Translucent pca ceramic, ceramic discharge vessel, and method of making

Abstract

A translucent polycrystalline material suitable for use in ceramic discharge vessels for metal halide lamps is produced by sintering an alumina powder doped with a MgO sintering aid in a nitrogen atmosphere containing a partial pressure of a vapor phase carbon-containing species. The sintered polycrystalline alumina has a grain boundary phase containing aluminum, oxygen and nitrogen. The formation of the Al—O—N grain boundary phase is believed to facilitate the transport of nitrogen from entrapped pores during sintering. Preferably, the PCA is sintered in a carbon-element furnace under flowing ultra-high-purity nitrogen.

1 - 7 . (canceled)  
     
     
         8 . A method of sintering ceramic body to translucency, the method comprising: 
 (a) placing a ceramic body comprised of aluminum oxide doped with MgO in a furnace containing a carbon source and a nitrogen atmosphere formed by a nitrogen gas source having a total impurity level of no more than about 0.005% by volume;    (b) sintering the ceramic body at a temperature from about 1800° C. to about 2000° C. to form a sintered translucent ceramic body.    
     
     
         9 . The method of  claim 8  wherein the furnace atmosphere during sintering contains from about 1×10 −12  atm to about 1×10 −7  atm of carbon.  
     
     
         10 . The method of  claim 8  wherein the furnace atmosphere during sintering contains less than about 0.2% hydrogen by volume.  
     
     
         11 . The method of  claim 9  wherein the furnace atmosphere during sintering contains from about 1×10 −3  atm to about 1×10 −4  atm CO.  
     
     
         12 . The method of  claim 9  wherein the furnace atmosphere during sintering contains less than about 0.2% hydrogen by volume.  
     
     
         13 . The method of  claim 8  wherein the ceramic body is sintered for about 1 hour to about 70 hours.  
     
     
         14 . The method of  claim 8  wherein the ceramic body is sintered at a temperature from about 1850° C. to about 1950° C. for about 4 hours to about 50 hours.  
     
     
         15 . The method of  claim 8  wherein the ceramic body is sintered at about 1900° C. for about 10 hours.  
     
     
         16 . The method of  claim 8  wherein the furnace is a carbon-element furnace and the carbon source is one or more components of the furnace.  
     
     
         17 . The method of  claim 16  wherein the furnace atmosphere during sintering contains from about 1×10 −12  atm to about 1×10 −7  atm of carbon.  
     
     
         18 . The method of  claim 17  wherein the furnace atmosphere during sintering contains less than about 0.2% hydrogen by volume.  
     
     
         19 . The method of  claim 18  wherein the ceramic body is sintered at a temperature from about 1850° C. to about 1950° C. for about 4 hours to about 50 hours.  
     
     
         20 . A method of sintering a ceramic body to translucency, the method comprising: 
 (a) placing a ceramic body comprised of aluminum oxide doped with about 100 ppm to about 1000 ppm MgO in a carbon-element furnace containing a nitrogen atmosphere formed by a nitrogen gas source having a total impurity level of no more than about 0.005% by volume;    (b) sintering the ceramic body at a temperature from about 1800° C. to about 2000° C. for about 1 hour to about 70 hours to form a sintered translucent ceramic body, the furnace atmosphere during sintering containing from about 1×10 −12  atm to about 1×10 −7  atm of carbon and less than about 0.2% hydrogen by volume.