Description:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electric lamps and has particular reference to an improved vitreous envelope for a fluorescent lamp or similar device that generates light by means of a low-pressure mercury-vapor discharge.
2. 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-lime-silicate 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 mercury-alkali amalgam deposits and the resultant loss of light output, barrier layers of finely divided refractory oxides such as A1 2 O 3 , Si0 2 and Ti0 2 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. for soda-lime-silicate glass) to vaporize the organic vehicle and affix 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 U.S. 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 U.S. 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 an organic binder such as nitrocellulose or ethyl-cellulose, 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.
A more specific object is the provision of a lamp envelope that contains an alkali-metal oxide constituent, such as Na 2 0 or K 2 0, and can be coated with phosphor and subjected to the other lamp-making operations without causing such constituents to subsequently form discoloring deposits within the finished lamp 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 2 0 3 ) particles if formed on the inner surface of the glass envelope which chemically reacts with the alkali constituents of the glass, such as sodium or potassium, that diffuse to the inner surface of the envelope during or after lamp fabrication and converts such alkalis into sodium aluminate (NaA10 2 ) or potassium aluminate (KAlO 2 ). The buffer film of rod-shaped A1 2 0 3 particles thus renders the inner surface of the envelope chemically stable and resistant to attack by the mercury ions in the electric discharge.
The integral film of rod-shaped gamma alumina particles 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 coating, coating the treated bulb with phosphor-containing lacquer in the regular manner and then baking the envelope at approximately 600° C. to remove the organic binder from the phosphor and thermally decompose the boehmite crystals and convert them into rodlike gamma alumina particles that are bonded directly to the glass surface. The colloidal boehmite crystals are, accordingly, converted in situ into rodlike gamma alumina fibrils during the normal sequence of operations required to make the lamp. The thermal conversion of the boehmite crystals into rodlike gamma alumina particles can, of course, also be achieved by heating the envelope before it is coated with phosphor and lehred. Satisfactory results have been obtained by treating the bulbs with a 0.5 to 5 percent aqueous solution of colloidal boehmite and a 2.5 percent solution is preferred. From 0.1 percent to 1 percent by weight of barium acetate (Ba(C 2 H 3 0 2 ) 2 . H 2 0) can 2 . 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 (BaS0 4 ).
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 EMBODIMENT
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 be described in this form.
With specific reference to the drawing, in FIG. 1 there is shown a fluorescent lamp 10 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 sealed to the envelope and support suitable electrodes 15 that are attached to the respective stems by lead wires 16 and 17. The electrodes 15 comprise tungsten wire coils coated with electron emissive material, as is well known in the art, and the lead wires 16 and 17 are sealed through the glass stems 14 and electrically connected to contact 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 rodlike 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 ultraviolet-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 percent by weight of Na 2 0 and up to 3 percent by weight K 2 0.
The film 22 of rodlike 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 A10(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 85 percent by weight A10(OH) crystals with 13 percent by weight acetic acid (CH 3 COOH) and 2 percent by weight H 2 0 attached to the crystals is commercially available under the trade name "Baymal" (DuPont de Nemours & Company). Satisfactory results have been obtained by using an 0.5 to 5 percent solution of the aforementioned colloidal alumina complex in distilled water and a 1 percent to 2.5 percent solution is preferred. Aqueous solutions containing more than 5 percent by weight of the aforesaid boehmite complex produces films of too great a thickness resulting in a glassy or smooth surface to which the phosphor 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 is 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 rodlike 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 coatings much 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 A10(OH) into gamma alumina some of the A10(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 NaA10 2 and other compounds which prevents the alkali ions from reacting with mercury ions and producing black alkali-mercury amalgams. The film of colloidal A10(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 percent solution of 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 X), the discrete rod-shaped particles 26 of gamma alumina formed on the inner surface of the envelope range from 2,000 A. to about 7,000 A. in length and are approximately 500 A. in width. In contrast, the photomicrograph (magnification 50,000 X and reproduced as FIG. 5) of a prior art barrier layer made in accordance with the teachings of the aforementioned Ray Patent U.S. Pat. No 3,067,356 shows that the finely divided particles 28 of prefired A1 2 0 3 are regularly shaped particles (mostly hexagonal) and about 500 A. in diameter. The barrier layer of prefired alumina also had a milky powdery appearance and the fine granules of A1 2 0 3 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. ##SPC1##
The data shown in table I was obtained on lamps that were treated with a 2.5 percent 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 percent after 100 hours burning and 1.2 percent after 1000 hours of burning.
Since the sodium on the inner surface of a soda-lime-silicate glass envelope is normally combined with sulfates to form Na 2 SO 4 . XH 2 0, 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 percent sodium sulfate. This is accomplished in accordance with the present invention by adding a small amount of barium acetate (Ba(C 2 H 3 0 2 ) 2 . H 2 0) to the colloidal solution of A10(OH) so that, upon lehring, nonreactive barium sulfate (BaS0 4 ) 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 A10(OH) and Ba(C 2 H 3 0 2 ) 2 . H 2 0 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 percent to 1 percent 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 2 0 3 to form BaA10 4 , 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 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 rodlike gamma alumina particles eliminates 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.