US2316999A

US2316999A - Quartz tungsten seal

Info

Publication number: US2316999A
Application number: US404535A
Authority: US
Inventors: Charles D Spencer
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1941-07-29
Filing date: 1941-07-29
Publication date: 1943-04-20
Anticipated expiration: 1960-04-20
Also published as: FR912949A; GB554330A

Description

April 1943- c. D. SPENCER 2,316,999

QUARTZ TUNGSTEN SEAL Filed July 29, 1941 lnventov: Chartes D. Spencev, b9

His Atrorneg.

Patented Apr. 20, 1943 UNITED STATES PATENT OFFICE QUARTZ TUNGSTEN SEAL tion of New York Application July 29, 1941, Serial No. 404,535

Claims.

This invention relates to seals for the conductors or the like of electric lamps or electric discharge devices where such conductors or the like pass through the Walls of the envelopes of the devices. The new seal is especially advantageous 5 in cases where the coefficient of expansion of the member to be sealed difiers considerably from that of the material in contact with it, and particularly where the coefficient of expansion of a metal member is much greater than that of associated vitreous material to which it must be sealed. An example of this arises in connection with a tungsten metal leading-in wire for a quartz-bulb mercury lamp; and the invention is explained and illustrated hereinafter with particular reference to this use. As will appear hereinafter, the invention is especially adapted for tungsten or molybdenum wires of moderate size,-of the order of 10 to 30 mils in diameter,-- such as are used in lamps rather than in high current devices like rectifiers.

Quartz, as is well known, has a relatively low thermalv coeflicient of expansion as compared with most metals, including both tungsten and the other tungsten metal, molybdenum; so that difiiculties with fused joints between quartz bulbs and metal lead-wires or electrodes sealed through the quartz would be anticipated. Ordinarily, such wires or electrodes are preliminarily provided with beads of seal glass which are fused on them, and are afterward fused to the quartz to form a seal for the metal member. This seal glass or beading glass is a glass whose softening temperature is sufficiently low to allow of its being easily worked, while its thermal coefficient of expansion is higher than that of quartz, and thus nearer to that of the metal: nevertheless, its coefficient of expansion, about 15 x 10 is very much lower than that of tungsten, which is about 45 x 10* (metric units). Actually, quartz lamps with tungsten metal lead-wires or electrodes sealed in with seal glass have heretofore shown an objectionably high percentage of failures due to cracks at the joints; and cracks in the preliminary glass beads formed on such wires as above described have also been very common.

As might be supposed from the widely different thermal expansion characteristic of quartz and seal glass on the one hand, and of tungsten metal and many other metals on the other hand, the vitreous material of a fused seal around a metal lead or electrode is in a rather highly stressed condition,alike in the preliminary bead of such material that may be applied to the metal member before sealing in, and in the completed lamp 5.5

or other device. The most important of these internal stresses, as I have found, is an inwardand-outward or radial tension in the vitreous material around the metal member,-radial, that is, with respect to a round wire. At the surface of the metal member, this stress may be of a very high order of magnitude. When this stress (as has undoubtedly often been the case in joints heretofore used) has exceeded the rather low average tensile strength generally characteristic of Vitreous materials, including seal glass and even quartz, cracking was only to be expected. Furthermore, the conditions of stress have been such that any crack once started in the seal or bead would naturally extend itself lengthwise of the metal member sufficiently to ruin the joint and produce failure of the lamp or discharge device.

I have discovered that such failure of sealed joints can be largely obviated or done away with in a very simple and inexpensive manner, without necessity for costly or troublesome changes, such as the use of special metal, or of specially designed metal parts, or of special vitreous materials. This improvement, I have found, can be accomplished by changing and controlling the distribution of vitreous material along the metal member at the joint in such a way as to keep the critical stress below the tensile strength of the vitreous material;-or, in ordinary joint-making practice, by changing and controlling the contour of the preliminary bead on the member. Besides a novel joint, my invention involves novelty in the joint bead and in its formation.

Other features and advantages of the invention will become apparent from the following description of species and forms of embodiment, and from the drawing.

In the drawing, Fig. 1 is a side view of a portion of a conductor with a joint bead thereon suitable for the purposes of my invention, a portion of the bead being shown broken away and in section in a vertical axial'plane; Fig. 2 is a diagram illustrating the favorable distribution of stress in this bead; Fig. 3 is a view similar to Fig. 1 illustrating a somewhat different form of bead also embodying the invention; Figs. 4 and 5 are side views of slightly different joint beads with associated electrodes and current leads for an arc tube, Fig. 5 also showing a fragmentary sectional view of the arc tube wall in position for the head to be sealed thereto; and Fig. 6 is a sectional view of the entire arc tube with beads like that of Fig. 4 sealed into its opposite ends, and a bead like that of Fig. 5 sealed into one side.

Fig. 7 is a view illustrating a stage in the making of a bead such as illustrated in Figs. 1, 3, 4 or 5.

Figs. 8 and 9 are views corresponding to Figs. 1 and 2 illustrating a bead such as commonly used prior to my invention, and the unfavorable distribution of stress therein.

As shown in Fig. 1, the glass bead on the tung sten metal conductor Wire W comprises a relatively thick main body portion B and reduced,

relatively thin end portions E, E, at either end of the body portion B. While the thickness of the main body B is shown many-fold that of the wire W, in the ends E, E the glass around the wire has a thickness of the same general order,

as the wire. The ends E, E are here represented as substantially cylindrical and uniformin size, except where they enlarge, on a concavely curved profile, to merge into the body B, and where they are rounded off at their extremities; and ideally the body 13 may also have the form of a short cylinder with conically reduced ends. However, practical glass-working may often result in somewhat less regular forms for the body B, as well as for, the ends E, E. Minor variations of form may be tolerated Within limits, especially when the extremities of the ends E, E are fairly even and not lopsided or on a bias.

Fig. 2 represents to scale the relative values of the outward or radial tensile stress at the surface of the wire W at various points in the length of the bead shown in Fig. 1. From zero at the transverse plane I of the extremity of the lefthand end portion E, the tensile stress increases to a relatively low Value at the transverse plane l2, and then remains substantially constant or uniform at the planes 13, I4, l5, l6 through the reduced end portion E. Where the thickness of the bead begins to increase at the plane [6, the tensile stress also starts to increase, reaching a maximum at the plane IT, a little to the left of the plane it where the maximum thickness or diameter of the bead begins. From the plane ll the stress rapidly falls off to a minimum at the middle plane 59 of the body B,which minimum, however, is several times greater than the low constant value in the left-hand end portion E. The right-hand half of the bead being, as shown, a duplicate of the left-hand half, the stress therein corresponds plane by plane to that in the left-hand half, as indicated by similar designations of these planes.

Important in this diagram Fig. 2 is the relatively low and'substantially constant value of the outward tensile stress at the surface at the wire W in the end portions E, E. Because the stress is low, there is relatively little tendency for cracks to start; because the stress is uniform along the wire, there is less tendency for a crack starting, say, at the extremity of an end E to extend itself toward the main body B of the bead. And, as proved by extensive observation, cracks almost always start at the ends of joint beads, and especially at or adjacent their very extremities.

Taken with Fig. 1, Fig. 3 illustrates the fact that variation in the lengths of the reduced bead ends is permissible: i. e., whereas the ends E, E in Fig. 1 are of a length equal to about five times the thickness or diameter of the member W, the ends E, E in Fig. 3 are of a length only about twice this thickness. Figs. 1 and 3 also represent practical limits of length for the end porthe thickness of the member W. would be rather difiicult for the glass worker to make with assurance of maintaining its essential distinctive character, and would scarcely be fully efiective in preventing cracks as above set forth. On the other hand, a length of five times the thickness of member W is about the greatest length for an end portion E that is necessary to safely assure full realization of the advantages of such an end portion; so that any greater length would only entail waste of material, and of the glass workers time in making the needlessly long bead.

The stress-diagram for the bead shown in Fig. 3 would be essentially like Fig. 2, except that the horizontal portions of the curve at either end would be shortened in correspondence to the reduced length of the ends E, E.

Figs. 4 and5 illustrate diversity of length as between the reduced ends E, E of the same bead, and also show a main body portion B shorter in proportion to its size, so that it is of substantially spherical form. Moreover, the ends E, E are proportionately thicker than the ends in Figs. 1 and 3: i. e., for the ends E, E in Fig. 4, the glass around the wire W has a thickness about threequartersthat of the wire itself; while for the ends E, E in Fig. 5, the thickness of glass around the wire W is about equal to that of the wire, and so the over-all thickness of the ends E, E is three times the thickness of the wire. Corresponding to the greater thickness of glass in the ends E, E in Figs. 4 and 5, the conical taper of these ends is more noticeable than in Figs. 1 and 3, where the rounding 01f of the thinner ends is prominent. The forms of the bead as shown in Figs. 4 and 5 are, as a whole, more irregular than in Figs. 1 and 3, and thus more nearly what may be expected in actual practice.

As regards the thickness or diameter of the end portions E, E, practical considerations may have considerable influence. Experience indicates that for a 20 mil wire W, a glass thickness of 10 mils in the ends E, E is about right; and this proportionof approximately half the thickness of member W for the glass on the ends (or twice the thickness of member W for the over-all thickness of end portions E, E), as shown in Figs. 1 and 3 may be regarded as the ideal for both larger and smaller wires than 20 mils. However, since beads withvery thin, long ends E, E are not easy to manipulate in lamp making, it may be found impracticable to follow the ideal norm for the smaller wires; and thicknesses of glass around the wire W approaching or even slightly exceeding the thickness of the wire Wmay have to be tolerated, as in Figs. 4 and 5. In this case, matters'may be improved by a prolonged taper of the extremities of the reduced ends E, E, as also exemplified in these figures. Generally speaking, it is to be remarked that any reduction in the ends of a bead improves the stress situation, es-

pecially at theextremities; While, compensativesealing-in operation and assure satisfactory seals.

Besides the bead already described, Fig. shows a portion of a vitreous arc tube with an opening in its side 20, the bead being shown in the position relative to the tube which it occupies just before being actually united to the vitreous material of the tube,after the parts have been heated to the proper temperatures. Fig. 6 shows the completed arc tube with one of the Fig. 5 beads and two of the Fig. 4 beads sealed into its side 20 and its ends 2|, 2|, with its exhaust tubulature sealed off at 23, and with a drop of mercury 24 enclosed in it. The distinctive shading of difierent parts makes clear the seals of the bead glass to the different glass or quartz of the side 20 and of the ends 2|, 2|, as well as the seals of the ends 21, M and side portion 20 to the main body of the tube. As shown, the reduced ends or sleeve extensions E, E of each bead which coat the corresponding member W extend outward and inward from the adherent vitreous portion of the envelope wall that has been formed by fusing the main body of the bead to said wall.

In practice, a bead such as shown in Figs. 1, 3, 4, and 5 may conveniently be made on the Wire W as now to be described. It should be done rapidly and skillfully, to avoid burning away the tungsten and to produce a clear glass coating, with the wire clean and bright beneath it.

The operations are best performed on a machine with a rotary chuck for holding the wire, revolving at a speed of about 900 R. P. M., and shiftable in an axial direction by the operator. An oxhydrogen torch with #52 tip may be used to heat the wire and the beading lass. The rates of flow of oxygen and hydrogen and the length of flame used naturally vary with the size of the wire, the kind of glass used, and the type of bead to be produced: For applying standard thin beads of ordinary seal-glass to tungsten wire ranging from .015 to .030 inch in diameter, the suitable flame is several inches long. The flame should have a short blue cone at the tip, and the wire should be in the hot region just ahead of this blue cone during the formation of the bead. A thin, uniform rod of seal glass (of about 2 to 3 mm. diameter) is held in the operators right hand, and is fed downward to the wire as the bead is formed on the wire by a wrapping action. The wire being heated where the bead is to start, and the tip of the glass rod being momentarily preheated, this softened tip is touched to the hot rotating wire, to which it at once adheres, and the wrapping begins. During the wrapping, the chuck is gradually moved to the left, and the glass is applied on the side of the wire away from the fire.

The first operation consists in coating the wire W uniformly with a thin layer of glass from one extremity to the other of the (ultimate) bead-ends E, E, controlling the thickness of the coating by the rate of feed of the glass rod and the rate of movement of the wire W. This requires careful working to insure a thin, uniform, clear coating, without lumps or unsymmetrical projections at the ends. When this uniform, thin coating has been formed, as shown in Fig. 7, the main bead-body B is built up at mid-length of the coated area, to about the form shown in Fig. 1, by further additions of glass from the rod. The wire with the completed bead is removed from the machine while the bead is still hot, and is dropped into a beaker of distilled water.

The cooled beads are inspected for proper dimensions, shape, cracks, and bubbles, under a low-power magnifier. The wire should be but very slightly reduced in diameter, if at all, and should be clean, bright, and shiny. The inspection may be facilitated by immersing the beaded wire in carbon tetrachloride. Any bubble in the glass close to the end of the bead or to the junction of a reduced end E with the body B manifests a condition likely to result in cracking, and is cause for rejection. Elsewhere, bubbles should not be larger than half the thickness of the glass on the wire.

The distinctive nature and advantages of my new bead and seal will be somewhat clearer from comparing Figs. 1, 2, 3, 4, and 5 with Figs. 8 and 9, which show a bead of seal glass such as used prior to my invention, and a corresponding stress diagram. Here the striking point is the high stresses at the ends of the bead in the absence of my reduced end portions E, E. Though the ends of the Fig. 8 bead are conoidally reduced or tapered, the stresses in them rise steeply from zero at the end planes II, II to maxima at planes 1'], l1 Well within the tapered portions-where the diameter is considerably less than that of the thick mid-portion,and then decrease rather moregradually than in Fig. 2 to a minimum at the plane [9 at mid-length of the thick midportion. As shown in Fig. 8, the thickness of the main body of the bead is the same as that of the main body B in Fig. 1, and the maximum and minimum values of radial tension in the main body of the head are the same as in Fig. 2. Fig. 9 makes strikingly apparent the reason for the numerous failures in beads and lamp seals of the old type: Any slightest crack starting at an extremity of the Fig. 8 bead will obviously extend itself readily toward mid-length of the bead, owing to the rapid increase of internal tensile stress there. This is justthe reverse of the conditions in my head, as represented in Fig. 2.

What I claim as new and desire to secure by Letters Patentof the United States is:

1. The combination with a vitreous envelope and a metal member penetrating its wall through a sealing body of vitreous material which is of substanstantially lower coefiicient of expansion than the metal member and is fused into the vitreous envelope wall, and is also fused about and adherent to said member, said sealing body having a thickness around said metal member that is many-fold that of the member and being thus under relatively high inward-and-outward tensile stress; of reduced sleeve extensions of said sealing body inside and outside the envelope wall, each said sleeve extension coating and adhering to said member and extending therealong a substantial distance with a vitreous thickness around the member not greater substantially than the thickness of said member, whereby the inward-and-outward tensile stresses in each sleeve extension are relatively uniform and low as compared with those in said main sealing body.

2. The combination with and a metal member penetrating its wall through a sealing body of vitreous material which is of substantially lower coeflicient of expansion than the metal member and is fused into the vitreous envelope wall, and is also fused about and adherent to said member, said sealing body having a thickness around said metal member that is many-fold that of the member and being thus under relatively high inward-andoutward tensile stress; of reduced sleeve extena vitreous envelope sions-of said sealing body inside and. outside the envelope wall, each said sleeveextension coating and; adhering to said member and. extending therealong a distance of from substantially twice.

toapproximately five times the thickness of the member, with a vitreous thicknessaround the latter not greater substantially than the thickness: of th member.

3. A metal member with a seal bead therearound, for sealing by fusion into a vitreous envelope wall; said seal bead consisting, of vitreous material which is of substantially lower coefficient of expansion than the metal member and is fused about and adherent to the latter, and

comprising both a main sealing body which has a thickness. aroundsaid metal member manyfoldthat of the member and is thus under relatiye'ly high inward-and-outward tensile stress, and also reduced sleeve extensions of said main sealing body extending opposite ways therefrom along said member, each said sleeve extension coating and adhering to said member and extending therealong a substantial distance with a vitreous thickness around the member not greater substantially than the thickness of said member, whereby the inward-and-outward tensile stresses in eachsleeve extension are relatively uniform and low as compared withthose in said main sealing body.

4. A metal member with a seal bead therearound, for sealing by fusion into a vitreous envelope wall; said seal bead consisting of vitreous material which is of substantially lower coeflicient of expansion than the metal member and is fused aboutv and adherent to the latter, and

comprising, both a main sealing body which has a thickness around said metal member and extending therealonga distance of the order of the over-all external sleeve dimension transversely through the member, with a vitreous thickness around the latter not greater substantially than. the thickness of the member.

5. A tungsten metal wire having a diameter of the order of 10 to 30 mils, with a seal bead there,-

, than that of tungsten, and of the order of 15x 10- and being fused about and adherent to the metal wire, and comprising both a main sealing body which has a thickness around the wire many-fold that of the latter and is thus under relatively high inward-and-eutward tensile stress, and also reduced sleeve extensions of said sealing body coating and adhering to said wire and extending from the body along the wire a distance of from substantially twice to approximately five times the wire thickness, with a vitreous thickness around the wire intermediate between the thickness and the radial dimension of the wire.

CHARLES D. SPENCER.