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Investigation of atmospheric pressure filamentary discharges and its application for film deposition on inner surface of tubes, cavities and flat surfaces

Fachliche Zuordnung Optik, Quantenoptik und Physik der Atome, Moleküle und Plasmen
Förderung Förderung von 2009 bis 2017
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 74729252
 
Erstellungsjahr 2016

Zusammenfassung der Projektergebnisse

A new plasma source for the deposition of hydrocarbon films on the inner surfaces of tubes, capillaries, cavities, and flat surfaces has been developed and investigated in the frame of this project. For the purpose of homogeneous inner coatings, a fundamental understanding of the transient filamentary discharge in argon under atmospheric pressure and its interaction with the tube walls, different diagnostic schemes have been developed. Small admixtures of methane and acetylene are used as precursor gases. The external plasma conditions, namely gas mixture, applied voltage and driving frequency are optimised for deposition of homogenous, hard films on inner surface of thin (1 mm diameter and lower) and long (1 m and longer) thermolabile capillaries which may be applied for biomedical applications like catheters or stents. The developed plasma source includes a conical shaped driven electrode, a grounded ring on the outer side of the tube which serves as the counter electrode and two short quartz tubes in electrodes regions, which are coupled with a long (up to 1 m) and thin (lower than 1 mm) treated capillary made of plastics. Obviously, for the deposition of homogeneous films a constant flux of precursor radicals to the inner surface of the capillary and, therefore, constant plasma conditions along the whole tube have to be established. These conditions are based upon the homogeneous distribution of the fundamental parameters, i.e. gas temperature and electron distribution function along the total length of the tubes. In order to evaluate the plasma parameters with temporal and spatial resolution, a small quantity of nitrogen was admixed to the working gas and emission spectra of molecular nitrogen are measured by using calibrated spectrometers. The gas temperature and the electron distribution function are determined by fitting numerically simulated spectra of nitrogen. Further on, the Boltzmann equation was solved under corresponding plasma conditions by a variable electric field strength. Based on these experiences, a new characterisation method for micro-plasma diagnostics with spatial and temporal resolution has been developed. With two different interference filters, stereoscopically measured calibrated ICCD camera images undergo Abel inversion. Afterwards, these results are compared with solutions of the Boltzmann equation in local approximation. Attainable resolutions of this method are in the micrometer and ten nanoseconds range. Therefore, the mentioned method is convenient for the characterisation of transient micro-plasmas like sparks, dielectric barrier discharges, guided streamers and others. In addition, based on the determined plasma parameters a plasma-chemical reaction scheme has been established and verified. For optimising purposes, the film hardness reached 7 GPa. As a new phenomenon, so called plasmoids have been investigated. These are small plasma entities, which finally exist without contact to any power supply. Even more surprisingly, these plasmoids produce traces, may etch deposited hydrocarbon films and can influence its roughness and permeability. The structure and physics of plasmoids are not fully clear up to now and need to be investigated in the near future. Further very interesting properties of plasmoids are investigated in this project, namely collection and transport of material, penetration through very small slits and holes, formation and deposition of crystals just by one contact with a substrate surface. Generally speaking, the properties of plasmoids depend on the surrounding gas composition and therefore can be influenced and even controlled during coating processes. It has been evaluated that for the deposition of high quality hydrocarbon films by means of filamentary discharges inside tubes or capillaries information about plasmoids properties turned out to be essential.

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