US2231459A - Electrical conductor for vapor electric devices - Google Patents

Description

lFeb. 11, 1941. w. T. ANDERSON, JR 2,231,459

ELECTRICAL CONDUCTOR FOR VAPOR ELECTRIC DEVICES Filed Nov. '7, 1939 mgmt o ATTORNEY Patented Feb. 11, 1941 UNITED STATES PATENT OFFICE ELECTRICAL CONDUCTOR FOR. VAPOR ELECTRIC DEVICES Jersey Application November 7, 1939, Serial No. 303,205

5 Claims.

This invention relates to the production of vacuum-tight seals between the walls of vessels of vitreous material and electrical lead-in conductors, and is concerned in particular with vacum-tight seals for lead-in conductors of electrical discharge devices.

It is one object of my invention to provide seals in vitreous materials such as glass, quartz and the like for metallic conductors, which seals are completely vacuum-tight.

It is another object of my invention to provide in electrical discharge devices, such as gas or vapor arc lamps, rectiiiers and the like, a conductor of high current carrying capacity sealed into the vitreous discharge vessel in a completely vacuum-tight manner.

It is still another object of my invention to provide a seal between a vitreous material having a relatively low coeiiicient of thermal expansion and a metallic conductor having a relatively high coeicient of thermal expansion, wherein the seal is vacuum-tight and the metallic conductor possesses a high current carrying capacity.

It is well known to provide for instance in electrical discharge devices, such as mercury vapor lamps having a quartz envelope, a lead-in conductor in the form of wire or thin foil of tungsten or moylbdenum. The sealing in of such conductors in the form of wire is accompanied, howover, by the shortcoming that it is not possible to provide a direct vacuum-tight seal between the conductor and the quartz envelope. The use of such conductors in the form of a thin foil permits the production of a vacuum-tight seal between theconductor and the quartz and the envelope but suffers from the drawback that the thin foil will carry the high current necessary in the operation of most electrical discharge devices only while completely embedded in the quartz or other vitreous material, but is unable to carry the heavy current once it emerges from the envelope into open air or into the gaseous or vaporous lling within the vessel due to overheating resulting for instance in burning through in air or oxygen containing gases or in reduction of strength in nonoxygen containing gases or vapors. In view of the fact that thin foils of tungsten, molybdenum and the like permit the production of a vacuum-tight seal thereof with surrounding quartz, attempts have been made to overcome the low current carrying capacity and low strength of such foils by complementing the thin foil with a wire having sufficient mass to carry without overheating the high current within and without the quartz embedded thin foil and having sufficient strength to serve as or carry an electrode within the electrical discharge device, the wire being sealed into the quartz at both ends and electrical contact between the wire and the foil being established by a pressure contact.

It' has been found, however, that the complementing of the foil with wire is not entirely satisfactory. The production of electrical contact by pressure introduces manufacturing difliculties; if the quartz is fused too tightly to the wire and to the foil, it may easily happen that the quartz cracks, While if the fusion is too light the junction between wire and foil will be heated by the passing current thus tending to cause fracture of the quartz about the junction. Also, lcoseness of the junction brings about increased electrical resistance and resultant production of heat which when transmitted to the thin quartz embedded foil raises the temperature of the foil thereby causing increased electrical resistance and lowering of the efficiency of the discharge device, in extreme cases even causing the foil to burn out. Substantially the same result is produced when the Wire is welded to the foil, in view of the relatively small contact area between the flat foil and the small area of 2D the wire available for making contact, thus producing hot spots.

It is, therefore, a further object of my invention to produce a vacuum-tight seal between vitreous materials such as quartz and electrical conductors of high fusing metals, such as molybdenum, tungsten and the like, wherein the electrical conductor is in the form of thin foil but has a high current carrying capacity and great mechanical strength without the shortcomings of overheating, variations in the electrical resistance and other difficulties heretofore encountered in connection with the sealing of conductors in the form of foil.

The diiculties inherent in the sealing of electrical conductors into vitreous materials are par 40 ticularly great when the coeflicient of thermal expansion of the vitreous material is considerably lower than that of the metal of the conductors, a phenomenon occuring in particular when the vitreeus material consists largely of silica, contain- 4.5 ing for instance 80% or more silica, and thus has a coefficient of thermal expansion less than 3 l06, such as fused quartz which has a coefficient of thermal expansion of 0.6Xl06, and the electrical conductor consists of tungsten, molybdenum, tantalum, or the like having a relatively large coefiicient of thermal expansion, tungsten and molybdenum having e. g. coeflicients of expansion of approximately 4.2 106.

It has been found that electrical conductors 50 Cil of tungsten, molybdenum and the like when inthe form of foil having a thickness not in excess of about 0.0008" can be sealed into quartz 0r into other glasses having a coemcient of thermal expansion less than 3X 10-c in such manner as to produce a vacuum-tight seal. As heretofore stated such thin foil is capable of carrying the necessary large electrical current only when completely embedded in the quartz. For instance, although the electrical resistance of molybdenum at 20 C. is 5.Z 10 ohm-centimeter, a molybdenum foil of 0.0006" thickness and 0.080" width when fused into fused quartz will carry 5 amperes of electric current indefinitely without overheating, due to the cooling action of the surrounding quartz. When such thin foil emerges into air it will burn up, in about one second incase of a current of 5 amperes and foil of the dimensions specitled above. When such foil emerges into an evacuated vessel or into a vessel lled with rare gas a current of 5 amperes would heat the same i'oil to incandescence, which would make it very brittle, reduce its already i-nherently low mechanical strengthy increase its electrical resistance, and would finally cause its destruction by evaporation of the metal, so that apart from its low life the thin foil would cause a lowering of the eiliciency of the electrical discharge device and would be incapable of serving as or carrying an electrode.

On the other hand foils of a thickness in excess of 0.0008" cannot be sealed in a vacuum-tight manner into quartz or other low expansion vitreous material due to the difference in thermal expansion, thus necessitating the use of glasses of intermediate coefficients of thermal expansion interposed between the quartz and the like and the metallic conductor, one of the drawbacks of such graded seals being the likelihood of fracture on the part of the intermediate glasses.

Electrical conductors sealed into quartz and other non-porous viteous materials having a high silica content must have a fusing point in excess of the temperature at which such viteous material is worked and for this reason such conductors consist of high fusing refractory metals such as tungsten, molybdenum, tantalum, and the like.

My invention consists in essence in providing in non-porous vitreous materials as described an electrical conductor consisting of a thin foil, usually of a thickness not in excess of 0.0008", joined at its ends by a thicker foil firmly secured thereto, the thin foil being firmly surrounded by the vitreous material, such as quartz or the like, in such manner as to produce a vacuum-tight seal, while the thicker foil is partially covered by the vitreous material without necessarily forming a vacuum-tight seal with the remainder of the thick foils emerging from the seal at both ends.

The objects of my invention and the nature of the electrical conductor discovered by me are further illustrated in the accompanying drawing, forming part hereof, and in which:

Fig. 1 is a sectional side view of the seal and the electrical conductor, and

Fig. 2 is a sectional top view of the seal and the electrical conductor, taken on line 2-2 of Fig. 1.

In accordance with my invention, I provide a thin foil I of a thickness such as between 0.0005" to 0.0007" of molybdenum or the like, but not in excess of a thickness of 0.0008" when sealed into quartz or the like high silica content vitreous material, to which I join at each end a heavier foil 2 and 3 also of molybdenum or the like of for instance a thickness of 0.002" to 0.004" or more. Instead of molybdenum I may use any other refractory high fusing point metal such as tungsten, tantalum, Iridium or other high fusing metals of groups V and VI and VIII of the periodic system having relatively high coemcients of thermal expansion.

In Fig. 2 I have shown the thicker foils 2 and 3 firmly secured to the thin foil I by spot welding as indicated at I before fusion thereof into the quartz, but it should be understood that any other method of rmly securing the foils together, for instance pressure, may be used, welding however being preferred as it results in a ilrm and secure bond.

In Fig. 1 I have shown my preferred method of providing a thicker foil, comprising the use of layers of foil, one layer on each side of the thin foil being shown at 2, and a plurality of layers on each side of the thin foil being shown at 3, each layer having for instance a thickness of 0.003".

The preferred arrangement of having the thin foil terminate between layers of heavier foil produces an extremely satisfactory and positive electrical contact between the foils.

In Figs. 1 and 2 I have shown the seal of an electrical discharge device of the mercury vapor type in which the heavy foil 2 leads to an electrode within the discharge device indicated at l', which electrode is carried by the foil 2. The thin foil I would be unable to carry the heavy electrode indicated at l due to lack of suiilcient strength and due to overheating which would take place as hereinbefore described if the thin foil were to emerge into an evacuated or gas or vapor filled vessel. The heavy foil 3 serves as connection for the electrical current for the operation of the electrical discharge device.

The end portions of the heavy foils 2 and 3 may overlap the ends of the thin foil I for a short distance as shown in Fig. 1. The electrical conductor is sealed into the quartz 5 or other vitreous material in any well-known manner, the vitreous material covering the thin foil I and the inner end portions of the heavy foils 2 and 3, including the overlapping parts thereof, as indicated in Fig. 1 at 6. The quartz or other vitreous material 5 forms a vacuum-tight seal with the thin foil I along the area of contact therewith but does not necessarily form a vacuum-tight seal along the area of contact with the heavy foils 2 and 3, the it between such heavy foils and such vitreous material being, however, quite snug, thus assisting in cooling the junction.

While the thickness of a thin foil of metal having a relatively high coefficient of expansion, such as molybdenum or tungsten, used in connection with a vitreous material having a relatively low coefllcient of expansion, such as quartz, is thus limited to a maximum of about 0.0008" the width of such foil is not so limited and will, in case of electrical discharge devices, comprise for instance from 0.02 to 0.18 and more. The width of the heavy foil will usually be of the same order as that of the thin foil. 'Ihe electrical conductor must have full rated current carrying capacity and the width of the thin foil and the width and thickness of the heavy foil are, therefore, selected so as to furnish suflicient current carrying capacity without overheating. I have found, for instance, in the case of conductors of molybdenum sealed into electrical discharge devices having an envelope of quartz and a filling of inert gas and mercury vapor that such conductors having a width of 0.040", and a thickness of 0.0006" for the thin foil and of 0.003" for four layers each of heavy Ioil leading to the thin foil, will carry a current of 2 amperes ontinuously and with 100% overload for at least several minutes; in another case a conductor of identical construction and dimensions except for a Width of 0.080" will carry a current of 4.5 amperes continuously and with 100% overload for about 8 minutes, whereas a conductor having a width of 0.120" will carry a current of 'I amperes continuously and a current of 15 amperes for about 10 minutes; still another conductor of the same type but having a width of 0.180 will carry a current of l2 amperes continuously. The total thickness of the foil or layers of foil leading from the thin foil into an electrical discharge device lled with gas or metal vapor or both, need not be as great as that of the foil or layers of foil leading to the thin foil, and if layers of identical thickness are used for both heavy foils there need not be as many layers leading from the thin foil as layers leading to the thin foil, a fact which I have indicated in Fig. l, due to the smaller tendency to heat up in a gaseous or vaporous medium than in air. The heavy foil leading into the electrical discharge device may serve as a current lead-in connecting mercury pool electrodes or solid or fllamentary electrodes, or it may perform a dual function of current lead-in and support for electrodes as indicated in the drawing at 1.

The complementing of the thin foil with heavy foil results in producing an electrical conductor having great current carrying capacity, excellent contact between the foils and possessing long life. Such conductors also have a great degree of ilexibility, which is particularly noticeable when the heavy foil consists of individual layers as shown in the drawing at 2 and 3. I do not wish to limit myself to the use of two or four layers as shown in the drawing, as a lesser or greater number can be used particularly for the purpose of increasing still further the flexibility of the conductor in which case a greater number of foils may be used accompanied with a decrease in the thickness of the individual layers.

My electrical conductor may be used on any vessel of non-porous vitreous material where a tight seal is desired. When used in connection with vapor electrical discharge devices it is often necessary to provide some current hunting means in the circuit, for instance in the form of an external impedance or resistor or of a resistance built into a vessel itself. as otherwise the current may exceed the safety limits for the thin foil thus causing rupture of the seal. It shouldalso be understood that while I have described my invention in detail in connection with conductors of molybdenum and the like sealed into quartz and the like, my invention may be used also in connection with conductors of other metals sealed into a.

great variety of other glasses. It should also be understood that the term metal as used hereinand in the appended claims shall include alloys containing the metals specified.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent, is:

l. An electrical conductor hermetically sealed into non-porous vitreous material, consisting of thin foil rmy secured at each end to at least two foils of substantially greater combined thickness partly overlapping such thin foil, and having the vitreous material vacuum-tightly fused against the free portion of the thin foil and fused firmly against the adjacent portions of heavy foils.

2. An electrical discharge device having an envelope. of non-porous vitreous material, provided with lead-in conductors sealed through the envelope, at least one of which is in the form of thin foil sealed vacuum-tightly into the vitreous material and connected at each end to overlapping layers of heavy foil partly sealed iirmly into the vitreous material and partly emerging at one end into air to serve as current connecter and partly emerging at the other end into the envelope and serving as electrode lead-in connecter.

3. In an.y electrical discharge device a vacuum tight seal of non-porous vitreous material having a coeillcient of thermal expansion below 3.0X 10-6 for an electrical conductor of refractory high fusing metal having a coeilcient of thermal expansion in excess of 3.0 10, in which the conductor consists of thin foil connected at each end to overlapping layers of heavy foil of substantially greater combined thickness, and having the vitreous material vacuum-tightly fused against the free portion of the thin foil and fused firmly against the adjacent portions of heavy foils.

4. In an electrical discharge device a vacuum tight seal according to claim 3, in which the thin foil has a. thickness not in excess of 0.0008".

5. An electrical discharge device having an envelope of non-porous vitreous material having a coeicient of thermal expansion below 3.0 106, provided with lead-in conductors of refractory high fusing metal taken from the group comprising tungsten and molybdenum sealed through the envelope, at least one of which is in the form of thin foil sealed vacuum-tightly into the vitreous material and connected at each end into operlapping layers of heavy foil partly sealed firmly into the vitreous material and partly emerging at one end into air to serve as current connecter and partly emerging at the other end into the envelope and serving as electrode lead-in connecter.

WILLIAM T. ANDERSON. Ja.

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