噴射式氦/氧大氣電漿模擬研究-物理化學機制與特性、操作參數關聯性之探討與驗證

Study of He/O2 atmospheric pressure plasma discharge by computer simulation and experimental characterization-physical/chemical mechanism and parametric analysis

余珮玟(Pei-Wen Yu)

指導教授 ： 柳克強

國立清華大學/原子科學院/工程與系統科學系/碩士(2010年)

摘要

噴射式大氣電漿 (Atmospheric pressure plasma jet, APPJ)近年來不僅發展迅速且備受重視，其主要優點除了不需使用昂貴的真空系統外，更讓人驚艷的乃在於其利用射頻所產生的非平衡電漿在產生極高的電子溫度與高活性電漿粒子(如:e-/1017 m-3以上,O，O2*，O3 等)密度的同時，其氣體溫度仍維持在室溫範圍，故特別適用於生醫美容、殺菌、熱敏感性材料(如:高分子材料)之表面處理與鍍膜等相關領域。而其中又以線性開槽式之噴射式大氣電漿，因可應用於傳輸式處理設備，且易於放大尺寸，最具工業應用潛力。
然而，由於目前鮮少實驗設備可直接量測位於大氣電漿內部之各項物理與化學的變化(如:電漿溫度、電漿粒子密度變化等)，造成許多電漿特性之形成機制無法釐清。因此，本論文藉由二維流體模型針對線性開槽式之He/O2 APPJ進行數值模擬計算(採用ESI CFD-ACE+ 模擬計算軟體)，探討He/O2 APPJ的α mode放電特性，並進一步藉由化學氣相反應速率的分析，指出潘寧游離效應(Penning ionization)對氧正離子形成的重要性與釐清臭氧(O3)的形成機制:
模擬結果顯示相較於由電子碰撞而產生之游離反應，氦氣與氧氣之間的潘寧游離效應 (He*+O2He+O2++e-,He*+OHe+O++e-)對於氧正離子(O2+ ，O+)的形成更為重要，此乃因He* (19.82 eV)比起其它亞穩態粒子更具有足夠的能量以游離氧氣之故。
其次，臭氧(O3)在放電區域(Discharge Region, DR) 與非放電區域(Effluent Region, ER)兩區域內的主要形成機制亦闡明於模擬結果中。研究發現於DR中，由氧負離子(O-)與亞穩態氧分子(O2*)碰撞生成臭氧的反應式(O-+ O2*O3 +e-)主導；然於 ER內，則轉以氦氣與氧氣之三體碰撞反應(He+O2+OHe+O3)為主，此乃由於氧負離子的產生在此區域受限於電子密度所致。
此外，進一步的研究更改變不同的電漿模擬參數(不同的功率輸入/流速/氧氣添加量)，針對平均電子溫度、平均電位與電漿粒子密度等作細節性的觀察。研究結果顯示輸入之功率愈高，將使得電場愈強，導致電子密度上升且電漿鞘層隨之縮小。同時，ER中主要的電漿活性粒子分別為O、O2*與O3，DR內則為O、O2*與O*，其中氧原子(O)之密度更高達2×1020 m-3以上。
然而電漿粒子在放電通道碰撞過程中，處於電場作用區的時間取決於氣體的流速，當氣體流速加快時，單位時間內APPJ腔體中存在著更多的中性粒子，造成吸收的能量不足以產生游離反應，即隨同部分已生成之電子被後方到來的中性粒子流帶走，使得電子密度下降，但其下降幅度僅約原電漿密度的5~10%，影響力相較於改變輸入功率為小。但以氧原子質量通量在ER中，隨著流速增加而增加之模擬結果而言，提升氣體流速有益於表面處理等相關之應用。
於改變氧氣添加量的模擬計算方面，研究結果顯示He/O2電漿的放電特性由氧氣之游離反應及其相競爭的氧氣吸附電子之吸附反應速率所決定。當氧氣添加量增加時，電子密度因氧氣吸附反應而減少，進而形成氧負離子(O-,O2-)。同時，氧正離子密度(O2+ , O+)因潘寧效應與氧氣游離能較低之故，當增加氧氣添加量時，其密度亦大量的增加,且亞穩態氦原子(He*)的密度呈現下降趨勢。值得特別注意的是，隨著氧氣的添加，氦/氧電漿的negative ion-dominated 特性更為明顯。
本研究探討之He/O2噴射式大氣電漿模擬計算分析，不僅有助於改善實際操作上He/O2 APPJ 的電漿特性與操控方法，更進一步指出潘寧游離效應對於氧正離子形成的相關性與臭氧的形成機制。故對於He/O2噴射式大氣電漿研究領域，本論文提供了具有參考價值的研究成果。

關鍵字

大氣電漿 ； 潘寧游離反應 ； 臭氧形成反應

並列摘要

Atmospheric Pressure Plasma Jets (APPJs) have been paid much attention and developed rapidly in recent years due to no necessity for expensive vacuum systems. As can be seen, APPJs produce non-thermal equilibrium plasmas as RF powers are applied. Extremely high electron temperature (around 1 eV) and active plasma particle densities (e.g., e- /above1017 m-3, O, O2* and O3) are generated, but the gas temperature is still maintained at room temperature. Therefore, they are widely used in biomedical applications, sterilization, heat-sensitive surface treatment, and depositing film on soft electric boards. Besides, among all type of APPJs, the linear slot structure APPJ is much suitable for industry applications which allows for continuous in-line processes and is easy to scale-up.
Nevertheless, a question, i.e., very rare experimental equipments could measure the plasma characteristics such as plasma temperature and plasma particle densities in the bulk plasma, arise. That’s why many physical/chemical mechanisms in APPJs can’t be explained clearly. Hence, in this thesis, we not only investigate the α mode discharge characteristics of a linear slot structure He/O2 APPJ by using a two-dimensional fluid model numerical analysis (Numerical simulation software, ESI CFD-ACE+ package, is employed.), but also indicate that the importance of Penning ionization and the O3 formation mechanism by calculating the gas phase reaction rates.
Simulation results point out that the penning ionization, i.e., He*+O2He+O2++e-, He*+OHe+O++e-, is a main source of producing oxygen positive ions. The main reason is that He metastables (19.82) have sufficient energy to ionize oxygen species comparing with other metastables.
Then, the main O3 formation mechanism is also clarified by gas phase reaction rates in our simulation. We find that O3 formation mechanism is different in the discharge region (DR) and Effluent Region (ER), respectively. In DR, O- and O2* are the most important species to ozone formation (O-+O2*O3+e-). However, in ER, the O is the key particle for ozone generation due to the decrease of electrons, i.e., He+O2+OHe+O3 is the dominant gas phase reaction for O3 formation in the effluent.
Further, we analyze the distribution of the average electron temperature and plasma particle density. Simulation results are presented by changing plasma parameters, such as different applied powers, flow rates and fraction of oxygen.
According to the simulation results, as the applied power increases, the electron density increases and the plasma sheath thickness shrinks due to higher electric field in the discharge gap. At the same time, the most dominant species are O(3P), O2*(1△g),O3 in the ER and O(3P) (around 2×1020 m-3), O2*(1△g) (around 6×1019 m-3), O*(1D) in the DR besides He and O2. The atomic oxygen (O) density is even up to 2×1020 m-3.
However, to increase the flow rate, the average electron density slightly decreases (5-10%) in the discharge gap due to the insufficient ionization energy. But, the mass flux of atomic oxygen increases with the flow rate. This simulation result is a great benefit to surface treatments.
Moreover, by changing simulation parameter, different fraction of O2, we observe the He/O2 discharge characteristics are determined by the ionization reactions and higher attachment rates of electrons with O2 and O. The electron density decreases to form negative ions (O-, O2-) due to attachment reaction. And, the competing reactions, the Penning and electron impact ionizations, produce a lot of electrons and positive ions (O2+, O+).
Besides, because the Penning ionization, He* number density decreases as the fraction of oxygen increases. Meanwhile, the positive oxygen ions increase markedly. One may notice that, an interesting phenomenon, the negative ion-dominated property of He/O2 discharge is more obvious as the fraction of oxygen increases which has shown in our simulations.
In summation, the simulation results not only provide an index to improve the He/O2 APPJ operation for better control, but also clarify some physical/chemical mechanisms, e.g., penning ionization and O3 formation mechanism, which few studies have been reported. We think the investigation of the thesis is a significant contribution to He/O2 APPJ.

並列關鍵字

Atmospheric pressure plasma ； Penning ionization ； O3 formation reaction

參考文獻

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