Metal Vapor Arc Lamp Having Thermal Link Diminishable in Heat Conduction

The invention relates to metal vapor arc lamps operating with an unvaporized excess of metal, and more particularly to high pressure sodium lamps utilizing an alumina ceramic envelope in which the cold spot temperature determines the vapor pressure within the lamp and the voltage drop across it.

The high intensity sodium vapor lamps with which the invention is most useful comprise a slender tubular ceramic arc tube which is generally mounted in an outer vitreous envelope or glass jacket. The arc tube is made of light-transmissive refractory oxide material resistant to sodium at high temperatures, suitably high density polycrystalline alumina or synthetic sapphire. The tube contains a discharge supporting filling comprising sodium together with mercury for improved efficiency, along with a rare gas to facilitate starting. Thermionic electrodes are contained within the tube whose ends are sealed by closure members through which connections are made to the electrodes. The outer envelope which encloses the ceramic arc tube is generally provided at one end with a screw base to which the electrodes of the arc tube are connected.

The high pressure sodium vapor lamp contains an excess amount of sodium-mercury amalgam, that is it contains more amalgam than is vaporized when the lamp reaches a stable operating condition. By having an excess, the quantity supplied is made noncritical and some of the excess amalgam is used to replace any lost during the life of the lamp as it ages, for instance by electrolysis through the alumina walls.

The lamp voltage, that is the voltage drop across the arc tube during normal operation, is dependent upon the vapor pressure and the vapor pressure in turn is determined by the lowest temperature in the arc tube which is dependent upon the thermal balance. A preferred lamp design utilizes an externally projecting metal exhaust tube which is sealed off and provides a reservoir for excess sodium mercury amalgam external to the arc tube proper. This arrangement has the advantage of placing the excess amalgam in a location removed from the direct heat of the arc and of the electrodes, so that arc tube blackening as the lamp ages has a minimal effect on sodium vapor pressure and on lamp voltage. Also the use of an external reservoir facilitates close adjustment of the heat balance in the lamp. Another lamp design avoids the need for an exhaust tube by inserting the charge of sodium-mercury amalgam into an arc tube closed at one end. Then, while the closed end is cooled, the other end is sealed off in a chamber containing an atmosphere of the inert starting gas intended for the lamp. In such a lamp, the heat balance is planned to make one end or the other the cold spot and the excess amalgam collects mostly in the corners where the end cap or plug is joined to the ceramic body. In both designs, lamp voltage increases as the lamp ages, and the end of life is reached when the ballast can no longer sustain the arc across the high voltage drop prevailing.

In high pressure sodium lamp manufacture, dimensions of parts, material quality and processing are carefully controlled in order to maintain lamp voltage within specified limits. Nevertheless over 10% of such lamps produced in the plants of applicant's assignee currently must be reworked because the voltage of the finished lamp falls above or below the specified limits. Reworking in order to salvage lamps is expensive and time consuming. It has meant breaking the outer envelope or jacket, cutting the ceramic arc tube out from the old stem assembly, welding it to a new stem assembly, and adding or removing exhaust tube radiation shields to correct the heat balance. Alternatively, the heat balance may be modified by grit-blasting the exhaust tube or by painting chrome green paint on it. The reworked arc tube must be sealed into a new outer envelope which must then be evacuated and the lamp must be rebased and reseasoned.

The object of my invention is to eliminate the foregoing rework procedures by allowing lamp characteristic voltage to be changed without opening the outer envelope.

In accordance with my invention, the lamps are provided with a thermal link joined to a metal member which forms part of the arc tube or is attached to the arc tube and the heat loss from this member influences significantly the temperature of the cold spot. In a lamp having an external metal exhaust tube serving as reservoir, the link is most conveniently made to the exhaust tube. The invention provides a thermal link such that the heat conduction thereof may be diminished in the completed lamp without breaking open the lamp jacket. Preferably heat conduction through the link lowers the mean of the lamp voltage distribution sufficiently to make the percentage of high voltage lamps manufactured negligible. The lamp production then consists almost entirely of lamps whose voltage is within specified limits, and an augmented percentage of lamps whose voltage is below the lower specified limit. The low voltage lamps are then adjusted upward in voltage by reducing the heat conduction of the thermal link.

In a preferred embodiment, the thermal link comprises a main part and a severable auxiliary part in the form of a wire extending from the exhaust tube to the metal frame of the mount assembly. The wire of the auxiliary part may be finer or may be longer than that of the main part in order to have lesser heat conduction. In low voltage lamps, the auxiliary wire is conveniently severed without breaking the outer envelope by cutting the wire with a laser beam aimed through the glass of the outer envelope. Another way of severing the auxiliary wire is to include a portion in it which melts at a lower temperature and to heat it by coupling high frequency currents into it.

In the drawings:

FIG. 1 shows a high pressure sodium vapor lamp embodying the invention and containing an auxiliary thermal link and FIG. 1a shows a fragment of the lamp with the auxiliary link severed.

FIG. 2 shows a portion of a similar lamp wherein the thermal links form a square loop to facilitate electromagnetic coupling of energy.

FIG. 3 is a plot of experimental data indicating the proportionality of lamp voltage rise to the square of the wire diameter in the auxiliary severable thermal link.

FIG. 4 shows a portion of a double wire arc tube embodying the invention.

FIG. 5 shows a portion of another double wire arc tube embodying the invention in a link to a heat shield.

A high pressure sodium vapor lamp 1 embodying the invention 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 the stem end which is shown uppermost. A re-entrant stem press 4 has a pair of relatively heavy lead-in conductors 5,6 extending through it whose outer ends are connected to the screw shell 7 and eyelet 8 of the base.

The inner envelope or arc tube 9 centrally located within the outer envelope comprises a length of light-transmitting ceramic tubing, suitably polycrystalline alumina ceramic which is translucent, or singly crystal alumina which is clear and transparent. The upper end of the arc tube is closed by an alumina ceramic plug 10 through which extends hermetically a niobium inlead wire 11 which supports the upper electrode. The lower end closure also comprises a ceramic plug 12 through which extends a thin-walled niobium exhaust tube 13. It serves as an exhaust and fill tubulation during manufacture of the lamp, and as support and current inlead for the lower electrode. In the finished lamp it forms an external reservoir for excess sodium mercury amalgam. The ceramic plugs are sealed to the ends of the tube, and the niobium conductors 11 and 13 are sealed through the plugs, by means of a glassy sealing composition comprising primarily alumina and calcia which is fused in place.

Electrodes of conventional construction (not shown), suitably close-wound coils of tungsten wire activated by dibarium calcium tungstate retained in the interstices between turns, are provided in opposite ends of the arc tube and supported from inleads 11 and 13. Reference may be made to U.S. Pat. No. 3,708,710--Symser et al for a detailed description of suitable electrodes. By way of example the illustrated lamp is a 400 watt size and the arc tube is 112 millimeters long by 7 millimeters in bore. The fill comprises a charge of 25 milligrams of amalgam of 25 weight percent sodium and 75 weight percent mercury, together with xenon at a pressure of 20 torr serving as a starting gas. The illustrated exhaust tube 13 is pinched off and hermetically sealed at the end 14 and has a flattened end portion 15 of sufficient volume to accommodate the excess amalgam. Such a flattened end portion is useful for a universal burning lamp subject to shock or vibration as disclosed in my copending application (LD 8432) Ser. No. 135,953, filed Mar. 31, 1980, now U.S. Pat. No. 4,342,938 entitled Universal Burning Ceramic Lamp and assigned like this application.

The arc tube is mounted within the outer envelope in a manner to allow for differential thermal expansion. A sturdy support rod 16 which extends substantially the length of the outer envelope is welded to lead-in conductor 5 at the stem end and braced by spring clamp 17 engaging nipple 18 in the dome end of the outer envelope. The arc tube is supported primarily by wire connector 19 which is welded across from niobium tube 13 to support rod 16. At the upper end, axial lead wire 11 extends through an insulating bushing 21 which is supported from rod 16 by means of metal strap 22. The aperture through the bushing allows free axial movement of inlead 11 and a flexible conductor 23 makes the electrical connection from the inlead to lead-in conductor 6. Differential thermal expansion of the alumina arc tube relative to the mounting is accommodated by axial movement of inlead wire 11 through bushing 21 and by flexing of curved conductor 23.

One may consider a thermal link extending from exhaust tube 13 to support rod 16 which comprises a main part and a severable auxiliary part. When the thermal conductance of the auxiliary part is small relative to that of the main part, a reasonable first approximation for the effect of severing the auxiliary part is that the change in lamp operating voltage is proportional to the thermal conductance of the severed part. The effective thermal conductance C of the auxiliary part is given by:

where

K=effective thermal conductivity of part,

A=cross-sectional area of part,

and

L=length of part.

I have conducted tests on lamps in which the thermal link comprised a main part of 35 mil niobium wire and an auxiliary part. FIG. 3 shows the effect on lamp voltage of severing the auxiliary conductor consisting of 20 mil, 30 mil or 35 mil niobium wire. It will be observed that the voltage rise is substantially linear with respect to the square of wire diameter. Either wire diameter or length may be varied to control the voltage rise occurring when the part is severed.

The thermal link shown in FIG. 1 between exhaust tube 13 and support rod 16 determines an embodiment of my invention which is preferred from the point of view of easy automated manufacture. A niobium wire 19, suitably of 35 mil diameter, is spot welded to support rod 16 and extends to niobium exhaust tube 13 to which it is spot welded on the flattend portion 15. At a point beyond the spot weld, the niobium wire is bent back to provide a longer portion 19a which is spot welded to support rod 16 at a point appreciably removed from the first spot weld. This arrangement allows the use of a single wire size and provides a thermal link in which the main part 19 is relatively short and the auxiliary part 19a has a lower conductance because it is considerably longer.

In a manufacturing process utilizing my invention, all lamps are made as illustrated in FIG. 1. The completed lamps are then briefly seasoned and tested for voltage. The lamps whose voltage falls below the lower specification limit are segregated and subjected to a laser pulse focused on the wire link 19a to sever it. I have found that a pulsed neodymium laser of 20 Joules rating with output wavelength at 1.06 micron is adequate to sever the wire when focused on it through the outer envelope. It is desirable to use niobium or a refractory metal having a low vapor pressure at the cutting temperature in order to avoid depositing a heat and light-reflecting film on the inside of the outer envelope 2. Niobium melts at 2468.degree. C. and when it is cut by the laser, tiny fragments are spat out or stick to the cut ends but no objectionable film is deposited anywhere.

There are other ways of opening the auxiliary part of the thermal link than through the use of a laser. Referring to FIG. 2, the thermal link between exhaust tube 13 and support rod 16 comprises a main part 31 consisting of 35 mil niobium wire and an auxiliary part which completes a square loop. The auxiliary part consists of 15 mil wire of which the portion 32 spot-welded to the exhaust tube is niobium, and the portion 33 welded to the support wire is another metal with lower melting point and low vapor pressure at its melting point, suitably aluminum. The two portions have been joined together by ultrasonic welding. With this embodiment, in order to open the auxiliary part of the thermal link after the lamp has been completed, a radio-frequency current is coupled into the rectangular loop formed by the two portions of the thermal link, the exhaust tube and the support rod. The current generates heat and since aluminum wire portion 33 has the least cross section and a lower melting temperature (660.degree. C.), it melts and opens the auxiliary portion of the link. An alternative method of accomplishing the same objective is to focus a heat lamp on the low melting temperature portion 33 of the link.

In the variant of the invention illustrated in FIG. 4, the lower end of the arc tube 9 is closed by a ceramic plug 40 through which extends hermetically a niobium inlead wire 41 supporting an electrode 42 shown in dotted lines. The wire seal may be similar to that at the upper end of the arc tube shown in FIG. 1. In such a lamp which has no exhaust tube and is symmetrical end for end, an amalgam charge is inserted into the arc tube prior to sealing the second end closure. The lower end of the arc tube is cooled and the seal is made in a chamber containing an atmosphere of the inert starting gas such as xenon intended for the lamp. A process suitable for making a lamp in this way is described in U.S. Pat. No. 3,609,437--Tol et al. The details of the wire seals themselves however are preferably in accordance with U.S. Pat. No. 3,992,642, McVey et al, which provides some thermal isolation of the lead wire seal from the electrode by means of a loop in the conductor between the electrode and the seal region. In such a lamp, the excess sodium-mercury amalgam collects for the most part in the corners 43 where the plug is joined to the ceramic body at the lower end of the arc tube. The thermal link from lead wire 41 to frame support rod 16 comprises heavier niobium wire 44 and lighter severable auxiliary niobium wire 45. The auxiliary wire 45 may be laser cut when needed in the finished lamp.

The variant of the invention shown in FIG. 5 has wire seals at both ends of the arc tube as in FIG. 4. The illustrated design is particularly suitable for smaller sizes of lamps, for instance 100 watts or less, wherein a heat shield may be provided at each end of the arc tube in order to achieve a sufficiently high cold spot temperature together with the needed heat balance. The heat shield at the lower end is illustrated in the drawing and comprises a metal reflector band 46, suitably of niobium, wrapped around the ceramic tube 9 with the ends spot-welded together and forming a radial tab 47 on the side next to the support rod 16. The shield may be retained in place by a wire cross-piece 48 welded to inlead wire 41 and by bent-over tabs 49 which together prevent any movement. Reference may be made to U.S. Pat. No. 4,034,252--McVey, for further details on such construction. In accordance with the present invention, a severable thermal link is provided to heat shield 46 in the form of a wire 50 attached to support rod 16 at one end and to tab 49 at the other. For those finished lamps which test low in voltage, thermal link 50 may be laser-cut to give an increment in voltage.

In lieu of a thermal link having a severable auxiliary part, one may use a thermal link which can be reduced in cross section. For instance one may use a flat band of niobium for a thermal link between the exhaust tube and the support wire. Any lamps needing it may be raised in voltage by using a laser to puncture one or more holes through the band as required to reduce the thermal conductance.