Abstract:
A method of making a component of a ceramic discharge chamber comprising the steps of forming a ceramic composition including an aluminum powder, a binder and a grain growth inhibitor. The alumina powder has a tap density greater than 1.0 gram per cc. The ceramic composition is then die pressed to form a component preform. Next, the component preform is heated to remove at least a substantial portion of the binder.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates generally to lighting and more particularly, to ceramic discharge chambers for a discharge lamp, such as a ceramic metal halide lamp or a high pressure sodium discharge lamp.  
           [0002]    The present invention relates generally to lighting, and more specifically, to a ceramic arc chamber for a discharge lamp, such as a ceramic metal halide lamp. This invention relates particularly to a method of manufacturing ceramic arc chambers, and more particularly, to a method for forming ceramic arc chambers.  
           [0003]    Discharge lamps produce light by ionizing a fill such as a mixture of metal halides and mercury with an electric arc passing between two electrodes. The electrodes and the fill are sealed within a translucent or transparent discharge chamber which maintains the pressure of the energized fill material and allows the emitted light to pass through it. The fill, also known as a “dose”, emits a desired spectral energy distribution in response to being excited by the electric arc.  
           [0004]    Previously, the discharge chamber in a discharge lamp was formed from a vitreous material such as fused quartz, which was shaped into a desired chamber geometry after being heated to a softened state. Fused quartz, however, has certain disadvantages which arise from its reactive properties at high operating temperatures. For example, at temperatures greater than about 950 to 1,000° C., the halide fill reacts with the glass to produce silicates and silicon halide, reducing the fill constituents. Elevated temperatures also cause sodium to permeate through the quartz wall. These fill depletions cause color shift over time, which reduces the useful life of the lamp.  
           [0005]    Ceramic discharge chambers were developed to operate at high temperatures for improved color temperatures, color renderings, and luminous efficacies, while significantly reducing reactions with the fill material. U.S. Pat. Nos. 4,285,732 and 5,725,827, for example, disclose translucent polycrystalline sintered bodies where visible wavelength radiation is sufficiently able to pass through to make the body useful for use as an arc tube.  
           [0006]    Typically, ceramic discharge chambers are constructed from a number of parts extruded or die pressed from a ceramic powder and then sintered together. For example, referring now to European Patent Application No. 0587238, five ceramic parts are used to construct the discharge chamber of a metal halide lamp. Two end plugs with a central bore are fabricated by die pressing a mixture of a ceramic powder and inorganic binder. A central cylinder and the two legs are produced by extruding a ceramic powder/binder mixture through a die. After forming the part, it is typically air sintered between 900-1400° C. to remove organic processing aids. Assembly of the discharge chamber requires tacking of the legs to the cylinder plugs, and the end plugs into the end of the central cylinder. This assembly is then sintered to form joints which are bonded by controlled shrinkage of the individual parts. Obviously, a simplified form of the product would be achieved by the reduction in the number of components separately formed. Moreover, the step of properly joining the compounds is time consuming, expensive and a potential point of failure.  
           [0007]    Typically, ceramic discharge chambers are constructed from a number of parts extruded or die pressed from a ceramic powder. For example, end plugs with the central bore may be fabricated by die pressing a mixture comprising a ceramic powder and an organic binder. A central cylinder, and the two legs may be produced by extruding a ceramic powder/binder mixture through a die. Assembly of the discharge chamber involves the placement and tacking of the legs to the end plugs and the end plugs into the ends of the central cylinder. This final assembly is then sintered to form four centered joints which are bonded by controlled shrinkage of the individual parts. The conventional ceramic discharge chamber method of construction has a number of disadvantages. For example, the number of component parts is relatively large and introduces the corresponding number of opportunities for variation and defects. Also, the conventional discharge chamber includes four bonding regions, each of which introduces an opportunity for lamp failure by leakage of the fill material if the bond if formed improperly. Each bonding area also introduces a region of relative weakness, so that even if the bond is formed properly, the bond may break during handling or be damaged enough in handling to induce failure in operation.  
           [0008]    Another disadvantage relates to the precision with which the parts can be assembled and the resulting effect in the light quality. It is known that the light quality is dependent to a substantial extent on the voltage across the electrode gap, which in turn is dependent on the size of the gap consistently achieve the gap size within an acceptable tolerance without significant effort devoted to optimizing the manufacturing process. Accordingly, it would be desirable to minimize the component parts necessary to manufacture the ceramic arc chamber. However, divergent shrinkage rates of variously shaped components and other factors have limited the ability to manufacture in a more efficient manner.  
         BRIEF SUMMARY OF THE INVENTION  
         [0009]    According to an exemplary embodiment of the invention, a method is provided for making a component of a ceramic discharge chamber by forming a ceramic composition including a ceramic powder, a binder, and a grain growth inhibitor. The ceramic powder has a tap density of greater than about 1.0 gm per cc. The ceramic composition is then die pressed to form the desired ceramic discharge chamber preform component. The preform component can then be assembled with additional preform components into a presintered discharge chamber and sintered to join the components via controlled shrinkage.  
           [0010]    The preform components of the present invention can facilitate the assembly of an arc discharge chamber from fewer components than demonstrated previously. Moreover, the present method allows intricate shaped components to be die pressed. More specifically, the end cap members can be die pressed in the shape of the disk shaped body and leg extension. This clearly provides a manufacturing simplicity versus conventional manufacturing steps which included the extrusion of leg portions which must then be secured to end caps or extensive machining.  
           [0011]    Exemplary embodiments of the invention can be used to improve the performance of various types of lamps, such as metal halide metals, high pressure mercury vapor lamps, and high pressure sodium vapor lamps. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    Other features and advantages of the invention will be readily understood upon reading the following detailed description. in conjunction with the drawings, in which:  
         [0013]    [0013]FIG. 1 illustrates a light source which includes a ceramic discharge chamber according to the exemplary embodiment of the invention; and  
         [0014]    FIGS.  2 A- 2 C illustrate components of a discharge chamber for a metal halide lamp. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]    [0015]FIG. 1 illustrates a discharge lamp  10  according to an exemplary embodiment of the invention. Discharge lamp  10  includes a discharge chamber  50  which contains two electrodes  52 ,  54  and fill material. Electrodes  52 ,  54  are connected to conductors  56 ,  58 , which apply a potential difference across the electrodes. In operation, the electrodes  52 ,  54  produce an arc which ionizes a fill material to produce a plasma in the discharge chamber  50 . The emission characteristics of the light produced by the plasma depend primarily on the constituents of the fill material, the voltage across the electrodes, the temperature distribution of the chamber, the pressure in the chamber, and the geometry of the chamber. For a ceramic metal halide lamp, the fill may typically comprise a mixture of Hg, a rare gas such as Ar or Xe and a metal halide such as NaI, TlI DyI 3 . For high pressure sodium lamp, the fill material typically comprises sodium, a rare gas, and Hg. Other fill materials are also well known in the art, and the present invention is believed to be suitable for operation with any of those recognized ionizable materials.  
         [0016]    As shown in FIG. 1, the discharge chamber  50  comprises a central body portion  60  and two leg portions  62 ,  64 . The ends of the electrodes  52 ,  54  are typically located near the opposite ends of the body portion  60 . The electrodes are connected to a power supply by the conductors  56 ,  58  which are disposed within a central bore of each leg portion  62 ,  64 . The electrodes are typically comprised of tungsten. The conductors typically comprise niobium and molybdenum which have thermal expansion coefficients close to that of alumina to reduce thermally induced stresses on the alumina leg portion  62 ,  64 .  
         [0017]    The discharge chamber  50 , is sealed at the ends of the leg portions  62 ,  64  with seals  66 ,  68 . The seal  66 ,  68  typically comprise a disprosia-alumina-silica glass that can be formed by placing a glass frit in the shape of a ring around one of the conductors, eg.  56 , aligning the discharge chamber  50  vertically and melting the frit. The melted glass then flows down into the leg  62 , forming a seal between the conductor  56  and the leg  62 . The discharge chamber is then turned upside down to seal the other leg  64  after being filled with the fill material.  
         [0018]    The leg portion  62 ,  64 , extends axially away from the center of the discharge chamber  50 . The dimensions of the leg portions  62 ,  64  are selected over the temperature of the seal  66 ,  68  by desired amount with respect to the center of the discharge chamber  50 . For example, in a 70 watt lamp, the leg portion portions have a length of about 10-15 mm, an inner diameter of 0.8-1.0 mm and an outer diameter of about 2.5-3.0 mm to lower the temperature at the seal  66 ,  68  to about 600 to 700° C., which is about 400° C. less than the temperature at the center of the discharge chamber. In a 35 watt lamp, the leg portions have a length of about 10-15 mm, an inner diameter of 0.7 to 0.8 mm and an outer diameter of about 2.0-2.5 mm. In a 150 watt lamp, the leg portions have a length of about 12-15 mm and an inner diameter of about 0.9-1.1 mm, and an outer diameter of about 2.5-3.0 mm. These dimensions, and others through the specification, are of course given as examples and are not intended to be limiting.  
         [0019]    The body portion  60  of the discharge chamber is typically substantially cylindrical. For a 70 watt lamp, the body portion typically has an inner diameter of about 7 mm and outer diameter of about 8.5 mm. For a 35 watt lamp, the body portion typically has an inner diameter of about 5 mm and an outer diameter of about 6.5 mm. For a 150 watt lamp, the body portion typically has an inner diameter of about 9.5 mm and an outer diameter of 11.5 mm.  
         [0020]    An exemplary embodiment of the invention is provided in FIGS. 2A, 2B and  2 C, demonstrating a discharge chamber formed from three components. FIGS.  2 A- 2 C illustrate components of a discharge chamber formed from three elements. In FIG. 2B, a body member  122  is shown which is substantially cylindrical. The body member  122  of FIG. 2B can be formed by injection molding, die pressing, or by any other technique known in the art. For example, the body member  122  can also be formed by extrusion. The composition used may comprise, for example, 75% by weight alumina powder, 22% by weight of water soluble polyacrylamide and 3% by weight of stearate. The alumina powder may also be doped with magnesia.  
         [0021]    The leg member  124  is depicted which includes a leg portion  112  and a transition portion  114 . Both the leg portion  112  and the transition portion  114  include a central bore  109  which houses one of the two electrodes and the conductor. Transition portion  114  may be generally in the form of a plug which fits inside the end of the body member  122 . Transition portion  114  typically has a circumference which is greater than the circumference of the leg portion  112 . Transition portion  114  typically includes a radially directed flange  115  which projects radially outwardly from transition portion  114 . The radially directed flange  115  provides a shoulder  117  which rests against the end  119  of the body member  100  during assembly and fixes the relative axial position of leg member  124  with respect to the body member  122 . “Axiar” refers to an axis through the central bores  107 ,  109  in leg portions  112 ,  126 . The radially directed flange  115  provides the advantage of the total length of the assembled discharge chamber, e.g. measured from the end  118  of leg member  120  to the opposite end  116  of leg member  124  can be maintained to within a tight dimensional tolerance. The total length of the discharge chamber typically effects the separation between the electrodes, since the electrodes are typically referenced to the ends  116 ,  118  of the leg portions,  120 ,  126  during assembly. For example, the conductor may be crimped at a fixed distance from the end of the electrode, which crimp rests against of the leg portion to fix the axial position of the electrode with respect to the leg portion. Because the axial position of the electrodes is fixed with respect to the leg portions, the separation of the electrodes is determined by the position of the leg member  124  with respect to the body member  122  which can be precisely controlled by the radially directed flange  115 . The radially directed flange  115  thus allows the electrodes to be consistently positioned to have a precise separation distance, which provides consistency and quality of the light produced.  
         [0022]    The leg members  120 ,  124  are constructed by die pressing a mixture of ceramic powder in a binder. Typically, the mixture comprises between about 80 and 99% by weight ceramic powder and about 1 and 20% by weight organic binder, preferably between about 95 and 98% and 2 and 5%, respectively. The ceramic powder may comprise alumina (Al 2 O 3 ) having a purity of at least 99.98% and a surface area of about 2-10 meters squared per gram. The alumina powder will have a tap density greater than 1 g/cc. Alumina powder may be doped with magnesia to inhibit grain growth, for example in an amount equal to 0.03%-0.2%, preferably 0.05% by weight of the alumina. Accordingly, the present ceramic powder mixture allows die pressing of the complex leg member shape without tacking the leg to the body or extensive machining.  
         [0023]    Other ceramic materials which may be used include non-reactive refractory oxides and oxynitrides such as yttrium oxide and hafnium oxide and compounds of alumina such as yttrium-alumina-garnet and aluminum oxynitride. Binders which may be used individually or in combination include organic polymers, such as polyols, polyvinyl alcohol, vinyl acetates, acrylates, cellulosics and polyesters.  
         [0024]    Subsequent to formation, the binder is removed from the green part, typically by thermopyrollisis, to form a bisque-fired part. The thermopyrollisis may be conducted, for example, by heating the green part in air from room temperature to a maximum temperature of about 900-1100° C. over 48 hours, then holding the maximum temperature from 1-5 hours, and cooling the part. After thermopyrollisis, the porosity of the bisque-fired part is typically about 40-50%. The bisque-fired part is then machined. For example, a small bore may be drilled along the axis of the solid cylinder which provides bore  107  in leg member  120 . Next a large diameter bore may be drilled along the portion of the axis to form the chamber  101 . Finally, the outer portion of the originally solid cylinder may be machined away along part of the axis, for example, by lathe, to form an outer surface of leg portion  126 . The machined parts are typically assembled prior to sintering to allow the sintering step to bond the parts together.  
         [0025]    According to an exemplary method of bonding, the densities of the bisque-fired parts used to form the body member  122  and the leg members  120 ,  124  are selected to achieve different degrees of shrinkage during the sintering step. The different densities in the bisque-fired parts may achieved by using ceramic powders having different surface areas. For example, the surface area of the ceramic powder used to form body member  122  may be 10-15 meters squared per gram, while the surface area of the ceramic body used to form the leg members  120  and  124  may be 2-4 meters squared per gram. The finer powder in the body member  122  causes the bisque-fired body member  122  to have a lower density than the bisque-fired leg members  120  and  124  made from the coarser powder. Because the bisque-fired body member is less dense than the bisque-fired leg members, the body portion shrinks to a greater degree (eg 3-10%) during sintering than the transition portion  114  to form a seal along transition portion  114 .  
         [0026]    The sintering step may be carried out by heating the bisque-fired parts in hydrogen having a dew point of about 10-15°. Typically, the temperatures increase from room temperature to about 1300° C. over a two hour period. Next, the temperature is held to about 1300° C. for about 2 hours. Next, the temperature is increased by about 100° C. per hour up to a maximum temperature of about 1850-1880° C. Next, the temperature is held at 1850-1880° C. for about 3.5 hours. Finally, the temperature is decreased from room temperature for two hours. The resulting ceramic material comprises densely sintered polycrystalline aluminum.  
         [0027]    An exemplary composition which has been used for die pressing the leg members  120 ,  124 , comprises 95% by weight alumina powder having a surface area of 3-5 meters squared per gram, available from Reynolds Chemicals, as product number RC-HPT. The alumina powder is preferably spray dried and is formed via dry milling. The alumina powder is typically doped with magnesia in the amount of 0-0.05% of the weight of the alumina. The composition also includes 4% by weight polyvinyl alcohol and 1% by weight Carbowax 600, available from Interstate Chemical.  
         [0028]    The alumina powder or other ceramic of choice, will have a tap density greater than 1.0 gram per cc as defined by ASTM B527-93 (1997). More preferably, the tap density will be in the range of 1.2 to about 1.5 grams per cc. The resultant ceramic powder composition can be die pressed according to a fill ratio of at least about 1.8.  
         [0029]    Die pressing at approximately 10,000 pounds per square inch is typically employed. Preferably, a Model 602 Double Action Press from Pentronix, Inc. may be used as the die pressing equipment.  
         [0030]    Although the invention has been described with reference to exemplary embodiments, various changes and modifications can be made without departing from the scope and spirit of the invention. These modifications are intended to fall within the scope of the invention, as defined by the following claims.