GB1561919A - High pressure vapour discharge lamp - Google Patents

Description

(54) HIGH PRESSURE VAPOUR DISCHARGE LAMP (71) We, GENERAL ELECTRIC COMPANY, a corporation organized and existing under the laws of the State of New York, United States of America, of 1 River Road, Schenectady 12305, State of New York, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement The invention relates to high pressure vapour discharge lamps utilizing alumina ceramic envelopes.

Alumina ceramic envelopes are of particular interest in connection with thigh pressure sodium lamps which are now widely used for outdoor lighting applications on account of their high efficacy, generally in excess of 100 lumens per watt. Alumina ceramic is resistant to sodium at high temperatures and both high density polycrystalline alumina and monocrystalline alumina or synthetic sapphire are utilized for lamp envelopes. The lamp fill comprises sodium along with a rare gas to facilitate starting, and mercury for improved efficiency. The ends of the alumina tube are sealed by suitable closure members affording connection to thermionic electrodes. The ceramic arc tube is generally supported within an outer vitreous envelope or jacket provided at one end with a mogul screw base.The electrodes of the arc tube are connected to the terminals of the base, that is to shell and center contact, and the inter-envelope space is usually evacuated in order to conserve heat.

One design of end seal for an alumina ceramic arc tube which is described in U.S.

patent 3,882,346- McVey, utilizes a ceramic plug sealed in the end of the arc tube and having a central perforation through which is sealed a lead wire of ceramiomatching metal, that is, of metal which approximately matches the co-efficient of expansion of the ceramic, preferably niobium or alternatively tantalum in the case of alumina ceramic. The sealing is effected through a glassy sealing composition which melts when the assembly is suitably heated and forms the ceramic-toceramic and ceramic-to-metal seals.

Such a seal design is more economical than a conventional one using a thin-walled niobium tube. But because a solid wire cannot give or deform as readily in response to thermally-induced stress as a thin walled tube, it is a more critical design. At sealing, the assembly is heated until the sealing composition or frit is molten and is driven by capilliary action into the ring-like gap between arc tube and ceramic plug and into the inlead hole through the plug. A problem which has been encountered with it is occasional fracture of the hermetic seal, particularly at the lead wire. Another problem which has been encountered with this seal design is unevenness in the distribution of sealing frit as between the peripheral seal of arc tube to plug and the central seal of plug to inlead which may cause weak seals and leakers.

We have determined that the seal failures are due to excessive temperature or temperature gradient along the bonded surfaces or across them where the lead wire extends through the apertured ceramic plug.

According to the invention, there is provided a high pressure vapour discharge lamp comprising: a tubular light-transmitting ceramic envelope having end closures and thermionic electrodes sealed into its ends and containing inert starting gas and a charge including an excess of vaporizable metal; the closure at one end of said envelope comprising a ceramic plug having a perforation and a ceramicmatching metal inlead wire sealed therethrough; and the electrode at said one end being attached to said inlead through an intervening curved or bent portion of a conductor serving to reduce stresses on the inlead seal as a result of thermal expansion when the lamp is in use.

A wire crosspiece attached to the inlead above the ceramic plug may be used as a hanger to support the inlead, electrode and plug assembly in the end of the arc tube during sealing. The crosspiece is disposed close to the surface of the plug and serves as a wick for feeding molten sealing fritby capillary action to the seals. The crosspiece permits the distrbution of sealing frit to be equalized as between the ring seal of arc tube to plug and the inlead aperture seal so that more uniform seals of greater reliability are achieved.

The invention will be further described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a side elevation view of a ceramic arc tube lamp embodying the present invention; Fig. 2 is a fragmentary pictorial view of the sectioned upper end of the arc tube; Fig. 3 shows the ceramic plug, lead, and electrode assembly ready to be lowered into the arc tube end for sealing; and Fig. 4 shows a variant of the lead and elec- trode assembly.

A high pressure sodium vapor lamp 1 embodying the invention in preferred form and corresponding to a 400-watt size is illustrated in FIG. 1. It comprises a vitreous outer envelope 2 with a standard mogul screw base 3 attached to one end and comprising a re-entrant stem press 4 through which extend, in conventional fashion, a pair of relatively heavy lead-in conductors 5, 6 whose outer ends are connected to the screw shell 7 and eyelet 8 of the base.

The arc tube 9 centrally located within the outer envelope comprises a piece of crystalline alumina tubing having its lower end closed by a metal end cap 10, suitably of niobium which matches the expansion coefficient of alumina ceramic. A metal tube 11 which may also be of niobium is hermetically sealed through the cap and serves as an exhaust and fill tubulation during manufacure of the lamp. The exhaust tube is sealed off at its outer end and serves as a reservoir in which excess vaporizable metal, sodium -mercury-amalgam in this case, condenses during operation of the lamp. The lower electrode within the lamp is attached to the inward projection of the tube 11. A short wire connector 12 is welded to tube 11 and to short support rod 13 which in turn is welded to lead-in conductor 5.Support rod 13 is braced to single side rod 14 welded to lead-in conductor 6, by means of a strap 15 attached to the side rod and wrapping around an insulator 16 threaded over support rod 13.

The upper end of the arc tube is sealed by a perforated alumina ceramic plug 17 best seen in Fig. 2. As illustrated, the plug is centrally perforated and a niobium inlead wire 18 extending through the hole is hermetically sealed by sealing composition indicated at 19.

The plug in turn is hermetically sealed into the end of arc tube 9 by a ring of sealing composition indicated at 20. The inlead supports the upper electrode within the arc tube, and its external portion passes through loop 21 in-transverse support wire 22 attached to side rod 14. This arrangement allows for thermal elongation of the arc tube during operation, and a resilient metal ribbon 23 assures a good electrical connection. The upper end of side rod 14 is braced by spring clamp 24 which engages inverted nipple 25 in the dome end of the outer envelope. The outer envelope or jacket is evacuated by pumping and flashing getter rings 26. A metal reflector band 27 may be desirable around the upper end of the arc tube in order to maintain the desired temperature, particularly in smaller sizes of lamps, for instance 250 watts or less.

The illustrated lamp is intended for base down operation and has the amalgam reservoir 11 lowermost. In a similar design for base up operation, the arc tube is reversed end for end relative to the outer envelope in order to have the amalgam reservoir lowermost, and the attachments or supports for the arc tube including the expansion slip loop 21 are reversed appropriately.

The hermetic seals, including that of the inlead wire through the ceramic plug and that of the inlead wire through the ceramic plug and that of the plug to the arc tube may be made using various sealing compositions, sometimes referred to as sealing glass, which comprise primarily aluminum oxide and calcium oxide. One composition which we have used successfully is designated G-54 and consists of approximately 54.0% A1203, 38.5% CaO, and 7.5% MgO by weight.

Other compositions which may be used are those described in U.S. Patent 3,281,309- Ross, 3,441,421Sarver et al., and patent 3,588,577-McVey.

The lamp utilizes self-heating thermionic electrodes 28 best seen in FIG. 2. The electrode comprises two helical layers 29 of tungsten wire wound around a tungsten shank 30. The inner helical layer has spaced turns and electron emitting material such as dibarium calcium tungstate Ba2CaWO6 is enclosed in the interstices between turns. The inner end of inlead 18 is bent sharply to a radial direction immediately beyond the hole through plug 17 and then curves into a ring-like loop 31 which terminates in an inwardly and downwardly directed extension to which the shank 30 of the electrode is welded at 32.

Loop 3 defines a plane surface and serves as a platform to support ceramic plug 17 prior to sealing. The inlead, electrode and ceramic plug assembly prior to-sealing is best seen in FIG. 3 wherein the plug and electrode are shown in phantom. A light wire crosspiece 33 is spot welded to inlead -18 just above plug 17 and serves to support the assembly when it is lowered into the open end of arc tube 9. The cross piece is bent or arched slightly about its midpoint in a horizontal plane traverse to said inlead in order to have its ends engage the edge of the arc tube in a vertical diametral plane whereby the assembly will hang vertically in the tube. A measured quantity of sealing composition or glass frit is placed on the ceramic plug suitably as a slurry and the assembly is then heated to the melting temperature of the glass frit so that the seals form upon cooling.The molten frit is drawn by capillary action into the ringlike crevice between arc like and plug and that between plug and inlead and the seals form upon cooling. This process advantageously favors degassing of the frit.

Prior to the invention, it would happen occasionally that the frit would collect at the peripheral seal of the arc tube to the plug and the quantity at the central seal of the inlead would be insufficient, or vice versa.

However, as illustrated, cross piece 33 is diposed close to the surface of ceramic plug 17 in order to strve as a wick for molten sealing frit during the sealing operation. The distribution of sealing frit between the peripheral seal of arc tube to plug, and the central seal of plug to inlead, is thereby equalized.

The surface tension of the molten frit causes it to form a thin wedge of liquid between the upper surface of the ceramic disk and the underside of the crosspiece as illustrated in FIG. 2. This permits molten frit to flow in either direction and the distribution of sealing frit between the peripheral seal of plug to arc tube and the central seal of inlead to plug is thereby equalized. As a result, more uniform seals of greater reliability are achieved.

FIG. 4 illustrates a variant of the lead and electrode assembly wherein the inner portion of the niobium inlead 181 is cut off at a point corresponding to emergence from the aperture through the ceramic plug. A small Ushaped connector piece 34, preferably of niobium, is welded at 35 to the distal end of the inlead to form a cross support or hanger.

The upper leg of connector 34 then serves to support the ceramic plug prior to and during sealing, and the electrode shank is welded at 36 to the lower leg.

The crosspiece 33 may also provide a conventional means for locking reflector band 27 in place. A bent-over tab may be used to prevent the band from sliding down the arc tube while the ends of the crosspiece overreach the band and prevent it from sliding off the end of the arc tube.

In both the single piece inlead construction of FIGS. 2 and 3, and the two piece inlead construction of FIG. 4, pressure may be used during the resistance welding of the tungsten electrode shank 30 to the end of niobium inlead 18 or to niobium connector piece 34.

This permits the hard tungsten to deform the relatively soft niobium whereby a large area bond is achieved having adequate strength to support the relatively massive tungsten electrode notwithstanding vibration and shocks during use.

The reliability of ceramic-to-metal seals is detrimentally affected by excessive temperature or temperature gradient along the bonded surfaces or across them. In the end structures of ceramic discharge lamps very high temperature gradients occur. Thus plasmas whose temperatures may exceed 3000"C are sustained by refractory electrodes whose inleads extend through ceramic-tometal seals. These seals may fracture and their lives are drastically shortened at temperatures in excess of 800"C. The electrode coil structure contains electron emission material and it must operate at a temperature high enough to effect a slow release of those elements that activate the electrode for efficient electronic emission.The conflicting requirements of a hot electrode and a much cooler electrode inlead seal are reconciled by loop 31 in the niobium inlead which lengthens the thermal conduction path from electrode shank 30 to the inlead seal. The U-shaped connector piece 34 performs the same function in the embodiment of FIG. 4.

Typical temperatures encountered in a 400-watt high pressure sodium lamp such as illustrated in FIG. 1 are given in Table 1 below. The corresponding temperature points are indicated in FIG. 2.

TABLE 1 Location Temperature A: electrode shank tip 1600"C B: electrode coil 1300"C C: shank rear end 1100 C D: inlead seal-inside 800"C E: inlead seal-outside 750"C F: ring seal-inside 750"C G: ring seal-outside 730"C It will be observed in the above table that the axial temperature drop along the length of the wire seal from inside to outside surface of the ceramic plug is only 50"C. The temperature drop from inside to outside of the ring seal between the arc tube and plug is only 20"C. Thus the requirement of an electrode coil or body at 1300"C and an inlead seal not above 800"C are reconciled and a small temperature gradient along the seal is achieved.

WHAT WE CLAIM IS:- 1. A high pressure vapor discharge lamp comprising: a tubular light-transmitting ceramic envelope having end closures and thermionic electrodes sealed into its ends and containing

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. above plug 17 and serves to support the assembly when it is lowered into the open end of arc tube 9. The cross piece is bent or arched slightly about its midpoint in a horizontal plane traverse to said inlead in order to have its ends engage the edge of the arc tube in a vertical diametral plane whereby the assembly will hang vertically in the tube. A measured quantity of sealing composition or glass frit is placed on the ceramic plug suitably as a slurry and the assembly is then heated to the melting temperature of the glass frit so that the seals form upon cooling. The molten frit is drawn by capillary action into the ringlike crevice between arc like and plug and that between plug and inlead and the seals form upon cooling. This process advantageously favors degassing of the frit. Prior to the invention, it would happen occasionally that the frit would collect at the peripheral seal of the arc tube to the plug and the quantity at the central seal of the inlead would be insufficient, or vice versa. However, as illustrated, cross piece 33 is diposed close to the surface of ceramic plug 17 in order to strve as a wick for molten sealing frit during the sealing operation. The distribution of sealing frit between the peripheral seal of arc tube to plug, and the central seal of plug to inlead, is thereby equalized. The surface tension of the molten frit causes it to form a thin wedge of liquid between the upper surface of the ceramic disk and the underside of the crosspiece as illustrated in FIG. 2. This permits molten frit to flow in either direction and the distribution of sealing frit between the peripheral seal of plug to arc tube and the central seal of inlead to plug is thereby equalized. As a result, more uniform seals of greater reliability are achieved. FIG. 4 illustrates a variant of the lead and electrode assembly wherein the inner portion of the niobium inlead 181 is cut off at a point corresponding to emergence from the aperture through the ceramic plug. A small Ushaped connector piece 34, preferably of niobium, is welded at 35 to the distal end of the inlead to form a cross support or hanger. The upper leg of connector 34 then serves to support the ceramic plug prior to and during sealing, and the electrode shank is welded at 36 to the lower leg. The crosspiece 33 may also provide a conventional means for locking reflector band 27 in place. A bent-over tab may be used to prevent the band from sliding down the arc tube while the ends of the crosspiece overreach the band and prevent it from sliding off the end of the arc tube. In both the single piece inlead construction of FIGS. 2 and 3, and the two piece inlead construction of FIG. 4, pressure may be used during the resistance welding of the tungsten electrode shank 30 to the end of niobium inlead 18 or to niobium connector piece 34. This permits the hard tungsten to deform the relatively soft niobium whereby a large area bond is achieved having adequate strength to support the relatively massive tungsten electrode notwithstanding vibration and shocks during use. The reliability of ceramic-to-metal seals is detrimentally affected by excessive temperature or temperature gradient along the bonded surfaces or across them. In the end structures of ceramic discharge lamps very high temperature gradients occur. Thus plasmas whose temperatures may exceed 3000"C are sustained by refractory electrodes whose inleads extend through ceramic-tometal seals. These seals may fracture and their lives are drastically shortened at temperatures in excess of 800"C. The electrode coil structure contains electron emission material and it must operate at a temperature high enough to effect a slow release of those elements that activate the electrode for efficient electronic emission.The conflicting requirements of a hot electrode and a much cooler electrode inlead seal are reconciled by loop 31 in the niobium inlead which lengthens the thermal conduction path from electrode shank 30 to the inlead seal. The U-shaped connector piece 34 performs the same function in the embodiment of FIG. 4. Typical temperatures encountered in a 400-watt high pressure sodium lamp such as illustrated in FIG. 1 are given in Table 1 below. The corresponding temperature points are indicated in FIG. 2. TABLE 1 Location Temperature A: electrode shank tip 1600"C B: electrode coil 1300"C C: shank rear end 1100 C D: inlead seal-inside 800"C E: inlead seal-outside 750"C F: ring seal-inside 750"C G: ring seal-outside 730"C It will be observed in the above table that the axial temperature drop along the length of the wire seal from inside to outside surface of the ceramic plug is only 50"C. The temperature drop from inside to outside of the ring seal between the arc tube and plug is only 20"C. Thus the requirement of an electrode coil or body at 1300"C and an inlead seal not above 800"C are reconciled and a small temperature gradient along the seal is achieved. WHAT WE CLAIM IS:-

1. A high pressure vapor discharge lamp comprising: a tubular light-transmitting ceramic envelope having end closures and thermionic electrodes sealed into its ends and containing

inert starting gas and a charge including an excess of vaporizable metal; the closure at one end of said envelope comprising a ceramic plug having a perforation and a ceramic-matching metal inlead wire sealed therethrough; and the electrode at said one end being attached to said inlead through an intervening curved or bent portion of a conductor serving to reduce stresses on the inlead seal, as a result of thermal expansion when the lamp is in use.

2. A lamp as claimed in claim 1, wherein the inner portion of said inlead wire is bent to a radial direction immediately beyond said perforation and then curves into a ring-like loop terminating in an inwardly and downwardly directed extension to which said electrode is attached.

3. A lamp as claimed in claim 1, wherein a connector piece is attached to the distal end of the inner portion of said inlead wire and said electrode is attached to said connector piece which forms said intervening portion.

4. A lamp as claimed in any one of claims 1 to 3, wherein the ceramic is alumina and said inlead is of niobium.

5. A lamp as claimed in any one of the preceding claims, wherein the electrode comprises a tungsten coil on a tungsten shank attached to the distal end of said intervening portion.

6. A lamp as claimed in any one of the preceding claims, wherein said plug is sealed to the arc tube and inlead by sealing frit, and a crosspiece is attached to said inlead immediately next to the plug and serves as a wick for molten sealing frit in order to equalize the distribution of frit between the seal of plug to arc tube and the seal of inlead to plug.

7. A lamp as claimed in claim 6, wherein said crosspiece is a fine wire welded to said inlead immediately above said plug and overreaching the arc tube walls in order to serve as a hanger to support said closure in the arc tube end during sealing.

8. A lamp as claimed in claim 7, wherein said cross-piece is bent on a plane transverse to said inlead sufficiently to engage the arc tube walls on a diameter whereby the closure assembly will hang vertically in the arc tube during sealing.

9. A lamp as claimed in claim 7 or claim 8, including a metal reflector band around said one end of the arc tube, said crosspiece having its ends overreaching said band so as to prevent the band from sliding off the end of the arc tube.

10. A high pressure vapor discharge lamp substantially as hereinbefore described with reference to and as shown in the accompanying drawing.