US3515929A - Short arc lamp seal - Google Patents

Description

June 2, 1970 p, K N ET AL 3,515,929

SHORT ARC LAMP SEAL Filed Jan. 24, 1968 FIG.

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WITNESSES United States Patent US. Cl. 313-219 6 Claims ABSTRACT OF THE DISCLOSURE A quartz short-arc lamp having a central bulbous envelope portion and a pair of oppositely disposed arm portions extending from the central envelope portion through which electrical current is carried to a pair of spaced electrodes. An electrode is mounted within one end of each arm and extends into the envelope portion. A lead-in conductor extends into the other end of each of the arms and is connected to the electrode mounted in that arm by a plurality of ribbon conductors. The juncture of the ribbon conductors with each lead-in conductor is encapsulated in a sealing composition having a coefiicient of thermal expansion less than that of quartz and preferably of lithium-aluminum-silicate powder and a suitable binder such as platinum paint.

BACKGROUND OF THE INVENTION In short-arc type lamps what is known in the art as a ribbon seal is employed to pass the current from the leadin conductors to their associated electrodes. Very often, due to lamp design or lamp application, a small portion of this ribbon seal is exposed to air. The sealed ribbon is always very thin, usually less than 0.002 inch in thickness and when the lamp is operated, in many instances, this exposed portion operates at a temperature high enough to cause the ribbon to oxidize. This oxidation continues to occur during the time the lamp is in operation and eventually the oxidation will work its way completely through the ribbon causing the ribbon to sever thereby occasioning discontinuity and lamp failure. The significant reduction in lamp life due to failure through oxidation is a problem which must be solved in the interest of providing a long lived short-arc lamp.

Attempts have been made to solve this problem by employing solder glass and other materials to accomplish the sealing off from the atmosphere of the ends of the ribbon seals at their point of association with the leadin conductor. Even though this point of interconnection as well as the outwardly extending portions of the ribbon seal are initially completely sealed 01f from the ambient atmosphere, through operation of the lamp and particularly during starting and stopping, because of the very high expansion coefficient of most of these materials, the expansion and contraction causes a working loose of the sealant and the material tends to push itself out of the opening or crevice and away from the ribbon and is subsequently replaced by air which eventually attacks the very thin walled ribbon.

SUMMARY OF THE INVENTION It is an object of this invention to eliminate oxidation of the ribbon seals in a quartz short-arc lamp.

Another object of this invention is to provide an airtight seal for the lead-in conductor and ribbon seal juncture area in a quartz short-arc lamp.

A further object of the present invention is to provide, in a quartz short-arc lamp, an encapsulating sealant having a low coeflicient of expansion to seal-01f the external ends of the lamp.

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These and other objects are accomplished in accordance with the present invention by providing a novel seat in a conventional quartz short-arc lamp. The lamp includes a central envelope or bulb portion and two outwardly extending arms. Each arm has an initially open end portion remote from the envelope and a pair of oppositely disposed electrodes mounted within the other end of the arm and extending into the envelope. A lead-in conductor extends into the open end of each of the arms and is connected electrically by means of conventional ribbon conductors to the electrode in that arm. A ceramic sealing composition is disposed wtihin the open end of the arm and encapsulates the connection between the ribbon conductors and the lead-in conductor. The ceramic sealing composition may be formed from a mixture of lithium-aluminum-silicate powder and a suitable binder such as for example platinum paint.

BRIEF DESCRIPTION OF THE DRAWING The foregoing objects, and others, along with many of the attendant advantages of the present invention will become more readily apparent and better understood as tthe following detailed description is considered in connection with the accompanying drawings in which:

FIG. 1 is an elevational View of one of the duplicate halves of a quartz short-arc lamp with a portion of the sealed-in end thereof encompassing the present invention broken away;

FIG. 2 is a sectional view taken along the line IIII of FIG. 1;

FIG. 3 is a sectional view taken along the line III--III of FIG. 2; and

FIG. 4 is an end view of one of the arm portions of the short-arc lamp with a portion of the sealing composition broken away.

DESCRIPTION OF THE PREFERRED EMBODIMENT For purposes of illustration FIGS. 1 and 2 illustrate only one of the duplicate halves of a short are lamp constructed in accordance with the present invention. The short arc lamp generally designated 10 includes a central spherical or bulbous vitreous envelope portion 12 preferably of quartz which normally encloses an arc sustaining atmosphere of mercury or Xenon but may include any are sustaining atmosphere and into which oppositely disposed electrodes 14 extend. The electrodes 14 are mounted in oppositely disposed arms 16 which generally consist of an outer hollow quartz element 18 and a relatively solid inner quartz dowel or body member 20. The electrodes 14 and lead-in conductors 22 are inserted into retaining slots 24 in the body member 20. The ribbon conductors 26 connect each electrode 14 with its associated lead-in conductor 22 substantially in the manner described in detail in US. Pat. 2,682,009 to H. D. Frazer for Seal and Method of Fabrication, issued June 22, 1954, and owned by the assignee of the present invention. The ribbon conductors 26 generally consist of two end sections 28 which may be of a thickness of about 0.002 inch connected by an intermediate section 30 which is approximately 0.0006 inch thick with the thicker end sections connecting the ribbon conductors to the electrode and lead-in conductor. Over the ledge defined by the intersection of the end of end sections 28 and intermediate ribbon 30 are welded a thin flap of molybdenum 32 or other suitable material of a thickness equivalent to the intermediate section 30. It has been found that cracks will form in the low thermal expansion glass such as quartz if it is fused onto a ledge of a greater than 0.001 inch depth. Since in the disclosed embodiment the ledge formed by the juncture of the intermediate section and the end sections of conductors 26 is over 0.001 inch in depth the flap 32 is used to gradually reduce the gradient and act as a cushion and thus prevent formation of cracks in the low thermal expansion glass at the ledge. In order to secure the conductive ribbons 26 within the arm 16 the outer cylindrical portion 18 is fused to the inner dowel member 20 by heating to a suitable deformation temperature.

FIG. 3 illustrates a sectional view through the juncture of the end section 28 with the intermediate ribbon 30 and the overlying molybdenum flap 32. When the outer cylindrical member 18 is fused to the inner dowel member 20 about these junctures in the ribbon seal 26 a raised or humped appearance indicated at 34 in FIGS. 1 and 3 is formed. 1 r

The critical points where oxidation as above-described may occur, if the ends of the arms 16 are not properly sealed, is that portion of end sections 28 which overly the end of the dowel member 20 and the interstices between and around the juncture at 36 where the ribbon conductor 28 enters the space between the dowel 20 and outer cylindrical member 18 adjacent the ends of intermediate ribbon 30 at the molybdenum flap 32. With the coefficient of expansion of quartz being about 5 10-"', to provide a good seal at the ends of the arms, it is contemplated by the present invention to use a sealing material which has a coefficient of expansion which approximates that of the quartz or preferably which is just slightly less than that of quartz.

In accordance with the present invention a mixture of lithium-aluminam-silicate powder for example that which is manufactured under the trade name Lithafrax by the Refractories Division of the Carborandum Corporation is one suitable powder. The fired, ground lithium-aluminum-silicate powders are mixed with a small amount of binder such as for example platinum paint, sodium silicate, potassium silicate, nitro-cellulose, or other similar binder type material. The amount and type of binder necessary is not critical but it is preferable that it be sufiicient to provide a paste like consistency to the mixture. Platinum paint, however, is preferred. In sealing ofi the lamp, a

small amount of the binder alone is put into the crevice or opening 36 to serve as a leader for the powder and binder mixture 40. The mixture of the lithium-aluminum-silicate powder and binder is then placed in the crevice or opening 38 to fill that opening. The material 40 is then warmed for a few moments by a heat gun in order to partially set the sealant and prevent it from falling out of the lamp. When the lamp is operated the sealant material will set itself completely. The coefficient of thermal expansion for the lithium-aluminum-silicate powders range from 5 10- to+3 10- which is slightly less than the coefiicient of thermal expansion of the quartz. When the lamp is operated the sealant is not alternately squeezed and pushed out of the crevice 38 to permit air to get to the conductive ribbons and cause oxidation as is the case with other high coeflicient of thermal expansion materials. Lamps constructed in accordance with the present invention have been operated at temperatures in excess of 300 C. and after 1200 hours of operation no failure has occurred and no oxidation could be detected.

As will be apparent from the foregoing the lamp of the present invention employs a ribbon type seal which regardless of lamp application prohibits the exposure to air of the ribbon conductors and eliminates the occurrence of any oxidation of the ribbons which can eventually cause failure of the lamp. Ribbon sealed lamps may now have a longer life since the sealant material no longer, due to generally very high expansion coeflicients, is pushed out of the crevice and away from the ribbons and replaced by air.

Since numerous changes may be made in the above described apparatus and different embodiments of the in vention may be made Without departing from the spirit thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

We claim as our invention:

1. A short-arc lamp comprising:

a central bulbous quartz envelope portion and a pair of oppositely disposed arm portions extending from said envelope portion;

an electrode mounted within one end of each arm portion extending into said envelope portion;

lead-in conductors extending into the other end of each of said arm portions;

a plurality of ribbon conductors electrically connecting said lead-in conductor to said electrode in each of said arms; and

a ceramic sealing composition surrounding the juncture of the ribbon conductors 'with each lead-in conductor and encapsulating said juncture, said sealing composition having a coefiicient of thermal expansion equivalent to or less than the coefiicient of thermal expansion of said quartz.

2. A short-arc lamp according to claim 1 wherein said sealing composition consists essentially of a lithiumaluminum-silicate powder and a binder material.

3. A short-arc lamp according to claim 2 wherein said binder material is platinum paint, sodium silicate, potassium silicate or nitrocellulose.

4. A short-arc lamp comprising:

a quartz envelope having outwardly extending arm portions and a central bulbous portion,

an electrode mounted within one end of each of said arm portions and extending into said bulbous portion,

a lead-in conductor extending into the other end of each of said arm portions,

a plurality of ribbon conductors within each arm portion electrically connecting one of said electrodes to one of said lead-in conductors and a ceramic sealing composition disposed within the said other ends of said arms encapsulating the connection between said ribbon conductors and said lead-in conductors, said ceramic material having a coeflicient of thermal expansion equivalent to or less than that of said quartz envelope.

5. A short-arc lamp according to claim 4 wherein said sealing composition consists essentially of lithium-aluminum silicate powder and a binder.

6. A short-arc lamp according to claim 5 wherein said binder is platinum paint, sodium silicate, potassium silicate or nitrocellulose.

References Cited UNITED STATES PATENTS 2,682,009 6/1954 Fraser 313220 3,211,826 10/1965 Holcomb et al. 17450.64

FOREIGN PATENTS 818,202 8/1959 Great Britain.

RAYMOND F. HOSSFELD, Examiner US. Cl. X.R.

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