US3412277A - Incandescent lamp with addition of fluorine compounds to the operating gas filling - Google Patents

Description

Nov. 19, 1968 w. E. THOURET 3,412,277

INCANDESCENT LAMP WITH ADDITION OF FLUORINE COMPOUNDS TO THE OPERATING GAS FILLING Filed April 8, 1966 2 Sheets-Sheet 1 OPERAT- A FILAMENT TE MPERAT. ND

FILAMENT LENGTH .'2 .4 .e v MM INNER EDGE OF CLAMP I0 I FIG. 20

'4 IQ FIG. 2b :2

I6 I 0 FIG. 2c "I FIG. 2d NW ///////////fil FlG.2e

FIG. 30 FIG. 3b FlG.3c 'FIG.-3d FIG. 3e

INVENTOR. WOLFGANG E. THOURET ATTOl QNEYS Nov. 19, 1968 w. E. THOURET 3,412,277

INCANDESCENT LAMP WITH ADDITION OF FLUORINE COMPOUNDS TO THE OPERATING GAS FILLING Filed April 8, 1966 2 Sheets-Sheet 2 INVENTOR. WOLFGANG E. THOURET ATTORNEYS United States Patent INCANDESCENT LAMP WITH ADDITION OF FLUORINE COMPOUNDS TO THE OPERAT- ING GAS FILLING Wolfgang E. Thou'ret, North Bergen, N.J., assignor to Duro-Test Corporation, North Bergen, N.J., a corporation of New Jersey Filed Apr. 8, 1966, Ser. No. 541,175 21 Claims. (Cl. 313-222) ABSTRACT OF THE DISCLOSURE An incandescent lamp with fluorine additive in which the ends of the filament are protected by a metal. which resists attack by the fluorine and/ or the filament ends are constructed to operate at a relatively higher temperature.

The useful life of incandescent lamps is relatively short because the tungsten Wire commonly used for the filaments cannot be manufactured with absolutely uniform cross-section. This nonuniformity results in so-called hot spots during lamp operation. At such hot spots, the tungsten evaporates faster, the local temperature rises still higher and the filament finally burns through.

It is known that the tungsten wire cross-section can be made more uniform and that hot spots can be avoided or their effect on lamp life minimized by adding to the lamp fill gas small amounts of fluorine compounds. For example, the addition of 0.1 to 2% of the gaseous compound nitrogen fluoride (NF to the argon-nitrogen lamp fill gas Will equalize the wire cross-section and eliminate hot spots or reduce their detrimental effect in the following manner: I

The NF molecules in the immediate vicinity of the hot filament operating at, for example, 2500 C. dissociate. They create a certain small concentration of free fluorine gas near the filament. The fluorine molecules react with the tungsten atoms of the filament according to Tungsten fluoride WF is a relatively stable compound. This means that it can exist in a certain not negligible concentration even at the high gas temperature prevailing in the immediate vicinity of the filament operating at 2500 C. However, the equilibrium between the components of the reaction described by Formula 1 is quite sensitive to temperature changes. This means that at filament hot spots the equilibrium is shifted more toward the left side of Formula 1 when compared to the thicker parts of the filament with slightly lower temperature. As a result, tungsten atoms are removed from the cooler filament areas and deposited at the hot spots until uniform filament temperature is achieved. This uniform temperature is not disturbed by the regular tungsten evaporation from the filament because the evaporation rate is much less sensitive to temperature changes than the above outlined fluorine-tungsten transport reaction. The dissociation of WF into tungsten and fluorine is much more dependent on temperature than the tungsten evaporation because the enthalpy of WF formation (approx. 1800 kj./mol, at C. and normal pressure) is much greater than the evaporation enthalpy of tungsten (approx. 800 kj./mol, at 25 C. and normal pressure).

The described process should lead to theoretically unlimited lamp life if the filament ends, which are basically much cooler than the main length of the filament, were not exposed to attack by the fluorine. FIG. 1 shows schematically the temperature distribution of a straight tungsten filament wire at its end, where it is clamped into the lead wire. The lead Wire usually consists of nickel,

"ice

copper, or nickel-plated copper. Directly at the lead Wire clamp the local temperature drops to about one third of the overall operating temperature. This means that the described fluorine transport reaction will quickly remove tungsten particles from the filament ends and distribute them over the main portion of the filament. As a result the filament ends are etched away and the lamp becomes inoperative.

The following measures have been proposed for the purpose of overcoming or at least reducing this detrimental effect of fluorine on the filament ends:

(1) PROTECTION OF THE FILAMENT ENDS THROUGH SLEEVES MADE OF FLUORINE RE- SISTANT MATERIAL Metals such as copper, nickel, aluminum or magnesium have been suggested as materials for such sleeves. These metals are either resistant to fluorine or fluorides at lower temperatures or, in the process, they are coated with a fluoride layer which prevents further fluorine attack. Of course, the operating temperature of the sleeves has to be kept below approximately 800 C. because above this temperature they are not fluorine-resistant any more. Sleeves made from high melting fluorides such as calcium fluoride or magnesium fluoride (melting points 1360 and 1400 C.) can be used to relatively high temperatures. However, the basic disadvantage of sleeves is the fact that they can be used with only relatively heavy filament wires. The wire has to be so stitf and rigid that it can be prevented from coming into contact with the sleeve ends. If the filament wire touches the sleeve end it is locally cooled down at this point to a temperature that makes it subject to fluorine attack. Therefore, the described sleeves are only useful in high watt-age or low voltage projection type lamps with very heavy filament Wire.

Due to these circumstances, the best possible protective sleeve cannot completely prevent the filaments ends from being attacked by fluorine. However, the attack can be slowed down considerably. With the use of sleeves, the lamp life becomes dependent upon the diffusion rate of the free fluorine or the used fluorine compound into the protective sleeves. This leads to the following possibility of further reducing the detrimental effect on the filament ends.

(2) LOWERING THE CONCENTRATION OF THE TRANSPORT GAS IN THE BULB TO A VALUE APPROXIMATELY IN THE ORDER OF THE TUNGSTEIN VAPOR PRESSURE AT FILAMENT OPERATING TEMPERATURE (10- TO 10 MM.)

This cannot be achieved directly by reducing the absolute quantity of transport gas in the bulb. The fluorine supply would be exhausted soon by gettering action of the inner lamp parts and the fluorine cycle would be interrupted. In order to secure a suflicient supply of fluorine but at the same time to keep the concentartion of fluorine and fluorides in the lamp as low as possible, solid fluorinating agents are introduced into the lamp. Solid fluorinating agents are fluorine compounds of polyvalent elements as, cobalt fluoride, silver fluoride, manganese fluoride, and others, which easily absorb or give off fluorine. For example, the inner bulb wall and the other inner lamp parts are coated with a thin layer of cobaltous fluoride CoF This reacts with surplus free fluorine relea ed through dissociation on the hot filament and is fluorinated to CoF Tungsten particles that evaporate from the filament will usually be fluorinated by free fluorine in the lamp to WF and transported back to the filament. If there is not sufi'icient free fluorine within the lamp volume, the tungsten particles will reach the bulb wall. They are fluorinaed there by the CoF and then transported back to the filament. The COP- is reduced to CoF but afterwards again fluorinated to CoF by the free fluorine becoming available through dissociation of the WF molecules at the filament. The concentration of fluorine and WF in the bulb is thus determined by the rate of evaporation of the hot tungsten filament and is only as large as required for the back transport of the evaporating tungsten. If some fluorine is removed from the cycle through gettering action, a sufficient supply of fluorine remains in the form of CoF on the bulb wall. The solid fluorine compounds introduced as fluorinating agents in the form of coatings on the inner bulb wall can be used in addition to gaseous fluorine compounds within the bulb volume.

While both of the measures suggested above for overcoming the detrimental effect of fluorine on the filament ends are effective to some extent, they have certain limitations. For example, the use of protective sleeves is limited to relatively thick tungsten wire. The sleeves cannot be made long and narrow enough in order to prevent completely the diffusion of fluorine atoms to the filament ends, even if the concentration of gaseous fluorine is at the minimum required.

It is the purpose of this invention to provide new and effective means in an incandescent lamp with a fluorine additive for preventing the filament ends from being attacked by fluorine. My invention is applicable to lamps where the fluorine is introduced in solid or gaseous form or as a combined solid and gas, and in both vacuum type and gas filled lamps. They consist mainly of the following two measures:

(I) The filament ends are coated with a dense layer of a metal with high melting point that is not subject to fluorine attack because its fluorides are dissociated fully or much more than tungsten fluoride is at the operating temperature of the filament ends.

(H) The operating temperature of the filament ends is increased substantially through the use of a new construction of lead wires and of new materials for the lead wire ends that come in direct contact with the filament ends.

Each of these measures can be used separately or in combination with the other. Each of them reduces the detrimental attack of fluorine on the filament ends to such extent that the fluorine transport reaction described in the foregoing paragraphs can be put to more practical use to increase the filament life of commercially produced incandescent lamps. This has not been possible before this invention. The fluorine transport reaction can be used either for increasing the luminous efiicacy considerably over the values presently obtained with conventional lamps or the useful lamp life can be prolonged to a multiple of the present life ratings. In addition, the initial lumen output can be maintained nearly at its full value over the entire life because no evaporated tungsten particles are deposited on the inner bulb wall. In this respect, the performance of incandescent lamps with fluorine transport reaction is similar to that of iodine-quartz incandescent lamps. However, the lamps with fluorine reaction have two decisive advantages over iodine-quartz lamps:

(1) Their life is prolonged much more than that of iodine lamps, because the fluorine transport reaction removes hot spots and maintains uniform cross-section of the filament. The iodine-tungsten reaction cannot achieve this, owing to the fact that tungsten iodide is fully dissociated at 1400 C., Le, well below the filament operating temperature.

(2) The introduction of the fluoride cycle does not require basic changes of design, dimensions and manufacturing methods. Changes concern only filament processing, lead wire design, support wire material, filament mounting tools, and introduction of the gaseous and/or solid fluorine compounds. In addition, internal bulb coatings for protection of the glass parts from attack by fluorine or fluorine compounds and external infrared absorbing coatings for the purpose of increasing the bulb operating temperature may be required. These are relatively simple changes or additions in view of the radically improved lamp performance.

For the purpose of illustrating, as far as is possible, the subject matter of the invention herein disclosed, the following figures are included as a part of this application:

FIGURE 1 is a chart showing the temperature gradient of a lamp filament at one end thereof;

FIGURES 2m to 2e inclusive diagrammatically illustrate structural modifications of the filament ends in accordance with important objects of this invention;

FIGURES 3a to 32 diagrammalically illustrate lead in wire constructions, likewise in accordance with a related phase of this invention;

FIGURE 4 is a vertical elevational view of an incandescent lamp embodying the novel constructions of this invention with a portion of its glass envelope broken away and a portion thereof in cross section; and

FIGURE 5 is a similar view of another form of incandescent lamp in accordance with this invention.

The principles and the practical application of the invention shall now be explained in detail:

(I) Coating of the filament ends with metal not subject to fluorine attack at operating temperature FIG. 1 shows that the temperature of the filament ends can drop to 33% of the overall filament temperature when conventional heavy lead wires are used. This means that, for example, with an overall operating temperature of 2550 C. the filament ends operate at approximately 850 C. At such low temperature the equilibrium of the reaction is entirely on the right side of the equation and tungsten is removed relatively fast from the filament ends. However, the equilibrium shifts definitely to the left side of the equation, without increase in temperature, if tungsten is replaced by, for example, one of the metals iridium, osmium, or rhenium. The fluoride of these metals are dissociated to a considerable degree and, therefore, unstable at temperatures of 850 C. or higher. Consequently, these metals are notor at least much less-subject to fluorine attack than tungsten. The fact that the compounds IrF OsF and ReF are beginning to dissociate at approximately 800 C. is illustrated by their heats of formation. These amount to approximately 500 kilojoule/ mol for IrF and OsF and to approximately 1100 kilojoule/mol for ReF The enthalpy of WF is approximately 1800 kj./mol. This illustrates the relatively great stability of the WF molecule at high temperatures and explains that a dense coating with one of the metals Ir, Os, Re, or with another metal with similar characteristics, will provide protection from fluorine attack for the filament ends. As shown on FIG. 1, filament ends are defined as those stretches of filament that operate below the overall operating temperature because they are cooled by the lead wire clamps.

Iridium, osmium and rhenium are particularly suited for the purpose of protecting the filament ends because their melting points are higher than the usual operating temperatures of incandescent lamp filaments. The vapor pressures of the mentioned metals are relatively high at the operating temperature of the filament ends and are much higher than the vapor pressure (evaporation rate) of tungsten at the same temperature. Therefore, the 0bjection could be made that the protective metal coating will evaporate off the filament ends relatively fast and the protection from fluorine attack will come to an end. This is not the case for the following reasons:

(1) The iridium, osmium or rhenium atoms (briefly called metal atoms from now on) that evaporate from the filament ends, react either in the lamp volume or on the bulb wall with fluorine and form the metal fluoride.

As metal fluoride molecules, they are transported back to some point of the filament because only in the closest vicinity of the filament, including the filament ends, is the temperature high enough to dissociate the meta fluoride molecules.

The regular operating temperature of the internal bulb surface at its coolest point is usually suflicient for the formation of the volatile meta fluorides. It amounts to between 50 and 150 C. depending upon the particular lamp design, wattage rating, and burning position. If the internal bulb wall temperature is not high enough for formation of the metal fluoride it can be raised to the required critical value by means of an infrared absorbing light transmitting coating on the bulb. Such coatings can be applied on the inner or outer bulb wall and consist, for example, of a thin layer of tin oxide or titanium dioxide. ln lamps with fluorine or fluorine compound addition it is, therefore, always possible to prevent tungsten or metal particles to remain deposited on the bulb wall or any other internal lamp part except the filament.

(2) The metal atoms are deposited onto the filament at approximately the same rate over the entire filament length because, due to their relatively low enthalpy, the temperature over the entire filament length is high enough to shift the dissociation equilibrium predominantly or completely toward dissociation.

(3) Due to the temperature gradient between main filament and filament ends, the deposited metal atoms evaporate much faster from the main filament than from the ends. Therefore, the concentration of metal atoms is much higher in the immediate vicinity of the filament ends than it is along the rest of the filament. As a result, the protective metal coating of the filament ends is maintained intact in spite of its relatively high evaporation rate. The metal atoms cannot settle anywhere within the lamp except on the filament because of the fluorine transport reaction. From the main filament they migrate to the ends because of the temperature gradient.

A basic reason for this behaviour is seen in the fact that the evaporation enthalpies of the metals are similar to the enthalpies of their fluorides and that all these values are much smaller than the enthalpy of WF (evaporation enthalpy of Ir, Os: approx. 450 kjoule/mol, of rhenium: 720 kjoule/mol. Enthalpy of IrR OsF approx. 500 kjoule/mol, of ReFr approx. 1050 kjoule/ mol).

The filament ends can be covered with the protective metal layer by any method that achieves a dense relatively thick coating not easily penetrated by fluorine. For example, the filament ends can be coated with rhenium, osmium, iridium, or another suitable metal by electrolytic deposition, by vapor deposition in vacuum, or by cathode-sputtering in a low pressure gas discharge. The filament ends can be prepared by cleaning processes only, without modification of the wire diameter. In this case, the wire cross-section at the filament ends isincreased by the cross-section of the protective layer. As a result, the electrical resistance of the filament is lowered at the ends and their operating temperature will drop below the level indicated by FIG. 1. This drop in temperature is undesirable because it increases--instead of decreasingthe need for protection against fluorine attack.

In order to avoid this, the tungsten wire diameter can be reduced at the filament ends before the protective metal layer is applied. Such reduction of diameter can be achieved by etching in molten sodium-hydroxide or molten sodium nitrite, by electrolytic etching in alkali hydroxide or acid solution, or by any other suitable means. The amount of tungsten removed at the filament ends can be replaced fully or only partially by the protective metal layer. Under full replacement it is understood that the electrical resistance of the filament ends is fully restored to the resistance value before diameter reduction through etching. Partial replacement means that the protective metal layer is made only thick enough to provide protection from fluorine attack but that the electrical resistance of the filament ends remains higher than it was originally before diameter reduction. Such increase of the electrical resistance may be desirable because it results in a higher operating temperature of the filament ends and, consequently, in a reduced exposure to fluorine attack. However, the temperature of the filament ends cannot be increased indefinitely. Above a certain temperature, the lead wire clamps (in conventional lamps consisting of copper, nickel, or nickel plated copper) will approach their melting point and lead wire material may diifuse into the tungsten filament ends. This often leads to embrittlement and breaking of the filament close to the lead wire clamps.

FIGS. 2a to 2e illustrate procedures of applying a protective metal layer to the filament ends. FIG. 2a shows a filament end 12 of the filament 10 with its diameter reduced through etching or other means. FIG. 2b shows the filament end after application of the protective metal layer 14 by partial replacement of the removed tungsten and FIG. 2c shows a filament end with the removed tungsten fully replaced with protective metal 16. It is assumed in this case that the protective metal has higher resistivity than tungsten. Therefor, the applied metal layer has to be thicker than the removed tungsten layer in order to maintain the original electrical resistance value per unit wire length.

FIGS. 2d and 2e illustrate additional methods of protecting the ends of the filament 10 against fluorine attack. The tungsten wire ends are left unchanged and a short piece of pure rhenium, osmium, or iridium wire 18 of the same or similar diameter is butt-welded to them. Thus, the filament is provided with ends that are not subject to fluorine attack and the entire length of tungsten wire operates at a uniform or nearly uniform temperature (FIG. 2d). The metal extensions 14, 16 and 18 are just long enough to take up the filament ends, that is those portions of the filament that operate cooler than the main filament length. For cost reduction, the filament extensions of pure metal (rhenium, osmium, iridium, or metal of similar characteristics) can be replaced by pieces of tungsten, tantalum or molybdenum wire 20 clad or densely coated with one or several of the metals 22 (FIG. 2e). The resistivity of the extensions can be adjusted in such manner that they operate at the highest temperature compatible with the dimensions and the material of the lead wire clamps.

A much simpler but equally practicable method of producing the protective metal layer on the filament ends is the following: The filament is coated with a thin layer of the metal along its entire length. This coating can be applied by electrolytic deposition, by vapor deposition in vacuum, by cathode sputtering or by any other suitable means. 'It is not important that the filament is evenly or densely covered with the metal. An'essential condition is only that the total amount of metal deposited on the filament is sufiicient to cover the ends with a layer thick enough and dense enough to provide protection against fluorine attack. The filament prepared in such manner is mounted into the lamp and the lamp is exhausted and filled with the conventional argon-nitrogen gas filling and the fluorine compound addition. After completion of the lamp, the filament is operated for a relatively short time (between 30* seconds and 15 to 30 minutes) at or close to its regular operating temperature. As explained above in an earlier paragraph, the metal evaporates relatively fast from the main filament length and is transported to the filament ends which represent the only location in the lamp where it can remain permanently. On all cooler parts of the lamp, that is on the bulb or on the inner lead wires, the metal is removed by fluorine reaction, from the hot main filament length it is removed by evaporation.

All described methods of producing the metal protection at the filament ends apply equally to straight wire, single coil and coiled coil filament designs. The methods characterized by etching and coating of the ends or addition of a piece of metal wire to the ends require that the last windings of coiled filaments are straightened out. This operation is inconvenient and not always compatible with regular filament production methods. The last described (automatic) method of coating the entire filament and using the temperature gradient in combination with the fluorine transport reaction for putting the end protection into place seems to be most suitable for coiled filaments. It does not require straightening of the last windings. However, with straight filament ends it is easier to achieve uniformly dense coating of the ends by the automatic method.

(II) Increasing the operating temperature of the filament ends through new lead wire design and materials According to the invention, this second measure can be used separately or in combination with the above described protective metal layer for the purpose of reducing or preventing the detrimental effect of fluorine on the filament ends. As illustrated by FIG. 1, the temperature of the filament directly at the point where it enters the lead wire clamp and within the clamp can be as low as 33% of the main filament operating temperature. The speed with which a low concentration of free fluorine or fluorine compound attacks the filament ends depends upon the temperature difference between the ends and the main filament length. Consequently, the detrimental effect of the fluorine can be reduced through raising the filament end temperature.

For a given tungsten Wire diameter and-in the case of coiled filamentsgiven winding data (pitch and mandrel diameter), the filament end temperature can be increased by reducing the heat capacity of the lead wire clamps. This is achieved generally through reduction of the lead wire diameter. However, the lead wires require a minimum thickness in order to give the filament suflicient support. Also, even if pure nickel is used as lead wire material, diameter reduction leads soon to the point where the filament end overheats the lead wire clamp and lead Wire material diffuses into the filament. This causes changes in the crystal structure of the filament wire ends, usually making them brittle and subject to breakage at the slightest shock or vibration.

Therefore, according to this invention, the lead wires are equipped with tips of a metal with the following characteristics (1) Melting point considerably higher than that of nickel.

(2) Relatively small detrimental influence on tungsten crystal structure if diffused into filament ends.

(3) Resistant to fluorine attack at least up to 1100 C.

A good example for such metal is thorium. Its melting point is at 1845 C. (melting point of nickel: 1455 C.). It does not appreciably damage the crystal structure of tungsten. Its fluoride is a very stable compound (enthalpy of ThF 1996 kj./mol) with relatively low evaporation rate up to 1100 C. In this connection it should be mentioned that nickel fluoride is also a relatively stable compound with relatively low vapor pressure up to 800 C. For this reason conventional nickel lead wires can be used in fluorine additive lamps if the operating temperature of the clamps is kept below this limit.

In order to raise the operating temperature of the filament ends from the standard value of approximately of the main filament temperature (see FIG. 1) to, for example, -60% of the main filament temperature, or higher, the lead wire constructions shown in FIGS. 3a to 3e are used. According to FIG. 3a, the conventional lead wire 24 (nickel or nickelplated copper of, for example, .02()" diameter) is equipped with a tip 26 of considerably smaller diameter wire (for example of thorium, .008".0l2 diameter) which is butt-welded to it. The small diameter wire tip is short (for example approximately A" long) and is formed into a clamp, shown in FIG. 3b, for mechanically supporting and electrically connecting the filament.

The heat capacity of a clamp formed of a certain diameter wire decreases at least with the square of the diameter. This illustrates the influence of the clamp size upon the filament end temperature. With the lead wire clamp design shown in FIGS. 3a and 3b, the filament end temperature can be increased from approximately 30% to 50 or 60% of the filament operating temperature. This means that the ends of a filament operating at, for example, 2500 C. will operate at 1250 to 1500 C. instead of 750 C. if the new lead wire design according to this invention is used.

For improved rigidity of the lead wire tips the thintipped composite leads can also be manufactured by the method illustrated in FIGS. 3c to 3e. The main lead wire 24 of nickel (or nickelplated copper) is first equipped by welding to it a short tip 26a (for example of thorium) of the same, relatively large diameter (FIG. 30). Then, the tip is etched into conical shape (see FIG. 3d) through dipping in molten sodium nitrite, or through electrolytic etching. Finally the thin clamp is formed around the filament on the filament mount machine as shown in FIG. 3e.

FIGS. 4 and 5 show examples of incandescent filament lamps designed, processed, and filled according to this invention. Both lamps have coiled coil filaments 40 and 52. However, the invention is also applicable to lamps with straight, single or triple coiled filaments. FIG. 4 shows a lamp with nearly conventional bulb shape and horizontally (vertically to the lamp axis) mounted coiled coil filament. The design is suitable for a wide wattage range, for example for wattages between 40 and 500 watts. However, the bulb dimensions can be chosen much smaller than is customary at present for conventional lamps because the fluorine transport reaction prevents any blackening tungsten deposit on the inner bulb wall. The bulb 32 is internally coated with a relatively thick but fully transparent layer 34 consisting of one or a mixture of several fluorine compounds. Also the glass surfaces of the stem 36, the ar-bor tube 38 and its button 35 are coated in the same manner. The coating of all internal glass surfaces with fluorine compounds has mainly two purposes:

(1) To prevent attack of the glass surfaces by the free fluorine present in the lamp.

(2) To replenish the fluorine concentration in the bulb volume in order to maintain the minimum concentration required for the tungsten transport reaction, as described above.

In order to meet the latter requirements, the coating has to consist of fluorine compounds of polyvalent elements like cobalt fluoride, silver fluoride, manganese fluoride, and others, which easily absorb or give off fluorine. The coating with such compound will, in many cases, also prevent attack of the glass surface by free fluorine and, thus, fulfill at the same time the first requirement. However, if this is not the case, an undercoating of one or more of the less volatile and more stable fluorine compounds as, for example, calcium fluoride, magnesium fluoride, lithium fluoride, can be used.

The filament 40 is of the coiled coil type. However, the secondary winding process has been performed in such manner that a single coiled center portion 42 has been maintained. This single coiled center portion and the ends of the filament are coated with a metal (e.g., rhenium, osmium, iridium) layer for protection against fluorine attack according to this invention. The different methods suitable for applying the protective metal coating have been described above in detail. The center portion of the filament has to be coated against fluorine attack in the same manner as the filament ends because the center Wire support loop 44 cools the filament at the point of contact and thus creates a small but not negligible length of filament with locally decreased temperature. In order to reduce the cooling effect as much as possible and in order to withstand fluorine attack, the support wire, which conventionally consists of molybdenum or tungsten, is constructed similar to the lead-wires described above and in FIGS. 3b and 3c. Its main length consists of nickel of relatively great diameter. It has a very thin tip of thorium, just long enough to form the support loop.

The inner lead wires 24 consist of nickel or nickel plated copper and, according to this invention, they have thin thorium tips 26 for the clamping ends as shown in FIG. 3b. The lamp has a conventional screw base 46, but can also be based with any other suitable base type.

FIG. shows another example of a lamp designed, processed, and filled according to this invention. A nearly spherical bulb 48 of hard glass is used in order to make it possible to increase the inner bulb wall temperature over the values attainable with soft glass and conventional bulb shapes. This is important when the internal wall temperature has to be raised for safe and immediate formation of the metal fluoride. In order to achieve a further increase in bulb temperature, an infrared absorbing light transmitting coating 50 is applied to the outer bulb surface. It consists for example, of a thin layer of tin oxide or titanium dioxide.

In addition to this outer coating the inside surface of the bulb has a coating of one or several fluorine compounds 34. The purpose and function of this coating has been described above. The lead wires 24 consist of nickel or nickel plated copper and, according to the invention, are equipped with thin thorium tips 26. The filament 52 is of coiled coil design in vertical (axial) arrangement in the lamp. The filament ends are coated with a protective metal layer of rhenium, iridium, osmium or similar metal according to this invention;

The screw lamp base 54 is made of brass or aluminum and has a relatively long cylindrical extension 56 in order to protect the socket from the unusually high operating temperature of the bulb 48.,The basing cement has a high content of silicone compounds in order to withstand the high temperature and in'order to maintain a firm bond between bulb and base extension over'the long lamp life.

Both sample lamps shown in FIGS. 4 and 5 have a total cold fill pressure of 600 to 700 torr of argon, krypton, nitrogen or mixtures of these gases. According to this invention, a small concentration (between 0.1 and 2% of the total fill pressure) of a gaseous fluorine compound like nitrogen fiuoride'.-NF is added to the gas filling. The lamps can also be dimensioned for operation as vacuum lamps because the fluorine transport reaction is eflective also without a higlrflpressure gas filling. In this case the lamp filling consists only of the above mentioned small amount of gaseous fluorine compound. The addition of the gaseous fluorine compound can be omitted both in gas-filled and in vacuum lamp types if an internal coating of polyvalent solid fluorine compounds can be relied upon to provide the minimum concentration of free fluorine that is required in order to maintain the fluorine transport reaction.

It will be at once apparent from the disclosure submitted above that the subject matter of this invention consists of many permutations and combinations of novel structural features capable of considerable variation by those skilled in the art within the spirit and substance of this disclosure. It is intended, therefore, that the disclosure is accepted solely as illustrative of the novel subject matter sought to be covered as defined by the appended claims.

What isclaimed is: p

1. An incandescent electric lamp comprising an envelope, a filament in said envelope, means for mounting said filament at the ends thereof, a fluorine additive within said envelope which dissociates into fluorine molecules which combine with and transport material to and from the filament, and means in electrical contact with at least one end of said filament and engaged by said mounting means resistant to attack by fluorine at the operating temperature of the filament.

2. An incandescent lamp as in claim 1 wherein the fluorine resistant means is a metal different from and which has a higher vapor pressure than the filament material and is resistant to fluorine at the operating temperature of the lamp.

3. An incandescent lamp as in claim 2 wherein said different metal is formed at least in part by material selected from the group consisting of iridium, osmium and rhenium.

4. An incandescent lamp as in claim 2 wherein said fluorine resistant means of a different metal comprises an extension secured to the end of the filament.

5. An incandescent lamp as in claim 2 wherein said fluorine resistant means of a diflerent metal comprises a sleeve surrounding the end of the filament and in contact therewith.

6. An incandescent lamp as in claim 2 wherein said fluorine resistant means of a different metal comprises a coating on the end of the filament.

7. An incandescent lamp as in claim 4 wherein said diflerent metal is formed at least in part by material selected from the group consisting of iridium, osmium and rhenium.

8. An incandescent lamp as in claim 5 wherein said diflerent metal is formed at least in part by material selected from the group consisting of iridium, osmium and rhenium.

9. An incandescent lamp as in claim 6 wherein said diiferent metal is formed at least in part by material selected from the group consisting of iridium, osmium and rhenium.

10. An incandescent electric lamp comprising an envelope, a filament in said envelope, a fluorine additive within said envelope which dissociates into fluorine atoms which combine with and transport material to and from the filament, and means for mounting the filament near its ends while limiting the temperature drop of at least one of the filament ends during lamp operation.

11. An incandescent lamp as in claim 10 wherein said mounting means comprises a pair of lead wires one for each end of the filament, the lead wires each having a reduced cross-sectional area portion for holding a respective filament end while limiting the filament end temperature drop during lamp operation.

12. An incandescent lamp as in claim 11 wherein said lead wires contain nickel.

13. An incandescent lamp as in claim 11 wherein said reduced cross-sectional area portions of said lead wire are of thorium material.

14. An incandescent lamp as in claim 11 wherein said lead wires contain nickel and said reduced cross-sectional area portion is of thorium.

15. An incandescent lamp according to claim 1 wherein said filament mounting means limits the temperature drop of the filament end during lamp operation.

16. An incandescent lamp according to claim 2 wherein said filament mounting means limits the temperature drop of the filament end during lamp operation.

17. An incandescent lamp according to claim 16 wherein said mounting means comprises a pair of lead wires one for each end of the filament, the lead wires each having a reduced cross-sectional area portion for holding a respective filament end while limiting the filament end temperature drop during lamp operation.

18. An incandescent lamp according to claim 16 wherein said different metal is formed at least in part by material selected from the group consisting of iridium, osmium and rhenium.

19. An incandescent lamp according to claim 17 wherein said dilferent metal is formed at least in part by material selected from the group consisting of iridium, osmium and rhenium.

' 1 1 1 Z 20. An incandescent lamp according to claim 2 where- FOREIGN PATENTS in said filament material comprises tungsten. 157,442 12/1963 U S,S.R,

21. An incandescent lamp according to claim 16 where- 1,150,153 4/ 7 ranc 1n said filament material comprises tungsten. 5 OTHER REFERENCES References Cited Kohl: Materials and Techniques for Electron Tubes, UNITED STATES PATENTS 1960 241-246 527-532 3,168,668 2 19 5 Rexer 13 213 X JAMES W. LAWRENCE, Primary Examiner.

3,263,113 7/1966 Schroder 3l3223 10 R. JUDD, Assistant Examiner.

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