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The group "Allgemeine Elektrotechnik und Plasmatechnik" at the faculty for engineering and information science.

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Investigation of the frequency dependent spatio-temporal dynamics and controllability of microdischarges in unipolar pulsed plasma electrolytic oxidation

The unipolar pulsed-plasma electrolytic oxidation (PEO) of aluminum has been replaced by bipolar pulsed methods that use a so-called 'soft-sparking' mode.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 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_1 = 50 Hz, f_2 =5kHz and f_3 = 100 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_2 = 5 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_3 = 100 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.

Release Date
Permanent Identifier (URI)
Is supplementing
Plasma Source Name
Plasma Source Application
Plasma Source Specification
Plasma Source Properties
Generator: Magpuls MP2-30 Current: I = 0.5 A (mean) Voltage: V = 0-1000 V Frequency: f = 50 -100,000 Hz
English (United States)
Plasma Source Procedure
Two electrodes are placed in an electrolytic cell containing an electrolyte. The cathode consists of stainless steel. An oxide coating is grown on an aluminum sample (anode). The plasma is ignited after the local dielectric breakdown of the oxide layer. The current induces evaporation of water and a bubble grows on the surface of the oxide. The plasma is ignited inside of the growing water vapor bubble.
Plasma Medium Name
Plasma Medium Properties
Electrolyte: 2 liter distilled water + 1 g/l KOH The pressure inside of the bubble is around 1-2 bar. The bubble size is in the region between a few microns to hundreds of microns.
Plasma Medium Procedure
First, add distilled water to the electrolytic cell. Then add 1 g/l KOH. The electrolyte is stirred by a magnetic stirrer and cooled with an external cooling circulator at 300 K.
Plasma Target Name
Contact Name
Patrick Hermanns
Plasma Target Properties
6061 aluminum cut to a size of 1 mm (thickness) x 10 mm (length) x 20 mm (height)
Plasma Target Procedure
Polishing, degreasing with acetone and isopropanol, rinsing with distilled water, and hot air drying.
Contact Email
Plasma Diagnostic Properties
Oscilloscope: LeCroy Waverunner 8254 Differential voltage divider: Tektronix ADP305 Current probe: LeCroy CP030 SEM: Zeiss Nano 40 EDS: Thermo Fischer Scienti c Ultra Dry EDS Detector ICCD Camera: Stanford Optics 4Picos and PCO HSFC Pro
Public Access Level
Plasma Diagnostic Name
Funding Agency

Data and Resources

Project Contact Name: 
Vera Bracht