Atmospheric-pressure plasma

Atmospheric-pressure plasma (or AP plasma or normal pressure plasma) is a plasma in which the pressure approximately matches that of the surrounding atmosphere – the so-called normal pressure.

Technical significance

Atmospheric-pressure plasmas have prominent technical significance because in contrast with low-pressure plasma or high-pressure plasma no reaction vessel is needed to ensure the maintenance of a pressure level differing from atmospheric pressure. Accordingly, depending on the principle of generation, these plasmas can be employed directly in the production line. The need for cost-intensive chambers for producing a partial vacuum as used in low-pressure plasma technology is eliminated.[1]

Plasma generation

Various forms of excitation are distinguished:

Atmospheric-pressure plasmas that have attained any noteworthy industrial significance are those generated by DC excitation (electric arc), AC excitation (corona discharge, dielectric barrier discharge, piezoelectric direct discharge and plasma jets as well as 2.45 GHz microwave microplasma).

Operating principle of a DC plasma jet

By means of a high-voltage discharge (5–15 kV, 10–100 kHz) a pulsed electric arc is generated. A process gas, usually oil-free compressed air flowing past this discharge section, is excited and converted to the plasma state. This plasma then passes through a jet head to arrive on the surface of the material to be treated. The jet head is at earth potential and in this way largely holds back potential-carrying parts of the plasma stream. In addition, it determines the geometry of the emergent beam.

Operating principle of a microwave plasma jet

Based on transistor amplifiers up to 200W power RF (radio frequency) and microwave sources are used to generate a microwave plasma. Most of the solutions work at 2.45 GHz. Meanwhile, is a technology developed which provide the ignition on the one hand and the high efficient operation on the other hand with the same electronic and couple network.[2] This kind of atmospheric-pressure plasmas is different. The plasma is only top of the electrode. That is the reason the construction of a cannula jet was possible.

Applications

The plasma jet is used, among other things, in industry for activating and cleaning plastic and metal surfaces prior to adhesive bonding and painting processes. Even sheet materials having treatment widths of several meters can be treated today by aligning a large number of jets in a row. In doing so the modification of the surface achieved by plasma jets is comparable to the effects obtained with low-pressure plasma.[3]

Depending on the power of the jet, the plasma beam can be up to 40 mm long and attain a treatment width of 15 mm. Special rotary systems allow a treatment width per jet tool of up to 13 cm.[4] Depending on the required treatment performance, the plasma source is moved at a spacing of 10–40 mm and at a speed of 5–400 m/min relative to the surface of the material to be treated.

A key advantage of this system lies in its capability of being integrated in-line. This means that it can usually be installed without any difficulty in existing production systems. In addition the activation achievable is distinctly higher than in potential-based pretreatment methods (corona discharge).

It is possible to coat varied surfaces by means of this technique. Thus, anticorrosive layers and adhesion promoter layers can be applied to many metals without the use of solvents and hence in an environmentally friendly manner.

See also

References

  1. ↑ Wolf, Rory A., Atmospheric Pressure Plasma for Surface Modification, Wiley, 2012
  2. ↑ Heuermann, Holger; et al. (June 2012). Various applications and background of 10-200W 2.45GHz microplasmas. 60th International Microwave Symposium. doi:10.1109/MWSYM.2012.6259386.
  3. ↑ Noeske M., Degenhardt J., Strudhoff S., Lommattzsch U.: Plasma Jet Treatment of five Polymers at Atmospheric Pressure: Surface Modifications and the Relevance for Adhesion; International Journal of Adhesion and Adhesives; 24 (2) 2004, pp. 171–177
  4. ↑ Buske C., Förnsel P.: Vorrichtung zur Plasmabehandlung von Oberflächen (Device for the plasma treatment of surfaces); EP 0986939
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