Abstract:
A plasma jet apparatus for performing plasma processing of an article includes: an elongated central electrode ( 2,15 ), an elongated cylindrical outer electrode ( 1 ) or two outer electrodes ( 15,16 ) surrounding the central electrode and being coaxial with the central electrode, or two electrodes substantially parallel to the central electrode. an electrical insulator ( 3 ) or insulators ( 18,19 ) are disposed between the outer electrode(s) and the central electrode, wherein a discharge lumen having a distal end and a proximal end is defined between the central electrode and the electrical insulator(s). A supply opening ( 6 ) is disposed at the distal end of the discharge lumen for supplying a plasma producing gas to the discharge lumen, A power source ( 9 ) provides a voltage between the central electrode and said outer electrode. The electrical insulator has a radial or outward extension ( 40,20 ) at the proximal end beyond the outer surface of the outer electrode(s).

Description:
FIELD OF THE INVENTION 
     The present invention is related to a plasma processing apparatus usable for plasma cleaning, surface modification and surface coating. More in particular, the present application is related to a novel plasma jet. 
     STATE OF THE ART 
     Atmospheric-pressure plasma jets are known in the art, e.g. as described by WO 98/35379 or WO 99/20809. These plasma jet devices comprise two coaxially placed electrodes defining a plasma discharge space between the outer diameter of the centrally placed electrode and the inner diameter of the outer electrode. A plasma jet can be generated at an open end of the device by introducing a flow of gas at a closed end of the device while a sufficient voltage is applied between the electrodes. Between said electrodes, a dielectric material can be placed to avoid arcing. The jet of plasma can be used to etch, clean or coat a surface. In the prior art devices, it is difficult to obtain a reasonably efficient plasma jet, due to several constraints of the currently known devices. For example, it is currently impossible to activate rubber sufficiently with a reasonably sized state-of-the-art classical plasma jet due to insufficient energy output. Most plasma jet devices therefore use nozzles to converge the plasma jet in order to obtain higher plasma densities. This however has the disadvantage that the treated spot is smaller and more devices, more time, or larger devices are necessary to treat a specific surface. 
     AIMS OF THE INVENTION 
     The present invention aims to provide a more efficient plasma jet device than known from the state of the art. 
     SUMMARY OF THE INVENTION 
     The present invention concerns an atmospheric-pressure 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. 
     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 central metal electrode and said 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. 
     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 jet 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 extension along the length of the electrode, while the outer electrode&#39;s front and back faces remain parallel to said central electrode.
       

     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. 
     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 and 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
       

     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 100 kV for the parallel configuration. 
     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 
         FIG. 1  represents a prior art plasma jet design. 
         FIG. 2  represents a schematic overview of the plasma jet device according to the present invention. 
         FIG. 3  represents a schematic overview of the parallel plasma jet device according to the present invention. 
         FIG. 4  represents a schematic overview of a special configuration of the embodiment with parallel electrodes. 
         FIG. 5  represents a number of possible cross-sections of parallel plasma jet devices according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. 
     The tubular embodiment of the present invention can be seen in  FIG. 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 . 
     The central electrode  2  and the outer electrode  1  can be cylindrical with a circular cross-section, 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 sides  72  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 . 
       FIG. 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  FIG. 5B to 5D . The devices shown in  FIG. 5B to 5D  are closed at the sides by suitable insulating materials (not shown). The parallel device of  FIG. 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 in this invention. Also, the embodiment where both the central electrode  15  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. 
       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. 
     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): 20-750 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 l/min for the tubular device, 10-4000 l/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: N 2 , Air, He, Ar, CO 2 +mixture of these gases with H 2 , O 2 , SF 6 , CF 4 , saturated and unsaturated hydrocarbon gases, fluorinated hydrocarbon gases.   Monomer flow: 1-2000 g/min (through canal  7  in the central electrode immediately into plasma afterglow).   Feed gas flow: 0.1-30 l/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: 6-80 mm.   Length of effective plasma afterglow: 5-100 mm. (dependent upon application).       

     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 by 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 
     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 
     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 inside 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: 
     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. 
     When using a U-shaped dielectricum such as in  FIG. 2 , more reactive plasma afterglow is obtained Parameters:
         Power: 400 Watt   Frequency: 70 kHz   Plasma gas: 65 l 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: &gt;75 dynes.   surface energy 1 week after plasma activation: 62 dynes.
 
Plasma Activation of PVC:
       

     PVC is thermal sensitive. The activation performed with the classical concept is not stable in time. After a few hours, activation was completely lost. 
     When using a U-shaped dielectricum, more reactive plasma afterglow is obtained.
         Power: 300 Watt   Frequency: 32 kHz   Plasma gas: 60 l N 2 /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: &gt;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
       

     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. 
     Examples 
     Plasma Activation of Polyethylene: 
     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 present 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 l N 2 /min   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.       

     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 Distance sample/plasma 
                 Broadness of homogenous 
               
               
                   
                 source (mm): 
                 activated spot (mm) (62 dynes): 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 2.5 
                 45 
               
               
                   
                 4 
                 41 
               
               
                   
                 6 
                 25 
               
               
                   
                 8 
                 22 
               
               
                   
                 10 
                 22 
               
               
                   
                 12.5 
                 22 
               
               
                   
                 15 
                 22 
               
               
                   
                 20 
                 18 
               
               
                   
                 30 
                 7 
               
               
                   
                 35 
                 3 
               
               
                   
                   
               
             
          
         
       
     
     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: 
     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 present 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 l 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.       

     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 Distance sample/plasma 
                 Broadness of homogenous activated 
               
               
                   
                 source (mm): 
                 spot (mm) (70 dynes): 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 2.5 
                 48 
               
               
                   
                 4 
                 45 
               
               
                   
                 6 
                 26 
               
               
                   
                 8 
                 22 
               
               
                   
                 10 
                 22 
               
               
                   
                 12.5 
                 22 
               
               
                   
                 15 
                 22 
               
               
                   
                 20 
                 20 
               
               
                   
                 30 
                 12 
               
               
                   
                 35 
                 4 
               
               
                   
                   
               
             
          
         
       
     
     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 
     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 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. 
     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.