US3818163A

US3818163A - Vacuum type circuit interrupting device with contacts of infiltrated matrix material

Info

Publication number: US3818163A
Application number: US00162881A
Authority: US
Inventors: A Robinson
Original assignee: English Electric Co Ltd
Current assignee: English Electric Co Ltd
Priority date: 1966-05-27
Filing date: 1971-07-15
Publication date: 1974-06-18
Anticipated expiration: 1991-06-18
Also published as: AT297833B; BE714487A; CH495618A; NL163353C; SE331756B; AU3701168A; GB1194674A; NL6806077A; DE1640039B1

Abstract

The specification discloses an electric circuit interrupting device in which the contacts are enclosed in an evacuated enclosure, and in which the contact material comprises a matrix of a low ductility semi-refractory metal infiltrated with a metal of high electrical conductivity. The metals are chosen so that during alternating current arcing (a) the loss of heat by vapourisation of the metals at the surface of a molten anode spot prevents the anode spot rising to a temperature at which substantial electron emission occurs, so that the ability to interrupt a high current arc at the next natural current zero is improved, and (b) precipitation alloying of the metals occurs, with consequent refinement of the matrix structure at the contact surface so that the performance of the interrupter is improved by conditioning of its contacts. Chromium and cobalt are specifically proposed as matrix metals, and copper, silver, and alloys of copper and silver are proposed as infiltrant metals.

Description

llnited States Patent 11 1 Robinson VACUUM TYPE CIRCUIT INTERRUPTING DEVICE WITH CONTACTS OF INFILTRATED MATRIX; MATERIAL [75] Inventor: Alfred Alexander Robinson,

Stafford, England [73] Assignee: The English Electric Company Limited, Stafford, England [22] Filed: July 15, 1971 [21] Appl. No.: 162,881

' Related [1.8. Application Data [63] I Continuation-impart of Ser. No. 871,431, Oct. 24, 1969, abandoned, which is a continuation-impart of Ser. No. 641,881, May 29, 1967, abandoned.

[30] Foreign Application Priority Data 2,137,281 11/1938 Hensel et a1. 200/166 C 2,975,256 3/1961 Lee et a1. 200/144 B 3,008,022 11/1961 Lee .l 200/144 B 3,042,474 7/1962 Aurand et al.... 75/173 X 2/1968 Kreiselm'aier 75/173 X 1451 June 18, 1974 3,514,559 5/1970 Ranheim v 200/166 C 3,551,622 12/1970 Takeuchi et a1. 200/166 C FOREIGN PATENTS OR APPLICATIONS 406,614 3/1965 Japan 200/144 B 403,937 0/1934 Great Britain 200/144 B [5 7] ABSTRACT The specification discloses an electric circuit interrupting device in which the contacts are enclosed in an evacuated enclosure, and in which the contact material comprises a matrix of a low ductility semirefractory metal infiltrated with a metal of high electrical conductivity. 1

The metals are chosen so that during alternating current arcing (a) the loss of heat by vapourisation of the metals at the surface of a molten anode spot prevents the anode spot rising to a temperature at which substantial electron emission occurs, so that the ability to interrupt a high current are at the next natural current zero is improved, and (b precipitation alloying of the metals occurs, with consequent refinement of the matrix structure at the contact surface so that the performance of the interrupter is improved by conditioning of its contacts. Chromium and cobalt are specifically proposed as matrix metals, and copper, silver, and alloys of copper and silver are proposed as infiltrant metals.

14 Claims, 5 Drawing Figures aim-SL153 PATENTEUJUN 1 8 m4 SHEET 1 BF 3 FIG.1

FIG.2

INVENTOR Alfred Alexander Robinson Misegades 8c Douglas ATTORNEYS P'ATENTEDJHMB 1914 3,818 163 SHEET 2 BF 3 l VACUUM TYPE CIRCUIT INTERRUPTING DEVICE WITH CONTACTS OF INFILTRATED MATRIX MATERIAL This application is a-continuation-in-part of application Ser. No. 871,431 filed Oct. 24, 1969 by A. A. Robinson relating to Electric Circuit Interrupting Devices, which was a continuation-inpart of application Ser. No..64l,88 l filed May 29, 1967 byA. A. Robinson Contacts, both of which have been abandoned.

The invention relates to vacuum interrupters, i.e., electric circuit interrupting devices comprising a pair of cooperating contacts in an evacuated enclosure, the contacts being relatively movable between closed and open positions to complete or interrupt anelectric power circuit under normal load current and fault current conditions.

Because of the vacuum conditions under which areing occurs between the contacts when they are separated to interrupt current, the mechanism of arc extinction in vacuum interrupters is somewhat different from the mechanism of arc extinction in other types of circuit interrupters in which the arcing occurs in a medium such as insulating gas or oil, and the tendency for the contacts to weld together is very much more severe. Therefore the choice of contact materials for other types of circuit interrupters and related devices, such as spark gaps, cannot be taken as relevant to the choice of Contact materials for vacuum interrupters.

Refractory metals, such as tungsten, molybdenum and their carbides, have been successfully used for the contacts of vacuum interrupters of relatively low current interrupting capability. In particular, such contacts have been constituted by a porous matrix of refractory metal particles metallurgicallybonded together (specifically, a sintered compact of such particles), the interstices of the matrix being infiltrated with a nonrefractory metal (usually, copper) of lower melting and boiling points and higher electrical and thermal conductivities. The refractory matrix provides high mechanical strength and good erosion resistance and, the matrix metal having little tendency to weld and being of low ductility, also maintains smooth contact surfaces and consequently high open-circuit dielectric strength. The presence of the non-refractory constituent reduces the current chopping level, which is excessively high with contacts of refractory metal alone. However, the current interrupting capability of vacuum interrupters having contacts of such infiltrated refractory matrix material is limited to about kilo-amperes (peak) owing to excessive thermionic emission from the refractory constituent at higher currents, whereas there is a demand for vacuum interrupters of very much higher current interrupting capability.

Until the present invention was made, the develop ment of vacuum interrupters of higher current interrupting capability had departed from the use of infiltrated matrix contact materials and had concentrated on the use of non-refractory alloys (typically copperbismuth) in which a major constituent metal is alloyed with a minor constituent which forms brittle films at the grain boundaries between the crystals of the major constituent. With this alloy material the excessive thermionic emission associated with refractory metals is avoided, and the effects of the excessive welding tendency and ductility of non-refractory'metals are relieved by what may be called inter-crystalline weakness enabling inter-contact welds to be easily broken without drawing spikes from the contact surfaces. However, the inter-crystalline weakness makes the contacts mechanically very much interior to those employing the infiltrated matrix material of low ductility. Being mechanically weak throughout the structure the material does not have the ability of the matrix material to retain contact profile during operation, for contact separation tends to rupture intercrystalline boundaries both near the original interface and further into the body of the material. This mechanical weakness also places many-limitations on the mechanical design of the contacts, which ideally is determined by the plasma physics of the arc. Furthermore, such materials of high thermalas well as electrical conductivity do not generally possess an advantage of the above-mentioned infiltrated matrix material, namely, a current chopping level substantially below that of either of the constituents.

In view of the enormous effort that has, on both sides of the Atlantic, been put into developing and exploiting the idea of using non-refractory alloy material having inter-crystalline weakness, despite its many disadvantages, it is apparent that a major inventive contribution to the vacuum interrupter art was needed to put the development of high-current interrupters back on to the right track leading towards the ideal contact material for both very high current interrupting capability and reasonably low current chopping level.

Such a contribution has been made by the present invention, according to which there is provided a vacuum interrupter comprising a pair of contacts having cooperating contact-making parts each of which is constituted by a porous matrix of metal particles metallurgically bonded together, the interstices of the matrix being infiltrated with another metal of lower melting and boiling points and higher electrical and thermal conductivities, wherein at least the bulk of said particles comprises particles of a low ductility semirefractory metal which has a melting point substantially higher than that of copper but lower than that of mo lybdenum and a boiling point not higher than 3,000C and forms a precipitation alloy with the infiltrated metal, but does not otherwise form alloys therewith, so that, in, surface regions of the contacts which are melted by arcing, the precipitation alloy forms and, on subsequent cooling, re-crystallises to re-establish the infiltrated matrix structure with the particle size of the semi-refractory metal particles not exceeding 250 microns,

The matrix may include minor amounts of particles of other metals if desired (say, up to about 5 percent by weight of the semi-refractory metal) provided that they are virtually insoluble in, or do not adversely modify the properties of, the major constituents of the infiltrated matrix.

The term metal is not necessarily limited to elementary metals; it includes suitable metallic alloys.

By low ductility is meant a ductility that is low in relation to that of copper or silver. The ductility of the matrix metal should be low enough to cause the infiltrated matrix to exhibit a percentage elongation in a tensile test of less than 5 percent. The percentage elongations, by way of comparison, of copper and silver lie in the range of 40 to 50 percent.

Theterms melting point and boiling point are defined as evaluated in Scientific Foundations of Vac- Lafferty. It will be appreciatedthat the'boiling point,

being the. temperature at whichthe vapour pressure equals atmospheric pressure (760mm Hg) is only an indirect guide to the materials behaviour in respect of vapour emission in the interior of a vacuum interrupter. However, the general similarity of the vapour-pressure/temperature relationship for the materials under consideration enables boiling point to be used as a convenient figure'of merit in respectof their vapour emitting properties and, consequently, also of their electron emitting properties which are temperature dependent. g

The American Society. for Metals Handbook, Vol. 1 p. 34, under the heading Definitions Relating to Metal and Metal Working, defines the word sinter as-follows: 1

Sinter to heat a mass of fine particles for a prolonged time, below the melting point, usually to cause agglomeration. Again, the American Society for Metals Handbook, Vol. 4, p. 455, under the heading Sintering," completes a further definition thus: or a compact'may be sintered for a short time and then infiltrated with a molten metal of lower melting point". This latter definition clearly indicates that sintering can produce a porous mass which can be infiltrated with another metal.

The precipitation alloying property of the two metals means that the matrix metal is soluble to a substantial extent in the infiltrated metal when the latter is liquid, but the solid solubility is low.(i.e., not more than a few percent). Thus the precipitation alloy will be formed in contact surface regions which are melted by arcingbe tween the contacts, and the surface structure will be refined and improved asa result of re-precipitation on subsequent cooling.

If, as is preferred, the particle size of the matrix metal particles'does not exceed 250 microns throughout the matrix, the performance of the interrupter will be'particularly good ab initio. However, it is possible to use a much larger particle size than 250 microns in manufacture since precipitation alloying during arcing may be relied upon to refine the surface structure and give improved interrupter performance. This may be achieved by a factory conditioning process before the interrupter is put into service, or by arcing during normal operation of the interrupter in service.

Preferably, the semi-refractory metal comprises chromium, though it may alternatively comprise, for example, cobalt or an alloy (or solid solution) consisting substantiallyof desired proportions as they are fully soluble in one another in all proportions. Another possibility is to use a mixture of. particles of more than one such semi-refractory metal (e.g., chromium particles and cobalt particles) each of which forms such a pre- 4 cludin'g eithe'r or both of copper and silver with which the matrix metal does not form an alloy other than a precipitation alloy. 3

Alloys of copper may include zirconium, tantalum or titanium, though only in small proportions; for example, the alloy-may consist of 99.7 per cent copper and 0.3 per cent zirconium by weight. Preferably the infiltrated metal'occupies between 10 and 35 per cent of the volume of the infiltrated matrix, giving particularly low inter-contact weld strength.

One vacuum interrupter embodying the present invention will now be described by "way of example and with reference to the accompanying drawings, in which; i

F IG. 1 showsa vertical cross-section through the interrupter, g

FIG. 2 shows diagrammatically andto a very much larger scale a vertical cross section through an arcing portion of one contact of the vacuum interrupter, FIG. 3 is a photomicrograph of part of a cross-section of a vacuum interrupter contact, showing an arcrefined surface layer on :the, unmodified substrate of a copper-infiltrated chromium matrix, and

- FIG. 4 is a graphical representation of test results showing the variation with percentage composition of the force required to break welds between experimental copper-infiltrated chromium matrix tipped test pieces simulating vacuum interrupter contacts.

FIG. 4A is a cross-sectional view of a weld test piece.

' Referring to FIG. 1, the'vacuum interrupter comprises a pair of

end plates

11, 12 bonded in a vacuumtight manner respectively to

cylinders

13, 14 of insulating material. The

cylinders

13, 14 are bonded to a flange 15 which is trapped between them and carries a shield 16 of generally cylindrically form.

The vacuum interrupter is provided with a pair of relatively

separable contacts

17,18, the movable contact 17 being capable of movement by means of an actuator (not shown) towards and away from the fixed contact 18. The movable contact 17 has its contact stem 21 reciprocable in a bushing 19, and a flexible conductor is provided which is attached to the contact stem 21. A bellows device .20 is secured-in a vacuum-tight manner to the contact stem 21 and to the base plate 12 to allow movement of the contact 17.

The contact stem 2l has a contact head 22 secured to it. The latter is recessed at its centre to afford a flat annular face 23, which co-operates with a similar face 24 on the cooperating contact 18 when the contacts are moved into engagement. The fixed contact 18 has a stem 26, which is secured to the base plate 11 and provides the other terminal of the circuit interrupter, and a head 27 which may conveniently be symmetrical with that of the movablecontact 17.

The contact heads 22, 27 are manufactured by compacting commercially-available pure chromium powder which is not excessively oxidised (e.g., powder as made by the Thermit process) of particle size not exceeding 250 microns, and then sintering the compact under high vacuum. The sintered compact is then infiltrated with molten copper 'under high vacuum and at a high temperature (about I,200 C) just above the melting point of copper. The copper occupies between 10 and 35 per cent of the volume of the infiltrated matrix material, as determined by the porosity of the sintered compact, and hence as determined by the degree of compaction applied to the compact. If necessary, the infiltrated compact may be shaped by normal machining methods. The ductility of chromium is low in relation to that of copper or silver, and the infiltrated matrix exhibits a low ductility giving a percentage elongation of about 2 to 3 per cent where the copper content is 30 per cent by volume.

Thus, contrary to the best beliefs of skilled metallurgists having regard to the substantial solubility of chromium in molten copper (about per cent at 1,200 C) which could well have been expected to result in destruction of the matrix structure during infiltration, the present inventor has succeeded in'bringing together matrix and infiltrant with chromium as the matrix and copper as the infiltrant,

The solid solubility of chromium in copper is low about 0.7 percent near the melting point of copper, less than 0.1 percent at 600 C, and asymptotic to 0.05 percent at low temperatures.

In place of chromium, other suitable'low ductility semi-refractory metals may be used such as, for example, cobalt or'an alloy (or solid solution) of chromium and cobalt or a mixture of chromium powder and cobalt powder; and in place of copper at number of other metals of good electrical conductivity may be used, including silver or an alloy including either or both of copper and silver, with which the matrix metal will not form an alloy other than a precipitation alloy.

Alloys of copper may include zirconium, tantalum or titanium, though only in small proportions; for examplc, the alloy may consist of 99.7 per cent copper and 0.3 per cent zirconium by weight.

In FlG. 2 a typical micro-structure of the contact material is shown, the scale marking representing 200 microns, and each chromium particle 30 is attached to the neighbouring particles of chromium 30 (shown hatched) and immersed in copper 31 which completely fills the interstices.

Since the chromium particle size is limited to 250 microns. the maximum size of asperities is limited, and when such contacts engage under pressure in a vacuum interrupter it is believed that micro-deformation occurs. resulting in a large number of well distributed contact points. It is considered that this condition would tend to result in low resistance between the contacts when closed, good distribution of the PR loss at high current flow immediately prior to arcing, and little tendency to weld.

When such vacuum interrupter contacts separate which is effected at high velocity by actuating means well known per se it is thought that there is a high probability of multiple arcs being formed between the

faces

23, 24, giving good distribution of the are energy and consequently low and uniform erosion.

Even after arcing, it is thought that the size of asperities tends to be limited to that of the maximum dimensions of the particles of thematrix, so that the local electric field stress concentrations and consequent field emission for'a given contact separation is low, and the breakdown voltage is higher and more predictable than 'for contacts of similar shapes made mainly of ductile materials.

FIG. 3 of the drawings is a photomicrograph of a secture in the lower part of the picture, and the similar but very much refined structure in the upper part where alloying and re-precipitation has taken place at the contact surface as a result of arcing. The refined surface layer was found to be about 1 millimetre thick. It can be seen that the particle size of the matrix metal particles in the surface layer is very much less than 250 microns, in fact only a few microns or less. Thus the arc-conditioned surface layer is very smooth and gives excellent vacuum-interrupter performance in respect of dielectric strength when the contacts are open, avoidance of serious contact welding when closed on to a fault, and avoidance of serious erosion due to explosive removal of 'asperities under through fault current and when the contacts are .parted to interrupt the current. By chemical analysis it was found that the proportions of chromium and copper in the arced surface layer were generally unchanged by the melting process, little preferential evaporation of copper occurring.

Thus it has been found that arcing between the interrupter contacts has the unusual effect of maintaining and even improving the good arcing and voltage breakdown characteristics of the arcing surfaces and hence the good performance of the interrupter, instead of the usual effect of degrading the arcing surfaces so that in time the performance of the interrupter falls to an unsatisfactory level.

Moreover, since the boiling point of the semirefractory metal is below 3,000 C, electron emission densities following a high-current arcing loop are reduced by several orders below that which occurs with a tungsten matrix when surface melting occurs, thus allowing a substantial improvement to be achieved in the recovery voltage performance.

Tests on contacts made in accordance with the invention from chromium compacts impregnated with 30 per cent copper by volume have shown that arcs up to at least kA peak can be interrupted satisfactorilywith low and uniform contact erosion and no detectable welding prior to arcing.

As stated above the chromium constituent of the contact material may be replaced by other metals such as cobalt.

The main properties of each such semi-refractory matrix metal are:

a. it is capable of being wetted by the impregnated metal during infiltration;

b. its melting point is higher than that of copper, and (with copper as the infiltrated metal) is preferably over l,200 C, so that it is not melted by the infiltration process;

c. its melting point is lower than that of molybdenum;

d. its boiling point is not greater than 3,000 C; and

e. it has low ductility in relation to that of copper and silver.

As stated above the copper constituent of the contact material may be replaced by other metals such as silver or an alloy-including either or both of copper and silver.

The main properties of each such infiltration metal are:

1. it has high electrical and thermal conductivity in relation to that of the matrix metal;

2. its melting point is below that of the semirefractory matrix metal and is preferably below l,200 C;

, jected or subjected to a different degree.

4. it has highductility' relative l-to' that'of the rnatrix metal; v r I it has low viscosity when'moltenso as to facilitate infiltration'at a temperature just above its melting point. s

with the resultthatfin places some of the matrix, material r rtight'be ejected under arcing'conditions or be torn iawayion parting of the contactsfThe effect offthis would be that'a considerable area of the r'noreductile For reference, the melting points and boiling points i of thefmetals referred to are given byc'e'rtairiauthorities as follows,indegrees Centigrade:

1 melting point boiling point a silver (Ag). 1 960.8 1 '2200 v per't-cu) 1083 2570 Y.

cobalt (Co) i492 3,000 chromium '(Cr) lSQO 2665 1 molybdenum (Mo) 2625 4800 3380' 6000 tungsten (W) lt will'be observed that contact materials according to the resentinventiom comprise a relatively semirefractorymetal of-low ductility and-a'rnetal of relatively good electrical and'thermal conductivity;

With such a contactmat'erial the boiling temperature of thesemi-refractory matrix metal -('e.g.; chromium) is substantially-belowthe temperature at which appreciable electron emission occurs. It, is believed that when during arcing the temperature of the anode hot spot approaches or even rises to the point at-which both metals boil, the-rate of vapourgeneration increases to a point atwhich the loss of heat from vaporisation of the metals substantially-balances the heatxinput to the hot spot fromthe are, so that the temperature vof the material would then be present, and thiscould givje rise to the drawing out of. spike formations 'when the contacts subsequently part, with a consequentialreduction in the voltage withstand ability of the interrupter under high source voltage conditions."

Degradation of the matrix structure by. alloying, if allowed, would also have'an adverseeffect'on the current chopping-performance, in-so far as thereduction of the volume of thematrixmaterial-adjacent-the arcing surface-relative to that oft'he surrounding infiltrated metal would enable more conduction of heat from the arcing surface and so .undesirablyreduce the arcing surface higher current value,

"It hasalso been found thativacuu'r n'interruptersfaccording to the present'invention exhibit very 'go od cur I 'rentchoppi'ng characteristics, which are, even better than those obtained with'interrupters 'havingcontacts made of copper-bismuth alloys or contacts'of tungsten infiltrated with copper. -It is believed that this results from the small size of the metal particles from which the matrix-is=compactedg this size "being'le'ss than thehot spot isheld at o'r nearthe boiling temperature of the jser'ni-refractoryv metal despite: the presence of higher temperatures in the arc itself. Since this boiling temperature of the matrix metal (e.g chrom-iurrrlis.

limit; the anodehot spot temperature-to thatwhich will enable arc extinction at-the next current zeroinstant to hold-off characteristics. Y 3

vlt should be observed that the matrix and infiltrant metals are chosen so that substantially no alloying other thanprecipitation alloying takes place-between them, the matrix metal-having low solid solubilityin the occur, and thereby provide good voltage recovery and high current values and to infiltrated metal. This is desirable since his desired to preserve, so far as possible, the principal characteristics of thetwo constituent metals. As the'prior art has shown, alloying. of minor. constituent metals with a majorconstituent metal results in a substantial modification of the properties of'the major constituent. lnthe present context alloying would result in-an undesirable lack of uniformity in the propertiesfof the electrodematerial, in that parts previously subjected toarcing would have different properties from parts not so subln particular,- alloying of the matrix metal with the infiltrate d metal would, if allowe d, 'undesi rably increase the electrical resistance of the infiltrated metal. It

would also tend to weaken the strong matrix structure.

particles 'of the matrix in the higher electrical conduc ti'vity-infiltrated metaljarid the poor thermal conductivity of the matrix metal which conducts little heat away fromthe arcing surfaceadjacent 'a cathode spot.

v.This achievement of low chopping current levels-is in clearcontradi'stinction to the technique proposed in the prior art for reducing current chopping. levels by providing in the cont'act'materials metals having ahigh .vapouripress'ure, such as antimony,-.bismuth, and tin. However, if azvery low current chopping level is required analloy of copperuwithl percentof, silver can beused as the infiltrated'rnetalm v :lndesigning contacts for a vacuum interrupter ac-. cording ,to the present invention care must-be takento keep the proportions of thematrix and infiltrant metals withinreasonable limits. If the contact material hasv too much matrix metal the voids in the'c ompactedmatrix will be inadequateto give effective infiltration ,of the infiltrant metal, and as a result not only will the contact material have too high an electricaland thermal resistance, butthere will be an insufficient-supply of vapour released.from'theacontact material to sustain the arc at' ensure good current" chop! ping performance. v a r v If on the other hand the contactmaterial'has too little matrix metal the Contact material will be physically weak, and there will beinsufficient of the lowconductivity, metal adjacent thearcing surface so that the current chopping performance) will be poor. Furthermore,

the arcing surface will have a high proportion of the more ductile material so that the arcingsurfaces will be degraded by the successive opening of the contacts since spike formations will be drawn fronithe contact surfaces. Hence the voltage withstand performance will be poor 1 a i FIG. 4 of the drawings shows the results of weldbreak tests on test pieces as illustrated in that figure,

having tips of copper-infiltrated chromium matrix material similar to that of a-vacuum interrupter in accordance with the present invention. It illustrates the excellence of the characteristics of the material in this respect where the infiltrated matrix contains 70 percent or more, by volume, of chromium (i.e., 30 percent or less, by volume, of copper). I

The performance of vacuum interrupters having contacts of copper-infiltrated cobalt matrix material is believed to be just as good as that which has been described with particular reference to copper-infiltrated chromium matrix material.

I claim:

1. A vacuum interrupter comprising a pair of contacts having cooperating contact-making parts each of which is constituted by a porous matrix of metal particles metallurgically bonded together by sintering under high vacuum, said metal particles also being defined as metal particles selected from a group consisting essentially of chromium, cobalt, an alloy of chromium and cobalt, and a mixture of chromium powder and cobalt powder; the interstices of the matrix being infiltrated under high vacuum with another metal of lower melting and boiling points and higher electrical and thermal conductivities, said another metal also defined as a metal selected from a group consisting essentially of copper, silver, an alloy including at least one of the group consisting essentially of copper, silver, an

alloy including at least one of the group copper and silver with which the matrix metal does not form an alloy other than a precipitation alloy, and an alloy of copper including at least one of the group consisting essentially of zirconium, tantalum and titanium; the infiltrated metal constituting between 10 and 35 percent of the volume of the infiltrated matrix and the matrix constituting between 65 and 90 percent thereof, wherein at least the bulk of said particles comprises particles of a low ductility semi-refractory metal which has a melting point substantially higher than that of copper but lower than that of molybdenum and a boiling point not higher than 3,000 C and forms a precipitation alloy with the infiltrated metal, but does not otherwise form alloys therewith so that, in surface regions of the contacts which are melted by arcing, the precipitation alloy conium.

forms and, on subsequent cooling, recrystallizes to reestablish the infiltrated matrix structure with the particle size of the semiwefractory metal particles not exceeding 250 microns.

2'. A vacuum interrupter according to claim 1, wherein the infiltrated metal constitutes about 30 percent of the volume of the infiltrated matrix.

3. A vacuum interrupter according to claim 1, wherein the semi-refractory metal comprises chromium.

4. A vacuum interrupter according to claim 3, wherein the infiltrated metal comprises copper.

5. A vacuum interrupter according to claim 3, wherein the infiltrated metal comprises silver.

6. A vacuum interrupter according to claim 3, wherein the infiltrated metal comprises an alloy of copper and silver.

7. A vacuum interrupter according to claim 3, wherein the infiltrated metal comprises an alloy consisting substantially of 99.7 percent and 0.3 percent zir- 8. A vacuum interrupter according to claim 3, wherein the infiltrated metal constitutes about 30 percent of the volume of the infiltrated matrix.

9. A vacuum interrupter according to claim 1, wherein the semi-refractory metal comprises cobalt.

10. A vacuum interrupter according to claim '9, wherein the infiltreated metal comprises copper.

11. A vacuum interrupter according to claim 9,

. wherein the infiltrated metal comprises silver.

12. A vacuum interrupter according to claim 9, wherein the infiltrated metal comprises an alloy of copper and silver.

13. A vacuum interrupter according to claim 9, wherein the infiltrated metal comprises an alloy consisting substantially of 99.7 percent copper and 0.3 percent zirconium.

14. A vacuum interrupter according to claim 1, wherein the semi-refractory metal is an alloy comprising chromium and cobalt.

Claims

2. A vacuum interrupter according to claim 1, wherein the infiltrated metal constitutes about 30 percent of the volume of the inFiltrated matrix.
3. A vacuum interrupter according to claim 1, wherein the semi-refractory metal comprises chromium.
4. A vacuum interrupter according to claim 3, wherein the infiltrated metal comprises copper.
5. A vacuum interrupter according to claim 3, wherein the infiltrated metal comprises silver.
6. A vacuum interrupter according to claim 3, wherein the infiltrated metal comprises an alloy of copper and silver.
7. A vacuum interrupter according to claim 3, wherein the infiltrated metal comprises an alloy consisting substantially of 99.7 percent and 0.3 percent zirconium.
8. A vacuum interrupter according to claim 3, wherein the infiltrated metal constitutes about 30 percent of the volume of the infiltrated matrix.
9. A vacuum interrupter according to claim 1, wherein the semi-refractory metal comprises cobalt.
10. A vacuum interrupter according to claim 9, wherein the infiltreated metal comprises copper.
11. A vacuum interrupter according to claim 9, wherein the infiltrated metal comprises silver.
12. A vacuum interrupter according to claim 9, wherein the infiltrated metal comprises an alloy of copper and silver.
13. A vacuum interrupter according to claim 9, wherein the infiltrated metal comprises an alloy consisting substantially of 99.7 percent copper and 0.3 percent zirconium.
14. A vacuum interrupter according to claim 1, wherein the semi-refractory metal is an alloy comprising chromium and cobalt.