Entwicklung, Charakterisierung und Anwendungen nichtthermischer Luft-Plasmajets

Abstract

The main applications for non thermal plasma jets are surface treatments with the purpose of cleaning, activation, modification or coating. For an effective plasma treatment, a high concentration of reactive plasma species and a sufficient reach of these species are absolutely required.In this work, a non thermal plasma jet operating with ambient air was developed. Here, especially the high oxygen content of air is challenging. Because oxygen is an effective quencher of excited states, it is not possible to generate long, luminous plasma plumes with air. A further problem is the generation of ozone. Naturally, all atmospheric plasmas produce a large amount of harmful ozone. One aim of this work was the generation of a high concentration of reactive plasma species and the maximization of the reach of these species. An additional aim was the minimization of the ozone production during the operation of the plasma jet.To achieve these aims, comprehensive analytical investigations for optimizing plasma sources accompanied by numerical simulations were carried out. As a result, extensive findings of the processes within the plasma and the correlation with external operating parameters were obtained.On basis of these findings, the implementation of the aims was accomplished as follows:. - The plasma jet developed here possesses a thin dielectric barrier made of MgO. MgO possesses a comparatively high secondary electron coefficient. Therefore, more secondary electrons are emitted during the discharge. This causes a higher electron density and hence more reactive plasma species are produced.. - The discharge gap is relatively small. This leads to an enhanced number of micro discharges. This improves the homogeneity of the plasma and increases the concentration of excited plasma species. In addition, the ozone concentration is reduced.. - The jet has a very short electrode. This leads to an increases voltage across the discharge gap although the total power remains constant. This causes an enhanced electron density and therewith a more dense plasma. Simultaneously, the ozone concentration is considerably reduced because the delay time of the gas molecules in the plasma becomes shorter than the formation rate of ozone. - The jet operates with a very high gas flow and possesses a very small nozzle. As a result the flow velocity is extremely high and the range of short living plasma species (especially O-atoms) is extended.The effectiveness of the plasma jet in comparison to other plasma jets was demonstrated by determining the increase of surface tension on PP- and PVC-substrates.The composition of the plasma and the plasma plume could be determined by analytical investigations and in addition by numerical simulations.