Abstract:
A new kinetic spray process is disclosed that enables one to secure a plurality of ceramic elements together quickly without the need for glues or other adhesives. The process finds special utilization in the formation of non-thermal plasma reactors wherein the kinetic spray process can be used to simultaneously secure the ceramic elements together and to form electrical connections between like electrodes in the non-thermal plasma reactor.

Description:
INCORPORATION BY REFERENCE  
       [0001]     The present invention comprises an improvement to the kinetic spray process as generally described in U.S. Pat. Nos. 6,139,913, 6,283,386 and the articles by Van Steenkiste, et al. entitled “Kinetic Spray Coatings” published in Surface and Coatings Technology Volume III, Pages 62-72, Jan. 10, 1999, and “Aluminum coatings via kinetic spray with relatively large powder particles”, published in Surface and Coatings Technology 154, pp. 237-252, 2002, all of which are herein incorporated by reference. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention is directed toward a method for securing the elements of a ceramic structure together, and more particularly, toward a method that both secures the ceramic elements together and provides for an electrical connection between the elements.  
       BACKGROUND OF THE INVENTION  
       [0003]     A new technique for producing coatings on a wide variety of substrate surfaces by kinetic spray, or cold gas dynamic spray, was recently reported in two articles by T. H. Van Steenkiste et al. The first was entitled “Kinetic Spray Coatings,” published in Surface and Coatings Technology, vol. 111, pages 62-71, Jan. 10, 1999 and the second was entitled “Aluminum coatings via kinetic spray with relatively large powder particles”, published in Surface and Coatings Technology 154, pp. 237-252, 2002. The articles discuss producing continuous layer coatings having high adhesion, low oxide content and low thermal stress. The articles describe coatings being produced by entraining metal powders in an accelerated gas stream, through a converging-diverging de Laval type nozzle and projecting them against a target substrate. The particles are accelerated in the high velocity gas stream by the drag effect. The gas used can be any of a variety of gases including air or helium. It was found that the particles that formed the coating did not melt or thermally soften prior to impingement onto the substrate. It is theorized that the particles adhere to the substrate when their kinetic energy is converted to a sufficient level of thermal and mechanical deformation. Thus, it is believed that the particle velocity must exceed a critical velocity high enough to exceed the yield stress of the particle to permit it to adhere when it strikes the substrate. It was found that the deposition efficiency of a given particle mixture was increased as the inlet air temperature was increased. Increasing the inlet air temperature decreases its density and thus increases its velocity. The velocity varies approximately as the square root of the inlet air temperature. The actual mechanism of bonding of the particles to the substrate surface is not fully known at this time. The critical velocity is dependent on the material of the particle. Once an initial layer of particles has been formed on a substrate subsequent particles bind not only to the voids between previous particles bound to the substrate but also engage in particle to particle bonds. The bonding process is not due to melting of the particles in the main gas stream because the temperature of the particles is always below their melting temperature.  
         [0004]     There is often a need in industry to secure a plurality of ceramic elements to each other. There are also ceramic structures that require establishment of electrical connections between elements on closely adjacent ceramic elements. Typically, ceramic elements are joined to each other by the steps of applying a glass adhesive to the various ceramic elements, assembling the ceramic structure formed from the elements, clamping or holding the structure together and then heating the entire structure in a furnace to cure the adhesive. This multi-step process is cumbersome and time consuming. In other applications ceramic elements are both bound together with an adhesive and regions are painted several layers of a silver paint to establish an electrical connection between the ceramic elements. It would be advantageous to develop a single step, rapid method to permit both binding of ceramic elements together and establishment of electrical connections between the ceramic elements.  
       SUMMARY OF THE INVENTION  
       [0005]     In one embodiment of the present invention a plurality of ceramic elements are secured to each other by at least a first band of a kinetic spray applied material.  
         [0006]     In another embodiment, the present invention is a non-thermal plasma reactor comprising a plurality of ceramic elements arranged in a stack, the stack including at least a first plurality of ceramic elements and a second plurality of ceramic elements; the first plurality of ceramic elements each having a ground electrode with a connector, the second plurality of ceramic elements each having a charge electrode with a connector; a first band of an electrically conductive material applied by a kinetic spray process and electrically coupling the connectors of the ground electrodes and a second band of an electrically conductive material applied by a kinetic spray process and electrically coupling the connectors of the charge electrodes; and the first and second bands securing the plurality of ceramic elements together.  
         [0007]     In another embodiment, the present invention is a method of securing a plurality of ceramic elements to each other comprising the steps of: providing particles of a material to be sprayed; providing a supersonic nozzle; providing a plurality of ceramic elements releasably held together and positioned opposite the nozzle; directing a flow of a gas through the nozzle, the gas having a temperature of from 600 to 1200 degrees Fahrenheit; and entraining the particles in the flow of the gas and accelerating the particles to a velocity sufficient to result in adherence of the particles to the ceramic elements upon impact, thereby forming at least a first band of adhered material on the ceramic elements and securing the ceramic elements together.  
         [0008]     In another embodiment, the present invention is a method of forming a non-thermal plasma reactor comprising the steps of: providing particles of an electrically conductive material to be sprayed; providing a supersonic nozzle; providing a first plurality of ceramic elements and a second plurality of ceramic elements, the ceramic elements releasably held together and positioned opposite the nozzle, with the first plurality of ceramic elements each having a ground electrode with a connector and the second plurality of ceramic elements each having a charge electrode with a connector; directing a flow of a gas through the nozzle, the gas having a temperature of from 600 to 1200 degrees Fahrenheit; and entraining the particles in the flow of the gas and accelerating the particles to a velocity sufficient to result in adherence of the particles to the ceramic elements upon impact, directing the accelerated particles at the connectors of the first plurality of ceramic elements forming a first band of adhered material electrically coupling the electrodes of the first plurality of ceramic elements together and directing the accelerated particles at the connectors of the second plurality of ceramic elements forming a second band of adhered material electrically coupling the electrodes of the second plurality of ceramic elements together, and the first and the second bands securing the ceramic elements together. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0010]      FIG. 1  is a generally schematic layout illustrating a kinetic spray system for performing the method of the present invention;  
         [0011]      FIG. 2  is an enlarged cross-sectional view of a kinetic spray nozzle used in the system;  
         [0012]      FIG. 3  is an exploded view of a cell of a non-thermal plasma reactor stack;  
         [0013]      FIG. 4  is an end view of a part of a non-thermal plasma reactor stack secured using the method of the present invention; and  
         [0014]      FIG. 5  is an end view of a part of a second embodiment of a non-thermal plasma reactor stack secured using the method of the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]     Referring first to  FIG. 1 , a kinetic spray system according to the present invention is generally shown at  10 . System  10  includes an enclosure  12  in which a support table  14  or other support means is located. A mounting panel  16  fixed to the table  14  supports a work holder  18  capable of movement in three dimensions and able to support a suitable workpiece formed of a ceramic structure to be coated. The work holder  18  is preferably designed to move a structure relative to a nozzle  34  of the system  10 , thereby controlling where the powder material is deposited on the structure. The enclosure  12  includes surrounding walls having at least one air inlet, not shown, and an air outlet  20  connected by a suitable exhaust conduit  22  to a dust collector, not shown. During coating operations, the dust collector continually draws air from the enclosure  12  and collects any dust or particles contained in the exhaust air for subsequent disposal.  
         [0016]     The spray system  10  further includes an air compressor  24  capable of supplying air pressure up to 3.4 MPa (500 psi) to a high pressure air ballast tank  26 . The air ballast tank  26  is connected through a line  28  to both a high pressure powder feeder  30  and a separate air heater  32 . The air heater  32  supplies high pressure heated air, the main gas described below, to a kinetic spray nozzle  34 . The pressure of the main gas generally is set at from 150 to 500 psi, more preferably from 300 to 400 psi. The high pressure powder feeder  30  mixes particles of a spray powder with high pressure air and supplies the mixture to a supplemental inlet line  48  of the nozzle  34 . Preferably the particles are fed at a rate of from 20 to 80 grams per minute to the nozzle  34 . A computer control  35  operates to control both the pressure of air supplied to the air heater  32  and the temperature of the heated main gas exiting the air heater  32 .  
         [0017]     The particles used in the present invention are preferably electrically conductive materials including: copper, copper alloys, nickel, nickel alloys, aluminum, aluminum alloys, stainless steels, and mixtures of these materials. Preferably the powders have nominal average particle sizes of from 60 to 106 microns and preferably from 60 to 90 microns. Depending on the particles or combination of particles chosen the main gas temperature may range from 600 to 1200 degrees Fahrenheit. With aluminum and its alloys the temperature preferably is around 600 degrees Fahrenheit, while the other materials preferably are sprayed at a main gas temperature of from 1000 to 1200 degrees Fahrenheit. Mixtures of the materials may be sprayed at from 600 to 1200 degrees Fahrenheit.  
         [0018]      FIG. 2  is a cross-sectional view of the nozzle  34  and its connections to the air heater  32  and the powder feeder  30 . A main air passage  36  connects the air heater  32  to the nozzle  34 . Passage  36  connects with a premix chamber  38  that directs air through a flow straightener  40  and into a chamber  42 . Temperature and pressure of the air or other heated main gas are monitored by a gas inlet temperature thermocouple  44  in the passage  36  and a pressure sensor  46  connected to the chamber  42 . The main gas has a temperature that is always insufficient to cause melting within the nozzle  34  of any particles being sprayed. The main gas temperature can be well above the melt temperature of the particles. Main gas temperatures that are 5 to 7 fold above the melt temperature of the particles have been used in the present system  10 . As discussed below, for the present invention it is preferred that the main gas temperature range from 600 to 1200 degrees Fahrenheit depending on the material that is sprayed. What is necessary is that the temperature and exposure time to the main gas be selected such that the particles do not melt in the nozzle  34 . The temperature of the gas rapidly falls as it travels through the nozzle  34 . In fact, the temperature of the gas measured as it exits the nozzle  34  is often at or below room temperature even when its initial temperature is above 1000° F.  
         [0019]     The mixture of high pressure air and coating powder is fed through the supplemental inlet line  48  to a powder injector tube  50  comprising a straight pipe having a predetermined inner diameter. The tube  50  has a central axis  52  which is preferentially the same as the axis of the premix chamber  38 . The tube  50  extends through the premix chamber  38  and the flow straightener  40  into the mixing chamber  42 .  
         [0020]     Chamber  42  is in communication with a de Laval type supersonic nozzle  54 . The nozzle  54  has a central axis  52  and an entrance cone  56  that decreases in diameter to a throat  58 . The entrance cone  56  forms a converging region of the nozzle  54 . Downstream of the throat  58  is an exit end  60  and a diverging region is defined between the throat  58  and the exit end  60 . The largest diameter of the entrance cone  56  may range from 10 to 6 millimeters, with 7.5 millimeters being preferred. The entrance cone  56  narrows to the throat  58 . The throat  58  may have a diameter of from 3.5 to 1.5 millimeters, with from 3 to 2 millimeters being preferred. The diverging region of the nozzle  54  from downstream of the throat  58  to the exit end  60  may have a variety of shapes, but in a preferred embodiment it has a rectangular cross-sectional shape. At the exit end  60  the nozzle  54  preferably has a rectangular shape with a long dimension of from 8 to 14 millimeters by a short dimension of from 2 to 6 millimeters.  
         [0021]     As disclosed in U.S. Pat. Nos. 6,139,913 and 6,283,386 the powder injector tube  50  supplies a particle powder mixture to the system  10  under a pressure in excess of the pressure of the heated main gas from the passage  36 . The nozzle  54  produces an exit velocity of the entrained particles of from 300 meters per second to as high as 1200 meters per second. The entrained particles gain kinetic and thermal energy during their flow through this nozzle. It will be recognized by those of skill in the art that the temperature of the particles in the gas stream will vary depending on the particle size and the main gas temperature. The main gas temperature is defined as the temperature of heated high-pressure gas at the inlet to the nozzle  54 . Since the particles are never heated to their melting point, even upon impact, there is no change in the solid phase of the original particles due to transfer of kinetic and thermal energy, and therefore no change in their original physical properties. The particles are always at a temperature below the main gas temperature. The particles exiting the nozzle  54  are directed toward a surface of a substrate to coat it.  
         [0022]     It is preferred that the exit end  60  of the nozzle  54  have a standoff distance from the surface to be coated of from 10 to 40 millimeters and most preferably from 10 to 20 millimeters. Upon striking a substrate opposite the nozzle  54  the particles flatten into a nub-like structure with an aspect ratio of generally about 5 to 1. Upon impact the kinetic sprayed particles transfer substantially all of their kinetic and thermal energy to the substrate surface and stick if their yield stress has been exceeded. As discussed above, for a given particle to adhere to a substrate it is necessary that it reach or exceed its critical velocity which is defined as the velocity where at it will adhere to a substrate when it strikes the substrate after exiting the nozzle  54 . This critical velocity is dependent on the material composition of the particle. In general, harder materials must achieve a higher critical velocity before they adhere to a given substrate. It is not known at this time exactly what is the nature of the particle to substrate bond; however, it is believed that a portion of the bond is due to the particles plastically deforming upon striking the substrate. Preferably the particles have an average nominal diameter of from 60 to 90 microns.  
         [0023]     In the present invention it is preferred that the nozzle  34  be at an angle of from 0 to 45 degrees relative to a line drawn normal to the plane of the surface being coated, more preferably at an angle of from 15 to 25 degrees relative to the normal line. Preferably the work holder  18  moves the structure past the nozzle  34  at a traverse speed of from 0.6 to 13 centimeters per second and more preferably at a traverse speed of from 0.6 to 7 centimeters per second.  
       Experimental Data  
       [0024]     The present invention will be described with respect to its utilization to form electrical connections and secure multiple ceramic elements in a non-thermal plasma reactor, however the present invention can be used to secure any plurality of ceramic elements together.  
         [0025]      FIG. 3  is an exploded view of a single cell  80  of a non-thermal plasma reactor. The cell  80  includes a first ceramic element  82 , a second ceramic element  84 , a third ceramic element  86 , and a fourth ceramic element  88 . A pair of spacers  89  are located between the second and third ceramic elements  84 ,  86 . The first ceramic element  82  includes a charge electrode  90  having a connector  92 . The second ceramic element  84  includes a charge electrode  91  having a connector  93 . The third ceramic element  86  includes a ground electrode  94  also having a connector  95 . The fourth ceramic element  88  includes a ground electrode  97  also having a connector  99 . The connectors  92 ,  93  of charge electrodes  90  and  91  are offset from the connectors  95  and  99  of ground electrodes  94  and  97  for reasons explained below. The electrodes  90 ,  91 ,  94 ,  97  and their connectors  92 ,  93 ,  95 ,  99  can comprise silver, tantalum, platinum, or any other conductive metal. They are applied to the ceramic elements  82 ,  84 ,  86  and  88  as is known in the art via any of a number of ways. These include painting, screen printing, and spray application. Each element  82 ,  84 ,  86 , and  88  has an edge  96 . Prior to the present invention the elements  82 ,  84 ,  86 ,  88  and the spacers  89  would need to be glued, clamped, and then fired to cure the glue. This was typically accomplished in the past by initially assembling the elements  82 ,  84 ,  86 ,  88  and spacers  89  using high temperature dielectric paste, clamping, and then firing to transform the paste into a sintered glass/ceramic dielectric bond layer.  
         [0026]     In  FIG. 4  an edge  96  view of an assembled non-thermal plasma reactor stack is shown at  100 . The components are as described above. Additionally, ceramic endplates  103  without electrodes are placed on either side of the stack  100  to insulate the stack  100 . Once the stack  100  is assembled it is clamped into work holder  18  and held in place. Then using the spray parameters described above a first band  98  of electrically conductive material was applied by the kinetic spray process described herein. The first band  98  replaces the previously used glue and serves to hold the elements of the stack  100  together. The first band  98  is applied over the set of connectors  92 ,  93  thereby electrically coupling all of the first and second element  82 ,  84  electrodes  90 ,  91  to each other. A second band  102  of electrically conductive material was applied by the kinetic spray process described herein. The second band  102  also replaces the previously used glue and serves to hold the elements of the stack  100  together. The second band  102  is applied over the other set of connectors  95 ,  99  thereby electrically coupling all of the third and fourth element  86 ,  88  electrodes  94 ,  97  to each other. Stack  100  may be further sprayed by the kinetic spray process described herein on the edge opposite edge  96  to further secure the elements together. The thickness of the first and second bands  98 ,  102  may vary from  1  millimeter to 2.5 centimeters depending on the stack  100  configuration. Generally, the material forming the bands  98 ,  102  is applied to the edge  96  at an angle of from 0 to 45 degrees relative to a line drawn normal to the edge  96 . More preferably the angle is from 15 to 25 degrees. In some embodiments it can be desirable to apply a corrosion resistant layer over bands  98 ,  102  either by kinetic spray applying a material such as tantalum or thermal spaying another ceramic. Such thermal spray methods are known in the art. The corrosion resistance layer is preferably form 20 microns to 1 millimeter in thickness.  
         [0027]      FIG. 5  also shows a stack  112  as described in  FIG. 4  with the difference that a first band  104  includes a conductive wire or ribbon  106  embedded in the band  104  while the kinetic spray process is occurring. The wire or ribbon  106  can be directly connected to a power source. Likewise a second band  108  includes a conductive ribbon or wire  110  that was embedded in the band  108  while the kinetic spray process was occurring.  
         [0028]     The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and do come within the scope of the invention. Accordingly, the scope of legal protection afforded this invention can only be determined by studying the following claims.