US2675497A - Quartz metal seal - Google Patents

Description

April 13, 1954 G. MEISTER ET AL QUARTZ METAL SEAL Filed Feb. 2'?, 1951 FEMA/rs al /f/xruze or .5% 7677710? Patented Apr. 13, V1954 ff l 2,675,497

QUARTZ METAL SEAL George Meister, Newark, and John E. Schiveree,

Bloomfield, N.

Electric Corporation, corporation of Pennsyl J., assignors to Westinghouse East Pittsburgh, Pa., a

Vania.

Application February 27, 1951, Serial No. 212,996

4 Claims.

This invention relates to seals for electron discharge devices and lamps and the like and, more particularly, to those between quartz of fused slica and treated minerals and those between metal and treated minerals.

High pressure vapor discharge lamps, long known to the art, operate at both high pressure and high temperature. Customarily the enclosing envelope of such a lamp is formed of quartz or a vitreous material having a high silica content to withstand the necessary high operating temperature and pressure. Because of the relatively high currents carried by the lead-in conductors for the electrodes and the resulting high temperatures they achieve during operation, said lead-in conductors are generally confined to refractory metals, such as tungsten and molybdenum, having melting points materially higher than the melting point of quartz. To make an air-tight seal directly between the lead-in conductor and the quartz envelope has long been an exceedingly diiicult problem because of the great difference in the coefficients of expansion. It has been usual practice in the past to make several intermediate-stage or graded seals between the lead-in conductor and the quartz envelope; as for example, fusing the conductor to a glass having a coefficient of expansion near that of the conductor, then fusing to this glass a second glass having a coefficient of expansion nearer to that of quartz than the first, and repeating this last operation until the final glass will seal to the quartz. Other forms of seals for joining a refractory metal conductor to a quartz envelope have been proposed, such as a single step seal or a direct fusion of the quartz to the conductor, but these seals have proven inadequate in actual practice.

Hence, it has been found advantageous, according to our invention, to employ a mineral known as petalite, primarily a lithiated aluminum silicate, for sealing to metals such as tungsten and also to fused quartz or a vitreous material having a high silica content such as Vycr. Natural powdered petalite, after it has been heated to 1100 C. for one hour and then fused, has practically the same coefficient of thermal expansion as fused silica or fused quartz and is substantially free of gas. Further heat treatment of the petalite at temperatures above 1325 C., degasifies the petalite and does not apprecably alter the thermal coefficient of expansion. The fused petalite is then readily beaded in a reducing ame onto, for example, a clean unoxidized tungsten lead-n conductor, having a silvery white metallic appearance. Despite the difference in the coei'licients of expansion of the tungsten and the petalite, a vacuum-tight seal of low stress is nevertheless formed, wherein our invention is contained, due, it is believed, to the presence of alkaline earth oxides in the petalite or the degasification of the petalite, or a combination of both. The petalite bead may be fur' ther built up to form a flare for sealing direct- 1y to the fused quartz envelope. This seal between two vitreous materials having approximately the same thermal coefficient of expansionV is easily consummated.

An alternative embodiment of the invention is the manufacture and use of synthetic petalite compositions, wherein a more favorable Valuminasilica ratio can be obtained for glass formation or the lithium may be partially or substantially replaced by alkaline earth metals, such as of beryllium or magnesium.

A further alternative embodiment of our invention is the preparation and use of mixtures of fused petalite, natural and synthetic, and fused quartz as a single seal between refractory type metal lead-in conductors and quartz.

Another alternative embodiment of our invention is the preparation and use of mixtures of fused petalite, natural and synthetic, and fused quartz as a single step seal between quartz and a second mixture of fused petalite and fused quartz.

In its general aspect, the present invention has the objective of overcoming the difficulties and defects of the prior art seals between refractory metals and fused quartz.

A specific object of the invention is the elimination of expensive graded seals between refractory metals metals and fused quartz.

A further object of the invention is the method of sealing degasied fused petalite mineral to refractory metals and to fused quartz.

Another object of the invention is the method of sealing degasied fused synthetic petalite of predetermined alumina-silica ratio and alkaline earth oxide content to refractory metals and to fused quartz.

Another object is the method of sealing predetermined mixtures of degasifled fused petalite, natural and synthetic, and fused quartz to refractory metals and to fused quartz.

A still further object of the present invention is the formation of a seal between a refractory metal and fused quartz by employing an intermediate bead of fused degasii'led petalite.

Another object of the present invention is the formation of a seal between a refractory metal and fused quartz by employing an intermediate bead of fused degasiiied synthetic petalite of predetermined alumina-silica ratio and alkaline earth oxide content.

Another object of the present invention is the formation of a seal between a refractory metal and fused quartz by employing an intermediate bead of a predetermined mixture of fused degasiiied petalite, natural or synthetic, and of fused quartz.

A further object of our invention is a single step seal comprising a mixture of fused degasined petalite and fused quartz between quartz and a second mixture of fused degasified petalite and fused quartz.

Other objects of the invention willV appear to those skilled in the art to which it appertains as the description proceeds, both by direct recitation thereof and by implication from the context.

In the accompanying drawing, in which like numerals of reference indicate similar parts throughout:

Fig. 1 is an elevational view of a gaseous discharge device of the high pressure mercury vapor type;

Fig. 2 is an elevational view of the inner envelope of the lamp of Fig. l, having a fragmentary portion thereof cut away to show the seal of our invention;

Fig. 3 is an elevational view showing an alternative embodiment of our invention;

Fig. 4 is an elevational view similar to Fig. 2, showing a further alternative embodiment of our invention;

Fig. 5 is an elevational view similar to Fig. 2, showing a still further alternative embodiment of our invention;

Fig. 6 is an elevational view of a refractory lead-in conductor beaded according to our invention, and showing the bead in section; and

Fig. '7 is an elevational View of a beaded refractory lead-in conductor and electrode assembly.

Referring now to the drawing in detail, a discharge device such as a high pressure mercury vapor lamp device I!) is shown in Fig. 1, as one practical embodiment of the present invention, as used in an electric translation device. While a vapor lamp is shown as a specic embodiment of the present invention, it will be understood that the present invention is not restricted solely to use with vapor lamps but may be used also with any lamp or electronic tube wherein a vacuumtightseal is required. This lamp device i cornsimilar to Fig. 2, of the seal prises an enclosing vitreous envelope ll pervious` to ultraviolet radiations or not, as desired. Secured to this envelope is a base i2 of the usual type to enable the lamp El) to be screwed into a conventional socket and thus connected to a source of supply of suitable household potential. The envelope l l is sealed at its based end to a reentrant stem i3 having a press i5, to which is sealed a pair of lead-in conductors I and I6 of rod-like form diverging immediately above the stem press and then extending in parallel relation longitudinally of the enclosing envelope Il. A pair of bridges il and S8 of suitable insulating material connect the leading-in conductors l5 and I8 together to form a rigid mount for a discharge lamp proper or arc tube iS.

, In Fig. 2, this lamp i9 as shown is of the high pressure type and comprises an envelope 2o of vitreous material, such as quartz or the like, having a high melting point so as to withstand the operating temperature of the lamp. Disposed at the ends of the envelope 2li are electrodes 2l and 22, which are supported by tungsten leading-in conductors 23 and 24 respectively. These conductors 23 and 26 are hermetically sealed to, and axially of, the envelope 28. They are connected respectively at their outer extremities to the rodlike leading-in conductors l5 and l5 by flexible connectors 25 and 2B.

To facilitate the starting of lamp le, a starting electrode 2l, shown in Fig. 6 separate from the lamp, is sealed also according to our invention to envelope 20, adjacent one of the main electrodes, and is connected through the usual impedance 28 to the leading-in conductor i5.

During the usual exhaust, lamp le is baked, electrodes 2l and 22 are treated, and envelope 2li is evacuated. The lamp is then filled with an ionizable medium, such as mercury vapor of just suicient quantity as to become completely vaporized and to produce a pressure of one to several atmospheres during operation of the device, and, in addition, a small quantity of rare or inert gas may be introduced prior to sealing off at the tip 29, to facilitate starting said lamp.

According to our invention, natural or synthetic petalite and mixtures of said petalite and glasses of low expansivity such as quartz, are vacuum-tight sealed to refractory lead-in conductors such as conductors 23, 2A and 2l' and also to the envelopes, such as quartz envelope 2e. The mineral petalite is the most recently discovered commercial source of lithium and its molecular formula is still in doubt. Recent evidence indicates that the ratio of lithia-alummasilica varies from 111:6 to 121:8. The approximate calculated percentages by weight for each of the two molecular formulae are shown below:

wenn

Since the favorable alumina-silica ratio in petalite is conducive to glass formation, small additions of powdered silica and alkalies of alkaline earths, including beryllium and magnesium oxides may be made to the natural petalite powder before processing to improve said silica ratio and the tungsten sealing properties of the petalite. Also, synthetic compositions of petalite, substantially the same as mineral petalite, may be made by mixing and fusing together at high temperature, as hereinafter described, powdered mixtures containing a desired lithia-aluminasilica ratio favorable to the formation of glass with a high temperature softening point and low expansivity.

To prepare the natural or synthetic petalite for sealing to a leading-in conductor, such as conductor 2l, pulverulent approximately 200 mesh petalite is pressed into rods about 1/4 in diameter and red uniformly and gradually to a temperature slightly greater than 1325 C. in an Oxy-hydrogen flame until the petalite is substantially degasied, as indicated in the absence of bubbles in the cooled mass, or until the petalite has formed a uniform mass of clear glass which will subsequently seal to quartz and to tungsten.

The rods may also be formed as practiced in the art of making fused quartz by dipping a hot `5 fused quartz rod into the iinely divided petalite powder and thus building up the petalite on the end of the quartz rod by proper manipulation to obtain a clear fused petalite glass.

Around 1000 C., petalite begins to dissociate into beta spodumene and a siliceous glass. Beta spodumene is the heat treated high form of the alpha or crystal (mcnoclinic) low form spodumene, a naturally occurring lithia mineral hav-` Firing at 1100 C. causes the coefficient of expansion of the petalite to change from about 19 107 in./in./ C. to approximately 5.5 107 in./in./ C., which is the same as that 0f fused silica, thereby indicating the above-mentioned dissociation is in progress.

Additional heat treatment at 1250o C. makes the expansivity appreoiably lower than that of fused silica. Firing to 1325 C., at which temperature the petalite becomes fused into glass or glass-like appearance, produces a slightly lower expansion than the 1250 C. heat treatment, and indicates that decomposition of the petalite is practically complete. The result of this decomposition is the low expansion silica residue and spodumene crystals of only. slightly greater expansion, both of which are substantially free of gas.

If the processed petalite rod is not heated above 1350 C. the temperature at which it begins to fuse and lose its shape, it Will retain its original configuration and can be used for beading a lead-in conductor such as conductor 23.

Petalz'te Said conductor 23 is cleaned for beading with the processed petalite rod by being electropolished in a suitable hot tri-sodium phosphate solution, or by being chemically cleaned with a sodium nitrite stick and the application of heat. After cleaning, the tungsten conductor 23 has a silvery White metallic appearance and is free from oxide. By heating the processed petalite rod and conductor 23 in close proximity slightly above 1350 C. in an Oxy-hydrogen flame to prevent oxidation of said conductor, and manipulating the rod about the conductor, a petalite bead 30 of general elliptical cross-section may be formed on this conductor as shown particularly in Fig. 6. As long as the oxy-hydrogen ame prevents oxidation of the conductor 23 during the beading, the processed petalite Will adhere to said tungsten despite the differences in expansivity, namely approximately 44 10'z in./in./ C. for tungsten and about 5.0 107 in./in./ C. for the processed petalite.

In turn, after electrode 2| has been aiiixed to conductor 23 as shown in Fig. 7, the petalite bead 30 on conductor 23 is readily sealed to the quartz envelope 20 which has approximately the same coefcient of expansion, namely, 5.5)(-7 in./in./ C. as said petalite bead.

In like manner, as described above, lead-in conductor 24 is sealed to a similar petalite bead 3l, and electrode 22 is afxed to an extremity of said conductor. Similarly, a petalite bead 32 is 6? formed on starting electrode '21,fas 'shown' in Fig. 6. Simultaneously, the beads 3| and 32 on conductors 24 and 21 respectively, are then sealed adjacently into quartz envelope 20, as shown in Fig. 2, thereby completing the unexhausted formof lamp I9.

In the usual manner, lamp proper i9 is exhausted as explained above, sealed into the enclosing envelope Il of high pressure mercury vapor lamp device l0, said lamp l0 is exhausted, and based. The exhaust of said lamp I 0 may consist of a bake, final evacuation of envelope l I and final tip off at 33.

Mixtures of petalite cmd quarte Further, according to our invention, a leadin conductor 23a may be beaded, as shown in Fig. 3, with a bead 30a consisting of a mixture of petalite and quartz. Suitable 400 mesh petalite and quartz ground in a sapphire mortar may be thoroughly mixed, pressed into a suitable rod, and then heated at a temperature greater than 1350 C. in an Oxy-hydrogen flame until the mixture is substantally degassed and sufficiently processed to form a uniform rod of clear glass as explained above. The cleaned tungsten leadin conductor 23 is then beaded with the petalite-quartz mixture rod to form bead 33a, and said bead 30a is sealed in turn to envelope 20a as before. In like manner, lead-in conductor 25a and conductor 21a are beaded with beads 3i and 32a, respectively, and sealed into` the other end of envelope 20a.

The following mixtures of quartz and petalite were investigated, as sealing glasses to tungsten and to quartz and formed a vacuum-tight seal in each instance:

[Percentage by weight] 90% petalite and 10% quartzpetalite and 20% quartz 70% petalite and 30% quartz 60% petalite and 40% quartz 50% petalite and 50% quartz 30% petalite and 70% quartz 20% petalite and 80% quartz 10% petalite and 90% quartz4 Mz'tures of petalite and other high temperature low epansion glasses Instead of quartz, according to our invention, other high temperature softening low expansivity glasses, such as Code No. 723, Corning 707GS1 glass (expansion 14x10-7 in./in./ C.) may be used to form a mixture. Code No. 723 glass is a borosilicate glass normally used in a graded seal between multiform glass, Corning No. 7932, and Corning No. 724A() glass. Vycor is the trade name for a 96% silica glass and is sometimes called synthetic quartz. In Fig. 4 a leading-in conductor 23b is shown with a bead 30b of a mixture by Weight of, for example, 50% petalite and 50% Code No. 723 glass and sealed to envelope 20". In a like manner, lead-in conductors 22b and 27b are beaded with similar beads 3lb and 32b respectively and sealed adjacently into quartz envelope 2Gb.

M ac-ture step seals When employing a mixture of petalite and quartz having a petalite Icontent less than 10% by weight to form a bead 30 on a conductor, such as conductor 23c in Fig. 5, it has been found desirable, according to our invention, to use a ring 34 of general frusto-conical configuration consisting of a second mixture of petalite and quartz as a step seal between the quartz envelope 20e and said bead 30. In actual practice, contrary to some allegations, pure quartz does not readily seal to tungsten unless extreme care is observed. Unless the step seal ring 34, suitably for example 30% petalite and 70% fused quartz, is sealed first to quartz envelope 20c and then to bead 30C, which may comprise 5% petalite and 95% fused quartz, and bead 30C on yconductor 23 will pull away from the conductor and crack.

Thus it will be seen from the foregoing description that the present invention has eliminated the expensive graded seals between refractory metal lead-in conductors and quartz envelopes. Specifically, our invention is the use of substantially degasified and suiciently processed fused natural or synthetic petalite for sealing in a protective ame to a tungsten leadingin conductor, such as `conductor 23 to a quartz envelope, such as envelope 20, as shown in Fig. 2. Eturther, instead of petalite, mixtures of substantially degasified fused natural or synthetic petalite and suitable low expansion high temperature softening glasses such as Code 723, fused quartz, or Vycor may be used for the same purpose. Also, in petalite-quartz mixtures wherein the petalite content is below 10% by weight it is necessary to employ a step seal ring 34 between quartz envelope 20C and the bead 36 which may have a composition of 5% petalite and 95% quartz.

Although preferred embodiments of our invention have been described, it will be understood that modifications may be made within the spirit and scope of the appended claims.

We claim:

1. A transition glass for making seals to fused quartz envelopes and refractory leading in conductors comprising substantially degasied mineral petalite dissociated into at least beta spodumene, said beta spodumene having about 6.78% LiOz, 28.42% A1203, 62.91% S102 and 1.89% alkali metals such as MgO and NazO.

2. A transition glass for making seals to fused quartz envelopes and refractory leading in conductors comprising a mixture of substantially degasied quartz and the mineral petalite dissociated into at least beta spodumene, said beta;

spodumene having about 6.78% LiOz, 28.42% A1203, 62.91% SiOz and 1.89% alkali metals such as MgO and NazO, said mixture comprising from 10% to 90% by Weight quartz and the balance petalite.

3. A seal for a high pressure gaseous discharge device comprising a lead in conductor assembly having a refractory metal lead in conductor and an electrode thereon, a fused quartz envelopel and a transition glass bead on said conductor and between said conductor and said envelope, said transition glass bead comprising a mixture of substantially degasied high temperature borosilioate glass and the mineral petalite dissociated into at least beta spodumene, said beta spodumene, having about 6.78% LiOz, 28.42% A1203, 62.91% Si02 and 1.89% alkali metals such as MgO and NazO.

4. A seal for a high pressure gaseous discharge device comprising a lead in conductor assembly having a refractory metal lead in conductor and an electrode thereon a fused quartz envelope, a step seal ring sealed at one end to said envelope, said ring comprising a mixture of 30% substantially degasil'led quartz and the balance of substantially degasified petalite dissociated at least into beta spodumene, and a transition glass bead on said conductor and between said conductor and said ring, said transition glass bead comprising a mixture of 95% substantially degasied quartz and the balance the mineral petalite dissociated into at least beta spodumene.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 1,558,524 Winninghof Oct. 27, 1925 1,716,140 Kirwer June 4, 1929 2,090,098 Berger Aug. 17, 1937 2,116,429 Gooskens May 3, 1938 2,330,072 Meister Sept. 21, 1943 2,340,460 Eitel Feb. l, 1944 2,414,504 Armistead Jan. 21, 1947

Claims (2)

1. 3. A SEAL FOR A HIGH PRESSURE GASEOUS DISCHARGE DEVICE COMPRISING A LEAD IN CONDUCTOR ASSEMBLY HAVING A REFRACTORY METAL LEAD IN CONDUCTOR AND AN ELECTRODE THEREON, A FUSED QUARTZ ENVELOPE AND A TRANSISTION GLASS BEAD ON SAID CONDUCTOR AND BETWEEN SAID CONDUCTOR AND SAID ENVELOPE, SAID TRANSITION GLASS BEAD COMPRISING A MIXTURE OF SUBSTANTIALLY DEGASIFIED HIGH TEMPERATURE BOROSILICATE GLASS AND THE MINERAL PETALITE DISSOCIATED INTO AT LEAST BETA SPODUMENE, SAID BETA SPODUMENE, HAVING ABOUT 6.78% LIO2, 28.42% AL2O3,
2. 62.91% SIO2 AND 1.89% ALKALI METALS SUCH AS MGO AND NA2O.

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