CONTACT ELECTRODE MATERIAL FOR VACUUM INTERRUPTER AND METHOD OF MANUFACTURING THE SAME

Abstract: A novel contact electrode material for vacuum interrup­ ters is disclosed, by which the chopping current value inherent in contact material can be reduced so that it is possible to stably interrupt small lagging current due to inductive loads without generating surge voltages. The material is equivalent or superior to the conventional Cu-0.5Bi material in large current interrupting capability and dielectric strength. The material consists essentially of copper, chromium, iron or molybdenum and metal carbide. The metallographical microstructure is such that copper is infiltrated into a porous matrix formed by mutually bonding chromium powder, iron or molybdenum powder and metal carbide powder in diffusion state. In its manufacturing process, firstly copper is placed onto a powder mixture of chromium, iron or molybdenum, and metal carbide, and then the copper and the powder mixture is heated within a nonoxidizing atmosphere at a first temperature lower than the copper melting point and thereafter again at a second temperature higher than the copper melting point.

Description: The present invention relates generally to contact electrode material used for a vacuum interrupter and to processes of manufacturing the contact electrode material. Contact electrode material exerts serious influences upon circuit interruption performance in a vacuum interrupter. Generally, the contact electrode is required to consistently satisfy the following various requirements: 1) Higher large-current interrupting capability, 2) higher dielectric strength, 3) excellent anti-welding characteristic, 4) higher small lagging- or leading-current interrupting capability, 5) higher electric conductivity, 6) lower electrode contacting electric resistance, 7) excellent anti-erosion characteristic. In the above requirements, the item 4), in particular will be explained in more detail hereinbelow. In the case where an inductive load is connected to a circuit to be interrupted, current lags as compared with voltage in phase. The current lagging as compared with voltage is called lagging current. On the other hand, in the case where a capacitive load is connected to a circuit to be interrupted, current leads as compared with voltage in phase. The current leading as compared with voltage is called leading current. In order to improve the above-mentioned lagging- or leading current interrupting capability, in particular the lagging-current interrupting capability, it is indispenable to reduce the chopping current value inherently determined in contact electrode material provided for a vacuum interrupter. The above chopping current value will be described in detail. When a small AC current is interrupted by an interrupter, a small-current arc is produced between two contact electrodes. When the small AC arc current drops near zero, there exists an arc current chopping phenomenon such that the current wave begins to vibrate and then is chopped (suddenly drops to zero) before the current reaches zero. An arc current I o at which vibration begins is called unstable current; an arc current I c , at which current is chopped is called chopping current. In practical use, since this chopping current generates surge voltage, there exists a danger that electrical devices connected to the circuit interrupter may be damaged. The reason why the arc current is chopped is explained as follows: When arc current reaches near zero, since the number of metal particles emitted from the cathode spots decreases below a particle density at which the arc can be maintained, the arc current becomes unstable, resulting in current vibration and further current chopping. Since the chopping current generates harmful surge voltages, it is preferable to reduce the chopping current so that it is as small as possible. The chopping current value decreases with increasing vapor pressure of the cathode material (low melting point material), because the higher the vapor pressure, the longer metal vapor necessary for maintaining an arc will be supplied. Further, the chopping current value decreases with decreasing thermal conductivity of cathode material, because if thermal conductivity is high, heat on the cathode surface is easily transmitted into the cathode electrode and therefore the cathode surface temperature drops abruptly, thus reducing the amount of metal vapor emitted from the cathode spot. Therefore, in order to reduce the chopping current value, it is preferable to make the contact electrode of a material having a low thermal conductivity and high vapor pressure (low melting point). In contrast with this, however, in order to improve the large-current interrupting capability, it is preferable to make the contact electrode of a material having a high thermal conductivity and low vapor pressure (high melting point). As described above, since the high current interrupting capability is contrary to the low chopping current value, various efforts have been made to find out special alloys suitable for the contact electrode for a vacuum interrupter. The mutually inconsistent relationship between large current interrupting capability and small-current interrupting capability has already been described. However, another mutually inconsistent relationship also exists between certain ones of the requirements already listed above with respect to the contact electrode material for a vacuum interrupter. For instance, US-A-3 246 976 discloses a copper alloy for contact electrode, which includes bismuth (Bi) of 0.5 percent by weight (referred to as Cu-0.5Bi hereinafter). Further, US-A-3 596 027 discloses another copper alloy for contact electrode, which includes a small amount of a high vapor pressure material such as tellurium (Te) and selenium (Se) (referred to as Cu-Te-Se hereinafter). The Cu-0.5Bi or the Cu-Te-Se including a high vapor pressure material is excellent in large-current interrupting capability, anti-welding characteristic and electric conductivity; however, there exists a drawback such that the dielectric strength is low, in particular the dielectric strength is extremely reduced after large current has been interrupted, in addition, since the chopping current value is as high as 10 amperes, surge voltages are easily generated when current is interrupted, and it is thus impossible to stably interrupt small lagging currents. That is to say, there exists a problem in that electrical devices connected to a vacuum interrupter may often be damaged by the surge voltages. On the other hand, in order to settle the above-mentioned problems resulting from the above Cu-0.5Bi or Cu-Te-Se, US-A-3 811939 discloses an alloy for a contact electrode, which substantially consists of copper of 20 percent by weight and tungsten of 80 percent by weight (referred to as 20Cu-80W hereinafter). Similarly, GB-A-2 024 257 discloses a copper alloy for contact electrode, which includes a low vapor pressure material such as tungsten (W) skeleton (high melting point material) for use at a high voltage. More specifically one or two different high melting point metal powders (above 1450°C) with diameters of (1) 80-300 µm and (2) less than 30 µm are distributed in amounts of 10 percent by weight or more in a copper matrix. The metal powders are selected from Cr, W, Mo, Ir and Co. The contact is formed by a melt-casting process at a temperature lower than the melting point of either of the high melting point metal powders. The resulting contact has high voltage tolerance, low melt bonding tendency, high current durability, and low chopping current. The 20Cu-80W or the copper-tungsten-skeleton alloy is high in dielectric strength; however, there exists a drawback in as much as is difficult to stably interrupt a large fault current produced by an accident. With these problems in mind, therefore, it is the primary object of the present invention to provide a contact electrode material used for a vacuum interrepter and a method of manufacturing the same by which chopping current value can be so reduced that small lagging current can stably be interrupted without generating surge voltages while satisfying other various requirements such as large current interrupting capability, dielectric strength, anti-welding characteristic, etc. In order to satisfy the above-mentioned object, there is provided in accordance with the resent invention a contact electrode material and process of manufacturing that material. The invention as defined in the claims. Advantageous developments and variations of the contact material and manufacturing processes are set forth in the subordinate claims. For the sake of completeness it is pointed out that DE-A-26 19 459 discloses contact material including compounds of alloys of metals with a boiling point above 2400°C of Sn, Cr₃C₂, and ZrCu₄, in order to keep the breaking current and the accompanying overvoltage four times the magnitude of the nominal voltages. US-A-4 032 301 proposes a contact material including a composite inclusion metal of at least two metal components. The first component has an electric conductivity of at least 10 m/ohm mm², 35-60% by volume. At least one component has a melting point of 1400°C. The porosity of the metal is less than 2% by volume. The contact metal is economical to manufacture. EP-A-0 101 024 describes contact material including 20-70 percent by weight Cu, 5-70 percent by weight Mo and 5-70 percent by weight Cr. A mixture of Mo and Cr powders are diffusion bonded into a porous matrix and then copper is infiltrated into the matrix. Alternatively, the materials are produced by sintering a mixture of three metal powders. The material is high in large current interrupting capability, in small lagging and leading current, interrupting capability, and in dielectric strength. EP-A-0 083 245 proposes a contact containing Cu and at least two of Cr, Mo, and W each in an amount not greater than 40% by weight. A low melting-point metal Bi (20% or less) can be added. The contact has a uniform fine-grained structure, improved breakdown voltage and large current characteristics. Finally, US-A-3 683 138 describes contacts containing a sintered metal carbide selected from WC, MoC, ZrC, TiC, VC, SiC and the combinations thereof and a wettable material composed of 0.1-5 percent by weight Ni, 0.1-1 percent by weight Cu, and 0.1-5 percent by weight Co. The sintered alloy is impregnated with at least one type of higher conductive metal of 10-60 percent by weight total weight. The contact improves arc maintenance characteristics during interruption of low currents. The features and advantages of the contact electrode material for a vacuum interrupter and the process of manufacturing the same according to the present invention over the prior-art contact electrode material will be more clearly appreciated from the following description taken in conjunction with the accompanying drawings in which: Fig. 1 is a longitudinal sectional view of a vacuum interrupter to which the contact electrode material according to the present invention is applied; Figs. 2(A) to 2(E) all are photographs taken by an X-ray microanalyzer, which show microstructures of a first test sample of a first embodiment of contact electrode material according to the present invention, the material thereof consisting essentially of 50 weight-percent copper, 5 weight-percent chromium, 40 weight-percent iron and 5-weight-percent chromium carbide; Fig. 2(A) is a secondary electron image photograph showing an insular porous matrix obtained by uniformly and mutually diffusion bonding chromium powder, iron powder and chromium carbide powder in black and copper infiltrated into the insular porous matrix in gray; Fig. 2(B) is a characteristic X-ray image photograph showing insular agglomerates indicative of the presence of chromium in gray; Fig. 2(C) is a characteristic X-ray image photograph showing insular agglomerates indicative of the presence of iron in white; Fig. 2(D) is a characteristic X-ray image photograph showing faint points indicative of the presence of carbon in white; Fig. 2(E) is a characteristic X-ray image photograph showing distributed parts indicative of the presence of copper infiltrated into the insular porous matrix in white; Figs. 3(A) to 3(E) all are photographs taken by an X-ray microanalyzer, which show microstructures of a second test sample of the first embodiment of contact electrode material according to the present invention, the material thereof consisting essentially of 50 weight-percent copper, 20 weight-percent chromium, 20 weight-percent iron and 10 weight-percent chromium carbide; Fig. 3(A) is a secondary electron image photograph showing an insular porous matrix obtained by uniformly and mutually diffusion bonding chromium powder, iron powder, and chromium carbide powder in black, and copper infiltrated into the insular porous matrix in gray; Fig. 3(B) is a characteristic X-ray image photograph showing insular agglomerates indicative of the presence of chromium in gray; Fig. 3(C) is a characteristic X-ray image photograph showing insular agglomerates indicative of the presence of iron in white; Fig. 3(D) is a characteristic X-ray image photograph showing faint points indicative of the presence of carbon in white; Fig. 3(E) is a characteristic X-ray image photograph showing distributed parts indicative of the presence of copper infiltrated into the insular porous matrix in white; Figs. 4(A) to 4(E) all are photographs taken by an X-ray microanalyzer, which show microstructures of a third test sample of the first embodiment of contact electrode material according to the present invention, the material thereof consisting essentially of 50 weight-percent copper, 40 weight-percent chromium, 5 weight-percent iron and 5 weight-percent chromium carbide; Fig. 4(A) is a secondary electron image photograph showing an insular porous matrix obtained by uniformly and mutually diffusion bonding chromium powder, iron powder, and chromium carbide powder in black, and copper infiltrated into the insular porous matrix in gray; Fig. 4(B) is a characteristic X-ray image photograph showing insular agglomerates indicative of the presence of chromium in white; Fig. 4(C) is a characteristic X-ray image photograph showing insular agglomerates indicative of the presence of iron in gray; Fig. 4(D) is a characteristic X-ray image photograph showing faint points indicative of the presence of carbon in white; Fig 4(E) is a characteristic X-ray image photograph showing distributed parts indicative of the presence of copper infiltrated into the insular porous matrix in white. With reference to the attached drawings, reference is now made to the embodiment of the contact electrode material accordimg to the present invention. Prior to the description of the contact electrode material, the structure of a vacuum interrupter to which the contact electrodes made of the material according to the present invention is applied will be explained briefly hereinbelow with reference to Fig. 1. In Fig. 1, a vacuum interrupter is roughly made up of a vacuum vessel 1 and a pair of contact electrodes 2A and 2B joined to a pair of stationary and movable contact electrode rods 3A and 3B, respectively. The vacuum vessel 1 is evacuated to a vacuum pressure of 6.67 mPa (5x10⁻⁵ Torr) or less, for instance. The vacuum vessel 1 includes a pair of same-shaped insulating cylinders 4A and 4B made of glass or alumina ceramics, a pair of metallic end disc plates 5A and 5B made of stainless steel, and four thin metallic sealing rings 6A, 6B and 6C made of Fe-Ni-Co alloy or Fe-Ni alloy. The two insulating cylinders 4A and 4B are serially and hermetically connected by welding or brazing to each other with two sealing metallic rings 6c sandwiched therebetween at the inner adjacent ends of the insulating cylinders 4A and 4B. The two metallic end disc plates 5A and 5B are also hermetically connected by welding or brazing to the insulating cylinders 4A and 4B with the other two sealing metallic rings 6A and 6B sandwiched therebetween at the outer open ends of the insulating cylinders 4A and 4B. A cylindrical metallic arc shield made of stainless steel 7 which surrounds the contact electrodes 2A and 2B is hermetically supported by welding or brazing by the two sealing metallic rings 6c with the shield 7 sandwiched therebetween. Further, a thin metallic bellows 8 is hermetically and movably joined by welding or brazing to the movable contact electrode rod 3B and the end disc plate 5B on the lower side of the vacuum vessel 1. The arc shield 7 and the bellow shield 8 are both made of stainless steel. One contact electrode 2A (upper) is secured by brazing to the stationary electrode rod 3A; the other contact electrode 2B (lower) is secured by brazing to the movable electrode rod 3B. The stationary electrode rod 3A is hermetically supported by the upper end disc plate 5A; the movable electrode rod 3B is hermetically supported by the bellows 8. The movable contact electrode 2B is brought into contact with or separated from the stationary contact electrode 2A. The first embodiment of contact electrode material according to the present invention will be described hereinbelow. The material is a composite metal consisting essentially of copper of 20 to 80 percent by weight, chromium of 5 to 45 percent by weight, iron of 5 to 45 percent by weight and chromium carbide of 0.5 to 20 percent by weight. This composite metal has an electric conductivity of 5 to 30 percent in IACS (abbreviation of International Annealed Copper Standard). The metallographical feature of the composite metal according to the present invention is such that: copper (Cu) is infiltrated into an insular porous matrix obtained by uniformly and mutually bonding powder particles of chromium (Cr), iron (Fe) and chromium carbide (Cr₃C₂) by sintering in diffusion state. The above diffusion bonding means here that powder particles are not bonded to each other on the surfaces thereof but bonded to each other in such a way that one particle diffusely enters into the other particle beyond the surfaces thereof. Further, the particle diameter of each metal powder (Cr, Fe, Cr₃C₂) is 60 mesh (250 µm) or less, but preferably 100 mesh (149 µm) or less. The process of manufacturing the above-mentioned contact electrode material according to the present invention will be described hereinbelow. The process thereof can roughly be classified into two steps: mutual diffusion bonding step and copper infiltrating step. In the mutual diffusion bonding step, chromium powder (Cr), iron powder (Fe) and chromium carbide (Cr₃C₂) powder are bonded to each other into a porous matrix in diffusion state. In the copper infiltrating step, melted copper (Cu) is infiltrated into the porous matrix. Here, it should be noted that the melting point of chromium is approx. 1890°C, that of iron is approx. 1539°C, that of carbon is approx. 3700°C and that of copper is approx. 1083°C (the lowest). Further, the process thereof can be achieved by three different methods as described hereinbelow. In the first method In this method, the metal powder diffusion bonding step and copper infiltrating step are processed within two different nonoxidizing atmospheres. In more detail, firstly, Cr powder, Fe powder, and Cr₃C₂ powder each having the same particle diameter are prepared. The selected particle diameter is 100 mesh (149 µm) or less. Secondly, predetermined amounts of three metal (Cr, Fe, Cr₃C₂) powders are mechanically and uniformly mixed. Thirdly, the resultant powder mixture is placed in a vessel made of material non-reactive to Cr, Fe, Cr₃C₂ or Cu (e.g. aluminum oxide or alumina). Fourthly, the powder mixture in the vessel is heated within a nonoxidizing atmosphere at a temperature (e.g. 600 to 1000°C) lower than the melting point of each metal powder for a predetermined time (e.g. 5 to 60 min.) in order that the owders (Cr, Fe, Cr₃C₂) are uniformly diffusion bonded to each other into a porous matrix. The nonoxidizing atmosphere is, for instance, a vacuum of 6.67 mPa (5x 10⁻⁵ Torr) or less, hydrogen gas, nitrogen gas, argon gas, etc. Fifthly, a copper (Cu) block is placed onto the formed porous matrix. Sixthly, the porous matrix onto which the Cu block is placed is heated again within another nonoxidizing atmosphere at a temperature (e.g. 1100°C) higher than the melting point of copper but lower than the melting points of other metal powders and the porous matrix for a predetermined time (e.g. 5 to 20 min) in order that the copper (Cu) is uniformly infiltrated into the porous matrix of Cr, Fe and Cr₃C₂. As described above, in this method, the porous matrix is formed before copper is infiltrated. However, being different from the above process it is also possible to manufacture the contact electrode material according to the present invention in such a manner that firstly the porous matrix is formed within a gas atmosphere (e.g. hydrogen gas) and then copper is infiltrated thereinto by evacuating the hydrogen gas. In the second method In this method, the diffusion bonding step and the copper infiltrating step are processed within the same nonoxidizing atmosphere. In more detail, firstly, Cr powder, Fe powder and Cr₃C₂ powder each having the same particle diameter are prepared. The selected particle diameter is 100 mesh (149 µm) or less. Secondly, predetermined amounts of three (Cr, Fe, Cr₃C₂) powders are mechanically and uniformly mixed. Thirdly, the resultant powder mixture is placed in a vessel made of material nonreactive to Cr, Fe, Cr₃C₂ or Cu (e.g. alumina). Fourthly, a copper block is placed onto the powder mixture. Fifthly, the powder mixture onto which the copper block is placed in the vessel is heated within a nonoxidizing atmosphere at a temperature (e.g. 600 to 1000°C) lower than the melting point of copper for a predetermined time (e.g. 5 to 60 min) in order that metal powders (Cr, Fe, Cr₃C₂) are uniformly diffusion-bonded to each other to form a porous matrix. Sixthly, the same powder mixture is heated within the same nonoxidizing atmosphere at a temperature (e.g. 1100°C) higher than the melting point of copper but lower than the melting points of other metal powders and the porous matrix for a predetermined time (e.g. 5 to 20 min) in order that the copper block is uniformly infiltrated into the formed porous matrix of Cr, Fe, and Cr₃C₂. As described above, in this method, the porous matrix is formed before copper is infiltrated within the same nonoxidizing atmosphere. In the third method In this method, copper powder is mixed with other powders instead of a copper block. In more detail, firstly, Cr powder, Fe powder, Cr₃C₂ powder and Cu powder each having the same particle diameter are prepared. Secondly, predetermined amounts of four (Cr, Fe, Cr₃C₂, Cu) powders are mechanically and uniformly mixed. Thirdly, the resultant powder mixture is press-formed into a predetermined contact electrode shape. Fourthly, the press-shaped contact material is heated within a nonoxidizing atmosphere at a temperature higher or lower than the melting point of copper but below the melting points of other metal powders. In this third method, it is also possible to place an additional copper block onto the press-shaped contact material. In this case, however, it is necessary to heat the press-shaped contact material onto which the copper block is placed to a temperature higher than the melting point of copper. In the above three methods, the particle diameter is not necessarily limited to 100 mesh (149 µm) or less. It is possible to select the metal powder particle diameter to be 60 mesh (250 µm) or less. However, in the case where the particle diameter exceeds 60 mesh (250 µm), the diffusion distance increases in diffusion bonding step of metal powder particles and therefore heating temperature should be high or heating time should be long, thus lowering the productivity. Here, when one metal is diffused from the surface of the other metal, the diffused metal is rich near the surface of the other metal but poor inside the other metal. Therefore, the diffusion distance indicates a distance from the metal surface to a position at which the concentration of diffused metal equals that of the other metal. On the other hand, in the case where the metal powder particle diameter is extremely small (e.g. 1 µm or less), it is rather difficult to uniformly mix each metal powder because power is not dispersed uniformly. In addition, since the small-diameter metal powder is easily oxidized, it is necessary to previously treat the metal powder chemically, thus necessitating a troublesome process and also reducing the productivity. Therefore, metal powders having particle diameter of 60 mesh (250 µm) or less should be selected taking various factors into consideration. Furthermore, it is preferable to heat the metal powder mixture within a vacuum (as nonoxidizing atmosphere). This is because it is possible to simultaneously degasify and evacuate the atmosphere when heating it. However, it is of course possible to heat the powder mixture within a nonoxidizing atmosphere other than a vacuum without bringing up practical problems with the contact electrode material for a vacuum interrupter. In addition, the heating temperature and the heating time required for the mutual diffusion bonding step of metal powders should be determined under consideration of various factors such as furnace conditions, shape and size of the porous matrix to be formed, productivity, etc., so that various performances required for contact electrodes can be satisfied. In the methods described above, heat treatment conditions in the mutual diffusion bonding step are typically 600°C in temperature and 1 to 2h (hours) in time, or 1000°C in temperature and 10 to 60 min (minutes) in time, for instance. The metallographical structure or the microstructure of the first embodiment of the composite metal contact electrode material according to the present invention will be described hereinbelow with reference to Figs. 2 and 4, the microphotographs of which are obtained by means of an X-ray microanalyzer. The contact electrode materials shown in Figs. 2 to 4 are manufactured in accordance with the second method in such a way that the metal powder mixture is heated within a vacuum of 6.67 mPa (5x10⁻⁵ Torr) or less at 1000°C for 60 min to form a porous matrix and further heated within the same vacuum at 1100°C for 20 min to infiltrate copper into the porous matrix. Each component composition (percent by weight) of three test samples corresponding to the first embodiment of the present invention shown in Figs. 2 to 4 is as follows: 1st Sample (Fig. 2): 50Cu-5Cr-40Fe-5Cr₃C₂ 2nd Sample (Fig. 3): 50Cu-20Cr-20Fe-10Cr₃C₂ 3rd Sample (Fig. 4): 50Cu-40Cr-5Fe-5Cr₃C₂ Figs. 2(A) to 2(E) show microphotographs of the first test sample. This sample has a composition consisting essentially of 50% copper, 5% chromium, 40% iron, and 5% chromium carbide each by weight. Fig. 2(A) is a secondary electron image photograph taken by an X-ray microanalyzer, which clearly shows a microstructure of the first test sample of the first embodiment. In the photograph, the clear black insular agglomerates indicate the porous matrix obtained by mutually diffusion bonding Cr, Fe and Cr₃C₂ powders; the distributed gray or white parts indicate copper infiltrated into the insular porous matrix. Fig. 2(B) shows a characteristic X-ray image of chromium (Cr), in which white or gray insular agglomerates indicate the presence of diffused chromium. Fig. 2(C) shows a characteristic X-ray image of iron (Fe), in which white insular agglomerates indicate the presence of diffused iron. Fig. 2(D) shows a characteristic X-ray image of carbon (C), in which faint white dots indicate the presence of a small amount of scattered carbon, Fig. 2(E) shows a characteristic X-ray image of copper (Cu), in which white distributed parts indicate the presence of copper infiltrated into the black insular porous matrix. When comparing these photographs with each other, excluding Fig. 2(D), it is clear that the insular agglomerates are the same in shape. This indicates that the insular agglomerates include chromium and iron but not copper. Although the carbon is not clearly shown, it is quite clear that chromium carbide is also distributed or diffused within the insular agglomerates. Fig. 2(B) clearly shows that chromium is uniformly diffused and black dots indicative of other metals (Fe, Cr₃C₂) are also uniformly diffused. Further, in Fig. 2(B), the white regions indicate that chromium is rich; the gray regions indicate that chromium is poor; the black regions indicates that no chromium is present. Here, it should be noted in particular that the edges or boundaries of insular agglomerates are not clear excepting Fig. 2(A). This indicates that an extremely small amount of copper is diffused into porous matrix near the boundary thereof in Figs. 2(B), (C) and (E). That is to say, copper is infiltrated not only into porous spaces of the porous matrix but also diffused into inner region of the porous matrix near the surfaces thereof. In summary, these photographs clearly indicate that (1) chromium, iron and chromium carbide are uniformly wnd mutually diffusion-bonded into the insular porous matrix and (2) copper is infiltrated between and into the porous matrix. Figs. 3(A) to 3(E) show microphotographs of the second test sample. This sample has a composition consisting essentially of 50% copper, 20% chromium, 20% iron and 10% chromium carbide each by weight. Fig. 3(A) is a secondary electron image photograph similar to Fig. 2(A). Figs. 3(B), 3(C), 3(D) and 3(E) are characteristic X-ray images of chromium, iron, carbon and copper, respectively, similar to Figs. 2(B), 2(C), 2(D) and 2(E). As compared with the first sample shown in Figs. 2(A) to 2(E), since the second sample material includes a greater amount of chromium than in the first sample material, the insular agglomerates shown in Fig. 3(B) is whiter than that shown in Fig. 2(B). However, since the material includes a smaller amount of iron, the insular agglomerates shown in Fig. 3(C) is a little blacker than that shown in Fig. 2(C). Figs. 4(A) to 4(E) show microphotographs of the third test sample. This sample has a composition consisting essentially of 50% copper, 40% chromium, 5% iron, and 5% chromium carbide each by weight. Fig. 4(A) is a secondary electron image photograph similar to Fig. 2(A). Figs. 4(B), 4(C), 4(D) and 4(E) are also characteristic X-ray images of chromium, iron, carbon and copper, respectively, similar to Figs. 2(B), 2(C), 2(D), and 2(E). As compared with the second example shown in Figs. 3(A) to 3(E), since the third test sample material includes a much greater amount of chromium, the insular agglomerates shown in Fig. 4(B) is more whiter than that shown in Fig. 3(B). However, since the material includes a much smaller amount of iron, the insular agglomerates shown in Fig. 4(C) is much blacker than that shown in Fig. 3(C). Further, in Fig. 4(B), some black spots (shown by Cu) located within a white insular agglomerate indicate positions at which copper is rich. This is because the similar black spots can be seen at the corresponding positions in Fig. 4(C) (this indicates a metal (e.g. Cu) other than iron) and the similar white spots can be seen at the corresponding positions in Fig. 4(E) (this indicates copper). Further, in Fig. 4(C), some large black spots (shown by Cr) located within an insular agglomerate indicate positions at which chromium is rich. This is because the similar black spots cannot be seen in Fig. 4(B) (this indicates chromium) and the similar white spots cannot be seen in Fig. 4(E) (this indicates a metal (e.g. Cr) other than copper). Anyway, these black spots indicate that each metal powder is not perfectly uniformly diffused. Various performances of the first embodiment of contact electrode material according to the present invention will be described hereinbelow. The test sample contact material is manufactured in accordance with the second method and machined to a disc-shaped test sample contact electrode. The test sample electrode is 50 mm in diameter and 6.5 mm in thickness having a chamber radius of 4 mm at the edges thereof. Further, various tests have been performed by assembling the test sample electrodes in a vacuum interrupter as shown in Fig. 1. Three kinds of performance test samples are made of three sample materials already described us the first sample (50Cu-5Cr-40Fe-5Cr₃C₂), the second sample (50Cu-20Cr-20Fe-10Cr₃C₂) and the third sample (50Cu-40Cr-5Fe-5Cr₃C₂), respectively. (1) Large-current interrupting capability Using the 1st, 2nd and 3rd test samples, it is possible to interrupt a large current of 12 kA (r.m.s.) under conditions that rated voltage is 12 kV, transient recovery voltage is 21 kV (JEC-181) (Japanese Electric Commission Standard); and interruption speed is 1.2 to 1.5 m/s. The above capability is equivalent to that of the conventional Cu-0.5Bi contact electrode material. (2) Dielectric strength For the 1st, 2nd and 3rd test samples, the dielectric strength is ± 110 kV (standard deviation ±10 kV) in impulse voltage withstand test with a 3.0 mm gap between stationary and movable contact electrodes. Further, although the same test is performed after a large current (12 kA) has been interrupted several times, the same dielectric strengths are obtained. Further, although the same test is performed after a small leading current of 80A (r.m.s.) has been interrupted many times, the dielectric strength is the same. In the case of the conventional Cu-0.5Bi contact electrode material, the same dielectric strength can be obtained when the gap between the electrodes is set to 10 mm. Therefore, in the contact material according to the present invention, it is possible to enhance dielectric strength as much as 3 times that of the conventional Cu-0.5Bi material. (3) Anti-welding characteristic With the 1st, 2nd and 3rd test samples, it is possible to easily separate two electrodes by a static force of 1961 N (200 kgf) after a current of 25 kA (r.m.s.) has been passed for 3s (seconds) under a pressure force of 1275N (130 kgf) (IEC short time current standard) (International Electric Commission Standard). The increase in electrode contacting electric resistance after electrodes separation is less than 4 to 10 percent of the initial value. Further, it is also possible to easily separate two electrodes after a current of 50 kA (r.m.s.) has been passed for 3s (seconds) under a pressure force of 9807N (1000 kgf). An increase in electrode contacting electric resistance after electrodes separation is less than 0 to 6 percent of the initial value. When compared with the conventional Cu-0.5Bi contact material, the anti-welding characteristic of the samples according to the present invention is about 70% of that of the conventional one. However, the above characteristic is sufficient in practical use. Where necessary, it is possible to increase the instantaneous electrode separating force a little when the movable electrode is separated from the stationary electrode. (4) Small lagging current (due to inductive load) interrupting capability For the 1st test sample, the chopping current value is 1.1A on average (the standard deviation σ n is 0.2A; the sample number n is 100) when a small lagging current test (84 × 1.5 3 kV, 30A) (JEC-181) is performed. For the 2nd test sample, the chopping current value is 1.4A on average (σ n =0.2A; n=100). For the 3rd test sample, the chopping current value is 1.3A on average (σ n =0.2A; n= 100). As compared with the conventional Cu-0.5Bi contact material, the chopping current value is as small as about 0.1 times that of the conventional one. Therefore, the chopping surge voltage is not significant in practical use. Further, the chopping current value does not change after the large current has been interrupted. (5) Small leading current (due to capacitive load) interrupting capability For the 1st, 2nd, 3rd test samples, no resignitions are generated when a small leading curcent test (84× 1.25 3 kV, 80A) (JEC-181 ) is being repeatedly performed 10,000 times. As compared with the conventional Cu-0.5Bi contact material, it is possible to interrupt a circuit including capacitive loads 2 times greater than that interruptable by the conventional one. (6) Electric conductivity For the 1st, 2nd and 3rd test samples, the electric conductivity is 8 to 11 percent (IACS %). (International annealed copper standard). (7) Hardness For the 1st, 2nd, and 3rd test samples, the hardness is 112 to 194 Hv, 9,807N (1 kgf). In the first embodiment described above, the composite metal consists essentially of 20 to 80% copper, 5 to 45% chromium, 5 to 45% iron and 0.5 to 20% chromium carbide each by weight. The above chromium carbide is Cr₃C₂. However, with respect to the chromium carbide, it is also possible to obtain similar good results even when Cr₇C₃ or Cr₂₃C₆ is used in place of Cr₃C₂. It is impossible to obtain satisfactory contact electrode performances in the case where the above mentioned weight percentages of the component composition in composite metal deviate out of the above predetermined ranges. In more detail, when the copper content is less than 20% by weight, the electric conductivity decreases abruptly; the electrode contacting electric resistance after short-time current test increases abruptly; Joule heat loss produced when a rated current is being passed increases, it being thus impossible to put the contact material into practical use. On the other hand, when the copper content is more than 80% by weight, the dielectric strength decreases and additionally the anti-welding characteristic deteriorates abruptly. When the chromium content is less than 5% by weight, the chopping current value increases and therefore the small lagging current interrupting capability deteriorates. When the chromium content is more than 45% by weight, the large current interrupting capability deteriorates abruptly. When the iron content is less than 5% by weight, the chopping current value increases. When the iron content is more than 45% by weight, the large current interrupting capability deteriorates abruptly. Furthermore, when the chromium carbide content is less than 0.5% by weight, the chopping curernt value increases abruptly. When the chromium carbide content is more than 20% by weight, the large current interrupting capability deteriorates abruptly. As described above the contact electrode material of the present teaching is equivalent to the conventional Cu-0.5Bi contact material in large current interrupting capability, but superior to the conventional one in dielectric strength. This arises since the contact material of the present teaching is a composite metal consisting essentially of copper, chromium, iron and chromium carbide, which is formed in such a way that copper is infiltrated into a porous matrix obtained by uniformly and mutually bonding the metal powders (Cr, Fe, Cr₃C₂) other than copper by sintering in diffusion bonding. In particular, since the chopping current value is reduced markedly for the presently proposed contact electrode material, it is possible to stably interrupt small lagging currents due to inductive loads without generating surge voltages; that is, without damaging electrical devices connected to the vacuum interrupter. Furthermore, in the method of manufacturing the contact electrode material according to the present invention, since the metal powders are uniformly bonded to each other in diffusion state into porous matrix and since copper is uniformly infiltrated into the porous matrix, it is possible to improve the mechanical characteristics as well as the above-mentioned electric characteristics and performances.

Claims: A contact electrode material for a vacuum interrupter, which consists essentially of: (a) copper of 20 to 80 percent by weight; (b) chromium of 5 to 45 percent by weight; (c) iron of 5 to 45 percent by weight; (d) chromium carbide of 0.5 to 20 percent by weight; and (e) said copper being infiltrated into a porous matrix composed of insular agglomerates in which powder particles of said chromium, said iron and said chromium carbide are mutually bonded to each other diffusely entering into the other powder particles beyond the surfaces thereof, said porous matrix including chromium rich and chromium poor regions in the insular agglomerates. The contact electrode material as set forth in claim 1, wherein the particle diameters of said chromium powder, said iron powder and said chromium carbide powder are 60 mesh (250 µm) or less. The contact electrode material as set forth in claim 2, wherein the particle diameters of said chromium powder, said iron powder and said chromium carbide powder are preferably 100 mesh (149 µm). The contact electrode material as set forth in claim 1, wherein said chromium carbide is selected from the group consisting of Cr₃C₂, Cr₇C₃, Cr₂₃C₆ and mixtures thereof. The contact electrode material as set forth in claim 1, wherein said chromium, said iron and said chromium carbide are non-uniformly distributed. Process of manufacturing a contact electrode material for a vacuum interrupter having a composition in accordance with any one of the preceding claims 1 to 5, the process comprising the following steps of: (a) preparing chromium powder, iron powder and chromium carbide powder each having powder particle diameters of 60 mesh (250 µm) or less; (b) uniformly mixing said chromium powder, said iron powder and said chromium carbide powder to obtain a powder mixture; (c) heating said powder mixture within a first non-oxidising atmosphere selected from the group consisting of a vacuum, hydrogen gas, nitrogen gas and argon gas for a first predetermined time at a first temperature lower than melting points of said chromium, iron and chromium carbide to obtain a porous matrix composed of insular agglomerates in which said chromium powder, said iron powder and said chromium carbide powder are bonded to each other diffusely entering into the other powder particles beyond the surfaces thereof, said porous matrix including chromium rich and chromium poor regions in the insular agglomerates; (d) placing copper onto said porous matrix; and (e) heating said porous matrix on which said copper is placed within a second non-oxidising atmosphere selected from the group consisting of a vacuum, hydrogen gas, nitrogen gas and argon gas for a second predetermined time at a second temperature higher than a melting point of copper but lower than melting points of said chromium, said iron, said chromium carbide and said porous matrix to infiltrate copper into said porous matrix. The process of manufacturing a contact electrode material as set forth in claim 6, wherein said first predetermined time is 5 to 60 min. The process of manufacturing a contact electrode material as set forth in claim 6, wherein said first temperature is 600 to 1000°C. The process of manufacturing a contact electrode material as set forth in claim 6, wherein said second predetermined time is 5 to 20 min. The process of manufacturing a contact electrode material as set forth in claim 6, wherein said second temperature is 1100°C. The process of manufacturing a contact electrode material as set forth in claim 6, wherein in step (c) said chromium, said iron and said chromium carbide are non-uniformly distributed. Process of manufacturing a contact electrode material for a vacuum interrupter having a composition in accordance with any one of the preceding claims 1 to 5, the process comprising the following steps of: (a) preparing chromium powder, iron powder and chromium carbide powder each having powder particle diameters of 60 mesh (250 µm) or less; (b) uniformly mixing said chromium powder, said iron powder and said chromium carbide powder to obtain a powder mixture; (c) placing copper onto said powder mixture; (d) heating said powder mixture on which said copper is placed within a non-oxidising atmosphere selected from the group consisting of a vacuum, hydrogen gas, nitrogen and argon gas for a first predetermined time at a first temperature lower than a melting point of copper to obtain a porous matrix composed of insular agglomerates in which said chromium powder, said iron powder and said chromium carbide powder are bonded to each other diffusely entering into the other powder particles beyond the surfaces thereof, said porous matrix including chromium rich and chromium poor regions in said insular agglomerates; (e) heating said porous matrix on which said copper is placed within said non-oxidising atmosphere selected from the group consisting of a vacuum, hydrogen gas, nitrogen gas and argon gas for a second predetermined time at a second temperature higher than the melting point of copper but lower than melting points of said chromium, said iron, said chromium carbide and said porous matrix to infiltrate copper into said porous matrix. The process of manufacturing a contact electrode material as set forth in claim 12, wherein said first predetermined time is 5 to 60 min. The process of manufacturing a contact electrode material as set forth in claim 12, wherein said first temperature is 600 to 1000°C. The process of manufacturing a contact electrode material as set forth in claim 12, wherein said second predetermined time is 5 to 20 min. The process of manufacturing a contact electrode material as set forth in claim 12, wherein said second temperature is 1100°C.

Inventors: KASHIWAGI, YOSHIYUKI, KITAKIZAKI, KAORU, NODA, YASUSHI
Applicants: KABUSHIKI KAISHA MEIDENSHA
Application year: 1985
Main classification: H01H1
IPC codes: H01H1, C22C32