Abstract:
Methods are disclosed for batch fabrication of vacuum switch tubes that reduce manufacturing costs and improve tube to tube uniformity. The disclosed methods comprise creating a stacked assembly of layers containing a plurality of adjacently spaced switch tube sub-assemblies aligned and registered through common layers. The layers include trigger electrode layer, cathode layer including a metallic support/contact with graphite cathode inserts, trigger probe sub-assembly layer, ceramic (e.g. tube body) insulator layer, and metallic anode sub-assembly layer. Braze alloy layers are incorporated into the stacked assembly of layers, and can include active metal braze alloys or direct braze alloys, to eliminate costs associated with traditional metallization of the ceramic insulator layers. The entire stacked assembly is then heated to braze/join/bond the stack-up into a cohesive body, after which individual switch tubes are singulated by methods such as sawing. The inventive methods provide for simultaneously fabricating a plurality of devices as opposed to traditional methods that rely on skilled craftsman to essentially hand build individual devices.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The United States Government has certain rights in this invention pursuant to Department of Energy Contract No. DE-AC04-94AL85000 with Sandia Corporation. 
    
    
     FIELD OF THE INVENTION 
     The invention generally relates to cold cathode vacuum switch tubes that by action of a trigger signal are capable of rapidly and repeatedly, switching large currents at high voltages, in extreme environments of shock, radiation and temperature. The invention further relates to methods for batch fabrication (e.g. a plurality fabricated simultaneously) of vacuum switch tubes providing lower cost and enhanced product uniformity over methods based on hand assembly of individual piece parts (e.g. individuals fabricated serially). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and form part of the specification, illustrate several embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings provided herein are not drawn to scale. 
         FIG. 1  is a schematic cross-sectional illustration of a non-limiting embodiment of a vacuum switch tube as can be fabricated by embodiments of methods according to the present invention. 
         FIG. 2  is a schematic cross-sectional exploded view of an embodiment of a stacked assembly of layers comprising a plurality of spacedly adjacent switch tube sub-assemblies as can be fabricated by methods according to the present invention. 
         FIG. 2A  is a detailed cross-sectional view of the stacked assembly of layers from  FIG. 2 . 
         FIG. 3  is a schematic plan view of the stacked assembly of layers comprising a plurality of spacedly adjacent switch tube assemblies from  FIG. 2 . 
         FIG. 4  is a schematic cross-sectional view of an embodiment of a stacked assembly of layers from  FIG. 2 , after joining and prior to singulation of individual vacuum switch tubes. 
     
    
    
     BACKGROUND 
     Vacuum switch tubes are needed in applications requiring stand-off of high voltages (e.g. on the order of 500V to 6 kV DC) and fast switching of large currents (e.g. on the order of 300 A to 20 kA). Such applications can include triggering of air-bags, the initiation of explosives, control of high energy physics equipment, power supplies and capacitive discharge units (CDUs). Vacuum switch tubes are typically configured to provide an open circuit, non-conducting condition between a current source and a load. The tube is activated (e.g. switch closed) by a triggering signal to affect an electric discharge within the tube, switching the tube to a closed circuit conducting condition, thereby allowing the passage of current from the source to the load. Further discussion of the operation of vacuum switch tubes can be found for example, in U.S. Pat. No. 5,739,637 to Boettcher, and in “Investigation into Carbon-Trigger Vacuum Switches for High-Voltage, High-Current Switch Applications”, by K. J. Bunch, et al., presented at the 7 th  IEEE International Vacuum Electronics Conference (IVEC) Apr. 25-27, 2006, Monterey, Calif., the entirety of each of which is herein incorporated by reference 
     The assembly of vacuum switch tubes typically requires piece-part hand assembly by highly skilled craft workers which makes them too expensive for many applications. Additionally, piece-part hand assembly of individual units (e.g. individuals fabricated serially) results in variations in assembly which can affect the part to part uniformity of the device&#39;s operational characteristics. What are needed are methods for batch fabrication (e.g. a plurality fabricated simultaneously) of vacuum switch tubes to 
     The present invention addresses this need for batch fabrication of vacuum switch tubes by providing methods that comprise stacking an assembly of layers comprising a plurality of tube sub-assemblies, aligned through one or more common layers, and heating the assembly of layers in a vacuum oven to affect joining (e.g. bonding) of the individual layers into a cohesive structure. Joining can be accomplished by methods such as; traditional metallization of ceramics followed by brazing, active metal brazing without the use of ceramic metallizations, or direct brazing methods, again not requiring the use of ceramic metallizations. The latter two approaches yield an additional reduction in the cost of units produced, by eliminating the processing steps and costs associated with producing metallized layers on bare ceramics. Additional descriptions of the traditional metallization and brazing, active metal brazing and direct brazing methods can be found for example in: “Comparison of Metal-Ceramic Brazing Methods”, by C. A. Walker et al., presented at the 36 th  International Brazing and Soldering Symposium, Chicago, Ill., Nov. 13-14, 2007, the entirety of which is incorporated herein by reference. 
     The bonded structure can then be singulated (e.g. by dicing, laser scribing, sawing etc.) to separate out the individual vacuum switch tubes. The vacuum joining process can produce an evacuated environment (e.g. on the order of 1×10(−7) mmHg) in the vicinity of the anode, cathode and trigger electrodes of the vacuum switch. The methods according to the present invention, by employing a stacked assembly of layers, provides for fabricating a plurality of switch tubes simultaneously in a batch fabrication approach, greatly eliminating hand assembly and piece part counts, thereby reducing the cost of producing a vacuum switch tube. Methods according to the present invention additionally reduce the spread in operational characteristics on a part to part basis, compared to methods based on traditional hand assembly of individual units. Methods according to the present invention can further reduce the cost of switch tubes by employing joining (e.g. brazing) processes that do not require the metallization of ceramic components. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic cross-sectional illustration of a non-limiting embodiment of a vacuum switch tube, as can be fabricated by embodiments of methods according to the present invention. Vacuum switch tube  100  comprises anode  102  and cathode comprising graphite block  104  separated by a gap  108  (e.g. on the order of 0.010″ to 0.100″) in a vacuum environment (e.g. less than atmospheric pressure) created within ceramic insulator  110  (i.e. in conjunction with other tube components as described below). Vacuum switch tube  100  can comprise a generally cylindrical (i.e. tubular) configuration symmetrical about the longitudinal axis or can as well comprise a rectangular shape or other external shape as a particular application may require with no effect on the practice of the methods of the invention. The cathode can comprise graphite block  104  inserted (e.g. press-fit as illustrated) within a metal (e.g. niobium, Nb) support  112  that can provide electrical connectivity to the graphite block  104 . Trigger electrode  106  (e.g. niobium) is connected to trigger probe  114  that can comprise a ceramic insulator  116  partially coated with a carbon source film  118  and press-fit into carbon block  104 . Trigger probe  114  comprises a conductor  121  extending through the body of the ceramic insulator  116 . Carbon film  118  can comprise a carbonaceous film deposited by methods such as chemical vapor deposition (CVD) or other method (e.g. sputtering, diamond like carbon (DLC) hand painting) onto ceramic insulator  116 . The carbon film  118  can be deposited onto the end face  128  and sidewalls of ceramic insulator  116  and subsequently scribed and/or partially removed by methods such as mechanical abrasion or laser ablation, to prevent the deposited carbon film  118  from extending down the entire length of the sidewalls of the ceramic insulator  116 . Conductor  121  can comprise a niobium wire press-fit through the ceramic insulator  116  and provides electrical connection from the trigger electrode  106 , through the carbon film  118  and cathode graphite block  104 , to the cathode support  112 . Trigger electrode  106  is electrically isolated from the cathode support  112  by ceramic insulator  120 . The anode-cathode gap  108  can be controlled for example, by fabricating the cathode structure of two conductive (e.g. niobium) layers  112  and  122  and adjusting the thickness of a layer e.g.,  122  to achieve a gap  108  of desired dimension. One or more braze alloy layers  124  can be used to join together and hermetically seal the various layers comprising switch tube  100 . 
     The vacuum switch tube  100  operates by creation of carbon plasma to initiate an arc breakdown between the anode  102  and cathode  104  that can have a voltage differential on the order of 500V to 6 kV DC. In the static state, no current flows between anode  102  and cathode  104  and the tube  100  is said to be in an open circuit non-conducting condition. A trigger signal (e.g. on the order of 10 to 100 volts) is applied to trigger electrode  106  causing current to flow through conductor  121  and across the carbon film  118  on the end face  128  of the ceramic body  116  to the graphite cathode block  104 . Resistive heating vaporizes a portion of the carbon film  118  on the end face  128  and creates a heated carbon vapor within the anode-cathode gap  108 . The high voltage between the anode  102  and cathode  104  strips electrons from atoms within the carbon vapor and quickly establishes an arc breakdown between the anode  102  and the cathode  104 , causing the tube to be in a closed circuit conducting condition. Ablated carbon from the deposited film  118  and carbon cathode  104  can deposit on the walls of the ceramic insulator  110  and lead to a conductive path being formed between the anode  102  and cathode  104  leading to a premature device failure. The configuration illustrated wherein the anode  102  has a “cupped” shape (e.g. as a niobium sheet formed by stamping into the cup shape illustrated) creates a shadowed region  126  on the walls of the ceramic insulator  110  effectively preventing the occurrence of this failure mechanism. 
     As illustrated in  FIG. 1 , by employing active metal brazing or direct brazing methods, no metallization layers are required on the ceramic insulators  110  and  120 , as would be required for traditional metallize and braze methods. This results in a considerable cost savings for the switch tubes produced. Active metal brazing methods employ braze alloys that include a reactive element (e.g. Ti, Zr) capable of reducing oxides on the surface of a ceramic component during the brazing process, thereby allowing the braze alloy to “wet” the ceramic without the need for metallized layers to effect such wetting. Examples of commercially available active metal braze alloy compositions include: {59% Ag, 27.25% Cu, 12.5% In, 1.25% Ti}, {63% Ag, 35.25% Cu, 1.75% Ti}, {62% Cu, 35% Au, 2% Ti, 1% Ni} and {97% Ag, 1% Cu, 2% Zr}. There are many more commercially available compositions these are merely cited here as non-limiting examples. Direct brazing methods employ traditional braze alloys that are found in combination with certain metal-ceramic couples (e.g. joints) to interact with the metal and ceramic layers to form an adherent metallic oxide layer on the ceramic. Some non-limiting examples of commercially available braze alloys suitable for use in direct brazing of niobium to ceramic (e.g. 94% alumina) include: {62% Cu, 35% Au, 3% Ni}, {82% Au, 8% Pd} and {50% Au, 50% Cu}. 
     As described above, one of the advantages of embodiments of the present invention is the capability for batch fabrication of vacuum switch tubes of the type illustrated by the non-limiting example in  FIG. 1 . It will be appreciated that other vacuum switch tube configurations can be produced by the methods of the present invention, and that the configuration in  FIG. 1  is merely used to exemplify the methods as described herein. 
       FIG. 2  is a schematic cross-sectional exploded view of an embodiment of a stacked assembly of layers comprising a plurality of spacedly adjacent switch tube sub-assemblies as can be fabricated by methods according to the present invention.  FIG. 2A  is a detailed cross-sectional view of the stacked assembly of layers from  FIG. 2  (without alignment layers as described below). It can be desired to perform one or more cleaning operations on the various components comprising the assembly prior to stacking the various layers. In an exemplary application, vapor degreasing of the components was followed by vacuum firing metallic (e.g. niobium) and graphite piece parts at approximately 1450° C. in a vacuum atmosphere less than 1×10(−6) mmHg for approximately 30 minutes and, air firing the ceramic (e.g. alumina) components at approximately 1000° C. for approximately 60 minutes in air. The assembly of layers can be placed on a supporting substrate  230 A (e.g. an alumina plate as an aligning layer) having clearances  240  for insertion of alignment pins (not shown) to aid in the registration and assembly of the stacked assembly. A metallic trigger electrode layer  206  is placed on the supporting substrate  230 A over which a braze alloy layer  224   a  is placed followed by a ceramic insulator layer  220  and then braze alloy layer  224   b . Insulator layer  220  (e.g. 94% alumina ceramic) separates and electrically isolates the cathode layer comprising metallic support structure  212  (e.g. niobium) from trigger electrode layer  206 . The cathode layer comprises a plurality of spaced clearances (e.g. through-holes) in the support structure  212  through which graphite cathode blocks  204  are inserted. In embodiments of the invention, it has been found structurally sufficient to press fit (e.g. interference fit) the graphite blocks  204  through the clearances of support structure  212 . While a brazing or other joining method has not been found necessary for joining the graphite blocks  204  into the support structure  212 , one could be used (e.g. a braze alloy) if desired, without effecting the practice of the present invention. 
     Trigger probe assemblies  214  comprising a ceramic insulator  216  having a carbon coating  218  on an end face  228  and extending down a portion of the sidewall of the ceramic insulator  216  are likewise press fit through clearances through the graphite cathode blocks  204 . A metallic conductor (e.g. niobium pin)  221  extends through the ceramic insulator  216  and provides eventual electrical connectivity from the trigger electrode layer  206  through the carbon film  218  and to the graphite block  204  of the cathode structure. It has been found that the pin  221  can be press fit through the ceramic insulator  216  and a slight deformation of the head of the pin  221  (e.g. by swedging or cold forming) at end face  228  can be used if desired to insure electrical connectivity to the carbon film  218 . Mechanically, the pin  221  is joined to the trigger electrode  206  by braze alloy layer  224   a . The carbon coated end face  228  of the trigger probe  214  can be arranged to be substantially even with the top surface of the graphite block  204  and is arranged to face the eventual anode  202 . The cathode layer can as well comprise a second metallic layer  222  (e.g. niobium) as described above to allow easy adjustment of the eventual anode-cathode gap  108 . 
     A second ceramic insulator layer  210  (e.g. 94% alumina) is disposed onto braze alloy layer  224   d  for eventual joining to the cathode layer, comprising support structure  212  gap adjustment layer  222  and graphite block  204 . The second ceramic insulator layer  210  comprises a plurality of clearances through which a plurality of anodes  202  are each placed into, substantially in alignment with a trigger probe  214 . The anode structure can comprise a niobium member (e.g. disk)  202  formed or stamped (e.g. cup shaped) to comprise a protrusion arranged to extend through the clearance gap in ceramic insulator  210  and a flange for joining the disk to the ceramic insulator  210  by means of braze alloy layer  224   e . A second niobium member (e.g. disk)  201  can be joined to the first niobium member  202  by means of braze alloy layer  224   f  to facilitate making electrical contact to the anode structure, i.e. by providing a flat surface for an electrical contact. 
     A second support substrate  230   b  (e.g. alumina plate, alignment layer) having clearances for the anodes and alignment through-holes  240  for insertion of alignment pins (not shown) can be placed on top of the stacked assembly and by means of corresponding through holes in the common layers (e.g. braze alloy layers, ceramic insulator layers, anode and cathode layers) serve to align the layers and components of the eventual switch tube sub-assemblies. Other alignment mechanisms can be employed as well such as aligning the edges of the various layers against mechanical stops, as a particular application may warrant. Parting lines “A”, “B”, “C” and “D” indicate approximate locations where the eventual joined stack-up can be sawn or cut to singulate individual vacuum switch tubes from the joined (e.g. bonded) assembly of layers. 
       FIG. 3  is a schematic plan view of the stacked assembly of layers comprising a plurality of spacedly adjacent switch tube assemblies from  FIG. 2 .  FIG. 3  illustrates a three by three array of nine vacuum switch tubes as can be batch fabricated by embodiments of methods according to the present invention. Parting lines “A”, “B”, “C” and “X”, “Y”, “Z” indicate approximate sawing lines for singulating the switch tubes. Batch fabrication of nine switch tubes is indicated, but any number as convenient for an application can be produced by the methods of the invention. 
       FIG. 4  is a schematic cross-sectional view of an embodiment of a stacked assembly of layers from  FIG. 2 , after joining (e.g. brazing) and prior to singulation of individual vacuum switch tubes  100 . After assembling the layers including the common layers, alignment layers and switch tube components as described above, the entire assembly of layers is placed into a vacuum oven, evacuated (e.g. to a pressure on the order of 1×10(−7) mmHg) and heated to effect reflow of the braze alloy layers  224 , thereby sealing a vacuum in the region of the anode-cathode gap  108 , and joining the layers into a cohesive structure. Placing weights on the top of the assembly to facilitate good contact between the layers can be employed during the brazing process. For an embodiment wherein the ceramic insulator layers comprise approximately 94% alumina, and are joined to niobium anode, cathode and trigger electrode layers using a commercially available direct braze alloy comprising approximately (62% Cu, 35% Au, 3% Ni) joining can be effected by heating the assembly under vacuum to approximately 1060° C., and allowing the assembly to cool while under vacuum, until the braze alloy solidifies. As described above, active metal braze alloys could be used as well with neither approach requiring the metallization of the ceramic insulator layers. After the joining operating, individual switch tubes  100  can be cut from the stacked assembly  200  by sawing along parting lines, e.g. “A”, “B”, “C” “D”, “W”, “X”, “Y” and “Z” as illustrated in  FIG. 3 , to produce switch tubes as exemplified by the embodiment illustrated in  FIG. 1 . 
     In a non-limiting exemplary embodiment of the invention, vacuum switch tubes of a configuration as illustrated in  FIG. 1  were fabricated having a diameter of approximately 0.225 inches, overall thickness of 0.160 inches, and anode cathode gap of 0.025 to 0.028 inches, for an application requiring the switching of 1500 volts and 5000 amperes (0.2 micro-farads) wherein the individual layer definitions (from  FIG. 2A ) were as given in Table 1. 
                                       TABLE 1                   Exemplary layer definitions.                    Layer Thickness           Layer ID   Material   (inches)   Description               206   Niobium   0.010   Trigger Electrode       224a   62% Cu, 35% Au,   0.002   Braze Alloy           3% Ni               220   94% Alumina   0.040   Insulator       224b   62% Cu, 35% Au,   0.002   Braze Alloy           3% Ni               214   Coated Alumina   0.060   Trigger Probe       204   Graphite   0.020   Cathode Graphite                   Block       212   Niobium   0.030   Cathode Support       224c   62% Cu, 35% Au,   0.004   Braze Alloy           3% Ni               222   Niobium   0.010   Cathode Gap                   Adjustment Layer       224d   62% Cu, 35% Au,   0.004   Braze Alloy           3% Ni               210   94% Alumina   0.050   Insulator       224e   62% Cu, 35% Au,   0.001   Braze Alloy Washer           3% Ni       0.160″ OD       202   Niobium   —   Anode, 0.160″ OD ×                   0.076″       224f   62% Cu, 35% Au,   0.001   Braze Alloy Disk           3% Ni       0.160″ OD       201       —   Anode Cap, 0.160″ ×                   0.005″ Thick                    
While it is not necessary for all braze alloy layers to comprise the same composition, it can be convenient in many applications to do so.
 
     The above described exemplary embodiments present several variants of the invention but do not limit the scope of the invention. Those skilled in the art will appreciate that the present invention can be implemented in other equivalent ways. The actual scope of the invention is intended to be defined in the following claims.