US 3851200 A
A heat and light reflective coating on a fused silica envelope, as on the ends of a quartz arc tube in a metal halide lamp. The coating comprises a mixture of a reflective metal oxide such as zirconium oxide and 20 to 60 percent by weight of a finely ground up glass substantially free of alkali metal, particularly sodium. The coating is fired on at a temperature in the range from 1,100 DEG to 1,200 DEG C sufficient to soften the glass and cause the oxide particles to be bound to each other and to the quartz. The coatings are white, hard and very scratch resistant and able to withstand thermal cycling throughout lamp life.
Description (OCR text may contain errors)
States Patent [191 Thoasson 51 Nov. 26, 1974 HEAT AND LIGHT REFLECTIVE COATING 0N QUARTZ LAMP  Appl. No.: 313,875
3,374,377 3/1968 Cook 313/17 3,490,984 l/197O Petticrew et a1 106/52 X 3,536,946 10/1957 Kopelman et ul 313/220 X 3,715,244 2/1973 Szupillo 117/35 R X 3,754,980 8/1973 Malmendier 106/52 X Primary Examiner-Alfred L. Brody Attorney, Agent, or Firm--Ernest W. Legree; Lawrence R. Kompton; Frank L. Neuhauser  ABSTRACT A heat and light reflective coating on a fused silica envelope, as on the ends of a quartz arc tube in a metal halide lamp. The coating comprises a mixture of a reflective metal oxide such as zirconium oxide and 20 to 60 percent by weight of a finely ground up glass substantially free of alkali metal, particularly sodium. The coating is fired on at a temperature in the range from 1,100 to 1,200C sufficient to soften the glass and cause the oxide particles to be bound to each other and to the quartz. The coatings are white, hard and very scratch resistant and able to withstand thermal cycling throughout lamp life.
5 Claims, 1 Drawing Figure HEAT AND LIGHT REFLECTIVE COATING ON QUARTZ LAMP BACKGROUND OF THE INVENTION The invention relates to heat and light reflective coatings on quartz lamp envelopes and is particularly useful in high intensity metal halide lamps having a high temperature quartz arc discharge tube enclosed in a larger glass envelope.
The metal halide lamps now in widespread use for industrial and outdoor lighting are disclosed in U.S. Pat. No. 3,234,421 Reiling, issued Feb. 8, 1966, and entitled Metallic Halide Discharge Lamps. In appearance, these lamps resemble a conventional high pressure mercury vapor lamp comprising a quartz arc tube mounted within a glass outer jacket provided with a screw base at one end. Therm'ionic electrodes are mounted in the ends of the arc tube which contains a quantity of mercury and metal halides along with an inert gas for starting purposes. One lamp in commercial production contains mercury, sodium iodide, thalous iodide and indium iodide, whereas another contains mercury, sodium iodide, scandium iodide and thorium iodide.
The portions of the arc chamber behind the electrodes, that is the ends of the arc tube, are the coolest regions in normal operation of such lamps. In the absence of special measures to raise the temperature of the ends, the metal halide such as sodium iodide rapidly condense on the envelope wall behind the electrodes, making the lamp ineffective. To prevent this, heat and light reflective coatings are generally applied to the ends of the arc tube, sometimes to the lower end only in vertically operated lamps. A coating which has been widely used is described in US. Pat. No. 3,374,377 Cook, Metal Vapor Lamp Coating, issued Mar. 19, 1968 and consists essentially of zirconium oxide ZrO While the zirconium oxide coating has been quite satisfactory in respect of reflectivity and avoidance of darkening or release of deleterious gases into the interenvelope space, it is quite fragile and will not withstand abrasion. Bumping of lamps during handling may cause the coating to flake off and this contributes to nonuniformity in color from lamp to lamp and is an appearance defect.
SUMMARY OF THE INVENTION The object of the invention is to provide a heat and light reflective coating suitable for-use on quartz and quartz-like glasses such as are tubes of metal halide lamps, having improved adherence and resistance to abrasion and able to withstand the thermal cycling normat] to the operation of the lamp.
In accordance with my invention, I provide a coating comprising a mixture of a reflective metal oxide and -60% by weight of a finely ground glass substantially free of alkali metal, particularly sodium, and having a softening point between 900-l ,200C. By substantially I have found experimentally that such coatings can be applied to quartz in the range from l,l00 to l,200C. The coatings are white, hard and very scratch resistant and able to withstand thermal cycling throughout lamp life.
Conventional lamp processing requires heat treatment of the arc tubes at about l,200C in order to degas them, and this provides a convenient occasion for application of the reflective coating. Thus. a glass which starts to soften below l,200C, but is not excessively reactive with quartz at this temperature is desirable. A preferred glass meeting these requirements is that known as GE 177 comprising Si0 62.3 percent. A1 0 16.7 percent, BaO 18.8 percent, CaO 2.2 percent, and including less than 0.05 percent alkali.
DESCRIPTION OF DRAWING The single FIGURE of the drawing is a side view of a metal halide arc lamp embodying the invention in its arc tube.
DESCRIPTION OF PREFERRED EMBODIMENT An ideal coating would be one which has sufficient adherence to resist flaking during lamp life and which is hard and scratch resistant enough to withstand the normal bumps and abrasions received in lamp finishing operations. The coating must also be sufficiently refractory and thermal shock resistant to withstand repeated cycling to temperatures of the order of 700C and it must not produce any adverse effects on the quartz. The release of gas by the coating during life must be negligible to avoid contamination of the outer jacket volume, and in particular release of gases which might contribute to are over at the mount must be avoided. Of course the coating must be a good reflector of visible and infrared radiation in order to perform its prime function.
I have found that desirable coatings can be made from a mixture of reflective metal oxide and ground up glass which does not contain any alkali or other materials which might induce quartz devitrification. The purpose of the glass is to bind the oxide particles to each other and to the quartz arc tube and bonding is accomplished by heating the coated arc tube to a high temperature in the range of about 1,100to l,200C for a time sufficient to soften the glass, a few minutes sufficing.
Glasses which I found satisfactory are GSC No. 4 quartz graded seal glass and GE I77 glass. The compo- Other glasses which may be used, and their compositions and properties are given in Table 2 below.
TABLE 2 Corning Corning O-l O-l Weight 1723 1717 EE2 EE SiO 56.3 66.5 61.5 63.7 A1 0 16.6 19.2 18.7 21.7 BaO 6.5 7.3 7.3 CaO 9.9 8 0 l 1.4 4.2 B 0 3.3 MgO 8 2 3 2 Alkali 0.1 .06 .14 Properties Exp. Coef. 46 35 43 31 X "/C Soft. PL, C 910 1107 955 1070 Anneal Pt.. C 710 861 761 819 Strain PL. C 670 804 714 772 Various coating compositions wherein the proportion of glass to metal oxide powder was varied from 20 to 60 percent by weight and using either aluminum oxide or zirconium oxide for the reflective oxide were made. The powders were combined with an ethyl cellulose binder and ball milled. The coatings were then applied to quartz tubes by spraying, air drying, and then firing at temperatures of 900, 1,000", l,100, and 1,200C for times ranging from 5 to minutes. The
best looking coatings were obtained with the 40 percent glass mixture and these coatings were hard, white and very scratch resistant.
' Coatings were tested under normal lamp operating conditions in metal halide lamps of otherwise conventional construction. As illustrated in the drawing, such a lamp 1 comprises an outer glass envelope 2 containing a quartz arc tube 3. The arc tube contains electrodes 4, 5 set in opposite ends and has sealed therein a filling comprising mercury, sodium iodide, thallium iodide, indium iodide and an inert starting gas such as argon. The electrodes are connected to inlead 6, 7 sealed through press 8 of stem 9 of outer envelope 2. The inleads are connected externally to the contact surfaces of screw base 10 attached to the neck end of the envelope.
The illustrated lamp is intended for base up operation and the reflective coating 11 has been applied to the lower end of the arc tube only. In a lamp intended for base down operation, the coating would be applied to the opposite end of the arc 2. The outer envelope 2 may be evacuated as a heat conservation measure, or it may be filled with an inactive gas such as nitrogen. In the larger sizes of lamps exceeding 400 watts, it is preferred to have a gas filling in the interenvelope space.
Lamps with coated arc tubes were put on standard life tests to determine if there would be any detrimental effects on the quartz. Tests extended to 4,000 hours life showed absolutely no change in the coating or in the quartz surface. I
Photometry tests indicated that when aluminum oxide was used for the reflecting oxide, generally higher coating reflectance and more heat insulation was required. This could be achieved by applying heavier layers but such heavy layers tend to crack and flake. The preferred solution is to use zirconium oxide which has a higher refractive index than aluminum oxide for the reflective oxide. Also GB 177 glass is preferred for the binder because of its low alkali content. For any given glass the melting point of-the glass sets an upper limit on the firing temperature which cannot be exceeded without encounteringexcessive attack on the quartz by the glass. Such attack eventually results in crazing or cracking of the quartz. For instance with the 40 percent GE 177 glass coating. a 1,200C firing temperature did not produce any quartz crazing. but in the case of the lower melting GSC No. 4 glass, 1,200C was sufficiently high to cause excessive reaction with the quartz. However GSC No. 4 glass produces good results without excessive reaction with the quartz when fired at 1,100C.-
Tests were run to determine whether there is an optimum particle size for the glass additive. Four different sieve fractions of GE 177 glass were prepared consisting of 65-100 mesh, -150 mesh, 150-325 mesh, and 325 mesh or less. By glass of 100-150 mesh is meant glass powder in which the particles can pass through a 100 mesh screen but not through a 150 mesh screen. By far the best coatings were obtained with the finest glass powder fraction, that is the fraction passing through a 325 mesh screen. The average weight of the coatings applied in a test on 1,000 watt arc tubes using ZrO and 40 percent by weight GE 177 glass was about 370 milligrams per lamp. This means that about 222 milligrams of Zr0 was coated on each lamp and this compared to about milligrams of ZrO when no glass binder is used as in US. Pat. No. 3,374,377 Cook. Thus my invention requires but a moderate increase in the quantity of ZrO used along with the relatively inexpensive glass powder for the great increase in adherence, durability and scratch resistance achieved.
What I claim as new and desire to secure by Letters Patent of the United States is:
1. An electric lamp comprising a sealed quartz envelope containing radiation generating means,
and a hard, scratch-resistant heat and light reflective coating on a portion of the outside thereof, said coating comprising a reflective metal oxide powder having an index of refraction greater than 1.75, and 20 to 60 percent by weight of finely ground up glass substantially free of alkali metal,
the glass particles in said coating having been heatsoftened and re-fused to cause the metal oxide particles to be bound to each other and to the quartz in a lightly sintered particulate coating.
2. A lamp as in claim 1 wherein said glass has a softening point between 900 and 1,200C.
3. A lamp as in claim 11 wherein the reflective metal oxide powder is zirconium oxide.
4. A lamp as in claim 1 wherein the glass is composed of approximately 62% SiO 17% A1 0 19% BaO, 2% CaO and not more than 0.05% alkali.
5. A lamp as in claim 1 wherein the glass is composed of approximately 80.5% SiO 1.5% A1 0 17% B 0 and not more than 0.9% alkali.