US2667595A

US2667595A - Ribbon lead construction

Info

Publication number: US2667595A
Application number: US244818A
Authority: US
Inventors: Edward B Noel; Paul A Dell
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1951-09-01
Filing date: 1951-09-01
Publication date: 1954-01-26
Anticipated expiration: 1971-01-26

Description

Jan. 26, 1954 RIBBON LEAD CONSTRUCTION Filed Sept. 1 1951 2 Sheets-Sheet l Ic.g-'M A Inventors: Edward B. Noel Paul A Dell Then" Attorney E. B. NOEL ET AL 2,667,595

Jan. 26, 1954 E. B. NOEL ET AL RIBBON LEAD CONSTRUCTION 2 Sheets-Sheet 2 Filed Sept. 1 1951 Inventors Edwar d 5. Noel Paul A. Dell byw Them Attorney Patented Jan. 26, 1954 UNITED STATES TENT OFFICE RIBBON LEAD CONSTRUCTION York Application September-1, 1951, Serial No. 2444818 7: Claims.

Thisinvention relates generally to electric lamps-comprising a sealed bulb of vitreous materialand containing an electric ener y'transla: tion'means, and-more particularly to lamps oper ating atzhigh temperatures and pressures. The invention is specificallyconcerned with the leadin wire and seal construction of such lamps for making: connection to the electrodes or other electric energy-translation means within the bulb.

Irl making a hermetic seal of alead-in wire oncon-dixctor into awvitreous bulb, it is necessary, either thatzthe-conductor have thesame coefficient of expansion' as the bulb into which it is: sealed, or thatit-beproportioned to yield to the material of the bulb during their unequalexpansions: The first method isthat'. generally'followedwith ordinary'incandescent and-low pres sure discharge-lamps, both of which, operate at relatively lowtemperatures. However where the lamp operates at: a very. high 7 temperature and; particularly where the vitreous bulb is made of quartz'or fused silica or of quartz-like high temperature'glasses, the first method is inapplicable. The reason for thisis thatthe Very'high fusing temperature of quartz restricts the choice: of" metals for-the lead-in-wires to a very few, such as tungsten and molybdenum, which can: withstand those temperatures; However the coeffi cient of thermab expansion of these two metals isvery much greater than that of quartz, asmuch as ten times greater, and accordingly,- recourse must generally'be had to the second method involving-the use of foil or ribbon seals;-

With a quartz bulb, a hermetic seal; that is onewhich'wiliwithstand a vacuum, can be formed with either tungsten; tantalum, or; molybdenum sheets orribbons-,- provided that the ribbon is sufiiciently-thin; Molybdenum is generallyused becausef its ductility'and'the fact that it is" readily-available. Owing-to the much greater coefiicient of expansion of molybdenum as compared with quartz, stresses will be set up-inthe seal at" different temperatures. Provided the metal is very. thin and properly bonded to'the quartz, such stressesmay be reduced tothepoint' Where they will not crack 'the quartz: In'other words; the metalhavingbecomebonded to the quartz at. av relatively high temperature; will merely. go into tension when the'b'ulb cools but will not rupture-nor crack the seal.

Up to the present time, molybdenum ribbon seals havegenerally. been produced by welding thingstrips. ofrfoil orribbon toxsuitabl lengths ofwire. The molybdenum foil is out into strips a few vmillimeters wide: and? of; the proper length? oil required for the seal, and thickermolybdenum wires are welded to each end of a strip to form' the current connection.

cleans the surface thoroughly and roughens' it slightly; The bath also dissolves away some-0f seal.

It will be understood thatmolybdenum ribbon lead wires made from welded sections require considerable manual labor and are necessarily expensive. wastage occurs due to the difficulty of controlling the welding temperature.

ual inspection, very-often, when they are subsequently assembled into lamps, it is found that they break atthewelded joints and't-he lamps are worthless. Such subsequent breakage during manufactureappears tube-due to the discontinuities occurring in the-leads by reason ofthe two welds; The welding temperature causes embrittlement of the metal-at those points so that-the leads break very easily.

lead-inwires, the seal frequentlyburnsout-atthe joints where the ribbon-portion of the lead" iswelded' on to the cylindrical'or wire portions:

An object of the present invention is to provide an improvedlead-in wire of a highly refractory metal for sealinginto a vitreous'bulb.

Another object of the invention is-to provide arr-improved lamp construction for operation at a high temperature wherein the lead-in Wires are made-of 'a' refractory metal having a coefiioient of" thermal expansion very much different from" that ofa quartz-like vitreous bulb into which they are sealed;

In accordancewith the present invention, the leadin wires are made ofa highly refractory metal such as molybdenum, tungsten'or tantalum and; consist of asingle piece of wire whereof anintermediate portion is foliatedby'fiattening' and longitudinal elongation so as. to provide a gradualchange in the shape of the cross section.

Such a leaclwire maybe made; for instance, by

rolling a piece-of molybdenum wire longitudinally The ribbon or foil is preferably etched in-an electrolytic bath which Moreover a considerable amount of Even though the welded ribbon leadsappear-satisfactory upon vis- These discontinuities are'not only mechanical but also thermal and between a pair of hardened cylindrical rolls. These ribbon leads have the very decided advantages that the cross section changes gradually from a circular one into a foliated one without the occurrence of any sharp discontinuities. Thus the concentrations of stress which occur with molybdenum ribbon leads made up of welded sections, are eliminated and the resulting seal is not only stronger but able to withstand a higher temperature and to carry a heavier current without danger of burning out or cracking the quartz. Moreover, when such a lead-in wire is sealed into a quartz bulb, there are no re-entrant cavities which the quartz has diificulty in entering, so that resistance heating losses are conducted away from every point on the surface of the wire, thereby assuring a greatly increased current carrying capacity.

For further objects and advantages and for a better understanding of the invention, attention is now directed to the following description and accompanying drawings. The features of the invention believed to be novel will be more particularly pointed out in the appended claims.

In the drawings:

Fig. 1 is a pictorial view of a fragment of an electrode for a lamp embodying the invention and comprising a one-piece molybdenum lead wire having a foliated ribbon portion rolled into it.

Fig. 2 is a pictorial view of a high pressure mercury vapor quartz arc tube constructed with one-piece ribbon seals in accordance with the invention.

Fig. 3a, b, c and d, are illustrative of successive steps in rolling a foliated portion into a molybdenum wire for making a one-piece ribbon lead.

Figs. 4a, b and c are reproductions of actual photomicrographs of various sections through a one-piece molybdenum lead constructed in accordance with the invention and showing its longitudinally elongated crystal structure.

Figs. 5a and b locate the sections of the photomicrographs with reference to a lead-in Wire.

Referring to Fig. 1, there is shown a rolled one-piece ribbon lead I constructed in accordance with'the present invention. The lead comprises generally cylindrical end portions 2 and 3 which are actually the wire from which the intermediate foliated or ribbon-like portion 4 is formed by rolling the wire forward and backward a number of times under tension between a pair of hardened rolls.

The maximum permissible thickness A of the foliated or ribbon portion 4 depends upon the material'of the lead and also upon the vitreous portion into which it is to be sealed. For a molybdenum seal into quartz, the optimum thickness is approximately .0005 inch; thicknesses greater than .001 inch may give trouble with leaks, whereas thicknesses less than .0005 inch are fragile and lack current carrying capacity. The width B of the ribbon portion depends upon the current to be carried and, in practice, varies from about 1 to 10 millimeters. The length depends upon the size of the seal, the temperature conditions of the lamp in operation, and the differences in the expansion coefficients of the lead material and of the surrounding vitreous portion. With very high temperature mercury arc quartz tubes, the seal may be as long as 25 millimeters. For a moderately high pressure mercury arc lamp such as is illustrated in Fig. 2, the width '3 and the length C of the seal may be approximately 2 and millimeters respectively.

In general a ribbon lead of the type illustrated in Fig. 1 may be used for eifecting a seal into a vitreous bulb where it is not feasible to rely on matching the expansion coefficients of the lead metal and of the vitreous bulb. As an illustrative example, there is shown in Fig. 2 a high pressure mercury vapor arc tube 5 comprising a vitreous envelope 6, into the opposite ends of which are sealed a pair of main electrodes 1 and 8 and an auxiliary starting electrode 9. In order to be able to withstand the high operating temperature, the envelope 6 is made of quartz or of a quartz-like glass, for instance a glass known in the trade as Vicor and containing approximately 96% quartz, or other high temperature resisting glasses.

The electrodes comprise the ribbon leads I which have been described, one cylindrical end serving as the external terminal and the other end supporting the active portion of the electrode within the arc tube. In the case of the main electrodes 1 and 8, small tungsten slipover coils H] are mounted on the'inner ends of the leads, and enclose a small sliver of thorium metal which serves to increase the electron emission of the electrodes. The electrode structure may more readily be seen in Fig. 1 wherein a portion of the thorium sliver H projects beyond the few turns of the slipover coils which have been illustrated. The tube may be filled with a starting gas such as argon and a controlled quantity of mercury. It will be obvious that the leads I may also be used to support or make connections to other electrical energy translating means within the bulb, than the discharge electrodes which have been illustrated.

The ribbon leads I, B, 9 may be sealed into a quartz tube by locating them inside a length of cylindrical quartz tubing corresponding to the bulb 8, heating th ends of the tubing while lo-- cated in an inert gas atmosphere, and thereafter collapsing those ends by mechanical means so as to lock the ribbon portion of the lead between the two fused layers of quartz. This may readily be visualized from Fig. 2, wherein the crack the edge of the flattened end of the bulb denotes the line of fusion of the opposite sides of the quartz tube 6.

and broken away near its middle in order to show the relativ thickness of its walls as compared with the size of the electrodes and of the lamp in general.

Although the lamp illustrated in Fig. 2 has been made through collapse of a quartz tube by mechanical means, there are other ways of making such lamps. For instance, the lamp may be made in stages, the first stage consisting of the vacuum collapse of a short length of quartz tubing about each of the ribbon leads. In the next stage, a pair of these ribbon lead seals may be fused to a suitable length of a larger size of quartz tubing which serves as the main bod or envelope of the lamp.

As is illustrated in the drawings, the ribbon lead I consists of a single piece of a highly refractory metal wire such as molybdenum or tungsten whereof the middle portion has been foliated by flattening and elongation so as I to produce a gradual transition from the cylindrical cross section of the end portions as illustrated at 15, to the flattened elliptical cross section of the middle portion as illustrated at M. The required configuration of the lead may be achieved through a rolling process such as is diagrammatically illustrated in Figs. 3a to 3d. Referring' to Fig. 3a, a short length I of a highly re- The tube 6 has been sectioned fractory metal wire such as molybdenum, tantalum or tungsten, is located between a pair of hardened rolls 29 and 2: in a direction transverse to their axes. The rolls are mounted steel semi-cylinders 22, 2.3 which are journaled in bearing blocks or

shoes

25 and 25. Th forward or mutually facing edges of the bearing blocks are shaped to a cylindrical curvature in order to serve as bearing surfaces for the semicylinders 22 and 23. The

rolls

25, 2| may be cylinders of a cemented tungsten carbide sold coinmercially under the trade name Carboloy.

The foliated or ribbon portion in the lead wire is formed by rocking the rolls 2t and M through a small angle while simultaneously applying pressure against the bearing

shoes

24, 25 so as to force the rolls together and squeeze the wire between them. In Fig. 3a, the lead wire I is shown at the beginning of the rolling cycle and is perfectly round throughout its length. At the beginning, the pressure on the rolls is preferably somewhat less than at the end, and the rocking arc is the least, as indicated the relatively short curved arrows 25. Figs. 3b and 3c illustrate intermediate stages in the rolling operation wherein the pressure exerted on the rolls has been increased and the rocking arc has been lengthened, as indicated by th arrows 2? and 28. In Fig. 3d, the pressure exerted on the rolls is a maximum and the arc of rock, indicated by the arrows 29, is also at its longest. Reference may be made to copending patent application No. 244,819, filed September 1, 1941, of Paul A. Dell, entitled Apparatus for Foliating Wire Ribbon Leads, and assigned to the same assignee as the present invention, for a complete description of a, preferred mechanism by means or" which the rolling operation may be performed.

In the illustrated embodiment, the leads are made of one piece of metal whereof the cross section changes gradually and continuously from a generally cylindrical one at the ends to a fiattened elliptical one at the center. Moreover, this gradual change exists not only in the configuration of the lead but also within the lead itself, that is, within its internal crystal structure. The result is a longitudinally elongated or streamlined crystal orientation within the lead which conforms generally to its external configuration and assures not only mechanical, but also thermal and. electrical continuity.

Referring to Figs. 4a to 4/3, the streamlined and smooth flowing crystal structure is readily seen in the reproductions of actual microphotographs of molybdenum leads produced in accordance with my invention. These photomicrographs are 100 diameter enlargements of one-piece molybdenum ribbon leads, made from 22 mil molybdenum wire by flattening down the middle portion to a thickness of approximately .0006 inch through a longitudinal rolling operation. The technique for taking the photomicrcgraphs involved plating the leads and casting a metal matrix about them in order to provide a firm backing for the lead so as to allow machining and polishing operations. Thereafter the matrix is cut through along a plan giving the desired section of the lead, the surface is polished smooth and then etched and photographed in the usual fashion.

The correlation of the different photomicrograph reproductions of Fig. l to an actual ribbon lead is shown in Fig. 5. Referring to Fig. 5c, the dotted circles 01; and b superimposed on the side section of the lead wire 1 correspond to the elliptical cross section which would cause discontinuities.

photomicrographs of Figs. 4a and 4b respectively. The dotted circle 0 superimposed on the plan section of the lead Wire I in Fig. 5b corresponds to the photomicrograph of Fig. 4a. In the photomicrographs, the lead metal appears as the lined and finely grained surface running smoothly right through the reproductions. The pitted and irregularly shaded surfaces both above and below are the matrix metal in which the lead was imbedded, as previously mentioned. It will be observed that there is a very pronounced crystal structure which flows smoothly in both planes from the round or cylindrical cross section or the ends of the lead into the flattened of the central ribbon portion.

The longitudinally elongated crystal structure illustrated in the photographs provides very definite mechanical advantages for this type of ribbon lead over the other types which have been used heretofore. The more common type of lead used heretofore comprises a fiat ribbon 0r foil portion to either end of which are welded suitable lengths of round wire. Another type of lead consists of a one-piece molybdenum wire into which a flat is formed in the middle portion either by swaging or by a transverse rolling operation. Swaging a flat into a wire, that is nattening it through repeated blows of a hammer on an anvil, does not secure the smooth streamlined crystal structure which is obtained by a longitudinal rolling operation. Likewise a transverse rolling operation produces an inferior lead because the grain structure in the ribbon portion is transverse to what it is in the cylindrical end portion. In a series of tests which were performed on longitudinally rolled one-piece ribbon leads produced in accordance with the invention and corresponding leads made up of welded sections in accordance with the prior art, the tensile strength of the longitudinally'rolled leads averaged from 1.8 to 2.2 times more than the tensile strength of welded leads. The superior tensile strength of the longitudinally rolled leads was measured after a high temperature cleaning process which had reduced their tensile strength by about 30 percent. When the tensile strength was measured previous to the cleaning process, it averaged 3.5 times greater than the tensile strength of welded leads. The average tensile strength of four welded lead samples which were measured was 1219 grams, whereas that of four similar rolled foil leads was 4054 grams previous to the cleaning process, and 2680 grams after the cleaning process. In the cleaning process, the lead was subjected to an atmosphere of hydrogen at a temperature of Q" C. for a fifteen minute period.

The streamlined and longitudinally elongated crystal structure also provides definite advantages due to the electrical and thermal characteristics of the lead resulting therefrom. Electrically, the leads have a lower resistance and can carry a heavier current from the same heat loss. More important yet, there are no points of localized high resistance, such as occur at the Weld points with welded leads, where the current must concentrate, so no hot spots developed. Also there are no reversals of the crystal structure, as with swaged or transversely rolled leads, Thermally there is better heat conduction from the fcliated portion into the cylindrical end portion so that the heat losses flow more readily to the outside where they can be dissipated. The net result is a greatly increased current carrying capacity, as

much as several times that of the prior art leads.

Another feature of the longitudinally rolled ribbon leads in accordance with the invention, are the fine lateral cracks 3| which may be seen in Fig. 1. These lateral cracks occur in the thin feather edges of the foliated portion of the lead and are very valuable in forming a hermetic seal into fused silica or quartz. During the process of sealing the lead into the quartz tubing, the quartz is heated to its fusion point and the walls of the tubing are collapsed inwardly and fiattened down on the leads. During the process some gas may be trapped and form an elongated hole or streamer next to the edges of the lead which would result in a defective seal. The slight irregularities and transverse cracks in the feather edges of the foliated or ribbon portion of the lead break up any streamers that might occur in the quartz near the edges in question, and thereby insure the making of a hermetic seal.

While certain specific embodiments have been shown and described, it will of course be understood that various modifications may be made without departing from the invention. Thus there is considerable scope for variation in the particular size and dimension of the lead wires, and the particular lead wire and lamp which have been described are given by way of illustrative example only. The appended claims are intended to cover any such modifications coming within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

l. A one-piece ribbon lead consisting of a length of wire composed of a highly refractory metal from the group comprising molybdenum, tantalum and tungsten, said wire having an intermediate foliated portion of a thickness in the range from 5 to 10 ten and a longitudinal crystal orientation running smoothly throughout its length.

2. A one-piece ribbon lead consisting of a length of wire composed of a highly refractory metal from the group comprising molybdenum,

tantalum and tungsten, said wire having an intermediate foliated portion of a thickness in the range from 5 to 10 ten and having a multiplicity of transverse cracks in its thinner edges, and said wire having a longiso smoothly throughout its length in general contudinally elongated crystal orientation running formity with its external configuration.

3. A one-piece ribbon lead consisting of a length of wire composed of a highly refractory metal from the group comprising molybdenum, tantalum and tungsten, said wire having an intermediate foliated portion of cal cross section of a thickness in the range from 5 to 10 ten thousandths of an inch and having a multiplicity of transverse cracks in its thinner edges, said wire having a crystal orientation running smoothly throughout its length in general conformity with it external configuration.

4. A ribbon lead made from a length of wire composed of a highly refractory metal from the group comprising molybdenum, tantalum and tungsten, said lead comprising round end portions and an intermediate foliated portion of flattened elliptical cross section produced by longitudinal rolling of said wire, said foliated portion having a thickness in the range from 5 to 10 ten thousandths of an inch and having feather edges with thousandths of an inch,.

thousandths of an inch flattened elliptii longitudinally elongated a multiplicity of fine transverse crack therein, said lead having a longitudinally elongated and streamlined crystal orientation running continuously throughout its length in general conformity with its external configuration and providing a transition, from said round end portions to said foliated portion, which is smooth and free from any discontinuities.

5. A high temperature operating lamp comprising an envelope of a high temperature resisting vitreous substance and leads sealed through said envelope and connected to electrical energy translating means therein, said leads comprising a length of wire of a highly refractory metal capable of withstanding the fusion temperature of said substance, said wire having an intermediate foliated portion of a thickness in the range from 5 to 10 ten thousandths of an inch, and a longitudinally elongated crystal orientation running smoothly throughout its length in general conformity with its external configuration, said envelope being bonded to said intermediate portion to provide a hermetic seal.

6. A high temperature operating lamp comprising an envelope of a high temperature resisting vitreous substance and leads sealed through said envelope and supporting electrodes therein, said leads comprising a length of wire of a highly refractory metal capable of withstanding the fusion temperature of said substance, said wire having an intermediate foliated portion of fiattened elliptical cross section of a thickness in the range from 5 to 10 ten thousandths of an inch and having a multiplicity of fine transverse cracks in its thinner edges, said wire having a longitudinally elongated crystal orientation running smoothly throughout its length in general conformity with its external configuration, and said envelope being bonded to said intermediat portion to provide a hermetic seal.

7. A high temperature operating lamp comprising an envelope of a high temperature resisting vitreous substance and leads sealed through said envelope and supporting electrodes therein, said leads comprising a length of wire of a highly refractory metal capable of withstanding the fusion temperature of said substance, said leads comprising an intermediate foliated portion of flattened elliptical cross section produced by longitudinal rolling of said wire, and foliated portion having a thickness in the range from 5 to 10 ten thousandth of an inch and having feather edges with a multiplicity of fine transverse cracks therein, said leads having a longitudinally elongated and streamlined crystal orientation running smoothly throughout their length in general conformity with their external configuration, said quartz being collapsed down upon said intermediate portion, and said cracks serving to break up elongated streamers of feather edges in order to assure a hermetic seal.

EDWARD B. NOEL. PAUL A. DELL.

Switzerland May 2, 1949 trapped gas along said