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Investigation of the frequency dependent spatio-temporal dynamics and controllability of microdischarges in unipolar pulsed plasma electrolytic oxidation
Journal of Physics D: Applied Physics ( IF 3.4 ) Pub Date : 2020-11-04 , DOI: 10.1088/1361-6463/abbde4
Patrick Hermanns 1 , Simon Boeddeker 1 , Vera Bracht 1 , Nikita Bibinov 1 , Guido Grundmeier 2 , Peter Awakowicz 1
Affiliation  

The unipolar pulsed-plasma electrolytic oxidation (PEO) of aluminum has been replaced by bipolar pulsed methods that use a so-called ‘soft-sparking’mode. This method results in an effective reduction of intense microdischarges, which are detrimental to the oxide layer. In a previous publication, we developed an in-situ multivariable microdischarge control scheme using unipolar pulsing. Using this method, it is possible to restrict the mean microdischarge size to well-defined limits, while at the same time influencing the mean microdischarge energy, number density or spectral emission behaviour. This method operates well inside a frequency range of $f = 1-20\ \mathrm{kHz}$. Although this method shows highly desirable plasma control properties, the mechanisms defining this frequency-dependent controllability are unclear. The aim of this study is to visualize the spatio-temporal behavior of microdischarges in higher frequency ranges. First, a wavelet transform was performed to estimate the temporal evolution of microdischarge lifetimes. Ceramic coatings were then deposited on aluminum alloy substrates in an aqueous solution using unipolar pulsed galvanostatic PEO. The aluminum samples were coated for 30 min at frequencies of $f_\mathrm{1} = 50\ \mathrm{Hz}$, $f_\mathrm{2} = 5\ \mathrm{kHz}$ and $f_\mathrm{3} = 100\ \mathrm{kHz}$. High-speed imaging was carried out utilizing four synchronized intensified charge-coupled device (ICCD) cameras, each with a 500 ns exposure time. At $f_\mathrm{2} = 5\ \mathrm{kHz}$, the microdischarges were still able to follow the electrical pulses. In this regime, the process can be divided into two stages, an initial charging of the substrate surface without plasma emission and a subsequent slower evolution of microdischarges. Equivalent circuit model descriptions are given for both processes. At $f_\mathrm{3} = 100\ \mathrm{kHz}$, microdischarges were not able to follow the pulse frequency, as the lifetimes and risetimes of the microdischarge characteristics were longer than the pulse length. Reignition at the same spatial location, clustering and permanent ignition through pulse periods were observed.



中文翻译:

单极脉冲等离子体电解氧化中随频率变化的时空动力学和可控性研究

铝的单极脉冲等离子体电解氧化(PEO)已被使用所谓的“软火花”模式的双极脉冲方法取代。该方法有效地减少了对氧化物层有害的强烈的微放电。在以前的出版物中,我们开发了使用单极性脉冲的原位多变量微放电控制方案。使用这种方法,可以将平均微放电尺寸限制在明确定义的范围内,同时又会影响平均微放电能量,数量密度或光谱发射行为。此方法在以下频率范围内运行良好$ f = 1-20 \ \ mathrm {kHz} $。尽管此方法显示出非常理想的等离子控制特性,但尚不清楚定义这种与频率相关的可控性的机制。这项研究的目的是可视化在更高频率范围内的微放电的时空行为。首先,执行小波变换来估计微放电寿命的时间演变。然后使用单极性脉冲恒电流PEO将陶瓷涂层以水溶液形式沉积在铝合金基材上。铝样品以$ f_ \ mathrm {1} = 50 \ \ mathrm {Hz} $$ f_ \ mathrm {2} = 5 \ \ mathrm {kHz} $和的频率涂覆30分钟$ f_ \ mathrm {3} = 100 \ \ mathrm {kHz} $。利用四个同步的增强型电荷耦合器件(ICCD)相机进行了高速成像,每个相机的曝光时间均为500 ns。在$ f_ \ mathrm {2} = 5 \ \ mathrm {kHz} $,微放电仍然能够跟随电脉冲。在这种情况下,该过程可以分为两个阶段,即在没有等离子发射的情况下对基板表面进行初始充电,以及随后缓慢释放微放电。给出了两个过程的等效电路模型描述。在时$ f_ \ mathrm {3} = 100 \ \ mathrm {kHz} $,微放电不能跟随脉冲频率,因为微放电特性的寿命和上升时间比脉冲长度长。观察到在相同空间位置处的点火,通过脉冲周期的聚集和永久点火。

更新日期:2020-11-04
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