US3889142A - Metal halide discharge lamp having heat reflective coating - Google Patents

Abstract

The arc tube ends of a metal halide arc discharge lamp have a heat reflective coating thereon. The coating comprises zirconium dioxide and zirconium diboride.

Description

United States Patent Keeffe June 10, 1975 METAL HALIDE DISCHARGE LAMP [56] References Cited HAVING HEAT REFLECTIVE COATING UNITED STATES PATENTS Inventor: William Keeffe, Rockport, Mass. 3,662,203 5/1972 Kuhl et a1. 313/221 [73] Assignee: GTE Sylvania Incorporated,

Danvers, M Primary ExaminerJames B. Mullins Attorney, Agent, or Firm-James Theodosopoulos [22] Flledz Mar. 25, 1974 [21] Appl. No.: 454,397 5 ABSTRACT The are tube ends of a metal halide arc discharge US. Cll 3 I 3 lamp havg a heat reflective coating thereon. The oat- [5 Int. Cl. omprises irconium dioxide; and irconium dibo- [58] Field of Search 313/17, 27, 44, 47, 220, i

4 Claims, 1 Drawing Figure METAL HALIDE DISCHARGE LAMP HAVING HEAT REFLECTIVE COATING THE INVENTION This invention relates to metal halide arc discharge lamps. Such lamps comprise an arc tube, usually made of fused quartz or other high silica glass, having electrodes disposed therewithin and containing a fill including an inert starting gas, mercury and a metal halide.

The are tube ends of such lamps usually have a heat reflective coating thereon in order to maintain said ends at a sufficiently high temperature so as to ensure adequate vapor pressure of the metal halides in the arc tube. Examples of such coatings are shown in US. Pat. Nos. 3,325,662 and 3,374,377, which disclose coatings of calcium pyrophosphate and zirconium dioxide.

One of the problems with prior art coatings is lack of adhesion to the arc tube. This problem is not severe when the arc tube is enclosed within an outer jacket. However, in unjacketed lamps, the rubbing-off of the coating can be detrimental to proper lamp operation.

This invention concerns zirconium dioxide heatrefiective coatings having improved adhesion and improved resistance to abrasion. l have found that a coating comprising ZrO and ZrB which has been fired at a suitably high temperature, yields said improvements.

The single FIGURE in the drawing is an elevational view, partly in section, of the arc tube of a metal halide discharge lamp in accordance with this invention.

The are tube comprises a tubular quartz envelope 1 having press seals 2 at each end thereof. Disposed within envelope 1 is the usual fill including an inert starting gas, mercury and metal halide. An electrode 3 is disposed at each end of the arc tube, each electrode being supported on metal rod 4 which is welded to molybdenum ribbon 5 wich, in turn, is welded to external lead-in wire 6. Press seal 2 completely embeds ribbon 5 and provides the support for rod 4.

Disposed on the exterior surface of the cup-shaped ends of envelope 1 is a heat reflecting coating 7. Coating 7 generally surrounds electrode 3 and extends onto press seal 2. The coating comprises ZrO and ZrB the addition of ZrB considerably improving the adhesion of the coating to the quartz.

In a specific example, 125 grams of powdered ZrO 3 grams of powdered ZrB and 3.2 grams of submicron powdered alumina were dispersed in 200 milliliters of isopropyl alcohol. The specific gravity of this coating suspension was about 1.9. Each end of sealed envelope 1 was dipped into the suspension up to about or slightly beyond the height of electrode 3 to form coating 7 thereon. The coating was then removed from the end of press seal 2 and then fired at a temperature of 550 to 800C in order to improve the adhesion thereof.

X-ray diffraction analysis of the bonded coating revealed a drop in ZrB intensity and an increase in ZrO intensity in comparison with the unfired coating. This suggests that the ZrB is being reduced and that the Zr metal thereby freed up is oxidized during the firing, thereby increasing the Zr0 concentration while reducing the ZrB concentration. The boron thus formed goes into a borosilicate glassy phase of the general form (x)SiO (xa)B O at the quartz interface which plays a role in the bonding mechanism. Scanning electron microscope microphotographs of the interface between coating 7 and quartz envelope 1 confirms the formation of the borosilicate glassy phase.

At a suspension specific gravity of about 1.9, the thickness of coating 7 was about microns, which is satisfactory for the purpose of this invention. If the coating is too thin, the heat reflectivity thereof is inadequate. If the coating is too thick, cracking thereof can result.

The ratio of ZrO to ZrB in the coating suspension should be between about 30:1 and 250:1. When the ZrB content is at a lesser ratio than about 250:1 there is insufficient ZrB present to improve adhesion. At greater ratios than about 30:1, the ZrB discolors the coating and reduces the visible light reflectivity thereof, thereby reducing lamp efficiency.

When the coating is fired at temperatures less than about 550C, the adhesion thereof to the arc tube is inadequate to prevent rub-off.

The purpose of the submicron alumina in the coating suspension is to increase the strength of the unfired coating.

When the arc tube is supported by metal supports 8 clamped on press seal 2, there should be a separation between clamp 8 and coating 7. The reason for this is to prevent electrolysis that can occur as a result of a potential difference between electrode 3 and coating 7, since coating 7 has some electrical conductivity at normal operating temperatures and since clamp 8 is usually a part of the electrical circuit of the lamp.

Lamps of this invention that have been life tested have shown no evidence of deleterious strains in the quartz as a consequence of the reaction of the ZrB with quartz.

I claim:

1. An arc discharge lamp comprising an arc tube, made of high silica glass and having press seals at each end, containing a filling including inert starting gas, mercury and metal halide, having electrodes sealed therein at opposite ends and having a heat reflective coating of ZrO and ZrB on the ends of said are tube.

2. The lamp of claim 1 wherein said coating surrounds said electrodes and extends onto said press seals.

3. The lamp of claim 1 wherein the ratio of ZrO to ZrB in said coating is between about 30:1 and 250:1.

4. The lamp of claim 1 including, in addition, metal supports clamped on said press seals and an uncoated space between said metal supports and said heat reflective coating.

Claims (4)

1. AN ARC DISCHARGE LAMP COMPRISING AN ARC TUBE, MADE OF HIGH SILICA GLASS AND HAVING PRESS SEALS AT EACH END, CONTAINING A FILLING INCLUDING INERT STARTING GAS, MERCURY AND METAL HALIDE, HAVING ELECTRODES SEALED THEREIN AT OPPOSITE ENDS AND HAVING A HEAT REFLECTIVE COATING OF ZRO2 AND ZRB2 ON THE ENDS OF SAID ARC TUBE.

2. The lamp of claim 1 wherein said coating surrounds said electrodes and extends onto said press seals.

3. The lamp of claim 1 wherein the ratio of ZrO2 to ZrB2 in said coating is between about 30:1 and 250:1.

4. The lamp of claim 1 including, in addition, metal supports clamped on said press seals and an uncoated space between said metal supports and said heat reflective coating.

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