Patent Publication Number: US-2011062851-A1

Title: Holding Rod

Description:
TECHNICAL FIELD 
     The invention relates to a holding rod for holding an anode or cathode in accordance with patent claims  1  and  5  and to a discharge lamp having at least one such holding rod. 
     PRIOR ART 
     Discharge lamps, in particular mercury vapor or xenon short-arc lamps, generally have two holding rods for holding their anode and cathode in a discharge chamber, said two holding rods consisting of tungsten doped with potassium. One disadvantage of this material composition is the fact that it is very brittle and thus, in the case of high-wattage discharge lamps, i.e. in particular in the case of lamps with a wattage greater than 2 kW, breakages occur again and again during transportation since such discharge lamps have very heavy anodes and long holding rod lengths. Mentioned by way of example is a conventional 5 kW mercury vapor short-art lamp which has an anode mass of approximately 1000 g and a holding rod length of approximately 100 mm. 
     One possibility for avoiding such breakages during transportation consists in increasing the size of the cross section of the holding rods such that the holding rods can accommodate the large anode masses. Owing to the compact design of the discharge lamps, however, such a geometric enlargement is only possible to a limited extent. 
     Another possibility for avoiding breakages during transportation is considered to be that of increasing the strength of the holding rods. A known measure is the use of thoriated tungsten in place of tungsten doped with potassium, which, however, has the disadvantage that the thorium used for this purpose is radioactive and therefore such a holding rod represents a radioactive load for the environment. 
     DESCRIPTION OF THE INVENTION 
     The invention is based on the object of providing a holding rod for holding an anode or cathode of a discharge lamp, which has a high strength in order to avoid breakages and whose material composition does not represent a radioactive load for the environment. The invention is likewise based on the object of providing a discharge lamp having at least one such holding rod. 
     This object is achieved as regards the holding rod by the features of patent claims  1  and  5  and as regards the discharge lamp by the features of patent claim  9 . Particularly advantageous embodiments of the invention are described in the dependent claims. 
     The holding rod according to the invention for a discharge lamp, in particular a mercury vapor or xenon short-arc lamp, for holding an anode or cathode in an interior of a discharge vessel contains molybdenum doped according to the invention. 
     An alternative solution according to the invention is provided by a holding rod, in the case of which the holding rod contains tungsten doped with at least one metal oxide compound. The metal oxide compound likewise results in an increase in the strength of the holding rod. Examples of metal oxide compounds are lanthanum oxide, yttrium oxide and rhenium oxide. 
     A discharge lamp according to the invention has at least one holding rod consisting of doped molybdenum or of tungsten which is doped with a metal oxide compound. 
     Doped molybdenum has the advantage that it has increased ductility compared with tungsten doped with potassium after a heat treatment or annealing during production of the discharge lamp and during operation of the discharge lamp. Owing to the ductility of the doped molybdenum, after the annealing process, as part of the production process, the strength up to the beginning of the plastic deformation (yield point) increases by approximately fourfold in comparison with tungsten doped with potassium. Furthermore, it is advantageous that molybdenum has a lower specific weight than tungsten, with the result that a corresponding discharge lamp can be designed to have a reduced weight. 
     Potassium is preferably used as the dopant, which has the advantage that molybdenum doped with potassium (MoQ) can be produced in a simple and cost-effective manner, and this material does not represent a radioactive load for the environment. For example, the volume content of the potassium is approximately 100 ppm to approximately 400 ppm, preferably approximately 280 ppm. 
     The ductility of the MoQ can be further increased if the holding rod is annealed prior to installation in a range above 1800° C., preferably at 2400° C. This recrystallization annealing leads to an MoQ with a low loss of strength, but the recrystallized structure is thermally stable, i.e. subsequent soldering of the holding rod to the anode or cathode does not change the properties of the MoQ. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in more detail below with reference to a preferred exemplary embodiment. In the drawings: 
         FIG. 1  shows a schematic illustration of a discharge lamp having holding rods according to the invention; 
         FIG. 2  shows an enlarged illustration of a holding rod from  FIG. 1 ; 
         FIG. 3  shows a testing arrangement for carrying out bending deformations of the holding rods from  FIG. 1 , and 
         FIG. 4  shows graphical results of the bending deformations from  FIG. 3 . 
     
    
    
     PREFERRED EMBODIMENT OF THE INVENTION 
       FIG. 1  shows a schematic illustration of a mercury vapor short-arc lamp (HBO) or a xenon short-arc lamp (XBO) having a base at two ends. Said lamp has a discharge vessel  4  consisting of quartz glass and having an interior  6  and two diametrically arranged, sealed bulb shafts  8 ,  10 , whose free end sections  12 ,  14  are each provided with a base shell (not illustrated). Two diametrically arranged electrodes  16 ,  18  protrude into the interior  6 , and a gas discharge is formed between them during lamp operation. An ionizable filling is enclosed in the interior  6  of the discharge vessel  4 , which ionizable filling essentially consists of a noble gas. 
     The electrodes  16 ,  18  are in the form of a two-part electrode system comprising a current-supplying holding rod  20 ,  22  and a discharge-side head electrode  24  (anode) or head electrode  26  (cathode), which is soldered to said holding rod  20 ,  22 . 
     In order to fit the electrode heads  24 ,  26  to the holding rods  20 ,  22 , the electrode heads  24 ,  26  are each provided with a blind hole  28 ,  30  on the side remote from the discharge, first end sections  32 ,  34  of the holding rods  20 ,  22  being fixed in said blind holes  28 ,  30 . 
     As shown in  FIG. 1 , the anode  24  is in the form of a barrel-shaped head anode which is subjected to a high thermal load and in the case of which the output power is improved by sufficient dimensioning of the electrode size. The cathode  26  is of multi-part design with a conical head cathode  36  in order to produce high temperatures, said head cathode  36  being fixed to a cylindrical base body  38  and ensuring that it is possible to achieve a defined arc attachment and sufficient electron flow owing to thermal emission and field emission (Richardson equation). 
     In order to hold the electrodes  16 ,  18  in the discharge vessel  4 , holding elements  40 ,  42  consisting of quartz glass are inserted into the bulb shafts  8 ,  10  and are provided with an axially extending through-hole  44 ,  46  for receiving the holding rods  20 ,  22 . 
     The holding rods  20 ,  22  of the electrodes  16 ,  18  are guided into the through-holes  44 ,  46  such that they reach into the interior  6  and bear the electrode heads  24  and  26 , respectively, there. On the base side, the holding rods  20 ,  22  are each extended beyond the holding elements  40 ,  42  and are inserted, with a second end section  72 ,  74 , into a receiving hole  45 ,  47  in an annular holding plate  48 ,  50 . 
     The holding plates  48 ,  50  are in each case adjoined by a quartz cylinder  52 ,  54 , which is fused into the bulb shaft  8 ,  10  and on whose outer circumference a plurality of molybdenum foils  56 ,  58  are arranged which are soldered to the holding plate  48 ,  50 , with the result that a gas-tight current bushing is formed. In this case, in order to additionally stabilize the anode  24 , the holding rod  22  is guided through the holding plate  50  into a hole  51  in the quartz cylinder  54 . 
     The molybdenum foils  56 ,  58  are soldered at in each case one end section  60 ,  62  to the edge of a contact plate  64 ,  66 , which is connected to a pin  68 ,  70  for the purpose of making electrical contact with the electrodes  16 ,  18 . 
     As shown in  FIG. 2 , the first end section  32  and the second end section  72  of the cathode-side holding rod  20  are stepped back radially in order to be received in the holes  28 ,  45 . The holding rod  20  is fixed in the holes  28 ,  45  in each case by means of soldering at approximately 1800° C. 
     The first end section  32  is stepped back to a lesser extent in the radial direction than the second end section  72 . The first end section  32 , as shown in the illustration in  FIG. 1 , is longer in the axial direction than the depth of the blind hole  28 , it being possible for the axial length, for example for reasons of stability, to also be selected such that the holding rod  20  touches the cathode  26  with a corresponding annular front face. In order to insert the first holding rod  20  easily into the blind hole  28 , the first end section  32  is beveled. 
     The second end section  72  is stepped back such that an annular front face  76  is formed, via which the holding rod  20  touches the holding plate  48  areally. In this case, the axial length of the second end section  72  is selected such that the holding rod  20  does not pass through the holding plate  48 . 
     The anode-side holding rod  22  (not illustrated in detail) has a similar design to the cathode-side holding rod  20  described in  FIG. 2 . The difference consists in the fact that the second end section  74  of the holding rod  22  is extended beyond the holding plate  50  in order to dip into the hole  51  in the quartz cylinder  54 . The extension has the advantage that the anode mass is likewise accommodated by the quartz cylinder  54 , and the discharge lamp  2  therefore has a more stable design. 
     According to the invention, the holding rods  20 ,  22  consist of molybdenum doped with potassium (MoQ), the volume content of the potassium being approximately 100 ppm to approximately 400 ppm, preferably approximately 280 ppm or approximately 300 ppm. The anode  24  and the cathode  26  primarily consist of tungsten doped with potassium (W-BSD) and the holding plates  48 ,  50  primarily consist of MoQ. 
     MoQ has a similar behavior to W-BSD in the non-recrystallized state. In this state, both materials have a very high strength and relatively good ductility. In the non-recrystallized state, W-BSD has an even higher bending strength Rm and a higher flexural yield point Rp than MoQ. In the crystallized state, however, W-BSD is very brittle, which is further intensified by the soldering at approximately 1800° C. 
     In contrast to this, MoQ has a very ductile structure after recrystallization annealing above 2000° C. and subsequent soldering at approximately 1800° C. This recrystallization annealing leads to a low loss in strength, but this recrystallized structure is thermally stable, with the result that the soldering of the holding rods  20 ,  22  to the electrodes  24 ,  26  and to the holding plates  48 ,  50  does not change the ductile properties of the MoQ. 
     Damage to the holding rods during transportation occurs in the case of conventional discharge lamps essentially directly in the holding rods and not in the region of the soldering of the electrodes  24 ,  26  or the holding plates  48 ,  50  and not in the region in which the bulb shafts  8 ,  10  are fused around the holding plates  48 ,  50 . In order to determine the strength of the holding rods  20 ,  22  according to the invention as compared with known holdings rods consisting of W-BSD, bending deformation tests have been carried out. A corresponding test arrangement is illustrated in  FIG. 3 . 
     The test arrangement comprises a known universal testing machine  78  having a bearing body  80 , which is arranged on a cross head  82 . The holding rods  20 ,  22  are positioned individually so as to lie over a prism-shaped cutout  84  in the bearing body  80  and are pressed against a stationary plunger  86  by the cross head  82  being displaced, said plunger  86  being connected to a load cell  88  in a load frame  90  for the purpose of measuring the load on the respective holding rod  20 ,  22 . The diameter of the holding rods  20 ,  22 , in accordance with conventional holding rod geometries, is 8 mm, and the width B of the cutout  84  amounts to 10 mm. The maximum displacement speed of the cross head  82  is 1000 mm/min. 
     The results of the bending deformations are shown in the upper graph A and the lower graph B in  FIG. 4 , in graph A the bending force F in kN being plotted over the bending distance S in mm, and in graph B the bending moment M in Nm being plotted over the bending distance S in mm. A known holding rod consisting of W-BSD is deformed elastically with cracks occurring at the same time owing to breakage. Before the theoretical bending strength Rm is arrived at, a brittle fracture occurs without any plastic deformation. The broken sample halves can be joined together at the breakage point such that the sample appears to be undeformed. The maximum bending strength Rm, with an annealing treatment at 1800° C., is at a value of 581 N (graph A, curves b and c), which corresponds to a bending moment (=load*free lever arm) of 16 Nm (graph B, curves b and c). At lower annealing temperatures, for example at 1500° C., the bending strength at Rm=888 N or 26 Nm is slightly higher (curves a). 
     On the other hand, holding rods  20 ,  22  consisting of MoQ according to the invention are deformed, in accordance with the upper and lower graphs, considerably beyond the flexural yield point Rp before the breakage sets in in the material (curves d). In this case, the deformation is so severe that it is barely possible to rejoin the samples and a residual bend of approximately 3-5 mm is visible. In accordance with the upper graph, an MoQ holding rod  20 ,  22 , which has been pre-annealed or recrystallized at 2400° C. with a holding time of 5 minutes, at 8 mm withstands a bending load of approximately 2632 N (graph A, curve d). This corresponds to a moment of 76 Nm (graph B, curve d). Only at a load of approximately 3500 N would the holding rod  20 ,  22  break. The shear strength (flexural yield point) is therefore approximately four times as great as the strength of known holding rods consisting of W-BSD. 
     Similar results as in the case of holding rods  20 ,  22  consisting of MoQ can be achieved with holding rods  20 ,  22  consisting of tungsten doped with metal oxide compounds. Advantageous metal oxide compounds are in this case lanthanum oxide, yttrium oxide and rhenium oxide. 
     The invention discloses a holding rod  20 ,  22  for a discharge lamp  2 , in particular a mercury vapor or xenon short-arc lamp, for holding an anode  24  or cathode  26  in an interior  6  of a discharge vessel  4 , the holding rod  20 ,  22  containing doped molybdenum or tungsten doped with at least one metal oxide compound. Furthermore, the invention discloses a discharge lamp having such a holding rod  20 ,  22 .