Abstract:
A dielectric barrier discharge uses three electrodes at an atmospheric pressure. A wide discharge gap can be used and an enhanced plasma density can be achieved so that thick materials can be processed and its processing speed can also be greatly improved.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an apparatus of a dielectric barrier discharge for generating glow discharge plasmas at atmospheric pressure; more particularly, relates to obtaining more stable glow discharge plasmas, wider discharge gap and a higher plasma density to speed up material processing and to process thick materials. 
       DESCRIPTION OF THE RELATED ART 
       [0002]    The word “plasma” is used to describe partially ionized gases containing many interacting free electrons, ionized atoms or molecules and free radicals. Non-thermal equilibrium plasma has many applications such as surface modification of polymers, cleaning, etching and thin film deposition. It is known that plasma sources operated at atmospheric pressure have many advantages such as free of vacuum chamber, the potential to achieve higher processing speed and lower processing cost, etc. 
         [0003]    The conventional dielectric barrier discharge (DBD) reactor consists of two electrodes with at least one dielectric barrier, high voltage power supplies, a gas flow system and diagnostic instruments. In which, the high voltage AC power supply is used to excite a capacitive load to generate plasma. The plasma generated using such a prior art is commonly employed for the surface treatment of sheet materials. However, DBD was usually operated in the filamentary mode and only under some special conditions, a glow discharge mode is available, which is highly desired for uniform surface treatment. Besides, there are many other issues in surface processing by DBD, such as narrow discharge gap and low plasma density. Hence, the prior art does not meet most of users&#39; requirements in practical applications. 
       SUMMARY OF THE INVENTION 
       [0004]    The main purpose of the present invention is to generate stable glow discharge plasmas, to speed up material processing and to process thick materials by increasing the atmospheric pressure plasma density with two properly correlated high voltage power supplies so that the device could be operated in glow discharge mode at a wide discharge gap. 
         [0005]    To achieve the above purpose, the present invention is an apparatus of triple-electrode dielectric barrier discharge (TDBD) at an atmospheric pressure, comprising a plasma chamber, a first power supply, a second power supply, a mass flow controller, a first electrode, a second electrode, a common ground electrode with respect to the first and the second electrode and a dielectric layer and a discharge gap; the first electrode together with the common ground electrode is connected to the first power supply; the second electrode together with the common ground electrode is connected to the second power supply; Therein, the mass flow controller provides and controls various gas in the plasma chamber flowing through the discharge gap to generate discharge plasmas on the surface of the first electrode; the charged particles on the surface of the 1 st  electrode are attracted by the 2 nd  electrode so that its discharge could be ignited at lower applied voltage and wider discharge gap than the conventional DBD. Thus, the discharge on the 2 nd  electrode becomes much easier due to the help of the pre-ionization plasmas generated on the surface of the 1 st  electrode; and, therefore, the discharge gap can be widened in order to process thick material and to speed up material processing. Accordingly, a novel apparatus of triple-electrode dielectric barrier discharge at an atmospheric pressure is obtained, which could be used to treat thick materials and to speed up its processing. 
     
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         [0006]    The present invention will be better understood from the following detailed description of the preferred embodiment according to the present invention, taken in conjunction with the accompanying drawings, in which 
           [0007]      FIG. 1  is the schematic showing the preferred embodiment according to the present invention; 
           [0008]      FIG. 2  shows the relative intensity of optical emission at various delay time; and 
           [0009]      FIG. 3  illustrates the ratio of plasma intensity and input power. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0010]    The following description of the preferred embodiment is provided to understand the features and the structures of the present invention. 
         [0011]    Please refer to  FIG. 1 , which is a schematic for the preferred embodiment according to the present invention. As shown in the figure, the present invention is an apparatus of triple-electrode dielectric barrier discharge at an atmospheric pressure, comprising a plasma chamber  11 , a first power supply  12 , a second power supply  13 ,  14 , a mass flow controller  15 , a first electrode  111 , a common ground electrode, a second electrode  112 , a plurality of dielectric layers  114  and a discharge gap  115 , where the first electrode  111  is together with the common ground electrode  113  is connected to the first power supply  12 ; the second electrode  112  together with the common ground electrode  113  is connected to the second power supply  113 ; The discharge gap  115  is located above the surface of the dielectric layer  114  on the first electrode  111 . 
         [0012]    The plasma chamber  11  is connected to the input of a mass flow controller  15 ; and, the substrate  16  to be treated is placed on the dielectric layer  114  in the discharge gap  115 . Plasma working gas  151  is filled into the plasma chamber  11  by the mass flow controller  15 . On operating the present invention, a surface discharge is formed on the surface of dielectric layer  114  of the first electrode  111 . Then charged particles on a surface of the dielectric layer  114  are attracted by the second electrode  112  to produce plasmas in the gap between the 2 nd  electrode and the common ground electrode. Thus, a novel apparatus of triple-electrode dielectric barrier discharge at an atmospheric pressure is obtained. 
         [0013]    When using the present invention, the first power supply  12  and the second power supply  13  are each connected to first electrode  111  and second electrode  112 ; when the first electrode  111  is applied with a power from the first power supply  12 , surface discharge plasma is formed on the surface of the dielectric layer  114 . After a delay time  183 , the second electrode  112  is applied with a voltage pulse from the second power supply  13  and the charged particles generated in the surface discharge plasma filled in the discharge gap  115  are to generate glow discharge plasma. Then, the plasma monitor  17  precisely figures out proper delay time  183  to enhance the plasma density. Thus, the discharge gap  115  can be widened to process thick materials. 
         [0014]    Please refer to  FIG. 2 , which shows the relative intensity of optical emission at various delay times. As shown in the figure, when applying the present invention, 2 slm of nitrogen is filled in a 10-mm discharge gap under an atmospheric pressure the relative intensity of optical emission line  3  for nitrogen molecules at a wavelength of 316 nm is measured. And it is clearly shown that the optical emission line  31  varies significantly for different delay time between the first pulse and the second pulse. As shown in the optical mission line  31 , when the pulse delay time is 40 μs, the intensity is about 90, which is much higher than 63.2 for a conventional two-electrode dielectric barrier discharge. Hence, triple-electrode dielectric barrier discharge has a higher discharge plasma density than the two-electrode one, where the total power intensity can be doubled by applying two closely correlated power sources at the same discharge area. 
         [0015]    Please refer to  FIG. 3 , which shows the ratio of the emission intensity and its total input power at various power for a conventional two-electrode DBD  41  and the (a) triple-electrode DBD  42  with the same total input power. As shown in the figure, the emission intensity per unit power  42  for the triple-electrode DBD ( 42 ) is higher than that  41  for the conventional two-electrode DBD ( 41 ). 
         [0016]    To sum up, the present invention is an apparatus of triple-electrode dielectric barrier discharge at an atmospheric pressure where a wide discharge gap can be used at an atmospheric pressure and its plasma density can be greatly enhanced for processing thick materials and speeding up the processing. 
         [0017]    The preferred embodiment herein disclosed is not intended to unnecessarily limit the scope of the invention. Therefore, simple modifications or variations belonging to the equivalent of the scope of the claims and the instructions disclosed herein for a patent are all within the scope of the present invention.