Patent Publication Number: US-2007096649-A1

Title: Electrode-mounted getter

Description:
BACKGROUND  
      A getter is a material that is formulated to absorb undesired chemical impurities in a sealed environment. Lamps, such as high pressure discharge lamps, typically use getters in order to enhance their performance and useful life. Getters in lamps are activated by high temperature and collect and capture undesirable contaminants while the lamp is operating. These contaminants can affect various performance characteristics of the lamp (e.g. ignition voltage, useful life) if they are not captured. In high pressure discharge lamps, for example, getters can be used to absorb hydrogen to limit deterioration of vacuum pressure or gas purity due to gas release from the hot lamp burner.  
      A getter is frequently a moldable material, which is often shaped into a pill or tablet. The tablet is then attached to a metal casing, which can be welded into place within the lamp. Getters also generally require high temperature for activation. Consequently, in a typical installation the getter is not located at the hottest spot within the lamp, with the result that its performance is not optimized. Being away from the hot spot, it may take longer for the getter to reach operating temperature, and the getter may never reach the proper temperature for optimum effectiveness.  
      To provide more thermal energy to the getter, some lamps have located the getter on one of the spider arms of a lamp cathode. While this helps heat the getter, it decentralizes the getter and can create blockage of light emission, particularly unbalanced blockage, hindering the operation and efficiency of the lamp.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Various features and advantages of the invention will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, which together illustrate, by way of example, features of the invention, and wherein:  
       FIG. 1  is a side, cross-sectional view of an electrode assembly of a high pressure projector lamp having an annularly shaped getter attached to the cathode;  
       FIG. 2  is a perspective view of an embodiment of an annularly shaped getter configured for attachment to a lamp electrode;  
       FIG. 3  is a plan view of another embodiment of an annularly shaped getter configured for attachment to a lamp electrode;  
       FIG. 4A  is a side, cross-sectional view of one embodiment of an annularly shaped getter configured for mounting on a lamp electrode;  
       FIG. 4B  is a side, cross-sectional view of one embodiment of an annularly shaped getter configured for mounting on a lamp electrode;  
       FIG. 5  is a detail cross-sectional view of the getter cup aperture rim of  FIG. 4A ;  
       FIG. 6A  is a perspective view of another embodiment of an annularly shaped getter; and  
       FIG. 6B  is a perspective view of yet another embodiment of an annularly shaped getter. 
    
    
     DETAILED DESCRIPTION  
      Reference will now be made to exemplary embodiments illustrated in the drawings, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Alterations and further modifications of the inventive features illustrated herein, and additional applications of the principles of the invention as illustrated herein, which would occur to one skilled in the relevant art and having possession of this disclosure, are to be considered within the scope of the invention.  
      As noted above, getters are often used in lamps, such as high pressure projector lamps, to absorb impurities in the sealed lamp environment. Some of these contaminants or impurities may be introduced during manufacture of the lamp. Others can be created during operation of the lamp, such as through outgassing from a hot lamp electrode. Impurities and contaminants in a high temperature lamp environment can cause plating, corrosion, and other deterioration of lamp components, which can affect the durability and performance characteristics of the lamp, such as its ignition voltage, brightness, and useful life. The getter is designed to collect and absorb these impurities to prevent this damage.  
      Unfortunately, the design and placement of the getter has a great influence on its effectiveness. Getters are heat activated. However, the minimum activation temperature of getter materials can vary. Some getters can be activated at temperatures as low as 300° C. if that temperature is maintained over a relatively long duration (e.g. 5 hours), while others require a much higher activation temperature (e.g. 750° C. to 900° C.). Both the cathode and anode of many projector lamps can reach temperatures above 1000° C. However, some areas of the lamp away from the electrodes may not reach these temperatures, or even reach the getter activation temperature, or may reach the getter activation temperature only slowly. If the getter does not reach an optimum operating temperature, the getter will not properly absorb impurities in the lamp environment, thus contributing to deterioration of the lamp components. Furthermore, if the getter does not reach its activation temperature quickly, damage can be done each time the lamp is turned on during the time interval after actuation of the lamp and before the getter reaches its activation temperature.  
      Shown in  FIG. 1  is a cross-sectional view of one embodiment of a high pressure Xenon projector lamp assembly  10 . This lamp generally includes a reflector housing  12 , and an anode  14  mounted in the housing. A lamp nose cap  16  is attached to the housing with an insulator material  18  therebetween, and a lamp window  20  is provided in the nose cap. The cathode  22  is mounted to a cathode strut  24  within the nose cap, and the cathode post extends to a point just opposite the anode.  
      As shown in  FIG. 1 , an annularly shaped getter  26  is mounted on the cathode post  22  in the lamp  10 . While the getter is shown mounted to the cathode, the designation of anode and cathode could be reversed, and the getter could also be attached to the anode. Consequently, the references herein to a lamp electrode are intended to encompass both cathodes and anodes.  
      An embodiment of a getter like that shown in  FIG. 1  is shown in greater detail in  FIG. 2 . The getter includes a substrate  28  with a central aperture  30  that is configured to press-fit upon the cathode post. The substrate can have an open-faced trough or cup shape, as shown in  FIG. 2 , though other shapes are also possible, as discussed below. The terms “getter cup” and “getter trough” are also used herein to refer to the getter substrate. When pressed down toward the base  32  of the cathode post (in the direction of arrows  34  in  FIG. 1 ), the getter is placed in a location that minimizes its potential obstruction of light produced by the lamp. However, since the getter is in direct mechanical contact with the cathode, and the cathode reaches a high temperature very quickly, the getter also heats up very quickly, reaching its activation temperature quickly and maintaining that temperature.  
      A closer view of one embodiment of an annularly shaped electrode mountable getter  26  is provided in  FIG. 2 . In this view the annular cup-like shape of the getter substrate is apparent. The getter cup can be of metal, such as nickel plated iron, steel, molybdenum, stainless steel, nickel steel (Kovar), tungsten, titanium, tantalum, or other thermally conductive materials capable of withstanding the high temperature lamp environment. A metal getter cup can be inexpensive and simple to manufacture, and also conducts heat very well.  
      The getter material is generally moldable and can be shaped into any desired shape. Getter materials are typically formed as a powder, and then pressed onto a metal substrate in some particular shape, such as a tablet. In the present application, the getter material can be pressed onto/into the getter cup in the desired shape. Other means of dispensing the getter may be possible depending upon the getter type and manufacturing process used. In the embodiment of  FIG. 2 , a cylindrical ring  36  of getter material, having a toroid or donut shape, is attached within the getter cup  28 . Alternatively, as shown in  FIG. 4B , the getter material can be shaped as a half toroid  38  to exactly or very nearly match the shape of the getter cup. Optimizing the exposed surface area of the getter material can be advantageous, depending upon the lamp hermeticity and design. Generally, the larger the surface area, the more rapidly the getter will absorb impurities. However, there is also a molecular migration within the getter material, so that the getter will continue to absorb impurities until the getter is fully saturated. Consequently, the amount or rate of absorption can remain relatively constant over the life of the getter.  
      Other configurations of the getter and its substrate are also possible, as illustrated in  FIGS. 6A-6B . As shown in  FIG. 6A , an alternative getter  60   a  can include a getter substrate that is a generally planar annular disc  62  having a central sleeve  64  that extends upwardly from the plane of the disc and is configured to slide over and press onto the lamp electrode. A substantially flat cylindrical disc  66   a  of getter material (essentially a tablet shape with a center portion punched out) is attached to the upper face of the getter substrate surrounding the sleeve. As shown in  FIG. 6B , another alternative getter  60   b  can include a lower flat cylindrical disc of getter material  66   b  that is attached to the lower face of the getter substrate, in addition to the upper disc of getter material  66   a , providing two layers of getter material attached to one getter substrate. It will be apparent that the diameter and thickness of the getter material and the getter substrate can vary.  
      The getter material can be any of a variety of materials that are used as getters. A particular material is selected for a particular application depending upon the undesirable impurities that are to be collected and captured while the lamp is operating. For example, one type of getter material may be suitable for absorbing hydrogen (H 2 ), while another material is suitable for absorbing carbon dioxide (CO 2 ). Getter materials that absorb multiple types of impurities are also available. Those skilled in the art will be able to select a proper getter material for a given application.  
      A wide variety of getters are commercially available. One commercial source of getters for a variety of applications is SAES Getters of Milan, Italy. For a sealed Xenon projector lamp such as that pictured in  FIG. 1 , the inventor has used an ST-101 getter from SAES Getters. The ST-101 getter is a zirconium-aluminum alloy that is configured to absorb hydrogen (H 2 ), oxygen (O 2 ), carbon monoxide (CO), carbon dioxide (CO 2 ), nitrogen (N 2 ), water (H 2 O), and methane (CH 4 ).  
      As noted above, the activation temperature for getter materials varies. For the electrode-mounted getter disclosed herein, higher activation temperatures are desirable because brazing, bake-out, and other operations during the lamp manufacturing process produce elevated temperatures. If the activation temperature of the getter material is relatively low, these higher temperature manufacturing processes could prematurely activate the getter, causing the getter to become saturated with impurities before the lamp is sealed. To help prevent premature saturation, a getter material with a higher activation temperature is desirable to resist activation during the manufacturing process.  
      The ST-101 getter requires a temperature of about 900° C. for 30 seconds to activate, but will activate faster if the temperature is higher. The ST-101 getter will also activate when exposed to a temperature of 800° C. for 5 minutes, or 750° C. for 20-30 minutes. Other getter materials have different activation time and temperature characteristics. Those skilled in the art will be able to select a proper getter material for a given temperature range.  
      The cross-sectional shape of the annular getter substrate can vary. For example, the getter cup  28  depicted in  FIG. 4A  has a generally squared shape, with the upper rim  31  of the central aperture  30  extending slightly higher than the outer rim  40 . Alternatively, as shown in  FIG. 4B , the outer rim  42  and inner rim  43  can be configured to have roughly the same height, with the getter cup  28   a  having a more rounded shape. It will be apparent that other shapes are also possible.  
      Referring to  FIGS. 3 and 4 , the central aperture  30  of the getter cup  28  has a diameter D g  that can be slightly smaller than the diameter D c  of the cathode. This allows the getter cup to be pressed onto the cathode  22  and obtain a snug fit. Where the getter cup is of metal, the malleability of the metal will allow the smaller getter aperture to deform slightly to conform to the size of the cathode and provide a secure fit. The sleeve  64  of the flat disc getter substrate  62  shown in  FIGS. 6A and 6B  can also include this smaller diameter aperture for providing a press fit.  
      Additionally, as shown in the cross-sectional views of  FIGS. 4A and 4B , the bottom of the getter cup can have a radius r g  that helps promote sliding of the getter onto the cathode. This radius on the under inside of the cup can assist alignment and pressing of the getter assembly into location. This radius can vary, as shown, for example, in the different embodiments of  FIGS. 4A and 4B .  
      The press-fit operation is illustrated in dashed lines in  FIG. 1 . Prior to placement, the getter  26  is aligned with the free end  44  of the cathode  22 , and with the getter aperture  30  aligned with the cathode, the getter is pressed down onto the electrode in the direction of arrows  34 . The radius r g  of the bottom side of the getter cup helps the getter slide down the cathode post. A point  46  on the free end of the cathode can also assist in this press-on operation.  
      The getter cup  28  can also be provided with a number of features that provide affixing structure to enhance the press fit and resist retraction of the getter cup from the cathode  22 . As shown in  FIG. 4A , the upper rim  31  of the central aperture  30  of the getter cup can have a lip or ridge  48  on its inside edge. This lip or ridge  48  can be shaped to dig in the cathode post, so that the getter will slide on in one direction, but resist retraction in the other direction. As seen more clearly in  FIG. 5 , this lip or ridge can be configured to have a beveled or knife edge  50  that has a sloped lower side  52  that allows sliding down onto the cathode, while a perpendicular top side  54  causes the knife edge to tend to dig into the cathode to resist retraction. This lip or ridge can be substantially or completely continuous around the inner edge of the annular aperture in the getter cup, or it can be discontinuous or in one or more discrete portions.  
      An alternative configuration for the retraction-resistant lip or ridge is shown in  FIG. 3 . In this embodiment, several locking tabs or wedges  56  extend inwardly from the inner edge of the getter cup aperture  30 . When the getter cup is pressed onto the cathode  22 , these tabs deflect to allow sliding of the getter post in the aperture. However, once in place, the tabs will naturally spring back toward their undeflected orientation, and will tend to dig into the cathode post to resist retraction. This will help secure the getter on the post. While three tabs are shown in  FIG. 3 , the getter cup can be configured with just one or any number.  
      The locking tabs  56  shown in  FIG. 3  can have a constant thickness. Alternatively, as shown in  FIG. 4B , the locking tabs can have a wedge shaped configuration similar to that shown in  FIG. 5  to help promote the pressing of the getter onto the cathode. Those skilled in the art will be able to determine the size, number, and configuration of the lip, ridges, tabs or other affixing structure for securing the getter cup to the cathode. It will be apparent that the various retraction resistant features shown and described with respect to the trough or cup shaped getter substrates of  FIGS. 1-4  can also be provided in the flat disc substrate embodiment shown in  FIGS. 6A and 6B .  
      This getter configuration provides a number of desirable features. Because the getter substrate is pressed onto the cathode post, the need for welding can be eliminated. Further, pressing the getter substrate on to the cathode post locates the getter close to the hot spot in the lamp. The avoidance of welding provides a simpler installation process, and eliminates a source of deposits and contaminants in the lamp, which can create undesirable lamp performance issues or necessitate a follow-up cleaning process. Additionally, by placing the getter substrate in intimate contact with the heated cathode electrode, the getter collects heat through the thermally conductive substrate and surroundings, so that a higher temperature can be reached and reached more quickly for activation of the getter material.  
      The getter is also shaped and sized to minimize shadows or blockage of light out of the reflector housing. Since the getter is located on the cathode shaft, which already creates some light blockage, and is positioned near the base of the cathode shaft, any additional blockage created by the getter is minimal, and the blockage is centralized for less unbalanced light obstruction and more uniform light out of the reflector housing.  
      It is to be understood that the above-referenced arrangements are illustrative of the application of the principles of the present invention. It will be apparent to those of ordinary skill in the art that numerous modifications can be made without departing from the principles and concepts of the invention as set forth in the claims.