US3441777A - Elements for incandescent devices - Google Patents

Description

April 29,.1969 R. STEINITZ "ELEMENTS FCR INCANDESCENT DEVICES Sheet Filed June 1, 1966 x CARBON ATOMS TANTALUM ATOMS INVENTOR.

ROBERT STEINITZ By a RNEX Fig. 3.

ATT

April 29, 1969 v R. sTalNrrz 3,441,777

ELEMENTS FOR INGANDESCENT DEVICES Filed June 1, 19,66 0 1 Sheet 3 of2 xlO' I, ATM. I 20 I TOC)( a. *1 o I 4 I I l l I I o 9 r v CARBON ATOMS TANTALUM ATOMS Fig. 2.

INVENTOR.

ROBERT STEINITZ Arra x United States Patent 3,441,777 ELEMENTS FOR INCANDESCENT DEVICES Robert Steinitz, Harrison, N.Y., assignor to General Telephone & Electronics Laboratories Incorporated, a corporation of Delaware Filed June 1, 1966, Ser. No. 554,422

Int. Cl. H01j 1/14, 19/06 US. Cl. 31'3 311 9 Claims ABSTRACT OF THE DISCLOSURE This invention relates to devices for operation at high temperatures in a state of incandescence and, more particularly, to elements for use in an incandescent lamp.

Incandescent lamps generally comprise an electrically incandescible element, such as a filament, mounted between spaced support elements in a sealed enclosing en velope of a light-transmitting material, such as glass. The brightness and efiiciency of the lamp is determined primarily by the properties of the electrically-incandescible element. The state of incandescence is reached by heating the element to temperatures in excess of 500 C.

While tungsten has been favored for use as the filament material, it is known that improved brightness can be obtained by making the filament of a more refractory material, such as tantalum carbide. Lamps utilizing a tantalum carbide filament are capable of operation at higher levels of total radiation due to the fact that tantalum carbide has a higher melting point than tungsten and the total radiant output of an incandescent lamp is a function of the fourth power of the absolute temperature at which the lamp filament is operated.

However, tantalum carbide has not generally replaced tungsten as an incandescent lamp filament since at high operating temperatures tantalum carbide (TaC) tends to become unstable. Due to this instability, an initially stoichiometric tantalum carbide filament decaburizes during operation. The carbon released in elemental form during decarburization is found to deposit on the support elements and on the walls of the enclosing envelope thereby decreasing the ability of the envelope to transmit light. In addition, the decarburization of the filament can result in the formation of TaC a subcarbide which is extremely brittle, has a lower melting point, and possesses an increased temperature coefficient of resistivity.

To limit this decarburization to acceptable levels, it has become generally accepted practice to start with a stoichiometric TaC filament and fill the envelope of the lamp with a protective atmosphere containing a volatile compound of carbon such as methane, ethylene or carbon tetrachloride. The gas dissociates within the envelope at the hot filament so that as carbon atoms are ejected from the filament, other carbon atoms bombard the filament surface and are retained. The amount of the volatile carbon 3,441,777 Patented Apr. 29, 1969 compound within the envelope has generally been at least that amount required to maintain the filament essentially stoichiometric.

In addition, the amount of carbon present in the gaseous mixture may be increased to a level such that a tantalum filament may be inserted in the lamp and carburized by a controlled heating of the lamp. In this method of manufacture, the volatile carbonaceous gas is added in quantities of three to four times the amount calculated to completely convert the filament to the stoichiometric carbide.

This method of carburization produces a distortion of the filament, notably a sag, during carbnrization. This sag may be partially eliminated during lamp manufacture by selectively applying a colloidal suspension of graphite to a tantalum filament and employing a carburizing gas in the manner described in US. Patent 3,113,893 to H. B. Sloan issued Dec. 10, 1963.

While the above methods have enabled tantalum carbide filaments to be made which are initially stoichiometric with a composition TaC, the use of protective carbon-containing environments in practice has been found to inhibit but not eliminate the decarburization of TaC. Decarburization of the stoichiometric TaC is still found to occur, with the released carbon depositing on the cooler surfaces of the envelope.

Also, stoichiometric TaC filaments exhibit a temperature coefiicient of resistivity 0c of approximately 25 x 10-*/ C. The temperature coefficient of resistivity is determined from the following expression wherein R is the electrical resistance at temperature T and R is the resistance at temperature T Thus, the coefficient a is indicative of the change of resistance with temperature. Since it is advantageous to eliminate current surges during the operation of a lamp, the coetficient should be minimized for lamp filaments.

The creep resistance of a lamp filament is an important parameter in the construction of a lamp since filaments by their nature are operated at temperatures of the order of 3500 K. The creep rate for a material is expressed as Al/ (l At) where Al is the change in length from the initial length 1 during a period A! for a given constant applied load and temperature. As known, the creep rate of a material increases with increasing temperature so that at high lamp temperatures the tendency for lamp filaments or supports therefor to sag and deform increases. The creep rate for fully carburized stoichiometric tantalum carbide at a load of 10,000 p.s.i. and a temperature of 1960 C. has been observed to be within the range of 2 to 5% per hour. This creep rate may result in an undesirable sagging and deformation of the tantalum carbide filament and/or support elements when the lamp is placed in operation.

Accordingly, an object of the present invention is a method of making an improved incandescent device containing a tantalum carbide element.

Another object is to provide an incandescent lamp having a tantalum carbide incandescent element in which the temperature coefiicient of resistivity is minimized.

A further object is to provide a device for operation at incandescent temperatures having a tantalum carbide element which exhibits reduced sag and deformation.

Still another object is to provide a lamp containing a tantalum carbide incandescent element wherein the amount of elemental carbon lost during operation is substantially reduced.

Yet another object is to provide an improved tantalum carbide support element for use in an incandescent lamp.

In accordance with the present invention, a device to be operated in a state of incandescence is initially provided with a tantalum carbide element which deviates from stoichiometry in a prescribed manner. In the formation of the device, the element is made of a nonstoichio metric carbide having the general compositional formula (Ta,A)C wherein x is less than 1.0 and A is a metal additive not in excess of 20 weight percent of tantalum. This additive A consists of at least one metal selected from the group consisting of zirconium, hafnium, niobium and titanium. The quantity x, which indicates the ratio of carbon atoms to the total number of metal atoms, is selected to be within the range of 0.98 and 0.78.

By initially incorporating a filament of nonstoichiometric composition in an incandescent lamp, the decarburization of the filament in vacuum or in a protective or inert atmosphere is substantially reduced and the electrical and mechanical properties thereof are maintained substantially constant. This result is obtained because the evaporation rate of carbon is found to decrease rapidly with a deviation from stoichiometry. The vapor pressure at 2500 C. for carbon of a tantalum carbide filament wherein x is 0.98, has been found to be about 15% of the vapor pressure for a stoichiometric carbide filament. As a result of this relatively low vapor pressure, fewer carbon atoms are available within the lamp and the initial blackening of the lamp envelope by elemental carbon depositing thereon is substantially reduced.

Further, it has been found that initially employing a tantalum carbide filament that deviates from stoichiometry in the prescribed manner provides a lamp which exhibits a reduced change in resistance during the period between energization and full operating temperature. This result is due to the low temperature coefiicient of resistivity of low carbon tantalum carbide filaments wherein the aforedefined x resides within the range of 0.98 to 0.78. The temperature coefficient for a filament wherein x is 0.92 is found to be one-third of that for a stoichiometric filament. An even more significant change is observed as x decreases to 0.82 wherein the coefiicient is about one-fiftieth that of the stoichiometric filament.

The initial placement of a tantalum carbide element deviating from stoichiometry in an incandescent lamp has also resulted in an improvement in the mechanical properties of the element. While tantalum carbide is known to be relatively brittle at room temperatures, the material becomes quite ductile at temperatures above 17 C. The creep rate for fully carburized or stoichiometric tantalum carbide elements is found to be substantially greater than that of tantalum elements having the aforementioned deviation from stoichiometry. As a result, tantalum carbide filaments and supporting elements therefor deviating from stoichiometry in the prescribed manner exhibit improved dimensional stability during the operation of the incorporating device.

Further features and advantages of the invention will become more readily apparent from the following detailed description of a specific embodiment of the invention when taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a representative incandescent lamp constructed in accordance with the invention;

FIG. 2 is a curve showing the variation in carbon vapor pressure of a tantalum carbide filament as a function of the deviation from stoichiometry x; and

FIG. 3 is a curve showing the variation of the temperature coefficient of resistivity a as a function of the deviation from stoichiometry x.

Referring now to FIG. 1, a tantalum carbide incandescent lamp is shown comprising a vitreous light transmissive envelope 11 and a base connection 12. A coiled filament 13 of tantalum carbide is supported within the envelope by support elements 14 and 15 which may also be formed of tantalum carbide but have a large diameter (i.e., low resistance) so that they do not become incandescent.

In the manufacture of an incandescent lamp, the support elements 14 and 15 are embedded in a glass plate or disc. The support elements generally are formed as rods and may be provided with a sleeve formed of a material, such as Kovar, having a temperature coefficient of expansion closely matching that of the glass. The glass plate is then softened by heating to form a stemlike structure containing the support elements.

The filament 13 is affixed by welding or the like to the ends of the support rods as shown in FIG. 1. The stem is then mated with an envelope 11. In the mating of the stem with the envelope, a passage is provided for the evacuation of the envelope atmosphere and for any back-fill that may be desired. This passage may be formed in the stem or the envelope.

The envelope is normally evacuated and back-filled with a carbon containing protective atmosphere to a pressure within the approximate range of 400 millimeters of mercury to one atmosphere. The gas may contain a hydrocarbon, such as methane, or ethane, or a cyanogen or another carbon-containing compound in order to provide the carbon atoms necessary to prevent the decarburization of the filament. However, the protective atmosphere may be omitted if desired and an inert atmosphere substituted therefor or the envelope may be maintained in vacuum.

The evacuation passage is sealed after back-filling, if desired, by localized heating and the application of pressure. The exposed portion of the stem is then provided with a conventional socket with the support elements being electrically connected to different portions thereof.

The filament 13 may be a straight wire, a coil or a coiled-coil, that is, a doubly coiled filament, and is supported by and electrically connected between support elements 14 and 15. The filament is formed of a nonstoichiometric tantalum carbide compound having the general formula (Ta,A)C wherein Ta is tantalum, C is carbon, and A is at least one metal selected from the group consisting of zirconium, hafnium, niobium and titanium. The amount of additive A is chosen to not exceed 20 weight percent of the tantalum. The parameter x indicates the deviation from stoichiometry of the compound and is the ratio of carbon atoms to the sum of metal atoms and is within the range of 0.98 to 0.78.

In accordance with the invention, a filament having the above composition is initially placed within the lamp during its manufacture. This is in contradistinction to the well known method of making an incandescent lamp with an initially stoichiometric filament and having it decarburize during use. One advantage obtained by initially employing a nonstoichiometric tantalum carbide filament is that the decarburization rate is greatly reduced with the result that substantially no elemental carbon is deposited on the envelope during normal use and the properties, such as the temperature coefficient of resistivity of the filament are maintained substantially constant during operation.

This advantage is shown graphically in FIG. 2 wherein the carbon vapor pressure for several tantalum carbide filaments having different values of x from 1.0 to 0.72 is plotted as curve 17. The data of curve 17 were calculated for a temperature of 2500" C. It shall be noted that the carbon vapor pressure decreases from in excess of 36x10 atm. for an initially stoichiometric filament having x essentially equal to 1, to less than 7 X 10* atm. for a filament having x initially equal to 0.98. A further improvement is apparent for filaments having an initial increased deviation from stoichiometry with the vapor pressure becoming as low as 1 10- atm. at x equal to 0.80. The evaporation rate of carbon from the filament is proportional to its vapor pressure.

By employing a tantalum carbide filament which deviates from stoichiometry in the prescribed manner, the decarburization process of the filament is greatly reduced as shown by the much reduced vapor pressures of FIG. 2. Consequently, the amount of elemental carbon given off by the filament and the deposition thereof on the envelope or the supporting rods 14, 15 is correspondingly decreased.

The use of a tantalum carbide filament having the initial prescribed low-carbon composition in the manufacture of an incandescent lamp provides a filament having a reduced temperature coefficient of resistivity. Since in normal use an incandescent lamp is used with a constant voltage source, it is desirable to maintain the resistance of the lamp constant during the interval from initial energization to the attainment of the normal high operating temperature to prevent current surges.

A lamp constructed in accordance with the invention is found to exhibit a substantially constant low temperature coeflicient of resistivity. This result is clearly shown in FIG. 3 wherein the coefficients for tantalum carbide filaments having different deviations from stoichiometry were measured at about +155 C. and again at the temperature of liquid nitrogen -l96 C. The temperature coefficient a was found to be the same for both high and low temperatures.

The temperature coefficient, as shown in FIG. 3, was found to decrease rapidly with increasing deviations from stoichiometry and found to reach a minimum value of about 0.5 l per C. at x equal to 0.82. It shall be noted that for x equal to 0.99 the average temperature coefiicient was found to be about 25 10-' per C. and that at x equal to 0.98 the coefiicient was found to be reduced to bout 20 10* per C.

The temperature coeflicient of FIG. 3 was found to increase again as x decreased below about 0.78. For tantalum carbide compositions having lower carbon content, i.e., increased deviation from stoichiometry, the material enters a two-phase region consisting of TaC and Ta C. This change in the crystal structure results in a material wherein each phase has its own properties, including a lower melting point and a high temperature coefiicient of resistivity, and the distribution of phases cannot be readily controlled or predicted. Therefore, filaments formed of TaC wherein x is less than 0.78 are generally not well suited for use in incandescent lamps.

In practice, it has been found desirable to insure a single phase filament and this may be provided b establishing a compositional limit for x of 0.80. While suitable filaments may be formed at x as low as 0.78, the use of the higher value is prefer-red to prevent the filament from decomposing to an additional phase in a few localized regions. In addition, the carbon vapor pressure shown in FIG. 2 becomes quite low, less than 1 l0 atm., at x equal to 0.80 and remains relatively constant for lower values of x.

The preferred compositional range for the tantalum carbide filaments employed in the present invention has been found to be where x may vary between 0.95 and 0.80. The upper value 0.95 is the point whereat the temperature coeflicient x of a filament is one-half that f the stoichiometric filament.

While the above discussion has referred to lamps initially containing a tantalum carbide filament having the defined deviation from stoichiometry, it will become apparent from the following discussion that the supporting elements 14 and 15 of FIG. 1 may likewise be formed of this material.

As known, tantalum carbide is a brittle material at room temperatures. However, at relatively high temperatures above about 1700 C., the material becomes quite ductile and shows elongation in short time tensile tests of about 40% at 2200 C. and about to at temperatures above 1900 C. The elongation rate under constant load (creep rate) was found to be decreased substantially for tantalum carbide having the prescribed deviation from stoichiometry. This property enables filaments and supports formed of this material to exhibit improved dimensional stability at high temperatures characterizing the state of incandescence. In tests performed at a temperature of 1960 C. and a constant load of 10,000 p.s.i., the creep rate for fully carburized TaC was found to reside within the 2 to 5% per hour range. However, for samples having the composition TaC and TaC the creep rates were found to be 0.3% per hour and 0.03% per hour respectively. Further, the hardness of the samples, i.e., the microhardness of the individual tantalum carbide grains, was found to increase from 1200 kgn/mm. for TaC to 24-00 kg./mm. for oas- The improved properties exhibited by filaments and support elements in a lamp apply to elements, both filamentary and supporting, having the prescribed composition which are incorporated in other types of devices to be operated in a state of incandescence. For example, the elements may be used in infrared sources, rocket nozzles, or other devices for operation at the high temperatures characterizing incandescence.

Although many methods can be employed to prepare tantalum carbide filaments and support elements, it has been found advantageous to preform the elements from tantalum sheet or wire and then carburize them to the desired non-st-oichiometric carbon content by heating in crucibles of tantalum carbide having the desired carbon to tantalum ratio. In the fabrication of the tested samples, the formed tantalum carbide elements were heated in this manner for 3 to 5 hours at temperatures of about 3000 C.

While the above description has referred to specific embodiments of the invention, it is apparent that many modifications and variations may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A filament for later inclusion in an incandescent lamp consisting essentially of a n-on-stoichiometric carbide having the general formula ('Ia,A)C wherein x is within the range of 0.98 to 0.78, Ta is tantalum, C is carbon and A is at least one metal selected from the group of metals consisting of zirconium, hafnium, niobium and titanium, in an amount not in excess of 20 weight percent of tantalum.

2. Apparatus in accordance with claim 1 in which x resides within the range of 0.95 to 0.80.

3. Apparatus in accordance with claim 1 in which x is approximately 0.82.

4. A filament support for later inclusion in an incandescent lamp consisting essentially of a non-stoichiometric carbide having the general formula (Ta,A)C wherein x is within the range of 0.98 to 0.78, Ta is tantalum, C is carbon and A is at least one metal selected from the group of metals consisting of zirconium, hafnium, niobium and titanium, in an amount not in excess of 20 weight percent of tantalum.

5. Apparatus in accordance with claim 4 in which x resides within the range of 0.95 to 0.80.

6. Apparatus in accordance with claim 4 in which x is approximately 0.82.

7. In the manufacture of an incandescent lamp of the type having a filamentary element mounted'on at least one support element, the step which comprises initially forming at least one of said elements therein of nonstoichiometric tantalum carbide having the general formula (Ta,A)C wherein x is within the range of 0.98 to 0.78, Ta is tantalum, C is carbon and A is an additive consisting of at least one metal selected from the group consisting of zirconium, hafnium, niobium and titanium in an amount not in excess of 20 weight percent of tantalum.

8. The method of claim 7 in which x resides Within the range of 0.95 to 0.80.

7 8 9. The method of claim 7 in which x is approximately 3,022,437 2/ 1962 Cooper 313-218 0.82. 3,022,438 2/1962 Cooper 313218 X 3,022,439 2/ 1962 Cooper 313-218 X References Clted 3,237,284 3/11966 Bird 313218 UNITED STATES PATENTS 5 3,277,330 10/1966 Cooper 313218 X 1,854,970 4/1932 Agte 313311 OHN HUCKERT P E 2,030,695 2/1936 Erber 3132-18 J xamfner 2,072,788 9 7 Andrews 313 2 X J. R. SHEW-MAKE'R, Asszstant Exannner. 2,596,469 5/ 1952 Cooper 313218 I 2,928,977 3/1960 Roth 3 13 3 11 X 10 3,022,436 2/1962 Cooper 3131-218 3513218; 316-1

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