GB2067449A

GB2067449A - Hermetic sealing

Info

Publication number: GB2067449A
Application number: GB8102062A
Authority: GB
Original assignee: VACUUM INTERRUPTERS Ltd
Current assignee: VACUUM INTERRUPTERS Ltd
Priority date: 1980-01-23
Filing date: 1981-01-23
Publication date: 1981-07-30

Abstract

A means for making a hermetic seal in an envelope 1 comprises a recess in a region of the outer surface of the envelope, the bottom of the recess 3 being provided with at least one aperture 2 communicating with the interior of the envelope and supporting a quantity of an alloy 4 composed substantially of silver and germanium with a proportion of gold, although the gold may be omitted, whereby on the envelope being evacuated and heated to a temperature at or above the liquidus of the alloy and subsequently being allowed to cool, the alloy fills the aperture or apertures and forms a hermetic seal to the envelope. <IMAGE>

Description

SPECIFICATION Hermetic sealing arrangement This invention relates to methods for making permanent gasqight seals in envelopes and particularly for permanently sealing evacuated envelopes after the last traces of gases therein have been removed by baking out at an elevated temperature. The invention finds use, for example, in the manufacture of vacuum interrupter devices.

It is known for the envelope of a vacuum interrupter device to be provided with a disc having a multiplicity of perforations communicating with the interior of the device, (for example 40 perforations of about 1 mm diameter) upon which a portion of an alloy of gold and indium is placed before the device is baked out. Baking out is done by heating the device for a prolonged period in an evacuated furnace at a temperature just below the solidus of the alloy in order to remove the last traces of adsorbed gases. The evacuated device is then sealed by raising thetemperture of the furnace to melt the alloy so that it fills the perforations in the disc and continues to do so after being allowed to cool to ambient temperature.

According to this invention means for making a hermetic seal for an evacuated envelope comprises a recess in the region of the outer surface of the envelope, the bottom of the recess being provided with at least one aperture communicating with the interior of the envelope and supporting a quantity of an alloy composed substantially of silver and germa niumwith or without a proportion of gold, whereby on the envelope being evacuated and heated to a temperature at or above the liquidus of the alloy and subsequently allowed to cool, the alloy fills the aperture or apertures and forms a hermatic seal to the envelope.

Satisfactory seals can be formed utilising an alloy having not more tha 80% by weight of silver, not more than 30% by weight of gold and between 10% and 20% by weight of germanium.

One alloy which we have found highly suitable substantially comprises 74.5% by weight of silver, 7.5% byweight of gold and 18% by weight of germanium.

This alloy exhibits a desired degree of wettability of nickel, nickel plated austenitic stainless steel or of the glass sealing alloy known as Nilo K and the recessed region of the envelope is conveniently formed of or coated by one such material, although other metals or alloys capable of being wetted by the sealing alloy can alternatively be exhibiting a short melting point range from 580 C to 650 C, this being desirable for the precise location of the alloy during the melting process.

An alternative alloy also found to be suitable substantially comprises 64.5% by weight of silver 18% by weight of gold, and 17.5% by weight of germanium, this having a melting point range of 535 C to 620or and also having the capability to wet nickel, nickel plated austenitic stainless steel, and Nilo K.

The aperture or apertures in the bottom of the recess must, of course, be sufficiently small to retain the molten alloy employed by virtue of its surface tension. in practice it has been found that the surface tension of the molten alloys above described is sufficient for it to fill holes 2 mm in diameter and to form a continuous fillet throughout a recess and aperture or apertures therein.

The sealing alloy used in accordance with the invention is an improvement over the gold-indium alloy disclosed in U.K. Patent 1281938 in that the largest holes which can be filled by melting the latter are of about 1 mm diameter, so that fewer apertures have to be provided in the present case to achieve the same rate of evacuation of an envelope.

An alloy according to this invention may be initially prepared in bulk quantities of 100 gm and then cast under vacuum into 3 mm diameter sticks.

Portions weighing approximately 0.85 gm may then be cut from these sticks and these remelted under vacuum to produce pellets suitable for use in sealing envelopes as described below.

This invention is particularly applicable to the sealing of the envelopes of vacuum electrical switches such as vacuum interruptors.

The invention is further described by means of an example with reference to Figures 1 to 4 of the accompanying drawings.

Figure 1 shows in sectional view a pellet of silver-gold-germanium alloy placed in a recess containing apertures according to the invention.

Figure 2 shows the arrangement of Figure 1 after the apertures have been sealed.

Figure 3 shows a plan view of a recess according to the invention.

Figure 4 shows a partially sectional view of a vacuum interruptor incorporating seal means according to the invention.

Figure 1 shows a recess in the exterior of a region of an envelope 1 with one of three apertures 2 which communicate with the interior of the envelope. The recess takes the form of a cylindrical depression, 7 mm diameter and 2 mm deep and the three apertures 2 are holes of 2 mm diameter equispaced inside the circumference of the floor 3 of the recess.

The wall of the recessed region of the envelope is composed of austenitic stainless steel and the surfaces of the recess 3 and apertures 2 are nickel plated to a minimum thickness of 0.0015 mm. Prior to sealing the envelope, a pellet 4 weighing 0.85 gm of an alloy comprising by weight 74.5% silver, 7.5% gold and 18% germanium is placed in the recess, care being taken not to block the apertures. The envelope is then placed in a vacuum furnace and evacuated. Heating of the envelope above the liquidus of the silver-gold-germanium alloy causes the latter to flow throughout the recess and apertures as shown in Figure 2, where it is retained due to its surface tension, and a hermetic seal is formed on cooling.

A vacuum interrupter with a vacuum seal-off according to the invention is shown in Figure 4, the features of the invention being labelled in accordance with Figures 1 to 3.

The envelope of such a vacuum interruptor comprises, in known manner, a generally cylindrical body mainly of ceramic glass 5 closed at its end by stainless steel end plates, one of which is shown at 6, carrying the interruptor contacts, one of which end plates is formed with an apertured recess 3 as above described. The envelope is sealed in accordance with the invention as follows. With the envelope supported with its axial vertical and the recessed end plate 6 uppermost a pellet of the silver-goldgermanium alloy is placed in the recess and the envelope is baked out in a furnace evacuated to a pressure of 10-5 torr at 5500C for 8 hours. The temperature is then raised to 7000C for 11/2 hours to melt the alloy and subsequent cooling causes a permanent vacuum-tight seal to be formed.

The same process can be used to seal a vacuum interrupter envelope with an alternative sealing alloy comprising by weight 64.5% silver, 18% gold and 17.5% germanium, although in this case the envelope should be baked out in a furnace evacuated to a pressure of 10-5torr at 5000 for 8 hours, and the temperature then raised to 5700C for 11/2 hours to melt the alloy and form the seal.

The procedure according to the invention is advantageous over the known method utilising a gold-indium alloy in that the alloy used is considerably cheaper than the gold-indium alloy. Also the requirement for a perforated disc insert is obviated and furthermore a higher temperature bake out is possible.

Conveniently a batch of vacuum interrupter envelopes is sealed simultaneously, by placing them in a furnace, each with a pellet of said sealing alloy in a respective apertured recess, and evacuating and heating as above described.

It will be understood that the invention includes within its scope vacuum interrupters, and other devices having envelopes sealed in the above described manner.

Claims

1. A means for making a hermetic seal for an evacuated envelope comprises a recess in a region of an outer surface of the envelope, the bottom of the recess being provided with at least one aperture communicating with the interior of the envelope and supporting an alloy composed substantially of silver and germanium with or without a proportion of gold, whereby on the envelope being evaucated and heated to a temperture at or above the liquidus of the alloy and subsequently being allowed to cool, the alloy fills the aperture or apertures and forms a hermetic seal to the envelope.

2. A means according to Claim 1 wherein said alloy comprises not more than 80% by weight of silver, not more than 30% by weight of gold and between 10 and 20% by weight of germanium.

3. A means according to Claim 2 wherein said alloy comprises approximately 74.5% by weight of silver, 7.5% by weight of gold and 18% by weight of germanium.

4. A means according to Claim 2 wherein the alloy comprises approximately 64.5% by weight of silver, 18% by weight of gold and 17.5% by weight of germanium.

5. A means according to any preceding claim wherein the recess is formed in a metal member forming part of the wall of the envelope.

6. A means according to Claim 5 wherein said metallic member comprises a plate of stainless steel constituting an end wall of the envelope, with at least the surfaces of the said recess and the aperture or apertures being nickel plated.

7. A means according to any preceding claim wherein said envelope comprises the envelope of a vacuum electrical switch.

8. A means substantially as hereinbefore described with reference to Figures 1 to 4 of the accompanying drawings.

9. A vacuum interrupter having an envelope incorporating a hermetic seal formed in accordance with any preceding claim.

10. A vacuum interrupter incorporating an evacuated envelope having a portion formed with a recess in its outer surface, the bottom of which recess is provided with at least one aperture communicating with the interior of the envelope for the extraction therethrough of gases from the envelope during bake-out of the interrupter, the envelope being sealed in a vacuum-tight manner by an alloy composed substantially of silver and germanium, with or without a proportion of gold, which alloy blanks off the aperture or apertures automatically when, on completion of bake-out, the temperature is raised above the liquidus of the alloy and is then allowed to cool to cause the molten alloy to enter and solidify within the aperture or apertures.