A SINTERED CONTACT MATERIAL FOR A VACUUM CIRCUIT BREAKER

Abstract: A contact for a vacuum circuit breaker contains copper and also at least two elements of the group of chromium, molybdenum, and tungsten, each of these other elements in an amount not greater than 40% by weight. Up to 20% of low melting-point metal e.g. Bi may be added. Such alloys have a uniform fine-grained structure, leading to improved break-down voltage and large current characteristics.

Description: This invention relates to a sintered contact material for a vacuum circuit breaker which has a high breakdown voltage and excellent large current characteristics. The sintered contact for the vacuum circuit breaker has to satisfy the following characteristic requirements: (1) The interrupting performance should be high. (2) The breakdown voltage should be high. (3) The contact resistance should be low. (4) The fusing force should be low. (5) Wear should be low. (6) The chopping current should be low. However, it is difficult to meet all these requirements with an actual contact, and generally contacts which can meet only particularly important characteristics are used for specific-applications while more or less sacrificing the other characteristics. Heretofore, copper-chromium alloys (hereinafter referred to as Cu-Cr, similar element symbol expressions being used for other elements and alloys of elements as well), Cu-Co, Cu-Bi, Cu-CrBi, Cu-Co-Bi etc have been used for vacuum circuit breaker contacts. As a result of experiments conducted by the inventors, however, it is found that contacts which do not contain a lowmelting metal such as Cu-Cr, and the like have a disadvantage that the fusing force is somewhat high even if they have performance, while contacts containing a low-melting metal, such as Cu- Bi and the like also have the disadvantages that the chopping current is somewhat high if the content of the low-melting metal is less than or equal to 1% by weight, and that the interrupting performance and breakdown voltage are sacrificed if the content is more than 1% by weight. Conventional contact alloys have been composed of Cu which is a good electric conductor, and such elements as Cr, Co Bi and the like, which do not form a solid solution with Cu in order to prevent the reduction of electric conductivity. As a result, when these alloys are produced by a dissolution technique, there result precipitation type metal structures having large size coarsely distributed grains. Generally, the finer and the more uniform in the metal structure of the contact alloy, the better are the interrupting performance, breakdown voltage, and chopping current. For this reason, the alloy abtained by the dissolution process is usually subjected to a heat treatment or to pulverization followed by sintering treatment in order to obtain an alloy which is uniform and fine metal structure. On the other hand, in the case of the sintering techniques an alloy having a uniform, fine metal structure is obtained by previously employing powders having small particle sizes as the raw material. However, these prior art contact alloys have limitations on their breakdown voltage, large current characteristics, chopping current, and uniformity and fineness of their metal structure. Thus, a strong need exists for a sintered contact alloy having better characteristics. Our British Patent specification 2024258 discloses a contact for a vacuum interrupter comprising a copper matrix, in which are uniformly distributed one or two high melting point metal powders, in particular chromium, tungsten, molybdenum, iridium and cobalt, preferably in an amount more than ten per cent by weight. The combination of copper and chromium is discussed in detail, the other alloy metals are merely mentioned. British Patent Specification 1 346 758 and swedish Patent Specification SE-A-427049 each describe contact materials comprising a relatively low percentage composition of copper (between 15% and 55% by weight) a relatively high percentage composition of chromium (between 37% and 84% by weight), and a low percentage composition of tungsten (between 0.5% and 15% by weight). The contact material described in SE-A-427049 is of the sintered matrix type. The object of the present invention is to provide a sintered contact material for a vacuum circuit breaker, having excellent breakdown voltage performance and large current characteristics, thereby overcoming the drawbacks of the prior art discussed above. In one aspect, the present invention resides in a sintered contact material for a vacuum circuit breaker, consisting of copper as a first element, chromium and tungsten, the chromium being present in an amount of at least 10% by weight, characterised in that chromium is present in an amount of 10% to 40% by weight, tungsten is present in an amount of 0.3% to 15% by weight, and the contact material consists of a fine grain structure with uniformly distributed grains, which contact material exhibits a high interrupting performance, provided that the contact material is not a contact material which consists of a porous matrix of an alloy powder forming 45% to 85% of the contact material and a copper filler agent forming 15% to 55% of the contact material, wherein the alloy powder consist of 82% to 99% chromium and 1% to 18% tungsten. We have manufactured alloys using Cu as a first element and various metals as second, third and further elements and conducted experiments by assembling these alloys in vacuum circuit breakers. It was found as a result that the present contact alloys are superior in the breakdown voltage and large current characteristics because more fineness and more uniformity of the grains are achieved in addition to the containment of high-melting metals. The present invention will be further described with reference to the accompanying drawings, in which: Fig. 1a is a picture showing the structure of a Cu-25 wt% Cr alloy manufactured by a prior art sintering process; Fig. 1b is a picture showing the structure of a Cu-24 wt% Cr-5 wt% W alloy according to an embodiment of the present invention; Fig. 2a is a picture showing the structure of a Cu-25 wt% Cr alloy manufactured by a prior art dissolution process; Fig. 2b is a picture showing the structure of a Cu-24 wt% Cr-5 wt% W alloy according to another embodiment of the present invention; Fig. 3 is a graph showing the relation between the hardness and the content of W of a Cu-25 wt% Cr-W alloy; Fig. 4 is a graph showing the relation between the breakdown voltage and the content of W of a Cu-25 wt% Cr-W alloy. Fig. 5 is a graph showing the relation between the contact resistance and the content of W of a Cu-25 wt% Cr-W alloy; Fig. 6 is a graph showing the relation between the fusion resistance and the content of W of a Cu- 25 wt% Cr-W alloy; Fig. 7 is a graph showing the relation between the interrupting capacity and the content of Cr of a Cu-base alloy. Hereinafter, there will be illustrated preferred embodiments of the invention. Fig. 1a shows a picture (with a magnification of 100 ×) of the structure of a prior art Cu-Cr alloy. This Cu-Cr alloy is obtained by mixing 75% by weight of Cu particles and 25% by weight of Cr particles and molding and sintering the mixture. It has large, coarsely distributed, cloud-like Cr grains. Fig. 1b shows a picture (with a magnification of 100 ×) of a Cu-Cr-W alloy according to an embodiment of the present invention. This Cu-Cr-W alloy is obtained by mixing 71% by weight of Cu particles 24% by weight of Cr particles, and 5% by weight of W particles and molding and sintering the mixture. Its Cr grains are again cloud-like, but they are far smaller and more uniformly distributed compared to in the alloy of Fig. 1a. Cu grains are also smaller and more uniformly distributed. The alloys shown in Figs. 1a and 1b are obtained by using the same lot of Cu and Cr particles as starting materials. Alloys obtained by the dissolution process show a similar trend. Fig. 2a shows a picture (with a magnification of 100 ×) of the structure of a Cu-Cr alloy obtained by the prior art dissolution process, and Fig. 2b shows a picture (with a magnification of 100 ×) showing the structure of a Cu-Cr-W alloy according to one embodiment of the invention. The alloy components of Fig. 2a correspond to those of Fig. 1a, and the alloy components of Fig. 2b correspond to those of Fig. 1b. It will be seen from these pictures that the component, W has a significant effect on both the uniformity and fineness of the grains. When the content of W is varied with Cu-25 wt% Cr as base, the uniformity and the fineness of the grain structure begin to develop from approximately 0.3% by weight of the W content. As, the grain structure becomes finer and more uniform with the increase of the content of W, the characteristics of the alloy gradually change. Hereinafter the relations between the content of W and various characteristics of alloy will be discussed. Fig. 3 shows the relation between the hardness and the content of W. It will be seen that the hardness is significantly increased compared to that of the prior art Cu-Cr alloy. Fig. 4 shows the relation between the breakdown voltage and the content of W. The breakdown voltage is increased with increasing W content. Fig. 5 shows the relation between the contact resistance and the content of W. The contact resistance increases with increasing W content. Fig. 6 shows the relation between the fusion resistance and the content of W. Improved fusion resistance can be obtained for a low W content range. However, the fusion resistance becomes inferior when the content of W is increased beyond about 15% by weight. It is thought from Fig. 5 that an increase in the W content increases the contact resistance to reduce the conductivity so as to increase the heat generation. The characteristics discussed above have been obtained by adding W to a base alloy containing Cu and Cr in weight proportions of 75: 25. Similar effects may be obtained by varying the content of Cr. Fig. 7 shows the relation between interrupting capacity and the content of Cr. As can be seen from the graph, Cr does not have an outstanding influence on the interrupting performance so long as its content is in a range of 10 to 40% by weight. While the above embodiments are concerned with alloys composed of Cu, Cr and W, similar effects on the uniformity and fineness of grain structure can be obtained with low chopping current vacuum circuit breaker contact which is obtained by adding low-melting metals such as Bi, Te, Sb, TL, and Pb to the alloys mentiones above. Further, it is found that the low-melting metals will not be coagulated but are uniformly and finely distributed and that low chopping current can always be maintained irrespective of the number of times that the load is broken. Further, similar effects may be obtained by incorporating Cr and W in the form of alloys or intermetallic compounds with other metals. The uniform, fine alloy structure is thought to be obtained when the following requirements are met. (1) The alloy contains Cu as a first component Cr and W. Cr and W each have a cubic system and entirely form a solid solution. (2) By the sintering process this occurs at temperatures above the melting point of Cu (1,083°C) as well as at temperatures below the melting point. In conclusion, the uniformity and fineness of grain structure is thought to be based on the formation of a complete solid solution of the elements consisting of Cr and W and also the effect of diffusion of these members.

Claims: 1. A sintered contact material for a vacuum circuit breaker, consisting of copper as a first element, chromium and tungsten, the chromium being present in an amount of at least 10% by weight, characterised in that chromium is present in an amount of 10% to 40% by weight, tungsten is present in an amount of 0.3% to 15% by weight, and the contact material consists of a fine grain structure with uniformly distributed grains, which contact material exhibits a high interrupting performance, provided that the contact material is not a contact material which consists of a porous matrix of an alloy powder forming 45% to 85% of the contact material and a copper filler agent forming 15% to 55% of the contact material, wherein the alloy powder consists of 82% to 99% chromium and 1% to 18% tungsten. 2. A sintered contact material for a vacuum circuit breaker according to claim 1, wherein chromium is present in an amount of 25% by weight. 3. A sintered contact material for a vacuum circuit breaker according to claim 1, wherein the tungsten is present in an amount of 5% by weight, the copper is present in an amount of 71% by weight, and the chromium is present in an amount of 24% by weight. 4. A sintered contact material for a vacuum circuit breaker according to claim 1, claim 2 or claim 3, wherein the contact material also consists of at least one of a low melting-point metal selected from bismuth, tellurium, antimony, thallium and lead and alloys and intermetallic compounds of these low-melting metals, in an amount not greater than 20% by weight.

Inventors: DEMIZU, MICHINOSUKE, HORIUCHI, TOSHIAKI, INAGAKI, KOUICHI, NAYA, EIZO, OKUMURA, MITSUHIRO, TAKEYA, YASUSHI
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1982
Main classification: H01H1
IPC codes: H01H33, H01H1