US 3789255 A
Incandescent lamp incorporates a non-sag tungsten filament having a microstructure comprising a plurality of elongated and interlocking grains disposed along the axial dimension of the filament. A large number of very minute inert gas bubbles, such as helium bubbles, are included within the filament, and a portion of the bubbles are generally aligned along the axial dimension of the filament and are positioned at the boundaries of the interlocking grains which comprise the filament. In the method for making the filament, inert gas atoms, such as helium atoms, are formed within the tungsten and when the filament is incandesced, the atoms migrate to form the very minute helium bubbles.
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Description (OCR text may contain errors)
FILAMENT FOR AN INCANDESCENT LAMP Inventors: Heinz G. Sell, Cedar Grove, N..I.;
Heinz-J. Stepper, Konigsbrunn, Germany Assignee: Westinghouse Electric Corporation,
Filed: Jan. 12,1972 Appl. No.: 217,162
US. Cl 313/217, 313/218, 313/222, 313/315, 313/344 Int. Cl. H0lk 1/04 Field of Search... 313/222, 315, 217, 218, 311, 313/344, 343
United States Patent 91 [111 3,789,255 Sell et al. i I r Jan. 29, 1974 NON-SAG INCANDESCENT TUNGSTEN Primary Examiner-Palmer C. Demeo Attorney, Agent, or Firm-A, T. Stratton et a1.
Incandescent lamp incorporates a non-sag tungsten filament having a microstructure comprising a plurality of elongated and interlocking grains disposed along the axial dimension of the filament. A large number of very minute inert gas bubbles, such as helium bubbles, are included within the filament, and a portion of the bubbles are generally aligned along the axial dimension of the filament and are positioned at the boundaries of the interlocking grains which comprise the filament. In the method for making the filament, inert gas atoms, such as helium atoms, are formed within the tungsten and when the filament is incandesced, the atoms migrate to form the ery minute helium bubbles.
ABSTRACT 6 Claims, 4 Drawing Figures PAIENTEB JAN 291974 SHEET 2 OF 2 FIG. 4
BACKGROUND OF THE INVENTION This invention generally relates to incandescent lamps and, more particularly, to an incandescent lamp which incorporates an improved non-sag tungsten filament. There is also provided a method for making the filament.
Non-sag tungsten filaments were initially developed prior to 1920 and the introduction of the non-sag tungsten filament, as described in US. Pat. No. 1,410,499 dated Mar. 21, 1922, had a great impact on the incandescent lamp industry. In explanation of the term nonsag, if a tungsten filament elongates when. it is operated, and thus sags, the individual turns between filament coils will tend to contact one another and short out, thereby shortening the life of the filament. In the practices of the prior art, non-sag tungsten has been fabricated by including therein a very small amount of so-called dopant, which dopant is added in the form In processing tungsten, the material is initially formed into a relatively massive ingot by conventional self-resistance sintering techniques. Thereafter, the relatively massive ingot is mechanically reduced in cross section, first by swaging and then by drawing into filamentary form. In accordance with the method for making the present filament, after the relatively massive sintered ingot has been formed, and before the resulting elongated tungsten filamentary member has been heated to a condition of incandescence, there is formed within the tungsten body a very large number of inert gas atoms. Thereafter, after the elongated filamentary member has been fabricated into the form intended for of alkali silicates. Because of this dopant addition,
when the filament is initially incandesced, the crystals or grains which are formed therein are elongated and interlocking and generally disposed along the axialdimension of the filament. Within recent years, it has been discovered that the non-sag characteristic which is imparted by the alkali silicate doping is due to the formation of veryminute potassium particles, which form potassium bubbles when the filament is incandesced. A substantial portion of these bubbles are generally aligned along the axial dimension of the filament and serve to inhibit the recrystallization and control the grain growth of the filament due to dislocation and grain boundary pinning by the potassium bubbles.
These potassium-formed bubblesare generally submicroscopic in nature and a representative diameter of same is in the order of 100 to 500A. While the performance of such a-filament is generally satisfactory, a major problem in the manufacture of the tungsten is the uniform incorporation of the doping impurity into the metal, and the attainment of the consistency of the required concentration, in orderto produce the best filament material.
'- It is disclosed by C. E. Ellis in ACTA Metallurgica, Volume 11, February 1963 that helium filled bubbles formed in samples of cold-worked aluminum by cyclotron alpha irradiation have an effect on subsequent recrystallation and grain growth in the aluminum samples. The helium bubbles also inhibit grain growth in beryllium and recrystallization in zirconium.
SUMMARY OF THE INVENTION In accordance with the present invention, there is provided, in combination with an incandescent lamp, an elongated tungsten filament having a microstructure which comprises a plurality of elongated and interlocking grains defined by boundries therebetween. The elongated dimension of the grains is disposed along the axial dimension of the filament. A very large number of very minute,'individual inert gas bubbles are included within the filament, and a portion of the individual bubbles are generally aligned along the axial dimension of the filament and positioned at boundries of the interlocking grains. The resulting structure operates with a non-sag characteristic.
its ultimate use, such as a coil or a coiled-coil, and also mounted in the environment in which it is intended to operate, the filamentary member is heated to a condition of incandescence which causes the inert gas atoms to coalesce into a very large number of very minute inert gas bubbles, which causes the filament to recrystallize with an interlocking and non-sag grain structure. In one method for forming the inert gas atoms coalesce to form helium bubbles.
BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention, reference should be had to the preferred embodiment, exemplary of the-invention, shown in the accompanying drawings in which! FIG. 1 is an elevational view, partly in section, illustrating an incandescent lamp which incorporates the non-sag tungsten filament which has been processed in accordance with the present invention; 9
FIG. 2 is a greatly enlarged sketch showing a crosssection of a recrystallized tungsten filament which includes helium bubbles in the body thereof, with a portion of the bubbles positioned at the boundries of the overlappinggrains which comprise the filament;
FIG. 3 is a transmission electron micrograph of alpha-particle irradiated pure tungsten annealed at l,900C for 45 minutes showing helium bubbles having a diameter of approximately 200A.; and
FIG. 4 is a photomicrograph (taken at 1,000 X) of alpha-particle irradiated pure tungsten ribbon annealed at 2,200C for 45 minutes showing the elongated, interlocking grain structure developed in the irradiated layer.
DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIG. 1, the generally conventional incandescent lamp 10 comprises a sealed radiationtransmitting envelope 12 which encloses a predetermined environment, such as an argon atmosphere, in which an incandescent filament 14 can be operated. Lead-in conductors 16 are sealed through and extend into the envelope 12 and these electrically connect to a conventional standard screw base 18. The envelope 12 can carry thereon an internal coating 20 of lightscattering material, if desired. The filament 14 has been fabricated in accordance with the teachings of the present invention, as explained in detail hereinafter. The filament 14 can be formed as a single coil of tungsten wire or as a coil which in turn is formd into a coil to form a so-called coiled-coil. Of course, other filament constructions are possible, as are well known in the art and the gaseous atmosphere can be replaced by a vacuum or by a combination of inert gas and halogen, and such operating atmospheres are well known.
In one method of making the non-sag filament of the present invention, a very small amount of boron (isotope B is mixed with the-tungsten powder prior to forming a compact of tungsten. As an example, the boron isotope can be included in an amount of about 0.5 percent by weight. The amount of added boron isotope does not appear to be critical. The formed tungsten compact is then pre-sintered by heating to a temperature such as 1,000C in a non-reactive atmosphere for about one-half hour. The pre-sintered compact is then self-resistance sintered in accordance with conventional techniques, in order to form a relatively massive sintered tungsten ingot. The sintering atmosphere is preferably hydrogen. The formed tungsten ingot is then swaged or otherwise mechanically worked until it is greatly elongated, with a relatively small diameter such as 0.083 inch (2.1 mm). The elongated material is then drawn to wire of desired size.
In accordance with the present invention, after the tungsten ingot has been mechanically reduced to a relatively small diameter, such as wire having a diameter of 0.1 inch (2.54 mm) it is passed through an irradiating chamber and irradiated with neutrons. The following nuclear reactions which occur can be described as follows:
W B l, L3 Li "LP," 2He After the foregoing irradiation, the tungsten is worked and drawn to wire of the desired size, and an example is 7 mils (0.178 mm) tungsten wire. The wire is formed into a coil or a coiled-coil as desired. This tungsten wire will exhibit a microstructure which comprises what is known in the art as a worked structure due to the swaging and the drawing operations, and the wire is ductile in that it can be wound into coils, with the formed helium atoms as well as lithium atoms distributed throughout the tungsten wire. When the tungsten filament is initially incandesced under-predetermined controlled conditions, it will begin to recrystallize and the helium atoms, as well as residual lithium atoms, which are distributed throughout the tungsten will begin to coalesce to form small bubbles of helium and lithium. These bubbles will be discrete and will be generally aligned in the direction of the axis of the wire.
As such, they will function similarly to the potassium bubbles of the standard dopedtungsten and will retard recrystallization and control same due to the effect of dislocation and grain boundary pinning.
An enlarged sketch of the present wire is disclosed in FIG. 2 wherein the inert gas bubbles such as helium bubbles 22 are dispersed throughout the body of the tungsten filament 14, with a portion of the bubbles generally aligned along the axial dimension of the filament and positioned at the boundries 24 of the overlapping elongated and interlocking grains 26 which comprise the filament. 1f alkali metal-formed bubbles 28 are also present, they will have the general appearance and orientation as the helium bubbles.
As a second method for forming the inert gas bubbles, after the filament has been drawn to a relatively fine size, such as a O. linch (2.54 mm), for example, it is irradiated with alpha particles which forms helium atoms within the body of the tungsten. Thereafter, the filament is drawn to its desired ,size and then recrystallized to an interlocking structure.
In FIG. 3 is shown a transmission electron micrograph of alpha-particle irradiated pure tungsten which has later been annealed at a temperature of 1 ,900C for approximately 45 minutes, with the magnification in this electron micrograph being 60,000. Each of the plurality of helium bubbles which are shown have a diameter of approximately 200A. The helium atoms were introduced into the tungsten by irradiating tunsten ribbon with 30 MeV alpha particles. The helium bubbles began to nucleate or coalesce at an annealing temperature of approximately 1,600C and the bubbles continued to grow in size as the annealing temperature was increased. After an anneal at approximately 2,200C, the average size of the bubbles increased somewhat with the majority of the bubbles having a diameter substantially less than one micron.
To show the difference between tungsten which had been irradiated with the alpha particles to form the helium atoms therein, as compared to substantially pure tungsten which contained no helium, a narrow zone of a tungsten ribbon was irradiated with a 30 MeV alpha particles, with the resulting structure shown in FIG. 4, which is a photomicrograph taken at a magnification of 1,000 X. The irradiated zone occurs in this center of this photomicrograph and comprises an elongated, interlocking crystal structure comprising a plurality of elongated and interlocking grains defined by boundries therebetween. A very large number of very minute individual helium bubbles are included within the tungsten, and a portion of the bubbles are generally aligned along the grain boundries. The portion of the tungston which was not irradiated with the alpha particles began to recrystallize at annealing temperatures in the order of 1,300C to 1,4()0C, but the recrystallization process was retarded in the irradiated band and initiation of recrystallization was not observed until an annealing temperature 2,200C was reached.
The concentration of the helium bubbles within the tungsten body does not appear to be particularly critical and can vary over a wide range. As an example, for helium bubbles having a diameter generally in the order of 2,600A., a bubble concentration per cubic centimeter of tungsten in the order of from about 4 X 10 to 5 X 10 provides very good results.
Inert gases other than helium can be formed within the tungsten body. As an example, if *Mg is included within the tungsten body and irradiated with neutrons, it will form within the tungsten a mixture of neon, helium and the alkali metal sodium. The latter will form sodium as bubbles in the tungsten body when the body is incandesced, in a manner similar to potassium. The Mg can be used to replace the boron isotope or it can be used to supplement same to form helium and a mixture of lithium and sodium. The alkali metal-formed bubble will be present in very large numbers and will be of a very minute size, such as in the order of to 500 A. As another example, Al irradiated with neutrons will form Na" and helium atoms. Detailed explanations of such nuclear transmutations are described in Nuclear Physics (2nd Ed), by Irving Kaplan, Addison- Wesley Publishing Co., Inc. (1964).
We claim: I
1. In combination with an incandescent lamp comprising a sealed radiation-transmitting envelope enclosing a predetermined environment in which an incandescent filament can be operated, and lead-in conductors sealed through and extending into said envelope, the improved incandescent lamp member which comprises:
a. an elongated tungsten filament supported within said envelope and electrically connected to said lead-in conductors, and
b. the microstructure of said filament comprising a plurality of elongated and interlocking grains defined by boundries therebetweeen and having their elongated dimension along the axial dimension of said filament, a very large number of very minute individual inert gas bubbles included within said filament, and a portion of said individual bubbles generally aligned along the axial dimension of said filament and positioned at the boundries of said grains which comprises said filament.
2. The incandescent lamp combination as specified in claim 1, wherein said inert gas is helium.
3. The incandescent lamp as specified in claim 2, wherein the major portion of said inert gas bubbles have a diameter substantially less than one miron.
4. The incandescent lamp combination as specified in claim 3, wherein the bubble concentration per cubic centimeter is in the order of about 4 X l0 to 5 X l0 bubbles.
5. The incandescent lamp combination as specified in claim 1, wherein a very large number of very minute alkali metal-formed bubbles are present and are disposed in a manner similar to said inert gas bubbles.
6. The incandescent lamp combination as specified in claim 5, wherein said alkali metal-formed bubbles are at least one of lithium-formed bubbles and sodiumformed bubbles.