Abstract:
An electrode assembly for an arrangement of electrodes used in a plasma surface treatment apparatus. The electrode assembly includes an electrode body formed of a dielectric material and having two spaced end walls. One or more flow channels are defined between the end walls for passage of cooling fluid to cool the electrode body. The flow channel(s) are configured such that a direct flow path exists for the cooling fluid from one of the end walls to the other of the two end walls to inhibit recirculation of the cooling fluid and therefore hot spots developing in the electrode body. Openings within the end walls are provided for passage of the cooling fluid. A high voltage conductor is located within the flow passage(s) so that heat produced at the high voltage conductor is dissipated by the cooling fluid.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to an electrode assembly that forms an arrangement of one or more electrodes of a plasma surface treatment apparatus in which a substrate is treated by a non-equilibrium plasma generated between an active electrode and a counter or ground electrode. More particularly, the present invention relates to such an electrode assembly in which a direct flow path is provided for cooling fluid within a cavity of the electrode assembly to inhibit recirculation of the cooling fluid within the cavity.  
       BACKGROUND OF THE INVENTION  
       [0002]     Non-equilibrium plasma, produced by uniform glow discharges, is utilized for the surface treatment of polymer films, fabrics, wool, metal, and paper to improve the physical and optical properties of the surface. Such properties include printability, wetability, durability, and adhesion of coatings.  
         [0003]     The non-equilibrium plasma is generated within a thin gap between two electrodes. The gap is generally less than about two millimeters. A high voltage is applied to an active electrode. The active electrode is formed of an electrode assembly having a electrode body made of a dielectric material. The electrode body contains and a high voltage conductor to which the high voltage is applied. The dielectric material, which can be a ceramic or glass, ensures the uniformity of the discharge. A grounded, counter electrode is positioned opposite to the active electrode and can be in the form of a rotating drum or a flat plate or can have a similar configuration to the active electrode. As such, the counter electrode can also be in the form of a high voltage conductor encased in a dielectric. A plasma medium, which can be helium or other gas or a mixture of gases, is injected into the region between the two electrodes to generate the non-equilibrium plasma.  
         [0004]     The substrate, which is in sheet form, is passed between the active and counter electrodes to be treated by the non-equilibrium plasma. The active components of the plasma interact with surface or bulk characteristic of the substrate. Potential surface interactions include chemical transformations of the surface functional groups, material deposition and surface cleaning or sterilization. Bulk interactions include the transformation or destruction of chemical or biological components.  
         [0005]     During generation and maintenance of the non-equilibrium plasma, the thermal cycle introduced by electromagnetic power deposition can lead to thermal failure of the electrode assembly, the substrate being processed, or the plasma itself. Potential types of thermal failure for the electrode assembly or substrate include fatigue cracking, thermal or mechanical distortion, thermal or mechanical stressing, binding failure and flow erosion. Uncontrolled thermal instabilities can cause the plasma to transition from a low temperature, non-equilibrium regime (in which the kinetic temperature of the electrons can exceed the neutral gas temperature by over two orders of magnitude) to a high temperature, thermal regime in which the plasma approaches local thermal equilibrium.  
         [0006]     Thermal instabilities can also cause the transition from a uniform flow discharge to a highly localized filamentary or arc plasma which is incapable of providing uniform surface or bulk treatment. Furthermore, improper control of plasma component temperatures directly impacts important operating characteristics, such as plasma number density, electric field strength, electron absorbed power density, surface and bulk reaction kinetics, concentration of reactive species (ions, electrons, neutrals, metastables), turbulent reaction flow and plasma processing rate.  
         [0007]     In order to maintain a stable non-equilibrium plasma discharge at near atmospheric pressure, the electrodes between which that plasma is generated must be cooled by dissipating the heat produced at the high voltage conductor. This cooling provides adequate thermal reliability for a particular duty cycle. In addition, thermal distortion of the electrodes is minimized while avoiding excessive heat. The particular cooling strategy utilized must provide a sufficient heat transfer rate to meet the system cooling requirements to optimize coolant velocities and thereby maximize heat transfer.  
         [0008]     U.S. Pat. No. 6,429,525 discloses a plasma surface treatment apparatus employing electrode assemblies in the form of two hollow, air cooled active electrodes located opposite to a rotating, cylindrical counter electrode. Each of the active electrodes has a body of generally rectangular cross-section that contains a strip-like high voltage conductor. Compressed air passes from a single inlet at one end of the hollow electrode and is discharged from a single outlet located at the other end thereof. The inlet and outlet are located in the side of the electrodes. As a result, the cooling fluid must change direction when flowing from the inlet to the outlet. This change in direction can produce recirculation of cooling fluid within the electrode.  
         [0009]     The single inlet and outlet configuration coupled with the lack of direct flow path for the cooling fluid results in recirculation zones within the electrode body and therefore uneven heating and hot spots. As will be discussed, the present invention solves this problem so that the electrode assembly is uniformly cooled by inhibiting the formation of recirculation zones within the electrode body of the electrode assembly.  
       SUMMARY OF THE INVENTION  
       [0010]     In one aspect, the present invention provides an electrode assembly for a plasma surface treatment apparatus for treating a surface of an article. In accordance with the present invention, an electrode body formed of a dielectric material is provided. The electrode body has two end walls spaced apart from one another. At least one flow channel is defined between the end walls for passage of cooling fluid to cool the electrode body. The at least one flow channel is configured such that a direct flow path exists for the cooling fluid from one of the two end walls to the other of the two end walls to inhibit recirculation of the cooling fluid and therefore hot spots developing in the electrode body. Openings are defined within the end walls to introduce and discharge the cooling fluid to and from the at least one flow channel, respectively.  
         [0011]     A high voltage conductor is located within the at least one flow passage and bounds part of the at least one flow passage so that the dielectric material forms a barrier between the surface of the article and the high voltage conductor when the electrode is in use and heat produced at the high voltage conductor is dissipated by the cooling fluid.  
         [0012]     The electrode body can be provided with ribs within the at least one flow channel such that the at least one flow channel is a plurality of flow channels. The openings are arranged so that each of said flow channels has a corresponding pair of openings for the flow of the cooling fluid within each of said flow channels. Alternatively, the at least one flow channel can be one flow channel.  
         [0013]     In any embodiment of the present invention, the at least one flow channel and the two end walls each have a rectangular transverse cross-section. In such embodiment the high voltage conductor is of plate-like configuration and the openings are arranged in an array across the rectangular transverse cross-section of the two end walls.  
         [0014]     The electrode assembly can further be provided with one or more flow deflectors situated in the one or plurality of flow channels, respectively, to deflect the flow of the cooling fluid toward the high voltage conductor. Further, preferably, the high voltage conductor is brazed to the electrode body.  
         [0015]     In another aspect of the present invention an arrangement of electrode assemblies for treating a substrate is provided. In such aspect, a high voltage is applied to an active electrode and a ground electrode spaced from the active electrode to allow the substrate to pass therebetween. Each of the active electrode and the ground electrode is formed of an electrode assembly that has the features described above. Further, the active electrode and the ground electrode can share the sidewalls of each electrode body associated therewith to form a slot-like opening between the sidewalls and the active electrode and ground electrode. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     While the specification concludes with claims distinctly pointing out the subject matter that Applicants regard as their invention, it is believed that the invention would be better understood when taken in connection with the accompanying drawings in which:  
         [0017]      FIG. 1  is a schematic perspective view of an electrode in accordance with the present invention in which portions have been broken away to illustrate internal components thereof;  
         [0018]      FIG. 2  illustrates an alternative method of using the electrode assembly shown in  FIG. 1 ;  
         [0019]      FIG. 3  is a schematic plan view of a manifold attached to the electrode assembly of  FIG. 1 , illustrated in a fragmentary plan, sectional view, that is used to feed the cooling fluid to such electrode assembly;  
         [0020]      FIG. 4  is a schematic, sectional view of an inlet manifold used in feeding cooling fluid to the electrode of  FIG. 2 ;  
         [0021]      FIG. 5  is a schematic, sectional view of a manifold used in connection with the electrode illustrated in  FIG. 2 ; and  
         [0022]      FIG. 6  is a sectional view of the flow channels utilized in the electrode assembly of either  FIG. 1  taken along line  6 - 6  of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION  
       [0023]     With reference to  FIG. 1 , an arrangement  1  of electrodes in accordance with the present invention is illustrated. Arrangement  1  includes an electrode assembly  10  which can be connected to a high frequency current source and a counter electrode that is formed of an electrode assembly  12  that is grounded. A substrate formed of sheet-like material to be treated is passed within a slot-like enclosed area  14  that is bounded by electrode assemblies  10  and  12  and sidewalls  16  and  18  thereof. Although not illustrated, the arrangement  1  of electrodes would be positioned within a chamber to which a plasma medium such as helium would be introduced. The chamber in a known manner would have a slot for passage of the substrate into the electrode assembly. The plasma medium through application of a high voltage to the electrode assemblies would in a known manner produce a non-equilibrium plasma to treat the substrate.  
         [0024]     Electrode assembly  10  has a electrode body of box-like configuration that is formed of dielectric material that is defined by two opposed plate-like elements  19  and  20 , a portion of sidewalls  16  and  18  and end walls  21  and  22 . Within a region of the electrode body defined between plate-like elements  18  and  20 , sidewalls  16  and  18 , and end walls  20  and  22 , a cavity  23  is formed that is bounded at the bottom by a plate-like high voltage conductor  24 . The high voltage is applied to high voltage conductor  24 .  
         [0025]     It is to be noted that high voltage conductor  24  can be attached to plate-like element  20  of the electrode body of electrode assembly  10  by a suitable adhesive or by braising. In this regard to obtain excellent hermetic properties and reduce problems related to voids and thermal expansion, the high voltage conductor  24  and dielectric barrier surfaces are assembled with the necessary brazing assembly materials. The brazing solder materials can be pre-applied to the individual piece in the quantities required for selected metal and dielectric materials. Typical materials used for an electrode assembly in accordance with the present invention and brazing solder combinations are listed in the table below.  
                       TABLE                       High voltage       Dielectric       conductor 24   Brazing-solder   Material                   Cu   AgCu 28%   SiO2       Fe/Ni   AgCu 15%   Si3N4       Kov   AgGe 13%   Al2O3       Fe/N142   AgSn 20%   TiO2, Ta2O5                  
 
         [0026]     For compatibility with highly diversified substrates during thermal expansion for thin electrodes, the high voltage conductors can be deposited directly on the dielectric surface using metal pastes such as Cu paste, Ag/Cu paste, and Ag/Pt paste etc. Selected powders used in the pastes can produce remarkably thick and dense film on the dielectric surfaces.  
         [0027]     The cavity  23  described above contains ribs  26  that form flow channels  28 . Flow channels  28  are bounded at opposite ends by the end walls  19  and  20 . A cooling fluid, which could be the plasma medium for instance, helium, is passed into flow channels  28  for dissipating heat produced at the plate-like high voltage conductor  24  in order to cool electrode assembly  10 . An array of a plurality of openings  30 , extending across end wall  19  are provided to introduce the cooling fluid in the direction of the arrowheads “A” into the flow channels  28 . The cooling fluid is expelled from an array of a plurality of openings  32  extending across end wall  20 . As illustrated one pair of openings  30  and  32  is associated with each of the flow channels  28 . In such manner, a direct flow path is produced within flow channels  28  that will inhibit recirculation of the cooling fluid within electrode assembly  10  since the flow enters flow channels  28  from end wall  19  and is discharged from end wall  20  that are located at opposite ends of the flow channels  28 .  
         [0028]     Electrode assembly  12  is of identical design to electrode assembly  10 . It is understood, however, that electrode assemblies in accordance with the present invention could be of any shape or incorporated into any arrangement of electrodes. For instance, electrode assembly  10  could be used in connection with a counter or ground electrode that was formed of a flat plate. Furthermore, electrode assembly  10  or  12  for that matter could be curved.  
         [0029]     With reference to  FIG. 2 , an alternative method of using arrangement  1  of electrodes in accordance with the present invention is illustrated which is identical to arrangement  1 . In such embodiment, however, the introduction and discharge of cooling fluid alternate across the end walls  19  and  20 . In this regard, openings  30  and  32  and openings  34  and  36  are all identical openings. Such numbering is used for purposes of illustration only. However, as indicated by the arrowheads “A” and “B”, the flow within flow channels  28  will be countercurrent due to the alternating inlet of cooling fluid into openings  34  and the discharge from openings  36 . The flow within electrode assembly  10  which is all in the same direction of arrowhead “A” and is thus, cocurrent. These flow patterns can be produced by manifolds described below.  
         [0030]     With reference to  FIG. 3 , in order to produce the cocurrent flow within electrode assembly  10  or electrode assembly  12  for that matter, electrode assembly can further be provided with an inlet manifold  40  having a manifold inlet  42  and manifold openings  44 . The manifold openings  44  are sized to balance the flow rates of the cooling fluid being discharged. Inlet manifold  40  can be connected the electrode body of electrode assembly  10  with manifold openings  44  in flow communication with to inlets  30 . As illustrated, manifold  40  would be attached to the electrode body of electrode assembly  10  by such means as brazing with manifold openings  44  in registration with inlets  30  of electrode assembly  10 . An outlet manifold could also be provided that would be identical to inlet manifold  40  except that manifold openings  44  would be in flow communication with outlets  32  and the manifold inlet  42  would serve as a manifold outlet.  
         [0031]     With reference to  FIG. 4 , an inlet manifold  50  can be provided for electrode assemblies  10 ′ and  12 ′. With additional reference to  FIG. 5 , a return manifold  60  is also provided. Inlet manifold  50  and return manifold  60  produce the countercurrent flow within arrangement  1 ′. Although not illustrated, inlet manifold  50  could be directly connected to end wall  21  of electrode assembly  10 ′ and return manifold could be connected to end wall  22  in a manner like that described with respect to inlet manifold  40 .  
         [0032]     Inlet manifold  50  is provided with a manifold inlet  52  for receiving the cooling fluid and a manifold outlet  54  for discharging the cooling fluid after having passed through either of the electrode assemblies  10 ′ or  12 ′. Also provided is a first set of alternating subsidiary outlets and inlets  55  and  56  which would be in flow communication with inlets  30  and outlets  32  of the electrode body of electrode assembly  10 ′ by being positioned in registration therewith. Internally, inlet manifold  50  is partitioned by partitions  58  and sub-partitions  59  to induce reversal of flow within inlet manifold  50  indicated by arrowheads “C”.  
         [0033]     Return manifold  60  is connected to the opposite side of electrode assembly  10 ′ (or electrode assembly &#39; 12 ) and is provided with a second set of alternating subsidiary inlets and outlets  62  and  64  which would be in flow communication with outlets and inlets  36  and  34  of the electrode body of electrode assembly  10 ′ by being positioned in registration therewith. Return manifold  60  is subdivided by partitions  66  and preferably sub-partitions  68  to help induce a reversal of flow within the partitioned regions of return manifold  60 .  
         [0034]     As can be appreciated, it is possible to provide an embodiment of the present invention without the manifold arrangements described directly above. In such case, however, individual pipes and conduits would be required to supply and discharge cooling fluid from the inlets and outlets of the electrode assemblies thereof. Additionally, although not illustrated it is also possible to cascade electrode assemblies in accordance with the present invention. For instance an inlet manifold  50  could be connected to inlets  30  of electrode assembly  12 . The outlets  32  of electrode assembly  10  would then be connected to the inlets  30  of electrode assembly  32 . The outlets of electrode assembly  12  would then be connected to an outlet manifold having the same design as inlet manifold  50 .  
         [0035]     With reference to  FIG. 6 , each of the flow channels  28  can optionally be provided with a flow deflector  70  to deflect the flow towards a high voltage conductor  24  located within the electrode assembly  10 . If the cavity  23  were hollow, without ribs  26 , a single large flow deflector could be employed within a single flow passage provided by cavity  23  alone. An equivalent for the flow deflector would be to appropriately shape plate-like element  19  to narrow the flow area near high voltage conductor  24 .  
         [0036]     While the present invention has been described with reference to preferred embodiment, as will occur to those skilled in the art, numerous changes, additions and omissions can be made without departing from the spirit and the scope of the present invention.