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.
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.
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.
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.
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.
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.
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.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Referring first to
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.
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.
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.
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.
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.
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.
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.
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.
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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.
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.