【作者】 尚万里；

【导师】 王德真；

【作者基本信息】 大连理工大学 ， 等离子体物理， 2009， 博士

【摘要】 近年来,大气压辉光放电(APGD)在许多领域发挥着重要的作用,其在刻蚀、沉积、材料表面改性、杀菌、臭氧生成以及新型光源等领域的应用,引起人们越来越多的关注。早在上世纪50年代,低气压放电中就被发现存在着两种不同的放电模式,称为α模式和γ模式;近年来,大气压辉光放电中也被证实存在着两种不同的放电模式。α模式下等离子体密度较低,但是稳定性好;γ模式下等离子体密度较高,但是稳定性不好,容易转化为电弧放电。深入了解两种不同机制下的放电特性,对于提高辉光放电等离子体的密度和保持其稳定性,以及其工业上的应用都有着极其重要的作用。本文基于一维流体模型,对平行板电极结构中的射频大气压辉光放电进行了数值模拟,深入探讨了其两种不同模式的放电特性。模拟结果显示,随着放电电流密度的增大,等离子体密度也随之增大,放电由α模式进入γ模式。第一种模式α模式下,放电电流密度相对较低,群等离子体电子从鞘层的收缩和扩张中获得能量。在第二种模式γ模式下,放电电流密度相对较高。焦耳加热和电子碰撞能量损失在鞘层区域达到最大。外加电压幅值为436V时,在阴极鞘层里的电子温度最高,达到6.44eV,阳极鞘层里最大的电子温度为2.96eV,而群等离子体中的电子温度为2eV左右。同样的,在鞘层中的电子产生源项也最大,其中阴极鞘层里的电子产生源项比阳极鞘层里的要大。在计算中发现,外加电压幅值为436V,二次电子发射系数0.05时,放电处于γ模式,而在其他条件不变,二次电子发射系数减小到0.05以下时,放电就不处于γ模式,而是处于α模式。可见二次电子发射在放电中起着至关重要的作用,二次电子发射系数越大,等离子体密度越大。模型的有效性通过别人的实验及模拟结果得到了验证。本文还基于电子、离子和亚稳态粒子的密度连续性方程,结合电流连续性方程,研究了一维同轴电极结构下,反应气体分别是纯氦气和含少量杂质氮气的混合气体时,射频大气压辉光放电中的放电模式及特性。同样的,在同轴电极放电中也存在着α模式和γ模式。模拟结果显示,不对称的放电结构对放电产生了较大的影响,上下半周期内的电流密度和电压图像出现了不对称。电流均方值为40mA/cm时(放电处于α模式),上半个周期和下半个周期的电流密度峰值并不相等,相差为2.3 mA/cm,气体电压幅值相差为50.3V。且无论是在哪个模式中,等离子体密度在放电空间并非均匀分布,而是从外电极到内电极,逐渐增大。杂质氮气对放电影响较大,能明显的降低发生模式转换所需要的电压。当反应气体为纯氦气时,α-γ模式转换点对应的电压均方值为289V,电流密度均方值为106mA/cm,而当氦气中加入10ppm的氮气杂质时,其α-γ模式转换点对应的电压均方值为214V,电流密度均方值为134mA/cm。总所周知,深入了解大气压射频辉光放电的物理机制对于其工业应用十分重要,然而一维模拟毕竟有较大的局限性。为了多角度深层次地研究大气压射频辉光放电,本文基于二维流体模型,对平行板电极结构中的射频大气压辉光放电进行了数值模拟。随着放电电流密度增大,鞘层电场强度增大,鞘层变薄。当电流密度增加到一定数值的时候,发生鞘层击穿,等离子体在极板中央区域形成一个极强的放电通道,群等离子体沿着极板方向发生收缩,随后放电进入了另外一种模式。这个现象和实验中的放电收缩极为相似,并在实验研究中广泛出现。该通道中电场强度极大,在本文的计算中最大值达到9kV/cm,要远大于模式转换之前的电场强度4kV/cm。在本文的模拟中,各种等离子体参数达到稳定所需要的时间不一样,一般需要几百个电压周期。二次电子发射对放电模式转换的发生作用明显。介质阻挡放电存在着三种不同的放电形式。本文运用二维流体模型,对柱对称电极结构中的大气压介质阻挡放电进行了数值模拟。在10kHz电压频率驱动下,放电出现了同心圆筒放电结构。随着时间的改变,放电圆筒会发生径向演化。原先放电的区域放电熄灭,原先未放电的区域开始放电,形成一个一个的放电环,交替放电。文中研究了电子密度和电流密度空间分布随时间的演化效果。介质板上的电荷积累对放电的影响至关重要。本文还从丝的分裂与合并来探讨了同心圆筒放电的产生和演化。更多还原

【Abstract】 In recent years,uniform atmospheric-pressure glow discharge(APGD) has attracted considerable attention because of its advantageous properties for industrial applications, including surface activation,etching,cleaning,decontamination,and thin film coating.In the last fifties,two discharge modes namedαandγmode were found in the low pressure glow discharge.And the existence of two different discharge modes has been verified in an rf (radio-frequency) APGD.It has been demonstrated that theαmode has better discharge stability,whereas theγmode produces more abundant plasma species including charged particles and metastables.The discharge inγmode turn to arc earily and this is not requisite.For continuous using APGD,particularly on an industrical scale,a through understanding of the operational characteristics of the two modes is essential.In this thesis,a one-dimensional self-consistent fluid model for rf APGDs is used to simulate the discharge mechanisms in theγmode in helium between two parallel metallic planar electrodes.The results show that as the applied voltage increases,the discharge current becomes greater and the plasma density corresponding increases,consequentially the discharge transits from theαmode into theγmode.In the first mode,referred toαmode,the discharge current density is relatively low and the bulk plasma electrons acquire the energy due to the sheath expansion.In the second mode,termed asγmode,the discharge current density is relatively high,the secondary electrons emitted by cathode under ion bombardment in the cathode sheath region play an important role in sustaining the discharge. The high collisionality of the APGD plasma results in significant drop of discharge potential across the sheath region,and the electron Joule heating and the electron collisional energy loss reach their maxima in the region.The validity of the simulation is checked with the available experimental and numerical data.The discharge in pure helium and the influence of small nitrogen impurities at atmospheric pressure are investigated based on a one-dimensional self-consistent fluid model between two coaxial electrodes.The simulation of the radio-frequency(rf) discharge is based on the one-dimensional continuity equations for electrons,ions,metastable atoms and molecules, with the much simpler current conservation law replacing the Poisson equation for electric field.Through a computational study of rf atmospheric glow discharges over a wide range of current density,this thesis presents evidence of at least two glow discharge modes,namely, theαmode and theγmode.The simulation results show the asymmetry of the discharge set exercises great influence to the discharge mechanisms compared to that with parallel-plane electrodes.It is shown that that the particle densities are not uniform in the discharge region but increase gradually from the outer to the inner electrode in both modes.The contrasting dynamic behaviors of the two glows modes are studied.Secondary electron emission strongly influences gas ionization in theγmode yet matters little in theαmode.Discharge mode transition is an important issue for widespread applications of radio-frequency atmospheric pressure glow discharges(APGD).This thesis reports a study of the mode transition(α-γ) in radio-frequency APGDs using two-dimensional fluid simulation. At theα-γmode transition point,the plasma is shown to undergo transverse contraction induced by an imbalance of transverse electron drift and diffusion.While transverse contraction often leads to irreversible mode transition experimentally,numerical results suggest that it is recoverable and with increasing discharge current the plasma starts to expand transversely and resume spatial uniformity.A numerical simulation of concentric-ring discharge structures has been performed within the scope of a two-dimensional diffusion-drift model at atmospheric pressure between two parallel circular electrodes covered with thin dielectric layers.With a relative high frequency the discharge structures present different appearances of ring structures within different radii in time due to the evolvement of the filaments.The spontaneous electron density distributions help to understand the formation and development of self-organized discharge structures. During a cycle the electron avalanches are triggered by the electronic field strengthened by the feeding voltage and the residual charged particles on the barrier surface deposited in the previous discharges.The accumulation of charges is shown to play a dominant role in the generation and annihilation of the discharge structures.Besides,the filaments split and unit to bring and annihilate filaments which form a new discharge structure.更多还原