BACKGROUND OF THE INVENTION

The present invention relates to alloy wire, lamp components made therefrom and lamps incorporating the components.

U.S. patent 1,602,526 to Gero describes the doping of tungsten oxide powders with potassium to promote a recrystallized structure having elongated interlocking crystals.

U.S. patent 3,236,699 to Pugh et al relates to a tungsten-rhenium alloy doped with potassium in the form of a filament having good ductile properties and sag resistance in the recrystallized state. U.S. patent 3,748,519 to Martin et al relates to supports for tungsten filaments and gettering. An alloy disclosed therein includes 92.5 percent tantalum and 0.5 tungsten.

U.S. patent 1,508,241 to Pace describes a non-sag filament which uses oxides of tantalum or niobium as dopants in place of sodium potassium silicates to produce a non-sag filament. SUMMARY OF THE INVENTION

The alloy wire composition of the present invention has metallurgical properties which permit its use as various components in various types of lamps. The metal¬ lurgical properties vary depending on the method of manufacture and use. The alloy wire may be used as a

non-sag filament, a vibration resistant filament, a filament support or gettering means.

In accordance with the present invention, there is provided an alloy wire consisting essentially of a single phase solid solution of tungsten and about 0.2 to about 6 percent by weight tantalum, said alloy including grain controlling additives uniformly distributed therein, said additives consisting essentially of from about 30 to- about 200 parts per million potassium and less than about 100 parts per million silicon.

There is also provided a filament for an incandescent lamp, an incandescent lamp and method for making the alloy wire. The stability of the fine grain structure at temperatures up to at least 2200 degrees centigrade make it suitable for use in incandescent lamps requiring a vibration resistance filament. The recrystallized structure having elongated grains, is suitable for use in high temperature lamps requiring a sag resistance filament. Due to the inclusion of tantalum, the alloy of the present invention has properties which make it suitable for use as a gettering component in lamps, such as tungsten-halogen lamps. BRIEF DESCRIPTION OF DRAWINGS

Figure 1 illustrates an incandescent lamp. Figure 2 shows cross section of grain characteristics conventional tungsten wire annealed at 2300C.

Figure 3 shows cross section of the grain character¬ istics of an alloy wire of the present invention annealed at 2300C. DETAILED DESCRIPTION

The alloy of the present invention consists essentially essential of tungsten and from about 0.2 to about 6 percent by weight tantalum. More preferably tantalum is present in an amount from about 1 to about 4 percent by weight based on total weight of the alloy. The

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alloy is intentionally doped with grain controlling additives to promote the formation of a favorable grain structure. The additives are preferably present in amounts less than about 300 parts per million and consist essentially of from about 20 to about 200 parts per million potassium and less than about 100 parts per million silicon. Silicon is present primarily to aid in the retention of potassium during processing. It has been found that potassium is particularly desirable and is more preferably present in an amount from about 30 to about 100 parts per million based on the weight of the final alloy composition.

Minor impurities may deleteriously affect the desired properties of the final alloy. It is desirable to maintain the impurities at amounts less than about 100 parts per million and preferably less than about 50 parts per. million by weight based on the total weight of the alloy. Typical impurities include aluminum, calcium, copper, iron, chromium, magnesium, manganese, nickel, tin, sodium and molybdenum. Impurities may be present despite all efforts to achieve high purity alloy material. It is most preferred that each of the impurities be less than about 5 parts per million.

The amount of minor ingredients including additives and impurities is based on the total weight of the alloy and is dependant on the metal source used, the temperature and time of sintering and other process steps. To achieve the desired level of dopant or additive in the final alloy, the amount of dopant employed in the presintered powder is at least equal to the amount desired in the final product and possibly up to 10 times the amount.

The alloys of the present invention are prepared by powder metallurgical techniques wherein component powders are intimately mixed to an extent to assure the homogenity

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of the final alloy. The powder mix is compacted to form an ingot and the ingot is sintered under conditions which result in the formation of a single phase solid solution of tungsten and tantalum. The sintered ingots are mechanically worked and fur¬ ther reduced in size by rolling, swagging, drawing and annealing to obtain a wire alloy having the desired metallurgical structure. According to one aspect of the present invention the alloy wire has a fine grain structure stable at a temperature of about 2300 centigrade. According to another aspect of the present invention, the fine grain structure is recrystallized to a grain structure having large grains extending in the longitudinal direction of the wire. A temperature of greater than about 2500 degrees centigrade is needed to promote the recrystallization to the desirable large grain structure.

As compared with conventional wires of the type comprising tungsten doped with potassium, the alloy wire of the present invention retains the fine grain structure at higher temperatures. Figure 2 illustrates the grain structure when conventional doped tungsten wire is annealed at 2300C to form large elongated grains. In Figure 3, the same anneal at 2300C does not result in the formation of a large grain structure but instead retains a fine grain structure. The number of grains across the cross section of the wire is very large. Retention of the fine grain structure at high temperatures is useful for lamp filaments requiring a vibration resistant structure.

At higher anneal temperatures; on the order of 2500 degrees centigrade or greater, the fine grain structure of the alloy wire of the present invention may be recrystal¬ lized to a large grain structure similar to the structure shown in Figure 2. The grain growth proceeds primarily in

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the longitudinal direction of the wire and results in coarse, interlocked grains and irregular grain boundaries which form, on the average, very small angles with the surfaces of the wire. This grain structure is effective in preventing sag in lamp filaments which is primarily offsetting of grains due to slip in grain boundaries forming large angles with the wire surfaces. The alloy of the present invention is ideally suited for sag resistant filaments for electronic lamps and tubes which are operated at temperatures above which recrystallization occurs.

In addition to the benefical metallurgical proper¬ ties, the inclusion of tantalum in the alloy of the pre¬ sent invention permits its use as lamp components which are operated at a suitable temperature to enhance the gettering properties of tantalum. It is known that tantalum reacts with oxygen; hydrogen and halides at high temperatures. It is theorized that the resulting com¬ pounds formed by tantalum gettering may further stabilize the alloy wire grain structure due to the formation of dispersed tantalum compounds which inhibit changes in grain structure.

In addition to the above properties, the alloy of the present invention possesses suitable ductility, tensile strength and electrical resistivity that contri¬ bute to the suitability for use as a component in a lamp.

Figure 1 is illustrative of an incandescent lamp and lamp components utilized therewith. The lamp has a hermetically sealed light transmitting envelope 2. A coiled filament 6 is supported within the envelope 2 by a pair of lead-in wire 10 extending through the envelope 2 and sealed into the flat pinch 4. The filament 6 which spans the inner ends of the lead-in wires 10 is clamped at positions 14 and 16. Lead-in wires 10 have terminal portions 22 which protrude endwise from the outer end of

the lamp. Outwardly of the flat pinch 4, the terminal portions 22 of the lead-in wires 10 are bent back to form double-legged contact members 24. For purposes of illustration a filament support is shown at 26. For applications requiring a vibration resistant filament, such as in automobile tail lamps, the fine grain structure of the present invention which is stable at filament temperatures up to 2200 degrees centigrade is particularly desirable when used as the filament compo- nent 8. ^•

In halogen lamps the envelope 2 is filled with an inert gas, such as argon, nitrogen, krypton or mixture thereof, and a halogen additive such as bromine, for example, in the form of hydrogen bromide. The total pressure of the admixed halogen and inert fill gas may range from 2 to about 7 atmospheres, at room tempera¬ tures, depending on the fill gas composition, voltage, lumen and life ratings for which the lamp is designed. The filament may be a coiled filament or a coiled coil filament which is operated at relatively high tem¬ peratures and which is desirably sag resistant. The elongated large grain alloy structure of the present invention is suitable for use in lamps of the halogen type and in lamps of the arc discharge type. When used as a filament in a halogen lamp, the alloy wire of the pre¬ sent invention is operated at temperatures above the temperatures at which the gettering properties of the alloy are most favorably utilized.

The gettering properties of the alloy of the present invention are utilized most effectively when the alloy is used as a lower temperature component of the lamp such as the lead-in wires 10 or filament support 26.

The alloy of the present invention prior to coiling into filament typically has a tensile strength of from about 200 to 300 kilograms per square millimeter. More

preferably, the tensile strength is greater than about 210 kilograms per square millimeter and most preferably greater than about 250 kilograms per square millimeter. The relatively high tensile strength contributes to the use of the alloy for applications relating to halogen lamps and contributes to the workability of the alloy material permitting the formation of wire.

Typically, the coefficient of expansion of the alloy of the present invention as measured at about 20°C is from about 4.3 to about 4.5 cm./cm./C x 10~ ⁶ and more pre¬ ferably the coefficient of expansion is from about 4.3 to about 4.4 cm./cm./C x 10 .

The alloy typically has an electrical resistivity of about 5.5 to about 6.0 microhm -cm. at 0°C. Preferably the electrical resistivity is less than about 5.7 microhm -cm. and more preferably less than about 5.6 microhm -cm.

For use in incandescent lamps, the lead wire pre¬ ferably has a circular cross section with a diameter of from about 0.25 millimeters to about 0.81 millimeters. The wire size depends to some extent on the power rating of the lamp with larger diameters being preferred for higher wattage lamps.

According to the process for preparing the alloy of the present invention, substantially pure tungsten powder doped with grain controlling additives consisting essen¬ tially of potassium and silicon is mixed with substan¬ tially pure tantalum powder, the resulting powder mix is compacted to form an ingot which is sintered in a hydro¬ gen atmosphere for a sufficient period of time and at a sufficient temperature to form a solid phase solution of tungsten and tantalum. The resulting ingot is mechani¬ cally worked into an alloy wire.

The dopants are preferably added to tungsten oxide prior to reduction to the tungsten powder. The dopants may be in any convenient form of potassium, aluminum and

silicon such as silicon dioxide, alumina and potassium chloride. Potassium silicate is a preferred dopant since it serves as a source for both potassium and silicon. The percent by weight of aluminum in the doping compounds as expressed in terms of equivalent aluminum trioxide is pre¬ ferably about 0.04% by weight of tungsten oxide. The per¬ cent by weight of potassium in the doping compounds, as expressed in terms of equivalent potassium oxide is pre¬ ferably about 0.3% by weight of tungsten oxide. The percent by weight of silicon in the doping compounds, as expressed in terms of equivalent silicon dioxide, is preferably about 0.4% by weight of tungsten oxide. After doping the chemically treated oxide is reduced to metallic tungsten by heating in hydrogen. Pure tantalum powder milled to obtain a fine particle size on the order of a Fisher Sub-Sieve Size of from about 5.0 microns to about 14.0 microns is mixed with the doped tungsten powder to produce powder blends having from about 0.2 to about 6 percent by weight tantalum. The blending operation is performed so as to yield a very uniformly blended doped tungsten-tantalum powder.

The resulting doped tungsten-tantalum is presintered in an inert atmosphere at about 1300°C. The ingot is next; sintered in an inert gas or hydrogen atmosphere by direct electric current resistance heating. The sintering is performed by a stepwise increase in current until a final temperature of about 2900°C is achieved. The final temperature is held for a sufficient period of time, typically on the order of about 15 minutes to achieve a single phase solution of tungsten and tantalum and densi- fication of ingot. It has been found that alloys of the present invention which have been sintered to at least about 90 percent, and more preferably to at least about 95 percent of their theoretical density (as calculated by the

rule of mixtures) are sufficiently sintered to yield the solid solution.

The resulting ingot is mechanically worked by known methods using multiple swagging steps which successively reduce the cross-sectional area and intermediate annealing steps which improve mechanical workability. Annealing steps are preferably performed in a hydrogen atmosphere. The material is further reduced by drawing, through a series of successive reductions. EXAMPLE.

A commercially available doped tungsten powder having an average particle size of about 4.2 microns, Sylvania AW 290, which is doped with potassium, aluminum and silicon. About 400 grams of commercially pure tantalum powder, KBI, Lot W1110, particle size of about 7.2 microns and about

19.6 kilograms of the doped tungsten powder are mixed in a blender for about one hour. A portion of the resulting mixture was compacted at a pressure of about 30,000 psi to form an ingot. The compacted ingot was presintered at a temperature of about 1300°C in a vacuum at a pressure of about 1.3 x 10 ^"" pascals in a furnace. The- ingot or rod was direct resistance sintered at 2700 to 2900°C for 15 minutes. The resulting density is about 17.6 g/cm . The ingot was swaged to a diameter of about 3 mm at temperature of 1600°C to 1300°C and annealed at about 2200°C at various intermediate sizes. Wire drawing from 3.3 mm diameter resulted in a size reduction to lamp wire sizes varying between 0.5 mm to 0.01 mm and was carried out at temperatures from 1000°C to 500°C in several drawing steps. The tensile strength of the wire at 0.5 mm was 237 kg/mm ² . Wire drawn to substantial smaller sizes for use as filament material will have tensile strength ranging from 220 to 400 kg/mm ² depending on the size and

the proximity of that size to an in-process anneal , Filament wires as small as 0. 01 mm are common.