that for negatively rectified AC- (NRA) driven plasma. The power consumption for PRA-driven plasmas is at least one order lower than that for NRA-driven plasmas. The result shows PRA-power is ideal to sustain the plasma. Finally, in order to control the gas composition, a bipolar pulse power source with adjustable positive and negative pulse width and amplitude was used to independently control the gas bubble formation and plasma ignition. Negative pulse with voltage from 0 to–80 V is used to generate the electrolytic gas bubble where plasma is formed and 600 V positive pulse is used to ignite the plasma. By this approach, the extension of plasma occurrence time in the positive pulse duration, the drop of peak electrolytic current, and the increase in relative H emission intensity are observed. These results suggest that by manipulating the driven power types, the gas composition and the discharge behavior can be effectively tailored.
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