EP3430864B1 - Plasma nozzle and method of using the plasma nozzle - Google Patents

Description

The invention relates to a plasma nozzle for generating a plasma jet with at least one electrode, a casing concentrically surrounding the electrode, a discharge chamber being formed between the electrode and the casing, and the discharge chamber having an inlet opening for a process gas and a nozzle opening for the exit of the plasma jet and the sheathing contains or consists of a dielectric and a first counter-electrode ring-shaped surrounding the discharge chamber in a first longitudinal section. The invention also relates to a method for using the plasma nozzle for treating a surface of a workpiece or a workpiece with the plasma jet thus generated.

In the case of such a treatment of a workpiece, surface activation and / or functionalization and ultra-fine cleaning of the treated surface by the plasma jet take place. The plasma can be operated with an inert gas or a reactive gas, so that physical and / or chemical processes take place on the surface of the workpiece to be treated. This improves the adhesion between materials, for example in the case of paintwork or adhesive bonds on plastic surfaces.

The application of plasmas to non-conductive workpieces of high material thickness or complicated geometries can be carried out, for example, with known plasma nozzles (jet systems) according to US 6,677,550 B2 or U.S. 5,961,772 be made. The sheathing itself forms the counter-electrode, so that thermal arc discharges form between the electrode and the counter-electrode after exposure to a high voltage. The resulting plasma is expelled from the nozzle by the gas flow.

Disadvantages of these jet systems are strong electrode burn-off that occurs during operation, high temperatures caused by the thermal arc discharges expelled from the nozzle opening as a plasma jet, and a lack of cleaning action or low efficiency. These disadvantages occur in particular at high process speeds, ie when the workpiece is passed over by the plasma jet at high speed. In these jet systems, the reactive species present in the plasma and intended for surface treatment, ie, excited molecules, atoms and / or ions, are not generated directly on the surface to be treated, since the high temperatures in thermal arc discharges would damage the surface. In order to avoid such physical effects of the plasma filaments of the arc discharges on the surface, the arc discharges are generated at some distance from the surface. These jet systems are therefore referred to as remote systems, and the plasmas generated in the process are referred to as "remote plasma". Since the reactive species are not generated directly on the workpiece surface, but at some distance from it, these species can react on the way to the surface, for example with the ambient air or the electrode walls of the remote system. So only a small proportion of this species can react with the surface. This reduces the efficiency of the plasma treatment. Further devices for generating a plasma jet by means of a dielectric hindered discharge can be found in the patents KR 2014 0101235 A , JP 2007 207475 A and CN 201 528 464 U known.

The use of known plasma jets enables activation, but not ultra-fine cleaning of the surface. High process gas consumption, the necessary cooling of the process gas and a small effective discharge area make this process a process that is only economical to a limited extent.

Alternatively, devices for generating sliding discharges on the surface of the workpiece to be cleaned or the formation of dielectrically impeded discharges on the surface of the workpiece to be cleaned are known from the prior art. In the case of sliding discharges, high-voltage electrodes arranged parallel to the surface to be treated are at different electrical potentials and use the workpiece to be treated as a bridging medium for generating discharge filaments. Depending on the process gas used, only sliding discharges can be achieved with discharge distances of around 1 mm. These methods are therefore only suitable for flat surfaces with low shape tolerances, since the plasma cannot penetrate into undercuts or depressions and the distance between the high-voltage electrodes must be precisely regulated. In the case of dielectrically impeded or barrier discharges, the workpiece held in the discharge gap must also be thin in order to generate and maintain the discharge with manageable electrical voltages.

The object of the present invention is thus to provide a cleaning method and a cleaning device for activating or ultra-fine cleaning of a surface, which allows an improved, in particular more efficient treatment of the surface and can be used universally on flat or even uneven surfaces of workpieces.

The object is achieved according to the invention by a device according to claim 1 and a method according to claim 10. Advantageous further developments of the invention can be found in the subclaims.

According to the invention, a plasma nozzle for generating a plasma jet is proposed. This contains at least one electrode. In some embodiments of the invention, the electrode can be elongated or rod-shaped. The electrode can be hollow or contain a cavity.

The at least one electrode is surrounded by a casing, so that a discharge chamber is formed between the electrode and the casing. In some embodiments of the invention, the casing and the electrode can be arranged concentrically with one another. According to the invention it is proposed that the sheathing contain or consist of a dielectric.

The discharge chamber further has at least one inlet opening for a process gas and a nozzle opening for the exit of the plasma jet.

Furthermore, the device according to the invention has at least one first counter-electrode, which surrounds the discharge chamber in a ring-shaped manner in a first longitudinal section. The first counter electrode is electrically insulated from the electrode by the sheathing. In addition, there is a second counter-electrode which surrounds the discharge chamber in a second longitudinal section in an annular manner. The counter-electrodes can also be arranged concentrically to the electrode and the casing.

A method according to the invention for using or operating a plasma nozzle according to the invention has the following method steps:
Supplying a process gas having a flow through the inlet opening into the discharge chamber and generating an electrical potential difference between an electrode and / or a first counterelectrode and / or a second counterelectrode, so that a plasma is formed in the discharge chamber by igniting dielectrically impeded discharges in the process gas in the discharge chamber, and generating a plasma jet from the flow plasma expelled from the nozzle opening.

A plasma jet is thus generated by means of a dielectrically impeded discharge (Dielectric Barrier Discharge or DBD) (DBD-Jet). The discharge filaments of the DBD are generated by applying a high electrical voltage between the electrode and the counter electrode or electrodes. In the case of a DBD, discharge filaments are generated in the process gas in which only low temperatures prevail relative to thermal arc discharges. The plasmas generated in this way are therefore non-thermal plasmas in which the reactive species generated are not in thermal equilibrium with their locally surrounding process gas. Due to the dielectric hindrance, the discharge process in the individual discharge filaments is so short in time that no thermal equilibrium can develop. Such plasmas, or a plasma jet generated from them, can therefore be used as directly acting plasmas for surface treatment. The possibility of generating plasma close to the surface without damaging the surface enables efficient treatment.

In contrast to known systems for plasma activation, which often use the workpiece as a dielectric barrier, thicker workpieces can also be processed with the device according to the invention, since the non-conductive workpiece becomes a counter potential for the second counter electrode. The plasma nozzle according to the invention thus combines the Advantages of well-known plasma jets such as high gap clearance with the efficiency of sliding discharges or barrier discharges on the surface of the workpiece.

The plasma nozzle according to the invention therefore also enables workpieces to be treated on an industrial scale, since greater spacing tolerances between the plasma nozzle and workpiece are possible than with known sliding discharges and greater workpiece thicknesses than with known barrier discharges. High spacing and shape tolerances, which occur especially with mass-produced goods such as semi-finished extrusions, require pretreatment systems in inline operation that implement efficient activation / functionalization in the highest possible tolerance ranges and, at the same time, perform ultra-fine cleaning of the surface. The plasma nozzle according to the invention can be used as such a pretreatment system. Environmentally harmful and costly wet chemical processes for surface treatment can thus be replaced. The use of the plasma nozzle according to the invention in a manner similar to a sliding discharge source enables two-dimensional applications. Due to the sufficiently high discharge distances that are made possible between the workpiece surface and the mouth of the nozzle opening, workpieces with shape deviations can be treated at high speeds without damage. In addition, there is good clearance for geometrically complex workpieces, e.g. with undercuts, grooves, etc.

In some embodiments of the invention, the second counter electrode is arranged on the nozzle opening in such a way that the nozzle opening runs centrally through the counter electrode. Discharge filaments of the DBD are not only created in the discharge chamber, but also directly in the area of the nozzle opening and can thus affect the surface of the workpiece to be treated. In this way, both chemical and physical influences can occur on the surface occur. In addition, the reactive species generated in this way can interact directly with the surface in the plasma jet without first coming into contact with the ambient air or other reaction partners.

In some embodiments of the invention, the second counter-electrode can have a ring surface that is widened in the shape of a plate. This has the advantage that discharge filaments can be generated directly on a surface to be treated. The normal to the plate surface should be roughly parallel to the direction of the plasma jet. With this embodiment of the invention, electrically insulating workpieces with material thicknesses of more than 10 mm can also be treated with a DBD, since the workpiece no longer has to be completely received in the discharge gap.

In some embodiments of the invention, the surface of the second counter electrode can be covered by the casing. The casing thus forms a continuous dielectric that shields the electrode surface from the discharge chamber and the workpiece. This allows simple design and manufacture and reliable operation of the plasma nozzle.

In some embodiments of the invention, the sheathing of a generic plasma nozzle is formed from a dielectric, the counterelectrodes being electrically insulated by the sheathing from the electrode for generating dielectrically impeded discharges in the process gas between the electrode and the counterelectrode. In some embodiments of the invention, the dielectric can contain a glass, a ceramic, or a polymer. In some embodiments of the invention, the polymer can be or contain polytetrafluoroethylene (PTFE).

In some embodiments of the invention, the electrode in the discharge space is coated with a dielectric. This additional coating causes an even more homogeneous discharge characteristic of the plasma generated by the DBD. The coating can contain or consist of a polymer or a ceramic. The coating can contain polytetrafluoroethylene or an oxide or an oxynitride.

In some embodiments of the invention, the inlet opening of at least one gas feed-through can be formed in the electrode. The gas feedthroughs can advantageously comprise a plurality of bores which are arranged radially symmetrically in the electrode, a uniform flow of the process gas through the discharge chamber is achieved. This also leads to a particularly homogeneous and stable plasma jet.

In some embodiments of the invention, a plasma nozzle housing can be present, the counter electrodes being fixed between the casing and the plasma nozzle housing by means of a potting compound. In this way, inexpensive production of the plasma nozzle is possible and parasitic discharges in the interior of the plasma nozzle are avoided.

In some embodiments of the invention, when the device is in operation, the first counter-electrode can be at a ground potential and the second counter-electrode can be at a floating potential. As a result, a plasma jet is expelled from the nozzle, which can have a range of more than 10 mm or more than 20 mm or more than 30 mm.

In some embodiments of the invention, when the device is in operation, the second counterelectrode can be at ground potential and the first counterelectrode can be at ground potential a floating potential. In this operating state, the plasma filaments slide over the underside of the plasma nozzle or over the partial surface of the casing that runs parallel to the plate surface of the second electrode. In this case, sliding discharges can be generated on the workpiece at a distance of about 1 mm to about 3 mm.

In some embodiments of the invention, the first counter-electrode and the second counter-electrode can be at a ground potential when the device is in operation. In this operating state, a discharge is generated between the first counter-electrode and the electrode in the interior of the discharge space, the plasma of which is expelled from the plasma nozzle. The second counter electrode forms a counter potential to the plasma emerging from the nozzle. In this way, sliding discharges are possible at a distance of about 3 mm to about 20 mm from the workpiece surface.

In some embodiments of the invention, the process gas can be selected from argon and / or helium and / or ambient air and / or synthetic air and / or gas mixtures of nitrogen and oxygen in a mixing ratio of 90/10 or 95/5 volume percent.

Possible surface treatments with the plasma jet of a plasma nozzle according to the invention are all previous areas of application in which plasma is used, e.g. surface functionalization, surface activation, layer deposition, ultra-fine cleaning and / or disinfection.

Fig. 1

einen Schnitt durch eine Ausführungsform einer erfindungsgemäßen Plasmadüse während deren Verwendung bei der Oberflächenbehandlung eines Werkstückes.

Fig. 2

die stiftförmige Elektrode der Plasmadüse aus Figur 1 als Detaildarstellung.

Fig. 3a

bis c erfindungsgemäße Plasmadüse aus Figur 1 in verschiedenen Betriebsmodi.

Fig. 4a

und b Qualitätsparameter von Arbeitsergebnissen bei der Oberflächenbehandlung von Werkstücken in Abhänggigkeit von Betriebsparametern des erfindungsgemäßen Verfahrens in jeweils einer Balkendiagrammdarstellung.

Fig. 5a

bis c photographische Aufnahmen des Plasmastrahls einer erfindungsgemäßen Plasmadüse.

Particular embodiments of the present invention are explained in more detail below with reference to the accompanying drawings. Show it:

Fig. 1

a section through an embodiment of a plasma nozzle according to the invention during its use in the surface treatment of a workpiece.

Fig. 2

the pen-shaped electrode of the plasma nozzle Figure 1 as a detailed representation.

Fig. 3a

to c plasma nozzle according to the invention Figure 1 in different operating modes.

Figure 4a

and b quality parameters of work results in the surface treatment of workpieces as a function of operating parameters of the method according to the invention, in each case in a bar diagram.

Figure 5a

to c photographic recordings of the plasma jet of a plasma nozzle according to the invention.

In Figure 1 a sectional representation of a plasma nozzle 1 according to the invention during its use in the surface treatment of a workpiece 3 with a plasma jet 5 is shown. The plasma nozzle 1 has a pin-shaped electrode 7 for applying a particularly pulsed high voltage. This pin electrode 7 is therefore also referred to below as the high-voltage electrode. The pin electrode 7 is concentrically surrounded by a casing 9 made of a dielectric material. The casing 9 has a cylindrical basic shape.

A discharge chamber 11 is formed between the electrode 7 and the casing 9. The discharge chamber 11 has an inlet opening 12 for a process gas, for example argon, helium and / or air, and a nozzle opening 14 for the plasma jet 5 to exit. The plasma jet 5 is made up of plasma generated in the discharge chamber 11 and expelled from the nozzle opening 14 by a flow of the process gas generated. The flowing process gas is shown symbolically in the figure by a block arrow in the area of the inlet opening 12.

Furthermore, a second metal counter-electrode 16 is provided which surrounds the discharge chamber 11 in a ring-shaped manner. The second counterelectrode 16 is electrically insulated from the electrode 7 by the dielectric sheathing 9 in order to generate dielectrically impeded discharges in the process gas between the pin-shaped electrode 7 and the counterelectrode 16. The second counter-electrode 16 can be connected to ground to generate the dielectrically impeded discharges, i. E. H. be electrically grounded, which is why it is also referred to below as a ground ring. The electrical contact is not shown in the figure.

The second counter electrode 16 is arranged in the region of the nozzle opening 14 in such a way that the nozzle opening 14 runs centrally through the second counter electrode 16. The second counter electrode 16 also has an annular surface 18 that is widened in the shape of a plate. In this case, widened in the shape of a plate is understood to mean a ring with a shape whose radial ring width is greater than its axial ring height. The ring surface 18 of the plate-shaped widened counter-electrode 16 facing away from the plasma jet direction is covered by the casing 9 and thus also shielded from the high-voltage electrode 7 by means of the dielectric of the casing 9. In this way, the plasma nozzle 1 has a plate-shaped widening (underside of the plate) in its lower area, ie in the area of the nozzle opening 14. The casing 9 is therefore also referred to below as a plate bar and the entire plasma nozzle 1 is also referred to below as a “disk jet”. The surface normals of the ring surface 18, which is widened in the shape of a plate, and the correspondingly shaped underside of the plate run approximately parallel to the exit direction of the generated plasma jet 5. The nozzle opening 14 extends radially centrally through the plate-shaped counter-electrode 16.

The plasma nozzle 1 furthermore has a first counter-electrode 20 made of metal, which can also be connected electrically to ground for operating the plasma nozzle 1. This first counter-electrode 20 also surrounds the discharge chamber 11 in a ring shape and is insulated from the rod-shaped electrode 7 by the casing 9. The first counter electrode 20 is arranged at the level of the rod-shaped electrode 7 and is also referred to below as the upper ground ring, whereas the second counter electrode 16 arranged in the area of the nozzle opening 14 is referred to as the lower ground ring. The electrical ground potential of the ground can also be replaced by any other potential and / or both ground rings can be placed on different potentials, i.e. electrically connected to different voltage sources. For safety and application reasons, however, the earth potential is to be preferred.

The plasma nozzle 1 can therefore be referred to as a "double-mass disk jet". A pre-discharge electrode is formed from the first counter-electrode 20 opposite the second counter-electrode 16 arranged in the region of the nozzle opening 14. That is, the plasma generated by discharges between the first counter-electrode 20 and the rod-shaped electrode 7 is transported by the flow of the process gas to the second counter-electrode 16 arranged in the area of the nozzle opening 14, which enables dielectrically impeded discharges to be ignited in the area of the last-mentioned second counter-electrode. ie a dielectrically impeded post-discharge is excited.

The high-voltage electrode 7 has gas feedthroughs 25 and thereby functions at the same time as a process gas feed, i.e. inlet opening 12. It is made of a conductive material, e.g. stainless steel, aluminum and / or brass. The plate bar serves as a dielectric and is made of a dielectric material e.g. ceramic, glass, and / or polymer. The upper ground ring is attached in the upper area of the plate rod sleeve, i.e. the cylindrical part of the casing 9 which forms the discharge chamber. Outside the discharge chamber 11, the ground rings are shielded from the high-voltage electrode 7 with an optional high-voltage-resistant potting compound 27 so that no parasitic discharges occur inside the part of the plasma nozzle 1 that forms an electrode head. The potting compound 27 also serves to fix the compound rings between the casing 9 and the housing 29 of the plasma nozzle 1.

On the basis of this arrangement, when the voltage is applied and the process gas supply is switched on, a plasma jet 5 with plasma generation from a dielectrically impeded discharge (DBD jet) can be ignited between the high-voltage electrode 7 and the upper ground ring 20. Depending on the process gas and applied electrical power, this DBD jet can be expelled from the nozzle opening 14 up to 50 mm. This DBD jet can, for example, also be ignited with compressed air as the process gas.

In the described discharge arrangement of the "disc jet", the concentric lower mass ring 16 forces filaments of discharges expelled from the nozzle opening 14 to ignite on the underside of the plate, ie on the underside of the casing 9, which is widened in the manner of a plate and faces the workpiece. In free operation, that is to say without a workpiece under the source, that is to say the nozzle opening 14, a discharge can thus ignite, which after acts on the principle of a repeater. In the latter principle, the discharge between the upper ground ring 20 and the high-voltage electrode 7, or the plasma of this discharge, generates a further discharge in the area of the lower ground ring 16 and is therefore repeated in the figurative sense. The filaments of the discharges in the area of the lower mass ring 16 can come into direct contact with the surface to be treated. Such surface discharges are more efficient than remote plasmas. The reactive species in the plasma react to a large extent on their way to the workpiece surface with remote plasmas, which results in great efficiency losses in the activation of the surface. The latter is not the case with surface discharges. In addition, the filaments come into contact with the surface in the event of surface discharges, which enables the surface to be finely cleaned.

One embodiment of the method according to the invention uses a DBD remote plasma as an ignition source and projects its potential onto the non-conductive surface of the workpiece. Since the non-conductive workpiece thus indirectly becomes a counter potential, also called floating potential, for the lower ground ring 16, tangential filaments of a discharge develop between the underside of the plate and the non-conductive workpiece. Depending on the distance between the underside of the plate and the workpiece, a largely homogeneous plasma discharge is formed over the surface of the underside of the plate (plate surface), supported by the back pressure of the continuously flowing process gas.

In Figure 2 the pin-shaped electrode 7 of the plasma nozzle is off Figure 1 shown as a detailed representation. The illustrated pin electrode 7 is designed to be radially symmetrical. On its electrode base 31 facing away from the tip 30 of the pin electrode 7, the pin electrode 7 has grooves 32, which serve to fix the pin electrode 7 in the casing 9 of the plasma nozzle. Furthermore, gas feedthroughs 25 are arranged radially symmetrically within the pin electrode 7 in the region of the electrode base 31. These form the inlet opening of the plasma nozzle for the process gas.

In the Figures 3a to c the plasma nozzle according to the invention is off Figure 1 shown in different operating modes during operation. Due to the electrode configuration with a high voltage electrode and two counter electrodes, namely the upper and the lower ground ring, the "Disk-Jet" can be operated in three different modes. In the Figures 3a to c the three modes are shown. The electrical contact is shown in the form of a circuit diagram.

If, with a high-voltage electrode connected to a high-voltage source, the ground is only applied to the upper ground ring, a pure DBD jet discharge is obtained with its plasma jet 40 in Figure 3a ("DBD-Jet" operating mode) is shown. This plasma jet 40 has a length of up to 50 mm for the process gases argon, helium or air.

On the other hand, as in Figure 3b If only the lower ground ring is in contact with ground, that is to say grounded, then discharge or plasma filaments 41 are obtained as a plasma jet which ignite directly from the high-voltage electrode onto the lower ground ring. In this mode, the plasma filaments 41 slide over the plate surface of the electrode. A clear interaction of the discharge filaments 41 with the workpiece surface is thereby possible. Realizable discharge distances to the workpiece to be treated are between 1 mm for air as the process gas and 3 mm for argon or helium as the process gas, depending on the process gas.

The most effective mode is in Figure 3c shown ("Disc-Jet" operating mode). Both earth rings are electrically connected to earth. As described, the lower ground ring acts as a "repeater". The remote plasma generated between the high-voltage electrode and the upper ground ring is blown out of the nozzle opening as a plasma jet and, with its floating potential, offers the lower ground ring a counterpotential. Discharge filaments 42 of a discharge ignite on the underside of the plate, which represents the dielectric for the lower ground ring. In this way, sliding discharges are possible with discharge distances from the workpiece to the nozzle opening of 3 mm with air as the process gas and up to 20 mm with argon or helium as the process gas. The discharge filaments interact both chemically and physically with the surface to be treated and cause surface activation / functionalization as well as ultra-fine cleaning of non-conductive workpieces of any thickness at the level of directly acting dielectrically impeded discharges.

Based on its design, the "Disc-Jet" can be easily integrated into existing processes and, thanks to its effectiveness, can also be used under industrial conditions. A plasma can be optimally applied using the "Disc-Jet" both on flat plate / foil material and on complex workpiece geometries or undercuts.

In the Figures 4a and b Quality parameters of work results in the surface treatment of workpieces as a function of operating parameters of the method according to the invention are shown in comparison in each case in a bar diagram. Figure 4a shows the peeling resistance of water-based paints on PVC in comparison with surface treatment in the "DBD-Jet" and "Disc-Jet" operating modes with argon process gas.

Comparisons between the "Disc-Jet" mode and a structurally identical DBD-Jet mode (ie the lower ground ring was omitted or not connected to ground, ie not electrically connected to ground potential) have shown that the "Disc-Jet" can for example be up to 100% better results achieved in the adhesive strength of water-based paints (the so-called adhesive pull strength [MPa]) on polyvinyl chloride (PVC) and polypropylene (PP). In Figure 4a the surface treatment results of the two operating modes at different process speeds are compared. The process gas is argon. At low treatment speeds, there are no significant differences in the adhesive strength of the lacquer layer. From 4 meters / min up to 32 meters / min there is a drastic decrease in the treatment results of the DBD-Jet mode, whereas the Disc-Jet mode shows significantly better results in terms of the adhesive strength of the paint. Its advantages are particularly evident at high treatment speeds, which are particularly relevant for industry. The discharge of the "Disc-Jet" has the character of a sliding discharge and can be realized in free operation, ie without a workpiece under the source, with discharge distances of up to 20 mm between the nozzle opening and the workpiece surface. A high level of clearance, ie penetration depth in surface gaps, is thus achieved on the basis of direct discharges, which enables the efficient treatment of profiled or structured surfaces or also workpieces with greater shape or position tolerances.

In Figure 4b results of the lift-off resistance of water-based paints on PVC in a surface treatment with a plasma nozzle according to the invention in disc-jet mode with two different process gases, namely argon and air, are compared. Profiles based on PVC were treated, which after treatment with water-based paint were painted. The use of argon as well as air enables a significant optimization of up to 300% of the lift-off strength of the paint compared to untreated surfaces at all treatment speeds.

In Figures 5a to c photographic recordings of the plasma jet of a plasma nozzle according to the invention are shown in the "Disc-Jet" operating mode.

Figure 5a shows the two-dimensional formation of the sliding discharge of the "disc jet" on a workpiece surface. In the tests carried out, the discharge distance is in accordance with Figures 4 constant 3 mm for flat treatment. A flat discharge over the workpiece surface can be clearly seen.

Figure 5b shows the discharge characteristics of the "Disk-Jet" in free operation (discharge without workpiece). The filaments emerge from the mouth of the nozzle opening and after a certain distance ignite again up to the plate surface of the "disk jet". Depending on how close the workpiece is to the underside of the source, ie the nozzle opening, the filaments slide over the surface of the workpiece and form a flat discharge that adapts to the size of the plate surface as the distance decreases.

Figure 5c shows the use of the "Disc-Jet" on a plastic profile with a 10 mm deep longitudinal groove with an undercut. Here, too, the plasma jet can be seen emerging from the nozzle mouth as a plasma bundle. This bundle forms a large base point on the bottom of the longitudinal groove, from which filaments ignite to the plate surface of the "disc jet".

Of course, the invention is not restricted to the embodiments shown. The above description is therefore not to be regarded as restrictive, but rather as explanatory. The following claims are to be understood in such a way that a named feature is present in at least one embodiment of the invention. This does not exclude the presence of further features. If the claims and the above description define "first" and "second" embodiments, this designation serves to distinguish between two similar embodiments without defining a priority. The invention is defined by the claims.

Claims (15)

1. Plasma nozzle (1) for generating a plasma jet (5) comprising at least one electrode (7),

   a sheathing (9) concentrically surrounding the electrode (7), wherein a discharge chamber (11) is formed between the electrode (7) and the sheathing (9), and wherein the discharge chamber (11) has an inlet opening (12) for a process gas and a nozzle opening (14) for emergence of the plasma jet (5) and the sheathing (9) contains or consists of a dielectric,

   a first counter-electrode (20) surrounding the discharge chamber (11) in a ring-shaped fashion in a first longitudinal section,

   wherein the first counter-electrode (20) is electrically insulated from the electrode (7) by the sheathing (9), and

   a second counter-electrode (16) is furthermore present, which surrounds the discharge chamber (11) in a ring-shaped fashion in a second longitudinal section,

   characterized in that the second counter-electrode (16) has a ring surface (18) widened in a plate-shaped fashion.
2. Plasma nozzle (1) according to claim 1, wherein the second counter-electrode (16) is arranged concentrically to the nozzle opening (14).
3. Plasma nozzle (1) according to claim 1 or 2, wherein the surface normal of the ring surface (18), widened in a plate-shaped fashion, of the second counter-electrode (16) runs approximately parallel to the exit direction of the producible plasma jet.
4. Plasma nozzle (1) according to any of claims 1 to 3, wherein the ring surface (18) is covered by the sheathing (9).
5. Plasma nozzle (1) according to any of claims 1 to 4, wherein the electrode (7) is coated with a dielectric, in particular a dielectric containing or consisting of polytetrafluoroethylene.
6. Plasma nozzle (1) according to any of claims 1 to 5, wherein the inlet opening (12) is formed by at least one gas passage (25) in the electrode (7).
7. Plasma nozzle (1) according to claim 6, wherein the gas passages (25) are arranged in the electrode (7) in a radially symmetrical fashion.
8. Plasma nozzle (1) according to any of claims 1 to 7, wherein the first counter-electrode (20) and the second counter-electrode (16) can be connected to different electric potentials.
9. Plasma nozzle (1) according to any of claims 1 to 8, wherein a plasma nozzle housing (29) is provided, wherein the counter-electrodes (16, 20) are fixed between the sheathing (9) and the plasma nozzle housing (29) by means of a sealing compound (27).
10. Method for using a plasma nozzle (1) according to any of claims 1 to 9, comprising the method steps of

    supplying a process gas having a flow through the inlet opening (12) into a discharge chamber (11), and

    generating an electric potential difference between an electrode (7) arranged in the discharge chamber (11) and a second counter-electrode (16) so that a plasma is formed in the discharge chamber (11) by igniting dielectrically hindered discharges in the process gas in the discharge chamber (11), and

    generating a plasma jet (5) from plasma expelled from the nozzle opening (14) by the flow,

    characterized in that

    the second counter-electrode (16) has a ring surface (18) which is widened in a plate-shaped fashion.
11. Method according to claim 10, wherein the plasma jet (5) acts on a surface of a workpiece (3).
12. Method according to any of claims 10 or 11, wherein furthermore a first counter-electrode is present, wherein the first counter-electrode (20) is at a ground potential and the second counter-electrode (16) is at a floating potential.
13. Method according to any of claims 10 or 11, wherein furthermore a first counter-electrode is present, wherein the second counter-electrode (16) is at a ground potential and the first counter-electrode (20) is at a floating potential.
14. Method according to any of claims 10 or 11, wherein furthermore a first counter electrode is present, wherein the first counter-electrode (20) and the second counter-electrode (16) are at a ground potential.
15. Method according to any of claims 10 to 12, wherein the process gas is selected from argon and/or helium and/or ambient air.

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