EP0123475A1

EP0123475A1 - Method of joining a contact to an electrode

Info

Publication number: EP0123475A1
Application number: EP84302428A
Authority: EP
Inventors: Vincent Joseph Santilli
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1983-04-14
Filing date: 1984-04-10
Publication date: 1984-10-31
Also published as: JPS59198625A; ZA842116B

Abstract

@ Method of joining an electrode to a contact having a material constituent with high vapor pressure in a final assembly stage of a vacuum device such as a vacuum interrupter. Attachment is effected during the vacuum-exhaust operation by providing an axially extending stem of a contact that is inserted into an axial bore in an electrode. A low vapor pressure, low melting point material such as tin (SN) or the like is placed in the bore prior to the insertion of the stem. Venting means in the electrode purge the bore during the vacuum exhaust stage which provides sufficient heat to liquefy the conductive material which flows between the stem and electrode, bore wall where it eventually solidifies upon cooling.

Description

This invention relates to a method of joining a contact to an electrode. More particularly, this invention provides a technique for the low temperature fabrication of electrode to contact bonds. This technique is particularly useful in the manufacture of vacuum devices, such as vacuum circuit interrupters.
Vacuum circuit interrupters are used to interrupt electric currents in electric power transmission and distribution systems. A conventional interrupter includes a generally cylindrical insulating envelope portion with a pair of contacts hermetically sealed through end plates sealed to the insulating envelope, with the device being evacuated. One of the contacts is movable along the device axis to make and break contact with the other, fixed contact from a mating closed contact, current carrying position, to an open contact, spaced apart circuit interrupting position.
The conventional structure for permitting contact movement is a generally cylindrical bellows which is hermetically sealed at one end to an end plate, and at the other end to the contact support rod or electrode which extends slidably through a guide bushing in the end plate.
It is the conventional practice to firmly attach the metal contact to the electrode by means of high temperature'vacuum brazing. This technique is illustrated in U.S.' Patent Specification Nos. 4,053,728 and 2,406,327. The electrode is usually copper while the contact may be copper, chromium, tungsten or a desired alloy thereof. Because a copper electrode is often being joined to a contact formed from a different material, the high temperature process of vacuum brazing makes a contact to electrode joint possible regardless of the disparity of the materials being joined.
Another technique for joining the contact to an electrode is disclosed in U.S. Patent Specification Nos. 4,272,661 and 2,406,327 whereby the contact is attached to the electrode with a threaded member such as a screw or bolt..
The type of material used in the contact is determined by the performance requirements of the particular vacuum interrupter device in which the contact is employed. As discussed above, vacuum brazing is routinely used in most vacuum interrupters and the completed contact-electrode junction is preserved for other subsequent tube assembly operations in' a vacuum environment. In certain cases, however, a contact is used in a vacuum interrupter which is necessary to obtain desired vacuum interrupter performance, but which is not compatible with high temperature-vacuum processing. For example, a contact heavily loaded with a material which has a low melting temperature and a high vapor pressure and which expands during solidification, such as bismuth (Bi), is disposed to failure. This failure is certain if the contact is present in an evacuated bottle that is brazed at high temperature because the bismuth, or a material with similar properties, generates a vapor which penetrates the braze joints rendering them brittle and full of voids. As a result the joint is not vacuum tight.
It is therefore an object of this invention to provide a technique for the assembly of an electrode and contact by means of a liquid-mechanical attachment.
It is another object of this invention to provide a method for forming a low resistance joint which bonds the electrode and contact of a vacuum device.
It is yet another object of this invention to provide a technique for the attachment of a contact heavily loaded with a metal having a low melting temperature and high vapor pressure to an electrode.
Accordingly, the present invention resides in a method of joining a contact to an electrode which comprises the steps of joining an electrically conductive stem to the contact; inserting said stem into an electrode having a bore therein adapted to receive said stem; providing a low vapor pressure, low melting point temperature conductive material in said bore; heating said electrode and conductor under a vacuum to at least a temperature sufficient to liquefy said conductive material whereby said material flows between said stem and said electrode bore; and cooling said conductor and electrode, solidifying said liquefied conductive material, forming a conductor joint between said electrode and said conductor.
Desirably, the electrode has a vent therein permitting purging of the bore during the final vacuum stage of assembly. The stem and bore may have complementing threads whereby a mechanical joint is formed when the contact mounted stem is threaded into the bore in the electrode.
In order that the invention can be more clearly understood, convenient embodiments thereof will now be described, by way of example, with reference to the accompanying drawing, which is an elevational view, partly in cross section, of a vacuum interrupter.
Vacuum circuit interrupter 10 includes a gener- .ally cylindrical, insulating envelope 12 with end plate closures 14 and 16. The end plate 14 is at the movable contact end, while end plate 16 is at the fixed contact end.
The fixed end plate 16 is an inwardly dished member with a cylindrical conductive support rod electrode 22 sealed through a central aperture 24 provided through the end plate 16.
- The movable end plate 14 is a planar member with a central aperture 26 therethrough. An annular guide bushing member 28 is disposed within the aperture 26 and a movable contact support rod electrode 30 is slidably fitted through the guide bushing member 28. A generally cylindrical bellows seal member 32 is hermetically sealed at one end 34 to the end plate 14, while the other end of the bellows seal member 32 is hermetically sealed to the contact support rod 30 at an axially inwardly extending portion 38.
The contacts 4C and 42 which are disposed at the extending ends of respective contact support rod or electrode 22 and 30, are of conventional design. A generally cylindrical center shield 48 is closely spaced from the insulating envelope 12, while annular end shields 50 and 52 protect respective end seals 18 and 20. A cup-shaped bellows shield 54 which is mounted from movable support rod electrode 30, extends about the bellows 32.
The annular bushing 28 is preferably an insulating member and includes a cylindrical portion 56 which extends coaxially within the bellows 32. The guide bushing 28 can also include an axially extending flange 58 by which the guide bushing 28 is radially aligned and seated within the end plate 14 aperture 26. The guide bushing 28 is secured to the end plate 14 by a retainer member 60 disposed over the bushing and engaged with the outer edge 62 of the end plate 14.
The present invention is directed to a technique ^r for the attachment of each contact 40 and 42 to the appropriate support rod electrodes 22 and 30 respectively. More particularly, the present technique permits the assembly of a contact which is loaded with a material which has a low melting temperature and a high vapor pressure such as bismuth (Bi), copper (Cu), chromium (Cr), and silver (Ag). What is meant by loaded is the use of such a material as a constituent of the contact in amounts greater than that of a trace element approximately 2 to 3 percent or more of the contact's weight. The present technique allows the evacuation of the envelope 12 at high enough temperature and vacuum to obtain desirable vacuum tube performance and yet maintains low enough bismuth vapor pressure that contact fall off and other disattach- ment problems are substantially eliminated.
The contact assembly includes the contact 40 (or 42) and a threaded stem 64. The threaded stems 64 consist of an electrically conductive material such as copper or the like which may be threaded at only one end as indicated at 66 or at both ends (not illustrated). The threaded stem 64 can include a flanged portion 68 which mechanically retains the contact 40 (or 42) against the electrode 22 (or 30). Additionally, each contact 40 (42) can be mounted on a stainless steel substrate, one of which is illustrated at 70 as an example thereof. Alternatively, the threaded stem 64 can be attached to the contact 40 (or 42) prior to the assembly of the vacuum tube by means of a braze joint or as will be explained below, by an adaptation of the method of this invention. The electrodes 22 and 30 each include an axial bore 72 having a threaded wall portion 74 adapted to receive the threaded portion 68 of the stem 66 axially extending from the contact. The bore 72 also includes a well portion 76 containing a supply 78 of a low vapor pressure, low melting point conductor, such as tin (Sn) a tin alloy, or the .like. A vent, such as the vent hole 80 is provided in each of the electrodes 22 and 30. Venting hole 80 provides communication between the electrode bore wells 76 and the environment exterior thereto which is the evacuated envelope 12 of the vacuum tube assembly 10. During the assembly of the vacuum tube, air is purged from the threaded surfaces of the stem and electrode bore which purging in turn facilitates the migration of the tin 78 along the threaded surfaces.
During final assembly of the vacuum interrupter structure 10, the contact and stem are joined together by brazing, in a hydrogen atmosphere, a brazed threaded joint or by means of a thin strip of tin (Sn), a tin alloy or the like characterized by low vapor pressure and a low melting point disposed between the stem and the contact, or the stem, substrate and contact as at 82. When the thin strip of tin is utilized, the joint between the contact and the stem is formed during vacuum exhaust. The electrode bore well 76 is filled with tin and the contact stem screwed tightly into the electrode bore well 76. Subsequent exhaust in vacuum heats the vacuum interrupter structure 10 to a temperature of about 450°C In reaching this temperature, the exhaust cycle passes through the melting point of tin, 231°C. The liquid tin wicks into the purged, mated threads, but does not evaporate because of its low vapor pressure. Moreover, the mated threads of the stem and electrode bore function as a mechanical stop which captures the melted tin. In addition, it is possible to provide other mechanical means such as a flute, wedge or step associated with the electrode to effect the mechanical capture of the tin and inhibit further migration either in addition to or in lieu of threads. The tin freezes in situ upon cooling to room temperature and is not attacked by corrosive bismuth vapor while in the liquid state. Alternatively, rather than providing tin, a tin alloy or the like in the electrode well, the stem and/or the electrode bore can be plated with the low vapor pressure, low melting point conductive material prior to the mechanical connection of the contact to the electrode.
During operation of the vacuum interrupter at appropriate current and temperature ranges the contact electrode joint formed according to the present technique has been found to be compatible with, and imperceptible as, a resistive element in series with the electrode.

Claims

1. A method of joining a contact to an electrode characterized by the steps of joining an electrically conductive stem to the contact; inserting said stem into an electrode having a bore therein adapted to receive said stem; providing a low vapor pressure, low melting point temperature conductive material in said bore; heating said electrode and conductor under a vacuum to at least a temperature sufficient to liquefy said conductive material whereby said material flows between said stem and said electrode bore; and cooling said conductor and electrode, solidifying said liquefied conductive material, forming a conductor joint between said electrode and said conductor.

2. A method according to claim 1, characterized in that the conductive material is tin or a tin alloy.

3. A method according to claim 1 or 2, characterized in that the electrode bore adapted to receive the stem therein has threaded sidewalls and the stem is provided with complementary threads and is threadedly received in the bore whereby the liquefied material flows between said threads.

4. A method according to claim 1, 2 or 3, characterized in that the electrode bore is purged through a vent in the electrode as the contact and electrode are heated in the vacuum.

5. A method according to any of claims 1 to 4, characterized in that the electrically conductive stem is joined to the contact by providing a low vapor pressure, low melting point temperature conductive material between the stem and the contact, liquefying said material and then solidifying the latter.

6. A method according to any of claims 1 to 5, characterized in that the contact consists of an alloy which includes bismuth.