EP2415331B1 - Method and beam generator for creating a bundled plasma beam - Google Patents

Description

The invention relates to a jet generator for generating a collimated plasma jet by arc discharge with supply of a flowing working gas with two in the flow of the working gas spaced electrodes and a voltage source for generating a voltage between the electrodes, wherein the voltage source is a voltage pulse with an ignition voltage for the arc discharge and generates a pulse frequency that causes the arc to extinguish between each successive voltage pulses. Moreover, the invention relates to a method for activating and coating substrate surfaces with a beam generator according to the invention.

When workpiece surfaces are to be coated, painted or glued, pretreatment is often required to remove contaminants from the surface and / or alter the molecular structure to better wet the surface with liquids such as adhesives, paints and the like can.

For surface treatment and cleaning beam generators are used to produce a collimated plasma jet used in which a plasma jet is generated by applying a voltage in a nozzle tube between two electrodes by means of a non-thermal discharge from a working gas. The working gas is preferably under atmospheric pressure. Preferably, air is used as the working gas.

The pre-treatment and cleaning by means of plasma has numerous advantages, of which in particular the high degree of degreasing, the environmental friendliness, the suitability for almost all Materials that stand out for their low operating costs and excellent integration into the various production processes.

From the EP 0 761 415 B9 as well as the DE 195 32 412 C2 a generic beam generator for generating a collimated plasma jet is known, which has a cup-shaped housing made of plastic with a lateral supply for the working gas. In the housing coaxially a nozzle tube made of ceramic is held. In the interior of the pot-shaped housing, a pin electrode made of copper is centrally arranged, which protrudes into the nozzle tube. The outer periphery of the nozzle tube is surrounded outside the cup-shaped housing by a jacket of electrically conductive material, which forms a ring electrode at the free end of the nozzle tube. At the same time, the annular electrode delimits a nozzle opening whose diameter is smaller than the inner diameter of the nozzle tube, so that a certain constriction is achieved at the outlet of the nozzle tube.

A disadvantage of the known jet generator is the high thermal load of the surfaces to be treated. The voltage source requires a very high ignition voltage in the order of 10 to 30 kV. Another disadvantage is the low efficiency. This is due in particular to a low degree of ionization in the plasma. In addition, the working gas exiting the jet generator has a high temperature, while the electrons have a fairly low temperature. However, for the operation of jet generators for surface treatment, it is desirable to produce non-thermal plasmas in which the electrons have a much higher temperature than the heavy particles (molecules, atoms, ions). However, engineered non-thermal plasmas usually have a low degree of ionization.

From the US Pat. No. 6,225,743 B1 There is already known a method for producing plasma with a plasma generator for the treatment of objects, in which an arc between an anode and a cathode is ignited and ionized with this gas, wherein the arc is operated with voltage pulses. In the pauses between the voltage pulses, the voltage applied to the anode-cathode path is lowered below the arc voltage of the arc, so that the arc extinguishes during these pauses. Although the plasma pulses have a very high temperature, even relatively sensitive materials are said to tolerate the plasma pulses without harm.

Based on this prior art, the invention is based on the object to provide a beam generator of the type mentioned above, which generates in particular a non-thermal plasma with low temperatures exiting the jet generator plasma jet and for activation and coating of substrate surfaces using a plasma jet can be used, which is particularly suitable for powder coating process temperature-sensitive substrate surfaces. Furthermore, a compact design of the beam generator is desired. Finally, a method for activating and coating substrate surfaces using the beam generator is provided.

This object is achieved by a beam generator having the features of claim 1 and a method having the features of claim 13.

A compact design of the jet generator with simultaneous homogeneous flow of the working gas is achieved in that one electrode is formed as a pin electrode and an electrode as an annular electrode, concentric with the Pin electrode is a hollow cylindrical, insulated from the pin electrode sheath of electrically conductive material is disposed on one end side of the annular electrode defining a nozzle opening whose diameter is smaller than the diameter of the hollow cylindrical shell and at the opposite end of the supply for the Working gas is arranged.

The jet generator according to the invention can be used for activating and coating substrate surfaces using a plasma jet, since at least one inlet for the introduction of powders, in particular with particle sizes of 10 nm to 100 μm, is arranged in the region of the nozzle opening. The electrons of the plasma jet sputter the powder particles fed in and melt them due to the still relatively high temperature there, in particular the high electron temperature, of the plasma. Due to the energy consumption for the melting and on the way of the plasma to the nozzle opening, it comes to a cooling, so that the fine-grained, the coating of the substrate surface forming powder relatively cool reaches the substrate surface. The jet generator according to the invention is therefore particularly suitable for powder coating processes of temperature-sensitive substrate surfaces.

The bundled plasma jet in the flowing working gas is generated by an arc discharge. The arc represents a gas discharge between the two spaced-apart electrodes to which a sufficiently high voltage is applied to generate by impact ionization required for the gas discharge high current density. The gas discharge forms the plasma in which the heavy particles are partially ionized.

The ignition voltage is the electrical voltage required to initiate the gas discharge between the two electrodes. The ignition voltage is generated by the voltage source or derived from the voltage source from a primary source. In principle, DC and AC voltage sources, but preferably DC voltage sources, come into consideration for the invention. The decisive factor, however, is that the voltage source generates a voltage pulse which causes the arc to extinguish between two consecutive voltage pulses. In this case, voltage pulse means that the voltage delivered by the voltage initially rises from a lower value, preferably zero, to a maximum value which is greater than or equal to the ignition voltage and shortly thereafter returns to the lower value, preferably zero, decreases. The periodic sequence of voltage pulses is called the voltage pulse.

During each voltage pulse, the voltage falls far below the required ignition voltage, so that with each voltage pulse of the arc extinguished until the ignition voltage is reached again in the next voltage pulse and a new arc discharge takes place between the electrodes. By extinguishing the arc forced with each voltage pulse, a low temperature of the effluent working gas from the jet generator is generated at high electrode temperatures. The sudden discharge of the electrons when the high ignition voltage is reached generates a large number of highly accelerated electrons in the plasma which have a high electron temperature. After reaching or exceeding the ignition voltage flows between the two electrodes for a very short period of time from one nanosecond to 1000 nanoseconds a current with a maximum current intensity of 10 to 1000 amperes. The resulting high current density has a positive effect the so-called pinch effect. The pinch effect refers to the contraction of the high electric current plasma into a thin, compressed plasma tube or filament due to the interaction of the plasma stream with the magnetic field generated by it.

The voltage source is designed to generate a pulse frequency of the voltage pulse, preferably in a range between 10 kHz to 100 kHz, in particular in a range between 20 kHz to 70 kHz. At these pulse frequencies, it is ensured that the plasma generation and the plasma jet are not interrupted. By this measure, an uninterrupted activation and coating, in particular with powder, of substrate surfaces can be made with the jet generator according to the invention. The maintenance of the plasma jet despite extinction of the arc with simultaneously very low heat load of the substrate surface is preferably achieved with pulse frequencies in the range between 20 kHz to 70 kHz.

The distance between the electrodes of the jet generator and the pressure of the working gas is determined so that the aforementioned currents in the plasma are achieved at ignition voltages between 2 kV to 10 kV. The basis for the determination of the electrode spacing is the Paschen law, according to which the ignition voltage is a function of the product of the gas pressure of the working gas and the impact distance, that is the distance between the electrodes. Depending on the shape of the opposing electrodes and the working gas used, preferably air, correction parameters must be taken into account in the calculation.

The voltage pulses generated by the voltage source may be the same or alternating.

A preferred embodiment of the voltage source is characterized in that the voltage source has a power supply unit with a connection for an output voltage and two outputs for the voltage converted in the power supply unit, wherein at least one capacitor is connected in parallel with the outputs and connected to the power supply unit via at least one resistor connected is. Optionally, one of the outputs can be connected to ground potential and the common ground can be used as reference potential and connection for the capacitor. The power supply is an assembly that converts the input voltage provided by the power supply into the output voltage required by the beam generator.

The circuit of capacitor and resistor forces the extinction of the arc, in which the power output from the power supply is stored in the capacitor. The power delivered by the power supply is initially stored by the capacitor until the ignition voltage for the arc discharge is reached. Upon reaching the ignition voltage, it comes to gas discharge and the energy stored in the capacitor flows within a nanosecond to 1000 nanoseconds with a high current intensity of 10 amps to 1000 amps. Due to the at least one charging resistor, via which the at least one capacitor is connected to the power supply, not enough current flows to maintain the fed from the capacitor arc. As a result, the arc extinguishes automatically and the charging of the capacitor for the next voltage pulse starts again.

In the interest of a compact design and a further increase in the efficiency of the beam generator according to the invention, the power supply of the voltage source is preferably designed as a switching power supply. The switching power supply is characterized by the fact that deviating from conventional power supplies with 50- or 60-Hz transformer, the Mains voltage is converted into an AC voltage much higher frequency and after the transformation is finally not rectified. The operation of the transformer with higher frequency has the consequence that at the same power, the mass of the transformer can be significantly reduced. As a result, switching power supplies are more compact and lighter with the same performance. Furthermore, their efficiency is higher than conventional power supplies.

Particularly space-saving, the capacitor of the voltage source in the form of a shielded cable run, in which a first electrode connecting the voltage source electrical line is surrounded by an insulator which covers at least part of an electrically conductive shield, which is part of the electrically conductive Connection between the voltage source and the other electrode is, wherein the shield encloses an outer insulator.

The capacitance of the capacitor is preferably in a range of 1 nF to 200 μF

A further reduction of the temperature of the working gas can be achieved by flow optimization. For this reason, the jet generator according to the invention has as a means for generating a turbulent flow of the working gas a sleeve inserted from the front side into the hollow cylindrical shell surrounding the pin electrode of electrically insulating material, on the surface of which at least one arranged as a helix web is arranged, which is between the inner wall of hollow cylindrical shell and the surface of the sleeve forms a channel for the working gas. The pitch of the helical land can effectively influence the temperature of the plasma jet. A larger slope cools the plasma jet stronger, while a smaller slope leads to a warmer plasma jet. At a greater slope is the residence time of the working gas at the same flow rate due to the shorter flow path through the jet generator shorter, whereby the cooling effect of the working gas is amplified. At a lower pitch of the web designed as a helix, the residence time of the working gas at the same flow rate due to the longer flow path through the jet generator is longer, whereby the cooling effect of the working gas is reduced.

At the same time, the sleeve forming the duct for the working gas fixes the pole electrode in the electrically conductive jacket and ensures the required electrical separation between the pole electrode and the jacket. The sleeve is not only easy to install, but also leads to the desired compact dimensions of the pin-shaped jet generator.

Preferably, the inlets for the powder are located on a conically tapered in the direction of the annular electrode portion of the hollow cylindrical shell of the jet generator. The substrate temperature increase during and after the coating process with the fine-grained powder is well below 100 degrees Celsius. Nevertheless, a good adhesion of the applied powder is achieved when using the jet generator according to the invention. The substrate surface needs no special pretreatment. The surface cleaning is carried out by the plasma jet of the jet generator itself. The powders are, for example, metals, ceramics, thermoplastics or mixtures thereof, which are applied as functional layers, such as protective, wear or insulating layers.

Figur 1

eine schematische Darstellung eines ersten Ausführungsbeispiels eines Strahlgenerators,

Figur 2

ein zweites Ausführungsbeispiel eines Strahlgenerators,

Figur 3

ein drittes Ausführungsbeispiel eines Strahlgenerators,

Figur 4

eine schematische Darstellung des Verlaufs von Spannung und Strom der Spannungsquelle eines Strahlgenerators sowie

Figur 5

ein viertes Ausführungsbeispiel eines erfindungsgemäßen Strahlgenerators zur Pulverbeschichtung von Substratoberflächen.

The invention will be explained in more detail with reference to the figures. Show it:

FIG. 1

a schematic representation of a first embodiment of a jet generator,

FIG. 2

A second embodiment of a jet generator,

FIG. 3

A third embodiment of a jet generator,

FIG. 4

a schematic representation of the course of voltage and current of the voltage source of a jet generator and

FIG. 5

A fourth embodiment of a jet generator according to the invention for powder coating of substrate surfaces.

The beam generator (1) for producing a collimated plasma jet (2) comprises two electrodes (4, 5) arranged in the flow of a working gas (3) and a voltage source (6) for generating a voltage between the electrodes (4, 5). The working gas (3) is channeled in a hollow cylindrical jacket (7). In the cavity (7) enclosed by the cavity, the electrodes (4, 5) at a distance (8) are arranged to each other.

The voltage source (6) has a switched-mode power supply (9) with a connection (10) for the input voltage, in particular the mains voltage, and two outputs (11, 12) for the voltage converted in the switched-mode power supply (9). Parallel to the outputs (11, 12), a capacitor (13) is connected, which is connected to the switching power supply (9) via a resistor (14), also referred to as a charging resistor.

In the switched-mode power supply (9), the line voltage applied to the connection (10) is first rectified by a rectifier (15). Subsequently, the DC voltage from an inverter (16), also referred to as an inverter, converted into an AC voltage much higher frequency before it is fed to the primary winding of a transformer (17). The on the secondary side of the transformer (17) tapped, compared to the mains voltage higher voltage is fed to a further rectifier (18) rectifying the transformed AC voltage.

• Figur 4 zeigt in der linken Bildhälfte in einem Spannungs-/Zeitdiagramm die Ausprägung eines Spannungsimpulses (21) sowie in einem darunter dargestellten Strom-/Zeitdiagramm den Verlauf des sich im Plasma einstellenden Stromes des Strahlgenerators (1).

The operation of the jet generator (1) will be described below with reference to FIG. 4 explained in more detail:

• FIG. 4 shows in the left half of the diagram in a voltage / time diagram, the expression of a voltage pulse (21) and in a current / time diagram shown below the course of the adjusting in the plasma current of the beam generator (1).

The power delivered by the switched-mode power supply (9) is first stored by the capacitor (13) until, between the electrodes (4, 5), the ignition voltage (19) for the formation of the arc is applied between the electrodes (4, 5). Upon reaching the ignition voltage (19), the air gap (8) between the electrodes (4, 5) becomes conductive and the total stored in the capacitor (13) energy flows within about 10 ns, as from the current / time diagram in FIG. 4 can be seen from. In this case, the voltage between the electrodes (4, 5) collapses and drops to a lower value near 0 volts.

When the ignition voltage (19) is reached, a maximum current (20) flows in the arc between the electrodes (4, 5). Through the resistor (14) of the switching power supply (9) does not flow enough charge to maintain the arc. For this purpose, the resistor (14) is to be dimensioned such that less power flows from the switched-mode power supply to the capacitor (13) than simultaneously flows out via the arc between the electrodes (4, 5). This has the consequence that the arc between each two successive voltage pulses extinguished before it is ignited again with the reaching of the ignition voltage (19) in the next voltage pulse (21). The pulse frequency is preferably in a range between 1 kHz to 100 kHz, in the illustrated embodiment at 60 kHz.

FIG. 2 shows a further embodiment of a jet generator (1). As far as this with the beam generator (1) after FIG. 1 is consistent, reference is made to the statements there. Differences arise with regard to the arrangement of the electrodes within the jacket (7). A first electrode is designed as a pin electrode (22), while the second electrode arranged at a spacing (8) is designed as an annular electrode (23). The jacket (7) of electrically conductive material is arranged concentrically with the pin electrode (22) and insulated from the pin electrode (22). At the end face opposite the annular electrode (23), the feed (24) for the working gas (3) is arranged. The supply for the working gas (3) has a sleeve (25) of electrically insulating material which is inserted into the hollow cylindrical jacket (7) and has a pin electrode (22) on the surface of which a web (26) designed as a helix is arranged. between the inner wall (27) of the hollow cylindrical shell (7) and the surface (28) of the sleeve (25) forms a channel for the working gas (3). The working gas passing through the helix thus enters the annulus between the pin electrode (22) and the inner wall (27) of the jacket (7) in a turbulent flow. This turbulence leads to a particularly advantageous bundling and channeling of the plasma jet (2), which itself along the pin electrode (22) in the direction of the annular electrode (23) therethrough.

FIG. 3 a shows a jet generator (1) accordingly FIG. 2 in which the switched-mode power supply (9) is indicated merely by a symbol for the sake of clarity. The capacitor is how out FIG. 3 b recognizable, in this embodiment formed in that the electrode (22) to the switching power supply (9) connecting electrical line (29) by an insulator (30) is surrounded, at least over a partial length (31) an electrically conductive shield (32 ), which is part of the electrically conductive connection between the switching power supply (9) and the further electrode (23). The shield (32) in turn encases an outer insulator (33).

In FIG. 3 c the capacitance (34) formed by the shield (32) and the electrical line (29) is shown as an equivalent circuit diagram. It can be seen that through the partially shielded cable, parallel to the outputs of the switched-mode power supply, there is a capacitor which is connected to the switching power supply (9) via the resistor (14).

FIG. 5 finally shows a beam generator (1) accordingly Figures 2 and 3 , which is intended according to the invention for coating a substrate surface (35) with fine-grained powders. The hollow cylindrical jacket (7) has an end face conically tapered in the direction of the annular electrode (23) portion (36) in which two inlets (37) are arranged. At each of the two inlets (37) sets a line (38) for the fine-grained powder to which a powder / gas stream (39) is supplied. Via the inlets (37), the powder particles (40) enter the plasma jet (2), with which they leave the jet generator (1) through the ring electrode (23). In which the beam generator (1) with aligned on the substrate surface (35) In the nozzle opening (41) in the direction (42) is moved, the powder particles (40) are deposited on the substrate surface (35). The deposited on the substrate surface layer (43) is in FIG. 5 indicated.

LIST OF REFERENCE NUMBERS

No. description No. description 1 ray generator 29 electrical line 2 plasma jet 30 insulator 3 working gas 31 partial length 4 electrode 32 shielding 5 electrode 33 outer insulator 6 voltage source 34 capacity 7 coat 35 substrate surface 8th distance 36 conical section 9 Switching Power Supply 37 inlet 10 connection 38 management 11 output 39 Powder / gas stream 12 output 40 powder particles 13 capacitor 41 nozzle opening 14 resistance 42 direction 15 rectifier 43 deposited layer 16 inverter 17 transformer 18 rectifier 19 ignition 20 maximum current 21 voltage pulse 22 pin electrode 23 annular electrode 24 Feed for working gas 25 shell 26 web 27 inner wall 28 surface

Claims (13)

1. Beam generator (1) for the generation of a bundled non-thermal plasma beam by means of arc discharge under a feed of a flowing working gas (3), having two electrodes (22, 23) arranged in the flow of the working gas (3) at a distance (8) from one another and a voltage source (6) for the generation of a voltage between the electrodes (22, 23), wherein one electrode is formed as a pin electrode (22) and the voltage source (6) generates a voltage pulse with an ignition voltage (19) for the arc discharge and generates a pulse frequency which causes the respective arcs between two consecutive voltage pulses (21) to be cancelled, characterized in that

   - a hollow cylindrical sheath (7) made of electrically conducting material which is insulated against the pin electrode (22) and arranged concentrically with respect to the pin electrode (22),

   - at whose one end face is arranged an annular electrode (23) which delimits a nozzle opening (41) whose diameter is less than the diameter of the hollow cylindrical sheath (7) and

   - at whose opposite end face is arranged the intake (24) for the working gas (3),

   - and in the region of the nozzle opening (41) there is arranged at least one inlet (37) for the feeding of a powder.
2. Beam generator (1) according to Claim 1, wherein the pulse frequency lies in a range between 10 kHz - 100 kHz.
3. Beam generator (1) according to Claim 2, wherein the pulse frequency lies in a range between 20 kHz - 70 kHz.
4. Beam generator (1) according to any one of the Claims 1 to 3, wherein after reaching the ignition voltage (19) a current with a maximum amperage (20) of 10 A - 1000 A flows between the two electrodes (22, 23) within a period of 1 ns - 1000 ns.
5. Beam generator (1) according to any one of the Claims 1 to 4, wherein the ignition voltage (19) is between 1 kV - 10 kV.
6. Beam generator (1) according to any one of the Claims 1 to 5, wherein the voltage source (6) has a power supply unit with a connector (10) for an input voltage and two outputs (11, 12) for the input voltage converted in the power supply unit, wherein at least one capacitor (13) is connected in parallel to the outputs, which capacitor (13) is connected to the power supply unit by way of at least one resistor (14).
7. Beam generator (1) according to Claim 6, wherein the power supply unit is a switched power supply unit (9).
8. Beam generator (1) according to Claim 6 or 7, wherein one of the electrical leads (29) connecting the electrodes (22, 23) to the power supply unit (9) is enclosed by an insulator (30) which is sheathed at least on a partial length (31) by an electrically conducting screen (32) which is a component of the other electrically conducting connection between the power supply unit (9) and the further electrode (23), wherein the screen (32) sheaths an external insulator (33).
9. Beam generator (1) according to any one of the Claims 6 to 8, wherein the capacity of the capacitor (13, 29, 30, 32) lies within the range of 10 nF - µF 200.
10. Beam generator (1) according to any one of the Claims 6 to 9, wherein the intake (24) for the working gas (3) has means for creating a turbulent flow of the working gas (3).
11. Beam generator (1) according to Claim 11, wherein the means for creating a turbulent flow of the working gas (3) comprises, inserted into the end face of the hollow cylindrical sheath (7), a sleeve (25) supporting the pin electrode (22) made of electrically insulating material on whose surface (28) is arranged at least one rib (26) shaped as a spiral, which rib (26) forms a channel for the working gas (3) between the internal wall (27) of the hollow cylindrical sheath (7) and the surface (28) of the sleeve (25).
12. Beam generator (1) according to Claim 13, wherein the hollow cylindrical sheath (7) has on its end face a section (36) tapering conically in the direction of the annular electrode (23) and each inlet (37) is arranged in this section (36).
13. Method for activating and coating substrate surfaces with a beam generator (1) for the generation of a bundled non-thermal plasma beam according to one or more of the Claims 1 to 12, characterized in that powder with a particle size of 10 nm to 100 µm is fed in through the at least one inlet (37).

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