Axel Sonnenfeld

Oxygen permeation, mechanical and structural properties of multilayer diffusion barrier coatings on polypropylene

To improve temperature durability for autoclaving of SiOx diffusion barrier coatings on polypropylene, plasma polymerized hexamethyldisiloxane (pp-HMDSO) is applied by plasma enhanced chemical vapour deposition as interlayer material and compared with results obtained with amorphous hydrogenated carbon-nitrogen (a-C : N : H) and a-Si : C : O : N : H interlayers. The influence of the O2/HMDSO ratio on the chemical structure and related mechanical and oxygen barrier properties is investigated by fragmentation tests, dilatometry, oxygen transmission rate, internal stress and mass density measurements as well as Fourier transform infrared and x-ray photoelectron spectroscopy. Carbon-rich, polymer-like coatings with low density, low internal stress and excellent adhesive and cohesive properties are found for pp-HMDSO at the expense of barrier performance. In the SiO x/pp-HMDSO coating a broad transition in chemical composition was observed, explaining improved mechanical properties responsible for good barrier performance after thermal cycling or autoclaving.

Comparison of calorimetric plasma diagnostics in a plasma downstream reactor

The energy influx in a non-equilibrium plasma in the afterglow of a plasma downstream reactor has been measured by two different calorimetric probe types. The radio frequency discharge is investigated for oxygen (10–100%)–argon (90–0%) mixtures at relatively high gas flow rates (750–3000 sccm) and pressures (100–350 Pa). The main process parameters influencing the energy influx are the plasma power, the system pressure and the total gas flow rate. Even though the size, geometry and material composition of the used probe types are different, comparable energy flux values are achieved. Depending on the process parameters, energy fluxes between around 100 and 3500 W m−2 are found and the variation of the pressure and total gas flow rate suggests a highly flow-dependent plasma density distribution in the reactor.

Discharge expansion in barrier discharge arrangements at low applied voltages

This study evaluates the spatial expansion of barrier discharges (BDs) in oxygen, carbon dioxide, synthetic air, and helium at atmospheric pressure. Despite being confined in narrow gas channels, the BD plasma only partially covers the available discharge area at low applied voltages as was experimentally observed. This is important for homogeneous surface treatment. The appearance of local discharge zones in the plasma development results in a variation of the capacitive behaviour of the equivalent electric circuit as a function of the applied voltage. This transient behaviour of the apparent dielectric capacitance is therefore investigated for various discharge areas and different gas compositions. Furthermore, a semi-empirical model describing the expansion behaviour is proposed and validated by means of the obtained experimental data. The results clearly show that a three times higher voltage excess is required for oxygen-containing gases compared with helium in order to develop full plasma expansion. For the discharge areas considered, the size of the available electrode plays a minor role in the plasma expansion process.

Influence of increased velocity on the statistical discharge characteristics of He and air barrier discharges

A detailed study of a description method for microdischarges based on the statistical analysis of single discharge bursts is presented and applied for small electrode arrays. As such, the electrical properties derived from the collective behaviour of the current bursts for microdischarges in helium and synthetic air for different pressure levels and variable voltage gradients are elaborately discussed. Motivated by the possible prolongation of the displacement distance of active species and thus the improved development of an atmospheric afterglow in a low frequency discharge, the interaction between high-speed gas flows and microdischarges in small ducts has been approached presenting first results. Intentionally, gases with different discharge mechanisms known as streamer (synthetic air) and Townsend (helium) have been considered. Similar velocity dependences of the discharge behaviour could be observed in both gases. For helium, subsonic channel velocity is sufficient to distinctively alter the discharge characteristics whereas transonic flow is needed to alter those of synthetic air. Subsequently, a simple model is proposed to explain these findings for elevated velocities in a dielectric setup.