US3484644A

US3484644A - Tungsten powder bonded filament connection for incandescent lamps and method of manufacture

Info

Publication number: US3484644A
Application number: US615808A
Authority: US
Inventors: Kenneth S G Pertwee
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1967-02-13
Filing date: 1967-02-13
Publication date: 1969-12-16
Anticipated expiration: 1986-12-16

Description

Dec. 16. 1969 K. S. G. PERTWEE TUNGSTEN.POWDER BONDED FILAMENT CONNECTION-TOR NCAN ESCENT LAMPS AND METHOD OF MANUFACTURE Filed Feb. 13, 1967 Irwvesrvtor: KTTOBC1IH 5.9.1 6 twee His m- -tne United States Patent O U.S. Cl. 313-331 4 Claims ABSTRACT OF THE DISCLOSURE The connection between a tungsten filament and a refractory metal lead wire is firmly bonded together by a mass of material consisting solely of fine tungsten powder particles sintered by a surface tungsten bond to each other and to the filament and the lead wire, and prepared by forming a suspension of very fine tungsten powder in a tungsten resinate solution containing a suitable binder, applying a quantity of the suspension to the connection and heating to a temperature of 1600l800 C. in a hydrogen atmosphere to decompose and drive oif all the organic material in the suspension.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to electric incandescent lamps comprising a sealed bulb or envelope containing a tungsten filament connected to current supply conductors through which current is conveyed from terminals at the exterior of the bulb. More particularly, the invention relates to the joint or connection between the ends of the filament and the current supply conductors.

Description of the prior art The filaments of incandescent lamps are generally connected to the current supply or lead-in conductors by a mechanical joint as opposed to a welded joint which tends to cause embrittlement and weakening of the tungsten filament adjacent to the joint, with consequent breakage. In certain types of lamps a satisfactory joint is made simply by securely clamping a hook in the end of the current supply conductor about the end of the filament. However, in other lamp types, particularly higher wattage lamps, such a joint is not feasible because the conductor is necessarily of larger size and is usually also made from more refractory and harder metal. The art has therefore resorted to joints of rather complicated structure in order to secure a satisfactory mechanical and electrical connection. However, in certain cases the frictional connection is unsatisfactory since it permits the filament to move and also permits a certain amount of arcing and local overheating at the joint. Such arcing produces a noise which, in the case of projection lamps used in movie or television studios, is picked up by nearby microphones.

More recently, as disclosed and claimed in my US. Patent 2,961,568, a firm conductive cohesion between the filament and conductor has been achieved by a bond formed by application of a tungsten-platinum slurry to the connection, followed by heating at a high temperature to melt the platinum which alloys with the tungsten 3,484,644 Patented Dec. 16, 1969 and becomes consolidated to firmly join together the filament and the conductor. While such a construction gives good results, the presence of the platinum makes it rather expensive. Also, in order to achieve proper flow and consolidation of the mixture, a quite high temperature (1850 C.) is required which can cause premature failure of the furnaces in which the product is heated. In some cases, accidental application of the slurry to portions of the filament proper which operate at very high temperatures, causes an attack or alloying of the platinum with the filament with resulting embrittlement.

SUMMARY OF THE INVENTION It is accordingly an object of the present invention to provide a firmly bonded connection of the filament and lead-in conductor which is less expensive than prior art bonds. It is a further object to provide a bond and method of manufacture thereof which may be formed at somewhat lower temperatures. It is a still further object to provide a bond wherein the bonding material consists solely of tungsten.

Therefore, in accordance with the invention, the filament and conductor are firmly joined or bonded by a mass of material consisting solely of fine tungsten powder particles sintered by a surface tungsten bond to each other and to the filament and the conductor. In further accord with the invention the bond is formed by applying to the connection a paste or suspension of the fine tungsten powder particles in a tungsten resinate solution, and heating the connection in a non-oxidizing environment to a temperature sufficient to decompose the resinate and effect a sintering of the tungsten particles to each other and to the filament and the conductor.

The absence from the bond of any metal other than tungsten not only results in a less costly construction and process, and avoids contamination of the tungsten filament, but it also makes the bond suitable for use in lamps of the halogen regenerative cycle type where it is desirable to avoid the presence of metals other than tungsten which could react with the halogen and interfere with the regenerative cycle.

Further features and advantages of the invention will appear from the following description of species thereof and from the drawing.

BRIEF DESCRIPTION OF THE DRAWING In the drawing, FIG. 1 is an elevation of a projection type incandescent lamp in which the invention may be embodied;

FIG. 2 is a fragmentary perspective view of the filament connection in the FIG. 1 lamp; and

FIG. 3 is a side view, in section and on an enlarged scale, of one end of a form of halogen cycle type lamp having a filament connection embodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The lamp shown in FIG. 1 is a conventional type high wattage projection lamp such as that shown in my aforesaid Patent 2,961,568, and comprises a sealed glass bulb or envelope 1 containing a planar type filament 2 composed of a plurality of coiled tungsten wire segments arranged in side-by-side relation. The serially connected segments of the filament 2 are suitably supported, and the filament terminates, at each end, in a coiled leg portion 3 which is fitted over an auxiliary current supply conductor or spud wire 4 of refractory metal such as molybdenum, although wires of tungsten, tantalum or columbium may also be used. In the particular case illustrated herein the spuds 4 have flattened end portions 5 (FIG. 2) fitted snugly in the coiled filament legs 3. The spuds 5 are connected to respective current supply or lead-in conductors 6 which have portions thereof sealed in and extending through a reentrant glass stem 7 which is sealed to the lower end of the bulb 1.

It is highly desirable, and in many applications necessary, that connection of the filament legs 3 to the spuds or conductors 4 be firmly fixed to avoid relative movement therebetween with consequent sparking or arcing and local overheating. In accordance with the present invention, the filament leg 3 and end portion 5 of conductor 4 are firmly bonded together by a mass of material indicated at 8 and consisting solely of finely divided tungsten powder particles firmly sintered by a. surface tungsten bond to each other and to the filament leg 3 and to the end 5 of conductor 4, all as more fully described hereinafter.

FIG. 3 is illustrative of a halogen regenerative cycle type lamp such as the iodine cycle type described and claimed in Patent 2,883,571 to Fridrich and Wiley. The lamp illustrated is of the double-ended tubular type comprising a tubular envelope 11 of high melting point vitreous material such as quartz or fused silica containing a tungsten filament 12 which may be of helically coiledcoil form having at each end thereof a helically singlecoiled leg portion 13 the end of which is fitted snugly over the end of an inner lead-in conductor or spud wire 14 of tungsten. The spud wire 14 is connected to a molybdenum foil 15, to which is also connected an outer lead wire 16 and which is hermetically sealed in a pinch seal 17 at the end of the envelope. The envelope 11 contains a filling of inert gas and a quantity of halogen such as iodine or bromine, preferably in the form of a compound such as hydrogen bromide or a hydrocarbon of bromine, which halogen reacts with tungsten vaporized from the filament 11 and returns it to the filament to prevent blackening of the envelope 11, as is well known in the art.

In accordance with the present invention, the filament leg 13 and the portion of the spud wire 14 enclosed thereby are firmly bonded together by the mass of material indicated at 18 and consisting solely of sintered tungsten particles. If desired, the tungsten bond may be extended over one or two turns of the filament leg 3 just beyond the inner end of spud 14 whereby to provide a more gradual temperature gradient between the coil turns of the filament leg 3 adjacent to and overlying the spud 14. This may be desirable in lamps containing bromine to minimize or prevent a tendency to an end attack or tungsten transfer phenomenon which otherwise tends to occur between adjacent coil turns across which there is a steep temperature gradient, as more fully described and claimed in application Ser. No. 609,592 of J. F. English, filed Jan. 16, 1967, now Patent No. 3,431,448.

The tungsten powder used to form the bond is extremely fine, preferably not greater than about 5 microns size, and desirably about 0.8 micron average particle size, ranging from 0.77 to 1 micron, which is the finest commercially available.

The tungsten resinate solution is preferably a 3.2% (by weight) tungsten resinate solution which is obtainable commercially from Englehard Industries, Inc., Hanovia Liquid Gold Division, Newark, N.J., and designated 3.2% Tungsten Resinate Solution #74. Such a resinate is made by reacting the tungsten with a natural rosin obtained from trees. The resinate is dissolved in a mixture of essential oil such as Oil of Rosemary and Oil of Lavender. The chief constituents of the rosin are resin acids of the abietic and primaric types having the general formula C H COOH and having a phenanthrene nucleus. They are unsaturated and hence reactive.

The mixture or suspension preferably also contains a suitable heat-decomposable organic binder which is soluble in the tungsten resinate solution. The binder is preferably a cellulosic type, such as ethyl cellulose. Where referred to hereinafter, it will be understood that the binder is ethyl cellulose of centipoise viscosity such as that marketed as Ethocell No. 100 viscosity, by Dow Chemical Company.

Good results have been obtained with a mixture or suspension prepared by adding 6 grams of ethyl cellulose binder (100 centipoise viscosity) to 200 milliliters of 3.2% Tungsten Resinate Solution in a closed jar, stirring vigorously, for example by placing the jar on a magnetic stirrer and allowing the binder to dissolve, (approximately three hours). 400 grams of 0.8 micron tungsten metal powder is added slowly while stirring by hand until the solution is thin enough to be handled elfectively by the magnetic stirrer, and is allowed to mix for one hour so that virtually each particle of tungsten powder is coated with the tungsten resinate solution. The ethyl cellulose binder is added to achieve a desired consistency, and the amount may be changed if a different consistency is desired.

The suspension is then applied to the filament joints where it soaks into the coil leg 3 (or 13) by capillary action. In the case of the FIG. 1 lamp, the filament assembly at that time includes the lead wires 6 and filament 2 and its supporting structure prior to scaling it to the stem 7. The assembly is placed in a furnace containing a non-oxidizing environment which might be a vacuum but is desirably a non-oxidizing gas, preferably hydrogen, where the filament connection is heated to a temperature in the range of about l600-l800 C., preferably 1700 C., for a time sufiicient to decompose and drive off the organic materials of the resinate solution and the binder, and leave a firm sintered metallic bond 8 consisting solely of tungsten.

The bond 8 of tungsten particles is different from ordinary sintering. The mixture or suspension shrinks about the connection, and the decomposition of the tungsten resinate solution results in the formation of new tungsten which is probably highly reactive and effects a firm molecular surface bonding of the tungsten powder particles to each other and to the filament leg 3 (or 13) and the conductor 4 (or 14). Attempts to sinter fine tungsten powder slurries without the resinate solution were to no avail; it did not adhere to the joint.

It will be evident to those skilled in the art that the proportions of the presently preferred composition described above may be varied somewhat. For example, the 3.2% tungsten resinate solution may be varied between about 100 to 300 milliliters per 400 grams of tungsten powder. 200 milliliters of the 3.2% tungsten resinate solution contains about 5.3 grams of tungsten which is in a ratio, by weight, of about 1:75 to the 400 grams of tungsten powder. For the stated range of resinate solution, the weight ratio of tungsten in the tungsten resinate solution to that of the tungsten powder is between 1:40 and 1:160.

Good results have been obtained by using a 22% tungsten resinate solution in the above-stated proportions; however, the cost is considerably increased. Good results have also been obtained by a suspension containing 1 gram of ethyl cellulose dissolved in 25 milliliters of 3.2% tungsten resinate solution to which was added 25 grams of fine tungsten powder. The viscosity may be adjusted by addition of a suitable solvent, such as butyl acetate, or xylene, or toluene, if necessary.

The tungsten resinate solution cannot be used alone. It does not provide sufiicient tungstein and does not stick; the tungsten powder is needed for bulk. Sufiicient resinate is needed to bind the tungsten powder particles together and promote sintering, presumably caused by the formation of new tungsten upon decomposition of the organic tungsten resinate.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In an electric incandescent lam-p, the combination of a current supply conductor of refractory metal, a tungsten filament connected at one end to said conductor and bonded thereto by a mass of material consisting solely of tungsten which is the heat reaction product of a suspension of finely divided tungsten powder particles in a tungsten resinate solution and in which the particles are firmly sintered by a surface tungsten bond to each other and to said filament and said conductor.

2. A lamp as set forth in claim 1 wherein the tungsten powder particles are of a size not greater than about 5 microns.

3. A lamp as set forth in claim 1 wherein the tungsten powder particles are of an average size of about 0.8 micron.

4. A lamp as set forth in claim 1 including a compact hermetically sealed envelope containing the filament and also containing a small quantity of halogen as a regenerative getter.

References Cited JOHN W. HUCKERT, Primary Examiner A. J. JAMES, Assistant Examiner U.S. Cl. X.R.