极微等离子体射流的放电机理与化学活性研究

Ultrafine plasma jets with the diameter<10 μm can be used for precisely localized single-cell treatment and maskless micro-pattern in plasma material processing. However, due to the few knowledge of the discharge mechanism and chemical activity of ultrafine plasma jets, it is difficult to control and optimize the plasma activity. In this work, based on a novel ultrafine plasma jet we designed lately, (1) the effects of external discharge parameters on the electrical, acoustic, and optical characteristics of ultrafine plasma jets are investigated first. Then the transition of the discharge mode is distinguished by the several conventional methods, such as current-voltage waveforms, breakdown characteristic curve, acoustic or optical signals, and the critical external parameters window is also determined simultaneously. Furthermore, to understand the breakdown of the charge quasi- neutrality and the wall effects on the ignition and propagation of ultrafine plasma jets, the dynamics of the nanosecond pulsed discharge are investigated by a optical emission system (OES) with high spatial and temporal resolution. In the last, a ionization mechanism is proposed according to these experimental results above. (2) Ultrafine plasma jets are rich in active species, such as charges, free radical, metastable species and excited species with short life time. First, by using the methods of the laser induced fluorescence diagnostics and OES, the absolute density and distribution of these active species are measured. Second, to understand the main production and loss of these active species, with the knowledge of the chemical reactions and the initial conditions of these active species, a chemical kinetic model is built for detailed simulation. (3) To reveal the relationship between the nanosecond pulsed discharge and the chemical activity, the effects of the discharge mode, working gas composition and driven frequency on the density and distribution of active species with long life time is investigated. Finally, this study explores the discharge law of atmospheric pressure plasma under extreme conditions, and provide scientific basis for the control and optimization of the ultrafine plasma activity.

直径<10 μm的极微等离子体射流不仅可对单细胞进行精准处理，而且可用于材料表面的微图形处理。但是目前极微等离子体射流的放电机理和化学活性仍然不清楚，本项目采用自主研发的新型极微等离子体射流源，(1)从它的电、声、光特性入手，通过变化外部放电参数，找到放电模式转变的临界参数窗口，在临界条件下诊断极微等离子体射流的纳秒放电过程，研究管壁效应和电荷准中性消失对它的产生和推进过程影响，并提出电离机制。(2)采用辐射光谱法和激光诱导荧光法，对极微等离子体射流中带电粒子、自由基、亚稳态和激发态活性粒子的浓度分布进行定量诊断，建立多种活性粒子的化学反应模型，阐明它的主要产生和损失机制。(3)研究放电模式、工作气体成分和放电频率对长寿命活性粒子的影响，揭示放电机理与化学活性之间的内在联系。上述机理研究探索极限条件下大气压等离子体的放电规律，为极微等离子体射流的活性调控与优化提供理论指导。

大气压微等离子体射流技术已被应用于生物医学和材料表面改性处理。针对极微等离子体射流的放电机理与化学活性，本项目提出了通过减小等离子体特征尺寸以提高电子密度的方法，首次成功研制了直径为4μm-10μm极微等离子体和3.4μm极微等离子体阵列，发现其长径比为5000以上，电子密度高达1016-1017 cm-3，比mm级等离子体高出2~4个数量级。微等离子体的电流密度、电子密度与火花放电中的接近，但是它的气体温度远低于火花放电的气体温度。随着介质管径减小，电流密度、电子密度和击穿电压显著升高。采用光电倍增管技术与高速摄像技术研究了极微等离子体的纳秒动态过程，发现了微等离子体的传播形状转变为连续的柱状模式，揭示了毛细管内部表面电荷、电场和扩散对高电离度、放电模式转变与传播速度的重要影响，证实了极微等离子体仍然遵循准中性原则。在化学活性方面，极微等离子体具有高电子密度、高电子温度和低气体温度特征。采用激光诱导荧光法，获得了微等离子体射流中OH自由基的绝对浓度及它的时空分布、O原子的时空分布，建立了OH自由基和氮分子亚稳态的化学反应动力学模型，研究了OH自由基及氮分子亚稳态的产生与损失机制，并明确了电源频率、电压幅值及气体组分对这些活性粒子的影响规律，探索了基底材料、放电动态过程与OH自由基时空分布的内在关联，并提出了微等离子体射流中OH自由基和O原子密度的调控方法。