Atmospheric pressure plasma
As atmospheric pressure plasma (also AD-plasma or atmospheric pressure plasma ) refers to the special case of a plasma , wherein the pressure of approximately that of the surrounding atmosphere - the so-called normal pressure - corresponds.
In the following, the method with a nozzle and a gas discharge with a low current density is essentially discussed. For further forms of plasma generation in normal atmosphere see also:
Technical importance
In contrast to low-pressure plasma or high-pressure plasma, atmospheric pressure plasmas do not require a reaction vessel that ensures that a pressure level or gas atmosphere that differs from that of atmospheric pressure is maintained. With low current densities, such plasmas are used to activate surfaces or to ionize and generate ozone. The production of nitrogen oxides, however, has lost its importance and is rather undesirable. The plasma cutter , which also works without a vessel, works with significantly increased pressure and higher current densities.
Plasma spraying (thermal coating process) is just as important . DC arc plasmas at atmospheric pressure are used for this.
The splicing of optical waveguides also works at atmospheric pressure in a normal atmosphere, but does not require a nozzle, instead a gas discharge with a low current density burns between the electrodes, which are cold compared to arc welding .
Generation of the plasma
There are different types of excitation:
- AC ( alternating current ) excitation
- low-frequency alternating currents
- Excitation with alternating currents in the radio wave range
- Microwave excitation
- DC ( direct current ) excitation
One notable industrial importance have only atmospheric pressure plasmas obtained, which are generated by AC excitation ( corona discharge and plasma nozzles). The plasma nozzle is discussed in more detail in the following section. Another important design of the plasma source is the dielectrically impeded discharge (DBE), which is used to generate ozone or to treat plastics. Further plasma generators can be found in Tendero et al., See literature .
Functional principle of a plasma nozzle
A pulsed arc is generated in the plasma nozzle by means of a high voltage discharge (5 - 15 kV, 10 - 100 kHz) . A process gas, usually oil-free compressed air, is used, which flows past this discharge section, is excited and converted into the plasma state. This plasma then passes through a nozzle head onto the surface of the material to be treated. The nozzle head is at ground potential and thus largely holds back potential-carrying parts of the plasma flow. In addition, it determines the geometry of the exiting jet.
Applications
Industrial application finds the plasma among other things, activation and cleaning of plastic - and metal surfaces prior to bonding - pressure - and varnishing processes. Even web products with treatment widths of several meters can be treated by lining up many nozzles. The modification of the surface achieved by the plasma nozzle can definitely be compared with the effects achieved in the low-pressure plasma.
The plasma jet can, depending on the power of the nozzle, be up to 40 mm long and achieve a treatment width of 15 mm. Special rotation systems allow a treatment width per nozzle tool of up to 13 cm.
Depending on the required treatment performance, the plasma source is moved at a distance of 10 - 40 mm at a speed of 5 - 400 m / min relative to the surface of the material to be treated.
A major advantage of these systems is their so-called inline capability, which means that they can usually be installed in existing production systems without any problems. The costs, however, are comparatively low.
In addition, the achievable activation is significantly higher than with potential pretreatment methods (corona discharge).
A wide variety of surfaces can be coated with these or similar systems. Corrosion protection layers and adhesion promoter layers can be applied to various metals without solvents and therefore very environmentally friendly.
Individual evidence
- ↑ gordonengland.co.uk: Plasma Spray - Thermal Spray Coating Process
- ↑ M. Noeske, J. Degenhardt, S. Strudthoff, U. Lommatzsch: Plasma Jet Treatment of five Polymers at atmospheric Pressure: Surface Modifications and the Relevance for Adhesion. In: International Journal of Adhesion and Adhesives. 24 (2), 2004, pp. 171-177.
literature
- C. Tendero, C. Tixier, P. Tristant, J. Desmaison, P. Leprince: Atmospheric pressure plasmas: A review. In: Spectrochimica Acta. Part B: Atomic Spectroscopy. 2005.
- R. Kovacs, N. Bibinov, P. Awakowicz, H.-E. Porteanu, S. Kühn, R. Gesche: An Integrated Atmospheric Microwave Plasma Source. In: Plasma Processes and Polymers. No. 6, 2009.