VACCUM INTERRUPTER AND METHOD OF ITS PRODUCTION

Abstract: A vacuum interrupter of more improved large current in­ terrupting capability and dielectric strength is disclosed. The interrupter has a pair of separable contact-electrodes (13, 24), a vacuum envelope (4) generally electrically insulating and en­ closing the pair therewithin, a contact-making portion (19) of 20 to 60% IACS electrical conductivity being a part of one con­ tact-electrode (13) of the pair and being into and out of en­ gagement with the other contact-electrode (24) of the pair, an arc-diffusing portion (20) of 2 to 30% IACS electrical conductiv­ ity being the other part of the one contact-electrode (13) and being electrically and mechanically connected to the contact-­ making portion (19) so as to be spaced from the other contact-­ electrode (24) when the contact-electrodes (13, 24) are into engagement, and means (14, 15) for applying an axial magnetic field in parallel to an arc established between the contact-elec­ trodes (13, 24) when separated.

Description: The present invention relates to a vacuum interrupter comprising a pair of separable contact-electrodes, each of which consists of a disc-shaped arc-diffusing portion and a contact-making portion projecting from a central portion of an arcing surface of the arc-diffusing portion; a vacuum envelope which is electrically insulating and encloses the contact-electrodes; and means for applying a magnetic field in parallel to an arc established between the contact-electrodes when said contact electrodes are separated. The invention has particular reference to a vacuum interrupter for an electric circuit of high power, for example, an alternating current circuit of high power. A vacuum interrupter of the initially named kind is disclosed in Figure 20 of US-A-3,946,179, no details are however given concerning the conductivity of the contact making and arc-diffusing portions. Recently, it has been required to provide a vacuum interrupter of much enhanced large current interrupting capability and dielectric strength to cope with increasing current and voltage of power lines with an expansion of an electric power supply network. In operation a vacuum interrupter employing an axial magnetic field, which includes a pair of contact-electrodes, restricts the electric arc to a space between the contact-electrodes with the applied axial magnetic field uniformly diffusing the arc in the space, when the contact-electrodes are separated, thus preventing any concentrating arc-spot of the contact-electrodes from locally overheating and thus enhancing the current interruption capability and dielectric strength of the vacuum interrupter. Generally, the contact-electrode itself is required to consistently satisfy the following requirements: i) low in electrical resistivity, ii) high in large current interruption capability, iii) high in dielectric strength, iv) high in anti-welding capability, v) high in leading and lagging small current interruption capabilities, vi) low in amount of chopping current, and vii) low in erosion. However, a contact-electrode which consistently satisfies all the above requirements has so far not been provided in the present state of the art. For example, a disc-shaped contact-electrode of copper which includes a plurality of radial slits has been proposed as a contact-electrode of a well-known vacuum interrupter of the axial magnetic field type. The disc-shaped and slitted contact-electrode has certain advantages in that it reduces eddy currents so as not to weaken the axial magnetic field. However, the small tensile strength of copper, which amounts to 20 kgf/mm 2 (196.1 MPa), and the plurality of slits cause the mechanical strength of the disc-shaped and slitted contact-electrode to be much reduced. Thus, the thickness and weight of the contact-electrode must be increased in order to prevent a deformation of the contact-electrode due to the mechanical impact and electromagnetic force from large currents which are applied to the contact-electrode when the vacuum interrupter is closed and opened. In addition, electric fields and multiple arcs are concentrated at edge portions of the slits which reduces the dielectric strength between the contact-electrodes, particularly the dielectric strength after an interruption of a large current (hereinafter, referred to as dynamic dielectric strength) and erodes the contact-electrode (see e.g. US-A-3,946,179). Furthermore, various contact-electrodes are known which are adapted for large currents of low voltage. These contact electrodes are made of copper alloyed with a minor constituent of a low melting point and a high vapor-pressure, such as a contact-electrode of copper alloyed with 0.5% bismuth by weight (hereinafter, referred to as a Cu-0.5Bi alloy) which is disclosed in the US-A-3,246,979, or a contact-electrode which is disclosed in the US-A-3,596,027. These pairs of contact-electrodes are however designed for use in a vacuum interrupter of an arc driving type but not for use in a vacuum interrupter of the axial magnetic field type. Such contact-electrodes of copper alloyed with a minor constituent of a low melting point and high vapor-pressure as a contact-electrode of Cu-0.5Bi alloy are excellent in large current interrupting capability, electrical conductivity and anti-welding capability, but are significantly low in dielectric strength, particularly in dynamic dielectric strength. In particular, the current chopping value of a pair of contact-electrodes of Cu-0.5Bi alloy amounts to 10A, which is relatively high, so that harmful chopping surges can be caused during current interruption. Thus, a pair of contact-electrodes of Cu-0.5Bi alloy do not have good lagging small current interrupting capability, which can lead to dielectric breakdown of electrical devices of inductive load circuits. For overcoming the drawbacks of the contact-electrode of copper alloyed with a minor constituent of a low melting point and a high vapor-pressure, various contact-electrodes are known of an alloy consisting of copper and a material of a high melting point and a low vapor-pressure, such as contact-electrodes of an alloy consisting of 20% copper by weight and 80% tungsten by weight (hereinafter-referred to as a 20Cu-80W alloy) which is disclosed in the US-A-3,811,939, or a contact-electrode which is disclosed in the GB-A-2,024,257A. Such contact-electrodes of alloys consisting of copper and a material of a high melting point and a low vapor pressure, e.g. the above described contact-electrode of 20Cu-80W alloy, have a relatively high static dielectric strength, but a relatively low large current interrupting capability. Attention should also be drawn to DE-A-29 47 090 and to EP-A-113 962 both of which disclose vacuum interrupters of the axial magnetic feld type. In DE-A-29 47 090 the arc contacts have a depressed circular center and the contact making portion and the arc-diffusing portion are formed by one and the same piece of material which is of high conductivity, e.g. a copper-chromium contact. In addition the reference describes a low conductivity backing disc butted onto the back surface of the contact. The backing disc has a smaller diameter than the arc-contact itself and cannot be considered part of the arc-contact because it has no arcing surface. EP-A-113 962, which is regarded as prior art only under the aspect of novelty, also describes an arc-contact in which the contact making portion and the arc-diffusing portion are formed by one and the same piece of material and therefore have the same conductivities. An object of the present invention is to provide a vacuum interrupter of the axial magnetic field type which has excellent large current interrupting capability and dielectric strength. Another object of the present invention is to provide a vacuum interrupter of the axial magnetic field type which possesses high resistance against mechanical impact and electromagnetic forces based on large currents, and therefore long period durability. In order to satisfy these objects the present invention is given in the characterizing part of claim 1. Advantageous embodiments of the invention are set forth in the appended subordinate claims. Other objects and advantages of the present invention will be apparent from the following description, claims and attached drawings and photographs, in which: Figure 1 is a sectional view through a vacuum interrupter of an axial magnetic field applying type according to the present invention. Figure 2 is a sectional view through the movable electrode assembly of Figure 1. Figure 3 is an exploded perspective view of the movable electrode assembly of Figure 2. Figure 4 is a comparison diagram illustrative of the relationship determined at 84 kV between the diameter D of the contact-electrodes and maximum interruption current I, for a contact in accordance with the present invention and for a contact in accordance with US-A-3,946,179. Figure 5 is a sectional view through an electrode assembly modified from the movable one of Figure 2; Figure 6 is a sectional view through another electrode assembly modified from the movable one of Figure 2. Figure 7A to 7D are all photographs by an X-ray microanalyzer of a structure of Example A 1 of a complex metal constituting an arc-diffusing portion, of which: Figure 7A is a secondary electron image photograph of the structure. Figure 7B is a characteristic X-ray image photograph of iron. Figure 7C is a characteristic X-ray image photograph of chromium. Figure 7D is a characteristic X-ray image photograph of infiltrant copper. Figures 8A to 8D are all photographs by the X-ray microanalyzer of a structure of Example A 2 of a complex metal constituting an arc-diffusing portion, of which: Figure 8A is a secondary electron image photograph of the structure. Figure 8B is a characteristic X-ray image photograph of iron. Figure 8C is a characteristic X-ray image photograph of chromium. Figure 8D is a characteristic X-ray image photograph of infiltrant copper. Figures 9A to 9D are all photographs by the X-ray microanalyzer of a structure of Example A 3 of a complex metal constituting the arc-diffusing portion, of which: Figure 9A is a secondary electron image photograph of the structure. Figure 9B is a characteristic X-ray image photograph of iron. Figure 9C is a characteristic X-ray image photograph of chromium. Figure 9D is a characteristic X-ray image photograph of infiltrant copper. Figures 10A to 10D are all photographs by the X-ray microanalyzer of a structure of Example C 1 of a complex metal constituting a contact-making portion, of which: Figure 10A is a secondary electron image photograph of the structure. Figure 10B is a characteristic X-ray image photograph of molybdenum. Figure 10C is a characteristic X-ray image photograph of chromium. Figure 10D is a characteristic X-ray image photograph of infiltrant copper. Figures 11A to 11D are all photographs by the X-ray microanalyzer of a structure of Example C 2 of a complex metal constituting the contact-making portion, of which: Figure 11A is a secondary electron image photograph of the structure. Figure 11B is a characteristic X-ray image photograph of molybdenum. Figure 11C is a characteristic X-ray image photograph of chromium. Figure 11D is a characteristic X-ray image photograph of infiltrant copper. Figures 12A to 12D are all photographs by the X-ray microanalyzer of a structure of Example C 3 of a complex metal constituting the contact-making portion, of which: Figure 12A is a secondary electron image photograph of the structure. Figure 12B is a characteristic X-ray image photograph of molybdenum. Figure 12C is a characteristic X-ray image photograph of chromium. Figure 12D is a characteristic X-ray image photograph of infiltrant copper. Figures 13A to 13D are all photographs by the X-ray microanalyzer of a structure of Example A 4 of a complex metal constituting the arc-diffusing portion, of which: Figure 13A is a secondary electron image photograph of the structure. Figure 13B is a characteristic X-ray image photograph of iron. Figure 13C is a characteristic X-ray image photograph of chromium. Figure 13D is a characteristic X-ray image photograph of infiltrant copper. Figures 14A to 14D are all photographs by the X-ray microanalyzer of a structure of Example A 7 of a complex metal constituting the arc-diffusing portion, of which: Figure 14A is a secondary electron image photograph of the structure. Figure 14B is a characteristic X-ray image photograph of iron. Figure 14C is a characteristic X-ray image photograph of chromium. Figure 14D is a characteristic X-ray image photograph of infiltrant copper. Figures 15A to 15E are all photographs by the X-ray microanalyzer of a structure of Example A₁₀ of a complex metal constituting the arc-diffusing portion, of which: Figure 15A is a secondary electron image photograph of the structure. Figure 15B is a characteristic X-ray image photograph of iron. Figure 15C is a characteristic X-ray image photograph of chromium. Figure 15D is a characteristic X-ray image photograph of nickel. Figure 15E is a characteristic X-ray image photograph of infiltrant copper. Referring to Figures 1 to 15 of the accompanying drawings and photographs, preferred embodiments of the present invention will be described in detail. As shown in Figure 1, a vacuum interrupter of a first embodiment of the present invention includes a vacuum envelope 4 which is evacuated to less than 10⁻⁴ Torr (13.4 mPa) and a pair of stationary and movable electrode assemblies 5 and 6 located within the vacuum envelope 4. The vacuum envelope 4 comprises, in the main, two insulating cylinders 2 of the same shape of glass or alumina ceramics which are serially and hermetically associated by welding or brazing to each other by means of sealing metallic rings 1 of Fe-Ni-Co alloy or Fe-Ni alloy at the adjacent ends of the insulating cylinders 2, and a pair of metallic end plates 3 of austinitic stainless steel hermetically associated by welding or brazing to both the remote ends of the insulating cylinders 2 by means of sealing metallic rings 1. A metallic arc shield 7 of a cylindrical form which surrounds the electrode assemblies 5 and 6 is supported on and hermetically joined by welding or brazing to the sealing metallic rings at the adjacent ends of the insulating cylinders 2. Further, metallic edge-shields 8 which moderate electric field concentration at edges of the sealing metallic rings 1 at the remote ends of the insulating cylinders 2 are joined by welding or brazing to the pair of metallic end plates 3. An axial shield 11 and a bellows shield 12 are provided on respective stationary and movable lead rods 9 and 10 which are electrically and mechanically joined to the respective stationary and movable electrode assemblies 5 and 6. The arc shield 7, the edge shield 8, the axial shield 11 and the bellows shield 12 all are made of austinitic stainless steel. The electrode assemblies 5 and 6 have the same construction and the movable electrode assembly 6 will be described hereinafter. As shown in Figures 2 and 3, the movable electrode assembly 6 comprises a movable contact-electrode 13, an electrical lead member 14 for a coil-electrode of which all portions are electrically and mechanically joined by brazing to the backsurface of the movable contact-electrode 13, a coil-electrode 15 which is mechanically and electrically joined by brazing to the inner end of the movable lead rod 10, spaced from the electrical lead member 14 for the coil-electrode, a spacer 16 positioned between the electrical lead member 14 for the coil-electrode and the coil-electrode 15, with the two ends of the spacer rigidly connecting together the central portions of the electrical lead member 14 for the coil-electrode and the coil-electrode 15, but with the lead member 14 and the coil-electrode being substantially electrically insulated from each other, electrical connectors 17 of columnar form which electrically connect the outer peripheries of the lead member 14 and the coil-electrode 15, and a reinforcement member 18 for the coil-electrode 15. The elements listed above will be successively described in particular. As shown in Figures 2 and 3, the movable contact-electrode 13 which generally takes the form of a thinned frustrum of a cone consists of a contact-making portion 19 and an arc-diffusing portion 20 electrically and mechanically joined by brazing to the contact-making portion 19. The contact-making portion 19 is made of material of 20 to 60% IACS electrical conductivity, for example, complex metal consisting of 20 to 70% copper by weight, 5 to 70% chromium by weight and 5 to 70% molybdenum by weight. In this case, the contact-making portion 19 can exhibit equivalently the same electrical contact resistance due to its thin disc-shape as a contact-making member of Cu-0.5Bi alloy. The contact-making portion 19 which is shaped as a frustrum of a circular cone is also fitted into a circular recess 21 which is formed in the central portion of the surface of an arc-diffusing portion 20, and projects from the surface of the arc-diffusing portion 20. For reducing the amount of eddy current created in the movable contact-electrode 13 as much as possible, the contact-making portion 19 has a diameter in the range from 20 to 60% of the diameter of the arc-diffusing portion 20. The arc-diffusing portion 20 is made of material of 2 to 30%, preferably, 10 to 15% IACS electrical conductivity, for example, material containing copper, iron and chromium. As examples of the latter material one can quote a complex metal of about 30 kgf/mm 2 (294 MPa) tensile strength consisting of 50% copper by weight and 50% austinitic stainless steel by weight, e.g., SUS 304 or SUS 316 (at JIS, hereinafter, at the same), and a complex metal of about 30 kgf/mm 2 (294 MPa) tensile strength consisting of 50% copper by weight, 25% iron by weight and 25% chromium by weight. The arc-diffusing portion 20 is shaped substantially as a frustrum of circular cone so that the surface of the arc-diffusing portion 20 has a slant associated with that of the surface of the contact-making portion 19. The arc-diffusing portion 20 also includes a circular recess 23 at the central portion of the backsurface thereof. An annular hub 22 of the electrical lead member 14 for the coil-electrode is fitted into the circular recess 23. The thickness t of the central portion of the movable contact-electrode 13 is made at most 10 mm in view of the generation of Joule heat produced when the stationary and movable contact-electrodes 24 and 13 are in contact. The electrical lead member 14 for the coil-electrode, an outer diameter of which is normally no more than the diameter of the movable contact-electrode 13, is made of material of high electrical conductivity such as Cu, Ag, Cu alloy or Ag alloy. The electrical conductivity of that material is much larger than that of a material of the arc-distributing portion 20. As shown in Figure 3, the electrical lead member 14 for the coil-electrode includes the hub 22, two radial webs 25 oppositely extending from the hub 22 and two arcuate bridges 26 extending in a common circumferential direction from the outer ends of the respective radial webs 25. The hub 22, radial webs 25 and angular bridges 26, as described above, all are electrically and mechanically joined by brazing to the backsurface of the movable contact-electrode 13. A circular recess 27 to which one end of the electrical connector 17 is brazed is provided in the backsurface of the distal end of each arcuate bridge 26. The electrical lead member 14 for the coil-electrode serves to carry most of the current which, in the absence of the electrical lead member 14 would flow through the movable contact-electrode 13 alone in a radial direction thereof thereby raising the temperature of the movable contact-electrode 13 due to Joule heating. Electrical lead member 14 thus suppresses such a rise in temperature. The coil-electrode 15 which serves to establish the major part of axial magnetic field is made of material of high electrical conductivity, e.g., Cu, Ag, Cu alloy or Ag alloy, as is the electrical lead member 14 for the coil-electrode. As shown in Figure 3, the coil-electrode 15 includes a circular hub 28, two radial webs 29 oppositely extending from the circular hub 28, and two partially turning segments 30 extending in a common circumferential direction from outer ends of the respective radial webs 29. The direction of an extension of the partially turning segments 30 is opposite to the direction of an extension of the bridges 26. An angular gap 31 is provided between the adjacent distal end of each partially turning segment 30 and each radial web 29. A circular hole 32 into which a part of the electrical connector 17 is secured by means of brazing is provided at the distal end of each partially turning segment 30. A circular recess 33 into which an outwardly extending flange 16a at one end of the spacer 16 is secured by means of brazing is provided in the surface of the hub 28. Moreover, a circular recess 34 into which the inner end of the movable lead rod 10 is secured by means of brazing is provided in the backsurface of the hub 28. The coil-electrode 15 of Figure 3 is a 1/2 turn type, however, may be of a 1/3, 1/4 or one turn type. The spacer 16 rigidly connects the electrical lead member 14 for the coil-electrode and the coil electrode 15 to each other in a manner to space them apart. The spacer 16 is also made of material of high mechanical strength, good brazability, and of such low electrical conductivity that the electrical lead member 14 for the coil-electrode and the coil-electrode 15 are effectively insulated from one another. Thus, for example, stainless steel or Inconnel may be used. Further, the spacer 16, which is shaped as a short cylinder having a pair of outwardly extending flanges 16a at the opposite ends thereof, is brazed at both the outwardly extending flanges 16a to the hubs 22 and 28 of the electrical lead member 14 for the coil-electrode and the coil-electrode 15. The reinforcement member 18 is made of material of high mechanical strength and low electrical conductivity, e.g., stainless steel, as well as the spacer 16. The reinforcement member 18 includes a hub 35 brazed to a periphery of the movable lead rod 10, a plurality of supporting arms 36 radially extending from the hub 35, and two limbs 37 which are integrated on to the outer ends of the supporting arms 36 and includes upward flanges. The limbs 37 are brazed to the partially turning segments 30 of the coil-electrode 15. A comparative performance test was carried out between a vacuum interrupter of the axial magnetic field applying type according to the first embodiment of the present invention, and a conventional vacuum interrupter of the axial magnetic field applying type (refer to US-A-3,946,179). The former interrupter includes a pair of contact-electrodes each of which consists of a contact-making portion of complex metal consisting of 50% copper by weight, 10% chromium by weight and 40% molybdenum by weight, and an arc-diffusing portion of complex metal consisting of 50% copper by weight and 50% SUS 304 by weight. The diameter of the contact-making portion is 20% of the diameter of the arc-diffusing portion. The latter interrupter includes a pair of disc-shaped contact-electrodes of Cu-0.5Bi alloy, each of the pair has six linear slits extending radially from an outer periphery and a 1/4 turn typed coil. Results of the performance comparison test will be described as follows: In the specification, amounts of voltage and current are represented as a rms value if not specified. 1) Large current interrupting capability Maximum interruption current I(kA) was measured at rated 84 kV when the diameter D(mm) of each contact-electrode was varied. Figure 4 shows results of the measurement. In Figure 4, the ordinate represents maximum interruption current I and the abscissa represents the diameter D of each contact-electrode. Line A represents a relationship between the maximum interruption current I and the diameter D of each contact-electrode relative to a vacuum interrupter of the present invention. Line B indicates the relationship between the maximum interruption current I and the diameter D of each contact-electrode relative to a conventional vacuum interrupter. As apparent from Figure 4, the vacuum interrupter according to the first embodiment of the present invention exhibits 2 to 2.5 times large current interrupting capability than that of the conventional vacuum interrupter. 2) Dielectric strength In accordance with JEC-181 test method, withstand voltages were measured for the vacuum interrupter of the first embodiment of the present invention and for the conventional vacuum interrupter, with a 3.0 mm gap between contact-making portions relative to the present invention but with a 10 mm gap between contact-making portions relative to the conventional vacuum interrupter. In this case, both the vacuum interrupters exhibited the same withstand voltage. Thus, the vacuum interrupter of the present invention possesses over 3 times the dielectric strength of the conventional vacuum interrupter. Withstand voltages for the first embodiment of the present teaching and for the conventional vacuum interrupter were also measured before and after large current interruption. The withstand voltage after large current interruption of the former interrupter decreased to about 80% of the withstand voltage before large current interruption thereof. On the other hand, the withstand voltage after large current interruption of the latter interrupter decreased to about 30% of the withstand voltage before large current interruption thereof. 3) Anti-welding capability The anti-welding capability of the contact-electrodes of the first embodiment of the present invention amounted to 80% of the anti-welding capability of those of the conventional vacuum interrupter. However, such decrease is not actually significant. If necessary, the disengaging force applied to the contact-electrodes may be slightly enhanced. 4) Lagging small current interrupting capability The current chopping value of the vacuum interrupter of the first embodiment of the present invention amounted to 40% of that of the conventional vacuum interrupter, so that chopping surge was almost insignificant. This value was maintained even after the contact-electrodes had engaged and disengaged more than 100 times for interrupting lagging small current. 5) Leading small current interrupting capability The vacuum interrupter of the first embodiment of the present invention interrupted twice the charging current of the conventional vacuum interrupter connected to condenser or unload line of condenser. Figure 5 shows an electrode assembly 40 of a modification to the first embodiment of the present invention. The electrode assembly 40 structurally differs from the movable electrode assembly 6 of Figure 2 in the aspect that it includes a contact-electrode 43 consisting of an arc-diffusing portion 41 including a centrally located circular hole 42 and a contact-making portion 19 of Figure 4 fitted into the hole 42, and an electrical lead member 45 for a coil-electrode including an annular hub 44. In this case, the axial length of the spacer 16 may be increased. A surface of the hub 44 is electrically and mechanically joined by brazing to the backsurface of the contact-making portion 19. On the other hand, the periphery of the hub 44 is electrically and mechanically joined by brazing to the wall defining the hole 42. The electrode assembly 40 advantageously makes, the electrical resistance between the contact-making portion 19 and the electrical lead member 45 for the coil-electrode, smaller than that of the same current path of the electrode assembly 6 of Figure 2. Figure 6 shows an electrode assembly 50 of another modification to the first embodiment of the present invention. The electrode assembly 50 structurally differs from the movable electrode assembly 6 of Figure 2 in that it includes a contact-electrode 52 consisting of an arc-diffusing portion 41 of Figure 5 and a contact-making portion 51 thickened and fitted into the hole 42 of the arc-diffusing portion 41. A backsurface of the contact-making portion 51 is electrically and mechanically joined by brazing to the hub 22 of an electrical lead member 14 for a coil-electrode of Figure 2. On the other hand, a periphery of the contact-making portion 51 is electrically and mechanically joined by brazing to a wall defining the hole 42. The electrode assembly 50 has the same advantages as that of the electrode assembly 40 of Figure 5. According to the first embodiment and the modifications thereto, the coil-electrodes for applying an axial magnetic field are each provided behind each coil-electrode. The present invention is also applicable to vacuum interrupters that include means for applying an axial magnetic field outside its vacuum envelope (refer to US-A-3,283,103), to vacuum interrupters that include a coil for applying an axial magnetic field one end of which is directly connected to the backsurface of a contact-electrode (refer to US-A-3,935,406), and to vacuum interrupters that include a coil for applying an axial magnetic field located surrounding a pair of contact-electrodes (refer to GB-A-1,264,490). The present invention is further applicable to such vacuum interrupters as include a contact-electrode consisting of a flat arc-diffusing portion and a contact-making portion projecting from a surface of the arc-diffusing portion at the central portion of the surface thereof. Other embodiments of the present invention will be described hereinafter in which changes were made to materials of the contact-making portion 19 and the arc-diffusing portion 20 of the pair of stationary and movable contact-electrodes 24 and 13. Figures 7A to 7D, Figures 8A to 8D and Figures 9A to 9D show structures of complex metals constituting arc-diffusing portions according to the 2nd to 10th embodiments of the present teaching. According to the 2nd to 10th embodiments of the present teaching, arc-diffusing portion 20 is made of material of 5 to 30% IACS electrical conductivity, at least 30 kgf/mm 2 (294 MPa) tensile strength and 100 to 170 Hv hardness (hereinafter, under a load of 1 kgf (9.81 N)), e.g., complex metal consisting of 20 to 70% copper by weight, 5 to 40% chromium by weight and 5 to 40% iron by weight. Processes for producing the complex metal may be generally classified into two categories. The process of one category comprises the step of diffusion-bonding a powder mixture consisting of chromium powder and iron powder into a porous matrix and the step of infiltrating the porous matrix with molten copper (hereinafter, referred to as an infiltration process). A process of the other category comprises the step of press-shaping a powder mixture consisting of copper powder, chromium powder and iron powder into a green compact and the step of sintering the green compact below the melting point of copper (about 1083°C) or at at least the melting point of copper but below the melting point of iron (about 1537°C) (hereinafter, referred to as a sintering process). The infiltration and sintering processes will be described hereinafter. Each metal powder was of less than 100 meshes (i.e. particle size no more than 0.149 µ). The first infiltration process. Firstly, a predetermined amount (e.g., an amount of one final contact-electrode plus a machining margin) of chromium powder and iron powder which are respectively prepared 5 to 40% by weight and 5 to 40% by weight but in total 30 to 80% by weight at a final ratio, are mechanically and uniformly mixed. Secondly, the resultant powder mixture is placed in a vessel of a circular section made of material, e.g., alumina ceramics, which interacts with none of chromium, iron and copper. A solid copper is placed on the powder mixture. Thirdly, the powder mixture and the solid copper are heat held under a nonoxidizing atmosphere, e.g., a vacuum pressure of at highest 5x10⁻⁵ Torr (6.67 mPa) at 1000°C for 10 min (hereinafter, referred to as a chromium-iron diffusion step), thus resulting in a porous matrix of chromium and iron. Then, the resultant porous matrix and the solid copper are heat held under the same vacuum at 1100°C for 10 min, which leads to infiltrating the porous matrix with molten copper (hereinafter, referred to as a copper infiltrating step). After cooling, a desired complex metal for the arc-diffusing portion is produced. The second infiltration process At first, chromium powder and iron powder are mechanically and uniformly mixed in the same manner as in the first infiltration process. Secondly, the resultant powder mixture is placed in the same vessel as that in the first infiltration process. The powder mixture is heat held in a nonoxidizing atmosphere, e.g. a vacuum pressure of at highest 5x 10⁻⁵ Torr (6.67 mPa), or hydrogen, nitrogen or argon gas at a temperature below the melting point of iron, e.g., within 600 to 1000°C for a fixed period of time, e.g., within 5 to 60 min., thus resulting in a porous matrix consisting of chromium and iron. Thirdly, in the same nonoxidizing atmosphere, e.g., a vacuum pressure of at highest 5x 10⁻⁵ Torr (6.67 mPa), as that of the chromium-iron diffusion step, or other nonoxidizing atmosphere, a solid copper is placed on the porous matrix, then the porous matrix and the solid copper are heat held at a temperature of at least the melting point of copper but below a melting point of the porous matrix, e.g., 1100°C for about a period of time of 5 to 20 min., which leads to infiltrating the porous matrix with molten copper. After cooling, a desired complex metal for the arc-diffusing portion is produced. In the second infiltration process, a solid copper is not placed in the vessel in the chromium-iron diffusion step, so that a powder mixture of chromium powder and iron powder can be heat held to a porous matrix at a temperature of at least the melting point (1083°C) of copper but below the melting point ( 1537°C) of iron. As an alternative in the second infiltration process the chromium-iron diffusion step may be performed in various nonoxidizing atmospheres, e.g., hydrogen, nitrogen or argon gas, and the copper infiltration step may be performed under an evacuation to vacuum degassing the complex metal for the arc-diffusing portion. In both the infiltration processes, vacuum is preferably selected as a nonoxidizing atmosphere, but not other nonoxidizing atmosphere, because degassing of the complex metal for the arc-diffusing portion can be concurrently performed during heat holding. However, even if a deoxidizing gas or an inert gas is used as a nonoxidizing atmosphere, the resultant process is satisfactory for producing the complex metal for the arc-diffusing portion. . In addition, a heat holding temperature and period of time for the chromium-iron diffusion step is determined on a basis of taking into account conditions of the vacuum furnace or other gas furnace, the shape and size of a porous matrix and workability so that desired properties as those of a complex metal for the arc-diffusing portion will be produced. For example, a heating temperature of 600°C determines a heat holding period of 60 min. or a heating temperature of 1000°C determines a heat holding period of 5 min. The particle size of a chromium particle and an iron particle may be less than 60 meshes, i.e., no more than 250 µm. However, the lower an upper limit of the particle size, generally the more difficult to uniformly distribute each metal particle. Further, it is more complicated to handle the metal particles and, when used, they necessitate a pretreatment because they are more liable to be oxidized. On the other hand, if the particle size of each metal particle exceeds 60 meshes, it is necessary to make the heat holding temperature higher or to make the heat holding period longer with a diffusion distance of each metal particle increasing, which leads to lower productivity of the chromium-iron diffusion step. Consequently, the upper limit of the particle size of each metal particle is determined in view of various conditions. Thus, in both infiltration processes, it is because the particles of chromium and iron can be more uniformly distributed to cause better diffusion bonding thereof, thus resulting in a complex metal for the arc-diffusing portion possessing better properties that the particle size of each metal particle is determined to be less than 100 meshes. If chromium particles and iron particles are badly distributed, then drawbacks of both metals will not be offset by each other and advantages thereof will not be developed. In particular, the more the particle size of each metal particle exceeds 60 meshes, the larger is the proportion of copper in the surface region of an arc-diffusing portion, which contributes to lowering the dielectric strength of the contact-electrode. Similarly, particles, iron particles and chromium-iron alloy particles which have large granulations are more likely to appear in the surface region of the arc-diffusing portion, so that the respective drawbacks of chromium, iron and copper are more apparent. The sintering process Firstly, chromium powder, iron powder and copper powder which are prepared in the same manner as in the first infiltration process are mechanically and uniformly mixed. Secondly, the resultant powder mixture is placed in a preset vessel and press-shaped into a green compact under a preset pressure, e.g., of 2,000 to 5,000 kgf/cm 2 (196.1 to 490.4 MPa). Thirdly, the resultant green compact which is taken out of the vessel is heat held in a nonoxidizing atmosphere, e.g., a vacuum pressure of at highest 5x 10⁻⁵ Torr (6.67 mPa), or hydrogen, nitrogen or argon gas at a temperature below the melting point of copper, e.g., at 1000°C, or at a temperature of at least the melting point of copper but below the melting point of iron, e.g., at 1100°C for a preset period of time, e.g., within 5 to 60 min., thus being sintered into the complex metal of the arc-diffusing portion. In the sintering process, conditions of the nonoxidizing atmosphere and the particle size of each metal particle are the same as those in both the infiltration processes, and conditions of the heat holding temperature and the heat holding period of time required for sintering the green compact are the same as those for producing the porous matrix from the powder mixture of metal powders in the infiltration processes. Referred to Figures 7A to 7D, Figures 8A to 8D and Figures 9A to 9D which are photographs by the X-ray microanalyzer, structures of the complex metals for the arc-diffusing portion 20 which are produced according to the first infiltration process above, will be described hereinafter. Example A 1 of the complex metal for the arc-diffusing portion possesses a composition consisting of 50% copper by weight, 10% chromium by weight and 40% iron by weight. Figure 7A shows a secondary electron image of a metal structure of Example A 1 . Figure 7B shows a characteristic X-ray image of distributed and diffused iron, in which distributed white or gray insular agglomerates indicate iron. Figure 7C shows a characteristic X-ray image of distributed and diffused chromium, in which distributed gray insular agglomerates indicate chromium. Figure 7D shows a characteristic X-ray image of infiltrant copper, in which white parts indicate copper. Example A 2 of the complex metal for the arc-diffusing portion possesses a composition consisting of 50% copper by weight, 25% chromium by weight and 25% iron by weight. Figures 8A, 8B, 8C and 8D show similar images to those of Figures 7A, 7B, 7C and 7D, respectively. Example A 3 of the complex metal for the arc-diffusing portion possesses a composition of consisting of 50% copper by weight, 40% chromium by weight and 10% iron by weight. Figures 9A, 9B, 9C and 9D show similar images to those of Figures 7A, 7B, 7C and 7D, respectively. As apparent from Figures 7A to 7D, Figures 8A to 8D and Figures 9A to 9D, the chromium and the iron are uniformly distributed and diffused into each other in the metal structure, thus forming many insular agglomerates. The agglomerates are uniformly bonded to each other throughout the metal structure, resulting in the porous matrix consisting of chromium and iron. Interstices of the porous matrix are infiltrated with copper, which results in a stout structure of the complex metal for the arc-diffusing portion. Figures 10A to 10D, Figures 11A to 11D and Figures 12A to 12D show structures of complex metals for the contact-making portions 19 according to the 2nd to 10th embodiments of the present invention. According to the 2nd to 10th embodiments of the present invention, the contact-making portion 19 is made of material of 20 to 60% IACS electrical conductivity and 120 to 180 Hv hardness, e.g., complex metal consisting of 20 to 70% copper by weight, 5 to 70% chromium by weight and 5 to 70% molybdenum by weight. The complex metals for the contact-making portion are produced substantially by the same processes as those for producing the arc-diffusing portion. Referred to Figures 10A to 10D, Figures 11A to 11D and Figures 12A to 12D which are photographs by the X-ray microanalyzer as well as Figures 7A to 7D, structures of the complex metals for the contact-making portion which are produced according to substantially the same process as the first infiltration process above, will be described hereinafter. Example C 1 of the complex metal for the contact-making portion possesses a composition consisting of 50% copper by weight, 10% chromium by weight and 40% molybdenum by weight. Figure 10A shows a secondary electron image of a metal structure of Example C 1 . Figure 10B shows a characteristic X-ray image of distributed and diffused molybdenum, in which distributed gray insular agglomerates indicate molybdenum. Figure 10C shows a characteristic X-ray image of distributed and diffused chromium, in which distributed gray or white insular agglomerates indicate chromium. Figure 10D shows a characteristic X-ray image of infiltrant copper, in which white parts indicate copper. Example C 2 of the complex metal for the contact-making portion possesses a composition consisting of 50% copper by weight, 25% chromium by weight and 25% molybdenum by weight. Figures 11A, 11B, 11C and 11D show similar images to those of Figures 10A, 10B, 10C and 10D, respectively. Example C 3 of the complex metal for the contact-making portion possesses a composition consisting of 50% copper by weight, 40% chromium by weight and 10% molybdenum by weight. Figures 12A, 12B, 12C and 12D show similar images to those of Figures 10A, 10B, 10C and 10D, respectively. As apparent from Figures 10A to 10D, Figures 11A to 11D and Figures 12A to 12D, the chromium and molybdenum are uniformly distributed and diffused into each other in the metal structure, thus forming many insular agglomerates. The agglomerates are uniformly bonded to each other throughout the metal structure, thus resulting in the porous matrix consisting of chromium and molybdenum. Interstices of the porous matrix are infiltrated with copper, which results in a stout structure of the complex metal for the contact-making portion. Measurements of IACS electrical conductivity which were carried out on Examples A1, A2 and A3 of the complex metal for the arc-diffusing portion established that they possessed 8 to 10% IACS electrical conductivity, at least 30 kgf/mm 2 (294 MPa) tensile strength and 100 to 170 Hv hardness. On the other hand, tests established that Examples C1, C2 and C3 possessed 40 to 50% IACS electrical conductivity and 120 to 180 Hv hardness. The contact-making portion of a 1st comparative is made of 20Cu-80W alloy. The contact-making portion of a 2nd comparative is made of Cu-0.5Bi alloy. Examples A 1 , A 2 and A 3 of the complex metal for the arc-diffusing portion and Examples C 1 , C 2 and C 3 of the complex metal for the contact-making portion, which are shown and described above, were shaped to substantially thinned frustrums of circular cone having 100 mm and 60 mm diameters respectively, as shown in Figures 2 and 3. Examples A 1 , A 2 , A 3 , C 1 , C 2 and C 3 , and a 20Cu-80W alloy and a Cu-0.5Bi alloy were all paired off, resulting in eleven contact-electrodes. A pair of contact-electrodes made up in the above manner was assembled into a vacuum interrupter of the axial magnetic field applying type as illustrated in Figure 1. Tests were carried out on performances of this vacuum interrupter. The results of the tests will be described hereinafter. A description shall be made on a vacuum interrupter of the 5th embodiment of the present invention which includes the pair of contact-electrodes each consisting of the arc-diffusing portion made of Example A 2 , and the contact-making portion made of Example C 1 . An arc-diffusing portion and a contact-making portion of a contact-electrode of a 2nd embodiment are made of respective Examples A 1 and C 1 . Those of a 3rd, of Examples A 1 and C 2 . Those of a 4th, of Examples A 1 and C 3 . Those of a 6th, of Examples A 2 and C 2 . Those of a 7th, of Examples A 2 and C 3 . Those of an 8th, of Examples A 3 and C 1 . Those of a 9th, of Examples A 3 and C 2 . Those of 10th, of Examples A 3 and C 3 . When performances of the vacuum interrupters of the 2nd to 4th and 6th to 10th embodiments of the present invention differ from those of the 5th embodiment of the present invention, then different points shall be specified. 6) Large current interrupting capability Interruption tests which were carried out at an opening speed within 1.2 to 1.5 m/s under a rated voltage of 12 kV, however, a transient recovery voltage of 21 kV according to JEC-181, established that the test vacuum interrupters interrupted 60 kA current. Moreover, interruption tests at an opening speed of 3.0 m/s under a rated voltage of 84 kV, however, a transient recovery voltage of 143 kV according to JEC-181, established that the test vacuum interrupters interrupted 49 kA current. Table 1 below shows the results of the large current interrupting capability tests. Table 1 also shows those of vacuum interrupters of 1st to 8th comparatives which include a pair of contact-electrodes each consisting of an arc-diffusing portion and a contact-making portion. The portions have the same sizes as those of the respective arc-diffusing portion and contact-making portion of the 2nd to 10th embodiments of the present invention. An arc-diffusing portion and a contact-making portion of a contact-electrode of the 1st comparative are made of Example A 2 and 20Cu-80W alloy. Those of 2nd comparative, of Example A 2 and Cu-0.5Bi alloy. Those of the 3rd comparative, of copper disc and Example C 1 . Those of the 4th comparative, of copper disc and 20Cu-80W alloy. Those of the 5th comparative, of copper disc and Cu-0.5Bi alloy. Those of the 6th comparative, of 6-radially slitted copper disc and Example C 1 . Those of 7th comparative, of copper disc of the same type of the 6th comparative and 20Cu-80W alloy. Those of the 8th comparative, of copper disc of the same type of the 6th comparative and Cu-0.5Bi alloy. Vacuum interrupters of the axial magnetic field applying type of the 3rd to 8th comparatives each are of a type in which an outer periphery of a backsurface of an arc-diffusing portion and a distal end of a partial turning segment of a coil-electrode are connected to each other by means of an electrical connector (refer to US-A-3,946,179). 7) Dielectric strength In accordance with JEC-181 test method, impulse withstand voltage tests were carried out with a 3.0 mm inter-contact gap. The vacuum interrupters showed 120 kV withstand voltage against both positive and negative impulses with a ±10 kV deviation. After 10 times interrupting 60 kA current of rated 12 kV, the same impulse withstand voltage tests were carried out, thus establishing the same results. After continuously 100 times opening and closing a circuit through which 80 A leading small current of rated 12 kV flowed, the same impulse withstand voltage tests were carried out, thus establishing substantially the same results. Table 2 below shows the results of the tests of the impulse withstand voltage at rated 84 kV which were carried out on the vacuum interrupters of the 5th embodiment. Table 2 also shows those of the vacuum interrupters of the 1st to 8th comparatives. 8) Anti-welding capability In accordance with the IEC rated short time current, current of 25 kA was flowed through the stationary and movable contact-electrode 24 and 13 which were forced to contact each other under 130 kgf (1275N) force, for 3 s. The stationary and movable contact-electrodes 24 and 13 were then separated without any failures with a 200 kgf (1961 N) static separating force. An increase of electrical contact resistance was then limited to within 2 to 8% In accordance with the IEC rated short time current, current of 50 kA was flowed through the stationary and movable contact-electrodes 5 and 6 which were forced to contact each other under 1,000 kgf (9807N) force, for 3 s. The stationary and movable contact-electrodes 24 and 13 were then separated without any failures with a 200 kgf (1961N) static separating force. An increase of electrical contact resistance was limited to no more than 5%. Thus, the stationary and movable contact-electrodes 24 and 13 actually possess good anti-welding capability. 9) Lagging small current interrupting capability In accordance with a lagging small current interrupting test of JEC-181, a 30A test current of 84x 1.5 3 kV was flowed through the stationary and movable contact-electrodes 24 and 13. Current chopping values had a 3.9A average (however, a standard deviation σ n =0.96 and a sample number n=100). In particular, current chopping values of the vacuum interrupters of the 6th and 7th embodiments of the present invention were respectively 3.7A (however, σ n =1.26 and n=100) and 3.9A (however, σ n =1.5 and n= 100) averages. 10) Leading small current interrupting capability In accordance with a leading small current interrupting test standard of JEC-181, a test leading small current of 80 A at 84x 1.25 3 kV was flowed through the stationary and movable contact-electrodes 24 and 13. Under that condition a continuously 10,000 times opening and closing test was carried out. No reignition was established. The following limits were apparent on a composition ratio of each metal in the complex metal for the arc-diffusing portion. Copper below 20% by weight significantly lowered current interrupting capability. On the other hand, copper above 70% by weight significantly lowered the mechanical and dielectric strengths of the arc-diffusing portion but increased the electrical conductivity thereof, thus significantly lowering the current interrupting capability. Chromium below 5% by weight increased the electrical conductivity of the arc-diffusing portion, thus significantly lowering the current interrupting capability and dielectric strength. On the other hand, chromium above 40% by weight significantly lowered the mechanical strength of the arc-diffusing portion. Iron below 5% by weight significantly lowered the mechanical strength of the arc-diffusing portion. On the other hand, iron above 40% by weight significantly lowered the current interrupting capability. The following limits were apparent on a composition ratio of each metal in the complex metal for the contact-making portion. Copper below 20% by weight significantly lowered the electrical conductivity of the contact-making portion but significantly increased the electrical contact resistance thereof. On the other hand, copper above 70% by weight significantly increased the current chopping value but significantly lowered the anti-welding capability and dielectric strength. Chromium below 5% by weight significantly lowered the dielectric strength. On the other hand, chromium above 70% by weight significantly decreased the electrical conductivity and mechanical strength of the contact-making portion. Molybdenum below 5% by weight significantly lowered the dielectric strength. On the other hand, molybdenum above 70% by weight significantly lowered the mechanical strength of the contact-making portion but significantly increased the current chopping value. According to the second to 10th embodiments of the present invention, the increased tensile strength of the arc-diffusing portion significantly decreases a thickness and weight of the contact-making portion and considerably improves the durability of the contact-making portion. According to them, decreased the electrical conductivity of the arc-diffusing portion significantly decreases amount of eddy current, thus obviating the need for any slit to considerably increase the mechanical strength of the contact-electrode. Accordingly the arc-diffusing portion and the contact-making portion are prevented from excessively melting, thus resulting in a significantly decreased erosion of both the portions, because the arc-diffusing portion is made of complex metal of high hardness and including uniformly distributed constituents, and because the arc-diffusing portion includes no slit. Thus, the recovery voltage characteristic is improved and there is little lowering of dielectric strength even after many interruptions. For example, lowering of dielectric strength after 10,000 interruptions amounts to 10 to 20% of dielectric strength before interruption, thus decreasing current chopping value too. Figures 13A to 13D and Figures 14A to 14D show structures of complex metals for the arc-diffusing portion. According to 11th and 28th embodiments of the present invention, arc-diffusing portions 20 are made of complex metal consisting of 30 to 70% magnetic stainless steel by weight and 30 to 70% copper by weight. For example, ferritic stainless and martensitic stainless steels are used as a magnetic stainless steel. As a ferritic stainless steel, SUS405, SUS429, SUS430, SUS430F and SUS405 may be listed up. As a martensitic stainless steel, SUS403, SUS410, SUS416, SUS420, SUS431 and SUS440C may be listed up. The complex metal above consisting of 30 to 70% magnetic stainless steel by weight and 30 to 70% copper by weight, possesses at least 30 kgf/mm 2 (294 MPa) tensile strength and 100 to 180 Hv hardness. This complex metal possesses 3 to 30% IACS electrical conductivity when a ferritic stainless steel used, while 4 to 30% IACS electrical conductivity when a martensitic stainless steel used. Complex metals for the arc-diffusing portion 20 of the 11th to 28th embodiments of the present invention were produced by substantially the same as the first infiltration process. Contact-making portions 19 of contact-electrodes of the 11th to 28th embodiments of the present invention are made of the same complex metals as those for the contact-making portions of contact-electrodes of the 2nd to 10th embodiments of the present invention. Contact-making portions of contact electrodes of the 9th and 10th comparatives of the present invention are made of Cu-0.5Bi alloy. Contact-making portions of contact-electrodes of the 11th and 12th comparatives of the present invention are made of 20Cu-80W alloy. Referred to Figures 13A to 13D and Figures 14A to 14D which are photographs by the X-ray microanalyzer, structures of the complex metals for the arc-diffusing portion which were produced according to substantially the same process as the first infiltration process, will be described hereinafter. Example A 4 of a complex metal for the arc-diffusing portion possesses a composition consisting of a 50% ferritic stainless steel SUS434 by weight and 50% copper by weight. Figure 13A shows a secondary electron image of a metal structure of Example A 4 . Figure 13B shows a characteristic X-ray image of distributed iron, in which distributed white insular agglomerates indicate iron. Figure 13C shows a characteristic X-ray image of distributed chromium, in which distributed gray insular agglomerates indicate chromium. Figure 13D shows a characteristic X-ray image of infiltrant copper, in which white parts indicate copper. As apparent from Figures 13A to 13D, the particles of ferritic stainless steel SUS434 are bonded to each other, resulting in a porous matrix. Interstices of the porous matrix are infiltrated with copper, which results in a stout structure of the complex metal for the arc-diffusing portion. Example A 7 of the complex metal for the arc-diffusing portion possesses a composition consisting of a 50% martensitic stainless steel SUS410 by weight and 50% copper by weight. Figures 14A, 14B, 14C and 14D show similar images to those of Figures 13A, 13B, 13C and 13D, respectively. Structures of complex metals of Figures 14A to 14D are similar to those of Figures 13A to 13D. Example A 5 of the complex metal for the arc-diffusing portion possesses a composition consisting of a 70% ferritic stainless steel SUS434 by weight and 30% copper by weight. Example A 6 , 30% ferritic stainless steel SUS434 by weight and 70% copper by weight. Example A 8 , 70% martensitic stainless steel SUS410 by weight and 30% copper by weight. Example A 9 , 30% martensitic stainless steel SUS410 by weight 70% copper by weight. Examples A 5 , A 6 , A 8 and A 9 of the complex metal for the arc-diffusing portion were produced by substantially the same as the first infiltration process. Measurements of IACS electrical conductivity which were carried out on Examples A 4 to A 9 of the complex metal for the arc-diffusing portion and Examples C 1 to C 3 above of the complex metal for the contact-making portion established that: Example A 4 , 5 to 15% IACS electrical conductivity Example A 5 , 3 to 8% Example A 6 , 10 to 30% Example A 7 , 5 to 15% Example A 8 , 4 to 8% Example A 9 , 10 to 30% Example C 1 , 40 to 50% Example C 2 , 40 to 50% Example C 3 , 40 to 50%. Respective measurements of tensile strength and hardness established that Example A 4 of the complex metal for the arc-diffusing portion possessed 30 kgf/mm 2 (294 MPa) tensile strength and 100 to 180 Hv hardness. Examples A 4 to A 9 of the complex metal for the arc-diffusing portion 20 and Examples C 1 to C 3 of the complex metal for the contact-making portion 19 are respectively shaped to the same shapes as those of the arc-diffusing portion and the contact-making portion of the 2nd to 10th embodiments of the present invention, and tested as a pair of contact-electrodes in the same manner as in the 2nd and 10th embodiments of the present invention. Results of the test will be described hereinafter. A description shall be made on a vacuum interrupter of the 11th embodiment of the present invention which includes the pair of contact-electrodes each consisting of the arc-diffusing portion 20 made of Example A 4 and the contact-making portion 19 made of Example C 1 . An arc-diffusing portion 20 and a contact-making portion 19 of a contact-electrode of a 12th embodiment are made of respective Examples A 4 and C 2 . Those of a 13th, of Examples A 4 and C 3 . Those of a 14th, of Examples A 5 and C 1 . Those of a 15th, of Examples A 5 and C 2 . Those of a 16th, of Examples A 5 and C 3 . Those of a 17th, of Examples A 6 and C 1 . Those of an 18th, of Examples A 6 and C 2 . Those of a 19th, of Examples A 6 and C 3 . Those of a 20th, of Examples A 7 and C 1 . Those of a 21st, of Examples A 7 and C 2 . Those of a 22nd, of Examples A 4 and C 3 . Those of a 23rd, of Examples A 8 and C 1 . Those of a 24th, sixth, of Examples A 8 and C 2 . Those of a 25th, of Examples A 8 and C 3 . Those of a 26th, of Examples A 9 and C 1 . Those of a 27th, of Examples A 9 and C 2 . Those of a 28th, of Examples A 9 and C 3 . Those of a 9th comparative, of Example A 4 and Cu-0.5Bi alloy. Those of a 10th comparative, of Example A 7 and Cu-0.5Bi alloy. Those of an 11th comparative, of Example A 4 and 20Cu-80W alloy. Those of a 12th comparative, of Example A 4 and 20Cu-80W alloy. When performances of the vacuum interrupters of the 12th to 28th embodiments of the present invention differ from those of the 11th embodiment of the present invention, then different points shall be specified. 11) Large current interrupting capability Interruption tests which were carried out at an opening speed within 1.2 to 1.5 m/s under a rated voltage of 12 kV, however, a transient recovery voltage of 21 kV according to JEC-181, established that the test vacuum interrupters interrupted, 63 kA current. Moreover, interruption tests at an opening speed of 3.0 m/s under a rated voltage of 84 kV, however, a transient recovery voltage of 143 kV according to JEC-181, established that the test vacuum interrupters interrupted 52 kA current. Table 3 below shows the results of the large current interrupting capability tests. 12) Dielectric strength In accordance with JEC-181 test method, impulse withstand voltage tests were carried out with a 30 mm inter-contact gap. The results showed 400 kV withstand voltage against both positive and negative impulses with a ±10 kV deviation. After 10 times interrupting 63 kA current of rated 12 kV, the same impulse withstand voltage tests were carried out, thus establishing the same results. After continuously 100 times opening and closing a circuit through which 80A leading small current of rated 12 kV flowed, the same impulse withstand voltage tests were carried out, thus establishing substantially the same results. Table 4 below shows the results of the tests of the impulse withstand voltage at rated 84 kV which were carried out on the vacuum interrupters of the 11th embodiment of the present invention, and the 9th to 12th comparatives. 13) Anti-welding capability The same as in item 8). 14) Lagging small current interrupting capability In accordance with a lagging small current interrupting test of JEC-181, a 30A test current of 84x 1.5 3 kV was flowed through the stationary and movable contact-electrodes 24 and 13. Current chopping values had a 3.9A average (however, a standard deviation σ n =0.96 and a sample number n=100). In particular, current chopping values of the vacuum interrupters of the 12th, 15th, 18th, 21st, 24th and 27th embodiments of the present invention had a 3.7A (however, σ n =1.26 and n=100) average, and current chopping values of the vacuum interrupters of the 13th, 16th, 19th, 22nd, 24th and 28th embodiments of the present invention had a 3.9 (however, σ n =1.5 and n=100) average. 15) Leading small current interrupting capability The same as in item 10). The following limits were apparent on a composition ratio of magnetic stainless steel in the complex metal for the arc-diffusing portion of the 11th to 28th embodiments of the present invention. Magnetic stainless steel below 30% by weight significantly increased the electrical conductivity to generate large eddy current but lowered the mechanical strength and durability of the arc-diffusing portion 20, so that the arc-diffusing portion 20 had to be thickened. On the other hand, magnetic stainless steel above 70% by weight significantly lowered interruption performances. The 11th and 28th embodiments of the present invention effect the same advantages as the 2nd to 10th embodiments of the present invention do. Figures 15A to 15E show structures of the complex metals for the arc-diffusing portion 20 of the 29th to 37th embodiments of the present invention. Arc-diffusing portions 20 of the 29th to 37th embodiments of the present invention are made of complex metal consisting of 30 to 70% austinitic stainless steel by weight and 30 to 70% copper by weight. As an austinitic stainless steel, SUS304, SUS304L, SUS316 or SUS316L may be, for example, used. The complex metal consisting of 30 to 70% austinitic stainless steel by weight and 30 to 70% copper by weight possesses 4 to 30% IACS electrical conductivity, at least 30 kgf/mm 2 (294 MPa) tensile strength and 100 to 180 Hv hardness. The complex metal for the arc-diffusing portion 20 of the 29th to 37th embodiments of the present invention were produced by substantially the same process as the first infiltration process. Contact-making portions 19 of the 29th to 37th embodiments of the present invention are made of complex metal of the same composition as that of the complex metal of the 2nd to 10th embodiments of the present invention. Referred to Figures 15A to 15E which are photographs by the X-ray microanalyzer, structures of the complex metals for the arc-diffusing portion which were produced by substantially the same process as the first infiltration process, will be described hereinafter. Example A₁₀ of a complex metal for the arc-diffusing portion possesses a composition consisting of 50% austinitic stainless steel SUS304 by weight and 50% copper by weight. Figure 15A shows a secondary electron image of a metal structure of Example A₁₀. Figure 15B shows a characteristic X-ray image of distributed iron, in which distributed white insular agglomerates indicate iron. Figure 15C shows a characteristic X-ray image of distributed chromium, in which distributed gray insular agglomerates indicate chromium. Figure 15D shows a characteristic X-ray image of distributed nickel, in which distributed gray insular agglomerates indicate nickel. Figure 15E shows a characteristic X-ray image of infiltrant copper, in which white parts indicate copper. As apparent from Figures 15A to 15E, the particles of austinitic stainless steel SUS304 are bonded to each other, resulting in a porous matrix. Interstices of the porous matrix are infiltrated with copper, which results in a stout structure of the complex metal for the arc-diffusing portion. Example A₁₁ of the complex metal for the arc-diffusing portion possesses a composition consisting of 70% austinitic stainless steel SUS304 by weight and 30% copper by weight. Example A₁₂ of the complex metal for the arc-diffusing portion possesses a composition consisting of 30% austinitic stainless steel SUS304 by weight and 70% copper by weight. Measurements of IACS electrical conductivity which were carried out on Examples A₁₀ to A₁₂ of the complex metal for the arc-diffusing portion and Examples C 1 to C 3 above of the complex metal for the contact-making portion established that: Example A₁₀; 5 to 15% IACS electrical conductivity Example A₁₁, 4 to 8% Example A₁₂, 10 to 30% Examples A₁₀ to A₁₂ of the complex metal for the arc-diffusing portion 20 and Examples C 1 to C 3 of the complex metal for the contact-making portion 19 are respectively shaped to the same as those of the arc-diffusing portion and the contact-making portion of the 2nd to 10th embodiments of the present invention, and tested as a pair of contact-electrodes in the same manner as in the 2nd and 10th embodiments of the present invention. Results of the test will be described hereinafter. A description shall be made on a vacuum interrupter of the 29th embodiment of the present invention which includes the pair of contact-electrodes each consisting of the arc-diffusing portion 20 made of Example A₁₀, and the contact-making portion 19 made of Example C 1 . An arc-diffusing portion and a contact-making portion of a contact-electrode of a 30th embodiment are made of respective Examples A₁₀ and C 2 . Those of a 31 st of Examples A₁₀ and C 3 . Those of a 32nd, of Examples A₁₁ and C 1 . Those of a 33rd, of Examples A₁₁ and C 2 . Those of a 34th, of Examples A₁₁ and C 3 . Those of a 35th, of Examples A₁₂ and C 1 . Those of a 36th, of Examples A₁₂ and C 2 . Those of a 37th, of Examples A₁₂ and C 3 . When performances of the vacuum interrupters of the 30th to 37th embodiments of the present invention differ from those of the 29th embodiment of the present invention, then different points shall be specified. 16) Large current interrupting capability Interruption tests which were carried out at an opening speed within 1.2 to 1.5 m/s under a rated voltage of 12 kV, however, a transient recovery voltage of 21 kV according to JEC-181, established that the test vacuum interrupters interrupted, 60 kA current. Moreover, interruption tests at an opening speed of 3.0 m/s under a rated voltage of 84 kV, however, a transient recovery voltage of 143 kV according to JEC-181, established that the test vacuum interrupters interrupted 50 kA current. Table 5 below shows the results of the large current interrupting capability tests which were carried out on the vacuum interrupters of the 29th to 37th embodiments. Table 5 also shows those of vacuum interrupters of the 13th and 14th comparatives which include a pair of contact-electrodes each consisting of an arc-diffusing portion and a contact-making portion each having the same sizes as those of the arc-portions of the contact-electrodes of the 29th and 37th embodiments of the present invention. The arc-diffusing portion and the contact-making portion of the 13th comparative are respectively made of Example A₁₀ and 20Cu-80W alloy. Those of the 14th comparative, of Example A₁₀ and Cu-0.5Bi alloy. 17) Dielectric strength In accordance with JEC-181 test method, impulse withstand voltage tests were carried out with a 30 mm inter-contact gap. The vacuum interrupters showed 400 kV withstand voltage against both positive and negative impules with a ±10 kV deviation. After 10 times interrupting 60 kA current of rated 12 kV, the same impulse withstand voltage tests were carried out, thus establishing the same results. After continuously 100 times opening and closing a circuit through which 80A leading small current of rate 12 kV flowed, the same impulse withstand voltage tests were carried out, thus establishing substantially the same results. Table 6 below shows the results of the tests of the impulse withstand voltage at rated 84 kV tests which were carried out on the vacuum interrupters of the 29th embodiment of the present invention and on them of the 13th and 14th comparatives. 18) Anti-welding capability The same as in item 8). 19) Lagging small current interrupting capability In accordance with a lagging small current interrupting test of JEC-181, a 30A test current of 84x 1.5 3 kV was flowed through the stationary and movable contact-electrodes 24 and 13. Current chopping values had a 3.9A average (however, σ n =0.96 and n=100). In particular, current chopping values of the vacuum interrupters of the 30th, 33rd and 36th embodiments of the present invention had a 3.7A average (however, σ n =1.26 and n=100), and those of the 31st, 34th and 37th embodiments of the present invention had a 3.9A average (however σ n =1.5 and n=100). 20) Leading small current interrupting capability The same as in item 10). The following limits were apparent on a composition ratio of austinitic stainless steel in the complex metals for the arc-diffusing portion of the 29th to 37th embodiments of the present invention. Austinitic stainless steel below 30% by weight significantly increased the electrical conductivity to generate large eddy current but lowered the mechanical strength and durability of the arc-diffusing portion 20, so that the arc-diffusing portion 20 had to be thickened. On the other hand, austinitic stainless steel above 70% by weight significantly lowered interruption performances. The vacuum interrupters of the 29th to 37th embodiments of the present invention possess more current interrupting capability than a conventional vacuum interrupter of the axial magnetic field type and dielectric strength as high as that of the vacuum interrupter of the 13th comparative. Arc-diffusing portions 20 of the 38th and 40th embodiments are each made of complex metal consisting of a porous structure of austinitic stainless steel including many holes of axial direction through the arc-diffusing portions 20 at an areal occupation ratio of 10 to 90%, and copper or silver infiltrating the porous structure of austinitic stainless steel. This metal composition possesses 5 to 30% IACS electrical conductivity, at least 30 kgf/mm 2 (294 MPa) tensile strength and 100 to 180 Hv hardness. Complex metals for the arc-diffusing portion of the 38th to 40th embodiments of the present invention were produced by the following process. The third infiltration process At first, a plurality of pipes of austinitic stainless steel, e.g., SUS304 or SUS316 and each having an outer-diameter within 0.1 to 10 mm and a thickness within 0.01 to 9 mm are heated at a temperature below a melting point of the austinic stainless steel in a nonoxidizing atmosphre, e.g., a vacuum, or hydrogen, nitrogen or argon gas, thus bonded to each other so as to form a porous matrix of a circular section. Then, the resultant porous matrix of the circular section is placed in a vessel made of material, e.g., alumina ceramics, which interacts with none of the austinitic stainless steel, copper and silver. All the bores of the pipes and all the interstices between the pipes are infiltrated with copper or silver in the nonoxidizing atmosphere. After cooling, a desired complex metal for the arc-diffusing portion was resultant. The fourth infiltration process In place of the pipes in the third infiltration process, a plate of austinitic stainless steel and including many holes at an areal occupation ratio of 10 to 90% is used as a porous matrix. On the same subsequent steps as those of the third infiltration process, a desired complex metal for the arc-diffusing portion was resultant. Contact-making portions of the 38th to 40th embodiments of the present invention are made of complex metal of the same composition as that of the complex metal of the 2nd to 10th embodiments of the present invention. Example A₁₃ of a complex metal for the arc-diffusing portion possesses a composition consisting of 60% austinitic stainless steel SUS304 by weight and 40% copper by weight. Example A₁₃ of the complex metal for the arc-diffusing portion 20 and Examples C 1 to C 3 above of the complex metal for the contact-making portion were respectively shaped to the same as those of the arc-diffusing portion 20 and the contact-making portion 19 of the 2nd embodiment of the present invention, and tested as a pair of contact-electrodes in the same manner as in the 2nd and 10th embodiments of the present invention. Results of the tests will be described hereinafter. A description shall be made on a vacuum interrupter of the 38th embodiment of the present invention which includes the pair of contact-electrodes each consisting of the arc-diffusing portion made of Example A₁₃, and the contact-making portion made of Example C 1 . An arc-diffusing portion and a contact-making portion of a contact-electrode of the 39th embodiment are made of respective Examples A₁₃ and C 2 . Those of the 40th, of Examples A₁₃ and C 3 . When performances of the vacuum interrupters of the 39th and 40th embodiments of the present invention differ from those of the 38th embodiment of the present invention, then different points shall be specified. 21 ) Large current interrupting capability Interruption tests which were carried out at an opening speed within 1.2 to 1.5 m/s under a rated voltage of 12 kV, however, a transient recovery voltage of 21 kV according to JEC-181, established that the test vacuum interrupters interrupted 65 kA current. Moreover, interruption tests at an opening speed of 3.0 m/s under a rated voltage of 84 kV, however, a transient recovery voltage of 143 kV according to JEC-181, established that the test vacuum interrupters interrupted 55 kA current. Table 7 below shows the results of the large current interrupting capability tests. Table 7 also shows those of the vacuum interrupters of the 15th and 16h comparatives which include a pair of contact-electrodes each consisting of an arc-diffusing portion and a contact-making portion each having the same sizes as those of the arc-diffusing portions and the contact-making portions of the contact-electrodes of the 3rd to 8th comparatives. The arc-diffusing portion and the contact making portion of the 15th comparative are respectively made of Example A₁₃ and 20Cu-80W alloy. Those of the 16th comparative, of Example A₁₃ and Cu-0.5Bi alloy. 22) Dielectric strength In accordance with JEC-181 test method, impulse withstand voltage tests were carried out with a 30 mm inter-contact gap. The results showed 400 kV withstand voltage against both positive and negative impules with a ±10 kV deviation. After 10 times interrupting 65 kA current of rated 12 kV, the same impulse withstand voltage tests were carried out, thus establishing the same results. After continuously 100 times opening and closing a circuit through which 80A leading small current of rated 12 kV flowed, the same impulse withstand voltage tests were carried out, thus establishing substantially the same results. Table 8 below shows the results of the tests of the impulse withstand voltage at rated 84 kV tests which were carried out on the vacuum interrupters of the 38th embodiment of the present invention and those of the 15th and 16th comparatives. 23) Anti-welding capability The same as in item 8) 24) Lagging small current interrupting capability The same tests as in item 19) established that the vacuum interrupters of the 38th, 39th and 40th embodiments of the present invention had respective 3.9A (σ n =0.96 and n = 100), 3.7A (σ n = 1.26 and n= 100) and 3.9A (σ n =1.5 and n=100) averages of current chopping value. 25) Leading small current interrupting capability The same as in item 10). In the complex metal for the arc-diffusing portion of the 38th to 40th embodiments of the present invention, the areal occupation ratio below 10% of many holes of axial direction in the plate of austinitic stainless steel significantly decreased the current interrupting capability, on the other hand, the areal occupation ratio above 90% thereof significantly decreased the mechanical strength of the arc-diffusing portion and the dielectric strength of the vacuum interrupter. The vacuum interrupters of the 38th and 40th of the present invention possess more improved high current interrupting capability than those of other embodiments of the present invention. A vacuum interrupter of an axial magnetic field applying type of the present invention, of which a contact-making portion of a contact-electrode is made of complex metal consisting of 20 to 70% copper by weight, 5 to 70% chromium by weight and 5 to 70% molybdenum by weight and of which an arc-diffusing portion of the contact-electrode is made of material below, possesses more improved large current interrupting capability, dielectric strength, anti-welding capability, and lagging and leading small current interrupting capabilities than a conventional vacuum interrupter of an axial magnetic feld applying type. There may be listed as a material for an arc-diffusing portion austinitic stainless steel of 2 to 3% IACS electrical conductivity, at least 49 kgf/mm 2 (481 MPa) tensile strength and 200 Hv hardness, e.g., SUS304 or SUS316, ferritic stainless steel of about 2.5% IACS electrical conductivity, at least 49 kgf/mm 2 (481 MPa) tensile strength and 190 Hv hardness, e.g., SUS405, SUS429, SUS430, SUS430F or SUS434, martensitic stainless steel of about 3.0% IACS electrical conductivity, at least 60 kgf/mm 2 (588 MPa) tensile strength and 190 Hv hardness, e.g., SUS403, SUS410, SUS416, SUS420, SUS431 or SUS440C, a complex metal of 5 to 9% IACS electrical conductivity, at least 30 kgf/mm 2 (294 MPa) tensile strength and 100 to 180 Hv hardness in which iron, nickel or cobalt, or an alloy as magnetic material including a plurality of holes of axial direction through an arc-diffusing portion at an areal occupation ratio of 10 to 90%, are infiltrated with copper or silver, a complex metal of 2 to 30% IACS electrical conductivity consisting of 5 to 40% iron by weight, 5 to 40% chromium by weight, 1 to 10% molybdenum or tungsten by weight and a balance of copper, a complex metal of 3 to 30% IACS electrical conductivity consisting of 5 to 40% iron by weight, 5 to 40% chromium by weight, molybdenum and tungsten amounting in total to 1 to 10% by weight and either one amounting to 0.5% by weight, and a balance of copper, a complex metal of 3 to 25% IACS electrical conductivity consisting of a 29 to 70% austinitic stainless steel by weight, 1 to 10% molybdenum or tungsten by weight, and a balance of copper, a complex metal of 3 to 25% IACS electrical conductivity consisting of a 29 to 70% ferritic stainless steel by weight, 1 to 10% molybdenum or tungsten by weight, and a balance of copper, a complex metal of 3 to 30% IACS electrical conductivity consisting of a 29 to 70% martensitic stainless steel by weight, 1 to 10% molybdenum or tungsten by weight, and a balance of copper, a complex metal of 3 to 30% IACS electrical conductivity consisting of a 29 to 70% austinitic stainless steel by weight, molybdenum and tungsten amounting in total to 1 to 10% by weight and either one amounting to 0.5% by weight, and a balance of copper, a complex metal of 3 to 30% IACS electrical conductivity consisting of a 29 to 70% martensitic stainless steel by weight, molybdenum and tungsten amounting in total to 1 to 10% by weight and either one amounting to 0.5% by weight, and a balance of copper, and a complex metal of 3 to 25% IACS electrical conductivity consisting of a 29 to 70% ferritic stainless steel by weight, molybdenum and tungsten amounting in total to 1 to 10% by weight and either one amounting to 0.5% by weight, and a balance of copper. The complex metal listed above are produced by substantially the same process as the first, second, third or fourth infiltration or sintering process. As used in this specification the term "IACS" will be understood to be an abbreviation of "international annealed copper standard", i.e., an annealed electrolytically refined copper processing a purity of 99,96 to 99,98, which exhibits a resistivity of 1,7241 (µ) ohm cm at 20°C. The conductivity of the IACS is defined as 100, and the percentage values used in this specification are percentages of this value. The term "IACS electrical conductivity" is used to define percentage conductivity. The term "percentage conductivity" may sometimes be more popular than the term "IACS electrical conductivity".

Claims: A vacuum interrupter comprising a pair of separable contact-electrodes (13, 24) each of which consists of a disc-shaped arc-diffusing portion (20) and a contact-making portion (19) projecting from a central portion of an arcing surface of the arc-diffusing portion (20); a vacuum envelope (4) which is electrically insulating and encloses the contact-electrodes (13, 24); and means for applying a magnetic field (14, 30) in parallel to an arc established between the contact-electrodes (13, 24) when said contact electrodes are separated, characterized in that said arc-diffusing portion (20) of at least one of the contact electrodes (13, 24) is made of a first material having an electrical conductivity in the range from 2 to 30 % IACS and said contact-making portion (19) of said at least on contact electrode is made of a second material having an electrical conductivity in the range from 20 to 60 % IACS, said conductivity of the arc-diffusing portion (20) being lower than said conductivity of the contact-making portion (19) said arc-diffusing portion being brazed directly or via an annular hub (44) to said contact making portion. A vacuum interrupter as defined in Claim 1, wherein said arc-diffusing portion (20) is made of complex metal consisting of 20 to 70% copper by weight, 5 to 40% iron by weight and 5 to 40% chromium by weight. A vacuum interrupter as defined in Claim 1, wherein said arc-diffusing portion (20) is made of material including copper, iron and chromium, and said contact-making portion (19) is made of complex metal consisting of copper, chromium and molybdenum. A vacuum interrupter as defined in Claim 1, wherein said arc-diffusing portion (20) is made of material of 10 to 15% IACS electrical conductivity. A vacuum interrupter as defined in Claim 1, wherein said contact-making portion (19) is made of complex metal consisting of 20 to 70% copper by weight, 5 to 70% chromium by weight and 5 to 70% molybdenum by weight. A vacuum interrupter as defined in Claim 1 or Claim 5, wherein said arc-diffusing portion (20) is made of complex metal consisting of 30 to 70% copper by weight and 30 to 70% by weight nonmagnetic stainless steel. A vacuum interrupter as defined in Claim 1, wherein said arc-diffusing portion (20) is made of complex metal consisting of 30 to 70% copper by weight and a 30 to 70% magnetic stainless steel by weight. A vacuum interrupter as defined in Claim 7, wherein said magnetic stainless steel comprises ferritic stainless steel. A vacuum interrupter as defined in Claim 7, wherein said magnetic stainless steel comprises martensitic stainless steel. A vacuum interrupter as defined in any one of Claims 7, 8 and 9, wherein said contact-making portion (19) is made of complex metal consisting of 20 to 70% copper by weight, 5 to 70% chromium by weight and 5 to 70% molybdenum by weight. A vacuum interrupter as defined in Claim 1, wherein said arc-diffusing portion (20) is made of complex metal consisting of a magnetic or nonmagnetic stainless steel including a plurality of holes of axial direction through said arc-diffusing portion (20) at an areal occupation ratio of 10 to 90%, and infiltrant copper or silver into the magnetic or nonmagnetic stainless steel, and wherein said contact-making portion (19) is made of complex metal consisting of 20 to 70% copper by weight, 5 to 70% chromium by weight and 5 to 70% molybdenum by weight. A vacuum interrupter as defined in Claim 1, wherein said arc-diffusing portion (20) is made of austinitic stainless steel of 2 to 3% IACS electrical conductivity. A vacuum interrupter as defined in Claim 1, wherein said arc-diffusing porton (20) is made of ferritic stainless steel of about 2.5% IACS electrical conductivity. A vacuum interrupter as defined in Claim 1, wherein said arc-diffusing portion is made of martensitic stainless steel of about 3.0% IACS electrical conductivity. A vacuum interrupter as defined in Claim 1, wherein said magnetic field applying means (14, 15) comprises a coil-electrode (15) positioned part from and behind said arc-diffusing portion (20) and an electrical lead member (14) for the coil-electrode, which is made of material of electrical conductivity higher than that of the material for said arc-diffusing portion (20), electrically connected to the coil-electrode (15) and all the portions (22, 25, 26) of which are mechanically and electrically connected to a backsurface of said arc-diffusing portion (20). A method of manufacturing a vacuum interrupter as defined in Claim 1, wherein said arc-diffusing portion (20) is produced by the steps of: a) placing together in a vessel powder of no more than 250 µm particle size of at least one metal possessing a melting point higher than that of copper and a copper bulk; b) heat holding the metal powder and the copper bulk at a temperature below the melting point of copper in a nonoxidizing atmosphere to produce a porous matrix from the metal powder; and c) heat holding the resultant porous matrix and the copper bulk at a temperature of at least the melting point of copper but below that of the porous matrix in a nonoxidizing atmosphere to infiltrate the porous matrix with molten copper. A method of manufacturing a vacuum interrupter as defined in Claim 1, wherein said arc-diffusing portion (20) is produced by the steps of: d) placing in a vessel powder of no more than 250 µm particle size of powder of at least one metal possessing a melting point higher than that of copper; e) heat holding the metal powder at a temperature below the melting point of the metal other than copper in a nonoxidizing atmosphere to produce a porous matrix; f) placing a copper bulk and the resultant porous matrix together in the vessel; and g) heat holding the porous matrix and the copper bulk at a temperature of at least the melting point of copper but below that of the porous matrix in a nonoxidizing atmosphere to infiltrate the porous matrix with molten copper. A method of manufacturing a vacuum interrupter as defined in Claim 1, wherein said arc-diffusing portion (20) is produced by the steps of: h) press-shaping into a green compact blended powders of no more than 250 µm particle size consisting of copper and other metal possessing a melting point higher than that of copper; and i) sintering the green compact at a temperature below the melting point of the other metal in a nonoxidizing atmosphere. A method of manufacturing a vacuum interrupter as defined in Claim 1, wherein said arc-diffusing portion (20) is produced by the steps of: j) heat holding in a nonoxidizing atmosphere a plurality of pipes, each of which is made of metal possessing a melting point higher than that of copper, placed in parallel to each other, at a temperature below the melting point of the metal other than copper to be bonded into a porous matrix; k) placing a solid copper bulk and the resultant porous matrix together; and l) heat holding the porous matrix and the solid copper bulk at a temperature of at least the melting point of copper but below that of the porous matrix in a nonoxidizing atmosphere to infiltrate the porous matrix with molten copper. A method of manufacturing a vacuum interrupter as defined in Claim 1, wherein said arc-diffusing portion (20) is produced by a step of heat holding in a nonoxidizing atmosphere a porous plate of metal possessing a melting point higher than that of copper and a solid copper bulk together at a temperature of at least the melting point of copper but below a melting point of the metal other than copper.

Inventors: KASHIWAGI, YOSHIYUKI, KITAKIZAKI, KAORU, NODA, YASUSHI
Applicants: KABUSHIKI KAISHA MEIDENSHA
Application year: 1984
Main classification: H01H33
IPC codes: H01H33, H01H11, H01H1