US3541377A

US3541377A - Fluorescent lamp having an envelope with a thin transparent buffer film bonded to its inner surface,and method of treating lamp envelopes to provide such a film

Info

Publication number: US3541377A
Application number: US776624A
Authority: US
Inventors: Rudolph Nagy
Original assignee: Westinghouse Electric Corp
Current assignee: Philips North America LLC
Priority date: 1968-11-18
Filing date: 1968-11-18
Publication date: 1970-11-17
Anticipated expiration: 1987-11-17
Also published as: JPS4945516B1

Description

Nov. 17,9970 R. NAGY 3,541,377

-LUORESCENT LAMP HAVING AN ENVELOPE WITH A THIN TRANSPAREN BUFFER FILM BONDED To ITS INNER SURFACE. AND MET OD 01 TREATING LAMP ENVELOPES T0 PROVIDE SUCH A FILM Filed Nov. 1.8, 1968 FIG.|.

FIGS.

WASH LAMP ENVELOPE H EAT ENVELOPE TO APPROX. I50C TO DRY FLUSH-COAT ENVELOPE WITH AQUEOUS SOLUTION OF COLLOIDAL ALUMINA HEAT ENVELOPE TO DRY COATING FLUSH ENVELOPE WITH PHOSPHOR LACQUER,

DRY AND LEHR W FIG. 5

FIG.4.

INVENTOR Rudolph Nagy WITNESSES G B A GENE United States Patent 'Office 3,541,377] Patented Nov. 17, 1970 U.S. Cl. 313109 9 Claims ABSTRACT OF THE DISCLOSURE The inner surface of a soda-lime-silicate glass envelope for a fluorescent lamp or similar mercury-discharge device is coated with a film of submicroscopic fibrous crystals of boehmite (AlO(OH)) prior to the phosphor coating operation. When the phosphor-coated envelope is subsequently lehred, the boehmite crystals are thermally decomposed in situ and transformed into fibrils and rodlike particles of gamma alumina (A1 that are bonded directly to the glass surface. The gamma alumina particles chemically react with the sodium and other alkali constituents of the glass to form inert compounds and thus provides a transparent film of buffer material that inhibits the formation of black alkali-mercury deposits on the inner surface of the envelope during lamp operation and enhances the lumen maintenance of the lamp during its useful life.

Deposition of the boehmite crystals is achieved by coating the envelope interior with a 0.5% to 5% aqueous solution of a colloidal boehmite complex that consists of 85% by weight crystalline boehmite, having 13% by weight of acetic acid and 2% by Weight of water attached to the boehmite fibrils. Transformation of the boehmite into rod-shaped gamma alumina is achieved by heating the envelope to a temperature of at least 400 C.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to electric lamps and has par ticular reference to an improved fluorescent lamp or similar device that generates light by means of a lowpressure mercury-vapor discharge confined by a vitreous envelope, and to a method of providing the envelopes for such devices with an integral film that inhibits bulb discoloration.

Description of the prior art As is well known, the light output of fluorescent lamps gradually decreases as the lamps are burned. It has been found that one of the contributing factors to this progressive loss of light output is the discoloration of the inner surface of the envelope produced by the reaction of mercury with sodium that is present within the soda-limesilicate glass from which the envelope is fabricated. The sodium apparently diffuses out of the glass to the inner surface of the envelope during the lehring operation when the envelope is heated to a temperature of around 600 C. in order to remove the binder from the phosphor coating. As the finished lamp is burned, mercury ions from the discharge combine with the sodium and (possibly other alkali ions) on the inner surface of the envelope and form a mercury-alkali amalgam that is brown-black in color and thus reduces the amount of light transmitted by the envelope.

In order to prevent the formation of such mercuryalkali amalgam deposits and the resultant loss of light output, barrier layers of finely divided refractory oxides such as A1 0 SiO and Ti0 in transparent thicknesses have heretofore been applied to the inner surface of the glass envelope. Such barrier layers are formed by suspending the refractory oxide particles in an organic vehicle, such as nitrocellulose or ethylcellulose, to form a lacquer which is coated onto the inner surface of the bulb and dried. The glass envelopes are then baked or lehred at a temperature just below the temperature at which the glass envelope deforms (550 C. to approximately 600 C. to soda-lime-silicate glass) to vaporize the organic vehicle and aflix a protective layer of refractory oxide particles to the glass. The inner surface of the bulb was then phosphor coated and lehred in the usual fashion. The barrier layer was thus interposed between the phosphor coating and the inner surface of the envelope and prevented the mercury ions from contacting and combining with sodium that may have diffused to the inner surface of the bulb during the high temperature lehring operations.

A fluorescent lamp having a barrier layer of the aforesaid type is disclosed in US. Pat. No. 3,067,356, issued Dec. 4, 1962 to J. G. Ray. A more recent proposal involves the use of a thinner barrier layer of titanium dioxide or zirconium dioxide that contains an additional material such as magnesium oxide, barium oxide, lead oxide or zinc oxide. The titanium or zirconium oxide is applied to the envelope in the form of a metallic-organic compound which is then converted to the oxide of the respective metal. A fluorescent lamp having such a modified barrier layer is disclosed in US. Pat. No. 3,377,494, issued Apr. 9, 1968 to R. W. Repsher.

While the aforementioned barrier layers achieve the desired result of physically shielding the alkali-containing inner surface of the bulb from the mercury ions in the discharge, they require a separate bulb-lehring operation, an organic binder such as nitrocellulose or ethylcellulose, or a plurality of refractory oxides and metallic-organic compounds, thus complicating the lamp manufacturing operations and increasing the cost of the lamps.

SUMMARY OF THE INVENTION It is accordingly the general object of the present invention to provide a simple and inexpensive means for preventing the envelope of a fluorescent lamp or similar mercury discharge device from becoming discolored as the lamp is burned.

Another object is the provision of a fluorescent lamp having an envelope that contains an alkali-metal oxide constituent such as Na O or K 0 but retains its transparency and, thus, enhances the light output of the lamp during its useful life.

It is a further object to provide a lamp envelope which has the aforementioned properties and can be coated with phosphor and subjected to the other lamp-making operations without impairing or destroying the ability of the envelope to inhibit the formation of discoloring deposits within the finished lamp.

Still another object of the invention is the provision of a simple inexpensive method for treating the inner surface of an alkali-containing glass envelope for a fluorescent lamp or similar device and rendering it resistant to discoloration in the presence of mercury ions.

The foregoing objects of the invention and other advantages which will become apparent are achieved by forming an integral film of material on the inner surface of the lamp envelope that provides a buffering action at the phosphor-glass interface. More specifically, a thin transparent film of rod-shaped gamma alumina (A1 0 particles is formed on the inner surface of the glass envelope which chemically reacts with the alkali constituents charge.

of the glass, such as sodium or potassium, that difi'use to the inner surface of the envelope during or after lamp fabrication and converts such alkalis into sodium aluminate (NaAlO or potassium aluminate-(KAlO The buffer film of rod-shaped A1 particles thus renders the inner surface of the envelope chemically stable and resistant to attack by the mercury ions in the electric dis- The integral film of rod-shaped gamma alumina par- .ticles is preferably deposited on the inner surface of the envelope by coating the latter with an aqueous colloidal solution of boehmite crystals, drying the resulting coat- ;ing, coating the treated bulb with phosphor-containing .lacquer in the regular manner and then baking the en- .velope at approximately 600C. to remove the organic binder from the phosphor and thermally decompose the boehmite crystals and convert them into rod-like gamma alumina particles that are bonded directly to the glass surface. The colloidal boehmite crystals are, accordingly, converted in situ into rod-like gamma alumina fibrils during the normal sequence of operations required to make the lamp. Satisfactory results have been obtained by treating the bulbs with a .5 to aqueous solution of colloidal boehmite and a 2.5 solution is preferred. From 0.1% to 1% by weight of barium acetate can also be added to the colloidal solution of boehmite to remove any sulfates that may be present on the glass surface by converting them to nonreactive barium sulfate (BaSO BRIEF DESCRIPTION OF THE DRAWING A better understanding of the invention will be obtained by referring to the accompanying drawing, wherein:

FIG. .1 is a front elevational view of a fluorescent lamp 'having an envelope that includes a transparent buffer film of rod-shaped gamma alumina particles in accordance with the present invention;

FIG. 2 is an enlarged fragmentary cross-sectional view of the phosphor-coated envelope taken along the line II-II of FIG. 1;

FIG. 3 is a block diagram illustrating the sequence of operations followed in forming the film of gamma alumina on the inner surface of the envelope in accordance with a preferred embodiment of the invention; and

FIGS. 4 and 5 are photomicrographs illustrating the physical characteristics of the rod-shaped gamma alumina particles formed in situ according to the present invention and the finely-divided alumina particles employed in the prior art barrier layers, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS While the present invention can be used in various types of mercury discharge devices which have vitreous envelopes that contain alkali-oxide constituents, it is especially adapted for use in conjunction with fluorescent lamps and has accordingly been illustrated and will b described in this form.

With specific reference to the drawing, in FIG. 1 there is shown a fluorescent larnp having a tubular glass.

envelope 12 that is hermetically closed at each end by the usual mount assemblies consisting of glass stems 14 that are scaled to the envelope and support suitable electrodes 15 that are attached to the respective stems by lead wires pins 18 carried by base members 20 fastened to the sealed ends of the envelope 12. Prior to being sealed, the envelope 12 is charged with a filling of suitable inert ionizable starting gas, such as argon at a pressure of 4 millimeters, and a small dose of mercury.

In accordance with the present invention, the inner surface of the envelope 12 is provided with a thin transparent film 22 of rod-like particles of gamma alumina that are bonded to the surface of the glass. As is shown more clearly in FIG. 2, the film 22 of gamma alumina is located between the inner surface of the envelope 12 and a layer 24 of a suitable ultra-violet-responsive phosphor such as calcium halophosphate activated by manganese and antimony. In accordance with standard lamp-making practice, the envelope 12 is composed of soda-lime-silicate glass that contains up to 16% by weight of Na O and up to 3% by weight K 0.

The film 22 of rod-like gamma alumina is formed by preparing an aqueous colloidal solution of boehmite crystals and flushing the envelope interior with this solution and drying it to form a thin film of boehmite crystals on the glass surface. Boehmite is a mineral found in bauxite and, more specifically, is an orthorhombic form of aluminum oxide and hydroxide AlO(OH). Boehmite is thus hydrous aluminous oxide. The crystals of boehmite are needle shaped, submicroscopic in size (0.1 micron or less) and fibrous and, when suspended in water, produce a colloidal solution having a positive ionic charge. The pore diameter of the boehmite crystals is only 47 A. and, by virtue of their small size and the positive charge on the colloid, a very thin coherent and microporous film of these crystals is produced on the glass. A colloidal boehmite .complex which has these properties, is soluble in water and consists of by weight AlO(OH) crystals with 13% by weight acetic acid (CH COOH) and 2% by weight H O attached to the crystals is commercially available under the trade name Baymal (Du Pont de Nemours & Company). Satisfactory resultshave been obtained by using an 0.5% to 5% solution of the aforementioned colloidal alumina complex in distilled water and a 1% to 2.5% solution is preferred. Aqueous solutions containing more than 5% by weight of the aforesaid boehmite complex produces films of too great a thickness resulting in a glassy or smooth surface to which thephosphor coating did not adhere readily.

After the thin film of boehmite crystals has been deposited on the inner surface of the envelope 12 as above described, the envelope is coated with a phosphor paint or lacquer consisting of a suitable vaporizable vehicle, such as ethylcellulose, and suspended phosphor particles. The phosphor lacquer is then dried and the bulb lehred or baked at a temperature of about 550 to 650 C. for about one minute to vaporize the ethylcellulose binder and thermally decompose the boehmite crystals and convert them. in situ into rod-like particles of gamma alumina. This material has an area of 300 to 350 square meters per gram and thus forms a very thin continuous film on the inner surface of the envelope 12. The thickness of the film 22 of gamma alumina does not exceed about 1 micron (10,000 A. units) and coatingsvmuch thinner than this can be readily formed by reducing the concentration of the colloidal boehmite crystals in the aqueous solution.

It is important to note that such thin films are possible as a practical matter in production by virtue of the fact that the fibrous boehmite crystals are decomposed in situ during the bulb-lehring operation and that no organic vehicles or binders or separate lehring operations are required to accomplish this. It should also be noted that since the boehmite is transformed in situ by heat into the rod-shaped gamma alumina particles, the latter are bonded directly to the glass surface and comprise an integral part of the envelope 12.

It is believed that during the thermal transformation of the AlO( OH) into gamma alumina some of the AlO( OH) chemically reacts with Na that has diffused to the inner surface of the envelope 12, and other alkali constituents such as K which may be there present, to form NaAlO and other compounds which prevents the alkali ions from reacting with mercury ions and producing black alkalimercury amalgams. The film of colloidal AlO(OH) and resulting film of gamma alumina thus act as buffers on the inner surface of the envelope 12 which prevent the formation of discoloring deposits within the finished lamp.

In FIG. 3 there is illustrated a specific example of the various steps involved in treating a lamp envelope to form a thin tenacious film of gamma alumina on its inner surface in accordance with the invention. As shown, the envelope is first washed to remove surface dirt and other contaminates. A 1% solutionof hydrofluoric acid can be used for this purpose. The washed envelope is then dried by heating it to approximately 150 C. This can be achieved by passing a stream of heated air through the envelope. The dried envelope is then flush coated with the aqueous colloidal solution of the boehmite complex, and the envelope is again heated to approximately 150 C. to dry the coating and form a thin film of boehmite crystals on the inner surface of the envelope. The envelope is then flushed with the phosphor lacquer, the resulting layer of binder and phosphor particles is dried and the bulb is then lehred at approximately 650 C. for one minute to vaporize the organic binder and transform the boehmite crystals into the rod-shaped gamma alumina particles.

As shown in the photomicrograph which constitutes FIG. 4 (magnification 40,000 the discrete rod-shaped particles 26 of gamma alumina formed on the inner surface of the envelope range from 2000 A. to about 7000 A. in length and are approximately 500 A. in width. In contrast, the photomicrograph (magnification 50,000X and reproduced as FIG. 5) of a prior art barrier layer made in accordance with the teachings of the aforementioned Ray Patent 3,067,356 shows that the finely-divided particles 28 of pre-fired A1 0 are regularly shaped particles (mostly hexagonal) and about 500 A. in diameter. The barrier layer of pre-fired alumina also had a milky powdery appearance and the fine granules of A1 0 could very easily be removed simply by rubbing the coating.

As indicated in Table I below, fluorescent lamps having envelopes provided with in situ formed films of gamma aluminum in accordance with the present invention have a higher light output compared to conventional lamps of the same type without such coatings.

The data shown in Table I was obtained on lamps that were treated with a 2.5% aqueous solution of Baymal. As will be noted, the use of the thin film of discrete particles of in situ formed gamma alumina increased the lamp efficiency by 2.1% after 100 hours burning and 1.2% after 1000 hours of burning.

Since the sodium on the inner surface of a soda-limesilicate glass envelope is normally combined with sulfates to form Na SO -XH O, it would also be desirable to remove such sulfates from the glass surface and also the sulfate which results from the oxidization of the ethylcellulose binder in the phosphor lacquer which may contain as much as 0.4% sodium sulfate. This is accomplished in accordance with the present invention by adding a small amount of barium acetate to the colloidal solution of A(OH) so that, upon lehring, nonreactive barium sulfate (BaSO is formed on the bulb surface. This compound is insoluble and very stable, even under very high energy radiation such as X-rays, and has thus been used in intensifying X-ray screens. Hence, the aforesaid mixture of colloidal AlO( OH) and Ba(C H O -H O in a water solution provides an inexpensive practical means for converting sodium and sulfates which may be present on the inner surface of the lamp envelope into inert compounds that do not impair lamp performance. As a specific example, from 0.1% to 1% by weight of barium acetate is added to the aqueous colloidal solution of the boehmite complex (Baymal). Any excess of barium salt that remains in the film will combine with A1 0 to form BaAlO an inert compound.

It will be appreciated from the foregoing that the objects of the invention have been achieved in that a very simple means and method for chemically stabilizing the inner surface of a fluorescent lamp envelope has been provided which prevents diffused sodium and other alkali constituents in the glass from combining with mercury ions and forming amalgam deposits which impair the light output and efficiency of the finished lamp. The use of an aqueous colloidal solution of boehmite to deposit a thin film of boehmite crystals on the inner surface of the bulb which is subsequently transformed in situ into an integral film of rod-like gamma alumina particles during the bulblehring operation eliminates the separate bulb-lehring operation, the organic vehicle and costly organo-metallic materials required to form the prior art barrier layers.

While one embodiment of the invention has been illustrated and described, it will be appreciated that various modifications in the types and quantities of materials used and in the method of treating the envelopes can be made without departing from the spirit and scope of the invention.

I claim as my invention:

1. In a low-pressure mercury discharge lamp having an envelope that is composed of a vitreous material that contains an alkali constituent, the improvement comprising;

a thin transparent film of gamma alumina on the inner surface of said envelope,

said gamma alumina being in the form of discrete rodlike particles that are bonded to and thus constitute integral parts of said vitreous envelope.

2. The lamp of claim 1 wherein said rod-like particles of gamma alumina are from about 2000 A. to about 7000 A. in length and approximately 500 A. in width.

3. The lamp of claim I wherein;

said envelope is composed of soda-lime-silicate glass,

and

said film of rod-like gamma alumina has a thickness of up to approximately 10,000 A.

4. The lamp of claim 3 wherein;

said lamp comprises a fluorescent lamp, and

said film of rod-like gamma alumina is coated with an adherent layer of ultraviolet-responsive phosphor.

5. The fluorescent lamp of claim 4 wherein;

said film of rod-like gamma alumina and the overlying phosphor coating extend over the entire inner surface of said envelope, and

the portion of said envelope that is in contact with said film of gamma alumina contains NaAlO 6. In the manufacture of a fluorescent lamp or similar mercury discharge device having a vitreous envelope that contains an alkali-metal oxide as a constituent, the method of inhibiting the formation of discoloring mercury-alkali deposits on the inner surface of the lamp envelope during the operation of the finished lamp, which method comprises;

coating the inner surface of the envelope with an aqueous colloidal solution of fibrous crystalline boehmite, drying the resulting coating to provide a thin film of boehmite crystals on the interior of said envelope, coating said film of boehmite crystals with a lacquer comprising a vaporizable vehicle and suspended phosphor particles, heating said envelope to a temperature suflicient to vaporize said vehicle and thermally decompose the boehmite crystals and convert them in situ into rodlike particles of gamma alumina that are located between the layer of phosphor particles and the inner surface of said envelope, and then sealing electrodes into the ends of said envelope and performing the other operations required to complete the fabrication of the lamp.

7. The method of claim 6 wherein said aqueous colloidal solution comprises a 0.5% to 5% solution of a colloidal boehmite complex, and

said boehmite complex consists essentially of 85% by weight of crystalline AlO(OH) with 13% by weight of CH COOH and 2% by weight of H 0 attached to the surface thereof.

8. The method of claim 6 wherein;

said aqueous colloidal solution comprises a 1% to 2.5%

solution of a colloidal boehmite complex, and

said colloidal boehmite complex consists essentially of 85% by weight of crystalline AlO(OH) with 13% by weight of CHgCOOH and 2% by weight of H 0 attached to the surface thereof.

3,013,901 12/1961 Bugosh 117124 X 3,094,641 6/1963 Gungle et a1. 313-109 3,141,990 7/1964 Ray 313-109 X 3,377,494 4/1968 Repsher 313-409 JAMES W. LAWRENCE, Primary Examiner D. OREILLY, Assistant Examiner US. 01. X.R. 117-435, 124; 313-321