脈衝式電源驅動之平板型常壓氮氣為主介電質電漿的流體模型模擬

Abstract

本論文考慮在交流60 kHz頻率的條件下，使用流體模型模擬脈衝式電源之平板型氮氣為主的介電質電漿。從數值模擬計算出的電流-電壓曲線，發現在計量上與實驗量測相當符合。 在純氮氣的數值模擬中，可看到N4+離子為電漿濃度最高的帶電粒子。另外改變介電質之間的距離超過1.0 mm時，氮氣電漿會從Townsend-like discharge轉變成filamentary-like discharge型式。實驗量測上也發現同樣的轉變現象。當增加輸入的電壓時，電漿密度與電漿內部吸收的能量也隨之增加。由結果可看到N4+離子獲得最多的能量並且超過電子吸收的十倍以上。介電質材質分析中，介電值壁面上的累積電荷正比於介電常數。由於累積電荷越多，可提供越多離子化時需要的電子，因此較容易維持電漿。另外增加介電質的厚度時對電漿密度與內部能量吸收也有相當大的影響。 當微量的氧氣加入氮氣電漿時，可看到N4+與O2-離子有相同的密度且為電漿濃度最高的帶電粒子。另外發現中性粒子N2(A3Σu+)與氮原子的模擬結果與實驗量測非常相近。由於電子會大量吸附在氧氣上，降低電漿內部的電子密度，而使電漿離子化的機率降低。因此可發現隨著氧氣濃度從0.003 %增加到0.1 %時，電子的密度從1017 m-3減少到1014 m-3，並且發現只要加入少量的氧氣就會明顯影響電漿的維持。在光譜的模擬結果上，選擇second positive system (SPS)、NOγ-system與ON2-excimer三組光譜與實驗量測作比較。兩者比較結果皆隨著氧氣含量的增加，光譜強度從快速的增加轉變成逐漸的降低。

A simulation of parallel-plate dielectric barrier discharge (DBD) using pure nitrogen and N2/O2 gas driven by a realistic distorted-sinusoidal voltage power source (60 kHz) is studied. The simulated current-voltage characteristic results quantitatively agree with experimental measurements. In the pure nitrogen simulations, N4+ ion density is the dominant charged species, which is unlike most glow discharges. The discharge transforms from Townsend-like to filamentary-like (microdischarge) as gap distance is more than 1.0 mm, which was also observed in the experiment. All densities of charged and neutral species increase exponentially with increasing applied peak voltages in the range of 6.2-8.6 kV. The higher permittivity of the dielectric material is, the larger the discharge current and the longer the period of gas breakdown are. In addition, the quantity of accumulated charge at each electrode increases with increasing permittivity of the dielectric material. Finally, the increase of dielectric thickness from 1.0 to 2.0 mm greatly reduces the densities of all species and also the plasma absorbed power. When trace amount of oxygen is introduced in nitrogen plasma, the dominate charged species are N4+ and O2- with densities about 1018 m-3. The neutral species densities of and atomic nitrogen are approximately 4 × 1019 m-3 and 1 × 1021 m-3 respectively, which agree well with experiments. The oxygen addition can significantly decrease the electron density from the order of 1017 m-3 down to 1014 m-3 as the fraction of trace oxygen increases from 0.003 % to 0.1 %. In addition, the calculated photon radiations are compared against the measured spectra. The spectral bands of second positive system (SPS) of N2, NOγ-system and ON2-excimer are selected for comparison. Results reveal that the simulations and experiments show the similar trend with oxygen addition, in which the quantity of radiation increases rapidly first, peaks at some oxygen addition and then followed by a slow decrease.