Zirconia reflector coating on quartz lamp envelope
United States Patent McVey et al.  
 [ Apr. 22, 1975 ZIRCONIA REFLECTOR COATING 0N 3.374.377 3/l968 Cook 313/17 QUARTZ LAMP ENVELOPE 3.533.827 Iii/I970 Rimback ll7/l06 R [75] Inventors: Charles I. McVey. Shaker Hts; 0. Pn-&#34;m&#34; Ewminer john Kominski Manna Cleveland both of ohm Attorney, Agent. or Firm-Ernest W. Legree&#39;. [73] Assignee: General Electric Company, Lawrence R. Kempton; Frank L. Neuhauser Schenectady, NY. 122 Filed: Oct. 9. 1973 {571 ABSTRACT The adherence of an optically reflective coating of zir- App! 404&#39;l78 conium oxide on a fused silica discharge tube surface is improved severalfold by an intermediate adhesion [52] U.S. Cl. 313/27; 106/52; 1 17/94; l y r nsisting f ery fine aluminum oxide and boric 3 l3/1 l3 oxide powders. The adhesion layer is first applied and [51] Int. Cl. H01] 7/24 ri h zir oni m xi ating i pplied. and the [58] Field of Search ll7/l06. 94. 124 A, 159, quartz tube is then lehred at a temperature greater 117/2! l; 313/17, 113 27, 116; 106/52 than 460C, the melting point of boric oxide The improved coating strength permits the use of a thicker [56] References Cited layer of zirconium oxide for higher optical and ther- UNITED STATES PATENTS 2.568.459 9/l95l Noel 313/221 4 Claims, I Drawing Figure g! 9 I/ a 7 6 ZIRCONIA REFLECTOR COATING ON QUARTZ LAMP ENVELOPE BACKGROUND OF THE INVENTION The invention relates to heat and light-reflective coatings on fused silica lamp envelopes operating at high temperatures and is particularly concerned with improving the coating strength and adherence.  
  High intensity metal halide lamps such as disclosed in U.S. Pat. No. 3,234,421 Reiling, are widely used for commercial, industiral, and outdoor lighting. 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. Thermionic eletrodes 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 commercially avail able lamp contains mercury, sodium iodide, thallium 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 are tube, are the coolest regions in normal operation of such lamps. In the absence of special measures to raise the temperature of the ends, too much of the metal halide such as sodium iodide may remain condensed on the envelope wall behind the electrodes. To prevent this and cause the lamp to achieve its proper efficiency, 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 U.S. Pat. No. 3,374,377 Cook, Metal Vapor Lamp Coating,&#34; issued Mar. 19, I968 and consists essentially of zirconium oxide ZrO While a zirconium oxide coating has been quite satisfactory in respect of reflectivity and avoidance ofdarkening or release of deleterious gases into the interenvelope space, it is quite fragile and will not withstand abrasion. Bumping of lamps during handling and even the mere heating and cooling from intermittent operation may cause the coating to flake off. This contributes to nonuniformity in color from lamp to lamp and creates an appearance defect. Also the coating is limited in thickness, and thicker coatings having greater reflectivity are desirable.  
 SUMMARY OF THE INVENTION The object of the invention is to improve the adherence and coating strength of an optically reflective coating of zirconium oxide on a fused silica discharge tube. By fused silica it is intended to include quartz and quartz-like glasses, such as those comprising 96 percent silica and up, some of which are sold under the trademark Vycor.  
  In accordance with our invention, adherence of the zirconium oxide coating is improved severalfold by an intermediate adhesion layer consisting of very fine aluminum oxide and boric oxide powders which are heated to a sufficient temperature, greater than 460C, the melting point of boric oxide, to react the boric oxide chemically with the silica surface, the fine aluminum oxide, and the ZrO reflector powder.  
  It is convenient to apply the adhesion layer by dipping the fused silica tube into a suspension of the aluminum oxide and boric oxide powders and allowing the coating to dry. The zirconium oxide coating is then applied and the quartz tube is lehred at a temperature greater than 460C. The improved coating strength prevents flaking off and permits the use of a thicker layer of zirconium oxide for higher optical and thermal reflection than previously possible.  
 DESCRIPTION OF DRAWING The single FIGURE of the drawing is a side view of a metal halide lamp in which the are tube is provided with an improved zirconium oxide reflector coating embodying the invention.  
 DETAILED DESCRIPTION The difficulty in achieving reliable adherence of ZrO to fused silica are tubes appears to be due at least in part to the mismatch in thermal expansion and the tremendous temperature range involved. The coefficient of thermal expansion of quartz is 0.56 X 10 cm/cm/C while that of ZrO is 7.5 X IO cm/cm/C, about 12 times greater. The are tube wall temperature at the hottest coated spot, located slightly above the tip of the electrode, may be as high as 925C. Thus in a lamp operating outdoors, the interface between the quartz and the ZrO coating may pass through a temperature swing of close to 1000C.  
  Aluminum oxide has a coefficient of thermal expansion of 8.0 X IO cm/cm/C and is almost a perfect match for ZrO The boric oxide B 0 melts at 460C and heating above that temperature permits reaction between B 0 and SiO and between B 0 and A1 0 We believe our invention thus provides an M 0 intermediate adhesion layer firmly attached to the fused silica. At the interfaces between the Al O particles and the ZrO particles, the rates of thermal expansion substantially match, resulting in a much stronger bond. However the improved adherence and thicker coatings achieved by our invention are facts irrespectively of the validity of the foregoing explanation.  
  The intermediate adhesion layer according to the invention may conveniently be applied as a wet coating by dipping the quartz arc tube or envelope into a suspension of the aluminum oxide and boric oxide powders in an organic vehicle. Table I below lists representative formulations which were tested and studied to determine and optimize the permissible range with regard to N 0,, to 3:0,, ratio, the liquid to solid ratio and the ratio of high volatile to low volatile components in the organic liquid vehicle.  
 The A1 0 used was very fine submicron size alumina such as is commercially available under the trademark ALON C&#34;. The B 0 used was in the hydrated form of boric acid H and the weight given above is the B 0 equivalent. Substantially all water present in the boric oxide and aluminum oxide is removed in subsequcnt heating of the quartz tube. For the highly volatile organic component, methanol of high purity (electronic grade) was used and for the nonvolatile component ethylene glycol monoethyl ether acetate commonly referred to as cellosolve acetate was used. The ingredients for each formulation were measured as indicated. placed in a A; liter porcelain ball mill contataining alumina pebbles, and intimately mixed by rolling for several hours.  
  The formulas were tested on the quartz arc tubes of metal halide lamps of conventional construction as illustrated in the drawing. The lamp l comprises an outer glass envelope 2 containing a quartz arc tube 3. The are 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 inleads 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 H 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 tube. The outer envelope 2 may be evacuated as a heat conservation measure, or it may be filled with an inactive gas. The illustrated lamp corresponds to a 400-watt size wherein the outer envelope is generally evacuated; in larger sizes an inactive gas, generally nitrogen, is provided in the interenveiope space.  
  A zirconium oxide suspension suitable for spraying was prepared by milling 870 grams of zirconium oxide in 870 cc of ethyl cellulose binder with l2 cc of surfactant for several hours until the average particle diameter measured 0.8 to 0.85 micron. The ethyl cellulose binder consisting of 2.9 percent solids by weight is made by the following formula:  
 ethyl cellulose 2) grams di butyl phthulate 44 grams xylol (xylene) H4 grams hutanol (hutyl alcohol) l3 grams The foregoing ingredients are rolled in a glass jug until the ethyl cellulose goes into solution.  
  The formulations in Table l were tested by dipping the end of the are tube into a suspension of the adhesion mix and allowing to air dry. The are tube was then wrapped with suitable masking paper exposing the portion ll desired to be coated. The are tube is heated to about 180C. and clamped in a fixture which rotated slowly before a spray gun. The previously prepared zirconium oxide suspension is sprayed during several revolutions of the arc tube. The are tube is then taken from the fixture, the masking paper is removed. and excess material is brushed off. The are tube is then baked in air for about 10 minutes at 600C.  
  Coating strength and adherence of the zirconium oxide coating were then measured following the scratehadhesion test designation F32-68 of the American Society for Testing and Materials. In this test a nee dle is drawn across the coated area in a manner forming two intersecting scratches. The test specimen is then blown with compressed air to dislodge any loosened coating and microscopically examined to appraise the degree of coating removal. In Table 2 below. coating strength is the weight in grams that must be applied to the needle or stylus to cut through the coating with only superficial scratching of the underlying base; adherence is inversely proportional to the extent of chipping and raggedness at the intersection of the scratches, and is measured on a scale from 0 to 4 by comparison with observational standards.  
 TABLE 2 Coating Strength Adherence Table 2 shows the results using formula 8 of Table 1 which is that preferred. The coating strength using the adhesion mix according to the invention has risen from 25 to 70, and the adherence has moved up from 2 to 4.  
  The quantity of 13 0 applied in the intermediate adhesion layer should be from 0.05 to 0.5 mg/cm and the quantity of A1 0 from 0.5 to 1.5 mg/cm. A preferred formulation is about 0.l mg. of B 0 and 0.3 mg. of M 0 per cm*.  
  The intermediate adhesion layer according to the invention achieves a threefold increase in coating strength and makes practical the application of thicker ZrO reflector coats which are desirable in metal halide Eamps for greater color uniformity. Prior to the invention, ZrO coatings heavier than 5 mg/cm had insufficient adherence and would flake off. With the invention. coating weights from 5 up to 30 mg/cm will adhere, and a preferred coating weight is now about 15 mg/cm.  
  What we claim as new and desire to secure by Letters Patent of the US. is:  
  1. A fused silica envelope having an optically reflective coating of finely divided ZrO adherent thereto and an intermediate adhesion layer consisting of very fine aluminum oxide and boric oxide heat-reacted with the silica surface, said coating of zirconium being heatreacted with said intermediate layer.  
  2. A fused silica envelope as in claim 1 wherein the weight of B 0 in said intermediate layer is from 0.05 to 0.5 mg/cm and the weight of A1 0 is from 0.05 to L5 mg/cm.  
  3. A fused silica envelope as in claim 2 wherein the weight of ZrO in said coating is from 5 to 30 mg/cm.  
  4. A fused silica envelope as in claim 1 wherein the weight of B 0 in said intermediate layer is about 0.1 mgicm and that of M 0 is about 0.3 mg/cm, and the weight of ZrO in said coating is from 5 to 30 mg/cm? k I