Atmospheric-pressure plasma
Encyclopedia
Atmospheric-pressure plasma (or AP plasma or normal pressure plasma) is the name given to the special case of a plasma
in which the pressure approximately matches that of the surrounding atmosphere
– the so-called normal pressure.
• AC (alternating current) excitation
• DC (direct current) and low-frequency excitation
• Excitation by means of radio waves
• Microwave excitation.
Only atmospheric-pressure plasmas with AC excitation, however, have attained any noteworthy industrial significance. These are generated by AC excitation (corona discharge and plasma jets). More detail about the plasma jet is presented in the following section. Other plasma generators can be found in Tendero et al. (see References).
By means of high-voltage discharge (5 – 15 kV, 10 – 100 kHz) in the plasma jet 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.
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. Today, special rotary systems allow a treatment width per jet tool of up to 13 cm.
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).
For some years now it has also been possible to coat the most varied surfaces by means of these systems. Thus, anticorrosive layers and adhesion promoter layers can be applied to many metals without the use of solvents and hence in highly environmentally friendly manner.
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Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
in which the pressure approximately matches that of the surrounding atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
– 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. Cost-intensive chambers for producing a partial vacuum as used in low-pressure plasma technology are eliminated.Generation of plasma
Various forms of excitation are distinguished:• AC (alternating current) excitation
• DC (direct current) and low-frequency excitation
• Excitation by means of radio waves
• Microwave excitation.
Only atmospheric-pressure plasmas with AC excitation, however, have attained any noteworthy industrial significance. These are generated by AC excitation (corona discharge and plasma jets). More detail about the plasma jet is presented in the following section. Other plasma generators can be found in Tendero et al. (see References).
Operating principle of a plasma jet
Operating scheme of a plasma jetBy means of high-voltage discharge (5 – 15 kV, 10 – 100 kHz) in the plasma jet 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.
Applications
The plasma jet is used inter alia 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 metres 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 certainly comparable to the effects obtained with low-pressure plasma.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. Today, special rotary systems allow a treatment width per jet tool of up to 13 cm.
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).
For some years now it has also been possible to coat the most varied surfaces by means of these systems. Thus, anticorrosive layers and adhesion promoter layers can be applied to many metals without the use of solvents and hence in highly environmentally friendly manner.
Web links
- Fraunhofer-Institut für Fertigungstechnik und Angewandte Materialforschung (IFAM): Plasmatechnik und Oberflächen (Plasma technology and surfaces) – PLATO
- Leibniz Institute for Plasma Science and Technology (INP Greifswald e.V.)
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