Atmospheric-pressure plasma jet

Aims of the invention

[0003] The present invention aims to provide a more efficient plasma jet device than known from the state of the art.

Summary of the invention

[0004] The present invention concerns an atmosphericpressure plasma jet comprising a cylindrical 2-electrode device or a parallel 3-electrode device. The 2-electrode device can be a tubular device comprising a central cylindrical metal electrode and an outer cylindrical metal electrode, said cylindrical metal electrodes being coaxial and defining a plasma discharge lumen, said device having an open (proximal) end and a closed (distal) end, said plasma discharge lumen being open to the atmosphere at said open end and comprising a gas flow feed opening at said closed end, a dielectric material interposed between said central cylindrical metal electrode and said outer cylindrical metal electrode and is characterised in that said dielectric barrier is radially extended at said open end.

[0005] One embodiment of the parallel device comprises a central flat or specially formed metal electrode and 2 outer metal electrodes, said electrodes being substantially parallel, i.e. at a constant (± 1 mm) distance and defining a plasma discharge lumen, said parallel device having an open (proximal) end and a closed (distal) end, said plasma discharge lumen being open to the atmosphere at said open end and comprising a gas flow feed opening at said closed end, a dielectric material interposed between said centrc.l metal electrode and said

PCI7BE2006/0 00008 outer metal electrodes and is characterised in that said dielectric barrier is outwardly extended at said open end. According to a specific embodiment, the outer electrodes are connected at the sides to form one electrode which is coaxial with the central electrode. This embodiment and the tubular embodiment are therefore two variations of the cylindrical device with one inner and one outer electrode.

[0006] The present invention concerns thus a plasma jet apparatus for performing plasma processing of an article. A cylindrical 2-electrode configuration and a parallel 3-electrode configuration are described. The cylindrical plasma j^et device comprises:

• An elongated central electrode, • An elongated cylindrical outer electrode surrounding said central electrode and being coaxial with said central electrode, • An electrical insulator coaxially disposed between said outer electrode and said central electrode, wherein a discharge lumen having a distal end and a proximal end is defined between said central electrode and said electrical insulator, • A. supply opening disposed at said distal end of said discharge lumen for supplying a plasma producing gas to said discharge lumen • A. power source for providing a voltage between said central electrode and said outer electrode wherein said electrical insulator extends in a radially placed ring at said proximal end beyond the outer surface of said outer electrode. The electrodes can be tubular and coaxial with a circular cross-section or the central electrode may be a flat, plate-shaped electrode, while the outer electrode has a front and a back side which are substantially parallel to the central electrode. In stead of a flat electrode, the parallel device may have a central electrode with — at the proximal end - a round extens.ion along the length of the electrode, while the outer electrode's front and back faces remain parallel to said central electrode.

[0007] According to a preferred embodiment, a supply canal is present through the central electrode for introducing reactive chemical compounds immediately into the plasma afterglow at the proximal end.

[0008] The 3-electrode parallel plasma jet device according to the invention comprises:

• A central electrode, for example a flat, plate-shaped electrode, • 2 outer electrodes at both sides of said central electrode and being substantially parallel to said central electrode, • 2 electrical insulators disposed substantially parallel between said outer electrodes and said central electrode wherein a discharge lumen having a distal end emd a proximal end is defined between said central electrode and said electrical insulators, • a supply opening disposed at the distal end of said discharge lumen, for supplying a plasma producing gas to said discharge lumen, • preferably, a supply canal through the central electrode for introducing reactive compounds immediately into the plasma afterglow at the proximal end, • a power source for providing a voltage between the central and the outer electrodes wherein said electrical insulators extend outwardly at the proximal end beyond the outer surface of the outer electrode.

[0009] In the plasma jet apparatus according to the present invention the electrical insulator preferably further extends towards the distal end at the outer surface of the outer electrode. Advantageously, the distance between an outer surface of the central electrode and the inner surface of the electrical insulator lies between 0,1 and 10 mm. The power source is preferably arranged to provide an AC or Pulse DC voltage between 1 and 10 kV for the tubular configuration and between 1 and 10 0 kV for the parallel configuration.

[0010] Another aspect of the present invention concerns a method for producing a plasma flow, comprising the steps of :

• Providing a plasma jet apparatus according to the present invention, • Providing a plasma gas flow through the supply opening, • Providing a reactive chemical compound (e.g. monomer) flow through the supply opening and/or through the central electrode introducing the reactive chemical compound in the plasma discharge at the open end of the plasma), and • Providing a voltage between 1 and 100 kV between the central electrode and the outer electrode.

Short description of the drawings

[0011] Fig. 1 represents a prior art plasma jet design.

[0012] Fig. 2 represents a schematic overview of the plasma jet device according to the present invention.

[0013] Fig. 3 represents a schematic overview of the parallel plasma jet device according to the present invention.

PCI7BE200 5/000008

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[0014] Fig. 4 represents a schematic overview of a special configuration of the embodiment with parallel electrodes .

[0015] Fig. 5 represents a number of possible crosssections of parallel plasma jet devices according to the invention.

Detailed description of the invention

[0016] State-of-the-art plasma jets, such as depicted in fig 1 usually comprise an outer electrode 11 and inner electrode 12, and a dielectric material 13 interposed there between.

[0017] The tubular embodiment of the present invention can be seen in figure 2 and concerns an atmospheric-pressure plasma jet with 2 coaxial, cylindrical electrodes (1, 2) and with one specifically formed electrical insulator in the form of a dielectric material 3. The dielectric barrier is extended at the proximal end of the plasma jet, preferably in the form of a U-shape extension 20. A plasma jet operates at temperatures between 30°C and 600°C and can be used for plasma cleaning, surface modification and surface coating. The U-shape dielectric material has major advantages for all these applications. A ring, so just a radial extension for the tubular configuration is also a preferable embodiment (without the return leg 21 of the 'U׳) . At the distal end of the device, is the supply opening 6, to supply plasma gas to the lumen defined between the central electrode and the dielectric material 3. Preferably, the central electrode 2 is connected to ground 8, while the outer electrode is connected to a voltage source 9. Electrode 1 connected to the ground and electrode 2 connected to a voltage source is also a possible embodiment. The embodiment where both electrodes are connected to a voltage source is also included in this invention. A supply canal 7 through the central electrode 2 can be present for introducing reactive compounds immediately into the plasma afterflow at the open end. The distance 4 between an outer surface of the central electrode and the inner surface of the electrical insulator lies between 0,1 and 10 mm. The distance 5 is the diameter of the homogenous plasma zone. The distance 50 is the height of said homogenous plasma zone, corresponding to the height of the external electrode 1.

[0018] The central electrode 2 and the outer electrode 1 can be cylindrical with a circular crosssection, i.e. tubular. Alternatively, the central electrode may be a flat electrode 2, while the outer electrode 1 comprises a front and backside 70, 71 (see fig. 5A) , connected at the sides72 ׳ to form one cylindrical outer electrode 1. The insulator 3 then also comprises front and backsides 73,74 parallel to the central electrode, and connected 75 at the sides to form one cylindrical insulator 3.

[0019] Figure 3 shows the plasma jet device according to the invention, equipped with 3 parallel electrodes. The device comprises a central electrode 15, and two parallel electrodes 16, 17 on either side of the central electrode. The figure shows a cut-through view of the device. The actual device is of course closed on the sides. Possible cross-sections are shown in figure 5B to 5D. The devices shown in figure 5B to 5D are closed at the sides by suitable insulating materials (not shown). The parallel device of figure 3 has two dielectric portions 18, 19 which are substantially parallel to the electrodes. At the distal end of the device, the supply opening 6 is present to supply a plasma producing gas to the discharge lumen defined between the central electrode and the insulators. A supply canal 7 through the central electrode 15 can be present for introducing reactive compounds immediately into the plasma afterflow at the open end. The central electrode 15 is connected to ground 8, while the outer electrodes 16,17 are connected to a voltage source 9. The embodiment where the outer electrodes 16, 17 are connected to ground and the central electrode 15 is connected to a voltage source is also included :.n this invention. Also, the embodiment where both the central electrode 1.5 as the outer electrodes 16, 17 are connected to a voltage source are included in this invention. At the proximal end of the device, the dielectric portions are produced with an outward extension 40, preferably in the shape of a U, or with a flat outward extension, so without the returning leg 41 of the 'U׳ . The distance 4 between an outer surface of the central electrode and the inner surface of the electrical insulator lies between 0,1 and 10 mm. The distance 5 is the width of the homogenous plasma zone. The distance 60 is the height of said homogenous plasma zone, corresponding to the height of the external electrodes. The distance 61 is the length of the plasma zone, corresponding to the length (depth) of the device.

[0020] Fig. 4 shows a possible special configuration of the parallel plasma jet device according to the invention. In this configuration, there is a round extension 30 along the entire length of the central metal electrode 15 at the said open end of the plasma jet. As shown in Fig. 4 both the specifically formed dielectric material (18,19) and the outer metal electrodes (16,17) have a special form in order to guarantee a constant (± 1 mm) distance between the outer surface of the central electrode and the inner surface of the electrical insulator. Reference 60 shows the height of the plasma jet, 5 the broadness of the homogenous effective plasma afterglow and 61 the length of the plasma zone in between the parallel electrodes. Because of the round extension

30, the concentration of the afterglow and thus the plasma density in the afterglow are increased.

[0021] In general, the following operating characteristics can be used when using the plasma jet according to the present invention:

- Electric power for the tubular device with an electrode height 50 of 10 cm (from here called tubular device) : 2 0 - 750 Watt;

- electric power for the parallel device (including parallel device with one outer electrode) with an electrode height (50,60) of 10 cm and an electrode length (61) of 10 cm (from here called parallel device): 100 - 5000 Watt. Applied power is dependent upon application.

- Electric voltage (8): 1 - 100 kV

- Plasma gas flow (6) : 1 - 400 1/min for the tubular device, 10 - 4000 1/min for the parallel device.

- Temperature preheated plasma gas: 20 - 400 °C. (This means the plasma gas can be preheated up to 400°C before being inserted in the plasma jet).

- Plasma gases: N₂, Air, He, Ar, C0₂ + mixture of these gases with H₂, 0₂, SF₆, CF₄, saturated and unsaturated hydrocarbon gases, fluorinated hydrocarbon gases..

- Monomer flow: 1 - 2 000 g/min (through canal 7 in the central electrode immediately into plasma afterglow) .

- Feed gas flow: 0.1- 3 0 1/min (through canal 7 in the central electrode immediately into plasma afterglow) .

- Inner gap distance (4): 0.1 - 10 mm (dependent upon plasma gas and application).

- Diameter (for tubular device) or broadness (5) (for parallel device) of the homogeneous plasma zone: ¢5 - 80 mm.

- Length of effective plasma afterglow: 5 - 100 mm. (dependent upon application).

[0022] When a high voltage AC or pulsed DC power is put on one of the electrodes, a dielectric barrier discharge takes place in between the dielectricum and the inner electrode. The active species from the plasma are blown out of the plasma jet t>y the plasma gas flow. This afterglow is directed against a sample and this way 3-D objects can be plasma treated. In case a pulsed DC power is used, the frequency is preferably comprised between 1 and 200 kHz, and advantageously between 50 and 100 kHz

[002 3] The advantages of the radially or outwardly extending dielectricum from the plasma jet apparatus according to the present invention can be summarised with the following 3 concepts: distance to the plasma source, width of activation and consumption of plasma gases.

Distance to the plasma source

[0024] It should be noted that radicals, and particularly ions, in the plasma discharge are extremely short lived, and can almost not be transported outside the discharge region. Metastable species produced insic.e the plasma, on the other hand, have longer lifetimes at atmospheric pressure, typically in the order of hundreds of milliseconds. This longer lifetime allows them to be carried out of the plasma volume with the plasma gas flow. Obviously the most reactive metastable species will be lost first. The closer to the plasma source the more reactive the plasma afterglow. With the novel plasma jet apparatus according to the present invention, samples can be brought up to 2 mm from the actual plasma source. Experiments have shown that stable activation of certain polymers can only be realised when using the described plasma jet configuration with the radially or outwardly extending dielectricum.

Examples

Plasma activation of rubber:

[0025] Rubber is impossible to activate sufficiently with the classical concept: the distance rubber/plasma source seems to be too large. The most reactive and in this case needed species of the plasma are lost before they hit the rubber sample.

[0 026] When using a U-shaped dielectricum such as in fig. 2, more reactive plasma afterglow is obtained Parameters:

- Power: 400 Watt

- Frequency: 70kHz

- Plasma gas: 65 1 air /min

- Precursor: none

- Temperature plasma after glow: 65°C

- distance rubber/plasma source: 4 mm

- surface energy before plasma activation: ±20 dynes.

- surface energy after plasma activation: > 75 dynes.

- surface energy 1 week after plasma activation: 62 dynes.

PCI7BE2006/000008

Plasma activation of PVC:

[0027] PVC is thermal. sensitive. The activation performed with the classical concept is not stable in time.

After a few hours, activation was completely lost.

[0028] When using a U-shaped dielectricum, more reactive plasma afterglow is obtained.

Power: 300 Watt

Frequency: 32kHz

Plasma gas: 601 N₂ / min.

precursor: none.

Temperature plasma afterglow: 60°C.

distance PVC/plasma source: 5-7 mm.

- surface energy before plasma activation: 45 dynes, surface energy after plasma activation: > 75 dynes, surface energy 1 week after plasma activation: 64 dynes, surface energy 1 month after plasma activation: 56 dynes .

surface energy 4 months after plasma activation: 54 dynes .

Width of activation

[0029] If flat samples are brought close to a plasma afterglow, the active species of the plasma afterglow are spread out over a certain region in between the plasma jet and the samples. This means that the activated spot can be much broader than the diameter of the plasma jet. The closer the samples are brought to the actual plasma source, the broader the activated spot will be. Experiments have confirmed that with the plasma jet according to the invention (with U-shaped dielectricum) this activated spot for the same plasma conditions is much broader than with the classical concept.

Plasma activation of polyethylene:

[0030] Increasing the broadness of the activated spot would decrease the overall working costs of a (multi-) plasma jet. When using a plasma jet according to the

Examples

<td> present</td><td> invention,</td><td> more</td><td> reactive plasma</td><td> afterglow</td><td> is</td>

<td colspan="2"> obtained and active region. 10 - Power: 200 Watt Frequency: 50 kHz Plasma gas: 50 1</td><td> species N₂ /min</td><td> are spread out</td><td colspan="2"> over a broader</td>

Precursor: none

- Temperature plasma after glow: 65°C

- diameter plasma jet: 15 mm surface energy before plasma activation: 32 dynes surface energy after plasma activation: 62 dynes

<td> Distance sample/plasma source (mm):</td><td> Broadness of homogenous activated spot (mm) (62 dynes):</td>

<td> 2,5</td><td> 45</td>

<td> 4</td><td> 41</td>

<td> 6</td><td> 25</td>

<td> 8</td><td> 22</td>

<td> 10</td><td> 22</td>

<td> 12,5</td><td> 22</td>

<td> 15</td><td> 22</td>

<td> 20</td><td> 18</td>

<td> 30</td><td> 7</td>

<td> 35</td><td> 3</td>

[0031] With the classical concept the broadness of homogenous activated spot was maximum 32 mm at 1,5 mm distance sample/plasma jet.

Plasma activation of polypropylene:

[0032] Increasing the broadness of the act.ivated spot would decrease the overall working costs of a (multi-) plasma j®t. When using a plasma jet according to the p!־esent invention, more reactive plasma afterglow is obtained and active species are spread out over a broader region.

Power: 200 Watt

Frequency: 50 kHz

Plasma gas: 50 1 air /min

Precursor: none

Temperature plasma after glow: 65°C diameter plasma jet: 15 mm surface energy before plasma activation: 36 dynes, surface energy after plasma activation: 70 dynes.

<td> Distance sample/plasma source (mm):</td><td> Broadness of homogenous act.ivated spot (mm) (70 dynes):</td>

<td> 2,5</td><td> 48</td>

<td> 4</td><td> 45</td>

<td> 6</td><td> 26</td>

<td> 8</td><td> 22</td>

<td> 10</td><td> 22</td>

<td> 12,5</td><td> 22</td>

<td> 15</td><td> 22</td>

<td> 20</td><td> 20</td>

<td> 30</td><td> 12</td>

<td> 35</td><td> 4</td>

PCI7BE2006/(O0008

[0033] With the classical concept the broadness of homogenous activated spot was maximum 33 mm at 1.5 mm distance sample/plasma jet.

Consumption of plasma gases / plasma power

[0034] As a consequence of the fact that the samples can be brought closer to the actual plasma zone, less reactive species are lost in the afterglow. So compared to 10 the classical plasma jet, the same effect can be obtained with a lower consumption of gas and/or power. This last advantage can be seen as an indirect consequence of the two former advantages.

[0035] It has been shown experimentally that one needs less gasses and/or power for the same plasma activation effect. Such experiments can be performed by the skilled person.