Incandescent lamps including a combined getter
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation of co-pending application Ser. No. 07/287,768 filed on Dec. 21, 1988 now abandoned which is a continuation-in-part application of U.S. patent application Ser. No. 153,862 now U.S. Pat. No. 4,810,221 of John W. Shaffer, filed Feb. 9, 1988 for &#34;Improved Method For Gettering Incandescent Lamps&#34;, the disclosure of which is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to incandescent lamps, and more particularly to the gettering of such lamps. 
     BACKGROUND OF THE INVENTION 
     The operating life of an incandescent lamp is greatly shortened by the presence of oxygen, carbon dioxide, and/or water vapor in the internal lamp atmosphere. Water vapor is particularly harmful because even trace amounts &#34;catalyze&#34; the evaporation of the tungsten filament coil by means of the well known &#34;water cycle.&#34; 
     In the water cycle, the temperature at the tungsten coil is thermally sufficient to decompose water vapor into hydrogen and oxygen. The resulting oxygen reacts with the tungsten in the coil to form volatile oxides which migrate to cooler parts of the lamp and condense. These oxide deposits are reduced by the gaseous hydrogen to yield black metallic tungsten and reformed water, which causes the cycle to repeat. 
     The problems introduced by excess oxygen in incandescent lamps are likewise well known. For example, in the tungsten-halogen cycle, oxygen is the primary agent of attack on the tungsten filament. This attack may result in etching and dendritic growth, and usually causes early filament failure. While an extremely small amount of oxygen is commonly accepted as a necessary constituent in the lamp, the amount which ends up in a finished tungsten-halogen capsule is generally recognized as being extremely variable and is always considered to be excessive. The presence of this &#34;necessary constituent&#34; has long been recognized as a major impediment to the fabrication of longer lived and more consistently performing tungsten-halogen lamps. 
     A commonly utilized solution to the oxygen problem in tungsten-halogen lamps is the introduction of one or more compounds into the lamp which will remove the excess oxygen and prevent its participation in the tungsten-halogen cycle. Such compounds are commonly referred to as &#34;oxygen getters&#34; or simply &#34;getters&#34;. 
     Various oxygen getters and/or gettering systems have been used previously. For example, metallic getters such as tantalum, zirconium, niobium, copper, hafnium, titanium, aluminum, or various combinations thereof, have been employed as oxygen getters. In application, metallic getters may be attached to a portion of the filament mount within the lamp, e.g., in the form of a crimped piece of metal. These metal getters may alternatively be incorporated as an alloy in the molybdenum leads which support the filament within the lamp. 
     U.S. Pat. No. 4,305,017 describes the use of the above-identified metals together with precious metals such as palladium, platinum and gold as oxygen getters. Metal flags, such as those described in the &#39;017 patent, tend to be difficult and expensive to attach to the internal structure of a tungsten-halogen lamp. Also, some metallic getters that are used in halogen-free incandescent lamps are not applicable for use in tungsten halogen lamps because they will react with the halogen and terminate the desired halogen cycle. Likewise, the fabrication of specialized getter alloys can also add considerably to the cost of manufacturing a tungsten-halogen lamp. In addition, in certain lamp types, it is desirable for the getter to be present across the entire range of locations within the lamp. Such positioning is impossible with metallic flag getters, and/or metal alloy gettering systems, which are generally limited to specific discrete locations. 
     Another commonly used oxygen getter for incandescent lamps is phosphorus. Phosphorus oxides which are formed by the gettering of oxygen are volatile, even at the cold spot temperatures found in hot operating incandescent lamps, including tungsten-halogen lamps. 
     Another oxygen getter which has been employed in incandescent lamps is the carbon getter. Carbon getters may be introduced to the lamp as part of a hydrogenated hydrocarbon gas or as carbon monoxide. However, in addition to deleteriously affecting filament life in certain lamp types, carbon has failed to perform as expected as an oxygen getter. 
     Yet another oxygen gettering system is described in U.S. Pat. No. 1,944,825. This patent teaches and claims the use of various gaseous fluoride compounds having water-absorbing properties. The list of fluoride compounds includes SiF 4 , BF 3 , AsF 3 , PF 3 , and salts thereof. 
     New gettering systems are constantly being developed. The present invention represents another such advance in this art. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided an incandescent lamp comprising an envelope, a tungsten filament mounted within the envelope, a fill gas, and a getter comprising phosphorus and a borane compound, or a partially halogenated derivative thereof. 
     In accordance with another aspect of the present invention, there is provided a method of gettering an incandescent lamp. The method comprises providing a getter comprising a borane compound, or a partially halogenated derivative thereof, and phosphorus in an unsealed lamp envelope; sealing the lamp envelope; and activating the getter located within the sealed envelope. 
     As used herein, the terms &#34;partially halogenated derivatives&#34; refers to borane compounds wherein one or more, but less than all, of the hydrogens have been replaced by a halogen, i.e., F, Cl, Br, and/or I. Preferably the replacement is accomplished by bromine. It must be noted that in the present invention, one hydrogen atom must always be present in the &#34;partially halogenated derivatives&#34; of the borane compounds. 
     During the getter activation step of the present method, the borane compound, or partially halogenated derivative thereof, and phosphorus are &#34;activated&#34; to combine with the impurities within the sealed envelope. 
     The getter &#34;activation&#34; conditions should be chosen such that the getter does not decompose prior to performing the desired gettering function. In fact, when temperatures sufficient to cause premature decomposition of the getter compound are employed, the getter compositions recited above fail to function. 
     During the getter activation step, the borane compound, or partially halogenated derivative thereof, and phosphorus react with residual impurities such as oxygen, water, etc., present in the sealed envelope, forming by-products. When a partially halogenated derivative of the borane compound is employed as a getter, such by-products further include halogen compounds, e.g., Br 2 , I 2 , and the like. 
     For a better understanding of the present invention, together with other and further advantages and capabilities thereof, reference is made to the figures accompanying this specification, the following detailed description and the claims appended hereto. 
    
    
     DETAILED DESCRIPTION 
     The present invention is directed to an incandescent lamp including an improved getter and a method for gettering such a lamp. 
     Use of a getter comprising a borane compound, or partially halogenated derivative thereof, and phosphorus, in accordance with the present invention, will provide lamps having a lamp life that is dramatically superior to that attainable by a lamp including a getter comprising either a borane compound, or partially halogenated derivative thereof, or phosphorus, separately. 
     The present invention will be particularly advantageous when used in tungsten halogen incandescent lamps and the fabrication thereof. 
     Incandescent lamps, including tungsten halogen incandescent lamps, are well known in the lighting art. Such lamps typically include an hermetically sealed light pervious envelope, such as quartz or hard glass, containing a fill gas including an inert gas. Typical fill gases for a tungsten halogen incandescent lamp further include a halogen and may further include hydrogen. The halogen in the fill gas may be used in its diatomic elemental form or as a volatile halide compound, such as a hydrogen halide compound. When the halogen is supplied to the lamp as a hydrogen halide, the hydrogen halide compound also furnishes hydrogen gas to the fill gas. The principal function of the fill gas in incandescent lamps is to retard evaporation of the coil. In some lamps the fill gas may perform the additional secondary function of suppressing the arc. The envelope also includes a tungsten filament wire or tungsten coil which is in connection with lead-in wires sealed into and extending internally and externally of the lamp envelope. Such lead-in wires may extend from opposite ends of the envelope (double-ended lamp) or from the same end of the envelope (single-ended lamp). Such lamps may further be enclosed within an outer envelope or a parabolic reflector and a lens. 
     In tungsten halogen incandescent lamps of the present invention, borane may react with halogen in the fill gas to from volatile haloboranes, which are as effective for gettering oxygen as borane itself. 
     Examples of borane compounds for use in the present invention include B 2  H 6 , diborane (6); B 4  H 10 , tetraborane (10); B 5  H 9 , pentaborane (9); B 5  H 11 , pentaborane (11); B 6  H 12 , hexaborane (12); as well as their various respective partially halogenated derivatives. 
     The incandescent lamps of the present invention include a getter comprising a borane compound, or partially halogenated derivative thereof, and phosphorus. The phosphorus may be initially included in the lamp as elemental phosphorus powder. For example, the phosphorus can be deposited in the lamp, on either the filament mount and/or the coil itself, e.g., by dipping the filament or coil mount in a suspension of red phosphorus or P 3  N 5  in a suitable solvent. Alternatively, phosphorus can be deposited on the filament by evaporative coating with red phosphorus. 
     The phosphorus need not be introduced directly into the lamp in its elemental form. Alternatively, phosphorus may be generated in situ. For example, phosphorus can be produced in the envelope from phosphine (PH 3 ) gas by thermal decomposition of phosphine into phosphorus and hydrogen. Such thermal decomposition may be effected by the heat of the coil at lamp light-up. 
     As used herein, &#34;phosphorus source&#34; means phosphorus and phosphorus containing materials having physical and chemical properties suitable for generating phosphorus in situ, e.g., phosphine gas. 
     Preferably the getter of the present invention includes a borane compound, or partially halogenated derivative thereof, in an amount from approximately 0.01 to 0.5 percent by volume of the fill gas and from approximately 5×10 -5  to approximately 5×10 -9  moles of Phosphorus per cubic centimeter (internal volume of capsule). Most preferably, the combined getter includes approximately 0.1 to approximately 0.2 volume percent a borane compound, or partially halogenated derivative thereof, and from approximately 5×10 -6  to approximately 5×10 -8  moles of phosphorus per cubic centimeter (internal volume of capsule). 
     In the method of the present invention, the unsealed envelope is provided with a getter comprising phosphorus and a borane compound, or partially halogenated derivative thereof. The getter can be provided in the unsealed envelope by various techniques. For example, a borane compound, or partially halogenated derivative thereof, and a gaseous phosphorus source can be introduced into the unsealed envelope with the fill gas in a single step by, for example, introducing a fill gas also including a borane compound, or partially halogenated derivative thereof, and a phosphorus source, e.g., phosphine (PH 3 ) gas, into the unsealed envelope, subsequent to which, the phosphine is converted into phosphorus and hydrogen during the getter activation step. 
     Alternatively, the phosphorus can be introduced into the envelope before the introduction of a borane compound, or partially halogenated derivative thereof, and the fill gas. This can be accomplished by introducing a phosphorus source into the unsealed envelope and converting it into elemental phosphorus, by, e.g, thermally decomposing the phosphine into phosphorus and hydrogen by lighting up the filament or coil, following which decomposition by-products are pumped out of the envelope, and the borane compound, or partially halogenated derivative thereof, and fill gas are then introduced into the unsealed envelope, after which the envelope is sealed or &#34;tipped off&#34;. Alternatively, elemental phosphorus may be included in the unsealed envelope as a coating on the filament or coil by conventional techniques with the borane compound, or partially halogenated derivative thereof, being introduced with the fill gas. 
     After the phosphorus (or phosphorus source), a borane compound, or partially halogenated derivative thereof, and fill gas are introduced into the envelope and the envelope is sealed, the getter located within the sealed envelope is activated. Such activation may be accomplished by heating the sealed envelope at a temperature and for sufficient time to activate the gettering properties of the borane compound, or partially halogenated derivative thereof, and phosphorus before the getter decomposes. Preferably the temperature is from about 100 to about 500 degrees C. For such temperature range, the heating period is preferably from about 1 minute to about 60 minutes. 
     The activation of the getter may also be carried out as a two step process by: (1) permitting the lamp including the borane compound, or partially halogenated derivative thereof, and phosphorus (or phosphorus source) to age, without heating, e.g., at room temperature, for a period of time sufficient to permit the borane compound, or partially halogenated derivative thereof, to react with the oxygen and/or water vapor impurities in the envelope; and (2) heating the phosphorus (or phosphorus) in the &#34;aged&#34; lamp to activate the phosphorus component of the getter. 
     A further alternative embodiment of the method of the present invention includes providing the borane compound, or partially halogenated derivative thereof, and phosphorus source in the lamp envelope, sealing the envelope, heating the sealed envelope at a temperature and for a period of time sufficient to activate the borane compound, or partially halogenated derivative thereof, component of the getter (but insufficient to convert the phosphorus source to phosphorus), and then energizing the coil to produce and activate the phosphorus component of the getter. This embodiment is particularly well-suited for use in an in-line manufacturing process in which lamp light-up is necessary. In fact, the activation of the borane compound, or partially halogenated derivative thereof, may be accomplished by ambient conditions in the manufacturing process without need for a separate heating step. 
     The borane compound, or partially halogenated derivative thereof, component of the getter should be activated prior to lamp light-up because the temperature of the coil during lamp light-up can cause decomposition of borane compounds before the desired gettering thereby occurs. 
     In accordance with the present invention, it is believed that borane compounds, also referred to in the art as boron hydride compounds, or partially halogenated derivatives thereof, and phosphorus will provide advantages similar to that obtained with getters comprising silane and phosphorus. For a general discussion regarding borane compounds, their chemistry and structures, see Cotton and Wilkinson, Advanced Inorganic Chemistry, John Wiley &amp; Sons (4th Ed. 1980) pp. 303-315, the content of which is hereby incorporated herein by reference. Elemental boron and silicon are analogous in that they both form volatile hydrides that are pyrophoric, that is, they ignite spontaneously upon contact with air. In fact, these boron and hydrogen containing compounds are expected to perform functionally superior to the silanes based upon their comparative thermochemistry. 
     The use of a getter comprising phosphorus and silane based compounds may be preferred, however, because they are less hazardous to work with, particularly in a non-laboratory, production oriented lamp manufacturing plant. The boranes have positive heats of formation (in contrast to silane) and are accordingly unstable chemicals that can violently decompose, especially in concentrated or pure forms. In addition to this instability, the boranes are significantly more toxic than are the silanes. For example, the threshold limit valves (TLV&#39;s) published in 1984-5 by the American Conference of Governmental Industrial Hygienists, lists time-weights average (TWA) exposures of 0.1 for diborane and 5.0 for silane. These values represent parts per million in air &#34;for a normal 8-hour workday and a 40 hour workweek, to which nearly all workers may be repeatedly exposed, day after day, without adverse effect.&#34; The 50-fold difference in TWA&#39;s shows the significantly higher hazard level in working with boranes. Nevertheless, boranes and haloboranes will function as highly effective oxygen getters in the present invention. It should be emphasized, however, that neither the silane nor borane component of the getter of the present invention would remain in the finished lamps after lamp light-up, and that no toxic or other hazards would exist in use or handling of the finished lamps. 
     While the following Examples are directed to showing the advantages of lamps including and fabricated by a method which includes introducing a silane and phosphorus getter into an incandescent lamp, the lamp and method of the present invention, employing a getter comprising a borane compound, or partially halogenated derivative thereof, and phosphorus will behave in an analagous manner. 
     LAMP TEST DATA 
     The advantages of the present invention are more easily appreciated by the following test data for lamps including a silane getter, lamps including a phosphorus getter, and lamps including a getter comprising silane and phosphorus. A comparison of the results of these three lamp tests clearly demonstrates the significant advantages obtainable by practice of the invention. 
     The increased efficiency of silane as a getter, relative to phosphorus, is described in U.S. patent application Ser. No. 153,863, suora the disclosure of which is hereby incorporated herein by reference. The following example further illustrates the high gettering efficiency of silane relative to phosphorus. 
     A lamp test was conducted using two groups of 45 watt tungsten halogen lamps. Each lamp included a hard glass capsule having a 2.3 cubic centimeter internal volume and a 84 volt filament. One group contained phosphorus from the thermal decomposition of one percent phosphine in nitrogen introduced into the capsule at 925 torr. The second group, made without phosphorus, had 0.083 percent by volume silane in the fill gas. The silane-containing capsules were baked at 450° C. for three minutes before light-up. The basic fill gas composition for both groups was 0.1 percent hydrogen bromide, 5.0 percent nitrogen, and the balance xenon. Table I, which follows, shows the life results for the above-described lamps: 
     
                       TABLE I                                                     
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               Phosphorus 0.083% Silane                                   
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1.   Moles of getter 1.15 × 10.sup.-6                               
                                  3.90 × 10.sup.-7                  
     Relative Quantity                                                    
                     1.00         0.34                                    
2.   No. Lamps        21           44                                     
3.   Average Life (hours)                                                 
                     1616         2344                                    
     Standard Deviation                                                   
                      447          706                                    
     Minimum Life     827          632                                    
     Maximum Life    2333         4627                                    
4.   Percent Life Gain                                                    
                       0 (reference)                                      
                                   46                                     
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     The data contained in Table I show a 46 percent life gain with the use of silane, even though only about one third the stoichiometric amount of getter was present in those lamps. 
     The relationship between silane concentration and lamp life was studied in another test. These 45 watt capsules were similar to those represented in Table I. 
     Silane concentration was geometrically increased from 0.05 percent by volume to 0.2 percent by volume. A silane concentration greater than about one-quarter percent results in some capsule discoloration. Such discoloration is believed to be caused by elemental silicon (from excess silane remaining after reacting with any oxygen present). Capsule discoloration is unacceptable for many commercial lighting products due to the resultant decrease in lamp lumen output. Of course, such higher silane levels are acceptable in lamps used in applications where decreased lumen output is not a concern. The silane containing capsules were baked for three minutes at 450° C. before light-up. 
     Table II summarizes the lamp test data for lamps including various silane concentrations. 
     
                       TABLE II                                                    
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                 SILANE LEVEL                                             
            P    0.05%   0.083%  0.13% 0.20%*                             
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1.  Moles (× 10.sup.-7)                                             
                  11.5   2.4   3.9   6.1   9.4                            
    Relative Quantity                                                     
                   1.00   0.21  0.34  0.53  0.82                          
2.  No. of Lamps  21     21    21    21    21                             
3.  Average Life (Hrs)                                                    
                  1945   2425  2492  2647  2617                           
    Standard Deviation                                                    
                  804    500   438   428   434                            
    Minimum Life  598    1101  1607  1910  1767                           
    Maximum Life  3957   3190  3460  3340  3358                           
4.  % Life Gain   0      +25   +28   +36   +35                            
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 *% values represent percent by volume of fill gas                        
 
    
     The test results summarized in Table II clearly show that the chemical type of getter used is far more important than the relative quantity of getter present with respect to lamp life attained. For example, in the 0.05 percent group, silane gave a 25 percent life increase relative to phosphorus, even though only one-fifth as much gettering agent was present stoichiometrically. The data also show that silane concentrations beyond an adequate amount (here approximately 0.13 percent) do not promote further increases in lamp life. 
     The optimum silane level in the fill gas of an incandescent lamp is affected by several parameters among which are internal volume, fill pressure, wall loading (watts per square centimeter of lamp surface), and total internal surface area of the lamp vessel and support structures. The important point being conveyed for the purpose of the present invention is that for any given lamp there is an optimum level of silane sufficient to getter all harmful contaminants in the lamp and above which there is no further expectable gain in life or possibly even a performance fall-off. 
     The demonstrated significant performance and life benefits afforded by the use of silane, relative to phosphorus are due to silane&#39;s superior ability to chemically getter or tie up harmful contaminants in the lamp atmosphere and the nonvolatile nature of the silicon dioxide formed. Also, while not wishing to be bound by theory, there is believed to be an ongoing gettering action promoted by traces of silicon halides which would form continually at the wall temperatures reached in operating tungsten halogen capsules. In light of this, it was completely unexpected that the addition of a less effective getter such as phosphorus to silane-gettered lamps would have any benefit whatsoever. 
     The simultaneous use of silane and phosphorus as the getter in incandescent lamps promotes a beneficial synergistic effect and a significant further filament life gain over either silane or phosphorus alone. 
     Table III shows the results of such a test, again using 45 watt tungsten halogen incandescent lamps including a 84 volt filament and a 2.3 cubic centimeter hard glass capsule. 
     The silane-containing lamps (0.083 volume percent silane) were baked at 450° C. for three minutes before light-up. 
     In the test lamps including a getter comprising silane and phosphorus, the getter was introduced into the lamp by a two-step process: first, phosphine contained in a stream of N 2  was introduced into a previously exhausted envelope, the coil was lighted to decompose the phosphine into phosphorus and hydrogen gas (which was pumped out of the unsealed envelope; and second, the silane was introduced into the envelope in mixture with the fill gas, and the envelope was sealed by conventional &#34;tipping-off&#34; techniques. The sealed lamps were then baked for three minutes at 450° C., after which heating step, they were lighted up. In each instance, the envelope was exhausted before introduction of the fill gas and/or getter. 
     
                       TABLE III                                                   
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              Phosphorus                                                  
                      Silane  P &amp; SiH.sub.4                               
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1.   Moles (× 10.sup.-7)                                            
                    11.5      3.9   (11.5 + 3.9)                          
     Relative Quantity                                                    
                     1.00      0.34  1.34                                 
2.   No. of Lamps   30        18    12                                    
3.   Average Life (Hours)                                                 
                    1818      2668  3325                                  
     Standard Deviation                                                   
                    791       464   444                                   
     Minimum Life   320       1332  2431                                  
     Maximum Life   3535      3375  4045                                  
4.   Percent Life Gain                                                    
                    0         47    83                                    
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     Some capsule discoloration was observed in each of the lamps in the combined getter test group of Table III after about 500 hours of operation. Such discoloration is believed to be the result of the amount of phosphorus used in the combined getter. A significant reduction in phosphorus content and/or an increase in halogen concentration in the lamp would correct the discoloration without adversely affecting lamp life. 
     The additional life increase for the getter combination is significant. A statistical &#34;Student&#39;s t&#34; test shows that there is less than one chance in 1,000 that the mean life of the combined getter group would differ so greatly from the silane-only group due to random (statistical sampling noise) effects. Also, the test results shown in Table II showed no hope of such an improvement by the mere increase of silane concentration in the fill gas. 
     The foregoing tests demonstrate the beneficial effect obtainable with the present invention. It appears that the phosphorus - borane combination will outperform either component alone regardless of their relative stoichiometric presence in the lamp. Most preferably, the amounts of the phosphorus and borane-based compound in the getter of the present invention combination are selected to be within the limits of sufficient optical clarity to provide an efficient light source. Determination of such limits can be routinely accomplished by the skilled artisan. 
     The present invention has been described in detail, including the preferred embodiments thereof. It will be appreciated that the skilled artisan, upon consideration of this disclosure, will be able to make modifications and/or improvements thereon, without departing from the spirit of the following claims.