Diagnostic Study of Pulsed Power Solution Plasmas

本研究為運用脈衝產生器(pulsing unit)產生脈衝式電壓，並以此電源供應器維持、控制水溶液電漿(solution plasma)系統。不同於使用直流或交流電源的水溶液電漿系統，脈衝式水溶液電漿系統擁有較佳的穩定性與再現性，並且可以調控系統中生成氣泡的行為表現，以利對此水溶液電漿系統有更深入的檢測與了解。 本實驗檢測共分為兩部分：第一部分以脈衝式電源激發電漿在硝酸鈉(NaNO3)水溶液中生成，藉由調控施加偏壓在50到550 V以及Ton在10到500 μs之間，數種不同的操作模式將出現在系統中。當施加偏壓小於或等於100 V，系統將呈現氣泡模式(bubble mode, BM)。在此模式中，許多直徑在數百微米大小的氣泡將連續地在電極表面生成然後藉由浮力脫附離開表面。增加施加偏壓至125 V會導致氣泡模式轉換為噴流模式(jetting mode, JM)，在噴流模式下，大量直徑小於100 μm的氣泡生成且被噴離表面。對多種實驗條件的檢驗顯示BM至JM的轉換發生於在Ton時間內，輸入功率和輸入能量同時超越4.4×10-3 ± 6×10-4 J和45 ± 5 W的兩個臨界值。在已知的電極表面積下，此臨界功率等價為229 ± 25 MW/m2的熱通量(heat flux)，為文獻中指出足以產生爆炸性汽化(explosive vaporization)的數值。前述的發現強烈地顯示BM至JM的轉換為電熱效應(electrothermal effect)所產生的結果。 第二部分為對水溶液電漿中產生之活性物質進行檢測，研究中運用脈衝式水溶液電漿系統在Ton = 0.1 ~ 1 ms的操作條件下，對一稱為酸性橙7(Acid Orange 7, AO7)的合成染料進行分解反應以及利用一稱為氫氧自由基分子探針(OH radical molecular probe)的對苯二甲酸二鈉鹽(disodium salt of terephthalic acid , NaTA)偵測水溶液中的氫氧自由基。在探討酸性橙7分解反應的部份中，反應動力學分析發現對酸性橙7的去色和降解反應符合擬一級反應動力學模型的描述，且分解反應的速率常數為Ton之函數。在偵測氫氧自由基的部分中，利用NaTA與氫氧自由基反應後產物，2-羥基對苯二甲酸二鈉鹽(disodium hydroxyterephthalic acid, HTA)，的高度螢光性質量測溶解於水溶液中的氫氧自由基。HTA的生成速率與水溶液中的氫氧自由基濃度相關並且為Ton之函數，與酸性橙7分解反應的結果類似。綜合前述的兩實驗結果可以得到水溶液中氫氧自由基濃度與酸性橙7的分解速率有直接的正比線性關係。In this study, the pulsed voltage is generated by a pulsing unit and used to sustain plasmas in NaNO3 electrolytic solution. The pulsed power solution plasma system owns the better stability and cycle to cycle reproducibility and is capable of control the complex bubble dynamics, which is advantage of the further investigation and understanding of the solution system. These experimental investigations include two parts: Firstly, plasmas in NaNO3 solutions sustained by pulsed power with Ton = 10 to 500 μs is studied. With an applied voltage equal or lower than 100 V, the bubble mode is observed. In this mode, bubbles hundreds μm in diameter are formed and detached continuously at the electrode surface because of the buoyant force. When the applied voltage increases to 125 V, the bubble to jetting transition occurs. The examination of various conditions shows that the transition occurs when the power and energy inputs simultaneously exceed critical values, 4.4×10-3 ± 6×10-4 J and 45 ± 5 W, respectively, within Ton are required. Given the electrode surface area, this critical power is equivalent to a heat flux of 229 ± 25 MW/m2, which is close to the heat flux required, 223 MW/m2, for the occurrence of explosive vaporization reported in the literature. Such an observation strongly supports the hypothesis that the bubble to jetting transition is induced by the electrothermal effect. Secondly, the decolorization and degradation of Acid Orange 7 (AO7), an anionic monoazo dye, and the detection of OH radicals formed with disodium salt of terephthalic acid (NaTA), a well-known OH radicals molecular probe, by plasmas in NaNO3 solutions sustained by pulsed power with Ton = 0.1 ~ 1 ms is investigated experimentally. In the study of decomposition of AO7, the kinetic analysis of the decolorization and degradation of AO7 show a pseudo-first-order reaction to AO7 concentration. The rate constants of decomposition of AO7 are a function of Ton. The study of OH radical detection includes using NaTA and the highly fluorescent property of the resulting 2-hydroxyterephthalic acid (HTA) to measure the OH radical. The production rate is related to the OH radical concentration as a function of Ton, which shows the similar tendency of the decomposition of AO7 as Ton increases. The combination of the above results gives the directly linear relationship between the decomposition rate of AO7 and the concentration of OH radical in solutions