Abstract:
A method of producing a ceramic-metal-halide (CMH) discharge lamp having a monolithic seal between a sapphire (single crystal alumina) arc tube and a polycrystalline alumina end cap. The method includes the steps of providing an arc tube of fully dense sapphire and providing an end cap made of unsintered compressed polycrystalline alumina powder. The end cap is heated until it is presintered to remove organic binder material at a low temperature relative to the sintering temperature. The presintered end cap is placed on an end portion of the arc tube to form an interface therebetween. The assembled presintered end cap and arc tube are then heated to the sintering temperature wherein the end cap is fully sintered onto the arc tube and the sapphire tube grows into the end cap. A monolithic seal is formed at the previous interface between the end cap and the arc tube as the sapphire tube grows into the polycrystalline alumina end cap.

Description:
This is a division of U.S. patent application Ser. No. 09/022,323, filed Feb. 11, 1998, now U.S. Pat. No. 6,126,889. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to sealing arc tubes for high-pressure discharge lamps and, more particularly, to sealing arc tubes composed of sapphire for high-pressure discharge lamps. 
     High-pressure discharge lamps, such as ceramic-metal-halide (CMH) lamps, commonly utilize ceramic arc tubes which are transparent or translucent. The ceramic tube should have high-corrosion resistance, high-temperature capabilities, and high light transmissivity. The opposite ends of the ceramic arc tube are closed and sealed by ceramic end assemblies such as plugs or caps. The end assemblies also support discharge electrodes made of molybdenum or tungsten. The electrodes extend through the end assemblies and are hermetically sealed therein. An arc discharge is formed within the tube between the electrodes when current is applied to the electrodes. 
     The metal halide arc tubes can be composed of polycrystalline alumina which has superior chemical attack resistance and higher practical operating temperatures than customary quartz metal halide arc tube materials. Polycrystalline alumina is a preferred arc tube material in current commercial practice. The polycrystalline alumina arc tubes are typically sealed with polycrystalline end plugs. 
     It has been proposed to use sapphire (single crystal alumina) instead of polycrystalline alumina as the arc tube material in order to gain an additional increase in lamp performance. The increased performance is primarily due to sapphire&#39;s increased level of transmission, compared to polycrystalline alumina. 
     An issue with fabricating sapphire (single crystal alumina) arc tubes, however, is sealing the ends of the arc tube. Conventional methods of sealing quartz and polycrystalline arc tubes have not proven to be satisfactory. Different crystal orientations of sapphire have different thermal coefficients of expansion. The crystal orientation of the sapphire arc tube, therefore, must be precisely oriented so that its thermal expansion coefficient closely matches the thermal expansion coefficient of the plugs or caps in the direction of greatest expansion and/or contraction. When the crystal orientation of the sapphire tube is not precisely oriented in this manner, rapid changes in temperature can crack the sapphire arc tube. Accordingly, there is a need in the art for an improved method of joining end assemblies to sapphire arc tubes. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of making a tube assembly for a ceramic-metal-halide discharge lamp. The method includes the steps of providing a tube made of sapphire or single crystal alumina and providing an end cap made of unsintered polycrystalline alumina. The end cap is heated until it is presintered to remove binder material. The presintered end cap is then placed on an end portion of the tube to form an interface therebetween. The presintered end cap and the tube are heated until the end cap is sintered onto the tube and the sapphire crystal of the tube grows into the end cap to form a monolithic seal at the previous interface between the end cap and the tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the present invention will be apparent with reference to the following description taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a side elevational view, in cross-section, of one end of a lamp assembly having a sapphire arc tube and a ceramic end cap prior to firing according to the present invention; 
     FIG. 2 is a side elevational view, in cross-section, similar to FIG. 1 but after firing to form a monolithic seal between the arc tube and the end cap; 
     FIG. 3 is a side elevational view, in cross-section, of one end of a lamp assembly having a sapphire arc tube and a ceramic end cap prior to firing according to a second embodiment of the present invention; and 
     FIG. 4 is a side elevational view, in cross-section, similar to FIG. 3 but after firing to form a monolithic seal between the arc tube and the end cap; 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an end of a ceramic metal halide (CMH) lamp assembly  10  according to the present invention. It is noted that both ends of the lamp assembly  10  are identical or substantially similar, therefore, only one end of the lamp assembly  10  is shown and described herein in detail. The lamp assembly  10  includes a high-pressure envelope or arc tube  12  which is transparent, end bushings or caps  14  sealing the open ends of the arc tube  12 , and electrode assemblies  16  extending through and supported by the end caps  14  to form an arc within the sealed arc tube  12  when electrical current is applied to the electrode assemblies  16 . 
     The transparent arc tube  12  is formed from sapphire (single crystal alumina) which is fully dense. The arc tube can be produced in any suitable manner. See, for example, U.S. Pat. Nos. 5,427,051, 5,451,553, 5,487,353, 5,588,992, and 5,683,949, for suitable methods of producing sapphire arc tubes, the disclosures of which are expressly incorporated herein in their entirety by reference. 
     The arc tube  12  is tubularly-shaped having annularly-shaped end surfaces  17  and cylindrically-shaped outer and inner surfaces  18 ,  20 . The wall thickness can be of any suitable size. 
     The end caps  14  are formed from a suitable polycrystalline ceramic material, preferably polycrystalline alumina, which is in an unsintered or “green state”. The end caps  14  most preferably include about 0.02 to about 0.2 percent by weight MgO with polycrystalline alumina powder. 
     The end caps  14  are preferably formed by cold die pressing a mixture of fine ceramic powder into the desired shape which is described in detail hereinafter. The end caps  14 , however, can alternatively be formed by compressing ceramic powder into a body or block and machining the desired shape from the block, by injection molding, or by any other suitable process. 
     Each end cap  14  has a disc-shaped main wall  22 , a cylindrically-shaped skirt or flange  24 , and a tubularly-shaped extension  26 . The main wall  22  has a planar inner surface  28  facing the end surface of the arc tube  12  and a planar outer surface  30  facing away from the end surface of the arc tube  12 . 
     The flange  24  axially extends inward toward the arc tube  12  from the outer periphery of the main wall  22 . The main wall  22  and flange  24  cooperate to form a cup or socket for receiving the end portion of the arc tube  12  therein. The flange  24  has a cylindrically-shaped inner surface  32  which has a diameter sized to form a sufficient monolithic seal with the outer surface  18  of the arc tube  12  as discussed in more detail hereinbelow. The length of the flange inner surface  32  is sized to provide a sufficient sealing area between the end cap  14  and the arc tube  12  as discussed in more detail hereinbelow. 
     The extension  26  axially extends outward from the outer surface  30  of the main wall  22  and is located generally at the center of the main wall  22 . The extension  26  and the main wall  22  cooperate to form an axially extending aperture or hole  34  which passes entirely through the end cap  14 . The aperture  34  is sized and shaped to form a sufficient hermetic seal between the electrode assembly  16  and the end cap  14  as discussed in more detail hereinafter. Preferably, the aperture  34  is cylindrically-shaped. The length of the extension  26  is sized to provide sufficient support for the electrode assembly  16  and to provide a sufficient sealing area between the end cap  14  and the electrode assembly  16 . 
     The electrode assembly  16  is of standard construction having a generally straight support  36  and a coil  38  secured to the inner end of the support  36 . The support  36  and the coil  38  are each formed from a high temperature and electrically conductive metal such as molybdenum or tungsten. 
     The “green” end caps  14  are initially heated to a prefiring or presintering temperature to remove organic or binder material and to develop green strength. The prefiring temperature is relatively low compared to the sintering temperature. Preferably, the prefiring temperature is in the range of about 900° C. to about 1100° C. The prefiring is preferably performed in air but alternatively can be any other suitable oxidizing atmosphere for burning-off the organic material. 
     Once cooled, the presintered end caps  14  are placed over the ends of the arc tube  12  with the end surfaces  17  of the arc tube  12  engaging the inner surfaces  28  of the end cap main walls  22  and the outer surface  18  of the arc tube  12  engaging the inner surfaces  32  of the end cap flanges  24 . The end caps  14 , therefore, close the open ends of the arc tube  12 . 
     As best shown in FIG. 2, the arc tube  12  and the end caps  14  are heated to a sintering and/or crystal growing temperature which creates a monolithic seal between the arc tube  12  and the end caps  14 . Preferably, the sintering temperature is in the range of about 1800° C. to about 1900° C. The sintering is preferably performed in hydrogen but alternatively can be in vacuum, helium, or any other suitable reducing atmosphere. The monolithic seal is created at both the previous interfaces, the first interface  40  between the arc tube end surfaces  17  and the end cap inner surfaces  28  and the second interface  42  between the arc tube outer surface  18  of end cap inner surfaces  32 . 
     Because, the end caps  14  are “green”, they shrink as they are heated to the sintering temperature. The sapphire arc tube  12  is fully dense so it does not shrink in size as it is heated to the sintering temperature. The arc tube  12  and the end caps  14  are preferably sized so that the shrinkage of the end caps  14  produces an inner diameter of the end caps  14  which is about 3% to about 7% smaller than the outer diameter of the arc tube  12  after sintering. The shrinkage of the end caps  14  creates stress which drives formation of the monolithic seal, as it facilitates an exaggerated grain growth process. The sapphire (single crystal alumina) of the arc tube  12  grows into the polycrystalline end caps  14  to form the monolithic seal. Continued heat treatment at the sintering temperature anneals out any stresses initially created at the interfaces due to the shrinkage of the end caps  14 . 
     In FIG. 2, the broken lines indicate the previous interfaces  40 ,  42  between the arc tube  12  and the end caps  14 . It is to be understood, however, that there is no longer a discontinuity between the components  12 ,  14  and the monolithic seal is completely continuous across the previous interfaces. It should also be understood that there is a visible boundary, which is not precisely at the previous interfaces, between the polycrystalline region having grain boundaries and the sapphire region which does not have grain boundaries. Such a boundary is shown in FIG. 2 of U.S. Pat. No. 5,451,553, the disclosure of which is expressly incorporated herein in its entirety by reference. 
     The end caps  14  can be doped with boundary mobility enhancing materials such as, for example, Gallium or Chromium. The dopants enhance pore removal at the interface and the growth of the sapphire (single crystal alumina) into the polycrystalline alumina. Alternatively, the interface region of the components  12 ,  14  can be painted with the boundary enhancing materials. 
     The electrode assemblies  16  are coated with a conventional sealant and frit and are inserted into the apertures. The assembly  10  is then refired to fuse the sealant and provide a hermetic seal between the ceramic end caps  14  and the metal electrode assemblies  16  in a known manner. 
     FIG. 3 illustrates an end of a ceramic metal halide (CMH) lamp assembly  44  according to a second embodiment of the present invention wherein like references numbers are used for like structure. The lamp assembly  44  is similar to the lamp assembly  10  described with reference to FIG. 1 except that the end caps  14  have an annularly shaped groove  46  rather than the flange  24  (FIG.  1 ).  45   
     The groove  46  axially extends outward into the main wall  22  from the inner surface  28  of the main wall  22 . The groove  46  forms a seat or socket for receiving the end portion of the arc tube  12  therein. The groove  46  is formed by an annularly-shaped bottom surface  48 , a cylindrically-shaped outer surface  50 , and a cylindrically-shaped inner surface  52 . The outer surface  50  has a diameter sized to form a sufficient monolithic seal with the outer surface  18  of the arc tube  12  and the inner surface  52  has a diameter sized to form a sufficient monolithic seal with the inner surface  20  of the arc tube  12 . The axial length or depth of the groove  46  is sized to provide a sufficient sealing area between the end cap  14  and the arc tube  12 . 
     Once the end caps  14  are presintered as discussed hereinabove with reference to the first embodiment, the end caps  14  are placed over the ends of the arc tube  12  with the end surfaces  17  of the arc tube  12  engaging the bottom surfaces  48  of the end cap grooves  46 , the outer surface  18  of the arc tube  12  engaging the outer surfaces  50  of the end cap grooves  46 , and the inner surface  20  of the arc tube  12  engaging the inner surfaces  52  of the end cap grooves  46 . 
     As best shown in FIG. 4, a monolithic seal is created between the arc tube  12  and the end caps  14  upon sintering. The monolithic seal is not created at all of the interfaces. The monolithic seal is created at the first interface  40  between the arc tube end surfaces  17  and the groove bottom surfaces  28 , and the second interface  42  between the arc tube outer surface  18  and the groove outer surfaces  50 , but not between the arc tube inner surface  20  and the groove inner surface  52 . Due to shrinkage of the “green” end caps  14  during the sintering step, an annularly shaped gap or space is created between the arc tube inner surface  20  and the groove inner surface  52  as the groove inner surface  52  pulls away from the arc tube inner surface  20 . This gap is preferably filled with a suitable glassy phase material  54  to further seal the end caps  14  to the arc tube  12 . 
     Although a particular embodiment of the invention has been described in detail, it will be understood that the invention is not limited correspondingly in scope, but includes all changes and modifications coming within the spirit and terms of the claims appended hereto.