CA1078678A - Refractory metal oxide reflector coating on lamp envelope - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The adherence of an optically reflective coating of refractory metal oxide particles such as ZrO2 or Al2O3 on a fused silica discharge tube surface is improved severalfold by an adhesion layer consisting of colloidal aluminum oxide and boric oxide powders. The adhesion layer may be first applied and dried, the refractory metal oxide coating then applied, and the quartz tube then heated to a temperature greater than 460°C, the melting point of boric oxide. The refeactory metal oxide particles may also be applied admixed with the colloidal aluminum oxide and the boric oxide. The improved coating strength permits the use of a thicker layer of refactory metal oxide for higher optical and thermal reflection.

Description

~ 7~ LD-7037 The invention relates to heat and light-reflective coatings on fused silica lamp envelopes operating at high temperatures and is particularly concerned wi~h improving the coating strength and adherence.
~ igh intensity metal halide lamps such as disclosed in U.S. Patent ~o. 3,234~421 dated February 8, 1966 - Reiling~
are widely used for commercial, industrial9 and outdoor lighting. In appearance these lamps resemble a conventional high pressure mercury vapor lamp comprising a quartz arc tube mounted within a glas~ outer jacket provided with a screw base at one end. Thermionic 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 pruposes~
One commercially available 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 ar~ 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 ~pecial 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 ~oating which has been widely used is des-cribed in U.S. patent 3,374,377 dated March 19, 1968 - Cook, "Metal Vapor Lamp Coating," and consi~t~ essentially of :~
zirconium oxide ZrO2.
While a zirconium oxide coating has been quite satis-factory in re3pect of reflectivity and avoidance o~ darkening ~ ' ' . :
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~7~ LD-7037 or release of deleterious gases into the interenvelope space, it is quite fragile and will not withstand adbrasion. Bump-ing of lamps during handling and even the mere heating and cooling from intermittent operation may cause the coating to flake off. ~his 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. An aluminum oxide coating o~ equal re~lectivity is even more fragileO
Thc o~ject of the invention is to improve the adherence and coating ~trength o~ optically reflective coatings or :::
- zirconium oxide or aluminum oxid~ on a fused silica discharge tube. By fused silica it is intended to include quartz and quartz-like gla~ses, such as those comprising 96~h silica and up, some o which are sold under the trademark Vycor.
In accordance with our invention, adherenoe of the refractory metal oxide coating is improved several~old by an adhesion layer consisting o~ very fine aluminum oxide (col-; loidal) 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 ~ilica surfa~e~ the colloidal aluminum oxide, and the re-~ractory metal oxide particles which may be ZrO2 or A1203.
The adhesion layer may be applied as a distinct in-: termediate layer and the refractory metal oxide particles applied thereover, or the adhesion components and the re-~ractory metal oxide particle~ may be mixed and applied to-gether. For instance, the fused silica tube may be dipped into a ~uspension of the colloidal aluminum oxide and boric oxide powder~ and the coating ~llowed to dry. The coating of refractory metal oxide particles i~ then applied and may he applied alone or may first be admixed with colloidal ~ 2 ~

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aluminum oxide and boric oxide po~Yders for even greater coat-ing strength if desired. The silica tube is then heated to a temperature greater than 460 C, The mixed adhesion com-ponent~ and refractory metal oxide particles may al80 be applied directly without a percoat. The improved coating strength prevents flaking off and permitQ the use of a thicker layer of ZrO2 or ~1203 reflective particles ~or higher optical and thermal reflection than previously possible~
lOIn the past zirconium oxide was preferred ~or the re-~lective coating on metal halide lamps because its higher index of refraction permitted a thinner layer to suffice than when~aluminum oxide wa~ used. This avoided the problem of lack o~ adequate adhesion and permitted a thin~er layer to be used. Our invention has made possible the good adherence of either aluminum oxide or zirconium oxide by means of an adhesion coat. For many applications, aluminum oxide is now prepared because it providesl adequate refle~tivity and i5 lower in eost. Also alumina is available in much purer form than zirconia at a reasonable price~ whereby it has less tendency to darken and a white coat throughout life is achieved.
The single figure of the drawing is a side view of a me~al halide lamp in which the arc tube is provided with an improved refractory metal oxide reflectory coating embodying ` the invention.
;The dificulty in achieving reliable adharence of ZrO2 . :
to ~used silica arc tubes appears to be due at least in part to the mismatch in thermal expansion and the tremendous temperature range involved. ~h coeficient of thermal ex-pan~ion of quartz is O.S6 x lO cm/cm/C w~ile that of Zr2 is 7.5 x 10 cm/cm/ Ci a~out 12 times greater. The ar~ ~

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tube wall tempexature at the hottest coated spot, located slig~tly above the tip of the electrode~ may be as high as 925C. Thus in a lamp operating outdoors, the inter~ace between the quartz and the ZrO2 coating may pass through a temperature swing of close to 1000C.

Aluminum oxide has a coefficient of th~rmal expansion o 8.0 x 10 cm/cm/C and is almost a per~ect match or ~

~rO2, ~he boric oxide B203 melts at 460C and heating ~.
above that temperature permits reaction between B~03 and SiO~ and between B2O3 and colloidal A12O3, We believe our invention thus provides an A12O3 intermediate adhesion material firmly attached to the ~used sili~a. At the interfaces between the colloidal A12O3 and the ZrO2 particles~ the rates of thermal expansion substantially match, resulting in a much stronger bond. When Al2O3 particles are used in lieu of ZrO~ the match is perfect, However the improvad adheren¢e and thicker coatings achieYed by our invention are facts irrespectively of the validity of the foregoing explanation.
~he adhesion layer according to ths inven~ion may conveniently ~e applied as a wet coating by dipping th3 ~uartz arc tube or enYelope into a ~uspension of the aluminum oxide and boxic oxide powders in an organic vehicles. Table I below lists representative formulations which were tested and ~tudied o determined and optimize the permissible range :~
with regard to A12O3 to B203 ratio9 the liquid to solid :
ratio and the ratio o~ high volatile to low volatile com-ponents ln the organic liquid vehicle, ":

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TABLE I

Formula MethanolCellosolve*Al 0 B 0 _ _- 2 3 2 3 1 60 cc20 cc 8.37 gm 5.32 gm ~-

2 60 20 4.78 4.04

3 60 12 7.40 4.71

4 60 12 4.23 2.70 12.56 2.67 6 60 20 7.17 1.52 7 60 12 11.09 2.35 8 60 12 6.32 1.34 ~- -9 60 16.26 6.50 2.91 The A1203 used was very fine submicron size (colloidal) alumina such as is commercially available under the trade mark "ALON C". The s203 used was in the hydrated form of boric acid HB0~ and the weight given above is the B203 equivalent. Substantially all water present in the boric oxide and aluminum oxide is removecl in subsequent 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 one-third liter porcelain ball mill containing . .
alumina pebbles r 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 1 comprises an outer glass envelope 2 containing a quartz arc tube 3. The arc tube contains ; 30 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 ~ ~
*Trade mark ~ -~ ' .- '. ,.

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~7~678 LD~7037 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 a tached 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 tube. The outer envelope 2 may be evacuated as a heat conservation measure, or it may be filled with an in-active gas. The illustrated lamp corresponds to a 400-watt size wherein the outer envelope is generally evacuated; in larger sizes an inactive g~s, generally nitrogen, is provided in the interenvelope space.
A zirconiwm oxide suspension suitable for spraying was prepared by milling 870 grams of zirconium oxide in 870 cc o ethyl cellulose binder with 12 cc of surfactant for several hours until the average particle diameter measured 0.8 to 0.85 micron. The ethyl cellulose binder consisting 0 of 2.9Dh solids by weight is made by the following formula:
ethyl cellulose - 29 grams di butyl phthalate - 44 grams xylol (xylene) - 914 grams butanol (butyl alcohol) - 13 grams The foregoing ingredients are rolled in a glass jug until the ethyl cellulose goes into solutio~.
The formulations in Table 1 were tested by dipping the end of the arc tube into a suspension of the adhesion mix and allowing to air dry. The arc tube was then wrapped with suitable masking exposing the portion 11 desired to be coated.
The arc tube is heated to about 180 C and cl~mped in a fixture which rotated slowly before a spray gun The pre-..... ~ .

~078~8 LD-7037 viously prepared zirconium oxide suspension is sprayed during several revolutions of the arc tube. The arc tube is then taken fxom the fixture, the masking paper is removed, and excess material is brushed off. The arc 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 scratch-adhesion test designation F32-68 of the American Society for Testing and Materi21s. In this test a needle is drawn across the coated area in a manner forming two intersecting scratches.
~he test specL~en is then blown with compressed air to dis-lodge any loosened coating and microscopically examined to appraise the degree of coating removal. In Table ~ below, "coating strength" is the weight in gram~ that must be applied to the needle or stylus to cut through the coating with only .:
super~icial scratching of the underlying base; "adherence"
is inversely proportional to the extent of chipping and rag~dness at the intersection of the scratches, and is measured on a scale from 0 to 4 by comparision with observa- -tional standards.

Coating 5trength Adherence ;~
Test Variables _ (qrams) ~Scale of 0 to 4) Zirconia without 25 2.0 adhesion mix Zirconia with adhesion mix 70 4.0 Table 2 shows the results using formula 8 of Table 1 which is that preferred. The coating strangth using the adhesion mix according to the invention has risen from 25 to 70, and the adherence has moved up from 2. to 4. ;~
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The quantity of ~23 applied in the intermediate adhesion layer should be from 0.05 to 0.5 mg/cm and the quantity of r-,~ 0~os' 2 Al203 from ~ to 1.5 mg/cm . A preferred formulation is about 0.1 mg. of B~03 and 0~3 mg. of A1203 per cm , The intermediate adhesion layer according to the invention achieves a threefold increase in coating strength and makes practical the application of thicker 2rO2 reflector co~ts which are desirable in metal halide lamps for greater color uniformity. Prior to the invention, ZrO2 coatings heavier ~han 5 mg/~m had insufficient adherence and would flake of.
With the invention, coating weights from 5 up to 30 mg/cm2, will adhere, and a preferred coating weight is now about `~
15 mg/cm .
An exampls of a reflective coating utilizing aluminum oxide is as ~ollows. ~he precoat may be applied in the same fashion as previously descri~ed. Thereafter the reflective metal oxide layer may be applied by spraying or alternatively by dipping. Since a thicker layer is desirable when A1203 particles are used, they are first a~mixed with colloidal ~ 20 A1203 and B203 for better adhesion. An experimental formu-; lation success~ully used for dipping with a suitable binder such as that earlier d2scribed is as follow~s 270 gram~ A1203 particles ~average particle size 0.5 micron) 5.0 grams colloidal alumina (Alon-C - .01 to .02 microns) 2.0 grams ~3 B03 - boric acid The composition is milled three hour in a 1.0 liter alumina ball mill with alumina stones. The material i~ ap-plied by dip coating dried and lehred above 460 C to react the materiais.
Coating the refractory metal oxide particles in this 10786~ LD-7037 way, that i~ by first precoating the fused silica surface with the colloidal alumina and boric oxide and thereafter overcoating with the refractory metal oxide particles ad-mixed again with colloidal alumina and boric achieves maximum adhesion. In such case the colloidal aluminum oxide and the boric oxide are present both in a layer intermediate the silica surface and the refractory metal oxide particles, and also dispersed between the refractory metal oxide particles. The method is particularly suitable for applying aluminum oxide coatings which need to be some~
what thicker in order to achieve the same reflectivity as æirconium oxide coatings. The aluminum oxide coatings are lower in cost and are more stable and resistant to darken-ing over the life of the lamp.
The reflective coating of refractory metal oxide particles may also be applied admixed with colloidal alumina and boric oxide to a surface which has not been precoated with colloidal alumina and boric oxide. Good adherence may be achieved in this way at reduced cost. The colloidal ~ -~
aluminwm oxide and boric is then dispersed between the re-~ractory metal oxide particles. We have found this method to be particularly suitable for applying a reflective coat-ing to alumina ceramic, for instance to the end of a poly-crystalline alumina ceramic tube such as is used in high pressure sodium vapor lamps.
With alumina ceramic arc tubes the mismatch between alumi~a ceramic and zirconia is not nearly so great and bonding between the surfaces is not as important as bonding o~e the re~lective particles to each other for greater im-pact resistance. The coating may be applied u~ing either alumina or zirconia particles admixed with colloidal alumina and boric oxide in a formulatio~ such as previously described, _ 9 _ :

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6~8 and excellent interoxide particle bonding is achieved upon heating above 4600C. The bonding can be demonstrated by peeling the coating after reaction with a razor sharp knife.
The coating can be sliced into small cut peels. The coating is somewhat fragile due to its low density and is readily removable form the alumina tube surface. Coat-ings without the b~ric oxide were attempted and the in-terparticle bonding was so weak that peeling was not possible; the coating simply crumbled into dust. Particles of aluminum oxide can be made to adhere to alumina ceramic in the same fashion as zirconium oxide.

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Claims (11)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An envelope of material selected from fused silica and alumina ceramic, said envelope having an optically reflective coating of refractory metal oxide particles selected from the group consisting of Al2O3 and ZrO2, said coating adhering to said envelope by means of an adhesion layer consisting of colloidal aluminum oxide and boric oxide heat-reacted with the surface of said envelope, said particles being heat-reacted with said adhesion layer.

2. The envelope of claim 1, wherein said envelope comprises a fused silica envelope, and said particles comprise Al2O3 particles.

3. The envelope of claim 2, wherein said colloidal aluminum oxide and said boric oxide are present primarily in a layer intermediate said surface and said Al2O3 particles.

4. The envelope of claim 2, wherein said colloidal aluminum oxide and said boric oxide are dispersed among said Al2O3 particles.

5. The envelope of claim 2, wherein said colloidal aluminum oxide and said boric oxide are present in a layer intermediate said surface and said Al2O3 particles and are also dispersed among said Al2O3 particles.

6. The envelope of claim 2, wherein the weight of boric oxide in said adhesion layer is from 0.05 to 0.5 mg/cm2, and the weight of colloidal aluminum oxide in said adhesion layer is from 0.05 to 1.5 mg/cm2.

7. The envelope of claim 3, wherein the weight of Al2O3 particles in said coating is from 5 to 30 mg/cm2.

8. The envelope of claim 2, wherein the weight of boric oxide in said adhesion layer is about 0.1 mg/cm2, the weight of colloidal aluminum oxide in said adhesion layer is about 0.3 mg/cm2, and the weight of Al2O3 particles in said coating is from 5 to 30 mg/cm2.

9. The envelope of claim 1, wherein said envelope comprises a fused silica envelope, said particles comprise ZrO2 particles, and said colloidal aluminum oxide and said boric oxide are dispersed among said ZrO2 particles.

10. The envelope of claim 9, wherein said colloidal aluminum oxide and said boric oxide are also present in a layer intermediate said surface and said ZrO2 particles.

11. The envelope of claim 1, wherein said envelope comprises an alumina ceramic envelope.