US3548135A

US3548135A - Contacts for vacuum interrupters

Info

Publication number: US3548135A
Application number: US641880A
Authority: US
Inventors: Allan John Wood
Original assignee: English Electric Co Ltd
Current assignee: English Electric Co Ltd
Priority date: 1966-05-27
Filing date: 1967-05-29
Publication date: 1970-12-15
Anticipated expiration: 1987-12-15
Also published as: CH460910A; SE325943B; BE714486A; AT294962B; AU3683968A; DE1640038A1; NL6806076A

Description

United States Patent 2,975,256 3/1961 Lee et al. 200/l66(C)X 3,042,474 7/1962 Aurand et al. 75/173 3,239,635 3/1966 Baude ZOO/144(2) 3,367,801 Z/l 968 Kreiselmaier 75/173X FOREIGN PATENTS 693,827 9/l 964 Canada ZOO/144.2

Primary ExaminerRobert S. Macon Attorney-Misegades and Douglas ABSTRACT: Contacts for vacuum interrupters have their faces, between which an arc may be struck in operation, formed of dissimilar materials, which materials, on arcing between the faces, become intermixed and form nonductile films. The materials are mechanically strong and have high electrical and thermal conductivities.

Preferably the face of one contact is formed of silver-silicon and that of the other is formed of silver-nickel, these materials on arcing forming nonductile layers of nickel silicides on the faces, which markedly reduces the tendency of the contacts to weld together.

In an alternative, the face of one contact may be formed of high-purity zone-refined copper and that of the other of highpurity vacuum outgassed titanium. In this case nonductile films of copper-titanium compounds are formed.

III/III! PATENTED 05m 5 I970 3548.135

INVENTOR Allan John Wood BY Misegades &:

Douglas ATTORNEYS CONTACTS FOR VACUUM INTERRUPIERS This invention relates to contacts for vacuum interrupters.

According to this invention a pair of cooperating contacts in or for a vacuum circuit interrupter has parts defining their respective cooperating faces, between which an arc is struck in operation, formed of dissimilar materials, each of which materials is mechanically strong and has high electrical and thermal conductivity, and which materials, on arching between said cooperating faces, become intermixed and form nonductile films.

Two embodiments of the invention will now be described by way of example, with reference to the accompanying drawing, which shows a vacuum interrupter which includes contacts in accordance with the present invention.

The vacuum interrupter has a pair of metal endplates ll, 12 to which are bonded respectively in a vacuumtight manner a pair of tubular insulators l3, l4. Bonded to and trapped between the insulators l3, I4 is a flange 15 which forms part of and supports a cylindrical shield 16.

The fixed contact 17 is secured in a vacuumtight manner to the endplate 11 and is provided with an external terminal 18 for connection in an external circuit; the movable contact 19 slides in a bush 20 fixed to endplate l2, and is connected to an actuator (not shown) which moves it between the open position shown, and the closed position in which it abuts the fixed contact 17. The movable contact 19 is connected, for example by means of a flexible conducting braid 21, to a second terminal; moreover, a flexible bellows device 22 is provided, one end of which is sealed to the movable contact 19, and the other end of which is sealed to the endplate 12.

in the first embodiment of the invention, a portion, including the arching face, of one of the

cooperating contacts

17, 19, is formed of silver-silicon material, and a portion including the arching face, of the other of the contacts is formed of silvernickel material. It is immaterial whether the fixed contact 17 is formed of silver-silicon material and the movable contact 19 is formed of silver-nickel material or vice versa.

Each of the contacts l7, 19, is of cylindrical form, and they abut one another (when closed) at their cooperating, flat, circular end faces 23, 24; the are normally runs between these flat end faces. A small radius is formed between the fiat, circular end faces 23, 24 and the respective cylindrical faces to avoid a sharp edge.

The silver-silicon contact was made by vacuum melting pure silver and pure silicon together in a pure graphic crucible. The high vacuum was maintained by diffusion pumps with liquid nitrogen vapor traps, and clean conditions were adhered to. The metals were heated by a radio frequency coil, using a molybdenum susceptor, for 30minutes at 1,050 C. and were then cast into ingots The preferred proportions are 4-6 percent silicon and 96-94 percent silver by weight.

The ingots so produced were machined by normal machining methods to the form of the contact.

The silver-nickel contact was made by compacting nickel powder of about microns particle size in a hydraulic press at a pressure of tons per square inch, using a steel die and plunger. The compact was then sintered at high vacuum for minutes at 800 C. and the sintered compact was then infiltrated with pure silver at high vacuum in a graphite crucible for ID minutes at l,050 C. The infiltrated compact was machined to the form of the contact by normal machining methods. Analysis of a contact formed in this way showed that the operating face contained between 70 and 80 percent by weight of nickel. The preferred nickel content is between 65 and 85 percent by weight.

Operation of a high-vacuum interrupter having one silversilicon contact and one silver-nickel contact made as described above showed that arcing between the cooperating faces of the contacts resulted in intermixing of the materials and the formation of nonductile films of nickel silicides on the cooperating faces. A large number of phases are believed to be formed, including Ni2Si, and Ni3Si and NiSi It was found that the contacts were mechanically strong and had high electrical and thermal conductivities, and that the tendency of the contacts to weld together, or for surface grains or metallic filaments of the contacts to be drawn out as a result of partial welding of the contacts, was markedly reduced.

It will be understood that the formation of such filaments, or whiskers, may affect the contact life, and may also seriously reduce the voltage withstand of the contacts when in the open position.

Although the invention is not limited to the use of the contact materials described above, it will be appreciated that in determining the suitability of materials it is necessary to take into account not only the properties discussed above, but also other factors well known to those skilled in the art such as the current chopping characteristics.

ln addition to the method described above, the silver-silicon contact may be formed by mixing silver powder and silicon powder (for example in the proportions percent silver, 10 percent silicon by weight); when the powders are thoroughly mixed, the resulting powder is compacted in a press at a pressure of 10 tons/sq. in., and the resulting compact is sintered in vacuo at 790 C. for 45 minutes. This method overcomes certain difficulties which may occur with the vacuum melting and casting method described above, especially where an appreciable proportion, for example over 5 percent, of silicon is present.

In the second embodiment of the invention a portion, including the arcing face, of one of the cooperating contacts l7, 19 is formed of high-purity zone-refined copper, and a portion, including the arcing face, of the other of the contacts is formed of high-purity vacuum-outgassed titanium.

Operation of a high-vacuum interrupter having one high-purity zone-refined copper contact and one high-purity vacuumoutgassed titanium contact showed that arcing between the cooperating aces of the contacts resulted in the formation of nonductile films of copper-titanium compounds, and that there was a reduced tendency of the contacts to weld together.

lclaim:

l. A vacuum circuit interrupter comprising an evacuated envelope and first and second contacts movable relatively to one another for contact engagement and disengagement within the envelope at cooperating contact regions, at said contact regions the first contact comprising nickel and the second contact comprising silicon; an arc struck between the contacts when separated during a circuit interruption causing the nickel and the silicon to mix with one another on the contact regions and to combine thereon to form surface films constituted by at least one nickel silicide, the material of the films forming a weld between the contacts when subsequently brought into engagement and being easily ruptured when the contacts are later separated.

2. A vacuum circuit interrupter comprising an evacuated envelope and first and second contacts movable relatively to one another for contact engagement and disengagement within the envelope at cooperating contact regions, at the said contact regions the first contact comprising silver-silicon and the second contact comprising silver-nickel.

3. A vacuum circuit interrupter according to claim 2, wherein at the respective contact region the first contact is a sintered particulate alloy of silver and silicon.

4. A vacuum circuit interrupter according to claim 3, wherein the sintered particulate alloy comprises approximately 90 percent silver by weight, the remainder silicon.

5. A vacuum circuit interrupter according to claim 2, wherein at the respective contact region the first contact is a cast alloy of pure silver and pure silicon.

6. A vacuum circuit interrupter according to claim 5, wherein the cast alloy comprises 94-96 percent silver by weight, the remainder silicon.

7. A vacuum circuit interrupter according to claim 2, wherein at the respective contact region the second contact is a sintered porous matrix of nickel having its pores infiltrated with silver.

9. A vacuum circuit interrupter according to claim 8, wherein the sintered porous matrix of nickel having its pores infiltrated with silver comprises -80 percent nickel by weight, the remainder silver.