Abstract:
A integral starting aid for high intensity discharge lamps is provided wherein the starting aid comprises a conductive, refractory metal nitride stripe which is directly applied to the surface of the ceramic arc tube. Preferably, the starting aid comprises titanium nitride or zirconium nitride and may be mixed a ceramic material to improve translucency.

Description:
BACKGROUND OF INVENTION 
     Conventional starting aids for high intensity discharge lamps, and in particular high pressure sodium (HPS) lamps, have traditionally consisted of a tungsten wire wrapped around, or positioned alongside, the ceramic arc tube. The tungsten wire is welded to a frame member or an electrode feedthrough during the manufacturing process to provide electrical contact with the lamp&#39;s power supply. This basic type of starting aid has been manufactured for many years and generally performs reliably over the life of the lamp. However, pure tungsten wire is relatively expensive and labor and time are required to form the welds. 
     More recently, lamp manufacturers have used an integral starting aid which consists of a printed stripe directly sintered to the ceramic arc tube. The stripe is made of either pure tungsten or a tungsten/alumina cermet. For example, U.S. Pat. No. 5,541,480, which is incorporated herein by reference, describes a polycrystalline alumina (PCA) arc tube having an integral tungsten ignition aid which is applied as a tungsten-containing paste prior to sintering the arc tube to translucency. These integral starting aids reduce the costs of manufacturing but may not perform as reliably as tungsten wire starting aids. In particular, because the starting aid is applied as a thin stripe directly to the surface, any mismatch in the thermal expansion coefficients of the ceramic arc tube and starting aid materials becomes important. The high temperature of the operating arc tube coupled with the on-off thermal cycling which occurs throughout the operating life of the lamp can cause the thin stripe to break and lose electrical continuity thereby rendering it inoperable. In addition, the geometry of the starting aid is also limited by thermal expansion mismatches. If the starting aid is applied too thickly, the induced stress from the thermal expansion mismatch can cause the arc tube to crack. Therefore, it would be desirable for the material comprising the starting aid to have a thermal expansion coefficient which closely matches that of the arc tube material. 
     SUMMARY OF INVENTION 
     It is an object of the invention to obviate the disadvantages of the prior art. 
     It is another object of the invention to provide an integral starting aid for high intensity discharge lamps wherein the thermal expansion coefficient of the starting aid material is closely matched to that of the ceramic arc tube. 
     It is still another object of the invention to provide an integral starting aid which is capable of withstanding the high temperatures and thermal cycling of the operating arc tubes of high intensity discharge lamps. 
     In accordance with one object of the invention, there is provided an integral starting aid comprised of a conductive, refractory metal nitride stripe applied directly to the surface of a ceramic arc tube. The metal nitride stripe may be applied by a number of conventional means including aerosol spraying, ink pen, ink-jet, or vapor deposition. In one aspect, the integral starting aid is comprised of a thin stripe containing titanium nitride or zirconium nitride. The starting aid is sintered with the ceramic arc tube to bond it to the arc tube surface. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is an illustration of a ceramic arc tube having an integral starting aid on the exterior surface of the arc tube. 
    
    
     DETAILED DESCRIPTION 
     Refractory metal nitrides such as titanium nitride and zirconium nitride are advantageous for integral starting aids because they are conductive, have high melting points, and their thermal expansion coefficients closely match that of the conventional polycrystalline alumina (PCA) arc tubes at the tube&#39;s operating temperature of about 1400K. The physical properties of titanium nitride (TiN) and zirconium nitride (ZrN) are compared in Table 1 with tungsten (W) and alumina (Al 2 O 3 ). Like tungsten, the metal nitrides, have melting points above that of alumina and possess low electrical resistivity. However, unlike tungsten, the nitrides possess thermal expansion coefficients which closely match that of alumina at 1400K. For this reason, it is expected that starting aids comprised of metal nitrides should outlast pure tungsten and W—Al 2  O 3  starting aids when subjected to lamp operating conditions and thermal cycling. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Linear 
                   
                   
                   
               
               
                   
                 Expansion 
                   
                   
                   
               
               
                   
                 Coefficient 
                 Electrical 
                 Melting 
                   
               
               
                   
                 at 1400 K 
                 Resistivity 
                 Point 
                   
               
               
                 Material 
                 (×10 31  /K) 
                 at ˜25° C. (μΩ) 
                 (° C.) 
                 Color 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Al 2 O 3   
                 10.1 
                 1 × 10 22   
                 2015 
                 White 
               
               
                 TiN 
                 10.5 
                 21.7 
                 2930 
                 Gold 
               
               
                 ZrN 
                 119.1 
                 13.6 
                 2980 
                 Gold 
               
               
                 W 
                 5.4 
                  5.7 
                 3410 
                 Black 
               
               
                   
               
             
          
         
       
     
     The metal nitride starting aid of this invention may be combined in a powdered form with a organic vehicle and applied as an aerosol spray or as an ink using a pen, brush, ink-jet, or similar printing means. Vapor deposition techniques such as vacuum sputtering and chemical vapor deposition (CVD) are also expected to be useful for applying the metal nitride starting aid. However, such means may prove impractical because of the high cost of vapor deposition equipment and the difficulties associated with applying vapor deposition to large-scale manufacturing. 
     In a preferred method, a prefired PCA arc tube is formed using standard ceramic fabrication techniques, e.g., isopressing or extruding of doped powders into a tubular shape and prefiring the tube in air to remove the binder material. A stripe containing the metal nitride is then applied directly to the porous tube via aerosol spray coating. The aerosol spray consists of the metal nitride and a carrier, e.g., TiN powder in an alcohol/acetone-based carrier. A titanium nitride-containing aerosol spray is commercially available as Traycoat TN Aerosol (ZYP Coatings, Inc., Oak Ridge, Tenn.). The stripe dimensions and shape are controlled by masking the arc tube surface tube except in the area for the desired stripe. The metal nitride may be blended with a ceramic material, preferably aluminum oxide or aluminum oxynitride, to improve the translucency of the starting aid. The prefired, striped arc tube is then sintered to full density, e.g., at 1880° C. for 1 hour in a flowing N 2  −8% H 2  atmosphere during which the metal nitride simultaneously sinters onto the PCA arc tube. The properties of a TiN starting aid are compared with conventional tungsten and tungsten-alumina starting aids in Table 2. The TiN sinters well without decomposition and provides an electrically conductive stripe. The sintered TiN starting aid adhered well to the PCA with no coloration of the PCA substrate, and yielded an arc tube having acceptable in-line and total transmittance. 
     
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 In-Line 
                 Total 
                 Resistance 
                   
                   
                 Thick- 
               
               
                   
                 Trans. 
                 Trans. 
                 of Stripe @ 
                 Length 
                 Width 
                 ness 
               
               
                 Material 
                 (%) 
                 (%) 
                 25° C. (Ω) 
                 (mm) 
                 (μm) 
                 (μm) 
               
               
                   
               
             
             
               
                 PCA/TiN 
                 5.7 
                 90.3 
                  9.3 
                 20 
                 1000 
                 10 
               
               
                 stripe 
               
               
                 PCA/W 
                 4.6 
                 94.6 
                  1.6 
                 75 
                  278 
                 27 
               
               
                 stripe 
               
               
                 PCA/W- 
                 5.6 
                 94.9 
                 21.0 
                 76 
                  210 
                 25 
               
               
                 25% Al 2 O 
               
               
                 cermet 3   
               
               
                 stripe 
               
               
                 PCA/no 
                 6.0 
                 95.0 
                 NA 
                 NA 
                 NA 
                 NA 
               
               
                 stripe 
               
               
                   
               
             
          
         
       
     
     FIG. 1 is an illustration of the integral starting aid applied to a ceramic arc tube for a high pressure sodium lamp. The arc tube  1  has a tubular body  3  comprised of polycrystalline alumina. The integral starting aid is comprised of longitudinal stripe  7  and transverse stripes  5 . The longitudinal stripe extends substantially along the length of the arc tube body and is connected at either end to a transverse stripe  5  which extends circumferentially around the tubular body  3 . The starting aid is applied to the exterior surface of tubular body  3  by a conventional ink dispensing means. 
     While there has been shown and described what are at the present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.