US2251062A

US2251062A - Hermetic seal

Info

Publication number: US2251062A
Application number: US363190A
Authority: US
Inventors: Lindwurm Leopold; Thouret Wolfgang
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1939-10-14
Filing date: 1940-10-28
Publication date: 1941-07-29
Anticipated expiration: 1958-07-29
Also published as: NL57687C

Description

y 1941- 1 LmbwuRM ETAL 2,251,062

HERMETI C SEAL Filed Oct. 28, 1940 lnven kcjrsz Leopold Lindvvur'm,

oL'F an Thouret b I WW Their" At'torneg.

Patented July 29, 1941 UNITED STATES PATENT OFFICE HERMETIC SEAL Leopold Lindwurm, Berlin,

and Wolfgang Thouret, Berlin-Wilmersdorf, Germany, assignors to General Electric Company, a corporation of New York Application October 28, 1940, Serial No. 363,190

- In Germany'octoher 14, 1939 2 Claims. (CL 176-126) The present invention relates to hermetic, electrically conducting seals for gaseous electric discharge devices and more particularly to seals 1 such as a metal foil. fused into a vitreous body having a low coemcient of expansion.

- comprising a thin, flat. elongated metal member,

The vitreous part of the seals to which the pres:

ent invention'pertains is slender and cylindrical in shape and the thin metal foil or strip is mounted with its largest dimension in or parallel to the longitudinal axis of the vitreous seal part. A pair of wires mounted in the same straight line in or parallel to the longitudinal axis of the vitreous part and passing through said .foil or strip is attached to the edge portions oi the foil in opposingpositions. Said wires terminate beyond the respective ends of the vitreous seal part. The distance between the, ends of the wires fastened to the strip is appreciable.

Seals of this type are useful for gaseous electric discharge lamps having a quartz or hard glass envelope containing a starting'gas and a quantity of mercury the vapor of which is at a pressure of about 10 atmospheres or higher during the operation of the lamp. Lamps of this type are eflicient light sources of high intrinsic brilliancy.

The seals of these high pressure discharge lamps I must be strong mechanically and resistant to heat to remain gas tight under high pressure.of the vapor atmwphere and thehigh temperature The seal usually protrudes from the envelope. Frequently the seal becomes so hot in service that the part of the metal conductor external to the seal and exposed to the oxygen containing atmosphere is oxidized. ;The oxide layer thus formed gradually creeps along the conductor and into the fused joint between the metal and the vitreous parts of the seal. The Joint then becomes pervious to gas to terminate the useful life of the lamp. I

The formation of an oxide layer on the exposed part of the electrical conductor of the seal occurs only at elevated temperatures when said part consists of tungsten or molybdenum, for

example. If the exposedpart of the conductor isbelow the oxidizing temperature thereof during the operation of the lamp dimculties caused by the oxide layer are avoided. Increasing the heat dissipating capacity of the seal, such as by plac-, ing heat radiating fins-in contacttherewith, or increasing the length of the seal, are helpful but not completely effective in maintaining the exposed part of the conductor below the oxidizing temperature thereof. Such structures are disof the envelope during the operation of the lamp.

advantageous since the mounting and transportation thereby.

"The object of the present invention is to pro-. vide a mechanically strong. hermetic, electrically conducting seal of small size and of simple structure for high pressure, high temperature dis-' charge devices. is to provide an electrically conducting foil or strip member having a low resistance to current flow for use in hermetic seals. Still further objects and advantages of the invention will be apparent to those skilled in the art from the following detailed description thereof and from the appended claims. We have discovered that the high temperatur of the seal is due not only to the high temperature of the lamp envelope during operation of the lamp but also due to the heat developed in the metal foil. which is about 20 microns, thick,

because of its appreciable resistance 'to the highcurrents. conducted thereby. By decreasing the electrical resistance of the foil the heat developed in the seal is reduced and the lamp is capable of operating at higher pressures and envelopL temperatures without causing the formation of a layer oi'.,oxide on the exposed part ofthe metal conductor. The resistance of the foil is decreased by increasing the cross-sectional area of v the center part'thereof. For example, one or more longitudinal metal strips, foils or wires are mounted on the center portion of the main foil so as to make a good electrical contact therewith. Preferably the auxiliary foil or' the like is welded or soldered to the main foil. An equivalent structure is a single foil which is thicker at the center portion thereof than at the end portiona The thin end portions of the foil both make an hermetic Joint withnthe vitreous part ofthe seal.

of this specification an embodiment of the in- I vention is shown, in which Fig. 1 is a side elevational, partly sectional view of the lamp is made more complicated Another object of the invention ing therefrom and contains two electrodes which are of the self-heating type. Each of said electrodes 5 consists of a perforated hollow body of high melting point metal, such as tungsten, having a material of high electron emissivity, such as thorium oxide, therein. During the operation of the lamp the electrodes 5 are at an elevated, arc discharge sustaining temperature. Said envelope I has therein a starting gas, such as argon or neon, at a pressure of about 1 or 2 centimeters and mercury in an amount sufficient to produce an unsaturated vapor atmosphere having a pressure of about 50 atmospheres when the lamp is at operating equilibrium.

'Each of said seals 2 comprises a metal conducting element consisting of a strip or foil 3 of high melting point metal, such as molybdenum, a metal wire 4, such as a tungsten wire, attached to said foil 3 and projecting beyond the external end of the vitreous part of the seal. A similar metal wire 8 is attached to the opposite end of the foil 3 and terminates in the space defined by the envelope I where it supports the electrode 5. The foil 3 is about 15 microns thick and is reinforced at the central portion thereof by a strip 6 of high melting point metal, such as molybdenum. We prefer to weld the reinforcing strip 6 to the foil 3 but it is possible to accomplish the objects of the invention by bringing the foil 3 and the strip 6 into contact with each other. The strip 6 increases the cross-sectional area of this part of the conducting element and substantially decreases the electrical resistance thereof to minimize the amount of electrical energy transformed into heat by said foil 3. We have demonstrated that the above structure so effectively reduces the internal resistance heating of the seal that the part of the wire 4 protruding from the vitreous part of the seal is below the temperature at which an oxide coating forms thereon even when the current input of the lamp is about 500 watts and the length of the vitreous part of the seal is such that an oxide coating would form on the protruding part of the wire 4 ii said strip 6 were omitted.

The-molybdenum foil 3 shown in Fig. 2 of the drawing is similar to those illustrated in Fig. 1 except that in this embodiment the foil 3 has a plurality of longitudinal strips I which are arranged in a staggered, overlapping pattern with a substantial distance between the strip I. The hermetic juncture between the foil 3 and the vitreous part of the seal is located at the end portions of the foil 3 extending beyond the'strips I and the portions of the foil 3 not covered by the strips 1.

A thicker molybdenum foil, such as a foil having a thickness of about 20 microns, is used, when desired. In the fabrication of seals comprising comparatively thick foils it is desirable to apply a relatively high pressure to the vitreous part of the seal when in a softened condition to force it into intimate contact with the foil. Even when thicker foils are used the greater cross-sect onal area at the center portion of the foil is desirable since this structure permits the use of higher current densities in the arc discharge and the use of comparatively narrow foils.

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

1. A hermetic, electrically conducting seal comprising a vitreous element having a low coefficient of expansion and an electrically conducting element fused into'said vitreous element, said vitreous element being an elongated cylindrical body, said conducting element being a thin, fiat, elongated metal member mounted with its greatest dimension in the direction of the longitudinal axis of said vitreous element, another element of electrically conducting material mounted on the center portion of said first mentioned conducting element to decrease the electrical resistance of said center portion, the edge portion of said first mentioned conducting element forming a hermetic joint with said vitreous element.

2. An electrically conducting element for fusion with a vitreous part of a hermetic seal,

said element consisting of a thin, elongated metal foil having a plurality of elongated strips mounted adjacent the center portion thereof in a staggered, overlapping pattern to minimize the heat generated in said foil by the passage of electrical energy therethrough and a pair of wires secured in opposing positions to the edge portion of said foil.

LEOPOLD LINDWURM.

WOLFGANG THOURET.