High pressure sodium lamp with sodium amalgam of controlled amount sealed therein

Abstract: A high pressure sodium lamp includes a ceramic arc tube of a diameter D (mm) having opposite plug portions. The one of the plug portions has a central aperture of a diameter d (mm) through which a niobium tube penetrates into the arc tube to support an electrode. The niobium tube is sealed by a glass solder filled in the central aperture. To prevent sodium amalgam condensed at the corner of the arc tube from contacting the glass solder, sodium amalgam including sodium of 10 - 30 (wt%) is sealed in the arc tube at a prescribed volume V (mm³) which substantially satisfies the following relationship; Vo/3 ≦ V ≦ Vo when WL is less than 200, or Vo/4 ≦ V ≦ Vo when WL is equal to or greater than 200, where Vo (mm³) is the volume of the sodium amalgam sealed in the arc tube when the shortest distance between the sodium amalgam condensed at the corner and the glass solder filled in the central aperture is indicated by the expression , and WL (W) is a lamp power.

Description: The present invention relates, in general, to high pressure sodium lamps. In particular, the invention relates to a relationship between an arc tube structure of such a lamp and an amount of sodium amalgam sealed in the arc tube. In general, high pressure sodium lamps typically include a ceramic arc tube in which a proper amount of xenon gas and a sodium amalgam are sealed. Sodium in sodium amalgam sealed in the arc tube gradually reacts on ceramic of the arc tube, and thus, some amount of sodium is lost during an operational life period of the lamp. To compensate the amount of the lost sodium, an excess amount of sodium amalgam is sealed in the arc tube in advance. It is believed that the greater the amount of sodium amalgam sealed in the arc tube, the better. FIGURE 1 shows one example of a conventional high pressure sodium lamp. A soldered alumina plug 11 includes a central aperture 13 through which a thin-walled niobium tube 15 penetrates for a short distance. Niobium tube 15 is hermetically sealed in central aperture 13 by sealing composition, e.g., glass solder, indicated by a thick line at 17. Niobium tube 15 acts as an exhaust tube and as an inlead. Plug 11 has its neck portion extending into an alumina arc tube 19. The contact portion between arc tube 19 and plug 11 is hermetically sealed by sealing composition, e.g., glass solder, indicated at 21 and 23. A coiled electrode 25 is fixed on the top portion of niobium tube 15 located in arc tube 19. The inside of niobium tube 15 is in fluid communication with the inside of arc tube 19 through an aperture 27 formed in the side wall of niobium tube 15. The outer end portion 29 of niobium tube 15 is squeezed shut after air is exhausted from arc tube 19 and niobium tube 15. At this time, an excess sodium amalgam 30 is provided into arc tube 19 and niobium tube 15. The sodium amalgam is accumulated at the inside of outer end portion 29 because of a low temperature of outer end portion 29. However, in the above-described high pressure sodium lamp, since the outer end portion 29 of niobium tube 15 outwardly extends from arc tube 19, temperature of outer end portion 29 hardly rises. Furthermore, since undesirable trenches or unevenness occur in the inner surface of niobium tube 15 during the forming process, liquidised sodium amalgam tends to move into arc tube 19 along such trenches by a capillary action, and therefore, lamp characteristics of the sodium lamp are adversely affected. FIGURE 2 shows another example of conventional high pressure sodium lamp. In this case, a high pressure sodium lamp includes a tube 31 composed of an alumina arc tube portion 33 and an alumina plug portion 35 integrally formed one with the other. Alumina plug portion 35 of tube 31 has a central aperture 37 through which a niobium tube 39 penetrates for a short distance. Niobium tube 39 and alumina plug portion 35 are hermetically sealed by sealing composition, e.g., glass solder, indicated by a thick line at 41. An electrode 43 is fixed on the penetrating end portion of niobium tube 39. In this high pressure sodium lamp shown in FIGURE 2, since no aperture fluidly communicating with the inside of niobium tube 39 and the inside of arc tube 31 is provided to the surface of niobium tube 39, sodium amalgam 45 is liquidised and stays in a ring-shaped state along an inner end corner 45 of tube 31, the temperature of which is lower than that of remaining portions thereof. In this case, since the low temperature portion is part of tube 31, the temperature thereof rises easily. Furthermore, since alumina tube 31 seldom has trenches or unevenness at the inner surface thereof, undesirable movement of sodium amalgam does not occur. Characteristics of the lamp is rarely changed. However, in this type of lamp described above, the end portion of tube 31 was intensely blackened after 3,000 hours elapsed in an operational life period when the diameter of monolithic tube 31 was reduced or the amount of sodium amalgam sealed in monolithic tube 31 was increased. Thus, the lamp voltage of the lamps having tube 31 greatly increased, and some of the lamps resulted in cycling. This is because a part of sodium amalgam 45 is in contact with glass solder 41, and thereby, sodium of sodium amalgam 45 reacts on a component of glass solder 41. JP-A-58-140963 discloses a high pressure sodium lamp assembly shown in FIGURE 3. A ceramic arc tube 51 includes a hollow body portion 53 and a plug portion 55 having a central aperture 57. A ring-shaped inner wall 59 extends from the edge of central aperture 57 toward the inside of arc tube 51. A niobium tube 61 penetrates through central aperture 57, and is hermetically sealed by a sealing composition 62, e.g., glass solder. An electrode 63 is fixed on the top portion of niobium tube 61. In the above-described sodium lamp, ring-shaped inner wall 59 prevents sodium amalgam from being in contact with electrode 63. Ring-shaped inner wall 59 also prevents sodium amalgam from being in contact with sealing composition 62. However, since the constitution of plug portion 55 having ring-shaped inner wall 59 is complicated, it is technically difficult to mass produce such a tube. US-A-3716743 discloses a high pressure metal vapour discharge tube. The tube is of alumina and is hermetically sealed at opposite ends by respective discs and each disc has a lead-in metal tube hermetically sealed through the disc. The surfaces between the tube and the discs and between the tubes and the discs are sealed with ceramic cement. A filling including sodium is present in the tube. Layers of thermally conductive heat durable metal are formed on the outer wall of the enclosure adjacent the ends thereof in order to increase the temperature of the coolest points of the tube and thus prevent condensation of the metal filling on this part of the tube. It is an object of the present invention to control the volume of sodium amalgam sealed in the arc tube for preventing the sodium amalgam from contacting the glass solder acting as a sealing material in a high pressure sodium lamp. According to the present invention a high pressure sodium lamp comprises a sealed light-permeable arc tube having a pair of electrodes spaced apart therein; the arc tube having a pair of opposite ends one of which includes a substantially flat surface with a central aperture therein, one of the electrodes forming part of an electrode structure which extends through the aperture; a sealing composition sealing the electrode structure within the aperture and a quantity of sodium amalgam including 10 - 30% (wt) of sodium within the arc tube; characterised in that the quantity of sodium amalgam is such that in use condensed sodium amalgam on said flat surface is spaced from the sealing composition which seals the electrode structure within the aperture. Conveniently the minimum distance between the condensed sodium amalgam and the sealing composition is given by the expression D-d / 4 where D is the inner diameter of the arc tube and d is the diameter of the central aperture. Furthermore the volume V (mm 3 ) of the sodium amalgam in the arc tube satisfies the following relationship. Vo = π 192 (5D 3 - 9D 2 d + 3Dd 2 + d 3 ) Vo 3 ≤ V ≤ Vo when W L is less than 200(W) or Vo 4 ≤ V ≤ Vo when W L is equal to or greater than 200(W) where W L is the lamp power, D is the inner diameter of the arc tube and d is the diameter of the central aperture. BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the present invention will be apparent from the following detailed description of the presently preferred embodiments taken in conjunction with the accompanying drawings, in which: FIGURE 1 is a fragmentary section view illustrating a first example of the prior art of the present invention; FIGURE 2 is a fragmentary sectional view illustrating a second example of the prior art of the present invention; FIGURE 3 is a fragmentary sectional view illustrating a third example of the prior art of the present invention; FIGURE 4 is a side view illustrating a high pressure sodium lamp of one embodiment of the present invention; FIGURE 5 is a fragmentary sectional view illustrating the relationship between glass solder filled in the central aperture of a plug and sodium amalgam condensed around the corner of an arc tube of the high pressure sodium lamp shown in FIGURE 4; FIGURE 6 is a fragmentary sectional view illustrating the arc tube and condensed sodium amalgam with no electrode and metal tube; FIGURE 7 is a fragmentary sectional view illustrating the relationship between glass solder filled in the central aperture of a plug and sodium amalgam condensed around the corner of a depressed portion of the plug in a second embodiment of the present invention; and FIGURE 8 is a fragmentary sectional view illustrating the relationship between glass solder filled in the central aperture of a plug including a step portion and sodium amalgam condensed around the corner of a depressed portion of the plug in a third embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be described hereafter by referring to the accompanying drawings. In FIGURE 4, an arc tube 71 of a high pressure sodium lamp 73 includes a bulb 75 and a pair of electrodes 77 and 79 individually disposed at each end of bulb 75. Bulb 75 has a translucent ceramic envelope, such as, e.g., alumina ceramic, containing a fill of a proper amount of starting rare gas, such as, xenon, mercury and sodium. To seal the opposite ends of bulb 75 airtightly, a pair of plugs 81 and 83 made of alumina ceramic are individually fixed to each end of bulb 75. In this case, arc tube 71 is a monolithic arc tube, and therefore, the pair of plugs 81 and 83 is integrally formed with bulb 75. Such a monolithic tube is made of alumina granules. Alumina granules with a binder are formed into a tube-shape by a press forming, and are also formed into a disk-shape. The tube-shaped bulb and the disk-shaped plug are individually sintered at 1,000 °C for 30 minutes to eliminate the binder therefrom. The disk-shaped plug is disposed at the open end portion of the tube-shaped bulb after reforming the bulb and the plug to a prescribed size. The assembled structure of the bulb and the plug is further sintered at 1,800 °C for 2 or 3 hours in a hydrogen atmosphere. A metal tube 85 made of niobium penetrates through a central aperture 86 of plug 83 at a short distance and is fixed by glass solder 87 to the plug, as shown in FIGURE 5. The penetrating end of tube 85 is closed, and one of the elctrodes 77 is welded thereto. The other electrode 79 disposed in bulb 75 is fixed to a lead wire 89 made of niobium. Lead wire 89 penetrates plug 81 and is fixed to plug 81 with the glass solder in an airtight state. During the manufacture of arc tube 71, a fill of starting rare gas, such as, xenon, mercury and sodium is sealed in arc tube 71. Mercury and sodium (sodium amalgam) are supplied in excess to arc tube 71, as compared with the vapored amount thereof needed for proper operation. The outer end of metal tube 85 is supported by a metal plate 91 firmly fixed to a supporting rod 93. Supporting rod 93 is supported by a stem 95 so that a voltage can be applied to electrode 77 through supporting rod 93, metal plate 91 and metal tube 85. One end of lead wire 89 is connected to electrode 79, as described above, and the other end thereof is connected to a lead 97 supported by stem 95. A voltage may be applied to electrode 79 through lead 97 and lead wire 89. A metal plate 99 is welded to support rod 93. An insulating bushing 101 is fixed at the center of metal plate 99. Lead wire 89 penetrates insulating bushing 101, and is supported by metal plate 99 through insulating bushing 101. More specifically, lead wire 89 loosely penetrates insulating bushing 101 so that lead wire 89 may move in the axial direction thereof without rolling in excess. As a consequence, when arc tube 71 expands in the axial direction thereof during the operation, lead wire 89 moves along insulating bushing 101 to absorb the expansion of tube 71. Arc tube 71 supported by supporting rod 93 is held in an outer envelope 103 made of hard glass. In this case, the inner diameter (D) of arc tube 71 is set at 4.5 mm, and the diameter (d) of central aperture 86 of plug 83 is set at 2.06 mm. The sodium amalgam includes 10 - 30 (wt%) of sodium, which is generally used in this type of the lamp. An amount (volume V) of sodium amalgam sealed in arc tube 71 is 2.39 mm 3 . In the above-described embodiment, the temperature of the corner portion 105 is defined by bulb 75 and plug 83 is maintained lower than that of other portions of arc tube 71 during the operation. This is because heat from electrode 77 is conducted to supporting rod 93 through metal tube 85 and metal plate 91. Therefore, sodium amalgam 107 sealed in arc tube 71 is condensed in a ring-shape at corner portion 105 of arc tube 71 while the lamp is operated, as shown in FIGURE 5. Since viscosity of sodium amalgam is relatively large, a longitudinal section of the condensed sodium amalgam 107 is a substantially triangle-shape. With the arrangement described above, no glass solder is disposed at the corner 105 of arc tube 71 where sodium amalgam is condensed because of monolithic structure of arc tube 71. Furthermore, since amount of sodium amalgam sealed in arc tube 71 is controlled at a prescribed value, sodium amalgam does not contact glass solder disposed around central aperture 86 of plug 83. Blackening of the end portion of arc tube 71 and extinction of arc tube 71 caused by the reaction between sodium amalgam and glass solder can be avoided. In the above-described high pressure sodium lamp, operational life experiment was carried out under the condition wherein the supply voltage was 200 V, and the operation cycle was 5.5 hours ON time and 0.5 hours OFF time. Five sample groups each consisting of twenty lamps were made by varying the amount of sodium amalgam, including sodium of 10 wt%, sealed in the arc tube. The other five sample groups each consisting of twenty lamps were also made by varying the amount of sodium amalgam including sodium of 15 wt%. The lamp voltage (VL) of each lamp of respective sample group was measured after 100 hour operation, 3,000 hour operation and 9,000 hour operation. Measured lamp voltage values in each sample group were averaged every each measuring time. Average increasing value of lamp voltage (VL) of each sample group between 100 hour operation and 3,000 hour operation were figured out. The other average increasing value of lamp voltage (VL) of each sample group between 100 hour operation and 9,000 hour operation also were figured out. TABLE 1 shows a result of the experiment described above. As can be understood from TABLE 1, increase in lamp voltage is small and extinction is not observed even after 9,000 hour operation when the sealed amount of sodium amalgam including sodium of 10 wt% is less than 14.6 mg or the sealed amount of sodium amalgam including sodium of 15 wt% is less than 11.2 mg. A small increase in lamp voltage indicates that a high pressure sodium lamp maintains a high lumen maintenance factor, and therefore, has a desirable operational life characteristics. When each volume (V) of amalgam is figured out with regard to 14.6 mg of sodium amalgam including sodium of 10 wt% and 11.2 mg of sodium amalgam including sodium of 15 wt%, it is found that each volume (V) of amalgam in former and latter cases is equal to one another, that is, substantially 2.4 mm 3 . During the operation of arc tube 71, electrode 77 is maintained at a high temperature. However, since a part of heat generated by electrode 77 is discharged through metal tube 85 by the heat conduction, or is discharged through bulb 75 by the heat radiation, the corner portion 105 defined by bulb 75 and plug 83 is maintained at a low temperature. Therefore, sodium amalgam sealed in arc tube 71 is condensed at the above-described corner portion 105 of arc tube 71 in a ring-shape, as described before. The cross section of sodium amalgam condensed is a substantially triangle-shape, as shown in FIGURE 5. The shortest distance between sodium amalgam condensed at corner portion 105 and glass solder 87 disposed around central aperture 86 of plug 83 is a half of the distance between corner portion 105 and central aperture 86 when volume (V) of sodium amalgam sealed in arc tube 71 is 2.4 mm 3 . This shortest distance is expressed as follows: wherein D (mm) is an inner diameter of arc tube 71, and d (mm) is a diameter of central aperture 86. Referring to FIGURE 6, a method for calculating the volume (Vo) of sodium amalgam sealed in arc tube 71 will be described hereafter when the shortest distance between sodium amalgam 107 condensed at corner portion 105 of arc tube 71 in the triangle-shape in cross section and the glass solder, i.e., edge of central aperture 86, satisfies the above-described expression (1). The volume (Vo) of sodium amalgam is figured out by subtracting a volume (Va) of frustrum of a cone 111 from a volume (Vb) of a cylinder 113. The volume (Vb) of cylinder 113 substantially satisfies the following equation (2): Vb = π D 2 × D-d 4 = π 16 (D 3 - D 2 d) The volume (Va) of frustrum of cone 111 substantially satisfies the following equation (3): Va = 1 3 π (R 2 + Rr + r 2 ) × h = π 192 (7D 3 - 7D - 4D 2 d + 5Dd 2 - d 3 ) wherein R is a radius of one of the base areas of frustrum of cone 111, and r is a radius of the other base area of frustrum of cone 111. Vo = Vb - Va = π 192 (5D 3 - 9D 2 d + 3Dd 2 + d 3 ) As can be understood from the above-described equation (4), the shortest distance between sodium amalgam 107 condensed at corner portion 105 of arc tube 71 and the glass solder 87 (edge of central aperture 86) is maintained at a distance expressed by the above-described equation (1) when the volume of sodium amalgam sealed in arc tube 71 is Vo (mm 3 ). The operational life experiment wherein an operational state of lamp is observed after 12,000 hours total operational period elapsed was carried out by varying the inner diameter D of arc tube 71 and the diameter r of central aperture 86 of plug 83 when volume of sodium amalgam sealed in arc tube 71 is Vo (mm 3 ). TABLE 2 shows each specifications of samples used in the above-described operational life experiment. In the above-described experiment, the increase in lamp voltage about 20 V was not observed during the operational life period. No extinction of lamp also was observed. This is because the distance between the sodium amalgam condensed and glass solder is maintained at a suitable range, and therefore, reaction between sodium amalgam condensed and glass solder can be avoided. As can be understood from the above-described consideration, a desirable result is obtained when volume of sodium amalgam sealed in an arc tube is less than Vo (mm 3 ) in each lamp listed in TABLE 2. With regard to the lower limit of amount of sodium amalgam sealed in the arc tube, decrease in lamp voltage is caused by the shortage of sodium, and therefore, an excess heat of a ballast may occur during the operation if amount of sodium amalgam sealed in the arc tube is excessively small. A desirable lower limit of sealed amount of sodium amalgam which causes the average increasing value of lamp voltage to be maintained under 20 V when a rated operational life period, i.e., 12,000 hours, has elapsed is as follows: Vo/3 when a lamp power (WL) is less than 200 (W), or Vo/4 when a lamp pwer (WL) is equal to or greater than 200 (W). As can be understood from the above description, a desirable range of sealed amount (volume V) of sodium amalgam including sodium of 10 - 30 wt% should satisfy the following relationship: Vo/3 ≤ V ≤ Vo when a lamp power (WL) is less than 200 (W), or Vo/4 ≤ V ≤ Vo when a lamp power (WL) is equal to or greater than 200 (W). A second and a third embodiments of the present invention will be described hereafter by referring to FIGURES 7 and 8. In FIGURES 7 and 8, similar construction parts with the one embodiment are designated by same numerals, and therefore, the detailed descriptions thereof are not repeated. The second and the third embodiments use an arc tube including a bulb portion and a pair of ceramic plugs fixed to the opposite open ends of bulb portion by a sealing composition respectively, instead of a monolithic tube used in the one embodiment. The second embodiment of the present invention is shown in FIGURE 7. An alumina ceramic plug 121 is provided with a depression 123 at a center thereof. Plug 121 is positioned at the open end of an alumina ceramic bulb 75 such that depression 123 of plug 121 is exposed to the inside bulb 75. The outer wall of plug 121 is airtightly fixed to the inner wall of bulb 75 by a glass solder 87a. Electrode 77 is supported by a niobium leadin wire 125 extending into bulb 75 through central aperture 86 of plug 121. Niobium wire 125 is airtightly fixed to central aperture 86 through glass solder 87b. A steel wire 127 welded to niobium leadin wire 125 extends to the edge of bulb 75 across plug 121 to support electrode 77 when manufacturing. In this case, sodium amalgam sealed in excess in arc tube 71 is condensed at corner portion 105 of the inner surface of depression 123. Therefore, the inner diameter D of depression 123 is used as symbol D in equation (4), instead of the inner diameter of arc tube 71. Similar result to the one embodiment may be achieved in the second embodiment described above when volume (V) of sodium amalgam sealed in arc tube 71 is set at less than volume (Vo) calculated by equation (4). Reaction between sodium amalgam 107 condensed at corner portion 105 of depression 123 and glass soler 87b filled in central aperture 86 of plug 121 may be avoided. Furthermore, since glass solder 87a exists at the outer wall of plug 121, sodium amalgam condensed in depression 123 of plug 121 does not contact glass solder 87a. The third embodiment of the present invention will be described. In FIGURE 8, a flange 131 is welded to niobium leadin wire 125. Flange 131 acts as a stopper to prevent electrode 77 from moving in excess by its gravity when manufacturing. To fix flange 131 to central aperture 86 of plug 83, a step portion 133 is formed around central aperture 86. Central aperture 86 including step portion 133 is filled with glass solder 86 when manufacturing. In this embodiment also, similar result to the one embodiment may be achieved in the above-described third embodiment when volume (V) of sodium amalgam sealed in arc tube 71 is set at less than volume Vo calculated by equation (4). However, in this case, the diameter d of step portion 133 shown in FIGURE 8 is used as sumbol D in equation (4), instead of the diameter of central aperture 86. In summary, the present invention overcomes the disadvantages of the prior art and provides an improved high pressure sodium lamp which may avoid reaction between glass solder used for fixing the electrode supporting element to the plug and sodium amalgam condensed at the low temperature portion of the arc tube by controlling volume of sodium amalgam sealed in the arc tube at a prescribed range. Many changes and modifications in the above-described embodiments can be carried out without departing from the scope of the present invention as defined in the appended claims.

Claims: A high pressure sodium lamp comprising a sealed light-permeable arc tube (71) having a pair of electrodes (77) spaced apart therein; the arc tube having a pair of opposite ends one of which includes a substantially flat surface with a central aperture (86) therein, one of the electrodes forming part of an electrode structure which extends through the aperture; a sealing composition (87) sealing the electrode structure within the aperture and a quantity of sodium amalgam (107) including 10 - 30% (wt) of sodium within the arc tube; characterised in that the quantity of sodium amalgam is such that in use condensed sodium amalgam on said flat surface is spaced from the sealing composition which seals the electrode structure (85) (125) within the aperture. A high pressure sodium lamp as claimed in claim 1, characterized in that in use the sodium amalgam condenses on the flat surface away from the sealing composition by a minimum distance D-d / 4 and the volume V(mm 3 ) of the sodium amalgam in arc tube satisfies the relationship Vo = π 192 (5D 3 - 9D 2 d + 3Dd 2 + d 3 ), Vo / 3 ≤ V ≤ Vo when W L is less than 200 W or Vo / 4 ≤ V ≤ Vo when W L is equal to or greater than 200 W, and D is the inner arc tube diameter in mm, d is the diameter of the central aperture in mm, W L is the lamp power. A lamp as claimed in any preceding claim characterised in that the sealing composition is a glass solder. A lamp as claimed in any preceding claim, characterised in that the electrode structure comprises a niobium tube (85) having a closed end within the arc tube and a coiled electrode (77) supported at the closed end. A lamp as claimed in any preceding claim, characterised in that the electrode structure comprises a niobium wire (125) supporting a coiled electrode (77). A lamp as claimed in any preceding claim characterised in that the substantially flat surface is the base of a central recess in a plug (83) disposed at said end of the tube. A lamp as claimed in claim 6 characterised in that there is a step (133) in said flat surface surrounding the aperture and the electrode structure has a flange (131) which rests on said step.

Inventors: ITO AKIRA, OKAMURA KAZUYOSHI, UCHIDA KAZUIKI, ITO, AKIRA, OKAMURA, KAZUYOSHI, UCHIDA, KAZUIKI, ITO, AKIRA C/O PATENT DIVISION, OKAMURA, KAZUYOSHI PATENT DIVISION, UCHIDA, KAZUIKI PATENT DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1988
Main classification: H01J61
IPC codes: H01J61