US3708272A

US3708272A - Method of providing a lead wire seal

Info

Publication number: US3708272A
Application number: US00103755A
Authority: US
Inventors: R Hamon
Original assignee: General Electric Co
Current assignee: RCA Licensing Corp
Priority date: 1971-01-04
Filing date: 1971-01-04
Publication date: 1973-01-02
Anticipated expiration: 1990-01-02

Abstract

A method for creating an improved seal between oxide-coated lead wires and the glass envelope of an evacuated electron discharge tube. A group of lead wires are disposed in a suitable array and then surrounded by a glass blank. As the lead wires and glass are heated to soften the glass preparatory to the formation of a seal, the heat causes an increased buildup of oxide upon the side of the lead wires remote from the surface of the glass. A reducing flame is then applied to the inner surfaces of the lead wires to diminish the thickness of the undesired oxide buildup. The softened glass is then forced about the lead wires, dissolving a portion of the remaining oxide to produce a hermetic seal.

Description

Hamon [451 Jan. 2, 1973 METHOD OF PROVIDING A LEAD WIRE SEAL inventor: Richard Eugene Hamon, Syracuse, NY. 13209 1 Assignee: General Electric Company, Syracuse,N.Y.

Filed: Jan. 4, 1971 Appl. No.: 103,755

US. 0|. ..65/59, 65/32, 65/154 Int. Cl ..c03 29/00 Field of Search ..65/59, 138, 147, 154, 32

References Cited UNITED STATES PATENTS Primary Examiner-Robert L. Lindsay, Jr.

Assistant Examiner-Kenneth Schor Attorney-Frank L. Neuhauser, Oscar B. Waddell, Joseph B. Forman, Marvin Snyder and W. J. Shanley, Jr.

[57] ABSTRACT A method for creating an improved seal between oxide-coated lead wires and the glass envelope of an evacuated electron discharge tube. A group of lead wires are disposed in a suitable array and then surrounded by a glass blank. As the lead wires and glass are heated to soften the glass preparatory to the formation of a seal, the heat causes an increased buildup of oxide upon the side, of the lead wires remote from the surface of the glass. A reducing flame is then'applied to the inner surfaces of the lead wires to diminish the thickness of the undesired oxide buildup. The softened glass is then forced about the lead wires, dissolving a portion of the remaining oxide to produce a hermetic seal.

6 Claims, 4 Drawing Figures PATENTEDJAH 2 I973 3.708.272

mvnu'ron RICHARD E. HAMW ATTOR N EY METHOD OF PROVIDING A LEAD WIRE SEAL BACKGROUND OF THE INVENTION The present invention relates to the manufacture of hermetically sealed envelopes for electrical discharge devices, and, more particularly, to a method for providing an improved seal between a metallic conductive member and a glass envelope.

The problem of providing a strong, reliable seal between metallic leads and the surrounding glass of an electron discharge tube envelope has long plagued the industry. Early problems included an incompatability between the thermal coefficients of expansion of the lead wire material and of the surrounding glass. Eventually lead wire materials were formulated which had a compatable coefficient of expansion; however, providing a reliable seal between the lead wire and the glass remained a problem. One means of solving this problem was to provide a portion of the lead wire with a metallic cladding, the oxide of which is soluble in the envelope glass. During the manufacturing process, the oxidized surfaces of the lead wires are surrounded by molten glass and the outer portion of the oxidized metal dissolves into the glass to form a continuous bond from the underlying metal cladding to the glass envelope.

In one widely used method embodying the above technique, a plurality of lead wires are disposed in a desired configuration, such as a circular array. A preformed glass blank is placed about the upstanding lead wires such that the inner surface of the glass blank is in close proximity to each of the lead wires. Heat is then applied to the assembly in order to heat the lead wires and soften the glass so that the glass can be pressed about the lead wires and into a desired configuration. The now-softened glass blank and associated lead wires are compressed in a mold, causing the glass to assume a desired configuration, flowing about the lead wires in sealing relationship.

A persistent problem occurring in this mode of manufacture has been an asymmetrical or nonuniform buildup of metal oxide upon the lead wires during the heating operation. As the lead wires and glass blank are heated, the lead wire cladding tends to oxidize, further causing an increase in the thickness of the oxide layer. As the glass blank softens, the inner surface bellies inwardly and contacts the upstanding lead wires so that the oxide on the side of each lead wire adjacent the glass blank dissolves into the glass, while the oxide at the opposite or remote side of the wire continues to increase in thickness. When the softened glass is pressed, the oxide upon the remote side of the wire ordinarily is not dissolved adequately, with the result that a thick layer of porous undissolved oxide is left between the lead wire and the surrounding glass. Prolonged heating of the assembly would cause the thickened oxide to be dissolved into the glass to the desired extent, but during this period the thinner oxide layer would be completely dissolved leaving a physically weak interface between the glass and the nascent cladding metal.

Another approach is to conduct the manufacturing process in an oxygen-free or a reducing environment. Producing and maintaining such an environment, however, is undesirably expensive and further limits access to and supervision of the process. It will thus be seen that it would be desirable to provide an economical method by which an uneven accumulation of oxide upon lead wire surfaces is prevented.

It is therefore an object of the present invention to provide a method for equalizing oxide thickness about a lead wire prior to sealing it within a glass envelope material.

SUMMARY OF THE INVENTION Briefly stated, in accordance with one aspect of the invention a plurality of lead wires are arrayed in a pattern corresponding to a desired final lead position. An apertured glass blank is placed about the arrayed lead wires such that each wire is in close proximity to the inner surface of the blank. Heat is applied to the assembly for heating the lead wires to form oxide layers thereon and for raising the temperature of the glass to a point at which the glass softens and is capable of being worked. The softened glass flows inwardly and contacts the outer surfaces of the lead wires at the regions adjacent to the glass, dissolving further oxide formed thereat. Oxidation continues to build up, however, at the regions of the lead wire remote from the glass, resulting in formation of a nonuniform oxide layer about each lead wire. A reducing hydrogen flame is then played on the lead wires such that it impinges most directly upon the regions of the oxide layers remote from the glass blank, the flame serving to reduce the accumulated oxide thereat, diminishing its thickness to a dimension which is substantially the same as that of the oxide adjacent the glass blank. The softened glass blank is then pressed by means of a suitable mold such that the glass envelops the peripheries of the lead wires, and assumes a desired configuration.

BRIEF DESCRIPTION OF THE DRAWING While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention will be better understood from the following description of the preferred embodiment taken in connection with the accompanying drawing in which:

FIG. I is a drawing of a cathode ray tube envelope showing lead wires extending through a stern assembly thereof;

FIG. 2 is a view of the lead assembly of FIG. 1 during manufacture, before a pressing operation has been completed;

FIG. 3 is an enlarged cross-sectional view of a lead wire having an asymmetrical oxide coating thereon; and

FIG. 4 is an end view of a complete stem assembly.

DESCRIPTION OF A PREFERRED EMBODIMENT In FIG. 1 there is shown a cathode ray tube 10 including a gun 11 mounted therein and connected to a plurality of lead wires 12. The lead wires extend through a glass wafer 13 and are hermetically sealed therein to prevent the leakage of air into the evacuated area within tube 10. A tubulation 14 extends through wafer 13 and has the distal end thereof sealed. The tubulation, lead wires and wafer are permanently united to comprise a stem assembly. A protective cap 15 may be placed over tubulation l4 and wafer 13 to add physical support and to prevent breakage.

Each of the lead wires 12 advantageously comprises three sections. A first section 18 extending within cathode ray tube may be made from a steel alloy having the desired physical and electrical characteristics. Similarly, an outer end 16 may be made from a similar alloy having the requisite conductivity and having the strength and rigidity to form a serviceable exterior contact. A third, central portion 17 of the lead wire comprises almost all of that portion of the wire which is enveloped by the glass of wafer 13. Ordinarily, the alloys suitable for

sections

16 and 18 of the lead wire have thermal coefficients of expansion which differ from that of the glass, preventing the creation of a seal therebetween which will withstand substantial variations in temperature. For that reason, the central section 17 of the lead wires may be made of a suitable metal, such as Dumet. Dumet, a metal familiar to those conversant with the art and commonly used for glassenclosed lead portions, has the requisite electrical characteristics for use as a lead wire material, and,

further, has a thermal coefficient of expansion in the radial direction which is substantially the same as that of glass. Each end of the Dumet section 17 is welded to the adjacent

lead wire sections

16 and 18. The junction between middle section 17 and the inner end of lead wire section 18 is enclosed by a glass fillet 19 which extends just far enough to cover the joint. Similarly, the junction between the Dumet portion 17 and the inner end of lead wire section 16 is covered by a glass button 20 which arises from the wafer. The lead wire thus constituted includes extending portions having the necessary strength and support, while most of the portion embedded in the glass wafer has a thermal coefficient of expansion compatible with thatof the glass for providing a reliable seal.

In FIG. 2 one step of a popular method of manufacturing the stem assembly is depicted. The desired number of lead wires 12 are disposed in an appropriate pattern and held by a clamping device (not shown). While a total of 12 lead wires are shown, arranged in a circle, it will be apparent that more or fewer lead wires could be utilized and disposed in various other configurations. An annular glass blank 21 is slipped over the lead wire array such that each lead wire is in close proximity to the inner periphery of the blank. An exhaust tubulation 14 is placed within the lead wire array and oriented generally parallel with the lead wires. Heat is then applied to the assembly preparatory to pressing the glass blank 21 for causing the softened glass to flow about the lead wires 12 and exhaust tubulation 14.

In order to provide a hermetic seal between the lead wire and the surrounding glass the middle portion 17 of each lead wire is clad with a coating of a metal such as copper, the outer surface of which is allowed to oxidize. As is well known, such oxides readily dissolve in molten glass to complete a bond which is essentially continuous from the surface of the lead wire into the glass itself. Since metals oxidize in the atmosphere as temperature is raised, as the assembly shown in FIG. 2 is heated the thickness of the oxide coating of lead ;wires 12 increases. However, as the glass blank softens it deforms in such a manner as to allow its inner the blank such that a relatively thin layer of oxide above copper coating 25 is maintained. At the remote or innermost side of each lead wire, corresponding in the present instance to those surfaces directed radially inward of the circular array, oxide buildup continues unmitigated by the presence of the glass solvent.

The resulting cross-sectional configuration is illustrated in exaggerated form in FIG. 3. The circular cross section of the middle element 17 of a lead wire is maintained during the heating process, but the thickness of the copper oxide 22 which is disposed about copper cladding 25 varies nonuniformly about the lead wire. That side of the lead wire in contact with glass blank 21 maintains a desirably thin depth of oxide layer at region 23. However, diametrically opposite from the edge of glass blank 21 the thickness of oxide at region 24 continues to increase. As the oxide thickness increases the oxide becomes porous and, if then enveloped by molten glass, will ordinarily afford a porous channel disposed axially along one side of the lead wire through which the atmosphere can enter the interior of an evacuated envelope. It is possible to dissolve most of the excess oxide at region 24 by the simple expedient of maintaining the enveloping glass at a high temperature for an extended period of time. When this is done, however, the molten glass soon dissolves all of the thinner oxide layer at region 23 so that the glass abuts either nascent cladding material or the inner Dumet portion of the lead wire, providing an unacceptably weak interface.

In FIG. 4, there is shown an end view of the stem assembly after the pressing operation. The molten glass blank has been pressed into a disk-like element 13, surrounding lead wires 12 and enveloping the periphery of exhaust tubulation 14. Further, the surfaces of the molding device (not shown) used to press the glass blank are advantageously provided with recesses registering about each of the lead wires 12 such that buttons 20 are formed about each lead wire as it arises from the disk. The assembly may then be inserted into a suitable envelope and the outer periphery of the disk 13 hermetically sealed to the envelope by glass-to-glass fusing. The interior of the envelope is then evacuated through exhaust tubulation 14, after which the tubulation is sealed.

It will be recognized that a layer of thick, porous oxide extending axially along the middle section of leads 12 will allow air to pass through the stem assembly and invade the evacuated envelope. In the past it has been the practice to regard porosity resulting from unduly thick oxide as being a defect inherent in the above-described manufacturing process. Stem assemblies were commonly manufactured in numbers in excess of that needed, then tested and defective ones destroyed, the unusable assemblies being considered simply as an added factor in the cost of production.

The present method comprises an inexpensive process by which the detrimental oxide buildup may be easily diminished without interrupting the manufacturing cycle. In practicing the inventive process, the lead wires 12 are arrayed in the normal position, as shown in FIG. 2. Exhaust tubulation 14 is secured in an appropriate position and an annular glass blank 21 disposed about the lead wires. Heat is then applied for heating the lead wires and softening the glass blank preparatory to the normal pressing operation. After a suitable temperature has been attained by the assembly but before the pressing operation a reducing agent such as a reducing flame, preferably constituted by a jet of hydrogen, is applied to lead wires 12 such that the flame impinges primarily upon the inner surfaces of the lead wires. By directing the jet in this manner it will be seen that the reducing flame directly contacts the oxide coating at the region of maximum thickness 24. While the reducing agent could be constituted in other ways, the flame of a gas such as hydrogen serves to maintain the requisite temperature and support necessary reduction process. As will be recognized by those skilled in the art the hydrogen flame serves to reduce the oxide. To put it another way, the application of the hydrogen or other reducing element essentially extracts the oxygen from the oxide to leave only the parent metal. In accomplishing this, the volume of the oxide is depleted and so the thickness of the oxide layer is deceased. After a suitable time the oxide layer at region 24 has diminished to a thickness which is substantially the same as the thickness of region 23, resulting in a substantially uniformly thick oxide layer about each respective lead wire. At this time, application of the reducing agent is terminated and the pressing operation commences. A suitable die or mold is applied to the glass blank and associated exhaust tubulation 14 and lead wires 12, causing the glass to flow inwardly of the lead wires, surrounding and sealing the lead wires and the exhaust tubulation. As the molten glass surrounds the lead wires 12 it encounters the portion of which is uncovered by glass oxide layer 22 thereabout, at which time the formerly uncovered portion of oxide begins to dissolve into the glass. Since the oxide is now distributed evenly about the periphery of the lead wire 12 the assembly may be allowed to cool at a rate suitable for allowing nearly all of the oxide layer to dissolve into the glass. Since the oxide layer is ofa substantially constant thickness the dissolving takes place at an essentially even rate about the periphery of each lead wire. It is thus possible to ascertain a point in time at which the oxide has dissolved adequately, without leaving thick, porous oxide areas.

While for purposes of illustration the process has been described using copper oxide and hydrogen for the bonding and reducing agents respectively, it will be appreciated that other oxides and reducing agents may be utilized in the process without substantially affecting the basic nature thereof. Further, while round lead wires have been represented the process is applicable to leads having various configurations, and it is therefore contemplated that other modifications or applications will occur to those skilled in the art. It is thus intended that the appended claims shall cover all such modifications and applications as do not depart from the true spirit and scope of the invention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

l. The method of effecting a seal between glass and a lead wire having an oxidizable outer surface, including the steps of? disposing a preformed blank of said glass adjacent to said surface along a portion of said surface;

heating the juxtaposed glass and lead wire to soften the glass and form an oxide layer about said wire,

said oxide layer being thicker on the region'of said wire remote from the glass than on the region of said wire adjacent the glass;

directing a reducing agent onto said oxide layer on the region of said wire remote from the glass to I diminish the thickness of said oxide layer thereat to the approximate thickness of said oxide layer on the region of said wire adjacent said glass, thereby rendering said oxide layer substantially uniformly thick about said wire; and

causing the softened glass to surround said wire and dissolve a portion of said oxide layer about said wire.

2. An improved method for effecting a hermetic seal between glass and a plurality of lead wires to extend therethrough, said lead wires having oxidizable outer surfaces, comprising the steps of:

arranging said lead wires in a desired pattern within the opening of an apertured blank of said glass, each of said lead wires being in close proximity to said glass; heating the juxtaposed glass and lead wires to soften the glass and form an oxide layer about each of said wires, each said oxide layer being thicker on the region of each respective wire remote from the glass than on the region of said wire adjacent the glass; directing a reducing agent onto each said oxide layer on the region of said wire remote from the glass to diminish the thickness of the oxide layer thereat [:o the approximate thickness of the oxide layer on the region of said wire adjacent said glass and thereby render the oxide layer substantially uniformly thick about said wire; and

causing the softened glass to surround each of said wires and dissolve a portion of each said oxide layer respectively disposed thereon.

3. The method as set forth in claim 2, wherein said reducing agent is a hydrogen flame.

4. An improved method for effecting a hermetic seal between glass and a plurality of lead wires to extend therethrough, a portion of each one of said lead wires being fabricated of conductive material having an oxidizable metallic coating disposed on the outer surface thereof, said conductive material having substantially the same thermal coefficient of expansion in the radial direction as that of said glass, said method comprising the steps of:

arranging said plurality of lead wires in a desired pattern within the opening of an apertured blank of said glass such that a portion of each said coating is in close proximity to the glass;

heating the juxtaposed glass and lead wires to soften the glass and form an oxide layer about each said coating, each said oxide layer being thicker on the region of its respective coating remote from the glass than on the region of its respective coating adjacent the glass;

allowing the softened glass to contact and cover a portion of each said oxide layer and dissolve said layer thereunder to a relatively thin depth above said respective coating;

directing a reducing flame upon the thicker region of each said oxide layer to decrease the thickness of each said oxide layer thereat; and

molding the softened glass about the remaining uncovered portion of each said oxide layer to dis-- solve said layer thereunder substantially to said relatively thin depth above said respective coating and thereby form a hermetic seal with each of said lead wires.

Claims

2. An improved method for effecting a hermetic seal between glass and a plurality of lead wires to extend therethrough, said lead wires having oxidizable outer surfaces, comprising the steps of: arranging said lead wires in a desired pattern within the opening of an apertured blank of said glass, each of said lead wires being in close proximity to said glass; heating the juxtaposed glass and lead wires to soften the glass and form an oxide layer about each of said wires, each said oxide layer being thicker on the region of each respective wire remote from the glass than on the region of said wire adjacent the glass; directing a reducing agent onto each said oxide layer on the region of said wire remote from the glass to diminish the thickness of the oxide layer thereat to the approximate thickness of the oxide layer on the region of said wire adjacent said glass and thereby render the oxide layer substantially uniformly thick about said wire; and causing the softened glass to surround each of said wires and dissolve a portion of each said oxide layer respectively disposed thereon.
3. The method as set forth in claim 2, wherein said reducing agent is a hydrogen flame.
4. An improved method for effecting a hermetic seal between glass and a plurality of lead wires to extend therethroUgh, a portion of each one of said lead wires being fabricated of conductive material having an oxidizable metallic coating disposed on the outer surface thereof, said conductive material having substantially the same thermal coefficient of expansion in the radial direction as that of said glass, said method comprising the steps of: arranging said plurality of lead wires in a desired pattern within the opening of an apertured blank of said glass such that a portion of each said coating is in close proximity to the glass; heating the juxtaposed glass and lead wires to soften the glass and form an oxide layer about each said coating, each said oxide layer being thicker on the region of its respective coating remote from the glass than on the region of its respective coating adjacent the glass; allowing the softened glass to contact and cover a portion of each said oxide layer and dissolve said layer thereunder to a relatively thin depth above said respective coating; directing a reducing flame upon the thicker region of each said oxide layer to decrease the thickness of each said oxide layer thereat; and molding the softened glass about the remaining uncovered portion of each said oxide layer to dissolve said layer thereunder substantially to said relatively thin depth above said respective coating and thereby form a hermetic seal with each of said lead wires.
5. The process set forth in claim 4, wherein said reducing flame is a hydrogen flame.
6. The process set forth in claim 5, wherein said oxidizable metallic coating is comprised of copper.