US2114869A - Quartz-to-metal seal - Google Patents

Description

' April 19, 1938. c. BOL ET AL QUARTZ-TO-METAL SEAL Filed May 26, 1936 Fig.5.

lmventors Cornehs Bol Hehdricus J. Lemmens by 5. Their Attorney.

Patented Apr. 19, 1938 PATENT OFFICE QUARTZ-TO -METAL SEAL Cornelis B01 and Hendricus J. Lemmens, Eindhoven, Netherlands, assignors to General Electric Company, a corporation of New York Application May 26,

1936, Serial No. 81,866

In Germany August 31, 1935 3 Claims.

The present invention relates toan improved quartz-to-metal seal particularly applicable to mercury vapor electric discharge devices of the type described in copending application, Serial No. 46,952, filed October 26, 1935, by Cornelis B01, Willem Elenbaas and Hendricus J. Lemmens.

The above-mentioned application discloses a new type of mercury vapor lamp comprising a capillary quartz envelope having an internal bore of less than 7 millimeters and preferably of from 1 to 5 millimeters diameter. These lamps are adapted to be operated at an extremely high specific loading and consequent high temperatures. The loading commonly is indicated in watts per centimeter of tube length. For example, an aircooled lamp may consume from 25 to watts per centimeter, while units equipped with special cooling means may utilize as high as from to 1000 watts per centimeter with a voltage drop of from 100 to 1000 volts per centimeter.

As a consequence of these operating conditions and the particular conformation of the envelope high vapor pressures may be developed during operation, pressures commonly-in excess of 10 atmospheres and, in extreme cases, even in excess of 300 atmospheres.

In view of the high temperatures encountered it is necessary to form the envelope of a refractory transparent material of which quartz is at present considered the most satisfactory example.

' A principal dimculty encountered in the manufacture of lamps of this material consists in the provision of a durable seal for the electrode leadin connections which is capable of remaining gastight under the conditions of operation. One method which has already been proposed for the solution of this difficulty comprises interposing between the quartz and the lead-in conductor one or more transition glasses having graded expansion' coefllcients which lie between the expansion coeflicients of the quartz and the metal.

The present invention increases the mechanical strength of seals of this type and/thereby raises the permissible mercury vapor pressure by providing an improved form for the body of transition glass employed. The nature of the improvement as well as the method by which it is produced may' best be understood by reference to the accompanying drawing in which Figs. 1, 2, 'and 3 are sectional views illustrating the successive steps in the sealing process; Fig. 4 shows a complete seal formed in accordance with the invention, and Fig. 5'shows a discharge device embodying our improved form of seal.

In carrying out the invention the end of a quartz tube i, preferably of less than-about 7 millimeters internal diameter, is filled. as by suction, with a stopper or plug 2 of transition glass which may be molten when introduced. One example of a glass suitable for use with tungsten lead-in conductors is described in copending application, Serial No. 43,230, filed October 2, 1935, by Cornelis Bol, Hendricus J. Lemmens, and Gottfried B. Jonas, and comprises approximately the following composition:

Per cent SiO2 .1 88.43 B203 8.4 A1203 2.9 CaO .4

The plug should be of such length that it is in contact with the interior of the quartz tube for a distance at least one and a half times the wall thickness of the latter and preferably from 2 to 4 times such thickness.

The plug 2, whlchshould be maintained at a sufficiently high temperature to preserve its plasticity is subjected to pressure at the inner end thereof in the direction indicated by the arrow in Fig. 2. Ordinarily this will consist of air pressure provided by the glass blower and should be of such nature as to cause a cup-like depression 4 to be formed in the inner end surface of the plug. For the purposes of the present invention, it is desired that the walls of the depression comprise a tapered rim of gradually diminishing thickness fused exteriorly' to the quartz tube.

. Preparatory to sealing a lead-in conductor 5 (of tungsten or similar refractory metal) into the structure so far described, the former is embedded in a second plug or solidcylinder 6 of a transition glass which may be of the same composition as the plug 2 or of a slightly different composition to give a more satisfactorily graded seal.

Thereafter, by continuing the application of heat and pressure to the plug 2 a central aperture 3 is formed completely therethrough of such size as to match approximately the external diameter of the cylinder 6. The latter then is inserted in the aperture 3 and heat applied until fusion of the two glasses takes place. We have found that best results are obtained when the cylinder 6 is so positioned that it protrudes very slightly from the stopper 2 at its inner end, thus leaving arounded' bead or knob surrounding the conductor 5 at the region where it projects into the bore of the quartz tube. i

-meters with a wall thickness of about the same magnitude. The electrodes 5 are spaced about 20 millimeters and are shown as being surrounded adjacent the lead-in connections with small globules of mercury 8 adapted to be at least partially vaporized during operation of the lamp. A small quantity of a readily ionizable gas, for example, neon or argon, is also enclosed in the envelope to facilitate initiation of the discharge.

Tests made on lamps constructed as described in the foregoing have resulted in a surprisingly small number of rejections and indicate that the particular form specified produces a minimum of strain between the edges of the transition glass and the quartz tube. There is little or no apparent tendency for the plug to crack free from the quartz surface at the superatmospheric pressures at which the seals have'been used, even at pressures as high as 300 atmospheres.

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

1. A quartz-to-metal seal comprising a quartz tube having an inside bore on the order of a few millimeters in diameter and a wall thickness not substantially greater than the diameter of the bore, a plug of transition glass projecting into the end of said tube and in sealing engagement with the interior wall thereof for a distance in excess of 1.5 times the wall thickness of the latter. and a metal lead-in wire fused into the plug, said plug terminating at its inner end in a hollow portion having walls of gradually diminishing thickness fused to the interior surface of said tube.

2. A quartz-to-metal seal comprising a quartz tube, a sealing plug of transition glass terminating within the tube in a hollow portion having walls of gradually diminishing thickness fused to the interior wall surface of the tube, a solid cylinder of a second transition glass passing axially through said plug and slightly protruding therefrom at the inner end thereof and a metal lead-in conductor embedded in said cylinder and projecting into the bore of said quartz tube.

3. An envelope for an electric discharge device comprising a tube composed of a vitreous material which has the characteristics of quartz, plugs of transition glass fused into the ends of. said tube and lead-in conductors sealed through the plugs, each of said .plugs terminating at its inner end in a hollow portion having walls of gradually diminishing thickness fused to the interior surface of said tube.

' I CORNELIS BOL.

HENDRICUS J. LEMMENS.

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